High Oxygen Sensitivity of TiO2 Thin Films Deposited by ALD

The gas sensitivity and structural properties of TiO2 thin films deposited by plasma-enhanced atomic layer deposition (ALD) were examined in detail. The TiO2 thin films are deposited using Tetrakis(dimethylamido)titanium(IV) and oxygen plasma at 300 °C on SiO2 substrates followed by annealing at temperatures of 800 °C. Gas sensitivity under exposure to O2 within the temperature range from 30 °C to 700 °C was studied. The ALD-deposited TiO2 thin films demonstrated high responses to O2 in the dynamic range from 0.1 to 100 vol. % and low concentrations of H2, NO2. The ALD deposition allowed the enhancement of sensitivity of TiO2 thin films to gases. The greatest response of TiO2 thin films to O2 was observed at a temperature of 500 °C and was 41.5 arb. un. under exposure to 10 vol. % of O2. The responses of TiO2 thin films to 0.1 vol. % of H2 and 7 × 10-4 vol. % of NO2 at a temperature of 500 °C were 10.49 arb. un. and 10.79 arb. un., correspondingly. The resistance of the films increased due to the chemisorption of oxygen molecules on their surface that decreased the thickness of the conduction channel between the metal contacts. It was suggested that there are two types of adsorption centers on the TiO2 thin films surface: oxygen is chemisorbed in the form of O2- on the first one and O- on the second one.

Epitaxy of III-Nitrides on β-Ga2O3 and Its Vertical Structure LEDs

β-Ga₂O₃, characterized with high n-type conductivity, little lattice mismatch with III-Nitrides, high transparency (>80%) in blue, and UVA (400–320 nm) as well as UVB (320–280 nm) regions, has great potential as the substrate for vertical structure blue and especially ultra violet LEDs (light emitting diodes). Large efforts have been made to improve the quality of III-Nitrides epilayers on β-Ga₂O₃. Furthermore, the fabrication of vertical blue LEDs has been preliminarily realized with the best result that output power reaches to 4.82 W (under a current of 10 A) and internal quantum efficiency (IQE) exceeds 78% by different groups, respectively, while there is nearly no demonstration of UV-LEDs on β-Ga₂O₃. In this review, with the perspective from materials to devices, we first describe the basic properties, growth method, as well as doping of β-Ga₂O₃, then introduce in detail the progress in growth of GaN on (1 0 0) and (−2 0 1) β-Ga₂O₃, followed by the epitaxy of AlGaN on gallium oxide. Finally, the advances in fabrication and performance of vertical structure LED (VLED) are presented.

Single-crystal GaN layer converted from β-Ga2O3 films and its application for free-standing GaN

Hexagonal GaN has been obtained by nitridating β-Ga₂O₃ film despite structural mismatch between β-Ga₂O₃ and GaN, and the conversion process has been investigated systematically.

The investigation of in situ removal of Si substrates for freestanding GaN crystals by HVPE

We investigate the etching of a Si substrate in the fabrication process of freestanding GaN crystal grown using a Si by HVPE. Followed by crystal growth, Si etching by vapor HCl at high temperature results in successful fabrication of the freestanding GaN. Due to the complicated vertical gas flows inside the reactor, careful design of the susceptor was implemented. The unintentional formation of Si_xN_y thin layer at the backside of the Si substrate after the epitaxial growth, which can cause the decreased etch rate and non-uniform etching of a Si substrate, was successfully prevented by N₂ purging during and after the etching of a Si substrate. We believe that this study will guide us to achieve the growth of freestanding GaN over 8-inch diameters in the efficient and practical way.

We investigate the etching of a Si substrate in the fabrication process of freestanding GaN crystal grown using in situ removal of the substrate by HVPE.

An extended application of β-Ga2O3 single crystals to the laser field: Cr4+:β-Ga2O3 utilized as a new promising saturable absorber

A Cr⁴⁺-doped β-Ga₂O₃ crystal has been grown successfully and demonstrated as a saturable absorber for the first time.

Theoretical and experimental investigation of optical absorption anisotropy in β-Ga2O3

The question of optical bandgap anisotropy in the monoclinic semiconductor β-Ga2O3 was revisited by combining accurate optical absorption measurements with theoretical analysis, performed using different advanced computation methods. As expected, the bandgap edge of bulk β-Ga2O3 was found to be a function of light polarization and crystal orientation, with the lowest onset occurring at polarization in the ac crystal plane around 4.5-4.6 eV; polarization along b unambiguously shifts the onset up by 0.2 eV. The theoretical analysis clearly indicates that the shift in the b onset is due to a suppression of the transition matrix elements of the three top valence bands at Γ point.

Nanoscale conductive pattern of the homoepitaxial AlGaN/GaN transistor

The gallium nitride (GaN)-based buffer/barrier mode of growth and morphology, the transistor electrical response (25-310 °C) and the nanoscale pattern of a homoepitaxial AlGaN/GaN high electron mobility transistor (HEMT) have been investigated at the micro and nanoscale. The low channel sheet resistance and the enhanced heat dissipation allow a highly conductive HEMT transistor (Ids > 1 A mm(-1)) to be defined (0.5 A mm(-1) at 300 °C). The vertical breakdown voltage has been determined to be ∼850 V with the vertical drain-bulk (or gate-bulk) current following the hopping mechanism, with an activation energy of 350 meV. The conductive atomic force microscopy nanoscale current pattern does not unequivocally follow the molecular beam epitaxy AlGaN/GaN morphology but it suggests that the FS-GaN substrate presents a series of preferential conductive spots (conductive patches). Both the estimated patches density and the apparent random distribution appear to correlate with the edge-pit dislocations observed via cathodoluminescence. The sub-surface edge-pit dislocations originating in the FS-GaN substrate result in barrier height inhomogeneity within the HEMT Schottky gate producing a subthreshold current.

Epitaxial growth of a self-separated GaN crystal by using a novel high temperature annealing porous template

A novel template with microporous structure was fabricated by the high temperature annealing method. The high temperature annealing porous (HTAP) templates were used for the growth of GaN crystals by HVPE. The GaN crystals were easily separated from the HTAP templates with the assistance of the microporous structure.