磊晶薄膜移植技術應用於光電元件之研究

Abstract

本篇論文之目的在於探討磊晶薄膜移植技術應用於光電元件時，所面臨的 一些重要課題.在這裡我們針對三個主要的課題進行探討：分別是移植薄 膜的物理性質之研究，低電阻垂直式電流導通的探究，以及實際的元件應 用.在移植薄膜的物理性質之研究部份，我們分別針對薄膜厚度對移植薄 膜的光學及結構性質之影響和移植過程對少數戴子元件電性之影響兩個主 題作詳細的探討. 本論文中我們以異質接面雙極性電晶體(HBT)作為少數 戴子元件的代表.關於結構性質的探討，我們主要是利用高解析度雙晶體 Ｘ光繞射(DCXRD)技術作鑑定；而光學性質的研究則利用變溫式光螢光( PL)光譜來鑑定.我們發現當把砷化鎵薄膜移植到矽基板上時，如果緩衝層 的厚度薄於1000nm，會有一個雙軸式壓縮應變的存在.這個應變主要是由 於移植薄膜與新基板間晶格大小差異所致.而由於移植薄膜與新基板間熱 膨脹係數差異的存在則會造成移植薄膜上砷化鎵/砷化銦鎵量子井的低溫 螢光譜相較於未移植前有往較長波長移動的現象.且此波長移動量也跟薄 膜厚度有關，即較薄的移植膜有較大的波長移動量.另外我們也針對相同 結構分別移植在矽基板，磷化銦基板及砷化鎵基板作低溫螢光光譜的比 較.在少數戴子元件電性影響研究方面，我們主要利用先作元件製程再作 薄膜移植以及先作薄膜移植再作元件製程兩種薄膜移植枝術對異質接面雙 極性電晶體移植於矽基板或磷化銦基板上來作移植前後的特性比較實驗結 果顯示不管是利用上述兩者之一技術作電晶體薄膜移植，其特性在移植後 並無任何退化現象這結果同時顯示出異質接面雙極性電晶體的薄膜品質及 電性並不因薄膜移植過程而受影響而針對一電流截取值(current gain)高 達550的元件作移植時，我們發現移植電流截取值並未改變,而且，此電流 截取值的大小將視原異質接面雙極性電晶體結構設計而定，並不受移植技 術所限.在垂直式導通行為的研究方面，我們成功地藉由先在矽基板上鍍 上Pd/Ge/Al或Pd/Ge/Pd等金屬複合層，再經由對移植砷化鎵薄膜.金屬層 和矽基板三者間作加熱退火處理，而得到一個垂直跨越移植薄膜與矽基板 的低電阻電流導通路徑而且所用的熱處理條件也同時促成了在界面處較強 金屬鍵結的形成同時我們也針對界面反應後形成的化合物作二次離子質譜 分析及ｘ光繞射分析等材料分析. 至於元件應用方面，除了上述的異質接面雙極性電晶體的移植研究外，我 們分別針對雷射二極體及反射式量子井光調變器作元件薄膜移植研究. 在雷射二極體元件薄膜移植的研究，我們主要是利用上述垂直式導通的特 性來製作一個具n型導通金屬於矽基板背面的扭曲層量子井薄膜雷射二極 體在未經封裝的情況下，移植雷射二極體可分別在脈衝(pulse)或連續波( cw)條件下操作，雖然移植後特性略微退化，但是藉此我們證實了垂直導 通的可行性.同時，由熱應力的影響使移植雷射二極體的發射波長對溫度 的敏感度反而獲得改善. 在反射式量子井光調變器元件薄膜移植的研究方面，我們主要是針對將反 射式量子井光調變器移植到事先鍍上各種不同反射面的矽基片上作特性的 比較及研究我們發現當針對相同之砷化鎵/砷化銦鎵多層量子井結構，利 用5週期層的矽/氧化鋁作為末桲僁鼓滬I向反射面及2週期層的砷化鋁/砷 化鋁鎵作為正向反射面可得到55%反射變化率及6V操作電壓的良好特性. The purpose of this dissertation is to study some important issues for ELOtechnique which is applicable in optoelectronics. Three primary issues are studied: the physical properties of ELO films, investigation of vertical conduction behavior, and device applications. In the study of physical properties, two subjects are studied. One is the influenceof film thickness on the optical and structural properties of ELO thin films. Theother is the influence of ELO process on the electrical properties of minority carrier device. Heterojunction bipolar transistors (HBT) stand proxy for the minority carrier device in our research. The structure was characterized by high-resolution double- crystal x-ray diffraction and the optical properties were measured by the temperature-dependent photoluminescence spectroscopy. A biaxial compressive strain was observedfor the samples bonded to Si with a buffer layer thinner than 1000nm. Due to different thermal expansion coefficient between the grafted thin film and the host substrate, the emission spectra of the quantum wells of the lifted-offthin films are red shifted compared to the as grown sple. The amount of the redshift is larger for thinner films. The ELO films bonded to Si, InP and GaAssubstrates have also been compared. Both preprocessed and postprocessed ELO HBT's on Si or InP were demonstrated.The characteristics of those HBT's on Si or InP with either technique show nearlyidentical characteristics of the HBT's on GaAs without transplantation.It indicates that the film quality and electrical properties of such a minority carrier device are maintained after transplantation. Deviceswith a high current gain of 550 were transplanted without any degradation, and the current gain is not limited by the ELO process. In the investigationof vertical conduction behavior, low-resistance ohmic conduction acrossan Epitaxially Lifted-Off (ELO) thin n-GaAs film and a Si substrate was obtainedby attaching the ELO film on the Si substrate coated with a Pd/Ge/ Al or a Pd/Ge/Pd multilayer. Good bonding and ohmic contacts to both GaAs and Si were achieved at the same time after annealing. The interface compound formation wasstudied by SIMS and XRD analyses. As for the device application, except the demonstration of ELO HBT's onto Si or InPsubstrate, ELO laser diodes (LDs) on Si and ELO Asymmetric Fabry-Perot modulators(AFPM) on Si were also demonstrated. For ELO LDs, we use the vertical conduction characteristics mentioned above to fabricate a ELO stripe geometry diode laser on Si with the back-side contact on Si substrate. Both conventional and ELO LDs were measured under pulse orcontinuous-wave (CW) operation without package. The emission characteristics ELO LDs did not degrade too much, and the temperature-dependent wavelength sensitivitywas even improved. For the ELO AFPM on Si, we present and compare the modulationcharacteristics of normally-on AFPM achieved by Epitaxially Lifting-Off(ELO)the GaAs/AlGaAs multiple quantum well structures onto a Si substrate coated with different types of external mirrors. A 40 periods multiple quantum well modulator with a five periods of Si/Al2O3 quarter wave stack as the back reflector and a two periods of AlAs/Al0.Ga0.9As quarter wave stack as the frontreflector showed excellent on/off characteristics with a 55% reflectivity changefor only a 6V voltage swing.