Hiroyuki Sazawa

Spontaneous polarization and band gap bowing in YxAlyGa1-x-yN alloys lattice-matched to GaN

First-principles calculations are carried out to estimate the spontaneous polarization and the energy band gap bowing in YxAlyGa1-x-yN alloys lattice-matched to GaN. The ground state properties of alloys are computed by using the pseudopotential-planewave method in conjunction with generalized gradient approximation to density functional theory. We find nonlinear behavior of the spontaneous polarization and the band gap energies in YxAlyGa1-x-yN alloys and the values depend on the atomic geometry in the unit cell, especially on that of yttrium.

High-performance GaAs-based metal–oxide–semiconductor heterostructure field-effect transistors with atomic-layer-deposited Al2O3 gate oxide and in situ AlN passivation by metalorganic chemical vapor deposition

GaAs-based metal–oxide–semiconductor heterostructure field-effect transistors (MOSHFETs) with Al2O3 gate oxide and in situ AlN passivation were investigated. Passivation with AlN improved the quality of the MOS interfaces, leading to good control of the gate. The devices had a sufficiently small subthreshold swing of 84 mV decade−1 in the drain current vs gate voltage curves, as well as negligible frequency dispersions and nearly zero hysteresis in the gate capacitance vs gate voltage curves. A maximum drain current of 630 mA/mm and a peak effective mobility of 6720 cm2 V−1 s−1 at a sheet carrier density of 3 × 1012 cm−2 were achieved.

Correlation between micropipes on SiC substrate and dc characteristics of AlGaN∕GaN high-electron mobility transistors

We report the influence of the size of hollow cores that extend from micropipes in SiC substrates on the dc characteristics of AlGaN∕GaN high-electron mobility transistors (HEMTs) fabricated on the substrates. Significant deterioration of the dc characteristics of HEMTs fabricated in the vicinity of hollow cores with a diameter of 5μm was observed, while no major deterioration was observed for HEMTs fabricated around hollow cores whose diameters were 1.5 and 3μm. A clear correlation between the size of hollow cores and free carrier densities at the peripheries of the cores was observed using micro-Raman imaging. The high densities of free carriers around relatively large hollow cores were suggested to be the cause of deterioration of the dc characteristics of HEMTs fabricated in the vicinity of the hollow cores. The device deterioration length with respect to the hollow core size was empirically deduced, and the effective decrease in the wafer yield was estimated as well.

Electrical properties of GaAs metal–oxide–semiconductor structure comprising Al2O3 gate oxide and AlN passivation layer fabricated in situ using a metal–organic vapor deposition/atomic layer deposition hybrid system

This paper presents a compressive study on the fabrication and optimization of GaAs metal–oxide–semiconductor (MOS) structures comprising a Al2O3 gate oxide, deposited via atomic layer deposition (ALD), with an AlN interfacial passivation layer prepared in situ via metal–organic chemical vapor deposition (MOCVD). The established protocol afforded self-limiting growth of Al2O3 in the atmospheric MOCVD reactor. Consequently, this enabled successive growth of MOCVD-formed AlN and ALD-formed Al2O3 layers on the GaAs substrate. The effects of AlN thickness, post-deposition anneal (PDA) conditions, and crystal orientation of the GaAs substrate on the electrical properties of the resulting MOS capacitors were investigated. Thin AlN passivation layers afforded incorporation of optimum amounts of nitrogen, leading to good capacitance–voltage (C–V) characteristics with reduced frequency dispersion. In contrast, excessively thick AlN passivation layers degraded the interface, thereby increasing the interfacial densi...

Influence of Micropipe and Domain Boundary in SiC Substrate on the DC Characteristics of AlGaN/GaN HFET

AlGaN/GaN HFETs were fabricated around micropipes and on a domain boundary in a semi-insulating silicon carbide (SI-SiC) substrate and the DC characteristics of the fabricated devices were measured. Devices around micropipe showed no pinch-off or large gate leakage. The devices on the domain boundaries showed no degradation in the performances, even though an X-ray topographic analysis indicated that crystal imperfections, due to the defects, propagated to the GaN layer across the hetero interface. Based on these results, we concluded that micropipe degrades the DC characteristics and that the domain boundary does not affect the DC characteristics. From Raman analysis on the devices around the micropipes, these degradations could be attributed to the free carriers introduced into the GaN crystal by the micropipes.

Electronic Structure and Spontaneous Polarization in ScxAlyGa1-x-yN Alloys Lattice-Matched to GaN: A First-Principles Study

We performed first-principles calculations of the spontaneous polarization and electronic band structures in ScxAlyGa1-x-yN alloys assuming their growth on freestanding GaN. We found an apparent deviation from the Vegards law of the lattice constants of the ScxAlyGa1-x-yN alloys lattice-matched to GaN. It was supposed that this deviation comes from the different bonding properties of IIIB and IIIA nitrides, resulting in different crystal structures, such as hexagonal and wurtzite structures. As was reported in our previous report on YxAlyGa1-x-yN [K. Shimada et al.: J. Appl. Phys. 110 (2011) 074114], we also found that in ScxAlyGa1-x-yN alloys, the superlattice-like structure of Sc atoms reduced the magnitude of spontaneous polarization [K. Shimada et al.: Semicond. Sci. Technol. 27 (2012) 105014]. The magnitude of the spontaneous polarization of ScxAlyGa1-x-yN is larger than that of YxAlyGa1-x-yN in a wide mole fraction range of Ga. We found the nonlinearity and dependence of the atomic arrangement of Sc in the alloys. The band-gap energies at Γ have the same characteristics as the spontaneous polarization. The band-gap energies of ScxAlyGa1-x-yN are also larger than those of YxAlyGa1-x-yN in the wide mole fraction range of Ga. The band structures of ScxAlyGa1-x-yN have a direct gap at Γ and form a flat band around the valence band top originating from the hybridization of the Sc 3d and N 2p electrons.