Na-doped ZnO Thin Films Research Articles

Influence of Wrinkle Structure on Properties of Na-doped ZnO Films

Un-doped ZnO and Na-doped ZnO (NZO) thin films with various Na doping concentrations were successfully fabricated on glass substrates by a sol-gel process. The effect of Na ions at concentrations of 0, 1, 2, 3, 4, and 5% on the crystalline structure, surface morphology, and optoelectronic properties of ZnO films was studied using appropriate measurement techniques. XRD analysis revealed a polycrystalline hexagonal wurtzite structure in the NZO films. The NZO films' average crystal size ranged from 17.7 nm to 19.6 nm. SEM micrographs showed that wrinkle network appeared in the pure and doped ZnO films due to solvent evaporation during the thermal annealing process. UV-Vis spectroscopy pointed out that the average transmittance of NZO films is as large as ca. 78%. The optical bandgap of the films varied slightly between 3.235 eV for the pure ZnO and 3.246 eV for the Na 5% doped NZO film. When Na ions were doped into the NZO films, the electrical conductivity improved. Moreover, the absorption figure of merit exhibited the largest value of 0.981 Ω-1cm-1 for the fabricated NZO films, which enables the use of these films for solar cells and for other optoelectronic applications.

Effects of substrate temperature and sodium source temperature on Na doping of ZnO by MOCVD

In this paper, Na-doped ZnO thin films were grown by MOCVD using cyclopentadienylsodium (CpNa) as the sodium source. The effects of substrate temperature and sodium source temperature on the properties of Na-doped ZnO thin films were studied. SIMS measurement verified that higher substrate temperature would result in higher Na doping content in ZnO:Na thin films. SEM and Hall measurements revealed that both surface morphology and electrical properties of ZnO:Na thin films showed nonlinear variation with substrate temperature and sodium source temperature. When the substrate temperature or the sodium source temperature was increased, the surface morphology of ZnO:Na thin films turned to sheet structures tilted in all directions first and then back to perpendicular hexagonal structure dominant morphology, and the hole concentration increased first and then decreased. This nonlinear variation trend was ascribed to be related to the variation of doping content and impurity position in ZnO:Na thin films, especially the Na impurity position.

Effects of Na Doping on Structural, Optical, and Electronic Properties of ZnO Thin Films Fabricated by Sol–Gel Technique

Undoped and Na-doped ZnO thin films were fabricated by sol–gel technique on quartz glass substrates and annealed at 500°C for 1 h. The structural properties of the films were characterized using x-ray diffraction analysis, which revealed hexagonal wurtzite structure with no peaks corresponding to Na2O or other Na phases being found. Surface morphology observations by scanning electron microscopy revealed that the crystallite size and topographical properties of the ZnO films were influenced by the Na doping concentration. X-ray photoelectron spectra revealed presence of Na+ in ZnO regime. The transmittance spectra indicated that the average transmittance of Na-doped ZnO film was above 80% in the visible range, superior to that of the undoped film. There was a blue-shift in the ultraviolet absorption edge with increase of the Na content. Photoluminescence spectra illustrated two peaks, corresponding to ultraviolet near-band-edge and visible emission.

Effect of Substrate Temperatures on the Structural and Optical Properties of Na-Doped ZnO Thin Films

Effect of Substrate Temperatures on the Structural and Optical Properties of Na-Doped ZnO Thin Films

Structural and optical properties of Na-doped ZnO films

Zn1-xNaxO (x = 0.0–0.05) solutions have been synthesized by the sol-gel technique using Zinc acetate dihydrate and Sodium acetate which were dissolved into solvent and chelating agent. Na-doped ZnO nanoparticles were obtained from solutions to find phase and crystal structure. Na-doped ZnO films have been deposited onto glass substrate by using sol-gel dip coating system. The effects of dopant concentration on the structure, morphology, and optical properties of Na-doped ZnO thin films deposited on glass substrate are investigated. Characterization of Zn1-xNaxO nanoparticles and thin films are examined using differential thermal analysis (DTA)/thermogravimetric analysis (TGA), Scanning electron microscope (SEM) and X-Ray diffractometer (XRD). Optical properties of Zn1-xNaxO thin films were obtained by using PG Instruments UV–Vis–NIR spectrophotometer in 190–1100 nm range. The structure, morphology, and optical properties of thin films are presented.

Effect of NaZn/Nai ratio on structural, optical, and electrical properties of Na-doped ZnO thin films

In this paper, Na-doped ZnO (NZO) thin films were prepared via a dip-coating sol-gel method with Na doping content in a range of 0 at. %–3 at. %. X-ray photoelectron spectroscopy analysis showed that the NaZn/Nai ratio (r) increased with Na doping contents from 1 at. %–3 at. %. The effect of the NaZn/Nai ratio on the structural, optical, and electrical properties of NZO thin films was investigated. As the NaZn/Nai ratio increased, the lattice constant exhibited a continuous increase and resulted in the presence of increasing compressive stress in the NZO films. When the NaZn/Nai ratio was less than 1 (r<1), the electron concentration of NZO films increased continuously. However, when r>1, it decreased. The Hall mobility was mainly limited by ionized impurity scattering and displayed a decrease trend with the increasing NaZn/Nai ratio. Due to the Burstein-Moss effect, the optical energy band gaps (Eg) of NZO films got slightly enlarged as compared with that of the undoped ZnO film. Considering the relationship between the Burstein-Moss shift and the carrier concentration, the enlargement of Eg was actually determined by the NaZn/Nai ratio as well as the doping content.

Na doping effects on the structural, conduction type and optical properties of sol–gel ZnO thin films

In this work we report on the sol–gel deposition of Na doped ZnO thin films on quartz substrates. The effects of Na doping concentrations (0, 3, 6 and 9 at.%) on structural, morphological, electrical and optical properties of the synthesized films were systematically investigated by X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), Hall-effect measurements, UV–Vis–NIR spectrophotometry and photoluminescence (PL) spectroscopy. XRD patterns showed that the prepared films were highly c-axis oriented exhibiting hexagonal wurtzite structure of ZnO. Raman spectra of All the ZnO films exhibited similar scattering peaks corresponding to the Raman active modes of ZnO wurtzite hexagonal structure. AFM images indicated that grain size and surface roughness of the films were affected by Na doping. From the Hall-effect measurements, it was found that carrier type is dependent on Na content. The UV–Vis–NIR spectroscopy analyses revealed that all the films were highly transparent in the visible region. Room temperature PL spectra demonstrated that UV emission of the Na-doped ZnO thin films depends on the Na concentration. In particular, the undoped sample shows the highest emission intensity, while the p-type Na doped ZnO film at 3 at.% exhibits the lowest one.

Effects of Na content on structural and optical properties of Na-doped ZnO thin films prepared by sol–gel method

In this paper, Na-doped ZnO (NZO) thin films with doping contents of 3–30at.% were synthesized on quartz glass substrates by sol–gel spin coating method. The structure, morphology, optical transmittance and photoluminescence properties of NZO films were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), atomic force microscope (AFM), UV–VIS spectrometer and fluorescence spectrophotometer, respectively. The results showed that all the NZO thin films exhibit a strong preferential c-axis orientation with a hexagonal polycrystalline structure. An apparent increase in crystallinity was observed with the increasing Na content. The surface of the heavily Na doped samples (30at.%) exhibits nanorods morphology. The RMS roughness changed from 16.219nm to 53.072nm with the increase of Na concentrations. As Na doping content increases to 24at.%, the average transmittance was still larger than 62% in the visible range. The optical band gap initially increased and then decreased nearly linearly with the increase of Na content. The refractive index of NZO films in the visible range was found to increase gradually with increasing Na dopants. Room temperature photoluminescence (PL) spectra showed sharp ultraviolet emissions centered at 388nm and broad green–yellow emissions (450–650nm). The violet emission centered at 424nm appeared in the spectra of 30at.% NZO. In addition, it was found that the low Na doping concentration contributed to the enhancement of INBE/IDLE.

Effect of Na contents on fabrication of p-type non-polar m-plane ZnO films

We report on the growth and characterization of Na-doped non-polar ZnO thin films, which have been prepared on m-plane sapphire substrates by plasma-assisted molecular beam epitaxy. The effects of Na contents on structural, morphological, electrical, and optical properties of Na-doped non-polar m-plane ZnO films are investigated. All the doped thin films have uniform m-plane orientation, which benefit from Na-doping. Na content plays a key role in determining the conduction of the ZnO films. An optimized result with a hole concentration of 5.3×1016cm−3, a Hall mobility of 0.22cm2V−1s−1, and a resistivity of 530Ωcm is achieved at beam equivalent pressure of the elemental Na source of 8.7×10−9Torr, and the films are electrically stable over several months.

Local p-type conduction of Na-doped ZnO thin films grown by MOCVD

Growth of Na-doped ZnO thin films by low-pressure metal–organic chemical vapor deposition method was realized using cyclopentadienylsodium as the sodium source. Secondary ion mass spectrometry measurements revealed the existence of Na in the ZnO thin films. Low-temperature scanning tunneling microscopy and scanning tunneling spectroscopy were used for obtaining local density of states (LDOS) of the Na-doped ZnO thin films at nanometer scale. The local measurements indicated a spatial distribution of the LDOS as well as the conductivity type that could be linked to spatial variations of Na doping level.

Influence of Film Thickness and Oxygen Partial Pressure on Cation-Defect-Induced Intrinsic Ferromagnetic Behavior in Luminescent p-Type Na-Doped ZnO Thin Films

In this article, we have investigated the effect of oxygen partial pressure (PO2) and film thickness on defect-induced room-temperature (RT) ferromagnetism (FM) of highly c-axis orientated p-type Na-doped ZnO thin films fabricated by pulse laser deposition (PLD) technique. We have found that the substitution of Na at Zn site (NaZn) can be effective to stabilize intrinsic ferromagnetic (FM) ordering in ZnO thin films with Curie temperature (TC) as high as 509 K. The saturation magnetization (MS) is found to decrease gradually with the increase in thickness of the films, whereas an increase in "MS" is observed with the increase in PO2 of the PLD chamber. The enhancement of ferromagnetic signature with increasing PO2 excludes the possibility of oxygen vacancy (VO) defects for the magnetic origin in Na-doped ZnO films. On the other hand, remarkable enhancement in the green emission (IG) are observed in the photoluminescence (PL) spectroscopic measurements due to Na-doping and that indicates the stabilization of considerable amount of Zn vacancy (VZn)-type defects in Na-doped ZnO films. Correlating the results of PL and X-ray photoelectron spectroscopy (XPS) studies with magnetic measurements we have found that VZn and Na substitutional (NaZn) defects are responsible for the hole-mediated FM in Na-doped ZnO films, which might be an effective candidate for modern spintronic technology.

Effect of Na-Doped Concentration on the Structure and Optical Properties of ZnO Thin Films

Na-doped ZnO thin films were deposited on microscope glass substrates by sol-gel spin coating method, the Na/Zn ratio were 0at.%, 5at.%, 7.5at.%, 10at.%, 15at.%. The crystal structures, surface morphology, and optical properties were analyzed by X-ray diffraction, scanning electron microscopy, ultraviolet–visible spectrophotometer, respectively. The results show that all the films are preferentially oriented along the c-axis perpendicular to the substrate surface. With the increase of the doping concentration, the roughness of the surfaces decrease and grain size grows from 17.1nm to 21.7nm, the sample with 10at.% Na exhibits best crystallinity and has lowest strain along the c-axis. The average optical transparency of the samples is higher than 70%, optical band gaps are between 3.213eV and 3.289eV.

Optical constants of Na-doped ZnO thin films by sol–gel method

The structural and optical properties of pure and Na-doped ZnO thin films have been investigated by X-ray diffraction (XRD), atom force microscopy and UV–Vis spectrophotometer. The crystal structure of all the thin films is the hexagonal wurtzite. The average grain size and surface roughness increases with the increase of the Na/Zn ratio. The optical band gap of the thin films decreases from 3.26 to 3.12 eV by increasing the Na/Zn ratio from 0.0 to 0.10. Transmittance spectra were used to determine the optical constants of the thin films, and the effect of Na/Zn ratio on the optical constants was investigated. With the increase of Na/Zn ratio, the refractive index decreases and the extinction coefficient increases in the 380–700 nm spectral range.

Effect of Na concentrations on microstructure and optical properties of ZnO films

Na-doped ZnO thin films were deposited on the glass substrates using sol-gel method. The effect of Na concentrations on the structural and optical properties of ZnO films was studied. As Na concentration increases from 0.0 at% to 16.0 at%, preferential c-axis orientation becomes more and more obvious, and the intensity of the diffraction peaks from (103) increases. The optical band gap Eg value increases from 3.261 to 3.286 eV first and then decreases as Na concentration increases from 0.0 to 2.0 at% and then beyond 2.0 at%. The intensity of all the emissions increases with increasing Na concentration and the origins of the violet emission (wavelength in the 400–407 nm) and the blue emission (wavelength at 473 nm) were discussed in detail.

Effect of preheating temperatures on microstructure and optical properties of Na-doped ZnO thin films by sol–gel process

The chemical composition, crystalline structure, surface morphology and photoluminescence spectra of Na-doped ZnO thin films with different heat treatment process were investigated by X-ray photoelectron spectroscopy, X-ray diffraction, atomic force microscopy and a fluorescence spectrometer. The results show that preferred orientation, residual stress, average crystal size and surface morphology of the thin films are strongly determined by the preheating temperature. The effects of preheating temperature on microstructure and surface morphology have been discussed in detail. The photoluminescence spectra show that there are strong violet & UV emission, blue emission and green emission bands. The violet & UV emission is ascribed to the electron transition from the localized level below the conduction band to the valence band. The blue emission is attributed to the electron transition from the shallow donor level of oxygen vacancies to the valence band, and the electron transition from the shallow donor level of interstitial zinc to the valence band. The green emission is assigned to the electron transition from the level of ionized oxygen vacancies to the valence band.

Enhanced photo-induced hydrophilicity of the sol–gel-derived ZnO thin films by Na-doping

Na-doped ZnO thin films with different Na/Zn ratio were prepared by sol–gel method. The microstructure, chemical composition, surface morphology, and wettability of the thin films were investigated by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, and water contact angle apparatus. The relation of wettability and Na/Zn ratio has been studied in detail. The wetting behavior of the thin films can be reversibly switched from hydrophobic to hydrophilic, through alternation of UV illumination and dark storage (or thermal treatment). Photo-induced hydrophilicity of the thin films increases with increasing Na/Zn ratio up to 0.08 and then decreases. The mechanism can be attributed to surface nanostructure and the concentration of Na doping.

Preparation and characterization of Na-doped ZnO thin films by sol–gel method

Undoped and Na-doped ZnO thin films were deposited on Si(1 1 1) substrate by the sol–gel method. Microstructure, surface topography and optical property of the thin films have been investigated. X-ray diffraction analysis showed that all the thin films had a polycrystalline hexagonal wurtzite structure. The 8 at% Na-doped ZnO thin films exhibited high c-axis preferred orientation. Surface topography revealed that average grain size of ZnO thin films annealed at 873 K increased initially, and then decreased with increasing Na concentration, and average grain size of 8 at% Na-doped ZnO thin films increased with increasing annealing temperature. PL spectra showed that all the thin films produced violet and yellow–green emissions in the visible region. In addition, the effects of Na concentration and annealing temperature on microstructure, surface topography and PL spectra are discussed.

Influence of Buffer Layers on the Structure and Optical Properties of Na-Doped ZnO Thin Films Prepared by Sol–Gel Method

Na-doped ZnO thin films on Si (100) substrates and on pure ZnO buffer layers prepared by sol-gel method, with the pure ZnO buffer layer annealing at 1073K for 90 min. X-ray diffraction (XRD) analyses revealed that all the films had a polycrystalline wurtzite structure and high c-axis preferred orientation. Photoluminescence(PL) spectra showed that the thin films produced strong emissions near ultraviolet(UV) and weak defect-related deep-level emission in visible regions. In addition, the effects of pure ZnO buffer layers on the structural and optical properties are discussed.

Influence of Annealing Condition on The Structure and Optical Properties of Na-doped ZnO Thin Films Prepared by Sol–Gel Method

Na-doped ZnO thin films were deposited on Si (1 0 0) substrates by sol–gel method. X-ray diffraction (XRD) analyses revealed that all the films had a polycrystalline wurtzite structure and high c-axis preferred orientation. PL spectra showed that the thin films produced strong emissions near ultraviolet (UV) and weak, defect-related deep-level emissions in visible regions. In addition, the effects of annealing condition on the structural and optical properties are discussed.

Structural, Electrical, and Optical Properties of Na-Doped ZnO Thin Films Deposited by Pulsed Laser Deposition

Na-doped ZnO thin films were deposited on quartz substrates at various temperatures by using pulsed laser deposition technique. An X-ray diffractometer and an atomic force microscope were used to investigate the structural and morphological properties of the thin films. A Hall effect measurement system was used to investigate the electrical properties of the thin films. A spectrophotometer was used to measure the transmittances of the thin films. The band gap energies of the thin films were calculated by linear fitting the sharp absorption edge for high-quality thin film. The band gap energies of the Na-doped ZnO thin films are nearly the same as the pure ZnO. A spectrometer was used to investigate the luminescent properties of the thin films. The thin film deposited at 200 degrees C had no near band edge emission and no deep-level emission. The NBE emission appeared and increased with increasing the growth temperature.