Semi-insulating GaN for vertical structures: role of substrate selection and growth pressure

Abstract

Samples comprising 1.3 μm-thick C-doped semi-insulating (SI) GaN layer sandwiched between two n-GaN layers were grown on sapphire or conductive GaN substrates by metal-organic chemical vapor phase epitaxy at varied reactor pressure between 100 and 20 mbar. Vertical cylindrical resistors with a radius of 50 μm were defined by ~ 1.6 μm-deep mesa etching down to the bottom Si-doped n-GaN layer. X-ray diffraction rocking curves revealed almost invariant crystallographic quality of homo-epitaxial structures grown on the GaN substrate, while dislocations in GaN on sapphire lead to curve broadening and pits formations. C concentration in SI GaN grown on the GaN substrate was found to increase from ~ 1 ✕ 1017 cm-3 to ~ 6 ✕ 1018 cm-3 as the growth pressure decreased from 100 mbar to 20 mbar. However, one order of magnitude lower C concentration and inhomogeneous distribution was found for the sapphire substrate.I–V characterization showed that the electrical strength of SI GaN could be as high as 2.8 MV/cm if grown at 20 mbar on the GaN substrate. In this case the non-destructive break-down voltage exceeds 350 V with a positive T coefficient pointing on impact ionization. Sample grown on the GaN substrate at 20 mbar demonstrated ln(I) ~ V dependence which is typical for a barrier-controlled leakage. Extracted barrier height of 0.41 eV at the n-GaN/SI GaN interface was explained by C-related compensation effect and by a shifting of the Fermi level. On the other hand, apart from SI GaN grown on GaN substrate at 20 mbar, all other structures showed I = f(Vn) dependence indicating a space-charge-limited current conduction mechanism. It is suggested that the optimized SI GaN grown on GaN substrate can be considered as a current blocking layer or as a channel in robust geometry vertical transistors.