By using a He-Cd laser in a chemical solution of H 3 PO 4 with a pH value of 3.5, Ga oxide films were directly grown on n-type GaN. From the energy-dispersive spectrometer (EDS) measurement and x-ray diffraction (XRD) measurement, the grown Ga oxide film was identified as (104) α-Ga 2 O 3 structure. A small amount of phosphors existed and bonded with oxygen on the grown films. The as-grown films were amorphous. From the XRD analysis, it is evident that annealing of the α-Ga 2 O 3 films led to a change in the microstructure from an amorphous to a polycrystalline phase. In addition, the as-grown low-density films gradually became dense films during the annealing process. Furthermore, the surface roughness of the annealed films also gradually decreased. Hexagonal pinholes on the grown films were observed. The density of the hexagonal pinholes was similar to the defect density of the n-type GaN. From the crosssectional transmission electron microscopy (TEM) micrographs, it is evident that the hexagonal pinholes originated from defects in the n-type GaN.
The ohmic performance of the ITO/ZnO multilayer deposited on n-type GaN layer was investigated. The best thermal annealing condition achieved for ohmic contact was 5 min at 500 °C, in hydrogen ambient. The measured specific contact resistance was 3×10−4 Ω cm2. Ohmic formation mechanisms would be attributed to the ITO/n-ZnO/n-GaN isotype conjunction and the reduction conduction band offset due to the quantum confinement effects in the thin ZnO layer.
Ohmic contacts to n-type GaN with low contact resistance were developed by (NH 4 ) 2 S x and KOH+(NH 4 ) 2 S x surface treatments prior to Ti/Al metal deposition. The lowest specific contact resistance of 3.0 × 10 -6 Ω-cm 2 was obtained for Ti/Al contacts in an (NH 4 ) 2 S x -treated GaN layer alloyed at 300°C for 3 min. To obtain the lowest specific contact resistance for a low temperature alloy, the (NH 4 ) 2 S x treatment conditions for both (NH 4 ) 2 S x and KOH+(NH 4 ) 2 S x -treated n-GaN layers have been investigated and the mechanism for ohmic formation in low temperature alloys analyzed.
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