Minority carrier behaviour in abrupt MBE grown HgCdTe heterostructures

Abstract

Steady-state lifetime of photogenerated minority carriers has been investigated in heterostructure HgCdTe devices fabricated on molecular beam epitaxy (MBE) grown material. A wider bandgap capping layer (Hg/sub 1-x/Cd/sub (x)/Te, x = 0.44) was grown on a narrower bandgap absorbing layer (Hg/sub 1-x/Cd/sub (x)/Te, x = 0.32, /spl lambda//sub ca,80 K/ = 4.6 /spl mu/m) material in an uninterrupted MBE growth to create an abrupt heterointerface. Steady-state lifetime as a function of temperature over the range 80 K to 300 K was extracted from photoconductive responsivity at an optical wavelength corresponding to the peak responsivity at that temperature. At 80 K, the photoconductors exhibit a specific detectivity of 4.5 /spl times/ 10/sup 11/ cmHz/sup -1/W/sup -1/ (chopping frequency of 1 kHz). For each measurement temperature, the steady-state excess carrier lifetime determined experimentally was compared to the theoretical bulk lifetime for material with x = 0.32 and effective n-type doping density of 3.7/spl times/ 10/sup 14/ cm/sup -3/. For temperatures below 180 K, the measured lifetime is in agreement with the calculated bulk lifetime for the absorbing layer of /spl sim/12 /spl mu/s; however, for higher temperatures there is evidence of an additional recombination mechanism which reduces the effective lifetime in the material. It is concluded that for temperatures above 180 K. there is significant thermally induced promotion of photogenerated carriers from the narrow bandgap absorbing layer into the wide bandgap capping layer, leading to a reduction in the responsivity of the detector due Io the lower excess carrier lifetime and relatively high doping of the wide bandgap layer.