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The authors report the growth of bixbyite In2O3 (111) epitaxial layers on c-plane sapphire substrates by a chemical vapor deposition route, in which growth takes place under the flow of oxygen and ammonia in a furnace. Indium metal is used as the source for indium. It has been found that In2O3 films with high epitaxial quality can be grown by optimizing the growth temperature and the flow rate of NH3. Ammonia plays a catalytic role in the growth process. At growth temperatures less than 550 °C, inclusion of a rhombohedral phase, which is known to be thermodynamically stable only at high pressure, has been detected in the film. X-ray photoelectron spectroscopy does not show the presence of nitrogen in these films. An x-ray diffraction study reveals a sharp increase of disorder in these films as the growth temperature increases beyond 550 °C. The bandgap of the material is also found to decrease with the increase of disorder.

(111) NiO epitaxial layers embedded with crystallographically oriented Ni-clusters are grown on c-sapphire substrates using pulsed laser deposition technique. It has been found that certain growth and post-growth cooling conditions can be adjusted to vary the size, shape and density of these clusters. (111) Ni epitaxial layer can also be deposited on c-sapphire substrate by this technique when no oxygen is supplied into the chamber during growth. Structural and magnetic properties of the clusters are examined by a variety of techniques including atomic probe tomography and superconducting quantum interference device magnetometry. All cluster embedded films are found to exhibit ferromagnetic behaviour even at room temperature. Magnetic characteristics of these films depend strongly on the density, size and shape of the clusters. It has also been observed that electrical conductivity of these samples enhances by several orders of magnitude when the cluster density crosses the percolation threshold. While, strong ferromagnetic behaviour is maintained even in highly resistive samples. Therefore, the material system has a potential to serve as an efficient spin-injector, the missing link for the realization of semiconductor based spintronics.

Coherent transmission of Cooper pairs through a non-superconducting medium is a key challenge for hybrid electronics with superconductors, normal metals and semiconductors. While superconductor-normal metal-superconductor (SNS) junctions have been known for quite sometime, including a low carrier density region or a two-dimensional electron gas (2DEG) in the path of superconducting electrons is relatively less explored. Indeed, this is due to the limited choice of materials that would make ohmic contacts to such systems, while simultaneously supporting a superconducting phase. In this paper we show a coherent transmission of supercurrent through a degenerate semiconductor over a length ≈2μm with a critical magnetic field B c ≈8T at 1.6K and T c ≈5K at zero magnetic field. This length scale is much larger than the typical thickness of a Josephson junction. Our system is a fragment of a GaN nanowall network that has been shown to support a high mobility 2DEG (μ n >10 4 cm 2 V −1 s −1 ). The current and voltage probes were superconducting tungsten-gallium composite electrodes and the measurements could be done in four-probe geometry. We demonstrate ballistic type carrier transport with a near ideal transparency of 1 and a critical current (I c ) large enough such that the Josephson coupling parameter . Some features in magneto-transport data suggest that there is possibly a small magnetic moment forming in the semiconductor fragment. In addition the combination of a T c typical of elemental metallic superconductors, but a critical field that appears to be higher than the Clogston-Chandrasekhar limit, may be indicative of the emergence of a triplet pairing mechanism in these structures.

(111) NiO epitaxial layers are grown on c-sapphire substrates by pulsed laser deposition technique. Structural and morphological properties of the films are studied using in-plane as well as out-of-plane high resolution x-ray diffraction and atomic force microscopy techniques as functions of growth temperature, oxygen pressure and the pulses count of the laser. The study shows that continuous epitaxial films of thickness as low as 3 nm with high crystalline quality, smooth surface and interface morphology can be grown by this technique. The study also reveals the co-existence of 60°-rotated (111) triangular domains of NiO in the film. The study also evidences the presence of a very low density of 60° dislocations in these films. Density of screw and edge dislocations are also estimated to be quite low. It has been found that growth-temperature, oxygen partial pressure and the film thickness can influence differently the density of various dislocation types. These parameters are also found to affect significantly the strain developed in the films.

Recently, the electron mobility in wedge-shaped c-GaN nanowall networks has been estimated to cross the theoretical mobility limit for bulk GaN. Significant blue-shift of the bandgap has also been observed. Both the findings are explained in terms of two-dimensional electron gas (2DEG) formed at the central vertical plane of the walls due to the polarization charges at the two inclined faces. Carrier concentration and mobility have earlier been determined from thermoelectric power and conductivity measurements with the help of a statistical model. Due to the network nature of the system, direct measurements of these quantities from Hall experiments are not possible. Search for a better way to estimate mobility in this system thus becomes important. Since, strain can also lead to the blue-shift of the bandgap, it is also imperative to evaluate carefully the role of strain. Here, using Raman spectroscopy, we have estimated carrier concentration and mobility in these nanowall networks with varied average tip-widths. Depth distribution of strain and luminescence characteristics are also studied. The study reveals that strain has no role in the bandgap enhancement. Moreover, the electron mobility, which is determined from the lineshape analysis of the A1(LO)-plasmon coupled mode in Raman spectra, has been found to be significantly higher than the theoretical limit of mobility for bulk GaN for the same electron concentration. These results thus corroborate the picture of polarization induced vertical 2DEG formation in these walls as predicted theoretically.

A systematic study of growth, structural, electrical, and luminescence properties of zinc oxide (ZnO) layers grown on c-oriented GaN/sapphire templates by the pulsed laser deposition technique is carried out. A thorough high-resolution x-ray diffraction study reveals that c-ZnO films with high crystalline quality can be grown under certain growth conditions. Screw and edge dislocation densities in these films are found to be as low as 7×108 and 3×1010cm−2, respectively. All layers are found to be unintentionally n-type with ∼1019cm−3 electron concentration and mobility as high as ∼50 cm2 V−1 s−1. Temperature and excitation intensity dependent photoluminescence (PL) studies as functions of the growth conditions are carried out to identify the transition processes behind various luminescence features found in these samples. At low temperatures, PL spectra are marked by sharp neutral donor bound excitonic transitions, their phonon replicas, and two broad luminescence bands at 2.2 and 2.9 eV. These broad bands are attributed to transitions from the (2+/0) oxygen vacancy (VO) and (2+/+/0) zinc-interstitial (Zni) levels, respectively, to the valence band. Thermal energy needed to depopulate these defects is found to be 11 and 385 meV, respectively, for the (2+/0) VO and (2+/+/0) Zni levels. Low temperature PL spectra for the samples grown with relatively high oxygen pressures are featured by the Zn-vacancy (VZn) related neutral acceptor bound excitonic transition, its phonon replicas, and a broad band at 2.75 eV. This band diminishes with increasing temperature and, instead, another broad feature appears at ∼2.1 eV. Our study attributes the 2.75 eV band to transition from the conduction band to (0/−) VZn levels and the 2.1 eV feature to the transition between (−/2−) VZn levels and the valence band. It has been found that all the defect related features can be minimized by adjusting the growth conditions.

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