InAs/GaAs单量子点中电子/空穴自旋弛豫

Electron-and Hole-spin Relaxations in InAs/GaAs Single Quantum Dots

利用分子束外延制备了三种类型量子点样品,它们分别是:未掺杂样品、n型Si调制掺杂样品和p型Be调制掺杂样品。在5K温度下,采用共聚焦显微镜系统,测量了单量子点的光致发光谱和时间分辨光谱,研究了单量子点中三种类型激子(本征激子、负电荷激子和正电荷激子)的电子/空穴自旋翻转时间。它们的自旋翻转时间常数分别为:本征激子的自旋翻转时间约16ns,正电荷激子中电子的自旋翻转时间约2ns,负电荷激子中空穴的自旋翻转时间约50ps。

Quantum dots （QDs） samples studied in the experiment were grown by molecular-beam epitaxy on a semi-insulating GaAs substrate. They are undoping QD sample, n-doping QD sample and p-doping QD sample. The samples grown in sequence are： 400 nm GaAs layer, 100 nm Al05Ga0.5As layer, 0.36 nm GaAs layer with the Si-doping （ 1.5 ×10^16 cm-3 ） ,or Be-doping （ 1 ×10^17 cm-3 ） , 10 nm GaAs layer, 2.35 monolayers （ML） InAs at a growth rate of 0. 001 ML/s,200 nm GaAs layer. For undoping QD sample, there is a non Si or Be doped 0.36 nm GaAs layer. The sample has an ultralow density of the QDs. Therefore, we can isolate single QDs without the use of the nanoscaled masks or mesas. In the experiment the sample was cooled to 5 K and a Ti ： sapphire laser operating at 750 nm is used as an excitation source. The scanning confocal microscopy with an objective （ NA ：0.5 ） is used to spatially resolve single QDs. The polarized photoluminescence （PL） and time-resolved PL measurements were carried out by time-correlated single-photon counting setup. The excitation light is a right circular polarization light. The polarization PL and time-resolved PL emissions were analyzed by a A/4 and A/2 wave plate and a linear polarizer to distinguish different polarization components. All the experiments were performed at low excitation power, which ensures the luminescence working in the single photon emission mode. The main conclusions were given ： （ 1 ） The exciton-spin relaxation time for undoping QD sample is about 16 ns; （2） The electron spin-flip time is about 2 ns for p-doping QD sample;（3） The hole spin-flip time is about 50 ps for n-doping QD sample.