xCdxTe Alloys Research Articles

Theoretical study of Urbach tail behavior in Hg1−xCdxTe in the 0.21 ≤ x ≤ 0.6 medium and far infrared optical ranges

We present a theoretical study of the optical absorption coefficient Urbach tail broadening parameter Γ behavior in the Hg1−xCdxTe alloy semiconductor in the 0.21 ≤ x ≤ 0.6 alloy composition interval. This x interval corresponds to the very attractive 0.10 ≤ EG ≤ 0.75 eV medium infrared (MIR) and far infrared (FIR) optical ranges. We compare two absorption coefficient nonparabolic models based on Kane 4-band formalism, one including the Burstein–Moss shift called the NPBM-model and the other one without and called the NP-model. By comparing the results of both models with existing experimental and theoretical data, we show the strong nonparabolic behavior of the absorption coefficient in Hg1−xCdxTe in agreement with previous studies. The best fitting is obtained with the NPBM-model, where Γ is used as an adjustable parameter varying with x, temperature (T), and photon energy (ħω) in the E ≤ EG sub-bandgap energy range. With decreasing x, Γ is found to increase first slightly with x in the 0.443 ≤ x ≤ 0.6 MIR range and then strongly and nonlinearly in the 0.21 ≤ x < 0.443 FIR range. These unusual Γ(x, ħω) dependences suggest a strong influence of nonparabolicity and band state mixing effects, which become strongly enhanced in the FIR range between strongly interacting and almost overlapping bands as x tends to 0.16 of the critical value, making Hg1−xCdxTe experience a semiconductor–semimetal transition.

Theoretical investigations of the structural, electronic and optical properties of Hg1−xCdxTe alloys

The structural, electronic and optical properties of mercury cadmium telluride (Hg1−xCdxTe; x = 0.0, 0.25, 0.5, 0.75) alloys are studied using density functional theory within full-potential linearized augmented plane wave method. We used the local density approximation (LDA), generalized gradient approximation (GGA), hybrid potentials, the modified Becke–Johnson (LDA/GGA)-mjb and Hubbard-corrected functionals (GGA/LDA + U), for the exchange-correlation potential (Eex). We found that LDA functional predicts better lattice constants than GGA functional, whereas, both functionals fail to predict the correct electronic structure. However, the hybrid functionals were more successful. For the case of HgTe binary alloy, the GGA + U functional predicted a semi-metallic behaviour with an inverted band gap of −0.539 eV, which is closest to the experimental value (−0.30 eV). Ternary alloys, however, are found to be semiconductors with direct band gaps. For the x = 0.25 and 0.50, the best band gaps are found to be 0.39 and 0.81 eV using LDA-mbj functional, whereas, the GGA-mbj functional predicted the best band gap of 1.09 eV for Hg0.25Cd0.75Te alloy, which is in a very good agreement with the experimental value (1.061 eV). The optical properties of the alloys are obtained by calculating the dielectric function ɛ(ω). The peaks of the optical dielectric functions are consistent with the electronic gap energies of the alloys.

New concepts in infrared photodetector designs

In 1959, Lawson and co-workers published the paper which triggered development of variable band gap Hg1−xCdxTe (HgCdTe) alloys providing an unprecedented degree of freedom in infrared detector design. HgCdTe ternary alloy has been used for realization of detectors operating under various modalities including: photoconductor, photodiode, and metal-insulator-semiconductor detector designs. Over the last five decades, this material system has successfully overcome the challenges from other material systems. It is important to notice that none of these competitors can compete in terms of fundamental properties. The competition may represent more mature technology but not higher performance or, with the exception of thermal detectors, higher operating temperatures (HOTs) for ultimate performance. In the last two decades, several new concepts for improvement of the performance of photodetectors have been proposed. These new concepts are particularly addressing the drive towards the so called HOT detectors aiming to increase detector operating temperatures. In this paper, new strategies in photodetector designs are reviewed, including barrier detectors, unipolar barrier photodiodes, multistage detectors and trapping detectors. Some of these new solutions have emerged as a real competitor to HgCdTe photodetectors.

Barrier infrared detectors

AbstractIn 1959, Lawson and co-workers publication triggered development of variable band gap Hg1−xCdxTe (HgCdTe) alloys providing an unprecedented degree of freedom in infrared detector design. Over the five decades, this material system has successfully fought off major challenges from different material systems, but despite that it has more competitors today than ever before. It is interesting however, that none of these competitors can compete in terms of fundamental properties. They may promise to be more manufacturable, but never to provide higher performance or, with the exception of thermal detectors, to operate at higher temperatures.In the last two decades a several new concepts of photodetectors to improve their performance have been proposed including trapping detectors, barrier detectors, unipolar barrier photodiodes, and multistage detectors. This paper describes the present status of infrared barrier detectors. It is especially addressed to the group of III-V compounds including type-II superlattice materials, although HgCdTe barrier detectors are also included. It seems to be clear that certain of these solutions have merged as a real competitions of HgCdTe photodetectors.

On the nature of a lattice vibrational mode at a frequency of 135 cm−1 in Hg1 − x Cd x Te alloys

The vibrational spectrum of a cadmium impurity atom in the HgTe crystal has been calculated using the microscopic theory of lattice dynamics in the approximation of a low impurity concentration. Within this theory, the behavior of the local and quasi-local modes induced upon substitution of the lighter Cd atom for the Hg atom in the region of the zero or very low one-phonon density of states in the HgTe crystal has been considered. It has been found that, apart from the local mode at a frequency of 155 cm−1, the calculated vibrational spectra exhibit a weak (but clearly pronounced) feature at a frequency of 134 cm−1, which coincides with the experimentally observed vibrational mode (the “minicluster” mode) at a frequency of 135 cm−1 in the Hg1 − xCdxTe (x = 0.2–0.3) alloys at 80 K.

Model of the two well potential for Hg-atoms in the Hg1 - xCdx Te alloy lattice

Model of the two well potential for Hg-atoms in the Hg1 - xCdx Te alloy lattice

Accurate evaluation of nonlinear absorption coefficients in InAs, InSb, and HgCdTe alloys

We present a full band structure calculation of temperature- and wavelength-dependent two-photon absorption (TPA) coefficients and free carrier absorption (FCA) cross sections in InAs, InSb, and Hg1−xCdxTe alloys. The wavelength dependence of the TPA coefficients agrees well with a widely used analytical expression. However, the magnitudes of the TPA coefficients obtained here are smaller by a factor of 1.2–2.5 than the analytical values. In addition, the TPA coefficient is found to depend sensitively on the photoexcited carrier density in small gap materials. The FCA is found to arise predominantly from hole absorption. The FCA cross section is found to be independent of the carrier density, but is strongly dependent on the temperature. The calculated TPA, FCA coefficients, and lifetimes are fitted to closed-form expressions and are used in solving the rate equations to obtain the transmitted pump and probe intensities as functions of incident intensity and sample thickness. The calculated pump transmission and time-dependent probe transmission in InAs agree very well with the measured values.

Photoexcited-carrier-induced refractive index change in small bandgap semiconductors

Using accurate band structures of InAs, InSb, and two Hg1−xCdxTe alloys, we calculate the change in refractive index caused by the photoexcited electrons and holes. The effects of both free-carrier absorption (FCA) and one-photon absorption are considered. We find that the change in refractive index varies nonlinearly with the density of photoexcited carriers and that the generally neglected FCA contribution is significant in InAs, owing to its weak spin-orbit coupling.

Understanding ion-milling damage in Hg1−xCdxTe epilayers

Transmission electron microscopy (TEM) is widely used for the characterization of the microstructure of Hg1−xCdxTe epilayers. Traditional TEM sample preparation methods, which usually involve argon ion milling, can easily cause damage to the material, and the size and density of the induced defects depend on the milling conditions. In this work, the structural damage caused by argon ion milling of Hg1−xCdxTe epilayers has been investigated. Multilayer samples with different Hg concentrations, as grown by molecular beam epitaxy, and p-n heterojunctions, as grown by liquid-phase epitaxy, have been examined. It is shown that, in addition to the milling conditions, the extent of the ion-induced damage depends sensitively on the Hg concentration of the Hg1−xCdxTe alloy as well as the epilayer growth conditions (i.e., Hg rich or Te rich). A possible mechanism that explains these results is briefly discussed.

The nature of the compositional dependence of p–n junction depth in ion-milled p-HgCdTe

The dependence of conductivity-type conversion depth in vacancy-doped Hg1−xCdxTe (MCT) alloys subjected to ion milling on alloy composition and treatment temperature is studied both experimentally and theoretically. It is shown that both in compositionally homogeneous crystals and in samples with a wide bandgap protective layer the dependence is defined by internal electric fields, which affect the diffusion of mercury interstitial atoms that are generated during the treatment. Results of the calculation of the effect of the potentials of a p–n junction formed by ion milling and of a varyband structure field (in samples with the protective layer) on the conversion depth fit both the original experimental data and those taken from the literature well. The data obtained confirms the validity of the diffusion model of the formation of the excessive mercury source in MCT subjected to ion milling, which was proposed by the authors previously. The results presented in the paper allow one to predict and control the conversion depth in MCT subjected to ion milling for p–n junction fabrication, which makes them useful in MCT infrared detector technology.

Modification of Hg1−xCdxTe and related materials by ion-beam treatment

Modification of ternary Hg1−xCdxTe alloys with 0.21≤x≤0.55 and quaternary Hg1−y−zZnyCdzTe alloys by treatment with low-energy (E<800eV) Ar ions was studied. It is shown that for conductivity type conversion in Hg1−xCdxTe with 0.28≤x≤0.39, it is essential to use a neutralised ion beam. The observed dependence of the conversion depth on x agrees with the diffusion conversion model. The effect of self-compensation of intrinsic acceptor defects by intrinsic donors during the annealing prior to ion treatment is confirmed. This effect provides the possibility of controlling the carrier concentration in the n-region of p–n junctions fabricated by ion-beam treatment in vacancy-doped Hg1−xCdxTe.

Fibre coupled Hg1−xCdxTe detectors

Fibre-coupled mercury–cadmium telluride Hg1−xCdxTe (MCT) high performance infrared radiation (IR) detectors have been developed and fabricated. This new product is originated from 25 years experience of MCT detectors and IR fibre optics technologies. The product includes single- and multi-element detectors designed for registration of optical signals in wide-band and narrow-band mode in spectral range from 2 to 18μm. Detectors are manufactured in an integrated or modular design and include MCT optical sensors, optical couplers and fibre pig-tails or cables. Registered infrared radiation is delivered to the sensitive area of the detector through either chalcogenide IR-glass (CIR-) fibre (2–6μm) or polycrystalline infrared (PIR-) fibre (4–18μm). The unique feature of Hg1−xCdxTe alloys enables tuning spectral responsivity of the detector element for the ordered spectral range.

Electron microscopy of surface-crater defects on HgCdTe/CdZnTe(211)B epilayers grown by molecular-beam epitaxy

Surface-void defects observed in Hg1−xCdxTe (x ∼ 0.2–0.4) alloys grown by molecular-beam epitaxy (MBE) have been investigated using scanning and high-resolution transmission-electron microscopy (HRTEM) as well as atomic force microscopy (AFM). These surface craters, which have been attributed to Hg-deficient growth conditions, were found to originate primarily within the HgCdTe epilayer, rather than at the CdZnTe substrate, and they were associated with the local development of polycrystalline morphology. High-resolution observations established the occurrence of finely spaced HgCdTe/Te intergrowths with semicoherent and incoherent grain boundaries, as well as small HgCdTe inclusions embedded within the Te grains. This study is the first time that high-resolution electron microscopy has been used to investigate this type of defect.

Analysis of the possibility to control complex semiconductors properties by shock wave treatment

This paper is dedicated to the experimental investigation of laser-induced shock waves impact on electrical, photoelectric and mechanical parameters of narrow-gap Hg1–xCdxTe alloys. A mechanism of defect structure rebuilding under the laser shock waves effect is developed. The proposed mechanism manifests itself in one of two dominant ways depending upon the processing mode. The two modes considered involve inducing shock waves by either a single laser pulse or a multi-spike laser pulse.

Kinetics of activation of group V impurities in Hg1−xCdxTe alloys

A kinetic model is developed for the activation of group V impurities in Hg1xCdxTe alloys as acceptors. The model assumes that mercury interstitials are introduced at the surface and diffuse into the alloy. There, they displace group V impurities residing on the metal sublattice and place them on the tellurium sublattice, generating tellurium interstitials. These tellurium interstitials then diffuse back to the top surface, or to climbing dislocations. The rate-controlling process is the out-diffusion of the tellurium interstitials. A key finding is that the conversion rate is inversely proportional to the impurity concentration. The model is found to be in good agreement with published data for the activation of arsenic.

Electronic and Positronic Distribution in the Ternary Alloy Hg1-xCdxTe

The electron and positron charge densities are calculated as a function of position in the unit cell for the Hg1–xCdxTe alloy system using the empirical pseudopotential method combined with the virtual-crystal approximation. The results show that the positron has a strong affinity to one sort of atoms in binary semiconductors. The results of the positron charge density should provide valuable insight into the effect of annihilation. The electron charge density and the ionicity character with respect to the variation of the mole fraction are discussed.

Optimization of the structural properties of Hg1−x CdxTe (x = 0.18−0.30) alloys: Growth and modeling

The conditions for the metalorganic molecular beam epitaxial growth of Hg1−xCdxTe (x = 0.18−0.32) alloys at very low growth temperatures (T ≤150°C) have been optimized by correlating the surface properties and crystalline perfection with the incident Hg flux. A window for growth has been defined for x = 0.18, 0.23, and 0.32. A thermodynamic model has been developed to account for void formation. A neural net model has been used for the first time to model the dependence of void density on the Hg flux and the x-ray rocking curve widths on growth parameters. The combination of these two complementary modeling techniques allows for a flexible process optimization to be carried out with a minimum effort spent in calibration runs.

In situ sensors for monitoring and control in molecular beam epitaxial growth of Hg1−xCdxTe

The current status of the implementation and refinement of two wafer state sensors forin situ monitoring and control during molecular beam epitaxial (MBE) growth of Hg1−xCdxTe will be reported. First a rapid scan spectral ellipsometer has been developed and employed for precisely measuring compositions of Hg1−xCdxTe alloys during growth. MBE films in the composition range x = 0.20 to 0.30 have been grown andin situ spectra taken at the growth temperature (180°C) and at room temperature. The MBE films were treated as single layers without the need to invoke any surface film (due to surface roughness, oxide, or of any different composition) as required for exsitu data. The least squares fit over the whole spectral range was used as a measure of the precision. The film composition was also determinedex situ by wavelength dispersive analysis of x-rays and by Fourier transform infrared (FTIR) spectrometry after verifying that there was no lateral variation. A precision of better than ±0.0015 has so far been demonstrated usingin situ spectral ellipsometry for Cd composition or CdTe mole fraction, x, measurements. This compares with ±0.003 for single wavelength ellipsometry. The composition of Hg1−xCdxTe films were also monitored during growth. A spectral pyrometer based on a FTIR spectrometer has also been developed for substrate temperature measurements during growth. The spectral pyrometer measures both the emission and reflectance to give the emissivity of a growing sample over a range of wavelengths spanning the peak of the grey body emission. From the reflectivity measurements, the thickness (in excess of 1 µm) of the growing film is also determined from the interference fringes. The spectral ellipsometer is only capable of measuring thicknesses up to C.a 5000°A (i.e. optically thin). Excellent agreement is obtained between thein situ (at growth temperature) andex situ (at room temperature) thickness measurements. The small discrepancy can be explained by the refractive index of Hg1−xCdxTe being 5% higher at the growth temperature than at room temperature. The combination ofin situ sensors now provides a means of continuously monitoring the composition and thickness of the growing Hg1−xCdxTe film.

Molecular beam epitaxial growth and properties of short-wave infrared Hg0.3Cd0.7Te films

Hg1−xCdxTe films with cut-off wavelengths ranging from 1.25 to 1.65 μm (at 300 K) in the short wave infrared band were prepared by molecular beam epitaxy. The structural perfection of the Hg1−xCdxTe films was strongly dependent on the degree of lattice matching provided by the substrate. First reports of in situ p-type doping of Hg1−xCdxTe (x≊0.7) alloys by molecular beam epitaxy is presented here. Electron and hole concentration in excess of 1×1017 cm−3 were measured in In and As doped films, respectively. Analysis of variable temperature Hall effect data assuming singly ionized impurities provided activation energies of 28.5 meV for As acceptors, and 2.95 meV for In donors.

An analytical approximation for the free electron density of the Hg1−xCdxTe alloy system for 0&amp;lt;x&amp;lt;1

An analytical approximation for the electron density in the conduction band of the entire Hg1−xCdxTe (MCT) alloy system (0&amp;lt;x&amp;lt;1) as a function of the composition, temperature and Fermi energy location, is proposed. A hyperbolic expression for the conduction band is shown to yield an error which is practically not larger than Kane’s model in the entire composition range of MCT. The analytical approximation is compared with a numeric calculation of the Fermi-Dirac integral using this hyperbolic band approximation, and shows a deviation of a few percents for temperatures in the range 2&amp;lt;T&amp;lt;300[K], compositions in the entire range 0&amp;lt;x&amp;lt;1 and electron densities up to n=1020[cm−3]. This analytical approximation can be extremely useful for numerical band diagram and transport simulations of graded and abrupt MCT heterojunctions and devices.