β-Ga2O3 (001) Schottky barrier diodes (SBDs) fabricated on a halide vapor phase epitaxy-grown epilayer showed anomalous reverse leakage characteristics, which could not be explained through thermionic field emission theory. A systematic investigation through the measurements and simulations of capacitance-voltage and current density-voltage characteristics suggested the presence of a thin surface layer on the epilayer with high density of oxygen vacancy states. This thin surface layer allowed the tunneling of electrons and caused anomalous reverse leakage properties (Thin surface barrier model). Annealing of the epilayer in an oxidative environment passivated the surface oxygen vacancy states and reduced the reverse leakage current enormously.

We demonstrate, for the first time, crack-free AlGaN/GaN high-electron-mobility transistors (HEMT) on 100 mm Si(111) by ammonia molecular beam epitaxy. High growth rate accelerates rapid transition from three-dimensional (3D) to two-dimensional (2D) growth and reduces the defect density. With increasing GaN buffer thickness, FWHM of GaN(002) XRD peak and dislocation density decrease. Highest electron mobilities of 1350 and 4290 cm2/V·s were measured at RT and 90 K, respectively. Submicron gate devices exhibited good pinch-off characteristics with a maximum drain current (IDmax) of 768 mA/mm at Vg= +1 V and a maximum extrinsic transconductance (gmmax) of 190 mS/mm at VD= 6 V.

Electrical characteristics of vertical Schottky barrier diodes (SBDs) fabricated on as-grown and oxygen annealed β-Ga 2 O 3 (001) epilayers were investigated. SBDs on as-grown epilayer showed anomalous reverse leakage characteristics. Annealing of β-Ga 2 O 3 epilayers in an oxygen-containing environment up to 40 min immensely reduced the reverse leakage current. The specific onresistance (R on ) of the SBD remained very close to that of the as-grown sample for annealing up to 20 min and increased almost by 25 times for annealing up to 40 min. Simulations of reverse tunneling characteristics, assuming oxygen vacancy type surface states, explained both the magnitude and the shape of anomalous reverse leakage. The diffusion of oxygen during annealing passivated the oxygen vacancy type surface states at first (for 20 min annealing)-resulting in two orders of reduction in leakage with minimal change in R on . Further annealing (up to 40 min) subsequently reduced the epilayer net carrier concentration from 3.3 × 10 16 to 2.9 × 10 15 cm −3 -resulting in immense change in both reverse leakage and R on . Thus, oxygen annealing proved to be a vital technique for the passivation of surface states and to reduce the net carrier concentration, which allows the modulation of reverse leakage of β-Ga 2 O 3 SBDs.

Surface states on (001) oriented halide vapor phase epitaxy (HVPE) grown β-Ga2O3 epilayers were explored through the determination of the Schottky barrier height (SBH) as a function of the metal work function using Cr, Cu, Ni, and Au Schottky barrier diodes. SBH is found to be nearly pinned between 1.2 and 1.35 eV in the HVPE grown epilayers. The position of the Fermi level pinning is closely matched with the energy level of the oxygen vacancy [VO(III)] state (EV + 3.57 eV) in the energy bandgap of β-Ga2O3, indicating that Fermi level pinning is due to oxygen vacancy type surface states on (001) oriented β-Ga2O3 epitaxial layers. The Fermi level is found to be relatively unpinned on the bulk β-Ga2O3 (001) substrate, suggesting the presence of lower density of oxygen vacancy states on its surface. Hence, the HVPE growth process was found to be responsible for the presence of oxygen vacancy states [VO(III)] in the epilayer. Moreover, this work highlights the role of these surface states in determining the SBH on β-Ga2O3 (001) epilayers and also explains the reason behind the scattered data of SBH values reported in the literature. In addition to these results, we also showed an increment in the built-in potential and the reduction of reverse leakage current for the epilayer with lower surface state density, which gives a direct evidence of the effect of surface states on the properties of β-Ga2O3 (001) Schottky barrier diodes.

The stress evolution of GaN/AlN heterostructure grown on 6H-SiC substrate by plasma assisted molecular beam epitaxy (PA-MBE) has been studied. AlN nucleation layer and GaN layer were grown as a function of III/V ratio. GaN/AlN structure is found to form buried cracks when AlN is grown in the intermediate growth regime(III/V∼1)and GaN is grown under N-rich growth regime (III/V<1). The III/V ratio determines the growth mode of the layers that influences the lattice mismatch at the GaN/AlN interface. The lattice mismatch induced interfacial stress at the GaN/AlN interface relaxes by the formation of buried cracks in the structure. Additionally, the stress also relaxes by misorienting the AlN resulting in two misorientations with different tilts. Crack-free layers were obtained when AlN and GaN were grown in the N-rich growth regime (III/V<1) and metal rich growth regime (III/V≥1), respectively. AlGaN/GaN high electron mobility transistor (HEMT) heterostructure was demonstrated on 2-inch SiC that showed good two dimensional electron gas (2DEG) properties with a sheet resistance of 480 Ω/sq, mobility of 1280 cm2/V.s and sheet carrier density of 1×1013 cm−2.