Near Infrared Superluminescent Diodes

半導體發光元件具有小體積化、高整合性、高效率，以及長壽命等優點。近年來，半導體的白光發射器，如寬頻的發光二極體，半導體光放大器和超螢光發光二極體等被廣泛的研究與推承。其潛在的應用範圍包括了固態照明、生物光學影像系統，與光纖通信等相關領域。為了提高元件的輸出功率和光頻寬，最直接與便利的方法就是增加其量子井的數目，以及結合不同的波長的量子井作為主動發光區。然而，在傳統型垂直架構的白光發光元件中，由於電洞的移動率與速度遠較電子低很多，縱使電子可以很快均勻地分散在每個量子井中，但對電洞而言卻非如此。電洞的分佈會傾向集中於較靠近P型區的量子井，並且隨著遠離而逐漸減少，若是量子井數目過多，甚至可能導致靠近N型區的量子井沒有電洞注入，因而使得光頻寬與發光效率被限制住。此外，這個載子分布不均的問題亦會導致不穩定的光頻譜，其頻譜峰值與形狀會隨著電流調動而劇烈地變化。為了克服此問題，在本論文中，我們提出了兩種全新類型的超螢光發光二極體元件，這些特殊的元件可同時結合了雷射的高功率，以及發光二極體的寬頻特性。 第一種元件為雙極性串接超螢光發光二極體 (Bipolar cascade superluminescent diode)。我們在多波長量子井主動區中插入了一個反置之高參雜且厚度甚薄的p-n層成為穿隧接面，當載子靠近此薄接面兩側會被其強電場吸引而穿隧道另一主動區，因而在不同主動區都有具有幾乎相同數目的載子注入，創造了一個載子均勻分布的環境。對於一個長度為0.5毫米的元件，在180毫安培的脈衝電流注入下，此元件可具有40奈米的光頻寬與5毫瓦的功率輸出。 第二種元件為橫向接面架構的超螢光發光二極體 (Transverse junction superluminescent diode)。我們利用局部鋅擴散的方式，在多重發光波長的砷化銦鎵多重量子井的主動區側邊製作了一P型區。在此架構下，從正電極注入P型區的電洞應具有相等位勢，而可均勻地且集中地從側壁的注入到主動區中，並與電子復合發光。此種載子傳輸機制遠不同於傳統的垂直型寬頻元件，其隨著外加電流的提升，電洞從P型區的量子井漸漸地流入靠N型區的量子井地傳輸。因此，在我們所提出的橫向接面超螢光二極體中，其光頻譜的峰值位置與頻寬不隨著電流改變而劇烈變化，提供了較大範圍且頻寬較穩定的電流調制區域。除此之外，其頻譜特性乃是取決何處頻率具有最高的材料增益以及最好的光場侷限。在常溫，且操作電流為連續之200毫安培的情形下，此元件可以具有20奈米的光頻寬與5.3毫瓦的光功率輸出。 為了讓橫向接面超螢光二極體具有更佳的特性，包括更寬的頻寬與較大的光功率輸出，我們亦採用了量子井層數弭補的設計，以及應力補償的量子井架構，來作為橫向接面元件的主動區。此種設計可提升短波長的總發光強度，避免大多數的較高能量的光子被輻射出長波長的量子井吸收；另者，應力補償的架構可使得我們可以成長厚度較厚、數量較多的量子井層成為主動區，並大幅地減少材料缺陷，因而提升元件特性。相較於我們之前的第一代元件，在第二代元件中，我們可以得到豐碩的結果，包括顯著地減少臨界電流，近四倍輸出功率的提升，以及較寬的光頻寬。 我們所示範的寬頻元件技術，包括雙極性串接式，與橫向接面超螢光二極體架構，提供了高性能半導體寬頻發光元件一個良好的前景與展望。Semiconductor light emitters can enjoy the advantages of compactness, ease of integration, high efficiency and long lifetime, in contrast to the bulky, low efficiency and short lifetime traditional light sources. Recently, semiconductor based white-light emitters, such as broadband light-emitting diodes (LEDs), semiconductor optical amplifiers (SOAs) and superluminescent diodes (SLDs) have attracted a lot of attention in terms of their potential applications in solid state lighting, bio-optical imaging and optical fiber communication. In order to improve the device output power and optical bandwidths, increasing the number of quantum wells (QWs) and using different wavelength emission QWs in the active region are attractive and straightforward approaches. However, in traditional vertical junction (VJ) white-light emitters, the much lower mobility and speed of the holes as compared to those of the electrons leads to non-uniform carrier distribution in each quantum well. Therefore, the recombination of the injected carriers and the emission density will be inhomogeneous in the active regime, the active area, and even the light-emitting volume of materials. This problem frustrates the development of high power and wider available bandwidth white-light emitters. In addition, it will result in unstable spectrum traces and a small bias modulation range. There could be significant variation in spectrum traces with an increase in the applied bias. In this thesis, we demonstrate two types of white-light emitters—the bipolar cascade (BC) SLDs and the transverse junction (TJ) SLDs, designed to minimize the above problems. Such devices combine the advantages of the high power of lasers with the broadband emission of LEDs by amplifying the superluminescence, or so-called amplified spontaneous emissions (ASE), with a low degree of coherence. The strained InGaAs/GaAs BC-SLDs are realized by epitaxially growing a pair of reversed heavily doped p-n AlGaAs layers that functions as a tunnel junction between a chirped multiple-quantum-wells (MQWs) in the active regime. This structure enforces virtually the same carrier recycling rate in each stage after each tunneling process so the optical spectrum thus can be broadened. The TJ-SLDs consisted of chirped InGaAs/GaAs MQWs are also devoted to provide a uniform carrier environment in the active region by utilizing the unique characteristic of a horizontally injected carrier flow from the sidewall of the MQWs instead of by well-by-well injection. The occurrence of the superluminescent phenomenon in TJ-SLDs is therefore governed by the highest material gain and optical confinement in the active regime rather than by the well closest to the topmost p-side cladding layer. In order to obtain improved performance, we further manipulate the QW number and recruit a set of InxGa1-xAs/GaAs0.9P0.1 strain compensated (SC) MQWs to compensate for the high compressive strain of the wells. We achieve successful results including a significant improvement in the threshold current, an enhancement of the output power, and wider optical bandwidths compared to TJ-SLDs without SC MQWs. Furthermore, compared with the high-performance ~1um VJ-SLDs, our novel BC- and TJ-SLDs exhibit comparable output power and 3-dB bandwidth performance with a more stable electroluminescence (EL) spectrum, which varies only negligibly under a wide range of bias currents. These techniques provide a promising means for attaining high performance semiconductor white-light generating devices