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大大小小的晶体。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——这是在一个非常小的区域内限制（或捕获）载流子的结果——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，并开发出可用于新型计算机或安全通信的全新光源。对于发出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。对于蓝色光谱区域的光发射，可以使用由氮化铟镓（InGaN）制成的量子点。除了它们方便的发射波长外，InGaN量子点可能相当灵活，因为它们的发射可以通过施加外电场来调节。此外，通过用最佳基质材料包裹InGaN量子点，可能会迫使它们在室温下表现出其独特的特性，而在红色中发射的量子点通常必须冷却到零下200度以上才能正常工作。不幸的是，InGaN量子点也有缺点。它们通常是在另一种半导体——氮化镓的层上形成的。氮化镓很难制造，而且在晶体中含有许多错误或缺陷。这些缺陷可能变成带电的，而这种带电的存在改变了量子点的性质。由于缺陷上的电荷随时间而变化，量子点的行为也会随之变化，从而导致量子点器件的操作问题。为了更彻底地了解InGaN量子点的性质，并改善量子点器件的性质，我们决定将量子点纳入光学腔中。光腔是一种可以将光限制在其中的结构。通过将量子点发出的光捕获在一个小体积内，我们可以迫使量子点和光强烈地相互作用，这可以导致量子点更有效地发射。通过理解光和量子点之间的相互作用，我们还可以利用空腔作为工具来探测量子点行为的细节，以及它与周围环境中任何缺陷的相互作用。我们希望利用这些空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破坏量子点的方法。由于这是一个非常复杂的问题，我们已经建立了国际合作，以便更有效地解决这个问题。两个在InGaN量子点方面具有专长的英国研究小组将与一个在空腔制造方面具有世界领先能力的美国研究小组合作。在一起，我们希望能够开发量子点-腔系统，使量子点和腔之间的相互作用非常强。在未来，这样的系统不仅将被用作研究量子点特性的探针，而且还将作为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.