碲化鋅/硒化鎂鋅量子點熱退火後熟成與內擴散競爭之機制研究

Abstract

利用分子束磊晶系統以 Stranski-Krastanov 成長模式在不同鎂濃度的硒化鎂鋅上沉積碲化鋅量子點，量子點的厚度分別為4.0、5.0、6.0與7.0原子層，另外在硒化鋅上成長4.0原子層厚度的碲化鋅量子點做為參考樣品。並利用快速熱退火、光激螢光光譜、變溫光激螢光光譜與時間解析光譜等實驗技術來探討碲化鋅/硒化鎂鋅量子點熱退火後熟成與內擴散競爭之機制研究。 對不同原子層厚度的碲化鋅量子點做攝氏350度到550度的快速熱退火分析，熱退火時間為30秒，經由光激螢光光譜發現兩個不同熱退火效應，分別為熟成機制和內擴散機制。在熱退火溫度大於470度時，原子層沉積厚度較小的量子點其光激螢光光譜峰值會發生紅移的現象，此為量子點熟成機制所致；然而在原子層沉積厚度較厚的量子點其光激螢光光譜的峰值隨著熱退火溫度上升而有藍移的現象，是因為內擴散機制所主導。而在中間原子層沉積厚度的量子點，上述兩個機制會互相達到平衡，光激螢光光譜的峰值並不隨熱退火溫度而有所變化。 另外從變溫光激螢光光譜觀察到三個不同的活化能，不同機制下活化 能的變化也會有所不同，藉此推測出熟成機制和內擴散機制能帶結構的變化；以及利用兩個複合時間擬合的時間解析光譜，了解不同熱退火機制下對於載子生命週期的影響。本研究闡述不同量子點覆蓋層厚度經由熱退火實驗可由熟成機制或內擴散機制來改變量子點的密度及大小。

ZnTe quantum dots were grown by Stranski-Krastnov mode on ZnMgSe of different Mg composition by Molecular Beam Epitaxy. The coverage thickness of quantum dots were 4.0, 5.0, 6.0, and 7.0 MLs. ZnTe quantum dots grown on ZnSe was investigated for reference. The physical characteristics were studied by rapid thermal annealing (RTA), photoluminescence (PL) spectroscopy, temperature dependent PL, and time resolved PL (TRPL). Different coverage of ZnTe quantum dots were annealed at temperature from 350°C to 550°C for 30 seconds by RTA. Two mechanisms, ripening and inter-diffusion, were observed in PL spectra of annealed samples. For the samples of annealing temperature larger than 470°C, a red shift of PL peak position was observed in quantum dots of smaller coverage. It is the signature of ripening of quantum dots. However, for the quantum dots with larger coverage, there was a blue shift of PL peak position with increasing annealing temperature. It results from the inter-diffusion of Mg from the barrier layers to the quantum dots. For the quantum dots with moderate coverages, two mechanisms balance and it results in no shift of PL peak position for different annealing temperatures. Three different activation energies were found from the Arrhenius fitting of the temperature-dependent PL. The activation energies also vary with ripening and inter-diffusion mechanisms, and the effect of ripening and inter-diffusion on the band structure could be understood. By using two recombination lifetimes to fit the TRPL spectra, the effect of annealing mechanisms on the carrier lifetime was studied. Current study showed that the dot density and size can be controlled by different coverage thickness of quantum dots and the ripening and inter-diffusion mechanisms of rapid thermal annealing.