Grain growth and structural transformation in ZnS nanocrystalline thin films

Fabrication of ZnS thin films having similar stoichiometry at different substrate temperatures (T_S) e.g. 200 °C, 300 °C and 400 °C by means of RF magnetron sputtering method is presented. The films grown at T_S of 200 °C are in cubic zinc-blende phase and textured along (111) plane. The films deposited at T_S of 300 °C and 400 °C are in hexagonal wurtzite phase. The surface roughness and grain size of the films increase with increasing T_S. The ultra-violet and visible absorption studies show that the bandgap of films can be tailored by varying T_S, taking advantage of the structural transformation.     

Intrinsic defects and structural phase of ZnS nanocrystalline thin films: effects of substrate temperature

Fabrications of ZnS nanocrystalline thin films at different substrate temperatures (T_S) of 200, 300 and 400 °C by means of pulsed laser deposition are presented. Thin film deposited at T_S of 200 °C is in cubic zinc-blende (ZB) structure while those deposited at T_S of 300 and 400 °C are in hexagonal wurtzite (W) phase. The grain size, surface roughness and bandgap of the films increases with increasing T_S. The zinc vacancies and interstitials in the films increases while sulfur vacancies decreases with increasing T_S. The variation of zinc and sulfur vacancies in ZnS films with T_S is responsible for structural phase transition from ZB to W which causes the change in energy bandgap.     

Magnetic phase transition in thin films

The theory of phase transitions in magnetic systems with surfaces is reviewed. Results of the phenomenological Landau-Ginzburg theory as well the scaling theory are briefly presented and compared with those obtained for particular microscopic models. Possibilities of special kinds of magnetic phase transitions in systems with surfaces are also presented.     

A study of structural transition in nanocrystalline titania thin films by X-ray diffraction Rietveld method

Structural and microstructural analyses of nanocrystalline titania thin films prepared by pulsed laser deposition have been carried out. At lower oxygen partial pressures (≤10⁻⁴ mbar), rutile films were formed, whereas at 1.2 × 10⁻³ mbar of oxygen partial pressure, the thin films contained both rutile and anatase phases. At 0.04 and 0.05 mbar of oxygen partial pressure, the film was purely anatase. Addition of oxygen has also shown a profound influence on the surface morphology of the as deposited titania films. Modified Rietveld method has been used to determine crystallite size, root mean square strain and fractional coordinates of oxygen of the anatase films. The influence of crystallite size and strain on the rutile to anatase phase transition is investigated.     

Preparation of nanocrystalline Mg doped CdSe thin films and their optical,photoluminescence, electrical and structural characterization

Phosphors used are mostly rare earth doped complex structures. A simple and unique material system of CdSe:Mg nanocrystalline thin films, which efficiently absorb UV (235 nm) and emit broad spectrum of green-yellow region has been prepared by chemical bath deposition method with average particle size of 52.3 nm, measured using AFM images. The optical absorption studies found that CdSe thin film has direct optical band gap, \({E_g}\) of 2.62 eV that shows a blue shift of 0.88 eV compared to the bulk \({E_g}\) value. Optical, electrical, structural and morphological properties were studied by UV–Vis–NIR spectrophotometer, photoluminescence (PL) emission spectra, dc two-probe method, X-ray diffraction (XRD), and atomic force microscope (AFM). Measured electrical resistivity decreased with increase of doping concentration. Activation energy was also calculated. The results confirm that the CdSe:Mg thin films are in the pure cubic phase. The magnesium concentrations also affect the nanocrystalline nature of the CdSe thin films. The optical band gap and surface roughness of CdSe thin films mostly decrease with 5% doping of Mg. The effect of Mg doping on refractive index, extinction coefficient and other optical parameters was also investigated.     

Preparation and characterization of ZnS nanocrystalline thin films by low cost dip technique

Nanocrystalline ZnS semiconducting nanopowder and thin films have been deposited by simple low cost technique based on combination of dip coating and thermal reaction process. The deposited films and the prepared nanopowder have been characterized in the structurally, optically and electrically point of views. The effect of preparation conditions has been also optimized for good quality films. X-ray diffraction analysis performed the ZnS cubic phase in the reaction temperatures in the range 473–593 K. Above 593 K mixed cubic and hexagonal crystallographic phases have been resolved. Crystallite size and micro strain have been calculated to be 2.65 and 0.011 nm, respectively. The deposited film surface and cross section morphologies show that neither cracks nor peels have been observed and good film adhesion with the substrate was performed. Energy dispersive X-ray measurements of the film agree well with the calculated concentrations of the precursor components. Optical measurements confirm the optical characteristics of nanocrystalline ZnS film such as absorption and dispersion properties. Copper doped ZnS reduces the band gap while indium doped ZnS increases the band gap. Electrical characterization shows that copper doped ZnS increases the resistivity by one order of magnitude due to electron compensation process while indium doped ZnS decreases the resistivity three orders of magnitude due to increase of the carriers concentration. Hot probe thermoelectric quick test of ZnS:Cu and ZnS:In show opposite sign of thermoelectric voltage due to bipolar p and n types, respectively.     

The phase transition of djurleite thin films

Thin films of djurleite have been examined by TED in order to study the phase transition at 86° C. The samples were heated during observation in the electron microscope. Under these conditions no intermediate phase was found, but we have found two different orientations of the djurleite supercell with respect to the high chalcocite cell. The transformation matrix was determined.     

Microcrystalline to nanocrystalline silicon phase transition in hydrogenated silicon-carbon alloy films

Different classes of interesting materials (such as protocrystalline, microcrystalline and nanocrystalline) have been grown under conditions very near to those for the microcrystalline phase. In spite of the importance of these materials, a clear picture regarding their phase transitions is missing. A smooth transition from the microcrystalline to the nanocrystalline silicon phase, distinctly different from an abrupt order-disorder phase transition, has been demonstrated, for the first time, in hydrogenated silicon-carbon alloy films, prepared from a silane-methane gas mixture highly diluted in hydrogen, by varying the rf power in a plasma enhanced chemical vapour deposition system. The study has also provided the signature of medium range order in hydrogenated silicon-carbon alloy films.     

Size-dependent optical and dielectric properties of nanocrystalline ZnS thin films synthesized via rf-magnetron sputtering technique

Nanocrystalline ZnS thin films have been synthesized by radio frequency magnetron sputtering technique on glass and Si substrates at a substrate temperature 300 K. X-ray diffraction and selected area electron diffraction studies confirmed the formation of nanocrystalline cubic phase of ZnS in the films, although the target material was hexagonal ZnS. The particle size, calculated from the XRD patterns of the thin films was found in the range 2.06-4.86 nm. TEM micrographs of the thin films revealed the manifestation of ZnS nanoparticles with sizes in the range 3.00-5.83 nm. UV-vis-NIR spectrophotometric measurements showed that the films were highly transparent (∼90%) in the wavelength range 400-2600 nm with a blue shift of the absorption edge. The direct allowed bandgaps have been calculated and they lie in the range 3.89-4.44 eV. The particle size, calculated from the shift of direct bandgap, due to quantum confinement effect lying in the range 3.23-5.60 nm, well support the TEM results. The room temperature photoluminescence spectra of the films showed two peaks centered around 315 and 450 nm. We assigned the first peak due to bandgap transitions while the latter was due to sulfur vacancy in the films. The composition analysis by energy dispersive X-rays also supported the existence of sulfur deficiency in the films. The dielectric property study showed high dielectric constant (85-100) at a higher frequency (>5 kHz).     

Investigation of the effect of temperature on growth mechanism of nanocrystalline ZnS thin films

In this paper the temperature effect on the growth mechanism of ZnS thin films prepared in a chemical bath containing zinc acetate, ethylenediamine, and thioacetamide aqueous solutions has been studied in the temperature range between 25 and 75 °C. These ZnS thin films possess a nanocrystalline structure, exhibit quantum size effects due to the small crystal size and produce a blue shift in the optical spectra. This blue shift was attributed to a decrease in crystal size by using X-ray diffraction and scanning electron microscopy. The growth mechanism of the thin films is suggested to proceed by two fundamental steps: in the first step, the ZnS nanocrystallites coalesce into small grains through homogeneous nucleation in the solution phase. In the second step, eventually, these small grains or large-sized clusters diffuse and stick to the surface of the substrate to form the ZnS thin film, in a way called a cluster-by-cluster manner, resulting in particulate thin film.     

Metal-semiconductor phase transition in transplanted thin polycrystalline vanadium dioxide films

A method for transfer of vanadium dioxide films from one substrate to another is proposed. This transplantation reveals a significant influence of elastic stresses arising during the film synthesis and as a result of the structural phase transition in the film on the temperature and shape of hysteresis of the optical reflectance.     

Optical properties of nanocrystalline ZnS:Mn thin films prepared by chemical bath deposition method

Nanocrystalline ZnS:Mn thin films were fabricated by a chemical bath deposition route on glass, silicon, and quartz substrates using a weak acidic bath, in which citrate ions acts as a nontoxic complexing agent for zinc ions and thioacetamide acts as a source of sulfide ions at 60 °C. The composition of films were characterized by energy-dispersive X-ray spectrometer, inductively coupled plasma atomic emission spectroscopy, Rutherford backscattering, and attenuated total reflection-Fourier transform infrared spectroscopy. X-ray diffraction pattern and transmission electron microscopy image confirm that the films have nanocrystalline nature. The band gap energy of ZnS:Mn films is blue-shifted by about 0.3 eV with respect to the bulk value (3.67 eV), probably due to the quantum size effect as expected from the nanocrystalline nature of the ZnS:Mn thin films. The dispersion and optical constants of the films were determined. These parameters changed with the deposition time.     

纳米硅薄膜及其太阳电池研究进展

纳米硅薄膜具有新颖的结构特征和一系列独特的物理性质,可望应用于新型光电子器件、量子功能器件、集成电路等领域.本文综述了纳米硅薄膜的研究现状及其优良的光电性能和纳米硅薄膜太阳电池的研究进展,指出在生产制备与性能方面纳米硅薄膜太阳电池所具有的优势,具有良好的发展前景.     

Electro-optical properties of thin ZnS films

The electro-optical properties of thin ZnS(Mn) films and d.c. luminescent diodes and triodes of the structure SnO_x-ZnS(Mn)-Cu_xS-ZnS(Mn)-Al have been studied. Cathodoluminescence and electroluminescence spectra of the films at various stages of production of the electroluminescent diodes were compared. It was found that the dependence of luminescence on voltage and time is exponential, as is the time dependence of the current.     

Size dependent structural, optical and morphological properties of ZnS:Cu thin films

ZnS:Cu thin films have been deposited on glass substrate by a simple neutral pH solution synthesis route and chemical bath deposition technique. The copper concentration was varied between 0 and 0.1 M%. The X-ray diffraction and scanning electron microscope studies show the average size of the nanoparticles are below 4 nm (Bohr diameter). The effect of film thickness on the optical and structural properties has been studied. The optical absorption studies show the band gap energy of ZnS:Cu films decreases from 3.68 to 3.43 eV as thickness varied from 318.3 to 334.1 nm. The structural estimation shows the variation in particle size from 2.67 to 3.14 nm with thickness. The insignificant change in band gap may be due to the increase in particle size and quantum size effect.     

Microstructure evolution and photoluminescence in nanocrystalline Mg(x)Zn(1 - x)O thin films

The effects of Mg concentration and annealing temperature on the characteristics of nanocrystalline Mg(x)Zn(1 - x)O thin films (where x = 0-0.4) were studied using electron microscopy and photoluminescence. The films were prepared by a sol-gel method. The solid solubility limit of MgO in ZnO for the sol-gel-derived Mg(x)Zn(1 - x)O films in the present study was determined to be ～ 20 at.%. Microstructural characterization of the films showed that the wurtzite crystallites decrease in size with increase in Mg concentration up to the solubility limit. Increasing Mg concentration beyond the solubility limit resulted in a decrease in crystallinity of the films. The bandgap energy was found to increase with Mg concentration whereas the linewidth first increased and then decreased when the Mg concentration was increased beyond the solubility limit. Photoluminescence properties have been correlated to the microstructure of the films. A growth mechanism for Mg(x)Zn(1 - x)O nanocrystalline films under the present processing conditions has also been proposed.     

Temperature and pressure-mediated structural transition of ZnS nanoparticles

We demonstrate that the structural transition of ZnS nanoparticles from sphalerite to wurtzite is influenced by high pressures and temperatures. Under the pressure of 1 GPa, the structural transition of ZnS nanoparticles commences at 250 degrees C, much lower than that 400-500 degrees C for ZnS nanoparticles under normal pressures. With the increase of the annealing temperature, the transition is enhanced then inhibited with a maximum transition fraction of 14% at 300 degrees C and disappears at 500 degrees C. At the annealing temperature of 300 degrees C, the structural transition of ZnS nanoparticles keeps almost invariable with the increase of the pressure from 0.6 GPa to 1 GPa. The mechanism for the phenomenon is discussed.