石谷 善博

We studied on the growth of N-polarity InN/AlInN MQWs by radio frequency plasma-assisted MBE. We found that the growth temperature greatly affected the structural properties of InN/AlInN MQWs. The growth temperature for achieving fine periodic MQWs was in the range from 550 to 580°C. By optimizing the growth conditions, we for the first time successfully fabricated InN-based MQWs with AlxIn1-xN (∼0.2 &lt x &lt ∼0.3) barrier layers. Clear satellite peaks up to the 3rd order in high resolution X-ray diffraction were observed, indicating that being fine periodic MQWs-structures with fairly fiat and sharp interfaces. Photoluminescence peaks ranging from 0.68 to 0.99 eV were observed depending on the well thickness and they were in good agreement with those estimated by simple theoretical calculation. © 2006 WILEY-VCH Verlag GmbH &amp Co. KGaA.

The optical properties of hexagonal InN crystal are investigated by reflectance, absorption and photoluminescence method. With use of the theoretical formula of these spectra, both of conduction and valence band properties are analysed. As a result, the average effective electron and hole mass values at the band edge are found to be about 0.046(±0.01)w 0 and 0.2(±0.05)m0) respectively. The photoluminescence emitted from the cleaved edge was strongly polarized. The agreement of the peak energies of π and σ polarized light means that A, B, and C band edges are closely located in energy space. © 2006 WILEY-VCH Verlag GmbH &amp Co. KGaA.

ZnO epilayers were grown on nitrided c-Al2O3 substrates by MBE and the effect of the deposition temperature for GaN buffer layers on the polarity of ZnO epilayer and their properties such as structural and optical properties were investigated. Zn-polar ZnO was normally obtained by using the low temperature GaN buffer layer at 650°C, while O-polar ZnO were obtained by the high-temperature GaN buffer layer at 850°C. PL spectra for Zn-polar and O-polar ZnO epilayers, PL spectra were dominated by donor-bound exciton at 3.361 eV and acceptor-bound exciton at 3.356 eV, respectively. It was found that the position of emission peak was affected by the strain in the ZnO layer. PL spectra observed in ZnO epilayers with different polarities are discussed together with their reflectance spectra and strains of ZnO epilayers with different polarities. © 2006 WILEY-VCH Verlag GmbH &amp Co. KGaA.

Strain-free Raman frequencies of the E2 high and A1 (LO) modes of hexagonal InN are determined to be 490.1±0.2 and 585.4±0.4 cm-1 by Raman measurements on a freestanding InN film grown by molecular beam epitaxy. The strain-free Raman frequencies are further confirmed by linear fits to Raman frequencies of the E2 and A1 (LO) modes of InN epilayers under different biaxial strains. Raman linear biaxial stress coefficients for the E2 and A1 (LO) modes of InN are obtained with values of 9.0±0.8 and 8.4±0.8 cm-1 GPa. The phonon deformation potentials are also obtained by using sets of available stiffness constants of hexagonal InN. © 2006 American Institute of Physics.

We have succeeded in the growth of very-fine-structure InNInGaN (3-16 nm9 nm) multi-quantum wells (MQWs) on GaN underlayer and characterized them by transmission electron microscopy (TEM), high-resolution x-ray diffraction (XRD), and photoluminescence (PL) at 13 K. Clear satellite diffraction peaks and sharp heterointerfaces were observed by XRD and TEM, respectively. A single PL-emission peak at 1.75 μm was observed in the ten-periods InN (16.1 nm) In0.67 Ga0.33 N (9.2 nm) MQWs. The use of the InGaN as a barrier layer instead of GaN resulted in remarkable reduction of lattice mismatch between the well and barrier, which was essential for the fabrication of MQWs with superior interface quality. This successful growth of fine-structure InNInGaN MQWs would be an important step for the application of InN in optical communication devices. © 2005 American Institute of Physics.

Epitaxy of InN films with N-polarity and In-polarity was investigated by RF-MBE with several in-situ monitoring/controlling systems. It was found that the epitaxy temperature for N-polarity growth could be as high as 600 degrees C and this was about 100 deg higher than that for In-polarity case. This temperature difference in two polarities drastically affected not only the growth behaviors but also the properties of InN epilayers, i.e. N-polarity growth was preferable in both view-points. The step-flow-like surface morphology was achieved for the InN films grown with N-polarity at 580 degrees C. The FWHMs of X-ray rocking curves for InN (002) and (102) of 5-8 mu m-thick InN films grown in N-polarity were about 200-350 and 650-950 arcsec, respectively. The highest Hall mobility was above 2000 cm(2)/V(.)s with a background carrier concentration of 1-2x10(18) cm(-3) at room temperature. For both polarity films, N-rich condition was necessary for the stable InN growth to obtain 5-8 mu m-thick InN films.

We have grown InN quantum dots (QDs) on nitrogen-polarity (N-polarity) GaN under-layer by the radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE), and systematically investigated growth mechanism of the InN dots. The InN QDs with the N-polarity could be grown at about 500 C, which was much higher than that of previous reports on InN dots grown by MBE. When the nominal coverage of InN became more than 1 mono-layer (ML), lattice relaxation of InN occurred and high density InN dots were uniformly formed. These results indicated that InN dots were formed by Stranski-Krastanov (S-K) growth mode. For the InN deposition above about 8ML, InN dots tended to coalesce and resulted in remarakable decrease of the dots density.

The effect of TMAl-preflow used for polarity change of MOVPE-grown GaN epitaxial layers on sapphire c face was studied by high resolution X-ray diffraction including 2θ-ω scan and reciprocal space maps. The a- and c-lattice parameters, as well as the in-plane and out-of-plane strains (of the order of 10-3 and 10-4, respectively) and biaxial stress are determined for GaN epilayers. In the GaN epilayers, in-plane strains are of compressive type, and this tendency is stronger with increasing TMAl-preflow time. High quality epilayers were obtained for 5 and 10 s TMAl-preflow. Their FWHMs of X-ray rocking curves were around 300-320 arcsec for (0 0 2) reflection and around 450-480 arcsec for (1 0 2) reflection. © 2005 WILEY-VCH Verlag GmbH &amp Co. KGaA, Weinheim.

We studied the fabrication of InN/GaN single-quantum well (SQW) and double hetero (DH) structures by the radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE) system with in-situ monitoring of spectroscopic ellipsometry (SE) and reflection high-energy electron diffraction (RHEED). By using this system, we could monitor and control the epitaxial growth front in real time and succeeded in growing the SQW and/or DH structures whose well layer thickness ranged from 5 nm to 40 nm. In the photoluminescence measurements at 13 K, single emission peak was observed in the range from 1.55 μm to 1.7 nm depending on the well layer thickness. The observed blue shift was ascribed to the quantum effect of InN/GaN SQW. © 2005 WILEY-VCH Verlag GmbH &amp Co. KGaA, Weinheim.

We investigated the growth behaviors of InN quantum dots (QDs) on N-polarity GaN by molecular-beam epitaxy. The N-polarity growth has been intentionally used to raise the temperature to facilitate formation of high-quality dots. It was found that the InN QDs could be grown up to 550 °C the Stranski-Kastanov growth mode with the wetting layer thickness of about 1 monolayer, which was confirmed by the simultaneous in situ observations of reflection high-energy electron diffraction and spectroscopic ellipsometry. The density and the diameter of typical InN QDs grown at 450-550 °C were the order of 1011 cm-2 and 15-20 nm, respectively. © 2005 American Institute of Physics.

The polarity of molecular-beam epitaxy grown ZnO films was controlled on nitrided c -sapphire substrate by modifying the interface between the ZnO buffer layer and the nitrided sapphire. The ZnO film grown on nitrided sapphire was proven to be Zn-polar while the O-polar one was obtained by using gallium predeposition on nitrided sapphire, which was confirmed by coaxial impact collision ion scattering spectroscopy and chemical etching effect. The Zn-polar ZnO film showed higher growth rate, slightly better quality, and different surface morphology in comparison to the O-polar one. © 2005 American Institute of Physics.

InN crystals are grown on sapphire substrates by a plasma assisted MBE system. The carrier concentration of the samples are 2×10 18-1×1019 cm-3. Optical transmission and reflectance measurements are performed on these samples in a temperature range of 8 - 300 K. The resultant spectra are analysed by theoretical spectra based on a LO-phonon plasmon coupling scheme for phonon related factor and nonparabolic conduction band structure for electronic transition factor. The observed absorption edge is estimated to mainly originate from the valence band to conduction band transition rather than defect or impurity related transition on the basis of the electron concentration dependence of the momentum matrix element. The bandgap energy is about 0.63 eV, and increases with the temperature decrease. © 2005 WILEY-VCH Verlag GmbH &amp Co. KGaA, Weinheim.

ZnO epilayers were grown on nitridated c-Al2O3 substrates by plasma-assisted molecular beam epitaxy and the effect of the deposition temperature for GaN buffer and/or intermediate layers on the polarity of ZnO epilayer was investigated. First, it was found that the polarity of the ZnO epilayers grown on low temperature-grown GaN buffer and/or intermediate layers was uncertain and it became often +c polarity, i.e., Zn-polarity. This seems strange because the polarity of the GaN-underlying layer was -c polarity, i.e., N-polarity, and then the polarity of the under layer was not kept unchanged by the following epilayer. On the other hand, when the growth temperature of the GaN buffer/intermediate layer was increased above 850 °C, it was found that the polarity of the GaN layer was followed by the ZnO epilayers the polarity became -c polarity, i.e., O-polarity. This reason was estimated by the effect of surface oxides on GaN which were formed just before the deposition of ZnO buffer layer on it. © 2004 WILEY-VCH Verlag GmbH &amp Co. KGaA, Weinheim.

ZnCdMgSe epilayers and ZnCdSe/ZnCdMgSe quantum well structures were grown on GaAs(111)A and B substrates by MBE using a low-temperature-grown CdS buffer layer. X-ray reciprocal space maps demonstrated that ZnCdMgSe epilayers grown on CdS buffer layers were predominantly hexagonal structures. In the quantum well structures, in-situ RHEED pattern also demonstrated the hexagonal structure during the growth. X-ray diffraction results showed that the crystalline quality of the ZnCdMgSe epilayers grown on GaAs(111)A surface was much better than that on the (111)B surface. Low temperature photoluminescence of ZnCdMgSe epilayers were dominated by the deep and shallow donor-acceptor pair emissions for the case of GaAs(111)A and B, respectively. The peak energy positions of the photoluminescence spectra increased with increasing the growth temperature. Although strong emissions from ZnCdMgSe barriers were observed, emissions from the ZnCdSe QW structure were very weak. © 2004 WILEY-VCH Verlag GmbH &amp Co. KGaA, Weinheim.

Rotation domains are easily and often introduced into ZnO epilayers grown on c-Al2O3 substrate, resulting in degraded epilayer quality. In this paper, effects of the sapphire surface properties on the epitaxy of ZnO were studied the surface was carefully treated by high temperature thermal cleaning (TC), atomic hydrogen (H*), oxygen radical (O*) treatments, decoration by a few monolayers gallium and nitridation, and then the ZnO epilayers grown on these surfaces were characterized. Three kinds of rotation domains were observed in total and the sub-domains were completely suppressed by Ga decoration and sapphire nitridation. The effect of the above treatments on polarities of ZnO epilayer was also studied. © 2004 WILEY-VCH Verlag GmbH &amp Co. KGaA, Weinheim.

InN crystals are grown on sapphire substrates using a plasma-assisted MBE system. The carrier concentrations of the samples are 2 × 10 18-1 × 1019 cm-3. Optical transmission and reflectance measurements are performed on these samples in the temperature range 5-300 K. The resultant spectra are analysed by theoretical spectra based on the LO-phonon-plasmon coupling scheme for the phonon-related factor and non-parabolic conduction band structure for the electronic transition factor. The observed absorption edge is estimated to originate from a valence band to conduction band transition rather than a defect- or impurity-related transition. It is estimated that InN has a bandgap energy in the range 0.59-0.65 eV. © 2004 WILEY-VCH Verlag GmbH &amp Co. KGaA, Weinheim.

InN epitaxial layers are grown on sapphire substrates. The investigated samples have electron concentration in a range of 1.8 × 1018 - 1.1 × 1019 cm-3. Optical reflection and transmission measurements are performed. The plasma edge energy position in the spectra is constant in a measurement temperature range of 5 - 300 K. The reflection and transmission spectra are calculated on the basis of the LO phonon-electron coupling scheme and non-parabolic conduction band structure. From this analysis we find that the observed absorption edge is attributed to valence band to conduction band transition rather than the valence band to defect (impurity) band transition, and the intrinsic bandgap energy of 0.64 (±0.03) eV. This bandgap energy increases by 40 - 50 meV as the temperature decreases from 295 to 10 K.

InN crystals with the electron concentration N-e of 4-11 x 10(18) cm(-3) are grown on sapphire substrates by molecular beam epitaxy. Optical reflectance and transmission measurements are performed in a temperature range of 5 to 296 K. The reflectance spectra are analysed by simulation including the longitudinal optical phonon-plasmon coupling scheme. At a N-e value of 4 x 10(18) cm(-3), the electronic states are found to be still degenerated. An anomalous temperature dependence of the bandgp energy E-s is found; with decreasing temperature E-g increases, and then decreases below 50 K. This decrease is smaller for samples with smaller N-e. In spite of this energy shift the decrease of N-e is too small to be detected. The effective electron mass of the intrinsic region is estimated to be 0.1m(0). (C) 2003 WILEYNCH Verlag GmbH & Co. KGaA, Weinheim.

The polarity manipulation and crystallinity of GaN epilayers grown by metalorganic vapor phase epitaxy (MOVPE) on deeply nitrided c-sapphire was studied by using TMAl preflow at high temperatures (HTTMAl preflow) of 1100 °C. It was found that the HT-TMAl preflow was very effective to change the surface polarity the successful change to Ga-polarity from N-polarity and the atomic-steps were observed when the Al layer thickness was above two-monolayers, though it was remained N-polarity without the Al layer. The crystallinity of GaN epilayer is increased using HT-TMAl preflow. High quality Ga-polarity GaN epilayers was grown by this technique and typical full width at half maximums (FWHMs) for X-ray rocking curves were 300-320 arcsec and 450-480 arcsec for (002) and (102) diffractions, respectively. © 2003 WILEY-VCH Verlag GmbH &amp Co. KGaA.

InN growth on 2-inch sapphire (0001) with thickness up to 5 μm was demonstrated by radio-frequency plasma-assisted molecular beam epitaxy (rf-MBE). The surfaces of the 2-inch samples were mirror-like and atomic growth steps were observed in large areas. The room temperature Hall mobility crossing the 2-inch InN wafers ranged from 900 to 1100 cm2/V s, with electron concentration of the order of 1018/cm3. The optical bandgap of the InN layers with electron concentration of the order of 10 18/cm3 was 0.70-0.74 eV measured by optical transmission/reflection spectroscopy. © 2003 WILEY-VCH Verlag GmbH &amp Co. KGaA.

Molecular-beam epitaxial growth of InN was investigated on Ga- and N-polar GaN templates. It was found that InN layers could be grown at higher temperature on N-polar GaN than on Ga-polar one, on which InN films succeeded the cation- or anion-polarity, respectively. The growth temperature of In-polar InN was limited below 500 °C while that of N-polar InN could be as high as 600 °C. In the upper-limit growth temperature range for N-polar InN, atomic-height growth steps could be observed on the surface. The possible mechanisms for the polarity dependence of InN growth behaviors were also discussed. © 2003 WILEY-VCH Verlag GmbH &amp Co. KGaA.

Effects of migration-enhanced-epitaxy-grown AlN (MEE-AlN) on polarities of GaN were investigated in low-pressure metal-organic vapor phase epitaxy (LP-MOVPE). AlN was grown on extensively nitrided sapphire substrate by alternatively supplying trimethyl-aluminium (TMA) and ammonia with 2-second purging in between. It was found that the polarities of GaN depend on the growth temperatures of MEE-AlN. With increase of the growth temperature of the MEE-AlN layer, the Ga-polar ratio in GaN epilayer decreased. At the same growth temperature of MEE-AlN, the formation of Ga-polarity could be promoted by prolonging the TMA pre-flow time in the first cycle of MEE-AlN growth. The experimental results well reveal the kinetic selection process of GaN polarity in the MOVPE growth. © 2003 WILEY-VCH Verlag GmbH &amp Co. KGaA.

Temporal developments of the photoluminescence (PL) intensity at temperatures of 7, 100, and 294 K are analyzed using the rate equations including the exciton dissociation and association terms for an Al0.53In0.47P/Ga0.52In0.48P/Al0.53In0.47P-quantum well structure. At 7 K, the nonexponential time dependence of the PL intensity is caused by the exciton dissociation process. At 7 and 100 K, PL intensity is dominated by the exciton recombination even if the exciton density is smaller than the dissociated carrier density. The thermally excited background carriers affect the recombination processes at 100 and 294 K. At 294 K, the rise part of the PL intensity is dominated by the exciton recombination, though the dissociated carrier density dominates. (C) 2001 American Institute of Physics.

The photoluminescence and the photo-current are simultaneously measured for indirect transition type undoped (Al0.7Ga0.3)(0.5)In0.5P/AlxIn1-xP (x=0.53 and 0.57) super lattices. The bias voltage is applied on the semi-transparent Au electrode on the epitaxial layer. From the measurement of the sample with x=0.53 (lattice matched to the GaAs substrate) under the bias voltage of +0.3 V the carrier transport little affects the PL decay curve, which is dominated by the radiative recombination of carriers. The surface electric field of the sample without electrodes is estimated to be less than 3x10(3) V/cm. For the sample with x=0.57, the energy states originating from the crystal defects act as the carrier traps under a low electric field and the levels which contribute to the tunneling of electrons through the barriers under a high electric field.

The photoluminescence (PL) intensity of AlGaZnP/GaInP-quantum well structures under a continuous excitation condition was measured as a function of the crystal temperature (10-450 K). The thermal emission of carriers from the well layers to the barrier layers was investigated by generating the carriers only in the wells. The dependence of the PL intensity on the crystal temperature was analyzed by fitting a model function to the experimental results. The rapid decrease in PL intensity at temperatures higher than 200 K was found to be probably due to the emission of carriers into the barrier layers. The effective barrier heights obtained by the fitting are in good agreement with the band-energy alignment schemes of these crystals. When AlInP barrier layers were inserted directly beside the quantum wells, the emission of carriers to the quaternary layers through the AlInP layers was reduced, resulting in stronger PL intensity at higher temperatures and a larger effective barrier height. (C) 1997 American Institute of Physics.

The optical processes in AlInP/GaInP/AlInP quantum wells free from long-range ordering are examined by photoluminescence (PL), photoluminescence excitation (PLE), and photoreflectance (PR) measurements. The PL method observes lower transition energy than the PLE and the PR methods which observe the space-averaged transition energy. This is because PL originates from localized lower-energy states to which an exciton state relaxes. By analyzing these measurements carried on 20- and 60-Å-wide wells, the reason for this deviation is ascribed to the fluctuation of the transition energy due to the local variation of the well width by one molecular layer. The plausible share of the band offset for the conduction band against the energy gap difference at the Γ point is obtained by comparing the experimentally obtained relative position of the energy levels in AlInP barriers and the 20 Å GaInP well with the calculated ones. This is found to be 0.75 (±0.06). © 1996 American Institute of Physics.

High concentration doping of silicon into Ga0.5In0.5P layers using disilane as the doping precursor in organometallic vapor phase epitaxy is performed at growth temperatures between 620 and 700°C. The electron concentration changes linearly with disilane flow rate, reaching a maximum at 1 × 1019 cm-3 and staturates thereafter, while the silicon concentration keeps increasing. Heavy doping raises the photoluminescence (PL) peak energy to 1.95 eV. The increment is 120 meV at lower growth temperatures on (100) substrates, but it remains 40 meV on (511)A. The former is due to the combined effect of electron concentration and atomic disordering, whereas the latter is due to electronic effects only. © 1995.

Short-wavelength stimulated emission from a GaInP/AlGaInP double-heterostructure (DH) grown on GaAs0.6P0.4 substrates, where lattice-matched Ga0.7In0.3P is the active layer with Γ band-gap energy beyond 2.1 eV is investigated. Laser oscillation is attained at a wavelength below 590 nm. This shows that the DH attains sufficient carrier confinement for lasing even though the minimum Γ band-gap energy is close to that in the X band. By applying high-reflectivity coating on both facets of the cavity to decrease the optical mirror loss, we achieve lasing operation by the DH devices under pulsed current injection up to 200 K. The device exhibits threshold currents of 115 mA at 77 K and 380 mA at 200 K, and an output power level up to 0.3 mW. The oscillation wavelength is 577 nm at 77 K and 588 nm at 200 K when the current is injected at 1.2 times the threshold.© 1995 American Institute of Physics.

The temperature variation of the photoluminescence peak energy of GaxIn1-xP shows a complex dependence on the structural order; however, such effects are not well explained even for the fully disordered material. The band-gap energy of disordered GaxIn1-xP was measured at temperatures from 10 to 300 K and the measured values were used to find the best parameters in a theoretical equation. Since a simpler expression does not fit the experimental data accurately (especially below 30 K), an expression consisting of three terms which represent the effect of thermal expansion, electron-optical-phonon coupling, and electron-acoustic-phonon coupling was adopted. Dividing an electron-phonon coupling term into two terms results in a better expression for InP over the entire range of measuring temperatures. This function was applied to disordered GaxIn1-xP (x = 0.5, 0.7). It is found that in InP the electron-acoustic-phonon coupling is stronger than electronoptical-phonon coupling, while in GaxIn1-xP the opposite is true.

Laser-induced-fluorescence spectroscopy has been applied to a neon discharge plasma. A disalignment rate coefficient for 2p2 (Paschen notation; 2p5 3p configuration) atoms for neon atom collisions was determined at several atom temperatures (77-640 K). From the temperature dependence of the rate coefficient we estimate the cross section which explicitly depends on collision energy. It is found that the cross section has a positive energy dependence, which suggests that the OMEGA = 0 and 1 molecular potential curves correlating to the 2P2 State of neon are repulsive by about 100 cm-1 where these curves split by 10-20 cm-1.

Laser-induced-fluorescence spectroscopy has been applied to neon and helium-neon discharge plasmas in magnetic fields lower than 10 T. Disalignment-rate coefficients of the neon 2p2 and 2p7 atoms (2p(5)3p configuration, Paschen notation) have been determined for neon and helium collisions in a field strength that is lower than the critical field at which the collision time is equal to the inverse Larmor frequency. The 2p7 atoms have rate coefficients independent of the magnetic-field strength, whereas the 2p2 atoms have a field-dependent rate coefficient for neon collisions.