Effects of N incorporation and InAs thickness on optical properties of InAs/GaAs quantum dots
|摘要:||本論文的主要內容為藉由變溫的PL量測，探討不同氮含量與InAs沉積厚度對InAs/GaAs量子點光性的影響。我們發現摻1%的N在In0.14Ga0.86As 量子井中，可將波長拉至1344nm，但PL強度卻減弱，而直接摻N在量子點中，品質變得更差。對於沉積厚度為1.98ML的樣品，在低溫時即有很大的半高寬，代表這些小size量子點侷限載子的能力較差，一旦溫度上升，明顯的紅移現象與半高寬的減少，起因於載子從小size量子點傳遞至大size量子點。增加沉積厚度至2.34ML和2.7ML，放射波長逐漸拉至1311nm。這些樣品在低溫時的半高寬很小且隨溫度上升並無明顯改變，暗示其侷限載子能力良好。然而，當沉積厚度再繼續增加並超過2.7ML時，量子點的放射波長不但不繼續增加，反而形成兩群不同波長的量子點同時存在，分別在1223nm和1300nm之處，而且這些短波長群量子點在低溫的發光品質與有近似波長的1.98ML樣品相等或甚至更好，但是當溫度增加至200K以上，半高寬劇烈地增加，我們推論是由於載子在高溫時被激發到激發態，此時wave function變寬，再加上受到起因於晶格鬆弛的缺陷影響，經歷非輻射復合所致。最後從PL強度隨溫度變化的曲線中得到每一片樣品的活化能，我們發現活化能與放射波長呈現反比的關係，而且活化能與Grundmann理論中的基態到第一激發態之能量間隔很接近，代表基態放射光隨溫度上升而衰減的主要原因是因為載子受熱從基態跑至第一激發態。|
Photoluminescence is used to study the optical properties of self-assembled InAs/GaAs quantum dots (QDs) with different N incorporation and InAs deposition thickness. The emission wavelength can be increased to 1344nm by incorporating 1% N into In0.14Ga0.86As quantum well, but PL intensity becomes weaker. Besides, incorporating N into QDs makes the quality much worse. For small deposition of 1.98ML, a large FWHM at 50K is observed, implying a relatively poor confinement for electrons in such small-size QDs. When temperature increase, we observe a significant red shift and a decrease of FWHM due to the transfer of the electrons from relatively small-size to large-size QDs. By increasing the InAs deposition to 2.34ML and 2.7ML, the emission wavelength increases to 1311nm. The small FWHM at 50K and its temperature insensitivity suggest a good electron confinement. However, when the InAs deposition thickness increases beyond 2.7ML, the QD emission wavelength shows no increase, instead, two groups of different wavelength QDs, one emits at 1223nm and the other at 1300nm, are observed. The quality of the short-wavelength QDs is comparable to the 1.98ML sample in which the QDs emit at a similar wavelength. Nevertheless, when the temperature increases beyond 200K, the FWHM drastically increases. We speculate this abnormal increase of FWHM by that the electrons excitation to first excited state at high temperature and undergo a nonradiative recombination through relaxation-induced defect states. Finally, from the relationship between PL intensity and temperature, we can obtain the activation energy which is found to be inverse proportional the emission wavelength. The obtained activation energy is consistent with the energy separation between the ground state and first excited state according Grundmann’s theory. Hence, we conclude that the decrease in intensity with increasing temperature is due to the carrier excitation from the ground state to first excited state.
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