砷化銦鎵量子點微碟雷射之研究

Study of InGaAs Quantum-Dot Microdisk Lasers

錢皓哲(Hao-Che Chien)

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(2012年)

https://doi.org/10.6342/NTU.2012.02248

摘要

微碟共振腔由於其迴音廊模態有良好的空間侷限性，因此擁有高品質係數與低模態體積之特性，結合量子點主動材料，可在砷化鎵材料系統實現光纖通訊用長波長雷射，並具有低臨界電流之潛力。在本論文中，我們研究了以砷化銦鎵量子點為主動材料的光激發式與電注入式之微碟雷射，針對這兩種元件，我們討論了其製程與量測分析的結果。
首先，在光激發式微碟共振腔雷射的研究上，我們利用光激放光量測，可以得知所製作出直徑為10微米的微碟雷射，其品質係數高達14000，同時，在製程中於微碟中心加入一小洞，可以有效壓抑其徑向上的共振模態。為了減少共振腔中的迴音廊共振模態數量，我們利用兩段式濕式蝕刻技術製作直徑約為3微米的微碟雷射，成功減少了模態的數量。在變溫實驗當中，我們也觀察到量子點放光頻譜和迴音廊模態波長的分佈重疊情形，將會影響雷射在不同溫度下的雷射閥值。
在電注入式微碟共振腔雷射的製作上，我們採取的是苯環丁烯聚合物包覆下的平坦化製程，並且透過光學量測，實現了世界上第一個室溫下操作的量子點電注入式微碟共振腔雷射，室溫下最低的臨界電流是0.45毫安培，其元件直徑是6.5微米。在變溫實驗當中，我們觀察到了量子點雷射的負特徵溫度現象，其最低的臨界電流出現在150 K，透過變率方程式的模擬，我們認為這個較高的臨界電流轉折溫度150 K正是微碟雷射元件品質係數較高的證據。
最後，我們分別對光激發式與電注入式微碟共振腔雷射來討論其動態的行為。透過時間解析下的光激放光實驗，我們分別量測了微碟雷射中共振波長與非共振波長下量子點的載子生命期，在普色效應下，其量子點的自發性放光速率被增強了4.3倍。在室溫下電注入式微碟共振腔雷射暫態行為的量測中，我們發現微碟雷射的起始時間很短暫，並且沒有觀察到弛逸震盪的現象，透過模擬可以了解到弛逸震盪被壓抑的原因是微碟雷射的自發性放光係數較大，同時較短的載子生命期使微碟雷射的起始行為相當迅速，在大訊號直接調變的實驗下，我們展示了此元件作為1 Gbps調變的可能。在變溫實驗中，我們看到在250 K以上電注入式微碟雷射在暫態與穩態下均為單一迴音廊模態雷射放光，因此證實了此元件可以應用在高度積體化光收發器模組中，做為高頻調變與單模操作的雷射光源。

關鍵字

半導體雷射 ； 微共振腔 ； 微碟 ； 量子點

並列摘要

Microdisk cavities have been exploited as high quality factor resonators due to the excellent confinement of whispering gallery modes (WGMs) and may be applied as low threshold laser cavities. Microdisks with embedded quantum dots (QDs) offer the potential of ultralow threshold semiconductor lasers due to the combination of the small mode volume and delta-like electron density of states. They can also be used to fabricate GaAs-based long-wavelength diode lasers for fiber communication systems. Both of the optically-pumped and current-injection microdisk lasers embedded with InGaAs QDs are studied in this dissertation. The fabrication techniques and measurement results of optical properties are discussed for these two kinds of devices.
In the study of optically-pumped microdisk lasers, we measured a 10-μm-diameter microdisk lasers by micro-photoluminescence (micro-PL) experiments. The corresponding quality factor can be larger than 14000. The radial modes can be greatly suppressed by introducing a small hole at the disk center. We also tried to fabricate microdisks with cavity size near 3 μm by two-step wet-etching in order to reduce the lasing modes inside the cavities. The numbers of lasing modes are reduced in these smaller microdisks. In temperature-dependent experiments, we have shown that the alignment of QD and whispering gallery mode energy may affect the lasing threshold.
In the study of current-injection microdisk lasers, we have fabricated our devices with Benzocyclobutene (BCB) polymer claddings. The first room temperature lasing phenomenon of QD injection microdisk lasers is demonstrated. The lowest threshold current at room temperature is 0.45 mA, which is measured from a 6.5-μm-diameter microdisk. In temperature dependent experiments, the negative characteristic temperature (T0) of QD microdisk lasers is observed. The transition temperature from negative T0 to positive T0 is around 150 K. From our rate-equation simulations, the experimental results indicate that the quality factor of our microdisk cavity is high.
Finally, the dynamic behaviors of both optically-pumped and current-injection microdisk lasers are studied. By the time-resolved micro-PL experiments, the QD carrier lifetimes corresponding to on-resonance and off-resonance wavelengths of a microdisk are determined. The average spontaneous emission rate is enhanced by a factor of 4.3, which is owing to the Purcell effect. The transient behaviors of current-injection QD microdisk lasers are studied at room temperature. Suppressed relaxation oscillations and fast turn-on behaviors are observed. The suppressed relaxation oscillation can be attributed to the enhanced spontaneous emission factor in microdisk lasers and the short turn-on time is due to the reduced carrier lifetime. A large-signal direct modulation at 1 Gbps is demonstrated. In temperature-dependent dynamic measurements, both of the transient lasing and steady-state lasing from side modes are suppressed at temperatures higher than 250 K. Therefore, the quantum-dot microdisk lasers have the potential to realize the single-mode operation under high-speed modulation at room temperature in highly integrated optical transceiver modules.

並列關鍵字

semiconductor laser ； microcavity ； microdisk ； quantum dot

參考文獻

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