Spectral variations in the optical transition matrix element and their impact on the optical properties associated with hydrogenated amorphous silicon

Using an empirical model for the density of states functions associated with hydrogenated amorphous silicon, in conjunction with an elementary model for the optical transition matrix elements, we aim to explore how variations in the matrix elements impact upon the spectral dependence of the optical properties associated with this material. We also wish to ascertain as to whether or not the hydrogenated amorphous silicon mobility gap result suggested by Jackson et al. [W.B. Jackson, S.M. Kelso, C.C. Tsai, J.W. Allen, S.-J. Oh, Phys. Rev. B 31 (1985) 5187] is consistent with the results of the experiment. We find that the mobility gap value suggested by Jackson et al. is too large. An upper bound on the mobility gap associated with hydrogenated amorphous silicon of 1.68 eV is suggested instead. Electrical measurements performed on undoped hydrogenated amorphous silicon yield a mobility gap value that is consistent with this bound.     

Defect absorption and optical transitions in hydrogenated amorphous silicon

Using an empirical model for the density of states functions associated with hydrogenated amorphous silicon, with defect states taken into account, we examine how the distributions of such states shape the optical response of this material. The contributions to this response attributable to the various types of optical transitions are also determined. Finally, we demonstrate that we are able to capture the spectral dependence of the optical absorption coefficient associated with a defect absorption influenced sample of hydrogenated amorphous silicon using our empirical formalism for the density of states functions associated with this material.     

Urbach edge and the density of states in hydrogenated amorphous silicon

Photoconductivity measurements of the optical absorption edge and time-of-flight measurements of the hole drift mobility, on the same amorphous silicon film, enable us to identify the origin of the Urbach edge in amorphous silicon with the density of states in the valence band tail.     

Ab initio calculations study of the electronic, optical and thermodynamic properties of NaMgH3, for hydrogen storage

The structural stability, electronic structure, optical and thermodynamic properties of NaMgH₃ have been investigated using the density functional theory. Good agreement is obtained for the bulk crystal structure using both the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange-correlation energy. It is found from the electronic density of states (DOS) that the valence band is dominated by the hydrogen atoms while the conduction band is dominated by Na and Mg empty states. Also, the DOS reveals that NaMgH₃ is a large gap insulator with direct band gap 3.4 eV. We have investigated the optical response of NaMgH₃ in partial band to band contributions and the theoretical optical spectrum is presented and discussed in this study. Optical response calculation suggests that the imaginary part of dielectric function spectra is assigned to be the interband transition. The formation energy for NaMgH₃ is investigated along different reaction pathways. We compare and discuss our result with the measured and calculated enthalpies of formation found in the literature.     

Quantum well model of hydrogenated amorphous silicon

A model of barrier-separated regions is proposed that leads to quantization and spatial correlation of carriers near the band gap of hydrogenated amorphous silicon. The size of these regions, which consist of pure Si bounded by potentials emanating from Si-H bonds, is estimated from a classical percolation picture. Near band gap localized states lie in these quantum well regions and are about 0.3 eV more widely separated than in unbounded Si, thereby accounting for a wide variety of hitherto uncorrelated experimental results. Dopants enhance conductivity by providing conduction paths through the barriers. Spatially coincident pairing of conduction with valence band localized states is speculated to be relevant to other amorphous semiconductors as well.     

Electronic structure and optical properties of RbPb2Br5

We report on density functional theory (DFT) calculations of the total and partial densities of states of rubidium dilead pentabromide, RbPb₂Br₅, employing the augmented plane wave+local orbitals (APW+lo) method as incorporated in the WIEN2k package. The calculations indicate that the Pb 6s and Br 4p states are the dominant contributors to the valence band: their main contributions are found to occur at the bottom and at the top of the band, respectively. Our calculations reveal that the bottom of the conduction band is formed predominantly from contributions of the unoccupied Pb 6p states. Data of total DOS derived in the present DFT calculations are found to be in agreement with the experimental X-ray photoelectron valence-band spectrum of this compound. The predominant contributions of the Br 4p states at the top of the valence band of rubidium dilead pentabromide are confirmed by comparison on a common energy scale of the X-ray emission band representing the energy distribution of the valence Br p states and the X-ray photoelectron valence-band spectrum of the RbPb₂Br₅ single crystal. Main optical characteristics of RbPb₂Br₅, such as dispersion of the absorption coefficient, real and imaginary parts of dielectric function, electron energy-loss spectrum, refractive index, extinction coefficient and optical reflectivity are explored for RbPb₂Br₅ by the DFT calculations.     

Electronic structure,optical dielectric constant and born effective charge of EuAlO3

EuAlO₃ (EAO) is synthesized by the sol–gel process. The Rietveld refinement of the X-ray diffraction data shows that the material has orthorhombic structure with Pbnm space group. The density functional theory calculations are initiated with the experimental lattice parameters. The full potential linearized augmented plane wave method and projector augmented wave method are used to investigate the ground state properties of EAO. An indirect band gap of 1.8 eV is observed with the valence band maximum at the Γ point and the conduction band minimum at the R point. The X-ray photoemission spectroscopy (XPS) spectra of EAO are obtained in the energy window of 0–1000 eV. Using the electronic density of states, the valence band (VB) spectrum of EAO is generated and compared with the observed VB-XPS spectrum. The optical dielectric constant and the refractive index of the material are calculated for the photon energy radiation. The optical properties show a considerable anisotropy in the material. The Born effective charge of various elements and the dielectric tensor of EAO have been calculated.     

Temperature and electric field dependences of transient photocurrents in a-Si:H

Temperature and electric field dependences of photocurrents measured at fixed times after pulsed-light excitation in hydrogenated amorphous silicon have been investigated. The photocurrents plotted against inverse absolute temperature for undoped samples exhibit activated behaviours. It is found that the activation energies for all different fixed times after photo-excitation and for all applied electric fields are the same. The results indicate the existence of quasi-equilibrium trapping level during the transit of excess electrons in undoped samples. However, in boron-doped samples the excess holes communicate with the effective trapping levels which move away from E_v (valence band mobility edge) during the transit within the time scale of measurement, which is consistent with the data that the width of valence band tail is wider than that of conduction band tail.     

First-principles study on electronic structure of Sr2Bi2O5 crystal

The electronic structure of Sr₂Bi₂O₅ is calculated by the GGA approach. Both of the valence band maximum and the conduction band minimum are located at Γ-point. This means that Sr₂Bi₂O₅ is a direct band-gap material. The wide energy-band dispersions near the valence band maximum and the conduction band minimum predict that holes and electrons generated by band gap excitation have a high mobility. The conduction band is composed of Bi 6p, Sr 4d and O 2p energy states. On the other hand, the valence band can be divided into two energy regions ranging from −9.5 to −7.9 eV (lower valence band) and from −4.13 to 0 eV (upper valence band). The former mainly consists of Bi 6s states hybridizing with O 2s and O 2p states, and the latter is mainly constructed from O 2p states strongly interacting with Bi 6s and Bi 6p states.     

Modelling amorphous silicon with hydrogenated defects: GW treatment of the ST12 phase

The ST12 phase of silicon is investigated as a possible model for amorphous silicon (a-Si). The structure is studied both with and without hydrogenated hole defects to model the properties of hydrogenated amorphous silicon (a-Si:H) as well as a-Si. A density functional theory model of ST12 Si is structurally relaxed, and the radial correlation function and phonon density of states are used to compare the structural properties of the model to those of a-Si. One-shot GW self-energy corrections are used to generate the band structure, and the corrected electronic structure is found to reproduce the experimental energy gap of a-Si. Introducing hydrogenated defects to the ST12 structure leads to a slight decrease in the band gap and a shift in the density of states, as the breaking of symmetry results in band splitting. The dielectric functions are calculated for both a-Si and a-Si:H, using the GW corrected band structures, with a density functional perturbation theory approach. The model ST12 Si is found to absorb strongly at slightly lower energies than experimental a-Si, whereas the spectrum of the hydrogenated ST12 closely matches that of a-Si:H.     

Metastability and microstructural changes in hydrogenated amorphous silicon

The effects of prolonged illumination and subsequent thermal treatment, on the valence band density of states in hydrogenated amorphous silicon have been investigated. Significant reversible changes are observed upon light soaking and annealing sequences. A microscopic model consistent with the experimental observations is proposed for the light-induced generation and thermal recovery of defects in a-Si:H.     

a-Si:H薄膜的表面光电压谱和光电化学特性研究

采用表面光电压谱和光电化学方法，对不同掺杂类型的氢化非晶硅(s-Si:H)薄膜的光伏响应和光电化学特性进行了研究，测定了a-Si:h薄膜的能带结构，为a-Si:H薄膜在太阳能电池上的应用提供了有用的基础数据和简单光伏测量方法。     

Comment on the optical absorption edge in a-Si:H

The optical absorption edge of undoped amorphous silicon hydride has been measured using optical transmission, photoconductivity, and photothermal deflection spectroscopy. The results obtained by these techniques agree in the exponential edge region. An apparent inconsistency pointed out by Redfield between the optical absorption edge and the valence band tail density of states as measured by drift mobility is attributed to the non-exponential behavior of the absorption edge above α～10³ cm^-1.     

Electrical and optical properties of GexFexSe100−2x chalcogenide thin films

Optical absorption at room temperature and electrical conductivity at temperatures between 283 and 333 K of vacuum evaporated Ge_xFe_xSe_100−2x (0≤x≤15) amorphous thin films have been studied as a function of composition and film thickness. It was found that the optical absorption is due to indirect transition and the energy gap increases with increasing both Ge and Fe content; on the other hand, the width of the band tail exhibits the opposite behavior. The optical band gap E_opt was found to be almost thickness independent. The electrical conductivity show two types of conduction, at higher temperature the conduction is due to extended states, while the conduction at low temperature is due to variable range hopping in the localized states near Fermi level. Increasing Ge and Fe contents were found to decrease the localized state density N(E_F), electrical conductivity and increase the activation energy for conduction, which is nearly thickness independent. Variation of the atomic densities ρ, molar volume V, glass transition temperature T_g cohesive energy C.E and number of constraints N_Co with average coordination number Z was investigated. The relationship between the optical gap and chemical composition is discussed in terms of the cohesive energy C.E, average heat of atomization and coordination numbers.     

Experimental elucidation of hydrogen local density of states in hydrogenated amorphous silicon

Ultraviolet and X-ray photoemission spectroscopies (UPS and XPS) are used to study the valence band in device quality hydrogenated amorphous silicon films (s-Si:H). Their results are combined to obtain estimated information about the hydrogen local density of states (HLDOS). The monohydride bonding configuration and the occurence of interhydride bonding between H atoms are confirmed by the experimental data.     

Microscopic identification of the origin of generation-recombination noise in hydrogenated amorphous silicon with noise-detected magnetic resonance

Spin-dependent changes in the noise power of undoped amorphous hydrogenated silicon ( a-Si:H) are observed under electron spin resonance conditions. The noise-detected magnetic resonance (NDMR) signal has the g value of holes in the valence band tail of a-Si:H. Both the sign of the NDMR signal and the frequency dependence of its intensity can be quantitatively accounted for by a resonant reduction of the generation-recombination noise time constant tau. This identifies hopping in the valence-band tail as the dominant spin-dependent step governing noise in this material.     

Some optical properties of amorphous and crystalline antimony trisulphide thin films

The change in optical properties accompanying the amorphous crystalline transition has been studied for antimony trisulphide thin films. The real and imaginary parts of the dielectric constant are found to be much lower for amorphous films at lower photon energies, possibly because of a large number of defect states which would mostly disturb the top of the valence band and the conduction band comprised, respectively, of chalcogen lone pair and antibonding states. The crystalline material shows some structure in the imaginary part of the dielectric constant that corresponds to interband transitions, and apparently the direct band edge is at 1.88 eV. In the edge region the power law absorption has been observed in the amorphous material from which the extrapolated optical gap has been found to be 1.7 eV.     

非晶硅锗电池性能的调控研究

采用射频等离子体增强化学气相沉积技术, 研究了非晶硅锗薄膜太阳电池. 针对非晶硅锗薄膜材料的本身特性, 通过调控硅锗合金中硅锗的比例, 实现了对硅锗薄膜太阳电池中开路电压和短路电流密度的分别控制. 借助于本征层硅锗材料帯隙梯度的设计, 获得了可有效用于多结叠层电池中的非晶硅锗电池. 关键词： 非晶硅锗薄膜太阳电池 短路电流密度 开路电压 带隙梯度     

Density of states modifications in amorphous and hydrogenated amorphous germanium and their effect on 3d core levels binding energy

By using photoemission and partial yield spectroscopies the conduction band edge, the valence band and the 3d core levels of amorphous germanium hydrogenated and nonhydrogenated are studied and compared to that of crystalline Ge. We measure a 0.25 eV shift of the 3d core levels of amorphous Ge as compared to crystalline Ge whereas a smaller shift and a slight line - broadening are found in hydrogenated amorphous samples with low H content. These results are discussed in term of the contribution to the self-energy due to relaxation of the valence electrons when a core hole is created.     

First-principles energy band calculation for undoped and N-doped InTaO4 with layered wolframite-type structure

The electronic structures of undoped and N-doped InTaO₄ with optimized structures are calculated within the framework of the density functional theory. Calculated lattice constants are in excellent agreement with experimental values, within a difference of 2%. The valence band maximum (VBM) is located near the middle point on the ZD line and the conduction band minimum (CBM) near the middle point on the DX line. This means that InTaO₄ is an indirect-gap material and a minimum theoretical gap between VBM and CBM is ca. 3.7 eV. The valence band in the range from −6.0 to 0 eV mainly consists of O 2p orbitals, where In 4d5s5p and Ta 5d orbitals are slightly hybridized with O 2p orbitals. On the other hand, the conduction band below 5.5 eV is mainly composed of the Ta 5d orbitals and the contributions of In and O orbitals are small. The band gap of N-doped InTaO₄ decreases by 0.3 eV than that of undoped InTaO₄, because new gap states originating from N 2p orbitals appear near the top of the valence band. This result indicates that doping of N atoms into metal oxides is a useful method to develop photocatalysts sensitive to visible light.