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The energy gap of zinc-blende epitaxial layers grown by molecular beam epitaxy has been determined over a wide range of composition using optical transmission and reflection and an empirical formula for the energy gap at room temperature is also given. In addition in situ spectroscopic ellipsometry was used in order to determine the temperature dependence of the energy gap between room temperature and the growth temperature of alloys with compositions x, y as used in cladding layers and waveguides for blue - green light-emitting devices.

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... eV with the increase in sulfur composition and is summarized in table 1. The dependence of the energy gap of a ternary A II B VI C VI alloy on the composition x at a fixed temperature can be described by the empirical equation [32]: The variation in band gap with composition is found to be nonlinear, showing a similar bowing; the degree of nonlinearity can be fitted to the following quadratic equation: ...

Keywords: thin films of ZnS x Se 1−x compounds, thermal evaporation of ZnS and ZnSe powders mixture, EXAFS spectroscopy, x-ray diffraction, Raman spectroscopy, band gap Abstract Interest to ZnS x Se 1−x alloys is due to their band-gap tunability varying S and Se content. Films of ZnS x Se 1−x were grown evaporating ZnS and ZnSe powder mixtures onto SiO 2 , NaCl, Si and ITO sub-strates using an original low-cost method. X-ray diffraction patterns and Raman spectroscopy, show that the lattice structure of these films is cubic ZnSe-like, as S atoms replace Se and film compositions have their initial S/Se ratio. Optical absorption spectra show that band gap values increase from 2.25 to 3 eV as x increases, in agreement with the literature. Because S atomic radii are smaller than Se, EXAFS spectra confirm that bond distances and Se coordination numbers decrease as the Se content decrea-ses. The strong deviation from linearity of ZnSe coordination numbers in the ZnS x Se 1−x indicate that within this ordered crystal structure strong site occupation preferences occur in the distribution of Se and S ions. The behavior is quantitatively confirmed by the strong deviation from the random Ber-noulli distribution of the three sight occupation preference coefficients of the strained tetrahedron model. Actually, the ternary ZnS x Se 1−x system is a bi-binary (ZnS+ZnSe) alloy with evanescent forma-tion of ternary configurations throughout the x-range.

... [37]. b Ref. [38]. c Ref. [39]. ...

In this paper we present optical and exciton properties of ZnxCd1-xS using a pseudopotential approach within an improved virtual crystal approximation which includes the alloy disorder effect. Features such as, refractive index, dielectric constants, exciton reduced mass, exciton binding energy and exciton Bohr radius have been investigated and their dependence on composition has been examined and discussed. All features of interest are found to vary monotonically and non-linearly with the Zn content. The results obtained in the present study may provide useful information for use of this alloy system in various device applications from visible to ultraviolet light.

A computational study of elastic and piezoelectric properties of crystalline ZnxCd1−xS obtained from a pseudopotential approach is presented. We report composition-dependent values of elastic constants, bulk modulus, shear modulus, Young’s modulus, Poisson ratio, piezoelectric stress constant, piezoelectric strain constant and electromechanical coupling constant. Our findings show a reasonable accord with data in the literature where available, and allow predictions where they are not. Our findings are expected to provide useful information for understanding the elastic and piezoelectric properties of ZnxCd1−xS and for applications in optoelectronics.

The structural characterisation of MOVPE-grown ZnMgSe and ZnSe/ZnMgSe multiple quantum wells (MQWs) is reported. Zn0.83Mg0.17Se epilayers and 6 periods ZnSe/Zn0.83Mg0.17Se MQWs having 4.5 nm thick ZnSe wells and 10.5 nm thick alloy barriers were deposited on (1 0 0) GaAs after a thin pseudomorphic ZnSe buffer. No macroscopic misorientations between ZnMgSe epilayers and GaAs lattice has been observed, but a broadening of the alloy (4 0 0) peak indicates the occurrence of extended defects. Reciprocal space map (RSM) measurements of a Zn0.83Mg0.17Se epilayer show a slight asymmetry of the alloy peak along the growth direction, ascribed to an inhomogeneous relaxation. Diffraction patterns recorded in asymmetrical configuration indicate almost no relaxation of the MQWs structure, within the technique sensitivity. RSMs in the vicinity of the (4 0 0) lattice point show a characteristic diffuse scattering around the MQW satellite peaks, indicating that the MQWs structure is beyond the onset of plastic relaxation.

This work assesses the Heyd-Scuseria-Ernzerhof (HSE) screened Coulomb hybrid density functional for the prediction of lattice constants and band gaps using a set of 40 simple and binary semiconductors. An extensive analysis of both basis set and relativistic effects is given. Results are compared with established pure density functionals. For lattice constants, HSE outperforms local spin-density approximation (LSDA) with a mean absolute error (MAE) of 0.037 A for HSE vs 0.047 A for LSDA. For this specific test set, all pure functionals tested produce MAEs for band gaps of 1.0-1.3 eV, consistent with the very well-known fact that pure functionals severely underestimate this property. On the other hand, HSE yields a MAE smaller than 0.3 eV. Importantly, HSE correctly predicts semiconducting behavior in systems where pure functionals erroneously predict a metal, such as, for instance, Ge. The short-range nature of the exchange integrals involved in HSE calculations makes their computation notably faster than regular hybrid functionals. The current results, paired with earlier work, suggest that HSE is a fast and accurate alternative to established density functionals, especially for solid state calculations.

Group BV(Bi, Sb)- and BVI(Te, Se)-doped Mg2Si compounds were synthesized by solid state reaction and mechanical alloying. Electronic transport properties (Hall coefficient,
carrier concentration and mobility) and thermoelectric properties (Seebeck coefficient, electrical conductivity, thermal conductivity
and figure-of-merit) were examined. Mg2Si powder was synthesized successfully by solid state reaction at 773 K for 6 h and doped by mechanical alloying for 24 h.
Powder was fully consolidated by hot pressing at 1073 K for 1 h. All doped Mg2Si compounds showed n-type conduction, indicating that the electrical conduction is due mainly to electrons. The electrical
conductivity increased greatly by doping due to an increase in the carrier concentration. However, the thermal conductivity
did not change significantly by doping, which was due to the much larger contribution of the lattice thermal conductivity
over the electronic thermal conductivity. Group BV (Bi, Sb) elements were much more effective at enhancing the thermoelectric properties of Mg2Si than group BVI(Te, Se) elements.
Keywordsthermoelectric–Mg2Si–mechanical alloying–solid state reaction–hot pressing

Based on the pseudopotential scheme, the electronic properties of zinc-blende Zn1−xMgxSe alloys have been predicted. The agreement between our calculated electronic band parameters and the available experimental data is generally reasonable. These parameters are found to depend non-linearly on alloy composition x. The electron valence and conduction charge densities derived from pseudopotential calculations for certain concentrations are also reported.

laser ablated films deposited on quartz substrates have been studied in order to obtain information on their structural and optical properties as a function of sulphur concentration (x). Transmittance and reflectance spectra at room temperature were carried out to obtain the energy gap modulation, the absorption coefficient and refractive index. By using the Newton-Raphson algorithm, the values of the real part of the refractive index in the transparent region were deduced including the effect of non-uniformity of the film thickness. The wavelength dependence of n was extrapolated for the weak and medium absorption range by means of two different models: the classical Cauchy formula and the single effective oscillator model proposed by Wemple and Di Domenico. From the latter theory, two physical parameters known as oscillator energy and oscillator strength, respectively, were obtained. These parameters have been related to the optical gap and structural properties of the deposited alloys. In the strong absorption region, the absorption coefficient and the optical gaps were calculated for each x-value. A quadratic trend was found for the x-dependence of the band gap, in agreement with the corresponding bulk alloy.

The structural and electronic properties of Zn1−xMgxSySe1−y quaternary alloys have been investigated using the full potential-linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT). We used the generalized gradient approximation (GGA) that is based on exchange-correlation energy optimization for calculating the total energy. Moreover, the Engel–Vosko GGA formalism is applied so as to optimize the corresponding potential for band-gap structure calculations. We investigated the effect of composition on lattice constant, bulk modulus and band gap. A non-linear dependence on the compositions x and y is observed for all the three properties. The energy gap Eg(x, y) has been determined over the entire compositions x and y. In addition, the energy gap of zinc-blende Zn1−xMgxSySe1−y quaternary alloys in conditions of lattice matched to GaAs substrate has been investigated.

We present experimental and theoretical studies of lattice-matched interfaces between Zn1−yMgySe and Zn1−xCdxSe wide-band gap semiconductors. Ab initio pseudopotential calculations predict lattice matching for y/x∼1.6 and a valence band contribution to the band gap difference Qv decreasing gradually from 0.34 to 0.27 with increasing x and y. Experimental x-ray diffraction and photoemission spectroscopy results are mostly consistent with such predictions, although non-negligible deviations of the band offsets from commutativity were observed for selected alloy compositions. © 2001 American Institute of Physics.

We determined the properties of Zn1−xMgxSe semiconductor alloys through a combination of optical and photoelectron spectroscopy, x-ray diffraction, Rutherford backscattering spectrometry, and ab initio pseudopotential calculations. The complementary character of the techniques and the good agreement between calculated and experimental trends allowed us to explain some of the discrepancies between the reported properties of these wide band gap alloys. © 2004 American Institute of Physics.

The electronic properties and optical gap of (Zn,Mg)(S,Se) wide-gap solid solutions are studied using ab initio techniques and starting from the previously determined atomistic structure of the alloy. Compositional disorder is shown to close the gap substantially with respect to the predictions of the virtual-crystal approximation. The bowing of the fundamental gap versus composition predicted by our calculations is in very good agreement with experiments available for the Zn(S,Se) pseudobinary alloy. At temperatures typical of molecular-beam epitaxy growth, the quaternary alloy displays a rather large amount of short-range order whose effect is to slightly but unmistakably open the gap. Our results agree well with recent experimental data for the quaternary alloy. © 1999 American Institute of Physics.

Spin-flip Raman scattering spectroscopy has been applied to the study of the wide band-gap semiconductor materials ZnSxSe1-x and Zn1-xMgxSe in order to determine the dependence on the composition, x, of the gyromagnetic ratio of electrons in the Γ6 conduction band and, thereby, to obtain a better understanding of the parameters underlying the band structure of these materials. The measured values are discussed in terms of the k⋅p perturbation theory for the band structure near the direct band gap, at different levels of approximation, and it is found that the observed dependence on composition can be reproduced well by the use of suitable interpolation schemes between the binary end members of the range of materials. Preliminary results for the related quaternary material Zn1-xMgxSySe1-y are discussed within the same model.

Electronic structures of Si, Ge, GaAs, BeSe, BeTe, MgSe, and MgTe resulting from the local-density approximation (LDA), exact-exchange (EXX) Kohn-Sham scheme and the many-body-GW approximation are compared. It is shown that although the EXX scheme gives good energy gaps between occupied and unoccupied bands, it underestimates by ∼10% the widths of upper valence bands. The GW approximation is applied perturbatively either on top of the LDA, or EXX band structures in the zero order. It is argued that such a perturbative scheme carries a built-in ambiguity: the quasiparticle band structure depends on an arbitrary constant in the Kohn-Sham potential. This ambiguity could be removed by a requirement of the energy alignment between the Kohn-Sham and GW schemes.

The low-pressure MOVPE growth of ZnMgSe on (1 0 0)GaAs is reported. ZnMgSe alloys were deposited after a thin pseudomorphic ZnSe buffer layer by using dimethylzinc: triethylammine (Me2Zn:Et3N), ditertiarylbutilselenide (Bu-t(2) Se) and bis(methylcyclopentadienyl)magnesium [(MeCp)(2)Mg]. Zn1-xMgxSe (0.10 <x < 0.46) epilayers were grown at 330 degreesC, 304 mbar and a high VI/II ratio in the vapour. Under these conditions the growth is limited by the mass transport of the II-group alkyls and good control of the composition was achieved. Rutherford backscattering spectrometry measurements allowed to determine the epilayer stoichiometry and deposited dose. The ZnMgSe solid-vapour distribution curve deviates from linearity due to the different Mg and Zn alkyl vapour diffusion coefficients, whose ratio turns out to be D-(MeCp)2Mg/D-Me2Zn = 0.410. The epilayer crystalline properties were studied by double-crystal X-ray diffraction (DC-XRD) and high-resolution reciprocal space mapping (RSM) measurements. Rocking curve FWHM values of 540-900 nm thick Zn1-xMgxSe were in the 5-12 mrad range, indicating the occurrence of extended defects in the epilayers. RSM measurements in the vicinity of the (400)-peak of a Zn0.844Mg0.156 Se/ZnSe(100)GaAs sample showed a slight asymmetry of the ternary alloy peak along the growth direction, ascribed to an inhomogeneous relaxation of the epilayer.

We report on the MOVPE of MgSe and ZnMgSe on (100)GaAs. Dimethylzinc:triethylammine, ditertiarylbutylselenide and bis(methylcyclopentadienyl)magnesium were used as Zn, Se and Mg sources, respectively. MgSe and ZnMgSe layers were grown either directly on GaAs or after a thin pseudomorphic ZnSe layer. To avoid the hygroscopicity of Mg-based chalcogenides, a ZnSe capping layer was grown on some samples. The crystallographic phase of as-grown layers was determined by both single- and double-crystal X-ray diffraction measurements. MgSe layers turned out to be mosaics made of (111)- and (100)-oriented MgSe crystals in their rocksalt phase. No signatures of the zincblend (ZB) phase was observed, irrespective of whether MgSe was grown on (100)GaAs or after the ZnSe buffer. Zn1−xMgxSe (0.07

Based on the pseudopotential scheme, the hydrostatic pressure dependence of the electronic properties has been predicted for zinc-blende ZnS. The material of interest is found to exhibit features of both direct and indirect band-gap semiconductor depending on the applied pressure. Detailed plots of the valence charge distribution along the [111] direction and in the (110) plane at different pressures are also presented and discussed.

As part of our development of an epitaxial lift-off process, utilising a sacrificial magnesium sulphide (MgS) layer, we have developed a MgS-rich ZnMgSSe alloy which provides excellent carrier confinement and resists both oxidation and acid attack. Here the optical transmission of the alloy has been measured and its bandgap determined as a direct transition at 4.19 ± 0.04 eV. Its composition has also been determined by X-ray interference (XRI) and comparison with simulations. For a range of alloy samples we obtain compositions of the Zn1−xMgxSySe1−y layers which are (x, y) = (0.80 ± 0.02, 0.645 ± 0.025). Using the alloy bandgap and composition we have determined direct bandgap transition energy for MgS by extrapolation. This is found to be 4.78 ± 0.14 eV.

The influence of compositional disorder on ZnxCd1−xS energy band gaps and carrier effective masses has been investigated. The calculations are performed using a pseudopotential approach under the virtual crystal approximation (VCA). A correction to the VCA has been introduced so as to take into consideration the disorder effect. Our results show that the theoretical model used in the present contribution should include the disorder effect in order to describe more accurately the fundamental band gap bowing parameter of the experimental dependence. However, the disorder effect upon electron, light-hole and heavy-hole effective masses in the material system of interest is shown to be weak. Moreover, the trend of the conduction Г, X and L valleys and the nature of the optical fundamental band gap are found to be weakly altered by the alloy disorder. The different photo-catalytic activity of wurtzite and zinc-blende CdS is discussed for a new understanding.

In a recent paper [Phys. Rev. B 57, 2257 (1998)], Okuyama et al. calculated band parameters for the quaternary alloy Zn1−xMgxSySe1−y. The energy gap of the ternary alloy Zn1−xMgxSe for x<~0.45 was investigated. In this composition range, a linear Eg(x) relation was observed. By assuming a linear dependence of the energy gap Eg for the entire composition range, Okuyama et al. obtained an extrapolated value of Eg(MgSe)=3.59 eV. However, the latter value is about 0.5 eV below an experimentally determined value [J. Cryst. Growth 159, 54 (1996); Semicond. Sci. Technol. 12, 970 (1997)]. Using the experimental values for the entire composition range, we demonstrate that the energy gap of the ternary alloy Zn1−xMgxSe exhibits a nonlinear dependence on composition.

ZnSe and ZnS are the prototype II-VI semiconductors and their cubic phase, which occurs naturally as a mineral, has been called the zinc-blende structure. ZnSe has received particular attention as a blue-lasing material and tremendous experimental efforts have been made to fabricate a sustainable ZnSe blue laser. On the other hand, the ternary system ZnS(x)Se(1-x) is a good candidate for developing optoelectronic devices such as light emitting diodes (LEDs) or semiconductor lasers. It is also used as waveguides and confinement layers in laser diodes (LDs). In the present study, we have computed the electronic and optical properties of ZnSe, ZnS and ZnS(x)Se(1-x) using the local model pseudopotential method under the VCA. A central aim of this study is to present the electronic and optical properties of the materials of interest in the zinc-blende structure. (C) 2009 Elsevier B.V. All rights reserved

Zn1−xMgxSe-based heterostructures were grown by molecular beam epitaxy on (100) GaAs substrates. In this communication we report on the structural and electronic properties of ZnMgSe alloys and ZnSe/ZnMgSe quantum wells and on the influence of the magnesium incorporation on the doping behaviour of ZnMgSe:N materials.

We report on theoretical study of the energy band gaps for the quaternary alloys Zn1-xMgxSySe1-y in conditions of lattice matching to GaAs substrates using simply the empirical pseudopotential method under the virtual crystal approximation which takes into account the effects of compositional disorder. Our results agree generally very well with the available experimental data. It is shown that the band-gap energies of Zn1-xMgxSySe1-y are expressed by the parabolic function of the composition considering the bowing parameter and that Zn1-xMgxSySe1-y can be a direct or an indirect semiconductor depending on the alloy composition. The Zn0.35Mg0.65S0.6Se0.4 is predicted to meet requirement of the cladding layer for fabricating blue double heterostructure laser diodes using ZnS0.06Se0.94 as the active layer.

Based on the pseudopotential scheme under the virtual crystal approximation that takes into account the compositional disorder effect, the electron and positron band structures and their related properties such as effective masses, ionicity and positron affinity in the zincblende Zn1−xMgxSySe1−y quaternary system lattice matched to GaAs have been determined. The positron wave functions are obtained using the point-core approximation for the ionic potential. Our results are generally in reasonable agreement with data available from the literature. The overall dependence of the electronic and positronic studied quantities on the alloy composition x is found to be non-linear.

The structural properties of the (Zn, Mg) (S, Se) solid solutions are determined by a combination of the computational alchemy and the cluster expansion methods with Monte Carlo simulations. We determine the phase diagram of the alloy and show that the homogeneous phase is characterized by a large amount of short-range order occurring among first-nearest neighbors. Electronic-structure calculations performed using the special quasirandom structure approach indicate that the energy gap of the alloy is rather sensitive to this short-range order.

We present an investigation of the electronic properties and optical
constants of zinc-blende ZnSxSe1-x semiconducting
alloys at normal and under hydrostatic pressure up to 20 kbar. For this
purpose, we used an empirical pseudopotential method within the virtual
crystal approximation. The effects of alloy composition are taken into
consideration in the calculation, which improves significantly the
bandgap bowing parameters with respect to the experiment. Results
regarding the composition and pressure dependences of energy bandgaps,
electron valence and conduction charge distributions, optical
high-frequency dielectric constant and its linear pressure coefficient
are presented and discussed. The information derived from the present
study may be useful for the development of opto-electronic devices that
operate in the blue/green spectral range.

Based on a pseudopotential scheme, the composition dependence of energy band-gaps of zinc-blende MgxZn1-xSe ternary alloys in the composition range of 0≤x≤1 are determined. The effect of deviation of lattice constants of the alloys of interest from Vegard's law on the optical bowing parameter and system transition between the direct and indirect structures is discussed.

The electronic and optical properties of the MgxZn1-xS semiconductor ternary alloys crystallizing in the Zinc Blende structure are calculated using the empirical pseudopotential method (EPM) coupled with the virtual crystal approximation (VCA). The composition dependence of the direct and indirect band gap energies as well as the antisymmetric gap are investigated in the composition range 0 up to 1. Other quantities such as refractive index and coefficient of reflection are also obtained by means of different existing models. Our results are generally in good agreement with those available in the literature. The obtained informations could be useful for the feature design of blue wavelength optoelectronic devices.

II–VI semiconductors have demonstrated convenient properties for application in photodetectors, solar cells, light-emitting diodes, laser diodes, solar blind detectors, radiation detectors, magneto-optical devices, biophotonics, etc. Molecular beam epitaxy (MBE) has proven to be a powerful technique for the elaboration of high-quality films and low-dimensional structures. This chapter presents a general overview and recent advances of the MBE growth of visible bandgap II–VI semiconductors for application in light-emitting devices and other novel quantum structures. A short description of the fundamentals of molecular beam epitaxy and derived techniques related to II–VI semiconductors is presented. A brief presentation of the layer-by-layer growth techniques and their applications to the growth of thin and ultra-thin quantum wells is included. This chapter contains abundant information about homo- and heteroepitaxial growth of II–VI semiconductors; particular emphasis is placed on the advantages of quaternary alloys for bandgap tuning and growth of lattice-matched systems. This chapter also contains an outline of recent results of II–VI quantum structures produced by MBE.

The quaternary semiconducting alloys open new possibilities for materials engineering as they enable independent control of three baSiC parameters, namely, the band-gap energy, lattice constant, and conduction- and valence-band offsets. We present here the optical constants of Zn1-x
Mgx
Sy
Se1-y
and Mgx
ZnyCd1-x-y
Se quaternary alloys.

The electronic and optical properties of (Zn,Mg)(S,Se) wide-gap solid solutions are studied using ab initio techniques and starting from the previously determined atomistic structure of the alloy. Compositional disorder is shown to close substantially the gap with respect to the predictions of the virtual-crystal approximation. The bowing of the fundamental gap vs. composition predicted by our calculations is in very good agreement with experiments available for the Zn(S,Se) pseudo-binary alloy. At temperatures typical for MBE growth, the quaternary alloy displays a rather large amount of short-range order whose effect is to slightly but unmistakably open the gap. Our results agree well with recent experimental data for the quaternary alloy.

The refractive index and the E 0 energy gap has been determined for a number of Zn 1-x Mg x S y Se 1-y alloys with x≤0.3 and y≤0.3 at temperatures between 5 and 300 K. It has been shown that the refractive index can be empirically expressed as a function of the E 0 energy gap. Furthermore we have determined the temperature dependence of the E 0 energy gap and hence the temperature dependence of the refractive index for these alloys. © 1995 American Institute of Physics.

The temperature dependence of the energy gap of zinc‐blende CdSe and Cd 1-x Zn x Se has been determined over the entire range of composition from optical transmission and reflection measurements at temperatures between 5 and 300 K. The experimental results can be expressed by the following modified empirical Varshni formula, whose parameters are functions of the composition x: E g (x,T)=E g (x,0)-β(x)T<sup>2</sup>/[T+γ(x)]. E g (x,0) exhibits a nonlinear dependence on composition, according to E g =E g (0,0)(1-x)+E g (1,0)x-ax(1-x). The parameters β(x) and γ(x) can be expressed by β(x)=β(0)(1-x)+β(1)x+bx(1-x) and γ(x)=γ(0)(1-x)+γ(1)x. © 1996 American Institute of Physics.

We report on the epitaxial growth and the characterization of Hg1−xMgxTe on (001), (111)B and (112)B Cd0.96Zn0.04Te substrates by molecular beam epitaxy. The band gap of this ternary compound is variable from −0.15 up to 3.49 eV at room temperature by changing the composition. At a fixed Mg concentration the band gap depends on the substrate orientation and additionally on the Hg to Te flux ratio. Possible reasons for this behavior such as phase separation, clustering or atomic ordering will be discussed.

ZnTe and CdSe with lattice constants of 6.10 and 6.09 Å are nearly lattice matched to InAs with a lattice parameter of 6.06 Å. InAs is available as a high quality substrate material for molecular beam epitaxy (MBE). Reflection high energy electron diffraction (RHEED) and X-ray diffraction studies have been performed to investigate the nature of the MBE growth on InAs substrates with and without the epitaxial growth of an InAs buffer layer. For the quaternary Zn1 − xMgxSeyTe1 − y (ZnMgSeTe), we could tune the band gap through the whole visible range. Lattice matched to the InAs substrate, rocking curve widths as low as 38 arcsec for ZnMgSeTe could be obtained. We will present results on structural and optical investigations of these layers and related quantum well structures. A pronounced curvature in the dependence of the band gap on composition could be found not only for ZnSeTe and MgSeTe, but also for ZnMgSe and ZnMgTe. A type-II band alignment between ZnTe and ZnMgSeTe allows us to measure band offsets directly via photoluminescence measurements in particular single quantum well structures.

We have grown the ternary alloy Cd1-xMgxTe as well as Cd1-xMgxTe/CdTe quantum well structures by molecular beam epitaxy - to our knowledge for the first time. Cd1-xMgxTe exhibits some very interesting features: The band gap has been determined as a function of Mg concentration, and a band gap of 3.0 eV was found for zincblende MgTe at room temperature. Cd1-xMgxTe thin films with Mg concentrations of up to 0.75 were fabricated, which corresponds to a band gap of 2.8 eV at low temperatures. Therefore, the whole visible band gap range (at room temperature) can be covered with Mg concentrations between 0.30 (red) and 0.75 (blue). Bulk MgTe crystallizes in the wurtzite structure, but zincblende MgTe could be grown on (100) oriented CdTe substrates up to a layer thickness of approximately 500 nm. The lattice mismatch between zincblende MgTe and CdTe was found to be as small as 1.0%. The growth of cubic MgTe could be followed by reflection high energy electron diffraction (RHEED) oscillations. In general, excellent structural quality could be reached, which is demonstrated by the FWHM of 22 arc sec for a Cd1-xMgxTe thin film with 0.44 Mg concentration on a Cd1-xZnxTe nearly lattice matched substrate. The Poisson number of Cd1-xMgxTe has been determined by X-ray diffraction as a function of Mg concentration. Cd1-xMgxTe/CdTe single quantum well structures have been fabricated with a large confinement energy of up to 0.8 eV. The photoluminescence spectra show exciton lines with very narrow linewidths. We are able to observe excited exciton states, and from the energetic difference between 1s and 2s heavy hole exciton lines we deduce exciton binding energies. Very bright luminescence could be seen even at room temperature, which is an indication of a large exciton binding energy and an effective radiative recombination.

Reflectivity measurements were made to determine the variation with composition of the lowest gap E0 together with its spin-orbit-split gap E0 + Delta0 in ZnSxSe1-x at room temperature. The variation of these gaps indicates that ZnSxSe1-x is an amalgamation-type alloy, at least for these gaps, contrary to a previous suggestion of a persistence type. The bowing parameters of the E0 and E0 + Delta0 gaps are determined to be 0.630 +/- 0.030 and 0.596 +/- 0.023 eV, respectively. The variation with composition of the spin-orbit splitting Delta0 tends to be convex as in the case of ZnSexTe1-x alloys. The negative bowing of Delta0 may be interpreted in terms of the virtual-crystal approximation.

Reflectivity measurements were made to determine the variation with composition of the lowest gap E0 together with its spin-orbit-split gap E0+Δ0 in ZnSxSe1-x at room temperature. The variation of these gaps indicates that ZnSxSe1-x is an amalgamation-type alloy, at least for these gaps, contrary to a previous suggestion of a persistence type. The bowing parameters of the E0 and E0+Δ0 gaps are determined to be 0.630 ± 0.030 and 0.596 ± 0.023 eV, respectively. The variation with composition of the spin-orbit splitting Δ0 tends to be convex as in the case of ZnSexTe1-x alloys. The negative bowing of Δ0 may be interpreted in terms of the virtual-crystal approximation.

The E0 band gap energies and the lattice constants of zinc‐blende Zn1−xMgxSe alloys grown by molecular beam epitaxy in the composition range of 0≤x≤0.95 are determined. A nonlinear dependence on the composition is observed for both the band‐gap energies and the lattice con‐ stants of the ternary alloys. To our knowledge this is an initial report of a bowing in the lattice constant of a ternary II–VI alloy. Considering the bowings, the band‐gap energy and the lattice constant of zinc‐blende MgSe are extrapolated to be about 4.0 eV and 5.91 Å, respectively. © 1996 American Institute of Physics.

Optical properties of Zn1 − yMgySxSe1 − x epilayers grown on GaAs substrates by low-pressure metalorganic vapour phase epitaxy (LP-MOVPE) were investigated with photoluminescence spectroscopy, reflection spectroscopy and X-ray diffraction. The sulphur (x) and magnesium (y) content in the samples measured by EPMA (electron probe micro analysis) was varied in the range 0 ≤ x, y ≤ 0.25, respectively. The photoluminescence study of these quaternary epilayers was performed in the temperature range between 14 and 300 K. Low and negligible concentration of deep levels was achieved. Comparison of the measured halfwidths of the band edge emissions with calculated minimum alloy broadening values allowed the estimation of the composition fluctuation of the majority of the epilayers to be less than 1%. However, at large x or large y (x, y ≥ 0.15) broadening in the band edge emission indicates an increased composition deviation from randomness. Bandgap reduction up to 75 meV with respect to the theoretically expected bandgap in the case of total randomness was observed. Enhanced phase separation and ordering effects especially at large x or large y seem to be the reason for the emission broadening as well as for the bandgap reduction.

The methods for calculation of the various optical constants in ZnS x Se 1-x ternary alloys are presented. The model used is based on an interpolation scheme, and the effects of alloy composition are properly taken into account in the calculation. The present model reveals distinct structures in the optical spectra at energies of the E 0 , E 0 +Δ 0 , E 1 , and E 1 +Δ 1 gaps. The optical constants and properties considered here are the complex dielectric constant ϵ=ϵ 1 +iϵ 2 , complex refractive index n*=n+ik, absorption coefficient α, and normal‐incidence reflectivity R. The refractive indices in the transparent region are also presented for a variety of waveguiding device applications.

Atomic layer epitaxy (ALE) is investigated for the binary semiconductor MgTe. Reflection high‐energy electron‐diffraction studies on MgTe atomic deposition, together with x‐ray diffraction, high‐resolution transmission electron microscopy, and photoluminescence experiments on ALE‐grown CdTe/MgTe superlattices are reported. They reveal that an autoregulated growth at 0.7±0.1 MgTe monolayer/ALE cycle can be achieved in a substrate temperature range between 260 and 300°C. New values of the zinc‐blende MgTe lattice parameter, a MgTe =6.420 ±0.005 Å, of the ratio of the elastic coefficients 2<sup>c 12 </sup> c11 (MgTe)=1.06, and of the 300 K MgTe band gap, E G =3.5 eV, are obtained by correlating x‐ray‐diffraction and optical results. © 1996 American Institute of Physics.

We have experimentally investigated refractive indices n of Zn x Mg 1-x S y Se 1-y , using the ellipsometry method and reflection‐spectrum measurement. The samples are epitaxial films of undoped Zn x Mg 1-x S y Se 1-y grown by molecular beam epitaxy on semi‐insulating GaAs substrates. The obtained dispersion relations of n in the transparent region are classified by the band‐gap energy E g . We have found that the refractive index n of Zn x Mg 1-x S y Se 1-y decreases as E g increases. These results will be available for the design of blue laser diodes containing ZnMgSSe.

A relation for the variation of the energy gap (Eg) with temperature (T) in semiconductors is proposed. Eg ≐ E0 - αT2/(T+β) where α and β are constants. The equation satisfactorily represents the experimental data for diamond, Si, Ge, 6H-SiC, GaAs, InP and InAs.

We propose a new material, ZnMgSSe, as the possible cladding layer of blue laser diodes. The band-gap energy can be varied from 2.8 to near 4 eV, maintaining lattice-matching to a (100) GaAs substrate. From the quarternary data, the band-gap energies of MgS and MgSe (zincblende structure) are estimated to be about 4.5 and 3.6 eV, and the lattice constants are 5.62 and 5.89 Å, respectively. The refractive index of ZnMgSSe lattice-matched to GaAs is smaller than that of ZnSSe lattice-matched to GaAs. ZnMgSSe therefore meets the requirements of the cladding layer of ZnSSe for fabricating blue laser diodes.