微小共振器ポラリトンレーザは新たな原理によるコヒーレント光源として興味深い。本研究では、独自のヘリコン波励起プラズマスパッタ法により、酸化亜鉛をベースとする微小共振器構築に必須な酸化マグネシウム亜鉛系半導体のエピタキシャル成長を行った。また、誘電体及び導電性分布ブラッグ反射鏡の設計と作製を行い、新規導電性酸化膜(アナターゼ相ニオブ添加酸化チタン)のエピタキシャル成長にも成功した。安価なスパッタ技術で先端エピタキシー技術と遜色の無い上記材料群のエピタキシャル成長が可能な事を示した成果を発展させることにより、ヘテロ構造、量子構造デバイス形成への道を拓くことができた

本年は、昨年までの成果に引き続き、ZnCdS/MgCdS超格子やZnSe/MgCdS超格子の研究について詳細な検討を行ってきた。昨年までに、基板温度の面内分布やルツボのシャッター開閉による変化が結晶性に与える影響について明らかにし、熱輻射の積極的利用によりそれらの悪影響を引き下げる技術を開発してきた。超格子構造の特性は周期構造によって変化する。しかし、設計と作製したサンプルの周期構造が一致しているかを検討する必要がある。また、界面がぼやけて混晶化してしまうと量子効果が発生しないことが考えられる。したがって、超格子は井戸層と障壁層の界面が急峻であることが重要である。熱輻射を積極的に応用することで電子顕微鏡による観察やX線回折による評価から、超格子が設計どおりに作製可能なこと(膜厚や組成が制御可能なこと)を明らかにした。その光学的特性も、設計と対応して変化することが明らかになった。さらに本手法を応用し、ZnTeのホモエピタキシー膜の高品質化に関する検討も行なった。ZnTeは一般にp型の伝導型のみを示すことが知られている。その価電子制御を行なう際に成長条件の安定化を図ることが重要である。本研究では価電子制御のために超格子構造を発展させた構造の作製を行なったが、その際にもシャッターの開閉に伴う基板温度の変動を抑えるため、輻射熱の積極的制御を行なった。そして、安定した成長条件のもとで高品質ZnTeの作製も可能になり、ZnTeのホモエピタキシーや伝導型制御に対しても超格子構造の応用が有効であるとの知見を得るに至った

近年の半導体デバイスの更なる高集積化に伴い、次世代CMOSのゲート絶縁膜として高誘電率材料の利用が検討されている。本研究では、候補材料であるハフニウムシリケート、ジルコニウムシリケート、ハフニア、ジルコニア等をプラズマ化学気相堆積法、上記の一部をスピンコート法より成膜した。薄膜試料は、分析機器による系統的な研究を行うのが難しいため、比較試料として、単結晶バルク試料であるイットリア安定化ジルコニア、ランタンアルミネート及びアルミナも用いた。フォトルミネセンス法(PL)、電子スピン共鳴法(ESR)、光電子スペクトル法により電子状態を調べた。
1.PL法による膜内欠陥および不純物の解析:シンクロトロン放射光を用いたPLスペクトル解析、PL寿命測定ならびに真空紫外域の光吸収測定より、ハフニア、ジルコニア、ハフニウムシリケート、ジルコニウムシリケートに共通するPL発光の原因は、バンド裾準位が関与した励起電子の緩和過程において生じることを明らかにした。
2.ランタンアルミネート及びアルミナ単結晶の光電子スペクトルにおける化学シフトの考察:光電子スペクトル測定からアルファアルミナの酸素1s電子の束縛エネルギーはランタンアルミネートより1.4eV大きいことが分かった。この化学シフトの原因の一つと考えられる緩和エネルギーの寄与を、理論式より見積もり、化学シフト+1.4eVのうち、+1.2eVであることを明らかとした。
3.イットリア安定化ジルコニアの紫外光誘起欠陥:上記試料に紫外光を照射すると、2種類の常磁性欠陥が生成され、3.3eV付近の吸収が増加することを見出した。さらに、これらの変化は光子エネルギー依存性を持ち、4.0eVより大きいエネルギーでのみ生じることから、伝導帯の裾に励起された電子により、これらの変化が生じていることを明らかにした。
本研究以前の研究も合わせ、シリカ、シリコンオキシナイトライド、シリコンナイトライド、ハフニウムシリケート、ジルコニウムシリケート、ハフニア、ジルコニア、ランタンアルミネート及びアルミナよりなる一連の無機絶縁性薄膜の禁制帯中局在準位および電子状態を系統的に明らかに出来たことになる。これは、所望の性質を具備した膜の堆積につながる優れた知見であると考えている。

これまで半導体を用いた二次元電子系においては量子ホール効果、分数量子ホール効果をはじめとした様々な電子物性や量子相関が明らかにされてきた。しかしながらさらに次元性の低い一次元、ゼロ次元電子系では電子物性研究の進展やデバイス応用の研究が著しく遅れている。この原因は平坦な二次元構造では原子スケールの構造精度がほぼ完全に達成されているのに対し、一次元・ゼロ次元構造ではそれが極めて不完全であることによる。本計画ではこれまで半導体ナノ構造の製造技術として開発を進めてきたMEE技術を進化・最適化することにより、おもにIII-V族化合物半導体を用いて、構造精度の極めて高い量子細線構造、量子ドット網構造などの製作技術を確立し、それらの構造を用いた新しい現象の探索を行った。III-V族化合物半導体の特性を生かすにはヘテロ接合が不可欠である。このため低次元構造の製作技術に関してはAlGaAs/GaAsをはじめ、InAs/GaAs、InGaAs/GaAsなどの諸材料を用いて行った。量子細線、量子ドット等の製作では、サイズと配列の不均一性・不規則性を排除するため、これまでの自己形成型の手法はとらず、電子線露光をベースに選択エピタキシャル成長の方法をとった。ナノ構造を設計どおりに実現するには横方向成長を阻止する必要があったが、MEE法を最適化することによりこれを実現した。その結果極めて均質性の高い一次元量子細線、ゼロ次元量子ドットの製作に成功すると共に、これらを用いた二次元格子配列構造の製作技術が確立できた。AlGaAs/GaAsを用いた量子ドット格子は、二次元フォトニック結晶として有効に機能することが明らかになった。さらに量子ドット-量子細線ネットワークの製作にも成功し、所期の性能を得た。一方InAs/GaAs量子ドット構造ではヒロックの発生に直面したが、結晶学的な解析によりその原因を究明し、この系においても構造精度の高い低次元構造の実現を可能にした

光波長多重通信網においては、光の分波機能を持つ光導波素子が必要となる。本研究は、光ファイバ型および平面型光導波素子にマイクロイオンビームや紫外光を導波素子に照射し、周期的に異なる屈折率を有する格子構造の形成を目指している。
1.イオンマイクロビーム照射によるシリカガラスの高密度化と屈折率変化:シリカガラスにH^+マイクロビーム照射を行い、顕微フォトルミネッセンス・ラマン分光、電子スピン共鳴法により評価した。注入イオンのエネルギー損失には原子衝突よりも電子励起の効果が大きいことをシミュレーションにより明らかにした。さらに、光ファイバ中に周期構造を実際に形成し、透過光損失特性を評価した。イオン照射されたシリカガラスの表面および側面に凹みが観測され、その主原因は高密度化であることを明らかにした。誘起屈折率変化は飛程付近で最大となり、回折格子作製に十分な値まで達することが分かった。マスクとして、電子線リソグラフィーによりシリコン窒化膜メンブレン上に周期2.65マイクロメートルのLine/Spaceパターンを作製し、イオン注入を行ったのちエッチングを行い、飛程の差に起因する凹凸の回折格子を形成した。フラウンホーファー回折により、回折格子の周期はLine/Spaceの周期と同じであると計算され、マスクパターン転写の成功が裏付けられた。
2.紫外光照射による屈折率増大:屈折率を1.45-2.00まで自在に変化できるシリコン酸窒化物は、光導波路材料の一つとして考えられている。紫外光エキシマレーザを照射することにより、屈折率を増加させることの可能なシリコン酸窒化物の作成方法および組成を明らかにした。屈折率上昇のメカニズムとして、高密度化に因る効果が大きいことを計算により導いた。さらに、周期1μmの平面回折格子を位相格子法により実際に作成し、光加工性の良さを示した。

半導体デバイスの絶縁膜や光ファイバなどとして使用され、現代のハイテク社会を担う基幹材料となっている非晶質シリカ薄膜について、膜中の不純物や空格子点などの点欠陥が膜の電気的・光学的特性に及ぼす影響を系統的に明らかにすることを目的とした研究を行った。不純物や空格子点などの存在はフォトルミネセンスやESR, FT-IR, 可視〜真空紫外分光, EPMA, SIMS等で明らかにし、電気特性は高電界伝導、絶縁破壊、C-V測定などより明らかにしている。酸素空孔の存在と定量、不純物添加による構造の安定化、成膜法の差による欠陥の多寡などが明らかになった。
平成9年度の研究において、協力な紫外-真空紫外光であるエキシマレーザ光やシンクロトロン放射光を用いることにより、a-SiO_2薄膜からでも充分な強度を有したフォトルミネセンスが得られることが証明され、今まで、有力な評価手段のなかったa-SiO_2薄膜の点欠陥に対する検出手法としてのフォトルミネセンス法が確立できた。さらに、この手法を用いて、シリコン熱酸化膜やSIMOX埋め込み酸化膜を対象として、その膜の製法に起因して膜が本質的に含有する欠陥とイオン注入や高温熱処理等のデバイス製造プロセスにより生じる欠陥の検出と欠陥構造の同定を行うことができた。
平成10年度は、引き続きSIMOX埋め込み酸化膜、フッ素添加SiO_2薄膜やシリコン窒化膜を対象として欠陥の検出と欠陥構造の同定をさらに進めた。また、絶縁破壊測定により、欠陥構造が絶縁破壊強度に与える影響を調査した。
平成11年度はイオン注入や加熱処理により誘起される構造欠陥が、a-SiN_x膜、a-SiO_xN_y膜やTa_2O_5薄膜の電気的・光学的性質に与える影響を明らかにした。さらに、これまでの研究の応用としてGe添加シリカガラスの欠陥を利用した光ファイバグレーティングの作成時に生じている光誘起構造変化を明らかにし、ついでイオン注入法による高効率で安定性の良いグレーティングの作成法を提案した。

(I)ゲルマニウム(Ge)ドープシリカガラスに紫外光を照射するとガラス内に構造変化が誘起され、発光や吸収に変化が生じることは以前から知られている。また、この光照射により常磁性欠陥であるE'centerやGEC(Germanium Electron-trapped Center)が誘起されることも報告されている。我々は、光パワー密度の大きく異なる2台のエキシマレーザや軌道放射光などを用いて誘起される構造変化について調べた。得られた主な成果は次の通りである。
(1)光パワー密度の異なる2台のKrFエキシマレーザのいずれにおいても、4.5eV、5.8eVをピークに持つ2つの吸収帯が誘起され、5.06eVをピークに持つ吸収帯がブリーチされる。また、高パワー密度光で誘起された吸収は、低パワー密度光でブリーチされる。このことから、KrFエキシマレーザ照射時には、2光子過程と1光子の過程が同時に生じ、それぞれ、欠陥の生成、消滅に寄与していることが分かった。
(2)いわゆるポンプープローブ法を用いて、励起状態よりの吸収を時間分解測定した。この結果、基底状態より3.1eV付近に存在する励起三重項状態の上約4eV以上に幅広い吸収があることがわかった。これにより、Geドープシリカガラスの欠陥のエネルギーダイアグラムがかなりはっきりしてきた。
(3)酸素欠乏性欠陥の発光体の発光励起スペクトルの温度依存性より、高エネルギー光子により伝導帯を経由した形で発光励起が生じる可能性があることを示唆する結果を得た。
(II)上述の研究遂行の過程で確立してきたフォトルミネッセンス手法をシリコン酸化膜に適用し、世界で初めてシリコン酸化膜中の酸素欠乏性欠陥からのルミネンスを検出し、さらに発光量とSiO_2構造との関連を明らかにした。
これらの成果は、Journal of Applied Physics,Phys. Rev. Bなどに掲載されたほか、さらに2〜3編むの論文を投稿予定である。

1.エキシマレーザによる常磁性欠陥の生成機構の確立
種々の条件下で合成された一連のシリカガラスにエキシマレーザ(6.4eV)照射をした際に誘起される常磁性欠陥種とその生成量をESRにより調べ、たとえば、E'中心はあらゆる型の試料において観測されるが、非架橋酸素ラジカルは酸素過剰型試料およびB2βと呼ばれる試料にのみ観測されることを見い出した。また、E'中心等の生成量の試料依存性を調べることにより、エキシマレーザ誘起欠陥の生成機構として、従来より考えられている自己捕獲励起子の非輻射緩和によるSi-O-Siの正常な結合からの欠陥生成と合成時に導入された前駆体からの欠陥生成の2つの機構のうち、後者が主たる欠陥生成機構であることを明らかにした。ついで、エキシマレーザ(6.4evあるいは7.9eV)照射したシリカガラスについて、分子科学研究所および東大物性研のシンクロトロン軌道放射光施設を利用することにより、SR光吸収スペクトルの測定を行った。同時に、さらに、ESRにより常磁性欠陥の生成種およびその生成量をレーザのパルス強度、光子エネルギー、および照射量の関数として調ベた。その結果、合成時に導入された酸素空孔、過酸化結合、およびSi-OH結合等の欠陥を前駆体として、二光子吸収により生成した電子あるいは正孔が前駆体欠陥において捕獲されるか、電子・正孔対の非輻射再結合によるかのいずれかにより、常磁性欠陥が生じていることを明らかにした。
2.欠陥生成におよぼす水素の役割の明確化
エキシマレーザ(6.4eVあるいは7.9eV)照射した2種類の酸素欠乏型試料([OH]〈1ppmおよび、[OH]=20ppm)について、ESRおよびSR光吸収スペクトルの測定を行い、OH基が多い試料では、OH基が少ない試料と比較してE'中心が1-2桁多く生成されると同時に、酸素空孔による7.6-eV帯の減少とE'中心による5.8-eV帯の生成が顕著であることを見い出した。さらに、この試料のESRにおいて、幅7.4mTの超微細相互作用分裂が生じ、水素原子の生成が観測されることから、酸素空孔からのE'中心生成機構として、酸素空孔における水素原子の捕獲による機構を新たに提案した。

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 >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]

我々はメラノーマの非侵襲的診断法としてハイパースペクトルイメージャ（HSI）を用いた新しい診断法の開発を進めている。HSIにより得られるハイパースペクトルデータ（HSD）は，病変の位置情報とスペクトル情報からなる。前にHSDに基づく鑑別指標を提案したが，今回新しい概念による指標を提案し，その有用性を示す。メラノーマ5例，脂漏性角化症12例，母斑細胞性母斑7例を解析した。病変毎に異なる部位のHSDを複数計測した。HSD毎，各画素のスペクトルに対応するスペクトル角度を求め，その角度の出現確率から計算されるエントロピー指標を新指標とした。新指標がメラノーマとそれ以外の病変を区別できる鑑別性能は非常に高く，感度，特異度はともに90％を超えた。単一で高い鑑別性能を示す新指標は，有用であると考えられた。なお，新旧指標のROC曲線下面積による比較では，新0.96 vs旧0.77となり，新指標の有用性が示された。

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.

色素性皮膚病変（PSLs）の形態は主にメラニンとヘモグロビン色素分子の分布および濃度の差異に帰せられる。その定性的かつ定量的差異は可視－近赤領域の拡散反射スペクトルを用いて客観的に解析できる。本論文ではスペクトルを用いて両色素分子の濃度分布の差異を数値化した。本指標は15症例のPSLsに対して，高い精度（感度85％，特異度91％）でメラノーマを他のPSLsから鑑別可能であった。本指標はPSLs辺縁部のirregularityと色の多様性を同時に反映していることが示唆された。

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 ( < 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 (< 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 (> 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\<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)-->Gamma(7c)) and BC (Gamma(9v),Gamma(7v)(l)-->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.