Chemical bath deposition and characterization of CdSe thin films for optoelectronic applications

Cadmium selenide (CdSe) thin films of high crystalline quality on glass substrate have been prepared by chemical bath deposition technique from an aqueous bath containing tetramine cadmium and sodium selenosulphate. Structural analysis using XRD shows that the film is single phase, crystallized in hexagonal structure with preferred growth in (111) direction. The energy band gap calculated from the absorption spectra of unannealed CdSe thin films shows an optical band gap of 1.8 eV and absorption coefficient near band edge (α)—0.58 × 10⁵ cm⁻¹. The conductivity of CdSe thin films is n-type.     

ZnO thin films prepared by pulsed laser deposition

Zinc oxide (ZnO) thin films were deposited on soda lime glass substrates by pulsed laser deposition (PLD) in an oxygen-reactive atmosphere. The structural, optical, and electrical properties of the as-prepared thin films were studied in dependence of substrate temperature and oxygen pressure. High quality polycrystalline ZnO films with hexagonal wurtzite structure were deposited at substrate temperatures of 100 and 300 °C. The RMS roughness of the deposited oxide films was found to be in the range 2-9 nm and was only slightly dependent on substrate temperature and oxygen pressure. Electrical measurements indicated a decrease of film resistivity with the increase of substrate temperature and the decrease of oxygen pressure. The ZnO films exhibited high transmittance of 90% and their energy band gap and thickness were in the range 3.26-3.30 eV and 256-627 nm, respectively.     

Nanocrystalline ZnO thin films: optoelectronic and gas sensing properties

Nanocrystalline Zinc oxide thin films have been deposited by sol–gel spin coating technique and then have been analyzed before and after a suitable thermal annealing in order to test their applications in various reducing and oxidizing gases. ZnO thin films were highly sensitive and selective for NH₃ gas. The spectrophotometric and conductivity measurements have been performed in order to determine the optical and electrical properties of zinc oxide thin films. The structure and the morphology of such material have been investigated by high resolution electron microscopy and small area electron diffraction. The average particle size is in 60–70 nm.     

Wide bandgap Mg-doped ZnAlO thin films for optoelectronic applications

Magnesium-doped ZnAlO thin films were grown on quartz substrate by ablating the sintered target with a KrF excimer laser. The effect of growth temperature from 30 °C to 700 °C on structural, optical, and electrical properties has been studied. These films are highly transparent in visible spectrum with average transmittance of 82%. The films grown at low temperature are amorphous while films grown at high temperature are crystalline in nature. These films are highly oriented along (0 0 2) direction. The electrical conductivity, carrier concentration, and electron mobility is found to increase with increase in temperature and then decreases with further increase in temperature. The bandgap is found to vary from 3.86 eV to 4.00 eV for various films.     

Influence of ZnO buffer layers on the optoelectronic properties in Ga-doped ZnO thin films prepared by RF magnetron sputtering on PET substrates

The influence of ZnO buffer layers on the optoelectronic properties in Ga-doped ZnO (GZO) thin films deposited on polyethylene terephthalate (PET) substrates by RF magnetron sputtering was investigated. The resistivity in GZO/ZnO bilayer films decreases significantly more than one order of magnitude than that in GZO film without a ZnO buffer layer. X-ray diffraction results show that the significant improvement of electrical properties is not related to the crystalline quality. Based on X-ray photoelectron spectroscopy analysis, it is suggested that the decrease in resistivity after the introduction of ZnO buffer layers is ascribed to the restraint of diffusion of moisture and gas from PET substrates to GZO thin films. The moisture and gas diffused into GZO films will be absorbed on the films’ surface in the form of negatively charged oxygen species acting as acceptors. Fifty nm buffer layer is thick enough to achieve the best effect, with further increase of the ZnO buffer layer thickness, the resistivity in the samples will increase due to the parallel effect of ZnO and GZO layers. The introduction of ZnO buffer layers has no obvious influence on the average transmittance in the visible range which is ~90 % high for all samples.     

ZnO thin films for VOC sensing applications

ZnO thin films were prepared by reactive RF sputtering on thermally oxidized Si for gas sensing applications. Three VOC vapors were chosen to investigate the response behavior of the prepared ZnO. Acetone, isopropanol and ethanol were tested, and the sensitivity of the sensor toward acetone was the highest (S ∼ 100) for 500 ppm acetone at 400 °C. The largest sensitivity was achieved at 400 °C for all the above vapors. The sensor shows a stable, reversible and repeatable behavior in the acetone concentration ranging from 15 up to 1000 ppm. The mechanism of the sensing was explained according to the ionosorption model.     

ZnO thin films grown on platinum (111) buffer layers by pulsed laser deposition

C-axis oriented ZnO layers were grown by pulsed-laser deposition on the surface of a platinum (111) epitaxial thin film supported by a c-sapphire substrate. The Pt bottom layer provides good in-plane lattice matching with c-ZnO, enabling epitaxial re-growth of the latter, as shown by X-ray diffraction data. Room- and low-temperature reflectance and photoluminescence measurements have been performed on such ZnO/Pt heterostructures for the first time. Intense resonances, corresponding to the A and B free excitons, are clearly evidenced in the reflectance measurements at 30 K, while the deconvolved full widths at half maximum of the bound excitonic lines, observed in the photoluminescence spectra at 28 K, range between 3 and 7 meV. This report clearly demonstrates that ZnO epitaxial thin films with very good structural and optical properties can be grown on a Pt bottom electrode and, thus, establishes the potential of this material system for use in ZnO-based optoelectronic devices.     

An economic CVD technique for pure SnO 2 thin films deposition: Temperature effects

A modified new method of CVD for formation of pure layers of tin oxide films was developed. This method is very simple and inexpensive and produces films with good electrical properties. The effect of substrate temperature on the sheet resistance, resistivity, mobility, carrier concentration and transparency of the films has been studied. The best sheet resistance obtained at substrate temperature of 500 °C was about 27 Ω/cm². X-ray diffraction showed that the structure of deposited films was polycrystalline with a grain size between 150–300 Å. The preferred orientation was (211) for films deposited at substrate temperature of about 500 °C. FESEM micrographs revealed that substrate temperature is an important factor for increasing grain size and modifies electrical parameters. UV-visible measurement showed reduction of transparency and bandgap of the layers with increasing substrate temperature.     

Fabrication of ZnO thin films by the photochemical deposition method

ZnO thin films were fabricated by the photochemical deposition (PCD) method. The deposition solution contains ZnSO₄, Na₂SO₃, Na₂S₂O₃ and a small amount of NH₄OH for pH adjustment. We blew oxygen or oxygen + ozone (O₃) gas into the solution to increase the dissolved oxygen content and enhance the oxidation reaction. The films were characterized by Auger electron and optical spectroscopy, and a photoelectrochemical (PEC) measurement. On an indium-tin-oxide (ITO) substrate, the films showed high optical transmission in the visible range. In a current-voltage measurement for films on a p-Si substrate, the O₃ bubbling sample showed rectification properties and photovoltaic effects.     

Structural and optoelectronic properties of indium doped SnO2 thin films deposited by sol gel technique

Indium doped tin oxide (SnO₂:In) thin films were deposited on glass substrates by sol–gel dip coating technique. X-ray diffraction pattern of SnO₂:In thin films annealed at 500 °C showed tetragonal phase with preferred orientation in T (110) plane. The grain size of tin oxide (SnO₂) in SnO₂:In thin films are found to be 6 nm which makes them suitable for gas sensing applications. AFM studies showed an inhibition of grain growth with increase in indium concentration. The rms roughness value of SnO₂:In thin films are found to 1 % of film thickness which makes them suitable for optoelectronic applications. The film surface revealed a kurtosis values below 3 indicating relatively flat surface which make them favorable for the production of high-quality transparent conducting electrodes for organic light-emitting diodes and flexible displays. X-ray photoelectron spectroscopy gives Sn 3d, In 3d and O 1s spectra on SnO₂:In thin film which revealed the presence of oxygen vacancies in the SnO₂:In thin film. These SnO₂:In films acquire n-type conductivity for 0–3 mol% indium doping concentration and p type for 5 and 7 mol% indium doping concentration in SnO₂ films. An average transmittance of >80 % (in ultra-violet–Vis region) was observed for all the SnO₂:In films he In doped SnO₂ thin films demonstrated the tailoring of band gap values. Photoluminescence spectra of the films exhibited an increase in the emission intensity with increase in indium doping concentration which may be due structural defects or luminescent centers, such as nanocrystals and defects in the SnO₂.     

Fabrication of a novel SnO2:Al/ZnO:F bi-layer for opto-electronic applications

Single layered aluminium doped tin oxide (ATO), fluorine doped zinc oxide (FZO) and bi-layered ATO/FZO thin films were deposited onto preheated glass substrates (T_s = 340 ± 5 °C) using a low-cost and simplified spray pyrolysis technique. The structural, optical, electrical and surface morphological properties of the bi-layered ATO/FZO thin films were studied and compared with that of the single layered films. The average optical transmittance of the bi-layer film in the visible range was found to be around 80 %. The bi-layered ATO/FZO films possessed both better transmittance in the visible range and sharp absorption edge, the unique desirable features of ATO and FZO films, respectively. The optical band gap (E_g) value of the bi-layer coating (3.22 eV) was found to lie between the E_g values of single layered ATO (3.71 eV) and FZO (3.20 eV) films. Sheet resistance values of ATO and FZO single layer films were 3.47 and 11.2 kΩ/sq., respectively. The bi-layered ATO/FZO thin films exhibited a sheet resistance of 4.42 kΩ/sq. which was very much close to that of ATO films and three times less than that of FZO film. The AFM images showed the good packing density and homogeneity of the surface of the bi-layer films. The annealing studies clearly showed that the ATO over layer remarkably improved the thermal stability of the bi-layered film.     

The compositional and structural properties of sprayed SnO2:F thin films

Thin films of SnO₂ doped with fluorine were produced by the conventional spraying method using Corning 7059 glass substrates. The electrical and optical properties of these films were determined as a function of the substrate temperature T_s and dopant concentration in the basic spraying solution. The visible and near-IR transmittance as well as the electrical resistivity of the films decreased with an increase in the fluorine concentration. The best electro-optical properties (an average transmittance of 75% and an electrical resistivity of 10⁻³ Ω cm) were achieved for 250°C⩽T_s⩽350°C and fluorine concentrations (relative to tin) in the spraying solution of around 2 at.%. The surface texture of the films was investigated by scanning electron microscopy and optical microscopy. The fluorine content of the samples was analysed by secondary ion mass spectroscopy, Auger electron spectroscopy and electron spectroscopy for chemical analysis. Structural changes were studied using X-ray diffractometry. A systematic change in the intensity of the X-ray diffraction lines as a function of the fluorine content was observed. Theoretical calculations of the structure factors associated with the SnO₂ lattice if fluorine atoms are introduced into substitutional and interstitial positions were performed.     

ZnO transparent thin films for gas sensor applications

Zinc oxide (ZnO) transparent thin films were deposited onto silicon and Corning glass substrates by dc magnetron sputtering using metallic and ceramic targets. Surface investigations carried out by Atomic Force Microscopy (AFM) and X-ray Diffraction (XRD) have shown a strong influence of deposition technique parameters on film surface topography. Film roughness (RMS), grain shape and dimensions are correlated with the deposition technique parameters as well as with the target material. XRD measurements have proven that the dc sputtered films are polycrystalline with the (002) as preferential crystallographic orientation. AFM analysis of thin films sputtered from a ceramic target has shown a completely different surface behavior compared with that of the films grown from a metallic target. This work demonstrates that the target material and the growth conditions determine the film surface characteristics. The gas sensing characteristics of these films are strongly influenced by surface morphology. Thus correlating the optical and electrical film properties with surface parameters (i.e. RMS and Grain Radius) can lead to an enhancement of the material's potential for gas sensing applications.     

ZnO nanocrystalline thin films: a correlation of microstructural,optoelectronic properties

The compositional, structural, microstructural, dc electrical conductivity and optical properties of undoped zinc oxide films prepared by the sol–gel process using a spin-coating technique were investigated. The ZnO films were obtained by 5 cycle spin-coated and dried zinc oxide films followed by annealing in air at 600 °C. The films deposited on the platinum coated silicon substrate were crystallized in a hexagonal wurtzite form. The energy-dispersive X-ray (EDX) spectrometry shows Zn and O elements in the products with an approximate molar ratio. TEM image of ZnO thin film shows that a grain of about 60–80 nm in size is really an aggregate of many small crystallites of around 10–20 nm. Electron diffraction pattern shows that the ZnO films exhibited hexagonal structure. The SEM micrograph showed that the films consist in nanocrystalline grains randomly distributed with voids in different regions. The dc conductivity found in the range of 10⁻⁵–10⁻⁶ (Ω cm)⁻¹. The optical study showed that the spectra for all samples give the transparency in the visible range.     

Evaluation of all-inorganic CdSe quantum dot thin films for optoelectronic applications

Exchanging the original organic ligands of colloidal CdSe quantum dots (QDs) with inorganic metal chalcogenide SnS(4) ligands resulted in absorption peak redshifts and complete photoluminescence (PL) quenching in QD solids. The SnS(4)-capped QDs, meanwhile, were able to retain strong excitonic absorption. After the ligand exchange, the ITO/QDs/Al structure showed much higher electrical conductivity and reduced space-charge limited current. Its photocurrent spectral response increased by over two orders of magnitude and closely resembled the absorption spectrum of the QDs. However, it was found that mild thermal treatment above 200 °C transformed the SnS(4)-capped QD film into to a more conductive assembly, degrading its absorption and photocurrent generation. These results suggest that the inorganic ligands considerably enhanced the inter-dot electronic coupling in QD solids, leading to facile charge separation and transport. Our study thus demonstrates the potential applicability of colloidal QDs with metal chalcogenide ligands processed at low temperatures for efficient photodetection and solar energy conversion.     

喷雾热解法制备SnO2:F+Sb薄膜的结构及性能研究

采用喷雾热解法在玻璃基板上制备了SnO2F+Sb薄膜,对薄膜的结构及性能进行了研究.用STM对薄膜表面进行表征,发现薄膜表面光滑平整,粗糙度Ra为16.283nm.四探针测试仪测定薄膜方阻为60Ω/□,电阻率为2.1×10-3Ω·Cm.用XRD表征薄膜结构,薄膜为四方相多晶SnO2结构,说明掺杂没有该变薄膜结构.对薄膜的光学性能进行了测试,可见光透过率达到80%,在2500nm处的中远红外区反射率由镀膜前的6%上升到36%.按国家标准测试了镀膜玻璃耐酸碱稳定性,实验前后薄膜的可见光透过率变化＜3%,符合国家标准.同时本研究还对镀膜玻璃的保温性能进行了测试,结果表明本实验制备的镀膜玻璃具有较好的保温性能.     

Optical characterizations of ZnO, SnO2, and TiO2 thin films for butane detection

The optogeometric properties of various sensitive thin films involved in gas sensing applications are investigated by using the m-line technique and atomic force microscopy. Variations of these optical properties are studied under butane and ozone exposure.     

Highly conducting and transparent tin-doped CdO thin films for optoelectronic applications

Highly conducting and transparent thin films of tin-doped cadmium oxide were deposited on quartz substrate using pulsed laser deposition technique. The effect of growth temperature on structural, optical and electrical properties was studied. These films are highly transparent (78-89%) in visible region, and transmittance of the films depends on growth temperature. It is observed that resistivity increases with growth temperature after attaining minimum at 150 °C, while carrier concentration continuously decreases with temperature. The lowest resistivity of 1.96 × 10^− 5 Ω cm and carrier concentration of 5.52 × 10²¹ cm³ is observed for the film grown at 150 °C. These highly conducting and transparent tin-doped CdO thin films grown via pulsed laser deposition could be an excellent candidate for future optoelectronic applications.     

Chemical solution deposition of thin TiO2-anatase films for dielectric applications

Thin films of TiO₂ have been prepared using chemical solution deposition on 6 n-type Si(1 0 0) wafers. Thin film thickness in the range from 70 to 210 nm could be achieved via control of the number of deposition sequences. The final annealing temperature of 700 °C resulted in an anatase phase structure with fine elongated grains and smooth surface topography. The capacity of the thin films is shown to depend on thickness, and could be interpreted assuming a series capacity with an SiO₂ interfacial layer. The resulting dielectric constant of the TiO₂ thin film has been calculated to be 23. The leakage current behavior and the break-down field also depend on film thickness. It is shown that thinner films are higher break-down fields.     

The variation of the features of SnO2 and SnO2:F thin films as a function of V dopant

V doped SnO₂ and SnO₂:F thin films were successfully deposited on glass substrates at 500 °C with spray pyrolysis. It was observed that all films had SnO₂ tetragonal rutile structure and the preferential orientation depended on spray solution chemistry (doping element and solvent type) by X-ray diffraction measurements. The lowest sheet resistance and the highest optical band gap, figure of merit, infrared (IR) reflectivity values of V doped SnO₂ for ethanol and propane-2-ol solvents and V doped SnO₂:F films were found to be 88.62 Ω–3.947 eV–1.02 × 10^?4 Ω^?1–65.49 %, 65.35 Ω–3.955 eV–8.54 × 10^?4 Ω^?1–72.58 %, 5.15 Ω–4.076 eV–6.15 × 10^?2 Ω^?1–97.32 %, respectively, with the electrical and optical measurements. Morphological properties of the films were investigated by atomic force microscope and scanning electron microscope measurements. From these analysis, the films consisted of nanoparticles and the film morphology depended on doping ratio/type and solvent type. It was observed pyramidal, polyhedron, needle-shaped and spherical grains on the films’ surfaces. The films obtained in present study with these properties can be used as front contact for solar cells and it can be also one of appealing materials for other optoelectronic and IR coating applications.