NOVEL SELF-POWERED SOLID-STATE NEUTRON DETECTORS
Solid-state neutron detectors with high neutron detection efficiency and low gamma sensitivity are required for a variety of applications including homeland security, nuclear safeguards and process monitoring. Our work has been targeted at neutron detector fabrication with hexagonal, parallelogram and trench designs. Each design with high aspect ratio can be achieved either by dry etch or wet etch. Converter materials such as boron, boron nitride and parylene can be used to fill high-aspect-ratio holes. With optimal design and filling, the detection efficiency of our neutron detectors can be up to ~48%.
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MOCVD OF II-VI SEMICONDUCTORS FOR SOLAR CELL APPLICATIONS
CdTe is an important solar cell material due to its ideal direct band gap of 1.5eV and high optical absorption coefficient. Recently, a record high efficiency of ~21.5% has been achieved surpassing the previous best of 20.4% using polycrystalline CdTe. However, the efficiency of CdTe solar cells is limited by low Open circuit voltage (Voc) as compared to the materials with similar bandgap. We work on MOCVD grown single-crystal CdTe to obtain high Voc solar cells due to its inherent advantages such as -negligible effect of grain boundaries, good doping control and high minority lifetime. We also grow polycrystalline CdTe for high Voc solar cells by optimizing doping and increasing lifetime of minority carriers.
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THIN FILM DEPOSITION FOR SURFACE PASSIVATION OF IR DETECTORS
Surface passivation of HgCdTe photovoltaic IR detectors improves their performance greatly. Metal-organic chemical vapor deposition (MOCVD) is used as the deposition technique for such passivation films. It has several advantages like excellent uniformity and conformality, which makes it favorable for deposition of thin films on complex device structures with high aspect ratios. CdTe is the most preferred material for passivation as it is near lattice matched to HgCdTe, satisfies the dangling bonds at the surface, has a high resistivity and has a higher bandgap and hence acts as a good minority carrier reflector. HgCdTe surfaces cannot be exposed to high temperatures as Hg is volatile and may deplete from the surface. We have developed a novel method to deposit CdTe films at temperatures as low as 135 C while facilitating cracking of precursors at a temperature as high as 600 C.Atomic layer deposition (ALD) of CdS is also being investigated for surface passivation of such devices.
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