The tri-service Vital Infrared Sensor Technology Acceleration (VISTA) program rapidly matured III-V semiconductor epitaxy to produce tactically viable detectors using Type II Superlattice (T2SL) structures. The T2SL material system allows tunable band gaps for creating lattice-matched heterojunction devices. Heterojunction devices are integral to suppressing sources of dark currents, such as internal Shockley Reed Hall (SRH) and device surface currents. Once the VISTA program demonstrated that T2SL detectors offered competitive performance to traditional indium antimonide (InSb) detectors at an operating temperature 40K to 50 K higher, many opportunities emerged. This elevation in operating temperature provides two benefits to infrared (IR) sensors. The first is to miniaturize the integrated Dewar-electronicscooler assembly (IDECA) such that it can support small aerial vehicle and soldier mounted sensors. The second is to increase the mean time to failure (MTTF) of an existing InSb IDECA. To benefit from T2SL higher operating temperature (HOT) detectors, the overall cost of the IDECA must be competitive with InSb. This drives a manufacturing capability that is equivalent to InSb. At the L3 Space and Sensors Technology Center (L3 SSTC), the III-V detector foundry processes 125 mm diameter InSb wafers. The development of 125 mm diameter T2SL detector wafers started with the gallium antimonide substrates. The greater size and weight of these substrates required extra care to avoid breakage. Leveraging the learning reported from the silicon industry, we developed a specification for the substrate thickness and edge bevel to provide a robust platform for wafer processing. Next, we worked with commercial III-V epitaxy suppliers to develop multi-wafer growth capability for 125 mm diameter substrates. The results of this effort, funded by the Office of the Secretary of Defense (OSD) Defense-wide Manufacturing Science and Technology (DMST) program through the Army Night Vision and Electronic Sensor Directorate (NVESD), we were able to improve focal plane array (FPA) yield from virtually zero to InSb manufacturing levels.
Rigorous electromagnetic field modeling is applied to calculate the quantum efficiency (QE) of various quantum well
infrared photodetector (QWIP) geometries. We found quantitative agreement between theory and experiment for linear
grating coupled QWIPs, cross-grating coupled QWIPs, corrugated-QWIPs, and enhanced-QWIPs. Also, the model
adequately explains the spectral lineshapes of the quantum grid infrared photodetectors. Equipped with a quantitative
model, we designed resonant cavities that are suitable for narrowband imaging around 8 - 9 microns. The results show
that with properly designed structures, the theoretical QE can be as high as 78% for 25-micron pixel pitch arrays and
46% for 12-micron pixel pitch arrays. Experimental efforts are underway.
Rigorous electromagnetic (EM) field modeling is applied to calculate the external quantum efficiency (QE) of various
quantum well infrared photodetector (QWIP) pixel geometries with thinned substrates. We found that for a 24 × 24 ×
1.5 μm3 cross-grating QWIP, the QE is peaked at 13.0, 11.0, and 8.4 μm, insensitive to the grating periods. These peaks
are identified as the first three harmonic resonances associated with the pixel resonant cavity. For a regular prismshaped
corrugated QWIP (C-QWIPs) with a 25-μm pitch, the QE oscillates about its classical value of 24.5% within the
calculated wavelength range from 3 to 15 μm. A peaked value of 32% occurs at 9.1 μm. For pyramidal C-QWIPs, the
maximum QE is 42%, and for cone-shaped C-QWIPs, it is 35%. In the presence of an anti-reflection coating, the
oscillation amplitude diminishes, and the average values generally rise to near the peaks of the oscillations. The
modeling results are compared with the experimental data for grating QWIP focal plane arrays (FPAs) and prismshaped
C-QWIP FPAs; satisfactory agreements were achieved for both. After verifying our EM approach, we explored
other detector geometries and found new types of resonator QWIPs (R-QWIPs) that can provide 30% QE at certain
wavelengths on a 1.5-μm-thick active material. Combining the high QE of a resonator and the high gain of a thin
material layer, the new R-QWIPs will have a conversion efficiency far higher than the existing QWIP detectors. The
present resonator approach will also have an impact on other detector technologies.