CZT material quality improvement has been achieved by optimizing the crystal growth process. N-type conductivity has
been measured on as-grown, undoped Cd0.9Zn0.1Te. Cd 0.85 Zn 0.15Te crystals have been grown for producing high
resistivity CZT radiation detectors. The best FWHM of 57Co 122KeV spectrum was measured to be 3.7% and (µτ)e was
3x10-3 cm2V-1. 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 TeCdVCd2 might be the
shallow and deep donors previously assigned to TeCd 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 9.4x1010cm x (Hz)1/2 x W-1 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 Cd0.90Zn0.10Te 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 Cd0.90Zn0.10Te crystals and the best detectors are fabricated from crystals grown with 1.5% excess Te. Detector resolution 57Co and 241Am radiation peaks are observed on all detectors expect the ones produced on Cd0.90Zn0.10Te 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 Cd0.96Zn0.04Te 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 Cd0.9Zn0.1Te crystals are produced by compensating the p-type crystals with indium at a low doping level of 1- 5x1015 cm-3. At room temperature, the high yield CdZnTe radiation detectors can resolve the six low energy peaks from the Am241 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.
An on focal plane digital readout development suggested by the Army Night Vision & Electronics Sensors Directorate is proceeding under a combined program with the development of two color HCT detector arrays. The on focal plane A/D process is based on the Amain patented multiplexed oversample A/D, MOSAD, technology. In the first year of the program, prototype on focal plane analog to digital converters for both staring arrays and scanning arrays were built and demonstrated. The prototypes included a 2 loop double ended switched MOSAD and a 1 loop single ended MOSAD. Results from the original experimental prototypes showed conclusively that better than 14 bits could be achieved and that well capacity could be increased to support high background HCT needs approaching 109 electrons. In the second year, a 64 X 64 staring array for HCT LWIR detectors, 50 micron centers, was built based on these original prototype designs. The layout of the per pixel MOSAD A/D staring array used Orbit 1.2 micron CMOS process and achieved a pixel size of 40 microns with a well capacity of 1.9 X 108 electrons. Integration capacitors were built using Orbit's normal double poly capacitors with a standard buffered direct inject TIA detector interface configuration. Preliminary testing has been completed indicating complete functionality. Fermionics LWIR HCT detectors with cutoff at 9 microns have been built for attachment to the readout but indium bumping was not completed in time to report system level testing results. However, some noise tests have been performed using on array current mirrors. These tests indicate that better than 12 bits has been achieved, but lower noise current sources will be required for a more accurate measurement.
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.
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