Sevag Terterian

Fabrication and performance of InAs/GaSb-based superlattice LWIR detectors

InAs/GaSb-based type II superlattices (T2SL) offer a manufacturable FPA technology with FPA size, scalability and cost advantages over HgCdTe. Work at Jet Propulsion Laboratory (JPL), Naval Research Laboratory (NRL), and Northwestern University (NWU) has shown that the performance gap between HgCdTe and T2SL FPAs has narrowed to within 5-10x over the last two years1,2,3. Due to the potential of T2SL technology for fabrication of large format (> 1k x1k) and dual-band arrays, HRL has recently resurrected efforts in this area4. We describe the progress on the FastFPA program funded by the Army Night Vision Labs towards the development of detectors and focal plane arrays (FPAs). Progress made in the areas of MBE growth, mesa diode fabrication, dry etch processing, and FPA fabrication over the last one year is presented.

HgCdTe on Si for monolithic focal plane arrays

HgCdTe was grown on Si substrates containing CCD and CMOS readout (R/O) circuits. Evaporated aluminum (Al) thin films were used to interconnect MWIR HgCdTe detector arrays with 1 X 64 scanned R/Os to demonstrate monolithic integration and eliminate indium bump bonds required to fabricate hybrid infrared focal plane arrays (IRFPAs). Conformal electroplated gold (Au) thin films on 32 X 64 staring arrays were used to integrate isolated MWIR HgCdTe detectors in each of the 100 micrometers X 100 micrometers unit cells to the input of the CMOS R/Os. Five micron wide Au thin films were used to make a conformal interconnect to 10 micrometers high HgCdTe layers in 40 micrometers X 40 micrometers unit cells within 256 X 256 arrays. Multiple thin film interconnects do not limit the size of the unit cell for dual band and multispectral staring arrays.

Advances in III-V based dual-band MWIR/LWIR FPAs at HRL

Recent advances in superlattice-based infrared detectors have rendered this material system a solid alternative to HgCdTe for dual-band sensing applications. In particular, superlattices are attractive from a manufacturing perspective as the epitaxial wafers can be grown with a high degree of lateral uniformity, low macroscopic defect densities (< 50 cm-2) and achieve dark current levels comparable to HgCdTe detectors. In this paper, we will describe our recent effort on the VISTA program towards producing HD-format (1280x720, 12 μm pitch) superlattice based, dual-band MWIR/LWIR FPAs. We will report results from several multi-wafer fabrication lots of 1280x720, 12 μm pitch FPAs processed over the last two years. To assess the FPA performance, noise equivalent temperature difference (NETD) measurements were conducted at 80K, f/4.21 and using a blackbody range of 22°C to 32°C. For the MWIR band, the NETD was 27.44 mK with a 3x median NETD operability of 99.40%. For the LWIR band, the median NETD was 27.62 mK with a 3x median operability of 99.09%. Over the course of the VISTA program, HRL fabricated over 30 FPAs with similar NETDs and operabilities in excess of 99% for both bands, demonstrating the manufacturability and high uniformity of III-V superlattices. We will also present additional characterization results including blinkers, spatial stability, modulation transfer function and thermal cycles reliability.

Advances in III-V bulk and superlattice-based high operating temperature MWIR detector technology

Barrier detectors based on III-V materials have recently been developed to realize substantial improvements in the performance of mid-wave infrared (MWIR) detectors, enabling FPA performance at high operating temperatures. The relative ease of processing the III-V materials into large-format, small-pitch FPAs offers a cost-effective solution for tactical imaging applications in the MWIR band as an attractive alternative to HgCdTe detectors. In addition, small pixel (5-10μm pitch) detector technology enables a reduction in size of the system components, from the detector and ROIC chips to the focal length of the optics and lens size, resulting in an overall compactness of the sensor package, cooling and associated electronics. To exploit the substantial cost advantages, scalability to larger format (2kx2k/10μm) and superior wafer quality of large-area GaAs substrates, we have fabricated antimony based III-V bulk detectors that were metamorphically grown by MBE on GaAs substrates. The electro-optical characterization of fabricated 2kx2k/10μm FPAs shows low median dark current (3 x 10-5 A/cm2 with λco = 5.11μm or 2.2 x 10-6 A/cm2 with λco = 4.6μm) at 150K, high NEdT operability (3x median value) >99.8% and >60% quantum efficiency (non-ARC). In addition, we report our initial result in developing small pixel (5μm pitch), high definition (HD) MWIR detector technology based on superlattice III-V absorbing layers grown by MBE on GaSb substrates. The FPA radiometric result is showing low median dark current (6.3 x 10-6 A/cm2 at 150K with λco = 5.0μm) with ~50% quantum efficiency (non-ARC), and low NEdT of 20mK (with averaging) at 150K. The detector and FPA test results that validate the viability of Sb-based bulk and superlattice high operating temperature MWIR FPA technology will be discussed during the presentation.

Fabrication of small pitch, high definition (HD) 1kx2k/5μm MWIR focal-plane-arrays operating at high temperature (HOT)

We describe our recent results in developing and maturing small pixel (5μm pitch), high definition (HD) mid-wave infrared (MWIR) detector technology as well as focal-plane-array (FPA) hybrids, and prototype 2.4 Megapixel camera development operating at high temperature with low dark current and high operability. Advances in detector performance over the last several years have enabled III-V high operating temperature (T≥150K), unipolar detectors to emerge as an attractive alternative to HgCdTe detectors. The relative ease of processing the materials into large-format, small-pitch FPAs offers a cost-effective solution for tactical imaging applications in the MWIR band. In addition, small pixel detector technology enables a reduction in size of the system components, from the detector and ROIC chips to the focal length of the optics and lens size, resulting in an overall compactness of the sensor package, cooling and associated electronics. An MBE system has been used to grow antimony-based detector structures with 5.1μm cutoff with low total thickness variation (TTV) across a 3” wafer, in order to realize high interconnect yield for small-pitch FPAs. A unique indium bump scheme is proposed to realize 5μm pitch arrays with high connectivity yield. Several 1kx2k /5μm hybrids have been fabricated using Cyan’s CS3 ROICs with proper backend processing and finally packaged into a portable Dewar camera. The FPA radiometric result is showing low median dark current of 2.3x10-5 A/cm2 with > 99.9% operability, and >60% QE (without AR coating).

Silver Nanowire-Based Infrared-Transparent Contacts for Future High-Density Format Focal Plane Arrays

We report the first demonstration of: 1) wide-wavelength range, infrared transparent conductors (ITCs) made of silver nanowires (Ag NWs), and 2) ITC contact-integrated prototype, InAsSb midwavelength IR (MWIR) detectors. The Ag NW-based ITCs show optical transmittance (T_λ) of ,,94% in the 0.9-2.5 μm wavelength range with a sheet resistance (R_s) of 19.1 Ω/□. T_λ of the Ag NW-ITC decreases slowly with increasing wavelength, resulting in T_λ ,,92%-87% at 2.5- 8 μm (MWIR) and T_λ ,,87%-82% at 8-15 μm (LWIR). The Ag NWbased ITC makes good ohmic contacts on InAsSb-based MWIR detectors with contact resistance of <;0.5 Ω · mm. The ITC contact-integrated prototype InAsSb IR detectors are front-side illuminated and show external quantum efficiency (QE) of >85% at 4.25 μm and 150K. The measured external QE remains at the same high level regardless of detector fill factor. These results indicate that Ag NW-ITCs may enable future pixel scaling for front-side illuminated, high-density-format focal plane arrays without compromising QE, responsivity, and detector performance.

Fabrication of InAs/GaSb type-II superlattice LWIR planar photodiodes

We have evaluated selective doping techniques for the fabrication of type II LWIR superlattice planar detectors. Ion-implantation and diffusion of dopants were evaluated for selective doping of the electrical junction region in planar photodiodes. Residual damage remains when superlattice structures are implanted with Te ions with an energy of 190 keV and a dose of 5x1013 cm-2, at room temperature. Controlled Zn diffusion profiles with concentrations from 5x1016 to > 5x1018 cm-3 in the wide bandgap cap layer was achieved through a vapor phase diffusion technique. Planar p-on-n diodes were fabricated using selective Zn diffusion. The I-V characteristics were leaky due to G-R and tunneling in the homojunction devices, for which no attempts were made to optimize the n-type absorber doping level. Work is underway for the implementation of planar diodes with the n-on-p architecture through selective Te diffusion. Due to increased minority carrier lifetimes for p-type InAs/GaSb superlattice absorber layers, planar device with the n-on-p architecture have the potential to provide improved performance as compared to the p-on-n counterparts.