熱退火於InAsSb/GaAs量子點光性及應力鬆弛引致缺陷之載子傳輸特性研究

Abstract

本論文主要是探討InAsSb/GaAs量子點經熱退火處理後之光電特性。由光性量測發現在室溫時，InAsSb QD基態與激發態的能階差約為60meV，但在低溫時能階差竟高達160meV，且激發態半高寬高達130meV。推測InAsSb 量子點在低溫下，並非單純量子點的基態與激發態訊號。 為了探討Sb摻雜至InAs QD中的效應，我們將樣品做650~7500C的熱退火處理。由低溫PL量測發現經熱退火處理的樣品，均勻性相較於as grown的樣品有逐漸便好的趨勢，因此推測InAsSb 量子點於長晶時存有尺寸不均勻的現象。 但熱退火7000C的樣品於室溫下PL特性變差，並有類似晶格鬆弛的現象。於是將該樣品作電性分析，在CV量測發現大量載子被空乏，並由DLTS量測觀察到兩個缺陷存在，其類型分別為threading dislocation 及misfit dislocation，而活化能分別為0.60eV及0.33eV。相較於已晶格鬆弛的InAsSb量子點，其活化能分別為0.64eV及0.35eV，兩者是相類似的。由實驗結果推測除了量子點成長時超過臨界厚度會發生晶格鬆弛的現象，當熱退火給予能量後，也有晶格鬆弛的情形。 接著針對2.8ML relaxed InAsSb QD樣品做650~7500C的熱退火處理。由PL量測發現熱退火後發光波長藍移且均勻性變差。在電性上由CV量測發現載子濃度銳減，而DLTS缺陷分析發現缺陷能階位置不變，但載子的傳輸機制產生變化。針對量子點附近的偏壓進行DLTS量測，發現缺陷的活化能由0.35eV變小至0.2eV，推測載子並非由缺陷跳至GaAs傳導帶的行為，而是趨向跳至量子點的量子能階。此外，DLTS量測在低溫下呈現對溫度沒有反應的缺陷訊號，這代表emission time是與溫度無關的，來源可能是Tunneling現象。由實驗結果推測載子趨向由缺陷跳至量子點的量子能階之後，再穿隧出去的傳輸機制。

The thesis is highlighted on the discussion over the optical and electrical characteristics of InAsSb/GaAs quantum dots before and after thermal annealing process. From the PL measurement, we observed the energy separation between the ground state and the excited state in InAsSb quantum dots is about 60 meV at room temperature. However, the energy separation at low temperature is increased up to 160 meV with a large FWHM of 130 meV for the excited-state emission. This abnormal large energy separation is explained by the non-uniformity of InAsSb quantum dots induced by the Sb incorporation, because, after 650~7500C thermal annealing, the broad excited-state emission is split into two well-separated peaks, suggesting a significant improvement in the uniformity. In a non-relaxed InAsSb sample (2.2 ML), annealing at 7000C was found to degrade the PL property. Electrical C-V profiling on this sample revealed massive carriers depletion around the QD region. Two defect traps with activation energies of 0.60eV and 0.33eV were found by DLTS measurements. These activation energies are similar as those (0.64eV and 0.35eV) of the traps associated with threading dislocation in the GaAs top cladding layer and misfit dislocation near the QD region previously observed in relaxed InAsSb QD samples. From these results, we infer that annealing can provide the necessary energy for the occurrence of strain relaxation and the generated defects are similar as those observed in the relaxed QD samples when the InAs thickness exceeds the critical thickness. Effect of annealing on the properties of relaxation-induced traps is also investigated in a relaxed 2.8ML InAsSb QD sample. After annealing at 650~7500C, the PL spectra show a blue shift and a degradation of uniformity. The C-V profiling displays a further reduction of carrier density in the QD. DLTS analysis reveals a decrease from 0.35eV to 0.2eV for the emission energy of the misfit-related trap. This is explained by an electron emission from the defect trap to the QD energy level, rather than to the GaAs conduction band. The DLTS spectra of this trap show a nearly temperature-independent constant signal at low temperatures, suggesting a tunneling effect at low temperatures. Thus, the electron escape from the misfit-related trap to the GaAs conduction band occurs through tunneling from the QD energy state.