Sg. Fujita

Efficient radiative recombination from -oriented InxGa1-xN multiple quantum wells fabricated by the regrowth technique

InxGa1−xN multiple quantum wells (QWs) with [0001], ⟨112¯2⟩, and ⟨112¯0⟩ orientations have been fabricated by means of the regrowth technique on patterned GaN template with striped geometry, normal planes of which are (0001) and {112¯0}, on sapphire substrates. It was found that photoluminescence intensity of the {112¯2} QW is the strongest among the three QWs, and the internal quantum efficiency of the {112¯2} QW was estimated to be as large as about 40% at room temperature. The radiative recombination lifetime of the {112¯2} QW was about 0.38ns at low temperature, which was 3.8 times shorter than that of conventional [0001]-oriented InxGa1−xN QWs emitting at a similar wavelength of about 400nm. These findings strongly suggest the achievement of stronger oscillator strength owing to the suppression of piezoelectric fields.

Radiative and nonradiative recombination processes in GaN-based semiconductors

Time-resolved optical characterization is an indispensable tool to study the recombination mechanisms of excitons and/or carriers based on radiative, non-radiative, localization and many-body processes. In this paper, we review the instrumentation of various spectroscopic techniques for the assessment of In x Ga 1 -x N-based semiconductors such as time-resolved photoluminescence (TRPL), time-resolved electroluminescence (TREL), transient grating (TG) method to probe photothermal processes, microscopic TRPL using optical microscope, submicroscopic TRPL using scanning near field optical microscopy (SNOM) and pump-and-probe spectroscopy for the measurement of transient absorption/gain spectra. The obtained results are cited in the references.

Dimensionality of excitons in InGaN-based light emitting devices

Temperature dependence of radiative and non-radiative recombination times has been investigated in InxGa1–xN based light emitting devices by employing time-resolved luminescence spectroscopy. The mean In-composition (x value) assessed in this study is 10%, 20% and 30% whose emissions at 300 K correspond to near ultraviolet (390 nm), violet (422 nm) and blue (471 nm), respectively. It was found that the degree of exciton localization was enhanced with increasing In-composition in InxGa1–xN active layers, and that the zero-dimensional feature was revealed best of all in the sample with x = 30%, where radiative recombination time was almost constant (4 to 6 ns) in the temperature range from 20 to 300 K. The internal quantum efficiency of this sample was estimated to exceed 70% at 300 K. It is likely that such high efficiency is a result of zero-dimensionality because capture cross-sections to non-radiative recombination centers are greatly reduced once excitons are trapped at deep localization centers.

ZnO growth on Si substrates by metalorganic vapor phase epitaxy

Diethylzinc(DEZn) and nitrous oxide (N 2O) as a source gas combination in the metalorganic vapor phase epitaxy (MOVPE) of zinc oxide (ZnO) has produced high-quality layers on sapphire, but no growth was confirmed on Si. This problem was overcome by using an underlying layer of ZnO grown directly onto the Si using nitrogen oxide (NO2 )a s a more reactive oxidation source. The main ZnO layer grown in this way on the ZnO/Si pretreated at 8001C possessed a c-axis orientation and exhibited bound exciton (BX) emission as narrow as 3 meV at full-width at half-maximum together with a free exciton (EX) at 9 K. These results demonstrated the high potential of MOVPE technology for the growth of ZnO on Si using appropriate surface treatments for optical and electrical applications. r 2002 Elsevier

Growth of Polycrystalline GaN on Silicon (001) Substrates by RF Plasma Chemical Vapor Deposition with ZnO Buffer Layer

Polycrystalline GaN was grown at 650 °C on silicon (001) substrates by radio frequency (rf) plasma enhanced chemical vapor deposition (PECVD) with ZnO buffer layer. X-ray diffraction (XRD) measurements revealed that the ZnO buffer layer was oriented along the (0002) planes while GaN thin films were along (0002) and (101-1) planes. The ZnO buffer layer was found to be effective for the enhanced photoluminescence (PL) from the polycrystalline GaN. The low temperature (24 K) PL spectrum was dominated by the band-edge luminescence peaking at about 3.35 eV without appreciable deep level emissions. Temperature-dependent PL measurements indicated that the band-edge PL spectrum red-shifted with increasing temperature, where the nonradiative recombination is considered to be more dominant. Slope between the PL photon energy and the measurement temperature higher than 200 K up to room temperature well agreed with that of polycrystalline GaN by molecular beam epitaxy (MBE).

Optical properties of GaP/AlP short-period superlattices grown by gas source molecular beam epitaxy

Optical properties of (GaP)m/(AlP)n superlattices (SLs), with m+n=14, grown on GaP(001) by gas source molecular beam epitaxy have been studied by photoluminescence (PL), electroreflectance (ER), and optical reflectance. The dependence of the PL peak energies and the relative intensity on the monolayer number of AlP is in agreement with those theoretically predicted for the case of type II band alignment of GaP/AlP SLs. The refractive index of GaP/AlP SLs has been studied for the first time and compared with that of an AlGaP alloy. The refractive index calculated for an AlGaP alloy using a single‐effective‐oscillator model is consistent with the experimentally obtained results.

Homoepitaxial Growth of ZnO by Metalorganic Vapor Phase Epitaxy

Homoepitaxial technique of metalorganic vapor phase epitaxy (MOVPE) was used for the growth of high quality epitaxial ZnO layers. Two conditions, proper thermal treatment of substrate prior to the growth for obtaining flat surface and high flow rate ratios of source materials, nitrous oxide (N 2 O) and diethylzinc (DEZn), were found to be important. Surface roughness below 1 nm as well as strong free exciton emission at 15 K of an MOVPE-ZnO layer on a bulk ZnO substrate have demonstrated the high potential of MOVPE for ZnO.

Ordering dependence of carrier lifetimes and ordered states of Ga0.52In0.48P/GaAs with degree of order <= 0.55

Photoluminescence (PL) properties and carrier lifetimes of spontaneously ordered Ga0.52In0.48P/GaAs are measured at low temperatures. Samples with values of degree of order S from 0.22 to 0.55 are used. The full width at half maximum of the PL increases from 9.36 to 30.6 meV with increasing S. The PL peak of the indirect transition is blueshifted with increasing excitation power, whereas the PL peak of the direct transition is not. The energy separation between the PL peaks for the direct and the indirect transitions becomes greater with increasing S. The carrier lifetime increases dramatically with increasing S, from 0.68 ns at S=0.22 to 13 800 ns at S=0.55 at the peak energy of the PL spectrum. The ordering dependence of these characteristics is interpreted using a simple physical model of the ordered state. Variations in the ordered state with increasing ordering are discussed.

Carrier dynamics in InGaN/GaN SQW structure probed by the transient grating method with subpicosecond pulsed laser

Carrier dynamics in GaN and InGaN/GaN SQW structures were observed by using the transient grating (TG) method with sub-picosecond pulsed laser at room temperature. The diffusion coefficients (D) of photo-created carriers were estimated by the decay rate time of TG signals and the photoluminescence (PL) lifetime. It was found that D depends on the emission wavelength (In composition). The relationship between the emission efficiencies and carrier diffusion was considered in terms of the spatial inhomogeneity of In composition.

Nondegenerated Pump and Probe Spectroscopy in InGaN-Based Semiconductors

Carrier dynamics in two types of InGaN-based semiconductors were investigated using nondegenerated pump and probe spectroscopy. The samples consist of an In 0.1 Ga 0.9 N active layer, 30 nm thick (sample a) or an In 0.1 Ga 0.9 N/GaN multiple quantum well with ten periods, 3 nm/10 nm thick (sample b). Pump and probe spectroscopy revealed that photo-generated carriers pumped at 370 nm (3.350 eV) rapidly reach the bottom of levels in the active layers within a time scale of several ps for both samples. For the sample a, a positive peak at 3.21 eV was sandwiched between two negative peaks at 3.29 and 3.17 eV in the spectra of photo-induced change of absorption coefficient (Aa) because Franz-Keldysh and Stark effects on excitonic absorption were almost eliminated due to the screening of internal electric field. However, for the sample b, only negative signals were observed in Δα, indicating that the band filling of localized states plays a more important role. This is because the effect of electric field on the optical transition property is different depending on well widths.