Thermal Stability in HgCdTe IR Photodiodes
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Thermal Stability in HgCdTe IR Photodiodes J. M. Dell, T. Nguyen, C. A. Musca, J. Antoszewski, L. Faraone, R. Pal1 School of Electrical, Electronic and Computer Engineering The University of Western Australia Crawley, 6009, Australia 1 Now at Solid State Physics Laboratory Timarpur, Delhi, India ABSTRACT Packaging of HgCdTe photodiode detector arrays in a dewar involves degassing at elevated temperatures for several days so as to achieve vacuum integrity. This sustained exposure to relatively high temperatures can influence the HgCdTe bulk material properties, p-n junction integrity, and the passivant-HgCdTe interface. This work investigates the effects of bake-out treatment on the performance of HgCdTe based photodiodes formed using a new plasma induced type conversion process. Experimental results of a series of experiments in both long-wavelength infrared (LWIR) and mid-wavelength infrared (MWIR) devices are presented. Bulk lifetime was used as an indicator of the change in the bulk material resulting from baking in vacuum and was measured by photoconductive decay. These measurements did not show any appreciable changes as a result of baking. Modification of the doping profile in the n-p junction may also result from high temperature baking. Doping profiles of the photodiodes were studied by measuring the junction capacitance-voltage relation before and after baking. The results of these tests after baking showed no changes to C-V measurements from those before bake. The effect of baking on the passivant/HgCdTe interface was also examined by carrying out surface recombination velocity measurements by photoconductive decay on samples with different passivation layers. Variable area HgCdTe photodiodes have also been fabricated and studied to understand the effect of the surface condition of the performance of the devices. Initial bake tests on LWIR devices show that the technology is stable when employing a double layer passivation technique. Bake tests on the more advanced MWIR technology indicates that the plasma induced type conversion process produces stable photodiodes with state of the art performance. INTRODUCTION Unlike photodiodes in the ultraviolet to near infrared wavelength ranges, photodiodes operating in the longer wavelength infrared (IR) regions rely on absorption outside the depletion region. As a result, a wide range of diverse material properties is important in determining overall device performance. In the infrared spectral region, Hg1-xCdxTe is the material of choice for high performance photodetectors. and have found significant application in the strategically important mid-wavelength infrared (MWIR, 3-5µm) and long-wavelength infrared (LWIR, 814µm) bands. Performance advantages offered by HgCdTe for photodetectors, include higher detectivity, higher operating temperatures, faster response times, tunable cut-off wavelength, lattice matching over the entire composition range, from CdTe to HgTe, and suitability for integration into focal plane array (FPA) configurations. There are, how
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