硒化鋅/碲化鋅半導體系統激子性質之研究

Abstract

本文主要是利用光激螢光頻譜 (photoluminescence, PL)與時間鑑別光激螢光頻譜(time resolved photoluminescence, TPRL)研究硒化鋅/碲化鋅(ZnSe/ZnTe)半導體系統在碲化鋅沉積厚度為2.0ML、2.5ML、3.0ML(ML為mono layer，為一個ZnTe原子層的厚度，約略為 )的光學性質與激子生命期(exciton lifetime)。其中 2.0 ML的樣品為只包含有碲化鋅濕層的樣品而2.5ML、3.0ML 的樣品則為同時包含有碲化鋅濕層及量子點的樣品。我們發現 2.0ML的樣品其生命期有隨溫度上先升會升後降。先上升的原因主要是因為表面粗造(surface roughness)使得在原侷限在較低位能處的的激子隨著溫度升高變為自由激子而使激子復合時間變長進而使生命期升高。在高溫時激子生命期下降的原因主要是因為激子與聲子(phonon)的交互作用增加使得激子游離成自由電子與電洞的機率增加而使得其生命期下降。我們發現在2.5ML的樣品中，激子復合幅射強度衰減速率最高可達2.0ML的兩倍.其原因為激子從濕層(wetting layer)不斷擴散至量子點使得量測到量子點中的激子復合幅射強度衰減速率變慢。而3.0ML量子點因為沉積厚度過厚，缺陷較多使得螢光強度與生命期相較2.0ML、2.5ML量子點來的微小許多。我們利用光激螢光強度隨溫度變化計算的活化能，與利用非輻射結合生命期隨溫度變化所得活化能數值相近，此活化能可被確認為硒化鋅/碲化鋅系統激子之束縛能。

Photoluminescence and time-resolved photoluminescence spectroscopies were used to study the properties of exciton in type-II ZnSe-ZnTe semiconductor heterostructures. The samples used for this study include a 2 monolayers (MLs, one ML is about 3.05A for ZnTe) sample that has 2MLs ZnTe wetting layer deposited on the ZnSe layers and a 2.5 sample that contains both the ZnTe wetting layer and ZnTe self-assembled quantum dots . The lifetime of the exciton for the 2.0ML sample was found to increases with increasing temperature at low temperature and then decreases when the temperature is above 70K. The initial increase of the exciton lifetime is attributed to the delocalization of exciton trapped by surface roughness and the decrease of the exciton lifetime at elevated temperature is attributed to the increasing electron-phonon interaction that leads to the dissociation of exciton. For the 2.5 MLs sample that contain both quantum dots and the wetting layer, the life time of the quantum dot related exciton can be twice longer than that of the lifetime of the exciton in the wetting layer observed for the 2MLs sample. Diffusion of exciton from the wetting layer to the quantum dots were observed directly for this sample and is used to explain the slower decaying rate of the PL intensity in this sample. The activation energy obtained from the variation of PL intensity with temperature is closer to the one obtained from the variation of the non-radiative life time of exciton with the temperature and could be identified as the binding energy of exciton of ZnSe/ZnTe system.