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Notes: Molecular dynamics simulation was carried out to investigate the stick-slip on an atomic scale by using the embedded atom method potential for Ni–Al. The analyses of the dynamic features of the atoms in the sliding block clearly show that the elastic deformation of the surface layers is the main cause for the stick-slip phenomenon, which is consistent with the macroscopic stick-slip. The simulation results also indicate that phonons are emitted during stick-slip, and a commensurate fit between the contacting surfaces is not significant for the stick-slip friction. © 2001 American Institute of Physics.

Notes: A modified atomic force microscopy (AFM) system, based on a force modulation technique, has been used to find an approximate value for the elastic modulus of a single peptide molecule directly from a mechanical test. For this purpose a self-assembled monolayer built from two kinds of peptides, reactive (able to anchor to the AFM tip) and nonreactive, was synthesized. In a typical experiment a single C3K30C (C=cysteine, K=lysine) peptide molecule was stretched between a Au(111) substrate and the gold-coated tip of an AFM cantilever to which it was attached via gold–sulfur bonds. The amplitude of the cantilever oscillations, due to an external force applied via a magnetic particle to the cantilever, was recorded by a lock-in amplifier and recalculated into stiffness of the stretched molecule. A longitudinal Young's modulus for the α-helix of a single peptide molecule and for the elongated state of this molecule has been estimated. The obtained values; 1.2±0.3 and 50±15 GPa, for the peptide α-helix and elongated peptide backbone, respectively, seem to be reasonable comparing them to the Young's modulus of protein crystals and linear organic polymers. We believe this research opens up a means by which scientists can perform quantitative studies of the elastic properties of single molecule, especially of biologically important polymers like peptides or DNA. © 2001 American Institute of Physics.

Notes: The formation energies of planar defects such as lamellar twins and intrinsic and extrinsic stacking faults in CdTe are calculated using first-principles total-energy calculations. We find that the formation energies, 16 erg/cm2 for lamellar twins and 34 and 31 erg/cm2 for intrinsic and extrinsic stacking faults, are very small. This explains why high densities of planar defects are always present in the fast-grown CdTe thin films. The effects of the planar defects on the formations of important point defects in p-type CdTe are also investigated. We find that the planar defects have negligible effects on Cd vacancies and substitutional Cu, whereas they lower the formation energy of Te antisites by about 0.1 eV compared to the perfect regions. © 2001 American Institute of Physics.

Notes: We have analyzed a series of data sets available from published literature for the temperature dependence of A and B exciton peak positions associated with the fundamental band gap of hexagonal GaN layers grown on sapphire. In this article, in contrast to preceding ones, we use the dispersion-related three-parameter formula Eg(T)=Eg(0)−(αaitch-theta/2)[(1+(π2/6)(2T/aitch-theta)2+(2T/aitch-theta)4)1/4−1], which is a very good approximation in particular for the transition region between the regimes of moderate and large dispersion. This formula is shown here to be well adapted to the dispersion regime frequently found in hexagonal GaN layers. By means of least-mean-square fittings we have estimated the limiting magnitudes of the slopes, S(T)≡−dEg(T)/dT, of the Eg(T) curves published by various experimental groups to be of order α≡S(∞)(approximate)(5.8±1.0)×10−4 eV/K. The effective phonon temperature has been found to be of order aitch-theta(approximate)(590±110) K, which corresponds to an ensemble-averaged magnitude of about 50 meV for the average phonon energy. The location of the latter within the energy gap between the low- and high-energy subsections of the phonon energy spectrum of h-GaN suggests that the weights of contributions made by both subbands to the limiting slope α are nearly the same. This explains the order of Δ(approximate)0.5–0.6 as being typical for the dispersion coefficient of the h-GaN layers under study. The inadequacies of both the Bose–Einstein model (corresponding to the limiting regime of vanishing dispersion Δ→0) and Varshni's ad hoc formula (corresponding to a physically unrealistic regime of excessively large dispersion Δ(approximate)1) are discussed. Unwarranted applications of these conventional models to numerical fittings, especially of unduly restricted data sets (T≤300 K), are identified as the main cause of the excessively large scatter of parameters quoted for h-GaN in various recent articles. © 2001 American Institute of Physics.

Notes: In this article, we present a theory of the pulse train generated by a rational harmonic mode locked ring fiber laser. The pulse width is calculated as a function of the rational harmonic order and the optical transfer function of the modulator. The theoretical work is based on a time domain analysis, which predicts that the pulse width decreases when the rational harmonic order goes up. The pulse width as a function of the modulation amplitude and bias level of the modulator was measured, and the experimental results agree with the theory. © 2001 American Institute of Physics.

Notes: Using a dual-layered photoconductor, we have investigated the primary photocarrier generation process in x-form metal-free phthalocyanine (x-H2Pc) over a wide range of illumination wavelengths. According to the results of quantum efficiency measurements, it has been established that the photocarrier generation mechanism in x-H2Pc occurs via two processes: (i) production of an intermediate that depends solely on the excitation energy, and (ii) subsequent free carrier production in the presence of an electric field. In addition, the spectral quantum efficiency and the electroabsorption spectrum were measured and compared. Based on that, the primary process efficiency was divided into four regions in terms of photon energy, which explained well the relationship between them. The excitation energy dependence of the primary efficiency was semiquantitatively validated based on the electron transfer theory. © 2001 American Institute of Physics.

Notes: The Hall effect and electrical resistivity of n-type CuInSe2 single crystals are measured between 4.2 and 300 K. Using a single conduction band model, the variation of the electron concentration with temperature above 100 K is explained in terms of the thermal activation of a shallow donor. The density of states effective mass me*=0.09me of the electrons, the activation energy of the donors around 7 meV, their concentration, and the compensation ratio are estimated. The temperature dependence of the electron mobility in conduction band is analyzed by taking into account the scattering of the charge carriers by ionized impurities and acoustic and polar optical phonon modes. The adjustable parameters, thus obtained, are compared with those reported earlier. On the other hand, by considering the two-band model with electrons in both the conduction and impurity bands, the change in the Hall coefficient with temperature between 300 and 40 K is explained. It is found that at the temperature where the Hall coefficient is maximum, the mobility in the impurity band is about 20% as compared to its value in the conduction band. The width of the impurity band is found to increase with increasing impurity concentration and the electron mobility below 20 K is explained by considering the effect of Mott-type variable range hopping conduction. © 2001 American Institute of Physics.

Notes: A theory for the current noise associated with carrier generation and recombination in the space-charge region of p–n junctions is presented. We propose a noise model based on the response of the electric field to fluctuations of the trapped charge in this region, and make use of a collective transport-noise theory. The effects of the fluctuations of the space-charge region borders due to the fluctuations of the trapped charge are now taken into account. This new contribution is negligible when generation–recombination current governs the diode current. However, it is significant when diffusion current dominates, allowing the analytical study of generation–recombination noise to be extended to wider ranges of bias and temperature. Experimental results at low and high temperatures are explained with our theory. Empirical formulas of current noise density are also explained according to this complete theory of current noise calculation. © 2001 American Institute of Physics.

Notes: WO3 films, either prepared by sputtering or evaporation under high or ultrahigh vacuum conditions, were irradiated with He+ and Ar+ ions (energy range 300–350 keV) at ambient and low temperatures (77–100 K). The resulting ion induced changes of the optical absorption as well as of the electrical conductivity could be determined on one and the same sample, which enables the variable range hopping (VRH) model to be tested under the assumption that the density of irradiation induced color centers is proportional to the electronic density of states contributing to the hopping conductivity. It is found that the data obtained at 300 K for He+ and Ar+ bombardment can be described within the VRH model by one common conductivity versus absorption curve, even though the effectiveness per projectile of the heavier ion for coloration as well as for increasing the conductivity is much higher. This is different at low temperatures. While the ion induced coloration is practically independent of the irradiation temperature for both projectiles, the effectiveness per projectile to enhance the conductivity is interchanged. This is attributed to the additional damage produced by the heavier ion at low temperatures resulting in strongly impeded hopping processes. Consistent with the VRH model, the temperature dependence of the conductivity of ion bombarded WO3 films follow the Mott "T−1/4" law, if the ion induced conductivity is not too high. For very high ion fluences clear deviations from the VRH model are observed for the conductivity versus absorption curves accompanied by a shift of the above power laws from T−1/4 towards T−1/2. © 2001 American Institute of Physics.

Notes: Transmission electron microscopy (TEM) and selected area electron diffraction pattern (SADP) measurements were carried out to investigate the ordered structures near ZnTe/GaAs heterointerfaces, and Auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS) measurements were performed to determine the compositions of the ZnTe/GaAs interfacial layer. The SADP showed two sets of {〈fraction SHAPE="CASE"〉12 〈fraction SHAPE="CASE"〉12 〈fraction SHAPE="CASE"〉12} extra spots with symmetrical intensity, and the corresponding high-resolution TEM image showed doublet periodicity in contrast of the {111} lattice planes. The results of the SADP and the high-resolution TEM measurements showed that a CuPt-type ordered (Cd, Zn)Te structure was observed near the ZnTe/GaAs heterointerface, and the AES and SIMS results showed that the ordered structure was formed due to the diffusion of Cd atoms into the ZnTe layer. Two variants, one for each direction of the doublet periodicity on the {111} lattice, were observed in the ordering, and each variant had its own domain structure with a similar probability. The formation of the CuPt-type ordered structure near the ZnTe/GaAs heterointerface originated from both the existence of the Cd residual impurities during the initial growth stage of the ZnTe epilayer and the strain relaxation of the ZnTe epilayer. These results can help to improve the understanding of the microstructural properties of the ZnTe/GaAs heterointerface. © 2001 American Institute of Physics.

Notes: Discontinuous multilayered Co80Fe20(t)/Al2O3(30 Å) thin films have been prepared by ion-beam sputtering. We report on structural, magnetic, and transport (for current in plane geometry) results obtained in this system. With growing nominal thickness t of the metal layers, which effectively characterizes the granular structure, a transition from tunnel to metallic conductance is observed, indicating the onset of infinite conducting paths at t〉18 Å. At t〈18 Å, that is within the range of tunnel regime, a different characteristic value t〉13 Å was detected from the magnetization data which display here a transition from superparamagnetic to ferromagnetic behavior. The measurements of tunnel magnetoresistance (MR) show that a sharp maximum of MR sensitivity to field takes place at this thickness, reaching ∼24%/kOe at room temperature. At least, MR itself as a function of t has a break at the same value. All these features suggest that some specific kind of percolation with respect to magnetic order occurs in our system when the disordered granular structure is still well separated, as confirmed by the data of high resolution transmission electron microscopy. Hence such magnetic percolation is clearly distinct from usual electrical percolation in these discontinuous layers. At the same time, the highest MR (∼6.5% at room temperature) in this series is attained with decreasing t only at t=10 Å. © 2001 American Institute of Physics.

Notes: We have investigated the behavior of high-critical-temperature (high Tc) direct-coupled superconducting quantum interference device (SQUID) magnetometers in static and fluctuating magnetic fields. The magnetometers consist of narrow-linewidth superconducting films to prevent flux trap during field cooling. Moreover, they have no superconducting films crossing the bicrystal lines of the substrates (except at the Josephson junctions); i.e., they have no flux dams. When one of these magnetometers was cooled in a static magnetic field Bcool, the low-frequency noise when Bcool〈100 μT was as low as that under zero-field cooling, but above 100 μT the noise increased substantially. On the other hand, when a field Bext of less than 4 μT was applied after zero-field cooling, the low-frequency noise increased in proportion to Bext. It returned to its original value reversibly when Bext was turned off. However, when Bext was greater than or equal to 4 μT, the output of the flux-locked-loop started to drift with time and the low-frequency noise increased further. This additional noise increase remained after turning off Bext. These results suggested that there are two modes of increase for the low-frequency noise induced by flux penetration due to the shielding current: a "reversible" mode and an "irreversible" mode. We found that the low-frequency noises of the two modes were additive with respect to their power, suggesting that the two noises derived from independent sources at different sites on the magnetometer. We also found that the reversible-mode noise could be reduced by improving the profile of the film edge. © 2001 American Institute of Physics.

Notes: Cluster glass and relatively high coercivity at low temperatures were found in disordered ultrafine nickel ferrite powders. High-energy mechanical milling of spinel NiFe2O4 led to formation of a wüstitelike structure. Our investigation suggested that ferrimagnetic clusters formed in an antiferromagnetic matrix. The strong ferri/antiferromagnetic exchange coupling resulted in a strong unidirectional anisotropy and a coercivity of over 10 kOe at 4.2 K. © 2001 American Institute of Physics.

Notes: We have studied the effect of Sm on the magnetic surface anisotropy (MSA) of Pd/Co100−xSmx/Pd (111) trilayers, where Sm content x was varied from 0 to 11. All samples show perpendicular anisotropy due to the strong MSA at the interfaces. The MSA significantly increases with x and attains a maximum value of 0.60 erg/cm2 at x(similar, equals)8.3, which is 36% larger than that of pure Co (x=0). The volume term of the magnetic anisotropy shows a similar behavior as the MSA. The appreciable increase in MSA is considered to be due to the enhancement of orbital moment of Co by the addition of Sm. © 2001 American Institute of Physics.

Notes: Ferroelectric twin-domain structures in epitaxial Pb(Zr, Ti)O3 (PZT) thin films grown on various single-crystal substrates such as MgO(001), KTaO3(001), and SrTiO3(001) were investigated by two-dimensional reciprocal space mapping using synchrotron x-ray diffraction. Each system showed a characteristic domain structure. PbTiO3 thin films grown on MgO(001) showed highly c-axis oriented domain structures consisting of a periodic array of 90° twinlike domains. Perfectly c-axis oriented films were obtained on SrTiO3(001), while the films grown on KTaO3(001) showed a-domain dominant structures with a small amount of c domains embedded in matrix a domains. Contributions of net elastic strain stored in each heteroepitaxial layer and its relaxation to the final domain structures were evaluated considering thermodynamic equilibrium relief of coherency strain by misfit dislocation generation at the film growth temperature. A comparison between theoretical consideration and experimental results clearly demonstrates that the nature of effective misfit strain and its relaxation during film growth play a critical role in the formation of domain structures in epitaxial PZT thin films. Moreover, it was verified that the control of such critical strain factors by changing film composition could modify dominant domain structures in a drastic way. In addition, it was found that the crystalline quality of the films is closely correlated to the tilting nature of the domain structure in each system and coherency strain across the 90° domain boundary is accommodated mainly by the domain tilt of the minor domain. © 2001 American Institute of Physics.

Notes: In this article an interesting result has been reported to highlight the physical implication of finite nonzero electron inertial delay in acoustic oscillations of an ion-beam plasma system. A simple fluid model has been used to demonstrate the excitation of relaxation type resonant instability on the electron inertial delay time scale. Physical arguments have been included to understand and explain the driving mechanism of the instability and its utility to comprehend the ion-beam driven oscillations in plasma sheath experiments. The physical nature and origin of the potential relaxation instability has been correlated to the electron inertial delay effect in the ion current carrying plasma system. © 2001 American Institute of Physics.

Notes: We report on the effect of Ar addition to an O2 plasma on photoresist etching in an inductively coupled, traveling wave driven, large area plasma source (LAPS). We also develop a simplified spatially varying O2/Ar mixture discharge model corresponding to the LAPS in a two-dimensional geometry in order to account for the effect of Ar addition. A photoresist etch kinetics model and spatially varying O2/Ar mixture discharge model are used to explain the experimental data. We find that the addition of 50% Ar increases the plasma density and etch rate approximately by a factor of 2. From the simulation we find that argon metastables (Ar*) play an important role in the mixture plasma. The simulation predicts an enhancement in O-atom density due to Ar addition, even in the presence of dilution of the feed gas. The experimental data and predicted etch rates from the simulation are generally in good agreement, indicating that the increase in the etch rate with Ar addition is due to both the increase in the plasma density and the enhancement in O-atom density attributable to the dissociation of O2 by Ar*. © 2001 American Institute of Physics.

Notes: An experimental investigation of a single-gap pseudospark was conducted using a flexible discharge chamber. The voltage breakdown characteristics were studied against a wide range of parameters such as gas pressure, gap separation, cathode cavity depth, cathode aperture size, external capacitance and applied voltage. An empirical formula, VB=(0.20±0.05) p−4.02±0.18 d−1.77±0.01, was obtained for the breakdown voltage VB in kV, given the gas pressure p in Torr and the gap separation d in mm. The electron beam extracted from this single gap was also studied and a current of up to 100 A was measured at 10 kV. © 2001 American Institute of Physics.

Notes: Micrometric, irregularly shaped Fe particles with a nanocrystalline structure have been prepared by mechanical attrition through ball-milling. Electron holography has been employed to visualize the stray field emerging from isolated Fe particles, both at 300 K and at selected temperatures T≤1200 K, from which indirect information on the magnetic domain configuration has been inferred. By complementary x-ray diffraction and transmission electron microscopy investigations a relationship has been established between the changes of the leakage field and of the microstructure upon annealing: it indicates that the structural evolution is accompanied by strong modifications in the interior magnetization pattern. This relationship finds explanation in the framework of the random anisotropy model, including temperature-induced reversible variations in the exchange correlation length and saturation magnetization. Moreover, the role played by the overall geometrical features of the particles in the determination of the actual domain configuration has been investigated. © 2001 American Institute of Physics.

Notes: Insulating nanogranular-type tunnel magnetoresistive thin films made of (Fe or Fe–Co)–(Mg-fluoride) have been investigated. The films were prepared by a tandem deposition method, using Fe, Co, or Fe+Co metal and MgF2 insulator targets. The granular structure was found to consist of Fe or Fe–Co based nanogranules surrounded by thin intergranules of Mg based fluoride with the MgF2 crystal structure. A magnetoresistance value of 13.3% at room temperature and 10 kOe, the largest values ever reported, were obtained at the compositions of 32 vol %(Fe0.51Co0.49)–(Mg–F). To increase the magnetic field sensitivity of the magnetoresistance, a granular-in-gap film consisting of an (Fe–Co)–(Mg–F) granular thin film filling a narrow gap in a soft magnetic Permalloy thin film was prepared. A remarkably high magnetoresistance of 4% or more at 1–2 Oe was obtained. © 2001 American Institute of Physics.

Notes: In this work we present a detailed structural characterization by Raman spectroscopy of hydrogenated amorphous silicon (a-Si:H) and of nanostructured silicon (ns-Si:H) thin films grown in radio-frequency plasma. The ns-Si:H thin films, also called polymorphous Si thin films, consist of a two-phase mixture of amorphous and ordered Si. The Raman spectra were measured at increasing laser intensities. Very low laser power densities (∼1 kW/cm2) were used to thoroughly analyze the structure of as-deposited thin films. Higher Raman laser powers were found to induce the crystallization of the films, which was characterized by the appearance of a sharp peak around 500 cm−1. This was attained faster in the ns-Si:H than in the conventional a-Si:H thin films because the silicon-ordered particles cause a heterogeneous nucleation process in which they act as seeds for crystallization. The laser power densities for film crystallization, crystal size, and surface temperature were determined from this Raman analysis. The validity and application ranges of the different models that can be used to calculate these parameters are critically discussed. © 2001 American Institute of Physics.

Notes: Porous silicon (PS) based metal/PS/p-Si structures with PS layer of different thickness were prepared on moderate- and high-resistivity substrates. Measurements of current–voltage (I–V) characteristics and impedance at various temperatures were used for the investigation of the electrical properties of these structures. Electrical properties of the structures with relatively thin (1 μm) PS layer significantly differ from those of thick structures. The exponential forward bias I–V dependencies for thin structures spread over several orders of magnitude with a low value of quality factor (close to 2) and have activation temperature dependencies with an activation energy equal to half the c-Si band gap. The reverse current has a square root dependence on the reverse bias voltage and the activation energy is equal to half the c-Si band gap. Therefore, it was concluded that the reverse and forward currents in thin PS-based device structures were determined by the generation and recombination of carriers in the depletion region of the c-Si substrate. It was shown that a large area spreading current exists in structures made on highly resistive substrates, which appears to be due to a highly conductive inverse (n-type) layer formed in the p-Si substrate at the PS/p-Si heterojunction. The spreading effect leads to high reverse currents and high capacitance of the device structures made on highly resistive substrates. © 2001 American Institute of Physics.

Notes: We report photoemission spectroscopic studies of zinc sulfide nanocrystallites in the quantum size regime. The nanocrystallites studied have average sizes of 1.8, 2.5, and 3.5 nm and narrow size distributions as determined from UV-visible absorption spectroscopy, as well as x-ray diffraction and high-resolution transmission electron microscopy. Analysis of sulfur core levels from the nanocrystallites show the presence of the three types of sulfur species corresponding to the core, the surface, and the capping layer of the nanocrystallites. We show that a quantitative analysis of these different sulfur components can be used to estimate the sizes of the nanocrystallites; thus, the obtained sizes are in good agreement with the sizes determined independently from small angle x-ray diffraction and high-resolution electron microscopy. © 2001 American Institute of Physics.

Notes: The size and depth distributions of pores in silica-based intermetal-dielectric materials were studied using monoenergetic positron beams. Doppler broadening spectra of the annihilation radiation and lifetime spectra of positrons were measured for methyl-silsesquioxane (MSSQ) spin-on-glass films. The size distribution of pores in the MSSQ films fabricated with 8% porogen load was found to be bimodal, with the major peaks located at 3 and 8 nm3. Increasing the porogen load from 8% to 40% caused the smaller pores (3 nm3) to disappear and 30-nm3 ones to appear; these pores were considered to be interconnected, and this structure makes it possible for positronium (Ps) atoms to find paths towards the surface and to escape into vacuum. The 8%-porogen MSSQ films had low porosity near the Si substrate. From measurements of the temperature dependence of the self-annihilation rate of ortho-Ps, we discuss the relationship between o-Ps emission into vacuum and the pore structure. © 2001 American Institute of Physics.

Notes: It has been shown theoretically and demonstrated experimentally that shear horizontal (SH) surface waves can exist when the surface of an isotropic substrate is perturbed by a strong corrugation, for instance consisting of deep grooves etched in the substrate, whereas these waves cannot exist without this perturbation. It is shown in this article that a periodic array of metallic electrodes (wires) exhibiting large aspect ratios deposited over a piezoelectric substrate give rise to surface acoustic waves with general polarization. The admittance of an interdigitated transducer, which is a basic tool for predicting the waves parameters, is calculated by a combination of finite element analysis and a boundary integral method. This approach has been extended to obtain the polarization of the acoustic waves. For different piezoelectric substrates, we predict various surface acoustic modes and their polarization. Along with mostly SH modes, we also find modes mostly polarized in the sagittal plane. We discuss the frequency behavior of the surface modes as a function of the electrode height compared to the period. © 2001 American Institute of Physics.

Notes: The nanocrystalline MnxFe3−xO4 (x=0, 1.18, 1.56, and 1.9) spinel ferrite thin films were investigated by means of the x-ray absorption spectroscopy and x-ray magnetic circular dichroism at the Mn and Fe L2,3 edges. The cationic distributions of thin spinel ferrite films for x=1.18 and 1.56 were determined using a crystal field atomic multiplets scheme for arbitrary symmetry. The results are compared with the distribution of cations obtained from the differential thermogravimetry analysis of fine powders of corresponding compositions. © 2001 American Institute of Physics.

Notes: A multitechnique approach, consisting of x-ray diffraction, differential thermal analysis, low frequency Raman scattering from the acoustic vibrations of nanoclusters, and transmission electron microscopy associated with selected area diffraction, has been used to study the nucleation and crystallization processes in SiO2–Ga2O3–K2O glasses. The specific aim was to determine the structure and the size distribution of nanoparticles embedded in the glass matrix. It has been found that nearly spherical nanocrystals of β-Ga2O3, with a size of ∼2–3 nm, nucleate during thermal treatments at 900 °C. Crystallization was observed after annealing at higher temperature. The amount of the crystalline phase and the mean size of the nanocrystals increased with heat treatment, time and temperature. β-Ga2O3 was the only crystalline phase to appear in all glass samples. © 2001 American Institute of Physics.

Notes: Optical absorption of nanocrystalline thin films can be influenced by the presence of both porosity and grain size effects. If both are present simultaneously, their effects are difficult to separate. In this study it is shown that the combination of uv-vis transmittance and reflectance measurements on porous CeO2 films provides enough data to make this separation. The CeO2 films were prepared by deposition of nanosized (∼5 nm) particles from a water colloidal suspension onto sapphire and subjecting these films to sintering temperatures sufficiently high to provide a series of films with a typical thickness of 0.6 μm with a wide range of grain sizes and porosity. X-ray diffraction, scanning electron microscopy, ellipsometry, and profilometry were used to characterize the films and to compare the observed grain sizes and porosity with that obtained from optical measurements. All of the techniques used gave results on porosity and grain size which were in good agreement, from 15% to 50% and 5 to 65 nm, respectively. For these porous films it was found that the changes in absorption which are normally explained by quantum confinement effects due to the small crystallite size can be attributed primarily to changes in porosity rather than in grain size. © 2001 American Institute of Physics.

Notes: The effect of thermal annealing on the properties of Al–AlOx–Al single electron tunneling transistors is reported. After treatment of the devices by annealing processes in forming gas atmosphere at different temperatures and for different times, distinct and reproducible changes of their resistance and capacitance values were found. According to the temperature regime, we observed different behaviors with regard to the resistance changes, namely the tendency to decrease the resistance by annealing at T=200 °C, but to increase the resistance by annealing at T=400 °C. We attribute this behavior to changes in the aluminum oxide barriers of the tunnel junctions. The good reproducibility of these effects with respect to the changes observed allows the proper annealing treatment to be used for postprocess tuning of tunnel junction parameters. Also, the influence of the annealing treatment on the noise properties of the transistors at low frequency was investigated. In no case did the noise figures in the 1/f regime show significant changes. © 2001 American Institute of Physics.

Notes: Electron-beam evaporation of metal contact is commonly used in the fabrication of semiconductor devices. We have observed that device irradiation by x-ray photons, which are generated by electrons striking the metal (titanium or gold) to be evaporated, has a strong impact on the characteristics of diamond devices used for radiation detection. It results in an improvement of the detector charge collection efficiency by a factor of 1.5 with respect to nonirradiated devices (standard thermal evaporation). Thermally stimulated current measurements showed that this effect is related to deep trap filling by free carriers generated in diamond by x-ray photons impinging the device during e-beam evaporation. Trap filling results in an increase of the free carrier drift distance before trapping. Study of contact annealing and the thermal stability of trap filling showed that charge detrapping at temperatures above 200 °C annihilates the observed detector sensitivity improvement. © 2001 American Institute of Physics.

Notes: The avalanche multiplication and excess noise properties of a range of submicron Si diodes have been measured and analyzed using a model for impact ionization which includes the effect of dead space, modified to allow for a gradual onset of ionization, with realistic threshold energies. Good agreement is achieved between the predictions of this "soft dead space" model and measurements of multiplication and excess noise, both on a range of submicron diodes with uniform electric fields and also on a p+n diode with a highly nonuniform electric field. © 2001 American Institute of Physics.

Notes: Instead of the conventional flat collimator a curved collimator was used in double-crystal x-ray topography. The curvature of the collimator was adjusted so that the Bragg condition for x-ray diffraction was uniformly satisfied over a wide area of the silicon wafer. The image area of the wafer was wide enough to characterize the local lattice distortion induced by test element groups of metal–oxide–semiconductor capacitors formed on the wafer. The lattice distortion was measured as variations in lattice plane spacing and in lattice plane orientation using local angular deviations from the Bragg condition. These angular deviations were determined by fitting the x-ray intensities measured at the same point on a series of topographs taken around the Bragg peak to the rocking curve of the sample. The lattice plane spacing changed abruptly by 10−6 at the boundary between the areas of gate oxide (11 nm thick) and the areas of field oxide (400 nm thick), and showed less variation within these areas. The lattice plane orientation changed monotonically in each area, with an inclination of the order of 10−5 rad within the largest gate oxide area (5×5 mm2). © 2001 American Institute of Physics.

Notes: Crystalline to amorphous transition and subsequent microstructural evolution in silicon induced by Ar+-ion implantation over a wide range of ion fluences (6×1013–1×1017 cm−2) have been investigated by spectroscopic ellipsometry. In the evaluation of the optical and microstructural properties of the damaged layer, the contribution of the surface overlayer to the measured dielectric spectra was separated by fitting a multilayer model with an effective medium approximation. The best fit to the dielectric spectra for disordered silicon could be obtained by taking our highest-fluence implanted (fluence=1×1017 ions/cm2) amorphous silicon (a-Si) data as reference data instead of a-Si data available in the handbook. The derivative spectra as a function of fluence show a distinct and sharp transition from the crystalline to amorphous phase. The threshold fluence for this transition is derived from fitting. Evaluation of standard sum rules and optical moments for imaginary part of the pseudodielectric function reveals no substantial change in various physical parameters below the transition indicating their insensitivity to point defects, while it shows a large change with fluence above the threshold for amorphization. The disorder induced changes in the effective dielectric constant, number of valence electrons per atom participating in optical transition, Penn gap energy, average bond length, coordination number, effective dispersion oscillator energy, an average strength of the interband optical transition with fluence is discussed on the basis of microstructural evolution and corresponding band structure modification. It is also shown that the dielectric functions of damaged silicon are well represented by a sum of six classical Lorentz oscillators. With increasing fluences, each of the oscillator amplitude decreases and linewidth increases except for the 3.3 eV transition which shows increasing amplitude with fluence. These results are discussed in the context of short-range order/disorder and effective band gap reduction along with flattening of the bands with increasing fluence above the amorphization threshold. © 2001 American Institute of Physics.

Notes: Since the prediction of Liu and Cohen [Science 245, 841 (1989)] of the potential extraordinary mechanical properties of crystalline β-C3N4, many authors have attempted its synthesis. However, in most cases, the obtained materials are amorphous phases with a complex bonding structure. Their characterization is complicated due to the absence of a reference compound, the lack of long-range order, and the poor knowledge about their bonding structure. In this article, we present 1H, 13C, and 15N solid-state nuclear magnetic resonance (NMR) measurements for the determination of the bonding types in amorphous CNx films. NMR measurements do not require long-range order and are able to clearly identify the signals from the sp2- and sp3-bonded phases. The analysis of the data obtained by other characterization techniques, such as infrared spectroscopy, x-ray photoelectron spectroscopy, electron energy-loss spectroscopy, and x-ray absorption near-edge spectroscopy on the same sample, based on the information acquired by NMR, enables the description of a structure model for the studied amorphous-CNx phase prepared by dc-magnetron sputtering and to revise the interpretation found in the literature. © 2001 American Institute of Physics.

Notes: Using a molecular-dynamics simulation, we study the buildup of damage in an a-Si specimen bombarded by Si atoms with energies between 10 and 150 eV for fluences up to 1.4×1015 cm−2, i.e., an equivalent of 2 monolayer growth. The production rate of overcoordinated atoms increases with the bombarding energy; we analyze its fluence and bombarding-energy dependence in detail. The number of undercoordinated atoms decreases for low-energy bombardment due to the saturation of dangling bonds at the surface; for higher bombarding energies, it increases slightly, but shows only little dependence on bombarding energy. The depth distribution of the damage, of the induced stress, and of the atom relocation in the target demonstrate that bombardment modifies the target at considerably greater depths than the ion range. © 2001 American Institute of Physics.

Notes: Single-shock (Hugoniot) equation-of-state data of shock-compressed C (graphite) are reported at pressures of 480 and 760 GPa (7.6 Mbar). Graphite is shock-compressed completely into a diamond-like phase at pressures below 80 GPa. At pressures of 80–800 GPa comparison of an ensemble of experimental Hugoniot data for shock-compressed graphite and diamond, and theoretical calculations of the Hugoniots of graphite and diamond, and the 0 K isotherm of diamond suggest diamond melts at ∼300 GPa on the Hugoniot of graphite and that the diamond phase is the ground-state structure of C up to at least 600 GPa. © 2001 American Institute of Physics.

Notes: We have studied the yield and the time evolution of pulsed laser induced photoluminescence in proton irradiated and thermally annealed amorphous hydrogenated silicon carbon alloys prepared by plasma enhanced chemical vapor deposition. Three major fluorescence decay channels have been observed with decay rates independent from proton irradiation and thermal annealing. Lack of correlation between yield and average decay time suggests a very simple phenomenological model which allows evaluation of the nonradiative time constant which is found to be linearly correlated with the photoluminescence yield. Our model suggests that radiative recombination occurs via exciton decay while the nonradiative recombination is driven by the trapping of carriers in defects states. © 2001 American Institute of Physics.

Notes: A detailed calculation of the three-dimensional elastic strain field and Gibbs free energy in and around InN/AlN wurtzite quantum dots is presented. The strain tensor is calculated by minimizing the Helmholtz free energy on a three-dimensional grid. The boundary conditions for a free surface are rigorously implemented to enable the strain field and Gibbs free energy at the surface to be modeled realistically. This has implications for the growth of additional layers of dots above a seed layer and can serve as an arbiter for determining possible nucleation sites. Results are presented for a single dot as well as coupled dots. The Gibbs free energy is seen to exhibit strong minima directly above a layer of seed dots, facilitating vertical ordering. Under certain conditions, satellite minima can also occur. Using the calculated strain field, the piezoelectric polarization field is also calculated. Because of the strong lattice mismatch, the strain field is quite large, particularly near the base and apex of the dots. This, in turn, leads to piezoelectric charges with magnitudes as high as 1014 cm−2 in regions of high strain. © 2001 American Institute of Physics.

Notes: The progress of multiple cracking in a silicon oxide (SiOx) film deposited onto a polyethylene terephthalate substrate was analyzed using Monte Carlo simulation. The finite-element analysis, assuming elastoplastic behavior of the polymer substrate, was conducted to calculate the stress distributions in film fragments and was used in the simulation. The Weibull parameters of the film were determined from the scatter of crack onset strain. The simulation predicted successfully the crack density and the distribution of fragment lengths during the progress of multiple cracking. The validity of the shear lag analysis based on the unique stress criterion in a previous study was also evaluated. © 2001 American Institute of Physics.

Notes: The optical and electronic properties of TiCxNy(x+y∼1,0〈y〈1) thin films have been investigated by spectroscopic ellipsometry in the 1.5–4.5 eV energy range and by valence band x-ray photoemission spectroscopy as a function of the composition. The dielectric functions measured in the energy range of intraband transitions are analyzed in terms of a Drude-like approximation. Both the free plasma energy and the damping constant are observed to depend on the nitrogen content of the samples, suggesting a certain tunability of the optical and electronic properties of these films. Analysis of the valence band reveals that the C 2p band shifts toward higher binding energies upon an increase of the nitrogen content, in good agreement with the shift observed in the minimum of the optical reflectivity associated with the threshold of the interband transitions. The enhancement of the metallic character of the films as the nitrogen content increases is also evidenced by x-ray photoemission spectroscopy as a continuous intensity growth of the conduction band at the Fermi level. © 2001 American Institute of Physics.

Notes: We have studied the magnetoresistance in an inclined magnetic field of a two-dimensional electron gas modulated by a two-dimensional array of thin cobalt nanostructures. At low fields, hysteretic behavior due to the magnetization of the elements was observed, and at higher fields magnetic commensurability (Weiss) oscillations were clearly seen. These magnetoresistance features only occurred when the in-plane component of the field was in the current flow direction. The commensurability oscillations behave exactly as predicted in a recent theory of two-dimensional surface superlattices. © 2001 American Institute of Physics.

Notes: In a recent article [S. Z. Karazhanov, J. Appl. Phys. 89, 332 (2001)], Karazhanov proposed a single-level recombination model as an explanation for the anomalous dependence of the carrier lifetime on injection level observed in cast multicrystalline silicon. This approach contrasts with previous models which involved the use of two distinct levels, one causing recombination and the other only trapping. The purpose of this Comment is to outline some critical considerations which suggest that only a two-level (or indeed a multilevel) model can satisfactorily explain the experimental observations. © 2001 American Institute of Physics.

Notes: An averaging procedure is applied to inelastic acoustic–phonon scattering which leads to lattice-temperature-dependent constants for the phonon energy and the square of the phonon wave vector. The resulting scattering rate depends on energy only thus facilitating the search of after-scattering states in full-band Monte Carlo simulations. The model still accurately reproduces the velocity–field characteristics over a wide range of lattice temperatures, but in silicon the hot-hole tail of the energy distribution is strongly enhanced compared with the elastic equipartition approximation. © 2001 American Institute of Physics.

Notes: A self-calibrating quantum efficiency measurement technique is applied to the Tm3+ 3H4→3F4 transition at 1460 nm. The method requires only relative fluorescence measurements to determine the absolute quantum efficiency. Overlapping spectral features are separated using phase-shifted fluorescence from different levels at high modulation frequency. The effect of energy transfer between Tm3+ ions is studied, and it is found that Tm3+ concentrations on the order of 1018 cm−3 are needed to avoid complications from Tm3+–Tm3+ cross relaxation. Application of the technique to Tm3+ doped fluorozirconate glass gives good agreement with the expected quantum efficiency based on multiphonon relaxation theory. © 2001 American Institute of Physics.

Notes: Lateral current spreading in shallow ridge processed unipolar semiconductor lasers is described using a two-dimensional flow model. In these devices, contrary to bipolar diode lasers, the density of carriers can be considered constant also in the active region. Therefore electron diffusion is a negligible effect and the spatial distribution of the current can be obtained by solving a two-dimensional differential equation for the electric potential. Our calculations prove that the major contribution to the current spreading takes place right before electrons enter the active region and is caused by the discontinuity of the conductivity at the cladding–active region interface. © 2001 American Institute of Physics.

Notes: Following the success of p-Ge hot-hole lasers, there is potential for using other semiconductor materials, notably III–V materials such as GaAs and InSb. Previous analysis had suggested that a large effective mass ratio between the heavy and light holes is advantageous, which implies that InSb would make an excellent hot-hole laser. Using our Monte Carlo simulation of both GaAs and InSb hot-hole lasers in combination with a rate equation model, we see that previously accepted criteria used to predict performance are not always reliable, and we suggest suitable alternatives. The simulation results include gain and gain bandwidth as a function of field strength and laser frequency, and alternative field orientations and photon polarizations are considered. Comparisons are made with bulk p-Ge systems. The optimum conditions predicted by our simulation could then be used in the design of quantum-well hot-hole lasers. © 2001 American Institute of Physics.

Notes: InGaAs strained epitaxial layers on GaAs are of considerable interest in semiconductor devices. An important feature is the critical thickness of the epitaxial layer beyond which relaxation occurs, affecting the device performance. With this in view, a series of such structures have been grown by organometallic vapor phase epitaxy and characterized by ion channeling, high resolution x-ray diffraction and Raman spectroscopy. The results of these three techniques are compared for the samples in this study which are fully strained, nominally and by experimental measurements. Beam steering effect that occurs at low energy channeling is also addressed. © 2001 American Institute of Physics.

Notes: The effects of proton irradiation on strained InAsxP1−x/InP-based quantum well solar cells (QWSCs) have been investigated by the electron beam induced current (EBIC) and cathodoluminescence (CL) techniques. From analysis of the EBIC data, capture rates within the quantum well region have been estimated, from which the open circuit voltages of the cells were calculated and shown to agree well with the measured values. Diffusion lengths have been estimated from analysis of both the EBIC and CL measurements. The location of the energy levels of proton-induced defects and their effectiveness as nonradiative recombination centers have been determined from Arrhenius plots of the total CL intensity emitted from the quantum wells following irradiation. The results suggest that deeper and narrower quantum wells increase the sensitivity of QWSCs to radiation damage. © 2001 American Institute of Physics.

Notes: Brillouin scattering has been used to investigate the elastic properties of CuxMo1−x thin films with thicknesses varying from 200 up to 430 nm, elaborated by ion beam sputtering. Two compositions around Cu30Mo70 and Cu70Mo30 and pure Cu and Mo films have been studied. The samples are essentially randomly oriented polycrystals, leading to an isotropic effective symmetry. By fitting all the Brillouin spectral lines, two independent effective elastic constants of these films have been determined, allowing the calculation of their elastic moduli. Results indicate that pure copper thin films' elastic properties are similar to the bulk copper ones whereas pure molybdenum thin films' Young's modulus is weaker (about 10%) than the bulk Mo one. Concerning Cu–Mo solid solutions, elastic constants values lie between pure copper and molybdenum ones. Finally, the effect of annealing at 760 K on the mechanical properties of the solid solutions in relation with the microstructural evolution is discussed. The annealed solid solutions show a hardening of the elastic moduli with respect to the as-deposited samples, which is supposed to reflect their full demixing. © 2001 American Institute of Physics.

Notes: Scanning anode field emission microscopy is used to map the electron emission current I(x,y) under constant anode voltage and the electron extraction voltage V(x,y) under constant emission current as a function of tip position on carbon based thin film emitters. The spatially resolved field enhancement factor β(x,y) is derived from V(x,y) maps. It is shown that large variations in the emission site density (ESD) and current density can be explained in terms of the spatial variation of the field enhancement β(x,y). Comparison of β(x,y) and I(x,y) shows that electron emission currents are correlated to the presence of high aspect ratio field enhancing structures. We introduce the concept of field enhancement distribution f(β), which is derived from β(x,y) maps to characterize the field emission properties of thin films. In this context f(β)dβ gives the number of emitters on a unit surface with field enhancement factors in the interval (β,β+dβ). It is shown experimentally for the carbon thin film emitters investigated that f(β) has an exponential dependence with regard to the field enhancement factor β. The field enhancement distribution function f(β) can be said to give a complete characterization of the thin film field emission properties. As a consequence, the emitted current density and ESD can be optimized by tuning f(β) of the emitting thin film. © 2001 American Institute of Physics.

Notes: We have studied the effect of grain boundary migration on hillock formation in unpassivated Al thin films during thermal cycling. Hillocking occurs more frequently in Al films that experience grain growth during thermal cycling than in films with stabilized grain structures. The hillocking frequency is at least four times greater in the films that experience grain growth, as judged by the number of hillocks observed per initial grain boundary triple junction. This latter measure takes account of the smaller initial grain size in the film that experiences grain growth and shows that grain boundary migration itself must enhance the hillocking frequency. © 2001 American Institute of Physics.

Notes: We proposed a simple lattice model to describe a solid–liquid interface of silicon based on experimental facts and molecular dynamics simulation results, and evaluated the relationship between the interface structure and the interfacial tension by comparing the model with experimental values. As a result, the entropy was found to give a major contribution to the interfacial tension, and it was revealed that the difference of entropy due to lattice disorder of bulk liquid and interface structure is the dominant factor of the entropy contribution. Moreover, the solid–liquid bond energy, which is crucial to estimate the contribution of the enthalpy, was successfully derived. The present model can be also applied to be the semiconductor material which has a diamond structure or a zinc blende structure. © 2001 American Institute of Physics.

Notes: A detailed model is proposed to explain the electroluminescence spectrum from metal–oxide–silicon tunneling diodes. This model includes phonon-assisted processes and exciton involvement. According to this model, the main peak and the low-energy tail of the electroluminescence spectrum are attributed to the transverse optical phonon and the two-phonon assisted recombination, respectively. With very few fitting parameters, the model accurately predicts the measured electroluminescence spectra. © 2001 American Institute of Physics.

Notes: A system of interdigital gates is used to create a periodic potential profile in a multilayer heterostructure. The electrostatic problem for the spatial distribution of the potential is solved and experimentally examined by measurements of current–voltage characteristics of resonant-tunnelling diodes embedded in the depletion region of the Schottky contact. It is shown that the position of the resonant peak voltage is sensitive to the spatial potential distribution and that with appropriate parameters of the heterostructure the sensitivity of the gates can be considerably enhanced. © 2001 American Institute of Physics.

Notes: Silicon carbide (SiC) powder is used in nonlinear field grading materials. The composite material, consisting of an insulating polymer matrix filled with the SiC-grains, is usually a percolated system with established conducting paths. In order to explain the properties, the electrical characteristic and conduction mechanisms of the SiC powder itself are of interest. SiC powders have been studied by current–voltage measurements and the influences of grain size and doping have been investigated. The macroscopic current characteristics of green and black SiC powders can be described by the transport mechanisms at the grain contacts, which can be modeled by Schottky-like barriers. The SiC is heavily doped and tunneling by field emission is the dominating conduction mechanism over the major part of the nonlinear voltage range. It is suggested that preavalanche multiplication influences the current at the highest voltages, especially for p-type black SiC. © 2001 American Institute of Physics.

Notes: While metastable B1-NaCl-structure δ-TaNx is presently used in a variety of hard coating, wear-resistant, and diffusion barrier applications, it is a complex material exhibiting a wide single-phase field, x(similar, equals)0.94–1.37, and little is known about its fundamental properties. Here, we report physical properties of epitaxial δ-TaNx layers grown as a function of x on MgO(001) by ultrahigh vacuum reactive magnetron sputter deposition. The room-temperature resistivity (ρ=225 μΩ cm), hardness (H=30.9 GPa), and elastic modulus (E=455 GPa) of δ-TaNx(001) are independent of x over the range 0.94–1.22. However, changes in the electronic structure associated with increasing x〉1.22 lead to an increase in ρ with a decrease in H and E. All δ-TaNx(001) layers exhibit negative temperature coefficients of resistivity between 20 and 400 K due to weak carrier localization. δ-TaNx is superconducting with the highest critical temperature, 8.45 K, obtained for layers with the lowest N/Ta ratio, x=0.94. Based upon the above results, combined with the fact that the relaxed lattice constant a0 shows only a very weak dependence on x, we propose that the wide phase field in δ-TaNx is due primarily to antisite substitutions of Ta on N (and N on Ta) sites, rather than to cation and anion vacancies. To first order, antisite substitutions in TaNx are isoelectronic and hence have little effect on charge carrier density. At sufficiently high N/Ta ratios, however, simple electron-counting arguments are no longer valid since large deviations from stoichiometry alter the character of the band structure itself. © 2001 American Institute of Physics.

Notes: We have measured the low-frequency voltage noise properties across mesa stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2Oy (BSCCO) single crystals as a function of bias voltage. The BSCCO mesas with area of 160×40 μm2 were fabricated on the surface of cleaved BSCCO single crystals using standard photolithography and Ar ion-milling techniques. The measured noise voltage spectrum density SV(f ) had a 1/f dependence on frequency. The magnitude of the 1/f noise spectra was found to increase monotonically with the branch number in the I–V characteristic at fixed bias current, and no anomalously enhanced 1/f noise was observed. © 2001 American Institute of Physics.

Notes: A micromagnetic model is established to analyze the permeability of Fe–Ta–N and Fe–Al–N thin films in a wide frequency range from 10 to 2200 MHz. The simulated real part of the permeability is about 2500 and 5000 at low frequencies for macroscopic Fe–Al–N and Fe–Ta–N films, respectively. It then decreases by a factor of 2 around 500 and 300 MHz. The corresponding imaginary part of the permeability increases from zero to a maximum value. In mesoscopic Fe–M–N films, huge magnetic poles on the edge of a layer play a significant role in the frequency response. As a result, the real part of the permeability in a mesoscopic film becomes much smaller than that in a macroscopic film. The spin patterns of mesoscopic Fe–M–N films in an external oscillatory field are also shown to understand the frequency response with strong magnetic poles. © 2001 American Institute of Physics.

Notes: Ultrathin Si oxynitride films grown by low-temperature remote plasma processing were examined by on-line Auger electron spectroscopy and angle-resolved x-ray photoelectron spectroscopy to determine the concentration, spatial distribution, and chemical bonding of nitrogen. The films were grown at 300 °C on Si(100) substrates using two radio-frequency remote plasma processes: (i) He/N2O remote plasma-assisted oxidation (RPAO) and (ii) two-step remote plasma oxidation/nitridation. A 5 min He/N2O RPAO process produces a 2.5 nm oxynitride film incorporating approximately 1 monolayer of nitrogen at the Si–SiO2 interface. The interfacial nitrogen is bonded in a N–Si3 configuration, as in silicon nitride (Si3N4). By comparison, a 90 s He/N2 remote plasma exposure of a 1 nm oxide (grown by 10 s He/O2 RPAO) consumes substrate Si atoms creating a 1 nm subcutaneous Si3N4 layer. The nitrogen areal density obtained via the two-step process depends on the initial oxide thickness and the He/N2 remote plasma exposure time. Moreover, as the oxide thickness is increased (by increasing the He/O2 remote plasma exposure), the nitrogen distribution shifts away from the Si–SiO2 interface and into the oxide. More nitrogen with a tighter distribution is incorporated using He versus Ar dilution. Insight into the remote plasma chemistry was provided by optical emission spectroscopy. Strong N2 first positive and second positive emission bands were observed for He/N2O and He/N2 remote plasmas indicating the presence of N2 metastables and ground-state N atoms. © 2001 American Institute of Physics.

Notes: Comparisons of buried InAs/GaAs and InAs/InP quantum dots (QDs) utilizing transmission electron microscopy display interesting parallels and differences between the two systems. The higher 7.2% misfit in the InAs/GaAs system produces small (∼20 nm diameter) QDs with a majority displaying a predominately round shape. The lower 3.2% misfit in the InAs/InP system produces larger QDs (∼35 nm diameter) with the majority also displaying a predominately round shape. In both systems, the size of the QDs can be varied by changes in growth procedures and the largest QDs in any population show evidence of faceting © 2001 American Institute of Physics.

Notes: Addition of N2 to Ar during Ti sputtering has been found to improve the thermal stability of TiSi2. For samples sputtered with a mixture of Ar and N2, TiSi2 was found to be stable after 1050 °C, 30 s annealing. Furthermore, the phase transformation temperature from the C49 to C54 phase was not affected with the addition of a small amount of nitrogen. The stuffing of grain boundaries of TiSi2 and TiN/TiSi2 interfaces by nitrogen atoms is thought to retard the transport of Si and Ti atoms. In addition, titanium nitride particles embedded in TiSi2 near the TiN/TiSi2 interface may also protect the TiSi2 films from plastic deformation and retard the grain growth during high temperature annealing. Smaller average grain size of C54–TiSi2 in samples prepared with the addition of N2 to Ar during Ti sputtering than that in pure Ti samples is also beneficial in enhancing the thermal stability. © 2001 American Institute of Physics.

Notes: We report a calculational procedure to obtain the rate of electron–hole recombination, mediated by the Shockley–Read–Hall (SRH) mechanism. Our method uses a combination of first-principles calculations and accurate empirical band structures. First, we use ab initio calculations to identify the point defects, their densities and energy levels in the gap. Then we parametrize the tight-binding interaction between defect and the host atoms in a Green's function approach to obtain the defect levels as identified by the first-principles calculations. Finally, the resulting tight-binding Hamiltonian is used to obtain the dipole matrix element between the conduction and valence band states, mediated through the defect levels in the gap, in second-order perturbation theory. The states are integrated over the entire Brillioun zone, subject to energy and momentum conservation, to obtain the limiting lifetimes of the carriers. This method is applied to study the minority carrier lifetimes in n-doped InAs. The calculated effective lifetimes that include Auger and SRH recombinations as functions of temperature agree reasonably well with experiment. Our calculation of lifetimes in 3.5×1016 and 2.0×1016 cm−3 n-doped InAs indicate that SRH is dominant at low temperatures and that the lifetimes vary between 10−8 and 10−7 s. © 2001 American Institute of Physics.

Notes: The electronic structures of the Ga1−xInxNyAs1−y/GaAs compressive strained quantum wells are investigated using 6×6 k⋅p Hamiltonian including the heavy hole, light hole, and spin-orbit splitting band. By varying the well width and mole fraction of N in the well material, the effects of quantum confinement and compressive strain are examined. The curves of dependence of transition energy on well width and N mole fraction are obtained. The valence subband energy dispersion curves and TE and TM squared optical transition matrix elements of three possible quantum well structures for emitting 1.3 μm wavelength are given. © 2001 American Institute of Physics.

Notes: Through the numerical solution of the time-dependent Schrödinger equation, the tunneling current through ultrathin gate oxides in metal-oxide-semiconductor (MOS) structures was computed. The tunneling current exhibited oscillatory behavior, which was attributed to wave interference. It is found that the oscillation amplitude decreases as the oxide thickness is increased. This amplitude decrease used to be attributed to different scattering mechanisms. However, we will show that the oscillation amplitude decrease as oxide thickness is increased is an intrinsic quantum-tunneling feature. Furthermore, we will also show that the oscillation amplitude decreases as well when the semiconductor effective mass in the MOS structures is increased, vanishing when the semiconductor effective mass reaches that of a metal, thus forming a metal-insulator-metal (MIM) structure. This result pinpointed why tunneling current oscillation has never been observed experimentally in MIM-like structures. © 2001 American Institute of Physics.

Notes: The inversion-layer mobility in future highly doped silicon-on-insulator (SOI) n-channel metal–oxide–semiconductor field-effect transistors (MOSFETs) has been examined for various SOI layer thicknesses (tSOI) using self-consistent calculation. Not only phonon scattering but also surface roughness scattering and ionized impurity scattering have been taken into account. It has been found for SOI MOSFETs with a highly doped channel that, whenever tSOI ((approximately-greater-than)2 nm) is reduced under the full-depletion condition, the inversion-layer mobility in SOI MOSFETs becomes higher than that in bulk MOSFETs. The increase in mobility with the reduction of tSOI to about 10 nm is mainly caused by the suppression of surface roughness scattering. Independent of acceptor concentration (NA), the mobility reaches its peak at tSOI of about 3 nm and then decreases drastically with decreasing tSOI. For SOI MOSFETs with NA higher than 5×1017 cm−3, the mobility increases monotonously to the peak with decreasing tSOI under the full-depletion condition due to the large suppression of ionized impurity scattering for the tSOI range between 10 and 5 nm. These results are different from those in the previous works for SOI MOSFETs with low channel impurity concentration. © 2001 American Institute of Physics.

Notes: Manganese–ferrite and nickel–ferrite ionic magnetic fluids (MFs) were investigated using static magnetic birefringence (SMB). Transmission electron microscopy (TEM) was used to obtain the particle diameter polydispersity profile of the MF samples. The model used in the present study to fit the SMB data includes the field dependence of the magnetic permeability associated to the magnetic structures (incoherent monomer and dimer) and allowed estimation of the magnetic surface anisotropy. The combined analysis of the SMB and TEM data support the contribution of incoherent monomer to the SMB signal in MFs and allows the estimation of its characteristic diameter. It was found that the incoherent monomer diameter is sensitive to the nanoparticle nature. Finally, the surface anisotropy depends upon the nanoparticle diameter and falls within the range of 0.1–1.5 erg/cm2. © 2001 American Institute of Physics.

Notes: The influence of hydrostatic pressure on magnetization, electrical conductivity and magnetoresistance of FeCr2S4 single crystals has been studied. Significant pressure influence on the temperature of a spin-glass like magnetization anomaly, dTm/dp (approximate) 2.7 K/kbar, on the Curie temperature, dTC/dp(approximate)0.9 K/kbar, as well as on the resistivity and magnetoresistance, was found. The negative magnetoresistance maximum at the Curie temperature is shifted under pressure towards higher temperatures. Pressure induced changes of the low field magnetoresistance and of the conductivity anomalies at Tm are correlated well with the changes in the magnetization. In the low temperature region the appearance of low symmetry anisotropy has been observed. This effect is distinctly enhanced by the hydrostatic pressure. The low symmetry component of magnetic anisotropy was suggested to be due to the static Jahn–Teller effect. © 2001 American Institute of Physics.

Notes: Phase transitions and microwave dielectric properties in the (1−x)Ca(Al0.5Nb0.5)O3–xCaTiO3 system were analyzed using x-ray and neutron powder diffraction, transmission electron microscopy, Raman spectroscopy, and dielectric measurements at microwave frequencies (2–8 GHz). Rietveld structural refinements demonstrated that both end compounds exhibit similar octahedral tilted frameworks, while in Ca(Al0.5Nb0.5)O3, tilting is superimposed onto NaCl-type ordering of Al and Nb on the B sites. Accordingly, the room-temperature structures of CaTiO3 and Ca(Al0.5Nb0.5)O3 are described by orthorhombic Pbnm and monoclinic P21/n symmetries, respectively, with similar lattice parameters, (square root of)2ac×(square root of)2ac×2ac (where ac is the lattice parameter of cubic perovskite). The (1−x)Ca(Al0.5Nb0.5)O3–xCaTiO3 system features both cation ordering and octahedral tilting phase transitions. The Ca(Al0.5Nb0.5)O3 structure remains ordered at least up to 1625 °C. However, the temperature of the order/disorder transition decreases rapidly with increasing Ti content, which correlates with a progressive increase of cation disorder in the specimens. A disordered structure is attained at x=0.5. For the "solid solutions," the nonlinear dependence of both permittivity ε and the temperature coefficient of the resonant frequency τf on Ti content corresponds to a linear dependence of the macroscopic polarizability on composition; that is, the oxide additivity rule was closely obeyed. Therefore, this rule can be used to predict ε and τf for any intermediate composition from the permittivities and temperature coefficients of permittivity of the end compounds. A zero temperature coefficient of the resonant frequency occurs at the composition x(approximate)0.5 with a relative permittivity of 50 and a Qf value of approximately 30 000 GHz (@4 GHz). © 2001 American Institute of Physics.

Notes: Substitutions by aliovalent cations on the Sr2+ and Bi3+ sites of ferroelectric SrBi2Ta2O9 (SBT) have been carried out, which result in modified dielectric and electrical properties of SBT. The substitution of 10 mole % Fe3+ for Sr2+ shows an increase of 80 °C in TC, whereas Ca2+ substitution for Bi3+ gives rise to diffused and frequency dispersive dielectric maxima with a relaxor-like behavior. The presence of Ca2+ in the (Bi2O2)2+ layers appears to enhance the mobility of charge carriers thus increasing the bulk conduction of the sample. On the other hand, Fe3+ addition in the perovskite-like units results in a sharp dielectric anomaly at the ferroelectric phase transition, with a bulk conductance similar to that of pure SBT compound, but with a reduced space charge relaxation time. The low temperature conductivity mechanism shows a frequency dependence, which can be ascribed to the space charge mainly due to the oxygen vacancies. The dielectric and conductivity properties of the Ca2+-doped SBT make it a promising material for the fatigue resistance in device applications. © 2001 American Institute of Physics.

Notes: The time dependent electroluminescence (EL) of c-Si (at 1.1 eV) is investigated at room temperature for a p-i-n structure under excitation with forward biased current pulses. The EL intensity increases by square law at shorter times (〈3 μs) and reaches a steady state value at longer times (〉10 μs). The parabolic dependence of the EL intensity on the current density at the shorter times points to the bimolecular recombination mechanism. The EL response time has been decreased to less than 200 ns for the given p-i-n structure by application of a reverse bias potential. The maximal EL quantum efficiency is of the order of 0.01% for the investigated p-i-n structure and possible ways to increase this value are discussed. © 2001 American Institute of Physics.

Notes: Hole-induced bulk and interface defect generation and breakdown in ultrathin silicon dioxide (2.0 and 3.0 nm) are studied using substrate hot-hole injection. The results show that although these substrate hot holes are effective in creating electrically active damage in the dielectrics, these defects are very ineffective in causing breakdown as compared to those defects created by constant voltage tunneling stress. Identical to hole trapping in thicker oxides, substrate hot-hole defect generation was independent of electric field, decreased with decreasing thickness, and increased with decreasing temperature. The defect generation and breakdown of ultrathin oxides by substrate hot-hole stress is significantly different from that observed for constant voltage tunneling stress. The results suggest that the degradation and breakdown of ultrathin silicon dioxide cannot be explained by the trapping of hot holes alone.

Notes: Group-III (B, Al, Ga, and In)-nitride quaternary alloys and group-III (Al, Ga, and In)-nitride-based mixed anion (As, P, and Sb) quaternary alloys are useful for blue and green light emitting devices and high-temperature, high-power, and high-frequency electronic devices. It is known that these alloys are very difficult to grow in certain compositional regions. The thermodynamical stability of these alloys is studied with respect to an unstable two-phase region in the phase field. The unstable two-phase region is predicted based on a strictly regular solution model. The interaction parameter used in our model is obtained analytically using the valence force field (VFF) model modified for wurtzite structures. The calculated interaction parameters, which are required to predict the unstable two-phase region, agree well with experimental results for related alloy systems. The modified VFF model can also be used to predict the microscopic crystal structure, such as first neighbor anion–cation bond lengths. In order to verify our calculations, we compare the calculated and experimental results for the first neighbor anion–cation bond lengths in the InGaN system. The calculated results agree well with the experimental results. From our calculation results, the unstable two-phase regions for four A1−x−yBxCyD type group-III-nitride quaternary alloys and nine A1−xBxC1−yDy type group-III-nitride mixed anion quaternary alloys are calculated. The predicted unstable two-phase regions agree well with experimentally observed regions of phase separation in ternary alloys, which suggests our model calculations can provide useful guidance in ternary and quaternary systems where there is no experimental data. © 2001 American Institute of Physics.

Notes: The influence of an interposed ultrathin Nb layer between Ti and Si on the silicide formation and the electrical contact between the silicide formed and the Si substrate is investigated. The presence of the Nb interlayer results in the formation of ternary alloy (Nb,Ti)Si2 in the C40 crystallographic structure adjacent to the Si substrate. Depending on the nature of the Si substrates and/or the amount of the initial Nb, the interfacial C40 (Nb,Ti)Si2 leads, in turn, to either epitaxial growth of a highly faulted metastable C40 TiSi2 or formation of the desired C54 TiSi2 at a lower temperature than needed for it to form in reference samples with Ti deposited directly on Si. On p-type substrates doped to various concentrations, the Nb also leads to a considerably lower specific contact resistivity than that obtained in the reference samples: a twofold to fourfold reduction in the contact resistivity is found using cross-bridge Kelvin structures in combination with two-dimensional numerical simulation. As C40 (Nb,Ti)Si2 forms at the interface when an interfacial Nb is present, the interface characterized is likely to represent the one between (Nb,Ti)Si2 and Si. For the reference samples, the interface studied is between TiSi2 and Si. © 2001 American Institute of Physics.

Notes: A single-electron transistor (SET) comprising highly resistive Cr thin-film strips (sheet resistance ∼4 kΩ) instead of traditional tunnel barriers is reported. Two such strips (∼1 μm long) connect two Al outer electrodes to an Al island 1 μm in length equipped with a capacitively coupled gate. This transistor with a total asymptotic resistance of 110 kΩ showed a perfect Coulomb blockade and strictly e-periodic reproducible modulation by the gate in wide ranges of bias (V) and gate (Vg) voltages. In the Coulomb-blockade region (|V|≤ about 0.5 mV), we observed a strong suppression of the transport current, allowing modulation curves V(Vg) with appreciable amplitude to be measured at a fixed bias current value I as low as 100 fA. The background-charge noise of our SET was found to be similar to that of typical Al/AlOx/Al tunnel-junction single-electron transistors, namely δQ(approximate)5×10−4e/Hz at 10 Hz. The electron transport mechanism is discussed. © 2001 American Institute of Physics.

Notes: We investigate the effect of repeated thermal cycles in some of the Pr0.5Ca0.5Mn1−xMxO3 (M=Cr, Co, and Ni and x=0.02–0.05) compounds which undergo a spontaneous (H=0 T) insulator to metal transition below the temperature Tp driven by percolation of ferromagnetic clusters in a charge ordered background. The zero field resistivity (ρ) shows an anomalous behavior upon thermal cycling between a high-(TS)- and a low (TF)- temperature limit: While ρ for T(very-much-greater-than)Tp is unaffected, it increases significantly for T≤Tp and Tp shifts down each time upon thermal cycling. In contrast, ρ at a given temperature decreases slowly with time. The anomalous increase of resistivity depends upon the ratio of the charge ordered to ferromagnetic phase fraction and decreases with increasing x and the applied magnetic field (H). We also show that a similar effect can be obtained for Pr0.5Ca0.5Mn0.97Al0.03O3 and Pr0.55Ca0.45MnO3 under H=5 T although these compounds do not exhibit insulator–metal transition in zero field. The low field magnetization study on Pr0.5Ca0.5Mn0.98Cr0.02O3 suggests that the ferromagnetic Curie temperature remains unaffected, whereas the ferromagnetic phase fraction decreases with repeated cycles. We suggest that the observed anomaly is caused by an increase of interfacial strain between charge ordered and ferromagnetic phases. © 2001 American Institute of Physics.

Notes: Two sensitive polarized spectroscopies, reflectance difference spectroscopy and photocurrent difference spectroscopy, are used to study the characteristic of the in-plane optical anisotropy in the symmetric and the asymmetric (001) GaAs/Al(Ga)As superlattices (SLs). The anisotropy spectra of the symmetric and the asymmetric SLs show significant difference: for symmetric ones, the anisotropies of the 1HH→1E transition (1H1E) and 1L1E are dominant, and they are always approximately equal and opposite; while for asymmetric ones, the anisotropy of 1H1E is much less than that of 1L1E and 2H1E, and the anisotropy of 3H2E is very strong. The calculated anisotropy spectra within the envelope function model agree with the experimental results, and a perturbation approach is used to understand the role of the electric field and the interface potential in the anisotropy. © 2001 American Institute of Physics.

Notes: We investigate optical properties of the deterministic network model for fractal metallic clusters at the percolation threshold. As input data we used the experimental bulk dielectric function of gold, aluminum, and nickel. Sharp absorption resonances in the optical spectra are observed only for gold. They are pronounced only for clusters consisting of particles with sizes larger than the mean free path of the conduction electrons. In smaller particles the peaks are heavily damped due to scattering of electrons by the particle surfaces. © 2001 American Institute of Physics.

Notes: The photoluminescence excitation (PLE) spectra of CuInS2 crystals grown by the traveling heater method (THM) and the iodine vapor transport method (IT) have been examined. THM crystals show significant structure with many peaks and dips in the PLE spectra, whereas IT crystals exhibits two clear peaks at two free exciton energies in the spectrum. The peak and dip structures of PLE spectra at the free exciton energy depend on the monitoring PL peak, the kind of crystals, and temperatures. A proposed relaxation process of excited electron–hole pairs explains peak and dip structures of PLE spectra at the free exciton energies. © 2001 American Institute of Physics.

Notes: It is shown that ferromagnetism and insulator to metal transitions in small A site cation manganites Pr1−xCaxMnO3 are induced by rhodium doping. Colossal magnetoresistance properties are evidenced for a large compositional range (0.35≤x〈0.60). The ability of rhodium to induce such properties is compared to the results obtained by chromium and ruthenium doping. Models are proposed to explain this behavior. © 2001 American Institute of Physics.

Notes: The performance of middle wavelength/long wavelength (MW/LW) dual-band HgCdTe photovoltaic detectors was examined theoretically. An original iteration scheme was used to solve the system of nonlinear continuity equations and the Poisson equation. The effect of composition and doping profiles on the heterojunction detector parameters is presented. It is assumed that the performance of photodiodes is due to thermal generation governed by the Auger mechanism. All quantities are functions of the electric potential and Fermi quasi-levels. The results of calculations are presented as maps showing spatial distribution of electrical potential, photoelectrical gain, sensitivity, and density of noise generation. The theoretical predictions of heterojunction device parameters are compared with available experimental data. © 2001 American Institute of Physics.

Notes: Polycrystalline diamond thin films deposited by electron cyclotron resonance-assisted chemical vapor deposition on Si (111) were investigated using spectroscopic phase-modulated ellipsometry from the near IR to UV range (830–270 nm). Analysis of the raw ellipsometry data [ψ(λi), Δ(λi)] by applying the conventional Bruggeman effective medium theory and linear regression analysis provided details about the film microstructure: (i) the multilayer structure and the component layer thickness of the films; (ii) the volume fraction of the constituents (sp3- and sp2- bonded carbon) and of voids (fv) in the bulk layer (L2); (iii) the inhomogeneity of the structure along the growth axis and its variation with the seeding density; and (iv) the surface roughness layer thickness (dS). A simplified three-layer structural model consisting of an interfacial layer, an intermediate (or bulk) layer, and a top surface roughness layer has been proposed that simulates the ellipsometry data reasonably well. The results obtained through ellipsometry modeling, such as surface roughness and overall film thickness, were compared with those from atomic force microscopy and profilometry, respectively, in order to validate the model employed. Typically, high surface roughness values around 60 nm were found for films grown under different substrate temperatures and oxygen-to-carbon ratios. It was also found that a combination of relatively high substrate temperature and O/C ratio can be used to reduce the surface roughness to around 25 nm. In general, the void fraction (fv) of the bulk layer decreases as a function of seeding density, indicating the formation of a denser film. The sp2-bonded carbon fraction (fsp〈sup ARRANGE="STAGGER"〉2 C) also varies with the process parameters. These results (fv and fsp〈sup ARRANGE="STAGGER"〉2 C) for the bulk layer and its behavior with respect to process parameters are discussed. © 2001 American Institute of Physics.

Notes: Erbium-doped gallium–arsenic–nitrogen thin films were prepared by cosputtering a crystalline GaAs target partially covered with small pieces of metallic erbium in an Ar+N2 atmosphere. The films were deposited near room temperature and under increasing partial pressures of nitrogen P(N2). The investigation of the films included ion beam analysis, optical spectroscopy in the infrared–visible–ultraviolet energy ranges, Raman scattering, and photoluminescence measurements. According to the experimental results, all films present an amorphous structure and nitrogen contents that scale with P(N2). Increasing amounts of nitrogen induce the widening of the optical band gap and a systematic redshift of the Raman signal of the films considered. The intensity of the Er-related light emission at ∼0.8 eV also increases at higher nitrogen contents. A detailed study of the absorption bands in the infrared region allowed the identification of different vibration modes involving gallium, arsenic, and nitrogen atoms. The combined analysis of the compositional data and of the infrared absorption bands provided a constant of proportionality between the nitrogen content in the films and the integrated absorption due to Ga–N bonds. Where applicable, comparisons between the optoelectronic and structural characteristics of amorphous and crystalline gallium–arsenic–nitrogen compounds were made. © 2001 American Institute of Physics.

Notes: The ground state and binding energies for a hydrogenic impurity in a spherical quantum dot within a uniform magnetic field, are calculated through the variational method in the frame of the effective-mass approximation. The trial wave functions used in the calculation are flexible enough to treat the cases of on-center, off-center, or edge impurities. Overall results show reasonable agreement when compared with other calculations. Interestingly enough, in the case of an off-center impurity, a critical point from which bulk states evolve to surface or edge states is also found. The dependence of this point on the magnetic field strength is discussed and interpreted. © 2001 American Institute of Physics.

Notes: In this article, the conduction mechanisms in nitride–oxide stacked structures on Si are investigated experimentally and theoretically. Amorphous silicon nitride films (3–5 nm thick) were deposited by low-pressure chemical vapor deposition. The ultrathin oxide layers (1–1.5 nm thick) were formed by reoxidization of the nitride layer at about 900 °C in wet ambient. The current–voltage characteristics for negative and positive gate polarities are asymmetric. An abrupt current increase under negative gate bias prior to dielectric breakdown is reported for this structure. This current–voltage phenomenon is attributed to trap-controlled single-carrier steady-state space-charge-limited conduction—the solid state analog of space-charge-limited current in a vacuum diode. Details of space-charge-limited conduction parameters depend on the thickness of the dielectric film and temperature. The study of those parameters can yield information about the traps inside nitride–oxide films. Such information can provide considerable insight into charge transport mechanisms and carrier trapping in these materials, which are important in understanding the physical processes involved in the basic film properties. © 2001 American Institute of Physics.

Notes: The nitridation of GaAs, InAs, and InSb by low-energy N2+ ion bombardment at room temperature was studied by near-edge x-ray absorption fine structure (NEXAFS) and x-ray photoelectron spectroscopy measurements. The formation of thin surface nitride layers, consisting mostly of GaN or InN but also containing minor amounts of mixed nitrides, was observed. Besides the nitride-related features, sharp peaks in the NEXAFS due to π* resonance at 401.0 eV and correlated peaks at 403.8 eV in N 1s core level spectra were detected. Both spectral features could be assigned to the presence of interstitial nitrogen, most likely molecular nitrogen. It was found that the amount of interstitial nitrogen in the surface layer strongly depends on the AIII–BV semiconductor system and may be affected by modification of the conditions during low energy ion bombardment. © 2001 American Institute of Physics.

Notes: We have determined the elastic constants of langasite-type crystals (La3Ga5SiO14, La3Ga5.5Nb0.5O14, and La3Ga5.5Ta0.5O14) from measurements of the sound velocity of acoustic waves. Starting with the elastic tensor determined from bulk acoustic waves, we optimized the data set by investigating the influence of the elastic tensor components on the angular dispersion of surface guided waves. This procedure is particularly useful for accurate determination of the nondiagonal elements of the elastic tensor. Phase velocity calculations based on the new set of constants show an increased agreement with experimental data compared to the data set derived from bulk waves and previously published material data. © 2001 American Institute of Physics.

Notes: Photoelectrochemical (PEC) etching has been used to study defects in heteroepitaxial GaN layers. In Ga-polar layers PEC etching reveals only dislocations in the form of filamentary etch features (whiskers). Transmission electron microscopy (TEM) confirmed a one-to-one correspondence between the whiskers and straight threading dislocations, which are mainly of edge and mixed type. In N-polar layers, apart from dislocations, inversion domains (IDs) also give rise to the formation of more complex etch features that also have been confirmed by TEM. IDs of nanometer diameter result in formation of whiskers similar to the dislocation-related ones. However, when the diameter of IDs exceeds a critical size (about 100 nm), crater-like deep etch features are formed during PEC etching. Based on the mechanism of PEC etching of GaN in aqueous KOH solutions, it is argued that inversion domain boundaries are electrically active defects. © 2001 American Institute of Physics.

Notes: Y2O3 films doped with Eu are grown by metalorganic chemical vapor deposition on Si(100) and yttria-stabilized zirconia (YSZ) (100). The mismatch is only 2.6% between the lattice constant of YSZ and half the lattice constant of Y2O3. The samples are characterized by x-ray diffraction, scanning electron microscopy, Rutherford backscattering, and photoluminescence. The films deposited on Si are polycrystalline. When films are deposited on YSZ, the Y2O3(100) direction is aligned with YSZ(100). The luminescence and the narrow x-ray diffraction lines indicate that a Y2O3:Eu film with high crystallinity is obtained without annealing. © 2001 American Institute of Physics.

Notes: The one-dimensional Schrödinger equation for an arbitrary potential with position-dependent mass is often solved by the transfer-matrix method. While the usual definition relates wave-function coefficients on two sides of an interface, this article presents an alternative approach, in which a propagation matrix evolves the wave function and its derivative between a pair of points. The formalism is developed without an a priori commitment to a breakdown of the potential into a series of flat, linear, or other types of segments. We obtain a Wick-expansion form for the matrix and also provide a geometrical interpretation based on the SL(2,R) group. Turning to a variably spaced discretized potential we show how this approach can be flexibly applied to any potential segments. We discuss explicitly the case of constant potential and the Wentzel–Kramers–Brillouin approximation, as well as the linear potential segment. For the latter, the obtained propagation matrix has definite advantages, from both speed and robustness standpoints. Applications to transport in the ballistic regime are discussed and explicit results are presented for a InP–InGaAs junction. © 2001 American Institute of Physics.

Notes: In our recent report, [Xu et al., Appl. Phys. Lett. 76, 152 (2000)], profile distributions of five elements in the GaN/sapphire system have been obtained using secondary ion-mass spectroscopy. The results suggested that a thin degenerate n+ layer at the interface is the main source of the n-type conductivity for the whole film. The further studies in this article show that this n+ conductivity is not only from the contribution of nitride-site oxygen (ON), but also from the gallium-site silicon (SiGa) donors, with activation energies 2 meV (for ON) and 42 meV (for SiGa), respectively. On the other hand, Al incorporated on the Ga sublattice reduces the concentration of compensating Ga-vacancy acceptors. The two-donor two-layer conduction, including Hall carrier concentration and mobility, has been modeled by separating the GaN film into a thin interface layer and a main bulk layer of the GaN film. The bulk layer conductivity is to be found mainly from a near-surface thin layer and is temperature dependent. SiGa and ON should also be shallow donors and VGa–O or VGa–Al should be compensation sites in the bulk layer. The best fits for the Hall mobility and the Hall concentration in the bulk layer were obtained by taking the acceptor concentration NA=1.8×1017 cm−3, the second donor concentration ND2=1.0×1018 cm−3, and the compensation ratio C=NA/ND1=0.6, which is consistent with Rode's theory. Saturation of carriers and the low value of carrier mobility at low temperature can also be well explained. © 2001 American Institute of Physics.

Notes: The time evolution of the quantum mechanical state of an electron is calculated in the framework of the effective-mass envelope function theory for an InAs/GaAs quantum dot. The results indicate that the superposition state electron density oscillates in the quantum dot, with a period on the order of femtoseconds. The interaction energy Eij between two electrons located in different quantum dots is calculated for one electron in the ith pure quantum state and another in the jth pure quantum state. We find that E11〉E12〉E22, and Eij decreases as the distance between the two quantum dots increases. We present a parameter-phase diagram which defines the parameter region for the use of an InAs/GaAs quantum dot as a two-level quantum system in quantum computation. A static electric field is found to efficiently prolong the decoherence time. Our results should be useful for designing the solid-state implementation of quantum computing. © 2001 American Institute of Physics.

Notes: A comparison is made of the electrical effects of carbon in n- and p-type in situ doped polycrystalline Si1−yCy and Si0.82−yGe0.18Cy layers. Values of resistivity as a function of temperature, effective carrier concentration and Hall mobility are reported. The n-type polycrystalline Si1−yCy and Si0.82−yGe0.18Cy films show dramatic increases in resistivity with carbon content, rising from 0.044 Ω cm to 450 Ω cm (0 and 0.8% C) and 0.01 Ω cm to 2.4 Ω cm (0 and 0.6% C), respectively. In contrast, the increase in B-doped films is much less severe, rising from 0.001 Ω cm to 0.939 Ω cm (0 and 7.9% C) and 0.003 Ω cm to 0.015 Ω cm (0 and 4% C) for the Si1−yCy and Si0.82−yGe0.18Cy layers, respectively. The grain boundary energy barrier, determined from the temperature dependence of the resistivity, is found to vary as the square of the C content in the n-type polycrystalline Si1−yCy and Si0.82−yGe0.18Cy layers, but linearly in the p-type Si1−yCy layers. The square law dependence seen in the n-type layers for C contents up to 0.9% is explained by an increase in the grain boundary trap density due to the presence of carbon, whereas the linear relationship seen in the p-type layers for C contents between 2% and 8% is explained by a shift in the grain boundary trap energy toward the valence band. Finally, lower values of grain boundary energy barrier are obtained in p-type Si0.82−yGe0.18Cy layers with a C content of 4% than in equivalent Si1−yCy layers, which could be explained by a larger shift in trap energy toward the valence band. © 2001 American Institute of Physics.

Notes: The influence of planar δ-doping on effective parameters of Schottky diodes has been studied by simulations of I–V curves within a drift-diffusion approximation. It is shown that an inserted δ layer with the same type of conductivity as that of the base semiconductor material has no significant influence on the effective Schottky diode parameters. The change of the potential barrier shape with the insertion of the δ-doped layer with the same type of conductivity as the base semiconductor material influences the diode current only slightly. On the other hand, significant changes were found for the Schottky diodes with the opposite type of δ-doped layer conductivity compared to the base semiconductor. The resultant barriers and the ideality factors both increase with increasing distance of the δ-doped layer from the metal–semiconductor interface. © 2001 American Institute of Physics.

Notes: Aluminum nitride insulating layers have been grown at room temperature with a film resistivity of 3.3×1016 Ω cm on mercury cadmium telluride substrates. Insulator breakdown fields of 640 MV/m were reached. Capacitance–voltage measurements with Al/AlN/Hg0.76Cd0.24Te metalinsulator-semiconductor devices demonstrate band bending at the semiconductor surface indicating that damage to the substrate was minimized during film growth. A fixed interface charge density (given here as total charge per unit area divided by the electron charge=Qss/q) of +2×1011 cm−2 and a slow interface state density of 4×1010 cm−2 were measured. The procedures for achieving these high quality insulating layers are reported. Frequency dependent dielectric constant and dielectric loss tangent measurements, carried out at room temperature and 100 K, are also presented. © 2001 American Institute of Physics.

Notes: The chemistry and electronic structure of interfaces between Al and copper hexadecafluorophthalocyanine (F16CuPc) are studied via x-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy (UPS), and current–voltage measurements. Electron injection barriers measured by UPS are reported. Analysis of the Al–F16CuPc reaction shows the formation of a phase of three-dimensional (F16CuPc)3Al species. The reacted region is extended at the Al-on-F16CuPc interface and narrow at the F16CuPc-on-Al interface. A series of metal/F16CuPc/metal structures is fabricated to study the impact of the interface chemistry and deposition sequence on device performances. It is found that (F16CuPc)3Al forms a low conductivity region, which has considerable bearing on the electron current. The interface fabrication sequence, which defines the thickness of the reacted region, therefore has a profound impact on device performance. © 2001 American Institute of Physics.

Notes: The real (χac′) and imaginary (χac″) components of the ac magnetic susceptibility of the polycrystalline GdAl2, DyAl2, and ErAl2 have been measured as functions of temperature, ac and bias dc magnetic field amplitude, and ac magnetic field frequency. Both χac′ and χac″ of the ferromagnetic DyAl2 and ErAl2 are strongly dependent on the ac magnetic field parameters, especially when compared with those of the ferromagnetic GdAl2. The observed behavior of the ac magnetic susceptibility is determined mainly by the domain dynamics, rather than by a change of the exchange interactions below Curie temperature. © 2001 American Institute of Physics.

Notes: Structured nanocomposite films consisting of five Fe layers embedded in an amorphous Al2O3 matrix (Fe:Al2O3) have been grown by sequential pulsed laser deposition. The formation of well isolated quasispherical nanocrystals is observed for samples with Fe content per layer close to 6.5×1015 atoms/cm2. Increasing the Fe content leads first to the formation of elongated nanocrystals and then to quasicontinuous layers. The evolution in the shape and size of the nanocrystals is reflected in the magnetic behavior of these systems. A crossover from a low temperature ferromagnetic regime to a high temperature superparamagnetic regime is observed at a temperature of 23 K in the samples containing isolated quasi-spherical nanocrystals. In this case, a reduced moment per Fe atom (1.4 μB/atom) with respect to the value for α-Fe (2.2 μB/atom) is estimated. This behavior is attributed to the presence of a Fe-oxide surface shell on the nanocrystals. The large values of the estimated effective magnetic anisotropy (1.4×106 J/m3) and the low temperature coercivity in these samples are attributed to a strong surface contribution to anisotropy, whereas the temperature dependence of coercivity is attributed to thermal activation. © 2001 American Institute of Physics.

Notes: A series of Permalloy films, of various shapes such as a square, a rectangle, a circle, and a rhombus, was made. The sample length to width ratio L/w varied from 1 (i.e., square) to 29 (i.e., rectangle). We studied how the planar Hall effect (PHE) signal was affected by adding a transverse field Hy along the easy-axis direction of the sample. It was found that in a certain range of Hy, the PHE sensitivity S might become inoperative, i.e., S changed linearly as a function of Hy from −Smax to +Smax (or vice versa), where Smax was the maximum sensitivity. This phenomenon is explained by considering the Zeeman-energy and the single-domain-rotation effects. In particular, for the square sample, the following data exist: (1) Smax is as high as 310 Ω/T at the film thickness t=500 Å, and (2) the inoperative range for Hy is the narrowest among all the samples. From this study, we conclude that the field of the Earth He—as long as its horizontal component is larger than 0.25 Oe—can be employed to stabilize the magnetic structure of a Permalloy element and to achieve the best PHE performance with Smax. © 2001 American Institute of Physics.

Notes: The phase transitions and structure of 7% La-modified Pb(Zr0.65Ti0.35)O3 (PLZT 7/65/35) at 40 K were investigated using high-resolution powder diffraction. The high-resolution data suggest the existence of an alternate phase, which is not expected from the widely accepted phase diagram. By structural analysis and Rietveld refinement, the space group of this phase was determined to be F1 (the standard setting is P1 by adding face-centered generators), rather than the expected R3c or the reported Cm phase. The unit cell of this phase is deduced by doubling the primitive cubic cell along three main axes, cell parameters area a=8.14546(13) Å, b=8.18539(15) Å, c=8.16785(11) Å, and interaxial angles are α=90.1925(24)°, β=89.8785(26)°, and γ=90.1564(34)°. The tilting system of oxygen octahedra is a¯b¯c¯ instead of a¯a¯a¯ in the R3c phase. The cation displacements and polarization are described in detail. The polarization direction was found to be along 〈112〉, not the main axes. The phase relationship and domain morphology are discussed in light of this triclinic phase, suggesting that this phase is important to understand the complex phase transitions and domain morphology in this series of materials. The experimental results suggest that this triclinic phase may extend to other compositions and temperature range. © 2001 American Institute of Physics.

Notes: A Ginzburg–Landau type theory of interaction of randomly distributed local dipoles in a paraelectric crystal is developed. The interaction is caused by the polarization of the host lattice generated by these dipoles. The obtained effective Hamiltonian of the dipole–dipole interaction is employed for the Monte Carlo simulation of ferroelectric properties of a system with off-center dopant ions producing local dipoles. The computer simulation shows that at low dopant ion concentration the paraelectric state transforms into a macroscopically paraelectric state consisting of randomly oriented polar clusters. These clusters amplify the effective dipole moment and dramatically increase the dielectric constant. The interaction between the clusters results in a spectrum of relaxation time and transition to the relaxor state. The real and imaginary parts of the susceptibility of this state are calculated. At intermediate dopant concentration, the material undergoes a diffuse phase transition into a ferroelectric state smeared within a temperature range. A further increase in the dopant concentration makes the transition sharper and closer to the conventional ferroelectric transition. The results obtained are compared with the behavior of the K1−xLixTaO3 relaxor ferroelectric. © 2001 American Institute of Physics.