Rie Togashi

Homoepitaxial growth of β-Ga2O3 layers by halide vapor phase epitaxy

Thick high-purity β-Ga2O3 layers of high crystalline quality were grown homoepitaxially by halide vapor phase epitaxy (HVPE) using gaseous GaCl and O2 on (001) β-Ga2O3 substrates prepared by edge-defined film-fed growth. The surface morphology and structural quality of the grown layer improved with increasing growth temperature. X-ray diffraction ω-rocking curves for the (002) and (400) reflections for the layer grown at 1000 °C had small full widths at half maximum. Secondary ion mass spectrometry and electrical characteristics revealed that the growth of high-purity β-Ga2O3 layers with low effective donor concentration (Nd − Na < 1013 cm−3) is possible by HVPE.

Temperature-dependent capacitance–voltage and current–voltage characteristics of Pt/Ga2O3 (001) Schottky barrier diodes fabricated on n––Ga2O3 drift layers grown by halide vapor phase epitaxy

We investigated the temperature-dependent electrical properties of Pt/Ga2O3 Schottky barrier diodes (SBDs) fabricated on n–-Ga2O3 drift layers grown on single-crystal n+-Ga2O3 (001) substrates by halide vapor phase epitaxy. In an operating temperature range from 21 °C to 200 °C, the Pt/Ga2O3 (001) Schottky contact exhibited a zero-bias barrier height of 1.09–1.15 eV with a constant near-unity ideality factor. The current–voltage characteristics of the SBDs were well-modeled by thermionic emission in the forward regime and thermionic field emission in the reverse regime over the entire temperature range.

Anisotropy, phonon modes, and free charge carrier parameters in monoclinic β -gallium oxide single crystals

There is growing interest in low-symmetry metal oxides because of their potential use in high-power electronics capable to sustain very high voltages. Very little is known about their fundamental physical properties, such as transverse and longitudinal optical phonon modes, dielectric constants, and how free charge carriers couple with lattice vibrations. This lack of information is partially due to the complexity by which these properties intertwine due to the low symmetry crystal systems. Here, the authors describe a general pathway to the analysis of long-wavelength experiments for monoclinic and triclinic crystal systems, and they report for the first time a complete set of phonon modes for the monoclinic phase of gallium oxide. These parameters may arrive just in time to support computational optimization of charge and heat transport for device designs. The concept for analysis of long wavelength properties in monoclinic and triclinic crystal systems can help access a widely uncharted field in condensed matter physics.

Ga 2 O 3 Schottky barrier diodes with n − -Ga 2 O 3 drift layers grown by HVPE

The new wide-bandgap oxide semiconductor, gallium oxide (Ga₂O₃), has gained attraction as a promising candidate for power device applications because of its excellent material properties and suitability for mass production. The Baligas figure of merit of Ga₂O₃ is expected to be much larger than those of SiC and GaN due primarily to Ga₂O₃s extremely large bandgap of 4.5~4.9 eV, which will enable Ga₂O₃ power devices with higher breakdown voltage (V_br) and efficiency than SiC and GaN devices [1]. The other important advantage of Ga₂O₃ is that large, high-quality bulk single crystals can be grown by using melt growth methods. Recently, we developed a homoepitaxial growth technique for high-purity Ga₂O₃ thin films on single-crystal Ga₂O₃ substrates by halide vapor phase epitaxy (HVPE) [2, 3]. This is the first report on Ga₂O₃ Schottky barrier diodes (SBDs) with epitaxial Si-doped n^--Ga₂O₃ drift layers grown by HVPE.

Theoretical Analysis for Surface Reconstruction of AlN and InN in the Presence of Hydrogen

We have investigated the surface structures of AlN and InN using ab initio calculations based on the density functional theory within generalized gradient approximation. We studied the surface energies obtained from total energy calculations for various (2×2) geometries of cation-terminated (0001) surfaces and anion-terminated (0001) surfaces of AlN and InN, with hydrogen in a carrier gas. It was found that the structures with N–H bonds were favorable under hydrogen ambient, while in the absence of hydrogen, the structures with metal adatoms and N adatoms tended to be stable under metal-rich conditions and N-rich conditions, respectively.

Electronic properties of the residual donor in unintentionally doped .BETA.-Ga2O3

Electron paramagnetic resonance was used to study the donor that is responsible for the n-type conductivity in unintentionally doped (UID) β-Ga2O3 substrates. We show that in as-grown materials, the donor requires high temperature annealing to be activated. In partly activated materials with the donor concentration in the 1016 cm−3 range or lower, the donor is found to behave as a negative-U center (often called a DX center) with the negative charge state DX− lying ∼16–20 meV below the neutral charge state d0 (or Ed), which is estimated to be ∼28–29 meV below the conduction band minimum. This corresponds to a donor activation energy of Ea∼44–49 meV. In fully activated materials with the donor spin density close to ∼1 × 1018 cm−3, donor electrons become delocalized, leading to the formation of impurity bands, which reduces the donor activation energy to Ea∼15–17 meV. The results clarify the electronic structure of the dominant donor in UID β-Ga2O3 and explain the large variation in the previously reported ...

Fabrication of vertical Schottky barrier diodes on n-type freestanding AlN substrates grown by hydride vapor phase epitaxy

Thick Si-doped AlN layers were homoepitaxially grown by hydride vapor phase epitaxy on AlN(0001) seed substrates. Following the removal of the seed substrate, an n-type AlN substrate with a carrier concentration of 2.4 × 1014 cm−3 was obtained. Vertical Schottky barrier diodes were fabricated by depositing Ni/Au Schottky contacts on the N-polar surface of the substrate. High rectification with a turn-on voltage of approximately 2.2 V was observed. The ideality factor of the diode at room temperature was estimated to be ~8. The reverse breakdown voltage, defined as the leakage current level of 10−3 A/cm2, ranged from 550 to 770 V.

Thermal stability of beta-Ga2O3 in mixed flows of H-2 and N-2

The thermal stability of β-Ga2O3(010) substrates was investigated at atmospheric pressure between 250 and 1450 °C in a flow of either N2 or a mixture of H2 and N2 using a radio-frequency induction furnace. The β-Ga2O3 surface was found to decompose at and above 1150 °C in N2, while the decomposition of β-Ga2O3 began at only 350 °C in the presence of H2. Heating β-Ga2O3 substrates in gas flows containing different molar fractions of H2 demonstrated that the decomposition was promoted by increasing the H2 molar fractions. Thermodynamic analysis showed that the dominant reactions are in N2 and in a mixed flow of H2 and N2. The second-order reaction with respect to H2 determined for the mixed flows agrees with the experimental results for the dependence of the β-Ga2O3 decomposition rates on the H2 molar fraction.

Study of the Decomposition Processes of (0001)AlN in a Hydrogen Atmosphere

The decomposition processes of AlN from its surface in a hydrogen atmosphere were studied using the ab initio calculation method based on the density functional theory. The activation energies of Al- and N-atoms desorption from (0001)AlN surface were calculated. It was found that Al-atoms desorb from (0001)AlN surface as AlH molecules, and N-atoms as NH3 molecules. Desorption of AlH molecules is a rate limiting reaction which conforms well with experimental data.

High-Temperature Heat-Treatment of c-, a-, r-, and m-Plane Sapphire Substrates in Mixed Gases of H2 and N2

Orientation dependent decomposition of sapphire substrates and resultant AlN formation during heat treatment in an atmospheric-pressure mixed gas flow of H2 and N2 (H2/N2= 3/1) was investigated within the temperature range 980–1480 °C. AlN was formed on sapphire in the temperature range 1030–1430 °C for c-, a-, and m-plane sapphire, and 980–1430 °C for the r-plane sapphire. At 1480 °C, AlN was not formed, and only sapphire was decomposed by H2 with the ranking of m- > r- > a- > c-plane. The ranking was contrary to that of the amount of AlN formation at 1380 °C, which occurred by the reaction of gaseous Al generated by the sapphire decomposition and N2. This discrepancy was due to the shape of AlN formed on sapphire; whisker-like AlN does not protect c- and a-plane sapphire from decomposing, while layer-like AlN protects r- and m-plane sapphire from decomposing.