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.     

Luminescence from praseodymium doped AlN thin films deposited by RF magnetron sputtering and the effect of material structure and thermal annealing on the luminescence

Thin films of Praseodymium doped AlN are deposited on silicon (111) substrates at 77 K and 950 K by rf magnetron sputtering method. About 500–1000 nm thick films are grown at 100–200 watts RF power and 5–8 mTorr nitrogen, using a metal target of Al with Pr. X-rays diffraction results show that films deposited at 77 K are amorphous and those deposited at 950 K are crystalline. Cathodoluminescence studies are performed at room temperature and luminescence peaks are observed in a wide range from ultraviolet to infrared region. The most intense peak is obtained in green at 526 nm from amorphous films as a result from ³P₁→³H₅ transition. In crystalline films the intense peak was obtain in red at 648 nm as a result from ³P₀→³F₂ transition. Films are thermally activated at 1300 K for half an hour in a nitrogen atmosphere. Thermal activation enhances the intensity of luminescence. Two peaks at 488 nm and 505 nm merged after thermal activation, giving rise to a single peak at 495 nm.     

Preparation and thermoelectric properties of BP films on SOI and sapphire substrates

The chemical vapor deposited (CVD) BP films on Si(100) (190 nm)/SiO _x (370 nm)/Si(100) (625 μm) (SOI) and sapphire (R-plane) (600 μm) substrates were prepared by the thermal decomposition of the B₂H₆–PH₃–H₂ system in the temperature range of 800–1050 °C for the deposition time of 1.5 h. The BP films were epitaxially grown on the SOI substrate, but a two-step growth method, i.e., a buffer layer at lower temperature and sequent CVD process at 1000 °C for 1.5 h was effective for obtaining a smooth film on the sapphire substrate. The electrical conduction types and electrical properties of these films depended on the growth temperature, gases flow rates and substrates. The thermal conductivity of the film could be replaced by the substrate, so that the calculated thermoelectric figure-of-merit (Z) for the BP films on the SOI substrate was 10⁻⁴–10⁻³/K at 700–1000 K. Those on the sapphire substrate were 10⁻⁶–10⁻⁵/K for the direct growth and 10⁻⁵–10⁻⁴/K for the two-step growth at 700–900 K, indicating that the film on a sapphire by two-step growth would reduce the defect concentrations and promote the electrical conductivity.     

Electrical properties of the CdS/InP solar cell for photovoltaic applications

n-CdS/p-InP solar cells have been prepared by deposition of n-CdS thin films using thermal evaporation technique onto p-type InP <100>. The I–V characteristics of the CdS/InP heterojunctions in dark condition were studied in the 298–350 K temperatures range for charge transport mechanism investigation. It has been established that in the entire temperatures range, the charge transport mechanism is determined by recombination processes in the depletion region. The CdS/InP heterojunction solar cells obtained using this technique and characterized under illumination condition have showed a conversion efficiency of 11% at I_sc = 10 mA/cm², V_oc = 0.7 V.     

Copper nitride films deposited by dc reactive magnetron sputtering

Copper nitride (Cu₃N) films were deposited on glass substrates by sputtering of copper target under various substrate temperatures in the range 303–523 K using dc reactive magnetron sputtering. The substrate temperature highly influenced the structural, mechanical, electrical and optical properties of the deposited films. The X-ray diffraction measurements showed that the films were of polycrystalline nature and exhibit preferred orientation of (111) phase of Cu₃N. The microhardness of the films increased from 2.7 to 4.4 GPa with the increase of substrate temperature from 303 to 473 K thereafter decreased to 4.1 GPa at higher temperature of 523 K. The electrical resistivity of the films decreased from 8.7 × 10⁻¹ to 1.1 × 10⁻³ Ωcm and the optical band gap decreased from 1.89 to 1.54 eV with the increase of substrate temperature from 303 to 523 K respectively.     

Conduction mechanism in doped polymethyl methacrylate (PMMA) films

The electrical transport behaviour of ferrocene mixed poly (methyl methacrylate) (PMMA) films (≈ 20 μm in thickness) deposited by the isothermal immersion technique has been studied in the temperature range of 333–373 K and field from (2·0–4·0)×10⁴ V/cm. It has been found that at higher fields and temperatures, the observed conduction behaviour could be consistently described by the Richardson-Schottky emission. The increase in current due to doping has been attributed to the formation of charge transfer complexes. The dopant molecules act as an additional trapping centre and provide a link between polymer molecules in amorphous region leading to the formation of charge transfer complex.     

The Viscosity of Aqueous Alkali-Chloride Solutions up to 623 K, 1,000 bar,and High Ionic Strength

An accurate viscosity (dynamic viscosity) model is developed for aqueous alkali-chloride solutions of the binary systems, LiCl–H₂O, NaCl–H₂O, and KCl–H₂O, from 273 K to 623 K, and from 1 bar to 1,000 bar and up to high ionic strength. The valid ionic strengths for the LiCl–H₂O, NaCl–H₂O, and KCl–H₂O systems are 0 to 16.7 mol · kg⁻¹, 0 to 6 mol · kg⁻¹, and 0 to 4.5 mol · kg⁻¹, respectively. Comparison of the model with about 4,150 experimental data points concludes that the average absolute viscosity deviation from experimental data in the above range is within or about 1 % for the LiCl–H₂O, NaCl–H₂O, and KCl–H₂O mixtures, indicating the model is of experimental accuracy. With a simple mixing rule, this model can be extrapolated to predict the viscosity of ternary aqueous alkali-chloride solutions, making it useful in reservoir fluid flow simulation. A computer code is developed for this model and can be obtained from the author: (maoshide@cugb.edu.cn).     

Dielectric relaxation behavior of CdS nanoparticles and nanowires

The dielectric relaxation and scaling behavior of CdS nanoparticles and nanowires were investigated in the frequency range 10²–10⁶ Hz and in the temperature range 373–573 K by complex impedance spectroscopy and electric modulus spectroscopy. Studies on the complex permittivity revealed that the dielectric relaxation in CdS nanostructures deviates from Debye like behavior. A detailed study on the grain and grain boundary charge transport was carried out. The charge carrier transport in CdS nanostructures was identified to be hopping of polarons. From the combined analysis of the variation of imaginary part of electric modulus and complex impedance with frequency, it was found that at high temperatures localized conduction is dominant in CdS nanoparticles where as the long range hopping process is dominant with nanowires. It was also found that the scaling behavior of CdS nanoparticles varied considerably from that reported earlier.     

Revised heat capacity and thermodynamic functions of GdVO<Subscript>4</Subscript>

The heat capacity of GdVO₄ has been determined by adiabatic calorimetry in the range 5–345 K. The present experimental data and earlier results have been used to evaluate the thermodynamic functions of gadolinium orthovanadate (C _p ⁰(T), S ⁰(T), H ⁰(T) − H ⁰(0), and Φ⁰(T)) as functions of temperature (5–350 K). Its Gibbs energy of formation is determined to be Δ_f G ⁰(GdVO₄, 298.15 K) = −1684.5 ± 1.6 kJ/mol.     

Characterization of CuAlO2 films prepared by dc reactive magnetron sputtering

Thin films of copper aluminum oxide (CuAlO₂) were prepared on glass substrates by dc magnetron sputtering at a substrate temperature of 523 K under various oxygen partial pressures in the range 1 × 10⁻⁴–3 × 10⁻³ mbar. The dependence of cathode potential on the oxygen partial pressure was explained in terms of oxidation of the sputtering target. The influence of oxygen partial pressure on the structural, electrical and optical properties was systematically studied. p-Type CuAlO₂ films with polycrystalline nature, electrical resistivity of 3.1 Ω cm, Hall mobility of 13.1 cm² V⁻¹ s⁻¹ and optical band gap of 3.54 eV were obtained at an oxygen partial pressure of 6 × 10⁻⁴ mbar.     

Temperature dependence of absorption edge in MOCVD grown GaN

We have studied the temperature dependence of absorption edge of GaN thin films grown on sapphire substrate by metal-organic chemical vapor deposition using optical absorption spectroscopy. A shift in absorption edge of about 55 meV has been observed in temperature range 273–343 K. We have proposed a theoretical model to find the energy gap from absorption coefficient using α = α_max + (α_min − α_max)/[1 + exp 2(E − E_g + KT)/KT]. Temperature dependence of band gap has also been studied by finding an appropriate theoretical fit to our data using E_g(T) = E_g(273 K) − (8.8 × 10⁻⁴T²)/(483 + T) + 0.088 (Varshni empirical formula) and E_g(T) = E_g(273 K)−0.231447/[exp(362/T)−1] + 0.082 relations. It has been found that data can be fitted accurately after adding a factor ∼0.08 in above equations. Debye temperature (483 K) and Einstein temperature (362 K) in the respective equations are found mutually in good agreement.     

Dispersion and rheology of nickel nanoparticle inks

Nickel nanoparticles were dispersed in α-terpineol solvents, and their rheological behaviour and suspension structure were examined using various organic surfactants, surfactant concentrations (0–10 wt.% of the powder) and solids loadings (φ=0.01–0.28 in volumetric ratios) over a shear-rate range 10⁰–10³ s⁻¹. A surfactant of oligomer polyester was found effective in the nanoparticle dispersion. An optimal surfactant concentration ca. 2–4 wt.% of the solids was found; beyond which, the apparent viscosity increased adversely. The oligomer-polyester molecules appeared to adsorb preferentially on the nanoparticle surface, forming a steric layer which facilitates the ink flow for the improved dispersion. A pseudoplastic flow behaviour was found as shear rate increased, and a maximum solids concentration (φ_m) was estimated as φ_m=0.32. The interparticle potential was dominated by van der Waals attraction in the terpineol liquid, and a reaction-limited cluster aggregation (RLCA) featuring with a fractal dimension (D _f) of 2.0 was calculated. This finding together with the reduced φ_m reveals that the nanoparticle inks were flocculated in character even with the presence of polyester surfactant. Additionally, a porous (electrically conductive) particulate network was expected to form if the inks were printed on a non-conductive substrate followed then by drying and sintering in practice.     

Influence of Zn doping on electrical and optical properties of multilayered tin oxide thin films

In this study, the electrical and optical properties of Zn doped tin oxide films prepared using sol-gel spin coating process have been investigated. The SnO² : Zn multi-coating films were deposited at optimum deposition conditions using a hydroalcoholic solution consisting of stannous chloride and zinc chloride. Films with Zn doping levels from 0–10 wt% in solution are developed. The results of electrical measurements indicate that the sheet resistance of the deposited films increases with increasing Zn doping concentration and several superimposed coatings are necessary to reach expected low sheet resistance. Films with three coatings show minimum sheet resistance of 1–479 kΩ/ in the case of undoped SnO₂ and 77 kΩ/ for 5 wt% Zn doped SnO₂ when coated on glass substrate. In the case of single layer SnO₂ film, absorption edge is 3.57 eV and when doped with Zn absorption edge shifts towards lower energies (longer wavelengths). The absorption edge lies in the range of 3.489-3.557 eV depending upon the Zn doping concentration. The direct and indirect transitions and their dependence on dopant concentration and number of coatings are presented.     

P-type semiconducting Cu2O–NiO thin films prepared by magnetron sputtering

P-type semiconducting thin films consisting of a new multicomponent oxide composed of Cu₂O and NiO were deposited on glass substrates by r.f. magnetron sputtering using Cu₂O–NiO mixed powder targets. The multicomponent oxide thin films deposited in an Ar atmosphere with a Ni content (Ni/(Cu + Ni) atomic ratio) in the range from 0 to 100 at.% were found to be p-type semiconductors. As the Ni content was increased in the range from 0 to about 30 at.%, the energy bandgap of the resulting films gradually increased as well as the obtained resistivity increased from 70 to 4 × 10⁴ Ω cm, a consequence of decreases in both the Hall mobility and the hole concentration. The films prepared with a Ni content of about 30–50 at.% exhibited a relatively constant resistivity and energy bandgap. The resistivity and the energy bandgap of films prepared with a Ni content above about 60 at.% considerably increased as the Ni content was increased. Furthermore, a pn thin-film heterojunction prepared by depositing undoped n-ZnO and p-multicomponent oxide (Ni content of 50 at.%) thin films exhibited a rectifying I–V characteristic.     

Preparation of titanium-dioxide films by heating titanium/silicon-dioxide structures on silicon in oxygen

 When titanium/silicon-dioxide (Ti/SiO₂) structures prepared by depositing titanium (Ti) on thermally oxidized silicon in vacuum were heated to temperatures of 800–1000°C in flowing oxygen gas, silicon surfaces were covered with a mixture films containing preferentially (110)-oriented Ti0₂ instead of the SiO₂ films. The thickness of the mixture films could be determined by that of the deposited Ti films. Titanium silicide grew only in the region near between the grown mixture film and the silicon substrate. The dielectric constants of the grown mixture films increased exponentially with increasing oxidation temperature and increased slowly with increasing the Ti film thickness, while the breakdown field strength increased slowly with increasing oxidation temperature and increased exponentially with increasing the Ti film thickness. The oxide films prepared at 1000°C had dielectric constants of (15–25)ɛ_o resistivities of 10¹⁰–10¹¹ Ω cm, and breakdown field strengths of about 10⁶ V/cm. Received: 10 February 1998 / Accepted: 10 March 1998     

Nickel diffusion in polycrystalline CuInSe2 thin films with a <112> fiber texture

Nickel diffusion in CuInSe₂ thin films was studied in the temperature range 430-520 °C. Thin films of copper indium diselenide (CuInSe₂) were prepared by selenization of CuInSe₂-Cu-In multilayered structure on glass substrate. A thin film of Nickel was deposited and annealed at different temperatures. Surface morphologies of the Ni diffused and undiffused CuInSe2 films were investigated using scanning electron microscope. The alteration of Nickel concentration in the CuInSe₂ thin film was measured by Energy Dispersive X-Ray Fluorescence (EDXRF) technique. These measurements were fitted to a complementary error function solution and the diffusion coefficients at four different temperatures were evaluated. The diffusion coefficients of Ni in CuInSe₂ films were estimated from concentration profiles at temperatures 430-520 °C as D = 1.86 × 10^− 7(cm²s^− 1)exp[− 0.68(eV)/kT].     

Thermodynamic investigations of liquid cerium–bismuth alloys

The thermodynamics of the low-level Ce dissolved in liquid Bi was determined by means of the electromotive force (EMF) measurement method using a cell consisting of molten chloride and liquid Bi at the temperature ranging from 735 to 937 K. The activity coefficients of Ce in Bi were deduced from the obtained EMF results. A considerable increase in the activity coefficient with temperature was observed in the Ce concentration range studied. The values of the molar excess formation free energy, the excess enthalpy change, and the excess entropy change of Ce dissolved in Bi were determined. The heat of formation of liquid Ce–Bi alloys () was deduced from the measured activity coefficient. There is a linear dependence of experimental ΔH _Ce–Bi ^M on the Ce concentration. The experimental results of ΔH _Ce–Bi ^M were compared with the values predicted by the Miedema’s model.     

Some physical properties of CuInSe2 thin films

Semiconducting thin films of CuInSe₂ have been grown by thermal annealing in air of evaporated layers of Cu, In and Se on glass substrates. The structure of the films has been studied using the X-ray diffraction (XRD). The films were polycrystalline and showed mixture phases (binary and ternary) depending on the annealing temperature. The electrical properties revealed resistivity range of 10¹–10⁴Ωcm, respectively. The resistivity influenced with the annealing temperature and decreased with increasing temperature. The films have been analyzed for optical band gap.     

Characterization of Tin disulphide thin films prepared at different substrate temperature using spray pyrolysis technique

Thin films of tin disulphide on glass substrates were prepared by spray pyrolysis technique using precursor solutions of SnCl₂·2H₂O and n–n dimethyl thiourea at different substrate temperatures varied in the range 348–423 K. Using the hot probe technique the type of conductivity is found to be n type. X ray diffraction analysis revealed the polycrystalline nature with increasing crystallinity with respect to substrate temperature. The preferential orientation growth of SnS₂ compound having hexagonal structure along (002) plane increased with the substrate temperature. The size of the tin disulphide crystallites with nano dimension were determined using the Full Width Half Maximum values of the Bragg peaks and found to increase with the substrate temperature. The surface morphology had been observed on the surface of these films using scanning electron microscope. The optical absorption and transmittance spectra have been recorded for these films in the wavelength range 400–800 nm. Thickness of these films was found using surface roughness profilometer. The absorption coefficient (α) was determined for all the films. Direct band gap values were found to exist in all the films deposited at different substrate temperatures. The value of room temperature resistivity in dark decreased from 5.95 × 10³ Ω cm for the amorphous film deposited at low temperature (348 K) to 2.22 × 10³ Ω cm for the polycrystalline film deposited at high temperature (423 K) whereas the resistivity values in light decreased from 1.48 × 10³ to 0.55 × 10³ Ω cm respectively, which is determined using the four probe method. Activation energy of these thin films was determined by Arrhenius plot.     

Percolation threshold for electrical resistivity of Ag-nanoparticle/titania composite thin films fabricated using molecular precursor method

To find the percolation threshold for the electrical resistivity of metallic Ag-nanoparticle/titania composite thin films, Ag-NP/titania composite thin films, with different volumetric fractions of silver (0.26 ≤ φ_Ag ≤ 0.68) to titania, were fabricated on a quartz glass substrate at 600 °C using the molecular precursor method. Respective precursor solutions for Ag-nanoparticles and titania were prepared from Ag salt and a titanium complex. The resistivity of the films was of the order of 10⁻² to 10⁻⁵ Ω cm with film thicknesses in the range 100–260 nm. The percolation threshold was identified at a φ_Ag value of 0.30. The lowest electrical resistivity of 10⁻⁵ Ω cm at 25 °C was recorded for the composite with the Ag fraction, φ_Ag, of 0.55. X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM), and transmission electron microscopic (TEM) evaluation of the effect of the morphology and the nanostructures of the Ag nanoparticles in the composite thin films on the electrical resistivity of the film revealed that the films consist of rutile, anatase, and metallic Ag nanoparticles homogeneously distributed in the titania matrix. It could be deduced that the electrical resistivity of the thin films formed at 600 °C was unaffected by the anatase/rutile content within the thin film, whereas the shape, size, and separation distance of the Ag nanoparticles strongly influenced the electrical resistivity of the Ag-nanoparticle/titania composite thin films.