II-VI Alloys Research Articles

Electronic band structure of semiconductor alloys: From ab initio to k[formula omitted]p via computational alchemy, on example of [formula omitted] alloy

First principles calculations are increasingly often used in device modelling. However, due to their complexity and high computational costs, simplified but reliable models are of great value. In this work, we present an original method of finding the 30-band k·p parameters on an example of Ge1-xSnx alloy. It allows to reproduce the electronic band structure in the full BZ and in the full composition range. The method uses the state-of-the-art supercell based ab initio calculations as the reference. A direct fitting of the k·p parameters from ab initio supercell band structures is difficult since the assignment of the k·p bands to correct eigenvalues of the reference band structures may be ambiguous. For this reason, the ab initio computational alchemy is used as an intermediate step. Owing to its properties, the supercell unfolded eigenvalues are approximated with continuous curves, which enables a straigthforward way to fit the 30-band k·p parameters. As an example of application, the optical gain in bulk Ge0.91Sn0.09 has been calculated. The advantage of using the 30-band over the often used 8-band k·p approach is clearly seen. The presented method can be applied to any mixed, isovalent and isostructural system, eg. III-V, or II-VI alloys.

Transforming nonisostructural wurtzite (ZnO) and rocksalt (MgO) II-VI compounds into stable alloy: The case of MgZnO

Abstract Structural disparity between fourfold-coordinated wurtzite ZnO and sixfold-coordinated rocksalt MgO II-VI semiconductors compounds disfavor their binding into a stable II-VI alloy. Here we search for a stable Zn–Mg–O system under ambient conditions by first-principles evolutionary searches. A new thermodynamically stable cubic phase Mg3ZnO4, space group Pm3m has been uncovered. Computed elastic constants, and phonon spectrum reveal the mechanic and dynamic stabilities of this phase. Using state-of-the-art quasiparticle-LDA-1/2 approach, we found a marked band-gap bowing parameter of ~4.2eV. The origin of the nonlinearity of the band gap emerges predominantly from electronic-charge-exchange effect.

Optical Signatures of Impurity-Impurity Interactions in Copper Containing II-VI Alloy Semiconductors.

We study the optical properties of copper containing II-VI alloy quantum dots (CuxZnyCd1-x-ySe). Copper mole fractions within the host are varied from 0.001 to 0.35. No impurity phases are observed over this composition range, and the formation of secondary phases of copper selenide are observed only at xCu > 0.45. The optical absorption and emission spectra of these materials are observed to be a strong function of xCu, and provide information regarding composition induced impurity-impurity interactions. In particular, the integrated cross section of optical absorption per copper atom changes sharply (from 1 × 10 -2 nm3 to 4 × 10 -2 nm3) at xCu = 0.12, suggesting a composition induced change in local electronic structure. These materials may serve as model systems to understand the electronic structure of I-III-VI2 semiconductor compounds.

II-VI Material Integration With Silicon for Detector and PV Applications

ABSTRACTHeteroepitaxial growth of high-quality II-VI-alloy materials on Si substrates is a well-established commercial growth process for infrared (IR) detector devices. However, it has only recently been recognized that these same processes may have important applications for production of high-efficiency photovoltaic devices. This submission reviews the process developments that have enabled effective heteroepitaxy of II-VI alloy materials on lattice-mismatched Si for IR detectors as a foundation to describe recent efforts to apply these insights to the fabrication of multijunction Si/CdZnTe devices with ultimate conversion efficiencies >40%. Reviewed photovoltaic studies include multijunction Si/CdZnTe devices with conversion efficiency of ∼17%, analysis of structural and optoelectrical quality of undoped CdTe epilayer films on Si, and the effect that a Te-rich growth environment has on the structural and optoelectronic quality of both undoped and As-doped heteroepitaxial CdTe.

Structural Origin of the Band Gap Anomaly of Quaternary Alloy Cd(x)Zn(1-x)S(y)Se(1-y) Nanowires, Nanobelts, and Nanosheets in the Visible Spectrum.

Single-crystalline alloy II-VI semiconductor nanostructures have been used as functional materials to propel photonic and optoelectronic device performance in a broad range of the visible spectrum. Their functionality depends on the stable modulation of the direct band gap (Eg), which can be finely tuned by controlling the properties of alloy composition, crystallinity, and morphology. We report on the structural correlation of the optical band gap anomaly of quaternary alloy CdxZn1-xSySe1-y single-crystalline nanostructures that exhibit different morphologies, such as nanowires (NWs), nanobelts (NBs), and nanosheets (NSs), and cover a wide range of the visible spectrum (Eg = 1.96-2.88 eV). Using pulsed laser deposition, the nanostructures evolve from NWs via NBs to NSs with decreasing growth temperature. The effects of the growth temperature are also reflected in the systematic variation of the composition. The alloy nanostructures firmly maintain single crystallinity of the hexagonal wurtzite and the nanoscale morphology, with no distortion of lattice parameters, satisfying the virtual crystal model. For the optical properties, however, we observed distinct structure-dependent band gap anomalies: the disappearance of bowing for NWs and maximum and slightly reduced bowing for NBs and NSs, respectively. We tried to uncover the underlying mechanism that bridges the structural properties and the optical anomaly using an empirical pseudopotential model calculation of electronic band structures. From the calculations, we found that the optical bowings in NBs and NSs were due to residual strain, by which they are also distinguishable from each other: large for NBs and small for NSs. To explain the origin of the residual strain, we suggest a semiempirical model that considers intrinsic atomic disorder, resulting from the bond length mismatch, combined with the strain relaxation factor as a function of the width-to-thickness ratio of the NBs or NSs. The model agreed well with the observed optical bowing of the alloy nanostructures in which a mechanism for the maximum bowing for NBs is explained. The present systematic study on the structural-optical properties correlation opens a new perspective to understand the morphology- and composition-dependent unique optical properties of II-VI alloy nanostructures as well as a comprehensive strategy to design a facile band gap modulation method of preparing photoconverting and photodetecting materials.

The Study and Simulation of the Electron Transport Phenomena on Bulk Hg1-xCdx Te Based on an Ensemble Monte Carlo Method

In this paper a comparative study of the electronic transport phenomenon in the semiconductor II-VI alloy HgCdTe is presented. In this study we have adopted Monte Carlo simulation. The model of the method used in this work takes into account the valleys, L and X of the conduction band, in which considered isotropic but not parabolic. This model provides a detailed description of the electronic dynamic and the electrons behavior at high electrical fields and high temperatures in these materials in each considered valleys. Furthermore, he permits two main functions: the calculation of the scatterings rates taken into consideration and the determination of the instantaneous quantities such as speed, energy etc. The obtained results, compared on many experimental reference frames, are satisfactory.

Infrared reflectance and transmission spectra in II-VI alloys and superlattices

Room temperature measurements of the far-infrared (FIR) reflectance spectra are reported for the polar optical phonons in a series of bulk Cd${}_{x}$Zn${}_{1\ensuremath{-}x}$Te (0 \ensuremath{\le} $x$ \ensuremath{\le} 1) and CdSe${}_{x}$Te${}_{1\ensuremath{-}x}$ (0 $x$ \ensuremath{\le} 0.35) crystals grown by Bridgman technique. The composition-dependent spectra exhibited many fundamental aspects of lattice vibrations, while the results of long-wavelength optical modes for the end member binary compounds displayed phonon values in good agreement with the existing inelastic neutron scattering and/or Raman spectroscopy data. Using a standard methodology of multilayer optics with effective-medium dielectric tensors, we simulated the FIR transmission and reflectivity spectra at oblique incidence (Berreman's effect) in many II-VI free-standing thin films, epilayers and superlattices. Berreman's approach provided us with a strong basis for analyzing the infrared experimental data and offered an effective means of estimating the zone-center transverse and longitudinal optical (${\ensuremath{\omega}}_{\mathrm{TO}}$, ${\ensuremath{\omega}}_{\mathrm{LO}}$) phonon frequencies in polar-semiconductor thin films and heterostructure materials of increasing technological importance.

Band-gap bowing coefficients in large size-mismatched II-VI alloys: first-principles calculations

Band-gap bowing coefficients in large size-mismatched II-VI alloys ${M}^{\mathit{II}}{X}_{1\ensuremath{-}x}^{\mathit{VI}}{\mathrm{O}}_{x}$ with ${M}^{\mathit{II}}=\mathrm{Zn}$ and Cd, and ${X}^{\mathit{VI}}=\mathrm{S}$, Se, and Te in the zinc-blende structure are calculated using first-principles methods. We show that in these systems, the bowing coefficients are large and composition dependent. The bowing coefficients increase as the size and chemical mismatch between the constituents increase. The bowing coefficients for the Zn alloys are larger than the corresponding Cd alloys, but smaller than the corresponding III-V alloys. We show that these results can be explained by the size and atomic eigenvalue differences between the constituents and the resulting band offsets and isovalent defect levels in these systems. Our results are compared with recent experimental data.

Effect of band inversion on the phonon spectra of the Hg1-x ZnxTe alloys

The narrow-gap II-VI and IV-VI alloys are convenient objects for studying the electron-phonon interaction. However, the concentration of free carriers in the IV-VI alloys is rather high (∼1018 cm−3), which complicates studying this effect, by the optical reflection method. The concentration of free carriers in the narrow-gap Hg1-xZnxTe alloys is considerably lower (∼1016 cm−3). Therefore, the plasma component exerts less effect on the lattice reflection spectrum. The reflection spectra of Hg1-xZnxTe crystals with x = 0.1–1 were studied in the far-IR region in the range of 30–700 cm−1 at 40–300 K. Using the dispersion analysis and the Kramers-Kronig method, the frequencies of the TO phonons of the HgTe-like and ZnTe-like modes were determined depending on the composition. It is shown that the reconstructed phonon spectrum involves two modes. The temperature dependences of the frequencies of the TO phonons and the damping parameter were measured for the narrow-gap alloy with x = 0.1 in the range of 80–200 K. A decrease in the frequency of the TO phonon of the soft mode in the vicinity of the inversion point of the bands at T = 110 K was for the first time found by optical methods. The damping parameter slightly increases in the vicinity of this temperature. The result obtained qualitatively agreed with the theoretical model of Kawamura et al., which makes allowance for the effect of the electron-phonon interaction on the frequency of the soft mode in the IV-VI compounds.

Ordering tendencies in octahedral MgO-ZnO alloys

Isostructural II-VI alloys whose components are either rocksalt stable (e.g., CaO-MgO) or zincblende stable (e.g., ZnS-ZnSe) are known to be thermodynamically unstable at low temperatures, showing a miscibility gap and nobulk ordering. In contrast, we show that heterostructural MgO-ZnO is stable, under certain conditions, in the sixfold-coordinated structure for Zn concentrations below 67%, giving rise to spontaneously ordered alloys. Using first-principles calculations, we explain the origin of this stability, the structures of their low-temperature ordered phases, short-range-order patterns, and their optical band-gap properties.

Beryllium substitution-mediated covalency engineering of II-VI alloys for lattice elastic rigidity reinforcement

Beryllium substitution-mediated covalency engineering of II-VI alloys for lattice elastic rigidity reinforcement

Raman and Infrared Spectroscopy of Optical Phonons in IIVI Alloys, Epilayers and Superlattices

The multimode behavior of the zone center optical phonons in bulk crystals and molecular beam epitaxy (MBE) grown epilayers of the tetrahedrally coordinated II–VI semiconductor alloys was investigated using Raman and infrared spectroscopy. Depending on the inter-atomic force constants and the relative nuclear masses of the constituents, the zone center optical modes of the ternary A1-xBxC exhibit either a two-mode behavior with distinct AC-like and BC-like TO and LO modes across the entire composition range, or an intermediate mode behavior when the masses of A and B are more closely matched. When B is much lighter than A, the BC-like TO and LO modes emerge from the localized vibrational mode of B in AC when x≈0 and the AC-like TO and LO modes converge into the gap mode of A in BC as x→1. A quaternary A1-x-yBxCyD may similarly exhibit a distinct three-mode or intermediate-mode behavior depending on the relative masses of the constituents. The two-mode behavior in bulk Zn1-xMgxTe and the three-mode behavior in bulk Cd1-x-yZnxMgyTe were studied with Raman scattering. Using infrared transmission at oblique incidence, i.e. the Berreman technique, the zone center TO and LO modes of MBE-grown zinc blende MnTe are observed as transmission minima. The observation of confined optical phonons in ultra-thin CdTe/ZnTe superlattices by exploiting the Berreman technique and Raman scattering permits their bulk dispersions to be delineated.

Raman and Infrared Spectroscopy of Optical Phonons in II-VI Alloys, Epilayers and Superlattices

The multimode behavior of the zone center optical phonons in bulk crystals and molecular beam epitaxy (MBE) grown epilayers of the tetrahedrally coordinated II–VI semiconductor alloys was investigated using Raman and infrared spectroscopy. Depending on the inter-atomic force constants and the relative nuclear masses of the constituents, the zone center optical modes of the ternary A1-xBxC exhibit either a two-mode behavior with distinct AC-like and BC-like TO and LO modes across the entire composition range, or an intermediate mode behavior when the masses of A and B are more closely matched. When B is much lighter than A, the BC-like TO and LO modes emerge from the localized vibrational mode of B in AC when x≈0 and the AC-like TO and LO modes converge into the gap mode of A in BC as x1. A quaternary A1-x-yBxCyD may similarly exhibit a distinct three-mode or intermediate-mode behavior depending on the relative masses of the constituents. The two-mode behavior in bulk Zn1-xMgxTe and the three-mode behavior in bulk Cd1-x-yZnxMgyTe were studied with Raman scattering. Using infrared transmission at oblique incidence, i.e. the Berreman technique, the zone center TO and LO modes of MBE-grown zinc blende MnTe are observed as transmission minima. The observation of confined optical phonons in ultra-thin CdTe/ZnTe superlattices by exploiting the Berreman technique and Raman scattering permits their bulk dispersions to be delineated.

MBE Growth of lattice-matched ZnCdMgSe quaternaries and ZnCdMgSe/ZnCdSe quantum wells on InP substrates

We report the growth and characterization of a new wide bandgap II-VI alloy, ZnxCdyMg1-x-ySe, grown lattice-matched to InP. High quality quaternary layers with bandgaps ranging from 2.4 to 3.1 eV were grown by molecular beam epitaxy. The bandgaps and lattice constants were measured using photoluminescence and single crystal Θ-2Θ scans. Quantum well structures with quaternary barriers and ZnCdSe wells were also grown, entirely lattice matched to InP. Their photoluminescence properties suggest that these materials are suitable for the design of visible semiconductor lasers spanning the blue, green, and yellow regions of the visible range. The absence of strain in these heterostructures is expected to improve the reliability of the materials in device applications.

Composition and temperature-induced effects on the phonon spectra of narrow-band-gap Hg1-xCdxTe.

Compositional and temperature-dependent Raman and infrared spectroscopies on the narrow-band-gap II-VI alloy, ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Cd}}_{\mathit{x}}$Te (0\ensuremath{\le}x\ensuremath{\le}0.29), are reported in this paper. Raman measurements over this composition range confirm the two-mode behavior of the optical phonons in the alloy, with the frequency positions exhibiting a monotonic change with the alloy composition x. A resonant enhancement of the HgTe-like forbidden longitudinal-optic mode is observed near the ${\mathit{E}}_{1}$ band gap of the alloy. Alloy induced perturbations on the electronic states lead to a smearing and a weakening of the resonance with increasing alloy composition. The optical phonons in this small band-gap alloy exhibit anomalous frequency shifts as a function of temperature vis-\`a-vis normal wide-band-gap semiconductors. This anomaly is attributed to the strong electron-phonon interaction in narrow-band-gap systems, which overshadows the dominant anharmonic (phonon-phonon) interaction in usual semiconductors. A detailed compositional dependent second-order Raman spectra is also reported.

Band offsets and optical bowings of chalcopyrites and Zn-based II-VI alloys

Using first-principles band-structure theory we have systematically calculated the (i) alloy bowing coefficients, (ii) alloy mixing enthalpies, and (iii) interfacial valence- and conduction-band offsets for three mixed-anion (CuInX2, X=S, Se, Te) and three mixed-cation (CuMSe2, M=Al, Ga, In) chalcopyrite systems. The random chalcopyrite alloys are represented by special quasirandom structures (SQS). The calculated bowing coefficients are in good agreement with the most recent experimental data for stoichiometric alloys. Results for the mixing enthalpies and the band offsets are provided as predictions to be tested experimentally. Comparing our calculated bowing and band offsets for the mixed-anion chalcopyrite alloys with those of the corresponding Zn chalcogenide alloys (ZnX, X=S, Se, Te), we find that the larger p−d coupling in chalcopyrite alloys reduces their band offsets and optical bowing. Bowing parameters for ordered, Zn-based II-VI alloys in the CuAu, CuPt, and chalcopyrite structures are presented: we find that ordered Zn2SeTe has bowing coefficients of 1.44 and 3.15 eV in the CuAu and CuPt structures, while the random ZnSexTe1−x alloy has a bowing of 1.14 eV. The band alignment between CuInSe2 and CuInSe2-derived ordered vacancy compounds are also presented.

Length mismatch in random semiconductor alloys. II. Structural characterization of pseudobinaries.

We have made an extensive study of the pseudobinary semiconductor compounds (${\mathit{A}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{B}}_{\mathit{x}}$)C, which includes both the III-V and the II-VI alloys. We use a Kirkwood model with all parameters derived from the elastic constants of the pure materials. It is shown that the mean lengths are linear in the composition x if the force constants for the two pure materials AC and BC are the same. We have derived analytic results for the mean lengths of AC and BC bonds and for their widths in the preceding paper. We construct an effective-medium theory for cases in which there is disorder in the force constants. This effective-medium theory is found to be in good agreement with the results of computer simulations of the same models. Mean values for the next-nearest-neighbor lengths are also given, and found to agree with extended x-ray-absorption fine-structure results.

Chemical ordering in Zn1−xFexSe alloys

We have observed chemical ordering in Zn1−xFexSe (x≊0.5) epilayers as well as in nominal (ZnSe/FeSe) superlattices along the [001] growth direction and the [110] direction using transmission electron microscopy. The ordered structure consists of alternating ZnSe and FeSe layers along the [001] and [110] directions. In nominal (ZnSe/FeSe) superlattices grown on (001) GaAs substrates, strain-induced interdiffusion between the layers takes place followed by ordering of the resultant Zn1−xFexSe alloys. Computer simulated images for a Zn0.5Fe0.5Se compound were obtained and compared with experimental images. To our knowledge, this is the first observation of ordering in a II-VI alloy.

Electrosynthesized thin films of group II–VI compound semiconductors, alloys and superstructures

II‐IV compound semiconductors such as CdTe and Hg1–x CdxTe are important in a wide range of optoelectronics applications ranging from solar cells and infrared detectors to „smart”︁ goggles. An alternative to the cost‐intensive vacuum‐based production techniques is electrodeposition which allows large‐area deposition, close control of the deposition conditions, and obviates the need for toxic, organometallic precursors. The advantages and disadvantages of the method are discussed.

New precursors for wide-gap II-VI semiconductors

Recent advances in the growth of wide-gap II-VI semiconductors by MOVPE have been greatly facilitated by the development of new precursor materials. For instance, the use of adducts such as Me2ZnNEt3 and Me2Zn (triazine)2 has served to eliminate a formerly severe pre-reaction during the growth of ZnSe and ZnS and has allowed the growth of high-quality layers at low to moderate temperatures. The use of adducts has also been extended to the growth of cadmium-based II-VI alloys. The bis-tetrahydrothiophene adduct of Me2Cd has been used as a precursor to CdS and CdSe. However, in this case, pre-reaction with the group VI hydride is only slightly inhibited. From a consideration of possible reaction mechanisms the author has developed a novel technique for the in situ control of gas-phase premature reactions.