第二型砷化銦鎵/砷銻化鎵“W”量子井之光學特性及光激發中紅外線雷射之研究

Abstract

此篇論文主要貢獻在探索與開發半導體中紅外線光源，有別於一般較昂貴且不成熟的銻化鎵及砷化銦基材，我們使用較普遍且散熱較佳的磷化銦基板成長第二型砷化銦鎵/砷銻化鎵 “W”量子井。不同於一般第二型電洞的量子井，此“W”結構藉由阻擋層(barrier layer)對電子侷限來增加電子電洞波函數耦合並提升光躍遷率。 我們透過理論計算改變各結構參數(例如量子井厚度及砷銻化鎵的比例成分)能得到2到3微米的發光範圍並且得知發光波長的延伸與光耀遷率的強度有一交替關係（trade-off），亦即發光波長越長通常伴隨光躍遷率的下降，此為“W”結構本質的特性。而在給定一個發光波長下，較薄量子井厚度及較高銻成分的結構設計組合能提供較高的電子電洞耦合。我們更進一步用八個能帶的ｋ．ｐ理論來計算能帶關係及不同載子濃度下的材料光增益，在適當的設計下單一層量子井增益即能達到10３cm-1，此已符合一般雷射元件的需求。我們也顯示出藉由適當增加電洞量子井的壓縮應變，能使電子電洞的等效質量較平衡進而降低透明載子濃度（transparency carrier density）及提高材料光增益。 在實驗上，我們使用分子束磊在（001）磷化銦基板上成長一系列的樣品，系統性地分別改變砷化銦鎵、砷銻化鎵的厚度以及砷銻化鎵合金中銻的比例，在低溫下樣品的光激發螢光光譜可顯示出2-2.5微米的波長範圍，其螢光強度與發光波長如前所述有一交換關係，並且如理論預測其螢光積分強度與計算的電子電洞波函數耦合平方成正比。樣品品質表現良好，螢光積分強度從低溫到室溫只約下降了10倍。在室溫下我們首次展示出其波長能超過3微米。我們透過變功率光激發螢光實驗來研究“W”量子井中第二型能帶排列的特徵，光譜的峰值位置隨激發功率增加而藍移，但不同於一般第二型量子井其藍移量並未與激發光功率的1/3次方成正比，對此現象我們提出局域的能態填充(localized states band filling)來解釋，其局域能態來自於合金成分的不均勻及磊晶層表面起伏所致，模擬結果與實驗值可以得到良好的吻合。 最後我們首次成功的展示室溫操作光激發2.56微米的“W”量子井中紅外線雷射，此為所知在磷化銦基板上波長最長的能帶間躍遷（interband transition）雷射，其閥值功率密度約為40kW/cm2，其特徵溫度T0在操作溫度小於250K時為487.8K，而從250K到室溫其T0為41.8K，此劇烈的T0變化及較小的室溫T0我們認為是歐傑複合所導致，經由不同雷射腔長的實驗萃取波導光損耗以及理論計算波導模態增益，我們得以估計歐傑常數約為1.67 x10-27 cm6/s。 最後我們提出並評估一方法能將“W”量子井發光波長延升至3-5微米波段，此即成長緩衝層將晶格常數轉至~5.94Ao，並接著使用In0.7Ga0.3As/GaSb/In0.7Al0.3As “W” 量子井的結構設計。

This dissertation mainly dedicates to explore and develop mid-infrared light sources. In difference with usual expensive and immature InAs-based and GaSb-based materials, we utilized more popular, better thermal conductive InP substrates and employed type-II InGaAs/GaAsSb “W” type quantum wells (QWs). Distinguishing from general type-II hole quantum well, the “W” structure includes barrier layers to confine electrons and to increase electron-hole wavefunction overlap, hence the optical transition rate. Through theoretical calculations changed with structure parameters (such as QWs thickness and GaAsSb composition), a range of 2-3µm emission wavelength can be derived. It also shows a trade-off situation between the extending of emission wavelength and the intensity of optical transition rate, meaning a longer emission wavelength generally accompanied with a decreased optical transition rate, which is an intrinsic feature for the “W” structure. However, at a given emission wavelength, the design with thinner InGaAs and GaAsSb layers and a higher Sb content in GaAsSb is more desirable, which could provide of a larger electron-hole wavefunction overlap. We further used the eight-band k.p theory to calculate the E-k relation and material gain as a function of carrier density. With a proper design, the material gain of a single “W” QW is able to reach above 10^3 cm-1, which is sufficient for general mid-IR lasers applications. We also pointed out that adopting proper compressive strain in hole QW makes a more balanced electron and hole masses that could reduce the transparency carrier density and increase the material gain. In experiments, we grew a series of samples systematically varied with thickness of InGaAs, thickness of GaAsSb and Sb mole fraction in GaAsSb. The photoluminescence (PL) spectra of samples cover the range of 2-2.5 µm at low temperature. The trade-off between optical transition rate and wavelength emission is confirmed as the prediction of the theoretical calculations, where the integrated PL intensity is proportional to the square of electron-hole wavefunction overlap. The samples showed good optical quality that integrated PL intensity only decreases an order from cryogenic temperature to room temperature (RT). For the first time we demonstrated PL emission wavelength longer than 3 µm at RT by the InP-based “W” structure. The type-II band alignment in the “W” structure has been characterized by the power dependent PL measurements. The peak position shifts to shorter wavelength as the excitation power (Pex) increases. It was found the amount of energy shifts does not follow the Pex^1/3 law as most type II structures. The localized states filling effect due to the surface roughness and alloy fluctuation is proposed to explain the observed phenomenon. The calculated results agree well with the experiment results. For the first time we demonstrated the room temperature optically-pumped mid-IR “W” type lasers on InP substrates. The lasing wave length is 2.56 µm, which is known as the longest for the InP-based interband transition, with a threshold pumping power density of ~40kW/cm2. The laser shows a characteristic temperature (T0) of 487.8K as operated below 250K and a T0 of 41.8K as operated near room temperature. This abrupt T0 change and the small T0 at room temperature are considered due to the dominated Auger processes. An Auger coefficient of 1.67x10-27 cm^6/s was estimated via different laser cavity length studies for the extraction of waveguide optical loss and theoretical calculations for the waveguide modal gain. Finally, we propose and evaluate an approach capable of extending the “W” QWs emission wavelengths into 3-5µm regime, which uses a metamorphic buffer layer to shift the lattice constant to ~5.94Å and then grows the designed “W” QW structure of In0.7Ga0.3As/ GaSb/In0.7Al0.3As.