砷化鎵(001)量子井的自旋動力學

Abstract

自旋動力學為一個由多學科交叉而形成的新興領域，以自旋自由度為研究對象並發展新型自旋電子元件為目標，在近十年間取得令人矚目的進展。在實現自旋電子元件中有兩個非常重要的問題：如何延長自旋鬆弛時間以及如何有效地控制自旋。 因此我們將三維空間自由運動的粒子束縛在一個平面區域，如量子井，由於砷化鎵和鋁砷化鎵存在的晶格不匹配，晶格對稱性遭到破壞，價帶能階產生了位移和分裂出重電洞和輕電洞兩個不同的能階，因為重電洞與輕電洞的能階分裂，極化率理論上可以達到接近100%， 這篇論文將利用自旋鬆弛時間解析光譜來探討二維砷化鎵/鋁砷化鎵量子井與三維砷化鎵的自旋特性差異，並根據本質、n型和p型砷化鎵量子井及塊材，來探討室溫下三大鬆弛機制：Elliott-Yafet, D'yakonov-Perel 和 Bir-Aronov-Pikus 的相對重要性。 我們發現到相較於三維塊材，量子井中受到自旋軌道耦合的影響更為明顯，在低激發強度下的本質量子井中，可觀察到自旋鬆弛行為由電洞所主宰，而(001)量子井中，因為自旋軌道耦合的影響，電子和電洞自旋鬆弛時間都比三維塊材來得短，最後，量子井中電洞自旋鬆弛行為較為明顯，可藉由控制電洞自旋鬆弛來達成高極化率，低摻雜N型(001)GaAs以較高的光激發密度激發，極化率可接近80%。

Spin dynamics in semiconductors have gained much interest in the past years due to the emerging field of semiconductor spintronics. In this thesis, we use a time-resolved photoluminescence polarization spectroscopy system to investigate electron and hole spin dynamics in GaAs/AlGaAs quantum wells grown on (001) wafers and compare the results with the bulk GaAs. We find that the DP mechanism plays an important role for spin relaxation in both GaAs quantum wells and in the bulk. The spin relaxation time of the electrons is only on the order of a few tens of ps. Due to the strong spin-orbit coupling effect the electron and hole spin relaxation times are much faster in GaAs quantum wells in comparing to the bulk. The spin relaxation is dominated by the holes in the undoped GaAs quantum wells at low excitation densities. It indicates that the spin-orbit coupling is stronger in quantum wells than in the bulk. In addition, the hole spin relaxation time is also determined with a rate of less than 300 fs. Due to the splitting of the heavy-hole and light-hole bands, as well as the much shorter hole spin relaxation time than that of the electrons, we are able to generate close to 80% spin polarization in the lightly n-doped quantum wells at high excitation densities.