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Journal ArticleDOI

Polycrystalline thin film ZnSexTe1-x: preparation and properties

J. Dutta, R. Pal, Subhadra Chaudhuri, A.K. Pal 
14 Jul 1994-Journal of Physics D (IOP Publishing)-Vol. 27, Iss: 7, pp 1538-1543

AbstractZnSexTe1-x films were prepared by co-evaporating ZnSe and ZnTe powders from a two-zone hot wall evaporation jig onto glass substrates. The optical band gaps for different x were determined and this showed a bowing behaviour. The refractive indices and extinction coefficients have been determined as a function of wavelength. Variations of surface roughness with composition and microstructural details were also reported. Grain boundary scattering effects were found to be a dominant factor controlling electron transport processes in these films.

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Citations
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Journal ArticleDOI
Abstract: Nanocrystalline thin films of ZnSe 1− x Te x (0.0 ≤ x ≤ 1.0) were deposited on glass substrate using electron beam deposition technique. The structure of the prepared films was examined using X-ray diffraction technique and revealed that the deposited films have polycrystalline zinc blend structure with lattice constant, a , increasing linearly from 0.55816 to 0.59989 nm as x varies from 0 to 1. The optical studies of the nanocrystalline ZnSe 1− x Te x films showed that the refractive index increases and fundamental band gap E g decreases from 2.58 to 2.21 eV as the tellurium concentration increases from 0 to 1. Furthermore, it was also found that the variation of E g with composition shows quadratic behavior with bowing parameter equal to 0.105. In addition, the thickness and annealing effects on the structure and optical properties of the deposited films were also investigated. The refractive index dispersion and its dependence on composition were discussed in terms of single oscillator model proposed by Wemple–DiDomenico.

34 citations

Journal ArticleDOI
Abstract: Polycrystalline ZnSe thin films have been deposited with different thicknesses by using sputtering evaporation method. Their nonlinear absorption behaviors were investigated by open aperture Z-scan and pump-probe techniques. Linear absorption measurements show redshift in energy as the film thickness increases. This can be attributed to Urbach tail effect. All films exhibit nonlinear absorption for 65 ps pulse duration at lower input intensities while they exhibit saturable absorption at higher input intensities. The life time of localized defect states on grain boundary was found to be ~ 3 ns from ultrafast pump-probe spectroscopy. Nonlinear absorption coefficients and saturation intensity thresholds were extracted from the fitting of the experimental data for 65 ps pulse duration. They increase with increasing film thickness. This behavior can be attributed to the increasing localized defect states on grain boundaries as the film thickness increases.

25 citations

Journal ArticleDOI
Abstract: Polycrystalline ZnSe1−xTex (0.0≤x≤1.0) thin films were deposited by the electron beam deposition technique on corning glass substrate. Two different techniques, Variable Angle Spectroscopic Ellipsometry (VASE) and spectrophotometry, have been applied and compared for characterization of the optical properties of ZnSe1−xTex films. The film thickness and optical constants (refractive index (n) and extinction coefficient (k)) of polycrystalline ZnSe1−xTex films were obtained by fitting the spectroscopic ellipsometric data (ψ, ∆) using a three-layer model system in the wavelength range from 400 to 1100 nm. Nevertheless, the optical band gap E g o p t determined from k values indicating a direct allowed transition. The optical studies of the polycrystalline ZnSe1−xTex films showed that the refractive index increases and the E g o p t decreases. It is worth noting that the obtained values of the optical band gap of the different Te doped films remain in between the reported optical energy gap values of the two ends of the solid solution,ZnSe and ZnTe thin films,. Furthermore, the transmittances spectra of the ZnSe1−xTex films are obtained experimentally from spectrophotometry measurements and theoretically calculated using Murmann's exact equation. Comparing the results yielding a fully agreement between experimental and fitted transmittance data.

18 citations

Journal ArticleDOI
Abstract: We studied the low-temperature growth and doping of polycrystalline ZnSe by MOCVD using ditertiary-butylselenide (DtBSe) and dimethylzinc-triethylamine (DMZn-TEN) as precursors. With these alkyls a deposition of ZnSe at less than 400°C is possible without using the toxic H 2 Se. Polycrystalline ZnSe grows in [1 1 1] direction, therefore also the deposition on GaAs(1 1 1) substrates was studied. Polycrystalline ZnSe remained semi-insulating after doping with Ga and Cl and can therefore not replace ZnO as front electrode material. The final goal of this study is the application of ZnSe as buffer material in polycrystalline Cu(In,Ga)Se 2 (CIGS) solar cells in order to substitute the usually used CdS buffer. By using MOCVD grown ZnSe as buffer layer Cd-free CIS-based solar cells with 11% efficiency have been fabricated.

8 citations

Journal ArticleDOI
Abstract: Here we report the influence of Sb doping on the structural and optical properties of Zn 1− x Sb x Se (0 ⩽ x ⩾ 0.15) thin films prepared by thermal evaporation technique on glass substrate. Various characterization techniques such as X-ray diffraction (XRD), EDS, Raman spectroscopy and spectroscopic ellipsometer are employed to assess the structural and optical properties of the deposited films. XRD analysis reveals the formation of polycrystalline cubic structure having preferred growth orientation along (1 1 1) plane without any evidence of secondary phases. Crystallographic parameters like grain size, micro strain, dislocation density, number of crystallites per unit area and texture coefficient point out the structural modification in ZnSe films with Sb inclusion. Raman analysis shows the existence of three 1LO, 2LO and 3LO phonon modes at 251, 511 and 745 cm −1 in pure ZnSe while 3LO mode disappears by the incorporation of Sb atoms in ZnSe matrix. Increase in FWHM of Raman peaks with Sb concentration also indicates the change in crystalline quality of ZnSe films which is in accordance with our XRD results. Spectroscopic ellipsometry results demonstrate a decreasing trend for the optical band gap energy (from 2.61 eV to 1.81 eV) with increasing Sb content.

3 citations


References
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Journal ArticleDOI
Abstract: Boron doses of 1×1012–5×1015/cm2 were implanted at 60 keV into 1‐μm‐thick polysilicon films. After annealing at 1100 °C for 30 min, Hall and resistivity measurements were made over a temperature range −50–250 °C. It was found that as a function of doping concentration, the Hall mobility showed a minimum at about 2×1018/cm3 doping. The electrical activation energy was found to be about half the energy gap value of single‐crystalline silicon for lightly doped samples and decreased to less than 0.025 eV at a doping of 1×1019/cm3. The carrier concentration was very small at doping levels below 5×1017/cm3 and increased rapidly as the doping concentration was increased. At 1×1019/cm3 doping, the carrier concentration was about 90% of the doping concentration. A grain‐boundary model including the trapping states was proposed. Carrier concentration and mobility as a function of doping concentration and the mobility and resistivity as a function of temperature were calculated from the model. The theoretical and ex...

2,539 citations

Journal ArticleDOI
Abstract: The transport properties of polycrystalline silicon films are examined and interpreted in terms of a modified grain‐boundary trapping model. The theory has been developed on the assumption of both a δ‐shaped and a uniform energy distribution of interface states. A comparison with experiments indicates that the interface states are nearly monovalent and peaked at midgap. Their density is 3.8×1012 cm−2, in accordance with carrier‐lifetime measurements performed on CVD films.

658 citations

Journal ArticleDOI
Abstract: Exponential absorption edges $\ensuremath{\alpha}=A{e}^{g(\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{-}\ensuremath{\hbar}{\ensuremath{\omega}}_{0})}$ have been observed in both ionic (Urbach's rule: $g=\frac{\ensuremath{\sigma}}{{k}_{B}{T}^{*}}$ and covalent materials. Arguments are given to show that a unified theory of exponetial absorption edges must (i) rely on electric microfields as the cause, (ii) include exciton effects and the final-state interaction between the electron and the hole, and (iii) ascribe Urbach's rule to the relative, internal motion of the exciton. An approximate calculation has been made in which the nonuniform microfields are replaced by a statistical distribution of uniform microfields; this calculation is a generalization to physically relevant intermediate-strength fields of previous strong- and weak-field theories of Redfield and Dexter. In contrast with the other microfield models, which obtain the exponential spectral shape by averaging over microfield distributions, the present theory obtains a quantitatively exponential edge as an inherent feature. The temperature dependences of the edges in various materials follow qualitatively from the nature of the microfield sources. The specific temperature dependence of Urbach's rule in ionic crystals is obtained from this model, with supplementary arguments to account for nonuniformity of the fields.

591 citations

Journal ArticleDOI
Abstract: Numerical calculations of the optical-absorption coefficient for direct, excitonic transitions in a uniform applied electric field are presented. The electron-hole scattering is treated within the effective-mass approximation and leads to an absorption coefficient which differs markedly in size and shape from the Franz-Keldysh absorption spectrum. A detailed numerical study of the shape of the absorption-edge spectrum at photon energies somewhat below the zero-field absorption threshold suggests that for small field strengths the dominant asymptotic form of the absorption coefficient is $\mathrm{exp}(\ensuremath{-}\frac{{C}_{0}|E\ensuremath{-}{{E}_{0}}^{\ensuremath{'}}|}{f})$, where $f=\frac{|e|\mathrm{Fa}}{R}$ is the electric field strength in units of exciton Rydbergs per electron-exciton Bohr radius. This result contradicts the existing belief that the electron-hole interaction does not alter the asymptotic form of the Franz-Keldysh shape: $\mathrm{exp}(\ensuremath{-}\frac{{{C}_{0}}^{\ensuremath{'}}{|E\ensuremath{-}{{E}_{0}}^{\ensuremath{'}}|}^{\frac{3}{2}}}{f})$. Physical arguments are presented to show why the exciton effects should be important. A discussion is presented of the interrelationships among the present treatment of electro-absorption and various one-electron, exciton, and many-body formalisms.

359 citations

Journal ArticleDOI
01 Apr 1992-Vacuum
Abstract: A new formulation and method are presented for evaluating bandgap, optical transitions and optical constants from the reflectance data for films deposited onto a non-absorbing substrate.

221 citations