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

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.