在(111)B砷化鎵基板上之砷化銦鎵/砷化鎵量子結構之研製及特性分析

Abstract

本篇論文之目的在(111)B砷化鎵基板上製作高品質之量子結構，並分析其特性。首先，我們提出一個新的量子點成長方法，利用(111)B砷化鎵分子束磊晶成長的特殊性質，使得量子點的形成非常容易而且均勻。與傳統的自組量子點採用應變鬆弛的特性不同，(111)B量子點成長的機制是由於2’2的表面重組所形成的三維成長特性。我們透過原子力顯微鏡(AFM)證實量子點的形成，而光激光(PL)的量測更得到相當優良的結果，在8K下的量測得到的半高寬僅有7.7meV，這是我們所知的報告中最好的結果。我們也嘗試不同的長晶條件，以找出可以形成量子點的成長參數範圍。 同時，對我們所製作的量子點加高磁場量測其光激光峰值的反磁位移(diamagnetic shift)，磁場加到42T所得的數據，可以計算出量子點中的激子(exciton)的束縛能是5.5meV，而參考用的量子井中的激子束縛能是2.8meV。在較低磁場下的數據則可以得到載子波函數的空間分佈，在長晶方向上量子點對波函數的局限較大，由此也可以證實量子點的形成及其對載子的束縛情形。 另外，我們量測了時間解析光激光的光譜，發現在(111)B晶面上的量子井中，載子鬆弛時間跟量子井在樣品中的位置有很大的關係。距離表面愈遠，則鬆弛時間愈長。同時成長的(100)晶面量子井並沒有這樣的現象，所以我們認為是(111)B樣品中的強大壓電電場造成的影響。在量子點與量子井的比較上，我們也發現量子點中的熱激子能量鬆弛較慢，結果與其他量子點研究報告一致。 最後，我們分析在磁場下砷化銦鎵/砷化鎵量子井的時間解析光激光，當磁場加到6T時，在(111)B量子井中的載子鬆弛時間明顯增長，而同時成長的(100)樣品並無此現象。我們仍將其歸因於強大的壓電電場，使得載子的自旋翻轉影響力增強，進而增加暗激子的數量，激子的鬆弛時間便因此增長。

In this dissertation, a simple in situ method is used to fabricate high-quality InGaAs/GaAs quantum dots on (111)B GaAs substrates. The formation of quantum dots was not due to strain relaxation, but due to the growth characteristics on (111)B GaAs under the 2’2 surface reconstruction. The formation of the quantum dots was verified by AFM images and the shift of photoluminescence. Excellent PL result was obtained with a linewidth of 7.7 meV at 8 K, which is the narrowest linewidth reported for quantum dots. We have also tried various growth condition to get the proper range of growth parameter for good quantum dots. Besides, we have used magneto-optical measurement in high magnetic fields to investigate InGaAs/GaAs quantum well and quantum dots grown on (111)B GaAs substrates. Photoluminescence were measured under magnetic fields up to 45 T in both Faraday and Voigt configurations. The exciton binding energies can be deduced from the high field diamagnetic shifts and the values are 2.8 meV for quantum well and 5.5 meV for quantum dots. In addition, the extents of carrier wave functions are also obtained from the low field diamagnetic shifts. The ECWFs in growth and lateral directions are both smaller for quantum dots as compared with the quantum well, showing stronger confinement for quantum dots. This is consistent with the result of high-field measurement. Comparing the in plane ECWFs of InGaAs quantum dots with various In composition, we found that it is smaller for higher In composition, showing stronger lateral confinement. While for quantum wells of various In composition, the in plane ECWFs are comparable. In addition, we have studied the carrier dynamic of the InGaAs/GaAs strained quantum wells located at different positions on (111)B GaAs substrates, together with the references of (100) samples grown side by side with the (111)B samples, by the measurements of the high resolution time-resolved photoluminescence. Longer lifetime of the PL peaks was found from the quantum wells deep inside the samples, as compared with that from the quantum wells near surface. Besides, we have also measured the time-resolved PL of the quantum well and quantum dots. The slower decay time of quantum dots is another proof of the increase in the growth direction when islands form in our quantum dots fabrication method. Finally, we have studied time-resolved photoluminescence of InxGa1-xAs/GaAs quantum wells grown on (111)B and (100) substrates. Magnetic fields up to 6 T in Faraday configuration were used during measurements. Spin-flip process was found to play an important role in the exciton relaxation in (111)B strained QWs because of the strong piezoelectric fields. The spin-flip time from the dark exciton level to the bright exciton level increases with the magnetic field. This is the first time that the spin-flip process under strong piezoelectric fields was studied.