InAs量子點掺入氮之電子放射與捕獲機制研究

Abstract

本篇論文主要是探討摻氮的砷化銦（InAs）自聚式量子點之光電特性研究，樣品的製程方式是用分子束磊晶系統在砷化鎵（GaAs）的基板上成長。在室溫下量測光激發螢光頻譜（PL）可顯示有一低能量訊號存在。電容─電壓（C-V）量測結果知道InAs量子點摻入氮可以使電子放射出來的時間變長，時間常數大約E-3~E-5秒。深層暫態能譜（DLTS）量測電子放射的訊號發現兩個峰值，其中一個峰值是量子點訊號活化能約0.2 eV，而另一個為缺陷訊號活化能大約0.7 eV。量子點訊號出現在約200 K且當電子的佔據數增加時，峰值會往低溫移動並飽和，類似於能帶填充，而這個能帶的平均範圍約0.18∼0.29 eV。DLTS量測訊號最強時候的活化能大約在0.21 eV，將此時的活化能對應到PL光譜電子放射強度最大時的波長（1200 nm）。而DLTS可以量到活化能較深能階的能量（約0.36 eV）應該與PL光譜低能量訊號有關。因此我們認為DLTS量到的活化能範圍從0.19∼0.36 eV可以對應到PL光譜量子點的放射訊號。電子要注入到量子點必須克服一個捕捉位能障，它的高度和電子從量子點放射的位能障高度差不多。當量子能階的電子佔據數愈多可以得到捕捉位能障的高度愈小。施加逆向偏壓能空乏樣品內的電子，一旦量子能階中的電子全都被空乏之後，再增加逆向偏壓可以空乏缺陷能階內電子。當缺陷內的電子開始被空乏時，我們觀察到量子點的位能障高度變小。缺陷內的電子被空乏愈多時，得到的量子點位能障高度會愈小。這結果顯示量子點的位能障高度會受到缺陷內電子數量影響，而電子在灌入缺陷之前會先注入量子點內，之後再回到缺陷能階。

In this thesis, electronic and optical properties of nitrogen (N) incorporation into self-assembled InAs quantum dots (QDs) grown on GaAs substrate by molecular beam epitaxy are investigated. Photoluminescence (PL) spectrum exhibits a low energy tail at room temperature. Capacitance-voltage (C-V) profile shows a long time constant about E-3~E-5 second for the InAs QDs with N incorporation. Deep-level transient spectroscopy (DLTS) spectra reveal two signals one is the QDs state about 0.2 eV, and the other is the defect level about 0.7 eV. The shallow emission is attributed to the QDs states since increasing electron occupation of the dots shifts the emission peak towards a lower temperature, suggesting a band-like filling of the state ranging from 0.18 to 0.29 eV. The maximum DLTS signal is observed at Ea = 0.21 eV which can be correlated with the PL peak at 1200 nm. And the wide activation energy extending to 0.36 eV can also be correlated with PL low-energy long tail. Therefore, we believe that the shallow level from 0.19 to 0.36 eV corresponds to the PL QDs emission. Injecting electrons into the QDs needs to overcome a capture barrier whose value is close to that of the emission barrier. A smaller capture barrier is observed for the QDs with higher electron occupation. Once the QDs are completely depleted of electrons, further increasing the reverse bias would start to deplete the electrons of the defect state. We observe a smaller capture barrier of the QDs when we start to deplete the electrons of the defect state. Further depleting the electrons of the defect state can further decrease the capture barrier of the QDs. This result shows that the capture barrier of the QDs is affected by the number of the electrons trapped on the defect state and electrons are injected into the QDs before they relax to the defect state.