Undoped CdO Research Articles

Liquefied petroleum gas sensing properties of sprayed nanocrystalline Ga-doped CdO thin films

Undoped and Ga-doped cadmium oxide thin films were prepared by chemical spray pyrolysis technique using (CH3COO)2Cd·2H2O as a precursor solution without or with (Ga (NO3)3−x·H2O) as a doping solution, respectively. The dopant concentration (at% Ga to Cd) was varied from 0 to 2.5at%. X-ray diffraction (XRD) analyses reveals all the films are of polycrystalline cadmium oxide, possessing cubic structure. The film shows the (200) preferential orientation for undoped CdO and Ga-doped CdO films. The crystallite size for the 1.5at% Ga-doped film is found to be minimum (15nm) among all the films. At a higher doping concentration of 2.5at%, the disorder produced in the lattice causes an increase in the resistivity of the film. It is found that compared to the undoped CdO film, Ga-doped films show high response to LPG. Among all the doped films studied, the 1.5at% Ga-doped CdO film shows the maximum response (63%) to 1040ppm of LPG in air at 673K. Further, the response and recovery times of the films to LPG become shorter at higher operating temperatures. A possible mechanism of LPG sensing has been explained. The photoluminiscence (PL) studies reveal UV emission in pure CdO which shifts to green emission on doping of Ga.

Optical properties of Al-CdO nano-clusters thin films

The aluminum doped cadmium oxide (CdO:Al) thin films were grown onto glass substrates by sol–gel spin-coating method. The structural properties of undoped and Al-doped CdO thin films were studied by atomic force microscopy. AFM results reveal that the studied CdO films were formed from the nano-clusters. The optical transmittance of undoped and Al-doped CdO is decreased with increasing Al contents. The optical band gaps of the CdO films were varied from 2.54eV to 2.32eV with increasing Al dopants. The width of localized states in the optical band gap of the films is increased with increasing Al content. The improvement of the optical constant of Al-doped CdO has potential applications as transparent conducting oxide for different optoelectronic device applications.

Interfacial modification in Si/CdO heterojunction by Ge doping for optoelectronic applications

The electrical properties of Ge-doped CdO (CdO:Ge) films grown on p-Si were studied in this work. The focussing was on the effect of optoelectrical properties of CdO:Ge film on the optoelectrical properties of the constructed p–n heterojunction, as well as to study the dependence of the optosensitivity and optoresponse of the heterojunction on the Ge% doping level. The characterisation of the transparent conducting oxide CdO:Ge layer was performed by the X-ray diffraction, SEM, electrical measurements, and spectral photometry. A strong optosensitivity was found especially for the p–n heterojunction that used 0.16 wt% Ge-doped CdO, attaining a great value of about 60,000% comparing to undoped CdO. In addition, a good value of optical response of 319.4 mA/W for the p–n heterojunction constructed with 0.25 wt% Ge-doped CdO. The results show that CdO:Ge/p-Si heterojunctions act as very good candidates for constructing high-efficiency photodetectors.

Morphological, electrical and optical properties of highly oriented undoped and doped zinc oxide and cadmium oxide films grown by atmospheric-pressure chemical vapor deposition

Morphological, electrical and optical properties of undoped and Ga-doped ZnO and undoped CdO films grown on sapphire (Al2O3) substrates by the atmospheric-pressure CVD method using Zn, Cd, H2O and GaCl3 as source materials were investigated. The influences of the deviation from the stoichiometric composition were clearly observed on photoluminescence (PL), photoluminescence excitation (PLE) and photoacoustic (PA) spectra of the undoped ZnO films. The carrier concentration n in the undoped ZnO film was found to be dominated by the hydrogen related donors rather than native defects. The Ga doping in ZnO film led to the degradation of surface flatness, the increase in carrier concentration n and the decrease in Hall mobility μ. With increasing carrier concentration n, the Ga-related neutral donor bound exciton line of the Ga-doped ZnO film shifted towards shorter wavelengths, accompanied by asymmetric broadening. This is probably due to the screening of the Coulomb potential by heavily introduced donors and free-electrons. Highly oriented CdO films with smooth surfaces were successfully grown on r-Al2O3 substrates. Transmittance values for the CdO/r-Al2O3 films were higher than 70% in the range of 700–2500nm. Under the assumption that the reflectance is neglected, the direct and indirect optical gap energies of the CdO films determined from the transmittance spectra were 2.34–2.38eV and 1.98–2.06eV, respectively. PA measurements for the CdO films revealed that the tail states associated with the residual impurities and/or the structural defects reduce the direct optical gap energy. A maximum Hall mobility μ of 178cm2/Vs was achieved on the CdO/r-Al2O3 film with the carrier concentration n of 4.2×1019cm−3.

Structural characterization, optical and magnetic properties of Ni-doped CdO dilute magnetic semiconductor nanoparticles

Abstract

Structural and electrical studies of Cu-doped CdO prepared by solid state reaction

This paper reports synthesis, crystal structure and electrical properties of Cu-doped CdO (CdO:Cu) powders. X-ray diffraction shows that majority of the samples are monophase and has the cubic structure. The limit solubility of Cu ions in CdO lattice is found to be 2mol% (after heating at 900°C), whereby the impurity phase was determined to be the monoclinic-CuO. For monophase CdO:Cu samples synthesized at 900°C, the lattice parameter decreased with increasing Cu concentration. Electrical conductivity of undoped CdO and 2mol% Cu-doped CdO (after heating at 900°C) were found to be 79 and 191Ω−1cm−1, respectively, at 100°C and 912 and 1549Ω−1cm−1, respectively, at 900°C. Thus, it appears that electrical conductivity slightly increases with Cu doping. Finally, the activation energy of monophase CdO:Cu (after heating at 900°C) is shown to decrease with Cu concentration.

Pyrolized Growth of (Al, N) Dual Doped CdO Thin Films and Study of Structural, Surface Morphology and Opto-Electrical Properties

Thin films of undoped and (Al, N) dual doped CdO were prepared on glass substrate at substrate temperature 350 oC by spray pyrolysis method. The films are characterized by XRD, AFM, Optical and electrical properties. The films are highly poly crystalline and found to be cubic. AFM images show the surface properties of the film. It increases with increasing concentration of Al & N in the CdO samples. Optical transmittance is made at room temperature within the wavelength range 400 nm to 1100 nm. Direct band gap of undoped CdO has been measured and found to be 2.58 eV and it decreases with (Al, N) dual doping in CdO thin film upto 2.52 eV. The resistivity of undoped CdO shows the metallic behavior upto 370 K and then followed by semiconducting behavior. (Al, N) dual doping shows the carrier compensation during temperature range (355 - 375) K.

Determining the nonparabolicity factor of the CdO conduction band using indium doping and the Drude theory

Due to their high intrinsic electron mobility, CdO-based materials are gaining interest as transparent conductive oxides. By creating model dielectric functions based on the Drude theory, accurate fits to the measured transmittance and reflectance of CdO and CdO : In thin films were achieved without using a frequency dependent Drude damping parameter. Difference in the model between undoped and In-doped CdO showed that the Burstein–Moss shift is not the only mechanism which improves the transparency in In-doped samples. Comparing the Drude analysis with Hall measurements revealed a nonlinear relationship between the free-electron effective mass and the carrier concentration, an effect which is caused by the nonparabolicity of the CdO conduction band. Analysis of 50 CdO : In thin films grown by pulsed filtered cathodic arc showed the nonparabolicity factor was C = (0.5 ± 0.2) eV−1 and the band-edge effective mass was (0.16 ± 0.05)me. Knowledge of the effective mass allows for optical measurements of carrier mobility, which was less than or equal to the measured Hall mobility in these films due to the large electron mean free path compared with the grain size.

Structural, optical and electrical properties of molybdenum-doped cadmium oxide thin films prepared by spray pyrolysis method

Molybdenum-doped cadmium oxide films were prepared by a spray pyrolysis technique at a substrate temperature of 300 °C. The effect of doping on structural, electrical and optical properties were studied. X-ray analysis shows that the undoped CdO films are preferentially oriented along the (111) crystallographic direction. Molybdenum doping concentration increases the films’ packing density and reorients the crystallites along the (200) plane. A minimum resistivity of 4.68×10−4 Ω cm with a maximum mobility of 75 cm2 V−1 s−1 is achieved when the CdO film is doped with 0.5 wt.% Mo. The band-gap value is found to increase with doping and reaches a maximum of 2.56 eV for 0.75 wt.% as compared to undoped films of 2.2 eV.

Controlling of crystal size and optical band gap of CdO nanopowder semiconductors by low and high Fe contents

The CdO:Fe nanopowder semiconductors were synthesized by the sol–gel calcination for the first time. The structural properties of Fe doped CdO samples were analyzed by AFM and XRD measurements. XRD patterns of the pure and Fe-doped CdO samples reveal that the pure and Fe doped CdO nanopowders are polycrystalline of cubic CdO structure. The crystallite size of undoped and Fe-doped CdO samples is changed unsystematically with a regular increase of Fe content. The optical band gaps of Fe doped CdO samples were determined for the first time by diffused reflectance measurements. The optical band gap of the samples is increased with the increase of Fe dopant inside the host matrix (CdO) up to 15 % followed by a decrease in its value. It is evaluated that Fe doped CdO nanopowder semiconductors can be producted by sol–gel calcination for advanced technological applications

Preparation of nanostructured Bi-doped CdO thin films by sol–gel spin coating method

The nanostructured Bi-doped CdO thin films were prepared using sol–gel spin coating method. The morphology and optical properties were investigated using atomic force microscopy (AFM) and UV visible spectroscopy. The AFM results indicate that the CdO films are formed from the nanoparticles and the grain size of the films is changed with bismuth doping level. The optical band gap values of undoped and Bi-doped CdO films were determined by optical absorption method. The band gap value of undoped CdO film was found to be 2.589eV, on the other hand, the band gap values of Bi-doped CdO films were found to be in the range of 2.657–2.684eV. The band gap value of the CdO films is increased with the content of Bi dopant. It is evaluated that the optical band gap and grain size of the CdO film could be controlled by Bi-doping.

Electrical Properties of Al-Doped CdO Thin Films Prepared by Thermal Evaporation in Vacuum

In this work, thin films of undoped and Al-doped CdO with 3, 5, 7 and 9wt.% were prepared by thermal evaporation in vacuum on glass substrate. From XRD patterns, doping CdO films with Al up to 7% causes small reduction in the intensity of the (200) plane, while small increase is observed in the intensity of (111) plane. However, intensity of all peaks rapidly decreases for the films with high Al content (9%). The formation of tubular nanostructures of undoped and Al doped-CdO films was observed by SEM technique. Energy gap value of the films was evaluated from the optical transmission spectra in the spectral region 300-1000nm. The best values of electrical conductivity, Hall mobility and electron concentration were obtained in the films with 5% Al doping. Temperature dependent resistivity measurements of the films doped with Al at 3, 5 and 7% showed the metal-semiconductor transition around 100, 150 and 205K respectively, which is rationalised by localisation of degenerate electrons in a weak-localisation regime. It was also found that the transition temperature is dependent on the Al concentration and is related to the increase in disorder induced by dopant addition.

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.

Current underestimation of the optical gap and Burstein-Moss shift in CdO thin films: A consequence of extended misuse of α2-versus-hν plots

This paper reports on the optical and transport properties of undoped and In-doped CdO films prepared by pulsed laser deposition. Film thickness (around 150 nm) was chosen to allow for an accurate measurement of the absorption coefficient spectrum up to 2-3 eV above the direct bandgap. The imaginary part of the dielectric function, as determined from absorption spectra, is consistent with ellipsometry results in the literature. The optical gap and transition width are determined from a gaussian fit to the first derivative of the absorption spectrum. CdO optical gap so determined increases from 2.3 to 3.4 eV for electron concentrations increasing from 1.8 × 1019 to 1.1 × 1021 cm−3 with a Burstein-Moss shift of 1.1 eV, much larger than those currently found in the literature for similar or larger carrier concentration ranges. We discuss this discrepancy and show that the origin of the current underestimation is related to an extended misuse of α2-versus-hν plots to determine the optical gap. A clear correlation between the optical transition width and electron mobility is also shown and discussed.

Behaviour of Charge Carriers in As-Deposited and Annealed Undoped TCO Films

We examine the structures, cut-off points of transmittance spectra and electric properties of undoped ZnO, SnO2 and CdO films by scanning electron microscopy, x-ray diffraction, spectrophotometer and Hall-effect measurements, respectively. The films are deposited by using an rf magnetron sputtering system from powder targets in argon and then annealed in vacuum. The structures and properties of the as-deposited films are compared with those of the annealed one. We try to explain the behaviour of charge carriers based on the semiconductor physics theory.

Effect of cerium doping on the structural and optoelectrical properties of CdO nanocrystallite thin films

Ce-doped CdO thin films with different Cd compositions were prepared on glass and Si wafer substrates by a vacuum evaporation technique. The effects of Ce 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 Ce 4+ ions substituted for Cd 2+ and the solubility of Ce in CdO is very limited and may be around ∼1.3 at%. The cerium doping influences all optoelectrical properties of CdO. The bandgap of Ce-doped CdO suffers narrowing by about 27% with a small (0.5 at%) doping level. The electrical behaviours show that all the prepared Ce-doped CdO films are degenerate semiconductors. Their dc-conductivity, carrier concentration and mobility increase compare with undoped CdO film. The largest mobility of 66.7 cm 2 V −1 s was observed for 3.8% Ce-doped CdO film. From transparent-conducting-oxide point of view, Ce is sufficiently effective for CdO doping.

Preparation of nanostructure Ni doped CdO thin films by sol gel spin coating method

The nanostructure Ni-doped CdO films have been prepared by sol gel spin coating method. Atomic force microscopy results indicate that the CdO films are formed from the nanoparticles and the grain size is changed with nickel content. X-ray diffraction patterns of the films indicate that the undoped and Ni-doped CdO films have polycrystalline structure with a cubic sodium chloride structure, showing two main characteristic peaks assigned to the (111) and (200) planes. The optical band gap values of undoped and Ni-doped CdO films were determined by optical absorption method. The Eg values of the CdO films were found to be in the range of 2.26–2.60 eV. The Eg values of the CdO films increase with the content of Ni dopant (up to 6% Ni). It is evaluated that the optical band gap and grain size of the CdO film can be controlled by doping with nickel atoms.

Influence of Yb-doping on optoelectrical properties of CdO nanocrystalline films

Yb-doped CdO thin films with different Yb contents were prepared on glass and silicon substrates by vacuum evaporation technique. The effects of Yb-doping on the structural and optoelectrical properties of the host CdO films were systematically studied. The X-ray diffraction study shows that some of Yb3+ ions substituted for Cd2+ and the solubility of Yb in CdO is very limited and may be around ~0.2 at.%. The Yb-doping influences all the optoelectrical properties of CdO. The bandgap of Yb-doped CdO suffers narrowing by about 20% with a very small (0.03 at.%) doping level. The electrical behaviours show that all the prepared Yb-doped CdO films are degenerate semiconductors. Their dc-conductivity, carrier concentration, and mobility increase compared with undoped CdO film. The largest mobility of about 87 cm2/V s was measured for 0.13 at.% Yb-doped CdO film. From transparent-conducting-oxide point of view, Yb is sufficiently effective for CdO-doping.

Nanopowder synthesis of aluminum doped cadmium oxide via sol–gel calcination processing

The structural, optical and electrical properties of undoped and Al-doped CdO powder nanostructures prepared by sol–gel calcinations method have been investigated. X-ray diffraction (XRD) results revealed that pure and Al-doped CdO have the polycrystalline with face centered cubic (FCC) structure. The crystallite size for undoped and 5, 10, 15, 20% of Al-doped CdO samples were found to be 17.2, 15.9, 16.1, 16.3 and 16.8 nm, respectively. The optical band gap of the samples were determined from the diffused reflectance spectra and E g values for undoped and 5, 10, 15, 20% of Al-doped CdO samples were found to be 1.89, 2.07, 2.03, 2.07 and 2.12 eV, respectively. The electrical conductivities of pure and Al-doped CdO were measured in the temperature range (290–420 K) and their temperature dependence was analyzed according to Arrhenius relation. The electrical conductivity results indicate that the electrical conductivity mechanism is controlled by thermally activated processes. The results showed that sol–gel calcinations technique can be effectively used to produce undoped and doped nanopowders.

Effect of indium doping in CdO thin films prepared by spray pyrolysis technique

Preparation of transparent and conducting indium doped CdO thin films by spray pyrolysis on glass substrate is reported for various concentration of indium (2–8 wt%) in the spray solution. The electrical, optical and structural properties of indium doped CdO films were investigated using different techniques such as Hall measurement, optical transmission, X-ray diffraction and scanning electron microscope. X-ray analysis shows that the undoped CdO films are preferentially orientated along (2 0 0) crystallographic direction. Increase of indium doping concentration increases the films packing density and reorient the crystallites along (1 1 1) plane. A minimum resistivity of 4.843×10 −4 Ω cm and carrier concentration of 3.73×10 20 cm −3 with high transmittance in the range 300–1100 nm were achieved for 6 wt% indium doping. The band gap value increases with doping concentration and reaches a maximum of 2.72 eV for 6 wt% indium doping from 2.36 eV of that of undoped film. The minimum resistivity achieved in the present study is found to be the lowest among the reported values for In-doped CdO films prepared by spray pyrolysis method.