Cd0.5Zn0.5Se wide range composite thin films for solar cell buffer layer application

Cd_0.5Zn_0.5Se composite thin films were obtained on glass substrate using aqueous alkaline solution at low temperature using cadmium acetate and zinc acetate as Cd²⁺ and Zn²⁺ and Se²⁻ ion sources. Different phases of individuals i.e. CdSe and ZnSe, spherical and needle shape surface morphology and good elemental chemical stoichiometric ratio were observed from X-ray diffraction, scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) studies, respectively. The band gap and electrical resistivity of the composite film were 2.35 eV and about 10⁷ Ω cm, respectively.     

A comparative study on Zn0.8Cd0.2O films deposited on different substrates

Highly (0 0 2)-oriented Zn_0.8Cd_0.2O crystal films were prepared on different substrates, namely, glass, Si(1 1 1) and α-Al₂O₃(0 0 1) wafers by the dc reactive magnetron sputtering technique. The Zn_0.8Cd_0.2O/α-Al₂O₃ film has the best crystal quality with a FWHM of (0 0 2) peak of 0.3700°, an average grain size of about 200 nm and a root-mean-square surface roughness of about 70 nm; yet the Zn_0.8Cd_0.2O/glass holds the worst crystal quality with a much larger FWHM of 0.6281°. SIMS depth profile shows that the Zn and O compositions change little along the film depth direction; the Cd incorporation also almost holds the line towards the top surface other than an accumulation at the interface between the film and the substrate. The Cd content in the film is nearly consistent with that in target.     

Annealing effect on the structural and optical properties of a Cd1−xZnxS thin film for photovoltaic applications

Herein is a report of a study on a Cd_1−xZn_xS thin film grown on an ITO substrate using a chemical bath deposition technique. The as-deposited films were annealed in air at 400 °C for 30 min. The composition, surface morphology and structural properties of the as-deposited and annealed Cd_1−xZn_xS thin films were studied using EDX, SEM and X-ray diffraction techniques. The annealed films have been observed to possess a crystalline nature with a hexagonal structure. The optical absorption spectra were recorded within the range of 350-800 nm. The band gap of the as-deposited thin films varied from 2.46 to 2.62 eV, whereas in the annealed film these varied from 2.42 to 2.59 eV. The decreased band gap of the films after annealing was due to the improved crystalline nature of the material.     

Structural,electronic and optical properties of CdxZn1 − xS alloys from first-principles calculations

Structural, electronic and optical properties as well as structural phase transitions of ternary alloy Cd_xZn_1 − xS have been investigated using the first-principles calculations based on the density functional theory. We found that the crystal structure of Cd_xZn_1 − xS alloys transforms from wurtzite to zinc blende as Cd content of x=0.83$x=0.83$. Effect of Cd content on electronic structures of Cd_xZn_1 − xS alloys has been studied. The bandgaps of Cd_xZn_1 − xS alloys with wurtzite and zinc blende structures decrease with the increase of Cd content. Furthermore, dielectric constant and absorption coefficient also have been discussed in detail.     

Anti-stokes luminescence of Zn<Subscript>0.6</Subscript>Cd<Subscript>0.4</Subscript>S solid solutions with adsorbed organic dye molecules and few-atom silver clusters

For microcrystals of Zn_0.6Cd_0.4S with adsorbed molecules of a number of organic dyes, we have observed sensitized anti-Stokes luminescence excited by radiation with wavelengths in the range 610–750 nm and flux density 10¹⁴–10¹⁵ photons/cm²·sec. The positions of the bands in the excitation spectra for such luminescence match those of the absorption spectra for the adsorbed dye molecules, which is evidence in favor of a cooperative mechanism for its appearance. We have shown that enhancement of the anti-Stokes luminescence is possible when silver atoms and few-atom clusters appear on the Zn_0.6Cd_0.4S surface in addition to the dye molecules. We hypothesize that its excitation in the latter case occurs as a result of two-photon optical transitions. These transitions occur sequentially, with transfer of an electron or the electronic excitation energy from the dye molecules to silver atoms and few-atom clusters adsorbed on the surface of Zn_0.6Cd_0.4S, creating deep localized states in the bandgap with photoionization energies 1.80–2.00 eV. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 617–621, September–October, 2007.     

Effect of zinc concentration on the structural,electrical and magnetic properties of mixed Mn–Zn and Ni–Zn ferrites synthesized by the citrate precursor technique

Mixed manganese-zinc and nickel-zinc ferrites of composition Mn_0.2Ni_0.8−xZn_xFe₂O₄ where x=0.4$x=0.4$, 0.5 and 0.6 have been synthesized by the citrate precursor technique. Decomposition of the precursor at temperatures as low as 500 °C gives the ferrite powder. The ferrites have been investigated for their electrical and magnetic properties such as saturation magnetization, initial permeability, Curie temperature, AC-resistivity and dielectric constant as a function of sintering temperature and zinc content. Structural properties such as lattice parameter, grain size and density are also studied. The mixed compositions exhibited higher saturation magnetizations at sintering temperatures as low as 1200 °C. While the Curie temperature decreased with zinc content, the permeability was found to increase. The AC-resistivity ranged from 10⁵–10⁷ Ω cm and decreased with zinc content and sintering temperature. The dielectric constants were lower than those normally reported for the Mn–Zn ferrites. Samples sintered at 1400 °C densified to about 94% of the theoretical density and the grain size was of the order of about 1.5 μm for the samples sintered at 1200 °C and increased subsequently with sintering temperature.     

Nano-star formation in Al-doped ZnO thin film deposited by dip-dry method and its characterization using atomic force microscopy, electron probe microscopy, photoluminescence and laser Raman spectroscopy

Zinc oxide doped with Al (AZO) thin films were prepared on borosilicate glass substrates by dip and dry technique using sodium zincate bath. Effects of doping on the structural and optical properties of ZnO film were investigated by XRD, EPMA, AFM, optical transmittance, PL and Raman spectroscopy. The band gap for ZnO:Al (5.0 at. wt.%) film was found to be 3.29 eV compared with 3.25 eV band gap for pure ZnO film. Doping with Al introduces aggregation of crystallites to form micro-size clusters affecting the smoothness of the film surface. Al³⁺ ion was found to promote chemisorption of oxygen into the film, which in turn affects the roughness of the sample. Six photoluminescence bands were observed at 390, 419, 449, 480, 525 and 574 nm in the emission spectra. Excitation spectra of ZnO film showed bands at 200, 217, 232 and 328 nm, whereas bands at 200, 235, 257 and 267 nm were observed for ZnO:Al film. On the basis of transitions from conduction band or deep donors (CB, Zn_i or V_OZn_i) to valence band and/or deep acceptor states (VB, V_Zn or O_i or O_Zn), a tentative model has been proposed to explain the PL spectra. Doping with Al³⁺ ions reduced the polar character of the film. This has been confirmed from laser Raman studies.     

Structure, refractive-index dispersion and the optical absorption edge of chemically deposited ZnxCd(1-x)S thin films

Zinc cadmium sulfide, Zn_xCd_(1-x)S, thin films have been deposited by a simple and inexpensive chemical bath deposition method from an aqueous medium using thiourea as a sulfide-ion source. The structure of the deposited films has been characterized by X-ray diffraction and transmission electron microscopy. It was observed from X-ray diffraction that the as-deposited films were amorphous in nature. However Zn_xCd_(1-x)S films annealed at 423 K for 1.5 h show a crystalline structure with a small scattering volume. The obtained results were confirmed throughout the transmission electron microscopy and the corresponding electron-diffraction patterns. The optical constants of Zn_xCd_(1-x)S films annealed at 423 K for 1.5 h in the compositional range 0≤x≤1 were estimated using transmission and reflection spectra in the wavelength range 300–2500 nm. The band gap varies non-linearly with the value of x. The dependence of the refractive index on the wavelength obeys the single-oscillation model, from which the dispersion parameters and the high-frequency dielectric constant were determined. A graphical representation of the surface and volume energy-loss functions was also given. Received: 23 February 2001 / Accepted: 26 February 2001 / Published online: 27 June 2001     

Optical properties of sol-gel fabricated Mn/SiO2 nanocomposites: Observation of surface plasmon resonance in Mn nanoparticles

Manganese nanoparticles were grown in silica glass and silica film on silicon substrate by annealing of the sol-gel prepared porous silicate matrices doped with manganese nitrate. Annealing of doped porous silicate matrices was performed at various conditions that allowed to obtain the nanocomposite glasses with various content of metallic Mn. TEM of Mn/SiO₂ glass indicates the bimodal size distribution of Mn nanoparticles with mean sizes of 10.5 nm and 21 nm. The absorption and photoluminescence spectra of Mn/SiO₂ glasses were measured. In the absorption spectra at 300 nm (4.13 eV) we observed the band attributed to the surface plasmon resonance in Mn nanoparticles. The spectra proved the creation of Mn²⁺ and Mn³⁺ ions in silica glass as well. The absorption spectra of Mn/SiO₂ glasses annealed in air prove the creation of manganese oxide Mn₂O₃. The measured reflection spectra of Mn/SiO₂ film manifest at 240-310 nm the peculiarity attributed to surface plasmons in Mn nanoparticles.     

Large Third-Order Optical Nonlinearity of Cadmium Sulphide Nanoparticles Embedded in Polymer Thin Films

A simple method for synthesis of well dispersed cadmium sulphide nanoparticles embedded in a polyethylene glycol matrix （PEG 400） in thin film form is presented. The large blue shift of the band gap energy of the CdS nanoparticles compared to the bulk semiconductors is observed via UV-vis absorption spectra. Photoluminescence spectra of CdS nanocomposite films show that the emission peaks shift towards the longer wavelength with the increase of annealing temperature. Transmission electron microscopic images as well as Raman scattering studies confirm the CdS nanometer size particle formation within the polymer matrix. The particle size is about 8 nm. Selected area electron diffraction （SAED） shows the cubic zinc blende polycrystalline rings. Third-order optical nonlinearity of the CdS nanopartieles embedded in polymer thin films is studied with the Z-scan technique under 1064 nm excitation. The results show that the CdS nanocomposite film exhibits negative nonlinear refraction index and positive absorption coefficient. The film shows large optical nonlinearity, and the magnitude of the third-order nonlinear susceptibility of the film is calculated to be 1.73 × 10^-9 esu. The corresponding mechanism is discussed.     

Optical properties of Cd<Subscript>0.6</Subscript>Mn<Subscript>0.4</Subscript>Te/Cd<Subscript>0.5</Subscript>Mg<Subscript>0.5</Subscript>Te quantum-well structures

Emission spectra of three Cd_0.6Mn_0.4Te/Cd_0.5Mg_0.5Te superlattices with Cd_0.6Mn_0.4Te quantum-well (QW) widths of 7, 13, and 26 monolayers, respectively, and the same thickness (46 monolayers) of the Cd_0.5Mg_0.5Te barriers have been studied. The QW width affects the shape and spectral position of the Mn²⁺ intracenter luminescence (IL) band as a result of the crystal field being dependent on the position of the manganese ion with respect to the interface. Measured in identical experimental conditions, the exciton luminescence as compared to the IL is substantially higher in intensity in a QW than in a bulk CdMnTe crystal. Some samples of superlattices and bulk crystals exhibit, in addition to the conventional IL band near 2.0 eV, a weaker band at about 1.45 eV. This band apparently derives from intracenter transitions in the Mn²⁺ ions in the regions where the crystal lattice has the rock-salt rather than the conventional zinc blende structure.     

Band gap energy and bowing parameter of In-rich InAlN films grown by magnetron sputtering

The crystal structure, band gap energy and bowing parameter of In-rich In_xAl_1−xN (0.7 < x < 1.0) films grown by magnetron sputtering were investigated. Band gap energies of In_xAl_1−xN films were obtained from absorption spectra. Band gap tailing due to compositional fluctuation in the films was observed. The band gap of the as-grown InN measured by optical absorption method is 1.34 eV, which is larger than the reported 0.7 eV for pure InN prepared by molecular beam epitaxy (MBE) method. This could be explained by the Burstein-Moss effect under carrier concentration of 10²⁰ cm⁻³ of our sputtered films. The bowing parameter of 3.68 eV is obtained for our In_xAl_1−xN film which is consistent with the previous experimental reports and theoretical calculations.     

Controlled synthesis and relationship between luminescent properties and shape/crystal structure of Zn2SiO4:MN phosphor

Mn-doped Zn₂SiO₄ phosphors with different morphology and crystal structure, which show different luminescence and photoluminescence intensity, were synthesized via a low-temperature hydrothermal route without further calcining treatment. As-synthesized zinc silicate nanostructures show green or yellow luminescence depending on their different crystal structure obtained under different preparation conditions. The yellow peak occurring at 575 nm comes from the β-phase zinc silicate, while the green peak centering at 525 nm results from the usual α-phase zinc silicate. From photoluminescence spectra, it is found that Zn₂SiO₄ nanorods have higher photoluminescence intensity than Zn₂SiO₄ nanoparticles. It can be ascribed to reduced surface-damaged region and high crystallinity of nanorods.     

Electronic properties of multi-quantum dot structures in Cd 1-xZn xS alloy semiconductors

In this paper, we present a theoretical study of the quantized electronic states in Cd_1-xZn_xS quantum dots. The shape of the confining potential, the subband energies and their eigen envelope wave functions are calculated by solving a one-dimensional Schr?dinger equation. Electrons and holes are assumed to be confined in dots having a flattened cylindrical geometry with a finite barrier height at the boundary. Optical absorption measurements are used to fit the bandgap edge of the Cd_1-xZn_xS nanocrystals. An analysis of the electron band parameters has been made as a function of Zn composition. Two main features were revealed: (i) a multiplicity in Cd_1-xZn_xS quantum dots with different crystalline sizes has been found to fit accurately experimental data in the composition range 0 ≤x ≤0.2; (ii) the fit did not, however, show a multiplicity for x higher than 0.4. On the other hand, we have calculated the energy level structure of coupled Cd_1-xZn_xS semiconductor quantum dots using the tight-binding approximation. As is found the Zn composition x = 0.4 is expected to be the most favorable to give rise a superlattice behavior for the Cd_1-xZn_xS quantum dots studied.     

Investigation on the structural properties and correlation of the optical constants of Cd1-xZnxS thin films with bath deposition temperature

Cd_1-xZn_xS thin films were deposited by chemical bath deposition (CBD) technique, which is simple and cost effective, in a chemical bath containing appropriate amount of cadmium acetate, zinc acetate, and thiourea as precursors, in a clean glass substrate. The deposition was carried out by varying the bath temperatures (70 °C, 75 °C, 80 °C, and 85 °C) of the precursor solution. The XRD results indicate the existence of hexagonal structures of Cd_1-xZn_xS with an average crystallite size of ∼ 27–41 nm. EDX studies confirm the presence of Cd, Zn, and S in the films. HRTEM and SAED patterns show the crystalline nature of the films with the coexistence of the hexagonal phase. The optical constants viz; optical band gap, Urbach energy, static refractive index, and optical conductivity were studied by using UV- Vis transmission spectra as a function of CBD temperature. It was observed that with the increase of bath temperature in the above range, there were concomitant decreases in optical band gap from ∼3.3 to 2.8 eV. The Urbach energy, optical conductivity, and static refractive index of the films increase with the increase in bath deposition temperature. FTIR studies confirm the formation of ternary Cd_1-xZn_xS thin films.     

Ellipsometric characterization of PbI2 thin film on glass

The optical properties of polycrystalline lead iodide thin film grown on Corning glass substrate have been investigated by spectroscopic ellipsometry. A structural model is proposed to account for the optical constants of the film and its thickness. The optical properties of the PbI₂ layer were modeled using a modified Cauchy dispersion formula. The optical band gap E_g has been calculated based on the absorption coefficient (α) data above the band edge and from the incident photon energy at the maximum index of refraction. The band gap was also measured directly from the plot of the first derivative of the experimental transmission data with respect to the light wavelength around the transition band edge. The band gap was found to be in the range of 2.385±0.010 eV which agrees with the reported experimental values. Urbach's energy tail was observed in the absorption trend below the band edge and was found to be related to Urbach's energy of 0.08 eV.     

Growth, structural and optical properties of CdxZn1−xS thin films deposited using spray pyrolysis technique

Cd_xZn_(1−x)S (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) thin films were deposited by the chemical spray pyrolysis technique using a less used combination of chemicals. Depositions were done at 573 K on cleaned glass substrates. The composition, surface morphology and structural properties of deposited films were studied using EDAX, SEM and X-ray diffraction technique. XRD studies reveal that all the films are crystalline with hexagonal (wurtzite) structure and inclusion of Cd into the structure of ZnS improved the crystallinity of the films. The value of lattice constant ‘a’ and ‘c’ have been observed to vary with composition from 0.382 to 0.415 nm and 0.625 to 0.675 nm, respectively. The band gap of the thin films varied from 3.32 to 2.41 eV as composition varied from x = 0.0–1.0. It was observed that presence of small amount of cadmium results in marked changes in the optical band gap of ZnS.     

Electronic and optical properties of Cd1-xZnxS nanocrystals

We report a numerical simulation of the conduction and valence band edges of Cd_1-xZn_xS nanocrystallites using a one — dimensional potential model. Electron — hole pairs are assumed to be confined in nanospheres of finite barrier heights. Optical absorption measurements are used to fit the bandgap of the Cd_1-xZn_xS nanocrystal material. A theoretical analysis is also made to calculate the energy location of bound excitons and the oscillator strength of interband transitions as a function of zinc composition. The aim of the latter study is to investigate the optical behavior of Cd_1-xZn_xS nanocrystals. An attempt to explain all the results is presented.     

Chemical bath-deposited ZnS thin films: Preparation and characterization

Chemical bath deposition of ZnS thin films from NH₃/SC(NH₂)₂/ZnSO₄ solutions has been studied. The effect of various process parameters on the growth and the film quality are presented. The influence on the growth rate of solution composition and the structural, optical properties of the ZnS thin films deposited by this method have been studied. The XRF analysis confirmed that volume of oxygen of the as-deposited film is very high. The XRD analysis of as-deposited films shows that the films are cubic ZnS structure. The XRD analysis of annealed films shows the annealed films are cubic ZnS and ZnO mixture structure. Those results confirmed that the as-deposited films have amorphous Zn(OH)₂. SEM studies of the ZnS thin films grown on various growth phases show that ZnS film formed in the none-film phase is discontinuous. ZnS film formed in quasi-linear phase shows a compact and a granular structure with the grain size about 100 nm. There are adsorbed particles on films formed in the saturation phase. Transmission measurement shows that an optical transmittance is about 90% when the wavelength over 500 nm. The band gap (E_g) value of the deposited film is about 3.51 eV.     

Electronic and optical properties of CdS/CdZnS nanocrystals

Cd1-x ZnxS nanocrystals are prepared by a co-precipitation method with different atomic fractions of Zn.The texture,structural transformation and optical properties with increasing x value in Cd1-x ZnxS are studied with scanning electron microscopy,electron diffraction patterning,and absorption spectra respectively.Quantum confinement in a strained CdS/Cd1-xZnxS related nanodot with various Zn content values is investigated theoretically.Binding energies on exciton bound CdS/CdxZn1-xS quantum dot are computed,with consideration of the internal electric field induced by the spontaneous and piezoelectric polarizations,and thereby the interband emission energy is calculated as a function of the dot radius.The optical band gap from the UV absorption spectrum is compared with the interband emission energy computed theoretically.Our results show that the average diameter of composite nanoparticles ranges from 3 nm to 6 nm.The X-ray diffraction pattern shows that all the peaks shift towards the higher diffracting angles with an increase in Zn content.The lattice constant gradually decreases as the Zn content increases.The strong absorption edge shifts towards the lower wavelength region and hence the band gap of the films increases as the Zn content increases.The values of the absorption edge are found to shift towards the shorter wave length region and hence the direct band gap energy varies from 2.5 eV for the CdS film and 3.5 eV for the ZnS film.Our numerical results are in good agreement with the experimental results.