K. Hazu

Impacts of Si-doping and resultant cation vacancy formation on the luminescence dynamics for the near-band-edge emission of Al0.6Ga0.4N films grown on AlN templates by metalorganic vapor phase epitaxy

Luminescence dynamics for the near-band-edge (NBE) emission peak at around 250 nm of c-plane Si-doped Al0.6Ga0.4N films grown on AlN templates by low-pressure metalorganic vapor phase epitaxy were studied using deep ultraviolet time-resolved photoluminescence and time-resolved cathodoluminescence spectroscopies. For the films with the Si-doping concentration, [Si], lower than 1.9 × 1017 cm–3, the doping lessened the concentration of cation vacancies, [VIII], through the surfactant effect or the aid of the reactant doping in a form of H3SiNH2. However, the room-temperature nonradiative lifetime, and, consequently, the equivalent value of internal quantum efficiency in the weak excitation regime steeply decreased when [Si] exceeded 1018 cm−3. Simultaneously, the intensity ratio of the deep-state emission band to the NBE emission abruptly increased. Because the increase in [Si] essentially gives rise to the increase in [VIII] (for [Si]>1.9×1017 cm−3) and the overcompensation of Si is eventually observed for ...

Time-resolved photoluminescence, positron annihilation, and Al0.23Ga0.77N/GaN heterostructure growth studies on low defect density polar and nonpolar freestanding GaN substrates grown by hydride vapor phase epitaxy

Time-resolved photoluminescence (TRPL) and positron annihilation measurements, as well as Al0.23Ga0.77N/GaN heterostructure growth by metalorganic vapor phase epitaxy were carried out on very low defect density, polar c-plane and nonpolar m-plane freestanding GaN (FS-GaN) substrates grown by hydride vapor phase epitaxy. Room-temperature photoluminescence (PL) lifetime for the near-band-edge (NBE) excitonic emission of the FS-GaN substrates increases with increasing positron diffusion length (L+); i.e., decreasing gross concentration of charged and neutral point defects and complexes. The best undoped c-plane FS-GaN exhibits record-long L+ being 116 nm. The fast component of the PL lifetime for its NBE emission increases with temperature rise up to 100 K and levels off at approximately 1.1 ns. The result implies a saturation in thermal activation of nonradiative recombination centers. The surface and interface roughnesses for a Si-doped Al0.23Ga0.77N/GaN/Al0.18Ga0.82N/GaN heterostructure are improved by th...

Femtosecond-laser-driven photoelectron-gun for time-resolved cathodoluminescence measurement of GaN

A rear-excitation femtosecond-laser-driven photoelectron gun (PE-gun) is developed for measuring time-resolved cathodoluminescence (TRCL) spectrum of wide bandgap materials and structures such as semiconductors and phosphors. The maximum quantum efficiency of a 20-nm-thick Au photocathode excited using a frequency-tripled Al(2)O(3):Ti laser under a rear-excitation configuration is 3.6×10(-6), which is a reasonable value for a PE-gun. When the distance between the front edge of the PE-gun and the observation point is 10 mm, the narrowest electron-beam (e-beam) diameter is 19 μm, which corresponds to one tenth of the laser-beam diameter and is comparable to the initial e-beam diameter of a typical W hair-pin filament of thermionic electron-gun. From the results of TRCL measurements on the freestanding GaN grown by the ammonothermal method and a GaN homoepitaxial film grown by metalorganic vapor phase epitaxy, overall response time for the present TRCL system is estimated to be 8 ps. The value is the same as that of time-resolved photoluminescence measurement using the same excitation laser pulses, meaning that the time-resolution is simply limited by the streak-camera, not by the PE-gun performance. The result of numerical simulation on the temporal e-beam broadening caused by the space-charge-effect suggests that the present PE-gun can be used as a pulsed e-beam source for spatio-time-resolved cathodoluminescence, when equipped in a scanning electron microscope.

Phonon scattering of excitons and biexcitons in ZnO

The phase relaxation time of biexcitons T2bi and that of excitons T2ex in a bulk ZnO have been measured by the use of femtosecond four-wave mixing as functions of excitation wavelength, excitation power, and temperature. The biexciton-acoustic phonon interaction coefficient has been determined to be βacbi≈53 μeV/K as an average value. For A excitons, the interaction coefficients with acoustic and optical phonons have been determined to be βacex≈8.4 μeV/K and βoptex≈14 meV, respectively.

Biexciton formation and exciton-exciton correlation effects in bulk ZnO

We demonstrate that exciton–exciton correlations strongly affect FWM signals. In particular, A exciton–B exciton correlation (bound and unbound AB-biexcitons), which can be generated even in co-circular polarizations, has a significant qualitative impact on the optical response of bulk ZnO.

Impact of the k-linear term on nonlinear optical response of the C-exciton manifold in ZnO

Time-integrated and spectrally resolved four-wave mixing (FWM) has been applied to study the impact of the k-linear term on nonlinear optical response of the C-exciton manifold in ZnO. From the excitation wavelength dependence of the FWM signal, we have found possible evidence that the mixing of the Γ1 and Γ5L states leads to a three-branch excitonic polariton dispersion but the oscillator strength transfer is small. The energy separation between the lower polariton branch and the extra branch due to the k-linear term has been found to be 3.5 meV. In addition, a binding energy of biexcitons consisting of two C excitons of 1.4 meV has been obtained.

Optical nonlinearities of excitons in ZnO single crystal

Time-integrated and spectrally-resolved four-wave mixing (FWM) has been used to study dephasing dynamics of excitons in a free-standing bulk ZnO. Clear FWM signals due to A⟩Γ5- and BΓ5-excitions have been observed. We discuss the dephasing dynamics based on the polariton dispersion and four-particle Coulomb correlations.

Nonlinear optical dynamics in GaN and related materials

The carrier dynamics in InGaN layers and InGaN multiquantum-well (MQW) structures were studied by employing the degenerate pump and probe, time-resolved photoluminescence (PL), and white-light pump and probe measurements. The results from degenerate pump and probe measurements showed that the internal field existed in undoped in InGaN but then was screened by the electrons supplied by the Si atoms in Si-doped InGaN. The rise time of Pl in InGaN MQW obtained using the upconversion method was very fast, below 1ps, the mechanism of which is due to the carrier-carrier interaction enhanced by the residual electrons. The ΔOD spectra in InGaN MQW observed in white-light pump and probe measurements indicated that the carrier temperature was substantially higher than the lattice temperature even at 40ps after pulsed photo-pumping.

Scattering processes and dynamics of exciton-biexciton system in GaN

Bound and unbound biexcitons in a free-standing bulk GaN are investigated by time-integrated and spectrally-resolved four-wave mixing measurements, where the formation of hetero-biexcitons that consist of A and B excitons (XXAB) as well as A-biexcitons (XXAA) and their unbound biexciton (XX*AA) are clearly observed. The FWM spectra and delay-time dependence are explained qualitatively and the interaction between A- and B-excitons gives rise to the phase shifts of the quantum beating and the energy shifts of the spectra, which is considered as the effect of the unbound state of XXAB (i.e. XX*AB). The unbound A-biexciton (XX*AA). Is also observed clearly in spectral and temporal domain and is found to play an important role in FWM signals for all polarizations.