Yuuki Ishida

Localized exciton dynamics in strained cubic In0.1Ga0.9N/GaN multiple quantum wells

Radiative and nonradiative recombination dynamics in strained cubic (c-) In0.1Ga0.9N/c-GaN multiple quantum wells were studied using temperature-dependent time-resolved photoluminescence (TRPL) spectroscopy. In contrast to hexagonal InGaN quantum wells, low-excitation photoluminescence peak energy increased moderately with decreasing well thickness L and the PL lifetime did not strongly depend on L. The results clearly indicated that the piezoelectric field was not acting on the transition process. The TRPL signal was well fitted as a stretched exponential decay from 10 to 300 K, showing that the spontaneous emission is due to the radiative recombination of excitons localized in disordered quantum nanostructures such as In clusters. The localized states were considered to have two-dimensional density of states at 300 K (quantum disk size), since the radiative lifetime increased with increasing temperature above 150 K.

Optical and structural studies in InGaN quantum well structure laser diodes

An InGaN multiple-quantum-well laser diode wafer that lased at around 400 nm was shown to have the InN mole fraction, x, of only 6% in the wells. Nanometer-probe compositional analysis showed that the fluctuation of x was as small as 1% or less, which is the resolution limit. However, the wells exhibited a Stokes-like shift (SS) of 49 meV at 300 K, which was approximately 65% of the luminescence linewidth, and effective localization depth, E0, was estimated to be 35 meV at 300 K. Since the effective electric field due to polarization in the wells was estimated to be as small as 300 kV/cm, SS was considered to originate from effective band-gap inhomogeneity. Because the well thickness fluctuation was insufficient to reproduce SS or E0 and bulk cubic In0.02Ga0.98N that does not suffer any polarization field or thickness fluctuation effect exhibited a SS of 140 meV at 77 K, the exciton localization is considered to be an intrinsic phenomenon in InGaN, which is due to the large band-gap bowing and In clusteri...

Raman studies on phonon modes in cubic AlGaN alloy

Cubic AlxGa1−xN alloy layers have been successfully grown for x=0−1 by gas-source molecular beam epitaxy on cubic-SiC/Si substrates, and the compositional dependence of the transverse-optic (TO) and longitudinal-optic (LO) phonon modes has been studied by Raman scattering. The LO-mode frequency of mixed crystals shows a systematic variation from the pure cubic AlN phase to the cubic GaN phase (one-mode type). On the contrary, there are two branches for the TO mode varying slowly in frequency with the composition (two-mode type). This behavior is explained within a random-element-isodisplacement model including the effect of polarization field. Our result indicates a strong polarization field acting on the cation-nitrogen bonds.

Arsenic surfactant effects and arsenic mediated molecular beam epitaxial growth for cubic GaN

Small amounts of As residual pressure were found to affect the structure of cubic GaN growing surfaces in molecular beam epitaxy growth, i.e., modification of surface reconstruction structures, stabilization of reconstructed flat surfaces at high substrate temperatures, and preferential growth of the cubic phase. These As surfactant effects are discussed in relation to the atomic arrangement of the As-passivated surface of GaN. It was shown that the quality of cubic GaN epilayers can be improved by utilizing a small amount of As residual pressure.

Optical Constants of Cubic GaN, AlN, and AlGaN Alloys

We have used spectroscopic ellipsometry to investigate, for the first time, the optical constants of cubic GaN, AlN, and AlGaN alloy epitaxial layers grown by molecular beam epitaxy. The refractive indices of cubic AlGaN were found to decrease with increasing Al content, as expected from the empirical observation that the refractive index decreases with increasing direct bandgap energy. In the transparent wavelength region, the refractive indices of cubic AlGaN with lower Al contents are somewhat larger than those of hexagonal modifications, while with higher Al contents, the refractive indices of cubic AlGaN and hexagonal AlGaN are almost equal. We also found that the direct bandgap energies of cubic AlGaN show a parabolic dependence on the Al content.

Influence of inn mole fraction on the recombination processes of localized excitons in strained cubic InxGa1-xN/GaN multiple quantum wells

Recombination dynamics of localized excitons in the best quality strained cubic InxGa1−xN/GaN multiple quantum wells (MQWs) grown on 3C-SiC (001) were studied as functions of InN mole fraction x and temperature T. The MQWs exhibited large Stokes-type shifts although the full width at half maximum of the photoluminescence (PL) peaks being 80–120 meV is 100–300 meV smaller than those reported previously. Time-resolved PL signal showed stretched exponential decay and spectral redshift with time after excitation up to 300 K. These results are fingerprints that the spontaneous emission is due to the radiative recombination of excitons localized in disordered quantum nanostructures forming extended and localized states. Effective localization depth increased with the increase in x, which gave rise to fast exciton localization. However, nonradiative lifetime in the free states decreased more rapidly with the increase in x and T, giving the emission efficiency maximum at particular x around 0.1.

Detection of defects in SiC crystalline films by Raman scattering

Defects in SiC films and bulk crystals are detected by Raman scattering. It is found that the observation of defect activated transverse optic bands at a Raman forbidden configuration is useful for the detection of small amount of defects. The shape of the defect activated Raman band is discussed in relation to structures of the defects.

Atomically Flat 3C-SiC Epilayers by Low Pressure Chemical Vapor Deposition

We have investigated the heteroepitaxial growth of 3C-SiC on Si by low pressure chemical vapor deposition (LPCVD) using a silane-propane-hydrogen reaction gas system. By the growth at low pressure below 10 Torr, several problems arising from atmospheric pressure CVD (APCVD) were solved, namely the growth of protrusions was suppressed and thickness uniformity was improved. Moreover, atomically flat surfaces were obtained. Although the growth temperatures in the case of LPCVD were lower than those in the case of APCVD, LPCVD epilayers showed excellent crystallinity and luminescence properties, comparable with those of APCVD epilayers.

Recombination dynamics of localized excitons in cubic InxGa1−xN/GaN multiple quantum wells grown by radio frequency molecular beam epitaxy on 3C–SiC substrate

Recombination dynamics of localized excitons in strained cubic (c-)InxGa1−xN/GaN multiple quantum wells (MQWs) grown on 3C–SiC (001) were summarized in terms of well thickness L, InN molar fraction x, and temperature T. Photoluminescence (PL) peak energy of c-In0.1Ga0.9N/GaN MQWs showed a moderate blueshift as L decreased, and the low-temperature PL lifetime did not change remarkably by changing L. These results proved that the quantum-confined Stark effect due either to spontaneous or piezoelectric polarization was inactive in cubic polytypes. Consequently, time-resolved PL (TRPL) data of c-InGaN MQWs reflect the intrinsic exciton dynamics. The TRPL signal showed stretched exponential decay and spectral redshift with time after excitation up to 300 K. The results are fingerprints that the spontaneous emission is due to the radiative recombination of excitons localized in disordered quantum nanostructures forming extended and localized states. Effective localization depth increased with the increase in x,...

Influence of stacking faults on the performance of 4H–SiC Schottky barrier diodes fabricated on (112̄0) face

The influence of stacking faults (SFs) on the performance of 4H–SiC (1120) Schottky barrier diodes fabricated on the epilayer grown on the substrate which was grown in [1120] direction by the sublimation method was investigated. The number of SFs under the Schottky electrode was determined by KOH etching of (1100) face cross section. SFs were found to have a severe influence on the leakage current of reverse characteristic and Schottky barrier height. The leakage current is increased even though a few SFs exist under the electrode. The Schottky barrier height is also affected by the SF under the electrode.