自聚式InAs量子點上覆蓋In0.14AlAs / In0.14GaAs複合層之研究

Abstract

本篇論文樣品是以MBE(分子束磊晶)長晶，藉由PL光性量測以及電流-電壓(I-V)、電容-電壓(C-V)、導納頻譜(G-T)等電性量測的方式探討在波長1.3μm的InAs/InGaAs dots in well結構中間覆蓋一層不同厚度（0、10、14、20Å）的In0.14AlAs所產生的影響。我們將四片樣品的電容-電壓量測結果換算成縱深分佈後發現，低溫80K時都有兩個載子堆積的峰值出現，且較深的峰值對頻率有響應（1K∼1MHz）；於是我們針對對頻率有響應的偏壓進行導納頻譜（G-T）量測，得到DWELL結構的活化能為57.2±5.1meV（1.4x10-14cm2）；另外我們從PL已知結構具有完整的量子點結構，DLTS在量測範圍內也沒有出現缺陷訊號，所以我們推測訊號可能來自於量子能階；其中較深的峰值來自於基態載子躍遷到激發態然後再從激發態直接穿隧出去，而我們得到載子從基態躍遷到激發態在溫度80K的時間常數約為10-6秒；至於較淺的峰值則可能來自於激發態載子直接從激發態穿隧出去，然而這個過程太快，導致我們一直到溫度30K都量不到載子對頻率的響應，也就是載子在溫度30K時時間常數仍小於10-6秒。 從光性PL量測結果發現當我們在DWELL中間加入一層In0.14AlAs後，量子點的基態跟激發態之間的能量差從78變成103meV；而電性量測更發現隨著In0.14AlAs厚度增加量子點基態到激發態的活化能越來越大，其值從57.2變成95.6 meV；藉由光性與電性量測結果的差異推算出∆Ege落在C.B與V.B上的比例，在DWELL結構是7.3:2.7，對於In0.14AlAs厚度10Å比例則為7.8:2.2。從光性與電性量測我們都發現載子不容易將基態填滿，推測這個原因是由於加了In0.14AlAs抑制了InAs 量子點內部的In segregation的情性，使得量子點密度變大。為了進一步證實這個推論，我們也從TEM (AFM)影像來估計量子點密度，發現三種量測結果很接近，加了一層In0.14AlAs後量子點密度相對於DWELL結構變大了1.5~3倍左右。

The electrical and optical properties of self-assembled InAs quantum dots(QDs) covered by an In0.14AlAs and In0.14GaAs combination layer are investigated by photoluminescence, current-voltage (I-V), capacitance-voltage (C-V), and admittance spectroscopy. Four samples with different In0.14AlAs deposition thicknesses of 0, 10, 14 and 20Å, are grown by molecular beam epitaxy (MBE). C-V profile shows two accumulation peaks at 80K, and the deeper one is frequency–dependent（1K∼1MHz）. From admittance spectroscopy measurement, we determine activation energy of 57.2±5.1meV（1.4x10-14cm2）. Quality of these quantum structures is good since no traps are observed by DLTS﹒ The two quantum peaks of C-V profile are probably originated from the ground and excited state of the QD, respectively﹒The electrons in the ground state are excited to the excited state of the QD then tunnel out of the potential well. The emission time of the electrons from ground to excited state is about 10-6 sec at 80K. On the other hand, the tunneling process for the excited state of the QD to the GaAs conduction band is too fast to be detected even at 30K (emission time<10-6sec at 30K ). By covering a 10Å In0.14AlAs layer, the PL and admittance spectroscopy data show an increase of energy separation between the quantum-dot ground and first-excited state, from 78.0 to 101.4meV for PL and from 57.2 to 79.1meV for admittance spectroscopy. From these results, the ratios of energy separation between the quantum-dot ground and first-excited state in conduction band and valence band are determined to be 7.3:2.7 for DWELL structure and 7.8:2.2 for the sample with 10Å In0.14AlAs, respectively. The electrical, optical and TEM(AFM) results show that covering the 10Å In0.14AlAs layer can increase the InAs quantum-dot density by 1.5 to 3 times. This result might be due to the effect of the reduction of indium segregation caused by the In0.14AlAs layer.