Materials Science Forum Vols. 717-720

Abstract: In this paper the structural and electrical evolution of Au/Ni contacts to p-type gallium nitride (GaN) upon annealing in different atmospheres was monitored. Rapid annealing of the contacts in an oxidizing atmosphere (N2/O2) resulted into a lower specific contact resistance (ρc) with respect to annealing in non-reacting ambient (Ar). The formation of a nickel oxide (NiO) layer was observed on the surface of the sample annealed in N2/O2 ,while was not present at the interface with p-GaN. The improvement of the contacts was associated with a reduction of the Schottky barrier height from 1.07 eV (Ar annealing) to 0.71 eV (N2/O2 annealing), as determined by the temperature dependence of the ρc. Local electrical measurements demonstrated the formation of inhomogeneous barriers. The electrical measurements were correlated with the different microstructure of the annealed contacts.

Abstract: This report describes the first to fabricate GaN p-n junction diodes on free-standing GaN substrates with a 3mm diameter. For the diode of 3 mm in diameter, the specific on-resistance and the breakdown voltage were 124 mΩ•cm2 (at 4.0 V) and -450 V, respectively. Consequently, combination of our material and device processing revealed a record fabricated device size with a high breakdown voltage and low forward leakage current in GaN vertical diodes.

Abstract: One normally-on N-channel AlGaN/GaN device and two types of normally-off GaN devices have been studied. The normally-on device with Sapphire substrate shows good I_dsat and breakdown characteristics, but the gate leakage current is quite large. The first normally-off GaN hybrid metal insulator semiconductor – high electron mobility transistor (MIS-HEMT) grown on Si substrate exhibits good performance with positive threshold voltage of 3V and the breakdown voltage of over 1800V. However the second normally-off GaN MOSFET structure is rather difficult to exhibit good blocking characteristic compared to GaN MIS-HEMT device due to inadequate device design.

Abstract: Simple, physics-based, and accurate circuit models are reported for GaN power HEMTs and inductors; these models are then used to design high-performance chip-scale synchronous buck (SB) power converters to provide agile point-of-load (POL) low-voltage ( down to 1V) high-current (up to 10A) power to portable mobile devices from a battery. Excellent agreement between the measured and simulated results is demonstrated for load regulation for a 19V/1.2V, 800 kHz SB converter; for comparison, the same converter performance using the best commercially available state-of-the-art silicon power MOSFETs is also evaluated. It is shown that the conventional approach used for estimating power loss of a SB power converter is in error; a new application-specific Figure of Merit (FOM) for power switches is proposed that accounts for both input and output switching losses.

Abstract: The chemical termination of diamond strongly impacts its electron affinity and thermal stability. We have performed density functional calculations examining up to a monolayer of selected transition metals (Ti, V, Ni and Cu) on the 2×1 reconstructed (001) surface. We find that addition of the carbide forming species, Ti and V, results in significantly higher binding adsorption energies at all surface coverages relative to those of the non-carbide-forming species. For monolayer coverage by Cu or Ni, and sub-monolayer coverage by Ti and V, we observe a negative electron affinity. We propose that based upon the electron affinities and binding energies, metal coated 2×1 reconstructed (001) diamond surfaces are promising candidates for electron emitters.

Abstract: Admittance spectroscopy (AS) and deep level transient spectroscopy (DLTS) have been applied to B-doped thin polycrystalline diamond films deposited on p+-silicon by hot filament chemical vapour deposition. Films with two boron concentrations (1.5×10^19 cm-3 and 4×10^19 cm-3) were selected to study the effect of B concentration on the electronic states in CVD-diamond. We have investigated whether these deep states arise from point or extended defects. DLTS and AS find two hole traps, E1 (0.29±0.03 eV) and E2 (0.53±0.07 eV), in both films. A third level, E3 (0.36±0.02 eV) was also detected in the more highly doped film. The defect levels E1 and E2 exhibited behaviour typical of extended defects, which we suggest may be due to B segregated to the grain boundaries. In contrast, the defect level E3 exhibited behaviour characteristic of an isolated point defect, which we attribute to B-related centres in bulk diamond.

Abstract: A field-plate structure is applied to vertical diamond Schottky barrier diode. A sputtered Al₂O₃ with 0.2 µm thickness is utilized for field-plate insulator. Fabricated diamond VSBD shows low leakage characteristics. Accordingly, the breakdown voltage is improved from 900V to 1,800V.

Abstract: ZnO as well as SiC are wide bangap materials with prospective electronic applications. Very good lattice matching of the materials allowed formation a n-ZnO/p-SiC heteroepitaxial junction. The n˗ZnO film was epitaxially grown onto the p-SiC substrate using the atomic layer deposition method. The fabricated p-n diode was studied by I-V and C-V characteristics as well as by impedance spectroscopy. The diode shows a high rectifying ratio of 10⁷ and an ideality factor of 1.21.

Abstract: We investigated the effect of the substrate temperature on the electrical and the optical properties of ZnO/4H-SiC structures. The n-type ZnO layer was grown on p-type 4H-SiC substrate by pulsed laser deposition to form p-n hetero-junction diode structure. The n-type ZnO thin films were deposited by pulsed laser deposition at different temperatures of 200, 400, and 600 °C, respectively. It was shown from transmission line method (TLM) and auger electron spectroscopy (AES) data that the sheet resistance of ZnO on SiC was increases from ~760 Ω/square to ~4000 Ω/square as the deposition temperature increases and the oxygen outdiffusion decreases. The I-V characteristics with and without illumination has also been studied.

Abstract: First-principles calculations are presented for CdGeN2 and ZnGe0.5Sn0.5N2 compounds with the orthorhombic structure derived from wurtzite. Lattice constants and internal parameters are obtained from local density approximation full-potential linearized muffin-tin orbital calculations and the quasiparticle self-consistent GW method is used to calculate the band structures. Corrections for zero-point motion and exciton binding energy are included. Both approaches lead to gaps in the blue-green region.