Next generation avalanche photodiodes: realising new potentials using nm wide avalanche regions

由于光纤网络的安装，目前英国家庭可以使用Mb/s的互联网数据速率。最近，一家主要的电信公司富士通概述了他们在英国铺设Gb/s光纤网络的计划，这可以将数据速率提高到10 Gb/s甚至更高。因此，光纤将在我们的生活中发挥越来越重要的作用，因此显然需要对用于将光信号转换为电信号的超快光学元件（如光电二极管）进行研究。在光电二极管中，来自光的能量用于从原子中释放电子，并且当电子被电场扫过时会产生可检测的电流。在特别设计的雪崩光电二极管（APD）中，电场增加，使得光电效应产生的单个电子可以产生电子和空穴的雪崩。因此，产生了大得多的信号，导致更好的信噪比。不幸的是，目前的商用APD只能工作到10 Gb/s，因此不能满足未来的需求。在这个提议中，我们将开发极薄的10-50 nm半导体层，以实现ps时间尺度的雪崩效应，使得我们的APD可以以Tb/s的比特率工作。该项目将开发的新半导体材料是AlAsSb和AlGaPSb，因为它们具有很大的潜力，可以承受极高的电场，同时保持低暗电流（这对于最大限度地减少数字信号中的误差至关重要）。至关重要的是，由于我们的材料只有nm厚，我们可以设计APD中的电场，以在电子和空穴行为中施加一定程度的相干性，从而使雪崩效应以最小的噪声发生。我们相信我们的APD可以设计为接近理想的无噪声APD的性能，具有高带宽的光通信。我们最近证明，雪崩效应在薄AlAsSb是相对免疫的温度变化。因此，除了超高速光通信之外，我们提出的纳米级AlAsSb和AlGaPSb雪崩层被设想用作对环境温度波动具有高免疫力的超快光子计数器。由于光子是光的基本单位，因此通过将雪崩增益增加到大约一百万来实现“最终”光传感器，使得APD作为光子计数器工作。我们的薄雪崩层有可能在几ps内记录光子计数，这至少比目前的APD光子计数器快一个数量级。如果成功的话，我们的光子计数器的主要影响之一将是改进称为量子密钥分配的数据加密技术，其中数据使用单个光子加密。这被认为是最安全的加密技术。任何未经授权的光子探测都会导致很大的错误率，因此会提醒发送者试图进行黑客攻击。因此，我们的APD的高热稳定性和快速响应时间将增强未来量子密码系统的鲁棒性。我们也相信我们的新技术将为医疗X射线成像带来重大改进，因为APD可以提高X射线检测系统的信噪比。通常，雪崩效应将由X射线吸收引起的电信号增加到电子电路噪声之上，从而提高图像质量。我们最近的工作表明，有一个薄的雪崩层是必不可少的高性能的X射线APD。因此，我们的工作将使新一代的X射线APD成像应用。为了实现上述目标，我们将通过先进的半导体晶体生长和优化的化学蚀刻工艺以及细致的测量来进行AlAsSb和AlGaPSb APD的非常系统的开发，以提取关键材料特性，用于利用nm雪崩区域设计高性能APD。

The internet data rate of Mb/s is currently available to UK homes thanks to installation of fibre network. Recently Fujitsu, a major telecom company, outlined their plan to lay Gb/s fibre network in UK, which can increase the data rate to 10 Gb/s and beyond. Therefore optical fibre will play an ever increasing importance in our life and hence there is a clear need to carry out research in ultrafast optical components such as photodiodes, used to convert optical signal to electrical signal. In photodiodes the energy from light is used to release an electron from an atom and a detectable current is generated when the electron is swept by an electric field. In a specially designed avalanche photodiode (APD) the electric field is increased such that a single electron generated by the photoelectric effect can produce an avalanche of electrons and holes. Consequently a much larger signal is produced, leading to a better signal to noise ratio. Unfortunately current commercial APD can only work up to 10 Gb/s and is therefore not future proof. In this proposal, we will develop extremely thin 10-50 nm semiconductor layer to achieve the avalanche effect at ps time scale such that our APDs can operate at bit rates of Tb/s. The new semiconductor materials that will be developed in this project are AlAsSb and AlGaPSb since they have great potential to withstand extremely high electric field while maintaining low dark current (essential to minimise errors in digital signal). Crucially since our materials are only nm thick, we can engineer the electric field in APD to impose some degree of coherence in the electron and hole behaviours so that the avalanche effect occurs with minimal noise. We believe our APDs can be designed to approach the performance of an ideal noiseless APD with high bandwidth for optical communications. We recently demonstrated that the avalanche effect in thin AlAsSb is relatively immune to temperature change. Therefore in addition to ultra high speed optical communication, our proposed nm scaled AlAsSb and AlGaPSb avalanche layers are envisaged to work as an ultra fast photon counter with high immunity to ambient temperature fluctuation. Since a photon is the basic unit of light, the "ultimate" light sensor is achieved by increasing the avalanche gain to approximately a million so that the APD works as a photon counter. Our thin avalanche layer has the potential to register a photon count in a few ps, which is at least an order of magnitude faster than current APD photon counters. If successful one of the major impacts of our photon counter will be to improve the data encryption technique called quantum key distribution in which the data is encrypted using a single photon. This is believed to be the most secure encryption technology. Any unauthorised detection of the photon will cause a significant error rate, and hence alerting the sender of the attempted hacking. Therefore the high thermal stability and fast response time of our APDs will enhance the robustness of future quantum cryptography systems. We also believe our new technology will bring significant improvement to medical X-ray imaging as the APD can improve the signal to noise ratio of X-ray detection system. Typically the avalanche effect increases the electrical signal, induced by the X-ray absorption, to above the electronic circuit noise and hence enhancing the image quality. Our recent work showed that having a thin avalanche layer is essential for high performance X-ray APD. Hence our work will enable a new generation of X-ray APDs for imaging applications. To achieve the goals discussed above we will carry out very systematic development of AlAsSb and AlGaPSb APDs via advanced growth of the semiconductor crystals and optimised chemical etching process as well as meticulous measurements to extract key material properties for design of high performance APDs utilising nm avalanche regions.