分子束磊晶成長硒化鎘量子點之成長機制 及光偵測器特性分析

Abstract

利用分子束磊晶系統成長不同原子層厚度之硒化鎘量子點樣品，從樣品表面形貌圖中觀察，發現量子點密度隨著原子層厚度增加先提升而後下降，量子點尺寸則是隨原子層厚度愈趨增加。從低溫光激螢光光譜中觀察，由於量子點尺寸對量子侷限效應之影響，發現隨著原子層厚度增加，量子點光激螢光峰值位置有紅移之趨勢，其譜峰值半高寬亦隨著量子點成長模式轉換而有變化。從變溫光激螢光光譜中觀察，發現譜峰值位置隨溫度變化呈S型走勢，譜峰值半高寬值也隨著溫度增加而顯著提升，以Varshni曲線作擬合，發現其S形走勢能以載子隨溫度上升獲得熱能使其在量子點中兩不同能態間重新分布之機制作解釋，比較硒化鎘薄膜近帶複合發光及束縛激子複合發光之能量，發現其能量差值與Varshni擬合曲線能態之能量差值十分接近，證實載子隨溫度增加獲得足夠熱能脫離量子點自由激子能態至束縛激子能態之重新分布現象。接著在硒化鋅覆蓋層及硒化鋅緩衝層摻雜氯，並在樣品表面鍍上鈦金電極，形成簡易光偵測器。發現電流-電壓曲線之階梯狀轉折可歸因於不同的侷限能態所貢獻的載子傳輸。在開光光照少之下，光電流值出現先隨時間遞減而後趨於穩定值，主要可能由於光激發載子受缺陷所捕捉。

CdSe quantum dots (QDs) of different coverage were grown by molecular beam epitaxy. From the investigation of surface morphology, the QD density increases initially with the increasing coverage and then decreases. While the size increases with the increasing coverage. The low temperature photoluminescence (PL) shows that the PL peak energy exhibits red shift with the increasing coverage due to the decreasing quantum confinement with the dot size. In addition, the full width at half maximum (FWHM) varies with the coverage due to transition of growth mode. The PL peak energy versus the temperature relation shows the “S” shape and could be fitted by the Varshini formula. The FWHM also increases abruptly with the increasing temperature. This phenomenon could be understood by the competition between two emission mechanisms, bound exciton emission and free exciton emission. The energy difference matches the result obtained from the CdSe thin film. In order to fabricate the photodetectors, the ZnSe buffer layer and capping layer were doped with Cl and Ti electrode was deposited on the sample surface. The I-V curve shows stair-type relation due to the transport of carriers from different confinement states of QD. Under the light on and off experiments, the photo-current decreases initially with time and reaches a steady value. It could result from the trap of phot-excited carriers by the defects.