氮化銦磊晶層之光學特性研究

Abstract

本論文中我們利用光激螢光光譜和時間解析光譜來研究氮化銦磊晶層之光學特性。從霍爾量測的結果知道，成長氮化銦時緩衝層的基板溫度不同及緩衝層材料的不同皆會影響氮化銦磊晶層內載子濃度的多寡。從光激螢光光譜及吸收光譜的觀察，我們發現載子濃度的增加會同時使發光能量及吸收位置藍移，這樣的現象是由Burstein-Moss 飄移所造成，量測到的實驗數據也與理論模型所計算的結果相符。從光激螢光光譜波形非常不對稱的情況，我們推測這是由於深層授子、淺層授子及Urbach tail存在於氮化銦磊晶層所造成。 從溫度變化的光激螢光光譜數據可得到兩個活化能，分別對應於另一個研究團隊所提出的深層授子束縛能（50~55毫電子伏特）及淺層授子和Urbach tail的束縛能（5~10毫電子伏特）。從時間解析的激發-探測實驗，可以觀察到在室溫時隨著載子濃度的增加，載子的生命週期會同時有縮短的趨勢，呈現的大約是一次方反比的關係，因此得知氮化銦的Auger結合是相對較微弱的。當樣品處於低溫時，由變化激發強度的光激螢光實驗得知，在低溫下載子複合是由發光結合所主導。另外我們利用時間解析光激螢光光譜去觀察不同發光波長之受侷限的載子生命週期，隨著偵測能量的降低可以看到載子生命週期明顯的增加，透過載子侷限理論模型得到侷限深度大約為10毫電子伏特，與前述之淺層授子和Urbach tail的束縛能（5~10毫電子伏特）相近。隨著溫度增加，被侷限的載子可獲得熱能而跳脫侷限使得生命週期變短。到目前為止，還沒有文獻報導氮化銦中授子及Urbach tail 在載子結合的動態扮演的角色。在本論文的研究中，我們觀察到淺層授子和Urbach tail對於發光機制及載子結合的影響。

In this thesis, photoluminescence (PL) spectroscopy and time-resolved spectroscopy were used to investigate the optical properties of InN epilayers. From the Hall measurements, the carrier density of InN epilayers is strongly dependent on the substrate temperature and the material of buffer layer. Blue shifts of absorption edge and PL peak energy are due to the strong Burstein-Moss shift which results from the increasing carrier concentration. Our experimental data are consistent with the theoretical calculation. Typical PL spectra of highly asymmetric line-shape were attributed to the existence of Urbach tail and the shallow and deep acceptor states. Two activation energies were obtained from integrated PL intensity versus inverse temperature plot. The small one is around 5~10 meV and the larger one is about 50~55 meV. These activation energies correspond to the binding energy of the shallow acceptors including the Urbach tail and the deep acceptors, respectively. Pump-probe technique was used to study the carrier recombination dynamics at room temperature. The room temperature carrier lifetime is inversely proportional to the carrier concentration. It reveals that the Auger recombination process does not contribute significantly. At low temperature, the power dependent PL study shows that the radiative recombination process dominates. The carrier lifetime at different emission energy was investigated by time resolved PL. The effect of exciton localization on the carrier recombination was observed. The depth of localization obtained by the theoretical fitting of localization model is about 10meV. Localization depth is very close to the binding energy of shallow acceptors and Urbach tail mentioned above. As the temperature was increased, the localized carriers may escape due to thermal excitation. As a result, the carrier life-time decreases abruptly with the increasing temperature. In this thesis, the influence of acceptors and Urbach tail in the recombination dynamics is shown to be crucial.