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

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.

Our proposed next generation avalanche photodiodes (APDs) can have major impact on the economy, introduce a key technological component in optical systems that have profound benefit to the society as well as providing an excellent platform for training of skilled workers in the area of optoelectronics.

Telecom: 100 Gb/s optical network has already been evaluated and launched in many countries. The continuing demand for high internet data will require future systems to operate at higher bit rates. Our pioneering APDs with THz gain-bandwidth product can both improve current systems at 40 and 100 Gb/s as well as cater for future systems. We will work very closely with our partner at Centre for Integrated Photonics (CIP) to accelerate the technology readiness level of our APDs and ensure UK's competitiveness. In addition to CIP, we will make the outputs from this project available to Oclaro Caswell who have the industry's most advance 3" InP wafer fabrication facility and therefore can potentially provide a mass-market exploitation route for this next generation of high speed APDs. Sheffield already has worked closely with CIP and Oclaro in previous projects.

Health: The X-ray APDs developed in this could lay the foundation for development of 2D arrays that are compatible with readout electronics such as Medipix. This could lead to highly portable and low cost X-ray imaging cameras for nuclear medicine (e.g: Scintigraphy and Positron Emission Tomography). Our collaborators at the Institute of High Energy Physics (IHEP), Russia, have expertise in developing X-ray medical scanners. We aim to develop high resolution medical imaging.

Environment: There is a strong interest in improving the sensitivity of Light Detection and Ranging (LIDAR) systems for monitoring o atmospheric aerosols at ~1540 nm due to the higher permissible laser power. The European Space Agency (ESA) has a strong interest in high performance APDs and SPADs for earth monitoring. By replacing InP with Sb alloy as the multiplication material, our proposed APDs/SPADs can be developed to operate at room temperature with low false count. SELEX-Galileo, the largest manufacturer of IR detection systems in the UK, is ideally suited to integrate our novel APDs into their product portfolio to ensure the UK holds the lead in IR imaging camera, range finder and LIDAR systems. We shall develop our APDs to be compatible with LIDAR requirements and transfer our knowledge to SELEX.

Security: X-ray APD arrays can be integrated with appropriate readout electronics to achieve compact and low cost X-ray imaging systems that are routinely used for security screenings at airports and security check points. The APDs and SPADs technologies developed will also bring unrivalled sensitivity to the imaging systems. We shall engage with UK companies such as Kromek, Todd Research and e2V to explore collaboration schemes to develop future X-arrays systems. We will also work with ID Quantique to incorporate our detectors in enhance quantum cryptography systems.

People:The work programme is structured to provide the post-doctoral researchers and PhD students with solid research skill development, leading to journal publications and enhanced their skill portfolios. They will gain valuable technical training in the areas of growth and fabrication of state-of-the-art semiconductor devices.

Knowledge:This research will uncover a range of new data and understanding of the growth, fabrication and high field carrier transports in wide band gap AlAsSb and AlGaPSb. These alloys are potentially suitable for design of high power double heterojunction bipolar transistors, quantum cascade lasers and intersubband optical switches.