Stephen P. Tobin

Simultaneous MW/LW dual-band MOVPE HgCdTe 64x64 FPAs

We report results for 64 X 64 simultaneous MW/LW dual-band HgCdTe Focal Plane Arrays (FPAs). The MW and LW average cutoff wavelengths at 78 K are in the 4.27 - 4.35 micrometer and 10.1 - 10.5 micrometer ranges respectively. The unit cell size is 75 X 75 micrometer2. These staring dual-band FPAs exhibit high average quantum efficiencies (MW: 79%; LW:67%), high median detectivities (MW: 4.8 X 1011 cm- (root)Hz/W; LW: 7.1 X 1010 cm-(root)Hz/W), low median NE(Delta) Ts (MW: 20 mK; LW: 7.5 mK for TSCENE equals 295 K and f/2.9), large dynamic ranges (MW: 77 dB; LW: 75 dB), and 87% stare efficiencies for both the MW and LW spectral bands. The dual-band HgCdTe detector array is fabricated from a four- layer P-n-N-P film grown in situ by MOVPE. The dual-band silicon CMOS input circuit utilizes a unique floating-direct- injection approach to achieve separate and simultaneous integration of both bands within each unit cell. There are two Compact Signal Averager circuits in each unit cell, to average subframes within every frame for each spectral band, allowing full stare efficiency in both spectral bands, as well as variable band-independent transimpedance gains. These data confirm that all key features of our P-n-N-P dual-band HgCdTe detector and our dual-band input circuit function as designed.

The relationship between lattice matching and crosshatch in liquid phase epitaxy HgCdTe on CdZnTe substrates

X-ray topography provides a very sensitive map of lattice mismatch between a HgCdTe LPE epitaxial layer and its (111) CdZnTe substrate. A well-defined Crosshatch pattern in the three «110» directions indicates a positive room-temperature lattice mismatch. For conditions of near-perfect lattice matching (±0.003% mismatch), the Crosshatch pattern disappears, presumably because there are few or no misfit dislocations present near the interface, and a region free of topographic contrast is observed. The crosshatch-free region occurs for a small positive room-temperature mismatch (about 0.02%); this is attributed to differences in the lattice matching condition at room temperature and the growth temperature. For negative mismatches, where the film is in tension, a mosaic pattern, rather than a crystallographically oriented Crosshatch, is observed in the topograph. Rocking curve full width at half maximum of the epitaxial layer is minimized in the crosshatch-free zone at a value nearly equal to that of the substrate. Etch pit density of the HgCdTe layer shows a strong minimum for perfect room temperature lattice matching, with values as low as 1 x 104 cm−2. For nearly lattice matched layers, Crosshatch is present throughout the thickness of the epitaxial layer except for a narrow graded-composition region near the substrate interface. Crosshatch contrast appears to result from long-range strain fields associated with a misfit dislocation network near the substrate interface. Spatial variations in topographic features and mismatch across relatively small lateral distances are caused by variations in substrate alloy composition. For truly lattice-matched substrates, better control over the substrate lattice parameter is required.

Update on the imaging sensor for GIFTS

Remote temperature sounding from the vantage point of Earth Orbit improves our weather forecasting, monitoring and analysis capability. Recent advances in the infrared hyperspectral sensor technology promise to improve the spatial and temperature resolution, while offering relatively quick re-look times to witness atmospheric dynamics. One approach takes advantage of a two-dimensional, imaging Fourier transform spectrometer to obtain a data cube with the field of view along one plane and multiple IR spectra (one for every FPA pixel) along the orthogonal axis. Only the pixel pitch in the imaging focal plane and the optics used to collect the data limit the spatial resolution. The maximum optical path difference in the Michelson FTS defines the spectral resolution and dictates the number of path-length interferogram samples (FPA frames required per cube). This paper discusses the unique challenges placed on the focal plane by the Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS) approach and how advanced focal plane technology is applied to satisfy these challenges. The instrument requires a midwave spectral band from 4.4 to 6.1m to capture the C02 and H20 absorption bands, and an optional VLWIR spectral band to cover from 8.85-14.6m. The paper presents performance data of Liquid Phase Epitaxy (LPE) fabricated HgCdTe detectors and design details of the advanced readout integrated circuit necessary to meet the demanding requirements of the imaging sensor for the GIFTS instrument. Point defects are removed by using a unique super-pixel approach to improve operability for the VLWIR focal plane. Finally, early focal plane performance measurements are reported, including Noise Equivalent Input, responsivity uniformity, output offset stability and 1/f noise knee.

Advances in liquid phase epitaxial growth of Hg1-XCdXTe for SWIR through VLWIR photodiodes

Hg1-XCdXTe photodiode arrays have assumed a critical importance for systems requiring sensitivity in any one of the infrared bands of interest extending from the SWIR 1-3 micrometer band to the VLWIR >14 micrometer band. As arrays have become larger, system requirements more stringent and cutoff wavelengths longer, more pressure has been placed on improving the Liquid Phase Epitaxial (LPE) Hg1-XCdXTe growth technique at BAE Systems. In this paper we will report on improvements made in each critical aspect of LPE growth, covering the entire range of Hg1-XCdXTe compositions required for photodiodes with cut-off wavelengths ranging from 3 to greater than 14 micrometers. Data presented will demonstrate that continual advances in LPE Hg1-XCdXTe growth techniques at BAE Systems promise high infrared system performance meeting SWIR to VLWIR needs.

Very long wavelength (>15 μm) HgCdTe photodiodes by liquid phase epitaxy

This paper reviews and assesses back-illuminated P-on-n photovoltaic HgCdTe detector technology, based on two-layer growth by Liquid Phase Epitaxy on CdZnTe substrates, for application at wavelengths beyond 15 μm in a new generation of spaceborne multispectral instruments for remote sensing. We review data that show feasibility of useful cutoff wavelengths as long as 18-19 μm. We recommend that that LPE photovoltaic HgCdTe technology be extended to the 20-25 μm wavelength region for single elements and small arrays for NASA remote-sensing applications.

High-operability SWIR HgCdTe focal plane arrays

SWIR HgCdTe photodiode test chips and 256x256 Focal Plane arrays with a 2.1 micron cutoff wavelength have been fabricated and tested. The base material was n-type HgCdTe. P-type junctions were created by ion implantation. Test chip arrays with 60-micron pixels exhibited an average RoA of 509 ohm-cm2 and internal quantum efficiency (QE) of 98% at 295 K; RoA and QE were uniform. Average RoA increased to 2.22x104 at 250 K and internal QE remained high at 93%. The mini-array of 30-micron pixels had lower RoA values, 152 and 6.24x103 ohm-cm2 at 295 and 250 K, but 100% internal quantum efficiency at both temperatures. There was no bias dependence of quantum efficiency, demonstrating that our junction formation process does not give rise to valence band barriers. FPA test data have demonstrated NEI operability greater than 98% at 220 K and greater than 97% at 250 K along with QE operability in excess of 99.9% at 220 K and in excess of 99.8% at 250 K.

Imaging sensor for the Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS)

Accurate high resolution temperature sounding through our atmosphere is paramount to improving our weather forecasting, monitoring, and analysis capability. From the vantagepoint of earth Orbit, remote temperature sounding is becoming a reality and its accuracy is bolstered by recent advances in infrared hyper-spectral sensor capability. One promising approach takes advantage of a two-dimensional, imaging Fourier transform spectrometer to obtain a data cube with the field of view along one plane and multiple IR spectra (one for every FPA pixel) along the orthogonal axis. The spatial resolution is limited only by the pixel pitch in the imaging focal plane and the optics used to collect the data. The maximum optical path difference in the Michelson FTS defines the spectral resolution and dictates the number of path-length interferogram samples (FPA frames required per cube. This paper discusses the unique challenges placed on the focal plane by the Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS) approach and how advanced focal plane technology is applied to satisfy these challenges. Two focal planes are required to provide spectral coverage from 4.4 to 6.1um and 8.85-14.6um. Currently, the GIFT’s LWIR focal plane is the longest wavelength two-dimensional PV HgCdTe array of this size (128 square on 60 um centers) planned for space deployment. The paper presents performance data of Liquid Phase Epitaxy (LPE) fabricated HgCdTe detectors and design details of the advanced readout integrated circuit necessary to meet the demanding requirements of the imaging sensor for the GIFTS instrument.

HgCdTe photodiodes with state-of-the-art performance in the near- infrared spectral region

We report on the technology we are developing to product photovoltaic devices of HgCdTe which are sensitive in the short wave region of the solar radiation and exhibiting detectivity performance close to theoretical limits imposed by the fundamental properties of the material.

Large-area x-ray topographic screening of II-VI substrates and epilayers

A crucial aspect of process control in II-VI based device fabrication is the detailed monitoring of material properties, particularly structural quality. We have developed a method of mapping structural defects over large wafer areas using synchrotron white beam x-ray topography and have used it to characterize large area single crystal CdZnTe substrates and LPE HgCdTe epilayers grown on them. The synchrotron white beam technique produces high resolution topographic images of whole wafers regardless of long range strain, and the multiple images generated as a result of the polychromatic white beam (Laue geometry) provide an automatic defect depth profiling. The topographs reveal various types of defect structure in the CdZnTe substrates, and we have compared these topographic images to IR micrographs and x-ray rocking curve maps. Defect structures as revealed by the x-ray topographs were then followed from the CdZnTe substrates to the LPE grown HgCdTe epilayers. Epilayer topographs were also compared to conventional optical micrographs as well as with x-ray rocking curve maps. Finally, a scanning stage was constructed to topographically image large wafers and boule slabs.