Peter W. Norton

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.

Commercialization of uncooled infrared technology

BAE Systems is the leading producer of uncooled microbolometer based thermal imaging engines in the world. Initial investments to develop and produce uncooled infrared (IR) technology were primarily driven by military applications, but it was the commercial market with the potential for large product volumes which provided BAE Systems with the business model required for investment in uncooled IR technology. This paper reviews the heritage of BAE Systems technology and current products and is an example of the success of a Dual-Use technology area which DARPA invested in during the 1990s.

Technology and application advancements of uncooled imagers

Having delivered over 30,000 uncooled microbolometer based thermal imaging engines, BAE Systems is the worlds leading producer. Advancements in technology include the demonstration of broadband microbolometers on a 46 μm pixel pitch which have excellent sensitivity in the MWIR (NETD ≈180 mK, 3-5 μm) and LWIR (NETD ≈ 15 mK, 8-12 μm) wavebands. Application advancements include the development of a family of thermal weapons sights for the military which will replace current cooled systems with lighter, lower power systems and the introduction of a new generation of handheld and pole mounted thermal imagers for commercial markets.

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.

Uncooled thermal imaging sensor and application advances

BAE Systems continues to advance the technology and performance of microbolometer-based thermal imaging modules and systems. 640x480 digital uncooled infrared focal plane arrays are in full production, illustrated by recent production line test data for two thousand focal plane arrays. This paper presents a snapshot of microbolometer technology at BAE Systems and an overview of two of the most important thermal imaging sensor programs currently in production: a family of thermal weapons sights for the United States Army and a thermal imager for the remote weapons station on the Stryker vehicle.

Predicted performance of HgCdTe photodiodes for 15-25 μm detection

This paper reviews and assesses progress in back-illuminated P-on-n photovoltaic HgCdTe detector technology, grown by two-layer Liquid Phase Epitaxy on CdZnTe substrates, for applications at wavelengths beyond 15 μm in a new generation of spaceborne multispectral instruments for remote sensing. We review data that demonstrate the feasibility of useful cutoff wavelengths as long as 17.5 μm. We recommend that LPE photovoltaic HgCdTe technology be extended to the 20-25 μm wavelength region for single elements and arrays for NASA remote-sensing applications.

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.

Application of IR microbolometers in border surveillance

BAE Systems led a collaborative study with New Mexico State University to investigate a series of ground based persistent surveillance solutions for potential use along the Southwest border of the United States. This study considered a wide range of system options for mobile and fixed site applications. This paper summarizes the findings of the study including the central role of the imaging subsystems in mobile ground based surveillance solutions and the suitability of uncooled IR Microbolometers within this subsystem. The paper also provides a discussion of the benefits of real time decision support applications when fielding a persistent surveillance solution.