A cavity polariton laser is attracting attention as a new generation coherent light source composed of a semiconductor microcavity. In the present research, epitaxial growth of single crystalline ZnO and MgZnO films exhibiting atomically flat surfaces and abrupt heterointerfaces was carried out using an uniquely designed 'helicon-wave-excited-plasma sputtering epitaxy (HWPSE)' method, in order to assess if ZnO microcavities can be prepared by the method. The epilayer properties resemble those of the films grown using conventional advanced epitaxial growth methods such as molecular beam epitaxy and metalorganic vapor phase epitaxy. In addition, anatase phase Nb-doped TiO_2 films, a new transparent conducting oxide having the refractive index close to GaN, were epitaxially grown. The findings that those new functional semiconductor epilayers can be grown by the inexpensive HWPSE method may cut open the way to fabricate semiconductor heterostructure quantum devices at a low price

Active compensation of the substrate temperature during the MBE growth of wide bandgap II-VI alloy materials were further applied for various complicated structures, namely superlattices, and delta-doping structures.Different from conventional III-V compound semiconductors such as GaAs, the materials property is significantly affected by the substrate temperature. II-VI wide bandgap compounds were usually grown under lower temperatures than Ga As and related materials, perturbation of the substrate temperature during the growth would cause the deterioration of the film quality of the epitaxial layer. In this research project, heater was introduced to the MBE chamber at the location of source material cells. The substrate temperature perturbation would take place when the cell shutter was opened so that the molecular beam would be introduced toward the substrate since the heat irradiation from the hot cell would take place. The additional heater was to cancel the heat irradiation from the source material cell when those shutters were closed.High quality ternary compound of ZnMgCdS layers were previously grown using this active compensation technique. This technique was also applied for the growth of superlattice structures, namely ZnMgS/ZnCdS, and ZnSe/MgCdS. Uniform layers structure was confirmed by TEM measurement and narrower X-ray diffraction FWHM were obtained by introducing the active compensation technique. This technique was further introduced for the growth of ZnTe homoepitaxial layer where complicated shutter sequence was introduced to form the delta co-doping structure. The PL properties were improved by using this active compensation technique.This active compensation of the substrate is a powerful tool to obtain the high quality II-VI epitaxial layers by MBE

For the coming sub-65 nm complementary metal-oxide-semiconductor (CMOS) technology, high-k materials such as hafnia (HfO_2), hafnium silicate (Hf_xSi_<1-x>O_y), zirconia (ZrO_2), and zirconium silicate (Zr_xSi_<1-x>O_y) are expected to take the place of SiO_2 as the basic material for gate dielectrics. However, there is a high possibility that a lot of localized states due to defects are present in the band gap of these high-k candidates. These localized states are assumed to cause various dielectric malfunctions such as high leakage current, low breakdown voltage, and threshold variation. Therefore, detailed and systematic research on the localized states is indispensable to understand the effects of the defects on electrical properties.
From this viewpoint, we have carried out basic studies on HfO_2, Hf_xSi_<1-x>O_y, ZrO_2, and ZrxSi_<1-x>O_y, using photoluminescence (PL) as a common tool that can provide fundamental information about the band gap energy and localized states. X-ray photoelectron spectroscopy (XPS) analyses were also carried out to investigate the band profiles. Furthermore, the effects of postannealing on the electrical properties of the high-k materials were also studied.
Energy band profiles were investigated by XPS for amorphous hafnia and hafnium silicate films sandwiched between an evaporated Au electrode and a Si substrate. Valence band offset and conduction band offset decrease until they become almost constant with an increase in hafnium content at both Si and Au sides. In literature, similar decrease in the valence band offset was reported in zirconium silicate.
Photoluminescence spectra induced by UV photons were measured for amorphous hafnia and zirconia deposited by plasma-enhanced chemical-vapor deposition, amorphous hafnia deposited by pulse laser deposition, and crystalline yttria-stabilized zirconia (YSZ). A PL peak appears around 2.7 to 2.9 eV similarly in all hafnia and zirconia samples, irrespective of the difference in crystallinity, oxygen deficiency, source alkoxide, deposition method, or the substrate material. The decay profile of the PL is also similar in all the samples. These results indicate that the PL is inherent in hafnia and zirconia, and is not due to impurities, oxygen vacancy, or interface defects between the sample and the substrate. From PL excitation (PLE) and vacuum-ultraviolet (VUV) absorption measurements, the PL was found to be excited by UV photons to tail states at the band edges. When the samples were annealed at 900℃, a new PL peak appears around 4.2 eV in all the samples except YSZ. The PLE and VUV absorption measurements indicate that the 4.2-eV PL is excited due to the interband absorption.
Mechanisms of PLs induced in hafnium and zirconium silicates were also discussed. A broad PL spectrum was observed from 2.0 to 5.0 eV similarly in both silicates. This PL has two components with peaks around 2.8 to 3.0 eV and 3.8 eV for hafnium silicates and those around 2.7 to 3.0 eV and 3.8 eV for zirconium silicates. Time-resolved PL and PL decay measurements indicate that the origin of the PL component around 2.7(2.8) to 3.0 eV is the same as that of the PL component around 2.7 to 2.9 eV in hafnia and zirconia. Furthermore, PLE and VUV absorption measurements show that both the PL components around 2.7(2.8) to 3.0 eV and 3.8 eV are excited to tail states at the band edges. Through these studies, it is assumed that hafnium silicate, zirconium silicate, hafnia, and zirconia have luminescent centers in their band gaps with their respective upper and lower states that have a certain fixed energy difference irrespective of the hafnium or zirconium content.
Furthermore, effects of postannealing on the electrical properties of hafnium and zirconium silicates were investigated. When the samples were postannealed in nitrogen monoxide (NO). leakage current and capacitance-voltage (C-V) hysteresis width are decreased drastically. From ESR measurement, it is assumed that paramagnetic defects at the interface between the sample and the Si substrate are responsible for the leakage current and the C-V hysteresis. Furthermore, depth profile by XPS shows that the postnitridation effectively terminates these interface defects and contributes to the improvement in electrical properties.

Important quantum correlation effects such as quantum Hall effect and fractional quantum Hall effect have been revealed using the semiconductor two-dimensional electron system. In this system, applications to practical devices such as high speed FETs and lasers have also been achieved. In the lower dimensional systems, however, no prominent progress has been achieved in both fundamental research and device applications. A major reason to this lies on their poor geometrical accuracy. In the two-dimensional structures, the thickness of each layer has atomic size accuracy. However, in one- and zero-dimensional structures, side walls are quite rough. Moreover, structures produced by bottom-up.self-assembly process suffer from inhomogeneity and random distribution. In the present research, therefore, we adopted area-selective epitaxy by combining with e-beam lithograph and migration-enhanced epitaxy(MEE). We have succeeded in fabricating high quality one-dimensional electron wires and quantum dot lattices with AlGaAs/GaAs, InAs/GaAs, and InGaAs/GaAs. Atomically flat side walls of nano-structures are formed by utilizing micro-facet structures. To fabricate accurately the structures with designed ones, lateral growth should be minimized. This has been accomplished by optimizing the MEE deposition sequence. One dimensional wires exhibit ballistic transport characteristics. Two dimensional quantum dot lattices show unique transport characteristics. They have proved useful also for two-dimensional photonic crystals

The refractive index change induced in a transparent material by UV photon or ion irradiation promises direct drawing of a waveguide that can be used as optical gratings. Structural changes in silica glass induced by ion microbeam were evaluated using microscopic photoluminescence and Raman scattering, and optical and atomic force microscopes (AFM) measurements. The AFM measurements on the microbeam irradiated area show the formation of a groove on the surface. In addtion, a cross sectional observation on the surface parallel to the incident plane reveals surface deformation along the ion tracks, which is deepest at the projected range of ions. Taking into account the possible structural changes of silica induced by energy deposition, the measured topological changes at the front and side surfaces result from internal compaction of silica glass. Refractive index changes were estimated from the Lorentz-Lorenz relationship using the distribution of the internal compaction estimated by the AFM measurements. A small refractive index change was observed along the ion tracks besides a much stronger index change at the projected range, suggesting that energy depositions by the ionization as well as by the atomic collision should be taken into account.
It has also become clear that the refractive index change can be induced by UV photons from a KrF excimer laser in a-SiO_x N_y : H films prepared by PECVD. Fabrication of a diffraction grating was performed using a phase mask made of a high quality fused silica plate. The surface of the fabricated grating observed by scanning electron microscopy (SEM). A clear square-toothed pattern with periodicity of around 1 μm can be observed. Furthermore, from the Fraunhofer diffraction pattern, the periodicity of the grating pattern was estimated to be around 1 μm, which agrees with the SEM image. This simply demonstrates that a-SiO_xN_y : H can be processed by UV photon irradiation.

Amorphous silica, a key material in the modern high-tech society. Is widely used as an insulating layer for a semiconductor device or as an optical fiber. In the present research, the effects of point defects such as impurities or vacaancies in the silica film on the electrical and optical properties have been systematically studied. The point defects were detected by photoluminescence, ESR, FT-IR, visible-VUV absorption, EPMA and SIMS. Electrical properties are examined by high-field conduction. Dielectric breakdown, and C-V measurement. Knowledge on the presence of oxygen vacancy and the estimation of its concentration, structural stabilization by impurity doping, and the relation between the defect concentration and the synthesis method has been obtained. In 1997, we demonstrated tha clear photoluminescence can be obtained in this amorphous SiOィイD22ィエD2 film by using an intense photon source such as an excimer laser or synchrotron radiation. This indicates that photoluminescence can be a strong tool to detect point defects in a-SiOィイD22ィエD2. Using this method, point defects in thermal silicon dioxide and SIMOX buried oxide and those induced in these oxides by ion implantation and heat treatment have been detected. The structures of these defects have also been identified. In 1998, researches were extended to fluorine-doped SiOィイD22ィエD2 film, SIMOX oxide, and silicon nitride film, and the structures of point defects in these materials were clarified. The effect of point defects on the dielectric strength was also examined by measuring the breakdown voltage. In 1999, researches were further extended to a-SiNィイD2xィエD2 a-SiOィイD2xィエD2NィイD2yィエD2, and TaィイD22ィエD2OィイD25ィエD2 films, focusing on the effects of ion implantation and heat treatment on the concentration of point defects. Photochemical reactions responsible for the formation of optical fiber grating in Ge-doped silica glass have been also clarified. Furthermore, stable and high-efficient fabrication method of optical fiber grating by use of ion implantation has bee proposed.

(1) The generation mechanism of the absorption changes, which cause a photorefractive change through the Kramers-Kronig relation in Ge-doped SiO_2 glass, has not been clarified yet. In the present paper, we examined the laser-power dependence of the absorption changes around 5 eV,induced by a KrF excimer laser. The induced absorption around 5 eV is composed of three different components, centering at 4.50,5.08, and 5.80 eV.The increasing behavior of each absorption component depends strongly on the energy density. The three absorption components reach different saturation levels, depending on the energy density. Furthermore the absorption induced by a high-power KrF excimer laser is bleached by a laser, the energy density of which is about one-twentieth of the inducing laser. Combining the results of mathematical analysis, it was found that a two-photon process and a one-photon process are, respectively, involved with the induction and the bleach of each absorption. It was also found that the precursor defects, which causes the absorption change, is of an oxygen-deficient type.
(2) Optical absorption change in the microsecond order in oxygen-deficient Ge-doped silica glass was measured as a function of time just after photon irradiation from a KrF excimer laser. The absorption above 3 eV was found to decay with the same time constant as that of the luminescence at 3.1 eV.From this, it is confirmed that the observed absorption change is due to the excitation of electrons from the lowest excited triplet state to an upper state.
(3) Defects in buried SiO_2 films in Si formed by implantation of oxygen ions were characterized by photoluminescence (PL) excited by KrF (5.0 eV) excimer laser and synchrotron radiation. Two PL bands were observed at 4.3 and 2.7 eV.The 4.3 eV band has two PL excitation bands at 5.0 and 7.4 eV,and its decay time is 4.0 ns for the 5.0 eV excitation and 2.4 ns for the 7.4 eV excitation. The decay time of the 2.7 eV PL band is found to be 9.7 ms. These results are very similar to those for the 4.3 eV and the 2.7 eV PL bands, which are observed in bulk silica glass of an oxygen-deficient type and attributed to the oxygen vacancy. Through the change in the PL intensity with the film thickness, the buried SiO_2 film is considered to contain the oxygen vacancy defects in a high amount throughout the oxide.

The purpose of this study is to clarify the structural and optical properties of point defects in a-SiO_2, and their formation mechanisms under ionizing radiations or laser photons. The emphasis is placed on (i) the characterization of preexisting diamagnetic defects and radiation-induced paramagnetic defects, and (ii) the establishment of the link between the diamagnetic defects and radiation-induced paramagnetic defects. The outline of the present study is as follows.
1.Defects and optical absorption bands associated with excess oxygen are identified. Two forms of excess oxygen are identified: peroxy linkage and interstitial oxygen. An optical absorption band at 3.8 eV is associated with peroxy linkages.
2.Characterization of new paramagnetic defects induced by 7.9-eV photons is performed. With the help of computer simulation analyses of the ESR spectra, the paramagnetic defects are identified to be ClO_x radicals (x=2,3).
3.Time-resolved photoluminescence (PL) measurements are carried out on a series of silicas under 7.9-eV pulsed laser excitation. Correlations of several PL bands with point defects are studied.
4.Paramagnetic defects induced by 6.4-eV excimer laser are studied on a series of silicas. Effects of the oxygen stoichiometry and hydroxyl content on the defect species and their concentrations are observed. It is shown that precursor transformation dominates in laser-induced defect formation.
5.Photon-energy dependence of the creation of paramagnetic defects is investigated by means of ESR and uv-vuv optical absorption measurements, using 6.4-eV and 7.9-eV photons, and gamma rays as irradiation sources. Correlations between the preexisting diamagnetic defects and laser-induced paramagnetic defects are investigated.
6.The effect of hydrogen on the photogeneration of E' centers is studied by means of ESR and uv-vuv absorption measurements. It is found that the creation of E' centers from neutral oxygen vacancies is enhanced in the presence of hydrogen.

本研究の目的はサファイア上に堆積された高品質GaN薄膜、InGaN/GaN単一量子井戸及びInGaN/InGaN多重量子井戸構造における励起子の性質とその光学的非線形性に及ぼす効果を調べることにあった。はじめに、研究コミュニティーからの要請により若干目的に変更が生じたことをお詫びしておく。研究実績は以下の通りである。1.Gan:0.83μm単結晶試料の10K-300Kにおける透過率・反射率の精密測定を行い、基礎吸収端近傍で吸収係数が1×105cm-1であることを見出した。この試料と4μmの試料で、Xeランプとガラスフィルタを組み合わせて得た3.3eV以上の白色スペクトルにより超弱励起PL測定を実施した。A,B励起子の完全分離、均一線幅の同定、A,B励起子第一励起状態からの発光同定ができた。更に膜厚変化に伴う残留歪による励起子発光ピークのシフト、欠陥密度の変化に起因すると思われる束縛励起子発光強度の変化等を見出した。吸収係数の絶対値が大きいことから、基礎吸収端でのポンプ・プローブ(P&P)法による透過率測定は満足のいく結果が得られなかったので、現在測定した吸収係数だけをもとにした光双安定性の計算を実行している最中である。ナノ秒P&P法では、吸収端より長波長側で、光照射後の比較的長い時定数を持つ屈折率変化を見出し、GaNの可変屈折率素子としての可能性を示唆した。2.InGaN単一・多重量子井戸:これら試料における最近の話題はピエゾ電場効果と混晶中のポテンシャル揺動に集中している。観測される電子系は両者の競合で決まっており、個々の効果の分離が難しい。本研究ではSiを高濃度に添加することで前者の効果をほぼゼロに抑制した試料を2つ成長条件を変えて作製し後者の効果の有無を調べた。結果はポテンシャル揺動が明らかに存在することを示した。これが無添加試料において励起子を局在させる一因であると考えられる。これらの試料でのゲインスペクトルを求めた結果、それは電子正孔プラズマモデルで説明できる一方で、如実にポテンシャル揺動を反映している事実を見出した。ポテンシャル揺動が弱い試料でアップ・コンバージョン法により蛍光の立ち上がり時間を測定した結果、それは60fs以下であることを見出した。n型試料でありかつ正孔・LOフォノン散乱が無視できる様にした励起波長での比較的弱励起条件下での結果であることから、実験結果は正孔が電子・正孔散乱により余剰エネルギーを冷たい電子へ移行して速やかに緩和することを反映したものと考えている。InGaN系における光励起正孔の初期緩和に関する研究はこれが始めてであることを付記しておく

固体ソースMBE法では、SiO2マスク上にGa原子が付着しやすいために多結晶が形成されやすく、選択成長は困難とされていたが、これまで我々はMEE法を用いた極めて制御性の良い選択成長法を確立してきた。今年度は、そのMEE法を用いた選択成長により、サブミクロン光導波路の作製に成功した。まず、電子ビームリソグラフィーを用いて、膜厚30nmのSiO2マスクで面を覆ったGaAs(001)基板上の[110]方向に沿った、長さ1μm、幅が200nmのサブミクロンチャネルパターンを作製した。その後、固体ソースMBE装置を使用し、SiドープGaAsの選択成長を行った。基板温度は590℃とし、GaとSiを1.0秒供給後2.0秒のアニール時間を取り入れ、その後As4を2.0秒供給し(成長速度0.2ML/cycle)、選択成長を行った。このチャネル周囲のSiO2マスク幅は約400nmとした。この成長層周囲のサブミクロンマスクにより、SiO2マスク上に供給されるGa原子の拡散による成長層への寄与が小さくなり、チャネル部分と電極との接続部分における膜厚の不均一性が改善された。また、SiO2マスク幅をGa原子の拡散長に近づけることで、完全なる選択性を得ることができ、成長速度の改善がされた。また、従来困難であったAIGaAs選択成長が可能となった。チャネル部分のPL発光強度の二次元パターンを観察したところ、成長層での発光はマスク下の基板からの発光に比べ極めて強く、均質であることが確認され、良質なチャネル構造が形成できたことが分かる。今後、基板面、チャネル方向、チャネルの幅、成長条件等を最適化するとともに光導波路としての特性調べる。さちに低次元の電気伝導特性も検討する。また、ゲート電極で制御することにより、バリスティック伝導および1次元伝導特性の観測が期待され、低消費電力FETへの応用も考えられる

Background Early detection and proper excision of the primary lesions of malignant melanoma (MM) are crucial for reducing melanoma-related deaths. To support the early detection of melanoma, automated melanoma screening systems have been extensively studied and developed. In this article, we present a hyperspectral melanoma screening system and propose a possible melanoma discrimination index derived from the characteristics of the pigment molecules in the skin, both of which have been derived from hyperspectral data (HSD). Methods The index expresses the disordered nature of each lesion including variegation in color based on variation in spectral information obtained from each lesion. Performance of the index in discriminating melanomas from other pigmented skin lesions has been studied in five cases of melanoma (41 HSD sets), one case of Spitz nevus (13 HSD sets), 10 cases of seborrheic keratosis (78 HSD sets), three cases of basal cell carcinoma (16 HSD sets), and nine cases of melanocytic nevus (21 HSD sets), obtained from patients and volunteers, all of whom were Japanese. Results Performance of the index, which reflects the disordered nature of a lesion, discriminates melanomas with a sensitivity of 90%, a specificity of 84%, and an area under the receiver operating characteristic curve of 0.93, on resubstitution. Conclusion An objective melanoma discrimination index at a molecular pigmentary level, derived from HSD, has been proposed, and its performance evaluated. This index was highly successful in discriminating MM from non-melanoma, although the statistical population was small.

Melanin is a ubiquitous biological pigment found in bacteria, fungi, plants, and animals. It has a diverse range of ecological and biochemical functions, including display, evasion, photoprotection, detoxification, and metal scavenging. To date, evidence of melanin in fossil organisms has relied entirely on indirect morphological and chemical analyses. Here, we apply direct chemical techniques to categorically demonstrate the preservation of eumelanin in two &gt;160 Ma Jurassic cephalopod ink sacs and to confirm its chemical similarity to the ink of the modern cephalopod, Sepia officinalis. Identification and characterization of degradation-resistant melanin may provide insights into its diverse roles in ancient organisms.

Spectroscopic and temporal four-wave-mixing (FWM) measurements are carried out on a strain-free bulk ZnO single crystal, in order to clarify the valence-band-ordering. Under the collinearly polarized lights with the electric-field component parallel to the c-axis, which can excite dipole-allowed Gamma(1)-excitons, the FWM signal appears only in the energies corresponding to the B-exciton. Under the cross-linear polarization configuration exciting both Gamma(5)- and Gamma(1)-excitons, the FWM signal arising from the two-photon-coherence is absent in the energies corresponding to A-exciton. Both the results indicate that Gamma(1)-exciton state belongs exclusively to B-exciton, meaning that the valence-band ordering is Gamma(9) - Gamma(7) - Gamma(7) in order of decreasing electron energy for the present strain-free ZnO single crystal. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4711103]

Mid-infrared vibrational spectroscopic study has been made on 5,6-dihydroxyindole (DHI) and DHI-derived eumelanin. It has been revealed for DHI monomer that measured infrared absorption spectrum is well reproduced by that predicted from ab initio calculations. Thus, vibrational modes of DHI monomer causing dominant absorption bands have been successfully assigned. It has been also reconfirmed that DHI-derived eumelanin includes indolequinone and/or quinone methide units in addition to DHI units. (C) 2011 Elsevier B.V. All rights reserved.

First-principles calculations are carried out to estimate the spontaneous polarization and the energy band gap bowing in YxAlyGa1-x-yN alloys lattice-matched to GaN. The ground state properties of alloys are computed by using the pseudopotential-planewave method in conjunction with generalized gradient approximation to density functional theory. We find nonlinear behavior of the spontaneous polarization and the band gap energies in YxAlyGa1-x-yN alloys and the values depend on the atomic geometry in the unit cell, especially on that of yttrium. (C) 2011 American Institute of Physics. [doi:10.1063/1.3651154]

The ABCD-rule of dermoscopy has increased diagnostic accuracy ; however, some training is required in using it efficiently, and the diagnosis based on morphology is to a large extent subjective as with any clinical diagnosis. Morphology mainly comes from spatial variegation in skin pigmentation, i.e. spatial change in pigment molecular species and their concentration. Optical spectra reflected from the skin carry information about it. An essential feature of melanoma is the variety in morphology of the lesion, which is equivalent to change in spectrum from portion to portion within the lesion. The hyperspectral imaging technique enables one to measure hyperspectral data (HSD) within a short period, which consist of both spatial and spectral information. In this paper, we propose a single melanoma discrimination index derived from HSD using the entropy concept, and demonstrate its usefulness.[Skin Cancer (Japan) 2011 ; 26 : 94-97]

The purpose of this study was to examine the efficacy of detecting oral squamous cell carcinoma (SCC) using photodynamic diagnosis (PDD), a highly reliable method for tumor diagnosis, through the use of photosensitizers such as talaporfin sodium. Talaporfin sodium (5mg/kg) was administered to 7 control rats and 43 rats that had ingested the carcinogen 4-nitroquinoline 1-oxide (4NQO). After 8h, tongues were examined by spectral imaging and measurements were made using a hyperspectral imaging system. On each spectral image, macroscopically lesions and normal regions of the tongue were selected. Lesions comprised squamous cell carcinoma (SCC) in 22 cases, carcinoma in situ (CiS) in 9 cases, and dysplasia in 12 cases. The value Δα was determined as the absorbance spectrum of a lesion compared with normal tissue, or as the difference between Δκ at the posterior of the lingual protrusion and Δκ at the anterior of the lingual protrusion in controls. We compared Δα for SCC, CiS, and dysplasia with controls. The value Δα was (2.4±1.78)×10-2 in SCC, (1.2±1.34)×10-2 in CiS, (-0.2±0.49)×10-2 in dysplasia, and (0.21±0.63)×10-2 in controls. Both SCC and CiS showed significant differences from controls (p=0.037 and p=0.021, respectively). Histopathological analysis revealed that proliferating cell nuclear antigen (PCNA) expression increased gradually in the following order: controls, dysplasia, CiS, and SCC. Development of PDD using a hyperspectral imaging system may represent a useful technique for detecting oral SCC. © 2010 Asian Association of Oral and Maxillofacial Surgeons.

The morphology of pigmented skin lesions (PSLs) comes from differences in distribution and in concentration of melanin and hemoglobin pigment molecules. Those differences may be analyzed qualitatively and quantitatively using diffuse reflectance spectra in the visible to near infrared wavelength region. We have expressed differences in concentration distribution of both pigment molecules numerically with the use of diffuse reflectance spectra varying from site to site. Performance of the parameter has been studied on fifteen cases of PSLs : six cases of melanoma, five cases of seborrheic keratosis, and four cases of nevus. It has been demonstrated that the parameter can discriminate melanoma from others with a sensitivity of 85% and a specificity of 91%. It has also been suggested that the parameter reflects both boundary irregularity and variation in colors of PSLs.[Skin Cancer (Japan) 2010 ; 25 : 81-84]

We report the heterodimerization of eumelanin precursors from the quantum chemical calculations using a general-purpose reactivity indicator. 5,6-Dihydroxyindole (DHI), its 2-carboxy derivative (DHICA), and related molecules are under consideration. Molecules participating in the reaction have been identified from the electron transfer probability. The heterodimerization has been demonstrated to be the electron-transfer-controlled reaction. The theoretical prediction is in good agreement with the result of the previous cooxidation of DHI and DHICA. (C) 2010 Elsevier B.V. All rights reserved.

Extremely radiative nature of high-quality AlN single crystalline epilayers was identified by means of far ultraviolet time-resolved photoluminescence using a frequency-quadrupled femtosecond Al(2)O(3):Ti laser. The gross radiative lifetimes of a free excitonic polariton emission as short as 10 ps at 7 K and 180 ps at 300 K were revealed, which are the shortest ever reported for bulk semiconductor materials.

The reactivity of 5,6-dihydroxyindole and its major dimers has been studied with the use of a recently proposed general-purpose reactive indicator (Anderson et al. J. Chem. Theory Comput. 2007, 3, 358-374) from ab initio density-functional theory calculations. Theoretical prediction has reasonably explained previously isolated oligomers up to tetramers. The oxidative polymerization is governed by the electron-transfer-control led reaction. The electrostatic interaction plays a regioselective role in the reactant complex and/or intermediates. A monomer-dimer coupling is able to form trimers, while a part of it is prevented by the exchange repulsion, i.e., steric hindrance. Therefore, a dimer-dimer coupling is also able to form tetramers.

The reactivity of 5,6-dihydroxyindole and its major dimers has been studied with the use of a recently proposed general-purpose reactive indicator (Anderson et al. J. Chem. Theory Comput. 2007, 3, 358-374) from ab initio density-functional theory calculations. Theoretical prediction has reasonably explained previously isolated oligomers up to tetramers. The oxidative polymerization is governed by the electron-transfer-control led reaction. The electrostatic interaction plays a regioselective role in the reactant complex and/or intermediates. A monomer-dimer coupling is able to form trimers, while a part of it is prevented by the exchange repulsion, i.e., steric hindrance. Therefore, a dimer-dimer coupling is also able to form tetramers.

We report the mid-infrared absorption spectrum of 5,6-dihydroxyindole-2-carboxylic acid (DHICA) solid samples in the wavenumber region ranging from 1000 cm(-1) to 4000 cm(-1) with resolution of 4 cm(-1). Vibrational modes for observed dominant absorption bands of DHICA have been successfully assigned with ab initio density-functional theory calculations. The experimental data also suggest the existence of intermolecular hydrogen bonds. (c) 2006 Elsevier B.V. All rights reserved.

A principal origin of the defect-insensitive emission probability of In-containing three-dimensional ( 3-D) bulk ( Al, In, Ga) N alloy films, such as InGaN, AlInN and AlInGaN, is proposed. In contrast to In-free GaN or AlGaN, In-containing films grown on sapphire substrates show significant emission probabilities, although the threading dislocation density generated due to lattice mismatch is six orders of magnitude higher than that in conventional ( Al, In, Ga)( As, P) light-emitting-diode films. According to the extremely short diffusion lengths of positrons ( &lt; 4 nm) and short radiative lifetimes of excitonic emissions, we conclude that capturing of holes by localized valence states associated with atomic condensates of In-N outrun the trapping by non-radiative recombination centres, which are defect complexes associated with group III vacancies, because holes and positrons have the same positive charge. The captured holes are considered to form localized excitons with surrounding electrons to emit the light. The enterprising use of atomically inhomogeneous crystals is proposed for future innovation in light emitters even when using defective crystals.

Group-III-nitride semiconductors have shown enormous potential as light sources for full-colour displays, optical storage and solid-state lighting. Remarkably, InGaN blue- and green-light-emitting diodes (LEDs) emit brilliant light although the threading dislocation density generated due to lattice mismatch is six orders of magnitude higher than that in conventional LEDs. Here we explain why In-containing (Al, In, Ga) N bulk films exhibit a defect-insensitive emission probability. From the extremely short positron diffusion lengths (&lt; 4nm) and short radiative lifetimes of excitonic emissions, we conclude that localizing valence states associated with atomic condensates of In-N preferentially capture holes, which have a positive charge similar to positrons. The holes form localized excitons to emit the light, although some of the excitons recombine at non-radiative centres. The enterprising use of atomically inhomogeneous crystals is proposed for future innovation in light emitters even when using defective crystals.

Prospective equivalent internal quantum efficiency (eta(int)) of approximately 34% at 300 K was demonstrated for the blue emission peak of nonpolar m-plane (1 (1) over bar 00) InxGa1-xN/GaN multiple quantum well light emitting diodes (LEDs) fabricated on freestanding m-plane GaN substrates. Although the eta(int) value is yet lower than that of conventional c-plane blue LEDs (&gt; 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 300 K. (c) 2006 American Institute of Physics.

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 ScAlMg O4 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 record long τnr (3.8 ns) at 300 K was demonstrated. Because the radiative lifetime (τr) is in principle constant in bulk and epitaxial ZnO, the increase in τnr gave rise to the increase in ηint. Rich structures originating from exciton-polaritons and excited states of excitons were eventually observed in the low-temperature PL spectrum of the improved ZnO epilayer on HITAB, of which ηint of the NBE emission was 6.3% at 300 K. © 2006 American Institute of Physics.

Recombination dynamics of the 268 nm photoluminescence (PL) peak in a quaternary Al0.53In0.11Ga0.36N/Al0.58In0.02Ga0.40N multiple quantum well (MQW) grown on relaxed AlGaN templates were studied. Although the polarization field in the compressively strained Al0.53In0.11Ga0.36N wells was as high as 1.6 MV/cm, the value of integrated PL intensity at 300 K divided by that at 8 K (eta(int)) was as high as 1.2%. The value was similar to that obtained for the 285 nm PL peak in an Al0.30Ga0.70N/Al0.70Ga0.30N MQW (1.3%), though the A1N molar fraction in the wells was higher by a factor of 1.7. According to these results and the fact that time-resolved PL signal exhibited a stretched exponential decay shape, the improved eta(int) of the AlInGaN wells was attributed to a beneficial effect of the exciton localization as is the case with InGaN alloys; doping or alloying with InN was confirmed to work also on AlGaN in improving eta(int) to realize deep UV optoelectronic devices.

In this paper, we discuss the conformational properties of the hydroxymethyl group of beta-glucopyranose in aqueous solution and its reorientation mechanism. First, using the values for the hydroxymethyl torsion (O5-C5-C6-06) angle obtained by our ab initio simulations, we reestimate the experimental ratio of the hydroxymethyl rotamer populations. The reestimated ratio is found to be in agreement with those previously reported in several computational studies, which probably partly explains the discrepancies between theoretical and experimental studies that have been discussed in the literature. Second, our time-frequency analysis on a reorientation in the hydroxymethyl group in an ab initio molecular dynamics trajectory suggests that, before the reorientation, the O6-H6 stretching mode is vibrationally coupled with a proton-accepting first-hydration-shell water molecule, whereas the C6-O6 stretching mode is vibrationally coupled with a proton-donating one. The amount of the total vibrational energy induced by these vibrational couplings is estimated to be comparable to typical values for the potential barriers between hydroxymethyl rotamers. To elucidate the vibrational couplings, we investigate the hydrogen-bonding properties around the hydroxymethyl group during the pretransition period. The implications, validity, and limitation of a possible reorientation mechanism based on these findings are also discussed.

Using the method of ab initio molecular dynamics, we examine the structural fluctuation and the low-frequency dynamics of beta-ribofuranose puckering in aqueous solution. Our analysis suggests that the distance between the anomeric and hydroxymethyl oxygens is a simple relevant geometrical parameter that dynamically correlates with the phase angle in the north region. The time-frequency analysis using the Hilbert-Huang transform also confirms the correlation, and most of the instantaneous frequencies for the phase angle and the above distance are found to be concentrated on the region below about 100 cm(-1). Our analysis of ab initio molecular dynamics trajectories suggests that the molecular origin of the hydration effects on the low-frequency dynamics of beta-ribofuranose puckering is closely related to this correlation and thus primarily attributed to the relatively local interactions among the anomeric and hydroxymethyl oxygens and the surrounding water molecules near them. Additionally, we discuss the difference in the low-frequency dynamics of P-ribofuranose puckering between two hydroxymethyl rotarners.

This paper examines the hydration structure on the surface of beta-ribofuranose in aqueous solution, using the ab initio molecular dynamics method. In particular, we focus on circular hydrogen bond networks involving two ribofuranose oxygens and three water molecules. In our simulations, the circular hydrogen bond networks near the ring oxygen of beta-ribofuranose are found to be significantly influenced by the orientation of the hydroxymethyl group. The arrangements of hydrogen bonds observed in the circular hydrogen bond networks are both homodromic and antidromic. To explain these observations, we analyze the electronic properties of the first-hydration-shell water molecules and the OH groups of beta-ribofuranose, using the centers of their maximally localized Wannier functions. The dipole moments of the proton-accepting first-hydration-shell water molecules in our well-defined circular hydrogen bond networks are found to increase by about 0.3 D compared with that of liquid water, indicating the relatively strong polarization effects created by the interactions between the OH groups of the solute and the surrounding water molecules. Our analysis also implies that circular H-bond networks cannot be fully explained from a simple geometrical point of view.

We have studied the exciton-polariton dynamics in a c-face bulk ZnO crystal grown by the seeded vapor transport method. We have performed two-pulse spectrally resolved four-wave mixing (SR-FWM) and three-pulse SR-FWM at T = 10 K. In two-pulse FWM experiment, obtained FWM signals showed the fast dephasing which has been attributed to the strong polariton dispersion of the upper polariton of B exciton. On the other hand, in three-pulse FWM experiment, the signal decay shows the free exciton population relaxation. Observed FWM signal has decayed with time constant similar to 12 ps which may represent exciton-polariton lifetime. (c) 2004 Elsevier B.V. All rights reserved.

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 (omega(LT)) 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 (1120) 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 (tau(nr)). The value of tau(nr) increased and density or size of Zn vacancies (V-Zn) decreased with increasing growth temperature (T-g) in heteroepitaxial films grown on a ScAlMgO4 substrate, and the use of homoepitaxial substrates further reduced VZn density. The value Of tau(nr) 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 V-Zn 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 V-Zn, such as V-Zn-defect complexes. As a result of defect elimination by growing the films at high T I followed by subsequent post-growth in situ annealing, combined with the use of high-temperature-annealed ZnO self-buffer layer, a record long tau(nr) 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.

Beneficial effects of the localized excitons were confirmed in nonpolar (11(2) over bar 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 300 K divided by that at 8 K were 25% and 17% for the peaks at 2.92 and 2.60 eV, respectively. (C) 2005 American Institute of Physics.

Room-temperature nonradiative lifetime (tau(nr)) of the near-band-edge excitonic photoluminescence (PL) peak in {0001} polar, (11 (2) over bar0), (1 (1) over bar 00), and (001) nonpolar GaN was shown to increase with the decrease in density or size of Ga vacancies (V-Ga) 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 V-Ga tended to decrease and tau(nr) 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 V-Ga, such as V-Ga-defect complexes. (C) 2005 American Institute of Physics.

We have investigated free excitons and biexcitons of bulk ZnO by time-integrated and spectrally-resolved four-wave mixing. The polarization- and wave-vector-dependent FWM signals for A and B excitons demonstrate clearly that the valence-band ordering is A - Gamma(9), B - Gamma(7), and C - Gamma(7). Next, we gave qualitative explanation of the FWM signals induced by excitons and biexcitons in ZnO. For negative delay times, bound and unbound AB-biexcitons (A exciton-B exciton correlation), which have been considered to play a minor role in GaAs heterostructures, was found to play an important role and give a significant qualitative impact to the optical response of ZnO. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The roles of point defects and defect complexes governing nonradiative processes in ZnO epilayers were studied using time-resolved photoluminescence (PL) and slow positron annihilation measurements. The density or size of Zn vacancies (V-Zn) decreased and the nonradiative PL lifetime (tau(nr)) increased with higher growth temperature for epilayers grown on a ScAlMgO4 substrate. Accordingly, the steady-state free excitonic PL intensity increased with increase in tau(nr) at room temperature. The use of a homoepitaxial substrate further decreased the V-Zn concentration. However, no perfect relation between tau(nr) and the density or size of V-Zn or other positron scattering centers was found. The results indicated that nonradiative recombination processes are governed not solely by single point defects, but by certain defect species introduced by the presence of V-Zn such as vacancy complexes.

The phase relaxation time of biexcitons T-2(bi) and that of excitons T-2(ex) in a bulk ZnO have been measured by the use of femtosecond four-wave mixing as functions of excitation wavelength, excitation power, and temperature. The biexciton-acoustic phonon interaction coefficient has been determined to be beta(ac)(bi)approximate to53 mueV/K as an average value. For A excitons, the interaction coefficients with acoustic and optical phonons have been determined to be beta(ac)(ex)approximate to8.4 mueV/K and beta(opt)(ex)approximate to14 meV, respectively. (C) 2004 American Institute of Physics.

Radiative and nonradiative excitonic transitions in nonpolar (11 (2) over bar0) 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. (C) 2004 American Institute of Physics.

Time-integrated and spectrally resolved four-wave mixing (FWM) has been applied to study the impact of the k-linear term on nonlinear optical response of the C-exciton manifold in ZnO. From the excitation wavelength dependence of the FWM signal, we have found possible evidence that the mixing of the Gamma(1) and Gamma(5L) states leads to a three-branch excitonic polariton dispersion but the oscillator strength transfer is small. The energy separation between the lower polariton branch and the extra branch due to the k-linear term has been found to be 3.5 meV. In addition, a binding energy of biexcitons consisting of two C excitons of 1.4 meV has been obtained. (C) 2004 American Institute of Physics.

We demonstrate that exciton-exciton correlations strongly affect FWM signals. In particular, A exciton-B exciton correlation (bound and unbound A B-biexcitons), which can be generated even in co-circular polarizations, has a significant qualitative impact on the optical response of bulk ZnO.

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 xless than or equal to0.7. 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 signals showed a biexponential decay at low temperature, and the slower component became longer with the increase in x (over 40 ns for x=0.49). Simultaneously, density or size of cation vacancies (V-III) and relative intensity of the deep-level emission over that of the near-band-edge one at 300 K increased as x increased to x=0.7. Consequently, certain trapping mechanisms associated with V-III where suggested, and excitons were then detrapped and transferred to the localized states before the radiative decay at low temperature; the increase in the slower lifetime and its dominance over the entire TRPL signal intensity with increasing x may reflect the increase of the depth and concentration of the trapping level. As the temperature was increased, the TRPL signal became single exponential due to the increasing dominance of nonradiative recombination processes in the free states, resulting in lower internal quantum efficiency (eta(int)) with increasing x for xless than or equal to0.7. Therefore, realization of AlGaN-based efficient deep-UV light emitters requires further reduction of the nonradiative defect density as well as the V-III-related trap density. (C) 2004 American Institute of Physics.

The polarized optical reflectance and photoreflectance spectra of an out-plane nonpolar ZnO epilayer grown by laser molecular-beam epitaxy was studied. The electric field component of its excitonic photoluminescence peak was polarized perpendicular to the [0001] axis. The monoenergetic positron beam line was used to determine the parameters in the ZnO epilayers. The negligible impact of growth direction on the defect incorporation suggests a potential use of epitaxial ZnO as polarization-sensitive optoelectronic devices operating in ultraviolet spectral regions.

Polarized optical reflectance and photoreflectance spectra of an out-plane nonpolar (11(2) over bar 0) 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 (11(2) over bar 0) ZnO were comparable to those of state-of-the-art polar (000(1) over bar) 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 (11(2) over bar 0) ZnO as polarization-sensitive optoelectronic devices operating in ultraviolet spectral regions. (C) 2004 American Institute of Physics.

Optical and structural properties of an InGaN double-quantum-well (DQW) laser diode (LD) wafer that lased at 450 nm were investigated to discuss an enormous impact of a polarization-induced electric field on the recombination dynamics in InGaN quantum structures. The quantum-well (QW) structure was shown to have the well thickness as thin as approximately I nm and InN molar fraction x of approximately 14%. The gross effective electric field in the QW (F-QW) was estimated to be 490kV/cm from the Franz-Keldysh oscillation (FKO) period in the electroreflectance (ER) spectrum, implying that an internal piezoelectric field (F-piz) of approximately 1.4MV/cm was cancelled by the pn junction built-in field (F-bi) and Coulomb screening due to carriers in the DQW. The magnitude of F-QW can be further weakened by applying reverse bias (V-R) on the junction; the decrease in the photoluminescence (PL) lifetime at low temperature measured under VR was explained to be due to a recovery of electron-hole wavefunction overlap for small VR (\V-R\&lt;4 V), and due mainly to the tunneling escape of carriers through the barriers for larger VR. By applying an appropriate VR smaller than 4V, electron-hole wavefunction overlap, which had been separated vertically along the c-axis due to quantum-confined Stark effect, could be partially recovered, and then the time-resolved PL signals exhibited a less-pronounced stretched exponential decay, giving a scaling parameter (beta) of 0.85 and effective in-plane localization depth (E-0) of 40-50 meV for the spontaneous emission. These values were closer to those of much homogeneous QWs compared to those reported previously for InGaN QWs having similar InN molar fractions. The use of very thin QWs is considered to bring easier Coulomb screening of F-QW and population inversion under high excitation conditions.

In this paper, using density functional theory, we show the effects of explicit intermolecular hydrogen bonds on infrared spectra of glucose-water complexes. The number of explicit water molecules ranges from four to nine, and we approximate the rest of other water molecules as a continuum medium. When eight or nine water molecules are considered explicitly, our computed spectra are in good agreement with the main features of the experimental one. However, in the cases of explicit water molecules less than eight, our computed spectra are not improved successfully. This suggests that in predicting infrared spectrum of glucose in water the effects of explicit water molecules around the glucose molecule are not negligible. We also discuss the numbers of intermolecular hydrogen bonds, spectral differences between alpha and beta anomers, and the main features of normal modes in the 1000-1200 cm(-1) region. (C) 2003 American Institute of Physics.

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. (C) 2003 American Institute of Physics.

Polarized Brillouin scattering measurements were carried out for a bulk ZnO single crystal. The whole set of elastic stiffness constants was determined to be c(11)=19.0, c(12)=11.0, c(13)=9.0, c(33)=19.6, c(44)=3.9, and c(66)=4.0 in units of 10(11) dyn/cm(2). The relationships between photoelastic constants at wavelength of 514.5 nm were also obtained: p(11), p(13), p(44), and p(66)=1.8, 2.3, 0.50, and 0.38, respectively, relative to p(12). (C) 2003 American Institute of Physics.

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. (C) 2003 American Institute of Physics.

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 (tau(nr)). Density or size of Zn vacancies (V-Zn) decreased and tau(nr) increased with increasing growth temperature in heteroepitaxial films grown on a ScAlMgO4 substrate. Use of homoepitaxial substrate further decreased the V-Zn density. However, tau(nr) was the shortest for the homoepitaxial film; i.e., no clear dependence was found between tau(nr) and density / size of V-Zn 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 V-Zn such as vacancy complexes. (C) 2003 American Institute of Physics.

Exciton-polariton structures in a high-quality bulk ZnO single crystal were resolved at 8 K by means of polarized photoreflectance (PR) and photoluminescence measurements. The energies of the PR resonances corresponded to those of the upper and lower exciton-polariton branches, where A, B, and C excitons couple simultaneously to an electromagnetic wave. Longitudinal-transverse splitting of ground-state exciton polaritons and resonances due to the first excited states of respective excitons were observed due to the large oscillator strength. Photoluminescence peaks due to the corresponding polarion branches were clearly resolved. The valence-band ordering was confirmed to be A-Gamma(9v), B-Gamma(7v)(u), and C-Gamma(7v)(l). (C) 2003 American Institute of Physics.

Infrared reflectance spectroscopy was carried out to study the properties of zone center optical phonons in ternary cubic InxGa1-xN free from phase separation. Raman spectra were also measured as auxiliary measures. Optical phonon behavior was confirmed to be type I. It is demonstrated that the behavior arises from both the small mass ratio of Ga to In atoms and the rather strong ionicity of Ga-N and In-N bonds. (C) 2003 American Institute of Physics.

Exciton-polariton emissions were observed at 8 K in the photoluminescence spectrum of a ZnO epitaxial film on the a-face of sapphire grown by radical-source molecular-beam-epitaxy. The resonance energies of corresponding photoreflectance structures agreed with those of longitudinal and transverse excitons, i.e. upper and lower polariton branches, where A, B and C-excitons couple simultaneously to an electromagnetic wave. In contrast to the results obtained for GaN, longitudinal-transverse splitting of the B-excitonic polariton was resolved, which is due to the large oscillator strength.

Exciton resonance energies in an AlN epilayer on (0001) sapphire substrate grown by low-pressure metalorganic vapor phase epitaxy were determined as a function of temperature by means of optical reflectance (OR) and cathodoluminescence measurements. The OR spectra exhibited distinct reflectance anomalies at the photon energies just above the multiple internal reflection fringes, and the spectral line shape was fitted considering A (Gamma(7v)(u)--&gt;Gamma(7c)) and BC (Gamma(9v),Gamma(7v)(l)--&gt;Gamma(7c)) exciton transitions. The fitting gave the values of them at 0 K to be 6.211 and 6.266 eV, giving the crystal- field splitting (Delta(cr)) of approximately 55 meV. The AlN film exhibited an excitonic emission even at 300 K, which is due to the small Bohr radius of excitons and large longitudinal optical phonon energies. The Einstein characteristic temperature Theta(E) was estimated to be 580 K. (C) 2002 American Institute of Physics.

We have studied the properties of coherent exciton states in a free standing wurtzite GaN using the degenerate four-wave-mixing (FWM) technique with subpicosecond time resolution. The polarization dependence of spectrally resolved FWM signals has demonstrated that excitation-induced dephasing is the dominant FWM process. The exciton energy renormalization has been found to dependent on the polarization configuration. FWM signals due to local field effect have been successfully discriminated from those due to other effects. These reveal an importance of four-particle correlations in two-exciton continuum states even for the low excitation power limit where no bound biexciton signal is detected. The temperature dependence of the homogeneous broadening of excitons has also been studied up to 60 K, to obtain the exciton-acoustic phonon interaction coefficient of 9 mueV/K.

Photoreflectance spectra of a high-quality ZnO epilayer on the ScAlMgO 4 (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 E of 380 K (33 meV). © 2002 American Institute of Physics.

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 Theta(E) of 380 K (33 meV). (C) 2002 American Institute of Physics.

Principal component analysis (PCA) has been used for quantitative analysis of infrared attenuated total reflection (ATR) spectra of chemical mixtures, so that the minute concentration of glucose in the chemical mixture would be evaluated. Spectroscopic calibration of chemical concentration has been, thus far, believed to require multidimensional least-squares methods, such as classical least squares (CLS) and partial least-squares (PLS) regression techniques. In the present study, however, the analysis of the infrared ATR spectra only by PCA has been found to be useful for the evaluation of concentration of specific minute species in a chemical mixture. The prediction accuracy by PCA only has proved to be comparable to that of PLS. The novel technique would enable us to measure the concentration of glucose in human blood with no invasion.