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.     

Dynamics of hydrogen in hydrogenated amorphous silicon

The problem of hydrogen diffusion in hydrogenated amorphous silicon (a-Si:H) is studied semiclassically. It is found that the local hydrogen concentration fluctuations-induced extra potential wells, if intense enough, lead to the localized electronic states in a-Si:H. These localized states are metastable. The trapping of electrons and holes in these states leads to the electrical degradation of the material. These states also act as recombination centers for photo-generated carriers (electrons and holes) which in turn may excite a hydrogen atom from a nearby Si-H bond and breaks the weak (strained) Si-Si bond thereby apparently enhancing the hydrogen diffusion and increasing the light-induced dangling bonds.     

Photoluminescence analyses of hydrogenated amorphous silicon nitride thin films

Silicon-rich hydrogenated amorphous silicon nitride (a-SiN_x:H) films were grown by plasma enhanced chemical vapor deposition (PECVD) with different r=NH₃/SiH₄ gas flow ratios. The optical absorption characteristics were analyzed by Fourier transform infrared (FTIR) and UV-visible transmittance spectroscopies. The recombination properties were investigated via photoluminescence (PL) measurements. As r was increased from 2 to 9, the PL emission color could be adjusted from red to blue with the emission intensity high enough to be perceived by naked eye at room temperature. The behaviors of the PL peak energy and the PL band broadness with respect to the optical constants were discussed in the frame of electron-phonon coupling and band tail recombination models. A semiquantitative analysis supported the band tail recombination model, where the recombination was found to be favored when the carriers thermalize to an energy level at which the band tail density of states (DOS) reduces to some fraction of the relevant band edge DOS. For the PL efficiency comparison of the samples with different nitrogen contents, the PL intensity was corrected for the absorbed intensity fraction of the incident PL excitation source. The resulted correlation between the PL efficiency and the subgap absorption tail width further supported the band tail recombination model.     

Structural and band tail state photoluminescence properties of amorphous SiC films with different amounts of carbon

Amorphous silicon carbide films are deposited by the plasma enhanced chemical vapour deposition technique,and optical emissions from the near-infrared to the visible are obtained.The optical band gap of the films increases from 1.91 eV to 2.92 eV by increasing the carbon content,and the photoluminescence(PL) peak shifts from 1.51 eV to 2.16 eV.The band tail state PL mechanism is confirmed by analysing the optical band gap,PL intensity,the Stocks shift of the PL,and the Urbach energy of the film.The PL decay times of the samples are in the nanosecond scale,and the dependence of the PL lifetime on the emission energy also supports that the optical emission is related to the radiative recombination in the band tail state.     

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     

Influence of holmium doping on the optical properties of quaternary InGaAsP epitaxial layers

Photoluminescence and photoconductivity measurements were used to study the influence of Ho doping on the optical properties of InGaAsP layers grown by liquid phase epitaxy (LPE). The full width at half maximum (FWHM) of the photoluminescence peak was found to decrease as the amount of Ho increases. When the amount of Ho is 0.11 wt%, the FWHM has a minimum value of 7.93 meV, about 46% lower than that of the undoped InGaAsP. The absorption tails observed in the photoconductivity were analyzed with the Urbach tail model and the Urbach energies were obtained from the fits. The Urbach energy decreases as the amount of Ho increases, indicating that Ho doping greatly reduces the amount of residual impurities in LPE-grown layers.     

Large supercell molecular dynamics study of defect formation in hydrogenated amorphous silicon

We have performed molecular dynamics simulations of the defect formation associated with the Staebler-Wronski (SW) effect in a-Si:H using 224 and 231 atom supercells and employing semiempirical Si-Si and Si-H total energy functionals. The role of hydrogen in the defect formation within the bond breaking model of the SW effect has been investigated for both large supercells. The results suggest that, within this model, H can be important in weakening the normal Si-Si bonds which break to produce defects in the SW effect.     

Variation of spark-processing parameters on the photoluminescence properties of spark-processed silicon

Electrical and physical parameters, which influence the photoluminescence (PL) properties of spark-processed silicon (sp-Si), were systematically varied in order to obtain optimal PL emission. Among these parameters are the average spark current, the pulse width of the spark events, the frequency of the pulses, the processing time, the electrode diameter, the distance between the electrodes, the spark-processing environment, and the gas ambient pressure. It was found that for optimal PL emission the processing current needs to be between 20 and 40 mA, and the pulse frequency of the sparks between 10 and 15 kHz. Further, the N₂/O₂ ratio of the processing environment needs to be about 7:3 and the ambient gas pressure and the processing time as large as feasible. The conditions that are favorable for green PL are a small pulse width, a small counter electrode diameter, a small gap between electrodes, a relatively large nitrogen concentration in the processing chamber, and a comparatively large spark frequency. In the opposite cases, a UV/blue PL is predominantly observed. The results are discussed in terms of various thermal effects on the resulting molecules or defects, which are believed to be important for the PL emission.     

Defect-induced increase in the phonon energy involved in the formation of Urbach tail in Cu-ternaries

From a combined analysis of the stoichiometric composition and Urbach tail in samples of CuInSe₂, CuInTe₂, and CuGaTe₂ of the I-III-VI₂ family of chalcopyrite semiconductors, it is found that the energy hν_p involved in the electron/exciton-phonon interaction is a linear function of a parameter Δz which is the sum of the deviations from ideal molecularity Δx and anion to cation ratio Δy. It gives evidence that in the copper ternaries hν_p is associated to the structural defects caused by cation-cation, cation-anion, and other intrinsic disorders. The high value of hν_p found in the studied samples, higher than the highest optical mode, is shown to come from the contribution of the additional phonon energy due to structural defects. This is in agreement with recently proposed models of the temperature dependence of the Urbach energy.     

Effect of annealing temperature on the optical and electrical properties of amorphous As45.2Te46.6In8.2 thin films

The optical absorption of the As-prepared and annealed As_45.2Te_46.6In_8.2 thin films are studied. Films annealed at temperatures higher than 453 K show a decrease in the optical energy gap (E_o). The value of E_o increases from 1.9 to 2.43 eV with increasing thickness of the As-prepared films from 60 to 140 nm. The effect of thickness on high frequency dielectric constant (?_∞) and carrier concentration (N) is also studied. The crystalline structures of the As_45.2Te_46.6In_8.2 thin films resulting from heat treatment of the As-prepared film at different elevated temperatures is studied by X-ray diffraction. An amorphous-crystalline transformation is observed after annealing at temperatures higher than 453 K. The electrical conductivity at low temperatures is found due to the electrons transport by hopping among the localized states near the Fermi level. With annealing the films at temperatures higher than 473 K (the crystallization onset temperature) for 1 h, the electrical conductivity increases and the activation energy decreases, which can be attributed to the amorphous-crystalline transformations.     

The effect of the Bi source on optical properties of Bi2Te3 nanostructures

nanostructures were synthesized by using different Bi sources via a simple solvothermal process, in which and BiCl₃ were used as the Bi sources. Optical properties of nanostructures prepared with and BiCl₃ as the Bi sources were investigated by micro-Raman spectroscopy. The Raman scattering spectrum of hexagonal nanoplates prepared by using as the Bi source shows that the infrared (IR) active mode A_1u, which must be odd parity and is Raman forbidden for bulk crystal due to its inversion symmetry, is greatly activated and shows up clearly in the Raman scattering spectrum. We attribute the appearance of the infrared active A_1u mode in the Raman spectrum to crystal symmetry breaking of hexagonal nanoplates. However, the Raman scattering spectrum of nanostructures with irregular shape prepared by using as the Bi source only exhibits the two characteristic Raman modes of crystals. Micro-Raman measurements on nanostructures with different morphologies offer us a potential way to tailor optical properties of nanostructures by controlling the morphologies of the nanostructures, which is very important for practical applications of nanostructures in thermoelectric devices.     

Temperature dependence of the photoluminescence spectra in AgGaS2

The Silver Gallium Sulfide (AgGaS₂) ternary compound is a wide bandgap semiconductor (about 2.7 eV) whose photoluminescence properties are characterized by excitons and donor-acceptor pairs recombinations. We have performed photoluminescence (PL) measurement exciting with the third harmonic (3.5 eV) of a Nd:YAG laser from room temperature down to 10 K at different excitation power. In this work we report the dependence of the ‘green band’ on the excitation power at various temperatures.     

Model based prediction of the trap limited diffusion of hydrogen in post‐hydrogenated amorphous silicon

The diffusion of hydrogen within an hydrogenated amorphous silicon (a‐Si:H) layer is based on a trap limited process. Therefore, the diffusion becomes a self‐limiting process with a decreasing diffusion velocity for increasing hydrogen content. In consequence, there is a strong demand for accurate experimental determination of the hydrogen distribution. Nuclear resonant reaction analysis (NRRA) offers the possibility of a non‐destructive measurement of the hydrogen distribution in condensed matter like a‐Si:H thin films. However, the availability of a particle accelerator for NRR‐analysis is limited and the related costs are high. In comparison, Fourier transform infrared spectroscopy (FTIR) is also a common method to determine the total hydrogen content of an a‐Si:H layer. FTIR spectrometers are practical table‐top units but lack spatial resolution. In this study, an approach is discussed that greatly reduces the need for complex and expensive NRR‐analysis. A model based prediction of hydrogen depth profiles based on a single NRRA measurement and further FTIR measurements enables to investigate the trap limited hydrogen diffusion within a‐Si:H. The model is validated by hydrogen diffusion experiments during the post‐hydrogenation of hydrogen‐free sputtered a‐Si. The model based prediction of hydrogen depth profiles in a‐Si:H allows more precise design of experiments, prevents misinterpretations, avoids unnecessary NRRA measurements and thus saves time and expense. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Effect of composition and thermal annealing on the absorption and electrical conduction of Se85−xTe15Sbx glasses

The optical absorption of the as-prepared and thermally annealed Se_85−xTe₁₅Sb_x (0≤x≤9) thin films was measured. The mechanism of the optical absorption follows the rule of non-direct transition. The optical energy gap (E₀) decreased from 1.12 to 0.84 eV with increasing Sb content of the as-prepared films from 0 to 9 at.%. The as-prepared Se₇₆Te₁₅Sb₉ films showed an increase in (E₀) with increasing the temperature of annealing in the range above T_g (363 K). The electrical conductivity of the as-prepared and annealed films was found to be of Arrhenius type with temperature in the range 300-360 K. The activation energy for conduction was found to decrease with increasing both the Sb content and temperature of annealing. The results were discussed on the basis of the lone-pair electron effect and of amorphous crystalline transformation.     

Bonds,bands, and band gaps in tetrahedrally bonded ternary compounds: The role of group V lone pairs

An interesting class of tetrahedrally coordinated ternary compounds has attracted considerable interest because of their potential as good thermoelectrics. These compounds, denoted as I₃–V–VI₄, contain three monovalent-I (Cu, Ag), one nominally pentavalent-V (P, As, Sb, Bi), and four hexavalent-VI (S, Se, Te) atoms; and can be visualized as ternary derivatives of the II–VI zincblende or wurtzite semiconductors, obtained by starting from four unit cells of (II–VI) and replacing four type II atoms by three type I and one type V atoms. We find that nominally pentavalent-V atoms are effectively trivalent and their lone (ns²) pairs play an active role in opening up a gap. The lowest conduction band is a strongly hybridized anti-bonding combination of the lone pair and chalcogen (VI) p-states. The magnitude of the gap is sensitive to the nature of the exchange interaction (local vs non-local) and the V–VI distance. We also find that the electronic structure near the gap can be reproduced extremely well within a local theory if one can manipulate the position of the filled d bands of Cu and Ag by an effectively large U.     

The effects of plastic deformation on band gap, electronic defect states and lattice vibrations of rutile

The extensive polygonization of 200 nm rutile crystals in high-energy dry milling allowed to study the spectral properties of grain boundaries and adjacent microstrained crystalline matter. Changes in UV, VIS, NIR, IR and FIR spectra during milling were followed. For the UV absorption edge the value of unstrained rutile was retained while residual traces of anatase, intergrown with the rutile phase, continued to act as traps for photoinduced charges. The evolving broad absorption in VIS and NIR could be attributed to electrons weakly bound to defects in the packing of oxygen anions at the grain boundaries, which may relax to face-sharing Ti³⁺-O octahedra. Among the IR-active lattice vibrations, the narrow E_u⁽²⁾ band showed a shift to higher frequencies by 15 cm⁻¹ which is definitively not due to phonon confinement or Fröhlich surface modes but probably to coupling of the bulk phonon to a plasmon at the grain boundary. At the external surface of the polygonized primary particle, the regular atomic order is destroyed by milling so that hydroxylation is replaced by physisorption of H₂O, as shown by IR and TG.     

Effect of substrate and annealing on the structural and optical properties of ZnO:Al films

ZnO:Al thin films with c-axis preferred orientation were deposited on glass and Si substrates using RF magnetron sputtering technique. The effect of substrate on the structural and optical properties of ZnO:Al films were investigated. The results showed a strong blue peak from glass-substrate ZnO:Al film whose intensity became weak when deposited on Si substrate. However, the full width at half maxima (FWHM) of the Si-substrate ZnO:Al (0 0 2) peaks decreased evidently and the grain size increased. Finally, we discussed the influence of annealing temperature on the structural and optical properties of Si-substrate ZnO:Al films. After annealing, the crystal quality of Si-substrate ZnO:Al thin films was markedly improved and the intensity of blue peak (∼445 nm) increased noticeably. This observation may indicate that the visible emission properties of the ZnO:Al films are dependent more on the film crystallinity than on the film stoichiometry.     

An ab initio analysis of electronic states associated with a silicon vacancy in cubic symmetry

The electronic orbitals localized in the vicinity of a vacancy in a silicon crystal are calculated by an ab initio method based on the density functional theory and analyzed in association with the elastic softening observed by the recent ultrasonic experiments, especially focused on an estimate of the electric quadrupole moments. The localized orbitals due to the existence of a vacancy show largely extended properties and the quadrupole moments calculated from the orbitals indicate the strong dependence on cell sizes up to 511 atoms in the basic cell. Asymptotic values of the quadrupole moments in the limit of large size are obtained by an extrapolating method. It is shown that the quadrupole moments are enhanced due to the extension of the orbitals and the ratio of the quadrupole moments of Γ₅ and Γ₃ symmetries agrees well with the value deduced from the experimental results.     

An origin of light induced centers in the visible range in ferroelectric oxides: possible role of the states of charge transfer vibronic excitons

A light induced absorption peculiarity connected with an absorption peak in the visible range for SBN/Ce was detected. The analysis of these data together with investigation results for a visible range center (VIS-center) absorption study in SBN/Ce and Cr, in nominally pure strontium barium niobate (SBN) as well as in Ba_0.77Ca_0.23TiO₃ crystals give support to charge transfer vibronic excitons (CTVEs) as origin of the phenomenon. While the fast (with relaxation time ∼1 ns at T∼300 K) response can be connected with transitions between excited branches of the adiabatic potential of the CTVE phase, the slow (with relaxation time ∼50 s at T∼230 K for SBN/Cr) response is connected with transitions between ground and first excited CTVE phase branches. The additional contribution to the latter effect due to the absorption of recharged oxygen ions which are in the framework of charge transfer induced by the CTVE cannot be disregarded. The explanation of the main results of the experiments shows that the VIS-center phenomena can be induced by the CTVE-states.     

Electrical conduction and optical properties of amorphous (Sb2Se3)100−xSnx thin films

The variation of DC electrical conductivity and the optical properties of thermally evaporated a- (Sb₂Se₃)_100−xSn_x thin films with temperature have been studied. It is found that the thermal activation energy decreases, while the optical gap first increases (up to x=1) and then decreases, with the increase in Sn content. These results have been explained by taking into consideration the structural modifications induced by the incorporation of Sn into the parent alloy. The variation in the conductivity prefactor (σ_o) with Sn addition indicates a change in the dominant conduction transport mechanism from extended states to localized states. An experimental correlation between the activation energy and the pre-exponential factor has been observed, indicating the validity of Meyer–Neldel rule in the studied samples.