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Anomalous variation of band gap with alloy composition: cation vs anion substitution in ZnTe
Abstract
The energy of the excitonic signature, Eg(x), in the wavelength modulated reflectivity spectrum of Zn1−xMgxTe shows a monotonic increase with x in contrast to a large downward bowing of Eg(x) in ZnSxTe1−x and ZnSexTe1−x. Free and bound excitonic signatures in the photoluminescence spectra of the same samples provide additional confirmation for these trends. Raman spectra of these ternaries display a remarkable resonance enhancement of the zone center longitudinal optical (LO) phonon and its overtones when the scattered photon energy approaches the free excitonic transition energy: for a fixed exciting photon energy, the intensities of Raman lines with shifts of 1LO, 2LO, 3LO,… are selectively enhanced in sequence as a function of x, but in the reverse order in Zn1−xMgxTe as compared to that in ZnSexTe1−x, underscoring the monotonic increase of Eg (x) in the former and a significant downward bowing in the latter.
... In another related investigation, García and co-workers [10] have shown in their TB calculations on the Zn 1−y Cd y Se 1−x Te x and Zn 1−y Cd y S 1−x Se x quaternary alloys that the bandgap energy possesses a bowing character when the mole fraction of anions (x) is varied and, on the other hand, it follows an almost linear variation when the composition of cations (y) is varied. Similar bowing and linear behaviors were also reported in the experimental work of Seong and co- workers [11] on the common-cation and common-anion II- VI ZnTe-based alloys. These behaviors were corroborated in the theoretical simulations of El-Haj Hassan and co- workers [12] using the density-functional theory (DFT) and Charifi and co-workers [13] using the linearized-augmented plane wave (LAPW) method. ...
... This excellent agreement made our claim valid while we recall that the calculations still rely on the validity of the VCA. For the ZnSe 1−x Te x ternary alloys, Seong and co- workers [11] reported wavelength-modulated reflectivity and Raman characterization of single crystals grown by the vertical gradient freezing technique. The PL measurements were done at low temperatures (8 K) and the results are shown on figure 4(b) by solid triangles. ...
... In another related experimental work, reported by Wu and co-workers [22], the ZnSe 1−x Te x alloys were grown by MBE and characterized using photomodulated reflectivity, optical absorption, and PL spectroscopies at RT. Their data [22] also represented further evidence for bandgap bowing in these alloys and are shown as solid circles in figure 4(b). The bowing character has also been corroborated by the TB calculations, , and PL4 correspond to [19], [11], [21], and [22] respectively, while CL is due to [19]. ...
We present an investigation into the existence and origins of bandgap bowing in compound-semiconductor common-cation ternary alloys. As examples, we consider CdSe(x)Te(1-x) and ZnSe(1-x)Te(x) alloys. A calculation, based on the sp(3)s(*) tight-binding method including spin-orbit coupling within the framework of the virtual crystal approximation, is employed to determine the bandgap energy, local density of states and atomic charge states versus composition and valence-band offset. The results show that (i) in the valence band, the top states are mainly contributed by Te atoms. The degree of ionicity of all atoms is found to vary linearly with mole fraction x. (ii) There is a strong competition between the anions (Se and Te) in trapping/losing charges and this competition is the main reason for the bandgap bowing character. (iii) There is a reasonable agreement between the calculated results and the available photoluminescence data. (iv) The bowing parameter is found to increase with increasing valence-band offset and increasing lattice mismatch.
... Since the band gap of Cd 1−x Mg x Te can be easily tuned by adjusting the composition, the sub-gap transmission through the top cell can be controlled to obtain current matched devices. Also, the addition of a small amount of Mg (x ≈ 0. 15) can open the band gap of CdTe to the required optimum value of 1.74 eV [2]. This suggests that the alloy can be prepared with minimum perturbation of the CdTe lattice. ...
... In our films we could not observe the MgTe LO modes probably due to the lack of resonant enhancement. The observed shift of the 1 LO band toward lower wavenumbers as x increases (figure 8(b)) is consistent with the two-mode behavior of the zone-center optical phonons in II-VI ternaries in which the heavy cation (Cd) is replaced with a lighter atom (Mg)[14,15]. ...
Cd1-xMgxTe is a potential candidate for the top cell in two-terminal tandem solar cells. The close match of the lattice constant of MgTe with CdTe and the apparent complete miscibility of MgTe in CdTe and the rapid increase in band gap with Mg content gives flexibility to prepare material with the appropriate band gap for current-matching in a tandem solar cell. We have chosen to deposit Cd 1-xMgxTe thin films by RF sputtering to maintain access to low growth temperatures. Substrate temperatures greater than 250degC produced highly adhering polycrystalline films with grain size in the range of 100-150 nm and a strong preferential (111) orientation. All the films were p-type and resistive. Energy Dispersive X-ray Spectroscopy (EDS) analysis showed that the Mg concentration in the film is significantly lower than that of the source target. Films were chloride-treated at temperatures in the range 350 to 387degC in a variety of ambients and the band gap showed narrowing at higher annealing temperature and time duration, particularly in oxygen containing ambients. XRD and AFM studies of the Cd1-xMgx Te films showed evidence of re-crystallization including grain growth and a more random crystallographic orientation of the grains. Prototype CdS/Cd1-xMgxTe solar cells were fabricated
... 18 Bandgap engineering in Cs 2 Na x Ag 1-x BiCl 6 nanocrystals with the bandgap varying linearly from 3.39 eV (x ¼ 0) to 3.82 eV (x ¼ 1) are reported by Lamba et al. 10 Interestingly, the cation and anion substitution is predicted to impart bandgap bowing, an effect where the bandgap varies non-linearly with alloying composition. 22 Han et al. calculated a large bandgap bowing parameter of 1.03 eV in Cs 2 Ag 1-x Na x BiCl 6 . 20 Experimental studies on Cs 2 SnX 6 have demonstrated similar bandgap bowing behavior. ...
Bandgap engineering in lead-free Cs2Ag1-xNaxBiCl6 (x = 0 to 1) double perovskite alloys synthesized through solution-based approach is investigated. The bandgap is shown to vary from 2.64 eV to 3.01 eV as Ag⁺ at B′ site gets replaced with Na⁺ cation. Despite a linear change in the lattice parameter according to Vegard's law, bandgap (Eg) changes in a nonlinear fashion for x = 0 to 1 with much lower Eg values observed than predicted by Vegard's rule. Further, we show the bandgap bowing effect in Cs2Ag1-xNaxBiCl6. Raman spectroscopic studies reveal that the changes in the vibrational mode positions arise due to the systematic variations in local distortions of [BiCl6]3– and [AgCl6]5– octahedra. The bandgap change, Raman mode frequency shift, Raman peak width, and the ratio of intensities of Raman modes all show a similar trend as a function of Na substitution concentration (x). The changes are minimal and linear for x from 0 to ∼0.6 and deviate sharply for higher Na concentration (x > 0.6). These observations strongly suggest that the sublattice distortion in the A2B′B″X6 lattice arises due to a mismatch in the octahedra. This imparts a nonlinear change in the bandgap. Thus, a strong interplay between the [Ag(Na)Cl6]⁵⁻ and [BiCl6]3– octahedra is shown to have a significant influence on the deviation of bandgap from Vegard's rule and further enforces the bandgap bowing effect in Cs2Ag1-xNaxBiCl6.
... The extent of such local atom displacements usually brings about nonlinear dependence on optical properties in ternary materials. 44,53 O'Brien et al. synthesize Bi 2À2x Sb 2x S 3 solid solutions from solvent less thermolysis of metal xanthate precursors and showed a slight deviation from linearity in the energy band gap on Sb substitution due to stoichiometric variations in the synthesized solid solution. 54 The Raman spectra of the particles are shown in Fig. 11. ...
The synthesis of the complete range of (Bi1-x Sb x )2S3 solid solutions, where 0 ≤ x ≤ 1, by the variation of the mole ratio of bismuth and antimony piperidine dithiocarbamate complexes is reported. There was a near linear expansion of a and c lattice parameters as the mole ratio of the antimony precursor was increased. The composition of the particles directionally followed the amount of precursor ratio used. When the composition of particles was compared to cell parameters, a slight deviation from Vegard's law was observed with a corresponding contraction of the b parameter and an approximately 3.5% reduction of the lattice volume. The nanorods obtained showed aspect ratios that depend on the composition of the material. The Bi and Sb rich materials had high aspect ratios of 16.58 and 16.58 respectively with a minimum aspect ratio of 2.58 observed for x = 0.50.
... 18,19 In ZnTe, tuning of band gap to harvest a wide portion of sun light is demonstrated using its nanostructures 20−22 and alloys. 23 −25 Among ZnTe-based alloys, ZnTe 1−x O x , a highly mismatched alloy of ZnTe (a = 6.104 Å) and ZnO (a = 4.565 Å) which exist in cubic zinc blende and wurtzite crystal structure, respectively, is of potential interest due to enhanced optical absorption of wide portion of sun light. ...
The ability to tailor the energy gap of semiconductors, as a function of their applications has led to Bandgap Engineering. From a materials perspective, Bandgap Engineering has been made possible, to a large extent, by Semiconductor Alloys. The applications of these alloys include solar cells, solid-state lasers, detectors, Light Emitting Diodes (LEDs), and Opto Electronic Integrated Circuits (OEICs). In this chapter, we discuss the electronic, optical, and elastic/mechanical properties of various semiconductor alloys.
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.
The electronic and optical properties of cubic Ga1-xIn xN ternary alloys are theoretically investigated using the sp 3s* tight-binding model, with the inclusion of spin-orbit interaction, versus composition and valence-band offset (VBO), stimulating the lattice-relaxation effects. Usually, the bowing of the band-gap energy in the common-anion ternary alloys should be vanishingly small as far as the virtual-crystal approximation remains valid. In contrary to this, the present alloys are shown to possess three unusual characteristics: (i) They possess rather strong bowing character with b > 2.0 eV; (ii) the bowing parameter is composition dependent b = b(x); and (iii) the Stokes shift between emission and absorption is so large that can even reach 200 meV when x ≈ 0.5. Two reasons are claimed to cause such behaviors; namely: the strong electro-negativity of nitrogen atoms and the large lattice relaxation as being induced by the large lattice-mismatch between the two constituents (GaN and InN). The favorable comparison with the available experimental data corroborates the above claims.
p-type doping of Zn1-xMgx Te has been investigated by metalorganic vapour phase epitaxy using tris-dimethylaminophosphorus as a dopant source. The photoluminescence (PL) and electrical properties of the as-grown and post-annealed layers have been clarified as a function of the dopant transport rate. In PL spectra at 4.2 K, the strong donor acceptor pair emission vanished and instead enhancement of P acceptor-related excitonic emission and/or appearance of free-to-bound emission were observed for all the layers by the post-annealing treatment. In accordance with the PL behavior, the hole concentration at room temperature of the annealed layers was significantly enhanced from 2×1015∼2×1016 cm-3 to 5×1016∼2×1018 cm-3 by a maximum factor of ∼600. The enhanced hole concentration shows saturation tendency with increasing dopant transport rate. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
A general expression for the pressure dependence of the energy gap of a series of group III-V and group II-VI ternary semiconductors have been derived based on Van Vechten's dielectric theory. The results obtained are in good accord with the available experimental data. The trends in the variation of the pressure dependence of the energy gap with the nearest neighbor distance and Phillips ionicity are explored qualitatively.
The binding energy of excitons bound to oxygen isoelectronic impurity in ZnSxTe1-x, shows a remarkable scaling with the difference between the electronegativity of oxygen and the concentration-weighted average of those of S and Te. The signatures of the free exciton, observed with wavelength modulated reflectivity, and those of shallow acceptor bound excitons, revealed in photoluminescence, show an unmistakable redshift with increasing x (0≤x≤0.13). In striking contrast, the excitons bound to the isoelectronic oxygen center in these alloys manifest a blueshift with increasing x.
The existence and origins of the bandgap bowing character in the wurtzite ZnO-based ternary alloys are theoretically investigated. The tight-binding method with minimal sp3 basis set is used to calculate the band structures, density of electronic states, and constituent charge states of both common-cation ZnO1−x
Sx
and common-anion Zn1−x
Mgx
O ternary alloys. The experimental data show that a well-pronounced bowing effect is induced in the common-cation case, whereas there is no apparent bowing effect in the common-anion case. The present investigation indicates that the bowing effect in the former case may result from competition between anions trapping charge made available by cations. The disappearance of this competition, in the case of common-anion alloys, eliminates the possibility of bowing, thus leading to the observed linear behavior. Furthermore, the electronegativity and its discrepancy between the two anion types are expected to play a dominant role in the magnitude of the bowing parameter. The excellent fit between our theoretical results and the available photoluminescence data support this model.
The photoluminescence and modulated reflectivity spectra of ZnSexTe1-x and ZnSxTe1-x with increasing x (0<~x<~0.17) show a distinct blueshift for excitons bound to the oxygen center in contrast to an unmistakable redshift for the free excitons. The binding energy of excitons bound to oxygen impurity present in these ternaries is proportional to the difference between the electronegativity of oxygen and the concentration-weighted average of those of Se (or S) and Te, both scaling with x. Due to the strong localization of the excitons bound to isoelectronic oxygen impurities, its no-phonon line exhibits a significantly faster alloy broadening compared to that associated with excitons bound to shallow acceptors. The Raman spectra show a contrasting mode behavior for their zone-center optical phonons, “one mode” for ZnSexTe1-x and “two mode” for ZnSxTe1-x. A highly accurate frequency determination of the LO phonon in ZnSexTe1-x over a large composition range exhibits a large bowing as a function of x.
Raman electron paramagnetic resonance (Raman-EPR) of the transitions due to the spin flip of the 3d electrons of Cr+ in Zn1−xCrxTe and Cd1−xCrxTe is observed at ℏωPM=g(Cr+)μBB with g(Cr+)=2.0041±0.0095 and 2.0039±0.0093, respectively. The appearance of Raman lines at ωLO±nωPM, n=1, 2, and 3 results from the strong Fröhlich interaction with the zone-center longitudinal-optic phonon. The intensity of the Raman-EPR of Cr+ can be enhanced through the photogeneration process by simultaneous illumination with photons having an energy close to the excitonic band gap; photoluminescence spectra reveal signatures of excitons bound to Cr+ acceptors in Zn1−xCrxTe specimens. The resonance profile of Cr+ Raman-EPR shows that the strong resonant enhancement is mediated via an exciton bound to a neutral acceptor.
We report on the dielectric properties of Zn1−xMnxTe (0≦x≦0.268) epilayers studied by capacitance and dissipation factor measurements at a temperature of 200 K<T<460 K and a frequency of 20 Hz<f<1 MHz. A Debye-like relaxation in the dielectric response has been observed, which is explained in terms of the presence of charge redistribution. The relaxation is found to be a thermally activated process, and the activation energies obtained from both dissipation factor and capacitance are in good agreement. It is also found that the activation energy decreases with increasing Mn content and this behavior is interpreted in terms of the four-center model, in which the number of Mn atoms appearing in the nearest-neighbor sites of a defect can have four possible configurations. In addition, we demonstrate that the mechanism responsible for the conduction of carrier hopping among structural defects can be attributed to the correlated barrier hopping model. © 2003 American Institute of Physics.
Zn1−xMnxTe and Zn1−xMgxTe ternary wide-gap semiconductor alloys were grown by molecular beam epitaxy on (100) GaAs substrates over a wide range of compositions (0⩽x⩽0.75 and 0⩽x⩽0.67, respectively). Values of the band gap were measured by photoluminescence at 12 K, and by optical reflectivity at room temperature. The wavelength dependence of the indices of refraction n of these ternary systems was also measured for these alloys at wavelengths below their respective energy gaps. The measurements were performed using a combination of the prism coupler method and reflectivity. Compilation of these results allows us to establish a set of empirical parameters for the two alloy families, that can be used to calculate the index of refraction for an arbitrary alloy composition and arbitrary wavelength. It is interesting that the values of n show a surprisingly linear dependence on the corresponding energy gaps for both these alloy systems. © 2002 American Institute of Physics.
The existence and the origins of the bowing character in the bandgap of compound semiconductor alloys are investigated using the sp3s* tight-binding method which includes the spin-orbit coupling effects. As examples, we consider several ternary alloys among the composition of the ZnyCd1−ySexTe1−x family. The results show that the common-cation ternary alloys (either y = 0 or 1) exhibit the bowing behavior as a consequence of the ionicity competition between the anions induced by the valence-band offset (VBO). On the other hand, the common-anion ternary alloys (either x = 0 or 1) do not possess the bowing behavior and their bandgaps vary almost linearly with composition (y). Such a linear behavior is claimed to be due to the lack of ionicity competition between the cations which is, in turn, consistent with the vanishing VBO. The favorable agreement between our theoretical results and the available experimental data corroborates our claim.
We present a theoretical investigation of the existence and origins of the bandgap bowing character in compound semiconductor alloys based on both the sp 3 s à -tight-binding (TB) method, which includes the spinÀorbit coupling, and the ¯rst-principle Full-Potential Linear Augmented Plane Wave (FP-LAPW) technique. First, we compare the bandgap variation versus composition in the IIIÀV direct-bandgap-GaAs-based ternary alloys, namely between the common-cation GaSb x As 1Àx and the common-anion Ga 1Àx In x As ternary alloys. The results show that the bowing behavior exists only in the common-cation (i.e., GaSbAs) alloys as a result of an existing competition between the anion atoms (Sb and As) in trapping the electronic charge. The bowing parameter is found to be proportional to the electronegativity characters of the competing anions (i.e., anion). The lack of such competition, in the case of common-anion alloys (i.e., GaInAs), makes the bowing be just absent and the variation of the bandgap become close to linear. Second, we have made a clear contrast in the bowing characters between the common-cation IIIÀV and the common-anion IIÀV ternary alloys by assessing the e®ect of anion electronegativity (as example, we consider the GaSb x As 1Àx and CdSe x Te 1Àx ternary alloys). The results show that the bowing character in the IIÀVI alloys is stronger than the one of IIIÀV alloys as to be simultaneously proportional to the electronegativity of anions (anion ave) and to the electronegativity mismatch between them ðÁ anion = anion ave Þ. Finally, the excellent agreement between our theoretical results and recent photoluminescence data has further corroborated our claims.
The band gap of Cd1−xMgxTe can be easily tuned from 1.48 to 3.5 eV (x = 0 to 1), and hence this material is a potential candidate for developing the wide band gap top cell in tandem solar cells. In this paper, we present an all-sputtered Cd1−xMgxTe/CdS top cell developed on commercial SnO2:F-coated (Tec-7) glass substrates with sheet resistance of ~7Ω/. The Cd1−xMgxTe (x = 0.05, 0.21, 0.26) films were deposited by RF sputtering at deposition temperatures in the range 250–300 °C. The films were characterized for structure, morphology and optical properties. Raman spectroscopic studies showed that the lattice perturbations increase with Mg content in the film. We have explored cadmium chloride vapor annealing of Cd1−xMgxTe thin films, and found that the film composition has a significant influence on stability. Films with a lesser amount of Mg show better stability during vapor chloride treatments at 387 °C. Longer annealing durations resulted in a decrease in band gap of the films. Recrystallization due to the vapor chloride treatments were studied using transmittance, XRD, AFM and Raman spectroscopy. Prototype devices of the configuration Tec-7/CdS/Cd0.95Mg0.05Te/metal show promising photovoltaic characteristics such as 630 mV open circuit voltage and 17 mAcm−2 short circuit current.
Zn1−xMgxS, Zn1−xMgxSe and Zn1−xMgxTe ternary wide-gap semiconductor alloys were investigated using the full potential–linearized augmented plane wave (FP-LAPW) method. We have studied the effect of composition on structural properties such as lattice constants, bulk modulus and bond ionicity. The bandgap and the microscopic origins of compositional disorder have also been explained in detail. In addition, from the obtained band structures, the electron (hole) conduction and valence effective masses are deduced. These parameters were found to depend non-linearly on alloy composition x, except the lattice parameter for Zn1−xMgxS, which follows Vegard's law. The calculated band structures for all three alloys show a direct bandgap in the whole range of x composition. We have paid special attention to the disorder parameter (gap bowing). Using the approach of Zunger and co-workers, we have concluded that the total bandgap energy bowing was mainly caused by the charge exchange effect for the alloys of interest.
The common-anion and common-cation II-VI ternary alloys of the family Cd(Zn)Se(Te) are theoretically investigated based on two different methods. Within the virtual-crystal approximation (VCA), both the sp3s*-tight-binding method, with the inclusion of spin-orbit coupling, and the first-principle full-potential linear augmented plane waves (FP-LAPW) technique are employed to determine the alloy constituents' charge states (ionicities) and degree of bond polarity. The results show that: (i) in the common-cation ternary alloys (i.e., CdSexTe1−x and ZnSexTe1−x), the anions alter a strong competition in trapping more charge. Such a competition builds up a compromised effect yielding the bowing behaviour. Whereas, (ii) in the common-anion ternary alloys (i.e., CdyZn1−yTe and CdyZn1−ySe), the absence of such competition would cause the complete absence of bandgap bowing behaviour. The variation of the bandgap is found to be rather close to linear. The obtained good agreement between our theoretical results and the recently available photoluminescence data does further corroborate our claims.
ZnS1−xTex ternary alloy epilayers were grown on GaAs (1 0 0) substrates by hot-wall epitaxy (HWE), and the dependence of their band gap energy and photoluminescence (PL) characteristics on the Te composition x was investigated. The spectrophotometer measurement results demonstrated that the room temperature band gap energy varied with the Te composition x. The band gap energy showed the strong bowing effect given by Eg(x)=3.73−5.27x+3.80x2. The PL peaks of ZnS1−xTex epilayers shifted to the lower energy with increasing Te composition x. For x>0.69, the PL peak energy was very close to the band gap energy. Bowing parameter, 3.80, was closer to the value of Bernard et al., 3.83, and was a little larger than the previous other results. Strong emission ranging from blue to yellow was observed at room temperature. Also, the relations between Ten (n=1,2,3) atoms of the ZnS1−xTex epilayers and the exciton emission at room temperature were investigated.
The effects of substrate temperature upon the optical property, composition and surface morphology have been investigated on nominally undoped Zn1−xMgxTe layers grown on (1 0 0) ZnTe substrates by atmospheric pressure metal organic vapor phase epitaxy (MOVPE). It was found that Mg composition increases with decreasing substrate temperature. The result of low temperature photoluminescence (PL) measurement suggests that the optical quality of Zn1−xMgxTe layers becomes better with decreasing substrate temperature. On the other hand, there is a narrow range of optimal substrate temperature for a smooth surface morphology. For all the layers, a two-mode behavior with ZnTe- and MgTe-like longitudinal optical phonon modes was confirmed by Raman scattering.Highlights► Mg composition increases with decreasing substrate temperature. ► Improvement of optical quality with decreasing substrate temperature was observed. ► There is narrow range of optimal substrate temperature for obtaining smooth surface. ► ZnTe-and MgTe-like longitudinal optical phonon modes were observed.
We investigate the structural and electronic properties of (Zn, Mg) (S, Te) solid solutions using the full potential-linearized augmented plane wave method within density functional theory. We use the generalized gradient approximation that is based on exchange–correlation energy optimization for calculating the total energy. Moreover, the Engel–Vosko GGA formalism is applied so as to optimize the corresponding potential for band gap calculations. We study the effect of composition on lattice constant, bulk modulus and band gap for pseudobinary as well as for quaternary alloys which showed nonlinear dependence on the composition. The bowing of the fundamental gap versus composition predicted by our calculations is in very good agreement with experiments available for pseudobinary alloys. In addition, the energy band gap of zinc-blende Zn 1−x Mg x S y Te 1−y quaternary alloys lattice matched to GaAs substrate is investigated. It should be noted that the present work is the first theoretical study of the quaternary alloys of interest.
The existence and origins of the bowing character in the bandgap variation of GaAs-based ternary alloys are theoretically investigated based on two different computational methods. Within the framework of the virtual crystal approximation (VCA), both the empirical sp3s * tight-binding (TB) method with, and without, the inclusion of the spin-orbit coupling effects, and the first-principle full-potential linear augmented plane wave (FP-LAPW) technique are applied on both the common-cation GaSbxAs1-x and the common-anion Ga1-xInxAs alloys. These methods are used to calculate the bandgap energy, the partial and total densities of states and the constituent charge ionicity versus the composition x. The results show that the bowing behavior exists in the case of common-cation alloys (GaSbxAs1-x) as a manifestation of a competition between the anion atoms (As and Sb) in trapping the made-available-cationic charges. The bowing parameter is found to be proportional to the electronegativity characters of the competing anions (χanion). Consistent with this in the case of common-anion alloys (Ga1-xInxAs), as due to the lack of anion competition, the bowing is just absent and the variation of bandgap energy is found to be rather linear. The excellent agreement between our theoretical results and recent photoluminescence data has corroborated our claim. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Photoluminescence spectra of strained structures Zn1 − x
Mn
x
Te/Zn1 − y
Mg
y
Te with magnetic quantum wells and nonmagnetic barriers are studied. The Zeeman splitting of the heavy exciton is found to
follow an unusual behavior: both spin components shift down in energy. The heavy-exciton photoluminescence Zeeman components
are observed to be inversely distributed in intensity, with the higher energy component being stronger than the lower energy
component. The Zeeman splitting of the exciton in a magnetic field is calculated. The data obtained permit refinement of some
parameters of the energy spectrum and magnetic properties of these structures.
The scope of the present investigation is to make a clear contrast between the bandgap bowing characters of III–V and II–VI compound-semiconductor common-cation ternary alloys. For this aim, both the sp
3s
* tight-binding method, with the inclusion of spin–orbit coupling, and the full-potential linear augmented plane-wave technique are used to calculate the partial and total densities of states, the constituent ionicity, and the total electron charge density for the common-cation GaSbx
As1−x
and CdSex
Te1−x
ternary alloys. The results show that the bowing is sensitive to competition between the anions for trapping/losing electric charges. The lack of this competition would result in complete absence of the bowing, as is the case for common-anion ternary alloys. In the common-cation ternary alloys studied herein, the bowing is found to be proportional to the electronegativity of the anions χ
anion (i.e., the 6-valency anions are more electronegative than the 5-valency ones, and consequently the former result in stronger intercompetition and yield stronger bowing in the II–VI alloys) and also proportional to the relative mismatch in electronegativity between the competing anions \(({\Updelta}\chi^{\rm{anion}}/\chi^{\rm{anion}}_{\rm{ave}}).\) The excellent agreement between our theoretical results and recent photoluminescence data corroborates our claim.
The electronic structures of Mg <sub>y</sub> Zn <sub>1-y</sub> Te <sub>1-x</sub> Se <sub>x</sub> alloys were studied by optical absorption and photoluminescence techniques under applied hydrostatic pressure. In samples with both x and y≠0, the band gap exhibits a strongly nonlinear pressure dependence which is similar to the effects observed previously in ZnTe <sub>1-x</sub> Se <sub>x</sub> and ZnTe <sub>1-x</sub> S <sub>x</sub> ternaries and that is well explained by the anticrossing interaction of the selenium localized electronic states with the conduction band of the matrix. In contrast, the pressure dependence of the band gap in Mg <sub>y</sub> Zn <sub>1-y</sub> Te (i.e., x=0) is not significantly changed in form from that of ZnTe; it is concluded that the effects of alloying MgTe with ZnTe can be well understood within the virtual crystal approximation. © 2002 American Institute of Physics.
The absence of bandgap bowing in the common-anion II–VI semiconductor ternary alloys is investigated. As examples, we consider the Cd1−xZnxTe and Cd1−xZnxSe alloys. The sp3s* tight-binding method with the inclusion of spin–orbit interactions is employed to calculate the alloy’s band structure and its corresponding constituents’ charge states (ionicities) as a function of composition. The variation is found to be nearly linear. The vanishingly small valence-band offset (VBO≃0) in common-anion compounds would yield a linear scaling of bandgap energy with composition, especially as the conduction-band edge state being a singlet state with spherical symmetry. Furthermore, the two cation atoms (Cd and Zn) are found not to compete in changing their charge states as the composition is varied. The absence of such competition is believed to be the main reason for the absence of bowing. The theoretical results are compared to the available experimental data and found to be in good agreement.
We report a strongly nonlinear pressure dependence of the band gaps and large downward shifts of the conduction band edges as functions of composition in ZnS xTe (1-x) and ZnSe (y)Te (1-y) alloys. The dependencies are explained by an interaction between localized A1 symmetry states of S or Se atoms and the extended states of the ZnTe matrix. These results, combined with previous studies of III-N-V materials define a new, broad class of semiconductor alloys in which the introduction of highly electronegative atoms leads to dramatic modifications of the conduction band structure. The modifications are well described by the recently introduced band anticrossing model.
Alloys of zinc chalcogenides exhibit both some of the smallest (for ZnS/sub x/Se/sub 1-//sub x/) and the largest (for ZnS/sub x/Te/sub 1-//sub x/) optical bowing observed in isovalent semiconductor systems. A theoretical analysis of this effect, by use of self-consistent band-structure techniques for ordered 50%-50% alloys in the CuAu-I structure, predicts correct bowing parameters and chemical trends, yet suggests a physical model which is altogether different from the virtual-crystal model.
Using the all-electron mixed-basis approach to the density-functional formalism for crystals, we calculate from first principles the electronic structure of zinc-blende ZnS, ZnSe, and ZnTe as well as that of their ordered pseudobinary alloys Zn2SSe, Zn2SeTe, and Zn2STe. For the latter we use as a model a CuAu I-like structure (space group P4¯m2), and analyze the observed optical bowing in terms of three contributions: (i) a volume deformation of the band structure due to the replacement of the lattice constants of the binary constituents by that of the alloy, (ii) a chemical-electronegativity contribution due to charge exchange in the alloy relative to its constituent binary subsystems, and (iii) a structural contribution due to the relaxation of the anion-cation bond lengths in the alloy. The total bowing effect [the sum of (i)-(iii) above] agrees well with observations, yet the present analysis suggests a physical mechanism for optical bowing which differs profoundly from that offered by the popular virtual-crystal approach. The maximum contribution of disorder to the optical bowing is calculated for ZnSxTe1-x using a cluster-averaging method, resulting in a reduction in the bowing of the fundamental gap. We further discuss the band structures, x-ray scattering factors, charge distribution, and deformation potentials of the binary zinc chalcogenides and their ordered alloys.
The low temperature (4. 2 and 77K) spectra of the edge absorption and the exciton reflection spectra of CdSe//xTe//1//-//x single crystals were studied in a wide range of values of x. The information obtained on the positions of the exciton reflection lines and of the fundamental absorption edge were used to determine the dependence of the forbidden-band width E//g on the composition (x). The dependence E//g(x) was found to be nonlinear with a minimum near x equals 0. 4.
Multiphonon LO lines were studied in the spectrum of secondary radiation from ZnTe crystals which were excited monochromatically in the fundamental absorption region. An experimental dependence of the 2LO line intensity on the excitation frequency was obtained, together with the dependence of the relative intensities of the multiphonon LO lines on the order of scattering. Thermal quenching of the LO lines was found. A theoretical treatment is given of the frequency dependence of the 2LO scattering cross section and the dependence of the relative intensities of the multiphonon LO lines, in two limiting cases, those of scattering via the exciton ground state and via states of free electron-hole pairs.
Major new advances in epitaxial growth techniques have contributed to an improved understanding of the properties of II–VI materials and their alloys; these advances have provided the creation of sophisticated heterojunctions, multiple-quantum wells, and strained-layer superlattices composed of II–VI/ II-VI and II-VI/III-V multilayers. The chapter presents an overview of the research activity in the area of II–VI-based structures, with significant recent achievements highlighted. From a materials-oriented point of view, much of the recent research activities have centered on epitaxial growth techniques especially molecular-beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), and hot-wall epitaxy (HWE), development of new alloys with higher-energy bandgaps, exploration of magnetically active alloys, growth on lattice-mismatched substrates, and the growth of superlattices that are lattice mismatched or have components with different crystal structures. The chapter highlights the significant advances in these exciting new areas of materials science, physics, and engineering. The MBE growth of II–VI compounds can utilize both compound and elemental sources.
These lectures are intended to provide a coherent account of the fundamental properties of both free and bound excitons in semiconducting crystals. The emphasis is upon a description of those properties most relevant to the exploitation of exciton-related properties in the derivation of parameters of the semiconductor, both intrinsic and the extrinsic properties related to impurity and defect states, rather than the more formal treatment which is already provided in other chapters of this book. The discussions naturally centre on the optical properties, from which most aspects of the behaviour of excitons in semiconductors have been discovered and interpreted. Section I concerns the properties of free excitons, Section II contains the main characteristics of weakly bound excitons other than those related to satellite optical transitions, caused by phonon coupling and excited electronic states of the centre binding the exciton which are the subject of Section III. Special effects related to high excitation intensity are briefly reviewed in Section IV, other than electron-hole drops. The latter do not involve exciton states are are treated separately in this book.
The multimode behavior of the zone center optical phonons in bulk crystals and molecular beam epitaxy (MBE) grown epilayers of the tetrahedrally coordinated II–VI semiconductor alloys was investigated using Raman and infrared spectroscopy. Depending on the inter-atomic force constants and the relative nuclear masses of the constituents, the zone center optical modes of the ternary A1-xBxC exhibit either a two-mode behavior with distinct AC-like and BC-like TO and LO modes across the entire composition range, or an intermediate mode behavior when the masses of A and B are more closely matched. When B is much lighter than A, the BC-like TO and LO modes emerge from the localized vibrational mode of B in AC when x≈0 and the AC-like TO and LO modes converge into the gap mode of A in BC as x→1. A quaternary A1-x-yBxCyD may similarly exhibit a distinct three-mode or intermediate-mode behavior depending on the relative masses of the constituents. The two-mode behavior in bulk Zn1-xMgxTe and the three-mode behavior in bulk Cd1-x-yZnxMgyTe were studied with Raman scattering. Using infrared transmission at oblique incidence, i.e. the Berreman technique, the zone center TO and LO modes of MBE-grown zinc blende MnTe are observed as transmission minima. The observation of confined optical phonons in ultra-thin CdTe/ZnTe superlattices by exploiting the Berreman technique and Raman scattering permits their bulk dispersions to be delineated.
The exciton spectrum of AgI, CuI, CuBr, and CuCl is discussed and various peaks attributed to several coexisting crystal modifications. The shift of the peaks with alloy concentration in the pseudobinary systems formed with these compounds is reported and interpreted. The absorption and reflection spectra of these materials and their alloys for energies between the energy gap and 10 eV is reported and the structure attributed to transitions between regions of the valence band lying below the highest maximum and parts of the conduction band higher than the lowest minimum. These spectra are compared with the corresponding spectra of other zincblende- and wurtzite-like materials.
A monotonic variation of band gap with composition occurs for many binary solid solutions. Of some Group II-Group VI systems, ZnS-ZnSe shows this type of variation of band gap with composition, whereas ZnSe-ZnTe, ZnS-ZnTe show an anomalous minimum in a plot of band gap versus composition of the solid solution.
The theory of the first-order lattice vibration Raman effect is given with particular reference to semiconductors. It is shown that the most important Raman scattering mechanism is always one in which the radiation interacts indirectly with the lattice via the electrons. The electron-lattice interaction is treated by the deformation potential approximation, and the additional long-range electrical interaction is included for the polar semiconductors. For both types of interaction, the proportion of incident photons Raman scattered by the optic phonons into unit solid angle is of order 1 in 10^6 or 10^7 for the conventional experimental geometry. The symmetry and frequency dependence of the scattering tensor are derived, and expressions are obtained for the width and temperature shift of the Raman lines. For acoustic phonons it is shown that the present theory correctly reproduces the previously established theoretical Raman intensities. Microscopic expressions for the elasto-optical and electro-optical coefficients are derived as by-products of the calculation.
Since approximately 1950 an increasing portion of experimental solid state physics research has been concerned with studying defects in crystals. This trend might appear to be a rather belated recognition that most of the materials we come into contact with have a random structure. In fact the theoretical understanding and controlled preparation of compounds with defects or random structure has been very slow in developing. The present paper examines and reviews our knowledge of the lattice vibrations associated with defects. The coverage is extremely broad, as shown by the table of contents. It includes localized and resonant modes of isolated defects as well as the modes in highly disordered mixed crystals and glasses. It is primarily a review of experimental work but theoretical results are included where the latter explain or predict significant features. In order to be self-contained several sections of the paper deal entirely with theoretical matters. There is a chapter on explicit solutions of the linear chain vibration problem and a short chapter on Green's function methods. The review emphasizes the infrared absorption and Raman scattering of defects. This is simply because other techniques have not yielded nearly so much information. Neutron scattering and electron tunneling are referred to only where they have shed light on certain systems. Extensive tables of defects and mode frequencies are included for each type of solid. The major solids which are reviewed include semiconductors, ionic compounds, organic compounds, and amorphous insulators.
We have observed nth order (n=1,2,...,9) Raman scattering in CdS under conditions of resonance between the laser frequency and the band gap or the associated exciton states. The frequency spectrum of these higher order processes demonstrates that the phonons involved are localized within a small volume of k space centered at Gamma. We believe that such spectral features are characteristic of direct-gap resonant scattering, and the kinematics of their production is discussed.
Reflectivity spectra of ZnSexTe1-x single crystals were measured from 2.0 to 6.6 eV through the whole range of mole fraction x at room temperature. The variation of the band gaps such as the E0, e1, E1, and E2 with x is reported. The E0, e1, and E1 gaps exhibit spin-orbit splitting. The lowest gap E0 shows a minimum in energy at x=0.33. The minimum value is 2.13 eV.
A series of ZnS, Se phosphor alloys activated with 1·5 × 10-3 Cu and 10-3 Cl was investigated for the wavelength dependence of the excitation and quenching of photoluminescence and the photoluminescence emission. Glow curves and the temperature dependence of the photoluminescence were also studied. The objective of this work is simply to show the position of the recombination and trapping levels that exist in these materials as a function of the varying base lattice composition and band edge. A modified Schön-Klasens(1,2) model is shown to accommodate the data with a satisfactory degree of consistency. The chemical nature of the centers that are actually responsible for the levels is not deduced.
We have investigated the photoluminescence, Raman, and infrared spectra of Cd1-xMgxTe and Cd1-x-yMgxMnyTe bulk crystals with the zinc-blende structure. From the band-gap (Eg) photoluminescence peak we deduce Eg=1.595+1.607x+1.592y eV at 10 K. The Raman spectra of the quaternary alloys exhibit a classic three-mode behavior with MgTe-like, MnTe-like, and CdTe-like LO-TO pairs of zone-center optical phonons, in decreasing order of frequency. Their composition dependence follows the modified random-element isodisplacement model. For very small x and/or y, the infrared absorption spectra recorded with a Fourier-transform spectrometer clearly reveal the local modes of Mn2+ and Mg2+. Mn2+ exhibits one and Mg2+ three local modes corresponding to their isotopic abundances; the latter have frequencies proportional to 1/ √MMg2+ .
Photoluminescence (PL) and absorption spectra of ZnTe doped with Ca, Sr, and Ba, substituting the cation Zn, exhibit features characteristic of excitons bound in a short-range potential generated by isoelectronic impurities. A no-phonon line (NPL) in both PL and absorption, phonon side bands as mirror images about the NPL, and a doublet splitting resulting from the exchange interaction between an electron and a hole characterize the spectra. Localized vibrational modes for ZnTe:Ca and ZnTe:Sr and a gap mode for ZnTe:Ba are observed in the phonon side bands and, more directly, in the Raman spectra. Ba centers show weaker phonon side bands than those of Ca and Sr. These spectra are compared with the case of the isoelectronic oxygen center in ZnTe, where the anion Te, rather than the cation, is replaced by oxygen.
Photoreflection (PR) measurements on ZnSexTe1-x epilayers were performed in the concentration range x<or=0.5 and 0.9<x. The dependence of the E0 band gap on the composition leads to a bowing parameter of b=1.37+or-0.03 eV. Additional photoluminescence (PL) measurements were used to investigate the localization energy of excitons caused by compositional fluctuations. It is shown that the large broadening of excitons in the PR spectra can be explained by localization effects.
It is shown that a complete series of mixed CdSxTe1-x semiconductors can be produced in the form of polycrystalline films by co-evaporation of CdS and CdTe. This finding is in contrast to previous results on single crystals. The films are homogeneous and highly oriented. The structure changes from cubic to hexagonal around x = 0.5. The energy gap depends highly nonlinearly on composition: it is constant between x = 0 and x = 0.5, and increases from 1.4 to 2.4 eV in the second half of the interval. The native resistivity is about 107 Ωcm between x = 0 and x = 0.5, decreases six orders of magnitude around x = 0.5 and stays constant at about 10 Ωcm for the rest of the interval.Es wird gezeigt, daß eine vollständige Reihe des Mischhalbleiters CdSxT1-x in Form dünner Schichten durch gemeinsame Verdampfung von CdS und CdTe erzeugt werden kann. Dieses Ergebnis steht im Gegensatz zu früheren Ergebnissen an Einkristallen. Die Schichten sind homogen und hochorientiert. Die Struktur wechselt von kubisch zu hexagonal bei etwa x = 0,5. Die Energielücke hängt stark nichtlinear von der Zusammensetzung ab: sie ist zwischen x = 0 und x = 0,5 konstant und steigt in der zweiten Hälfte des Intervalls von 1,4 auf 2,4 eV an. Der spezifische Widerstand der nicht zusätzlich dotierten Schichten liegt bei 107 Ωcm zwischen x = 0 und x = 0,5, sinkt bei x = 0,5 um sechs Größenordnungen auf etwa 10 Ωcm ab und bleibt im Rest des Intervalls konstant.
High purity MgXZn1−XTe alloys in the range of 0 < x < 0.45 have been prepared by the Bridgman technique. Wavelength modulated reflectivity measurements
have been performed at 300 K. The structure of the spectra near the fundamen-tal edge is similar to the one for ZnTe. The
bandgap va-riation versus the composition x is Eg (x) = 2.271 + 0.7 x + 0.6 x2. Cathodoluminescence at 300K and 110K shows a single bandedge narrow peak. The variation with composition of the emission
peak energy Ep (X) follows the same law as Eg (x). Undoped crystals are highly resistive (> 105Ωcm) but when phosporus is introduced into the melt, the resis-tivity decreases to 103 Ωcm.
The electron and/or hole ionization energy of imperfections in single crystals of cadmium sulfo‐selenide solid solutions have been measured by a variety of photoelectronic techniques including optical absorption, optical quenching of photoconductivity, thermal quenching of photoconductivity, photoconductivity decay, and thermally stimulated conductivity. Both the optical and thermal hole ionization energy of sensitizing center acceptors undergo an abrupt decrease from a value characteristic of cadmium sulfide (∼1 eV) to a value characteristic of cadmium selenide (∼0.6 eV) between 30% and 50% cadmium selenide. The same transition is observed as an abrupt increase in the energy of the imperfection absorption edge, corresponding to the optical electron ionization energy of these centers. The electron ionization energy of electron traps varies approximately proportionally to the band gap throughout the solid solution range.
We present the results of experimental investigations on bandgap bowing and spontaneous ordering in MOVPE grown ZnSe$_{x}$Te$_{1-x}$. The bowing parameter of b = 1.40 ± 0.05eV found from different optical measurements on our epilayers is slightly larger than values found earlier from bulk material, indicating the possibility of spontaneous ordering in our samples. Ab-initio calculations for Zn$_{2}$SeTe in the ordered CuAu- and CuPt-structures lean support to this possibility. In fact X-ray measurements performed on our samples with x = 0.39 and x = 0.54 show peaks, forbidden in the zincblende structure, which give evidence for spontaneous ordering in the CuPt structure. Author Keywords:
- Klein
- Vogelgesang