Multi-parameter controlled quantum dots and its application to ultrafast optical functional devices

本研究旨在开发一种新的技术来控制量子点的性质，重点是波长和偏振依赖，利用多个参数，如点的形状和原子组成，以及施加在点上的应力。由于量子点在半导体光放大器（soa）和开关等光通信器件中的实际应用具有重要意义，因此提出了这一目标。利用等离子体氮源，在砷化镓（GaAs）衬底上普通sk模式生长InAs量子点之后，引入原子层氮化步骤，开发了一种独特的量子点MBE生长技术。经过优化，该工艺在室温下表现出波长为1.3□m的强发射。透射电镜结果表明，在点表面存在氮化层，优化氮化时间可以抑制位错的产生。在大多数传统soa中，在水平器件结构中使用量子点时，量子点形状的控制对于确定量子点的极化特性非常重要。我们成功地通过优化柱状量子点的形状来调整偏振方向，其中多个点层堆叠最终形成球形点。在边缘发射中，已经证明了在TE和TM极化强度相等的情况下，最佳的衬垫层数。通过改变盖层成分（GaAs或InGaAs）来控制点上的应力也被证实有助于控制极化特性。在光增益下，这种偏振控制也被证明是有效的，这支持了该技术在实际设备中的应用。因此，量子点多参数控制是开发长波域实用量子点光学器件的重要技术。

This research was conducted for developing a novel technique of controlling quantum dot properties with an emphasis on the wavelength and polarization dependence by utilizing multiple parameters such as the dot shape and atomic composition, and stress applied to the dot. This target was taken up because of its importance in real world application of quantum dots to optical communication devices such as semiconductor optical amplifiers (SOAs) and switches.A unique MBE growth technique of quantum dots has been developed by introducing an atomic-layer nitridation step using plasma nitrogen source just after ordinary SK-mode growth of InAs quantum dots on GaAs substrates. This process has been optimized to exhibit intense emission at the wavelength of 1.3□m at room temperature. Transmission electron microscopy has shown existence of nitrided Layer on top of the dot surface, and the generation of dislocation has been shown to be suppressed when the nitridation time is optimized.Control of quantum dot shape has been found to be extremely important in determining the polarization properties of quantum dots when used in horizontal device structures as seen in most of conventional SOAs. We have succeeded to adjust the polarization direction by optimizing the shape of columnar quantum dot in which multiple dot layers are stacked to eventually make a spherical dot. Optimum staking layer numbers have been demonstrated for equal TE vs. TM polarization intensities in the edge emission. The control of stress on the dot by changing the capping layer composition (GaAs or InGaAs) has also been confirmed to be useful in controlling the polarization properties. Such polarization control has been also demonstrated to be effective under optical gain, which supports the application of this technique in real devices.Thus multiple parameter control of quantum dots is promising technique in developing practical quantum dot optical devices in the long wavelength region.