Fast Switching zincblende-GaN LEDs

如今，每个人都希望无论走到哪里都能访问移动数据。这意味着大量的信息必须以无线方式传输，通常使用无线电波。为了让每个人的信息快速、独立和私密地传播，需要在不同的无线电频率上有许多不同的频道，而越来越多的不同频率无法满足我们所有的数据需求。这个问题的一个解决方案是通过其他类型的电磁波传输数据，而不仅仅是无线电波。光波是一个非常好的选择，因为不同颜色（或波长）的光可以组成许多额外的通道，从而可以传输大量额外的数据。在这样的光学无线通信系统中，数据通过改变光的强度来传输。为了实现快速数据传输，能够非常快速地打开和关闭用于数据传输的光源非常重要，理想情况下每秒超过10亿次。大多数标准光源的速度比这个慢得多，但是微小的发光二极管（led），被称为微型led，直径只有几十微米，提供了所需的快速开关和卓越的能源效率。led已经广泛应用于照明领域。不幸的是，对于这些基于氮化镓（GaN）的器件，原子在材料中的排列方式（晶体结构）的本质使得很难在整个可见波长范围内实现快速切换。这限制了可以打开的通信信道的数量，因为在某些波长没有任何快速切换设备可用。然而，本提案的申请人已经开发出一种以替代晶体结构生长GaN的方法，称为立方（或锌闪锌矿）结构，它可以克服通常六方（或纤锌矿）结构的固有局限性。基于锌闪锌矿氮化镓的led还处于起步阶段，但越来越多的证据表明，它们可以用来制造跨可见光谱的快速开关微型led。为了使这一愿景成为现实，材料的许多方面都需要优化。我们需要了解晶体中的缺陷（或错误）不仅会影响开关速度，还会影响微型led的效率和发射光的颜色纯度。（颜色纯度很重要，因为如果LED发出整个颜色范围，就很难用它来创建许多单独的信息通道，每个通道都有不同的颜色）。如果晶体中的缺陷导致这些方面的任何问题，我们需要改变我们制造材料的方式，要么减少缺陷密度，要么使LED对缺陷的存在更坚固。我们还需要优化微型led的设计方式。LED材料被沉积成不同化学组成的多层，每层都需要有合适的厚度和成分，并在合适的温度和压力条件下沉积，以确保其具有最佳性能。我们将使用最先进的显微镜来探索材料的制造方式、结构和性能之间的重要联系，并利用这些研究的见解来指导进一步改进材料的设计和制造。我们还将开发新的工艺，将沉积材料层转化为微小的微型led，适当地与外界连接，以测试其高速开关性能。总的来说，这个项目将使我们能够采用一种新兴材料——锌铀矿氮化镓——并将其发展成为一种真正的光学无线通信技术。我们将设计，开发和测试用于高频应用的微型led，并与工业合作伙伴合作，加速技术向现实世界的应用。

These days, everyone expects to be able to access mobile data wherever they go. This means that an enormous amount of information must be transmitted wirelessly, typically using radio waves. To keep everyone's information moving quickly, separately and privately, requires many distinct channels at different radio frequencies, and increasingly there just aren't enough different frequencies to fulfil all our data needs. One solution to this problem is to transmit data on other types of electromagnetic waves, not just radio waves. Light waves are a very good option, because different colours (or wavelengths) of light can make up lots of extra channels so that a large amount of extra data can be transmitted. In such an optical wireless communication systems, data is transmitted via changes to the intensity of the light. For fast data transfer, it's thus important to be able to turn the light source used for data transmission on and off very quickly, ideally more than a billion times per second. Most standard light sources are much slower than this, but tiny light emitting diodes (LEDs), known as microLEDs, only a few tens of micrometres across, offer both the required fast switching and excellent energy efficiencies. LEDs are already widely used in lighting. Unfortunately, for these devices, which are based on gallium nitride (GaN), the very nature of how the atoms are arranged in the material (the crystal structure) makes it difficult to achieve fast switching across the whole visible wavelength range. This limits the number of communication channels that could be opened up, because there aren't any fast-switching devices available at some wavelengths. However, the applicants in this proposal have developed a way to grow GaN in an alternative crystal structure, known as the cubic (or zincblende) structure, which can overcome the inherent limitations of the usual hexagonal (or wurtzite) structure. LEDs based on zincblende GaN are in their infancy, but evidence is building that they can be used to make fast switching microLEDs right across the visible spectrum.To make this vision a reality, many aspects of the material need to be optimised. We need to understand how defects (or mistakes) in the crystal affect not only the switching speed, but also the efficiency of the microLED and the colour purity of the emitted light. (Colour purity is important because if the LED emits a whole range of colours, it becomes difficult to use it to create many separate information channels, each at a distinct colour). If defects in the crystal cause problems with any of these aspects, we need to change the way we make the materials, to either reduce the defect density or to make the LED more robust to the presence of defects.We also need to optimise the way the microLED is designed. LED materials are deposited as many layers of different chemical makeup, and each layer needs to have the right thickness and composition, and to be laid down under exactly the right conditions of temperature and pressure, to ensure it has optimum properties. We will use state-of the art microscopes to explore these vital links between how the material is made, its structure and its properties, and use insights from these studies to guide further improvements to the design and fabrication of the material. We will also develop new processes to transform the layers of deposited material into tiny microLEDs, appropriately connected to the outside world to allow testing of their high-speed switching performance.Overall, this project will allow us to take an emerging material - zincblende GaN - and develop it into a real technology for optical wireless communications. We will design, develop and test microLEDs for high frequency applications and work with industrial partners to accelerate the technology towards real world applications.