Kazuki Kodama

Temperature dependence of electron concentration and mobility in n-GaN measured up to 1020 K

Temperature dependence of Hall electron concentration and mobility in n-GaN has been measured up to 1020 K. The electron concentration increased monotonically with temperature and did not saturate. The measured values were fitted with the calculated ones for the whole temperature range. It is found that following two assumptions have to be made in order to obtain the best fit for both electron concentration and mobility: (i) two donor levels and one acceptor level (including dislocation) have to be taken into account; and (ii) one donor level lies in the conduction band. The obtained results in this study will contribute to the design of GaN devices operating at high temperatures.

A model for calculating impact ionization transition rate in wurtzite GaN for use in breakdown voltage simulation

A model for calculating impact ionization transition rate (IITR) in wurtzite GaN has been developed for use in breakdown voltage simulations. The characteristic feature of the model is to calculate energy-dependent IITR by taking a conduction band index into account. Depending on the band index, the IITR values calculated by the proposed model show spreading by three orders of magnitude in the electron energy range from 6.5 to 8 eV, while this spreading is totally disregarded in the conventional model. An impact ionization coefficient is calculated based on a full band Monte Carlo simulation which incorporates IITRs by the proposed model. The calculated impact ionization coefficients by the proposed model exhibit better agreements with those by the rigorous model. The proposed model is applied to the calculation of breakdown characteristics for AlGaN/GaN HEMTs and demonstrates a higher breakdown voltage by about 30% than that by the conventional model.

Low-temperature (≥400 °C) growth of InN by metalorganic vapor phase epitaxy using an NH3 decomposition catalyst

In this paper, we report the metalorganic vapor phase epitaxial (MOVPE) growth of InN using a NiO-based pellet-type NH3 decomposition catalyst. The use of the catalyst significantly changes the growth behavior of InN, which is dependent on the growth temperature (T g). Continuous InN films without the incorporation of metallic In and a cubic phase are grown at T g = 400–480 °C. An InN film grown at T g ≈ 450 °C has a full-width at half maximum (FWHM) of 376 arcsec in the X-ray rocking curve for InN(0002) reflection. At T g ≥ 500 °C, the deposition rate of InN rapidly decreases and the deposited films become discontinuous with large (ca. 1 µm) pyramidal grains of InN. Depositions are scarcely obtained at T g ≥ 600 °C. Such changes in the growth behavior of InN are governed by the NH3 decomposition.

MOVPE growth of thick (∼1 µm) InGaN on AlN/Si substrates for InGaN/Si tandem solar cells

In order to develop key technologies for InGaN/Si two-junction tandem solar cells, MOVPE growth of thick InGaN on Si p-on-n cell structures has been studied. By clarifying the phase separation behavior of MOVPE InGaN, a thick (~1 µm) InxGa1−xN (x ~ 0.5) without phase separation is successfully grown on AlN/Si(111) wafers. A sufficient current flow for the operation of the tandem cell is obtained through n-InGaN/AlN/p-Si structures by employing the annealing of AlN/Si wafer at around 1000 °C in NH3 flow just before InGaN growth. The annealing of AlN/Si wafers also brings about the degradation of the underlying Si pn junction. The optimization of the annealing conditions is required to balance such favorable and unfavorable effects.

Low-temperature (≤600 °C) growth of high-quality In x Ga1− x N (x ∼ 0.3) by metalorganic vapor phase epitaxy using NH3 decomposition catalyst

In x Ga1− x N (x ~ 0.3) films on GaN/sapphire templates were grown by NH3 decomposition catalyst-assisted metalorganic vapor phase epitaxy (CA-MOVPE). NiO-based pellets were used as a catalyst. Even at a temperature lower than 500 °C, single-crystal In0.3Ga0.7N films were grown without the incorporation of metallic components (In, Ga) or the cubic phase. In contrast with the case of InN growth using the same catalyst [A. Yamamoto et al., Jpn. J. Appl. Phys. 55, 05FD04 (2016)], no marked grain growth or hydrogen etching was observed in In0.3Ga0.7N. Samples grown at a temperature ≤500 °C showed a full-width at half-maximum of the (0002) X-ray rocking curve as small as 10 arcmin or smaller. The carrier concentration in nominally undoped In0.3Ga0.7N grown using the catalyst was higher by about 4 orders of magnitude than that in conventional MOVPE samples. Secondary ion mass spectroscopy analysis revealed that such a higher carrier concentration was due to the marked reduction in carbon contamination level in the films.

Analysis of electron transport in AlGaN based on empirical pseudopotential method

This paper describes calculation of band structures in AlGaN based on an empirical pseudopotential method. The pseudopotential parameters were adjusted so that the Al composition dependent bandgap energy exhibited reasonable agreements with the experimental data. The full-band model of AlGaN was applied to the electron transport calculation in AlGaN/GaN structures. Current-voltage characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) were calculated based on a full-band Monte Carlo algorithm.

Optical and Structural Properties of Zn--Cd--Mn--Se Double Quantum Well Systems

Double quantum well (DQW) structures consisting of a ZnCdSe well and a ZnCdMnSe well separated by a ZnSe barrier are grown with molecular beam epitaxy (MBE). The DQW structures are characterized by using X-ray diffraction measurement and simulation. Thickness of each well layer is designed so that the lowest energy level of ZnCdMnSe well is close to the excited level of the ZnCdSe well. Optical properties of the DQWs are studied with photoluminescence (PL) and reflection spectra in external magnetic fields up to 8 T in the Faraday geometry. Exciton transfer from ZnCdMnSe well to ZnCdSe well is observed in magneto PL with energy selective photoexcitation. Exciton energies in ground and excited states are estimated from PL excitation spectra and reflection spectra.