應力與高濃度摻雜對低溫分子束磊晶生長砷化鎵的砷析出過程之影響

Abstract

分子束磊晶在200至300℃低溫下所生長之砷化鎵或相關化合物薄膜內含許多過量的砷元素 (1~2%) 。在經過600至900℃退火之後，這些過量的砷元素即在砷化鎵晶體中形成析出物，使退火後之砷化鎵呈現了極高的電阻僅以及其他獨有的特性，這些特性均可延伸至各種元件的應用上。因此控制砷析出物的大小及分佈是相當重要的。在本論文中，我們發展出以高摻雜濃度及異質結構兩種方法來控制退火後低溫材料中砷的析出，並且相信這種砷的偏析現象與應力效應有直接的關係。低溫磊晶的微結構性質是以穿透式電子顯微鏡及雙晶Ｘ射線繞射技術來分析。在未摻雜及高濃度摻雜交互成長的低溫砷化鎵（砷化鋁鎵）超晶格結構中，砷析出物會聚集在每一個鈹摻雜層及其後的未摻雜層之間的界面上。此析出過程與摻雜濃度及每一層個別的厚度都有關連。在碑化銦鎵╱砷化鎵之多層量子井異質結構中砷析出物不只出現在砷化銦鎵層裡，在每一個砷化銦鎵╱砷鎵化界面附近都會形成二維的砷析出物陣列。當砷化銦鎵層的厚度超過臨界值時，砷析出物束縛在界面的現象即不再出現。我們提出了應力驅動機制來解釋上述砷析出物的偏析現象。本論文的結果將對低溫材料的研究與未來在元件上的應用提供重要的訊息。

GaAs or related compounds thin films grown by Molecular Beam Epitaxy (MBE) at low growth temperatures (LTs) of 200-300℃ contains a high volume of excess arsenic (1~2 at %). Upon post-growth annealing at 600~900℃, the excess As forms precipitates in the GaAs matrix and the annealed LT GaAs exhibits an extremely high resistivity and other unique properties that can be exploited for various device applications. Therefore the factors controlling the final sizes and distributions of the As precipitates are important. In this dissertation, we develop two methods: heavy doping and heterostnicture to control the As precipitation in post-growth annealed samples and believe that the strain effect have direct correlation with the preferential segregation of As precipitates. The transmission electron microscope and double-crystal X ray-diffraction, technique are used to characterize the layers microstructure property. The preferential accumulation of As precipitates in superlattice structures of alternate-ly undoped and heavily doped (~10^19 cm-3) LT GaAs (AlGaAs) are observed in each interface of Be-doped GaAs (AlGaAs) and the following undoped GaAs (AlGaAs) after annealing at high temperature. The precipitation process in found to be dependent on doping concentrations and thickness of individual layer. In the InGaAs/GaAs multiple quantum well heterostructure, the precipitates are not only confined in the InGaAs wells and further formed two-dimensional precipitate arrays near each InGaAs/GaAs interface. When the thickness of InGaAs layer is beyond the critical thickness, the confinement of precipitates disappears. A strain-driven mechanism is proposed to account for the preferential segregation of As precipitates. Our work provides important information for the study of LT material and their device application in the future.