This paper presents the infrared detector performance improvement accomplishments by Raytheon Vision Systems (RVS) and by AVYD Devices Inc (AVYD). The RVS-AVYD collaboration has resulted in the demonstration of very large imaging focal plane arrays with respectable operability and performance which could potentially be useful in a variety of promising new applications to advance performance capability for future near and short wave infrared imaging missions. This detector design concept potentially permits ultra-small pixel large format imaging capabilities for diffraction limited resolution down to 5μm pitch focal planes. In this paper, we report on the work performed at the RVS's advanced prototype engineering facility, to fabricate planar detector array wafers with a combination of RVS's Hg1-xCdxTe production material growth and detector fabrication processes and AVYD's p-type ion-implantation process. This paper will review the performance of a 20μm pitch 1,024 x 1,024 format SWIR focal plane array. The detector array was fabricated in Hg1-xCdxTe material responsive from near-infrared to 2.5μm cutoff wavelength. Imaging capability was achieved via interconnect bump bond connection of this detector array to an RVS astronomy grade readout chip. These focal plane arrays have exhibited outstanding quantum efficiency uniformity and magnitude over the entire spectral range and in addition, have also exhibited very low leakage current with median values of 0.25 electrons per second. Detector arrays were processed in engineering grade Hg1-xCdxTe epitaxial layers grown with a modified liquid phase epitaxy process on CdZnTe substrates followed by a combination of passivation/ion implantation/passivation steps. This paper will review the detector performance data in detail including the test structure current-voltage plots, spectral cutoff curves, FPA quantum efficiency, and leakage current.
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.
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.
Mechanisms of incorporation of native defect and dopants in HgCdTe alloys are reviewed. Origin of the native defect related deep centers in limiting the minority carrier lifetime is explored. Primary and secondary mechanisms operative in the activation of n type and p type dopants in HgCdTe are discussed along with implications for fabrication of high performance detectors.
Hg1-xCdxTe films with x values varying from 0.2 to 0.23 have been grown and characterized. N-type carrier concentrations in the range of 1 X 1015 cm-3 to 3 X 1015 cm-3 have been obtained. Hall effect measurements before and after anneals at 250 degrees Celsius have led to the evaluation of the Hg vacancy concentration in the samples. Dislocation density less than 105 cm-2 and X-ray rocking curve width less than 25 arc- secs measured in some of the films attests to the excellent crystallinity of the material.
`The demand for high detectivity LWIR IR focal plane arrays that operate at low backgrounds is shown to drive the HgCdTe technology toward increased detector performance. Reduced operating temperature together with advanced material technology and detector design are presented as solutions. High performance MWIR, MLWIR and LWIR HgCdTe detector test arrays and variable area test structures were recently demonstrated through the joint collaboration of Aerojet Electronic Systems Division and Rockwell International. These devices are based on the innovative buried planar heterostructure (BPH) detector architecture grown by liquid phase epitaxy of HgCdTe on II-VI substrates. The major features of the BPH design include planar geometry, heterostructure wide gap p-type on narrow gap n-type HgCdTe and a buried LWIR electrical junction. Excellent 78K median R(omicron )A performance across the IR spectrum from 5.2 micrometers to 12 micrometers is reported and shown to follow the diffusion trend line. Excellent 40K median R(omicron )A performance for devices with cutoffs ranging from 9 micrometers to 19 micrometers are also presented. LWIR R(omicron )A statistical performance data at both 78K and 40K from fanout test arrays are presented with median R(omicron )A values of 100 ohm-cm2 at 78K and > 106 ohm-cm2 at 40K for cutoffs of 10.4 micrometers and 11.4 micrometers respectively. The 90% test array operability was found to exceed 5 x 105 ohm-cm2 at 40K. Devices with median R(omicron )As of 20 ohm-cm2 at 78K and 7 x 105 ohm-cm2 at 40K were measured for cutoffs of 12 micrometers and 13 micrometers respectively. Uniform and high quantum efficiencies were measured at 40K with a median of approximately equals 70%.