A. Bittar

Crystallization and diffusion in progressively annealed a‐Ge/SiOx superlattices

Raman spectroscopy is used to investigate the structural changes in isochronally annealed a‐Ge/SiOx superlattices. The Ge crystallization temperature is found to be higher in superlattices with thinner Ge layers, which can be interpreted in terms of the retardation of nucleation and growth of Ge microcrystallites near the Ge/SiOx interface. Before and after the Ge layers crystallize there is Si diffusion into the Ge layers. The Si appears to be uniformly distributed throughout the Ge layers.

Ion beam analysis of ion-assisted deposited amorphous GaN

Rutherford backscattering spectrometry and nuclear reaction analysis measurements were done using a 920 keV deuteron beam to investigate amorphous GaN films prepared on Si substrates by an ion assisted deposition technique. Elemental concentrations and depth profiles of O, N, Ga were measured. Films deposited at 750 eV ion energy show that the Ga/N ratios are within 10% of unity and the Ga/O ratios are within 10–15% in the films having thicknesses ranging from 75 to 270 nm. Ga/N and Ga/O ratios in the films deposited at 800 and 900 eV were found to vary more than 10%. The Ga/N ratios of films produced at 400–750 eV are optimal for the production of single-phase a-GaN. There are only minor differences in the optical responses of these films and none at all in the Raman spectra within these levels of variation. Hydrogen analysis was also performed on the films using elastic recoil detection analysis with a 2.5 MeV 4 He þ beam. This showed a surface enrichment of H in the thick films and low H concentration values of around

Comparison between experiment and calculated band structures for DyN and SmN

Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106-7079, USA(Dated: March 20, 2008)We investigate the electronic band structure of two of the rare-earth nitrides, DyN and SmN.Resistivity measurements imply that both materials have a semiconducting ground state, and bothshow resistivity anomalies coinciding with the magnetic transition, despite the different magneticstates in DyN and SmN. X-ray absorption and emission measurements are in excellent agreementwith densities of states obtained from LSDA+U calculations, although for SmN the calculationspredict a zero band gap.

Raman spectroscopy of nanocrystalline and amorphous GaN

We report Raman measurements on thin films of strongly disordered GaN and GaN:O prepared by ion-assisted deposition. The incident photon energies used in the experiments ranged from 1.95 to 3.8eV, spanning the interband edge. Under subgap excitation the signal resembles the crystalline GaN vibrational density-of-modes, with significant broadening as expected for disordered material. There is a strong resonant behavior at the interband edge of the same mode for which a strong resonance is found in crystalline GaN, with a width suggesting that the entire vibrational branch contributes to the signal. Even nanocrystalline material is found to display Raman spectra characteristic of very short-range (<1n) translational symmetry, in agreement with x-ray diffraction evidence for the random stacking nature of the 3nm diameter crystallites. The presence of oxygen at even 25at.% has only a subtle effect on Raman spectra at the network vibrational frequencies below 800cm−1, but its presence is signaled by the appear...

Raman spectroscopy of amorphous Ge/SiOx superlattices

Raman spectroscopic studies have been performed on a series of vacuum evaporated amorphous Ge/SiO x superlatticefilms with germanium layer thicknesses between 8 and 200 A and silicon oxide layer thicknesses between 12 and 300 A. The results show the presence of a Ge y (SiO x )1−y interface alloy of thickness (0.7±0.2) nm and average composition of y∼0.35. This alloy region is identified as the residence of the observed germanium bond angle disorder, leaving the pure germanium layers in the superlattices essentially undistorted from the bulk a‐Ge structure.

Amorphous Thin Films: Insulators, Metals, and Semiconductors**

Amorphous thin films of some materials can be prepared by various deposition techniques onto ambient temperature substrates. The process results in material which resembles the crystalline counterpart in many respects, although they show defect-related electronic states which influence some of their properties. Nonetheless they have been exploited in some technologies. Researchers at Victoria University and Industrial Research Ltd. have worked in the field for many years, and this paper reviews some of that work. The most recent studies have focused on amorphous GaN, a wide bandgap semiconductor, and the review emphasizes this ongoing research. Rapid progress in reducing the density of defect states is being made and a range of structural, conducting, and optical properties are being investigated. The best films so far have unobservable mid-gap absorption and a resistivity of 104 Ω cm.

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.

Single phase nanocrystalline GaMnN thin films with high Mn content

The authors gratefully acknowledge financial support from the New Zealand Foundation for Research Science and Technology through its New Economy Research Fund, and through a postdoctoral fellowship of one of the authors B.J.R.. The work of the MacDiarmid Institute is supported by a New Zealand Centre of Research Excellence award. Another author S.G. wishes to thank Education New Zealand for financial support of the EXAFS measurements.

Conductivity, photoconductivity and optical properties of amorphous GaN films

It has been predicted that amorphous GaN has a low density of states in the gap, and therefore has potential as a useful opto-electronic material in the blue-green spectral region. We have synthesised amorphous GaN films on various substrates by ion assisted deposition and investigated the effects of sample preparation conditions on the conducting and optical properties. The room temperature resistivity ρ 0 of stoichiometric (Ga:N of 1:1) films is above 10 5 Ω cm, and these films exhibit a complex form for the temperature dependence of the resistivity. Films having an excess of Ga show a much lower ρ 0 . The optical absorption shows ar 0 band-gap of 3 eV, with the gap falling below that value when the amorphous network incorporates homopolar (Ga-Ga) bonds. The best films are thus transparent across the visible region with a low density of gap states, undetectable in optical absorption. The photoluminescence spectra obtained from these a-GaN films consist of a broad green light emission peaking at 528 nm. Preliminary photoconductivity measurements show sensitivity in the UV.

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.