Akira Uedono

Correlation between the photoluminescence lifetime and defect density in bulk and epitaxial ZnO

Influences of point defects on the nonradiative processes in ZnO were studied using steady-state and time-resolved photoluminescence (PL) spectroscopy making a connection with the results of positron annihilation measurement. Free excitonic PL intensity naturally increased with the increase in the nonradiative PL lifetime (τnr). Density or size of Zn vacancies (VZn) decreased and τnr increased with increasing growth temperature in heteroepitaxial films grown on a ScAlMgO4 substrate. Use of homoepitaxial substrate further decreased the VZn density. However, τnr was the shortest for the homoepitaxial film; i.e., no clear dependence was found between τnr and density / size of VZn or positron scattering centers. The results indicated that nonradiative recombination processes are not solely governed by single point defects, but by certain defect species introduced by the presence of VZn such as vacancy complexes.

Improvements in quantum efficiency of excitonic emissions in ZnO epilayers by the elimination of point defects

The internal quantum efficiency (ηint) of the near-band-edge (NBE) excitonic photoluminescence (PL) in ZnO epilayers was significantly improved by eliminating point defects, as well as by the use of ZnO high-temperature-annealed self-buffer layer (HITAB) on a ScAlMgO4 substrate as epitaxial templates. Negatively charged Zn vacancy (VZn) concentration was greatly reduced by high-temperature growth, and slower postgrowth cooling (annealing) under minimum oxygen pressure further reduced the gross concentration of positively and negatively charged and neutral point defects, according to the suppression of nonequilibrium defect quenching. The nonradiative PL lifetime (τnr) at room temperature was increased by decreasing the gross concentration of point defects, as well as by decreasing the concentration of VZn. Accordingly, certain point defect complexes incorporated with VZn (VZn-X complexes) are assigned to the dominant nonradiative recombination centers. As a result of the elimination of point defects, a re...

Limiting factors of room-temperature nonradiative photoluminescence lifetime in polar and nonpolar GaN studied by time-resolved photoluminescence and slow positron annihilation techniques

Room-temperature nonradiative lifetime (τnr) of the near-band-edge excitonic photoluminescence (PL) peak in {0001} polar, (112¯0), (11¯00), and (001) nonpolar GaN was shown to increase with the decrease in density or size of Ga vacancies (VGa) and with the decrease in gross density of point defects including complexes, leading to the increase in the PL intensity. As the edge threading dislocation density decreased, density or size of VGa tended to decrease and τnr tended to increase. However, there existed remarkable exceptions. The results indicate that nonradiative recombination process is governed not by single point defects, but by certain defects introduced with the incorporation of VGa, such as VGa-defect complexes.

Defects in ZnO thin films grown on ScAlMgO4 substrates probed by a monoenergetic positron beam

Zinc oxide (ZnO) thin films grown on ScAlMgO4 substrates were characterized by means of positron annihilation. We measured Doppler broadening spectra of annihilation radiation and photoluminescence spectra for the ZnO films deposited by laser molecular-beam epitaxy and single-crystal ZnO. Although the lifetime of positrons in single-crystal ZnO was close to the lifetime of positrons annihilated from the free state, the diffusion length of positrons was shorter than that for typical defect-free materials. We attribute this to the scattering of positrons by native defects. For the ZnO films, we observed a correlation between the defects and the lifetime of bound exciton emissions τEx; the main defect species detected by positron annihilation was Zn vacancies or other related defects. Isochronal annealing at 750–850 °C was found to introduce additional vacancy-type defects into the film, although the value of τEx was scarcely changed by the annealing.

Radiative and nonradiative processes in strain-free AlxGa1−xN films studied by time-resolved photoluminescence and positron annihilation techniques

Radiative and nonradiative processes in nearly strain-free AlxGa1−xN alloys were studied by means of steady-state and time-resolved (TR) photoluminescence (PL) spectroscopy, and the results were connected with that of positron annihilation measurement. The results of steady-state optical reflectance and PL measurements gave the bowing parameter b of approximately −0.82 eV. Values of the full width at half maximum (FWHM) of the near-band-edge PL peak nearly agreed with those predicted by the classical alloy broadening model. However, the Stokes-type shifts (SS) were as large as 100–250 meV and both SS and FWHM of the PL increased with the increase in x for x⩽0.7. Simultaneously, the luminescence redshift due to the increase in temperature T from 8 to 300 K decreased with increasing x and approached zero for x=0.5. These results indicated the presence of compositional fluctuation forming weakly bound states in the alloys, and the localized excitons tended to delocalize with the increase in T. The TRPL signa...

Study of defects in GaN grown by the two-flow metalorganic chemical vapor deposition technique using monoenergetic positron beams

Defects in GaN grown using metalorganic chemical vapor deposition were studied through the use of monoenergetic positron beams. For Mg-doped GaN, no large change in the diffusion length of positrons was observed before and after activation of Mg. This was attributed to the scattering of positrons by potentials caused by electric dipoles of Mg–hydrogen pairs. For Si-doped GaN, the line-shape parameter S increased as carrier density increased, suggesting an introduction of Ga vacancy due to the Fermi level effect. Based on these results, we discuss the effects of the growth polar direction of GaN on optical properties in this article. Although the optical properties of a GaN film grown toward the Ga face direction exhibited excitonic features, a film grown toward the N face (−c) direction exhibited broadened photoluminescence and transmittance spectra, and a Stokes shift of about 20 meV was observed. This difference was attributed to extended band-tail states introduced by high concentrations of donors and ...

Low temperature buffer growth for modulation doped SiGe/Ge/SiGe heterostructures with high hole mobility

Characterization of low-temperature (LT) Si and SiGe on the LT-Si grown by molecular beam epitaxy was carried out. Positron annihilation spectroscopy showed that the size of point defects in LT-Si grown at 4008C was much larger than divacancies. These vacancy clusters were considered to play an important role in strain relaxation of SiGe layers on the LT-Si. Although Si0.7Ge0.3 layer grown on the LTSi had excellent qualities compared with the graded buffer Si12xGex layer, it was found to degrade when Ge content was increased up to x , 0:5 but be improved again above this content. The surface morphology also changed at this Ge content from crosshatch pattern to smooth surface with increasing Ge content. p-type modulation doped structures with pure-Ge channels were fabricated on the LT-buffers and the transport properties were found to significantly depend on the surface morphology. The reason that the rough surface gave rise to rather high mobility was discussed in the terms of roughness scattering. q 2000 Published by Elsevier Science S.A. All rights reserved.

Relation between Al vacancies and deep emission bands in AlN epitaxial films grown by NH3-source molecular beam epitaxy

Intensity ratios of characteristic deep cathodoluminescence (CL) bands at 4.6, 3.8, and 3.1eV to the near-band-edge emissions at 11K of AlN epilayers grown by NH3-source molecular beam epitaxy were correlated with the change in the S parameter of positron annihilation measurement, which represents the concentration or size of Al vacancies (VAl). Since the relative intensities of 3.1 and 3.8eV bands increased remarkably with lowering supply ratio of NH3 to Al (V/III ratio) and growth temperature (Tg), they were assigned to originate from VAl-O complexes. The VAl concentration could be decreased by adjusting V/III ratio and Tg, resulting in observation of fine excitonic features in the CL spectra. From the energy separation between the ground and first excited states, the binding energy of A exciton was determined to be 48meV.

Impact of growth polar direction on the optical properties of GaN grown by metalorganic vapor phase epitaxy

The growth polar direction during metalorganic vapor phase epitaxy of wurtzite GaN was shown to affect the optical properties in terms of impurity and vacancy incorporation during the growth. The GaN film grown toward the Ga (0001) face (+c polarity) exhibited clear excitonic features in its optical absorption and luminescence spectra up to room temperature. Conversely, the film with the N (0001) face (−c polarity) exhibited a broad emission band, which locates in the broad absorption tail. The difference between the two was explained in terms of the presence of impurity-induced band-tail states in −c GaN due to increased impurity density and incorporation of large volume vacancy-type defects, which were confirmed by secondary ion mass spectrometry [Sumiya et al., Appl. Phys. Lett. 76, 2098 (2000)] and monoenergetic slow positron annihilation technique.

Exciton-polariton spectra and limiting factors for the room-temperature photoluminescence efficiency in ZnO

Static and dynamic responses of excitons in state-of-the-art bulk and epitaxial ZnO are reviewed to support the possible realization of polariton lasers, which are coherent and monochromatic light sources due to Bose condensation of exciton–polaritons in semiconductor microcavities (MCs). To grasp the current problems and to pave the way for obtaining ZnO epilayers of improved quality, the following four principal subjects are treated: (i) polarized optical reflectance (OR), photoreflectance (PR) and photoluminescence (PL) spectra of the bulk and epitaxial ZnO were recorded at 8 K. Energies of PR resonances corresponded to those of upper and lower exciton–polariton branches, where A-, B- and C-excitons couple simultaneously to an electromagnetic wave. PL peaks due to the corresponding polariton branches were observed. Longitudinal–transverse splittings (ωLT) of the corresponding excitons were 1.5, 11.1 and 13.1 meV, respectively. The latter two values are more than two orders of magnitude greater than that of GaAs being 0.08 meV. (ii) Using these values and material parameters, corresponding vacuum-field Rabi splitting of exciton–polaritons coupled to a model MC mode was calculated to be 191 meV, which is the highest value ever reported for semiconductor MCs and satisfies the requirements to observe the strong exciton–light coupling regime necessary for polariton lasing above room temperature. (iii) Polarized OR and PR spectra of an out-plane nonpolar ZnO epilayer grown by laser-assisted molecular beam epitaxy (L-MBE) were measured, since ZnO quantum wells (QWs) grown in nonpolar orientations are expected to show higher emission efficiencies due to the elimination of spontaneous and piezoelectric polarization fields normal to the QW plane. They exhibited in-plane anisotropic exciton resonances according to the polarization selection rules for anisotropically-strained wurzite material. (iv) Impacts of point defects on the nonradiative processes in L-MBE ZnO were studied using time-resolved PL making a connection with the results of positron annihilation measurement. Free excitonic PL intensity at room temperature naturally increased with the increase in nonradiative lifetime (τnr). The value of τnr increased and density or size of Zn vacancies (VZn) decreased with increasing growth temperature (Tg) in heteroepitaxial films grown on a ScAlMgO4 substrate, and the use of homoepitaxial substrates further reduced VZn density. The value of τnr was shown to increase with the decrease in gross density of positively and negatively charged and neutral point defects including complexes rather than with the decrease in VZn density. The results indicate that the nonradiative recombination process is governed not by single point defects, but by certain defects introduced with the incorporation of VZn, such as VZn-defect complexes. As a result of defect elimination by growing the films at high Tg followed by subsequent post-growth in situ annealing, combined with the use of high-temperature-annealed ZnO self-buffer layer, a record long τnr for spontaneous emission of 3.8 ns was obtained at room temperature. By using progressively improving epitaxial growth methods, the polariton laser effect is expected to be observed at room temperature in the near future.