S. F. Chichibu

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.

Prospective emission efficiency and in-plane light polarization of nonpolar m-plane inxGa1-xN/GaN blue light emitting diodes fabricated on freestanding GaN substrates

Prospective equivalent internal quantum efficiency (ηint) of approximately 34% at 300K was demonstrated for the blue emission peak of nonpolar m-plane (11¯00) InxGa1−xN∕GaN multiple quantum well light emitting diodes (LEDs) fabricated on freestanding m-plane GaN substrates. Although the ηint value is yet lower than that of conventional c-plane blue LEDs (>70%), the results encourage one to realize high performance green, amber, and red LEDs by reducing the concentration of nonradiative defects, according to the absence of the quantum-confined Stark effects due to the polarization fields parallel to the quantum well normal. The electric field component of the blue surface emission was polarized perpendicular to the c axis with the in-plane polarization ratio of 0.58 at 300K.

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.

Improved quantum efficiency in nonpolar (112̄0) AlGaN/GaN quantum wells grown on GaN prepared by lateral epitaxial overgrowth

Radiative and nonradiative excitonic transitions in nonpolar (1120) AlxGa1−xN/GaN multiple quantum wells (MQWs) grown on the GaN template prepared by lateral epitaxial overgrowth (LEO-GaN) were investigated. The structural advantages of using nonpolar orientations were confirmed by a moderate shift of the photoluminescence (PL) peak energy and negligible change in low-temperature PL lifetime with decreasing GaN well width, both of which are the results of eliminating quantum-confined Stark effects due to the polarization fields that exist in polar (0001) MQWs. Appearance of the correct in-plane light polarization and improved internal quantum efficiency for the PL peak in the MQWs on LEO-GaN were attributed to the reduction in densities of nonradiative defects and bound states.

Photoreflectance spectra of a ZnO heteroepitaxial film on the nearly lattice-matched ScAlMgO4 (0001) substrate grown by laser molecular-beam epitaxy

Photoreflectance spectra of a high-quality ZnO epilayer on the ScAlMgO4 (0001) substrate grown by laser molecular-beam epitaxy exhibited clear excitonic resonances due to three excitons associated with uppermost valence bands (A, B, and C excitons). The oscillator strengths of the B and C excitons are larger than that of the A exciton. Their broadening was interpreted to be due to the contribution by exciton–polaritons in terms of large longitudinal–transverse splitting of respective excitons. Dependence of the exciton energy on temperature was well fitted assuming the Bose–Einstein statistics giving the Einstein characteristic temperature ΘE of 380 K (33 meV).

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.

Localized exciton dynamics in nonpolar (112¯0) InxGa1−xN multiple quantum wells grown on GaN templates prepared by lateral epitaxial overgrowth

Beneficial effects of the localized excitons were confirmed in nonpolar (112¯0) InxGa1−xN multiple quantum wells (QWs) grown on GaN templates prepared by lateral epitaxial overgrowth. Due to the absence of the polarization fields normal to the QW plane, the photoluminescence (PL) peak energy moderately shifted to the higher energy and the radiative lifetime did not change remarkably with the decrease in the well thickness. Similar to the case for polar InGaN QWs, time-resolved PL signals exhibited the nonexponential decay shape, which can be explained by thermalization and subsequent localization of excitons. Although the growth conditions were not fully optimized, values of the PL intensity at 300K divided by that at 8K were 25% and 17% for the peaks at 2.92 and 2.60eV, respectively.

Radiative and nonradiative excitonic transitions in nonpolar (112̄0) and polar (0001̄) and (0001) ZnO epilayers

Polarized optical reflectance and photoreflectance spectra of an out-plane nonpolar (1120) ZnO epilayer grown by laser molecular-beam epitaxy exhibited anisotropic exciton resonance structures according to the polarization selection rules for anisotropically strained hexagonal material. Consistently, the electric field component of its excitonic photoluminescence (PL) peak was polarized perpendicular to the [0001] axis. Different from the case for GaN, nonradiative PL lifetime at 293 K and the S parameter, which is a measure of Zn vacancy-related defect density obtained by positron annihilation spectroscopy, of the (1120) ZnO were comparable to those of state-of-the-art polar (0001) and (0001) epilayers. Since the polar epilayers exhibited pronounced exciton–polariton emissions, the negligible impact of growth direction on the defect incorporation suggests a potential use of epitaxial (1120) ZnO as polarization-sensitive optoelectronic devices operating in ultraviolet spectral regions.

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.

Recombination dynamics of localized excitons in Al1−xInxN epitaxial films on GaN templates grown by metalorganic vapor phase epitaxy

Recombination dynamics of excitons in nearly strain-free Al1−xInxN alloys on the GaN template were studied. Their band-gap energy showed a nonlinear dependence on the InN molar fraction x, and the bowing parameter was determined to be approximately −3.1 eV. Most of the alloys exhibited an extremely diffused band-edge, and consequently exhibited huge Stokes-type shifts up to 1–2 eV and full width at half maximum of the luminescence peaks up to 0.5 eV. The results suggested enhanced material inhomogeneity in AlInN compared to InGaN alloys. Since the time-resolved photoluminescence signal showed a pronounced stretched exponential decay, the spontaneous emission was assigned as being due to the radiative recombination of excitons localized in disordered quantum nanostructures. The integrated PL intensity at 300 K was as strong as 29% of that at low temperature, showing a potential use of AlInN alloys as infrared-to-UV light emitters.