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The precise control of Mg concentration ([Mg]) in p-type GaN layers from 2.3 × 1016 to 2.0 × 1019 cm−3 was demonstrated by halide vapor phase epitaxy (HVPE) on n-type GaN (0001) freestanding substrates. [Mg] in GaN layers could be controlled well by varying the input partial pressure of MgCl2 formed by a chemical reaction between MgO solid and HCl gas under the thermodynamic equilibrium condition. In the sample with [Mg] of 2.0 × 1019 cm−3, a step-bunched surface was observed because the surface migration of Ga adatoms was enhanced by the surfactant effect of Mg atoms. The samples show high structural qualities determined from x-ray rocking curve measurements. The acceptor concentration was in good agreement with [Mg], indicating that almost all Mg atoms act as acceptors. The compensating donor concentrations in the samples were higher than the concentrations of Si, O, and C impurities. We also obtained the Mg acceptor level at a sufficiently low net acceptor concentration of 245 ± 2 meV. These results show that the HVPE method is promising for fabricating GaN vertical power devices, such as n-channel metal–oxide–semiconductor field-effect transistors. [ABSTRACT FROM AUTHOR]

GaN metal-insulator-semiconductor field-effect transistors (MISFETs) with Ni/Al2O3/GaN gate stack structures formed on a vicinal Ga-polar (c-plane) or nonpolar (m-plane) surface were fabricated. Current–voltage characteristics and carrier transport properties of the channels in the MISFETs with and without post-metallization annealing (PMA) were systematically investigated. GaN layers grown on freestanding substrates with vicinal surfaces led to good transistor behaviors for the m-plane as well as the c-plane. It was found that PMA improves the electron mobility in the MIS channel (μEFF) for both the c- and m-planes, and that μEFF for the m-plane is slightly higher than that for the c-plane both with and without PMA. Temperature dependences of μEFF clarified that μEFF for the m-plane is dominantly limited by phonon scattering, while both Coulomb and phonon scatterings are dominant for the c-plane. Consequently, we achieved high performance for the GaN MISFETs fabricated on the m-plane because of the reduced numbers of Coulomb scattering centers at the Al2O3/GaN interface. [ABSTRACT FROM AUTHOR]

A normally-off GaN double-implanted vertical MOSFET (DMOSFET) with an atomic layer deposition (ALD)-Al2O3 gate dielectric film on a free-standing GaN substrate fabricated by triple ion implantation is presented. The DMOSFET was formed with Si ion implanted source regions in a Mg ion implanted p-type base with N ion implanted termination regions. A maximum drain current of 115 mA/mm, maximum transconductance of 19 mS/mm at a drain voltage of 15 V, and a threshold voltage of 3.6 V were obtained for the fabricated DMOSFET with a gate length of 0.4 μm with an estimated p-type base Mg surface concentration of 5 × 1018 cm−3. The difference between calculated and measured Vths could be due to the activation ratio of ion-implanted Mg as well as Fermi level pinning and the interface state density. On-resistance of 9.3 mΩ·cm2 estimated from the linear region was also attained. Blocking voltage at off-state was 213 V. The fully ion implanted GaN DMOSFET is a promising candidate for future high-voltage and high-power applications. [ABSTRACT FROM AUTHOR]