V. N. Ovsyuk

Microbolometer detector arrays for the infrared and terahertz ranges

This paper presents the results of the development of photodetectors for the IR and terahertz ranges, based on uncooled microbolometer detector arrays fabricated by the sol-gel technology. Detectors with a 320×240-element format are demonstrated, having a temperature resolution better than 100mK and a threshold power of 160pW/pixel in the 8-14-μm range, with a response time of 12-16ms. The threshold power at a wavelength of 130μm was 33nW/pixel for a terahertz image visualized in a 160×120 format at a rate of 90frame/sec. A design was developed to match an antenna with an uncooled high-resistance integrated microbolometer, allowing the detector sensitivity to be significantly increased in the terahertz range.

Application of uncooled microbolometer detector arrays for recording radiation of the terahertz spectral range

The mechanism of occurrence of sensitivity of microbolometer detector arrays based on vanadium oxide to terahertz radiation are analyzed. Experimental data are given showing the possibility of increasing the sensitivity of microbolometer detectors in the terahertz range by using an additional thin metal absorbing layer applied on the microbolometer membrane. A polarization dependence of the sensitivity of microbolometers in the terahertz and far infrared spectral ranges is found. It is shown that the sensitivity of microbolometers in the terahertz range is due to the absorption of radiation in the narrow metal runs placed on the support legs of the microbolometers and playing the role of an ohmic contact between the heat-sensitive layer and the processing curcuit.

Focal plane arrays based on HgCdTe epitaxial layers MBE-grown on GaAs substrates

Heterostructures HgCdTe/CdTe/GaAs grown by molecular beam epitaxy were used for LWIR FPA fabrication. The technology was developed and 32 by 32 and 128 by 128 photodiode arrays with indium bumps of 15 micrometer height in each pixel were fabricated. Mean NEP is 1.7 by 10-13 W/Hz1/2 and 1.1 by 10-14 W/Hz1/2 for 128 by 128 photodiode arrays with (lambda) c value of 10.4 micrometer and 5.2 micrometer correspondently. The technology of hybrid assembling with continuous control of cold welding on the measuring stand was demonstrated on the example of 32 by 32 LWIR FPA. Mean NEP value of 5.4 by 10-14 W/Hz1/2 with (lambda) c equals 10.6 micrometer at 80 K operation were obtained. using an infrared camera system the infrared image was successfully demonstrated. The NETD value of 0.077 K was obtained under 293 K background condition.

Charge transport in HgCdTe-based n+-p photodiodes

It is shown that dark currents measured at 77 K in Hg1−xCdxTe (x⋍0.21) homojunctions can be adequately described by the balance equations with allowance made for the two main charge-transport mechanisms, i.e., tunneling assisted by traps in the band gap and recombination via these traps; the above homojunction may find application in microphotoelectronics in the infrared spectral range of 8–12 µm. Other charge-transport mechanisms are included in the consideration as additive terms. A comparison between the experimental current-voltage characteristics and dynamic resistance of HgCdTe diodes with the results of calculations was carried out. A good agreement was obtained between experimental data and the results of calculations, in which the donor and acceptor concentrations in the n and p regions of diodes, the concentration of traps and the position of their levels in the band gap, and the lifetimes of charge carriers for recombination via these traps were used as adjustable parameters.

Electrical activation of boron implanted in p-HgCdTe (x = 0.22) by low-temperature annealing under an anodic oxide

Abstract The low-temperature electrical activation of boron atoms implanted in p-HgCdTe has been observed for the first time. The bulk crystals of p-Hg 0.75 Cd 0.22 Te were implanted at room temperature and at 250 °C with 150 keV B + ions up to a dose of 3 × 10 16 cm −2 through an anodic oxide with a thickness of 90 nm. The same control samples were implanted with N + ions under analogous conditions. The two-step post-implant annealing was carried out at 250 °C for 2–4 h and then at 200 °C up to 44 h in a nitrogen atmosphere. Differential Hall effect measurements at 77 K, optical reflection and secondary ion mass spectroscopy were used for implanted surface layer studies. It was established that the doping efficiency of boron atoms amounts of 0.03–14% decreasing with the increasing boron ion dose and becoming higher in the case of ion implantation at 250 °C. Mercury loss was found not to occur from the surface of HgCdTe through an anodic oxide cap during such heat treatment.

IR photodetectors based on MBE-grown MCT layers

A complete technological cycle has been designed to produce photodetector arrays, which involves MBE growth of Hg1-xCdxTe (MCT) heteroepitaxial layers, fabrication of MCT-based photodetector structures, manufacture of silicon array multiplexers and hybrid assembly of a photodetector module consisting of a photodetector and multiplexer by means of indium micro bumps. Photoelectric parameters are given of photodetector array modules on the basis of photodiodes for the middle (3 - 3.5 μm) and far (8 - 12 μm) infrared ranges, operating at 78 - 80 K and 200 - 220 K temperatures.

MCT heterostructure design and growth by MBE for IR devices

The molecular beam epitaxy (MBE) set-up with analytical control equipment of growth process was designed and fabricated for growing A2B6 compounds including the mercury-containing ones. A technology was elaborated for growing mercury-cadmium-telluride (MCT) solid solution heteroepitaxial structures (HS) by molecular beam epitaxy (MBE) method with a given variation of MCT composition throughout the thickness. HSs MCT MBE on CdZnTe/GaAs substrate with different variation composition in layers at MCT film interfaces were designing and growing. These structures were used for manufacture of high quality single, linear and array photoconductors (PC) and photodiodes (PD) operating at 77 K and 200 - 250 K temperature in the wavelength range of 3 - 5 micrometer and 8 - 12 micrometer, up and over 20 micrometer. Widegap layers at MCT film interfaces are used as passivating coating. Narrowgap layer at MCT film/CdZnTe buffer layers interface is used for decrease of PD series resistance. For decrease of dark currents of photodiodes array operating at 200 K HS MCT MBE were used with special composition distribution of composition throughout the thickness.

Properties of 2x64 linear HgCdTe MBE-grown LWIR arrays with CCD silicon readouts

Mercury-Cadmium-Telluride (MCT) 2 X 64 linear arrays with silicon readouts were designed, manufactured and tested. NCT layers were grown by MBE method on (103) GaAs substrates with CdZnTe buffer layers. 50 X 50 mm n-p-type photodiodes were formed by 80 divided by 120 keV boron implantation. The dark current at 100 mV reversed biased diodes was within 15*30 nA and zero bias resistance-area product was within R0 approximately equals 20 divided by 50 Ohm X cm2. Silicon read-out circuits were designed, manufactured and tested. Read-outs with skimming and partitioning functions were manufactured by n-channel MOS technology with buried or surface channel CCD register. The parameters of LWIR MCT linear arrays with cutoff wavelength (lambda) co 10.0 divided by 12.2 micrometers and Si readouts were tested separately before hybridization. The HgCdTe arrays and Si readouts were hybridized by cold welding In bumps technology. With skimming mode used for integration time of 24-30 ms for such MCT n-p-junctions, the detectivity D*(lambda ) approximately equals 4 X 1010 cmXHz1/2/W. Dark carrier transport mechanisms in these diodes were calculated and compared with experimental data. Two major current mechanisms were included into the current balance equations: trap-assisted tunneling and Shockley-Reed-Hall generation-recombination processes via a defect trap level in the gap. Other current mechanisms (band-to-band tunneling, bulk diffusion) were taken into account as additive contributions. Tunneling rate characteristics were calculated within k-p approximation with the constant barrier electric field. Relatively good agreement with experimental data for diodes with large zero resistance-area products (R0A > 10 OhmXcm2) was obtained.

Method for the characterization of electron, light- and heavy-hole concentrations and mobilities in narrow-gap p-type HgCdTe

Abstract A novel simple characterization method based on magnetotransport measurements has been developed to unambiguously determine the electrical parameters of the electrons, light and heavy holes in the narrow-gap p-type HgCdTe (x~0.2). No mathematical fitting procedure is required in this method for reliably calculating the mobility and density of each type of carrier. The technique comprises simultaneous measurements of the Hall coefficient RH and magnetoresistance at five values of the low magnetic flux density B and the conductivity σ at B = 0. This method allows the determination of the electron, light- and heavy-hole mobilities and concentrations at temperatures below 77 K, where the dependence RH(B) is positive and the bulk mixed-conduction regime still takes place. When the electrons freeze out at low temperatures, the parameters of heavy and light holes can be determined by measuring three values of Hall coefficient and magnetoresistance. It has also been shown that a more accurate value of heavy-hole mobility can be obtained by multiplying together RH and σ measured at the high magnetic field at which the transverse magnetoresistance becomes saturated. The simple equations have been derived for the electron and light-hole mobilities and concentrations from the measurement of the differential magnetoresistance. The developed method has been employed for the experimental determination of electron, light- and heavy-hole parameters in p-type HgCdTe (x~0.2).

Mobility of minority charge carriers in p-HgCdTe films

Temperature dependences of electron mobility in p-Hg1−xCdxTe films (x=0.210–0.223) grown by molecular beam epitaxy are investigated. In the temperature range 125–300 K, mobility was found by the mobility-spectrum method, and for the range 77–125 K, it was found using a magnetophotoconductivity method suggested in this study. The method is based on the measurement of the magnetic-field dependence of photoconductivity. The magnetic field is parallel to the radiation and normal to the sample surface. The electron mobility is determined using the simple expression μn [m2/(V s)]=1/BH[T]. Here, BH is the induction of the magnetic field corresponding to a half-amplitude of the photoconductivity signal under zero magnetic field. In the temperature range 100–125 K, the results obtained by the magnetophotoconductivity and mobility-spectrum methods coincide. For the samples investigated, the electron mobility at 77 K is in the range 5–8 m2/(V s).