Library

Notes: The stability of an amorphous GexSi1−xO2 in contact with an epitaxial (100)GexSi1−x layer obtained by partially oxidizing an epitaxial GexSi1−x layer on a (100)Si substrate in a wet ambient at 700 °C is investigated for x=0.28 and 0.36 upon annealing in vacuum at 900 °C for 3 h, aging in air at room temperature for 5 months, and immersion in water. After annealing at 900 °C, the oxide remains amorphous and the amount of GeO2 in the oxide stays constant, but some small crystalline precipitates with a lattice constant similar to that of the underlying GeSi layer emerge in the oxide very near the interface for both x. Similar precipitates are also observed after aging for both x. The appearance of these precipitates can be explained by the thermodynamic instability of GexSi1−xO2 in contact with GexSi1−x. In water at RT, 90% of GeO2 in the oxide is dissolved for x=0.36, while the oxide remains conserved for x=0.28.

Notes: We propose and study a new GaSb/InAs/GaSb/AlSb/InAs broken-gap interband tunneling diode by varying the thicknesses of the InAs layer. A twice-higher peak current density and a three-times-higher peak-to-valley current ratio in the proposed structure with a 30-A(ring)-thick InAs layer were observed relative to the structure with no InAs layer. This result indicates that the characteristic of the negative differential resistance can be improved simply by placing a thin effective InAs barrier on the GaSb side of the GaSb/AlSb/InAs single-barrier structure. The increase of the peak current is interpreted as the result of forming a quasi-bound state in the GaSb well. This interpretation is supported by the observation that the current-voltage characteristic of the proposed structure is similar to that of a conventional GaSb/AlSb/GaSb/AlSb/InAs double-barrier interband tunneling structure.

Notes: Sol-gel derived ferroelectric thin films of lead zirconate titanate have been annealed through the rapid thermal annealing (RTA) technique to investigate the effect of various annealing temperature-time combinations. Crystallization of the film into the perovskite phase required 10 s at 600 °C and a mere 1 s at 700 °C. Rapid thermally annealed films recorded weak-field permittivities greater than 1000, dissipation losses of 0.02–0.05, maximum remanent polarization of 29 μC/cm2, and coercive field around 40 kV/cm. RTA films are distinguished by superior breakdown strengths, and morphologically smoother surfaces. The frequency dependent dielectric constants have been discussed in terms of a lumped circuit model.

Notes: We report on the experimental studies of an interband tunneling effect between GaSb valence-band and InAs conduction-band quantum wells in a GaSb/AlSb/InAs/AlSb/GaSb/AlSb/InAs triple-barrier interband tunneling device. Multiple negative differential resistances were observed both at room temperature and 77 K. By varying the InAs well width to adjust the alignment of the conduction-band and the valence-band quasi-bound states, we observed more than one order of magnitude variation in the peak current density, indicating a significant quantization effect. Possible current conduction mechanisms were discussed based on the dependence of the peak-current density on the InAs well width.

Notes: We present quantum mechanical self-consistent calculations on the transfer characteristics of a new resonant tunneling transistor. The model structure consists of a source, a tunneling barrier, a quantum-well drain, a thick insulator, and a backgate. The tunneling barrier consists of a double-barrier structure. We demonstrate that based on energy and momentum conservations, the transistors display oscillatory negative transconductance, as the gate can control the resonant tunneling probability between source and drain. With the inclusion of a realistic energy relaxation time of ∼0.1 ps, the double-barrier resonant tunneling transistor shows an enhancement of the tunneling current density and the negative transconductance feature is only slightly changed. We also find that the quantum-well drain is not able to completely screen the electric field imposed by the backgate bias as a result of limited density of states of two-dimensional systems.

Notes: Having been exposed to hydrogen plasma, Te-doped GaAs wafers were deposited with metal Ti, to form Ti/n-GaAs Schottky barrier diodes (SBD). It was found that due to hydrogenation the Schottky barrier height (SBH) of the SBD decreases from 0.76 to 0.58 eV and the effective Richardson constant A** decreases from 11 to 0.03 A/cm2/K2. Annealing the hydrogenated SBD at different reverse biases at 100 °C produces SBHs that have a one-to-one correlation with the biases of reverse-bias annealing (RBA), i.e., the SBH can be controlled, within the range 0.58–0.74 eV, by the bias of RBA. When the reverse bias is equal to or larger than 3 V, the SBH and the effective Richardson constant of the hydrogenated SBDs after RBA are very near to those of SBDs without hydrogenation. This means that a RBA with a bias higher than a critical value, 3 V in our case, can remove the effects of hydrogenation, but if the SBD is annealed again without a bias at 100 °C for 1 h or more, its SBH and effective Richardson constant recover their hydrogenated values. The main experimental facts can be explained qualitatively by a simple theoretical model.

Notes: Au, Cu, Ag, and Al are deposited on Te-doped n-GaSb layers directly grown on lattice mismatched GaAs semi-insulating substrates. Sb4/Ga beam equivalent pressure ratios are found to profoundly influence the electrical properties of the Schottky diodes investigated here. The fact that both breakdown voltage and barrier height decrease with increasing Sb4/Ga ratios is attributed to the increase in surface state densities for samples grown at higher Sb4/Ga ratios. This suggestion is further confirmed by the model of surface state densities employing the relationship of barrier height to metal work function. The surface state densities are in the range of 2.3×1014 to 1.2×1015 states/cm2/eV corresponding to Sb4/Ga ratios of 2 to 9, respectively. X-ray rocking peaks of samples grown at various Sb4/Ga ratios, and subsequently subjected to annealing, indicate different interactions at the interfaces which might support the observations.

Notes: Several etchants have been applied to polar (111)Cd and (111)Te surfaces of CdTe. Induced surface layers were analyzed by x-ray photoelectron spectroscopy and grazing-incidence x-ray diffraction for studying compositions, chemical states, and crystalline phases of respective layers. A bromine/methanol etch led to a layer composed of mainly tellurium oxides on both (111) surfaces. Oxidation depth, however, was larger at the (111)Te side. Etching with HCl subsequently removed the oxides. A film made of crystalline tellurium was, however, produced with the thickness being larger on the (111)Te surface than on the other surface. A fairly thick layer consisting of TeO2 and crystalline Te for etching with N solution (H2O2:H2O:HF=2:2:3 v/v), formed on both surfaces with the cadmium having been severely depleted, especially on the (111)Te surface.

Notes: The formation and decomposition of formate species on the clean and on potassium-modified Ru(001) surfaces have been investigated with time-resolved vibrational spectroscopy and thermal desorption mass spectrometry (TDMS). Utilizing Fourier transform-infrared reflection absorption spectroscopy (FT-IRAS) we have characterized chemisorbed formate produced by the decomposition of formic acid on clean Ru(001), Ru(001)–((square root of)3×(square root of)3)R30° K and on a K-multilayer adsorbed on Ru(001). The vibrational spectra show that formate is adsorbed on both clean Ru(001) and Ru(001)–((square root of)3×(square root of)3)R30° K with C2v symmetry indicative of a bridged or bidentate species. There are, however, characteristic differences in the vibrational spectra, which indicate that for the Ru(001)–((square root of)3×(square root of)3)R30° K surface the formate is directly bound to potassium. The vibrational spectrum of the latter species is found to be in good agreement with that of bulk potassium formate adsorbed on Ru(001).Based on the agreement with literature data for bulk formate, we propose a bonding model for the potassium formate monolayer, which also accounts for the observed contraction of the potassium monolayer resulting from the compound formation. The thermal decomposition of the various formate overlayers has been monitored by simultaneous thermal desorption mass spectrometry and time-resolved FT-IRAS. This combination allows us to correlate the desorbing gas-phase products with the appearance and disappearance of surface intermediates. In the case of formate adsorbed on the clean Ru(001), the C–H and C–O bond cleavage reactions occur simultaneously, leading to the production of equal amounts of CO and CO2. The simultaneous observation of desorbing CO2 (TDMS) and of adsorbed CO (IR) confirms earlier work, which postulated a mechanism involving a coupling of the C–H and C–O bond cleavage reaction channels of two neighboring formates. The presence of potassium changes dramatically the reaction pathway of the formate as it suppresses the C–H bond cleavage channel, leaving CO and OH as the main decomposition products. Compound formation with potassium also leads to thermal stabilization of the formate in comparison to formate adsorbed on the clean surface. However, formate adsorbed on the potassium-modified ruthenium substrate is found to be thermally less stable than formate adsorbed on clean Ru(001).

Notes: We demonstrate the utility of a simple, inexpensive, microchannel plate (MCP) detector for monitoring the x-ray fluorescence (XRF) yield in the soft x-ray region. The detector consists of a dual MCP array, and appropriately biased grids. We compare the Al 2p XRF yield to the total-electron yield (TEY) (obtained with the same detector) of single-crystal sapphire. These measurements show that the XRF yield has the following advantages over TEY for monitoring absorption spectra in the soft x-ray region: (1) a greater bulk sensitivity, (2) an insensitivity to charging.