GaN epitaxial thin films were grown on a nitridated sapphire at low temperature (550°C) using remote plasma enhanced ultrahigh vacuum chemical vapor deposition system and these films were investigated by Rutherford backscattering spectroscopy (RBS), X-ray diffraction(XRD) θ-rocking technique and the Ultraviolet-Visible-Nearinfrared (UV-VIS-NIR) absorption spectrum. The FWHM of the X-ray θ-rocking curve was about 0.4 degree using the (0002) reflection from the GaN layer with 5000Å thickness grown on the nitridated sapphire. An analysis of XRD and the UV-VIS-NIR absorption spectrum showed that the crystalline and optical qualities of GaN are dependent on the nitridation time of the sapphire even at low temperature when a plasma source is used for nitridation. This means that the density of protrusion, which is formed by a relaxation of the elastic energy caused by the lattice difference between the sapphire and AlxO1-xN, with the sapphire nitridation time plays a key role in the crystalline and optical properties of grown GaN films. The RBS channeling data and the FWHM value of the θ-rocking curve for GaNr(0002) also indicated that the truncated hexagonals are tilted towards each other. These results showed that the GaN epitaxial film can be successfully grown on nitridated sapphire by RPE-UHVCVD even at low temperature.

We report a new Ni/Pt/Au (20/30/80 nm) metallization scheme to achieve a low ohmic contacts to p-type GaN with a carrier concentration of 9.4 × 1016 cm-3. A Mg-doped GaN layer (0.5 μm) was grown on (0001) sapphire substrate by metalorganic chemical vapor deposition (MOCVD). All metal thin films were deposited on the p-GaN layer in an electron-beam evaporation system. Samples were annealed by a rapid thermal annealing (RTA) process at a range of temperatures from 300 °C to 850 °C under a flowing Ar atmosphere. A circulartransmission line model (c-TLM) was employed to calculate the specific contact resistance, and current-voltage (I-V) data were measured with HP4155A. The Ni/Pt/Au contacts without the annealing process showed nearly rectifying characteristics. The ohmic contacts were formed on the samples annealed at 500 °C for 30 sec and the I-V data showed a linear behavior. The specific contact resistance was 2.1 × 10-2 Ωcm2. However with increasing the annealing temperature above 600 °C, ohmic contacts were again degraded. Auger electron spectroscopy (AES) depth profiles were used to investigate the interfacial reactions between the trilayer and GaN. AES results suggested that Pt plays a significant role in forming ohmic contact as an acceptor at the interface. Atomic force microscope (AFM) also showed that the samples with good ohmic contact have very smooth surface.

A remote plasma enhanced-ultrahigh vacuum chemical vapor deposition (RPE-UHVCVD) system equipped with a radio frequency-inductively coupled plasma (RF-ICP) which produces the reactive nitrogen species was employed to study the nitridation process at low temperature. The sapphire surface nitridated under various conditions was investigated with x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The nitridation process seems to be mostly affected by the RF power even at low temperature since the intensity of the N1s, peak was not dependent on the substrate temperature but on the RF power. The AFM images showed that the protrusion density on the sapphire surface decreased rapidly when the nitridation temperature was decreased. This result suggests that the formation of the protrusions is closely related to the process temperature, indicating that the formation of such protrusions is caused by the change of an elastic strain energy due to the thermal stress. It was possible to nitridate the sapphire surface without protrusion at a very low temperature. The crystallinity of GaN grown at 450 °C was found to be much improved when the sapphire substrate was nitridated at low temperature prior to the GaN layer growth.

This study investigated the low temperature growth of GaN on a nucleation layer in a remote plasma enhanced-ultrahigh vacuum chemical vapor deposition (RPE-UHVCVD) system which is equipped with an rf plasma cell for a nitrogen source. It was found that the growth temperature and the film thickness of the nucleation layer and the nitrogen flow rate for GaN growth play important roles in the improvement of crystallinity of the GaN layer. The nitridation of sapphire was also found to enhance the formation of facet shaped nuclei on the nucleation layer. As the temperature of the nucleation layer increased, islands with hexagonal and other facet shapes were formed on the grown GaN surface. This facet formation was related with the surface morphology and crystallinity of GaN. The best crystallinity was measured in a GaN layer with hexagonal facets on the surface and such GaN layers could be grown on a nucleation layer grown at 375 °C. Nitridation of sapphire and the growth temperature of the nucleation layer were also found to change the island shapes which enhances the formation of columnar structures in the GaN layer, resulting in the growth of a high crystalline GaN layer at low temperature.