Fast Switching zincblende-GaN LEDs

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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.