Article

Optical properties of ZnTe/Zn1−xMgxSeyTe1−y quantum wells and epilayers grown by molecular beam epitaxy

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Abstract

We have investigated optical properties of ZnTe epilayers, Zn1−xMgxSeyTe1−y epilayers, and ZnTe/Zn1−xMgxSeyTe1−y quantum wells (QWs) grown on (100)-InAs substrates by molecular beam epitaxy. We observed several sharp photoluminescence lines close to the excitonic position and no detectable luminescence from deep levels in ZnTe epilayers. Bright luminescence has been obtained from Zn1−xMgxSeyTe1−y epilayers which are lattice matched with InAs. The band alignment of ZnTe/Zn1−xMgxTe QWs was found to be type I. The reduction of the band gap energy of the ZnTe layer due to a tensile strain was confirmed in this structure. Nearly lattice-matched ZnTe/Zn1−xMgxSeyTe1−y QWs have been fabricated. A type II band alignment was observed for many of these QWs. We estimated bowing parameters not only of the band gap but also of the valence band for ZnSeyTe1−y. © 1997 American Institute of Physics.

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... It is also found that molecular-beam epitaxy (MBE) or metalorganic vapour phase epitaxy (MOVPE) growth of ternary or quaternary alloys of magnesium chalcogenides [5,6,[10][11][12][13][14] and beryllium chalcogenides [65][66][67][68][69] on GaAs-InAs-InP substrates as well as their characterizations by SE, X-ray diffraction (XRD), photoluminescence (PL) techniques, etc. established them as suitable quantum well materials for visible radiation emitters and for designing laser diodes capable of operating in UV-VIS spectral range. ...
... Also, available experimental and previously calculated structural data for some of the specimens are also presented in table 1 for comparison. Table 1 shows that our calculated a 0 for magnesium chalcogenides as well as a 0 , B 0 and B 0 data for beryllium chalcogenides are in good agreement with the respective experimental investigations for MgS, MgSe and MgTe [4][5][6][10][11][12][13] and BeS, BeSe and BeTe [29,30]. Compared to some previously calculated a 0 , B 0 and B 0 data for MgS, MgSe and MgTe [16][17][18]20,21,26,[70][71][72][74][75][76] as well as BeS, BeSe and BeTe [39,41,42,44,45,48,50,52,60,78,79,81,83,85], good agreement is observed between our calculated results and some of the respective previously calculated data. ...
... The substitution of Mg atom(s) of higher radius (r Mg = 1.45 Å) with beryllium atom(s) of lower radius (r Be = 1.12 Å) decreases the volume of the cubic unit cell and Experimental data ⇒ a Ref. [4], b Ref. [5], c Ref. [6], d Ref. [10], e Ref. [11], f Ref. [12], g Ref. [13], h Ref. [29], i Ref. [30]. Earlier theoretical data ⇒ a1 Ref. [18], b1 Ref. [17], c1 Ref. [16], d1 Ref. [26], e1 Ref. [20], f1 Ref. [21], g1 Ref. [72], h1 Ref. [71], i1 Ref. [75], j1 Ref. [74], k1 Ref. [70], l1 Ref. [76]. ...
Article
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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.
... Several experimental studies on the electronic properties [20][21][22][23][24], optical properties [25,26], elastic properties [27,28] etc. on ZnSe and ZnTe have established their superiority in different areas of microelectronic and optoelectronic applications. 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]. ...
... Experiments were performed for preparation and characterization of structural, morphological, electrical and optical properties of nano-crystalline cubic MgSe thin films [33,34], molecular beam epitaxial (MBE) growth of ZnSe thin films and epilayers on GaAs substrates and their optical characterizations [35][36][37], molecular beam epitaxial (MBE) growth of cubic MgTe thin films as suitable materials for optoelectronic applications in the entire visible range [38]. Moreover, MBE growth of ZnTe epilayers on InAs substrates and their optical and structural characterizations [30], structural and optical characterizations of molecular beam epitaxially (MBE) grown ZnTe epilayers on GaAs substrates with ZnSe/ZnTe strained superlattices buffer layers [39], electrical characterizations of ZnTe thin films, deposited by thermal evaporation method on n-type Si substrate [40], studies of Raman spectroscopy and photoluminescence of molecular-beam epitaxially (MBE) grown ZnTe thin films on GaAs substrates [41], etc. have also been performed for their different potential applications. ...
... 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. ...
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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.
... Consequently we had to rely only on the results of optical measurements. A way to determine Mg content is through the graph presented in Appendix E and using the fit of the data reported at [67]. According to this fit, the emission at 2520 meV corresponds to a Mg content near 19% -20% . ...
... The determination of MgTe emission energy is very difficult due to the fact that this compound is unstable in standard conditions. There are numerous works reported in literature where the authors tried to extrapolate it, by studying the emission of Zn x Mg 1−x Te and Cd x Mg 1−x Te alloys for different Mg content [100,126,127,67]. In order to analyze our data we addressed to all these works as it was necessary to deduce an accurate fit for calculating Mg content for both core -shell ZnTe -ZnMgTe nanowires and the ZnMgTe nanowires containing a ZnTe quantum dot. ...
Thesis
In this work we investigated the valence band ground state properties of nanowire quantum dots based on the II-VI materials. The main objective was to prove experimentally the stabilization of a light hole ground state in the nanowire quantum dot and understand which parameters influence the purity of the valence band ground state. The two main factors which determine the switching between heavy and light hole and their mixing is confinement and mismatch induced strain. These parameters can be tuned by modifying the length to diameter aspect ratio of the quantum dot and by choosing properly the material which surrounds it in order to maintain confinement of the hole inside the quantum dot.The effect of strain and confinement was studied extensively by $vec{k}cdot vec{p}$ theory on nanowire quantum dots similar to those we studied with optical measurements. More specifically we investigated the hole ground state properties of both compressive CdTe quantum dots in ZnTe nanowire and tensile ZnTe quantum dots surrounded by ZnMgTe. Strain was tuned by modifying the aspect ratio of the quantum dot and by depositing an external ZnMgTe shell to the ZnTe core. The effect of confinement was investigated by changing the valence band offset between the core and the dot and switching from a strong type I to a strong type II. Additionally, for the CdTe quantum dots we carried out calculations also under the presence of an exchange field, in order to study the spin properties of the ground state through the giant Zeeman shift. These calculations revealed a strong renormalization of the light hole Land'e factor due to a combined effect of elastic strain and spin-orbit coupling.The nanowires were grown by molecular beam epitaxy in our group and the electronic properties of the quantum dots inserted in them, were studied by low temperature micro-photoluminescence spectroscopy. The study of the excitonic properties (identification of confined excitons, cathodoluminescence, autocorrelation) and the degree of polarization, allowed us to identify without ambiguity the presence of light holes in the valence band ground state, in agreement to what is expected from theoretical predictions.In order to investigate the spin properties of a light hole ground state, we carried out measurements on magnetic quantum dots containing Mn atoms (concentration in the order of 10%). These quantum dots were characterized by magneto-optical spectroscopy under strong magnetic fields, up to 11 T. This study was carried out for different magnetic field configurations, using both a uniaxial and a vectorial magnet (magnetic fields applied parallel and perpendicular to the nanowire axis, rotating magnetic fields). The presence of a light hole ground state was confirmed through a quantitative study of the excitonic giant Zeeman shift. Light hole presence was manifested through the formation of an exciton magnetic polaron characterized by anisotropic magnetic properties, which were observed for the first time. The experimental data were fitted in very good agreement with a quantitative model which was developed, using the results obtained from numerical calculations.
... h Ref. [60]. i Ref. [61]. j Ref. [23]. ...
Article
Ab initio calculations based on density functional theory using the full-potential linearized augmented plane wave method have been carried out to find the structural stability of different crystallographic phases, the pressure-induced phase transition and the electronic properties of the magnesium chalcogenides MgS, MgSe and MgTe. The zinc blende (B3), wurtzite (B4), rock salt (B1), CsCl (B2), NiAs (B8), β-BeO, 5-5 and TiP crystal structures are considered and the exchange and correlation potential is treated by the generalized-gradient approximation using the Perdew–Burke–Ernzerhof parameterization. Moreover, the modified Becke-Johnson (mBJ) scheme is also applied to optimize the corresponding potential for the band structure calculations. Results show that the wurtzite phase is the stable structure in the ground state adopted by MgSe and MgTe compounds while MgS adopts the rock-salt one. Moreover, the band structure calculations reveal a metallic behavior in the CsCl structure for all the compounds, whereas for the other structures, a semiconducting behavior is observed.
... Since the Zn 1−x Mg x Te has a larger lattice constant than ZnTe, the quantum wells are placed under biaxial tensile strain. The strain-splitting parameter S was obtained in each case from reflectivity spectra (see Sec. II B): the values of S are about one-half of those predicted by using the parameters of Ref. 15 and by assuming that the wells are fully strained to fully relaxed bottom barriers. ...
... The potential of such materials for use in different application areas has been established based on experimental studies of the electronic structure [22][23][24][25], optical properties [26,27], and elastic properties [28,29] of bulk CdSe, CdTe, ZnSe, and ZnTe. Experimentally, growth and optical characterization of CdSe thin films [30], characterization of the photosensitivity of CdSe thin films [31], photovoltaic applications of CdTe thin films [32], optical characterization of nanocrystalline CdTe thin films for solar cell applications [33], optical characterization of ZnSe thin films [34], deposition as well as structural and electrical characterization of ZnTe thin films [35], photoluminescence study of ZnTe thin films [36], structural and optical characterization of ZnTe epilayers [37], etc. have been performed for use in different potential applications. ...
Article
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The structural and optoelectronic properties of technologically important CdxZn1−xSeyTe1−y quaternary alloys have been calculated using the density functional theory (DFT)-based full potential (FP)-linearized augmented plane wave (LAPW) approach. The exchange–correlation potentials are calculated using the Perdew–Burke–Ernzerhof (PBE)-generalized gradient approximation (GGA) scheme for the structural properties and both the modified Becke–Johnson (mBJ) and Engel–Vosko (EV)-GGA schemes for the optoelectronic properties. A direct bandgap (Γ–Γ) is observed for all the examined compositions in the CdxZn1−xSeyTe1−y quaternary system. At each cationic (Cd) concentration x, the lattice constant decreases while the bulk modulus and bandgap increase nonlinearly with increasing anionic (Se) concentration y. On the other hand, a nonlinear increase in the lattice constant but a decrease in the bulk modulus and bandgap are observed with increasing cationic concentration x at each anionic concentration y. The contour maps calculated for the lattice constant and energy bandgap will be useful for designing new quaternary alloys with desired optoelectronic properties. Several interesting features are observed based on the study of the optical properties of the alloys. The compositional dependence of each calculated zero-frequency limit shows the opposite trend, while each calculated critical point shows a similar trend, with respect to that found for the compositional dependence of the bandgap. Finally, the results of these calculations suggest that ZnTe, InAs, GaSb, and InP are suitable substrates for the growth of several zincblende CdxZn1−xSeyTe1−y quaternary alloys.
... Since the Zn 1−x Mg x Te has a larger lattice constant than ZnTe, the quantum wells are placed under biaxial tensile strain. The strain-splitting parameter S was obtained in each case from reflectivity spectra (see Sec. II B): the values of S are about one-half of those predicted by using the parameters of Ref. 15 and by assuming that the wells are fully strained to fully relaxed bottom barriers. ...
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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.
Article
Density functional calculations of structural and optoelectronic properties of cubic Zn1-x-yBexMgySe quaternary alloys are carried out considering their nearly lattice matching to GaAs substrate. Calculations ensure that each quaternary alloy is a direct band gap (Γ−Γ) semiconductor. The mBJ-GGA based computed minimum band gap of each alloy is larger that with EV-GGA scheme. Enhancement in beryllium or magnesium composition nonlinearly reduces the lattice constant, but enhances the bulk modulus and minimum band gap of quaternary alloys. Lower effective mass of electrons compared to holes confirms dominant role of electrons in carrier transportation in each specimen. Electronic transitions from occupied Se-4p state of valence band to unoccupied Zn-5s, Mg-3p, Mg-4s, Be-2p and Be-3s states of conduction band collectively contribute intense peaks in ε2(ω) spectra of each quaternary alloy. Quaternary semiconductor with higher band gap possesses lower value of zero-frequency limits in ε1(ω), n(ω) and R(ω) spectra, but requires higher critical point energies in ε2(ω), k(ω), σ(ω) and α(ω) spectra and vice versa. Computed oscillator strength of each quaternary alloy confirms the presence of sufficient number (>200) of electrons in the unoccupied states of conduction band above 27.0 eV of incident photon energy during any optical excitation.
Article
Mechanical properties of zinc-blende MgxZn1−xSyTe1−y alloys and cationic (Mg) and anionic (S) composition dependence of these properties have been investigated theoretically through First-principles calculations. The elastic stiffness constants increase nonlinearly with increasing sulfur concentration at each fixed magnesium concentration, while each of them reduces with increasing magnesium concentration at each fixed sulfur concentration in any binary–ternary/ternary–quaternary system. Hardness of specimens increases nonlinearly with increasing sulfur concentration at each fixed magnesium concentration, while it reduces with increasing magnesium concentration at each fixed sulfur concentration in any binary–ternary/ternary–quaternary system. Each of the considered specimens is mechanically stable, ductile, elastically anisotropic, fairly compressible and plastic in nature. Blending of covalent and ionic bonding with domination of covalent, bending over stretching in chemical bonds and central character of force between atoms have been observed in each specimen. Computed Debye temperature confirms MgS as the hardest and ZnTe as the softest among all the considered specimens. Computed Gruneisen parameter of each specimen demonstrates anharmonic character of atom–atom interactions in each crystal. Thermal conductivity and melting temperature of each of the considered specimens have also been computed in the present study.
Article
Elastic and thermal properties of zinc-blende MgxZn1-xSySe1-y quaternary alloys and their constituent binary/ternary compounds have been computed through first principles calculations. Elastic stiffness constants of specimens have been increased almost linearly with increasing sulfur composition at any fixed magnesium composition, while reverse trends have been observed with increasing magnesium composition at any fixed sulfur composition in each binary–ternary/ternary–quaternary system. Hardness of specimens has been increased almost linearly with increasing sulfur composition at any fixed magnesium composition, while it has been decreased with increasing magnesium composition at any fixed sulfur composition in each system. Mechanical stability, elastic anisotropy, compressibility, ductility and plasticity have been observed in each compound. Mixture of covalent and ionic bonding with prominent role of covalent nature, dominating role of bond bending over stretching and central nature of interatomic forces have been investigated in each compound. Interaction between the atoms in any compound has been observed to be anharmonic in nature via calculated Gruneisen parameter. Computed Debye temperature, Debye frequency, thermal conductivity and melting temperature of all the specimens have also been reported.
Article
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.
Article
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.
Article
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.
Article
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.
Article
Structural, electronic, and optical properties of alloys BexMg1-xX (X = S, Se, Te) in the assortment 0 < x < 1 were theoretically reported for the first time in zinc-blende (ZB) phase. The calculations were carried out by using full-potential linearized augmented plane wave plus local orbitals (FP-LAPW+lo) formalism contained by the framework of density functional theory (DFT). WuCohen (WC) generalized gradient approximation (GGA), based on optimization energy, has been applied to calculate these theoretical results. In addition, we used Becke and Johnson (mBJ-GGA) potential, modified form of GGA functional, to calculate electronic structural properties up to a high precision degree. The alloys were composed with the concentrations x = 0.25, 0.5, and 0.75 in pursuance of special quasi-random structures (SQS) approach of Zunger for the restoration of disorder around the observed site of alloys in the first few shells. The structural parameters have been predicted by minimizing the total energy in correspondence of unit cell volume. Our alloys established direct band gap at different concentrations that make their importance in optically active materials. Furthermore, density of states was discussed in terms of the contribution of Be and Mg s and chalcogen (S, Se, and Te) s and p states and observed charge density helped us to investigate the bonding nature. By taking into consideration of immense importance in optoelectronics of these materials, the complex dielectric function was calculated for incident photon energy in the range 015 eV.
Article
First-principles calculations based on the full potential muffin-tin orbitals method (FP-LMTO) within the local density approximation (LDA) and generalized gradient approximation (GGA) used to study the structural, electronic and optical properties of ternary alloy are presented. The lattice parameter, bulk modulus, energy gap, refractive index, optical dielectric constant and effective masses for ternary alloy with compositions x = 0, 0.25, 0.5, 0.75, 1 are investigated. The refractive index and optical dielectric constant using specific models are verified. Our calculated results are in good agreement with the available theoretical and experimental data.
Article
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.
Article
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.
Article
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.
Article
A firs t-princi pal technique is empl oyed to investigate the concen tration dep endenc e of the structu ral, electronic band str ucture, optical and che mical bon ding properties of Zn1 - xMgxS, Zn1 - xMgxSe and Zn1 - xMgxTe alloys. Structu ral parameters such as lattice constants and bulk mod uli are found to vary non-lin early wit h chan ging concen tration x and deviating from Ve gard's law. Parent binarie s as well as ternary alloys have a direct band gap ( Γ– Γ) which in creases non-l inearly wit h increment in concen tration. Chemic al bonding nature chan ges from strong covalency to part ial ionic character in increasing Mg-contents. The direct band gap and high optical ac tivity in visi ble and ultraviolet range reveal the impli cation of these all oys in the optoelectronic devices appl ications
Article
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.
Article
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.
Article
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.
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Very recently, in the 2011 version of the Wien2K code, the long standing shortcome of the codes based on Density Functional Theory, namely, its impossibility to account for the experimental band gap value of semiconductors, was overcome. The novelty is the introduction of a new exchange and correlation potential, the modified Becke-Johnson potential (mBJLDA). In this paper, we report our detailed analysis of this recent work. We calculated using this code, the band structure of forty one semiconductors and found an important improvement in the overall agreement with experiment as Tran and Blaha [ Phys. Rev. Lett. 102, 226401 (2009)] did before for a more reduced set of semiconductors. We find, nevertheless, within this enhanced set, that the deviation from the experimental gap value can reach even much more than 20%, in some cases. Furthermore, since there is no exchange and correlation energy term from which the mBJLDA potential can be deduced, a direct optimization procedure to get the lattice parameter in a consistent way is not possible as in the usual theory. These authors suggest that a LDA or a GGA optimization procedure is used previous to a band structure calculation and the resulting lattice parameter introduced into the 2011 code. This choice is important since small percentage differences in the lattice parameter can give rise to quite higher percentage deviations from experiment in the predicted band gap value.
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Full-text available
We propose a new device structure for obtaining visible light emission from wide band gap semiconductors. This heterojunction structure avoids ohmic contacting problems by using only the doping types which tend to occur naturally in II‐VI semiconductors, while using a novel injection scheme to obtain efficient minority carrier injection into the wider band gap semiconductor. To verify this proposal we have fabricated green light emitting structures using n‐CdSe and p‐ZnTe regions separated by a graded Mg x Cd 1-x Se injection region. Room temperature electroluminescence spectra from these devices demonstrate the effectiveness of the injection scheme, while the current‐voltage characteristics show the merits of avoiding difficult ohmic contacts. We further show how the structure can be extended to blue wavelengths and beyond by opening up the band gap of the ZnTe recombination region with a Mg y Zn 1-y Te alloy.
Article
We present the optical characterization of MOVPE grown ZnSe xTe 1- x epilayers with 0<x<0.35 and ZnSe xTe 1-x single quantum wells (SQWS) with Lz=2.0-8.5 nm. The structures were investigated using photoreflectance and photoluminescence spectroscopy. Transmission electron microscope analysis was used to determine quantum well thickness. The studied ZnSe 0.3Te 0.7 SWQs show a bright emission band with a halfwidth ≈ 10 meV in the spectral region of 100-250 meV below the band gap of the corresponding mixed crystal. We suggest a type II bands alignment in ZnSexTe1- x/ ZnTe single quantum wells.
Article
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.
Article
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.
Article
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 ZnMgSeZnTe 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.
Article
A complete set of elastic, piezoelectric, and dielectric constants is presented for the sulfides, selenides, and tellurides of zinc and cadmium. The piezoelectric constants for the hexagonal crystals in this group are markedly higher than for the cubic crystals. An elementary model theory applied to these data leads to electric charges on the metal atom increasing from +0.066 e for ZnTe to +0.84 e for CdS. The elastic compliance increases regularly with increasing anion and cation weight, with no break between the cubic and hexagonal crystals. Pyroelectric constants are given for CdS and CdSe. The quality factor of elastic resonances of CdS plates is measured as function of electric conductivity.
Article
ZnSe x Te 1-x epilayers were investigated by means of luminescence, reflectivity, and temperature dependence in the concentration range 0≪x≪0.4. The studied ZnSe x Te 1-x epilayers with thicknesses of about 1.5 μm were grown on GaAs substrates by metalorganic vapor phase epitaxy. It was found that the luminescence and reflectivity spectra of the mixed crystals are strongly affected by the compositional disorder. A continuous transition from the recombination through free and bound exciton states to the recombination of excitons localized by the compositional fluctuations of the mixed crystal was observed in the concentration region of about x=0.25. The position of the excitonic band edge was derived from the photoluminescence excitation spectra and from temperature dependence of the emission spectra.
Article
We have studied the strain in ZnTe epilayers grown by atmospheric‐pressure metalorganic vapor‐phase epitaxy on (001) GaAs and GaSb substrates. Reflectivity and absorption measurements are performed at 2 K using single‐crystalline layers with thicknesses of 0.2–2 μm. The biaxial strain in the samples caused by the lattice mismatch of layer and substrate is deduced from the splitting of the degenerate heavy‐ and light‐hole exciton. A polariton model is used to describe the reflectivity structure at the E 0 gap and to determine the transverse exciton energies. The deformation potentials obtained from an analysis of the absorption structures are a=-5.5 eV and b=-1.4 eV. The critical thickness for ZnTe/GaSb is lower than 0.8 μm near thermodynamic equilibrium. It also slightly depends on growth temperature which has its optimum at 345 °C.
Article
We present optical investigations on CdTe/(CdMg)Te single quantum wells (QWs) and demonstrate the high structural quality of the pseudomorphic grown QWs structure which shows high photoluminescence efficiency up to room temperature. Due to the large band‐gap difference between CdTe and Cd 0.51 Mg 0.49 Te of more than 0.8 eV remarkable strong confinement effects are observable. A strong enhancement of the exciton binding energies is found by decreasing well width. In the 50‐Å‐wide QW the binding energy is more than two times larger compared with that of bulk CdTe. In addition, a strong functional dependence of the localization energy of donor bound excitons on the well thickness is found. A valence‐band offset of 30% in the strain‐free limit is determined from the energy difference between heavy‐ and light‐hole excitons which is consistent with the strong blue shift of exciton energies by decreasing well widths and the observed effective electron‐hole confinement.
Article
We report a systematic study of the optoelectronic properties of ZnSe 1-x Te x alloys grown by molecular beam epitaxy over the entire range of compositions. The band‐gap energy as a function of the composition presents a minimum at x≂0.65. The main luminescence emission observed at 5 K becomes narrower and closer to the band‐gap energy as we increase the Te content. The linewidth and the difference between the emission peak and band‐gap energy decrease significantly with increasing x and present a break in the slope at x≂0.65.
Article
We report the successful growth of ZnTe on nearly lattice‐matched III‐V buffer layers of InAs (0.75%), GaSb (0.15%), and on GaAs and ZnTe by molecular beam epitaxy. In situ reflection high‐energy electron diffraction measurements showed the characteristic streak patterns indicative of two‐dimensional growth. Photoluminescence measurements on these films show strong and sharp features near the band edge with no detectable luminescence at longer wavelengths. The integrated photoluminescence intensity from the ZnTe layers increased with better lattice match to the buffer layer. The ZnTe epilayers grown on high‐purity ZnTe substrates exhibited stronger luminescence than the substrates. We observe narrow luminescence linewidths (full width at half maximum ≊1–2 Å) indicative of uniform high quality growth. Secondary‐ion mass spectroscopy and electron microprobe measurements, however, reveal substantial outdiffusion of Ga and In for growths on the III‐V buffer layers.
Article
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.
Article
A self-consistent tight-binding (SCTB) calculation of the deformation potentials is performed for ZnTe, ZnSe, and ZnS. Tight-binding bulk parameters reproducing photoemission and reflectivity experiments are given for these compounds. Then the strain effects are considered in the SCTB model, which allows the calculation of the hydrostatic deformation potential and of the nonlinear variation of the fundamental E0 gap with the relative change in the lattice constant Deltaa/a0. The method is applied to the determination of the offset in the strained ZnS-ZnSe system. Results are in good agreement with experimental and other theoretical works.