T. Parodos

Understanding ion-milling damage in Hg1−xCdxTe epilayers

Transmission electron microscopy (TEM) is widely used for the characterization of the microstructure of Hg1−xCdxTe epilayers. Traditional TEM sample preparation methods, which usually involve argon ion milling, can easily cause damage to the material, and the size and density of the induced defects depend on the milling conditions. In this work, the structural damage caused by argon ion milling of Hg1−xCdxTe epilayers has been investigated. Multilayer samples with different Hg concentrations, as grown by molecular beam epitaxy, and p-n heterojunctions, as grown by liquid-phase epitaxy, have been examined. It is shown that, in addition to the milling conditions, the extent of the ion-induced damage depends sensitively on the Hg concentration of the Hg1−xCdxTe alloy as well as the epilayer growth conditions (i.e., Hg rich or Te rich). A possible mechanism that explains these results is briefly discussed.

HgCdTe MWIR back-illuminated electron-initiated avalanche photodiode arrays

This paper reports performance data for back-illuminated planar n-on-p HgCdTe electron-initiated avalanche photodiode (e-APD) 4×4 arrays with large-area unit cells (250×250 μm2). The arrays were fabricated from p-type HgCdTe films grown by LPE on CdZnTe substrates. The arrays were bump-mounted to fanout boards and were characterized in the back-illuminated mode. Gain increases exponentially with reverse bias voltage, and gain versus bias curves are quite uniform from element to element. The maximum gain measured is 648 at -11.7 V for a cutoff wavelength of 4.06 μm at 160 K. For the same reverse bias voltage, the gain at 160 K for elements with two different cutoff wavelengths (3.54 and 4.06 μm at 160 K) increases exponentially with increasing cutoff wavelength, in agreement with Becks empirical model for gain versus voltage in HgCdTe e-APDs. Spot scan data show that both the V=0 response and the gain at V=-5.0 V are quite uniform spatially over the large junction area. To the best of our knowledge, these are the first spot scan data for avalanche gain ever reported for HgCdTe e-APDs. Capacitance versus voltage data are consistent with an ideal abrupt junction having a donor concentration equal to the indium counterdoping concentration in the as-grown LPE film. Calculations predict that bandwidths of 500 MHz should be readily achievable in this vertical collection geometry, and that bandwidths as high as 3 GHz may be possible with careful placement of the junction relative to the compositionally interdiffused region between the HgCdTe LPE film and the CdZnTe substrate.

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.

HgCdTe LWIR p-on-n photodiodes formed by arsenic diffusion from the vapor phase

We report current-voltage data for back-illuminated mesa photodiode test structures fabricated by arsenic-diffusion into n-type LPE HgCdTe films. Arsenic diffusion was carried out in a sealed quartz ampoule containing a source of both Hg and As. The arsenic-diffused p-on-n photodiodes were characterized at 70 K and 80 K. The cutoff wavelength was about 11 μm at 80 K. The data for 400 μm diameter photodiodes fabricated by the arsenic diffusion process are very similar to those from a conventional two-layer LPE P-on-n process for material with approximately the same cutoff wavelength. We outline process and doping level changes that should improve detector performance.