Mikhail Fadeev, Alexander Troshkin, Alexander Dubinov, Vladimir Utochkin, Anna Razova, Vladimir Rumyantsev, Vladimir Aleshkin, Vladimir Gavrilenko, Nikolay Mikhailov, Sergey Dvoretsky, Sergey Morozov
Heterostructures with quantum wells (QWs) based on HgCdTe are promising systems for making compact semiconductor sources of the mid-infrared range. This range has paramount practical importance due to the presence of absorption lines of common pollutant gases. Stimulated emission (SE) from HgCdTe heterostructures has been observed previously at the wavelengths up to 3.7 μm at temperatures above 200 K, but the optimal design of the QW is still debated. We present the spectra of SE from a HgCdTe-based heterostructure obtained at room temperature (RT). We investigate the effect of optical excitation wavelength on the characteristics of SE and discuss possible routes toward improving the design of the active region of the sample. Obtained results demonstrate that HgCdTe heterostructures can be used for the creation of lasers operating at RT at the wavelengths in the vicinity of at least 2.5 μm.
Temperature-driven photoconductivity spectra are studied in HgCdTe thin films and quantum well (QW) heterostructures grown by molecular beam epitaxy (MBE). It is shown that the absorption edge steepness in narrow gap HgCdTe epilayers approaches the fundamental limit. The corresponding Urbach energy is 1.5 to 4 meV at 4.2 to 77 K, which is an order of magnitude lower than values reported previously, indicating a significant progress in the quality of structures grown by MBE. Auger-suppressed multi-QW heterostructures that can be used for development of long-wavelength lasers/detectors are shown to have the comparable steepness of the absorption edge. The corresponding “Urbach” energy is much less than the threshold energy of the Auger recombination, which means that furthering the operating wavelengths beyond 20 μm is feasible for optoelectronic devices based on HgCdTe structures.
F. Teppe, S. Ruffenach, S. Krishtopenko, M. Marcinkiewicz, C. Consejo, J. Torres, M. Orlita, W. Knap, D. Smirnov, S. Morozov, V. Gavrilenko, N. Mikhailov, S. Dvoretskii
HgCdTe compounds can be engineered to fabricate “gapped-at-will” structures. Therefore, 1D, 2D and even 3D massless particles can be observed in topological phase transitions driven by intrinsic and external physical parameters. In this work, we report on our experimental results, obtained by temperature-dependent Terahertz and Mid-Infrared magneto-spectroscopy, of topological phase transitions in HgCdTe-based quantum wells and bulk samples. These transitions are accompanied with the appearance of 2D and 3D massless particles called Dirac and Kane fermions, respectively.
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.
We report on a Terahertz magnetospectroscopy study of a set of five HgTe quantum wells of different thickness, from below to above the critical thickness dc. In quantizing magnetic fields up to 11 T, both intraband and interband transitions have been observed. In samples with inverted band structures, we confirm the observation of the crossing avoiding of the zero-mode Landau levels at a critical value of the magnetic field. In samples with non-inverted band structures, close to the critical thickness, we report on the square root dependence of the intraband transition energy on the magnetic field, as expected in the single-particle model of massless Dirac fermions. The obtained results are compared with the allowed transition energies calculated using the 8 × 8 Kane model.
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.
Modern technology advances combined with unique physical properties of mercury cadmium telluride (MCT)
material, empower low-signal applications in technical vision systems. Properties of MCT detectors manufactured from
LPE and MBE epilayers and their dependences on thickness and doping both for IR and THz regions are discussed. It is
shown by comparison of experimental data and modeling of I-V dark current characteristics that MCT photodiode
ultimate electrical characteristics are limited by diffusion current in n+-n--p junctions and by current via the deep traps
in the gap with position Et= 0.7 Eg above the valence band and concentrations Nt = (1.0-5.5)•1015 cm-3 which are
comparable with donor concentration in n--region Nd = (1.1-1.8)•1015 cm-3. Detector array parameters for a wavelength
range 8-12 microns are: detectivity D*=1.9•1011 cm•Hz1/2/W, noise equivalent difference temperature NEDT ≈ 9 mK,
dynamical resistance R ≈ 4·109 Ohm for the reverse biases ΔV = 0.1-0.2 V.
Also, it is shown that MCT layers can be successfully used as sub-mm or mm wave ambient temperature or
moderately cooled hot electron bolometers. Thus, in addition to the wavelength range from SWIR to VLIR, where the
MCT detectors are employed mainly as photodiodes or photoresistors, they can be used as sub-mm or mm wave
detectors in the range from 190 microns to 8 mm. They can be employed here as semiconductor hot electron bolometers
(SHEB). Measurements performed at electromagnetic wave frequencies ν = 37, 55, 77 GHz, and also at 0.89 and 1.58
THz with non-optimized Hg0.8Cd0.2Te bolometer prototype, has confirmed the basic concepts of SHEB. At ν = 0.89
THz, 77 GHz and 37 GHz the signal temperature dependencies were measured too. At 77 K the SHEB sensitivity at ν =
37 and 77 GHz is increasing up to two orders compared to room temperature data. The sensitivity Sν ≈ 2 V/W at 300 K,
and calculated both Johnson-Nyquist and generation-recombination noise values give estimations of SHEB NEP ~ 4•10-10
W at band-width Δf = 1 Hz and ν = 37 GHz.
The measurement signal (S) to noise (N) ratio (S/N) of novel 128×128 FPA in temperature range 77 -130K was carried
out. FPA for spectral range 8-12 μm was fabricated by B+ implantation process into graded MCT P-p heterojunction with potential barrier. MCT P-p heterojunction with specific MCT composition throughout the thickness was grown by MBE
on GaAs substrate by ellipsometric control in situ. The potential barrier was determined by the difference of MCT
composition at absorber and p-n junction location layers and equal to ΔXCdTe = 0,025. It was shown that based on P-p
heterojunction FPA operated temperature and wavelength increases over routine one without P-p heterojunction.
Keywords: graded gap layers, heterojunction, MCT, photodiodes, FPA.
Ultimate performance small-pitched infrared focal plane arrays (FPA) are of the great interest for development and production of state-of-the-art special and common use thermal imagers. Novel MBE-grown Hg1-xCdxTe/CdZnTe/GaAs heterostructures (MCT/CZT on GaAs HS) are considered perspective for implementation of sophisticated FPA concepts including multi-color and HOT (high operating temperature). Performance of MWIR and LWIR photoconductive (PC) and photovoltaic (PV) infrared detectors fabricated on the base of a. m. heterostructures are presented. The main feature of developed technology is formation of multi-layer device structure in single MBE growth run with precise control of thickness and alloy composition "x" across individual layers and hence throughout heterostructure. Flexible HS design results in half-finished products of PC and PV detectors with optimized parameters of absorber and perfect interfaces between absorber and blocking layers providing effective suppression of surface recombination and surface leakage currents. Giant peak responsivity RV (λco=10.5 μm, 500 K, 1200 Hz) over 6,0×105 V/W was reached on LWIR PC. Average values D* = 1.8×1011 cmHz1/2W-1 and NEDT = 9 mκ were measured on 4×288 PV FPA with λco = 11.2 μm at T=78 K just as 23 mK and 19 mκ on 320×256(240) PV FPA with λco = 5.5 and λco = 10.2 μm at T=78 K.
In the paper experimental results on boron implantation of the CdxHg1-xTe epilayers with various composition near surface of the material are discussed. The electron concentration in the surface layer after irradiation vs irradiation dose and ion energy are investigated for range of doses 1011 - 3•1015 cm-2 and energies of 20 - 150 keV. Also the results of the electrical active defects distribution measurement, carried out by differential Hall method, after boron implantation are represented. Consideration of the received data shows, that composition gradient influence mainly on the various dynamics of accumulation of electric active radiation defects. The electric active defects distribution analysis shows, that the other factors are negligible.
The electro-physical and photo-electrical properties of the HgCdTe/SiO2/Si3N4 and HgCdTe/anodic-oxide film MIS structures is experimentally studied. The heteroepitaxial graded-band films Hg0.78Cd0.22Te were produced on the GaAs substrates by molecular-beam epitaxy. It was established of features of electrical properties were related with conduction type of the semiconductor and to the presence of near-surface graded-band layers. The test measurements of the electro-physical and photoelectric performances of MIS-structures in base of graded-band HgCdTe are held and the following parameters are found: resistances of volume, voltage of flat bands, densities of mobile and fixed charges, spectrums of surface states. It is shown that low-temperature double insulator SiO2/Si3N4 is perspective for passivation of surface of focal plane arrays in base of HgCdTe-photodiodes.
CdTe and HgCdTe layers were grown by MBE on GaAs(310) and Si(310) substrates. The dependences of microrelief height and macroscopic defects densities on the growth conditions of CdTe layer are plotted. CdTe buffer layers with the average height of surface relief of 2 nm are obtained. HgCdTe layers on GaAs(310) substrates with V-shaped defects density of 200-300 cm-2 were grown. When Si(310) substrates are used, the boundaries between antiphase domains are an additional reason for formation of V-shaped defects. It is shown, that the optimization of surface preparation processes and the growth conditions allows to grow one-domain films of CdTe buffer layers on Si(310) substrates and to lower the density of V-shaped defects.
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 %.
The first results on a radiation stability investigation of mercury cadmium telluride (MCT) films, grown by molecular beam epitaxy (MBE) are represented. The samples were irradiated by high energy electron beams and gamma rays. Electrophysical and photoelectric parameters of MCT epilayers were measured. Volume material was measured too for the checking with MBE-grown one. MBE epifilms were irradiated on a pulsed electron accelerator with electron energy 1-2 MeV and current density less than 1 μA/cm2 for several fluences. Also MCT epitaxial heterostructures were irradiated by Co60 gamma rays. The same experiments were carried out for volume material. The analysis of dependence of Hall coefficient and conductivity from temperature and magnetic field (B) for p- and n- type samples was made. The irradiation of epilayers and volume MCT in the investigated range of irradiation fluences does not give both creation of electrical active damages in high concentrations and reconstruction of initial defects. Thus, MBE films of MCT have the high radiation stability to an electron and gamma irradiation. The obtained first results allow us to speak about high performance of explored MCT epilayers.
The surface microrelief of CdHgTe layers grown by molecular-beam epitaxy (MBE) method has been studied by means of atomic-force microscopy. A periodic surface microrelief in the form of an ordered system of extended waves with the characteristic period 0.1-0.2 μm has been detected on epilayers grown at increased temperatures. Angular dependencies of the conductivity at 77 K have been measured and the conductivity anisotropy has been detected with a minimum in the direction transverse to microrelief waves. A feature of the transmission system and the spectrum change after film annealing are observed. It is assumed that walls growing in the direction from the substrate to the surface are formed under microrelief waves slopes. Such structure can cause the observed feature of the transmission spectrum if the adjacent walls have different composition. In this work a calculation of spectral characteristics taking into account the influence of variable-gap composition and nonuniformity of the composition through the depth has been carried out.
View of basic and specific physical and chemical features of growth and defect formation in mercury cadmium telluride (MCT) heterostructures (HS's) on GaAs substrates by molecular beam epitaxy (MBE) was made. On the basis of this knowledge a new generation of ultra high vacuum set, ultra-fast ellipsometer of high accuracy and automatic system for control of technological processes was produced for reproducibility of MCT Hs's growth on substrates up to 4" in diameter. The development of industrially oriented technolgoy of MCT HS's growth by MBE on GaAs substrates 2" in diameter and without intentional doping is presented. The electrical characteristics of n-type and p-type of MCT HS's and uniformity of MCT composition over the surface area are excellent. The residual donor and acceptor centres are supposed as hypothetically tellurium atoms in metallic sublattice ("antisite" tellurium) and double-ionised mercury vacancies. The technology of fabricating focal plane arrays is developed. The high quality characteristics of infrared detectors conductance and diode mode are measured. Calculations of detector parameters predicted the improvement in serial resistance and detectivity of infrared diode detectors based on MCT heterostructures with graded composition throughout the thickness.
Molecular beam epitaxy of MCT makes it possible to dissolve the problems of producing MCT heteroepitaxial structures with uniformity parameters on the large size alternative substrates for IR PD of existent and new generation and the growing of MCT layers on Si -substrates. The information about characteristics of geteroepitaxial structures on GaAs-substrates, a new generation equipment for controlled growing of MCT layers by MBE and technological conditions allowed to grow the epitaxial buffer CdTe layers on Si-substrates are represented in this work.
New generation of ultra high vacuum set, ultra-fast ellipsometer of high accuracy and automatic system for control of technological processes was produced for reproducibility growth of mercury cadmium telluride (MCT) solid solution heterostructures (Hs's) by molecular beam epitaxy (MBE). This system allows to grow MCT HS's on substrate up to 4" in diameter and used for future development technology of growth on Si substrate. The development of industrially oriented growth MCT HS's MBE on GaAs 2" in diameter is presented. The electrical characteristics of n-type and p-type MCT HS's MBE and uniformity of MCT composition over the surface area is excellent and satisfied for fabricating multielements arrays of high quality infrared devices.
We present a novel technique for estimation of temperature dependence of optical constants from ellipsometric measurements. We suggest making measurements during short-time impulse heating using high-speed single-wavelength ellipsometer. In this way we can eliminate or reduce significantly the influence of thermal oxidation or surface decomposition on measured results. For temperature monitoring we have used a ZnTe/GaAs heterostructure with high sensitivity of ellipsometric parameters to the temperature. Our measurements show impulse-to-pulse reproducibility of the temperature well within ±5°C Using this technique we determined the temperature dependence of optical constants for mercury cadmium telluride (MCT) compounds from room temperature up to 250°C.
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. HS's 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.
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. HS's 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 77K and 200-250K temperature wavelength range of 3-5 micrometers and 8-12$ mum, up and over 20micrometers . 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.
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.
We used the heterostructures of HgCdTe/CdZnTe/GaAs grown by molecular beam epitaxy for fabrication of photoconductor devices. The composition of MCT films throughout the thickness was controlled in situ by ellipsometry during the growth process. There were wide band gap layers at the interface and at the surface of the MCT films for decreasing the surface recombination which strongly influences on devices characteristics. The use of n-type material for LWIR photoconductors (77 K, the cutoff wavelength is more than 13 mkm) with good performance was demonstrated. The detectivity in maximum of wavelength dependence varies in interval (1.5 divided by 5)(DOT)1010 cmHz1/2 W-1. P-type material was used for MWIR photoconductors that operated at room and near room temperatures with the close to the theoretical value detectivity.
The results of MBE growth of CdHgTe epilayers and fabrication of photosensitive in 8 - 10 mkm region small p-n junctions using planar technology are presented. During MBE epitaxy the growing dynamic, composition and surface roughness were controlled in situ using build in high energy electron diffractometer and ellipsometer. Small area photosensitive diodes (50 X 70 mkm) were fabricated using planar technology and annealing under anodic oxide film. The measurements of V-I, spectral response and noise characteristics showed that the photodiodes on MCT epilayers grown by MBE have an acceptable parameters for fabrication of the linear and 2D photodiode arrays.
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