應力鬆弛引發之缺陷效應下的InAs/InGaAs量子躍遷機制

Abstract

本論文主要是藉由光性及電性的量測，包括光激發螢光頻譜(PL)、電容電壓(C-V)、導納頻譜(C-F&G-F)、深層能階暫態頻譜儀(DLTS)及暫態電容(transient)的量測、還有TEM表面分析技術，來探討InAs/InGaAs這種DWELL結構的量子點在應力鬆弛後所展現的特殊現象，主要著重在量子躍遷機制的改變。樣品是InAs量子點成長3.3 ML的厚度，並予以熱退火650℃和700℃1分鐘作為進一步研究。 由TEM得知此3.3ML的樣品因為量子點成長超過臨界厚度，應力鬆弛後在量子點內部及底層產生差排缺陷。而DLTS的量測顯示此缺陷捕捉載子濃度小於TEM觀測到的缺陷濃度及量子點濃度，表示缺陷不足以完全空乏量子點中的電子；且與2.7 ML的樣品比較下，3.3 ML樣品的PL頻譜在低溫仍有很明顯量子點訊號存在，顯示應力鬆弛並未完全破壞量子點特性，量子躍遷仍可被觀測到。 在量子點區域附近隨著偏壓的加深，C-F高溫量測到活化能從0.091 eV變化到0.213 eV一系列寬的能帶頻譜，表示載子分別依序由較淺的激發態和較深的基態跳出，而C-V量測所轉的縱深濃度圖也估算出載子至少填滿到第一激發態的QD能階，且此能帶頻譜的寬度與PL頻譜之FWHM是可比擬的，這些初步證明了C-V和C-F所量的量子訊號。 C-F的分析顯示：電子躍遷在高溫是熱激發跳躍，而低溫載子熱能不足，被迫以穿遂形式出去。由低溫穿遂時間帶入公式所擬合的能障(barrier height)，與高溫活化能差不多相符，高溫活化能亦與低溫能障對穿遂時間的關係式相符，這啟發了載子在高溫是直接跳上GaAs導帶的說法，而此論述也由捕捉截面積所估算出載子穿遂前的能障來支持。由縱深濃度圖來看，正常無缺陷的量子點是由基態躍遷至激發態再穿遂出去，相較於應力鬆弛而在下方GaAs引發缺陷的樣品，後者產生很大的載子空乏區，增加了空乏的寬度，阻止量子點中的電子穿遂到下方GaAs層，所以高溫才會直接跳到GaAs導帶。 熱退火處理樣品的比較是另一個主要的研究重點，發現底層缺陷產生之空乏效應仍存在，影響著C-V與C-F的量測結果。由C-F量測結果得知，熱退火會減低低溫穿遂時間與高溫活化能，與PL藍移結果相符，這是由於原子在量子點界面的擴散效應造成的量子點能階提升，實驗結果也更加證實C-F量測數據的來源是量子躍遷。

We investigate the properties of strain relaxed InAs/InGaAs dot-in-well (DWELL) quantum dots (QDs) by optical and electrical measurement. This research emphasizes the mechanism of the electron emission from the QDs containing a misfit defect state. The QD samples are grown by molecular beam epitaxy (MBE) with and without rapid thermal annealing (RTA). Because the InAs thickness of the 3.3 ML sample exceeds the critical thickness, strain relaxation is observed to introduce misfits in the QD and neighboring GaAs bottom layer. The DLTS spectra show the concentration of the defect state is not high enough to completely deplete the electrons in the QD states. Besides, the photoluminescence quality for 3.3 ML sample is comparable to that of the non-relaxed 2.7 ML sample, and the quantum emission (Q.E.) in the 3.3 ML sample can be measured, suggesting that relaxation doesn’t degrade the QD. From C-F measurements, electron emission from the 3.3 ML QDs exhibits a relatively long emission time with a very broad energetic spectrum from 0.091 to 0.213 eV due to the depopulation of the QD first excited and ground states. The spectral broadness is also comparable to the broadness of the PL spectrum. Moreover, from the area under the peak of the depth profiles, electrons are filled up at least to the QD first excited state. From C-F analysis, electron emission from the QDs shows a thermal emission at high temperatures, and because of the insufficient thermal energy, the tunneling emission prevails at low temperature. The energy barrier height evaluated from the tunneling time is consistent with the thermal emission energy which also agrees with the formula for the tunneling barrier versus tunneling time, suggesting that electrons are thermally activated from the QD states to the GaAs conduction band. Furthermore, the relationship between the capture cross section observed from the thermal emission at high temperatures and electric field yields a negligible triangular barrier height seen by the electrons before tunneling. In contrast to defect-free QD samples which show a thermal activation from the QD ground state to the first-excited state and then tunneling to the GaAs conduction band, the relaxation-induced defects create additional carrier depletion which significantly increases the depletion thickness in the neighboring bottom GaAs layer, preventing the electrons in the QDs from tunneling to the bottom GaAs layer. As a result, electrons escaping out of the QD states would have to occur via a thermal emission to the GaAs conduction band at high temperature. The results of the C-V and C-F show that the effect of tunneling suppression due to the additional carrier depletion still exists after annealing 650℃ and 700℃. RTA is found to decrease the electron-emission time and emission energy, consistent with the optical blueshift due to the interdiffusion of atoms across the QD interface.