Doped ZnO Films Research Articles

Investigation of thickness effect on NO2 gas sensing properties of ZnO/Na thin films

This present communication studies the dependences of film thickness, electrical properties, and film morphology on the gas detection capability of Na/ZnO thin films. The phase formation, microstructure, surface composition, and electrical parameters of these films were analyzed by XRD, FESEM-EDAX, and Hall measurement. XRD measurements reveal that the crystal quality of Na doped ZnO films is enhanced with the increase in the sensing layer thickness. The morphology of samples shows a transition of grain shape from spherical to irregular as the films become thicker. The film thickness shows variations in the range from 132 nm to 356 nm. Hall measurement concludes that Na doping concentration induces p-type conductivity in ZnO. The Na/ZnO film of a thickness of 132 nm emerged as the suitable NO2 sensor with better sensitivity and the fastest res/rec property. The obtained discrepancy in responsivity is attributed to the difference in grain size and carrier concentration with a change in film thickness.

Deposition of Sol-Gel ZnO:Mg Films and Investigation of Their Structural and Optical Properties.

This work presents a facile sol-gel method for the deposition of ZnO and ZnO:Mg films. The films are spin coated on silicon and quartz substrates. The impact of magnesium concentrations (0, 0.5, 1, 2 and 3 wt%) and post-annealing treatments (300-600 °C) on the film's structural, vibrational and optical properties is investigated. Undoped ZnO films crystallize in the wurtzite phase, with crystallite sizes ranging from 9.1 nm (300 °C) to 29.7 nm (600 °C). Mg doping deteriorates the film crystallization and shifting of 002 peak towards higher diffraction angles is observed, indicating the successful incorporation of Mg into the ZnO matrix. ZnO:Mg films (2 wt%) possess the smallest crystallite size, ranging from 6.2 nm (300 °C) to 25.2 nm (600 °C). The highest Mg concentration (3 wt%) results into a segregation of the MgO phase. Lattice constants, texture coefficients and Zn-O bond lengths are discussed. The diminution of the c lattice parameter is related to the replacement of Zn2+ by Mg2+ in the ZnO host lattice. The vibrational properties are studied by Fourier transform infrared (FTIR) spectroscopy. IR lines related to Mg-O bonds are found for ZnO:Mg films with dopant concentrations of 2 and 3 wt%. The optical characterization showed that the transmittance of ZnO:Mg thin films increased from 74.5% (undoped ZnO) to about 89.1% and the optical band gap energy from 3.24 to 3.56 eV. Mg doping leads to a higher refractive index compared to undoped ZnO films. The FESEM (field emission scanning electron microscopy) technique is used for observation of the surface morphology modification of ZnO:Mg films. The doped ZnO films possess a smoother grained surface structure, opposite to the wrinkle-type morphology of undoped sol-gel ZnO films. The smoother surface leads to improved transparency of ZnO:Mg films.

Influence of oxygen partial pressure on the strain behaviour of reactively co-sputtered Ga doped ZnO thin films

Gallium doped ZnO films (GZO) were deposited by reactive co-sputtering of Zn and GaAs at different partial pressures of oxygen in Ar-O2 mixture. The GZO films were deposited at 375 °C, with GaAs area coverage of 0 - 3 % and 4 - 10 % O2 in sputtering atmosphere. The composition of GZO films was studied by X-ray photoelectron spectroscopy, which showed that the films deposited at 4 % and 5 % O2 display Ga/Zn ratio of ∼ 0.1 and contain oxygen vacancies, while those deposited at 6 % and 7 % O2 display Ga/Zn ratio of ∼ 0.01 and contain chemisorbed/interstitial oxygen. X-ray diffraction measurements supported by Raman studies revealed that undoped ZnO films are strongly c-axis oriented and display decrease of crystallite size along c-axis, along with substantial increase of hydrostatic tensile strain with increase of O2 percentage, which are attributed to the presence of interstitial oxygen. In sharp contrast, GZO films deposited at 4 % and 5 % O2 under limited oxygen availability, display smaller and mis-oriented crystallites along c-axis and large compressive strain, which is also hydrostatic, owing to the incorporation of Ga. The GZO films deposited at 6 % and 7 % O2 display improved crystallinity and decrease of compressive strain, which are attributed to reduced Ga content of the films and sufficient availability of oxygen during deposition. This study reveals the critical influence of the partial pressure of oxygen on the stoichiometry, doping concentration, crystallinity and the nature of strain in reactively sputtered GZO films.

Effect of Na doping and heat treatment on the characteristics of ZnO thin films and ZnO/Si cells

• Undoped and Na doped ZnO films and ZnO/Si cells fabricated by chemical spray pyrolysis. • Band gap energy and resistivity of ZnO thin films decreased with Na doping and thermal annealing. • Power conversion efficiency of ZnO/Si heterojunction cell remarkably increased with Na doping and thermal annealing. Undoped and sodium doped ZnO thin films were grown on glass and Si substrates by spray pyrolysis. Post thermal annealing was applied for 1 h at 350 °C. Effect of Na doping and thermal annealing on the properties of ZnO thin films were investigated via XRD, UV–Visible spectrometry and resistivity measurements. Resistivity of the Na doped and annealed ZnO film was decreased to 7.15x10 3 Ω.cm from 3.26x10 4 Ω.cm where band gap energy was decreased to 3.31 eV from 3.36 eV. Current-voltage measurements held for all ZnO/Si heterostructures in dark and under illumination and device parameters were determined. Again best result obtained for the device in which both Na doping and annealing were applied as efficiency was increased to 4.57% from 2.01%.

Highly selective ppm level LPG sensors based on SnO2-ZnO nanocomposites operable at low temperature

The nanoscaled SnO2, ZnO and SnO2-ZnO nanocomposite powders were prepared by microwave aided ultrasonication technique by employing centrifuge for the collection of dried powder of the materials. The fabrication of thick films of the pure ZnO and SnO2-ZnO nanocomposite powders were made by simple screen printing technique. The crystallographic phases of the pure ZnO and SnO2 doped ZnO are hexagonal in nature. The average crystallite size of 0 (pure), 1, 3, 5, 7 and 9 wt% of SnO2 doped ZnO films was found to be 30.2, 30.3, 109.7, 37.9, 36.8 and 45.6 nm, respectively. The SEM micrographs depict the porous nature of the thick films. The EDS analysis reveals that the pure ZnO and SnO2 doped ZnO films are oxygen deficient and show a semiconducting nature. The symmetric I-V characteristics depict the ohmic and non-ohmic natures of the thick films. The electrical resistivity measurements indicate a negative temperature coefficient of resistance. The gravimetric analysis shows that the average thickness of the pure ZnO thick film is 28 µm. The LPG response versus doping concentration plot suggests that the (1 wt%) SnO2 doped ZnO thick film exhibits crucial response to 100 ppm LPG at 50 °C and 100 °C. Therefore, the SnO2 doped ZnO thick film can be applicable in the field of LPG sensing with rapid response and recovery.

Magneto-Optical and Muliferroic Properties of Transition-Metal (Fe, Co, or Ni)-Doped ZnO Layers Deposited by ALD.

ZnO doped with transition metals (Co, Fe, or Ni) that have non-compensated electron spins attracts particular interest as it can induce various magnetic phenomena and behaviors. The advanced atomic layer deposition (ALD) technique makes it possible to obtain very thin layers of doped ZnO with controllable thicknesses and compositions that are compatible with the main microelectronic technologies, which further boosts the interest. The present study provides an extended analysis of the magneto-optical MO Kerr effect and the dielectric properties of (Co, Fe, or Ni)-doped ZnO films prepared by ALD. The structural, magneto-optical, and dielectric properties were considered in relation to the technological details of the ALD process and the corresponding dopant effects. All doped samples show a strong MO Kerr behavior with a substantial magnetization response and very high values of the Kerr polarization angle, especially in the case of ZnO/Fe. In addition, the results give evidence that Fe-doped ZnO also demonstrates a ferroelectric behavior. In this context, the observed rich and versatile physical nature and functionality open up new prospects for the application of these nanostructured materials in advanced electronic, spintronic, and optical devices.

Magnetic and electric properties of Co doped ZnO films via in-situ growth

Co content plays a crucial role in the magnetic and electric properties in Co doped ZnO (ZnO:Co) films. In this work, the method of RF source-assisted molecular-beam vapour deposition was used to prepare ZnO:Co films with various Co contents. The doped Co2+, the metal Co, and the oxygen vacancies (Vo) were found in the deposited films. The surface roughness and the crystalline size of the ZnO:Co films also increased by using bigger Co content. The prepared ZnO:Co films show n-type conductivity and room temperature ferromagnetism. By increasing the Co content, the content of the metal Co exceeded that of the doped Co2+ gradually, which led to the higher electric conductivity, carrier concentration, mobility, and saturation magnetization.

The effect of Bi doping on the thermal conductivity of ZnO and ZnO:Al thin films

The dissipation of heat generation has been one of the largest obstacles in the design of semiconductor devices and reducing the thermal conductivity is vital for improving thermoelectric efficiency. This work focuses on the Bi doping effect on ZnO, and ZnO:Al thin films produced by magnetron sputtering with thickness varying between 500 and 900 nm. The approach introduces Bi ions, a higher mass element, into the ZnO metal-oxide matrix, to hinder phonon-mediated heat conduction and, consequently, reduce thermal conductivity. Atom probe tomography (APT) was employed to survey Bi doping distribution in ZnO:Al:Bi and ZnO:Bi thin films and to study the morphology of the grain boundaries. The thermal properties of the thin films were measured by frequency-domain thermoreflectance. Based on thermal conductivity results, it is concluded that the doping of ZnO films with Al has a significant effect on thermal conductivity, being reduced from 6.0 W m−1 K−1 in its undoped state to 3.3 W m−1 K−1 for ZnO with ∼3 at.% of Al, mainly due to alloy scattering of phonons in the wurtzite cell. Further doping with Bi contributes to a slight reduction in the thermal conductivity of ZnO:Al.Bi films (2.9 W m−1 K−1), due to grain boundary scattering by Bi/Bi2O3 phases. This result is understood as the confluence of two counteracting effects. On the one hand, the thermal conductivity of the film decreases because Bi, unlike Al, is segregated to grain boundaries and does not substitute Zn in the wurtzite crystal lattice, which is unequivocally demonstrated by APT results. On the other hand, the simultaneous presence of Al and Bi triggers a morphological change with the film's microstructure becoming more columnar. This change in microstructure from 3D island growth in ZnO:Al and ZnO:Bi to a more regular columnar structure in ZnO:Al,Bi limits further reduction in the thermal conductivity.

Robust ferromagnetism of (Co, Er) co-implanted ZnO-based diluted magnetic semiconductor

The present study reports room temperature ferromagnetism observed in Co implanted, Er implanted and (Co, Er) co-implanted ZnO films deposited by filtered cathode vacuum arc (FCVA) on sapphire substrates. The microstructures, surface morphologies, electrical and magnetic properties of pure and implanted ZnO films were investigated. X-ray diffraction results confirm that the implantation of Co and Er ions cause the substitution of Zn ions without the formation of secondary phases. The results of X-ray photoelectron spectroscopy indicate that the Er ions and Co ions are in trivalent and divalent states, respectively. After ions implantation, the electrical properties of the films are improved and the carrier concentration of Er implanted ZnO film is higher than that of Co implanted ZnO film. Substantial intrinsic ferromagnetism has been observed for all the implanted ZnO films. The saturation magnetization for the Er implanted ZnO film is higher than that of the Co implanted ZnO film and the magnetization is further enhanced by (Co, Er) co-implanting. The carrier mediated exchange mechanism can well describe the origin of ferromagnetism in the doped ZnO films. Our results can provide a reference for preparing high-quality diluted magnetic semiconductors (DMSs) by RE elements and TM elements co-doping in ZnO films. The saturation magnetization for the Er implanted ZnO film is higher than that of the Co implanted ZnO film and the magnetization is further enhanced by (Co, Er) co-implanting. • A filtered cathode vacuum arc (FCVA) deposition system was used to grow ZnO films on sapphire substrates. • The saturation magnetization for the Er implanted ZnO film is higher than that of Co implanted ZnO. • The magnetization is enhanced by Co-Er co-doping compared with Co doped and Er doped film. • The magnetization shows carrier concentration-dependent behavior.

Tuning Optical and Electrical Properties of Ultra-Fast Prepared Nanoflower Mg:ZnO Films by MWCNTs Coating

Mg doped ZnO films were coated multi-walled carbon nanotubes (MWCNTs) via fast chemical bath onto ZnO seed layers. XRD analysis showed preferential orientation shift from high-energy (002) peak to low-energy (101) peak with MWCNTs coating. Average crystalline size of Mg doped ZnO samples are 15 nm. diameter and 50% percent reduction has been observed with MWCNTs coating. SEM images reveals the presence of high accumulative nanoflower forms on Mg:ZnO surfaces and homogenous net-shaped coating has been achieved by MWCNTs inclusion so active surface area may increase. No major difference of optical absorption edge is detected in both films however MWCNTs coating cause an increase direct band gap. Due to Burstein-Moss effect, Mg:ZnO and Mg:ZnO/MWCNTs films with using Tauc plot calculated band gap values are 3.04 eV and 3.34 eV, respectively. From FTIR spectra, no obvious change is not detected the functional groups of the samples. Electrical studies show that MWCNTs decrease the resistance and the resistance of films at room temperature were calculated 29.85 and 8.53 k for Mg:ZnO and Mg:ZnO/MWCNTs films , respectively.

Synergistically enhanced ultraviolet emission of Yb doped ZnO films by using a capping of ultrathin Al and SiO2 microspheres.

We studied the enhancement effects of ultraviolet (UV) emission from rare earth ytterbium (Yb) doped ZnO films, by using capping layers of Al and SiO2 micro-spheres. The films were deposited on Si substrates with magnetron sputtering followed by high temperature (∼1000°C) heat treatment, and then capped with a nanoscale ultrathin aluminum (Al) layer and/or SiO2 micro-spheres on the surface of the films. The photoluminescence (PL) results indicate that compared to the case without any capping, the UV emission is enhanced by a factor ranging from several to dozens times, the films capped with 2.0 nm Al layer and 5.0 µm SiO2 microspheres have the longest highest PL intensity among the samples. The PL enhancements are discussed in terms of increased optical (or electrical) fields around the surface of the films combined with defect passivation after the capping. Our work has proposed a strategy to enhance the UV emissions of ZnO, which will broaden the application potential of ZnO in UV photonics.

Modification of the structural, linear and nonlinear optical properties of zinc oxide thin films via barium and magnesium doping

In the current work, the structural, linear and nonlinear optical properties of zinc oxide thin films have been modified via barium/magnesium doping for modern optoelectronic applications. Ba/Mg-doped and undoped ZnO thin films were successfully prepared by the spray pyrolysis route. The surface morphology, structures' modification and structural parameters were explored using a field emission scanning electron microscope, FT-IR spectrophotometer and X-ray diffraction techniques, respectively. The XRD analysis shows that Ba/Mg-doped and undoped ZnO thin films possess a crystalline nature with a hexagonal wurtzite structure. The crystallite size of the undoped ZnO nanoparticles is 18.5 nm and varies to 21.4 nm (Ba:ZnO), 23.1 nm (Ba/Mg:ZnO) and 16.4 nm (Mg:ZnO). The optical properties (transmittance, absorbance and reflectance) were measured using a UV–visible spectrophotometer. The optical bandgap (Eg) of ZnO thin film decreases from 3.42 eV to 3.35 eV (Ba:ZnO), 3.25 eV (Ba/Mg:ZnO) and 3.31 eV (Mg:ZnO). The decrease in the Eg values of the doped ZnO films as compared with that of the undoped one is mainly attributed to the created defects and localized states in the bandgap of the host ZnO due to Ba/Mg-doping. Moreover, the major optical parameters (refractive index, distinction coefficient, dielectric constant) are also affected as a result of Ba/Mg-doping. Furthermore, a great effect of Ba/Mg-doping on the nonlinear optical constants (third-order optical susceptibility (χ(3)) and nonlinear refractive index (n2)) and first-order linear optical susceptibility (χ(1) is achieved. The obtained results show that the linear and nonlinear optical properties of ZnO film could be enhanced via Ba/Mg-doping for updated applications in multifunctional modern optoelectronics and high-speed communications.

Structural and optical properties of copper-doped ZnO thin films at different weight percentage

ZnO thin films have been synthesized by chemical vapor deposition technique at substrate temperature 400˚C and doped with Cu at (1,3,5 wt.%). The morphologies of pure and doped ZnO films were investigated by SEM and the grain size was become larger by increasing the amount of Cu doped. From X-ray diffraction measurements, the maximum intensity peak was (002) for ZnO:Cu films at diffraction angle 34.4˚ and crystallized of hexagonal phase. Based on XPS Measurements on the surface of ZnO:Cu films, it has been found that the peak was shift to higher diffraction angle and full width half maximum of the films were become wider with increasing Cu doping. The band gap of ZnO film was 3.3 eV and it was decreased as an increasing of Cu dopant.

Structural, optical and electrical properties of Sb-doped ZnO/ZnO homojunction thin film structures prepared by sol-gel based coating process under nitrogen annealing atmosphere

This investigation focuses on structural, optical and electrical properties of Sb-doped/undoped ZnO double layer structure annealed in nitrogen atmosphere. The double layer film was prepared by sol-gel spin coating technique onto ITO substrate then annealed in nitrogen atmosphere. The crystal structure demonstrates hexagonal ZnO without phase impurity. The deterioration in the crystal structure due to Sb is incorporated in ZnO lattice and nitrogen annealing atmosphere and the decrease in diffraction intensity due to the different thickness of the ZnO film. The existence of Sb atom in the doped ZnO film is confirmed by depth profile measurement. The transmittance spectra exhibit high transparency with interference fringe and the photoluminescence spectrum shows the near-band-edge emission of ZnO without any emission in the visible region. The electrical properties demonstrate diode characteristics and photocurrent under dark and UV irradiation, respectively.

Effect of N doping on the Optical and Electrical properties of Thermal Spray pyrolysed ZnO Thin Films

Nitrogen doped ZnO (ZnO:N), thin films have been deposited on glass substrate at the temperature of 350ºC by low cost homemade thermal spray pyrolysis (TSP) technique at a normal environmental conditions. In this study the average reflectance, transmittance and absorbance were measured (20%), (35%), and (45%), respectively. Absorption coefficient is 3.5x104cm-1 for N doped ZnO. Direct bandgap energy varies from 3.08-2.99eV and indirect band gap energy varies from 2.86-2.67eV for N doped ZnO. XRD analysis shows the (002) plane is present of samples and the average grain size decrease with increasing N concentration. Surface morphology of N doped ZnO films is studied by Scanning Electron Microscopy (SEM). It is seen that hexagonal crystal grains few voids are present for N doped ZnO samples. The surface exhibits more or less uniform surface morphology with some clusters on the whole surface. Hall Effect study confirms that Nitrogen doped ZnO (ZnO:N), thin films using Vander pauws method were made at room temperature at a constant field of 9.75 KG. Experimentally (1, 2, 3, and 4) % N doped ZnO thin films have shown in negative Hall Constant (RH). Which exhibited n-type characteristics. Hall Constant (RH), and Hall concentration (n), increases with increasing N doping concentration. Initially Hall mobility (μH), increases linearly for (1-2) % N doping concentration then it decreases for the rest of the doping concentration. We also found that the resistivity (ρ) decreases and the conductivity (s) increases with increasing N doping concentration which exhibits the semiconducting nature.

SYNTHESIS OF Mg-DOPED n(ZnO) THIN FILMS USING SOL-GEL 6 DIP-COATING METHOD AND CHARACTERIZATION OF ITS STRUCTURAL AND OPTICAL PROPERTIES

Mg-doped ZnO alloy thin films having hexagonal Wurtzite structures were prepared using the Sol-Gel method. The doping results into, increased surface roughness and reduced grain size. Photonic applications are made easier by altering the optical bandgap of ZnO nano-structured thin films doped with various components. Mg metal atom doped ZnO films are an excellent example for study. In this work, the thin film of Mg-doped ZnO is formed on glass substrate via dip coating method with varied concentration of dopant. Dip-coating method is one of the cheapest methods. SEM, Raman spectroscopy and UV-vis spectroscopy are used to characterize the thin films. The spectroscopic analysis revealed a more uniformed crystalline nano-structured surface with fewer structural deffects. As the Mg atom concentration is increased, the results show a linear increment in the band gap of Mg-doped ZnO thin film in comparsion to the pure ZnO nanostructured thin film, as well as there occurs an improvement in crystalline character. The reduction in intensity of visible radiation (enhancement in radiation in UV region) with increase in Mg concentration indicates for a decrease in intrinsic defects. The changing trends in optical and structural parameters as with increased Mg content reveal a nonlinear and non-monotonic relations. The work provides insights to analyse the dependence of optical bandgap on crystal structure and defects.

Aluminum–nitrogen co‐doping improves the blue emission of ZnO films: A combined theoretical–experimental study

AbstractThe investigation on visible emission of ZnO is necessary due its potential application in electrochemical communication, biosensor, and white light diodes, and so forth. Previous report indicates that the visible light of ZnO is related to the defect type and concentration. It can be regulated by various methods. However, quenching of light emission occurs if too many defects are introduced. In this work, combined with theoretical calculation about energy band structure, aluminum–nitrogen co‐doped ZnO is conducive to form p‐type semiconductor, but aluminum doped ZnO films tend to form a deep n‐type semiconductors. Therefore, in the experiment, aluminum–nitrogen co‐doped ZnO films are prepared using radio frequency magnetron sputtering. The substitution of oxygen using nitrogen impurity is expected to introduce more oxygen vacancies and interstitial zinc defects related to visible light emission. At the same time, annealing in vacuum is also a highly non‐equilibrium processes with low oxygen and high zinc contents. Results indicate that combined with aluminum–nitrogen co‐doping and an appropriate annealing temperature, the blue and blue‐green emission of ZnO films can be greatly improved.

Role of defects in the electronic properties of Al doped ZnO films deposited by spray pyrolysis

Role of defects in the electronic properties of Al doped ZnO films deposited by spray pyrolysis

Boron Doped ZnO Films Deposited by DC Reactive Sputtering Using Zn:B Target: Influence of the Deposition Temperature on the Structural, Electrical and Optical Properties

ZnO-based transparent and conducting oxides (TCOs) are commonly used as a window layer in thin-film solar cells. However, TCOs with high transparency in the visible and near-infrared parts of the electromagnetic spectrum, plus excellent electrical properties are required in this application. In this study, TCOs based on ZnO:B films deposited by DC reactive sputtering using Zn:B alloy target were investigated. The impact of deposition temperature on the growth and physical properties of the films was examined. Structural, optical and electrical properties of these films were investigated by means of x-ray diffraction (XRD), Ultraviolet-Visible-Near Infrared (UV-VIS-NIR) spectroscopy, and Hall effect measurement, respectively. The XRD analysis revealed that all films are of hexagonal wurtzite structures, with a preferred orientation along the c-axis. The optical spectroscopy results indicated that all the ZnO:B films had optical transparency above 90% in the visible region which then slightly decreased in the near-infrared region. The highest carrier concentration, conductivity, and mobility were obtained at the deposition temperature of 300 °C–due to improvement in crystal growth–while higher temperatures slightly deteriorated the electrical properties, possibly due to a slight decrease in the crystallite size.
 Keywords: ZnO; Transparent and conducting oxides; ZnO:B, DC reactive sputtering; deposition temperature

Effects of electron doping on the d0 magnetism in N-implanted ZnO and ZnAlO films

In this paper, N ions are implanted into ZnO and ZnAlO films with different carrier concentrations prepared by radio frequency reactive magnetron sputtering on sapphire substrates. The structural, electrical, optical and magnetic properties of N-doped and (Al, N) co-doped ZnO films is investigated, and the particular emphasis is placed on the effects of carrier concentration on the defects induced magnetism in the films. Our results show that all the doped ZnO films are ferromagnetic at room temperature. A trace amount of additional Al doping has a significant effect on the improvement of ferromagnetic properties, and a maximum saturation magnetization of 73 emu/cm3 is obtained for (Al, N) co-doped ZnO films. The optical band gaps of ZnO films increase with the increasing Al doping content, which is owing to the combined effect of high carrier concentration and the Burstein Moss effect. The average transmittance of all ZnO films exceeds 80%. Our results confirm that appropriate electron doping can effectively enhance the magnetic moment in N implanted ZnO which may also apply to other ZnO systems with d0 magnetism.