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Radiation tolerance of Cu(In,Ga)Se2 (CIGS) solar cells has been investigated using high-fluence proton beam irradiation for application to devices in extremely-high-radiation environments. CIGS solar cells deteriorated after high-energy proton irradiation with non-ionizing energy loss of 1 × 1016 MeVneq cm−2, however, the CIGS solar cells could generate power after high-fluence irradiation. The ideality factors increased from 1.3 to 2.0, and series resistance increased, indicating that the concentration of recombination centers increased in CIGS layers. After heat-light annealing, the conversion efficiencies gradually recovered, and the recombination centers were confirmed to be partly passivated by annealing at 90 °C. The short-circuit currents for 10 μm thick CIGS solar cells were recovered by dark annealing in the same manner as for 2 μm thick CIGS solar cells. Dark annealing on irradiated CIGS solar cells has beneficial effects on passivate the recombination centers, even using thicker CIGS layers.

Mg ions were implanted in 1 μm thick AlN layers grown on sapphire substrates. The Mg implantation with a total dose of 5 × 1014 cm−2 introduced Al-vacancy related defects, which were decreased by annealing at temperatures over 1400 °C in an N2 ambient. We found that annealing temperatures over 1400 °C were necessary for an electrically conductive Mg-implanted AlN layer. The Mg-implanted AlN layer annealed at 1500 °C showed 1.1 nA at a bias of 100 V at room temperature and 7 nA at a bias of 10 V at 300 °C.

Solid-state materials with a large bandgap energy have potential for high-temperature, high-power, radiation-hardened, and ultraviolet-light applications. α-Al2O3, with a bandgap energy of ∼9 eV, is one of the most attractive such materials. However, α-Al2O3 suffers from a high resistivity. Here, we show that heavily Si-doped α-Al2O3 (10 1 ¯ 0) layers grown by molecular beam epitaxy have a layer resistivity of 166 Ωcm at room temperature. We found that α-Al2O3 layers with Sn or Si concentrations over 1 × 1018 cm−3 are electrically conducting and the layer resistivity of the Si-doped α-Al2O3 layer is dramatically reduced by post-thermal annealing at 1400 °C in a N2 ambient.

Photoconductivity transients in an unintentionally doped (UID) n-type β-Ga2O3 layer are investigated at temperatures ranging from 90 to 210 K. Illumination of the β-Ga2O3 layer with a 600 nm light pulse induces photoconductivity, which persists after the light is turned off. The origin of persistent photoconductivity (PPC) is probed using the temperature dependencies of the photoconductivity buildup and decay kinetics. Upon excitation by 600 nm light, the photoconductivity in the UID β-Ga2O3 sample is related to the photoionization of two distinct deep levels with thermal ionization energies of 1.76 ± 0.07 eV (Franck–Condon energy D FC: 0.78 ± 0.24 eV) and 2 ± 0.08 eV (D FC: 0.52 ± 0.15 eV). When the light is turned off, PPC is observed due to thermal capture barriers preventing the photo-generated electrons from relaxing back to deep levels. Capture barriers of 35 meV and 165 meV have been estimated using the temperature dependence of the PPC decay time constant. The experimentally detected deep levels are ascribed to deep donors, such as oxygen vacancies.

We report on the critical layer thickness for (AlGa)2O3 hetero-epitaxial growth on β-Ga2O3 (010) substrates via plasma-assisted molecular-beam epitaxy and on the electrical properties of heavily tin-doped (AlGa)2O3 layers. The aluminum composition in the (AlGa)2O3 layers was reproducibly controlled within 19% by changing aluminum fluxes. We achieved the pseudomorphic growth of the 1050 nm thick (Al0.12Ga0.88)2O3 and 420 nm thick (Al0.15Ga0.85)2O3 layers on β-Ga2O3 (010) substrates. The electron concentration, contact resistivity, and sheet resistance of the (Al0.10Ga0.90)2O3 layer with a tin concentration of 4 × 1019 cm−3 were 1 × 1018 cm−3, 3 × 10−5 Ω cm2, and 9 × 102 Ω/⎕, respectively.

Photo-induced conductivity transients are reported for unintentionally doped (UID) β-Ga2O3 and n-type β-(Al0.16Ga0.84)2O3. The illumination of (UID) β-Ga2O3 and β-(Al0.16Ga0.84)2O3/Ga2O3 heterojunction with a sub-bandgap light ranging from 400 to 1000 nm (1.2–3.1 eV) increases their conductivity. The increase in the conductivity still remains after the light is turned off, and then slowly exhausts. From the transient photoconductivity, the optical cross-sections of the photo-ionized defects have been measured as a function of the photon energy, and the optical absorption peaks of the ionized defects have been calculated. Thus, the measured photoconductivity in both β-Ga2O3 and (Al0.16Ga0.84)2O3 are induced by broad optical absorption peaks that have been estimated to be 2.52–2.88 eV and 2.61–3.11 eV.

We investigated dry and wet etchings of β-Ga2O3 and fabricated vertical Schottky barrier diodes (SBDs) with mesa termination using the optimal etching condition. Using an inductively-coupled plasma reactive-ion etching with a nickel-hard mask, a β-Ga2O3 (010) mesa structure with a smooth sidewall is obtained at an etching rate of 77 nm min−1 in BCl3/Cl2 mixture gas. By immersing β-Ga2O3 (001) vertical SBDs with mesa termination in hot phosphoric-acid solution, the specific on resistance and ideality factor of the SBDs are reduced to 0.91 mΩcm2 and 1.03, respectively. Current density at reverse bias is in good agreement with thermionic field emission model.

This paper demonstrates a modulation-doped fisseld-effect transistor (MODFET) and a metal-semiconductor field-effect transistor (MESFET) using β-(AlGa)2O3 (010). Ohmic contacts on Sn-doped (Al0.15Ga0.85)2O3 exhibit a fairly linear behavior, which has a specific contact resistivity and sheet resistance of 9 × 10−5 Ω cm2 and 75 kΩ/, respectively. The MODFET with Sn-doped (Al0.08Ga0.92)2O3 barrier layer showed a breakdown voltage of 610 V for gate-drain spacing (L gd) of 8 μm, while the (Al0.16Ga0.85)2O3-channel MESFET exhibited a breakdown voltage of 940 V for L gd of 20 μm. These results show the great potential of (AlGa)2O3 transistors for high-power applications.

We fabricated an AlN ridge structure using a chlorine-based inductive-coupled plasma reactive-ion etching (ICP-RIE) and a tetramethylammonium hydroxide (TMAH) solution. The ICP etching of single-crystal AlN was systematically investigated by varying ICP power, chamber pressure, and Cl2/BCl3/Ar mixture gas composition. The selectivity and anisotropy for all samples with a Ni mask were more than 20 and ~1, respectively. Etching of AlN in a Cl2/BCl3 mixture gas yields a higher etch rate compared with a Cl2/Ar mixture gas. The etch rate of AlN increases with increasing ICP power, reaching 286 nm min−1 for 400 W. The TMAH solution has an anisotropic characteristic for an AlN etch. A ridge structure with smooth {} sidewalls was achieved by dipping AlN in the TMAH solution after ICP-RIE.

We report on the electrical characterization of Si-ion implanted AlN layers and the first demonstration of metal-semiconductor field-effect transistors (MESFETs) with an ion-implanted AlN channel. The ion-implanted AlN layers with Si dose of 5 × 1014 cm-2 exhibit n-type characteristics after thermal annealing at 1230°C. The ion-implanted AlN MESFETs provide good drain current saturation and stable pinch-off operation even at 250°C. The off-state breakdown voltage is 2370V for drain-to-gate spacing of 25μm. These results show the great potential of AlN-channel transistors for high-temperature and high-power applications.

β-Ga2O3(010) homo-epitaxial growth was performed by plasma-assisted molecular beam epitaxy. Under Ga-rich conditions and for growth temperatures above 650 °C, the growth rate was independent of the Ga/O ratio (>1). A high growth rate of 2.2 nm/min for β-Ga2O3(010) was achieved by optimizing the O flux between 650 and 750 °C. Under Ga-rich conditions between growth temperatures of 500–900 °C, smooth surfaces with rms roughness below 1 nm were realized. We found that the slightly Ga-rich conditions between 650 and 750 °C were optimal for β-Ga2O3(010) growth with a smooth surface and a high growth rate.

To obtain a high-crystalline-quality AlN/GaN short-period superlattice with higher average GaN mole fraction, the effects of the thicknesses of AlN barrier and GaN well layers on the superlattice growth were investigated. Coherent growth with an average GaN mole fraction of 20% was realized by reducing the AlN barrier layer thickness to 8 bilayers (BL) while keeping the GaN well layer thickness at 2 BL. Further reduction in the AlN barrier layer thickness resulted in lattice relaxation and degradation of the crystalline quality. Grown layers with various well and barrier thicknesses were investigated by transmission electron microscopy, X-ray diffraction, and atomic force microscopy.

The optical properties of wurtzite AlN under large compressive strain are investigated by photoluminescence and optical reflectivity measurements with two different geometries. The AlN layer was coherently grown on 6H-SiC(0001), resulting in strains of exx=eyy=-9.6×10-3 and ezz=5.12×10-3, as confirmed by high-resolution X-ray diffraction. Free exciton transitions were clearly observed. The transition energy of A free exciton (with Γ1 symmetry) was estimated to be 6.246 eV at 10 K. The large energy shift of the free exciton transition with respect to the transition in unstrained AlN was well explained by the reported deformation potentials of AlN.

We report on the critical thickness of AlN on SiC(0001). AlN was directly grown on 6H-SiC(0001) at 650 °C by rf-plasma-assisted molecular beam epitaxy. The growth layer had a relatively low threading dislocation density of 4×108–4×109 cm-2. Although the critical thickness of AlN on SiC(0001) is estimated to be 3.5 nm using a Matthews–Blakeslee model, the critical thickness in our experiment was over 700 nm. Low dislocation density, a layer-by-layer growth mode, and low growth temperature may contribute to such a large critical thickness. Sharp and intense free exciton emission was observed in low-temperature PL measurements of the AlN layer.

We demonstrate the coherent growth of AlN/GaN short-period superlattice (SPSL) on 6H-SiC(0001) substrates by molecular beam epitaxy. A high-quality 5-nm-thick AlN layer was grown on SiC as a template layer, followed by the growth of AlN (12 BL)/GaN (2 BL) SPSL, which consists of 40 periods (total thickness: 140 nm). The SPSL was coherently grown on SiC, and its threading dislocation density (TDD) was as low as 8×108 cm-2. The SPSL, which had 3-BL-thick GaN layers, was relaxed, and the TDD increased to 8×1010 cm-2.

300-nm-thick AlN layers were grown directly on 6H-SiC(0001) with six Si–C bilayer-height (1.5 nm) steps by rf-plasma-assisted molecular-beam epitaxy (MBE). To avoid unintentional active-nitrogen exposure, AlN was grown just after the nitrogen plasma ignition. By combining optimized Ga pre-deposition and no active-nitrogen exposure, layer-by-layer growth was realized from the first layer of AlN. Screw-type and edge-type threading dislocation densities in the AlN layer were reduced to 6×104 and 4×108 cm-2, respectively. Most of the edge-type dislocations were located at the step edge of the SiC substrate. The dislocation density of the AlN grown on the terrace of the SiC substrate was as low as 8×107 cm-2.

The thermal decomposition and hydrogen etching of a 6H-SiC(0001) Si-face were directly monitored using an in situ gravimetric monitoring system. The monitoring of the weight change of the 6H-SiC Si-face using this system clarified the dependences of the thermal decomposition and hydrogen etching rates on the substrate temperature. Although the thermal decomposition of the 6H-SiC Si-face above 1400 °C generated a graphite layer since only the Si atom directly desorbs from the surface, the etching of the 6H-SiC Si-face by hydrogen did not form this layer, and both Si and C atoms react with hydrogen. Moreover, the surface reaction of the 6H-SiC Si face with H2 and the resultant surface morphology were found to change at approximately 1250 °C.