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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.

Funding Information: This work was supported by JSPS KAKENHI Grant No. 16H06424 and 19H02166, and the ARPA-E PNDIODES program. S. Suihkonen acknowledges the financial support of the Academy of Finland (grant 297916) and the Foundation for Aalto University Science and Technology. A part of the research was performed at the OtaNano—Micronova Nanofabrication Centre of Aalto University. This work was carried out through the use of MIT’s Microsystems Technology Laboratories, AIST NPF, and open facility in the University of Tsukuba. Publisher Copyright: © 2022 The Author(s). Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd. We report on impurity diffusion in ion implanted AlN layers after thermal annealing. Silicon, tin, germanium, and magnesium ions were implanted into single-crystal Al-polar AlN layers grown on sapphire substrates. By annealing at 1600 °C, silicon and magnesium atoms were diffused in the AlN layer, while less change was observed in the distribution of germanium atoms. Silicon implantation introduced vacancy-related defects. By annealing at temperatures over 1300 °C, the vacancy-related defects were reduced, while oxygen atoms were diffused from the substrate due to sapphire decomposition. We reproducibly achieved silicon-implanted AlN layers with electrical conductance by controlling the annealing temperature and distribution of silicon and oxygen concentrations.

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.

This letter reports the demonstration of N-polar Al0.8Ga0.2N/AlN continuously-graded-channel polarization-doped field-effect transistors (PolFETs) on SiC. A PolFET with a source to drain distance of $12~\mu \text{m}$ exhibited a maximum drain current of 62.8 mA/mm and an ON/OFF current ratio of $1.1\times 10^{{4}}$ . The maximum drain current was stable between 20 °C and 250 °C operating temperatures. With the addition of 30-nm-thick Al2O3 gate insulator the maximum drain current increased to 126 mA/mm.

We present the effect of miscut angle of SiC substrates on N-polar AlN growth. The N-polar AlN layers were grown on C-face 4H-SiC substrates with a miscut towards 〈1¯100〉 by metal-organic vapor phase epitaxy (MOVPE). The optimal V/III ratios for high-quality AlN growth on 1° and 4° miscut substrates were found to be 20,000 and 1000, respectively. MOVPE grown N-polar AlN layer without hexagonal hillocks or step bunching was achieved using a 4H-SiC substrate with an intentional miscut of 1° towards 〈1¯100〉. The 200-nm-thick AlN layer exhibited X-ray rocking curve full width half maximums of 203 arcsec and 389 arcsec for (0 0 2) and (1 0 2) reflections, respectively. The root mean square roughness was 0.4 nm for a 2 μm×2μm atomic force microscope scan., Academy of Finland (grant 297916), Aalto University Science and Technology, Japan Society for the Promotion of Science. Grant-in-Aid for Scientific Research (Grant no. 16H06424), Japan Society for the Promotion of Science. Grant-in-Aid for Scientific Research (Grant no. 17K14110)

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.

This letter demonstrates vertical GaN junction barrier Schottky (JBS) rectifiers fabricated with novel ion implantation techniques. We used two different methods to form the lateral p-n grids below the Schottky contact: 1) Mg implantation into n-GaN to form p-wells and 2) Si implantation into p-GaN to form n-wells. Specific differential ON-resistances ( ${R}_{ \mathrm{\scriptscriptstyle ON}}$ ) of 1.5–2.5 $\text{m}\Omega ~\cdot $ cm2 and 7–9 $\text{m}\Omega ~\cdot $ cm2 were obtained in the Mg-implanted and Si-implanted JBS rectifiers, respectively. A breakdown voltage of 500–600 V was achieved in both devices, with a leakage current at high reverse biases at least 100-fold lower than conventional vertical GaN Schottky barrier diodes. The impact of n-well and p-well widths on the ${R}_{ \mathrm{\scriptscriptstyle ON}}$ and BV was investigated. Fast switching capability was also demonstrated. This letter shows the feasibility of forming patterned p-n junctions by novel ion implantation techniques, to enable high-performance vertical GaN power devices.

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 present the electrical characterization of N-polar AlN layers grown by metal-organic vapor phase epitaxy and the demonstration of N-polar AlN-channel metal-semiconductor field-effect transistors (MESFETs). A high concentration of silicon is unintentionally incorporated during the high-temperature growth of N-polar AlN, causing a high buffer leakage current. The silicon concentration decreases from 2 x 10(18) to 9 x 10(15) cm(-3) with decreasing growth temperature, reducing the buffer leakage current to 5.6 nA/mm at a 100 V bias. The N-polar AlN MESFET exhibits an off-state drain current of 0.27 nA/mm and a transistor on/off ratio of 4.6 x 10(4) owing to the low leakage of AlN buffer layers. (C) 2018 The Japan Society of Applied Physics

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.

III-nitride semiconductors are well-suited for solid-state lightening, such as light-emitting diodes (LEDs) and laser diodes (LDs). GaN (3.4 eV) alloyed with InN (0.7 eV) and AlN (6.1 eV) can cover a continuous range of a direct band-gap energy. A wide spectrum range from red to deep ultra-violet has been achieved using III-nitride LEDs12. Advanced optoelectronic devices demand higher power output and lower energy consumption. Conventional GaN-based optoelectronic devices suffer from rather high p-type contact resistance in the mid-10−4 Ωcm2, leading to high threshold voltage for high current optoelectronic devices. Generally, Mg is used as p-type dopant of GaN. Mg-doped GaN (GaN:Mg) has an intrinsically low ionized-acceptor concentration (1017-1018 cm−3) due to the large ionization energy Ea of Mg (∼0.18 eV) and high compensation by donor-like defects such as nitrogen vacancy. Mg doping level beyond ∼1020 cm−3 causes generation of V-shaped pyramidal extended defects into GaN layers and polarity inversion of their wurtzite crystal structure. These low effective-acceptor concentrations (N a -N d ) prevent ptype contact resistance from decreasing through field-emission (FE) tunneling. Recently, the formation of compensation defects in GaN:Mg was hindered thanks to the low growth temperature of 740 °C by molecular-beam epitaxy (MBE) using ammonia gas. This allows achieving high acceptor concentrations over 1019 cm−3 3 4. Here we report on the electrical properties of heavily Mg-doped GaN and carry out systematic investigation of p-type contact resistance.

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.

Mg-doped GaN with a net acceptor concentration (NA-ND) in the high 1019 cm−3 range was grown using ammonia molecular-beam epitaxy. Electrical properties of NiO contact on this heavily doped p-type GaN were investigated. A potential-barrier height of 0.24 eV was extracted from the relationship between NA-ND and the specific contact resistivity (ρc). We found that there is an optimum NA-ND value of 5 × 1019 cm−3 for which ρc is as low as 2 × 10−5 Ω cm2. This low ρc is ascribed to hole tunneling through the potential barrier at the NiO/p+-GaN interface, which is well accounted for by the field-emission model.

300-nm-thick AlN layers without a nucleation layer were grown on 6H-SiC (0001) vicinal substrates with 3-bilayer-height steps by rf-plasma-assisted molecular-beam epitaxy. A Ga beam was supplied for 0 to 30 seconds just before growth of AlN. The Ga pre-irradiation for 7 seconds (∼1.6 ML) was found to be effective for realization of the AlN layer-by-layer growth mode at an earlier stage of the growth and reduction of threading dislocation densities (TDD) in the AlN layers. The screw-type and edge-type TDDs were ∼106 cm-2 and 6×108 cm-2, respectively. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

250 nm thick AlN layers without a nucleation layer were grown directly on 6H-SiC(0001) with 3-bilayer-height steps by rf-plasma-assisted molecular-beam epitaxy. The structure and morphology of the AlN layers have been studied using atomic force microscopy, X-ray diffraction and transmission electron microscopy. Two different types of unique defect structures were observed. Rows of pure-edge-type threading dislocations were observed along the pre-existing step-edges of the SiC substrate for AlN grown on as-gas-etched SiC sub- strates, while the planar defects threading through the AlN layer were observed at the step-edges of the substrate for AlN on SiC with sacrificial oxidation. We concluded that these planar defects were the stacking mismatch boundaries due to the difference in stacking sequence of AlN layers on different SiC terraces.

Impact of step height of silicon carbide (SiC) substrates on heteroepitaxial growth of aluminum nitride (AlN) was investigated. Step-and-terrace structures with various step heights, 6 monolayer (ML), 3ML and 1ML, were formed on 6H–SiC (0 0 0 1) vicinal substrates by high-temperature gas etching. 2H–AlN layers were grown on the substrate by plasma-assisted molecular-beam epitaxy (MBE) and then these layers were characterized by atomic-force microscopy (AFM) and X-ray diffraction (XRD). High-quality AlN can be grown on SiC substrates with 6ML- and 3ML-height step, while AlN grown on SiC substrates with 1ML-height step exhibited inferior crystalline quality. A model for high-quality AlN growth on SiC substrates with 3ML-height step is proposed.

We grew heavily Mg-doped GaN using ammonia molecular-beam epitaxy. The use of low growth temperature (740 °C) allows decreasing the incorporation of donor-like defects (

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.

N-face GaN was grown on free-standing GaN (0001¯) substrates at a growth rate of 1.5 μm/h using plasma-assisted molecular beam epitaxy. Difference in growth rate between (0001¯) and (0001) oriented GaN depends on nitrogen plasma power, and the (0001¯) oriented GaN had only 70% of the growth rate of the (0001) oriented GaN at 300 W. Unintentional impurity concentrations of silicon, carbon, and oxygen were 2 × 1015, 2 × 1016, and 7 × 1016 cm−3, respectively. A growth diagram was constructed that shows the dependence of the growth modes on the difference in the Ga and active nitrogen flux, ΦGa − ΦN*, and the growth temperature. At high ΦGa − ΦN* (ΦGa ≫ ΦN*), two-dimensional (step-flow and layer-by-layer) growth modes were realized. High growth temperature (780 °C) expanded the growth window of the two-dimensional growth modes, achieving a surface with rms roughness of 0.48 nm without Ga droplets.

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.