應力誘發之雙模態InAs/InGaAs量子點特性

Abstract

本論文使用MBE系統成長出超越臨界厚度的砷化銦量子點樣品，藉由應力鬆弛的方式誘發形成雙模態量子點，並透過光性與電性量測分析來探討雙模態量子點形成的機制與特性。由穿透式電子顯微鏡下所觀察到錯位缺陷和光激螢光光譜中約70 meV的藍移現象，我們可知應力鬆弛過程中所產生的兩群量子點可分為：利用生成錯位差排缺陷來達到應力釋放的低能量量子點與藉由將銦原子往外擴散而使所承受應力減輕的高能量量子點。 利用分析3.06 ML樣品的變溫光激螢光光譜(photoluminescence, PL)可以觀察到兩群量子點間的載子轉換。3.06 ML樣品中低能量量子點隨溫度變化的PL積分強度可分為兩個溫度區間：(1) 當溫度上升至110 K左右，強度有一異常增加的現象直到 (2) 溫度160K時再度下降。而高能階量子點的半高寬在110 K時則會下降，結合兩群量子點隨溫度變化的特性並配合模擬驗證，我們可知在溫度上升的過程中，高能階量子點中的載子會透過兩群量子點間的量子井傳輸至低能階量子點中。在3.3ML樣品的導納頻譜量測中，我們同樣在此溫度區間(78 K~140 K)內發現載子躍遷速率有一轉折，可以證明是由載子轉移所造成。同時利用模擬的方式驗證雙模態量子點的載子躍遷機制。 最後我們透過光激發下的電性量測觀察量子點樣品的內部性值。在不同光能量激發下，載子躍遷速率與光電容有相似的變化，可知載子的躍遷速率與空乏區寬度、摻雜濃度、電場有正比的關係。

In this work we study a method for controlling of the structure of the InAs quantum dots (QDs) with InGaAs capping layer fabricated by molecular beam epitaxy (MBE) deposition. When the InAs deposition exceeds of 3 ML, strain in the InAs QDs is relaxed. At the same time, the bimodal QDs start to form. The characteristics of the bimodal QDs are studied by optical and electrical measurements. The interfaces of the GaAs buffer layer and the InAs QDs contain misfit dislocations as determined from transmission electron microscopy (TEM). The photoluminescence (PL) spectra of the bimodal QDs shows an abnormal PL blueshift of 70 meV. We observe the existence of two types of QDs in the strain-relaxed QDs system: a low energy QD family whose strain is relaxed by the generation of misfit dislocations, and a high energy QD family whose strain is mainly relieved by indium outdiffusion. The effect of interdot carrier transfer on temperature dependent PL is investigated. The integrated-PL intensity of low energy QDs shows two regimes (i) an unusual increment begins about 110 K (ii) and then drops rapidly above 160 K. The full width half maximum (FWHM) of the high energy QDs first decreases about 110 K and reaches a minimum value at about 200 K. The phenomenon can be attributed to the carrier transfer between the bimodal QDs from the high to the low energy QDs through the InGaAs quantum well. Accordingly the carrier emission time determined by G-F measurement exhibits a V-shape versus the similar temperature dependence (78 K~140 K) due to carrier transfer between bimodal QDs in 3.3 ML sample. Based on G-F data analysis, the mechanism of carrier emission in a large electric field is likely phonon-assisted tunneling when temperature increased. Furthermore, we investigate the energy level distribution and defect states in the QD sample by electrical measurements under optical ionization. The variation of electron emission time under illumination suggests the emission rate is proportional to the depletion region, concentration, and electric field.