First-principles study on the electronic and optical properties of Si and Al co-doped zinc oxide for solar cell devices

Electronic and optical properties of co-doped zinc oxide ZnO with silicon (Si) and aluminum (Al), in Zn_1?2x Si _x Al _x O (0 ≤ x ≤ 0.0625) original structure forms, are investigated by the first-principles calculations based on the density functional theory (DFT). The optical constants and dielectric functions are investigated with the full-potential linearized augmented plane wave (FP-LAPW) method and the generalized gradient approximation (GGA) by WIEN2k package. The complex dielectric functions, refractive index and band gap of the pure as well as doped and co-doped ZnO were investigated, which are in good agreement with the available experimental results for the undoped ZnO. Thus, the maximum optical transmittance of the co-doped ZnO of about 95 % was achieved; it is higher than that of pure ZnO. Thus, we showed for the Si–Al co-doped ZnO with x = 0.0315 that the optical transmittance can cover a larger range in the visible light region. In addition, an occurrence of important energy levels around Fermi levels was showed, which is mainly due to doping atoms that lead to an overlap between valence and conduction bands, and consequently to the significant conductor behavior of the Si–Al co-doped ZnO. The original Zn_1?2x Si _x Al _x O structure reveals promising optical and electronic properties, and it can be investigated as good candidates for practical uses as transparent and conducting electrodes in solar cell devices.     

Structural stabilities and optical properties of SixGeyHz nanocrystals

Based on the high-throughput first-principles calculations with structural recognition, we have ystematically investigated the structural stabilities and optical properties of Si_xGe_yH_z nanocrystals(H-SiGeNCs), including various sizes, shapes and compositions. The total energies of H-SiGeNCs can be simply estimated by the bond energy model in high accuracy, where the error of test set is less than 0.5 meV per atom. According to the energy difference of Si/Ge in various bonding environments, we have determined the ground state structures by the geometry analysis, as is confirmed the convex hulls of formation enthalpy from the first-principles calculations. In addition, the energy gaps of H-SiGeNCs are modulated by the atomic distributions, as well as the vibrations of Si-H and Ge-H bonds at room temperature which is revealed by ab initio molecular dynamics simulations.     

Theoretical study on the mono and multiply oxygenated Si60H60 fullerene

We have applied density functional theory (DFT) calculations to study the structures, stabilities, electronic and magnetic properties of mono and multiply oxygenated Si₆₀H₆₀ fullerenes (Si₆₀H_60–2nO_n, n = 1, 3, 6, 9, 10, 12, 18, 20, 21, 27 and 30). DFT results show that rearrangement between the closed [6,6] and [5,6] isomers of Si₆₀H₅₈O follows a two-step pathway involving an intermediate and two transition states. Preserving the C₃ symmetry in the cage structure, extra epoxidation of Si₆₀H₆₀ has been accomplished. Based on our results, formation energies per oxygen atom for the multiple additions of oxygen atoms on Si₆₀H₆₀ cage are positive (endothermic character), and increase with the increasing of the number of oxygen atoms. In general, the oxygenation of Si₆₀H₆₀ cage leads to an increase in the electrophilicity of the Si₆₀H_60–2nO_n oxides. The oxygenation of Si–Si bonds not only introduces a substantial broadening of the NMR pattern but also yield individual peaks, indicating different electrostatic environments of silicon nuclei in the Si₆₀H_60–2nO_n oxides.     

Experimental and theoretical studies of interactions between Si<Subscript>7</Subscript> clusters

The possibility of using magic Si₇ clusters to form a cluster material was studied experimentally and theoretically. In experiments Si₇ clusters were deposited on carbon surfaces, and the electronic structure and chemical properties of the deposited clusters were measured using X-ray photoelectron spectroscopy (XPS). A non bulk-like electronic structure of Si₇ was found in the Si 2p core level spectra. Si₇ is suggested to form a more stable structure than the non-magic Si₈ cluster and Si atoms upon deposition on carbon surfaces. Theoretically it was possible to study the interaction between the clusters without the effect of a surface. Density functional theory (DFT) calculations of potential curves of two free Si₇ clusters approaching each other in various orientations hint at the formation of cluster materials rather than the fusion of clusters forming bulk-like structures.     

Effect of Ce and Cu co-doping on the structural,morphological, and optical properties of ZnO nanocrystals and first principle investigation of their stability and magnetic properties

Ce, Cu co-doped ZnO (Zn_1−2xCe_xCu_xO: x=0.00, 0.01, 0.02, 0.03, 0.04 and 0.05) nanocrystals were synthesized by a microwave combustion method. These nanocrystals were investigated by using X-ray diffraction (XRD), UV–visible diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM). The stability and magnetic properties of Ce and Cu co-doped ZnO were probed by first principle calculations. XRD results revealed that all the compositions are single crystalline. hexagonal wurtzite structure. The optical band gap of pure ZnO was found to be 3.22 eV, and it decreased from 3.15 to 3.10 eV with an increase in the concentration of Cu and Ce content. The morphologies of Ce and Cu co-doped ZnO samples confirmed the formation of nanocrystals with an average grain size ranging from 70 to 150 nm. The magnetization measurement results affirmed the antiferro and ferromagnetic state for Ce and Cu co-doped ZnO samples and this is in agreement with the first principles theoretical calculations.     

Si–Si optical phonon behavior in localized Si clusters of Si<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> alloy nanocrystals

Raman spectra acquired from Si _x Ge_1−x -nanocrystal-embedded SiO₂ films show dependence of the Si–Si optical phonon frequency on Si content. The frequency upshifts, and peak intensity increases as the silicon concentration increases. For a given Si content, the frequency remains unchanged with annealing temperature. Spectral analysis and density functional theory calculation reveal that the optical Si–Si phonon is related to the formation of localized Si clusters surrounded by Si/Ge atomic layers in the Si _x Ge_1−x nanocrystals and the intensity enhancement arises from the larger cluster size. The synergetic effect of surface tensile stress and phonon confinement determines the Si–Si optical phonon behavior.     

Si80笼状分子的结构研究

采用基于密度泛函的第一性原理计算方法,构建了Si₈₀笼状分子的模型,并对其结构和电学特性进行了考察. 研究发现,经过结构优化计算,Si₈₀分子从I_h高对称性下降为T_h,但仍然保持较好的笼状结构. 对Si₈₀笼状分子的稳定性、轨道分布和电荷分布等性质进行了分析和讨论. 关键词： 80')" href="#">Si₈₀ 富勒烯 结构     

DFT study on electronic structure and optical properties of N-doped,S-doped,and N/S co-doped SrTiO3

The electronic structures and optical properties of N-doped, S-doped and N/S co-doped SrTiO₃ have been investigated on the basis of density functional theory (DFT) calculations. Through band structure calculation, the top of the valence band is made up of the O 2p states for the pure SrTiO₃. When N and S atoms were introduced into SrTiO₃ lattice at O site, the electronic structure analysis shows that the doping of N and S atoms could substantially lower the band gap of SrTiO₃ by the presence of an impurity state of N 2p on the upper edge of the valence band and S 2p states hybrid with O 2p states, respectively. When the N/S co-doped, the energy gap has further narrowing compared with only N or S doped SrTiO₃. The calculations of optical properties also indicate a high photo response for visible light for N/S co-doped SrTiO₃. Besides, we find a new impurity state which separates from the O 2p states could improve the photocatalytic efficiency and we also propose a model for light electron-hole transportation which can explain the experiment results well. All these conclusions are in agreement with the recent experimental results.     

Influence of Si substitution on structural,electronic and magnetic properties of Fe<Subscript>2</Subscript>MnGa Heusler compound

We investigate the electronic and magnetic properties of Fe₂MnGa_1?x Si _x alloy (x = 0, 0.25, 0.5, 0.75, and 1) using first-principles density functional theory within the generalized gradient approximation method. The lattice constant decreases linearly whereas bulk modulus increases with increasing Si content. The total magnetic moment varies linearly with increasing Si content, which follows the Slater-Pauling rule. Electronic band structure calculations indicate that the Fe₂MnGa_1?x Si _x exhibits half-metallic character for all the concentrations studied and the spin polarization and the spin-down band gap both increase with the Si content. Based on the magnetic properties calculations, the Heisenberg exchange coupling parameters give Fe-Mn ferromagnetic coupling and Mn-Mn antiferromagnetic coupling. The T _C first decreases and then increases with Si content, which is in well agreement with the experimental results.     

Ab-initio calculations of luminescence and optical gain properties in silicon nanostructures

Density-functional and many body perturbation theory calculations have been carried out in order to study the optical properties both in the ground and excited state configurations, of silicon nanocrystals in different conditions of surface passivation. Starting from hydrogenated clusters, we have considered different Si/O bonding geometries at the interface. We provide strong evidence that not only the quantum confinement effect but also the chemistry at the interface has to be taken into account in order to understand the physical properties of these systems. In particular, we show that only the presence of a surface Si–O–Si bridge bond induces an excitonic peak in the emission-related spectra, redshifted with respect to the absorption onset, able to provide an explanation for both the observed Stokes shift and the near-visible PL experimentally observed in Si-nc. For the silicon nanocrystals embedded in a SiO₂ matrix, the optical properties are discussed in detail. The strong interplay between the nanocrystal and the surrounding host environment and the active role of the interface region between them is pointed out, in very good agreement with the experimental results. For each system considered, optical gain calculations have been carried out giving some insights on the system characteristics necessary to optimize the gain performance of Si-nc. To cite this article: E. Degoli et al., C. R. Physique 10 (2009).     

Electronic and optical properties of the H2O adsorbed the B-N-C nanotubes

We investigate the structures and properties of boron/nitrogen co-doped carbon nanotube with water molecules adsorbing. The electronic and optical properties of the systems are calculated by using the first-principles theory in detail. The results reveal that the doped nanotubes show hydrophilic behavior when the oxygen atoms are close to the nanotubes. The Mulliken charges redistribute and transfer between the doped carbon nanotubes and the water molecules. The band gaps of the nanotubes vary with the positions of H₂O. The positions and intensities of the reflectivity peaks are affected by the distributions of boron/nitrogen atoms and the positions of water molecules. The investigations are beneficial to further biological applications of co-doped nanotubes.     

Electronic structures of chemisorption systems on Si(111) surface I. Si(111)7 × 7/H,Cl

Electronic structures of chemisorption on Si(111)/H,C1 are investigated by the first principle DV-Xα cluster method. The calculations are carried out for chemisorption on different sites, based on the Si₁₃H₁₅ cluster, and the effect of surface vacancy and buckling on the electronic structure is examined in detail. The present calculation shows that the Si₁₃H₁₅ surface cluster reproduces very well the more sophisticated band calculation for the Si(111) surface. It is concluded that the vacancy model with chemisorbed atoms at appropriate sites is reasonable to interpret the observed UPS of Si(111) 7 × 7/H,C1. The charge transfer between the substrate atom and the adatom depends strongly both on the chemisorption sites and on the electronegativitv difference.     

A computational investigation of electronic structure as well as 19F and 29Si chemical shielding tensors in the fluorinated silicon fullerenes SinFn (n≤60)

Density functional theory (DFT) calculations are performed to investigate the electronic features of the structures of fluorinated polysilanes Si_nF_n (n=4, 6, 8, 10, 12, 20, 24, 28, 30, 32, 36, 50, and 60). Among all of these fluorinated polysilanes, Si₂₀F₂₀ has the highest binding energy and, thus, stability. The binding energy then shows a very slow (monotonically) decrease as the size of the fluorinated silicon fullerene n≥20 increases which can be related to an increase in fluorine–fluorine repulsion. Following an irregular pattern, the HOMO–LUMO energy gap strongly depends on the size of the cage. On the other hand, ²⁹Si CS parameters detect equivalent electronic environment for silicon atoms within Si_nH_n polysilanes with n≤20 while ²⁹Si NMR pattern indicates a few separated peaks for Si_nH_n polysilanes with n≥20. Seeking correlation between these peaks and local structures around silicon sites, Si_α, Si_β, Si_γ observed in these models shows that δ_iso(Si_γ)<δ_iso(Si_β) <δ_iso(Si_α). Obtaining similar values (458.8–478.7 ppm) of ¹⁹F calculated chemical shieldings for all the fluorinated polysilanes means the same tendency of the silicon atoms on the surfaces of all cages for contribution to chemical bonding with fluorine atoms.     

Charge effects in silicon-doped heterofullerenes

We provide information on the modification induced on the structural and electronic properties by the inclusion of an extra positive and negative charge on C₃₀Si₃₀. The calculations are performed based on the framework of Car–Parrinello molecular dynamics within the spin density version of density functional theory. Adding or extracting an electron from the C₃₀Si₃₀ does not dramatically alter the topology of the neutral network either structurally or electronically. However, negative C₃₀Si₃₀ is characterized by an accumulation of negative charge on the inner part of the Si region, while the distribution of charge in positive C₃₀Si₃₀ is closer to the neutral case. PACS 61.48.+c; 71.20.Tx     

Formation of Ge nanocrystals and evolution of the oxide matrix in as-deposited and annealed LPCVD SiGeO films

SiGeO films were deposited by LPCVD using Si₂H₆, GeH₄ and O₂ as reactive gases and furnace annealed to segregate the possible excess of Si and Ge in the form of nanocrystals embedded in an oxide matrix. For low GeH₄:Si₂H₆ flow ratios and deposition temperatures of 450 ^°C or lower, the deposited film consists of a SiO₂ matrix incorporating Ge. No Ge oxides and no nanocrystals are detected. After annealing of the samples with SiO₂ matrices at temperatures of 600 ^°C or higher, quasi-spherical isolated Ge nanocrystals with diameters ranging from 4.5 to 9 nm and homogeneously distributed throughout the whole film thickness are formed. In the samples deposited with low GeH₄:Si₂H₆ flow ratios, the original SiO₂ matrix holds its composition.     

掺杂半导体β-FeSi2电子结构及几何结构第一性原理研究

采用基于第一性原理的密度泛函理论全势线性缀加平面波法，使用广义梯度近似处理交换相关势能，首先计算了β-FeSi₂的电子结构及其各元素各亚层电子的能态密度，β-FeSi₂的电子能态密度主要由Fe的d层电子和Si的p层电子的能态密度确定；其次通过计算不同掺杂系统的总能量确定了掺杂原子在β-FeSi₂中的置换位置，在β-FeSi₂中，Co置换Fe_Ⅱ位置的Fe原子，Al置换Si_Ⅰ位置的Si原子，这种择位置换与现有的计算结果完全一致；最后计算了Fe_1-xCo_xSi₂和Fe(Si_1-xAl_x)₂的电子结构，对它们的电子结构特征进行了分析，并探讨了电子结构对其热电性能(塞贝克系数、电传输及热传输性能)的影响. 关键词： 第一性原理 电子结构 热电性能 2')" href="#">FeSi₂     

Embedded silicon nanocrystal interface structure and strain

The structure of nanocrystal-matrix interface and strain in embedded nanocrystals are studied using large-scale atomistic simulations, with the examples of Si nanocrystal embedded in amorphous matrix of SiO₂. Photoluminescence from silicon nanocrystals embedded in a dielectric matrix like SiO₂ and Si₃N₄ are promising for Si-based optical devices. The nanocrystal-matrix interface plays a crucial role in understanding its optical and electrical properties. Nanocrystals with diameters varying from 2.17 to 4.56 nm are studied. A detailed quantitative analysis of the variation of Si/SiO₂ interface structure and strain distribution with nanocrystal diameter is reported. A linear variation of the interface width with nanocrystal diameter is observed with thinner interfaces for larger nanocrystals. Local deformation analysis reveals that the smaller nanocrystals are highly strained, whereas the strain in the larger ones shifts to the interface. This is in accordance with observed increase in total percentage of defect states in the interface from 39 to 70% for diameter increasing from 2.17 to 4.56 nm. Moreover, based on the atomic arrangements at the interface, optically active defects like Pb centres, E centres and non-bridging oxygen centres are identified and a dominance of Pb centres is observed for all the nanocrystals. The detailed structural characterization-related investigations using the proposed simulation approach will find useful application in designing system-level response of embedded nanocrystals and also to correlate various experimental observations.     

Electronic structure and magnetism of MnFeP1−xSix alloys from first-principles calculations

First-principles calculations of electronic structure and magnetic properties based on density-functional theory were performed for MnFeP_1−xSi_x (0.44?x?0.60) alloys which are considered as promising magnetocaloric refrigerants. We used the full-potential APW+lo method and treated the random order of P(Si) atoms in the ZrNiAl-type structure in a virtual-crystal approximation. A non-monotonic behavior of the alloy magnetization as a function of x was obtained, in qualitative agreement with experiment, and explained in terms of the spin-polarized densities of states.     

Room-temperature below bulk-Si band gap photoluminescence from P and B co-doped and compensated Si nanocrystals with narrow size distributions

Thin films consisting of the layers of phosphorus (P) and boron (B) co-doped Si nanocrystals (Si-ncs) and glass spacer layers were prepared and their photoluminescence properties were studied. Cross-sectional TEM observations revealed the growth of Si-ncs with narrow size distributions. The samples exhibited PL below the band gap energy of bulk Si crystal at room temperature. The low-energy PL is considered to arise from the transitions between donor and acceptor states in compensated Si-ncs. The successful formation of narrow size distribution co-doped Si nanocrystals promotes the study of the optical properties of compensated Si nanocrystals.     

p，n型掺杂剂与Mn共掺杂GaN的电磁性质

采用基于密度泛函理论的第一性原理平面波赝势法计算Mg,Zn,Si,O和Mn共掺GaN,分析比较共掺杂后的电子结构和磁学性质,并分别用平均场近似的海森伯模型和Zener理论估算共掺杂后体系的居里温度（T_C）.计算表明：共掺杂后体系均在能隙深处产生自旋极化杂质带,具有半金属性,能产生自旋注入.p型共掺杂（GaN：Mn-Mg＼Zn）后体系具有较GaN：Mn更稳定的FM态且能使T_C升高;而n型共掺杂（GaN：Mn-Si＼O）后体系FM态稳定性 关键词： Mg Zn Si O和Mn共掺GaN 第一性原理 T_C）')" href="#">居里温度（T_C）