Materials World Network to Optimize the Growth of InGaN Quantum Dots within High Quality Optical Micro-Cavities

材料科学家多年来一直在研究晶体--无论大小。然而，非常微小的晶体-只有几个原子的晶体-表现出一些真正令人惊讶的性质，我们才刚刚开始了解。就其光学性质而言，这些非常小的晶体，我们称之为量子点，其表现出的行为更类似于单个原子，而不是大晶体。这一令人惊讶的观察结果--是载流子被限制（或捕获）在一个非常小的区域内的结果--不仅仅是一个奇怪的学术好奇心。科学家们希望利用量子点来提高激光二极管等光源的性能，并开发出全新的光源，用于新型计算机或安全通信。对于发射红光或红外线的光源，研究人员已经开始使用一种叫做砷化铟镓的材料来实现其中的一些目标。然而，对于蓝光发射-这与高密度数据存储和基于卫星的通信网络等应用特别相关-需要由不同材料制成的量子点。对于蓝色光谱区域中的光发射，可以使用由氮化铟镓（或InGaN）制成的量子点。除了它们方便的发射波长之外，InGaN量子点可能相当灵活，因为它们的发射可以通过施加外部电场来调整。此外，通过用最佳基质材料包围InGaN量子点，可以迫使它们在室温下表现出其独特的特性，而发射红光的量子点通常必须冷却到冰点以下200度以上的温度才能正常工作。不幸的是，InGaN量子点也有缺点。它们通常形成在另一种半导体氮化镓层的顶部。氮化镓很难制造，晶体中含有许多错误或缺陷。缺陷可能会带电，这种电荷的存在会改变量子点的性质。由于缺陷上的电荷随时间而变化，量子点的行为也随时间而变化-导致量子点设备的操作出现问题。为了更深入地了解InGaN量子点的性质，并改善量子点器件的性能，我们决定将量子点引入光学腔中。光学腔是光可以被限制在其中的结构。通过将量子点发射的光捕获在一个小体积内，我们可以迫使量子点和光强烈相互作用，这可以导致量子点更有效的发射。通过理解光与量子点之间的相互作用，我们还可以使用腔作为工具来探测量子点行为的细节及其与周围环境中任何缺陷的相互作用。我们希望利用腔来调整量子点的特性，以便在未来的应用中更容易开发。然而，制造空腔是非常具有挑战性的，特别是因为我们必须找到不损坏量子点的路线。由于这是一个非常复杂的问题，我们已经建立了一个国际合作，以便更有效地解决这个问题。两个在InGaN量子点方面具有专业知识的英国研究小组将与一个在腔体制造方面具有世界领先能力的美国研究小组合作。我们希望能够共同开发量子点-腔系统，该系统允许量子点和腔之间的非常强的相互作用。在未来，这样的系统不仅将被用作研究量子点特性的探针，而且将被用作新型光源的主要组成部分。

Materials scientists have been studying crystals - large and small - for many years. However, very tiny crystals - crystals only a few atoms across - exhibit some really surprising properties, which we are only just starting to understand. In terms of their optical properties, these very small crystals, which we call quantum dots, exhibit behaviour more similar to that of an individual atom, than that of a large crystal. This surprising observation - which is a consequence of the confinement (or trapping) of charge carriers within a very small region - is more than just a weird academic curiosity. Scientists hope to exploit quantum dots to allow improved performance in light sources such as laser diodes, and to develop completely new light sources which might be used in novel computers or in secure communication. For light sources emitting in the red or infra-red, researchers are already starting to realise some of these goals using a material called indium gallium arsenide. However, for light emission in the blue - which is particularly relevant to applications such as high density data storage and satellite-based communications networks - quantum dots made from different materials are required. For light emission in the blue spectral region, quantum dots made from indium gallium nitride (or InGaN) could be used. Quite apart from their convenient wavelength of emission, InGaN quantum dots might be rather flexible, since their emission can be adjusted by applying an external electric field. Also, by surrounding the InGaN quantum dots with an optimal matrix material, it may be possible to force them to exhibit their peculiar properties at room temperature, whereas quantum dots emitting in the red usually have to be cooled down to temperatures more than 200 degrees below freezing before they work properly. Unfortunately, InGaN quantum dots also have disadvantages. They are usually formed on top of layers of another semiconductor - gallium nitride. Gallium nitride is quite difficult to make, and contains many mistakes, or defects, in the crystal. The defects may become electrically charged, and the presence of this charge alters the properties of the quantum dot. Since the electrical charge on the defect varies with time, so does the behaviour of the quantum dot - leading to problems with the operation of a quantum dot device. In order to try to understand the properties of the InGaN quantum dots more thoroughly, and to improve the properties of quantum dot devices, we have decided to incorporate the quantum dots into optical cavities. An optical cavity is a structure within which light may be confined. By trapping the light emitted by the quantum dot within a small volume, we can force the quantum dot and the light to interact strongly, and this can lead to more efficient emission from the quantum dot. By understanding the interactions between the light and the quantum dot, we can also use the cavity as a tool to probe the details of the quantum dot's behaviour and its interactions with any defects in its immediate surroundings. We hope to use the cavities to tailor the quantum dots' properties so that they are easier to exploit in future applications. However, making the cavities is very challenging, particularly since we have to find routes to do this which do not damage the quantum dot. Since this is a very complex problem, we have set up an international collaboration in order to attack it more effectively. Two British research groups with expertise in InGaN quantum dots will collaborate with an American research group which has world-leading capability in cavity fabrication. Together, we hope to be able to develop quantum dot - cavity systems which allow very strong interactions between the quantum dot and the cavity. In the future such systems will be used not only as a probe to study the quantum dot properties but as a major building block of novel light sources.