砷化銦量子點雷射靜態與動態之溫度特性研究

Temperature Dependences of Static and Dynamic Properties in InAs Quantum Dot Lasers

吳德清(Der-Chin Wu)

指導教授 ： 毛明華

國立臺灣大學/電機資訊學院/電子工程學研究所/博士(2009年)

https://doi.org/10.6342/NTU.2009.03282

摘要

半導體量子點目前已被廣泛大量的研究，且由於它們獨特的物理特性，現今被視為下一世代光通訊使用的光源。因此，在本論文當中我們不僅是研究砷化銦量子點的基礎物理特性，我們更專注在量子點雷射靜態與動態的特性。
就量子點物理特性而言，我們專注在光場與載子波函數的交互強度。躍遷偶極矩(Interband Transition Dipole Moment)用來表示為光電場與載子波函數的交互耦合躍遷的共振強度，我們以量測吸收係數的方法來估算出躍遷偶極矩約為32±2 Debye。線寬增強因子(Linewidth Enhancement Factor)不只是影響半導體雷射直流下的線寬也影響在高速調變下的頻率漂移 (chirp)。我們利用Hakki-Paoli方法估算出量子點雷射1260 nm雷射操作下的線寬增強因子數值約2。
至於量子點雷射靜態與動態的模擬方面，我們首先提出以變率方程式模型為基礎，推導出的小訊號等效電路來模擬量子點雷射的光學響應。我們發展出的小訊號等效電路模型是根據變率方程式模型，此包含著載子的復合、鬆弛過程、非均勻寬化(inhomogeneous broadening)、非線性增益壓縮(gain compression)與多模態頻譜雷射光。等效電路模型也可以幫助我們以電路的思考角度來了解量子點雷射的動態特性，並結合SPICE電路軟體，把量子點雷射電路嵌入在傳輸光源模組，搭配雷射驅動電路來作整體設計。
模態群組效應(mode grouping effect)包含2-5 nm的週期性雷射光譜變化，此週期比Fabry-Perot模態週期大上數十倍。由於量子點材料本身非均勻寬化，模態群組的雷射頻譜常被觀察到。我們以動態頻譜解析技術來研究各個群組的雷射動態。我們發現個別群組動態的特性會隨著電流注入大小而有所改變。同樣的技術，我們也應用在非對稱量子井雷射與InGaAsN/GaAs量子井雷射的觀察。
量子點雷射同時由基態(ground state)與激發態(excited state)雙穩態的雷射光起源於包立不相容原理造成較慢的鬆弛時間(relaxation time)。較高的基態載子填滿率會延長從激發態到基態的鬆弛時間，因此導致雙穩態雷射發生。在實驗上我們不僅是觀察到雙穩態雷射的發生，也首次觀察到同時三穩態量子點雷射的發生。在雷射靜態的隨溫度特性測量，我們發現到基態與激發態隨溫度變化的雷射臨界電流(threshold current)呈現出互補的特性。我們也成功的提出多重聲子鬆弛過程(multi-phonon relaxation)搭配著速率方程式來模擬出此臨界電流隨著溫度變化的特性。
當我們持續增加電流在雙穩態雷射的樣品，基態雷射光強度會下降，最終只剩下激發態雷射光。我們並且對雙穩態雷射也做了分光時間解析實驗，我們發現隨著電流的增加，基態光雷射啟發時間(turn-on time)會減少而後異常的增加，然而激發態光雷射啟發時間符合預期的持續減小。隨著加入電流加熱效用在速率發程式模擬上，我們也成功的模擬出異常的基態光雷射啟發時間與衰退的靜態特性。
最後，我們設計出新型的在脊狀波導的中間開了一個沒有覆蓋金屬的窗，我們發現此新型量子點雷射結構可以產生自動脈衝(self-pulsation)的雷射光。我們在此開窗的雷射不僅是加入直流電流，並外加飛秒雷射外加光源來作激發，用以觀察雷射動態特性。我們從動態的量測發現光飛秒雷射入射後會有短暫的雷射光強度下降，之後會再有強烈的鬆弛震盪發生。

關鍵字

量子點雷射 ； 砷化銦量子點 ； 半導體雷射

並列摘要

Semiconductor quantum dots(QDs) have been widely explored because of their unique fundamental properties and also because they are considered to be promising candidates for a new generation of semiconductor laser used in optical communication as light sources. In this thesis, we not only study the fundamental of InAs quantum dots but also investigate static and dynamic properties of quantum dot lasers. For the fundamental studies of quantum dots, we focus on the strength of the interaction between a self-assembled QD and a light field.
Transition dipole moment represents the oscillation strength between optical waves and electronic wavefunctions. We obtained the transition dipole moment around 32±2 Debye for InAs/InGaAs QD structures with different number of layers from the modal absorption coefficient method. The linewidth enhancement factor affects not only the linewidth of a semiconductor laser under continuous wave operation but also the frequency chirp under directly high-speed direct current modulation. For the lasing wavelength of 1260 nm,we obtain an alpha-factor of 2 for our quantum dot laser samples by Hakki-Paoli method.
As for the static and dynamic simulation, we first proposed the small-signal equivalent circuits based on a rate equation model to demonstrate the optical response frequency of quantum lasers. We developed the QD lasers equivalent circuits based on the rate-equation model that includes carrier recombination and relaxation, inhomogeneous broadening, nonlinear gain compression, and multi-mode spectral emission. The circuit model may also help us to directly describe the QD laser dynamic properties by the electric circuit means. This circuit model can be further easily incorporated into a complete circuit of for SPICE analysis and design of transmitter modules.
The mode-grouping effects induce 2-5 nm of periodic wavelength modulation which wavelength spacing is several ten times of the Fabry-Perot mode spacing. Strong mode grouping in the lasing spectra is observed due to larger inhomogeneous broadening of self-assembled quantum dots. We analyzed the spectrally-resolved dynamic behaviors of individual lasing groups within the same state at room and cryogenic temperatures. The injection-level dependence of mode-grouping lasing dynamics is proposed to interpret the transient behaviors of carriers in QD lasers. We also demonstrated non-identical quantum well and InGaAsN/GaAs single quantum well lasers for the mode grouping transients.
The simultaneous lasing emissions from ground states (GS) and excited states (ES) are due to the finite relaxation time largely caused by the Pauli exclusion principle. The higher carrier population in GS causes longer relaxation times from ES to GS and hence result in simultaneous two-state lasing. We not only observed the two-state lasing but also demonstrated the simultaneous three-state lasing of QD lasers for the first time. From the static properties of laser characteristics, it’s interesting to find that temperature trend of excited-state threshold Jth,ES is opposite to that of ground-state threshod Jth,GS .
We successfully simulated the trend of thresholds consistent to experimental results with the assumption of multi-phonon relaxation process. While increasing the applied current further above the two-state lasing condition, the ES gradually becomes the only surviving lasing state with the GS lasing state vanishing. We have demonstrated the spectrally-resolved dynamics measurement to study the two-state lasing properties. With the incorporation of current heating effect into our simulation, we successfully simulated the behaviours of two-state lasers not only in static L-I characteristics but also in their transients with an abnormal turn-on delay of GS.
Finally, we demonstrated a new device structure with a central window on the quantum-dot laser strip to generate the self-pulsation phenomenon. We apply the same device to study laser dynamics under femtosecond optical pulse excitation. Initial step-like decrease and pronounced relaxation oscillations occurred after the ultrafast pulse excitation.

並列關鍵字

quantum dot lasers ； InAs quantum dots ； semiconductor lasers

參考文獻

43. A. R. Kovsh, R. S. Hsiao, G. Lin, D. A. Livshits, I. F. Chen, Y. T. Wu, L. P. Chen, J. F. Chen, and J. Y. Chi, "Long-wavelength ridge-waveguide InGaAsN/GaAs single quantum well lasers grown by molecular beam epitaxy," in CLEO/Pacific Rim 2003(Taipei, Taiwan, 2003), p. 497.

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