ALBERT

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 At sufficiently high N/Ta ratios, however, simple electron-counting arguments are no longer valid since large deviations from stoichiometry alter the character of the band structure itself.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: Epitaxial zinc blende structure metastable (GaAs)1−x(Si2)x alloys have been grown with 0〈x〈0.3 on As-stabilized GaAs(100) substrates by a hybrid sputter deposition/evaporation technique. The films, typically 2–3 μm thick, were deposited at 570 °C with growth rates between 0.7 and 1 μm h−1. Alloys with 0〈x〈0.12 were defect-free as judged by plan-view and cross-sectional transmission electron microscopy (TEM and XTEM) with x-ray diffraction peak widths approximately the same as that of the substrate, 30 arcsec 2θ. XTEM lattice images showed smooth abrupt interfaces. (GaAs)1−x(Si2)x alloys with x〉0.12 exhibited increasing evidence of interfacial defects associated with lattice strain when grown on GaAs. However, defect-free alloys with x up to 0.3 were obtained using (GaAs)1−x(Si2)x/GaAs strained-layer superlattice buffer layers to provide a better lattice match.

Notes: 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.

Notes: The microstructure of single-crystal zincblende-structure (GaAs)1−x(Si2)x metastable semiconducting alloys with 0≤x≤0.40 has been investigated using triple-crystal x-ray diffraction (XRD), plan-view and cross-sectional transmission electron microscopy (TEM and XTEM), scanning transmission electron microscopy, and convergent-beam electron diffraction. The alloys, typically 1–3 μm thick, were grown using a hybrid sputter-deposition/evaporation technique on As-stabilized GaAs(001) and (GaAs)1−x(Si2)x/GaAs(001) strained-layer superlattices, (SLS). Alloy XRD peak widths were approximately equal to those of the GaAs substrates, 30 arcsec, and lattice constants, uncorrected for strain, obeyed Vegard's "law'' and decreased linearly with increasing x. TEM and XTEM examinations of (GaAs)1−x(Si2)x alloys with 0≤x≤0.20 grown on GaAs revealed no evidence of dislocations or other extended defects. Film/substrate lattice misfit strain in alloys with 0.11〈x〈0.20 was partially accommodated by the formation of a thin interfacial spinodal layer whose average thickness increased with x to (approximately-equal-to)70 nm. The spinodal region, which remained epitaxial, consisted of lenticular platelets extending along the [001] direction with a compositional modulation in orthogonal directions. Films with x≥0.20 exhibited, together with the interfacial zones, inhomogeneously distributed a0/2〈110〉-type threading dislocations. Antiphase domains were observed in alloys with x≥0.23. The use of (GaAs)1−x(Si2)x/GaAs SLS buffer layers extended the composition range to x=0.3 over which dislocation-free alloys, with no evidence of interfacial spinodal decomposition, could be obtained.

Notes: A low-energy, high-brightness, broad beam Cu ion source is used to study the effects of self-ion energy Ei on the deposition of epitaxial Cu films in ultrahigh vacuum. Atomically flat Ge(001) and Si(001) substrates are verified by in situ scanning tunneling microscopy (STM) prior to deposition of 300 nm Cu films with Ei ranging from 20 to 100 eV. Film microstructure, texture, and morphology are characterized using x-ray diffraction ω-rocking curves, pole figure analyses, and STM. Primary ion deposition produces significant improvements in both the surface morphology and mosaic spread of the films: At Ei〉37 eV the surface roughness decreases by nearly a factor of 2 relative to evaporated Cu films, and at Ei(approximately-equal-to)35 eV the mosaic spread of Cu films grown on Si substrates is only (approximately-equal-to)2°, nearly a factor of 2 smaller than that of evaporated Cu. During deposition with Ei(approximately-equal-to)25 eV on Ge substrates, the film coherently relaxes the 10% misfit strain by formation of a tilt boundary which is fourfold symmetric toward 〈111〉. The films have essentially bulk resistivity with ρ=1.9±0.1 μΩ cm at room temperature but the residual resistance at 10 K, ρ0, shows a broad maximum as a function of Ei, e.g., at Ei(approximately-equal-to)30 eV, ρ0=0.5 μΩ cm. © 1996 American Institute of Physics.

Notes: Surface morphological and compositional evolution during the initial stages of Si growth on Ge(001)2×1 by cyclic gas-source molecular beam epitaxy from Si2H6 has been investigated using in situ reflection high-energy electron diffraction (RHEED), Auger electron spectroscopy, electron-energy-loss spectroscopy, and scanning tunneling microscopy, combined with post-deposition high-resolution cross-sectional transmission electron microscopy. The layers were deposited using repetitive cycles consisting of saturation Si2H6 dosing at room temperature, followed by annealing for 1 min at 550 °C. Film growth was observed to proceed via a mixed Stranski–Krastanov mode. Single-step-height two-dimensional growth was obtained for nominal Si deposition thicknesses tSi up to (approximately-equal-to)1.5 monolayers (ML). However, the upper layer remained essentially pure Ge which segregated to the surface through site exchange with deposited Si as H was desorbed. At higher tSi, the Ge coverage decreased slowly, the surface roughened, and two-dimensional multilayer island growth was observed for tSi up to (approximately-equal-to)7.5 ML, where bulk reflections in RHEED patterns provided evidence for the evolution of three-dimensional island formula.

Notes: A single-grid ultra-high-vacuum-compatible ion source was used to provide accelerated In+-dopant beams during Si(100) growth by molecular-beam epitaxy. Indium incorporation probabilities σ, determined by secondary ion mass spectrometry, in films grown at Ts=800 °C were too low to be measured for thermal In (σIn was 〈3×10−5 at Ts〉550 °C) . However, for accelerated In+ doping, σIn+ at 800 °C ranged from 0.03 to ∼1 for In+ acceleration energies EIn+ between 50 and 400 eV. Temperature-dependent Hall-effect and resistivity measurements were carried out on In+-doped Si films grown at Ts =800 °C with EIn+=200 eV . Indium was incorporated substitutionally into electrically active sites over a concentration ranging from 2×1015−2×1018 cm−3, which extends well above reported equilibrium solid-solubility limits. The acceptor-level ionization energy was 156 meV, consistent with previously published results for In-doped bulk Si. Room-temperature hole mobilities μ were in good agreement with the best reported data for B-doped bulk Si and were higher than previously reported values for annealed In-implanted Si. Temperature-dependent (77–400 K) mobilities μ(T) were well described by theoretical calculations, with no adjustable parameters, including lattice, ionized-impurity, neutral-impurity, and hole-hole scattering. Lattice scattering was found to dominate, although ionized-impurity scattering was still significant, at temperatures above ∼150 K where μ varied approximately as T−2.2 . Neutral-impurity scattering dominated at lower temperatures. Plan-view and cross-sectional transmission electron microscopy observations showed no indications of dislocations or other extended defects. Considering the entire set of results, there was no evidence of residual ion-bombardment-induced lattice damage.