Pressure Coefficient Of Band Gap Research Articles

HgGa2Se4 under high pressure: An optical absorption study

High‐pressure optical absorption measurements have been performed in defect chalcopyrite HgGa2Se4 to investigate the influence of pressure on the bandgap energy and its relation with the pressure‐induced order–disorder processes that occur in this ordered‐vacancy compound. Two different experiments have been carried out in which the sample undergoes either a partial or a total pressure‐induced disorder process at 15.4 and 30.8 GPa, respectively. It has been found that the direct bandgap energies of the recovered samples at 1 GPa were around 0.15 and 0.23 eV smaller than that of the original sample, respectively, and that both recovered samples have different pressure coefficients of the direct bandgap than the original sample. A comprehensive explanation for these results on the basis of pressure‐induced order–disorder processes is provided.

First principles study on structural and electronic properties and defect formation enthalpies of cubic Hf3N4 under high pressure

Using the self-consistent density functional method, we investigate the structural and electronic properties of cubic Hf3N4 with Th3P4 structure at ambient and high pressures. The lattice parameters, cell volume, bulk modulus, and pressure derivative at ambient pressure are obtained, which are in excellent agreement with the available measurements. The change of bond lengths for two different types of Hf–N bond with pressure suggests that the tetrahedral Hf–N bond is slightly less compressible than the octahedral ones, which agree well with the Zr3N4 and Zn3N2 results. The band gap pressure coefficient for c-Hf3N4 are fitted, which are 8.5 × 10−2 eV/GPa and –7.0 × 10−5 eV/(GPa)2, respectively. Based on the density of states analysis, band structure suggests that the investigated material can be used as a semiconductor optical material. Mulliken population analysis shows that the charge density of the Hf atom is less sensitive to pressure variation than that of the N atom. At last, the defect formation enthalpies of the cubic Hf3N4 are calculated.

Residual stress induced crystalline to amorphous phase transformation in Nb2O5 quantum dots

Nb2O5 quantum dots (QDs) were grown using a simple technique of vacuum thermal evaporation. QDs were found to be crystalline in nature by selected area electron diffraction (SAED) in TEM. Samples with thickness up to 20 nm did not show any significant residual strain. Residual stress effect on band gap of crystalline Nb2O5 was studied for films thicker than 20 nm. Residual strain was determined using SAED of the films with reference to powder X-ray diffraction (XRD). Films thicker than 45 nm become amorphous as analyzed by both SAED and XRD. The optical absorption of films in the range 25–60 nm indicates significantly varying optical band gap of films. The varying band gap with film thickness scales linearly very well with the variation of residual stress with film thickness. The residual stress dependence of band gap of crystalline films yields stress free band gap as 3.37 eV with pressure coefficient of band gap (∂Eg/∂P)T = −29.3 meV/GPa. From this study, the crystalline to amorphous transformation in tetragonal form of M-Nb2O5 has been determined to be at about 14 GPa. Both pressure coefficient of band gap and crystalline to amorphous transition for tetragonal M-Nb2O5 have been determined for the first time in the literature.

Residual stress dependant anisotropic band gap of various (hkl) oriented BaI2 films

The thermally evaporated layer structured BaI2 grows in various completely preferred (hkl) film orientations with different growth parameters like film thickness, deposition rate, substrate temperature, etc. which were characterized by structural, morphological, and optical absorption measurements. Structural analysis reveals the strain in the films and the optical absorption shows a direct type band gap. The varying band gaps of these films were found to scale linearly with their strain. The elastic moduli and other constants were also calculated using Density Functional Theory (DFT) formalism implemented in WIEN2K code for converting the strain into residual stress. Films of different six (hkl) orientations show stress free anisotropic band gaps (2.48–3.43 eV) and both positive and negative pressure coefficients. The negative and positive pressure coefficients of band gap are attributed to the strain in I-I (or Ba-Ba or both) and Ba-I distances along [hkl], respectively. The calculated band gaps are also compared with those experimentally determined. The average pressure coefficient of band gap of all six orientations (−0.071 eV/GPa) found to be significantly higher than that calculated (−0.047 eV/GPa) by volumetric pressure dependence. Various these issues have been discussed with consistent arguments. The electron effective mass me*=0.66m0 and the hole effective mass mh*=0.53m0 have been determined from the calculated band structure.

First-principles study of the structural, electronic and thermal properties of CaLiF3

Density functional theory calculations have been performed to study the structural, electronic and optical properties of CaLiF3 cubic fluoroperovskite. Our calculations were carried out by means of the full-potential linearized augmented plane-wave method. The exchange–correlation potential is treated by the local density approximation and the generalized gradient approximation (GGA) (Perdew, Burke and Ernzerhof). Moreover, the alternative form of GGA proposed by Engel and Vosko is also used for band structure calculations. The calculated total energy versus volume allows us to obtain structural properties such as the lattice constant (a0), bulk modulus (B0) and pressure derivative of the bulk modulus (B'0 ). Band structure, density of states and band gap pressure coefficients are also given. Our calculations show that CaLiF3 has an indirect band gap (R–Γ). Following the quasi-harmonic Debye model, in which the phononic effects are considered, the temperature and pressure effects on the lattice constant, bulk modulus, thermal expansion coefficient, Debye temperature and heat capacities are calculated.

Structural, Elastic, and Electronic Properties of CuClxBr(1-x) Compounds under Pressure

We have applied the pseudo-potential plane wave (PP-PW) method to study the structural, elastic, and electronic properties of CuClxBr(1-x) under high pressure using the generalized gradient approximation (GGA). The effect of Cl substitutional impurities (x) on the lattice parameters for both the GGA and virtual crystal approximation (VCA) approaches were studied. The pressures at which the compounds CuClxBr(1-x) undergo a structural phase transition from ZnS type to NaCl type Pt were calculated. The elastic constants at various pressures in the 0 ＜ x ＜ 1 composition range were presented. The longitudinal-wave mode speed (VL) and transverse-wave mode speed (VT) in CuClxBr(1-x) propagating in the [100], [110], and [111] directions at zero pressure for various Cl compositions (x) in the range 0-1 were investigated. The band structure and band gap-pressure coefficients are also given. The calculated effective masses of electrons and heavy and light holes in the material under study for different concentrations x are presented.

Pressure effect on the electronic and optical properties of the FeP2 and FeAs2 compounds

First principles calculations, by means of the full potential linearized augmented plane wave method within the local density approximation, were carried out for the pressure dependence of the electronic and optical properties of the orthorhombic compounds FeP2 and FeAs2. The variation of the energy gaps has the same trend in both compounds. All the first order band gap pressure coefficients are positive except the T−T one in FeP2 and the one corresponding to smallest indirect gap which are negative. The structures in the optical spectra shift towards higher energies when pressure increases. The static dielectric components decrease with pressure and show anisotropic behavior.

Pressure effect on structural, elastic and electronic properties of (B3) BSb compound

In this paper we present the results obtained from first-principles calculations of the pressure effect on the structural, elastic and electronic properties of (B3) boron-antimonide, using the pseudopotential plane wave method (PP-PW) based on density functional theory within the Teter and Pade exchange–correlation functional form of the local density approximation. The lattice parameter constant, crystal density, independent elastic constants, bulk modulus, shear modulus, zener anisotropy parameter and linear and quadratic pressure coefficients of the energy bandgaps under high pressures are presented. In the investigation of the stability criteria, the results show a phase transition from the zincblende (B3) to rock-salt (B1) phase (or to amorphous state) at around 0.72 Mbar, which is generally in good agreement with the available theoretical data reported in the literature.

Hydrostatic pressure dependence of excitonic optical transitions in strained wurtzite GaN/AlN quantum disks

In the framework of the effective mass approximation, the effects of hydrostatic pressure on optical transitions associated with the excitons confined in strained wurtzite (WZ) GaN/AlN quantum disks (QDisks) with the confinement potential of finite depth are investigated by using a variational technique, with considering the influences of the built-in electric field (BEF) and the biaxial strain dependence of material parameters. The Schrödinger equation via the proper choice of the exciton trial wave function is solved. The behaviors of the excitonic optical transition are examined at different pressures for different QDisk sizes. In our calculations, the effective masses of electron and hole, dielectric constants, phonon frequencies, energy gaps, and piezoelectric polarizations are taken into account as functions of biaxial strain and hydrostatic pressure. Numerical results show that the hydrostatic pressure and the QDisk size have a remarkable influence on exciton states. The calculated pressure coefficient of optical transition energy shows a negative value if the QDisk height L>3.2nm, in contrast with the positive pressure coefficient of the GaN band gap. The peculiar pressure behavior is related to the pressure-induced increase of the built-in electric field. For a fixed pressure, the optical transition energy has a red-shift if the QDisk height and radius increase and QDisk height has a more obvious influence on Eph than QDisk radius. Furthermore, the relationship between the radiative decay time and hydrostatic pressure (QDisk height) is also investigated. It is found that the radiative decay time increases with pressure and the increment tendency is more prominent for the large height QDisks. The radiative decay time strongly increases by three orders of magnitude reaching microsecond order if the QDisk height increases from 1nm to 3nm.

High-pressure optical absorption in InN: Electron density dependence in the wurtzite phase and reevaluation of the indirect band gap of rocksalt InN

We report on high-pressure optical absorption measurements on InN epilayers with a range of free-electron concentrations ($5\ifmmode\times\else\texttimes\fi{}{10}^{17}$--$1.6\ifmmode\times\else\texttimes\fi{}{10}^{19}$ cm${}^{\ensuremath{-}3}$) to investigate the effect of free carriers on the pressure coefficient of the optical band gap of wurtzite InN. With increasing carrier concentration, we observe a decrease of the absolute value of the optical band gap pressure coefficient of wurtzite InN. An analysis of our data based on the k\ifmmode\cdot\else\textperiodcentered\fi{}p model allows us to obtain a pressure coefficient of 32 meV/GPa for the fundamental band gap of intrinsic wurtzite InN. Optical absorption measurements on a 5.7-\ensuremath{\mu}m-thick InN epilayer at pressures above the wurtzite-to-rocksalt transition have allowed us to obtain an accurate determination of the indirect band gap energy of rocksalt InN as a function of pressure. Around the phase transition (\ensuremath{\sim}15 GPa), a band gap value of 0.7 eV and a pressure coefficient of \ensuremath{\sim}23 meV/GPa are obtained.

Red-green luminescence in indium gallium nitride alloys investigated by high pressure optical spectroscopy

We performed optical absorption and photoluminescence experiments under high pressure up to 10 GPa on two good quality InGaN epilayers with ∼40% indium. The pressure coefficient of about 30 meV/GPa for the absorption edge is close to the bandgap pressure coefficients of InN and GaN, indicating similar pressure dependence of the fundamental band gap in the whole composition range. In contrast, the pressure coefficient of the photoluminescence peak energy shows much weaker pressure dependence which we attribute to an increasing role of highly localized defects when the conduction band approaches the Fermi level stabilization energy at higher indium contents.

Time Dependence of Current–Voltage Characteristics of Pb/p-Si Schottky Diode under Hydrostatic Pressure

Abstract The effect of time on the characteristic parameters of Pb/p-Si Schottky diodes has been presented as a function of hydrostatic pressure. Current-voltage curves of the Pb=p-Si Schottky diodes have been measured at immediate, 15, 30, 60, and 120 min intervals under 1, 2, and 4 kbar hydrostatic pressure. It has been found that the values of the ideality factor have been approximately unchanged with increasing time. On the other hand, the barrier height of the Pb=p-Si structure slowly increase with increasing time, while these parameters also change with hydrostatic pressure. The diode shows nonideal current-voltage behaviour with an ideality factor greater than unity that can be ascribed to the interfacial layer and the interface states. In addition, the Schottky barrier height increases with a linear pressure coefficient of 92 meV=kbar, which is higher than the pressure coefficient of the silicon fundamental band gap.

Structural, electronic and thermodynamic properties of cubic Zn 3N 2 under high pressure from first-principles calculations

The structural, electronic and thermodynamic properties of cubic Zn 3N 2 under hydrostatic pressure up to 80 GPa are investigated using the local density approximation method with pseudopotentials of the ab initio norm-conserving full separable Troullier–Martin scheme in the frame of density functional theory. The structural parameters obtained at ambient pressure are in agreement with experimental data and other theoretical results. The change of bond lengths of two different types of Zn–N bond with pressure suggests that the tetrahedral Zn–N bond is slightly less compressible than the octahedral bond. By fitting the calculated band gap, the first and second order pressure coefficients for the direct band gap ofthe Zn 3N 2 were determined to be 1.18×10 −2 eV/GPa and −2.4×10 −4 eV/(GPa) 2, respectively. Based on the Mulliken population analysis, Zn 3N 2 was found to have a higher covalent character with increasing pressure. As temperature increases, heat capacity, enthalpy, product of temperature and entropy increase, whereas the Debye temperature and free energy decrease. The present study leads to a better understanding of how Zn 3N 2 materials respond to compression.

Trends in the band-gap pressure coefficients and bulk moduli in different structures of ZnGa2S4, ZnGa2Se4 and ZnGa2Te4

We have performed a first-principles investigation for the family of compounds ZnGa2X4 (X = S, Se, Te). The properties of two possible structures, defect chalcopyrite and defect famatinite are both calculated. We reveal that ZnGa2S4 and ZnGa2Se4 have direct band gaps, while ZnGa2Te4 has an indirect band gap. The local density approximation band gaps are found to be very different in two structures, while the lattice parameters and bulk moduli are similar. We extend Cohen's empirical formula for zinc-blende compounds to this family of compounds. The pressure coefficients are calculated and metallization pressures are discussed. We find that aig remains fairly constant when the group-VI element X is varied in ZnGa2X4(II–III2—VI4).

Anomalous composition dependence of the band gap pressure coefficients in In-containing nitride semiconductors

The pressure-induced changes in the electronic band structures of In-containing nitride alloys, ${\text{In}}_{x}{\text{Ga}}_{1\text{\ensuremath{-}}x}\text{N}$ and ${\text{In}}_{x}{\text{Al}}_{1\text{\ensuremath{-}}x}\text{N}$ are examined experimentally as well as by ab initio calculations. It is found that the band gap pressure coefficients, $d{E}_{g}/dp$, exhibit very large bowing with $x$, and calculations with simulation of clustered distributions of the In atoms over the cation sites show a strong enhancement of this effect. This relates well to the experimental data obtained from pressure dependent photoluminescence measurements for ${\text{In}}_{x}{\text{Ga}}_{1\text{\ensuremath{-}}x}\text{N}$ and ${\text{In}}_{x}{\text{Al}}_{1\text{\ensuremath{-}}x}\text{N}$ layers, performed in this work, and combined with existing data for ${\text{In}}_{x}{\text{Ga}}_{1\text{\ensuremath{-}}x}\text{N}$ layers. We discuss possible explanations of the anomalously large magnitude of the $d{E}_{g}/dp$ bowing in these nitride alloys.

In‐clustering effects in InAlN and InGaN revealed by high pressure studies

AbstractElectronic band structure calculations of InAlN and InGaN under pressure are presented for two different arrangements of the In atoms, uniform and clustered. The band gap pressure coefficients exhibit strong bowing, and the effect is especially large when indium atoms are clustered. The theoretical results are compared with the results of photoluminescence measurements performed at high hydrostatic pressures on InAlN and InGaN quasi‐bulk epilayers. We discuss the modification of the uppermost valence band due to formation of In clusters which, together with the related lattice relaxations, may be a possible source of the observed phenomena.

First-principles calculations of pressure-induced phase transformation in AlN and GaN

Ab initio total energy calculations have been carried out to study pressure-induced wurtzite and zinc-blende to rocksalt phase transformation in AlN and GaN. The effects of d electrons on the phase transition pressure and pressure coefficients of band gap have been studied. It is shown that the presence of Ga 3d electrons do have a certain effect on the transition pressure, while other factors such as the variation of charge distribution with pressure should also be considered to explain the higher phase transition pressure for GaN than AlN. Our calculations also show that Ga 3d electrons affect the pressure coefficients of band gap slightly.

Influence of the electronic states anisotropy on the band gap pressure coefficient of InxGa1−xN alloys

Using the first-principle methods, we investigate the structural and electronic properties of the wurtzite InxGa1−xN (0<x<1) alloys under hydrostatic pressure. We find that the pressure coefficient decreases with the In concentration and becomes constant when the In concentration is higher than some critical value, which is in agreement with experimental results. Our calculations demonstrate that the axial ratio c/a of InxGa1−xN increases with the hydrostatic pressure when x<0.25, while it decreases when x>0.25. The densities of px and py orbitals are higher than that of pz orbital at the valence band top. This anisotropy induces the pronounced bowing of the pressure coefficient.

Structural, elastic, electronic, optical and thermal properties of c-SiGe2N4

We have investigated the structural, elastic, electronic, optical and thermal properties of c-SiGe2N4 by using the ultrasoft pseudopotential density functional method within the generalized gradient approximation. The calculated structural parameters, including the lattice constant, the internal free parameter, the bulk modulus and its pressure derivative are in agreement with the available data. The independent elastic constants and their pressure dependence, calculated using the static finite strain technique, satisfy the requirement of mechanical stability, indicating that c-SiGe2N4 compound could be stable. We derive the shear modulus, Young’s modulus, Poisson’s ratio and Lame’s coefficients for ideal polycrystalline c-SiGe2N4 aggregate in the framework of the Voigt-Reuss-Hill approximation. We estimate the Debye temperature of this compound from the average sound velocity. Band structure, density of states, Mulliken charge populations and pressure coefficients of energy band gaps are investigated. Furthermore, in order to understand the optical properties of c-SiGe2N4, the dielectric function, refractive index, extinction coefficient, optical reflectivity and electron energy loss are calculated for radiation up to 40 eV. Thermal effects on some macroscopic properties of c-SiGe2N4 are predicted using the quasi-harmonic Debye model in which the lattice vibrations are taken into account. We have obtained successfully the variations of the primitive cell volume, volume expansion coefficient, heat capacities and Debye temperature with pressure and temperature in the ranges of 0–40 GPa and 0–2000 K. For the first time, the numerical estimates of the elastic constants and related parameters, and the thermal properties are performed for c-SiGe2N4.

FP-APW + lo study of the elastic, electronic and optical properties of the filled skutterudites CeFe 4As 12 and CeFe 4Sb 12

Using a full-relativistic version of the full-potential augmented plane wave plus local orbitals (FP-APW + lo) method within the local density approximation (LDA), we have studied the elastic, electronic and optical properties of the filled skutterudites CeFe 4As 12 and CeFe 4Sb 12. Structural parameters, including lattice constant, internal free parameters and, bulk modulus and its pressure derivative were calculated. We have determined the full set of first-order elastic constants, Young’s modulus, Poisson’s ratio and the Debye temperature of these compounds. Band structures, density of states, pressure coefficients of energy band gaps are also given. It is found that both CeFe 4As 12 and CeFe 4Sb 12 are indirect band gap semiconductors. The valence band maximum (VBM) is located at Γ point, whereas the conduction band minimum (CBM) is located at N point. Optical constants, including the dielectric function, optical reflectivity, refractive index and electron energy loss were calculated for radiation up to 30 eV. This is the first quantitative theoretical prediction of the elastic and optical properties for these compounds, and it still awaits experimental confirmation.