Structural inheritance and transport response in MOCVD/PA-MBE regrowth of AlGaN/GaN heterostructures on GaN/sapphire and thin-buffer SiC/Si templates

Abstract

In this study, the process of forming a nanometer-scale active region via plasma-assisted molecular beam epitaxy (PA-MBE) with nitrogen plasma activation was applied to two distinctly different nitride template architectures. These templates were initially fabricated by the metal–organic chemical vapor deposition (MOCVD) method and served as controlled materials science probes to investigate the inheritance of the structural state of the original buffer during the subsequent regrowth of AlGaN/GaN heterostructures. A conventional GaN/c–Al₂O₃ template with an MOCVD-GaN buffer approximately 1.5 μm thick was compared to a significantly thinner Al_0.1Ga_0.9N/AlN/SiC/Si(111) template. On both templates, a nominally identical active sequence—AlN/GaN/AlN/AlGaN/GaN—was formed. The structures were characterized using Hall measurements, scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray diffraction/reciprocal space mapping (XRD/RSM), X-ray reflectivity (XRR), Raman spectroscopy, and photoluminescence. The results demonstrate a pronounced inheritance of the initial architecture. The reference GaN/sapphire template exhibited a more uniform morphology, narrower diffraction distributions, a higher XRD-estimated lateral coherence length (~ 130 versus ~ 41 nm), and a lower volume-averaged biaxial stress (~ 0.55 versus ~ 3–4 GPa). Cross-sectional TEM of the SiC/Si template directly revealed contrast from extended defects traced from the MOCVD-AlGaN layer through the MOCVD/PA-MBE interface into the PA-MBE-GaN layer, whereas the structure on GaN/sapphire showed a more uniform microstructure in the active region. Raman spectroscopy further indicated a structurally inhomogeneous conversion SiC interlayer with contributions characteristic of several SiC polytypes. At room temperature, Hall measurements showed effective sheet electron concentrations of approximately 3–4 × 10¹³ cm⁻² for both structures, with effective mobilities of about 510 and 690 cm²/(V·s) for the SiC/Si and GaN/sapphire templates, respectively. Since Hall data alone cannot exclude parallel conduction in GaN or at the regrowth interface, the measured sheet concentration is not interpreted as the definitive concentration of a single two-dimensional electron gas (2DEG) channel. Meanwhile, the thin-buffer SiC/Si template retains about 74% of the effective Hall mobility of the thick-buffer GaN/sapphire reference. Overall, the data indicate that the inherited inhomogeneity of the initial SiC/Si architecture significantly contributes to the observed structural limitations and suggests specific directions for further optimization.