Issues associated with the development and exploitation of infrared (IR) and terahertz (THz) radiation detectors based on a narrow-gap “HgCdTe” semiconductor have been discussed. This mercury–cadmium–telluride (MCT) semiconductor can be applied for two-color detector operation in IR and sub-THz spectral ranges. Two-color uncooled and cooled down to 78 K narrow-gap MCT semiconductor thin layers grown using the liquid phase epitaxy or molecular beam epitaxy methods on high-resistive “CdZnTe” or “GaAs” substrates, with bow-type antennas, have been considered both as sub-THz direct detection bolometers and 3 to 10 μm IR photoconductors. Their room temperature noise equivalent power at the frequency ν≈140 GHz and signal-to-noise ratio at the spectral sensitivity maximum under monochromatic (spectral resolution ∼0.1 μm) globar illumination reached the following values; ∼4.5×10−10 W/Hz1/2 and ∼50, respectively. Aspheric lenses used for obtaining the images in the sub-THz spectral region were designed and manufactured. With these detectors, about 140 and 270 GHz imaging data have been demonstrated.
Development of infrared and sub-terahertz radiation detectors at the same sensitive elements on the base of mercurycadmium- telluride (MCT) is reported. Two-color un-cooled and cooled to 78 K narrow-gap MCT semiconductor thin layers, grown by liquid phase epitaxy or molecular beam epitaxy method on high resistivity CdZnTe or GaAs substrates, with bow-type antennas were considered both as sub-terahertz direct detection bolometers and 3 to 10 μm infrared photoconductors. Their room temperature noise equivalent power (NEP) at frequency ~ 140 GHz and signal-to-noise ratio (S/N) in the spectral sensitivity maximum under the monochromatic (spectral resolution of ~0.1 μm) globar illumination were reached NEP ~4.5*10-10 W/Hz1/2 and S/N~50, respectively.
It is shown that electron heating by electromagnetic radiation in mercury-cadmium-telluride (MCT) layers can be used for designing of uncooled terahertz (THz)/sub-THz detectors with appropriate for active imaging characteristics (noise equivalent power ∼2.6×10 −10 W/Hz 1/2 at ν∼140 GHz ) and these detectors can be manufactured within well established MCT technologies. This narrow-gap semiconductor can be considered as a material for THz/sub-THz detectors with possibility to be assembled into arrays. The characteristics of those detectors can be controlled and improved by selection of parameters of initial layers, substrate properties, and antenna configuration. For field effect transistor detectors, even for transistors with rather long channels (∼1 μm ), rather similar characteristics at ν∼140 GHz can also be obtained.
It is shown that electron heating by electromagnetic radiation in MCT layers can be used for designing of uncooled
THz/sub-THz detectors with appropriate for active imaging characteristics (NEP ~2.6•10-10 W/Hz1/2 at ν ~ 140 GHz)
and these detectors can be manufactured within well established MCT technologies. This narrow-gap semiconductor
can be considered as a material for THz/sub-THz detectors with possibility to be assembled into arrays. The
characteristics of those detectors can be controlled and improved by selection of parameters of initial layers, substrate
properties and antennas configuration. For FET detectors, even for transistors with rather long channels (~ 1 μm) rather
similar characteristics at ν ~ 140 GHz can be obtained too.
Direct detection thin-film bipolar narrow-gap Hg1-xCdxTe semiconductor is considered as a waveguide THz/sub-THz bolometer. The response of such thin layer detectors was calculated and measured in ν=0.037-1.54 THz frequency range at T~70-300 K. Noise equivalent power of such detectors can reach NEP300K~4×10-10 W/Hz1/2 and NEP100K~10-11 in sub-THz frequency range.
Direct detection mm/sub-mm wave warm-carrier bipolar narrow-gap Hg1-xCdxTe semiconductor bolometers
that can be used as picture elements in THz sensitive arrays, are considered. The response of Hg1-xCdxTe warm-electron
bolometers was measured in v=0.037-1.54 THz frequency range at T=68-300 K. Bipolar semiconductor warm-electron
bolometer theoretical model was developed. In the detector considered the electromagnetic wave propagates in
semiconductor waveguide, heats electrons and holes, creates their excess concentrations, as well as, the electromotive
forces. These effects cause the bolometer response voltage. Experimental results confirm the model main conclusions.
Because of response time defined by carrier recombination time in HgCdTe layers (τ~10-8-10-6 s) and the noise
equivalent power that can reach NEP300 K~4×10-10 W/Hz1/2 in mm-wave region, the arrays on the base of HgCdTe
bolometers can make them promising for active relatively fast frame rate sensitive applications. At liquid nitrogen
temperature NEP can lowering up to NEP77K~10-11 W/Hz1/2. Embeded p-n-junctions in HgCdTe can increase the
detectors responsivity by an order.
Narrow-gap direct detection mercury cadmium telluride (MCT) THz semiconductor hot electron bolometer (SHEB)
is considered. Device operation takes into account the phenomena in semiconductor bipolar plasma and hot-carrier
effect at uncooled or slightly cooled conditions. To examine the SHEB detector in the wide range of operation
frequencies (ν=0.037-1.58 THz) the simplest dipole antennas were used in prototype arrays.
The experiments were performed at T = 300 K at ν= 37, 50, 75 GHz, 0.89 and 1.58 THz with a MCT SHEB devices
with intrinsic conductivity. At ν=1.58, 0.89, 0.078 and 0.037 THz the signal temperature dependencies were measured
too. The sensitivity was Sv~2.5 V/W (estimated NEP~4×10-10 W/Hz1/2) at T=300 K; and Sv~2×103 V/W (estimated
NEP~3×10-12 W/Hz1/2) at T=78K and ν=37, and 78 GHz. The signal temperature dependences at ν=0.89 THz are
different compared to those at ν=37 and 78 GHz. Temperature phase dependent signals are discussed with their
dependence on energy relaxation time. A model of such a detector is developed. The radiation entrance through
semiconductor surface and metal contacts are both modeled.
Current-voltage characteristics and differential resistance of n+ -p Hg1-xCdxTe (x=0.2236±0.0015) LWIR (long wavelength infrared) photodiodes forming 128-element array were measured in the temperature range 77-95 K. Experimentally obtained characteristics were fitted by the special nonlinear fitting program based on the carrier- balance equation method which interconnects two processes of transport through the trap level in the band gap: trap-assisted tunneling and thermal Shockley-Reed-Hall generation-recombination process. Other essential current mechanisms (bulk diffusion, band-to-band tunneling, etc.) were included in the model as independent and additive. By the fitting procedure we determine the concentration of donor, acceptor and trap centers, carrier lifetimes, and trap level energy position in the band gap. A good agreement with the experimental data in the whole temperature range of measurements was found assuming that the energy of traps is Et= 0.75 eV above the top of the valence band and does not depend on temperature, unlike the band gap energy. This level seems to be a donor metal vacancies nature of traps in HgCdTe.
Investigations of designed and manufactured mercury cadmium telluride (MCT) multipixel arrays for long-wavelength infrared (LWIR) applications with n+-p-diodes at T≈80 K are performed. As main performance parameters the volt-ampere characteristics and differential resistances from LWIR-photodiodes were investigated using microprobe technique at T≈80 K before and after various doze of gamma-radiation exposure. The current mechanisms for those structures described within the framework of the balance equation model.
4×288 MCT LWIR linear arrays with 28X25 μm diodes and silicon ROICs were designed, manufactured and tested. MCT layers were grown by MBE technology on (013) GaAs substrates with CdTe/ZnTe buffer layers and λco = 11.2±0.15 μm at T = 78 K. CCD and CMOS “hybrid” technology for design and manufacture of silicon ROICs was used. The design rules 2.5 μm for CCD technology and 2.0 μm design rules for CMOS technology happened to be sufficient to realize most of the functions for 288×4 MCT TDI array. Analog functions were realized by CCD elements. An amplification of the output signals is realized by CMOS buffer amplifier. Decoding and deselection code storing functions are accomplished by digital CMOS elements. 288 information channels were attached to 4 analog outputs operating in the frequency range f≤4 MHz clock. Total consumption power measured is 50 mW at T = 298 K and 70 mW at T = 78 K. Before hybridization the parameters of MCT linear arrays and Si readouts were tested separately. With aperture 280×640 the detectivities Dλ ≈ 1.8.1011 cm.Hz1/2/W were achieved (λco ≈ 11.2 μm, λmax 10.0 μm) with standard deviation about 15 % and operability close to 100 %.
MCT 2×64 and 4×288 linear arrays with silicon readouts were designed, manufactured and tested. (013) MCT MBE layers were grown on GaAs substrates with ZnTe and CdTe buffer layers. 2×64 arrays were also manufactured on the base of LPE layers on CdZnTe (111) substrates. 50×55 and ≈30×30 μm area n-p-type photodiodes were formed by 50 ÷ 120 keV boron implantation. The dark currents at V ≈ 100 mV reversed biased diodes used in arrays with cutoff wavelength λco ≈ 10.0 - 12.2 μm were within 15 - 50 nA and zero bias resistance-area products were within R0A ≈ 5 ÷ 20 Ohm×cm2. Designed silicon readouts with skimming and partitioning functions were manufactured by n-channel MOS technology with buried or surface channel CCD register. For achievement with the silicon readouts the deselection function, the “composite” technology approach was considered. In this case both the technology of n-channel CCD and CMOS technology were applied, which allow to weaken considerably the technological design rules for realization of 288×4 readouts with deselection of “dead” elements. It is shown that 2.5 μm design rules for CCD and 2.0 design rules for CMOS technologies allow to realize most of the functions needed for 288×4 MCT array operation with deselection function. Before hybridisation the parameters of MCT linear arrays and Si readouts were tested separately. HgCdTe arrays and Si readouts were hybridised by cold welding In bumps technology. In dependence of FOV with skimming mode used for integration time of 8 - 20 μs detectivities within D*λ (0.4 - 1.7)×1011 cm×Hz1/2/W were achieved in dependence of the array format. Dark carrier transport mechanisms in MCT diodes were calculated and compared with experimental data.
For 288x4 mercury-cadmium telluride (MCT) diode array silicon readouts with deselection function, the "composite" technology approach, which simplifies the technology of their manufacturing, is considered. Both technology of n-channel CCD devices and the CMOS technology are applied, which allow to weaken considerably the technological requirements for realization of 288x4 readouts with deselection of "dead" elements (generally the 0.8 micron design rules technology is applied). It is shown that the design rules 2.5 μm for CCD technology and 2.0 design rules for CMOS technology are sufficient to realize most of functions needed for 288x4 MCT array design and manufacture. All analog functions (including TDI as the most complex function for realization in CMOS basis) are realized by CCD elements. Four-phase TDI register was realized using semi-buried channel by phosphorus ion implantation. An amplification of the output signals is realized by CMOS buffer amplifier. Decoding and deselection code storing functions are realized by digital CMOS elements. The parameters of the 288x4 silicon readout device: direct injection input circuits, 4 elements TDI function, 4 outputs; 4 MHz maximum information output frequency; 2 MHz maximum clock frequency; 3 V swing output voltage; not less than 1.6 pC maximum charge capacity per each input; 3.0 pC maximum charge capacity at multiplexor input; 75 dB dynamic band; 28 output pins.
x4×288 heteroepitaxial mercury-cadmium telluride (MCT) linear arrays for long wavelength infrared (LWIR) applications with 28×25 micron diodes and charge coupled devices (CCD) silicon readouts were designed, manufactured and tested. MCT heteroepitaxial layers were grown by MBE technology on (013) GaAs substrates with CdZnTe buffer layers and have cutoff wavelength λco ≈ 11.8 μm at T = 78 K. To decrease the surface influence of the carriers recombination processes the layers with composition changes and its increase both toward the surface and HgCdTe/CdZnTe boundary were grown. Silicon read-outs with CCD multiplexers with input direct injection circuits were designed, manufactured and tested. The testing procedure to qualify read-out integrated circuits (ROICs) on wafer level at T = 300 K was worked out. The silicon read-outs for 4×288 arrays, with skimming and partitioning functions included were manufactured by n-channel MOS technology with buried or surface channel CCD register. Designed CCD readouts are driven with four- or two-phase clock pulses. The HgCdTe arrays and Si CCD readouts were hybridized by cold welding indium bumps technology. With skimming mode used for 4×288 MCT n-p-junctions, the detectivity was about (formula available in paper) for background temperature Tb = 295 K.
Investigations of performance of mercury cadmium telluride (MCT) multipixel arrays at T≈80 K are considered. MCT hybrid arrays for long-wavelength infrared (LWIR) applications with n+-p-diodes and n-channel charged coupled devices (CCD) silicon readouts were designed, manufactured and tested. For testing procedure the measurements of noise and signal-to-noise ratio (SNR) are the key issues to determine performance parameters to characterize IR-sensors. That puts certain requirements to the registration system and methods of measuring used. The noise of the signals from LWIR-photodiodes with CCD readouts or CCD readouts itself was measured using several different techniques. To find out and eliminate noise sources the spectral noise power of signals was analyzed. It allowed the possibility to implement actions for reducing of the registration system noise, and to define the software noise filters to be used. The testing procedure of FPA performance characteristics includes the measurements of detectivity D*, noise equivalent temperature difference NETD, cut-off wavelength and some other parameters of the arrays.
Dark carrier transport mechanisms in narrow-gap Hg1-xCdxTe multilayer structures and Pb1-zSnzTe/PbTe1-yS(Se)y heterojunctions at T~80 K for applications in IR arrays are analyzed and compared with homojunction mercury-cadmium telluride (MCT) photodiode characteristics in the temperature range T~70-150 K. In the analysis procedure two major current mechanisms were included into the current balance equations: trap-assisted tunneling (TAT) and Shockley-Reed-Hall (SRH) generation-recombination processes for a defect trap level. Other current mechanisms (e.g., band-to-band tunneling, bulk diffusion) were taken into account as additive contributions. For TAT the tunneling rate characteristics were calculated within the k-p-approximation. Using donor and acceptor concentrations, trap level energies and concentrations, and in-junction trap level lifetimes as fitting parameters, good agreement with experimental data for HgCdTe and PbSnTe heterojunction and homojunction diodes was obtained, which allows one to predict the diode parameters from the known material characteristics. Photodiode or array parameters itself, or with CCD readouts, or CCD readouts separately were tested to study the influence of readout cascade on the diodes' properties.
Two X sixty-four linear photodiode arrays on the base of HgCdTe MBE grown layers with CCD silicon readouts were designed, fabricated and tested. It is shown that detectivity for the given arrays even with skimming mode used for long integration times that is need for large square n-p-junctions used and cut-off wavelength of 12.2 micrometer was near the ultimate performance limit.
Strong magnetic field Hall coefficient and magnetoresistivity dependencies in PbTe/PbS semimetallic SLs with periods of 12.0 divided by 100.0 nm were observed. From the analysis of these dependencies by Monte-Carlo fitting procedure the band-offset (Delta) Ev equals 0.32 plus or minus 0.05 eV was obtained (T equals 77 K) and it was proved that these SLs are type II 'misaligned structures.' The band structure calculations of PbTe/PbS SLs in the envelope wave function approximation showed that in such SLs a semimetal-semiconductor transition should occur for layer thicknesses of approximately 6.0 nm.
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