JOURNAL OF SCIENCES (ISLAMIC AZAD UNIVERSITY)
Year:2011 | Volume:21 | Issue:79 (PHYSICS ISSUE)|Papers Count:12

Introduction: Today, Silicon carbide (SiC) is widely used in semiconductor industry, especially in single crystal structure, it can be used in fabrication of high-temperature, highpower, and high-frequency electronic devices. Also, this semiconductor has variety of applications in laser diodes, photo-diodes and different sensors. SiC is a wide band-gap indirect semiconductor. It has high hardness and its electrical and optical properties differ greatly from one-type to another. In all SiC poly-types chemically consist of 50% carbon atoms covalently bonded with 50% silicon atoms. Due to the arrangement of Si and C atoms in the SiC lattice structure, there are different types of crystal structures, each has its unique physical properties. But only the most common poly-types of SiC presently being developed for electronics are the cubic 3C-SiC, the hexagonal 4H-SiC and 6H-SiC. Frequently, two structures, 4H-SiC and 6H-SiC, have attracted economically the attention of semiconductor industry.Aim: This paper has been investigated effects of hydrogen implantation on the surface structure of 6H-SiC crystal before and after thermal oxidation at oxygen ambient. H ions were implanted in SiC with dose of 1×1016 and energy of 15keV.Materials and Methods: SiC surface has been investigated by AFM (Atomic Force Microscopic) technique in implanted and un-implanted regions separately. Also, crystal quality and effect of H implantation on the SiC crystal have been determined by RBSChanneling (Rutherford Back Scattering) and distribution of H ions in the SiC by ERD technique. Furthermore, bonding between SiC atoms have been investigated in implanted and un-implanted regions by FTIR technique.Results: AFM results show roughness in the implanted area increases in compare with un-implanted area but, we could not observe any evidence of implantation effect in the nonoscratchs that were created during polishing process. Moreover, we can see in the RBSChanneling that the collision of H ions has affected a little on the SiC atoms. In addition, FTIR results in two areas show a significant difference between two regions both before and after oxidation at oxygen ambient.Conclusion: Distribution of hydrogen atoms in SiC was determined accurately using ERD and compared to simulation results. It has been shown the maximum distribution of hydrogen atoms in depth is about 200nm. So, the comparison indicates proper agreement between experimental and simulation results.The result of RBS channeling Analysis showed difference on oxide layer in both implanted and un-implanted regions. Although there is no remarkable difference among spectra obtained from two reigns before oxidation, the results of FTIR indicated intensity of transition peak related to Si-O bond of two regions after oxidation is very different. In addition, the FTIR spectra show obviously the presence of carbon and hydrogen in implanted region.

Introduction: During the last decades several classical techniques such as the wellknown Luttinger-Tisza method, vertex model, and so on have been introduced to solve classical Hamiltonian exactly. In a very recent work, T. Kaplan have used a kind of cluster methods, simply LK method, which is based on a block of three spins to solve his frustrated model. He claimed that the LK method is not limited to one dimension or to translationally invariant Hamiltonians and extend his approach to determine the phase diagram of Frustrated classical Heisenberg and XY models in d=2 dimensions. Actually, these are our stimulating reasons to take a quit well known Frustrated Ferromagnetic Model with Added Dzyaloshinskii-Moriya interaction.Aim: In this work we are going to use LK method to determine the phase diagram of Frustrated Ferromagnetic Model with Added Dzyaloshinskii-Moriya exactly.Material and Method: LK method is a classical approach which is able to minimize the three cluster of spins energy and relate it to whole system energy minimization condition and solve the problem rigorously.Results: Disappearing the spiral phase in the presence of Dzyaloshinskii-Moriya interaction was the most interesting result which have been achieved.Conclusion: We have tried to find the phases of our model in co-planar and noncoplanar states by using Kaplan classical approach. In the either case the presence of anisotropic interaction causes to spontaneous disappearing of ferromagnetic phase. With the help of a mapping we have converted our model to anisotropic frustrated ferromagnetic model and could capture three ferromagnetic, spin-liquid and incommensurate phases.

Interoduction: We consider the 1D spin-1/2 quantum compass model. At zero temperature, the behavior of the system is in the rule of quantum fluctuations. At this temperature, the model on the critical line is considered and a transverse magnetic field is applied. Using the numerical Lanczos method the ground state of the system and magnetization for chains up to 20 spins is calculated. We have showed the depend on the exchange couplings the magnetization remains zero up to a critical magnetic field. At high magnetic field the magnetization saturate.Aim: Effects of transverse magnetic field in the quantum compass model has been investigated.Material and Method: In order to study the ground state properties of the system we have used Lanczos numerical method.Results: It has been shown that implementing transverse magnetic field causes to increase magnetization slowly till a point which magnetization reach its saturation value.Conclusion: With increasing transverse magnetic field in the quantum compass model, magnetization will increase gradually up to a saturation value.

Introduction: Although there are exact solution to atmospheric radiative transmission equations, they are not suitable for use in atmospheric general circulation models (GCM). The parameterization of atmospheric radiation for use in an atmospheric GCM is to provide a simple, accurate and fast method of calculating the total radiative flux.Aim: A review of some techniques used to compute radiation flux for use in numerical GCMs and intercomparison output of some GCMs include various methods of radiation parametrization.Materials and Methods: we review the various methods used to compute both the radiative fluxes of longwave (LW) and shortwave (SW) for use in numerical climate models.This paper discusses various methods of integration of absorption over frequency, parameterization of scattering and absorption due to atmospheric gases, separately, at both the SW and LW, surface absorption and aerosols. Then, output data from some atmospheric GCMs with different radiation parametrizations are compared.Results: Line by line is one of exact radiation transmission methods for calculating of gases absorption in inhomogeneous atmosphere which considers all absorption lines. Adding- Doubling, discrete ordinate and Monte Carlo are some of exact techniques in calculation of radiation intensity of SW that have been used as a reference of parametrization. There are four integrations of all zenith angles, z-direction fluxes, du spectral interval and optical absorption path in LW equations. K-distribution, band methods and emissivity methods are among approximation methods in estimation of spectral interval. For each gas, depending on absorption band, different absorption coefficient and therefor different transfer function are available. Non-scattering atmosphere approximation, single scattering approximation, two stream solutions and Eddington approximation estimate SW radiative flux. Radiative transmission parametrization methods are investigated in some GCMs. Finally, data outputs containing absorbed Sw radiative flux in surface and atmosphere and LW radiative flux in surface are compared together and with observation data.Conclusion: radiation parameterization schemes run in GCMs may generally be different, but there are the same solutions for some parts of radiation flux calculation employed at some schemes. Global mean radiation budget calculated by GCMs especially at the earth’s surface are different. Compared to observations, the GCMs include a tendency towards more excessive surface insolation and a smaller longwave downward radiation at the surface.

Introduction: Ion implantation technique is a well-known technique to improve physical properties of metals and semiconductors. In the last two decades ion implantation has been used to produce new structural properties in the near surface region of metals.Aim: The study of crystalline structure, optical reflection, surface morphology and microhardness properties of silver surface after nitrogen ion implantation has been the main purpose of this paper.Materials and Methods: In this paper the effect of nitrogen ion implantation at the energy of 50 keV and doses in the range between 1017 to 1018 ions/cm2 on silver substrate has been discussed. XRD, AFM, Vickers and spectrophotometry analysis were used to characterize microstructure, roughness, microhardness and diffused reflectrion properties of implanted samples, respectively.Results: Direct 50 keV nitrogen ion implantation into silver can produce silver-azide.There is a relationship between creation of this phase and implantation dose.Conclusion: Formation of AgN3 was confirmed by XRD results. AgN3 with orthorhombic structure has been formed on silver surface with cubic structure. By increasing the ion dose more than 1×1017 ions/cm2 roughness increases. The results show that by increasing the ion dose up to 1×1018 ions/cm2 hardness enhances. The results of spectrophotometry shows reduction in diffused reflection spectrum of nitrogen implanted samples.

Introduction: In a cold cathode electron source electron beam is created by electrical gas discharge. It is based on electron emission from a cathode surface by ions and energetic neutral impact. Different parameters such as material and geometry of cathode surface, pressure and plasma gas type are effective in electron beam characteristics.Aim: How cathode material, pressure and plasma gas affect on electron beam formation Materials and Methods: Helium, Hydrogen and Argon were used for low pressure electrical gas discharge. Different cathodes such as aluminium, stainless steel, copper and brass were tested. In order to investigate the cathode geometry we used various curvatures 60- 8-100 mm and flat. Voltage-current characteristic, Pachen curve and electron beam current in above condition were measured and analyzed.Results: For helium and argon, breakdown occurs in a pressure which is ten times smaller than argon. The argon atoms in comparison to other gases' atoms produce larger sputtering effect from cathode surface that is inconvenient. Aluminium has two properties to be a nice cathode in comparison with copper. It has smaller sputtering yield and higher secondary electron emission coefficient.Conclusion: The results show that between various cathode material, aluminium and copper have maximum and minimum electron beam current respectively, and concave cathode has positive effect. The discharge confirm Pachen curve and the facility works on the left side of this curve. The electrical discharges of gases such as helium, hydrogen in comparison to argon are more stable to form the electron beam.

Introduction: Structural and magnetic properties of ferromagnetic Fe thin layer films on Si substrates have been reported. We present topography features of these nanostructures films by atomic force microscopy (AFM). Characterization of magnetic domains and walls between them can be performed by using magnetic force microscopy (MFM) and alternating gradient field magnetometer (AGFM) instruments. The Fe thickness ranges from 50 to 150 nm. Our focus is to study systematically the effects of film thickness on magnetic anisotropy in thin films grown on semiconductor substrates. MFM images reveal stripe domain structure for the 100nm thick Fe on Si as well as hysteresis curves. The effective anisotropy shows oscillation for two types of Si substrates when the Fe films thickness increased.Aim: Our focus is to investigate systematically the effect of film thickness on magnetic hysteresis and effective anisotropy, especially in thin films grown on semiconductors substrates. For revealing stripe domain structure of Fe/Si (100) and Fe/Si (111), we use MFM pictures as well as hysteresis curves.Material and method: Fe thin films were deposited by thermal evaporation. Iron ingots from a 99.99 % purified Fe powder evaporated using an electron gun. The deposited Fe layer thicknesses vary from 50 to 150 nm. The crystalline structure of the deposited structures, X-ray diffraction method was used for investigation of nanostructure. Magnetic properties were also measured with MFM and experimental hysteresis loops were obtained by AGFM unit.Results: For Fe layer with 50nm thickness, Fe on Si (100) and Si (111) substrates, the Fe grows with (110) texture and has a bcc structure. The coercive field measured for the Fe films is about 100Oe. The measured hysteresis loops show that the coercive field decreases while the saturation field increases and such uniaxial anisotropy increases with increasing step density.Conclusion: Hysteresis diagrams show good ferromagnetic characteristics of these samples. The effective anisotropy shows oscillation, for two substrates Si types, when the Fe films thickness increased.

Introduction: Laser anti-reflection coatings (LAR) are designed for the most effective anti-reflection results at a specifically defined wavelength. As such, they are ideal for applications involving lasers or other monochromatic light sources. Composite MgF2 (low refractive index) and TiO2 (high refractive index) films are fabricated by reactive ion-assisted coevaporation deposed on BK-7 glass substrate.Aim: The purpose of the present paper is to produce a high reflection mirror for Ar+ laser, multilayers of MgF2 (low refractive index) and TiO2 (high refractive index) have been deposed on BK-7 glass substrate.Material and method: A vacuum system capable of reaching pressures with of 10-7 mbarn (Balzers) and an optical thickness monitor (GSM-420 Balzers), as well as vibrating quartz crystal thickness monitor were mounted in this vacuum system to be able to check the obtained results. Substrate was circular glass with 20mm diameter and 1 mm thickness (BK-7 glass). Pure research grade MgF2 and TiO2 was used and both evaporated the from tantalium crucible separately, at pressures about 2-3×10-5 mbar. MgF2 was coated with 1 nm/s deposition rate and for deposition of TiO2 partial pressure of 5×10-5 mbar was used and coating rate was 0.5 nm/s.Results: Substrate temperature play an important role on refraction index and packing density of thin films has affect on R value. After annealing although reflection power increased by 5%. Increasing of R forl=500nm, by 5% but at other wavelength l<400nm and l>600nm (outside the spectral band width), also reflectivity rises 20 to 30 percent as well.Conclusion: After eleven thin film layers of l/4 (G/ (HL)5/TiO2/Air), mirror with 95% power of reflection over spectral band width of 160nm and nearly 5% of transmission was produced. These parameters are good enough for making front mirror of Ar+laser.

Introduction: Recently, interplay between nonlinear dynamics and geometry has received particular attention in physics of intrinsic localized modes. Apart from its conceptual and theoretical interest, this has real importance in science and technology.Aim: I aim at focusing on the nonlinear aspects of the transverse (vertical) dust-lattice mode on the two dimensional (hexagonal) lattices. I am interested in investigating the conditions for the occurrence of discrete multisite lattice excitations in a nonlinear Klein- Gordon-like chain, which is characterized by an inverse dispersion law.Material and Method: The occurrence of discrete vibrational modes in 2D hexagonal dusty plasma lattices is investigated. The system is described by a Klein – Gordon hexagonal lattice. Discrete equation of motion was found and it change to discrete nonlinear Schrodinger type equation.Results: The discrete dispersion relation was found. It is demonstrated that highly localized structures involving vertical oscillations of charged dust grains may exist in a 2D hexagonal dust lattice.Conclusion: The results show, the difference in structure from the usual nonlinear Klein– Gordon equation used to describe oscillator chains is: the phonons (here) are stable only in the presence of the electric field force.