R.L. Henry

Current collapse and the role of carbon in AlGaN/GaN high electron mobility transistors grown by metalorganic vapor-phase epitaxy

The two deep traps responsible for current collapse in AlGaN/GaN high electron mobility transistors grown by metalorganic vapor-phase epitaxy have been studied by photoionization spectroscopy. Varying the growth pressure of the high resistivity GaN buffer layer results in a change in the deep trap incorporation that is reflected in the observed current collapse. Variations in the measured trap concentrations with growth pressure and carbon incorporation indicate that the deepest trap is a carbon-related defect, while the mid-gap trap may be associated with grain boundaries or dislocations.

Influence of MOVPE growth conditions on carbon and silicon concentrations in GaN

Abstract Impurity incorporation is studied as a function of metalorganic vapor phase epitaxy growth conditions. The same GaN growth conditions were used initially, resulting in films with approximately the same dislocation density, after which a single growth parameter was varied and the impurity concentrations measured using SIMS. The C concentrations were found to decrease with increasing growth temperature, pressure, and ammonia flow, and to increase with increasing H 2 carrier and trimethylgallium flow. The Si concentrations for both unintentionally doped (UID) and intentionally doped (ID) films increased with increasing growth pressure. The UID and ID Si concentrations varied inversely with the GaN growth rate, suggesting an independent source for UID Si within the reactor. Moreover, the NH 3 flow rate influenced the Si-doping concentration, even though the GaN growth rate remained constant. A H 2 /NH 3 etching mechanism is proposed to explain the growth parameter influence on the observed C and Si concentrations. The reduction in the ID Si concentrations at high NH 3 flows is explained by NH 3 site blocking, similar to that proposed for increased Ga vacancies at high NH 3 flows.

Current collapse induced in AlGaN/GaN high-electron-mobility transistors by bias stress

Current collapse is observed to be induced in AlGaN/GaN high-electron-mobility transistors as a result of short-term bias stress. This effect was seen in devices grown by both metalorganic chemical vapor deposition (MOCVD) and molecular-beam epitaxy (MBE). The induced collapse appears to be permanent and can be reversed by SiN passivation. The traps responsible for the collapse have been studied by photoionization spectroscopy. For the MOCVD-grown devices, the same traps cause the collapse in both unstressed and stressed devices. These effects are thought to result from hot-carrier damage during stress.

Nondestructive analysis of threading dislocations in GaN by electron channeling contrast imaging

Threading dislocations in metal-organic chemical-vapor grown GaN films were imaged nondestructively by the electron channeling contrast imaging (ECCI) technique. Comparisons between ECCI and cross-sectional transmission electron microscopy indicated that pure edge dislocations can be imaged in GaN by ECCI. Total threading dislocation densities were measured by ECCI for various GaN films on engineered 4H-SiC surfaces and ranged from 107to109cm−2. A comparison between the ultraviolet electroluminescent output measured at 380nm and the total dislocation density as measured by ECCI revealed an inverse logarithmic dependence.

Lowered dislocation densities in uniform GaN layers grown on step-free (0001) 4H-SiC mesa surfaces

We report that very low threading dislocation densities (8×107∕cm2) were achieved in uniform GaN layers grown by metalorganic chemical vapor deposition on (0001) 4H-SiC mesa surfaces 50μm×50μm in area that were completely free of steps. Transmission electron microscopy (TEM) indicated that all observable GaN film threading dislocations were of edge type. TEM analysis of the defect structure of the nucleation layer (aluminum nitride, AlN) revealed a lack of c-component dislocations, and the clean annihilation of lateral, a-type dislocations within the first 200 nm of growth, with no lateral dislocations developing threading arms. These results indicate that the elimination of steps on the initial (0001) 4H-SiC growth surface may play an important role in the removal of mixed and c-type dislocations in subsequently grown AlN and GaN heteroepitaxial layers.

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.

High-reflectance III-nitride distributed Bragg reflectors grown on Si substrates

Distributed Bragg reflectors (DBRs) composed of an AlN/AlGaN superlattice were grown of Si (111) substrates. The first high-reflectance III-nitride DBR on Si was achieved by growing the DBR directly on the Si substrate to enhance the overall reflectance due to the high index of refraction contrast at the Si/AlN interface. For a 9x DBR, the measured peak reflectance of 96.8% actually exceeded the theoretical value of 96.1%. The AlN/AlGaN superlattice served the added purpose of compensating the large tensile strain developed during the growth of a crack-free 500 nm GaN / 7x DBR / Si structure. This achievement opens the possibility to manufacture high-quality III-nitride optoelectronic devices without optical absorption in the opaque Si substrate.

Photoconductive response of granular superconducting films

The application of a DC magnetic field to disordered or granular films of Y-Ba-Cu-O is shown to lead an enhancement of a nonbolometric photoresponse at temperatures near and below T/sub c/. The disorder is evidenced by the broadened resistive transition to superconductivity and by the higher normal state resistance of the films. This observation is consistent with a nonequilibrium effect, which is described by the flux-flow model of the low-temperature photoresponse of granular superconductors. >

Thin film growth of YBa2Cu3Ox from nitrate solutions

Abstract Thin films of YBa2Cu3Ox have been grown by spraying nitrate solutions of Y, Ba, and Cu onto a heated substrate. The new orthorhombic phase of the deposited film has unit cell parameters of a = 4.701 A , b = 5.747 A , and c = 16.245 A . Annealing the film at 950°C causes the crystal structure to become the superconducting orthorhombic 1:2:3 phase with a = 3.860 A , b = 3.877 A and c = 11.663 A . Results are presented for SEM and X-ray diffraction measurements.

Growth of (100) GaAs by vertical zone melting

Abstract The vertical zone melt growth technique (VZM) was used to grow undoped, semi-insulating GaAs. Single crystals 34 mm in diameter were grown in the direction. A molten zone length of 23 mm resulted in a solid-liquid interface which deviated only slightly from planarity to give a convex growing interface. With a vertical temperature gradient of 9δ C cm -1 (measured in the furnace), the dislocation density was a uniform (2−5) × 10 3 cm -2 throughout the crystal. Hall measurements, carbon determinations and EL2 determinations all indicated that the GaAs had a relatively uniform impurity content in the first 80% of the crystal. VZM was also used to zone level Sn dopant to give an n-type carrier concentration varying by approximately 20% over 80% of the crystal length.