InAs量子點中缺陷效應影響之量子躍遷機制

Abstract

本論文利用電性量測去探討InAs量子點在應力鬆弛下所引發的缺陷，對於電子躍遷機制的影響，利用改變ac訊號的導納頻譜(C-F)於低溫之量測，再配合與理論曲線之擬合，可以推得在基態的低溫穿遂能障會等於高溫活化能，從而證明電子由應力鬆弛量子點躍遷之機制為高溫熱激發形式，低溫為穿隧機制。 利用C-F高溫量測3.3 ML InAs 量子點，我們可以得到捕捉截面積(Capture cross section)，對應約四個電子填入量子點中，捕捉截面積也由10-16 to 10-20 cm2減少，我們將此效應歸因於庫侖排斥力，一個電子填入量子點約略會減少捕捉截面積一個數量級。我們與正常的量子點作比較，發現應力鬆弛對於量子點之捕捉截面積大約會減少兩個數量級，考慮到缺陷能階在空間上位置處在量子點旁邊，推測捕捉截面積數量級減少是由於缺陷能階中之電子影響所致。 在C-F低溫量測中，我們在120~140 K發現電子躍遷速度對應溫度升高，有不尋常的減少之現象，我們將此歸因於在兩群不同的量子點間有載子轉移的現象，使我們量測到不同的電子躍遷速度。與PL頻譜(Photoluminescence spectra)相比較，可推得這兩群不同的量子點分別為鬆弛量子點與未鬆弛量子點，PL頻譜顯示，在低溫時，鬆弛量子點具有很大的強度，而隨著溫度升高，其強度卻變弱，因此，可以印證在C-F量測上看到的不尋常現象為兩群量子點間的載子轉移所造成。 此外，我們發現了在光激發下的量子躍遷行為，在電容電壓(C-V)量測上，可以看到在光照射下電容平台會跟著增加；而由C-F量測，我們發現電子由量子點躍遷速度在光照射下明顯變快。由以上量測可以推測在光照射下有更多的電子注入至量子點中，由於空乏區中之電子數量增加，使得多餘的電子注入至量子點中所導致。

Electron emission mechanism from InAs quantum dots (QDs) containing a misfit-related defect state induced by strain relaxation is investigated by electrical measurements. By changing oscillation level in the Capacitance-Frequency (C-F) measurement at a low temperature and fitting with a theoretical curve, we find that the tunneling barrier height is equal to the thermal activation energy in the ground state. Therefore, Electron escaping from the QDs shows a thermal activation process at high temperatures and a tunneling emission at low temperatures. The capture cross section for relaxed QDs grown with InAs deposition thickness of 3.3 ML, obtained from the thermal emission in C-F measurement, is decreased from 10-16 to 10-20 cm2 with increasing electron occupation to about four electrons in each QD. This effect is explained by a Coulomb repulsive force which averagely reduces the capture cross section by one order of magnitude with increasing one electron occupation in the QDs. A comparison with non-relaxed QDs shows that strain relaxation can reduce the capture cross section of the electron ground state by about two orders of magnitude. In view of the defect state which is spatially localized near the QDs, we attribute the capture cross section reduction to a Coulomb repulsive effect due to the electrons trapped in the defect state. From C-F measurements, the electron emission rate from the 3.3 ML QDs exhibits a unusual reduction with increasing temperature at about 120~140K, which is attributed to a carrier transfer between two different groups of QDs with different emission rates. Comparing with Photoluminescence (PL) spectra, the two groups of QDs are considered to be the relaxed and non-relaxed QDs. The PL spectra show a strong intensity for relaxed QDs at low temperature, and as temperature increases, the intensity becomes weaker. Hence, the unusual phenomenon in the C-F measurement reflects the carrier transfer between the two groups of QDs. Furthermore, the electron emission of the 3.3 ML QDs under illumination is investigated. The value of the capacitance plateau in the C-V spectra has increased during illumination and, from the C-F measurement, the electron emission rate become faster during illumination. These observations suggest that more electrons are injected into the QDs upon illumination, and the amount of electrons in the depletion region are increased, pushing additional electrons into the QDs.