鍺 量 子 點 之 拉 曼 光 譜 分 析

Raman scattering of self-organized Ge quantum dot

程子玹(Tzu-Hsuan Cheng)

指導教授 ： 徐子民

國立中央大學/理學院/物理學系/碩士(2014年)

摘要

本論文藉由拉曼散射(Raman scattering)量測系統來分析不同直徑的鍺量子點(Quantum dot)樣品之聲子特性與結晶品質等晶格資訊。由室溫拉曼光譜結果可知，當鍺從塊材變為奈米結構下的量子點後，聲子訊號峰值頻率隨直徑變小而有增加的趨勢。先前的研究中指出量子侷限效應(Quantum confinement effect)與應力作用(Strain effect)皆會對塊材與量子點間的相對頻率偏移量造成貢獻。進一步利用此頻率偏移量來分析鍺量子點所受到的應力，並比較兩種不同結構之樣品來觀察其應力來源。進行變溫拉曼量測，可由光學聲子訊號的峰值頻率與半高寬隨溫度的變化關係擬合出非諧和作用所造成的影響。從鍺塊材與量子點的本質頻率差值來探討量子點所受應力，其結果與室溫下所觀察到的相同；而比較非諧和係數後，可以發現量子點確實有受到侷限作用，雖然在聲子頻率沒有很大的變化，但在非諧和振盪作用卻有明顯的貢獻。利用拉曼訊號半高寬可以推算出光學聲子(Opitcal phonon)生命期，其分為本質生命期與衰變生命期。本質生命期可以反應出樣品結晶的好壞，在低溫時本質生命期為總生命期的主要貢獻；隨著溫度的升高，衰變生命期與本質生命期已經相當接近，因此非諧和振盪也會影響總生命期，而使高溫時的光學聲子總生命期較短。

關鍵字

鍺量子點 ； 拉曼 ； 應力 ； 非諧和效應 ； 生命期

並列摘要

By applying Raman spectroscopy, we have analyzed the phonon properties and crystal structure of different diameters of Ge quantum dot. When the germanium bulk transformed into quantum dot, we can observe that the Raman peak frequency increases as the diameter decreases via the Raman spectra. The reference indicates that the Ge-Ge mode frequency of Ge nanocrystals can be changed by quantum confinement and strain effect. And, Strain tensor in the Ge quantum dots can be further calculated by the Raman shift, and by comparing the different structures of the two samples, their strain sources would be found.The Raman spectra from various sizes of Ge quantum dot shows changes in peak position and linewidth with temperature. These temperature-dependent changes can attribute to the anharmoicity in the vibrational potential. Whether we calculcated the strain tensor from the Raman shift at 300K or intrinsic frequency, we can get the same result. Compared with the anharmonic coefficients, we find that Ge quantum dots were indeed influenced by quantum confinement effect. Quantum confinement effect plays a role in anharmonic oscillations, even though there is no significant change in the phonon frequency.The lifetime of the phonons can be obtained through the result from the temperature-dependent Raman peak, which is divided into the intrinsic lifetime and the decay lifetime. The former lifetime corresponds to the crystal quality, and it is the main contribution to the lifetime of the phonons at low temperature. The latter lifetime will be close to the value of the former lifetime with the rising temperature. Therefore, we conclude that as the temperature rises, the anharmonic effect would be raised.

並列關鍵字

quantum dot ； Raman ； strain ； anharmonic effect ； lifetime

參考文獻

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