N-doped ZnO Thin Films Research Articles

Investigations of structural, optical, electrical and photocatalytic behavior of ZnO:N thin films for p-type substrate: Influence of annealing temperature

This study used a simplified, automated spray pyrolysis setup with a perfume atomizer to prepare nitrogen (N) doped ZnO thin films. The deposited thin films were annealed at 300 ºC to 500 ºC under a nitrogen atmosphere. In the present work, detailed investigations are carried out on the effect of annealing on structural, optical, and surface morphological, photoluminescence, and electrical properties of p-type ZnO:N thin films. The effect of annealing temperature on the photocatalytic activity of N-doped ZnO thin films was studied, the degradation efficacy was and 91%. The XRD diffractograms depicted that all the prepared films have a wurtzite structure, high crystallinity, and C-axis orientation. The increased annealing temperature received a red shift in the band gap. A flake-like morphology was observed from SEM images. The p-type to n-type conductivity transformation is kept at the maximum applied annealing temperature of 500 ºC. A resistivity received ∼ 21.7 Ω cm was obtained for 400 ºC annealing temperature. Photocatalytic studies confirmed that higher degradation efficiency was exhibited for higher annealing temperatures. The present investigations reveal that the properties of p-ZnO:N thin films are highly suitable for optoelectronic applications.

Influence of nitrogen dopant source on the structural, photoluminescence and electrical properties of ZnO thin films deposited by pulsed spray pyrolysis

The N-doped ZnO thin films have been deposited using pulsed spray pyrolysis from Zinc Acetate (ZA) precursor along with the N dopants of N2 carrier gas (N2 – series) and Ammonium Acetate (AA – series) on glass substrates at the optimized substrate temperature of 300 °C with different spraying pulse intervals. The X-ray powder diffraction studies confirmed the polycrystalline structures with the presence of mixed compressive and tensile strain along ‘a’ and ‘c-axes’ respectively for the N2 doped films and the presence of compressive strain alone along both ‘a’ and ‘c-axes’ for the AA doped films. The XPS analysis revealed that the N2 gas source led to the incorporation of elemental N into the film and the AA source led to the incorporation of both elemental and molecular N into the film. The Micro Raman Analysis confirmed the N-doping and its contributed carrier localization by exhibiting A1(LO) and A1(TO) modes. Photoluminescence studies exhibited the active band gap of ~3.19 eV with additional peaks related to hole traps at ~3 eV and electron traps at ~2.8 eV without exhibiting peaks correspond to oxygen vacancy defects. The Seebeck measurements confirmed the establishment of intrinsic p-type conductivity in both the cases at room temperature (RT) and the films deposited with pure elemental doping from N2 source found exhibiting better p-type conductivity than those films deposited using AA source.

Effect of surface carbon contamination on the chemical states of N-doped ZnO thin films

Nitrogen-doped ZnO thin films [ZnO:N] and intentional surface carbon-contaminated ZnO:N thin films [ZnO:N@C] were grown on quartz substrates by radio frequency magnetron sputtering deposition method. The structural, electrical and optical properties as well as chemical states of elements were investigated by means of X-ray diffraction (XRD), Hall effect measurement (Hall), UV–Vis–Near infrared spectrophotometer and X-ray photoelectron spectroscopy (XPS). The results indicate that surface carbon contamination almost does not affect the band gap of ZnO:N thin films but has a strong impact on the crystal quality of ZnO:N thin film surface and results in a significant increase in tensile stress. The XPS analysis shows that the effect of surface carbon contamination treatment on the chemical states of ZnO:N thin films is remarkable. Because the stability of Zn–N bonds in N-rich local environments is nowhere near that of those in O-rich local environments, the N atoms in N-rich local environments easily bond with surface carbon atoms to form undesirable C–N bonds, thus resulting in a decrease of NO acceptors in N-rich local environments. Obviously, it is unfavorable to subsequently prepare high stability of N-doped p-type ZnO thin films.

Tribological behavior of N-doped ZnO thin films by metal organic chemical vapor deposition under lubricated contacts

N-doped ZnO thin films were deposited on 304L stainless steel through the pyrolysis of zinc acetate and ammonium acetate in different ratios at a temperature of 420 °C using metal organic chemical vapor deposition. Compositional and structural analyzes of the films were performed by using Rutherford backscattering spectroscopy and X-ray diffraction. The frictional behavior of the thin films and 304L stainless steel substrate was evaluated using a ball-on-flat configuration with reciprocating sliding under marginally lubricated and fully flooded conditions. Al alloy (2017) was used as ball counterface, while basestock synthetic polyalfaolefin oil (PAO10) without additives was used as lubricant. The flat and ball counterface surfaces were examined to assess the wear dimension and failure mechanism. Under marginally lubricated condition, N-doped ZnO thin films provided significant reduction in friction, whereas the films have minimal or no effect in friction under fully flooded condition. N-doped ZnO thin films showed a significant effect in protecting the ball counterface as wear volume was reduced compared with that of the substrate under the marginally lubricated condition. Under the fully flooded condition, with the exception of one of the films, the wear volume of the N-doped ZnO thin films ball reduced compared with that of the substrate. In all the ball counterfaces for N-doped ZnO thin films under both conditions, wear occurred through abrasive mechanism of various degrees or mild polishing. Thus, superfluous lubrication of N-doped ZnO thin films is not necessary to reduce friction and wear.

Optical property and AC conductivity RF sputtered N-doped ZnO thin films

Optical property and AC conductivity RF sputtered N-doped ZnO thin films

Study of Structural Properties of N-Doped ZnO Thin Film Prepared by Reactive Gas-Timing RF Magnetron Sputtering

We examined structural properties of nitrogen doped (ZnO:N) thin films prepared by reactive RF magnetron sputtering technique in conjunction with gas timing method. The deposited films were polycrystalline ZnO in wurtzite structure. Morphology of the ZnO:N films could be modified by adjusting gas timing conditions. The x-ray photoelectron spectroscopy (XPS) and extended x-ray absorption fine structure (EXAFS) analysis showed that incorporation of nitrogen may cause structural distortion in the ZnO:N crystal.

High quality nitrogen-doped zinc oxide thin films grown on ITO by sol–gel method

Highly transparent N-doped ZnO thin films were deposited on ITO coated corning glass substrate by sol–gel method. Ammonium nitrate was used as a dopant source of N with varying the doping concentration 0, 0.5, 1.0, 2.0 and 3.0at%. The DSC analysis of prepared NZO sols is observed a phase transition at 150°C. X-ray diffraction pattern showed the preferred (002) peak of ZnO, which was deteriorated with increased N concentrations. The transmittance of NZO thin films was observed to be ~88%. The bandgap of NZO thin films increased from 3.28 to 3.70eV with increased N concentration from 0 to 3at%. The maximum carrier concentration 8.36×1017cm−3 and minimum resistivity 1.64Ωcm was observed for 3at% N doped ZnO thin films deposited on glass substrate. These highly transparent ZnO thin films can be used as a window layer in solar cells and optoelectronic devices.

Influence of annealing temperature on photoluminescence properties and optical constants of N-doped ZnO thin films grown on muscovite mica substrates

A sol–gel spin-coating method was used to synthesize N-doped ZnO (NZO) thin films on muscovite mica substrates; the films were then annealed at 200, 300, 400, and 500°C. The effects of the annealing temperature on their photoluminescence properties and optical constants were investigated. All the films had strong UV emissions in their photoluminescence spectra, but the green emissions at ~2.4eV were observed only for the annealed films. The average transmittance of all the films was about 80% in the visible range and the absorption edges in the UV range at 375nm depended strongly on the annealing temperature. The optical band gap of the films decreased gradually as the annealing temperature was increased up to 400°C, and the Urbach energy decreased significantly as the annealing temperature increased. Finally, the various optical constants, the dielectric constant, and the optical conductivity were measured for the un-annealed film and the film annealed at 500°C.

Effect of N and Al-N dual doping on optical, photoluminescence and transport properties of ZnO films

N-doped and Al-N dual-doped ZnO thin films have been synthesized on glass substrate by low-cost spray pyrolysis technique. Introduction of N and Al in ZnO affects the surface morphology and electro-optical properties of ZnO. SEM micrographs clearly show the formation of nano-scale semi-spherical and hexagonal crystalline grains. 3D AFM images revealed the formation of crystalline grains with array structure. Transparency of the films increased substantially for N and Al-N dual-doping in ZnO lattice in the measured optical spectrum range (380 to 1000 nm). Optical band gap calculations reveal that the un-doped, N-doped and Al-N dual-doped ZnO thin films are direct band gap semiconductors. The carrier concentration of undoped and Al-N dual-doped ZnO samples is found to be in the order of ∼1015 cm−3. Highly N (>6%) doped ZnO samples shows p-type behavior. Photoluminescence study confirms both near band edge (NBE) and the deep level (DLE) emissions.

원자층 증착법으로 성장된 ZnO 박막의 질소 도핑에 대한 연구

For feasible study of opto-electrical application regarding to oxide semiconductor, we implemented the N doped ZnO growth using a atomic layer deposition technique. The p-type ZnO deposition, necessary for ZnO-based optoelectronics, has considered to be very difficulty due to sufficiently deep acceptor location and self-compensating process on doping. Various sources of N such as N2, NH3, NO, and NO2 and deposition techniques have been used to fabricate p-type ZnO. Hall measurement showed that p-type ZnO was prepared in condition with low deposition temperature and dopant concentration. From the evaluation of photoluminescence spectroscopy, we could observe defect formation formed by N dopant. In this paper, we exhibited the electrical and optical properties of N-doped ZnO thin films grown by atomic layer deposition with NH3OH doping source.

High-efficiency micro-energy generation based on free-carrier-modulated ZnO:N piezoelectric thin films

The free-carrier-modulated ZnO:N thin film-based flexible nanogenerators (NZTF-FNGs) are proposed and experimentally demonstrated. The suggested flexible nanogenerators (FNGs) are fabricated using N-doped ZnO thin films (NZTFs) as their piezoelectric active elements, which are deposited by a radio frequency magnetron sputtering technique with an N2O reactive gas as an in situ dopant source. Considerable numbers of N atoms are uniformly incorporated into NZTFs overall during their growth, which would enable them to significantly compensate the unintentional background free electron carriers both in the bulk and at the surface of ZnO thin films (ZTFs). This N-doping approach is found to remarkably enhance the performance of NZTF-FNGs, which shows output voltages that are almost two orders of magnitude higher than those of the conventionally grown ZnO thin film-based FNGs. This is believed to be a result of both substantial screening effect suppression in the ZTF bulk and more reliable Schottky barrier formation at the ZTF interfaces, which is all mainly caused by the N-compensatory doping process. Furthermore, the NZTF-FNGs fabricated are verified via charging tests to be suitable for micro-energy harvesting devices.

Properties of p-type ZnO thin films with different orientations

The stable properties of N-doped p-type ZnO thin films with preferential nonpolar (100) plane orientation relative to polar (002) plane orientation are investigated. The two kinds of oriented thin films are fabricated by the methods of post heat treatment and double sources in situ, respectively. The Hall investigations demonstrate that N-doped p-type ZnO thin films with preferential nonpolar (100) plane orientation are more stable,and the results are also proved by build-in electric field model and electronic structure calculations of the films based on the first principle.

Influence of ${\hbox{N}}_{2}$ Gas Flow on the High-Frequency Magneto-Electrical Properties of ZnO Thin Films

Undoped and N-doped ZnO (ZnO:N) thin films were prepared with different N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> flow rates on Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (0001) substrates by rf magnetron sputtering methods. The structure and high-frequency magneto-electrical properties of the ZnO:N films varied drastically with the variation of N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> flow rate. With the introduction of N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas during deposition, short hexagonal-like nanorods grown at grain surface were observed. In comparison with the undoped ZnO film, Raman spectra of the ZnO:N films revealed four anomalous peaks at 276, 510, 586 and 644 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , which are attributed to nitrogen-related defect complexes. Complex impedance spectra of all the films were analyzed by an equivalent circuit, employing two sets of parallel resistance and capacitance components in series to represent the oxide grain and grain boundary contributions, respectively. The analyzed results have implied that the N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> flow rate can effectively alter the defect concentration of the films, and consequently adjust the ac conductivity, magneto-dynamical and dielectric relaxation behaviors of the oxide-based magnetic semiconductor polycrystalline films.

N-doped ZnO thin film for development of magnetic field sensor based on surface plasmon resonance

Magnetic-field-dependent optical properties of nitrogen-doped ZnO (ZnO:N) thin films were investigated using surface plasmon resonance (SPR) and a highly sensitive (4.65/Tesla) magnetic field sensor has been realized. The refractive index (RI) of ZnO:N film increases from 1.949 to 2.025 with increase in N doping from 0% to 10% demonstrating tunable RI. In contrast to pure ZnO, SPR curves for ZnO:N films exhibit a shift toward lower angles with increasing applied magnetic field from 0 to 35mT due to change in reflectance of light upon reflection from ferromagnetic surface. Results indicate promising application of ferromagnetic ZnO:N film as a magnetic field sensor.

Nitrogen and vacancy clusters in ZnO

Abstract

Influence of the substrate and nitrogen amount on the microstructural and optical properties of thin r.f.-sputtered ZnO films treated by rapid thermal annealing

N-doped ZnO (ZnO:N) thin films, intended to be used as one of the layers in solar cell applications were deposited by r.f. sputtering, using ZnN target (99.9% purity), on silicon and fused silica substrates. In the gas flow composition, Ar was kept constant (50%) and the O2/N2 ratio was varied as: 40%/10%, 25%/25% and 10%/40%. After deposition, rapid thermal annealing (RTA) at 400 and 550°C for 1min in N2 ambient has been performed. The RTA impact on the optical and microstructural properties of ZnO:N thin films have been investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM) coupled with selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDX), UV–vis–NIR spectroscopy, UV–vis–NIR spectroscopic ellipsometry (SE) and infrared ellipsometry (IR-SE). The as-deposited (ad) ZnO:N films are polycrystalline with preferentially oriented columnar crystals. After RTA we found ZnO:N films with improved crystallinity and fewer boundary defects. We report optical constants of ZnO:N from UV to IR spectral range as well as the infrared active phononic modes.

Investigation of structural, optical and luminescent properties of sprayed N-doped zinc oxide thin films

N-doped ZnO (NZO) thin films are synthesized via spray pyrolysis technique in aqueous medium treating zinc acetate and N,N-dimethylformamide as precursors. Influence of N doping on structural, optical and luminescence properties have been investigated. Films are nanocrystalline having hexagonal crystal structure. Raman analysis depicts an existence of NZnO structure in NZO thin film. XPS spectrum of N 1s shows the 400eV peak terminally bonded, well screened molecular nitrogen (γ-N2). Lowest direct band gap of 3.17eV has been observed for 10at% NZO thin film. The UV, blue, and green deep-level emissions in photoluminescence of NZO films are due to Zn interstitials and O vacancies.

Photocatalytic degradation of toluene using sprayed N-doped ZnO thin films in aqueous suspension

Thin films of N-doped ZnO are synthesized via spray pyrolysis technique in aqueous medium using zinc acetate and ammonium acetate as precursors. Influence of N doping onto photochemical, structural, morphological, optical and thermal properties have been investigated. Structural analysis depicts hexagonal (wurtzite) crystal structure. The effect of N doping on the photocatalytic activity of N-doped ZnO in the degradation of toluene is studied and results are compared with pure ZnO. The results show that the rate of degradation of toluene over N-doped ZnO is higher as compared to that of pure ZnO and increases with increasing N doping up to 10at.% and then decreases. The enhancement of photocatalytic activity of N-doped ZnO thin films is mainly due to their capability for reducing the electron hole pair recombination. The photocatalytic mineralization of toluene in aqueous solution has been studied by measuring COD and TOC. Possible reaction mechanism pathways during toluene degradation over N-doped ZnO has been proposed.

On the origin of an additional Raman mode at 275 cm−1 in N-doped ZnO thin films

A systematic investigation on the optical properties of N-doped ZnO thin films was performed in order to understand the origin of an additional Raman mode at 275 cm−1. This Raman peak was observable only at N2 pressures lower than 30 Pa during pulsed laser deposition. Its intensity decreased with an increase of N2 pressures and eventually vanished at pressures above 30 Pa. N substitution of O (NO) was identified by photoluminescence and x-ray photoelectron spectroscopy and correlated well with the Raman intensity. The electrical measurements showed significant changes in resistivity, charge carrier concentration, and mobility due to the presence of N acceptors. Investigations on undoped ZnO films grown in Ar without N2 further confirm that N doping plays a key role in the Raman scattering. The experimental data indicate that the Raman mode originates from NO related complexes, likely in the form of Zni-NO. These investigations help to understand the doping mechanisms and underlying physics of the additional Raman mode in the ZnO films.

Effect of In Doping on Thermoelectric and Magnetoresistive Properties of ZnO Films Prepared by RF Magnetron Sputtering

n-doped ZnO (ZnO:In) thin films with 0~9at.% In content have been prepared by RF high magnetron sputtering on glass substrate. All the films were annealed in N2at 473K for 1h. The effect of In doping concentration on the structure, surface morphology, electrical properties,Seebeck and magnetoresistive effect of the films were investigated. It was found that the crystal quality of the films degrades, and the surface gets rough with increasing In content. The lowest resistivity of the ZnO:In thin films 6.1×10-4Ω.cm occurs at 5at.% in the target doping concentration. All the films show a striking Seebeck effect and the absolute value of the Seebeck coefficient decreases under magnetic field. Meanwhile, a positive magnetoresistance of the films was found. ZnO thin films with 1at.% In-doped is best for the thermoelectric material, which has the largest power factor of 2.1×10-4W/K2m at room temperature.