砷化銦/砷化鎵量子點中載子自旋動力學

Abstract

之前我以時間解析光譜儀，在室溫下研究砷化銦/砷化鎵自組裝量子點的載子捕捉與釋熱過程。因此，在原有之時間解析系統上，建構出一套擁有分析光波極化方向能力的新系統。藉此研究量子點、溼潤層和基板內自旋載子的產生與弛豫。在77K的低溫下，電子和電洞的自旋機制已被探知，電子生命週期約100ps；電洞生命週期約20ps。室溫下砷化鎵塊材內自旋電洞的弛豫速度極快，大約在100fs，但自旋電洞在量子點內保存的時間較長。 由於upconversion系統擁有高時間解析度，正可藉由實驗來研究自旋載子在砷化銦/砷化鎵量子點內的動態行為。在分析數據時，發現自旋極化率弛豫的時間曲線無法以一個指數來分析(exponential fit)，必須以兩個指數(雙指數函數)。得到的兩個生命週期(lifetime)中，較快的自旋弛豫趨勢主要藉由庫倫力產生的載子—載子散射所貢獻；而較長的自旋弛豫趨勢則由自旋載子與聲子作用使然。

I have investigated the carrier capture and relaxation processes in InAs/GaAs self-assembled quantum dots at room temperature by time-resolved photo- luminescence techniques previously. So a polarization-resolved photo-luminescence up-conversion spectroscopy was set up from the original time-resolved system. Then we can use it to study carrier spin capture and relaxation in quantum dots, wetting layers, and barrier. The electron and hole spin lifetimes are found (electrons spin: ~100ps; hole spin: ~20ps) at 77K. The rapid relaxation of hole spin is about 100fs in bulk GaAs at room temperature, but it is preserved in quantum dots longer time. Because up-conversion system is performed with high time resolution, we can do experiments to study carrier spin dynamics in InAs/GaAs quantum dots. The time-resolved measurements reveal that spin relaxation time can not be fit with monoexponential but can be fit with biexponential. The fast decay time is primarily attributed to the carrier-carrier scattering by coulomb interaction. And the slow decay time corresponds to the spin flip process with phonons.