Type-II strained layer superlattices (SLS) are a rapidly maturing technology for infrared imaging applications,
with performance approaching that of HgCdTe1,2,3,4. Teledyne Imaging Sensors (TIS), in partnership with the
Naval Research Laboratory (NRL), has recently demonstrated state-of-the-art, LWIR, SLS 256 × 256 focal
plane arrays (FPAs) with cutoff wavelengths ranging from 9.4 to 11.5 μm. The dark current performance of
these arrays is within a factor of 10-20 of (state-of-the-art) HgCdTe. Dark current characteristics of
unpassivated and passivated devices exhibit bulk-limited behavior, essential for FPA applications. TIS has
also demonstrated rapid substrate thinning processes for increased infrared transmission through the GaSb
substrate. In addition to this work, this presentation will discuss the recent developments of 1K x 1K LWIR
Performance of HgCdTe detector technology surpasses all others in the mid-wave and long-wave infrared spectrum. This technology is relatively mature with current effort focused on improving uniformity, and demonstrating increased focal plane array (FPA) functionality. Type-II superlattice (InAs-GaSb and related alloys) detector technology has seen rapid progress over the past few years. The merits of the superlattice material system rest on predictions of even higher performance than HgCdTe and of engineering advantages. While no one has demonstrated Type-II superlattice detectors with performance superior to HgCdTe detectors, the difference in performance between these two technologies is decreasing. In this paper, we review the status and highlight relative merits of both HgCdTe and Type-II superlattice based detector technologies.
InAs/GaSb superlattices are a promising technology for long-wave and very-long-wave infrared photodetectors. Present detectors at these wavelengths are mostly built using bulk HgCdTe (MCT) alloys, where the bandgap is controlled by the mercury-cadmium ratio. In contrast, InAs/GaSb heterostructures control the bandgap by engineering the widths of the layers making up the superlattice. This approach is expected to have important advantages over MCT, notably the tighter control of bandgap uniformity across a sample and the suppression of Auger recombination. InAs/GaSb superlattices have a potential advantage in temperature of operation, uniformity and yield. To realize their inherent potential, however, superlattice materials with low defect density and improved device characteristics must be demonstrated. Here, we report on the growth and characterization of a 9.7 μm cutoff wavelength InAs/GaSb superlattice detector, with a resistance-area product of R<sub>0</sub>A = 11 Ωcm<sup>2</sup> at 78 K, and an 8.5 μm cutoff diode with a resistance-area product of R<sub>0</sub>A = 160 Ωcm<sup>2</sup> at 78 K. The devices are p-i-n diodes with a relatively thin intrinsic region of depth 0.5 μm as the active absorbing region. The measured external quantum efficiencies of 7.1% and 5.4 % at 7.9 μm are not yet large enough to challenge the incumbent MCT technology, but suggest scaling the intrinsic region could be a way forward to potentially useful detectors.
InAs-GaSb strained layer superlattices (SLSs) form a narrow band gap material whose cut-off wavelength can be tuned from 3 um to beyond 30 um. Theory predicts that in the LWIR and VLWIR, the SLS narrow bandgap layer structures can be engineered to reduce Auger recombination, relative to other narrow bandgap materials, such as HgCdTe. This should result in the SLS diodes having better performance than currently available detectors. A key to achieving this improved performance is knowing the detailed layer structure of the superlattice, and being able to accurately model this layer structure. Having an accurate model to guide the improved performance is essential to optimizing this material system.
Cross-sectional scanning tunneling microscopy data will be presented which shows that the actual layer structure differs significantly from the intended layer structure, due to the detailed dynamics of MBE growth and the very thin layers in the superlattice. Specifically, cross-sectional scanning tunneling microscopy demonstrates that the InAs contains excess antimony, and the GaSb excess indium, due to segregation from the underlying arsenide-on-antimonide, or antimonide-on-arsenide, heterojunctions respectively.
These deviations from the intended structure have a significant impact on the predicted properties of the superlattice. The predicted behavior of the intended and actual superlattice structures will be compared to measured performance.
AlGaAs/GaAs Pnp HBTs have the potential for high frequency performance approaching that of Npn HBTs. To achieve this performance, it is necessary to dope the base as heavily n-type as possible. This heavy base doping results in large degeneracy in the base, which reduces the heterobarrier to reverse injection of electrons from the base into the emitter. High A1 content in the emitter is desirable to maintain good injection efficiency. Incorporating a gradient in the base doping can introduce fields to sweep injected holes across the neutral base region, which reduces base transport time. DC and RF characteristics of Pnp HBTs with 40% and 75% Al in the emitter will be presented. ft of 17 GHz and fmax of 39 GHz has been achieved in 2 ?m x 11 ?m HBTs fabricated using a self-aligned ohmic contact process. Further improvement in performance should be possible.
The microwave power performance of AlGaAs/GaAs self-aligned HBTs from 10 to 35 GHz is described. A record value of 68% power added efficiency was obtained at 10 GHz. At 18 GHz, 16.3 dB associated gain was achieved with 1.83 W/mm power density and 40% efficiency. At 35 GHz, a 15 dB small signal gain was observed. The tested HBTs have 2 micron feature size. Further improvement is expected with optimization of the HBT structure.