Microstructure and enhanced morphology of planar nonpolar m-plane GaN grown by hydride vapor phase epitaxy

Abstract

Nonpolar ($1\bar 100$) m-plane gallium nitride has been grown heteroepitaxially on (100) γ-LiAlO₂ by several groups. Previous attempts to grow m-plane GaN by hydride vapor phase epitaxy (HVPE) yielded films unsuitable for subsequent device regrowth because of the high densities of faceted voids intersecting the films’ free surfaces. We report here on the growth of planar m-plane GaN films on (100) γ-LiAlO₂ and elimination of bulk and surface defects. The morphology achieved is smooth enough to allow for fabrication of m-plane GaN templates and free-standing substrates for nonpolar device regrowth. The GaN films were grown in a horizontal HVPE reactor at 860–890°C. Growth rates ranged from 30 µm/h to 240 µm/h, yielding free-standing films up to 250-µm thickness. The m-plane GaN films were optically specular and mirror-like, with undulations having 50–200-nm peak-to-valley heights over millimeter length scales. Atomic force microscopy revealed a striated surface morphology, similar to that observed in m-plane GaN films grown by molecular beam epitaxy (MBE). Root-mean-square (RMS) roughness was 0.636 nm over 25-µm² areas. Transmission electron microscopy (TEM) was performed on the m-plane GaN films to quantify microstructural defect densities. Basal-plane stacking faults of 1×10⁵ cm⁻¹ were observed, while 4×10⁹ cm⁻² threading dislocations were observed in the g=0002 diffraction condition.