GaAsN/GaAs量子井中 光激發引致電滯曲線於不同溫度與電場之分析

Abstract

本論文主要探討電滯曲線的成因與變化，由於量子井(quantum well, QW)在照光後藉由外加偏壓可以累積壓降，因此可將QW視為一電容器，當我們對其做不同掃動方向的電容電壓量測(capacitance–voltage measurement, C-V)，可觀察到明顯的電滯曲線開口，又開口大小會與QW累積電荷量有關，因此我們希望能藉由電滯曲線來了解QW的充放電速率。首先利用光激發螢光量測(photoluminescence, PL)與深層能階暫態頻譜量測(Deep level transient spectroscopy, DLTS)，確定摻氮濃度為1.8%的QW能有較好的電子侷限能力，使電子不易逃脫量子井，接下來透過模擬與實驗的比較得知電滯曲線成因，原因為：QW具有與電容器相同的特性，使其在充放電過程皆需要一時間常數τ來達到穩態，造成C-V量測結果呈現反對稱性，進而發生電滯曲線。然而，QW並非能夠無限制的填充電荷累積壓降，QW可能受溫度或電場大小等因素而使得充進QW內的載子逃脫，甚至是激發出的電流源並非能完全流進QW內，因此最後我們探討QW的漏電流如何改變電滯曲線，並藉由改變照光強度、溫度與電場大小，發現由Top GaAs layer提供的電流源越大，能使開口越大，而漏電流越嚴重會導致開口變小，也就是我們能透過這些外在實驗條件，進而調控電滯曲線的開口大小，改變QW的充放電速率。

In this study, we discuss the hysteresis mechanism and how it change. GaAsN/GaAs quantum well (QW) will produce potential drop after illumination, so we can consider QW to be a capacitor. When the voltage was swept in two directions, a clear clockwise hysteresis is observed. The hysteresis opening is related to the quantity of carriers in QW. As a result, we can know the charging and discharging rate of QW through discussing the hysteresis. First, we have studied N=1.8% SQW sample by photoluminescence (PL) and deep level transient spectrum measurement (DLTS). These experimental evidence indicate that QW with good carrier confinement behaves as a capacitor. According to the analysis of the simulation and experiments, hysteresis results from the RC time constant due to the QW charging and discharging rate. Therefore, the hysteresis opening caused by the antisymmetric C-V curve will be influenced by voltage sweeping rate. However, some photoexcited carriers cannot be captured by QW, and the carriers stored in QW will escape through thermal or tunneling process. We must to consider the effects of leakage current for hysteresis. The leakage current will be larger under high temperature or electric field so that the hysteresis opening is reduced. Moreover, top GaAs layer will supply more carriers for QW to enhance hysteresis opening after illumination with high intensity. Thus, hysteresis can be modulated by temperature, electric field and light power.