Polycrystalline cadmium selenide, CdSe, thin films were prepared by chemical bath deposition (CBD) technique. The films were deposited using cadmium chloride as a Cd+2 ion source, and sodium selenosulphite as a Se-2 ion source. Annealing the films in air for 10 minutes at 100 0C-350 0C affected the grain growth. The annealed films were returned to room temperature either by quenching or slow cooling. The energy band gap (Eg) decreased with increasing thicknesses and substrate temperatures. Value of Eg calculated from UV/visible absorption spectra ranged between 2 and 1.83 eV. The used film thickness was ~10μm. Annealing and rate of cooling that present the best photoluminescence (PL), photo and dark currents for the film electrodes are discussed here. Covering CdSe thin films with metalloporphyrine complex, embedded inside polymeric polysiloxane matrices, enhanced the electrode efficiency and stability. The deposited films were investigated by optical PL and UV absorption spectra.
Monday, January 18, 2010
FP-LAPW CALCULATIONS OF THE STRUCTURAL PHASE TRANSFORMATIONS OF ZnSe AND ZnS UNDER HIGH PRESSURE
ABSTRACT FP-LAPW CALCULATIONS OF THE STRUCTURAL PHASE TRANSFORMATIONS OF ZnSe AND ZnS UNDER HIGH PRESSURE At present, calculations of ground–state energy of II–VI and III–V compounds are computed relatively slowly and expensively, even on supercomputers. Thus, an efficient, inexpensive and accurate method of calculations is very important. The Full-Potential Linearized Augmented Plane-Wave (FP-LAPW) method have been used to investigate the structural phase transformations of ZnS and ZnSe under high-pressure. In these calculations, the local density approximation (LDA) for the exchange-correlation potential have been used. Equations of states (EOS's) of the zinc-blende (ZB), rock-salt (RS), cinnabar and simple cubic-16 (SC16) of the ZnS and ZnSe have been calculated. From these EOS's, the structural phase transformations of both ZnS and ZnSe under high pressure were investigated by treating the 3d-electrons of Zn and Se as valence states. Moreover, the structural properties and the electronic structures of the ZB, RS, cinnabar and SC16 phases of ZnS and ZnSe have also been calculated. The most important results are: (1) our calculations agree very well with the available experimental data and the other theoretical calculations. (2) the relaxation of the internal parameters of the (ZnS and ZnSe) in cinnabar and SC16 structures have important effects on its calculated equation of state. (3) the ZB and RS forms of ZnS are found to be semiconductors up to the transition pressure. (4) the ZB form of ZnSe is found to be semiconductor while the RS of ZnSe is found to be semimetal with narrow energy band-gap of 0.5eV. (5) the cinnabar and SC16 structures of ZnS and ZnSe which lies between ZB and RS are expected to be semiconductors. (6) SC16 is a stable phase and relatively more stable than RS and cinnabar phases for ZnS and ZnSe.
Ga1−xMnxN Magnetic Semiconductors: First-Principles Study
Abstract
We present the results of First-Principles calculations of the magnetic semiconductors for systems taking the concentrations (0.0,0.125, 0.25, 0.50, 0.75 , 1.00) in the Zinc-blende Structure (ZB-Structure), using a self-consistent full-potential linearized augmented plane-wave (FP-LAPW) method implemented by the WIEN2K package. The local spin density approximation (LSDA) as well as the generalized gradient approximation (GGA) are used to treat the exchange correlation potential, and taking into account spin polarization.
In order to design new or employ the existing diluted magnetic semiconductor materials, the underlying mechanisms of magnetism must be understood. The total energy versus lattice constant is obtained using the spin density functional theory (DFT). It is found that the equilibrium lattice parameters strongly depend on the concentration of the Mn-dopant (x ). Also we found that the energy band gaps ( ) for these systems depend on (x ), in other words the energy band gap decreases by increasing the Mn concentration.
We mainly studied the Bulk parameters of our system, band structures, and magnetic properties. We made numerical investigations of the structural, magnetic properties for simple cases under pressure, in other part of our results we report an analysis of structures, magnetic properties of the , i.e GaN doped with Mn, with different concentrations, 0.125, 0.25, 0.5, 0.75 and so for major, minor compounds (GaN, MnN).
XO(X=Be, Zn) COMPOUNDS UNDER HIGH PRESSURE
Abstract
The structural phase transformations of semiconductors under high pressure have recently attracted a lot of attention. Experimental studies in this field are very difficult and expensive, the computational physics programs make these studies very easy, very accurate and inexpensive. The computational approach enables us to know the atomic structures of materials, the electronic properties and give the chance to modify the bonding between atoms to create novel materials with predetermined properties. In the present study the Full-Potential Linearized Augmented Plane-Wave (FP-LAPW) (which is included in a computer code WIEN2K) method depending on the Density Functional Theory (DFT) were used to investigate the structural phase transformations of BeO and ZnO compounds under high pressure. In these calculations, the local density approximation(LDA), the gradient generalized approximation (GGA) and the modified Wu- Cohen-GGA approximation for the exchange correlation potential have been used. For BeO the equations of state (EOS’s) of wurtzite(WZ), zinc-blende(ZB) and rock salt (RS) have been calculated. From these (EOS’s) the transition under high pressure is occurred from wurtzite to rock salt and from zinc-blende to rock salt structures, the transition pressure and the structural properties have also been calculated. The energy band gap for all phases of BeO have been calculated and a large
band gap was found to be (6 ~ 8 eV) which is indicating that BeO is a good insulator.
The same work was done for ZnO using the same method and the same approximations. A number of transition phases is predicted for ZnO, wurtzite to rock salt, wurtzite to cesium chloride, zinc-blende to rock salt, zinc-blende to cesium-chloride and rock salt to cesium-chloride. The transition pressure for each case was calculated. The structural properties have also been calculated and finally the energy band gap for each phase was investigated.
Small energy band gap (0.3 ~1.5eV) is found, which means that ZnO behaves as a semiconductor.
The most important results of this study are:
1- The present calculations agree very well with the available experimental data and the other theoretical calculations.
2- The transition from structure to another is possible under high pressure.
3- BeO behave as an insulator in all its structures.
4- Wurtzite found to be the ground state for BeO compound at zero temperature.
ZnO behave as a semiconductor in all its structures except in cesium-chloride structure it behaves as a semi-metalEffects of a Uniform Applied Magnetic Field and Temperature on the Magnetic Properties of the Dipolar Anti-ferromagnetic planar System: Parametric
Abstract
The effects of a uniform external magnetic field, with strength parameter of h, on the magnetic properties of a two-dimensional square dipolar antiferromagnetic planar system, with sizes (104 × 104,64 × 64,32 × 32), have been determined for both zero and finite temperatures. In this study, the classical spins are confined to the plane of the system and interact through a nearest neighbor antiferromagnetic exchange interaction, the long-range dipolar interaction, and a uniform external magnetic field along the axis of the lattice. Throughout, the strength of the exchange interaction is assumed to be antiferromagnetic and fixed at −1.2g, where g is the strength of the dipolar interaction. At zero temperature, the ground state calculations show that the system switches from ferromagnetic phase (FE phase) to the dipolar antiferromagnetic phase (AF phase) at ho = 6.00g as the applied field is decreased. As the applied field is decreased further, the spin configuration starts to turn antiferromagnetically perpendicular to the applied field in a continuous manner. As the applied field goes to zero, the system favors the dipolar antiferromagnetic in which the spins are aligned perpendicular to the field (AF1 phase). At finite temperature, the magnetic phase diagram for the system has been determined as a function of both h and T using Monte Carlo simulations. At low temperatures, the results from simulations show that the system exhibits a first order transition from the ferromagnetic phase to the dipolar phase (AF phase) as the field is decreased. When the applied field goes to zero, the system favors the dipolar phase in which the spins are ordered at with the axis of the lattice (AF2 phase). At low fields, the Monte Carlo results indicate that the system exhibits a second order transition from the dipolar antiferromagnetic phase to the paramagnetic phase as the temperature is increased. However, at high fields and for low temperatures the system favors the ferromagnetic phase. As the temperature is increased the system gradually disorders. In addition, Monte Carlo simulation results show that there exists a range of the magnetic field values in which the system exhibits a first order reorientation transition from the dipolar antiferromagnetic phase to the ferromagnetic phase as the temperature is increased.
First Principle Electronic Structure Calculations of Ternary alloys. BNxP1-x, GaxB1-x N and BxIn1-x N in zinc blende structure
Abstract
In this work full-potential linearized augmented plane wave method (FP-LAPW) within the density functional theory (DFT) and within generalized gradient approximation (GGA) are used to investigate, electronic band structure, structural properties and thermodynamic properties of III (In, B, Ga) - V (N, P ) compounds and their ternary alloys of BNxP1-x, GaxB1-xN, BxIn1-xN in zinc blende structure.
The present DFT-GGA calculations have shown direct band gap energy in zinc-blende phase for InN, GaN, and indirect band gap energy for BN and BP. Here, the conduction band minima of both InN and GaN are located at Γ point, while that of BN is at a position lying along Γ–X direction and BP at Γ–∆min .
In our work we have found that the band gap engineering of BNxP1-x, GaxB1-xN, BxIn1-xN alloys have been studied for range of constituents (x= 0.25, 0.50, 0.75). The downward band gap bowing seems the largest in GaxB1-xN alloy comparable with the other alloys considered in this work. Even for a small amount of contents (x), adecrease of the electronic effective mass around Γ point appears for BNxP1-x, GaxB1-xN, BxIn1-xN alloys manifesting itself by an increase of the band curvature .
The calculated cross over from indirect to direct band gap of ternary alloys has been found to be consistent with the experimental measurements.
At last, the determinations of the band gaps of alloys as a function of contents, the concentration range of constituents leading to metallic character of the alloys, the change of the electronic effective mass around (Γ) as a function of the cross over from indirect to direct band gap of the alloys which are direct on one end, indirect on the other end are main achievements in this work.
We have found also that the optimize volume and the thermo- dynamic properties were different with different concentrations of the component of their ternary alloys above and we have also analyzed the relative stability, the bulk modulus, and the minimized energy of these ternary compounds.
Electronic, Structural and Magnetic Properties of Al1−xMnxN in Zincblende Structure:
Abstract We apply a First-Principles method to calculate the electronic structure and magnetic properties of the semiconductors Al1-xMnxN alloys by taking the concentrations (0.0, 0.25, 0.50, 0.75 , 1.00) in the zincblende structure (ZB), using a self-consistent full-potential linearized augmented plane-wave (FP-LAPW) method within the local-spin-density functional approximation (LSDA) and the generalized gradient approximation (GGA). Local spin density approximation (LSDA) and the generalized gradient approximation (GGA) are used to treat the exchange correlation energy. We studied the evolution of the band structure and magnetic moment as a function of the lattice parameter of the MnN compounds and the ternary alloys Al1-xMnxN. The binary compound MnN and also the ternary alloys with(x=0.5 and 0.75) magnetization increases as the lattice parameter increase and tend to saturate at the value 4µB for MnN and 8 µB for the ternary alloys, as the material lattice (MnN) expansion the material goes from paramagnetic to ferromagnetic phase. We also found that the ternary alloy with x=0.25 is ferromagnetic and candidate to be half-metallic material, the majority spin states are metallic and the minority spin stats are insulating, for the other concentrations(x=0.5 and 0.75) these are found to be ferromagnetic semimetals, the bands are crossing the Fermi energy for both spin up and spin down (majority spin and minority spin) (the Fermi level lies in the band). The total energy versus lattice constant is obtained using the spin density functional theory. It was found that the equilibrium lattice parameter and the total magnetic moment strongly depend on concentration of Manganese atoms.
Electronic and Structural Properties of SCSb and Sc punder high pressure
Abstract In the present study, the Full-Potential Linearized Augmented Plane Wave method that depends on the Density Functional Theory was used to investigate the structural phase transformations of ScSb and ScP compounds under high pressure. In these calculations, the local density approximation (LDA) and the gradient generalized approximation (GGA) for the exchange correlation potential have been used . For ScSb the equations of state (EOS's) of rock- salt(RS), cesium chloride(CsCl), zincblende(ZB) and wurtzite(WZ) have been calculated, from these (EOS's), it is found that a transition under high pressure is occurred from rock salt structure to cesium chloride structure. The transition pressure and the structural properties have also been calculated, the energy band gap for all phases of ScSb have been calculated and (-0.873,-0.683, 1.434, 1.481) eV band gaps were found and indicating that ScSb is semimetals and semiconductors respectively. The same work was done for ScP using the same method. A number of transition phases are predicted for ScP ; rocksalt to cesium chloride and wrutzite to zincblende. The transition pressure for each case was calculated. The structural properties have also been calculated, Finally the energy band gap for each phase was investigated. (-0.787,-0.583, 1.578, 1.6249)eV energy band gaps are found, which means that ScP behaves as metal, semimetal and semiconductor respectively. The most important results of this study are: 1. the structural parameters agree very well with the available experimental data and the other theoretical calculations. 2. the transition from structure to another is possible under high pressure, for ScSb the transition pressure from rocksalt to cesium chloride was found to be (31.5)GPa by LDA method and 35.4 GPa by GGA method. 3. For ScP the transition pressure from rocksalt to cesium chloride was found to be 69 GPa and from wurtzite to zincblende was found to be 84GPa by LDA method, while the transition pressure from rocksalt to cesium chloride was found to be 73.4 GPa and from wurtzite to zincblende was found to be 88 GPa by GGA method . 4. ScSb behaves as semimetal for rocksalt, metal for cesium chloride and semiconductor for zincblende and wurtzite phases using both LDA and GGA methods. 5. ScP behaves as semimetal for rocksalt, metal for cesium chloride and semiconductor for zincblende and wurtzite phases using both LDA and GGA methods. 6. Rocksalt was found to be the ground state for ScSb and ScP compound at ambient conditions.
FP-LAPW Study of Phase Changesin An(A = Al,IN) under high Pressure
Abstract In the last few years no other class of material of semiconductors has attracted so much scientific and commercial attention like the group III-nitrides( AlN, BN, and InN). The increasing interest is due to its extraordinary physical properties, which can be used in many new electronic and optoelectronic devices. The AlN is stable to very high temperatures in inert atmospheres. Another stable material in inert and reducing atmospheres is BN. It is a very good electrical insulator. It offers very high thermal conductivity and good thermal shock resistance. InN has attracted considerable attention due to the repeated observation of an effective band gap in the range around 0.7 eV by optical techniques, this smaller band gap value would extend the possible emission range of optoelectronic devices based on III-nitrides from the deep-UV down to the near-IR region. Very prominent examples are the short wavelength Light emitting diodes (LED’s) and laser diodes, which take advantage of the wide band gap of AlN. InN also has been expected to be a suitable material for electronic devices such as high mobility transistors due to its small effective mass. The effect of pressure on the electronic properties of (AlN, BN, and InN) are investigated using both experimental and theoretical methods. In this study, we carry out all-electron full potential linearized-augmented plane waves (FP-LAPW) (which is included in a computer code WIEN2K) approach within the density functional theory (DFT) in the local density approximation (LDA), and the generalized gradient approximation (GGA) for the exchange correlations functional, which used to calculate ground-state energies, the lattice parameters, the bulk modulus and its derivatives, transition pressure and the band structures. The equation of state of wurtzite (WZ), zincblende(ZB) and rocksalt(RS) structures for (AlN, BN, and InN) compounds have been calculated. In this study, the most important results are: 1. The present calculations agree very well with available experimental data and other theoretical calculations. 2. AlN compound behaves as an insulator in (WZ, ZB, and RS) structures. 3. BN compound behaves as a semiconductor for RS and ZB in LDA calculation and an insulator for RS and ZB in GGA calculation. 4. InN compound behaves as a semimetal in (WZ, ZB, and RS) structures. 5. The energy band gap for (WZ, RS, and ZB ) structures of AlN are found to be (4.42, 4.032, 2.7) eV respectively, using LDA method, and (4.17, 4.34, 3.275) eV respectively, using GGA method. 6. The energy band gap for (ZB and RS) structures of BN are found to be (4.36, 2.193) eV respectively, using LDA method, and (4.43, 1.71) eV respectively, using GGA method. 7. The energy band gap for (WZ, RS, and ZB ) structures of InN are found to be ( -0.264, 0.0838, -0.3896) eV respectively, using LDA method, and (-0.3643, -0.277, -0.5136) eV respectively, using GGA method. 8. For AlN the transition pressure from wurtzite to rocksalt was found to be (10) GPa and from zincblende to rocksalt was found to be (4.64) GPa using GGA method, while the transition pressure from wurtzite to rocksalt was found to be 9.3 GPa and from zincblende to rocksalt was found to be 3 GPa using LDA method. 9. For InN the transition pressure from wurtzite to rocksalt was found to be 16.6 GPa and from zincblende to rocksalt was found to be 18.5 GPa using GGA method. 10.The transition pressure for BN compound from zincblende to rocksalt was found to be 500 GPa using GGA method.
Confined hydrogen atom in spherical cavity in N - dimensions
Abstract
In this research the Schrödinger equation for a confined hydrogen atom in a spherical cavity in N dimensional spatial space has been solved for N . The eigen functions as well as the eigen values have been determined.
We show that the Schrodinger equation here doesn’t differ from that of the free hydrogen atom in N dimensions; therefore they have similar wave functions namely
“ l (ρ) = A e 1F1 (l+ – λ; 2l + N – 1; ρ)”, while they differ in energy. A series solution of the Schrödinger equation is adopted here, and then, by applying the boundary conditions to the wave functions we found the energy eigen-values.
The dependence of the ground state energy eigen-values of a confined hydrogen atom for l = 0 for certain values of N, on the radius of the cavity S, has been examined. We found that they depend on the radius of the cavity S, we show that for a given N, if S increases the ground state energies decreases until they approach a limiting value which approaches the energy eigen value of that N of the free hydrogen atom. While as S decreases, the ground state energy eigen values increases up until it approaches zero at a minimum value of S that is called the critical cage radius (Sc ) at which the total energy of the confined hydrogen atom equals zero.
The critical values Sc are calculated for dimensions from (2-10), whose values are 0.722890, 1.835247, 3.296830, 5.088308, 7.200250, 9.617367, 12.35000, 15.36350, 18.68200 respectively, (all the values here are multiples of Bohr radius (𝑎0), where (𝑎0) = 0.529x meters.
It is shown here that Sc increases as N increases.
It is also shown that for a given S, the energy eigen-values for l=0 depend on the dimensionality of space N, that is, as N increases, the ground state energy eigen-values increase.
The dependence of bound states of a confined H-atom, for a given S, as a function of N is investigated, and it is found that it decreases as N increases, while if we choose a larger value of S, the number of the bound states increases for each value of N.
We found it interesting to compare the energy eigen-values of a confined hydrogen atom in a spherical cavity of a certain radius, with those energies of the corresponding energy eigen-states of a free hydrogen atom in the same dimension N. We found that the effect of confinement becomes more profound for larger N.
Finally, I considered the behavior of pressure on the cavity as the radius S is varied.
It has been shown that the pressure exerted on the atom increases as S decreases up to a certain maximum value which occurs at a radius value called SP max, but then it decreases within a small range of S.