| Summary: | 博士 === 國立臺灣大學 === 光電工程學研究所 === 98 === 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.
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