半導體自聚性量子點之載子動力學

Study of Carrier Dynamics in Semiconductor Self-Assembled Quantum Dots

蘇立杰(Li-Chieh Su)

指導教授 ： 毛明華

國立臺灣大學/電機資訊學院/光電工程學研究所/博士(2010年)

https://doi.org/10.6342/NTU.2010.02603

摘要

在本論文中，利用理論模擬以及實驗分析來研究半導體量子點內的載子動力學。量子點結構可以對載子造成三維的侷限，而這樣的侷限可以產生獨特的能態密度和不連續的能階，同時增進了一些物理上的特性。因此，利用量子點作為主動層的半導體元件，將具有許多不錯的特性，非常適合用於高速的光通訊系統。
首先，我們針對量子點雷射的雙能態激發放光現象來進行理論計算以及實驗驗證。在電訊號動態量測中，我們發現激發態量子點比基態量子點還要早達到激發放光。這個現象是由於量子點內的能態數目是有限的，造成當量子點雷射的光學損耗接近基態飽和增益時，載子較難找到未填滿的量子點進入，使得量子點的捕捉時間以及鬆弛時間將會變長，而讓激發態的載子能夠累積先達到激發放光。我們成功解釋雙能態激發放光量子點雷射獨特的動態特性。
對於量子點元件特性來說，載子的生命週期是一項很重要的參數。因此我們建立一套新的時間解析系統-簡併激發偵測光激發螢光系統來量測量子點的載子生命週期。首先，我們證明了這個系統是可以使用在發光波長長於1.1微米的近紅外範圍。在這個波長範圍內，很難找到同時具有高速以及高敏感度的光子計數元件。我們並利用這個時間解析系統，研究p型摻雜對於具有垂直耦合與未耦合量子點生命週期的影響。藉由改變溫度量測載子生命週期的實驗，我們可以清楚的證明：(i)在p型摻雜量子點樣品裡，因為這些摻雜造成載子生命週期由非輻射放光機制主導，(ii)在垂直耦合量子點樣品裡，因為較小的振子強度而使得載子生命週期變長。
最後，我們建立一套時間解析上轉換系統來研究量子點的捕捉時間，這個量測系統的時間解析度為280飛秒。我們觀察到在垂直耦合量子點內不管是否具有p型摻雜，都有較快的捕捉時間(6.5 - 6.7皮秒)，可能是因為較薄的分隔層產生較大的載子濃度而提高了藉由歐傑過程釋放能量的機率，使得載子更容易掉入量子點內。另外，我們也觀察到在單純只有p型摻雜量子點的捕捉時間(8.0皮秒)與未摻雜量子點的捕捉時間(8.0皮秒)相近，造成這個現象的原因可能還需要進一步探究。

關鍵字

量子點 ； 載子動態

並列摘要

This dissertation studies the carrier dynamics in semiconductor quantum dots by means of numerical simulations and various experimental techniques. Quantum dots exhibit a structure of three dimensional confinements for carriers. This kind of confinement results in the unique density of states and some improved optical properties. Hence, semiconductor devices based on quantum dots have many superior performances and are suitable for using in optical communication.
In the first part of this dissertation, we study the two-state lasing phenomenon in quantum-dot lasers experimentally as well as theoretically. The onset of excited-state lasing prior to ground-state lasing was observed in dynamics measurements under electrical excitation. The explanation for this phenomenon is due to the finite states in quantum-dot lasers. When the optical loss level is close to ground-state saturation gain, the carrier capture time into ground states becomes longer and results in the establishment of excited-state population. We successfully explain the origin of the unique dynamics phenomenon in two-state lasing quantum-dot lasers.
In semiconductors, carrier lifetime is an important parameter determining the device performances. We build up a novel time-resolved system by using a degenerate pump-probe photoluminescence technique, to measure carrier lifetimes of different quantum-dot samples. We demonstrate that this technique can be used in the infrared region with wavelength longer than 1.1 um. In this wavelength region, it is difficult to find high-speed and high sensitivity photon counting devices. We also study the p-doping impact on carrier lifetime in vertically coupled and uncoupled quantum dots by unsing this degenerate pump-probe photoluminescence technique. From the temperature dependent carrier lifetime measurement, we can clearly demonstrate (i) the dominant nonradiative mechanism induced by the extra p-type dopants and (ii) the smaller oscillator strength in the vertically coupled quantum dots.
Finally, we study carrier capture times of vertically coupled and p-doped quantum dots by using a home-made time-resolved up-conversion system. The time resolution of this measurement is 280 fs. Faster capture times in the vertically coupled quantum-dot samples (6.5 ps to 6.7 ps) are observed. It may be a consequence of the enhanced Auger-assisted relaxation mechanism due to high carrier density caused by the thinner spacer layers. However, no capture time difference (8.0 ps) has been observed between the p-doped and undoped quantum-dot samples. Further studies are still needed to reveal the reason for this phenomenon.

並列關鍵字

quantum dot ； carrier dynamics

參考文獻

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