利用暫態理論與時間解析量測分析光激發下GaAsN/GaAs量子井結構之載子暫存機制

Abstract

本論文主要針對MBE所成長的N=2.7% 80 Å GaAsN/GaAs量子井結構樣品的暫態電流(Transient current)及暫態電容(Transient capacitance)進行分析，進而探討GaAsN量子井在持續照光下瞬間中斷外加激發光源引致的電荷暫存現象。首先，從光激發螢光量測光譜圖(PL)可以發現，未經熱退火的樣品有兩個主要訊號，分別是高能量位置1.16 eV的GaAsN 量子井訊號，以及由深層能階(Deep-level defect)與量子井復合所產生的1.03 eV的低能量訊號。此深層能階訊號的來源為GaAs本身VGa (Gallium vacancy)的點缺陷(Point defect)。由GaAsN量子井樣品進行瞬間照光後又瞬間關光的暫態電容量測發現，瞬間外加光激發源時，光電流會對GaAsN量子井進行充電而產生壓降，使得暫態電容值遞升；當瞬間中斷外加光源後，暫態電容值以指數型式遞減，此為量子井進行放電過程而壓降減小所致，利用暫態測量可測得量子井的充電與放電時間常數約為秒的等級。我們亦發現了以瞬間照光及中斷光源的實驗方式，對於未經熱退火的量子井樣品是測量不到充放電的行為，故推測對於未經熱退火的量子井而言，大部份由光激發產生的電子/電洞對選擇了深層能階作為復合路徑，以產生螢光(PL)的形式消失，也就是說熱退火(RTA)可減少樣品中的點缺陷，有助於提升GaAsN量子井電子能階訊號的載子侷限能力(Carrier-confinement ability)。此外，固定偏壓下中斷照射光源時，其電容值竟回不到未照光前的電容值大小，這是因為當光源中斷時量子井內部仍有少量的電荷儲存，且外加偏壓越大時量子井內可儲存的電荷量越少。這樣的一個結果提供了元件設計時的具體想法，或許可以提出相關機制，再應用於科技產業，作為光記憶元件(Optical memory device)的研發。

In this study, we analyze the transient current and transient capacitance measurement of the N=2.7% 80 Å GaAsN/GaAs quantum well (QW) samples grown by MBE, and discuss the dynamic processes of carrier storage in QW for the change of light situation. At the moment of light-on suddenly, the QW is charging by the photocurrent, and a potential drop of charged QW is producing. So the measured capacitance of depletion increases with the potential drop of charged QW. By contrast, the QW is discharging at the situation of light-off, and the measured capacitance of depletion decreases with time. However, we can’t detect the charging process of QW for the as-grown sample in the transient measurements. The optical properties of as-grown sample are studied by photoluminescence (PL). An additional peak (1.03 eV) can be observed, which is due to the recombination between the deep level defect and QW. Most of electron-hole pairs in QW will recombine with deep level. Therefore, the as-grown QW behaves poor carrier confinement. Rapid thermal annealing (RTA) can reduce point defect effectively and enhance the carrier confinement ability in QW. In some case of light-off, the measured capacitance of depletion behaves a persistent capacitance phenomenon. The persistent capacitance is caused by that some carriers are still confined in QW after light-off, and the amount of stored carriers can be controlled by applied reverse bias. This mechanism of written by light and erased by reverse bias provides an idea for optical memory device.