A hybrid photodetector based on a Gen 3 photocathode and electron-bombarded silicon, non-pixilated, position sensitive, Avalanche Photo Diode (APD) is being developed. The device promises gains of over 106 and sub-millimeter spatial resolution. Signals read at the output of the device can be used to build up images, integrated over the time scales relevant to the process being studied. This integration as a post-process allows significant flexibility in investigation at very low light levels. A design and fabrication process is being developed that can be readily adapted for fast-turnaround proof-of-concept prototypes using a variety of solid state detectors. This process approach also facilitates the parallel development of high Quantum Efficiency (QE), low dark count III-V based photocathodes with a broad range of spectral response from UV to NIR. The Imaging Hybrid Avalanche Photo Diode (IHAPD) is targeted to bioluminescence, chemoluminescence and other low light level spectral imaging. A discussion of a reflection mode hybrid APD development is included as well.
To achieve the DoD objective of low cost high performance infrared focal plane arrays a manufacturing technique is required which is intrinsically flexible with respect to device configuration and cutoff wavelength and easily scaleable with respect to volume requirements. The approach adopted is to fully develop the technology of molecular beam epitaxy (MBE) to a level where detector array wafers with a variety of configurations can be fabricated with first pass success at a reduced cost. As a vapor phase process, MBE lends itself directly to: (1) the inclusion of real-time monitoring and process control, (2) a single or multiple wafer growth mode, (3) nearly instantaneous changes in growth parameters. A team has been assembled to carry out the program. It is composed of four industrial organizations -- Rockwell International, Hughes Aircraft Company, Texas Instruments, and Lockheed-Martin, and a university -- Georgia Tech Research Institute. Since team members are committed suppliers and users of IRFPAs, technology transfer among team members is accomplished in real-time. The technical approach has been focused on optimizing the processes necessary to fabricate p-on-n HgCdTe double layer heterostructure focal plane arrays, reducing process variance, and on documenting flexibility with respect to cutoff wavelength. Two device structures have been investigated and fabricated -- a 480 by 4 and a 128 by 128.
The electrical and optical properties of iodine doped n-type HgCdTe alloys and superlattices grown by metalorganic molecular beam epitaxy using ethyliodide are reviewed. The rationale for the use of iodine rather than indium as the dopant species and the incorporation kinetics of iodine at the growth surface are discussed. The electrical and optical properties of iodine- doped CdTe and HgCdTe (x equals 0.24) are presented for carrier concentrations between 1015 and 1018 cm-3, as determined by Hall effect measurements and low- and room-temperature photoluminescence spectroscopy. These samples show strong room temperature excitonic effects due to free exciton and band to band recombination as determined by photoluminescence excitation spectroscopy. The electrical and optical properties of iodine-doped HgCdTe-CdTe superlattices also are discussed based on magnetoluminescence measurements in tilted magnetic fields of up to 7 Tesla in Voigt and Faraday geometry.
A brief review is given of recent results to assess the capability of metalorganic molecular beam epitaxy for the low-temperature growth of high-quality low-carrier-concentration CdTe and HgCdTe alloys. In particular, studies of this technique to produce highly uniform HgCdTe material and the extension of the gas source doping of CdTe and HgCdTe with ethyliodide so as to obtain back-doped electron concentrations from 1015 to 1018 cm-3 are reported. Some preliminary results on the growth of ternary CdTe/HgCdTe superlattices and the p-type doping of CdTe with As will also be presented. The electrical and optical properties of these materials were determined by resistivity and Hall effect, photoluminescence, and IR transmission measurements between 300 and 10 K.
A brief review is given of the development of a metalorganic molecular beam epitaxial system for Hg-based II-VI semiconductors. Recent results on the growth of HgZnTe, HgCdTe, and iodine-doped CdTe epitaxial layers are presented and demonstrate the potential of this technique for the growth of high-quality materials.
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