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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.

We aim to achieve a breakthrough in the performance of "dilute nitride" semiconductor materials to enable the development of novel light sources and photodetectors which can operate in the mid-infrared spectral range. The 3-5 um wavelength range is technologically important because it is used for applications including; remote gas sensing, range-finding and night vision, bio-medical imaging for diagnosis in healthcare and sensitive detection in optical spectroscopy. However, the development of instrumentation is limited by the availability of efficient, affordable light sources and photodetectors, which is directly determined by the semiconductor materials which are currently available. By introducing small amounts (~ 1%) of N into InAs(Sb) we have shown that it is possible to access the mid-infrared using a new (dilute nitride) semiconductor and we are now seeking to engineer its band structure in order to significantly enhance the material's optical properties and increase quantum efficiency for light detection and emission. To enable the development of new photodetectors we will exploit the sensitivity of the conduction band to the resonant interaction of the N-level with the extended states of the host InAsSb crystal lattice to tailor the photoresponse and create a near ideal situation for electron acceleration and avalanche multiplication, resulting in a much larger detectable signal. To minimise the unwanted processes causing excessive noise and dark current, which compete with the avalanche multiplication and light detection in the detector, we shall arrange for the avalanche multiplication to be initiated by only one carrier type (electrons in our case). Many applications rely on the detection of very weak signals consisting of only a few photons. Conventional photodiodes have a limited sensitivity, especially if high speed detection is needed. In applications which are "photon starved", avalanche photodiodes (APDs) can provide an effective solution. However, at present effective avalanche multiplication in the mid-infrared spectral range can only be obtained by using exotic CdHgTe (CMT) semiconductor alloys. The resulting detectors require cooling, thus making CMT-based APDs prohibitively expensive for all except military applications. Simpler fabrication, low noise, low operating voltage, inexpensive manufacturing and room temperature operation, together with monopolar electron ionisation are all significant advantages of APDs based on the dilute nitride materials compared to existing technologies. Similarly, we shall enable the development of more efficient mid-infrared light sources. By adjusting the N content within InAsN(Sb) quantum wells and carefully tailoring the residual strain and carrier confinement, we shall be able to defeat competing non-radiative recombination processes whilst simultaneously enhancing the light generation efficiency. These novel quantum wells would then form the basis of the active region from where the light is generated, either within an LED or a diode laser. Currently mid-infrared LED efficiency is low at room temperature, and with the improvements which we shall deliver; we envisage that devices with significantly higher dc output power will be developed following our lead. Mid-infrared diode lasers incorporating our strained dilute nitride quantum wells are also expected to exhibit a reduced threshold current and could offer an affordable alternative to existing technology, especially in the 3-4 um spectral range. We will produce prototype photodetectors and LEDs and use these to demonstrate the above-mentioned avalanche behaviour and quantum efficiency improvements respectively. We shall validate our dilute nitride materials and structures in close collaboration with our collaborators at NPL, SELEX, CST and INSTRO to evaluate performance for use in practical applications and help ensure uptake of our technology.