Papers by Keyword: Formation Energy

Abstract: The study of formation energy and the vibrational effect on the vacancy concentration in the gray tin (α-Sn) using density functional theory has been successfully carried out. The vacancy is modeled by using a supercell consisting of 64 atoms. The vibrational effect is obtained by calculating the phonon density of states (P-DOS) of perfect and vacancy gray tin through the Gaussian-function approach. It is found that the formation energy of Sn-vacancy is 1.89 eV, and the vacancy concentration at the melting point is 3.68 × 10³ cm^-3. However, the vacancy concentration increases significantly to 6.48 × 10⁹ cm^-3 when the vibrational effect is considered. It is expected due to the softening of P-DOS on the vacancy case.

Abstract: A study on the formation and stability of new quaternary compounds with the general chemical formula Gd₃TAl₃Ge₂ (T = Mn, Cu) has been undertaken by experimental investigations (SEM-EDX, DTA and XRD) and density functional theory (DFT) calculations. These compounds crystallize in the hexagonal Y₃NiAl₃Ge₂-type structure (hP9, P–62m, Z = 1) (an ordered, quaternary derivative of the ternary ZrNiAl or of the binary Fe₂P prototypes), with lattice parameters values a = 7.0239(2) Å and c = 4.2580(1) Å for Gd₃MnAl₃Ge₂ and a = 7.0434(1) Å and c = 4.2089(1) Å for Gd₃CuAl₃Ge₂. DTA suggests a peritectic reaction for the formation of these compounds (at 1245°C for Gd₃CuAl₃Ge₂). The existence and stability of these phases has been explained on the basis of DFT calculations, and a comparison of ground state properties of the studied compounds with the earlier known Gd₃CoAl₃Ge₂ phase is outlined. The negative formation energies in all three cases govern the stability of compounds from theory as well, predicting Gd₃MnAl₃Ge₂ as the most stable phase with highest formation energy (–13.01 eV/f.u.). The total DOS are generic in nature and suggest the robust magnetism, with the Gd-f moments of ≈7 μ_B. An antiparallel coupling among Gd-f and T-d states is observed for all compounds, as usually seen in rare earth (R) - transition metal (T) compounds. Preliminary magnetization measurements on Gd₃MnAl₃Ge₂ show two ferromagnetic/ferrimagnetic (FM/FIM) like transitions at T_C1 = 142 K and T_C2 = 97 K, with another anomaly seen at ≈15 K. Isothermal magnetization data show no hysteresis even at 5 K, and the magnetization does not saturate up to 50 kOe, further suggesting a possible FIM behavior.

Abstract: In the work we propose a method for determining of the formation energy of bivacancy using molecular dynamics method. The key moment of the method for determining of the formation energy of bivacancy is the use of the value ζ, the minimum work that must be spent to remove one atom to infinity from the kink in the monatomic step on the surface of the crystal, calculated indirectly through the experimental data on the formation energy of the vacancy and the sublimation energy. The energy of migration of bivacancy in the work was determined from the temperature dependence of the diffusion coefficient when one bivacancy was introduced into the calculation block.

Abstract: We present results of defect formation energies and charge state thermodynamic transition levels of Mg and Te interstitials in MgTe wurzite structure. We use the generalized gradient approximation and local density approximation functionals in the framework of density functional theory for all calculations. The formation energies of the Mg and Te interstitials in MgTe for both the tetrahedral and hexagonal configurations were obtained. The Mg and Te interstitials in MgTe depending on the functional, introduced transition state levels that are either donor or acceptor within the band gap of the MgTe. The Te interstitial exhibit charge states controlled metastability, negative-U and DX centre properties. The Mg interstitial acts as deep or shallow donor and there is no evidence of acceptor levels found for the Mg interstitial.

Abstract: The current work is devoted to studying the evolution of deep level defects in the lower half of the 4H-SiC bandgap after high temperature processing and ion implantation. Two as-grown and pre-oxidized 4H-SiC sets of samples have been thermally treated at temperatures up to 1950 °C for 10 min duration using RF inductive heating. Another set of as grown samples was implanted by 4.2 MeV Si ions at room temperature (RT) with different doses (1-4×10⁸ cm^-2). The so-called “D-center” at E_V+0.6 eV dominates and forms after the elevated heat treatments, while it shows no change after the ion implantations (E_V denotes the valence band edge). In contrast, the concentration of the so-called HK4 level at E_V+1.44 eV increases with the implantation dose, whereas it anneals out after heat treatment at ≥ 1700 °C.

Abstract: With the extended applications of hexagonal silicon carbide (h-SiC) in the various fields, particularly in the application of the electronic devices, more and more attentions have been focused on the micro structures as well as their physical properties of h-SiC surface. In this study, we have performed the first principal calculations to compare the formation energies of four typical defects (Vc, Vsi, C_I and Si_I) on the 4H-SiC (0001) surface as well as in the interior layers. Due to the surface reconstruction and the reduced lattice constrain, the optimized structures of the defects on/near the 4H-SiC (0001) surface are quite different from the ones in the deeper layers. The distinguished formation energies as function of chemical potential indicate that we may control the defects concentrations in different layers by tuning the environmental conditions. This theoretical work provides a significant understanding to the formation mechanism of the point defects on the 4H-SiC surface, and paves a way to the modification of the SiC surface via electron irradiation or ion implantation with micro-defects introduced.

Abstract: One of promising candidates as dopants for forming ultra-shallow and high conductive source/drain in future silicon devices has been proposed to be dimer or trimer co-dopants containing pairs or trios of different impurity elements. Making choice of a combination of the impurity elements with a small ionization energy is essential for the appropriate dimers and trimers. In this work we calculated total energies of silicon with substitutional or ineterstitial impurity elements and derived formation energies for the substitutional and interstitial impurity elements for the atomic number 1 to 83 with the exception of inert gas and lanthanum series elements. We present here the periodic table with the formation energies of the substitutional and interstitial impurity elements for determining the most stable site of the impurity elements in Si lattice. We can use this table as a database for calculating ionization energies of the impurity element.

Abstract: One of promising candidates as dopants for forming ultra-shallow and high conductive source/drain in future silicon devices has been proposed to be dimer or trimer co-dopants containing pairs or trios of different impurity elements. Making choice of a combination of the impurity elements with a small ionization energy is essential for the appropriate dimers and trimers. In this work we calculated total energies of silicon with substitutional or ineterstitial impurity elements and derived formation energies for the substitutional and interstitial impurity elements for the atomic number 1 to 83 with the exception of inert gas and lanthanum series elements. We present here the periodic table with the formation energies of the substitutional and interstitial impurity elements for determining the most stable site of the impurity elements in Si lattice. We can use this table as a database for calculating ionization energies of the impurity element.

Abstract: The stability and electronic structures of Al or/and P doped single-walled SiC nanotubes (SWSiCNTs) are investigated by the first-principles theory. It is found that the ones with P atom located at Si site are most energetically favorable both for armchair and zigzag SWSiCNTs, which means that P atom is prone to substitute Si atom. In the same time, we found that the formation energy of Al substituting Si atom is lower than that of Al substituting C atom. The energetic disadvantages of P or Al atom located at C site in SWSiCNTs may be due to the obviously structural distortion in view of that the Al and P atoms are much bigger than C atom. The SWSiCNTs can be routinely modified ranging from p-type semiconductor to n-type by Al and P substitution doping. These results are expected to give valuable information in building nanoscale electronic devices.

Abstract: The binding energies, formation energies and DOS (density of state) of several carbides(Fe₃C, Fe₂C, Fe₅C₂, NbC) in Nb micro-alloyed steels are investigated using the first-principle pseudo potential plane-wave method. The results show that the structure type with the strongest alloying ability and the highest structure stability is NbC with fcc structure. After compared the DOS of these different structure types, the results show that the discrepancy in structural stability of carbides can be attributed to the difference in the bonding electron numbers at Feimi level. The less the valence electrons at Feimi level are, the better the structural stability of carbides. Therefore, the theory predicts that the existence form of Nb in medium and high carbon steels are the same as low carbon steels. Nb is still the effective alloying element in medium and high carbon steels.