石谷 善博

InN layers have electron accumulation structure around the surfaces and interfaces with the substrates. Infrared (IR) spectroscopy enables the analysis of the inside bulk region in spite of the high sheet electron density of the order of 10(13) cm(-2) in the accumulation layers using the dispersion of the penetration depth. The polarization spectroscopy shows the anisotropic electron and hole scattering rates. The higher scattering rates of electron and hole plasmons vibrating along the c axis are attributed to edge-type dislocations, while the scattering by Mg or related complex dominates the damping processes of hole plasmons vibrating vertical to the c axis in highly Mg-doped p-InN. The smaller photoluminescence (PL) intensity of p-InN is attributed to the smaller activation energy of carriers to be captured by deep levels than that from the captured carriers to the ground state. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We investigate the nonradiative carrier recombination (NR) process in Mg-doped p-InN films having lower photoluminescence (PL) intensity than n-InN films. The NR activation energy in the p-type films is found to be in a range of 9-15 meV, which is smaller than that in n-InN films (40-65 meV). We also investigate the effect of the greater mean free path of minority carriers in p-InN. At room temperature the collision rate of minority carriers with NR centers within the radiative lifetime in p-InN is found to be three orders of magnitude greater than that in n-InN. © 2011 American Institute of Physics.

In this work we study the optical properties of two high quality fifty-periods of In-polarity InN/In0.73Ga0.27N MQWs samples, grown by radio-frequency plasma-assisted molecular beam epitaxy, with different well (0.5-1 nm) and barrier thicknesses (3-4 nm). We employ spectroscopic ellipsometry at room temperature in the energy range from 0.6 to 6 eV, and incidence angles of 60 and 70°. Ellipsometric data were successfully modelled using the model dielectric function approach and a multilayer model assuming the MQWs as a homogeneous layer. The E0, A and E1 MQWs transition energies were determined and found to exhibit a blueshift with decreasing the well thickness. © 2011 WILEY-VCH Verlag GmbH &amp Co. KGaA, Weinheim.

We report a strong circular photogalvanic effect (CPGE) in ZnO epitaxial films under interband excitation. It is observed that CPGE current is as large as 100 nA/W in ZnO, which is about one order in magnitude higher than that in InN film while the CPGE currents in GaN films are not detectable. The possible reasons for the above observations are the strong spin orbit coupling in ZnO or the inversed valence band structure of ZnO. (C) 2010 American Institute of Physics. [doi:10.1063/1.3467835]

InN is still the least studied material among III-nitrides and there are several problems to be overcome for better understanding of its material properties and also its material control for device application. In particular, successful p-type doping is a quite important issue but quite difficult in InN. In this article, we present a study on the latest advances in material control paying special attention to the p-type doping of InN films using Mg-acceptors by MBE, the demonstration of successful p-type control and p-type conduction, and their electrical and optical properties as well. Furthermore, as a prerequisite of successful p-type doping of InN, epitaxy behaviors of InN on GaN and effects of threading dislocations on high-purity undoped InN epilayers grown on GaN template are also discussed. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Effect of Mg doping on terahertz (THz) emission from InN with different lattice polarities was studied. Strong enhancement of THz emission was observed from InN with appropriate Mg-concentrations (10(18) cm(-3)), which is independent of lattice polarity. The buried p-type layers show stronger THz emission than the n-type ones. The dominant mechanism for THz emission was found to be photo-Dember effect and the emission intensity was inversely proportional to the conductivity, which is beneficial to investigate THz emission from InN since the conductivity can be more accurately measured than the carrier concentration and mobility due to the electron accumulation on surface.

Photoluminescence (PL) spectra of In-polar n-type InN films with different dislocation and residual electron densities are investigated in the temperature range 15-300 K. The dependence of PL intensity on temperature is analyzed by using a model function that is based on rate equations for photoexcited hole density. By considering the relation between the dislocation densities estimated from the widths of the peaks of x-ray omega-rocking curves and the parameters obtained from the rate equations, two kinds of nonradiative carrier recombination processes are identified. One process is independent of threading dislocations and is thermally activated, while the other takes place in the vicinity of edge-type dislocations and requires no activation energy.

One monolayer (1 ML) InN/GaN multiple quantum wells (MQWs) grown on bulk-GaN and MOVPE-GaN/ sapphire substrates with threading dislocation densities (TDDs) of about 106 cm-2 and 108 cm-2, respectively, were characterized by SEM-CL measurements to study the effects of TDs on structural quality of InN/GaN MQWs. For the sample grown on bulk-GaN substrate, step-flow surface morphology and quite uniform spatial emission were observed as compared to the sample grown on MOVPE-GaN/sapphire. CL peak energy at 86 K was observed at around 3.2 eV for both MQWs, but CL emission spectra from MQWs on bulk-GaN substrate were five times stronger with smaller peak energy distribution, and full width at half maximum than those on MOVPE-GaN substrate. This was caused by low-TDD of bulk-GaN substrate. Further, effects of growth temperature on 1 ML InN/GaN MQWs were investigated and it was found that CL peak energy tended to discretely shift to lower energy side, for e.g., from 3.18 eV to 2.9 eV when the growth temperature was decreased from 660 °C to 620 °C. © 2009 WILEY-VCH Verlag GmbH &amp Co. KGaA, Weinheim.

An LED structure with a 1-2 monolayer (ML) thick InN quantum well (QW) with InGaN barrier grown by radio-frequency plasma assisted molecular beam epitaxy (rf-MBE) is proposed for a new active layer of blue-green light emitters. Compared with previously reported 1-2 ML thick InN/GaN QWs, extended emission wavelength up to pure green region (∼530 nm) is expected for these QWs with InGaN barriers. It is found that for the InN/InGaN QW structure, in which the InGaN layer is used as a barrier instead of GaN, very thin InN well layers are basically formed in the same manner as the InN/GaN QWs. In photoluminescence spectra at 15 K, emission peak wavelengths are observed from 412 nm to 480 nm when In contents in InGaN barriers are changed from 0.005 to 0.14. The emission wavelength is longer than that of 1-2ML thick InN/GaN QWs (380nm-430nm), and green emission can be obtained by further increase of In content in InGaN barrier to more than 0.2. The 1-2 ML thick InN/In0.08Ga0.92N(10nm) 5-QW LED structure was successfully fabricated and bright EL emission was observed at 419 nm, which is longer than that of a 1-2 ML InN/GaN 5-QW LED. © 2009 WILEY-VCH Verlag GmbH &amp Co. KGaA, Weinheim.

Infrared reflectance and ellipsometry measurements are applied in order to study the influence of Mg-doping on the properties of hexagonal InN films. Reflectance spectrum characteristics reveal the large effective mass and large plasmon damping rate just in the region where net acceptors have been observed by electrolyte capacitance-voltage technique. The numerical spectrum analysis accounting for the modulation of the normal mode energies of longitudinal optical phonon-plasmon coupling (LOPC) by the large hole scattering rate yields the hole density of (0.1-1.2)×1019 cm-3 and optical mobility of 25-70 cm2/Vs for the direction vertical to the c axis. The properties of the bulk-like part of the films are determined by the optical techniques. Infrared ellipsometry on the study of anisotropy of LOPC mode broadening indicates that threading dislocations or columnar grain boundaries cause the significantly larger scattering rate for holes vibrating along the c axis than those vibrating vertical to the c axis. © 2009 WILEY-VCH Verlag GmbH &amp Co. KGaA, Weinheim.

Initial stage of MBE-epitaxy processes of InN on c-plane GaN template and successful p-type doping were studied. It was found that InN epitaxy proceeds through SK-mode and 1-ML-thick InN can be coherently grown on GaN. Successful p-type doping of InN was achieved by Mg doping levels from 1018 to about 3×1019 cm-3. Overdoped Mg's introduced new donors in InN resulting in n-type conduction. Novel structure InN-based QWs consisting of coherent 1-ML-thick InN wells embedded in GaN matrix were proposed and fabricated. It was confirmed that extremely fine structure InN QWs with quite sharp and flat hetero-interface were fabricated by self-limiting growth mode under In-polarity growth regime at remarkably higher growth temperatures up to 650 °C. © 2008 Elsevier B.V. All rights reserved.

We propose the fabrication of asymmetric QaN/InN/InGaN/QaN quantum wells (QWs) composed of the following layers in succession: an ∼1-monolayer-thick InN well grown on a GaN barrier layer, In0.15-0.20Ga 0.85-0.80N layer, and a GaN barrier layer, these QWs can be used for the development of blue-green light emitters. An asymmetric-QW light-emitting diode (LED) emits at around 500 nm and almost no blue shift is observed in the electroluminescence spectra of the LED at different current levels, indicating that the asymmetric QWs can be used in blue-green light emitters, and the quantum-confined Stark effect can be reduced by employing an ultrathin-InN QW structure. © 2009 The Japan Society of Applied Physics.

The introduction of vacancy-type defects into InN by Mg-doping was studied using a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured for Mg-doped InN (N-polar) grown on GaN/sapphire templates using plasma-assisted molecular beam epitaxy. The concentration of In-vacancy (VIn) related defects was high near the InN/GaN interface, and the defect-rich region expanded from the interface toward the surface with increasing Mg concentration [Mg]. Using electrolyte-based capacitance-voltage analysis, we determined that the conduction type of InN with low [Mg] (1× 1018 cm-3) was still n -type. It became p -type with increasing [Mg] (3× 1018 -2× 1019 cm-3), but turned into n -type again above 3× 1019 cm-3. The point defects introduced at the conductivity transition from p -type and n -type were found to be complexes between In-vacancy (VIn) and N-vacancy clusters such as VIn (VN) 3. Below [Mg] =4× 1019 cm-3, an observed behavior of positron annihilation parameters was well explained by assuming the trapping of positrons by N-vacancy clusters such as (VN) 3. This fact suggests that, although isolated VN is positively charged, a VN cluster could be a positron trapping center because of the increased electron concentration in the local In-rich region. © 2009 American Institute of Physics.

We report an effective and nondestructive method based on circular photogalvanic effect (CPGE) to detect the lattice polarity of InN. Because of the lattice inversion between In- and N-polar InN, the energy band spin splitting is opposite for InN films with different polarities. Consequently under light irradiation with the same helicity, CPGE photocurrents in In- and N-polar layers will have opposite directions, thus the polarity can be detected. This method is demonstrated by our CPGE measurements in both n - and p -type InN films. © 2009 American Institute of Physics.

Mg-doped InN films grown by plasma-assisted molecular beam epitaxy were characterized by infrared reflectance. Signatures of p-type conductivity in the spectra were obtained in the same doping density range where the existence of net acceptors was found by electrolyte capacitance-voltage measurements. Numerical spectrum analysis, which takes into account the large broadening factor of the normal mode energies of longitudinal optical phonon-plasmon coupling yielded high hole densities in the range of (0.1-1.2)x10(19) cm(-3) and optical mobilities in the range of 25-70 cm(2)/V s.

The energy broadenings of the higher energy branch of the longitudinal optical (LO) phonon-plasmon coupling modes for E(1)(LO) and A(1)(LO) are analyzed for InN films by infrared reflectance and spectroscopic ellipsometry. Larger broadening for the vibration parallel to c of A(1)(LO)-plasmon coupling is found with the decrease in electron density. This phenomenon is caused by the plasmon lifetime anisotropy, and is attributed to the reduction in crystal defects causing isotropic carrier scattering and the remaining of defects along the c axis such as threading dislocations and columnar grain boundaries. (C) 2008 American Institute of Physics.

Spatially resolved luminescence properties of InN/GaN multiple quantum wells (MQWs) consisting of nominally one monolayer (1-ML)-thick InN QWs embedded in a GaN matrix are studied by cross-sectional and plan-view cathodoluminescence measurements. First it is confirmed that the dominant emission peaks observed at around 390 nm to 430 nm in the MQWs samples are attributed to the effects of inserting similar to 1-ML-thick InN wells in the GaN matrix, resulting in efficient localization of GaN excitons at InN QWs. Furthermore, it is revealed that the detailed structure of the MQWs, such as the thickness distribution and interface sharpness, is very sensitive to the presence of surface defects such as hillocks around screw-component threading dislocations, resulting in different emission wavelengths/energies. This is because the epitaxy process for depositing such thin InN wells is seriously affected by the atomic-level surface structures/properties of the growth front. It will be concluded that it is necessary to use lower dislocation density GaN bulk templates to obtain much higher structural quality InN/GaN MQWs good enough for characterizing their optical properties.

The hydrostatic pressure dependence of photoluminescence, d EPL /dp, of Inx Ga1-x N epilayers has been measured in the full composition range 0&lt x&lt 1. Furthermore, ab initio calculations of the band gap pressure coefficient d EG /dp were performed. Both the experimental d EPL /dp values and calculated d EG /dp results show pronounced bowing and we find that the pressure coefficients have a nearly constant value of about 25 meV/GPa for epilayers with x0.4 and a relatively steep dependence for x&lt 0.4. On the basis of the agreement of the observed PL pressure coefficient with our calculations, we confirm that band-to-band recombination processes are responsible for PL emission and that no localized states are involved. Moreover, the good agreement between the experimentally determined d EPL /dp and the theoretical curve of d EG /dp indicates that the hydrostatic pressure dependence of PL measurements can be used to quantify changes of the band gap of the InGaN ternary alloy under pressure, demonstrating that the disorder-related Stokes shift in InGaN does not induce a significant difference between d EPL /dp and d EG /dp. This information is highly relevant for the correct analysis of pressure measurements. © 2008 American Institute of Physics.

The infrared reflectance spectra of InNGaN structures are analyzed in two energy regions for higher and lower energy branches of the longitudinal optical phonon-plasmon coupled modes. For samples with smaller residual electron density than 5× 1017 cm-3 and the film thickness of 0.7-4 μm, the spectra down to 200 or 250 cm-1 shows the existence of the electron accumulation with sheet electron density of 1013 cm-2 in the vicinity of the InNGaN -interface. The effect of the surface electron accumulation on the spectra is not identified, which is possibly caused by the small mobility of the order of 10 cm2 V s or less for the surface electrons. The electron density of the inside bulk region is obtained with account of this interface accumulation. However, for samples with higher electron density and thickness of several micrometers a model with uniform electron density well reproduce the experimental spectra. With this spectrum fitting we extract the electron properties inside the bulk region for samples with the thickness of about 700 nm or larger. © 2008 American Institute of Physics.

We demonstrate the effect of conduction band shape evolution of InGaN with increasing In content and applying hydrostatic pressure. The influence of conduction band filling on the hydrostatic pressure dependence of photoluminescence in In0.7Ga0.3N and InN is investigated. It is found that the PL pressure coefficient dEPL/dp of InN changes from ∼27 meV/GPa to ∼21 meV/GPa when the electron concentration increases from 3.6×1017 cm-3 to 1.1×10 19 cm-3. In contrast, no significant change of dE PL/dp with electron concentration was observed for In 0.7Ga0.3N. We conclude that the pressure sensitivity of the Fermi level, which is responsible for the lowering of dEPL/dp with respect to dEG/dp in InN, is much less prominent in In 0.7Ga0.3N than in InN. We attribute this difference to the larger band gap of In0.7Ga0.3N, which lowers the pressure sensitivity of m*. © 2008 Wiley-VCH Verlag GmbH &amp Co. KGaA.

Mg-doped p -type InN layers with different thicknesses were grown under the same growth/doping conditions so that their net acceptor concentrations were almost the same (3-6) × 1018 cm-3, which were confirmed by electrolyte capacitance-voltage measurements. The conductivity of p-InN region embedded under high density surface electrons could be extracted through the slope of total sheet conductivity against thickness, which was about 8.1 -1 cm-1. Then, corresponding hole mobility was determined to be about 17-36 cm2 V s for the hole concentrations of about (1.4-3.0) × 1018 cm-3 obtained by providing the hole effective mass and Mg acceptor activation energy as 0.42 m0 and 61 meV, respectively. © 2008 American Institute of Physics.

The authors propose and demonstrate fine structure novel InN/GaN multiple quantum wells (MQWs) consisting of ultimately thin InN wells around 1 ML inserted in a GaN matrix grown under In-polarity growth regime by molecular beam epitaxy. Since the critical thickness of InN epitaxy on the c -plane GaN is about 1 ML and also the growth temperature for 1 ML InN insertion can be remarkably higher than conventional one, the proposed MQW structure can avoid new generation of misfit dislocation at the heterointerface, in principle, and results in high quality MQW structure due to the effects of enhanced surface migration at higher temperatures. It is shown that demonstrated 1 ML InN/GaN MQW structures indicate surprisingly higher structural quality/properties than those former-reported InN-based heterostructures. Self-ordering mechanism arising from immiscibility nature in between InN and GaN will also contribute for depositing sharp and atomically flat InN well. The proposed MQW structure has physically and practically important meanings leading to room temperature operating GaN-based excitonic devices and also efficient photonic devices working in short wavelength visible colors. © 2008 American Vacuum Society.

The LO phonon-plasmon coupling of InN for the E-1(LO) mode has been found; however, the decoupling for the A(1)(LO) mode has been reported in many issues. Several reports have discussed the electron- and phonon-scattering mechanisms, while there is no detailed investigation on the broadening factors of mode energies even for E-1(LO) phonon-plasmon coupled scheme. In this issue, the studied InN layers grown by the molecular-beam epitaxy method have residual electron density obtained by Hall measurements in a range from 8x10(17) to 1x10(19) cm(-3). The electron density measured by infrared reflectance measurements is almost constant with a temperature variation from 5 to 300 K. The dependence of the observed broadening factors of the E-1(LO) phonon-plasmon coupled modes on the electron density is found to show the characteristic properties of the level anticrossing, in particular, for the increase of these broadening factors around the level anticrossing region with the temperature increase from 5 to 300 K. Compared to the recent report on the LO phonon-plasmon coupled modes for A(1)(LO), smaller broadening for E-1(LO) is observed here.

C-axis oriented InN nanocolumns (NCs) were grown on (0 0 0 1) GaN templates by molecular beam epitaxy. In situ spectroscopic ellipsometry and reflection high-energy electron diffraction demonstrated that InN NCs epitaxy was initiated from self-organized growth of InN quantum dots in Stranski-Krastanov mode, which was further confirmed by ex situ measurement of atomic force microscopy. InN NCs were of high crystal quality. The full-width at half-maximum (FWHM) value of omega-scan for (002) InN NCs showed a comparative value with that of GaN template in a range of 500-600 arcsec. (c) 2006 Elsevier B.V. All rights reserved.

The authors propose and demonstrate the fabrication of InNGaN multiple quantum well (MQW) consisting of 1 ML and fractional monolayer InN well insertion in GaN matrix under In-polarity growth regime. Since the critical thickness of InN epitaxy on GaN is about 1 ML and the growth temperature for 1 ML InN insertion can be remarkably higher, the proposed MQW structure can avoid/reduce generation of misfit dislocation, resulting in higher quality MQW-structure nature in principle than former InN-based MQWs. The proposed InNGaN MQWs are potentially applicable to room temperature operating excitonic devices working in short-wavelength visible colors. © 2007 American Institute of Physics.

In-rich (XIn &gt 0.5) InGaN ternary alloys were grown on N- and Ga-polarity GaN templates by radiofrequency plasma assisted molecular beam epitaxy (RF-MBE) and their polarity dependence was investigated. In-rich InGaN alloys with In-content ranging from 0.5 to 1.0 could be grown without phase separation and sharp single diffraction peak was observed for these samples in X-ray diffraction (XRD) 2θ-ω scan. Compared to N-polarity samples, In-polarity InGaN showed better crystalline quality in spite of about 100 °C lower growth temperature. Moreover, the dependence of crystalline quality on V/III ratio was investigated for In-polarity samples. It was found that the phase separation in the InGaN tends to occur under metal-rich condition but this can be prevented in N-rich and around unity stoichiometry conditions as well. Precise control of V/III ratio to around unity stoichiometry was realized by using a shutter control method, which resulted in improvement of crystalline quality and surface morphology of In-rich InGaN films. © 2007 WILEY-VCH Verlag GmbH &amp Co. KGaA.

After the finding of the bandgap energy of InN as less than 1 eV, the re-examination of the bandgap of InGaN alloys and its bowing parameter is an issue of interest. We study the absorption spectra of In-rich InGaN layers by taking account of the Burstein-Moss effect and bandgap renormalization with the analysis of photoluminescence and infrared reflectance spectra. We find the far larger broadening of optical transition and lattice vibration energies than those expected from the alloy composition fluctuation based on the x-ray diffraction patterns. We estimate that local potential fluctuation by the alloy composition fluctuation give rise to the strong localization of carriers and relaxation of the momentum conservation rule in optical transition. The average bandgap energies of the films show the bowing parameter less than 1.4 eV the value down to 0.7 eV is possible. © 2007 WILEY-VCH Verlag GmbH &amp Co. KGaA.

In-polar InN films with atomically flat surface are grown on Ga-polar GaN templates by molecular beam epitaxy. Densities of threading dislocations with screw and edge components in these films are about 108 and low 1010 cm-2, respectively. It is found that the screw-component threading dislocation is the dominant cause for macroscopic surface defects appearing as growth-spiral hillocks their densities (their growth temperature dependences) are almost the same (similar) with each other. Further, it is shown that the residual electron concentration in InN is almost the same with the density of dangling bonds at the edge-component threading dislocations. © 2007 American Institute of Physics.

Mg doping into In-polar InN layers for different Mg fluxes is performed on GaN templates by molecular beam epitaxy, and their electrical and optical properties are investigated. Mg concentration is linearly proportional to Mg-beam flux, indicating that the Mg-sticking coefficient is almost unity. With Mg doping, electron concentration decreases by the effect of carrier compensation, but it begins to increase with further increasing Mg flux because of Mg-related donorlike-defects formation. For the partially carrier-compensated Mg-doped InN, two photoluminescence peaks are observed one is originated from free-to-acceptor emission with an acceptor activation energy of about 61 meV and the other is similar to the conventional band-to-band emission. © 2007 American Institute of Physics.

To investigate the Mg-dopability in In-polar InN epilayers grown by molecular beam epitaxy, polarity inversion dependence on Mg-doping level is studied. A multiple-InN layer-structure sample with different Mg-doping levels is grown and analyzed by transmission electron microscopy. Formation of high density V-shaped inversion domains is observed for the Mg-doped InN with Mg concentration ([Mg]) of 2.9× 1019 cm-3. These domains lead to polarity inversion from In to N polarity. Further study for Mg-doped InN epilayers shows that polarity inversion takes place when [Mg] increases above 1.6× 1019 cm-3. It is also shown that the Mg-sticking coefficient is almost independent of the polarity. © 2007 American Institute of Physics.

p -type conductivity control of In- and N-polar InN layers grown on GaN templates with different Mg concentrations ([Mg]) was systematically investigated by using electrolyte-based capacitance-voltage (ECV) analyses. With increasing [Mg], p -type conduction was confirmed for [Mg]s from ∼ 1018 to ∼3× 1019 cm-3. The conduction was reversed to n type at [Mg]s above 1020 cm-3, however, due to overdoping effects introducing shallow donors in InN. Further, it was found that charges at the interface states located within forbidden band of InN greatly affected the ECV measurements resulting in overestimation of net acceptor concentrations, and some calibration was necessary to estimate them. © 2007 American Institute of Physics.

In-polarity In0.7Ga0.3N films were grown by radio-frequency plasma-assisted molecular beam epitaxy (rf-MBE) and the dependences of crystalline quality on V/III ratio and growth temperature were investigated. To precisely control V/III ratio, In and Ga beams were periodically interrupted by closing their shutters, which is called a shutter control method in this study. With a change in V/III ratio, the crystalline quality of the In0.7Ga0.3N films markedly changed. It was found that the phase separation in the InGaN films tends to occur under metal-rich conditions but this can be prevented in N-rich and shutter-controlled growth as well. A sample grown by the shutter control method at 550 °C showed the best crystalline quality. The X-ray rocking curve full width at half maximum (FWHM) for (0002) diffraction was 287 arcsec, which is much smaller than that of a standard MBE-grown In0.7Ga0.3N film, whose FWHM is about 600 arcsec. © 2006 The Japan Society of Applied Physics.

Unintentionally doped n-type InN epilayers were grown on sapphire by using plasma-assisted molecular beam epitaxy and were characterized by using Hall effect and X-ray diffraction. The electron density was in a narrow range of 3.6 - 4.5 × 10 18 cm -3. Hall effects showed that two scattering mechanisms due to ionized impurities and space charges dominate at temperatures lower than room temperature. The lattice-limited mobility was calculated at 5800 cm 2/V·s at 300 K. Despite the high density of edge dislocations, scattering due to those dislocations turned out to be negligible. Instead, space-charge scattering was significant and likely originated from randomly clustered dislocations, typically in InN layers. Two donor states were observed, one above and the other below the conduction band minimum. The thermal activation energy of the lower state donor was high due to the Fermi energy level being located deep in the conduction band, and the state seemed to be related to the dislocations.

N-polar InN films were grown as a function of temperature (∼440-620°C) on 500 nm thick GaN template by plasma-assisted molecular beam epitaxy. InN films grown at different temperatures showed different morphologies that could be roughly divided into two regions. Dendritic morphologies could be observed at temperatures lower than 540°C while step-flow-like morphology could be obtained at temperatures higher than 540°C. Crystalline quality became better with the increase of growth temperature at temperatures lower than 540°C and almost saturated at higher temperatures. Strong photoluminescence was observed at room temperature and 13 K with the emission peaks at 0.67 and 0.7 eV, respectively. Based on our results, the growth temperature above 540°C was recommended for N-polar InN epitaxy. © 2006 WILEY-VCH Verlag GmbH &amp Co. KGaA.

We studied on rf molecular beam epitaxy (RF-MBE) growth of AlInN ternary alloys on N-polarity GaN templates. The growable highest temperature for the AlInN ternary alloy with mid-composition range was about 600°C, which was very similar to that of N-polarity InN epitaxy. The compositional and structural qualities of AlInN ternary alloys were quite poor, however, for growth temperatures above 580°C. AlInN ternary alloys without apparent phase separation in the whole composition range could be grown at 550°C, and their crystalline, electrical, and optical properties were characterized. The bowing parameter for the optical bandgap of AlInN ternary alloys was found to be 4.96 ± 0.28 eV. Further we for the first time fabricated InN/AlInN multiple quantum wells (MQWs). © 2006 The Japan Society of Applied Physics.

InN epilayers grown by radio-frequency plasma-assisted molecular beam epitaxy under different surface stoichiometries were characterized by in situ spectroscopic ellipsometry in the range from 0.731 eV (1697 nm) to 4.95 eV (250 nm). Nitrogen polarity InN epilayers were grown at 600°C on GaN-underlayer/sapphire substrate. The surface stoichiometry during growth was changed by varying the indium-beam flux under the same nitrogen-beam flux. It was found that the pseudodielectric functions were drastically affected by the surface stoichiometry. The dielectric functions of InN grown under different stoichiometries were obtained. Both real and imaginary parts of the dielectric functions tended to be larger with increasing In-beam flux. © 2006 American Institute of Physics.

In-rich InGaN films (XIn&gt 0.5) and InN/InGaN multi-quantum wells were grown on Ga- and N-polarity GaN templates by radio-frequency plasma-assisted molecular beam epitaxy. The In-polarity InGaN films grown at 450°C showed superior crystalline quality and smoother surface morphology compared to the N-polarity samples, which were grown at 500-550°C. By using the In-polarity In0.7Ga0.3N as a barrier layer, the InN/InGaN multi-quantum wells were successfully fabricated on the III-element polarity GaN templates at 450°C. Fine periodic structures and strong photolumincscencc emission around optical communication wavelength were obtained from the In-polarity MQWs. These results indicate that high quality InGaN films and the InN/InGaN MQWs can be obtained in the In-polarity growth regime in spite of its lower growth temperature. © 2006 Materials Research Society.

20 periods InN/In0.75-0.80Ga0.25-0.20N MQWs were fabricated on In0.75-0.80Ga0.25-0.20N underlayers by radio frequency plasma-assisted molecular-beam epitaxy. Fine periodic MQWs structures were confirmed by X-ray 20-ω scans, which showed clear satellite peaks up to 2nd order. From reciprocal space mappings for (10-14) diffraction, it was found that the InGaN underlayer completely relaxed for all MQWs samples, and when the InN well thickness was 2.2 nm or less, the MQWs could be coherently grown on the InGaN underlayer. In PL measurement at 15 K, near-infrared emissions from the MQWs were observed. With decreasing the InN well thickness from 18.7 nm to 2.2 nm, the emission peak shifted to 1.63 um. This result indicates a quantum confinement effect in the InN/InGaN MQWs. © 2006 WILEY-VCH Verlag GmbH &amp Co. KGaA,.

Epitaxial growth of N-polarity InN films under atomic-hydrogen irradiation was systematically investigated. It was found that any difference could not be observed in electron concentrations of InN films with and without the atomic-H irradiation, where typical residual carrier concentrations were from 3 to 5×1018 cm-3. These results suggest that the atomic H may not be an origin of residual donors at least in the InN films with carrier concentrations of 3-5×1018 cm-3. On the other hand, the atomic-H irradiation was effective in improving surface morphology of the InN films, which suggested an effect of surfactant on InN epitaxy. © 2006 WILEY-VCH Verlag GmbH &amp Co. KGaA.

In-polar and N-polar InN nanocolumns were grown on Ga-polar and N-polar GaN templates by molecular beam epitaxy, respectively. High N/In flux ratio was preferred for successful growth of the InN nanocolumns in both polarities. The maximum growth temperature of N-polar InN columns was about 100°C higher than that of In-polar ones. The density and diameter of columns could be controlled by varying the growth temperature. The InN columns in different polarities showed different morphologies. In comparison to InN film, the InN nanocolumns showed higher growth rate and stronger photoluminescence. XRD results indicated that InN nanocolumns were in good crystal quality with small tilt. © 2006 WILEY-VCH Verlag GmbH &amp Co. KGaA.

In-polarity InN films were grown by radio frequency plasma-assisted molecular-beam epitaxy and their structural properties as a function of III/V beam-flux ratio were investigated. It was found that the window for the surface stoichiometry to achieve high quality InN films was very narrow only 11% change in In beam-equivalent pressure drastically affected surface morphology and structural property. This is because the upper-limited growth temperature of the In-polarity growth is as low as about 500°C. Under the growth condition around the unity stoichiometry, a step-flow-like surface with a 2 mono-layer step height was obtained in the In-polarity growth. Best values for the measurements of structural and electrical properties were as follows full width at half maximum of X-ray rocking curves were 645 arcsec and 1425 arcsec for (0002) and (10-12) reflections, respectively and the electron concentration and mobility were 1.3×1018 cm-3 and 1650 cm 2/V s, respectively. Furthermore, we tried to fabricate In-polarity InN/GaN multi-quantum wells with a 1 mono-layer InN well thickness. A result of X-ray 2θ-ω scan showed clearly satellite peaks up to 2nd order and fringe peaks, which indicates that a fine periodic structure is achieved in the In-polarity MQWs. © 2006 WILEY-VCH Verlag GmbH &amp Co. KGaA.