Resonant cavity enhanced HgCdTe structures have been grown by molecular beam epitaxy, and photoconductors
have been modelled and fabricated based on these structures. Responsivity has been measured and shows a
peak responsivity of 8 x 104 V/W for a 50 X 50 μm2 photoconductor at a temperature of 200K. The measured
responsivity shows good agreement with the modelled responsivity across the mid-wave infrared window (3-5μm).
The measured responsivity is limited by surface recombination, which limits the effective lifetime to ~15ns. The
optical cut-off of the detector varies with temperature as modelled from 5.1 um at 80K to 4.4 um at 250K.
There is strong agreement between modelled and measured peak responsivity as a function of temperature from
Heterostructured Hg(1-x)Cd(x)Te photodetectors are important for the next generation of high performance Infra-Red (IR) sensing applications. The measurement of the composition and thickness of each layer in double layer HgCdTe heterostructures is examined in this paper, in particular, the use of infrared transmission and Secondary Ion Mass Spectroscopy techniques. Several authors have published models of the optical absorption coefficient and refractive index in HgCdTe, and these models have been assessed on their suitability for use in modelling the infrared transmission characteristics of multilayer HgCdTe films. No data is available for the refractive index of HgCdTe for photon energies around the bandgap energy, so a modified Sellmeier equation has been used to approximate the refractive index in this range. A versatile
mathematical model of the infrared transmission of multilayer HgCdTe
films is presented, based on the characteristic matrix of each layer.
The model is then fit to experimental data, where the composition and thickness of each of the HgCdTe layers are fitting parameters. While some film parameters may be determined with high accuracy from infrared transmission, Secondary Ion Mass Spectroscopy (SIMS)
is useful as a complimentary technique which enables the measurement of the composition of the wider bandgap HgCdTe layer in double layer HgCdTe films, as well as measurement of the interface abruptness and layer uniformity. A method of calibrating SIMS data is presented, which gives results consistent with those obtained from infrared transmission fitting. Room temperature infrared transmission spectra and SIMS depth profiles of HgCdTe layers grown by Molecular Beam Epitaxy at the University of Western Australia are presented, and are compared with theoretical composition vs. depth profiles which have been determined using elements of each measurement technique.
Current infra-red detectors are limited to detect broad windows in
transmission. By adding Fabry-Perot filtering to these detectors
multi- and hyper-spectral detectors can be fabricated. However,
filtering will reduce the signal available to the detector. In
order to decrease the noise (thereby increasing the signal to
noise ratio), the detector can be moved into the resonant cavity
of the filter. The design of the mirrors is changed by placing the
detector with the resonant cavity. Materials for the design of a
resonant cavity enhanced mercury cadmium telluride detector are
investigated in this paper.
Steady state minority carrier lifetime has been investigated in heterostructure HgCdTe devices fabricated on Molecular Beam Epitaxy (MBE) grown material. Wider band gap Hg(1-x)Cd(x)Te x = 0.44 was MBE grown on narrower gap Hg(1-x)Cd(x)Te x = 0.32 material to create an abrupt interface. Both layers were unintentionally doped n-type as determined by Hall measurements, which show two distinct electron species corresponding to the two HgCdTe layers. Steady state lifetime as a function of temperature over the range 80K to 300K was extracted from responsivity and noise measurements performed on variable area photoconductor structures fabricated on the sample. At 80K, the photoconductors exhibit a specific detectivity at 1kHz of 4.5 x 1011cmHz-1W-1. 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 x 1014cm-3. For temperatures below 180K, measured lifetime is in agreement with bulk lifetime of ~12μs, however, for higher temperatures there is evidence of an additional mechanism which reduces the apparent lifetime in the material. It is concluded that for temperatures above 180K, there is significant thermally induced promotion of photogenerated carriers from the narrow bandgap material into the wide bandgap material, leading to a reduction in the responsivity of the detector due to the relatively high doping of the wide bandgap layer.