Recent advances over the last several years in III-V strained-layer superlattice-based infrared detectors have lead this material system to emerge as a solid alternative to HgCdTe for dual-band focal plane arrays (FPAs). Rapid development of superlattice-based detectors has been realized by capitalizing on mature, III-V foundry-compatible processing. Furthermore, superlattice-based epitaxial wafers exhibit a high degree of lateral uniformity with low macroscopic defect densities (< 50 cm<sup>-2</sup>) and can achieve dark current levels comparable to HgCdTe detectors. In this paper, we review our recent efforts towards producing HD-format (1280x720, 12 μm pitch) superlattice-based, dual-band MWIR/LWIR FPAs. For a representative FPA, characterization was conducted in a pour-fill dewar at 80K, f/3 and using a blackbody range of 22°C to 32°C. For the MWIR band, the noise equivalent temperature difference (NETD) was 14.9 mK with a 3x median NETD operability of 99.91%. For the LWIR band, the median NETD was 28.1 mK with a 3x median NETD operability of 99.66%. To illustrate the manufacturability of superlattice technology, we will present results on 1280x720, 12 μm pitch MWIR/LWIR FPAs built over the last year at HRL through multiple fabrication lots utilizing 4" epiwafers.
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<sup>-5 </sup>A/cm<sup>2</sup> with > 99.9% operability, and >60% QE (without AR coating).
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<sup>-2</sup>) 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.
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<sup>-5</sup> A/cm<sup>2</sup> with λ<sub>co</sub> = 5.11μm or 2.2 x 10<sup>-6</sup> A/cm<sup>2</sup> with λ<sub>co</sub> = 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<sup>-6</sup> A/cm<sup>2</sup> at 150K with λ<sub>co</sub> = 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.
Recent efforts in developing InAs/GaSb strained-layer superlattices for LWIR detectors are
described. The structural properties of the devices grown by MBE at HRL were evaluated using
optical microscopy, x-ray diffraction, and atomic force microscopy. Epilayer roughness and surface
morphology are briefly described. Small format focal plane arrays were fabricated to serve as a
baseline for device study, and to determine the effects of underfill epoxy on detector performance. A
novel approach for epilayer transfer on silicon is also presented.
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 years<sup>1,2,3</sup>. 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 area<sup>4</sup>. 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.
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 5x10<sup>13</sup> cm<sup>-2</sup>, at room temperature. Controlled Zn
diffusion profiles with concentrations from 5x10<sup>16</sup> to > 5x10<sup>18</sup> cm<sup>-3</sup> 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
CZT material quality improvement has been achieved by optimizing the crystal growth process. N-type conductivity has
been measured on as-grown, undoped Cd<sub>0.9</sub>Zn<sub>0.1</sub>Te. Cd <sub>0.85</sub> Zn <sub>0.15</sub>Te crystals have been grown for producing high
resistivity CZT radiation detectors. The best FWHM of <sup>57</sup>Co 122KeV spectrum was measured to be 3.7% and (µτ)<sub>e</sub> was
3x10<sup>-3</sup> cm<sup>2</sup>V<sup>-1</sup>. The microscopic gamma ray response using a beam size of 10µm has been used to map the entire 4 mm
x 4 mm detector. Several black spots indicating no signal responses were observed while all other areas showed an
average of 65-70% collection efficiency. The black spots suggest that at those locations, the Te precipitates are larger
than 10μm. Detailed microscopic infrared transmission measurement on the sample found that most Te precipitates have
sizes of 4-6μm. Theoretical analysis of the results suggests that singly and doubly ionized TeCdVCd<sup>2</sup> might be the
shallow and deep donors previously assigned to Te<sub>Cd</sub> by us.
High performance LWIR and VLWIR focal plane arrays have been produced on advanced HgCdTe/ZnCdTe materials. The ZnCdTe substrates typically are n-type with an infrared transmission over 65%. Mesa and planar ion-implantation-isolated heterojunction processes have been used to produce the arrays. The operability of 128x128, 10.6mm arrays reached 99.27% based on the criteria that the signal output is within +/-30% of the mean. Several 320x256 arrays with wavelength from 12mm to 13.75mm at 77-85K have also been produced. Excellent imaging pictures have been obtained at these temperatures.
The effect of the location of the high resistivity region within the crystal boule is investigated for 10% zinc with 1.5% excess Te. By varying the indium doping concentration in several CdZnTe boules, the region of high resistivity is changed along the vertical length of the crystal. The variation of the zinc concentration within the crystal boule is compared with the location of the high resistivity region along the length of the crystals. The concentration of zinc is extracted from FTIR measurements, and the segregation coefficient is calculated using data obtained from the CdZnTe crystals. The zinc distribution is plotted in terms of the location along the crystal length in order to correlate the concentration with detector performance. Radiation spectra obtained from these boules reveal a strong dependence between detector performance, and the relative location of the high resistivity region within the crystal. Initial results suggest that there are three semi-distinct regions along the length of the boule that give very different characteristics,
where it can be said that the best detector performance is in the middle region. It is determined that this middle region has a zinc concentration of ~9-11%, which varies slightly from the original concentration of 10%. The differences in the performance characteristics is discussed, and defect distribution within the crystal as the main source of the variation is suggested. Also, based on the results, it is believed that the role of indium is essentially to compensate the vacancies in the crystal, and therefore, secondary to the crystalline properties and impurities within the boule.
The advanced planar ion-implantation-isolated heterojunction process, which utilizes the benefits of both the boron implantation and the heterojunction epitaxy techniques, has been developed and used to produce longwave and very longwave HgCdTe focal plane arrays in the 320v256 format. The wavelength of these arrays ranges from 10.0-17.0μm. The operability of the longwave HgCdTe arrays is typically over 97%. Without anti-reflection coating and with a 60° FOV cold shield, the D* of the 10.0μm array is 9.4x10<sup>10</sup>cm x (Hz)<sup>1/2</sup> x W<sup>-1</sup> at 77K. The 14.7μm and 17.0μm very longwave HgCdTe array diodes have excellent reverse characteristics. The detailed characteristics of these arrays are presented.
Boron implantation and heterojunction epitaxy have been the standard techniques for the production of HgCdTe focal plane arrays for a variety of applications. Each of these techniques has its special advantageous features. In this paper, we will describe an advanced HgCdTe junction formation technique, the planar ion-implantation-isolated heterojunction process, which utilizes the benefits of both the boron implantation and the heterojunction epitaxy
techniques. HgCdTe arrays in the format of 320x256 and 640x512 have been produced by this method. The characteristics of these arrays are reported.
Spectrometer grade, room-temperature radiation detectors have been produced on Cd<sub>0.90</sub>Zn<sub>0.10</sub>Te grown by the low-pressure Bridgman technique. Small amount of indium has been used to compensate the uncompensated Cd vacancies for the crystals to be semi-insulating. The properties of the detectors are critically dependent on the amount of excess Te introduced into the growth melts of the Cd<sub>0.90</sub>Zn<sub>0.10</sub>Te crystals and the best detectors are fabricated from crystals grown with 1.5% excess Te. Detector resolution <sup>57</sup>Co and <sup>241</sup>Am radiation peaks are observed on all detectors expect the ones produced on Cd<sub>0.90</sub>Zn<sub>0.10</sub>Te grown from the melt in the stoichiometric condition. The lack of resolution of these stoichiometric grown detectors is explained by a p/n conduction-type inhomogeneity model.
Shortwave, midwave, and longwave HgCtTe focal plane arrays with a format of 320x256 have been produced by both heterojunction epitaxy and boron implantation techniques. In general, the heterojunction diodes and arrays with a p-on-n polarity have high diode RoAs at high temperatures, while the boron implanted diodes and arrays with an n-on-p polarity have high diode RoAs at lower temperatures and better array operability because of excellent diode surface passivation. Diodes with wavelength longer than 20 micrometers have been produced. The 320x256 HgCdTe arrays have been fabricated and hybridized to readout integrated circuit chips ISC 9705 and ISC 9809 designed by Indigo Systems Inc. Imaging pictures were taken by cameras equipment with these array hybrids. The array operability depends on the hybrid operating temperature. For heterojunction arrays, the best operability of 2.5micrometers arrays at 200K is over 98%, while the best operability of 9.7micrometers arrays at 77K is over 96%. The operability of n-on-p arrays hybridized to ISC9809 cannot be determined because the readout circuit is not specifically designed for arrays with this polarity. However, testing results indicate that with proper readout chips, array operablity over 99% can be achieved with boron- implanted arrays.
The n-type conduction of CdTe and Cd<SUB>0.96</SUB>Zn<SUB>0.04</SUB>Te crystals grown from melts with excess tellurium indicates that the origin of the donors with an energy level at 0.01 eV below the conduction band are most likely singly ionized tellurium antisites instead of cadmium interstitials. Based on this model, the deep level at 0.75 eV below the conduction band is therefore doubly ionized tellurium antisites. After increasing the zinc content over 7%, CdZnTe turns to p-type. The conduction type variation of CdZnTe crystals as a function of zinc contents is explained by the compensation between the donors of Te-antisites and the acceptors of Cd vacancies. High resitivity Cd<SUB>0.9</SUB>Zn<SUB>0.1</SUB>Te crystals are produced by compensating the p-type crystals with indium at a low doping level of 1- 5x10<SUP>15</SUP> cm<SUP>-3</SUP>. At room temperature, the high yield CdZnTe radiation detectors can resolve the six low energy peaks from the Am<SUP>241</SUP> source, a performance comparable to the best reported CdZnTe detectors.
Recently, it was reported that p-type, Au-doped HgCdTe epilayers have a carrier lifetime two to three times higher than the Hg-vacancy doped epilayers with the same condition type. Analysis of the temperature dependent Hall measurement results indicates the existence of vacancy complexes in the vacancy doped HgCdTe epilayers but not in the Au-doped epilayers. Therefore, it is very likely that the defect complexes are generation-recombination centers, which reduce the carrier lifetime. Shortwave, midwave, and longwave HgCdTe diodes arrays have been produced in the Au-doped HgCdTe epilayers by the ion implantation technique. The n- type conversion by implantation is explained by the formation of tellurium antisites. Excellent array performances have been observed. Comparing these arrays to the heterojunction HgCdTe arrays, the arrays formed by ion implantation perform similar to or even better than the heterojunction array at liquid nitrogen temperature, but are inferior to the heterojunction arrays at a temperature over 150K.
The technology of producing HgCdTe materials, detectors, and arrays is rapidly maturing. In this paper, the performance and producibility of 2.5 - 14 micrometers LPE HgCdTe epilayers, as well as PC and PV detectors, are presented. The diodes show no degradation after a 95 degree(s)C baking for a period of two weeks. Fully operational linear arrays are produced at Fermionics Corporation. Most of the arrays show excellent uniformity. From the cost and quality point of view, these products are currently very competitive. These results demonstrate the producibility of HgCdTe materials, detectors, and 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.