Traditionally, photoconductive (PC) HgCdTe detectors have been used in interferometers for detection in the 14-15 .tm range. In this paper we present recent quantum efficiency and junction impedance data which demonstrate that P-on-n HgCdTe photovoltaic (PV) detectors with 16-17 m cutoff wavelengths at 70 K are suitable for use in spaceborne remote sensing interferometrjc instruments such as IMG-2, ATRAS, TES and CCOSM. Theperformance of these large-area detectors is of particular importance for interferometers because they have higher linearity at higher fluxes than PCHgCdTe arrays. The linearity requirement of 1% for the IMG-2 instrument, with background fluxes in excess of 1016 ph/cm2-s, is only marginally met by PC HgCdTe detectors. We present, for the first time, data showing better than 1% linearity at fluxes of l.3x10'7phlcm2-s for PV HgCdTe detectors with 60 K cutoff wavelengths of 15.5 pm. Keywords: HgCdTe, infrared detectors, photovoltaic detectors, linearity, interferometers,mercury cadmium telluride, IMG-2

There has been significant progress made during the past several years in PV HgCdTe technology for advanced long wavelength remote sensing applications. Useful cutoffs wavelength shave been extended to beyond 17.0 micrometers . Junction quality has been improved to the point that D* > 3 X 1011 cm-(root) Hz/W can be achieved at temperatures of 60-65 K. The Atmospheric Infrared Sounder (AIRS) instrument, scheduled for launch in the year 2000 as part of the NASA EOS program, uses long linear multiplexed arrays of PV HgCdTe detectors with cutoff wavelengths extending as far as 15.0 micrometers at 60 K. PV HgCdTe offers many advantages over PC HgCdTe for multiplexers are possible, backside-illuminated 2D arrays of closely spaced elements, 10X-100X better linearity, dc coupling for measuring the total incident photon flux, and a (root) 2 higher BLIP D* limit. In this paper we compare the relative merits of PV and PC HgCdTe for advanced remote sensing instruments, we review recent data for linear arrays of PV HgCdTe with cutoff wavelengths as long as 17.5 micrometers at 70 K, and we project that PV HgCdTe should be able to meet or exceed the present demanding GOES LW Sounder D* requirements at T equals 100 K, with the additional benefits of negligible 1/f noise and better linearity.

We are developing two-layer LPE P-on-n HgCdTe photovoltaic detector arrays with cutoff wavelengths out to 17 μm for a NASA spaceborne infrared radiometer. These bilinear multiplexed arrays will operate at 60 K, and must achieve sensitivities approaching the background limit for a background photon flux of 2×1015 photons/cm2-sec. The detectors must operate at reverse bias voltage to interface with silicon CMOS multiplexer circuits, and must exhibit low 1/f noise. This paper reviews progress toward these demanding requirements. The limiting junction current mechanisms for HgCdTe photodiodes at these very long cutoff wavelengths are reviewed. Data are presented for both CdTe-passivated and ZnS-passivated arrays at 60 K with cutoff wavelengths of 15.4–16.9 μm. Average R0A products of 13 ohm-cm2 and quantum efficiencies of 89% have been achieved for cutoff wavelengths of 15.4 μm at 60 K. These array data demonstrate the potential for VLWIR PV HgCdTe to meet the requirements for advanced NASA applications.

High-resolution x-ray diffraction has been used to measure the composition difference between P and N layers in HgCdTe heterojunction photodiode material grown by liquid phase epitaxy. The composition (band gap) difference is a critical parameter in long wavelength photodiodes because it affects dark current and the formation of photocurrent collection barriers. We find that symmetric 333 reflections cannot resolve the small composition differences of interest. However, by making use of the asymmetric 246 reflection, small composition differences (0.03) can be resolved. There is good agreement between rocking curves and secondary ion mass spectroscopy composition depth profiles, both in the value of the composition difference and in the extent of compositional grading in the top layer. High-resolution x-ray diffraction shows promise as a nondestructive, relatively rapid technique for screening as-grown heterojunction material for carrier collection barriers.

The authors review those characterization techniques that have played significant roles in the development of HgCdTe infrared detector technology. They focus on the two specific HgCdTe devices that have achieved widespread application for infrared detection in the LWIR (8-12 mu m) and VLWIR (12-20 mu m) spectral regions: the simple n-type photoconductor and the P-on-n LPE heterojunction photodiode. They review the device physics of these two detectors, relate device performance to starting material properties and processing parameters, and describe the most important characterization techniques that have had a role in their development.

The field and temperature dependence of the Hall coefficient has been used to simultaneously extract information about the p and n layers in very long wave length infrared P/n HgCdTe heterojunctions. The field dependence allows the effects of high mobility electrons to be separated from those of low mobility holes. The higher the magnetic field, the higher the sensitivity to the parameters of the P layer. For a maximum magnetic field of 8000 gauss, the hole sheet concentration must be at least five times the electron sheet concentration to obtain accurate results for the P layer. This criterion is satisfied for typical liquid phase epitaxy (LPE) heterostructures. The analysis determines the hole sheet resistance (concentration times mobility), rather than the hole concentration or mobility separately. Independent knowledge of the P layer thickness and the relationship between hole concentration and resistivity are needed to convert the Hall measurement results to hole concentrations. Analysis of the field-dependent Hall data is complicated by the finding that at least three electrons of different mobilities are needed to fit the field dependence of the Hall coefficient in n-type LPE HgCdTe layers. These results are consistent with previous conclusions that electrons with different mobilities are needed to model bulk n-HgCdTe, and with a range of mobilities in the graded composition interface between the LPE layer and CdTe substrate. Consistent results are obtained for the concentrations and mobilities of the three types of electrons in the n-HgCdTe layer with and without the P layer present. N and P type carrier concentrations are also consistent with dopant concentrations measured by secondary ion mass spectroscopy.

The capability of growing long-wavelength infrared HgCdTe liquid-phase epitaxy P-on-n heterojunction films with state-of-the-art photodiode performance, with excellent thickness uniformity (±10%), and with excellent cutoff wavelength uniformity (e.g., 10.5±0.1 μm) across 2.5 cm×4.0 cm wafers has been demonstrated. In addition, we have extended the region of HgCdTe photodiode operation to wavelengths of 18–19 μm at 80 K. Both measured carrier lifetime and photodiode data show that the n-type HgCdTe base layers are of excellent quality, with 77 K carrier lifetimes at the calculated Auger-1 limit for film carrier concentrations above 4×1014 cm−3. The R0A products for large-area diodes (10−3 cm2 ) with cutoff wavelengths of 11–19 μm are consistent with n-side diffusion current calculated using the film Auger-1 lifetime. Smaller diodes of area 1×10−5 cm2 have typical R0A values of 12 Ω cm2 at 80 K for a 12.2 μm cutoff wavelength. Large area diodes with an 80 K cutoff wavelength of 18–19 μm have R0A products of 0.14 Ω cm2 at 80 K and 0.4 Ω cm2 at 70 K. Quantum efficiency values of 60% are observed with no antireflection coating (corresponding to 75% internal quantum efficiency) and the spectral response data are classical.

This paper reports recent results on two-layer P-on-n LPE HgCdTe heterojunction photodiodes with cutoff wavelengths beyond 19μm. These results demonstrate the potential of photovoltaic HgCdTe detectors to satisfy the detector requirements of advanced NASA satellite instruments out to wavelengths of 17μm.