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

本研究是为了开发一种新的技术，控制量子点的性能，重点是波长和偏振依赖性，通过利用多个参数，如点的形状和原子组成，并施加到点的应力。本文采用分子束外延技术（MBE），在普通的GaAs衬底上生长InAs量子点后，引入等离子体氮源进行原子层氮化。该工艺已被优化，在室温下在1.3□m的波长处表现出强发射。透射电子显微镜观察到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.量子点形状的控制对于确定量子点的偏振特性非常重要，当量子点用于水平器件结构时，正如大多数传统SOA所见.我们已经成功地通过优化柱状量子点的形状来调整偏振方向，在柱状量子点中，多个量子点层堆叠在一起，最终形成球形量子点。在边缘发射中，对于相等的TE与TM偏振强度，已经证明了最佳的堆叠层数。通过改变覆盖层成分（GaAs或InGaAs）来控制点上的应力也已被证实在控制偏振特性方面是有用的。这种偏振控制在光增益下也是有效的，这为该技术在真实的器件中的应用提供了理论依据，因此量子点的多参数控制在长波长量子点光学器件的开发中具有广阔的应用前景。

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.