Control of threading dislocation density at the initial growth stage of AlN on c-sapphire in plasma-assisted MBE

The temperature and concentration dependences of electron mobility in AlGaN/GaN hetero-structures are studied. The mobility for the samples under study at T = 300 K lies in the range of 450–1740 cm2/(V s). It is established that scattering at charged centers is dominant for samples with low mobility (lower than 1000 cm2/(V s) right up to room temperature. These centers are associated with a disordered piezoelectric charge at the heterointerface because of its roughness or with a piezoelectric charge similarly to the Al-GaN barrier because of alloy disorder, as well as with the deformation field around dislocations. Scattering at optical phonons is dominant for samples with mobility exceeding 1000 cm2/(V s) at T = 300 K. Scattering at alloy disorders, heterointerface roughness, and dislocations are dominant at temperatures lower than 200 K. A decrease in the influence of scattering at roughness with improvement of the heterointerface morphology increases room-temperature mobility from 1400 cm2/(V s) to 1700 cm2/(V s).

Electron scattering in AlGaN/GaN heterostructures with a two-dimensional electron gas

The impact of the sharpness of the AlGaN/GaN interface in high-electron mobility transistors (HEMTs) is investigated. Two structures, one with an optimized AlGaN/GaN interface and another with an unoptimized, were grown using hot-wall metal-organic chemical vapor deposition. The structure with optimized sharpness of the interface shows electron mobility of 1760 cm2/V $\cdot $ s as compared with 1660 cm2/V $\cdot $ s for the nonoptimized interface. Gated Hall measurements indicate that the sharper interface maintains higher mobility when the electrons are close to the interface compared with the nonoptimized structure, indicating less scattering due to alloy disorder and interface roughness. HEMTs were processed and evaluated. The higher mobility manifests as lower parasitic resistance yielding a better dc and high-frequency performance. A small-signal equivalent model is extracted. The results indicate a lower electron penetration into the buffer in the optimized sample. Pulsed- IV measurements imply that the sharper interface provides less dispersive effects at large drain biases. We speculate that the mobility enhancement seen AlGaN/AlN/GaN structures compared with the AlGaN/GaN case is not only related to the larger conduction band offset but also due to a more well-defined interface minimizing scattering due to alloy disorder and interface roughness.

Performance Enhancement of Microwave GaN HEMTs Without an AlN-Exclusion Layer Using an Optimized AlGaN/GaN Interface Growth Process

GaN-based heterostructures have been used in high-power radio frequency applications for a number of years due to the wide bandgap of GaN and high values of spontaneous and piezoelectric polarization, resulting in high breakdown voltages and high 2D carrier concentrations However, the accurate modeling of low-field electron mobility within such structures remains a topic of interest This paper presents a comprehensive numerical model for calculating the carrier mobility within a 2D electron gas in Al xGa 1 − xN/GaN high electron mobility transistors The model is based on solving the Schrodinger and Poisson equations self-consistently, taking into account the polarization charges at material interfaces and performing semi-classical numerical calculations of low-field electron mobility within the momentum relaxation time approximation, taking into account all relevant scattering mechanisms Both intra- and inter-subband transitions are considered, and the differences in intra- and inter-subband scattering rates are analyzed for some of the key scattering mechanisms The importance of including inter-subband transitions in the calculations is demonstrated by comparing the calculated results with experimentally measured mobilities

A. V. Tikhonov

Papers

Electron scattering in AlGaN/GaN heterostructures with a two-dimensional electron gas

TEM study of defects in AlxGa1−xN layers with different polarity