Bandgap variation in grain size controlled nanostructured CdO thin films deposited by pulsed-laser method

Cadmium oxide (CdO) thin films were prepared by pulsed laser deposition technique. Their structure, surface morphology, optical and electrical properties have been investigated. With a decrease in the laser energy density, the average grain size of the CdO film can be adjusted from 108 to 25 nm. High-resolution TEM observation showed that more crystalline defects like lattice distortion, dislocation and amorphous structure existed in the small grained (25 nm) CdO film, and X-ray photoelectron spectroscopy analysis confirmed that the film had more oxygen vacancies. The electrical and optical properties of the films significantly depended on the grain size. With the grain size decreasing to 25 nm, the optical band gap energy of the CdO film increased obviously from 2.82 to 3.33 eV. This change in the nature of material from semimetal to a wide band gap semiconductor, combining with its higher optical transmission (92 %) in visible light region, higher carrier concentration (1.25 × 10²¹ cm^?3) and lower electrical resistivity (2.8 × 10^?4 cm^?3), makes the nano-grained CdO film very useful in optoelectronic applications.     

Improved characteristics of sprayed CdO films by rapid thermal annealing

We report the influence of rapid thermal annealing (RTA) on optical, electrical and structural properties of sprayed undoped CdO thin films. XRD investigation revealed that as-grown films are polycrystalline in nature with cubic structure; and pronounced improvement in crystallinity of the films have been noticed after annealing. RTA-treated films showed preferred orientation in the (2 0 0) direction. The optical band gap was deduced from transmittance data for as-grown and annealed films and found to be 2.51 eV for as-grown film and ranged between 2.25 and 2.48 eV for annealed films. Figure of merit was calculated and found its maximum value (1.2 × 10^?2 Ω^?1) was for film annealed with of 6 min. Thermoelectric power (TEP) measurements revealed that the CdO films are having n-type conductivity. Furthermore, the activation energy was calculated for films from TEP data. The lowest electrical resistivity was found to be 6 × 10^?4 Ω cm for CdO film annealed with time ≥6 min. The observed changes demonstrated that RTA is a viable technique for improving characteristics of sprayed CdO films.     

Structural, optical, and electrical properties of indium-doped cadmium oxide films prepared by pulsed filtered cathodic arc deposition

Indium-doped cadmium oxide (CdO:In) films were prepared on glass and sapphire substrates by pulsed filtered cathodic arc deposition (PFCAD). The effects of substrate temperature, oxygen pressure, and an MgO template layer on film properties were systematically studied. The MgO template layers significantly influence the microstructure and the electrical properties of CdO:In films, but show different effects on glass and sapphire substrates. Under optimized conditions on glass substrates, CdO:In films with thickness of about 125 nm showed low resistivity of 5.9 × 10^?5 Ωcm, mobility of 112 cm²/Vs, and transmittance over 80 % (including the glass substrate) from 500 to 1500 nm. The optical bandgap of the films was found to be in the range of 2.7 to 3.2 eV using both the Tauc relation and the derivative of transmittance. The observed widening of the optical bandgap with increasing carrier concentration can be described well only by considering bandgap renormalization effects along with the Burstein–Moss shift for a nonparabolic conduction band.     

Characterization of ITO/CdO/glass thin films evaporated by electron beam technique

Abstract

A thin buffer layer of cadmium oxide (CdO) was used to enhance the optical and electrical properties of indium tin oxide (ITO) films prepared by an electron-beam evaporation technique. The effects of the thickness and heat treatment of the CdO layer on the structural, optical and electrical properties of ITO films were carried out. It was found that the CdO layer with a thickness of 25 nm results in an optimum transmittance of 70% in the visible region and an optimum resistivity of 5.1×10^?3 Ω cm at room temperature. The effect of heat treatment on the CdO buffer layer with a thickness of 25 nm was considered to improve the optoelectronic properties of the formed ITO films. With increasing annealing temperature, the crystallinity of ITO films seemed to improve, enhancing some physical properties, such as film transmittance and conductivity. ITO films deposited onto a CdO buffer layer heated at 450 °C showed a maximum transmittance of 91% in the visible and near-infrared regions of the spectrum associated with the highest optical energy gap of 3.61 eV and electrical resistivity of 4.45×10^?4 Ω cm at room temperature. Other optical parameters, such as refractive index, extinction coefficient, dielectric constant, dispersion energy, single effective oscillator energy, packing density and free carrier concentration, were also studied.     

Thickness dependent electrical properties of CdO thin films prepared by spray pyrolysis method

A large number of thin films of cadmium oxide have been prepared on glass substrates by spray pyrolysis method. The prepared films have uniform thickness varying from 200–600 nm and good adherence to the glass substrate. A systematic study has been made on the influence of thickness on resistivity, sheet resistance, carrier concentration and mobility of the films. The resistivity, sheet resistance, carrier concentration and mobility values varied from 1·56–5·72×10⁻³ Ω-cm, 128–189 Ω/□, 1·6–3·9×10²¹ cm⁻³ and 0·3–3 cm²/Vs, respectively for varying film thicknesses. A systematic increase in mobility with grain size clearly indicates the reduction of overall scattering of charge carriers at the grain boundaries. The large concentration of charge carriers and low mobility values have been attributed to the presence of Cd as an impurity in CdO microcrystallites. Using the optical transmission data, the band gap was estimated and found to vary from 2·20–2·42 eV. These films have transmittance around 77% and average reflectance is below 2·6% in the spectral range 350–850 nm. The films aren-type and polycrystalline in nature. SEM micrographs of the CdO films were taken and the films exhibit clear grains and grain boundary formation at a substrate temperature as low as 523 K.     

High transmittance–low resistivity cadmium oxide films grown by reactive pulsed laser deposition

High transmittance, low resistivity, and highly oriented cadmium oxide thin films were prepared by pulsed Nd:YAG laser ablation of Cd target in the presence of oxygen as reactive atmosphere. The structural, optical, and electrical properties of CdO films were dependent on the background oxygen pressure PO₂. The XRD data show that the grown CdO film at 350 Torr oxygen pressure exhibited preferential orientation along (111) crystal plane. The average transmittance of the CdO films in the visible region was found to be in the range of 65–85% and the corresponding optical energy gap found to be in the range of 2.5–2.8 eV depending on oxygen pressure. The lowest electrical resistivity was found to be 7.56 × 10⁻³ Ωcm for CdO film prepared at 350 Torr of oxygen ambient without using post-deposition heat treatment.     

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.     

Nanocrystalline copper selenide thin films by chemical spray pyrolysis

Thin films of copper selenide were deposited onto amorphous glass substrates at various substrate temperatures by computerized spray pyrolysis technique. The as deposited copper selenide thin films were used to study a wide range of characteristics including structural, surface morphological, optical and electrical, Hall Effect and thermo-electrical properties. X-ray diffraction study reveals that the films are polycrystalline in nature with hexagonal (mineral klockmannite) crystal structure irrespective of the substrate temperature. The crystalline size is found to be in the range of 23–28 nm. The SEM study reveals that the grains are uniform with uneven spherically shaped and spread over the entire surface of the substrates. EDAX analysis confirmed the nearly stoichiometric deposition of the film at 350 °C. The direct band gap values are found to be in the range 2.29–2.36 eV depending on the substrate temperature. The Hall Effect study reveals that the films exhibit p-type conductivity. The values of carrier concentration and mobility for the film are found to be 5.02 × 10¹⁷ cm^?3 and 5.19 × 10^?3 cm² V^?1 s^?1; respectively for film deposited at 350 °C.     

Structural, optical and electrical measurements on boron-doped CdO thin films

Several boron-doped CdO with different boron composition thin films have been prepared on glass substrate by a vacuum evaporation technique. The effects of boron doping on the structural, electrical and optical properties of the host CdO films were systematically studied. The X-ray diffraction study shows that some of B³⁺ ions occupied locations in interstitial positions and/or Cd²⁺-ion vacancies of CdO lattice. The band gap of B-doped CdO suffers narrowing by 30–38% compare to undoped CdO. Such band gap narrowing (BGN) was studied in the framework of the available models. Furthermore, a phenomenological evaluation of the dependence of band gap on the carrier concentration in the film samples is discussed. The electrical behaviours show that all the prepared B-doped CdO films are degenerate semiconductors. However, the boron doping influences all the optoelectrical properties of CdO. Their dc-conductivity, carrier concentration and mobility increase compare to undoped CdO film. The largest mobility of 45–47 cm²/V s was measured for 6–8% boron-doped CdO film. From near infrared transparent-conducting oxide (NIR-TCO) point of view, boron is effective for CdO donor doping.     

Preparation and characterization of indium oxide (In2O3) films by activated reactive evaporation

The electrical and optical properties of In₂O₃ films prepared at room temperature by activated reactive evaporation have been studied. Hall effect measurements at room temperature show that the films have a relatively high mobility 15 cm²v⁻¹s⁻¹, high carrier concentration 2·97 × 10²⁰/cm³, with a low resistivityρ = 1·35 × 10⁻³ ohm cm. As-prepared film is polycrystalline. It shows both direct and indirect allowed transitions with band gaps of 3·52eV and 2·94eV respectively.     

Quality enhancement of AZO thin films at various thicknesses by introducing ITO buffer layer

Due to the simultaneously superior optical transmittance and low electrical resistivity, transparent conductive electrodes play a significant role in semiconductor electronics. To enhance the electrical properties of these films, one approach is thickness increment which degrades the optical properties. However, a preferred way to optimize both electrical and optical properties of these layers is to introduce a buffer layer. In this work, the effects of buffer layer and film thickness on the structural, electrical, optical and morphological properties of AZO thin films are investigated. Al-doped zinc oxide (AZO) is prepared at various thicknesses of 100 to 300 nm on the bare and 100 nm-thick indium tin oxide (ITO) coated glass substrates by radio frequency sputtering. Results demonstrate that by introducing ITO as a buffer layer, the average values of sheet resistance and strain within the film are decreased (about 76 and 3.3 times lower than films deposited on bare glasses), respectively. Furthermore, the average transmittance of ITO/AZO bilayer is improved nearly 10% regarding single AZO thin film. This indicates that bilayer thin films show better physical properties rather than conventional monolayer thin films. As the AZO film thickness increases, the interplanar spacing, d₍₀₀₂₎, strain within the film and compressive stress of the film in the hexagonal lattice, decreases indicating the higher yield of AZO crystal. Moreover, with the growth in film thickness, carrier concentration and optical band gap (E_g) of AZO film are increased from 4.62?×?10¹⁹ to 8.21?×?10¹⁹ cm^?3 and from 3.55 to 3.62 eV, respectively due to the Burstein-Moss (BM) effect. The refractive index of AZO thin film is obtained in the range of 2.24–2.26. With the presence of ITO buffer layer, the AZO thin film exhibits a resistivity as low as 6?×?10^?4 Ω cm, a sheet resistance of 15 Ω/sq and a high figure of merit (FOM) of 1.19?×?10⁴ (Ω cm)^?1 at a film thickness of 300 nm. As a result, the quality of AZO thin films deposited on ITO buffer layer is found to be superior regarding those grown on a bare glass substrate. This study has been performed over these two substrates because of their significant usage in the organic light emitting diodes and photovoltaic applications as an enhanced carrier injecting electrodes.     

Study of optical and electrical assessments of the quaternary MgZnSnO system containing different Mg content

Magnesium zinc tin oxide (MZTO) quaternary systems were prepared using solution-processed method. Assessment of the Mg dopant content of the MZTO quaternary system was done to obtain a better performance of the films in the device applications. The combination of silicon and MZTO system exhibited rectifying behavior. It has been observed that the current in the reverse direction is increased by illumination of 30 mW/cm². The capacitance-time characteristics were measured by transient photocapacitance spectroscopy. The composition of MZTO was studied by absorbance, transmittance and reflectance measurements. It has been seen that the band gap (~4.07 eV) of MZTO films remains almost constant with regard to the increasing Mg dopant content while the thin film of ZTO ternary system without Mg has band gap of 3.9 eV.     

Enhancement of nonlinear optical susceptibility of CuPc films by ITO layer

In the present study, the Copper Phthalocyanine (CuPc)/ITO thin film was fabricated using thermal evaporation method. The structural property was analyzed by X-ray diffraction study and confirms that the thin film has been preferentially grown along (200) plane. The atomic force microscope study was carried out on deposited film and quality of thin films is assessed by calculating the roughness of the films. The direct and indirect band gap, linear and nonlinear optical characteristics of grown films were calculated by using UV–Vis–NIR spectrometer studies. The calculated values of the first direct and indirect band gaps (E_g1(d) & E_g1(ind)) are 1.879 and 1.644 eV as a fundamental gap, while the values of second direct and indirect band gap (E_g2(d) & E_g2(ind)) are 1.660 and 1.498 eV as an onset gap for CuPc. The values of nonlinear refractive index (n₂) and third order nonlinear optical susceptibility (χ³) are found to be 5 × 10⁻⁸ and 8 × 10⁻⁹ (theoretical) and 5.2 × 10⁻⁸ and 1.56 × 10⁻⁷ (experimental) respectively. The optical band and third order nonlinear properties suggest that the as-prepared films are may be applied in optoelectronic and nonlinear applications.     

Highly Transparent and Conductive Zn<Subscript>0.86</Subscript>Cd<Subscript>0.11</Subscript>In<Subscript>0.03</Subscript>O Thin Film Prepared by Pulsed Laser Deposition

Zn_0.86Cd_0.11In_0.03O alloy semiconductor film was deposited on quartz substrate by pulsed laser deposition technique. Cd is used to change the optical band gap and In is used to increase the carrier concentration of the ZnO film. XRD studies confirm that the structure of Zn_0.86Cd_0.11In_0.03O is hexagonal wurtzite structure without CdO phase appeared. FE-SEM shows that the grain size of Zn_0.86Cd_0.11In_0.03O film is smaller than that of ZnO. These films are highly transparent (∼85%) in visible region. Most importantly, the electrical properties of Zn_0.86Cd_0.11In_0.03O film highly improved with In doped. It has low resistivity (4.42×10⁻³ Ω cm) and high carrier concentration (5.50×10¹⁹ cm⁻³) that enable this film a promising candidate for window layer in solar cells and other possible optoelectronic applications.     

Surface morphology and optoelectronic studies of sol–gel derived nanostructured CdO thin films: heat treatment effect

Morphological dependence of the optoelectronic properties of sol–gel derived CdO thin films annealed at different temperatures in air has been studied. After preparing, the films were investigated by studying their structural, morphological, d.c. electrical and optical properties. X-ray diffraction results suggest that the samples are polycrystalline and the crystallinity of them enhanced with annealing temperature. The average grain size is in the range of 12–34?nm. The root mean square roughness of the films was increased from 3.09 to 6.43?nm with annealing temperature. It was observed that the electro-optical characteristics of the films were strongly affected by surface roughness. As morphology and structure changed due to heat treatment, the carrier concentration was varied from 1.13?×?10¹⁹ to 3.10?×?10^19?cm^?3 with annealing temperature and the mobility increased from less than 7 to 44.8?cm² V^?1 s^?1. It was found that the transmittance and the band gap decreased as annealing temperature increased. The optical constants of the film were studied and the dispersion of the refractive index was discussed in terms of the Wemple–DiDomenico single oscillator model. The real and imaginary parts of the dielectric constant of the films were also determined. The volume energy loss increases more than the surface energy loss at their particular peaks.     

Effect of deposition conditions on the InP thin films prepared by spray pyrolysis method

InP thin films were prepared by spray pyrolysis technique using aqueous solutions of InCl₃ and Na₂HPO₄, which were atomized with compressed air as carrier gas. The InP thin films were obtained on glass substrates. Thin layers of InP have been grown at various substrate temperatures in the range of 450–525°C. The structural properties have been determined by using X-ray diffraction (XRD). The changes observed in the structural phases during the film formation in dependence of growth temperatures are reported and discussed. Optical properties, such as transmission and the band gap have been analyzed. An analysis of the deduced spectral absorption of the deposited films revealed an optical direct band gap energy of 1.34–1.52 eV for InP thin films. The InP films produced at a substrate temperature 500°C showed a low electrical resistivity of 8.12 × 10³ Ω cm, a carrier concentration of 11.2 × 10²¹ cm⁻³, and a carrier mobility of 51.55 cm²/Vs at room temperature.     

Structural and optical properties of sulfurized Cu2ZnSnS4 thin films from Cu–Zn–Sn alloy precursors

This study reports the preparation of Cu₂ZnSnS₄ (CZTS) thin films by magnetron sputtering deposition with a Cu–Zn–Sn ternary alloy target and sequential sulfurization. The effects of substrate temperatures on the structural, morphological, compositional as well as optical and electrical properties were characterized. The results showed the CZTS thin films prepared by sulfurization at substrate temperature of 570 °C yielded secondary phases along with CZTS compound. The relatively good properties of CZTS thin film were obtained after sulfurization at substrate temperature of 550 °C. This CZTS film showed compact structure with large grain size of 900 nm, direct optical band gap of 1.47 eV, optical absorption coefficient over 10⁴ cm^?1, resistivity of 4.05 Ω cm, carrier concentration of 8.22 × 10¹⁸ cm^?3, and mobility of 43.38 cm² V^?1 S^?1.     

Growth of CZTS thin films by sulfurization of sputtered single-layered Cu–Zn–Sn metallic precursors from an alloy target

Cu₂ZnSnS₄ (CZTS) thin films were prepared by sulfurizing single-layered metallic Cu–Zn–Sn precursors which were deposited by DC magnetron sputtering using a Cu–Zn–Sn ternary alloy target. The composition, microstructure and properties of the CZTS thin films prepared under different sputtering pressure and DC power were investigated. The results showed that the sputtering rate of Cu atom increases as the sputtering pressure and DC power increased. The microstructure of CZTS thin films can be optimized by sputtering pressure and DC power. The CZTS thin film prepared under 1 Pa and 30 W showed a pure Kesterite phase and a dense micro-structure. The direct optical band gap of this CZTS thin film was calculated as 1.49 eV with a high optical absorption coefficient over 10⁴ cm^?1. The Hall measurement showed the film is a p-type semiconductor with a resistivity of 1.06 Ω cm, a carrier concentration of 7.904 × 10¹⁷ cm^?3 and a mobility of 7.47 cm² Vs^?1.     

Preparation and characterization of novel CuClSe2 semiconductor thin film

CuClSe₂ was synthesized by solid-state reaction between copper chloride and selenium at 300 °C. CuClSe₂ thin film was prepared on a glass substrate by pulsed laser deposition (PLD) method. XRD (X-ray diffraction) analysis revealed that the CuClSe₂ thin film has a preferred surface orientation parallel to (006). The transmittance and reflectance spectra of the film indicated that the compound is an indirect band gap material; the energy band gap is about 1.45 eV; its absorption coefficients are in the range of 10⁴-10⁵ cm^− 1 when the wavelength is shorter than 720 nm. The melting point of CuClSe₂ is about 328 °C. These results show that CuClSe₂ is a potential absorber layer material applied in solar cells.     

Electrical,optical and magnetoresistive behavior of nanostructured ZnO:Cu thin films deposited by sputtering

Copper doped ZnO (ZnO:Cu) nanostructured films with magnetoresistive behavior were produced by growing ZnO/Cu/ZnO arrays at room temperature (RT) by the sputtering technique on corning glass substrates. The arrays were made with two electrical insulating ZnO films of 50 and 105 nm, and a Cu film of 5 nm, both materials were deposited at RT by the RF- and DC-sputtering technique, respectively. The processing method involves two stages that proceed in the course of the growth process, the main one is originated by the non-equilibrium regime of the sputtering technique, and the second is the diffusion-redistribution of the intermediate Cu film towards the neighborhood ZnO layers aided by the nanocrystalline films character. The influence of applying an additional annealing stage to the arrays in N₂ atmosphere at 250 and 350 °C by periods of 30 min were studied. The resistivity of the ZnO:Cu films can be varied from 0.0034 to 2.83 Ω-cm, corresponding to electron concentrations of 1.12?×?10²¹ and 7.85?×?10¹⁷ cm^?3 with carrier mobility of 1.6 and 2.8 cm²/V s. Measured changes on the magnetoresistance behavior of the films at RT were of ?R?~?3% for annealed samples with electron concentration of 1.12?×?10²¹ cm^?3. The X-ray diffraction measurements show that the films are comprised of nanocrystallites with dimensions between 13 and 20 nm in size with preferred (002) orientation. The transmittance of the films in the visible region was of 83% with an optical band gap of ~?3.3 eV for the low-resistivity samples.