Novel InGaAs/InAlAs travelling wave avalanche photodiode for ultra high speed photonic applications

互联网的爆炸性使用增加了对高速光电探测器的需求，以将光信号转换为2.5Gb/s， 10Gb/s或40Gb/s光通信系统的电信号。高灵敏度的光电探测器可以检测到非常低的光水平，这对于提高信号质量和增加传输距离，从而降低这些系统的成本是重要的。人类基因组测序、医学成像和未来的量子计算也需要它们。半导体雪崩光电二极管（apd）可以提供这些应用所需的高灵敏度，并且坚固，廉价，紧凑和高效。在apd中，电子-空穴对可以引发电子和空穴的雪崩（就像雪崩效应）。这种倍增过程提供了一个内部增益，提高了apd的灵敏度。在大多数半导体中，在乘法过程中存在显著的统计波动，从而产生不必要的多余噪声。低多余噪声很重要，这可以通过使用一种电子比空穴更容易繁殖的材料来实现（反之亦然）。在光通信系统中，使用波长为1550nm的红外光来传输信息，以尽量减少光纤中的损耗。因此，我们将不得不使用使用称为InGaAs的半导体作为吸收层来检测1550nm的红外光，并使用另一种半导体InAlAs作为倍增层来产生雪崩效应。与目前可用的1550nm的商用apd相比，InAlAs产生的多余噪声更少，因为这种材料中的电子比空穴更容易繁殖。从我们的研究中，我们知道我们可以通过使用非常薄的亚微米倍增层（<头发直径的1/50）并仔细设计InAlAs倍增层中的电场分布来进一步减少多余的噪声。我们已经证明，这些技术可以减少导致更高灵敏度APD的多余噪声。在这个项目中，我们将发展、制造和描述许多不同的设计，以尽量减少apd中的多余噪音。apd的另一个重要参数是带宽，因为它们的数据速率非常高，最高可达40Gb/s。为了实现高带宽，我们将结合以下创新：首先，我们将使用非常薄的亚微米吸收和倍增层来减少电子和空穴的传递时间。为了确保红外光被有效地吸收，我们将光限制在一个叫做光波导的特殊结构中。通过将APD与波导集成，可以有效地吸收红外光，从而产生高速高灵敏度的APD波导。其次，我们将波导apd设计成一种具有传输线特性的行波apd结构。这种结构将确保高速信号有效地从我们的APD传输到外部电路。我们将使用理论模型来预测行波的特性，以确保它们能够在高达40GHz的频率下产生高灵敏度。我们将进行几个实验，以使我们了解生产高速高灵敏度光电探测器所需的知识。在apd上进行测量，以监测从室温到-250摄氏度的温度范围内乘法如何变化，以及当信号频率增加到40GHz时乘法如何变化。这将为我们提供生产用于光通信系统的高灵敏度光电探测器所需的数据和理解，以及许多其他应用，如医学成像、环境污染物监测、制造业缺陷监测和许多其他影响我们日常生活的领域。

The explosive use of internet has increased the demand for high speed photodetectors to convert optical signal to electrical signal at 2.5Gb/s, 10Gb/s or 40Gb/s optical communication systems. High sensitivity photodetectors that can detect very low light level are important to improve the signal quality and increase the transmission distance and hence lower the cost of these systems. They are also required in sequencing of the human genome, in medical imaging and in future quantum computing. Semiconductor avalanche photodiodes (APDs) can offer the high sensitivity required for these applications and are robust, cheap, compact and efficient. In APDs an electron-hole pair can trigger an avalanche of electrons and holes (like the snow avalanche effect). This multiplication process provides an internal gain which improves the sensitivity of APDs. In most semiconductors, there is significant statistical fluctuation in the multiplication process giving rise to unwanted excess noise. Low excess noise is important and this can be achieved by using a material in which electrons can multiply much easier than holes (or vice versa).In optical communication systems infrared light with a wavelength of 1550nm is used to transmit information to minimise loss in the optical fiber. Because of this we will have to use APDs fabricated using a semiconductor called InGaAs as an absorption layer to detect infrared light of 1550nm and another semiconductor, InAlAs, as the multiplication layer to produce the avalanche effect. InAlAs produces less excess noise compared to currently available commercial APDs at 1550nm because electrons can multiply much easier than holes in this material. From our research we know that we can further reduce the excess noise by using very thin sub-micron multiplication layer (< 1/50 of the diameter of our hair) and carefully engineer the electric field profile in the InAlAs multiplication layer. We have shown that these techniques can reduce the excess noise leading to higher sensitivity APD. In this project we will grow, fabricate and characterise a number of different designs to minimise the excess noise in our APDs. Another important parameter of APDs is the bandwidth since they operate at very high data rate up to 40Gb/s. To achieve high bandwidth we will incorporate the following innovations; Firstly, we will use very thin sub-micron absorption and multiplication layers to reduce the electron and hole transit times. To ensure that the infrared light is absorbed efficiently we will confine the light in a special structure called optical waveguide. By integrating the APD with a waveguide the infrared light will be efficiently absorbed to yield high speed high sensitivity waveguide-APD. Secondly, we are going to design the waveguide-APD into a structure called travelling wave-APD which has characteristics of an electrical transmission line. This structure will ensure that high speed signals are transmitted efficiently from our APD to the external circuit. We will use a theoretical model to predict the characteristics of the travelling-wave to make sure that they can produce high sensitivity at frequency up to 40GHz.There are several experiments that we will perform to give us the understanding we need to produce a high speed high sensitivity photodetector. Measurements on the APDs to monitor how the multiplication changes with temperature ranging from room temperature down to -250 degree Celcius as well as how the multiplication changes when the signal frequency is increased up to 40GHz will be carried out. This will provide us the data and understanding required to produce very high sensitivity photodetectors for optical communication systems as well as many other applications such as for medical imaging, environmental pollutant monitoring, defects monitoring in manufacturing and many other areas that affect our daily lives.