Room-temperature-operating CdZnTe radiation detectors have high energy resolution, linear energy response and are capable of operating in normal counting and spectroscopic modes, hence are highly desirable for medical diagnosis, nondestructive industrial evaluations, homeland security, counterterrorism inspections and nuclear proliferation detection to ensure national and international nuclear safety. HgTe/HgCdTe superlattices can be designed to selectively transport one carrier species while hindering transport of the other. Specifically, one designs a large carrier effective mass for undesired carriers in the electric field direction, which results in low carrier velocities, and yet a density of states for undesired carrier that is lower than that of a comparable bulk semiconductor, which results in low carrier concentrations, hence a low current density under an electric field. The opposite carrier species can be designed to have a large velocity and high density of states, hence producing a large current density. By employing HgTe/HgCdTe superlattices as contact layers intermediate between CdZnTe absorbers and metal contacts, leakage currents under high electric fields are reduced and improved x-ray and γ-ray detector performance is anticipated. Pixilated CdZnTe radiation detectors arrays were fabricated and characterized to evaluate the effectiveness of HgTe/HgCdTe superlattices in reducing leakage currents. Current-voltage characteristics show that HgTe/HgCdTe superlattice contact layers consistently result in significantly reduced leakage currents relative to detectors with only metal contacts.
An approach to the fabrication of CdZnTe-based heterojunction detectors is presented with the primary goal of reducing
leakage currents, permitting increased bias voltages and therefore improving x-ray and gamma-ray detector
performance. The p-i-n detector architecture is theoretically superior to traditional CdZnTe detectors, and our modeling
predicts that superlattice contact layers result in leakage current reductions relative to bulk semiconductor contacts. The
benefits arise because the superlattices can be designed to have large carrier effective masses along the electric field
direction yet a density of states less than that of a comparable bulk semiconductor.
An overview of the properties of the absorption coefficient of mercury cadmium telluride
that may make this material useful for intrinsic hyperspectral detection is presented. A review of
recent work on modeling the absorption coefficient is provided, and new directions for achieving an
analytical representation with higher fidelity are suggested.
Detectors in the far infrared based on HgTe/Hg<sub>1-x</sub>Cd<sub>x</sub>Te superlattices (SL's) are superior to those
based on Hg<sub>1-x</sub>Cd<sub>x</sub>Te alloys. This is based on their band structure, however, it was concluded in an
investigation of the hydrostatic pressure dependence of photoluminescence (PL) peaks investigation
reported in Phys. Rev. B 48, 4460 (1993) that either the band structure model was incorrect or the
observed PL peaks were related to impurities. In contrast, in optical absorption experiments, the
hydrostatic pressure dependence of intersubband transitions agrees with theory, which corroborates
the validity of the band structure model.
The first advantage is the required precision of the growth parameters for the desired band gap
or cut off wavelength (λ<sub>co</sub>). More important is the possibility to significantly reduce leak currents
by the appropriate choice of barrier thickness. Furthermore the absorption edge is much steeper
and therefore the SL can be much thinner. Due to Auger suppression in these type III SL's, carrier
lifetimes are significantly enhanced. Finally the SL is significantly less susceptible to a Burstein-Moss shift of the adsorption edge; at least an order of magnitude greater electron concentration is
necessary in order to produce the same Burstein-Moss shift. A method for <i>in situ p</i> type doped
quantum wells (QW's) and SL's with nitrogen and arsenic by means of molecular beam epitaxy
(MBE) is discussed.