Bibliothek

Notizen: Epitaxial undoped and Sb-doped Si films have been grown on Si(001) substrates at temperatures Ts between 80 and 750 °C by ultrahigh-vacuum Kr+-ion-beam sputter deposition (IBSD). Critical epitaxial thicknesses te in undoped films were found to range from 8 nm at Ts=80 °C to (approximately-greater-than)1.2 μm at Ts≥300 °C, while Sb incorporation probabilities σSb varied from unity at Ts(approximately-less-than)550 °C to (approximately-equal-to)0.1 at 750 °C. These te and σSb values are approximately one and one to three orders of magnitude, respectively, higher than reported results achieved with molecular-beam epitaxy. Temperature-dependent transport measurements carried out on 1-μm-thick Sb-doped IBSD layers grown at Ts≥350 °C showed that Sb was incorporated into substitutional sites with complete electrical activity and that electron mobilities in films grown at Ts≥400 °C were equal to the best reported results for bulk Si. © 1996 American Institute of Physics.

Notizen: Epitaxial Si1−xGex(001) alloy films, with 0.15≤x≤0.30, were grown on Si(001) at temperatures Ts ranging from 300 to 550 °C using hyperthermal Si (average energy 〈ESi〉(approximately-equal-to)18 eV) and Ge (〈EGe〉(approximately-equal-to)15 eV) beams. The deposition rate was 0.1 nm s−1 and film thicknesses ranged from 30 nm to 0.8 μm. The energetic Si and Ge beams are generated by bombarding Si and Ge targets with 1 keV Kr+ ions from double-grid, multiaperture, broad ion-beam sources in a system geometry established based upon TRIM simulations of energy-dependent angular distributions of sputtered and backscattered particles. A combination of high-resolution plan-view and cross-sectional transmission electron microscopy, high-resolution x-ray diffraction, Rutherford backscattering spectroscopy, channeling, and axial angular-yield profiles demonstrated that the films are of extremely high crystalline quality. Critical layer thicknesses hc for strain relaxation in these alloys were found to increase rapidly with decreasing growth temperature. For Si0.70Ge0.30, hc ranged from 35 nm at Ts=550 °C to 650 nm at 350 °C compared to an equilibrium value of (approximately-equal-to)8 nm. At even lower growth temperatures, hc becomes larger than critical epitaxial layer thicknesses, (approximately-greater-than)1 μm at 300 °C. In addition, atomic force microscopy studies showed that strain-induced roughening, which occurs at elevated growth temperatures, is strongly suppressed at Ts between 300 and 400 °C with no indication of kinetic roughening. © 1996 American Institute of Physics.

Notizen: The evolution of surface roughness in epitaxial Si0.7Ge0.3 alloys grown on Si(001) as a function of temperature (200–600 °C), thickness (t=7.5–100 nm), and substrate miscut were investigated by atomic force microscopy and quantified in terms of the height-difference correlation function G(ρ), in which ρ is lateral distance and [G(ρ→∞)]1/2 is proportional to the surface width. The films were deposited by ultrahigh vacuum ion-beam sputter deposition at 0.1 nm s−1. Strain-induced surface roughening was found to dominate in alloys grown on singular Si(001) substrates at Ts(approximately-greater-than)450 °C where [G(ρ→∞)]1/2 initially increases with increasing t through the formation of coherent islanding. The islands are preferentially bounded along 〈100〉 directions and exhibit 105 faceting. This tendency is enhanced, with much better developed 〈100〉 islands separated by deep trenches—of interest for growth of self-assembled nanostructures—in films grown on Si(001)-4°[100]. Increasing the film thickness above critical values for strain relaxation leads to island coalescence and surface smoothening. At very low growth temperatures (Ts≤250 °C), film surfaces roughen kinetically, due to limited adatom diffusivity, but at far lower rates than in the higher-temperature strain-induced regime. Si0.7Ge0.3 alloy surfaces are smoother, while the films exhibit larger critical epitaxial thicknesses, than those of pure Si films grown in this temperature regime. There is an intermediate growth temperature range, however, over which the alloy film surfaces remain extremely smooth even at thicknesses near critical values for strain relaxation. This latter result is of potential importance for device fabrication. © 1996 American Institute of Physics.

Notizen: Secondary-ion-mass spectrometry (SIMS) was used to determine the concentration and depth distribution of B incorporated into Ge(001)2×1 films grown on Ge(001) substrates by gas-source molecular-beam epitaxy using Ge2H6 and B2H6. B concentrations CB (3×1016–4×1019 cm−3) were found to increase linearly with increasing flux ratio JB2H6/JGe2H6 (8.2×10−3–1.7) at constant film growth temperature Ts (300–400 °C) and to increase exponentially with 1/Ts at constant JB2H6/JGe2H6 ratio. The difference in the overall activation energies for B and Ge incorporation over this growth temperature range is (approximately-equal-to)0.22 eV while B2H6 reactive sticking probabilities ranged from 8×10−4 at 300 °C to 2×10−5 at 400 °C. SIMS depth profiles from B modulation-doped samples and two-dimensional δ -doped samples grown at Ts〈350 °C were abrupt to within instrumental resolution with no indication of surface segregation. Structural analysis by in situ reflection high-energy electron diffraction combined with postdeposition high-resolution plan-view and cross-sectional transmission electron microscopy showed that all films were high-quality single crystals with no evidence of dislocations or other extended defects. B doping had no measurable affect on Ge deposition rates. © 1995 American Institute of Physics.

Notizen: The microstructure of nominally undoped epitaxial wurtzite-structure α-GaN films, grown by gas-source molecular-beam epitaxy, plasma-assisted molecular-beam epitaxy, and metalorganic chemical-vapor deposition, has been investigated by transmission electron microscopy (TEM) and high-resolution TEM. The results show that undoped α-GaN films have an ordered point-defect structure. A model of this defect-ordered microstructure, based upon a comparison between experimental results and computer simulations, is proposed. © 1994 American Institute of Physics.

Notizen: B-doped Si(001)2×1 films were grown on Si(001) substrates by gas-source molecular beam epitaxy using Si2H6 and B2H6. B concentrations CB (5×1016–5×1019 cm−3) were found to increase linearly with increasing flux ratio JB2H6/JSi2H6 (9.3×10−5–2.5×10−2) at constant film growth temperature Ts (600–950 °C) and to decrease exponentially with 1/Ts at constant JB2H6/JSi2H6 ratio. The B2H6 reactive sticking probability ranged from (approximately-equal-to)6.4×10−4 at Ts=600 °C to 1.4×10−3 at 950 °C. The difference in the overall activation energies for B and Si incorporation at Ts=600–950 °C is (approximately-equal-to)0.34 eV. A comparison of quantitative secondary-ion mass spectrometry (SIMS) and temperature-dependent Hall-effect measurements showed that B was incorporated into substitutional electrically active sites over the entire B concentration range investigated. SIMS B depth profiles from modulation-doped samples were abrupt with no indication of surface segregation to within the instrumental resolution limit and initial δ-doping experiments were carried out. Structural analysis by in situ reflection high-energy electron diffraction combined with post-deposition high-resolution plan-view and cross-sectional transmission electron microscopy showed that all films were high-quality single crystals with no evidence of dislocations or other extended defects. Temperature-dependent (20–300 K) hole carrier mobilities were equal to the best reported bulk Si:B values and in good agreement with theoretical maximum values. © 1995 American Institute of Physics.

Notizen: The growth rates RGe of epitaxial Ge films deposited on Ge(001)2×1 and Si(001)2×1 substrates from Ge2H6 by gas-source molecular beam epitaxy were determined over a wide range of temperatures Ts (300–800 °C) and impingement fluxes JGe2H6(0.1–1×1016 cm−2 s−1). Steady-state RGe(Ts, JGe2H6) curves were well described at both low and high growth temperatures (Ts≤325 °C and Ts(approximately-greater-than)500 °C) using a model based upon dissociative Ge2H6 chemisorption followed by a series of surface decomposition reactions with the rate-limiting step being first-order hydrogen desorption from Ge monohydride for which the activation energy was found to be 1.56 eV. At intermediate temperatures, however, experimental RGe results exhibited a large positive deviation from model predictions due, as demonstrated by temperature programmed desorption measurements and transmission electron microscopy (TEM) observations, to kinetic surface roughening. Extensive (113) faceting resulted in both an increase in the number of active surface sites and higher reactive sticking probabilities. With increased growth temperatures, the facets became more rounded and film surfaces appeared sinusoidal in cross section. The zero-coverage Ge2H6 reactive sticking probability on Ge(001) in the high-temperature flux-limited regime was found to be 0.052, more than two orders of magnitude higher than that for GeH4. In situ reflection high-energy electron diffraction and post-deposition TEM examinations showed that Ge films deposited on Ge(001) at Ts≤325 °C grew in a layer-by-layer mode exhibiting a smooth flat surface. © 1995 American Institute of Physics.

Notizen: A short history of the roots of DSM-III and the politics of its creation help to explain the extensive occasions it offers for fulfilling criteria for multiple diagnoses. With use of standardized interviews, it becomes possible to discover all the diagnoses a single person can qualify for. This should prompt rethinking how diagnoses are selected and what rules for preemptions accord with the nature of psychopathology.

Notizen: Reactive-ion molecular-beam epitaxy has been used to grow epitaxial hexagonal-structure α-GaN on Al2O3(0001) and Al2O3(011¯2) substrates and metastable zinc-blende-structure β-GaN on MgO(001) under the following conditions: growth temperature Ts=450–800 °C; incident N+2/Ga flux ratio JN+2/JGa=1–5; and N+2 kinetic energy EN+2=35–90 eV. The surface structure of the α-GaN films was (1×1), with an ≈3% contraction in the in-plane lattice constant for films grown on Al2O3(0001), while the β-GaN films exhibited a 90°-rotated two-domain (4×1) reconstruction. Using a combination of in situ reflection high-energy electron diffraction, double-crystal x-ray diffraction, and cross-sectional transmission electron microscopy, the film/substrate epitaxial relationships were determined to be: (0001)GaN(parallel) (0001)Al2O3 with [21¯1¯0]GaN(parallel)[11¯00]Al2O3 and [11¯00]GaN(parallel)[12¯10]Al2O3, (21¯1¯0)GaN(parallel)(011¯2)Al2O3 with [0001]GaN(parallel)[01¯11]Al2O3 and [01¯10]GaN(parallel)[21¯1¯0]Al2O3, and (001)GaN(parallel)(001)MgO with [001]GaN(parallel)[001]MgO.Films with the lowest extended defect number densities (nd(approximately-equal-to)1010 cm−2 threading dislocations with Burgers vector a0/3〈112¯0(approximately-greater-than)) and the smallest x-ray-diffraction ω rocking curve widths (5 min) were obtained using Al2O3(0001) substrates, Ts≥650 °C, JN+2/JGa≥3.5, and EN+2=35 eV. Higher N+2 acceleration energies during deposition resulted in increased residual defect densities. In addition, EN+2 and JN+2/JGa were found to have a strong effect on film growth kinetics through a competition between collisionally induced dissociative chemisorption of N2 and stimulated desorption of Ga as described by a simple kinetic growth model. The room-temperature resistivity of as-deposited GaN films grown at Ts=600–700 °C with EN+2=35 eV increased by seven orders of magnitude, from 10−1 to 106 Ω cm, with an increase in JN+2/JGa from 1.7 to 5.0. Hall measurements on the more conductive samples yielded typical electron carrier concentrations of 2×1018 cm−3 with mobilities of 30–40 cm2 V−1 s−1. The room-temperature optical band gaps of α-GaN and β-GaN were 3.41 and 3.21 eV, respectively.