M. Fatemi

X‐ray rocking curve measurement of composition and strain in Si‐Ge buffer layers grown on Si substrates

The level of strain and the fraction of Ge in SiGe layers grown on Si can be found rapidly and unambiguously using double‐crystal x‐ray diffraction and a simple application of the linear elasticity theory combined with Vegard’s law. The method gives excellent results for 0.4‐μm‐thick buffer layers of SiGe/Si containing 5%–50% germanium. It is shown that lattice relaxation rises abruptly at x(Ge)≥15%, and that some strain remains for x(Ge) as high as 50%.

Structure of stacking faults formed during the forward bias of 4H-SiC p-i-n diodes

Using site-specific plan-view transmission electron microscopy (TEM) and light emission imaging, we have identified stacking faults formed during forward biasing of 4H-SiC p-i-n diodes. These stacking faults (SFs) are bounded by Shockley partial dislocations and are formed by shear strain rather than by the condensation of vacancies or interstitials. Detailed analysis using TEM diffraction contrast experiments reveal SFs with leading carbon-core Shockley partial dislocations as well as with the silicon-core partial dislocations observed in plastic deformation of 4H-SiC at elevated temperatures. The leading Shockley partials are seen to relieve both tensile and compressive strain during p-i-n diode operation, suggesting the presence of a complex inhomogeneous strain field in the 4H-SiC layer.

Surface segregation and structure of Sb-doped Si(100) films grown at low temperature by molecular beam epitaxy

Abstract Sb surface segregation and doping during Si(100) molecular beam epitaxy were studied for growth temperatures of 320–500°C. Surface segregation was analyzed by depth profiling with secondary ion mass spectrometry and the results indicate the existence of several distinct dopant concentration- and temperature-dependent surface segregation regimes: (1) For dilute Sb surface concentrations the measurements reveal a region where bulk and surface concentrations are linearly related, and the surface segregation is described by a constant. However, the experimentally determined temperature dependence of the segregation does not follow simple kinetics theory, and appreciable surface segregation is observed at temperatures ≤ 400°C. (2) At temperatures ≥ 350°C, the surface segregation reaches a maximum for Sb surface concentrations of 0.5 monolayers. (3) For surface concentrations near 1 monolayer, the surface segregation decreases with increasing surface Sb coverage due to dopant interaction within surface and subsurface layers. In cases where films were grown under very high dopant fluxes, we have identified cone-like defects and stacking faults that are the result of the apparent surface concentration exceeding 1 monolayer.

Structural and optical properties of thick freestanding GaN templates

Structural and optical properties of thick (larger than 160 μm) freestanding hydride vapor phase epitaxy GaN templates have been investigated. AFM measurements showed that flat and smooth surface could be fabricated. High-resolution X-ray diffraction studies carried out with different spectrometer slit for the symmetric and asymmetric diffractions show that the linewidth increases with increasing slits width, indicating that a considerable degree of tilting and twisting of the individual grains are still present in these thick samples. Raman scattering measurements performed in a few samples indicate good crystalline quality and reduced strain. Very sharp and intense exciton related lines (FWHM less than 1 meV) have been observed in the low temperature photoluminescence spectra. Variable-temperature photoluminescence experiments were performed on both the growth surface and interface to identify the nature of the recombination processes observed in the luminescence spectra. FTIR absorption measurements show the presence of at least two donors with binding energy of 30.5 and 33.6 meV.

Enhancement of electrical and structural properties of GaN layers grown on vicinal-cut, a-plane sapphire substrates

We report the observation of significant enhancement in the electrical and crystalline properties of GaN layers grown on vicinally cut, a-plane sapphire substrates. Room-temperature Hall mobility and x-ray rocking curve data show a nearly twofold improvement, independent of the processing conditions, for layers grown on substrates having vicinal angles of 1.5° compared to on-axis substrates. Transmission electron microscopy shows reduced edge dislocation density and better alignment of the grains in layers grown on vicinally cut substrates. Preliminary photoluminescence measurements also indicate pronounced differences in the yellow band spectra between the on-axis and off-axis cut substrates. These findings contrast the relatively modest improvements observed in layers grown on c-plane substrates with vicinal angles as high as 10°.

Influence of AlN nucleation layer temperature on GaN electronic properties grown on SiC

GaN electronic properties are shown to depend on the AlN nucleation layer (NL) growth temperature for GaN films grown on 6H– and 4H–SiC. Using identical GaN growth conditions except AlN NL growth temperature, 300 K electron mobilities of 876, 884, and 932 cm2/Vs were obtained on 6H–SiC, 4H–SiC, and 3.5° off-axis 6H–SiC. An AlN NL temperature of 1080 °C was used for the planar and 3.5° off-axis 6H–SiC, while an AlN NL temperature of 980 °C was used for 4H–SiC. Atomic force microscope images of the AlN NL grown at 1080 °C reveal smaller AlN grains on the 6H–SiC than those on 4H–SiC, suggesting that the AlN morphology influences GaN film formation and subsequent electron mobility. Transmission electron microscope cross section measurements reveal the absence of screw dislocations in the AlN and a low screw dislocation density near the AlN/GaN interface, consistent with the high electron mobilities achieved in these films.

Fabrication of GaAs and InAs wires in nanochannel glass

A newly developed porous glass, nanochannel glass, was used to fabricate uniform, high‐density GaAs and InAs micro‐ and nanowires with high aspect ratios. The fabrication process utilized reactions between organogallium and organoindium compounds with arsine to produce polycrystalline GaAs and InAs with crystallite sizes of approximately 50–130 A when annealed at 400–500 °C. At the higher annealing temperatures, the InAs wires exhibited an increase in surface porosity and grain size, whereas the GaAs wires maintained a uniform, smooth texture.

Organometallic chemical vapor deposition and characterization of indium phosphide nanocrystals in Vycor porous glass

Indium phosphide has been deposited in 40 and 150 A Vycor porous glass by the reaction of trimethylindium with excess PH3. X‐ray diffraction (XRD) of the red, transparent samples confirms the presence of crystalline indium phosphide. The amount of III‐V material deposited within the glass matrix (loading) and annealing conditions influence the crystallinity and particle size of the nanocrystallites. The absorption spectra of vacuum‐annealed samples is blue shifted with respect to bulk InP, with maxima ranging from 500 to 700 nm for both the 40 and 150 A samples and is consistent with size quantization. The nonlinear optical properties of these nanocrystallite composites have been evaluated by Z‐scan measurements and are characterized by a defocusing refractive nonlinearity influenced by a substantial nonlinear loss presumed to be caused by two‐photon absorption.

Properties of Bulk AlN grown by thermodecomposition of AlCl3⋅NH3

Self-nucleated bulk AlN crystals were grown by thermodecomposition of AlCl3⋅NH3 vaporized in the low-temperature zone of a two-zone furnace. X-ray diffraction of the AlN crystals show single lines with a small linewidth indicating high single-crystalline quality. Polarized Raman scattering experiments of these samples confirm the x-ray results based on the detection of a small linewidth for all allowed optical phonons. Low-temperature cathodoluminescence spectra show very sharp emission bands close to the optical band gap, which have been assigned to free-excitons A and B, and exciton-bound to shallow neutral impurity. The latter has a full width at half maximum smaller than 1.0 meV.Self-nucleated bulk AlN crystals were grown by thermodecomposition of AlCl3⋅NH3 vaporized in the low-temperature zone of a two-zone furnace. X-ray diffraction of the AlN crystals show single lines with a small linewidth indicating high single-crystalline quality. Polarized Raman scattering experiments of these samples confirm the x-ray results based on the detection of a small linewidth for all allowed optical phonons. Low-temperature cathodoluminescence spectra show very sharp emission bands close to the optical band gap, which have been assigned to free-excitons A and B, and exciton-bound to shallow neutral impurity. The latter has a full width at half maximum smaller than 1.0 meV.

Properties of Si-doped GaN films grown using multiple AlN interlayers

Electrical, optical, and structural properties of Si-doped GaN films grown on multiple AlN interlayers (IL) sandwiched between high-temperature (HT) GaN are presented. We show that as the number of AlN IL/HT GaN layers increases, the electron mobility increases in the top Si-doped GaN layer, showing a near doubling from 440 to 725 cm2 V−1 s−1. Cross-sectional transmission electron microscopy images reveal a significant reduction in the screw dislocation density for GaN films grown on the AlN IL/HT GaN layers. The symmetric and off-axis x-ray linewidths increase as the number of AlN IL/HT GaN layers increase, indicating a greater relative misalignment of the adjacent HT GaN layers. Photoluminescence spectra of undoped and Si-doped GaN films on the multiple AlN IL/HT GaN layers have small yellow-band intensity. Analysis based on a single-donor/single-acceptor model for the electrical conduction suggests that the improved electron mobility is the result of a reduced acceptor concentration in the top GaN film...