Transverse Effective Charge Research Articles

Electronic and optical properties of MgTe quantum dots: size effect

The size-dependent electronic and optical properties of magnesium telluride quantum dots are investigated using a pseudopotential scheme. The emphasis is focused on the effect of the size of the quantum dots on the undertaken properties. The quantum dot radius is considered to be varying in between 1 and 10 nm. It is found that the direct- and indirect band gaps as well as the electron and hole effective masses decrease non-linearly as the quantum dot radius is increased. However, the refractive index, the static- and high frequency dielectric as well as the transverse effective charge increase as the quantum dot size is augmented. The quantum confinement is found to play a major role for the studied physical quantities as long as the quantum dot radius remains less than 5 nm. This permits to tailor the electronic and optical properties of our material according to the desired application.

Lattice Vibration, Optical and Mechanical Properties of AlP Semiconductor under the Influence of Pressure

The lattice vibrations, optical, and mechanical properties of zinc-blende AlP semiconductor have been investigated. Research has been carried out to determine how pressure affects various properties, including the refractive index, optical and static dielectric constants, longitudinal and transverse sound velocities, reflectivity, susceptibility, phonon frequencies, ionicity, transverse effective charge, and micro-hardness. The pseudo-potential method (EPM) was used to perform the computations in this article. A fair degree of agreement is seen when comparisons are made with the available experiment and other theoretical calculations.

Mechanical, optical, and lattice vibrational properties of gallium antimonide (GaSb) semiconductor under the influence of temperature

We have determined the optical, mechanical, and lattice dynamic features of the zinc-blende GaSb compound. It has been investigated how temperature affects longitudinal and transversal sound velocities, reflectivity, phonon frequencies, micro-hardness, and transverse effective charge. Additionally, the dependences of the effective charge, ionicity, bending, stretching force constants, susceptibility, Cauchy, and Born ratios on the temperature of zinc-blende GaSb material, have been calculated. The pseudo-potential method (EPM) has been used to perform the computations in this paper. Comparative analysis with the existing experiment and other theoretical calculations reveals a respectable degree of agreement.

Effect of hexagonality on the pressure-dependent lattice dynamics of 4H-SiC

The pressure-dependent lattice dynamics of 4H-SiC is investigated using diamond anvil cell, and compared with those of 3C- and 6H-SiC. It is found that both the zone-center longitudinal optical (LO) and transverse optical (TO) modes shift to higher frequencies with the increase of the applied pressures. This indicates that polymorph transitions are unlikely to happen under the (quasi-)hydrostatic pressure. The LO–TO splitting is described well by the cubic function with respect to the applied pressure. A decrease in the LO–TO splitting is observed above 33 GPa. The change of transverse effective charge and thus the ionic character of 4H-SiC exhibits a cubic dependence on the pressure due to the nonequivalent lattice dynamics parallel and perpendicular to the c-axis of 4H-SiC. Compared to what happens in 6H-SiC, the high pressure exerts higher effect on the ionic character of 4H-SiC because less nonequivalent bilayers are evolved. At last, the mode-Grüneisen parameters of the LO and TO modes at the Γ point are determined. Given the hexagonal lattice of 4H-SiC, the LO mode are softer than the TO mode.

Electronic, optical, and mechanical properties of AlxIn1−xP alloys under temperature and pressure

In this work, the optoelectronic and mechanical properties of AlxIn1−xP combinations in the zinc-blende structure were studied for different Al concentrations. The energy band gaps left( {{text{E}}_{{text{g}}}^{{text{L}}} ,,{text{E}}_{{text{g}}}^{Gamma } ,,{text{E}}_{{text{g}}}^{{text{X}}} } right), refractive index (n), high frequency and static dielectric constants (ɛ∞, ɛo), elastic parameters (C11, C12, C44) were investigated. Other mechanical properties such as bulk (Bu), shear (Cs), Young’s (Y0) moduli, Poisson ratio (σ), linear compressibility (C_{o} ), Cauchy (C_{a} ) ratio, isotropy factor (A), bond stretching parameter (alpha ,), bond-bending force parameter (beta ,), internal-strain parameter (xi ), and the transverse effective charge (e_{T}^{*} ) were calculated. Also, the temperature and pressure dependences of these properties were studied. Our estimations were made with the empirical pseudo-potential method combined with the virtual crystal approximation incorporated the compositional disorder impact. There was a reasonable agreement between our determined outcomes and the accessible experimental values for the binary materials AlP and InP which give help for the consequences of the ternary combinations.

Pseudopotential study of wide band-gap GaN at high pressures

A pseudopotential approach is used to study the lattice and elastic properties of the wide band-gap GaN at zero and high pressures up to 120 kbar. When the pressure is 0 kbar, our findings are generally in agreement with the data reported in the literature. The pressure dependence of lattice constant, polarity, transverse effective charge, elastic constants and their related mechanical parameters, and microhardness has been examined and discussed. Our results show that all these features exhibit a monotonic behaviour against pressure. Upon compression up to 120 kbar, our results suggest that the material in question remains mechanically stable with higher stiffness, becomes more resistant to the deformations or deflections and its chemical bond and rigidity become stronger.

Electronic and optical properties of InSb quantum dots from pseudopotential calculation

The electronic and optical features of InSb spherical quantum dots have been investigated by a pseudopotential approach as a function of their radius taken in the range 1-10 nm. The direct- and indirect band gaps along with the electron and heavy hole effective masses are all found to be diminished as the quantum dot radius is increased. However, the refractive index, the static- and high frequency dielectric constant as well as the transverse effective charge are shown to be augmented by increasing the quantum dot radius. It is noted that the quantum confinement is of great impact on all the studied quantities for quantum dot radius below 6 nm. This could result in more opportunities to obtain desired optoelectronic properties that cannot be obtained in the bulk InSb materials.

Comparative study of the pressure dependence of optical-phonon transverse-effective charges and linewidths in wurtzite InN

We investigate the hydrostatic pressure dependence of the zone center optical phonons of $c$-plane and $a$-plane wurtzite InN epilayers grown on GaN substrates. The longitudinal to transverse mode splitting for the ${A}_{1}$ and ${E}_{1}$ modes was found to increase with increasing pressure, whereas the associated transverse effective charge decreases for both modes as ${e}_{T}^{*}({A}_{1})=2.93--9.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}P$ and ${e}_{T}^{*}({E}_{1})=2.80--10.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}P$ (in units of elementary charge and $P$ in GPa). These observations are well in line with results for other II--VI, III--V, and group-IV semiconductor compounds as far as the relation between the magnitude and sign of the pressure derivative of ${e}_{T}^{*}$ and the bond ionicity is concerned. As the latter increases so does $|\ensuremath{\partial}{e}_{T}^{*}/\ensuremath{\partial}P|$ with a sign change from positive to negative for bond ionicities around ${f}_{i}=0.46$ for compounds with anions belonging to the first row of the Periodic Table. A comparison of the results for InN and other nine tetrahedrally bonded compounds indicate that the pressure behavior of the transverse effective charge is mainly determined by the strength of the Pauli repulsion between cation valence electrons and those of the anion core. We also perform ab initio calculations in order to address the origin of the observed increase in linewidth of the ${E}_{2}^{\text{high}}$ mode which is found to arise from a pressure-induced increase in the rate of two-phonon decay processes. This broadening is associated with tuning into resonance of a steep edge in the two-phonon density of states around 460 ${\mathrm{cm}}^{\ensuremath{-}1}$ with the frequency of the ${E}_{2}^{\text{high}}$ mode.

Raman shifts and photoluminescence of the InSb nanocrystals ion beam-synthesized in buried SiO2 layers

The InSb nanocrystals embedded in buried SiO2 layers were obtained by the In+ and Sb+ ion implantation into the SiO2 layers thermally grown on Si wafers followed by the hydrogen transfer of a Si film and high-temperature annealing at 800–1100 °C. Transmission electron microscopy, Raman spectroscopy and photoluminescence were used to study the sample properties. The spherical-shaped InSb nanocrystals distributed close to the implanted atom profiles were obtained in buried SiO2. The InSb TO- and LO-like phonon modes at 187 cm−1 and 195 cm−1 were observed in the Raman spectra. The effect of both phonon quantum confinement and stresses on the phonon frequency shift was calculated. The TO-LO splitting obtained from the ion-beam synthesized InSb nanocrystals was 3 cm−1 less than that from the unstressed bulk monocrystalline InSb. The obtained effect is discussed in the frames of decreasing the transverse effective charge, as well as that of the surface phonon influence. The photoluminescence peak at 1524 nm (0.81 eV) was seen in the low-temperature PL spectra from the samples annealed at 900–1000 °C. Its energy position corresponds to the localized charge carrier energy in the InSb nanocrystals.

Electron states, effective masses and transverse effective charge of InAs quantum dots

The electron energy levels, direct energy band gaps, electron and hole effective masses as well as the transverse effective charge of InAs spherically shaped quantum dots have been studied as a function of the quantum dot radius considered as varying from 1 to 10 nm. The direct energy band-gap as well as the electron and heavy hole effective masses decrease non-linearly with increasing the quantum dot radius. Nevertheless, the transverse effective charge is found to increase with increasing the quantum dot radius. It is concluded that the quantum confinement has a strong influence on all the studied physical quantities for quantum dot radius below 6 nm. The results of the present contribution show that more opportunities can be offered to tailor desired optoelectronic properties surpassing those presented by bulk InAs materials.

Energy levels and optical properties of GaN spherical quantum dots

Using the pseudopotential approach, we have undertaken a detailed study of the energy levels and optical properties of zinc-blende GaN spherical quantum dots as a function of their radius taken in the range 1–10nm. Features such as energy levels, energy band gaps, electron and heavy hole effective masses, refractive index, dielectric constants and transverse effective charge have been investigated. The quantum confinement is found to be of major effect on all the studied quantities for quantum dot radius below 5nm. Beyond 5nm, all properties of interest tend towards retrieving the bulk properties.

Raman scattering study of InAs nanowires under high pressure

The pressure-dependent phonon modes of InAs nanowires have been investigated by Raman spectroscopy under high pressure up to ∼58 GPa. X-ray diffraction measurements show that InAs nanowires at 21 GPa exhibit a phase transition from a wurtzite to an orthorhombic crystal structure, with a corresponding drastic change in the first-order Raman spectra. In the low-pressure regime, a linear increase in phonon frequencies is observed, whereas splitting between longitudinal and transversal optical phonon modes decreases as a function of applied pressure. The calculated mode Grüneisen parameters and Born’s transverse effective charge indicate that the wurtzite InAs nanowires exhibit a more covalent nature under compression.

Investigation of lattice dynamical and dielectric properties of MgO under high pressure by means of mid- and far-infrared spectroscopy

We investigate the lattice dynamical and dielectric properties of MgO single crystals and powders by measurements in the mid- and far-infrared frequency region under high pressures, ranging up to 21.7 GPa. The shift of the restrahlen region is used to determine the pressure dependence of the transverse and longitudinal optical modes. The analysis of the refractive index in the mid- and far-infrared region allowed us to obtain the pressure behavior of the static and electronic dielectric constants. The transverse effective charge slowly decreases under high pressure, reflecting the stability of MgO. As a consequence, the pressure dependence of the static and electronic dielectric constants is mainly determined by the pressure dependence of the polar phonon frequency and Penn gap, resulting in a pronounced decrease of the former and a moderate decrease of the latter.

Optical Properties of Si Quantum Dot Potential under Pressure Effect

Further study of the quantum dot potential under hydrostatic pressure for Si is presented. This potential has been calculated by means of our recent empirical model. The indirect energy gap (Γ-X) is calculated using the full potential-linearized augmented plane wave (FP-LAPW) method. The Engel-Vosko generalized gradient approximation (EV-GGA) formalism is used to optimize the corresponding potential for energetic transition and optical properties calculations of Si. The refractive index and transverse effective charge are predicted under pressure effect. The pressure effect is used to test the validity of our model. The results are compared with others and showed reasonable agreement.

Lattice Dynamics and Crystalline Properties of Wurtzite Zn1–xMgxO Powders under High Pressure

This investigation systematically studies the lattice dynamics and crystalline properties in Zn1–xMgxO using high-pressure Raman spectroscopy. The incorporation of Mg and the application of external pressure cause distinct phonon vibrational behaviors in ZnO. Accordingly, the 202.7, 332.7, and 511.5 cm–1 phonons, which have been controversially assigned, can be conclusively identified. Detailed Raman spectra reveal that the metallic phase transition of ZnO is complete by around 13.2 GPa, which pressure is found to decrease as the Mg content increases. Upon pressure release, an unusual hysteresis effect (>10.0 GPa) in Zn1–xMgxO is observed. The degree of crystal ionicity and anisotropy importantly affects the phase transition pressure of Zn1–xMgxO. Under ambient conditions, ZnO becomes more ionic upon the incorporation of Mg and becomes more covalent under higher pressure. These results are caused by the interplay between the pressure dependence of the high-frequency dielectric constant and Born’s transverse dynamical effective charge. The E1–A1 splitting of the longitudinal and transverse optical phonons is analyzed, yielding insight into the pressure-dependent crystal anisotropy of Zn1–xMgxO.

High-pressure study of the infrared active modes in wurtzite and rocksalt ZnO

We present a high-pressure study of ZnO carried out in the mid- to far-infrared frequency domain with the aim of characterizing the optic modes of wurtzite and rocksalt ZnO. We obtained the pressure coefficients of the E1(TO), E1(LO), A1(TO), and A1(LO) modes of the low-pressure wurtzite phase and compare them with previous Raman measurements. The optical modes of the high-pressure rocksalt phase are infrared active, so we were able to determine their wave numbers and pressure dependencies. In the wurtzite phase, high pressure induces a slight decrease in both longitudinal and transverse effective charges. The decrease is more pronounced in the rocksalt phase.

Investigated optical studies of Si quantum dot

Abstract Further study of the quantum dot potential for Si is presented. This potential has been calculated by means of our recent empirical model. The indirect energy gap (Γ–X) is calculated using the full potential-linearized augmented plane wave (FP-LAPW) method. The Engel–Vosko generalized gradient approximation (EV-GGA) formalism is used to optimize the corresponding potential for energetic transition and optical properties calculations of Si. The refractive index and transverse effective charge are predicted as a function of dot diameter that is in turn used to test the validity of our model. The obtained results show a reasonable agreement in comparison with experimental data and theoretical results.

Phonon deformation potentials in wurtzite GaN and ZnO determined by uniaxial pressure dependent Raman measurements

We report the phonon deformation potentials of wurtzite GaN and ZnO for all zone center optical phonon modes determined by Raman measurements as a function of uniaxial pressure. Despite all the structural and optical similarities between these two material systems, the pressure dependency of their vibrational spectra exhibits fundamental distinctions, which is attributed to their different bond ionicities. In addition, the LO-TO splitting of the A1 and E1 phonon modes is analyzed which yields insight into the uniaxial pressure dependency of Born’s transverse effective charge eT∗.

Reduction of the transverse effective charge of optical phonons in ZnO under pressure

From Raman scattering on a-plane wurtzite ZnO crystals we obtained a decreasing splitting between longitudinal and transversal optical phonons with A1 and E1 symmetry as a function of hydrostatic pressure up to 5.5 GPa. Consequently, the transverse effective charge (eT∗) exhibits a strong reduction with increasing pressure, yielding 2.17–14.6×10−3 P/GPa and 2.04–13.7×10−3 P/GPa (in units of the elementary charge) for the A1 and E1 phonons, respectively. We find a clear systematic in the linear pressure coefficient of eT∗ with bond polarity for the series of wide-band gap semiconductors SiC, AlN, GaN, and ZnO.

Band structure and lattice vibration properties of III-P ternary alloys

Electronic band structure, optical and vibrational properties of zinc-blende GaP x Sb 1− x and GaAs 1− x P x ternary alloys are obtained from pseudo-potential calculations. Comparisons are made with the available experimental values and with data obtained in previous theoretical studies. These comparisons show generally good agreement between the present results and experiment. The direct and indirect band-gap energies, the transverse effective charge, and the longitudinal and transversal optical phonon energies show a non-linear behavior with varying the composition x. The ionicity of the materials of interest is discussed in terms of the antisymmetric gap.