InAs量子點應力鬆弛所引發缺陷對量子躍遷之影響

Abstract

本論文主要利用光激發螢光頻譜(PL)、電壓-電容(C-V)、導纳頻譜(G-F)、深層能階暫態導纳頻譜(DLTS)等電性量測、光性量測及TEM表面分析技術去探討InAs/InGaAs 量子點在應力鬆弛狀態下所引發的缺陷，對於量子躍遷所造成的影響。研究樣品分別為三片不同厚度的InAs量子點分別為2.0 ML、2.3 ML及3.3ML，量子井(InGaAs)厚度均為60Å。 由實驗上得知當InAs量子點超過一臨界厚度(介於2.7~3.0ML之間)，會發生晶格鬆弛而產生差排缺陷(misfit dislocation)。從DLTS、TEM、C-V量測上，我們發現到位於底層GaAs的差排缺陷，會使量子點區的載子放射時間拉長，因為在另一片2.8 ML InAsSb 量子點鬆弛樣品，觀測到其量子躍遷無頻率響應情形，而從TEM上得知此樣品的缺陷分佈只有在量子點內部與底層GaAs，代表這些缺陷對於穿遂效應完全無抑制作用，所以只有當缺陷分佈於底層GaAs才會抑制穿遂效應，而其機制為差排缺陷會在底層GaAs處形成載子空乏區，或對載子產生散射效應，而抑制載子直接從量子點激發態穿遂至底層GaAs導帶，導致載子是從量子能階受熱激發躍遷至GaAs導帶，增加其躍遷時間。另外，從3.3 ML 應力鬆弛量子點 DLTS量測所獲得此缺陷捕捉載子的濃度比較於從TEM量測所得到的差排缺陷濃度小兩個數量級，這暗示著此差排缺只有百分之一為active trap。利用此缺陷拉長量子躍遷時間的作用，可以從G-F與C-V量測中，獲得躍蹮能量0.160 eV與0.068 eV分別為量子點基態與激發態相對於GaAs導帶的侷限能量﹔相對於沒有應力鬆弛的樣品，只能觀察到一對應於量子點基態躍蹮至第一激發態60 meV的放射能。為了更進一步了解缺陷對於量子躍遷的影響，我們把這鬆弛樣品做700℃ 1min.熱退火，我們發現到熱退火後，缺陷載子的躍遷時間常數變短，推測是因為In與Ga之間的擴散使得量子點能帶結構改變而造成。

The electrical and optical properties of relaxation-induced lattice misfits and their effects on the electron emission of InAs QD are investigated by photoluminescence (PL), current-voltage (CV), admittance spectroscopy, deep-level transient spectroscopy (DLTS), and cross-sectional transmission electron microscopy (TEM). Three samples with different InAs deposition thicknesses of 2.0, 2.3, 3.3 ML are grown by molecular beam epitaxy (MBE). From previous experiments, when the InAs thickness exceeds the critical thickness (between 2.7 and 3.0 ML), strain is relaxed by introducing misfits in the QD and neighboring GaAs bottom layer. In a relaxed 3.3 ML sample, capacitance-voltage profiling shows an electron accumulation peak at the QD with a long emission time, followed by additional carrier depletion caused by the misfits in the GaAs bottom layer. By comparison with anther 2.8 ML relaxed InAsSb QD which displays no emission-time increase because the relaxation-induced misfits are not located in the GaAs bottom layer but are located in the QD, the emission-time lengthening in the relaxed sample is explained by the suppression of tunneling for the QD excited states due to the additional carrier depletion caused by misfits in the bottom GaAs. As a result, electrons are thermally activated from the QD states to the GaAs conduction band, consistent with the CV and conductance-frequency (G-F) measurements which show emission energies of 0.160 and 0.068 eV, close to the confinement energies of the QD electron ground and first-excited states, respectively, relative to the GaAs conduction band. This is in contrast to non-relaxed samples in which emission energy of 60 meV is observed, corresponding to the emission from the QD electron ground state to the first-excited state. Thus, placing the misfit dislocations in the GaAs bottom layer can have the effect of lengthening the emission time for electrons escaping from the QD. DLTS reveals a trap at 0.41 eV in the bottom GaAs layer, which is attributed to the misfits. Its intensity is about two orders of magnitude less than the misfit intensity, suggesting that most of the misfits are not active traps. The effect of post-growth annealing on the 3.3 ML relaxed sample is also studied. After annealing at 700 ℃ for 1min, a blueshift of both the ground-state and first-excited-state emissions is observed, suggesting an interdiffusion between In and Ga after annealing. Annealing is found to increase the emission time foe the electrons trapped on the misfit-related traps.