高銦氮化銦鎵薄膜之成長與特性

Abstract

本論文中，我們使用有機金屬化學氣相沉積(MOCVD)系統成長兩個系列之氮化銦鎵(InGaN)薄膜，一為變溫成長系列，另一為變TMIn莫耳流率系列樣品。在變溫成長系列中，我們將磊晶溫度從750℃降低至650℃，InGaN薄膜銦含量呈現明顯變化之趨勢，由低銦組成0.18增加至高銦組成0.40，值得注意的是，當磊晶溫度高於700℃時，光譜上呈現兩個波峰的存在，經由倒置空間圖與陰極激發光譜深度分析證實上述高、低能量波峰分別來自於下層之應力層及上層之鬆弛層所致。700℃以下之樣品，由於銦組成成分較高，與氮化鎵緩衝層晶格不匹配較大，整個InGaN薄膜幾無應力層存在，故皆呈現單一波峰螢光光譜。光激螢光光譜的峰值能量亦隨著磊晶溫度下降隨之下降，鬆弛層之譜峰能量由2.44eV下降至1.68eV。 為了進一步獲得較長發光波長之InGaN薄膜，我們隨之在650℃磊晶溫度，調變TMIn莫耳流率，探討In/III族氣相與固相組成之相互關係。當TMIn莫耳流率從∼3.2 μmol/min 增加至∼16.0 μmol/min，銦含量由0.16增加至0.44，氮化銦鎵薄膜發光波長可進一步延伸至950nm，且X光繞射頻譜証實整個晶體皆為單相狀態。一旦TMIn莫耳流率增為22.3 μmol/min，薄膜表面即析出大量之金屬銦顆粒，同時InGaN薄膜之銦組成亦呈現驟降之現象，此顯示銦顆粒之析出並不利於高銦組成InGaN薄膜之成長。 從吸收光譜得知高組成0.44 InGaN薄膜之能隙與螢光光譜峰值的能量差將近500 meV。透過變溫光激螢光光譜，量測光譜峰值隨溫度變化呈現兩大特點，一為光譜峰值的轉折點靠近室溫位置，另一為紅移能量為70meV。若考慮現有的能帶尾端(band tail)之侷限態解釋，以氮砷化鎵為例，其光譜峰值的轉折點約在50K，紅移能量僅5-10meV，去侷域化的溫度位置約在150K。這些強烈對比使得我們考慮在InGaN薄膜內具有強烈的侷限化效應。因此我們引入一熱激發遷移模型(thermal activation and transfer model)，定量的擬合光譜峰值隨著溫度變化的行為。從擬合結果可知侷限態分佈寬度σ約30-50meV，然而上述能隙與螢光光譜峰值的能量差將近500 meV，因此我們認為富銦的簇(In-rich clusters)主導氮化銦鎵薄膜的發光行為，而造成侷限化的主因為InGaN薄膜存在微結構紊亂(disorder)。

In this thesis, two series of InGaN epilayers were grown using metal organic vapor phase deposition (MOCVD) system. The one is grown at different epitaxial temperatures；The other one is grown at different TMIn flow rates. In the series of samples growing at different epitaxial temperatures, the epitaxial temperatures decreased from 750℃ to 650℃. The In content shows significant changes in the trend, increasing from low In content 0.18 to high In content 0.40. It is noteworthy that when the epitaxial temperature is higher than 700℃, the PL spectra shows the presence of double peaks. We confirms that the above high and low energy peaks comes from strained phase in the lower part and relaxed phase in the higher part by using the X-ray reciprocal space mapping (RSM) and depth resolved Cathodoluminescence (CL). When the epitaxial temperature is lower than 700℃, the lattice mismatch is larger owning to the high In content, leading to almost relax in the whole InGaN film. Therefore, the PL spectra shows single peak. The PL peak energy of the relaxed layer also decreases from 2.44eV to 1.68eV with decreasing epitaxial temperatures. In order to obtain a longer emitting wavelength of InGaN epilayers, we subsequently followed the epitaxial temperature at 650℃. TMIn molar flow rates modulation of growth of InGaN films is conducted to discuss the composition relationship between the In/III gas phase and solid phase. When the TMIn molar flow rate is increased from∼3.2 μmol/min to ∼16.0 μmol/min, the In content is increased from 0.16 to 0.44. The emission wavelength of InGaN film can be further extended to 950nm and all samples showed single phase in the XRD (002) θ-2θ patterns. Once the TMIn molar flow rate is increased to 22.3 μmol / min, the precipitation of a large number of In droplets appears on the surface of the InGaN film, simultaneously the In content of the InGaN film shows a deep decrease. It is indicated that the precipitation of In droplets is not conductive to grow the high In content of InGaN films. The energy difference between the bandgap obtained from the absorption and PL peak is almost 500 meV. Through the temperature dependent PL, we didn’t observe the S-shape behavior of the PL peaks as the temperature increases. This contrast allows us to consider that there is a strong localized effect in the InGaN film. Therefore, we introduce a thermal activation and thansfer model, fitting the temperature-dependent behavior of the PL peaks quantitatively. Fitting results suggest that the localized state distribution width σ is about 30-50meV. Therefore, we believe that the fluorescence emission comes from localized states caused by In-rich clusters and it is attributed InGaN film to exist microstructure disorders.