Semiconductor HgCdTe Research Articles

Fast transient infrared detection for time-domain astronomy

Multi-messenger astronomy requires the use and development of telescopes and detectors for temporal observations across all the electromagnetic spectrum. The interest of our team is towards time-domain (or longitudinal, in the jargon of storage ring diagnostics) detectors able to make observations over the time rather than transversely by making photographs or spectrographs. The FAIRTEL (Fast InfraRed TELescope) experiment wants to build a very fast detector in the Mid InfraRed (MIR) to be used in time-domain astronomy. The experiment is oriented mainly toward the search of fast astronomical transients in the Galaxy, similarly to what has been observed in different electromagnetic ranges with the discovery of the FRBs (Fast Radio Burst) and the GRBs (Gamma-Ray Burst). In the last years, time-domain astronomy is growing in interest and intends to study events on time scales that can go from hundreds of milliseconds to microseconds and even shorter. The detector proposed by the FAIRTEL experiment, is based on HgCdTe semiconductors and on the experience done in the diagnostics for storage ring developed at DAFNE, the Italian e+/e− circular collider. The detection system design is in progress at the LNF (Laboratori Nazionali di Frascati) of INFN (Istituto Nazionale di Fisica Nucleare) while the telescope will be a standard reflecting type, like a Newtonian, a Cassegrain or a Ritchey–Chrétien telescope. The detection apparatus is under test at the IR synchrotron beamline SINBAD at DAFNE and it is foreseen to be implemented at the OPC (Osservatorio Polifunzionale del Chianti). It should be able to observe events with transients down to the nanosecond. A further development of the project involves the extension from ground-based observations to those by using balloons for scientific studies.

Optimization of infrared detector HgCdTe in photoconductive‏ ‏in the spectral region 2-6 µm‎ ‎

In this paper a new method in design and modeling of infrared detector HgCdTe in photoconductive mode ‎is presented. In this method after scrutiny mechanical, optical, electrical properties of semiconductor ‎HgCdTe and solving the photoconductive equations, software of equations is programmed and with initial ‎value and repeat cycle, their changes to key parameters such as thickness, wavelength and amount of ‎impurities are calculated and finally, performance of detector is optimized.‎ Modeling of photoconductive detector has been done in temperature of 300K and 2-6 µm wavelength range‎‎, according to obtained results, optimal specific detectivity is 3×109 cmHz1/2W-1‎‏ ‏in 5.77 µm wavelength.‎

HgCdTe-Based 640 × 512 Matrix Midwave Infrared Photodetector

Matrix photosensitive elements based on a HgCdTe semiconductor solid solution on silicon substrates with 640 × 512 elements at a pitch of 25 μm with a long-wavelength sensitivity of 5 μm at half maximum are designed and produced. The scheme and topology are developed according to which matrix multiplexers with 640 × 512 elements at a pitch of 25 μm, which ensure operating modes at a clock frequency up to 10 MHz, are manufactured. Using a hybrid assembly method on indium bumps, a matrix photodetector with 640 × 512 elements at a pitch of 25 μm is produced. The best photodetector specimens are characterized by the following parameters: average NETD value 99.5%.

Fast rise time IR detectors for lepton colliders

Diagnostics is a fundamental issue for accelerators whose demands are continuously increasing. In particular bunch-by-bunch diagnostics is a key challenge for the latest generation of lepton colliders and storage rings. The Frascati Φ-factory, DAΦNE, colliding at 1.02 GeV in the centre of mass, hosts in the main rings few synchrotron radiation beamlines and two of them collect the synchrotron radiation infrared emission: SINBAD from the electron ring and 3+L from the positron ring. At DAΦNE each bucket is 2.7 ns long and particles are gathered in bunches emitting pulsed IR radiation, whose intensity in the long wavelength regime is directly proportional to the accumulated particles. Compact uncooled photoconductive HgCdTe detectors have been tested in both beamlines using dedicated optical layouts. Actually, the fast rise time of HgCdTe semiconductors give us the chance to test bunch-by-bunch devices for both longitudinal and transverse diagnostics. For the longitudinal case, single pixel detectors have been used, while for the transverse diagnostics, multi-pixel array detectors, with special custom design, are under test. This contribution will briefly describe the status of the research on fast IR detectors at DAΦNE, the results obtained and possible foreseen developments.

Mercury–cadmium–telluride thin layers as subterahertz and infrared detectors

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-tel- luride (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 photo- conductors. 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∕Hz 1∕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. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE) (DOI: 10 .1117/1.OE.54.12.127102)

Hysteresis Phenomena in mis Structures Based on Graded-Gap MBE Hgcdte with a Two-Layer Plasma-Chemical Insulator SIO2/SI3N4

Capacitance-voltage characteristics (CV characteristics) of MIS structures based on graded-gap MBE Hg 1–x Cd x Te (x = 0.22–0.23) with a two-layer plasma-chemical insulator SiO2/Si3N4 are studied under variation of the voltage sweep direction. Special features are revealed in the CV characteristics at the forward and reverse voltage sweeps for MIS structures based on n-HgCdTe and p-HgCdTe. The results obtained suggest that the electron capture by states of the transition layer between the HgCdTe semiconductor and SiO2 insulator plays an important role in the hysteresis phenomena. It is found that if an insulator is applied, the impurity-defect centers of donor type are formed near the interface, whose density of states in the transition layer is (0.6–2.1)·10 11 сm –2 .

Dynamic Curvature and Stress Studies for MBE CdTe on Si and GaAs Substrates

Infrared focal plane arrays (IRFPA) based on HgCdTe semiconductor alloys have been shown to be ideal for tactical and strategic applications. High density (>1 M pixel), high operability HgCdTe detectors on large area, low-cost composite substrates, such as CdTe-buffered Si or GaAs, are envisioned for next-generation IRFPAs. Thermal expansion mismatch is among various material parameters that govern the structural properties of the final detector layer. It has previously been shown that thermal expansion mismatch plays the dominant role in the residual stress characteristics of these heteroepitaxial structures (Jacobs et al. in J Electron Mater 37:1480, 2008). The wafer curvature (bowing) resulting from residual stress, is a likely source of problems that may occur during subsequent processing. This includes cracking of the film and substrate during post-growth annealing processes or even certain characterization techniques. In this work, we examine dynamic curvature and stress during molecular beam epitaxy (MBE), of CdTe on Si and GaAs substrates. The effect of temperature changes on wafer curvature throughout the growth sequence is documented using a multi-beam optical sensor developed by K-Space Associates. This monitoring technique makes possible the study of growth sequences which employ annealing schemes and/or interlayers to influence the final residual stress state of the heteroepitaxial structures.

Uncooled Wide-Range Spectral Optoelectronic Devices on the Base of HgCdTe Semiconductor

Issues associated with the development and the exploitation of infrared (IR) and sub-terahertz (THz) radiation detectors based on HgCdTe semiconductor are discussed. It is shown that this mercury cadmium telluride (MCT) semiconductor can be applied to the development of bi-colour detectors operating in the IR and sub-THz spectral ranges.

Unified carrier density approximation for non-parabolic and highly degenerate HgCdTe semiconductors covering SWIR, MWIR and LWIR bands

We propose a unified approximate analytical expression for the carrier density of HgCdTe semiconductors that have non-parabolic energy bands and are highly degenerate. The proposed expression is without any adjustable parameter. It can be applied to HgCdTe for the case where electron densities are very high and the material is strongly degenerate, e.g., a highly accumulated surface due to passivant-induced negative fixed charge density in an n-HgCdTe photoconductor device. The proposed expression is simultaneously valid for SWIR, MWIR and LWIR bands.

Carrier density approximation for non-parabolic and highly degenerate HgCdTe semiconductors

We propose simple, analytical, approximations of carrier density for HgCdTe semiconductors that have non-parabolic energy bands and are highly degenerate. The proposed expressions are in the form similar to the classical Boltzmann's approximation without any adjustable parameter. These relations can be applied to HgCdTe for the case where electron densities are very high and the material is highly degenerate, e.g. highly accumulated surface due to passivant induced negative fixed charge density in n-HgCdTe photoconductor device. The expressions are valid in the LWIR and MWIR bands, and can be tuned further to apply to other bands.

Development of a five-band multi-spectral infrared radiometer for low temperature measurements

A five-band infrared radiometer has been developed for the measurement of spectral radiance and radiance temperature at low temperatures. The optical system of this radiometer consists of a scanning plane reflecting mirror, five narrow-band interference filters in the 6–12 μm band, a mirror-type rotating chopper, a cold source, a hot source, and a HgCdTe semiconductor detector. Measurement of radiance temperature and spectral radiance using this radiometer is carried out automatically using a personal computer. The calibration of the output signal for each spectral channel of the radiometer is carried out exactly using a blackbody source with an accurate temperature controller. The short-time stability of the radiometer is estimated to be within 0.2% mean deviation for the main spectral channel. The temperature detection sensitivity of a radiometer is evaluated as the noise equivalent temperature difference (NETD) for the optical and measuring system; the NETD for the main spectral channel is estimated to be about <0.2 °C. The minimum detectable radiance for the main spectral channel is also estimated to be about <0.002 mW/cm2 sr μm. For confirmation of the long-time stability of the radiometer, the measurement of the radiometer output ratio between the blackbody source at a temperature of 15 °C and the hot source at a constant temperature of 40 °C is carried out over 3 h; the long-time stability of measurement for the main spectral channel is estimated to be within ±0.3 °C mean deviation. The variation over time of the spectral radiance and the radiance temperature of the cloud in the sky was actually measured using the radiometer, and its usefulness was clarified.

Subnanosecond, time-resolved, broadband infrared spectroscopy using synchrotron radiation

A facility for subnanosecond time-resolved (pump-probe) infrared spectroscopy has been developed at the National Synchrotron Light Source of Brookhaven National Laboratory. A mode-locked Ti:sapphire laser produces 2 ps duration, tunable near-infrared pump pulses synchronized to probe pulses from a synchrotron storage ring. The facility is unique on account of the broadband infrared from the synchrotron, which allows the entire spectral range from 2 cm−1 (0.25 meV) to 20 000 cm−1 (2.5 eV) to be probed. A temporal resolution of 100 ps, limited by the infrared synchrotron-pulse duration (full width at half maximum), is achievable. A maximum time delay of 170 ns is available without gating the infrared detector. To illustrate the performance of the facility, a measurement of electron–hole recombination dynamics for an HgCdTe semiconductor film in the far- and mid-infrared range is presented.

Two-dimensional electron gas in semimagnetic semiconductor HgMnTe with inverted bands

The two-dimensional electron gas in the surface layers of HgMnTe with inverted bands is studied for the first time experimentally and theoretically. It is shown that the structure of the investigated capacitance magnetooscillations in HgMnTe MOS structures is entirely similar to that observed in the non-magnetic gapless semiconductor HgCdTe and the sole effect of the exchange interaction is the temperature shift of beat nodes. The information about the exchange parameters is obtained only from modeling the oscillations, because no pronounced changes in the position of oscillations are observed and the separate spin components are not resolved. For the description of the spectrum in the magnetic field, we propose a theory that takes the exchange and spin-orbit interactions into account for materials with direct and inverted bands. A comparison between experiment and theory for different temperatures and exchange interaction parameters is reported. The modeling shows that the spin-orbit splitting by far exceeds the contribution of the exchange interaction. The calculated amplitudes of “partial” oscillations for different spin branches of the spectrum are essentially different in accordance with the difference in the intensities of the corresponding lines in the Fourier spectra of the experimental oscillations.

Spin effects in the magneto-oscillations of the capacitance of a two-dimensional gas of semiconductors with Kane and Dirac spectra

The results of experimental investigations and computer modeling of the magneto-oscillations of the capacitance of surface layers with a two-dimensional gas in the narrow-gap semiconductor HgCdTe with direct and inverted band structure are compared. The structure of the Landau levels is calculated by a model that has a clear physical interpretation and is based on the reduction of matrix equations to Schrodinger-like equations with an effective potential in which the terms responsible for nonparabolic and spinor-type effects are easily separable. An analytic approach is developed for describing magneto-oscillation phenomena in the two-dimensional gas of materials with a quasirelativistic spectrum, and the emergence is noted of new theoretical parameters (compared to the parameters of materials with parabolic bands). Finally, the parameters of level broadening in spin-orbit split subsubbands are determined and the dominant scattering mechanisms are discussed.

Many-body effects and electron tunneling in metal-insulator-p-type semiconductor structures

Investigation of the tunneling conductivity σd(V) of structures made on a highly doped, narrow-gap p-type semiconductor HgCdTe reveals an abrupt increase in this quantity at voltages corresponding to the start of tunneling into the conduction band. It is shown that the observed functions σd(V) cannot be described in the framework of a model based on single-particle tunneling. It is proposed that the abrupt increase in σd(V) is attributable to tunneling into exciton states.

The vertical photoconductor: A novel device structure suitable for HgCdTe two-dimensional infrared focal plane arrays

A novel photoconductive device structure is proposed and described that has been designed specifically as a sensing element for high density two-dimensional infrared focal plane array (IRFPA) applications. Although the design concept can be applied to a variety of epitaxially grown HgCdTe material, optimum performance can be achieved using n-type HgCdTe semiconductor material consisting of epitaxially grown heterostructure layers in which a two-dimensional mosaic of vertical design photoconductors are fabricated. The heterostructure layers provide high performance devices at greatly reduced power dissipation levels, while the vertical design allows for the high density integration of photoconductors in a two-dimensional array geometry with high fill factor. The salient feature of the proposed device structure is that the bias field is applied in the vertical direction such that it is parallel to the impinging infrared radiation. A comprehensive one-dimensional model is presented for the vertical design photoconductor, which is subsequently used to determine the optimum design parameters in order to achieve maximum responsivity at the lowest possible power dissipation level. It is found that the proposed device structure has the potential to be used in the fabrication of long wavelength IRFPAs approaching 10 6 pixels using 25 × 25 μm 2 detector elements. Furthermore, this is achieved with individual device detectivities that are background limited and for a total array power dissipation of less than 0.1 W using a pulsed biasing scheme. Performance issues such as response uniformity, pixel yield, fill factor, crosstalk, power dissipation, detector impedance, array architecture, and maximum array size are discussed in relation to the suitability of the proposed vertical photoconductor structure for use in IRFPA modules. When considering IRFPA operability, it is found that in many cases the proposed technology has the potential to deliver significant advantages, such as higher yield, higher fill factor, better uniformity, less crosstalk, and larger potential array size, when compared to photovoltaic technology.

Spin-orbit splitting of the spectrum of 2D electrons in the surface quantum well of gapless semiconductors

Spin-orbit splitting of the two-dimensional subbands in the surface quantum well of gapless semiconductor HgCdTe has been measured by tunneling spectroscopy. The results have been analysed in the framework of the three band Kane model of the energy spectrum.

New radiation thermometer for near room temperature

The authors have developed a new radiation thermometer for the measurement of near room temperature with high signal-to-noise ratio ( S N ). The thermometer, utilizing an HgCdTe semiconductor sensor, is compact and insensitive to temperature changes of the surroundings. Its major breakthrough owes much to the fact that the sensor is cooled and controlled by Peltier thermoelectric devices and that the sensor serves as reference radiator. A well designed optical system with a chopper and a concave mirror shuts off stray radiation, thus improving the S N . On-line experiments in a steel sheet manufacturing process using a prototype thermometer proved the validity of this new radiation thermometer.

Investigation of Phonons in HgCdMnTe Using Far-Infrared Reflectivity

The quaternary alloy Hg1-x y Cdx Mny Τd is an interesting diluted magnetic semiconductor because of possibility of additional control of its material parameters. Important physical properties such as band gap, lattice constant, magnetization can be changed independently of each other by proper choosing its molar compositions x and y [1]. HgCdMnTe is a competitive material to the widely used HgCdTe semiconductor for infrared detection [2].

New light modulation device: optical transistor

The plasma reflectivity edge in the infrared reflectivity spectrum measured on a narrow-gap semiconductor HgCdTe has been found to be very sensitive to the variation of carrier concentration under certain circumstance, based on which a new way of infrared modulation is studied, where the estimated modulation gain in power is impressively high. Thus, infrared radiation in far- and mid-infrared can be effectively modulated by electromagnetic waves with shorter wavelengths, and hence a less sensitive room-temperature infrared detector could be as sensitive as the existing cooled infrared detectors. The suggested new optoelectronic device, which can be termed an optical transistor, working with light beams instead of electronic currents, is quite promising for fiber communication and infrared detection as well as for investigating the relaxation processes of excess carriers in semiconductors. The detectivity for the plasma-edge detector, which is a detector equipped with an optical transistor, is calculated.