三五族半導體材料的載子與自旋動力學

Abstract

半導體自旋電子學是一門研究如何操控自旋的自由度，以發展出節能、低放熱、高速運作的電子元件，其研究包含自旋的注入、相位維持及偵測等議題，都與自旋動態特性密切相關，因此半導體自旋動力學的研究是開發實用的自旋電子元件的重要基礎。 本篇論文從三維的GaAs深入至零維度材料-InAs/GaAs量子環的自旋動態特性探討，以了解巨觀與介觀材料的自旋特性差異。在研究自旋動力學前，必須先了解材料的載子動態特性，因此將論文分成兩大部分 :「半導體奈米結構的載子動力學」及「半導體塊材及量子環的自旋動力學」。 第一部份，我們利用解析度約200 飛秒( fs ) 的Up-conversion超快時間解析光譜系統，探討未摻雜與調變摻雜自組裝量子環的載子捕捉與鬆弛過程。我們發現當在低光激發密度時，摻雜的量子點基態和激發態的載子捕捉速率較未摻雜的量子環快，這是由於摻雜電子或電洞的量子環存在內建的冷載子，加速載子-載子散射與捕捉電洞或電子進入量子環的激態及基態。 第二部份，我們利用自旋鬆弛時間解析光譜系統來探討本質、n型及p型GaAs塊材的自旋動力學。根據GaAs塊材在室溫下的自旋極化時間解析 及分析三大自旋鬆弛機制 : Elliott-Yafet ( EY ), D'yakonov-Perel ( DP ) 和 Bir-Aronov-Pikus ( BAP ) 的相對重要性，我們發現自旋極化鬆弛時間只有大約幾十皮秒 ( picosecond, ps ) 且DP機制扮演重要的角色。除此之外，我們也量測出不到三百飛秒 ( femtosecond ) 的電洞鬆弛時間。最後我們探討砷化銦/砷化鎵量子環的自旋動態特性，由於量子環具三維的量子侷限，預期可以比塊材維持更長的自旋相位時間。然而量子環基態與激發態的自旋鬆弛時間不到1ps，這是由於載子激發在量子環 barrier 上，載子在抵達激發態及基態前必須經由很多冷卻過程 ( 如載子-載子散射、載子-聲子散射、次能帶內散射 )，導致自旋快速地鬆弛。我們應該將載子直接激發在量子環內，再一次檢驗量子環的自旋動力學。

Semiconductor spintronics has open a research field devoted to the manipulation of the spin degree of freedom and the developing of electronic devices with energy conservation, low heat disspiation, and high operation speed. The studies of spin-based devices including spin injection, phase coherence and spin detection, are closely related to spin dynamics. Therefore, the research of spin dynamics in semiconductors is essential to the development of pratical spin-based electronic devices. This thesis begins with the investigations on spin dynamics in bulk GaAs and gradually moves toward the dynamics in InAs/GaAs quantum rings in order to understand the difference of spin properties between macroscopic and mesoscopic materials. However, before the exploration on spin dynamics, we must fully understand the carrier dynamics in the same materials. Therefore, this thesis is divided into two parts: the first part deals with the carrier dynamics in semiconductor nanostructures, and the second part investigated the spin dynamics of bulk and semiconductor quantum rings. In the first part, we investigate the carrier capture and relaxation processes in undoped and modulation-doped self-assembled InAs/GaAs QRs using time- resolved photoluminescence up-conversion spectroscopy with a time resolution of 200 fs. We find that carrier capture in the ground state and the excited states of the charged QRs are faster in compared to the undoped rings even at a low excitation level. This is attributed to the presence of the build-in cold carriers in the charged rings, which lead to the accelerated carrier-carrier scattering and capturing of holes or electrons into the excited and ground states. In the second part, we use a time-resolved photoluminescence polarization spectroscopy system to investigate the spin dynamics of electrons and holes in p-, n-, and undoped bulk GaAs. From the time-resolved polarization decay rate, we analyze the relative importance of spin relaxation mechanisms, including Elliott-Yafet (EY) model, D'yakonov-Perel (DP) model and Bir-Aronov-Pikus (BAP) model at room temperature for GaAs. We find that the spin relaxation time of the electrons is only on the order of a few tens of ps and the DP mechanism plays an important role for spin relaxation in bulk GaAs. In addition, the hole spin relaxation time is also determined with a rate of less than 300 fs. Finally, we study the spin dynamics in InAs/GaAs QRs in which a longer spin coherence time than the GaAs is predicted due to the three-dimensional quantum confinement. However, the spin relaxation time in excited and ground was found to be less than 1 ps. This is because that the carriers are excited above the barriers and they have to go through many cooling stages (such as, carrier- carrier, carrier-phonon, and inter-subband scatterings) before they can arrive at the excited or ground states in the ring, which leads to the observed ultrafast spin relaxation. The spin dynamics in quantum rings should be carefully re-examined by direct excitation of the carriers inside the rings.