The ability to detect very low light level in the infrared (IR) wavelengths, down to a single photon has numerous applications ranging from enabling highly secured communication that relies on detection of a single photon, measurement of very weak fluorescence in biomolecule identification to high resolution 3 dimensional imaging based on laser ranging. Conventional semiconductor photodiodes do not have the sensitivity required for these photon-starved applications. Therefore it is necessary to use photodiodes designed with internal amplification or gain, called avalanche photodiodes (APDs), to convert the signal from a few photons to a large current that can be detected by an external electronics. In most semiconductors this amplification process also introduces excess noise. However Silicon APDs were able to produce high gain with low excess noise and therefore have been used in many applications to provide detection down to a single photon in the visible wavelengths. This is because, in Silicon the gain is provided predominantly by the electron multiplication process which reduces the excess noise. Unfortunately no commercial IR APD with performance similar to, or better than, Silicon is available despite various proposals to achieve Silicon-like APDs over the last 20 years. This exciting proposal will address this void by developing a new class of APDs based on InAs, a semiconductor with unique band structure features, to achieve high gain with negligible excess noise that is lower than that of Silicon. This proposal aims to provide IR APDs with extremely high performance, capable of detecting a single photon in the wavelength range of 1100 nm to 3000 nm. For instance they can provide low cost high performance large format imaging arrays for IR applications such as LIDAR, a technique that can provide excellent images and range measurements, non-invasive blood glucose sensing, atmospheric CO2 concentration monitoring as well as eye-safe free space optical communication. We therefore expect our APDs to generate new applications and provide highly competitive IR APDs. Based on the understanding of the InAs bandstructure, our APDs will be designed such that only electron will undergo impact ionisation to produce high avalanche gain with negligible excess noise. In addition to excellent gain, our devices can be operated at low voltage, making them compatible with off-the-shelf readout circuits. This could pave the way to a highly sensitive and affordable IR camera. To enhance the exploitation and the gain characteristics we will grow a novel InAsSb APDs on GaAs substrate which is significantly larger and cheaper than InAs substrate. This, if successful, will enable integration with commercial GaAs electronics. To propel our InAs APDs towards exploitation in the applications mentioned above we will;I) Optimise the crystal growth method to achieve high quality InAs materials with low level of impurities.II) Develop fabrication and surface passivation techniques to yield devices with low leakage current, leading to higher sensitivity.III) Pioneer techniques to implant ion species and to perform dopant diffusion to control the electric field in the InAs devices leading to high reliability.IV) Control growth conditions such as temperature and atomic pressure to achieve low crystal defect formation during the growth of InAsSb APDs on GaAs.This exciting project will be carried out by a highly skilled research team, comprising UK universities (Sheffield, Heriot-Watt and Surrey), American university (Virginia) and UK companies (Selex-Galileo and Thales Optronics) with years of experience in research and development of sensing applications. Thus, one of the outputs of the project is to provide a leading IR sensor technology to the research communities to facilitate new research and to the industry to maintain a lead in the IR sensor market.

Potential Impact:
IR wavelengths of 2000-4000nm have been used to monitor industrial emission including NH3, CO2, CO and CH4 using the differential absorption LIDAR (DIAL) technique. InSb pin diodes cooled to 77K have high detectivity and hence are usually the preferred IR detector. However despite this high detectivity InSb pin diodes can only produce a single electron-hole pair per photon and hence limits the sensitivity of DIAL systems, for instance to a maximum range of 1 km with a sensitivity of 50 ppb when used to detect CH4. Our minimally cooled InAs APDs will provide a significant enhancement to the DIAL sensitivity at the IR wavelengths up to ~3500nm since high gain can be obtained with negligible excess noise factors. A TE-cooled InAs pin diode without gain can provide detectivity similar to that of InSb. Therefore our high gain InAs APDs can be expected to provide orders of magnitude increase in the detectivity, enabling detection at the single photon level. The National Physical Laboratory (NPL) has developed one of the best DIAL systems for industrial volatile organic compound emission. We have initiated preliminary discussion with the NPL to evaluate InAs APDs for incorporation into their DIAL system. It is expected that at the end of the project, high performance InAs APDs could be supplied to NPL for evaluation. A satellite borne DIAL system can provide very accurate measurement of CO2 levels in the atmosphere. CO2 is strongly absorbed at the wavelength of 2000nm. The European Space Agency (ESA) has a strong interest in high performance IR APDs for this application. Our APDs can be developed to meet the detector specifications for future ESA CO2 monitoring programmes. We have engaged with ESA in recent years in this topic and have a clear exploitation route for satellite based research via ESA and ESA contractors, Surrey Satellite Technology Ltd. The performance of conventional long range passive long wave IR imaging systems have limited resolution and are prone to vibration and interference from surroundings. Active imaging systems at shorter IR wavelengths can overcome these limitations. Of special interest is the wavelength of 1550nm because it is relatively eyesafe, has a good atmospheric transmission characteristics and availability of high quality laser sources. Therefore laser gated imaging (LGI) provides an excellent 3-D imaging capability when a focal plane array is used. Selex-Galileo is a leading manufacturer in LGI systems using an array of highly sensitive 1550nm photodiodes and provides a clear exploitation path for detectors developed in this project. To overcome the noise floor of the system, a gain mechanism is required to amplify photocurrent generated by returning photons. Hence our high gain InAs APDs with minimal cooling and operating with low reverse bias will be ideally suited for cost effective laser gated imaging systems. Thales Optronics has a wide range of surveillance and thermal imaging products. InAs APDs can provide low cost high performance detectors for applications such as low-light imaging and spectral imaging. Hence they can be complementary to Thales' existing range of imaging products. Quantum Key Distribution (QKD) has received intense interest as highly secure communication system. However, despite much publicity, current systems are limited by system transmission range (usually 10's km) and key exchange rate (kbits-1) which is caused by detector dark counts, and particularly afterpulsing effects (these are false detections) in the detectors used, typically InGaAs/InP APDs. Although NbN nano-wire superconductors are used in some high-performance QKD demonstrations, such detectors require expensive and bulky cooling system to achieve the operating temperature of a few Kelvin. Therefore InAs devices can provide the much needed high performance detectors for QKD systems.