UC Santa Barbara

InAlN has received significant attention due to its great potential for electronic and optoelectronic applications. In particular, $In_{0.18}Al_{0.82}N$ presents the advantage of being lattice-matched to GaN and simultaneously exhibiting a high spontaneous polarization charge, making $In_{0.18}Al_{0.82}N$ attractive for use as the barrier layer in high-electron-mobility transistors (HEMTs). However, in the case of InAlN growth by plasma-assisted molecular beam epitaxy (PAMBE), a strong non-uniformity in the in-plane In distribution was observed for both N-face and metal-face $In_{0.18}Al_{0.82}N$. This compositional inhomogeneity manifests itself as a columnar microstructure with AlN-rich cores (5-10 nm in width) and InN-rich intercolumn boundaries. Because of the large differences between the bandgaps and polarization of InN and AlN, this non-uniformity in InAlN composition could be a source of scattering, leading to mobility degradation in HEMTs.

In this work, the growth conditions for high quality lattice-matched InAlN layers on free-standing GaN substrates were explored by plasma-assisted molecular beam epitaxy (PAMBE) in the N-rich regime. The microstructure of N-face InAlN layers, lattice-matched to GaN, was investigated by scanning transmission electron microscopy and atom probe tomography. Microstructural analysis showed an absence of the lateral composition modulation that was previously observed in InAlN films grown by PAMBE. Using same growth conditions for InAlN layer, N-face GaN/AlN/GaN/InAlN high-electron-mobility transistors with lattice-matched InAlN back barriers were grown directly on SiC. A room temperature two-dimensional electron gas (2DEG) mobility of $1100\,cm^2V^{-1}s^{-1}$ and 2DEG sheet charge density of $1.9\times10^{13}\,cm^2$ was measured on these devices. However, the threading dislocation density (TDD) of GaN grown directly on SiC by PAMBE ($\approx2\times10^{10}\,cm^{-2}$) is two orders of magnitude higher than GaN grown by MOCVD on SiC or sapphire ($\approx5\times10^8\,cm^{-2}$). This high TDD can severely degrade the 2DEG mobility, especially at lower 2DEG sheet densities.

Relatively low TDD ($\approx5\times10^8\,cm^{-2}$) on MOCVD-grown GaN substrates motivated us to study the growth of N-face GaN-based HEMT structures with InAlN backbarriers on such substrates. Since on-axis GaN-on-sapphire substrates with low threading dislocation density are not available in the N-face orientation, we explored the growth of InAlN on vicinal ($4^{\circ}$ miscut along GaN $10\bar{1}0$) GaN-on-sapphire substrates. The microstructure of $In_{0.18}Al_{0.82}N$ layers grown by PAMBE at different temperatures was studied using scanning transmission electron microscopy (STEM). The cross-sectional and plan-view STEM images revealed lateral variations in the InAlN composition along $10\bar{1}0$ (perpendicular to the step edges), in addition to step bunching in InAlN layers thicker than 10 nm. N-face HEMTs with lattice-matched InAlN backbarriers were then grown on these vicinal substrates with different InAlN thicknesses.

Transmission line measurements showed that step bunching and lateral variation of InAlN composition degraded the 2DEG mobility in the directions parallel and perpendicular to the steps. A 2DEG charge density of $1.1\times10^{13}\,cm^{-2}$ and mobility of $1850\,cm^2V^{-1}s^{-1}$ were achieved on a GaN/AlN/InAlN/GaN structure with 7.5 nm thick $In_{0.18}Al_{0.82}N$. By designing a double backbarrier ($In_{0.18}Al_{0.82}N$(7.5 nm)/$Al_{0.57}Ga_{0.43}N$(7 nm)), a 2DEG charge density of $2\times10^{13}\,cm^{-2}$ and mobility of $1360\,cm^2V^{-1}s^{-1}$ were attained, which resulted in a sheet resistance of $230\,\Omega/\square$.

Two good measures of the device quality concerning the power loss in power switch and high frequency switch applications are Huang material figure of merit , and Baliga high-frequency figure of merit, respectively, which shows that for any fixed material system, power loss reduces by increasing the mobility of the 2DEG. Therefore, it is very important to understand the source of scattering mechanisms which affect the 2DEG mobility. In this work, we studied effect of decreasing channel thickness or increasing gate reverse bias on charge density and 2DEG mobility in N-face HEMT structure. Our calculations showed that increasing the gate reverse bias and decreasing the channel thickness both reduce the 2DEG mobility. This trend has been observed by experiment as well. Previously, it was believed that increasing the gate reverse bias or decreasing the channel thickness in N-face GaN-based HEMT structures lead to deeper penetration of the 2DEG wavefunction into the barrier, and consequently, higher interface roughness and alloy scattering rates. Although this statement is true, our calculations revealed that the penetration of the 2DEG into the barrier and, therefore, 2DEG mobility limited by alloy and interface roughness scattering mechanisms do not vary significantly by increasing gate reverse bias or decreasing the channel thickness. therefore, these two scattering mechanisms are not enough to explain the significant drop in the 2DEG mobility observed in experiments. We believe that the charged trap states at the AlGaN-GaN interface, where the 2DEG forms, are responsible for this 2DEG mobility reduction.