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

这项研究是为了开发一种新的技术来控制量子点的性质，重点是通过利用点的形状和原子组成以及施加在点上的应力等多个参数来控制量子点的波长和偏振相关性。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，这一目标被人们所重视。在普通SK模式生长InAs量子点的基础上，引入了利用等离子体氮源的原子层氮化步骤，开发了一种独特的量子点分子束外延生长技术。这一过程已经被优化，在室温下表现出1.3m波长的强烈发射。透射电子显微镜显示，在量子点表面存在氮化层，当氮化时间优化时，位错的产生被抑制。量子点形状的控制被发现对于决定量子点在水平器件结构中的偏振特性是极其重要的，就像在大多数传统的SOAS中所看到的那样。我们通过优化柱状量子点的形状成功地调整了偏振方向，在柱状量子点中堆积了多层量子点，最终形成了一个球形点。证明了在边缘发射中TE和TM极化强度相等时的最佳堆积层数。通过改变盖层成分(GaAs或InGaAs)来控制点上的应力也被证实在控制偏振特性方面是有用的。这种偏振控制也被证明在光学增益下是有效的，这支持了该技术在实际器件中的应用。因此，量子点的多参数控制在开发实用的长波长量子点光学器件方面是很有前途的技术。

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.