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Optical properties of II-VI compound semiconductor quantum structures
|Authors: ||Tung-Uuan Lu|
|Contributors: ||NTOU:Institute of Optoelectronic Sciences|
PL;TRPL;QW coupled QDs;separation layer;CdTe;ZnTe
|Issue Date: ||2011-06-22T08:37:06Z
|Abstract: ||本論文用光激螢光光譜、時間解析螢光光譜及光激螢光激發光光譜來研究ZnTe/ZnSe第二類交錯式(type-II staggered)量子點及CdTe第一類型量子點耦合量子井結構的光學特性。 ZnTe/ZnSe第二類交錯式量子點的光激螢光光譜有兩個主要的峰值，由螢光光譜的峰值皆小於ZnTe(2.4eV)塊材的能隙值、光激螢光激發光沒有ZnTe濕層吸收的訊號，顯示ZnTe/ZnSe為第二類交錯式量子點。當自聚性量子點(self- assembly)成長至一定大小時，為釋放所累積的應力，而形成大小不同的量子點群，二組峰值分別代表大小不同的兩量子點群的光激螢光訊號峰值。 在變溫光激螢光的實驗中，發現大量子點群與小量子點群在低溫(50K到90K)有不同的變化。小量子點群的積分光激螢光強度隨溫度的升高而逐漸變小，為正常的溫度對光激螢光抑制（quench）現象，而大量子點的積分光激螢光強度在50K到90K之間，呈現隨溫度升高螢光強度先增加的特殊情況，在90K以上溫度螢光強度便如一般的情況隨溫度升高而逐漸變小。我們發現大量子點群光激螢光特殊的溫度變化可以由一個假設在大量子點群附近存在缺陷的模型所解釋。同時，由大、小量子點群的時間解析光激螢光光譜所量測的生命期大小差異的分析，與我們所引用模型的假設相一致，並且可以解釋小量子點的活化能大於大量子點的活化能。 CdTe量子點耦合量子井樣品，隨著間隔層厚度減少有光激螢光訊號的半高寬有變窄的趨式，而光激螢光訊號強度則有增強的趨式。光激螢光訊號的半高寬變窄可以用間隔層厚度減少會使CdTe量子點受ZnTe間隔層的應力影響來解釋；光激螢光訊號強度增強則為間隔層厚度對穿隧效率的影響。同時，時間解析光譜量測CdTe量子點耦合量子井樣品時，會因間隔層厚度變薄而使樣品的載子生命週期因穿隧效應及量子點受伸張應力影響而改變。|
The .optical properties of the type-II staggered ZnTe/ZnSe quantum dots(QDs) and CdTe/ZnTe quantum well(QW) coupled quantum dots(QDs) nanostructures were studied by photoluminescence(PL), time-resolved photoluminescence(TRPL) and photoluminescence excitation(PLE) measurements. The PL and PLE spectra indicated the existence of the type-II staggered ZnTe/ZnSe QDs. It was found that the self-assembly QDs could split at certain thickness they were grown in order to relieve the built-up strain, and therefore, two categories of QDs with different averaged sizes were created. Two main peaks corresponding, to the two populations of the QDs with different sizes were found in the PL spectra. The temperature dependent PL spectra of the larger QDs showed peculiar behaviors by increasing the PL intensity with the increase in the temperature ranged between 50K and 90K, while that of the smaller QDs show the normal decrease in the PL intensity with the increase in the temperature. We found that the abnormal behavior for the larger QDs can be understood by assuming the existence of the defects near the larger QDs,. From the TRPL measurements, the comparisons of the lifetimes for the carriers in larger QDs and the smaller QDs supported the assumption and was able to explain the discrepancy in activation energies between the larger and the smaller QDs. In the case of the CdTe/ZnTe QW coupled QDs, it was found that the full width of the half maximum(FWHM) of the PL spectra decreased and the intensity of the PL spectra increased as the separation larger between the QW and QDs were decreased. In additions, TRPL measurements, showed that the QW coupled QDs with the thinnest separation layer exhibited the shortest lifetime in PL decay process. We point out that all the phenomena can be understood by the effects of the strain induced by the ZnTe separation layer and the tunneling of carriers from QW to QDs in the QW coupled QDs nanostructures.
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