HgCdTe is one of the most important materials for fabrication of infrared detectors and focal plane arrays (FPAs) deployed in environments where high energy particle, such as protons and neutrons, are present. We designed and fabricated HgCdTe-based FPAs that can be used in high neutron radiation environment and we measured their characteristics. The influence of the radiation on the infrared FPAs and cameras are presented. HgCdTe material and devices are capable of maintaining high performances under high energy neutron irradiation environment. For the MWIR FPA directly facing 2.59×10<sup>8</sup> n/cm<sup>2</sup>·s neutron flux beam (with the highest energy 66 MeV) for 1 hour, the noise equivalent differential temperature (NEDT) increased ~ 8 times after irradiation. However NEDT decreased to 33 mK (compared to the original value of 21 mK) after one warming-up (to room temperature) and cooling-down cycle. The NEDT for the MWIR FPAs mounted parallel to the beam did not degrade (16 mK and 28 mK before irradiation, changed to 18 mK and 26 mK after irradiation, respectively).
The combination of HgCdTe detectors and Fabry-Pérot filters (FPFs) is highly desirable for hyperspectral
detection in the infrared band over a broad wavelength range. The results of comprehensive modeling of distributed-
Bragg-reflector-based tunable FPFs that can be used with HgCdTe array detectors for hyperspectral imaging modules are
presented, focusing on the impact of FPF non-idealities on optical performance. The effects of surface and interface
roughness on the spectral resolution and transmissivity of the cavity was explored to determine if certain thin film
deposition techniques are suitable to economically fabricate FPFs. The impact of varying field-of-view (FOV) and
incident angles are also discussed. Finally, the impact of FPF bowing on spectral resolution is discussed.
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.
Mid wavelength infrared (MWIR) HgCdTe heterostructures were grown on 3-inch dia Si (211) substrates by the molecular beam epitaxy technique and p+n format devices were fabricated by arsenic ion implantation. Very long wavelength infrared (VLWIR) layers have been employed as interfacial layers to block the propagation of detects from the substrate interface into the HgCdTe epilayers. Excellent material characteristics including the minority carrier lifetime of 7.2 usec at 200K and 2 usec at 80K in the n-HgCdTe absorber layer with 5 um cut-off wavelength at 80K were achieved. The photovoltaic detectors fabricated on these MWIR heterostructures show excellent zero-bias resistance-area product (R0A) on the order of 10<sup>8</sup> ohm-cm<sup>2</sup> and peak dynamic impedances on the order of 109 ohm-cm<sup>2</sup>. A two-step arsenic activation anneal followed by the 'Hg' vacancy filling anneal (third step) is shown to produce the best R0A values, since the intermediate temperature annealing step seems to control the diffusion of arsenic, assisted by the implantation-induced defects. The experimental R0A values are compared with that predicted by theory based on a one-dimensional model, indicating g-r limited performance of these MWIR devices at 80K.
Very long wavelength infrared (VLWIR, λ<sub>c</sub> approximately 20 to 50 μm) HgTe/HgCdTe superlattices were grown by molecular beam epitaxy (MBE). The layers were characterized by means of X-ray diffraction and Fourier transform infrared spectroscopy. Photoconductive interdigitated electrode detectors for heterodyne applications in the Far-infrared wavelengths (FIR) regions were designed and fabricated. Spectral response measurements exhibit the ability of these detectors to function in the long wavelength (LWIR) to VLWIR regions. Detectivity observed at 77 K is very encouraging and could be enhanced further at lower operating temperatures.
II-VI intrinsic very long wavelength infrared (VLWIR, λ<sub>c</sub>~20 to 50 μm) materials, HgCdTe alloys as well as HgCdTe/CdTe superlattices, were grown by molecular beam epitaxy (MBE). The layers were characterized by means of X-ray diffraction, conventional Fourier transform infrared spectroscopy, Hall effect measurements and transmittance electron microscopy (TEM). Photoconductor devices were processed and their spectral response was also measured to demonstrate their applicability in the VLWIR region.
In this paper, photoluminescence (PL) measurements were performed on several series of single-side Si doped MBE pseudomorphic high electron mobility transistors (p-HEMTs) quantum well samples, with different spacer layer widths, well widths and Si (delta) -doped concentrations, under different temperatures and excitation power densities. PL signals from the transitions of the second electron subband to the first heavy-hole subband (e2-hh1) and the first electron subband to the first heavy-hole subband (e1-hh1) have been observed with good symmetry and narrow full with at half maximum indicating high sample quality compared with previous reported results. The dynamic competitive luminescence mechanism between the radiations of e2-hh1 and e1-hh1 was discussed in detail. The confining potential, subband energies, corresponding envelope functions, subband occupations and transferring efficiency have been calculated by self-consistent definite differential method at different temperatures in comparison with our experiment results. The relative variation of the integrated luminescence intensity of the two transitions (e1-hh1 and e2-hh1) was found to be dependent on the temperature and the structure's properties, e.g. spacer layer width, dopant concentration and well width, which is an efficient characterization method before p-HEMTs device fabrication.
The 4 X 4 element arrays based on high Tc superconducting infrared microbolometers have been fabricated using the micromachining technology. The detectivities D* for various bolometric elements are ranged from 1.2 X 10<SUP>8</SUP> to 7.2 X 10<SUP>8</SUP> cmHz<SUP>1/2</SUP>w<SUP>-1</SUP> at the operating temperature approximately 88 K.
The Fourier transform and double modulation Fourier transform photoluminescence measurements were performed on HgCdTe films from liquid helium temperature to room temperature in the infrared band to 10 micrometers where the influence from room temperature background blackbody emission is very strong. From the band to band transition photoluminescence peak, which dominated in HgCdTe films with the small cadmium composition, the cadmium composition, crystal-quality-related band tail energy, and the active energy of the non-radiative Shockly-Read center, are obtained. The photoluminescence characterization method is also used to investigate the intentionally doped impurity behavior in HgCdTe. The amphoteric impurity behavior of As implanted in HgCdTe is discovered with the donor and acceptor energy level of 8.5 meV and 31.5 meV, respectively. The Ag impurity level of 70 meV in MBE HgCdTe is also found.
The optical properties of four electron trapping materials are studied, such as photoluminesce excitation spectra and emission spectra, writing/stimulating/readout spectra, temperature dependence of the relative writing/stimulating efficiencies. According to the experimental results, the optical storage mechanisms of these materials are given.