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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.

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... The wide energy band gap magnesium selenide (MgSe) and telluride (MgTe) semiconductors have diverse technological importance due to their use in optoelectronic devices [23]. Many comprehensive studies have been conducted for MgSe and MgTe in the rock salt, wurtzite, and zincblende (ZB) phases which predict the energy band gap variations between ground state and high pressure structural phases [24][25][26][27][28]. At ambient conditions, MgTe [29] and MgSe [24] crystallize stably in wurtzite and ZB phases, respectively; however the ZB phase for MgTe has also been achieved [25,26]. ...

... Many comprehensive studies have been conducted for MgSe and MgTe in the rock salt, wurtzite, and zincblende (ZB) phases which predict the energy band gap variations between ground state and high pressure structural phases [24][25][26][27][28]. At ambient conditions, MgTe [29] and MgSe [24] crystallize stably in wurtzite and ZB phases, respectively; however the ZB phase for MgTe has also been achieved [25,26]. ...

... Since even the fourth-order gradient expansion of exchange energy available in WC-GGA is not sufficient to accurately predict the electronic structure of wide band gap semiconductors and HFM materials [10,12,38], we have computed the electronic and magnetic properties of Mg 1−x V x Y (Y = Se and Te) compounds with the mBJLDA at the optimized lattice parameters presented in Table 1. It is important Table 1 The optimized lattice parameter, a 0 , and bulk modulus, B 0 , for Mg 1−x V x Y (Y = Se and Te) at doping concentrations x = 0, 0.25, 0.50, 0.75, and 1 Results are obtained with WC-GGA to point out here that the electronic structure of the all-end binary compounds (MgY and VY (Y = Se and Te)) have been explored in numerous earlier studies [24][25][26][27]; however, since the aim of this work is to explore HMF in the V-doped MgY, in our discussion, we restrict ourselves to the case where 0 < x < 1. ...

In this study, we have explored the structural, electronic, and magnetic properties of V-doped zincblende MgSe and MgTe compounds using density functional calculations. The Wu-Cohen generalized gradient approximation is used for optimizing the structural properties, while the modified Becke and Johnson local (spin) density approximation functional has been employed to compute the electronic and magnetic properties. The spin dependent band structures, electronic density of state, and magnetic moments calculated for V-doped MgSe and MgTe semiconductors exhibit occurrence of 100 % spin polarization at the Fermi level which confirms stable half-metallic ferromagnetism in these materials. The spin-down gaps and the half-metallic gaps are analyzed in terms of V-3d and Se-4p (Te-5 p) hybridization, where it is observed that the V-3dstates play a key role in generating spin polarization and the magnetic moment in these compounds. The exchange constants N
0αand N
0β have been calculated to demonstrate the effects resulting from exchange splitting process. Furthermore, spin-polarized charge density calculation is presented for elucidating the bonding nature, while pressure dependence of total magnetic moment for three concentrations of V-doped MgSe and MgTe are also discussed.

... In addition, the lattice parameters of zinc-blende MgSe [29,30] and MgTe [24,31] as well as rock-salt MgSe [17] and MgTe [18] were investigated. Experimental studies on their electronic properties indicate that ZnSe and ZnTe as well as MgSe and MgTe are wide direct fundamental band gap (Γ-Γ) semiconductors in their B3 phase [31,32]. ...

... on InAs substrates and their structural, electronic and optical characterizations [30], MBE growth of quaternary alloys Zn 1−x Mg x Se y Te 1-y on InAs substrate and their structural characterizations [32], MBE growth of Mg x Zn 1−x Se y Te 1−y quaternary alloys on InP substrate and their structural, electronic and optical characterizations [49], MBE growth of Zn 1−x Mg x Se y Te 1−y alloys on ZnTe substrates and photoluminescence studies of their optical properties [50], etc. ...

... Therefore, in the introductory stage, we have adopted the eight-atom 1 × 1 × 1 cubic zinc-blende unit cell of these binary compounds in our calculations. These have been designed by using the experimental lattice parameters of ZnSe [21], ZnTe [21], MgSe [32] and MgTe [32]. The anionic ternary alloys ZnSe y Te 1−y and MgSe y Te 1−y at y = 0.25, 0.50 and 0.75 have been designed by successive substitution of Te atom(s) with Se atom(s) in the 1 × 1 × 1 unit cell of ZnTe and MgTe, respectively. ...

Structural and optoelectronic properties of technologically important MgxZn1−xSeyTe1−y quaternary alloys are calculated employing DFT-based FP-LAPW approach. Computations of exchange–correlation potentials are performed with PBE-GGA for structural properties and both the mBJ and EV-GGA for optoelectronic properties. Each specimen within MgxZn1−xSeyTe1−y system is a direct band gap (Γ–Γ) semiconductor. At each cationic (Mg) concentration x, lattice constant decreases, while bulk modulus and band gap increase nonlinearly with increase in anionic (Se) concentration y. Again, nonlinear increase in lattice constant and band gap, while decrease in bulk modulus is observed with increase in cationic concentration x at each anionic concentrations y. Calculated band gap bowing for few ternary alloy systems are in good agreement with corresponding experimental data. The calculated contour maps for lattice constants and energy band gaps would be very useful for designing new quaternary alloys with desired optoelectronic properties. Optical properties of the said specimens within MgxZn1−xSeyTe1−y quaternary system show several interesting features. Composition dependence of each calculated zero-frequency limit shows opposite trend, while each calculated critical point shows similar trend of composition dependence of band gap. Finally, suitability of ZnTe and InAs as substrates for the growth of several zinc-blende MgxZn1−xSeyTe1−yquaternary alloys has been investigated.

... g Ref. [39]. h Ref. [47]. i Ref. [50]. ...

... GGA [20] 4.259 [21] 160.00 [21] 4.247 [22] 169. 10 The formation energy indicates the stability of the alloys in regard to decomposition into its bulk constituents. The formation energy is calculated using the formula: The calculated MgO 1−x Se x band gaps using GGA-PBEsol and TB-mBJ approach, with experimental values [31][32][33], and other calculations [20,25,[34][35][36][37][38] are listed in Table 2, where we can observe that the calculated band gap energies for MgO and MgSe using TB-mBJ approaches are found to be 7.081 and 2.972 eV, respectively. These results confirm that TB-mBJ approach is a very useful approach to study the band structure properties. ...

We report on the investigation of the structural, electronic, and optical properties of binary compounds (MgO and MgSe) and their ternary \(\hbox {MgO}_{1-{x}}\hbox {Se}_{{x}}\) (\(x=0.25, 0.5, 0.75\)) alloys within the density functional theory based on the full-potential linearized augmented plane wave method as implemented in the WIEN2k code. We have used the revised Perdew–Burke–Ernzerhof generalized gradient approximation (GGA-PBEsol) to calculate the structural properties and analyze the effect of the Se composition on the lattice constant and the bulk modulus of \(\hbox {MgO}_{1-{x}}\hbox {Se}_{{x}}\). The calculated electronic properties by employing the GGA-PBEsol and TB-mBJ approaches show that \(\hbox {MgO}_{1-{x}}\hbox {Se}_{{x}}\) alloys have a direct band gap \(\Gamma \)–\(\Gamma \) for \(x = 0, 0.25, 0.5\) and 0.75, suggesting the possibility of their use in the long wavelength optoelectronic applications. The optical properties such as the real and imaginary parts of the dielectric function, the refractive index, and the reflectivity of \(\hbox {MgO}_{1-{x}}\hbox {Se}_{{x}}\) are computed by using the accurate TB-mBJ potential. The wide band gaps larger than 3.1 eV mean that \(\hbox {MgO}_{1-{x}}\hbox {Se}_{{x}}\) alloys can be used in the applications of the ultraviolet region of the spectrum. Our data for all studied bowing parameters of \(\hbox {MgO}_{1-{x}}\hbox {Se}_{x}\) may serve as references for future experimental studies.

... Specifically, magnesium selenium (MgSe) is an attractive binary semiconductor of the II-VI family, which can excite in several phases [30], such as zinc blende with in experimental wide direct band gap of 4 eV at 0 K [31] and lattice constant of 5.89Å [32]. Used in high-temperature, high-power blue and ultraviolet wavelength optics [32][33][34][35] in ZnSe-based laser diodes and light-emitting diodes (LEDs) as a cladding material [35]. Furthermore, some theoretical studies are available in the literature which reported the magnetic properties of MgSe semiconductor doped with 3d TM using the first-principles calculation [36][37][38][39]. ...

In this paper, we have investigated the structural, electronic, and magnetic properties of magnesium selenium (MgSe) doped with transition metal manganese (Mn) impurity in the cubic diluted magnetic semiconductor (DMS) zinc blende structure. The compounds which we are interested are as Mg1−xMnxSe where x change between 0 and 1 by step 0.25. All properties are studied, using first-principles calculation of density functional theory under the framework of the full-potential linearized augmented plane waver (FP-LAPW). In our study, we employed the Wu-Cohen generalized approximation (WC-GGA) to optimize the crystal structure, whereas Tran-Blaha modified Becke-Johnson potential (TB-mBJ) as a new functional was applied to compute the electronic and magnetic properties in order to get some better degree of precision. The electronic band structures and density of state plots reveal ferromagnetic semiconducting behavior in these compounds, and the exchange constants N0α and N0β are calculated to validate the effects resulting from exchange splitting process. Moreover, for each concentration x, the value of total magnetic moment has been estimated to equal to 5 μB. The important magnetic moments values obtained in these compounds indicate the potential for their use in spintronic devices.

... It should be mentioned here that several studies to explore structural, electronic, optical and various other physical properties of ternary alloys of strontium chalcogenides, formed by doping of suitable atom(s) from different groups of periodic table into respective unit cell of SrS, SrSe and SrTe at different concentrations, have been performed experimentally [64,65] as well as theoretically [34,[66][67][68][69][70][71][72] considering their B1 phase. Similar studies on such physical properties of doped-MgS, MgSe and MgTe ternary alloys have also been performed experimentally [73][74][75][76][77] as well as theoretically either considering their B1 [78,79] or B3 [80][81][82][83][84][85][86] phase. Therefore, alloying of these two types of respective compounds having same chalcogen, performed by doping of magnesium atom(s) into the unit cells of SrS, SrSe and SrTe at different concentrations in B1 phase, might be an effective approach of preparation of a series of new ternary alloys having optoelectronic properties intermediate or completely different from their parent binary compounds SrY and MgY (Y ¼ S, Se & Te). ...

This document is part of subvolume C2 'Optical Properties', of volume 34 'Semiconductor quantum structures' of Landolt-Börnstein, Group III, Condensed Matter. Data for CdSe quantum wells are provided.

The growth of Zn1-xMgx Sey Te1-y layers was performed on (100) ZnTe substrate by metalorganic vapour phase epitaxy using dimethylzinc, bis-methylcyclopentadienyl-magnesium, diethyltelluride and diethylselenide. The effects of substrate temperature upon the compositions of Mg and Se have been investigated. The Mg composition in Zn1-xMgx Sey Te1-y layer is significantly enhanced at low substrate temperature. Although the Se composition decreases with decreasing the substrate temperature, Zn1-xMgx Sey Te1-y layer with a relatively high Se composition of 0.3 is obtainable at a low substrate temperature as low as 380 ºC. For all the layers, a two-mode behaviour with ZnTe- and MgTe-like longitudinal optical phonon modes is confirmed by Raman scattering. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This document is part of subvolume C2 'Optical Properties', of volume 34
'Semiconductor quantum structures' of Landolt-Börnstein, Group III,
Condensed Matter. Data for(Hg,X)Te quantum wells are provided.

This document is part of subvolume C2 'Optical Properties', of volume 34
'Semiconductor quantum structures' of Landolt-Börnstein, Group III,
Condensed Matter. It contains the Introduction and the basic properties
of bulk II-VI compounds.

This document is part of subvolume C2 'Optical Properties', of volume 34
'Semiconductor quantum structures' of Landolt-Börnstein, Group III,
Condensed Matter. Data for ZnTe quantum wells are provided.

This document is part of subvolume C2 'Optical Properties', of volume 34
'Semiconductor quantum structures' of Landolt-Börnstein, Group III,
Condensed Matter. Data for (Cd,Zn)Se quantum wells are provided.

The present work performs self-consistent ab initio full potential-linearized augmented plane wave method to study the structural, electronic and thermodynamic properties of ZnS x Se 1Àx , ZnS x Te 1Àx and ZnSe x Te 1Àx semiconductor alloys. The ground-state properties were determined for the bulk materials (ZnS, ZnSe, and ZnTe) and for their alloys in cubic phase at various concentrations (x ¼ 0:25, 0.5 and 0.75). A marginal deviation of the lattice parameter from Vegard's law was observed for ZnS x Se 1Àx and ZnSe x Te 1Àx alloys, while the lattice bowing of ZnS x Te 1Àx alloy was found to be significant. This is mainly because of the large mismatch of the lattice parameters of the binary compounds ZnS and ZnTe. A large deviation of the bulk modulus from linear concentration dependence was observed for all the three alloys. We have also investigated the effect of composition on bonding properties and correlated it to the charge-exchange effect in the optical bowing. Using the approach of Zunger et al. [21], the microscopic origins of the gap bowing were explained. The disorder parameter (gap bowing) for the alloys of interest was found to be mainly caused by the structural relaxation contributions. The charge-exchange contributions for all the three alloys were also found to be significant. The calculated phase diagram showed a broad miscibility gap for all the alloys of interest with a high critical temperature.

X-ray diffraction, photoluminescence and high-resolution transmission electron microscopy (HRTEM) were used to show that several as-grown Zn1−xMgxSe crystals prepared by the high-pressure Bridgman method exhibit polytypism for x in the range from 0.17 to 0.21. The largest contribution of polytypes is found in two unannealed crystal plates characterised by x=0.17 (polytype 8H) and 0.18 (polytype 4H). Crystal domains with disordered stacking were also found. A small quantity of 4H or 8H is detected in other sphalerite type crystals of various x values lower than 0.17 and for wurtzite type crystals for x from 0.18 up to about 0.25. Exciton transition energy for the given Mg content depends on the polytype. With Mg content increasing along the crystal, the polytype sequence observed at diffractograms (in agreement with HRTEM observations and with luminescence spectra) is 3C→8H→4H→2H. The presence of higher polytypes (4H and 8H) depends on thermal history of the crystal growth. Annealing of the as-grown crystals containing these two polytypes results in full or almost full transformation to 3C, 2H or their mixture, depending on the Mg concentration..

The correlated function expansion (CFE) methodology has been used to estimate the energy band gap and the alloy bond length of Zn1−xMgxSeyTe1−y over the entire composition space . The lattice matching condition was obtained by optimizing the alloy bond length to the bond length of the substrates (ZnTe, InAs and InP), and the corresponding band gap and its bowing effect were identified. Excellent agreement was obtained between the CFE-estimated band gap and the recent photoluminescence data for the alloy grown on ZnTe and GaAs. As the CFE only requires ternary input data, it can accelerate efforts to find desirable quaternary alloy compositions.

The present first-principle calculations have established the possibility of tuning of structural, optoelectronic and transport properties of direct-band-gap and optically active zinc-blend magnesium chalcogenides through doping of Hg atom(s) at different compositions. The process results a set of thermodynamically stable, direct-band-gap and optically active HgxMg1-xS, HgxMg1-xSe and HgxMg1-xTe semiconductor ternary alloys. Non-linear enhancement in lattice constant (a0), but reduction fundamental band-gap (Eg) takes place in each system with increasing Hg-composition (x). Electrons and light holes possess much smaller effective-mass compared to heavy holes in each specimen. Calculations show positive Seebeck coefficient and hence p-type conduction property of ternary alloys. Calculated electronic figure-of-merit shows that ternary alloys are suitable for efficient thermoelectric applications beyond 900 K temperature. Electronic excitations from valence chalcogen-p level to Mg-4s, 3p and Hg-7s levels of conduction region of band-structure near Fermi-level put significant contributions in diverse optical phenomenon in the UV region, which lead to their compatibility in fabricating diverse UV optoelectronic devices. Oscillator strength, optical energy-gap, skin-depth, optical electronegativity difference, electronic polarizability and diamagnetic susceptibility of each of the considered specimens are also calculated in the present study.

We report on molecular beam epitaxy of CdSe/CdMgSe heterostructures on InAs(001) substrates and studies of their optical and structural properties. The CdMgSe energy gap versus composition dependence is determined. The zinc-blende MgSe band-gap energy and optical bowing parameter are estimated to be 4.05 eV and 0.2 eV, respectively. The CdSe quantum wells embedded into CdMgSe barriers demonstrate intense photoluminescence. Effective mass approximation calculations of electron-heavy hole optical transitions in CdSe quantum well are in a good agreement with the experimental data obtained.

This document is part of subvolume C2 'Optical Properties', of volume 34 'Semiconductor quantum structures' of Landolt-Börnstein, Group III, Condensed Matter. Data for CdTe quantum wells are provided.

Structural and elastic properties as well as lattice dynamics of ternary MgSxSe1-x alloy have been studied using first-principles calculations. These are done using density functional theory (DFT) and density functional perturbation theory (DFPT) within the local density approximation (LDA) and employing the virtual-crystal approximation (VCA). We found that the lattice parameter, the elastic constants and the phonon frequencies follow a quadratic law in x.

This document is part of subvolume C2 'Optical Properties', of volume 34 'Semiconductor quantum structures' of Landolt-Börnstein, Group III, Condensed Matter. Data for ZnSe quantum wells are provided.

The photopumped lasing characteristics of double heterostructures with a BeZnSeTe active layer grown on InP substrates were systematically investigated. Green-to-yellow lasing emissions from 538 to 570 nm were observed at room temperature (RT). The threshold excitation power density (Pth) was approximately 30 kW/cm2. From the temperature dependence of Pth, stable lasing emissions were obtained up to 353 K. The characteristic temperatures of Pth were 106 to 140 K above RT. The relationship between the threshold gain (Gth) and the threshold carrier density (Nth) was estimated from the cavity length dependence of Pth and by waveguide analysis. Using the relationship between Gth and Nth, the threshold current densities (Jth) of electrically pumped BeZnSeTe laser diode structures were calculated to be less than 1.3 kA/cm2. Jth decreases as the lasing wavelength increases from 538 to 570 nm. The above results demonstrate that BeZnSeTe is a promising active-layer material for high-performance green-to-yellow LDs.

This document is part of subvolume C2 'Optical Properties', of volume 34 'Semiconductor quantum structures' of Landolt-Börnstein, Group III, Condensed Matter. Data for zincblende sulphide/selenide type-II quantum wells are provided.

This document is part of subvolume C2 'Optical Properties', of volume 34 'Semiconductor quantum structures' of Landolt-Börnstein, Group III, Condensed Matter. Data for telluride/selenide quantum wells are provided.

Density functional calculations are performed to study the structural and electronic properties of technologically important Zn1-xMgxSeyTe1-y quaternary alloys using the full potential-linearized augmented plane wave method. We use both Perdew-Burke-Ernzerhof and Engel-Vosko generalized gradient approximations of the exchange-correlation energy that are based on the optimization of total energy and corresponding potential, respectively. Our investigation on the effect of composition on lattice constant, bulk modulus, and band gap for pseudobinary as well as for quaternary alloys shows nonlinear dependence on the composition. The bowing of the fundamental gap versus composition predicted by our calculations is in good agreement with experimental data available for pseudobinary alloys. The presented contour maps of energy band gap and lattice constants versus concentrations could be very useful for designing new structures with desired optical properties. In addition, the energy band gap and natural band offset of zinc-blende Zn1-xMgxSeyTe1-y quaternary alloys lattice matched to ZnTe and InP substrates is investigated. The obtained results show that the quaternary alloys of interest could be an appropriate material for designing heterostructures with desired optical and interfacial properties.

Energy band gaps and electron effective mass as well as their composition dependence are the most critical parameters for band structure calculations of semiconductor alloys. Therefore, an accurate knowledge of these parameters is very important. Unfortunately, there is a limited experimental and theoretical information in the literature regarding the electronic band parameters for zinc blende ZnSxSe1−x. This has incited us to carry out such calculations. For this purpose, we have used the empirical pseudo potential method within the Virtual Crystal Approximation and the effect of compositional disorder is treated as an effective potential. The band gap variation versus sulfur concentration x shows two different behaviors: clear diminution of gap energy for low concentrations, and quasi-linear behavior with a small bowing for large values of x. Furthermore, the calculated effective mass shows that the disorder is not only compositional but also structural.

MgSe is an important component in Mg-based ternary or quaternary alloys, but is less understood due to its hygroscopy and unstable zinc blende structure. In order to get a clear knowledge of this material, we have investigated the growth of MgSe films of ZB,(zinc blende), structure on ZnTe substrates by MBE (molecular beam epitaxy). The surface reconstruction of MgSe under different flux ratios and growth temperatures have been studied systematically by RHEED (reflected high-energy electron diffraction). (4×1) reconstruction is observed under VI/II flux ratios of 16–30, while (2×1) reconstruction appears at higher VI/II ratios. The oscillation of RHEED intensity at the first stage of MgSe growth indicated that 2D growth was achieved, and thereafter the growth mode changed to 3D growth due to the lattice mismatch. The structure of the MgSe layer is further confirmed by XRD (X-ray diffraction). The XRD results indicated the growth of a MgSe film having a ZB structure for a layer below 2000Å thick. No diffraction is observed from 2000 to 4000Å thick MgSe films. Thicker samples show the formation of MgSe layers having a rock salt structure.

Measurements of electric resistivity, Hall coefficient and mobility as a function of temperature were performed over the temperature range 11–300 K in Cd1−xMgxSe and Zn1−xMgxSe mixed crystals grown by high-pressure Bridgman method. Free electron concentrations of the order of 10¹⁷ cm⁻³ can be easily achieved in Cd1−xMgxSe with 0≤x≤0.4 without any intentional doping. The obtained results show two competing conduction mechanisms; conduction-band conductivity at high temperatures and impurity-band conduction at low temperatures. The low-temperature mobility is influenced by potential barriers due to statistical distribution of charged impurities and fluctuations of composition.

We present a careful and detailed ab initio study of the crystal structure and electronic band structure of different crystalline phases of MgSe. Calculations were performed using the full-potential linear augmented plane wave method. To determine the crystal phase of the ground state of MgSe, we computed the total energy as a function of volume for the rock-salt, zinc-blende, wurtzite, and NiAs phases. From the optimized volume, and by using the Birch–Murnaghan equation of state the lattice parameters , the bulk moduli, and its first pressure derivative () for the different phases of MgSe were found. In our calculations, we have used the local density approach for the exchange–correlation part of the total energy. It was determined that in this approach the sequence of phase transformations under pressure is rock-salt–NiAs–wurzite–zinc-blende. However, we also found that under ambient conditions, the different studied phases for MgSe are energetically available. Using the modified Becke–Johnson exchange correlation potential, the calculated values of the bandgap were improved, and the results are comparable to existing experimental values for the zinc-blende phase.

ZnCdMgSe epilayers and ZnCdSe/ZnCdMgSe quantum well structures were grown on GaAs(111)A and B substrates by MBE using a low-temperature-grown CdS buffer layer. X-ray reciprocal space maps demonstrated that ZnCdMgSe epilayers grown on CdS buffer layers were predominantly hexagonal structures. In the quantum well structures, in-situ RHEED pattern also demonstrated the hexagonal structure during the growth. X-ray diffraction results showed that the crystalline quality of the ZnCdMgSe epilayers grown on GaAs(111)A surface was much better than that on the (111)B surface. Low temperature photoluminescence of ZnCdMgSe epilayers were dominated by the deep and shallow donor–acceptor pair emissions for the case of GaAs(111)A and B, respectively. The peak energy positions of the photoluminescence spectra increased with increasing the growth temperature. Although strong emissions from ZnCdMgSe barriers were observed, emissions from the ZnCdSe QW structure were very weak. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This document is part of Subvolume A 'Growth and Structuring' of Volume 34 'Semiconductor Quantum Structures' of Landolt-Börnstein - Group III 'Condensed Matter'. It contains examples of quantum wells and superlattices based on ZnTe and its alloys. Parent documents: SpringerMaterial s Volume III/ 34A Introduction to semiconductor quantum structures

Structural, mechanical and optoelectronic features of cubic BexMg1-xS,BexMg1-xSe and BexMg1-xTe alloys have been explored by DFT-based FP-LAPW approach. Nonlinear reduction in lattice constant, but increment in bulk modulus and each of the elastic constants C11, C12 and C44, occurs with increasing Be-concentration x in each system. All the specimens exhibit elastic anisotropy. Specimens at x=0.0, 0.25 and 0.50 show ductility, but remaining specimens at x=0.75 and 1.0 show brittleness. Each ternary alloy is a direct (Γ-Γ) band gap (Eg) semiconductor. Almost linear decrease in Eg with increase in x is observed in each alloy system. Ionic bonding exists among the constituents of all specimens. The occupied valence chalcogen-p as initial and unoccupied conduction Be-3s, 2p and Mg-4s, 3p as final states play a key role in optical transitions. Nature of variation of zero-frequency limit in each of the ε1(ω), n(ω) and R(ω) spectra with x is opposite to, while critical point in each of the ε2(ω), k(ω), σ(ω) and α(ω) spectra with x is similar to, the nature of variation of Eg with x.

Structural, mechanical and optoelectronic features of cubic MgxCd1−xS, MgxCd1−xSe and MgxCd1−xTe are calculated using density functional FP-LAPW approach. Exchange-correlation potentials are calculated for structural and mechanical properties with WC-GGA and optoelectronic properties with mBJ, EV-GGA and PBE-GGA schemes. Each ternary specimen exhibits thermodynamic stability. In each system, nonlinear decrease in each of the a0, B0, C11, C12 and C44 with increase in Mg-concentration x is observed. Each specimen exhibits elastic anisotropy, ductility and dominancy of ionic bonding. Each binary and ternary specimen is a direct (Γ-Γ) band gap (Eg) semiconductor. In each system, calculated Eg increases nonlinearly with increase in x. Optical excitations from chalcogen-p to Cd-6s, 5p and Mg-4s, 4p contribute intense peaks in each ε2(ω) spectra. Nature of variation of zero-frequency limit in each of the ε1(ω), n(ω) and R(ω) spectra and critical point in each of the ε2(ω), k(ω), σ(ω) and α(ω) spectra with x is opposite and similar, respectively, to the nature of variation of Eg with x.

Density functional calculations of structural and optoelectronic properties of cubic MgxZn1−xSyTe1−y quaternary alloys are performed with mBJ, EV-GGA and PBE-GGA functional. Each quaternary specimen is direct band gap (Γ-Γ) semiconductor. Lattice constant reduces, while bulk modulus and band gap enhances nonlinearly with increase in S-concentration y at each Mg-concentration x. Again, nonlinear increment in lattice constant and band gap, but reduction in bulk modulus is found with increase in Mg-concentration x at each S-concentrations y. Contour plots for lattice constants and band gaps would be useful in fabricating new quaternary alloys with preferred optoelectronic features. Chalcogen-p→Zn-5s,4p; Mg-4s,4p optical excitations contribute intense peaks in each ε2(ω) spectra. Composition dependence of each calculated zero-frequency limit shows opposite trend, while each calculated critical point shows similar trend of composition dependence of band gap. Calculations also suggested the possibility of growth of several said quaternary alloys on ZnTe, InAs and InP substrates.

In this work, we use the full potential linearized augmented plane wave (FP-LAPW + lo) method under the framework of spin polarization density functional theory (SP-DFT) to study the structural properties of diluted magnetic semiconductors alloys, Mg1−xCrxSe (x = 0.25, 0.50 and 0.75), in the rock–salt ferromagnetic phase by using the WC-Cohen GGA approximation. To compute the electronic and magnetic properties of our compounds, we have used the Tran Blaha modified Becke–Johnson potential (TB-mBJ) combined with PBE approximation. The results of electronic properties show that our compounds have a half-metallic behavior with a spin polarization of 100% at the Fermi level. Moreover, at all concentrations, the magnetic moment has been estimated as equal to 4.0 (μB). Furthermore, to validate the effects resulting from the exchange splitting process, we calculate the values of the spin-exchange constants N0α and N0β, respectively. Finally, we present in detail the effects of the temperature and the pressure on some macroscopic thermodynamic parameters by using the quasi-harmonic Debye model.

Alkaline earth chalcogenides (AECs) are very important compounds because of these possess semiconducting properties besides having large band gap mostly of the order of 7-10 eV which is the characteristic properties of insulators. These compounds are having many important optoelectronic properties, which serves its role in the production of many electronic devices. These are found in many crystallographic phases such as rock salt (B1), zinc blende (B3), wurtzite (B5) and nickel arsenide (B8) phase. A de-Launay angular force (DAF) model has been used to study the interatomic interactions and phonons of MgX (X=S, Se, Te) in zinc blende structure. The interatomic interaction in the form of central and angular forces up to second nearest neighbors has been considered. The interatomic interaction Mg-X is found to be strongest and its value is highest for MgS compared to others. This is because of small bond length in MgS compared to others. Zone centre phonons have been calculated for MgX and are in agreement with other available results. The phonon dispersion curves in three high symmetric direction are calculated for MgX (X=S, Se, Te) and are interpreted in light of other existing results.

The band gaps and optoelectronic properties of binary calcium chalcogenide semiconductors have been modified theoretically by doping magnesium atom(s) into their respective rock-salt unit cells at some specific concentrations x = 0.0, 0.25, 0.50, 0.75 and 1.0 and confirmed such modifications by studying their structural, electronic and optical properties using DFT based FP-LAPW approach. The WC-GGA functional is used to calculate structural properties, while mBJ, B3LYP and WC-GGA are used for calculating electronic and optical properties. The concentration dependences of lattice parameter, bulk modulus and fundamental band gap for each alloy system exhibit nonlinearity. The atomic and orbital origin of different electronic states in the band structure of each compound are explored from its density of states (DOS). The microscopic origin of band gap bowing for each of the alloy systems is explored in terms of volume deformation, charge exchange and structural relaxation. The chemical bonds between the constituent atoms in each compound are found as ionic in nature. Optical properties of each specimen are calculated from its computed spectra of dielectric function, refractive index, extinction coefficient, normal incidence reflectivity, optical conductivity, optical absorption and energy loss function. Several calculated results have been compared with available experimental and other theoretical data.

Density functional theory (DFT) based full-potential linearized augmented plane wave (FP-LAPW) methodology has been employed to investigate theoretically the structural, electronic and optical properties of MgxBa1−xS, MgxBa1−xSe and MgxBa1−xTe ternary alloys for 0 ≤ x ≤ 1 in their rock-salt (B1) crystallographic phase. The exchange-correlation potentials for the structural properties have been computed using the Wu-Cohen generalized-gradient approximation (WC-GGA) scheme, while those for the electronic and optical properties have been computed using both the WC-GGA and the recently developed Tran-Blaha modified Becke-Johnson (TB-mBJ) schemes. The thermodynamic stability of all the ternary alloys have been investigated by calculating their respective enthalpy of formation. The atomic and orbital origin of different electronic states in the band structure of the compounds have been identified from the respective density of states (DOS). Using the approach of Zunger and co-workers, the microscopic origin of band gap bowing has been discussed in term of volume deformation, charge exchange and structural relaxation. Bonding characteristics among the constituent atoms of each of the specimens have been discussed from their charge density contour plots. Optical properties of the binary compounds and ternary alloys have been investigated theoretically in terms of their respective dielectric function, refractive index, normal incidence reflectivity and optical conductivity. Several calculated results have been compared with available experimental and other theoretical data.

The growth of undoped and phosphorus (P)-doped Zn1−xMgxSeyTe1−y layers on (100) ZnTe substrates by metalorganic vapor phase epitaxy was carried out. The compositions of Mg and Se, surface morphology, roughness and Raman property were characterized as a function of substrate temperature. Not only the compositions of Mg and Se but also the crystal quality of undoped Zn1−xMgxSeyTe1−y layer strongly depended upon the substrate temperature. Furthermore, the growth of Zn1−xMgxSeyTe1−y layer nearly-lattice-matched to ZnTe substrate was achieved independent of the transport rate of trisdimethylaminophosphorus. Undoped Zn1−xMgxSeyTe1−y layer nearly-lattice-matched to ZnTe led to improvement of surface roughness. On the other hand, P doping brought about deterioration of crystalline quality.

Zn1-xMgxSeyTe1-y layers have been grown on (100) ZnTe substrates by low-pressure metalorganic vapor phase epitaxy using dimethylzinc, bis-methylcyclopentadienyl magnesium ((MeCp)2Mg), diethylselenide and diethyltelluride as source materials. The Mg mole fraction of Zn1-xMgxSeyTe1-y layer can be controlled successfully by varying (MeCp)2Mg transport rate. P-type doping of this material has been tried using tris-dimethylaminophosphorus as a dopant source. The influence of annealing temperature or dopant transport rate upon the electrical property has been investigated. All the layers show p-type conduction even without annealing. Through this study, a maximum carrier concentration of 2.5×10¹⁸ cm⁻³ is obtained.

The photopumped lasing characteristics of double heterostructures with a BeZnSeTe active layer grown on InP substrates were systematically investigated. Green-to-yellow lasing emissions from 538 to 570 nm were observed at room temperature (RT). The threshold excitation power density (Pth) was approximately 30 kW/cm². From the temperature dependence of Pth, stable lasing emissions were obtained up to 353 K. The characteristic temperatures of Pth were 106 to 140 K above RT. The relationship between the threshold gain (Gth) and the threshold carrier density (Nth) was estimated from the cavity length dependence of Pth and by waveguide analysis. Using the relationship between Gth and Nth, the threshold current densities (Jth) of electrically pumped BeZnSeTe laser diode structures were calculated to be less than 1.3 kA/cm². Jth decreases as the lasing wavelength increases from 538 to 570 nm. The above results demonstrate that BeZnSeTe is a promising active-layer material for high-performance green-to-yellow LDs.

We report on pseudomorphic MBE growth of CdTe/Zn(Mg)(Se)Te quantum dot (QD) structures on InAs(100) substrates and studies of their structural and optical properties. The QDs were fabricated by using a thermal activation technique comprising deposition of a strained CdTe 2D layer, covering it with amorphous Te, followed by fast thermal desorption of the Te layer, which results in a 2D-3D RHEED pattern transition. The QDs exhibit the surface density as low as ~10¹⁰ cm⁻². The influence of MBE growth parameters and the structure design on photoluminescence properties of the QDs are discussed. Single QD photoluminescence was observed at T=8 K from the 200-nm-wide mesa-structures made of the CdTe QD structures, and the antibunching effect with g⁽²⁾(0) = 0.16±0.04 was demonstrated. The peculiarities of MBE growth of ZnTe/MgTe/MgSe short-period superlattices nearly lattice-matched to InAs, which could serve as wide gap barriers for efficient electron and hole confinement in the CdTe/Zn(Mg)(Se)Te QDs, are also described.

The correlation between the optical (such as complex refractive index, energy band gap) and thermal (such as diffusivity, effusivity and conductivity) properties of Zn1−xMgxSe (with 0 ≤ x ≤ 0.50) crystals are presented. The Zn1−xMgxSe mixed crystals were grown by the modified high-pressure Bridgman method and were studied using spectroscopic ellipsometry as well as photopyroelectric technique. We found that when Mg content in Zn1−xMgxSe increases the value of Eg increases. Moreover, the values of thermal conductivity and refractive index of studied materials decrease with increasing Mg content in Zn1−xMgxSe. Therefore, from the performed measurements, one can conclude that the incorporation of Mg as constituent into ZnSe crystals plays a crucial role in the observed optical and thermal response.

The structural, electronic and optical properties of MgxPb1−xS, MgxPb1−xSe and MgxPb1−xTe alloys for 0 ≤ ≤ 1 in their rock-salt (B1) crystallographic phase have been calculated using the full-potential linearized augmented plane wave (FP-LAPW) method under the framework of density functional theory (DFT). Using the Wu-Cohen generalized-gradient approximation (WC-GGA) induced exchange-correlation potential scheme, the ground state structural parameters such as equilibrium lattice constants, bulk modulus and its pressure derivatives are calculated and deviations of the lattice constants from Vegard’s law and the bulk modulus from linear concentration dependence have been observed for the alloys. Electronic band structures and density of states have been calculated using Tran-Blaha modified Becke-Johnson (TB-mBJ) parameterization scheme to study the electronic properties of the binary compounds and their ternary alloys. Using the approach of Zunger and co-workers, the microscopic origins of band gap bowing have been discussed in term of volume deformation, charge exchange and structural relaxation. Optical properties of the binary compounds and their ternary alloys have been calculated in terms of their respective dielectric function, refractive index, reflectivity and optical conductivity. Few calculated results are compared with available experimental and other theoretical data.

Forming solid solutions in PbTe based materials can simultaneously reduce lattice thermal conductivity and engineer the band structure to enhance the electrical properties. In this work, quarternary alloys of Pb1-xMgxTe0.8Se0.2 are designed to improve the figure of merit zT. The significant roles of MgTe in enhancing electrical properties and reducing thermal conductivity of PbTe0.8Se0.2 are investigated. A maximum zT of ∼ 2.2 at 820 K has been achieved in PbTe0.8Se0.2 with 8% MgTe. Subsequently, a large dimension bulk (∼200 g, Φ42 mm × 18 mm) is fabricated and its homogeneity and the repeatability of high zT values are checked. It is shown that high zT ∼ 2.0 can also be achieved even in such a large sample. These results highlight the multi-functional roles of quarternary alloying with Mg and Se, and demonstrate the realistic prospect of large-scale commercial fabrication in high performance PbTe-based thermoelectric materials.

We study the chemical ordering in Bi2Te3-x Se x grown by molecular beam epitaxy on Si substrates. We produce films in the full composition range from x = 0 to 3, and determine their material properties using energy dispersive x-ray spectroscopy, x-ray diffraction and Raman spectroscopy. By fitting the parameters of a kinetic growth model to these results, we obtain a consistent description of growth at a microscopic level. Our main finding is that despite the incorporation of Se in the central layer being much more probable than that of Te, the formation of a fully ordered Te-Bi-Se-Bi-Te layer is prevented by kinetic of the growth process. Indeed, the Se concentration in the central layer of Bi2Te2Se1 reaches a maximum of only ≈75% even under ideal growth conditions. A second finding of our work is that the intensity ratio of the 0 0 12 and 0 0 6 x-ray reflections serves as an experimentally accessible quantitative measure of the degree of ordering in these films.

In the case of technologically important quaternary alloys, structural and optoelectronic properties have been calculated with density functional theory (DFT)-based full-potential linearised augmented plane-wave (FP-LAPW) approach. The Perdew–Burke–Ernzerhof generalised gradient approximation (PBE-GGA) for structural properties and both the modified-Becke–Johnson (mBJ) and Engel and Vosko GGA (EV-GGA) for optoelectronic properties are employed to calculate the respective exchange-correlation potentials. Each specimen within the quaternary system is a direct band-gap – semiconductor. The lattice constant decreases, while bulk modulus and band gap increase nonlinearly with increasing anionic (S) concentration at each cationic (Mg) concentration . On the other hand, nonlinear increment in lattice constant and band gap, but decrement in bulk modulus is found with increase in cationic concentration at each anionic concentration . Calculated contour maps for lattice constants and energy band gaps would be useful in fabricating new quaternary alloys with preferred optoelectronic features. Optical properties of the specimens within the quaternary system show several interesting features. , 4p and 4p optical excitations contribute intense peaks in each spectrum. The composition dependence of each calculated zero-frequency limit shows opposite trend, while each calculated critical point shows similar trend of composition dependence of band gap. Moreover, calculations suggest the possibility of growth of several cubic quaternary specimens on GaAs and InP substrates.

Novel MgSe/ZnSeTe II–VI compound superlattice quasi-quaternaries (SL-QQs) were grown on InP substrates by molecular beam epitaxy with p-type doping using a RF-radical nitrogen source. The SL-QQs with various equivalent Mg compositions (xMg) were prepared by changing the MgSe layer composition in the superlattice. The photoluminescence peak energy at 15K increased from 2.11 to 2.68eV with increasing xMg from 0 to 0.76. A hole concentration over 2×1018cm−3 was obtained for xMg of 0.14, and the doping level monotonically decreased with increasing xMg. By changing the doping process, the doping property was improved, and a hole concentration of about 3×1017cm−3 was realized even for xMg of 0.4. ZnCdSe/MgZnCdSe(Te) light emitting diodes were fabricated on InP substrates using MgSe/ZnSeTe SL-QQs as p-side cladding layers. Yellow light emissions around 577nm were observed under a pulsed current injection at 77K.

We have used x‐ray photoelectron spectroscopy to measure the valence‐band offsets for the lattice matched MgSe/Cd 0.54 Zn 0.46 Se and MgTe/Cd 0.88 Zn 0.12 Te heterojunctions grown by molecular beam epitaxy. By measuring core level to valence‐band maxima and core level to core level binding energy separations, we obtain values of 0.56±0.07 eV and 0.43±0.11 eV for the valence‐band offsets of MgSe/Cd 0.54 Zn 0.46 Se and MgTe/Cd 0.88 Zn 0.12 Te, respectively. Both of these values deviate from the common anion rule, as may be expected given the unoccupied cation d orbitals in Mg. Application of our results to the design of current II‐VI wide band‐gap light emitters is discussed.

We present high resolution spectra of excitons, shallow donors and acceptors in ZnTe epilayers grown on GaAs and GaSb by atmospheric pressure metal-organic vapor-phase epitaxy (MOVPE). Resonant excitation made the observation of selective-pair luminescence (SPL), two-electron transitions (TET) and two-hole transitions (THT) possible. The investigation of donor states in I-doped layers yields m∗e = 0.117m0 and a static dielectric constant ϵst = 9.4. The Luttinger parameter γ1 = 3.8 was obtained from 1s-and 2s-free exciton transitions. As-acceptor states were observed in strain-free layers. A fit to calculations of Baldareschi and Lipari leads to γ2 = 0.72 and γ3 = 1.3. Level shift and splitting in magnetic fields corroborated the present assignments. The magnetic parameters KA = -0.27 and qA = -0.015 were obtained from As-acceptor bound excitons and the first excited acceptor state.

The valence band offset between ZnMgSe and ZnTe is studied as a function of the Mg content. The band alignment was deduced from photoluminescence measurements on multi-quantum-well structures consisting of alternated layers of Zn1−xMgxSe and ZnTe. The thickness of each layer was 100 Å to reduce errors due to strain or confinement in the wells. We measured a valence band offset of 880 meV between ZnSe and ZnTe and observed a transition from a type II to type I line-up in the ZnMgSeZnTe system for a Mg concentration of about 60%. Our results indicate that the increase in bandgap caused by the addition of Mg to ZnSe changes only the position of the conduction band edge, while the valence band remains constant with respect to the vacuum level.

We present photoluminescence spectra from CdZnj_Te /ZnTe and ZnSe,,Tei_ /ZnTe strained
layer superlattices grown by MBE, and analyze the band alignments and strain effects. Our results
are based on fitting the dominant photoluminescence peaks to the superlattice band structure obtamed
by k "theory. We find that the valence band offset of the CdZniTe /ZnTe system is quite
small. On the other hand, the photoluminescence data from the ZnSeTei_ /ZnTe superlattices
suggest that the band alignment is type II, with a large valence band offset. We also investigate
the band gap bowing in the ZnSeTej_ alloys, and determine the individual components of the
bowing in valence and conduction bands. Based on our results for band alignments, we evaluate the
prospects for minority carrier injection in wide bandgap heterostructures based on ZnSe, ZnTe, and
CdTe.

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.

The pressure dependence of the low-temperature photoluminescence of CdTe/ZnTe strained-layer superlattices is reported, up to the phase transition of the structure at about 6 GPa. The superlattices can be simultaneously type I for heavy holes and type II for light holes. A theoretical fit to the pressure dependence of the type-I and type-II luminescence lines confirms a number of bulk and superlattice parameters. In particular, the results are consistent with a treatment of the band structure in the framework of the envelope-function approach. The ground valence-band state is found to be the first light-hole valence subband. The valence-band offset has been fitted to be pressure dependent; we found 75+4.5P meV. Under pressure, a type-I-type-II transition is observed, due to a crossover in the valence band of the superlattice.

The problem of doping in II-VI semiconductors is examined using an ab initio pseudopotential approach with emphasis on ZnSe, ZnTe, MgSe, and zinc blende MgTe. The n-type doping of ZnTe and MgTe is found to be hindered by the formation of localized DX-like deep donor centers. DX centers are found to be energetically unfavorable in ZnSe and MgSe. The possibility of obtaining large band gap MgZnSeTe semiconductor alloys that can be doped low-resistance n- and p-type is discussed.