A. Koo

Stabilization of amorphous GaN by oxygen

We have investigated experimentally the structure of disordered GaN films. The results suggest that it is not possible to stabilize an amorphous network in stoichiometric films, and the GaN instead consists of random-stacked nanocrystals of some 3-nm diameter. However, incorporation of 15% or more oxygen stabilizes an amorphous phase, which we attribute to the presence of nontetrahedral bonds centered on oxygen. The ionic favorability of heteropolar bonds and its strikingly simple constraint to even-membered rings are the likely causes of the instability of stoichiometric a-GaN.

Filled and empty states of disordered GaN studied by x-ray absorption and emission

X-ray absorption and emission spectroscopies are used to study the effects of short-ranged ordering on the electronic states of disordered GaN. Nanocrystalline samples with crystallites as small as 3nm exhibit an electronic structure resembling a broadened version of that in crystalline GaN. The electronic structure is even more heavily broadened in amorphous GaN films containing oxygen impurities or excess gallium. The oxygen containing films show an additional peak in the density of states just above the conduction band edge, and a downward shift of the valence band edge. The signature of molecular nitrogen trapped within the films is evident in both the absorption and emission spectra.

Optical response of DyN

We report measurements of the optical response of polycrystalline DyN thin films. The frequency-dependent complex refractive index in the near IR-visible-near UV was determined by fitting reflection/transmission spectra. In conjunction with resistivity measurements, these identify DyN as a semiconductor with an optical energy gap of 1.2 eV. When doped with nitrogen vacancies it shows free carrier absorption and a blue-shifted gap associated with the Moss-Burstein effect. The refractive index of 2.0 ± 0.1 depends only weakly on energy. Far infrared reflectivity data show a polar phonon of frequency 280 cm−1 and a dielectric strength of Δe=20.

Compositional and structural studies of amorphous GaN grown by ion-assisted deposition

Amorphous GaN films have been deposited onto various substrates by ion-assisted deposition. The films were deposited at room temperature using nitrogen ion energies in the range 40-900 eV. Rutherford backscattering spectroscopy and nuclear reaction analysis show that the Ga:N atomic ratio is approximately one for films grown with ion energy near 500 eV; these films have the highest transparency. Films grown with ion energies below 300 eV are Ga rich, and show reduced transparency across the visible. Raman spectroscopy, x-ray diffraction, and transmission electron microscopy confirm the amorphous nature of the films. Annealing studies on a-GaN establish that the films begin to crystallise at a temperature of about 700 C. To investigate the local bonding environment of the Ga or N atoms, we have measured the extended x-ray absorption fine structure (EXAFS) of the transparent GaN films. The EXAFS results indicate that the films are dominated by heteropolar tetrahedral bonding, with a low density of homopolar bonds.

Annealing amorphous GaN - a way to nano-crystalline state

The effects of thermal annealing on structural and optical properties of amorphous GaN thin films obtained by Ion Assisted Deposition (IAD) have been investigated. Thermal annealing. was carried out in flowing nitrogen and respectively under water vapour pressure (1.7-2.4 torr), at temperatures up to 800/spl deg/C. From SEM observations of the annealing processes in real time it appears that formation of nano-crystallites in the amorphous environment occurs at about 400/spl deg/C. XRD and Raman scattering performed on the annealed films prove the presence of nano-crystallites (2-4 nm). Optical measurements show a weak absorption edge in the as prepared film, and the deepening edge and reduced sub-band absorption after annealing.

Nanostructured amorphous group III nitrides: potential for refractive index engineering

Amorphous and nanocrystalline group III nitrides can be formed with relative ease in comparison with fully single crystalline material, and they show at least some properties in common with the crystalline form. Here we report experiments with the preparation of these materials, focussing especially on the potential to form patterned films with structures whose dimensions are comparable to visible wavelengths. There are clear opportunities to form both self-assembled and controlled patterns.

Amorphous GaN: Optoelectronic properties and device potential

This paper describes the preparation of an amorphous form of hetero-polar tetrahedrally bonded GaN by ion-assisted deposition. The structure, including composition and bonding configurations, has been subjected to thorough investigation to establish the optimum deposition conditions. The optoelectronic properties of the best films have been measured to evaluate their potential in UV-blue detectors and/or emitters. Currently the best films have shown clear photoconductivity, with sensitivity peaking in the UV, but no useful luminescence has yet been found.