Development of Type II Superlattice IR Detectors Monolithically Integrated on Silicon Substrates.

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BackgroundThe state-of-the-art CdHgTe (CMT) infrared photodetectors is currently dominating the market due to the near ideal optical properties for infrared sensing and well-established techniques. The current major challenges for CMT detectors are the very high cost and difficulty to obtain the required uniformity for long wavelengths. Moreover, the limited size of CdZnTe substrates widely used for growth of CMT detectors sets the upper limited of the pixel size of CMT focal plane arrays (FPA). Type II superlattice (T2SL) detectors have advantages such as lower leakage currents and greater uniformity than CMT detectors due to the advance in epitaxy growth techniques. In the type II structures, the InAs and GaSb layers can be engineered in a way that their thickness have opposite effect on the detection wavelength. Therefore, an improved tolerance to growth uncertainty and hence better uniformity can be expected from the type-II superlattice (T2SL). Despite a short history, T2SL detectors currently are approaching the performance of the state-of-the-art CMT detectors, which have been developed for over 50 years. Although T2SL photodetectors have shown great potential to be an important alternative to CMT detectors, GaSb substrates are either expensive or in limited size. The promise of long excess carrier lifetime and low leakage current is yet to be fulfilled. More importantly, similar to CMT, the FPA fabrication is very complex, consisting of over 50 individual steps, which increases the cost of T2SL FPAs. The development of T2SL detectors on Si substrates will greatly benefit from the mature Si-based IC technology. A single Si wafer can accommodate a number of large format FPAs. The possibility of monolithic integrating the read-out circuits and FPA on Si substrates also significantly reduces the fabrication cost.3. MethodologyMolecular beam epitaxy (MBE) is used to direct growth III-V T2SLs on silicon wafers. MBE is able to manipulate the deposition of materials at accuracy of sub-atomic layer per second. New thin film growth techniques will be developed and implemented in the project for hetero-epitaxy of high quality III-V materials on silicon surfaces. The expected dislocation density in the epitaxial layers is on the order of 1.0E6 per cm2 in order to fabricate well-performed devices. Standard microfabrication will be involved in this project to develop high performance photodetectors on silicon wafers. Particularly, surface passivation of the devices will be carried out to minimise the surface leakage current pathways and to distinguish the impact of threading dislocations on dark current. 4. ObjectivesThe objective of this project is to develop monolithic integration of FPAs based type II superlattice photodetectors on Si substrates. It aims to address a few scientific and technical challenges, including maximizing the quantum efficiency of type II superlattice detectors, solving growth issues of III-V materials on Si substrates, and understanding the impact of strain-induced dislocations to the device performance.5. Relevance to EPSRC's remitsThe proposed project is closely relevant to the following EPSRC's Themes and Research Ares: 1) Energy: Sensors and Instrumentation; Solar Technology; Materials for Energy Applications.2) Engineering: Sensors and Instrumentation; Materials for Energy Applications.3) Physical sciences: Photonic Materials; Materials for Energy Applications.