| Among theⅢ-Ⅴsemiconductors,the nitrides(BN,AlN,and GaN) have attracted both scientific and technological interest in recent years. This is due to the fascinating mechanical properties of them,such as hardness,high melting point,high thermal conductivity,large bulk modules.These materials can therefore be used for short-wavelength light-emitting diodes,laser diodes,and optical detectors,as well as for high-temperature,high-power,and high-frequency devices,zinc-blende zinc sulfide(ZnS) and zinc selenide(ZnSe) are wide bandⅡ-Ⅵsemiconductor.They are extremely useful in the manufacture of semiconductor devices.They are also the most important materials for image display applications.Based on the density-function theory(DFPT) and density-function perturbation theory(DFPT),we report first-principles calculation of structure,lattice-dynamical and dielectric properties for zinc-blende and wurtzite AlN and GaN,i.e.(1) the lattice constants,the bulk moduli,and the elastic constants.A linear-response approach is used to derive the Born effective charge,the high-frequency dielectric constant,and the frequencies in ground state.(2) the structure,elastic properties and lattice-dynamical properties under hydrostatic pressure for zinc-blende and wurtzite AlN and GaN.(3) the thermodynamic properties of zinc-blende BN including the PTV equation of state and thermal expansion coefficient and heat capacity at constant pressure with temperature for wurtzite GaN.(4) the phonon frequencies with pressure, the thermal expansion coefficient and heat capacity at constant pressure with pressure for zinc-blende ZnS and ZnSe.Above these properties are performed using a pseudopotential plane wave method。The results of this paper show that:(1) The results for lattice constants,elastic constants,Born effective charges,dielectric constant are in good agreement with the experimental data available.Owing to different electronegative and atomic radius for atom Al and Ga,which resulting in different relative and absolute strengths of the ionic versus the covalent bonding,this results in different properties for zinc-blende and wurtzite AlN and GaN in ground state.(2) The C11,C12 in zinc-blende AlN and GaN,and the C11,C12,C13 and C33 in wurtzite AlN and GaN depend significantly on hydrostatic pressure.Much weaker dependence on pressure has been observed for C44 elastic constant in both zinc-blende and wurtzite phases.It is possible to obtain elastic constants of wurtzite material from the zinc-blende ones applying the so-call Murtin's transformation.Comparing the results of Murtin's transformation and DFPT,the result using DFPT in this calculation is advantages to one of Martin's transformation.(3) Except for the Born effective charge along c axis for wurtzite AlN, the other Born effective charges and high-frequency dielectric constants for both zinc-blende and wurtzite AlN and GaN decrease with rising pressure.This exception is likely due to the dehybridization that accompanies the tetrahedron deformation.In general,the pressure(and hence,volume)dependence is more pronounce for GaN because of stronger covalent bonding in AlN.(4) The longitudinal optic(LO) and transverse optic(TO) phonon frequencies in Brillouin zone center for the zinc-blende and wurtzite AlN and GaN are monotonic increase with rising hydrostatic pressure.The pressure dependences of all vibrational modes related to optical phonon branches are rather similar and independent of the polytype of the nitride. Small deviation between GaN and AlN can be traced back to stronger covalent bonding in AlN.(5) We obtain the PTV equation of state of zinc-blende BN from the free energy using phonon dispersion.The room temperature values of lattice constant is in better agreement with the experimental than T=0Kcalculated value.The effect of pressure is to reduce the thermal expansion coefficient.(6) The anisotropic thermal expansion of wurtzite GaN is studied by density-function perturbation theory.The thermal expansion coefficient along a axis is always larger than one along c axis.For zinc-blende ZnS and ZnSe,wurtzite GaN,the thermal expansion coefficient are negative at low temperature,which can be explained as follow:in the quasiharmonic approximation(DHA),the thermal expansion coefficient at low temperature,the excited phonon modes are predominantly of transverse acoustic type which have negative Gr(u|¨)neisen parameter, giving negative values of thermal expansion coefficient.(7) The calculated result of heat capacity at constant pressure of zinc-blende ZnS,ZnSe and wurtzite GaN are compared with the available experimental data in a wide temperature range.Generally in low temperature range,they have good agreement.But in high temperature range,due to anharnonic effect and the samples themselves forming more defects,lead to large errors between theoretical results and available experimental data.
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