MCT detectors: from IR to sub-mm and mm wavelength bands

Abstract

Modern technology advances combined with unique physical properties of mercury cadmium telluride (MCT) material, empower low-signal applications in technical vision systems. Properties of MCT detectors manufactured from LPE and MBE epilayers and their dependences on thickness and doping both for IR and THz regions are discussed. It is shown by comparison of experimental data and modeling of I-V dark current characteristics that MCT photodiode ultimate electrical characteristics are limited by diffusion current in n+-n--p junctions and by current via the deep traps in the gap with position Et= 0.7 Eg above the valence band and concentrations Nt = (1.0-5.5)•1015 cm-3 which are comparable with donor concentration in n--region Nd = (1.1-1.8)•1015 cm-3. Detector array parameters for a wavelength range 8-12 microns are: detectivity D*=1.9•1011 cm•Hz1/2/W, noise equivalent difference temperature NEDT ≈ 9 mK, dynamical resistance R ≈ 4·109 Ohm for the reverse biases ΔV = 0.1-0.2 V. Also, it is shown that MCT layers can be successfully used as sub-mm or mm wave ambient temperature or moderately cooled hot electron bolometers. Thus, in addition to the wavelength range from SWIR to VLIR, where the MCT detectors are employed mainly as photodiodes or photoresistors, they can be used as sub-mm or mm wave detectors in the range from 190 microns to 8 mm. They can be employed here as semiconductor hot electron bolometers (SHEB). Measurements performed at electromagnetic wave frequencies ν = 37, 55, 77 GHz, and also at 0.89 and 1.58 THz with non-optimized Hg0.8Cd0.2Te bolometer prototype, has confirmed the basic concepts of SHEB. At ν = 0.89 THz, 77 GHz and 37 GHz the signal temperature dependencies were measured too. At 77 K the SHEB sensitivity at ν = 37 and 77 GHz is increasing up to two orders compared to room temperature data. The sensitivity Sν ≈ 2 V/W at 300 K, and calculated both Johnson-Nyquist and generation-recombination noise values give estimations of SHEB NEP ~ 4•10-10 W at band-width Δf = 1 Hz and ν = 37 GHz.