Article

Epitaxy of Zn1 − xMgxSeyTe1 − y on (100)InAs

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Abstract

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]. ...
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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.
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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
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.
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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.
Article
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.
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
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.
Article
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.
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.
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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.
Article
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.
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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
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.
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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.
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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
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
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.
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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.
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
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.