N-face GaN/AlN/GaN/InAlN and GaN/AlN/AlGaN/GaN/InAlN high-electron-mobility transistor structures grown by plasma-assisted molecular beam epitaxy on vicinal 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 degrees miscut along GaN < 10 (1) over bar0 >) GaN-on-sapphire substrates. The microstructure of In0.18Al0.82N layers grown by plasma-assisted molecular beam epitaxy 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 (1) over bar0 > (perpendicular to the step edges). Also, step bunching was observed in InAlN layers thicker than 10 nm. N-face highelectron-mobility transistor structures 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 two-dimensional electron gas (2DEG) mobility in the directions parallel and perpendicular to the steps. A 2DEG charge density of 1.1 x 10(13) cm(-2) and mobility of 1850 cm(2) V-1 s(-1) were achieved on a GaN/AlN/InAlN/GaN structure with 7.5 nm thick In0.18Al0.82N. By designing a double backbarrier (In0.18Al0.82N(7.5 nm)/Al0.57Ga0.43N(7 nm)), a 2DEG charge density of 2x10(13) cm(-2) and mobility of 1360 cm(2) V-1 s(-1) were attained, which resulted in a sheet resistance of 230 Omega/square.