Structural, electronic, and optical properties of nanocrystalline As-doped ZnO films on quartz substrates determined by Raman scattering and infrared to ultraviolet spectra

Nanocrystalline As-doped ZnO films with different laser power energy (40 mJ and 55 mJ) and As doping concentrations (C_As from 1% to 3%) have been grown on quartz substrates by pulsed laser deposition. The average grain size of the films was calculated from the (002) peak of x-ray diffraction patterns and is estimated to vary from 9 to 13 nm. Electronic transitions and optical properties of the films have been investigated by Raman scattering, far-infrared reflectance, and infrared-ultraviolet spectral transmittance technique. With increasing doping concentration, the A₁ longitudinal optical phonon mode shifts towards the lower energy side and can be described by (564-75C_As) cm^-1 owing to the increment of free carrier concentration. The E₁ transverse optical phonon frequency is located at about 415 cm^-1 and does not show an obvious decreasing trend with the C_As. The optical constants in the photon energy range of 0.5-6.5 eV have been extracted by fitting the experimental data with the Adachi's model. The refractive index dispersion in the transparent region can be well expressed by a Sellmeier's single oscillator function. Due to different doping concentration and hexagonal crystalline structure, the optical band gap of the films grown at 40 mJ linearly decreases with increasing As concentration. The phenomena agree well with the results from the theoretical calculations.     

Growth and characterization of ZnMgSrO thin films lattice-matched to ZnO and with deep-UV energy band gap

Quaternary Zn_1-x-yMg_xSr_yO films were grown and characterized in detail, which were observed to be lattice matched to the ZnO by the X-ray diffraction (XRD). Cathodoluminescence measurement showed that near-band UV emission peaks of the samples move toward higher energy as concentration of Mg and Sr increases, to 3.67 eV for the Zn_0.87Mg_0.08Sr_0.05O and to 4.02 eV for the Zn_0.72Mg_0.17Sr_0.11O. It was also observed by the scanning electron microscopy and the XRD that the films are single crystalline. It is believed that the ZnMgSrO films would be one of the important candidate materials for the high quality deep-UV optoelectronic devices.     

Effect of vacuum magnetic annealing on the structural and physical properties of the Ni and Al co-doped ZnO films

About 300 nm-thick Zn_0.87Al_0.06Ni_0.07O, Zn_0.83Al_0.06Ni_0.11O and Zn_0.81Al_0.04Ni_0.15O films were deposited on glass substrates at 300 K by co-sputtering ZnO:Al and Ni targets. The films were annealed in vacuum at 673 K for 2 h under a magnetic field of 4.8 × 10⁴ A/m applied along the film plane and then were cooled down to room temperature without magnetic field. All the films have a wurtzite structure and consist of thin columnar grains perpendicular to the substrate. The annealing promotes the (002) orientation growth in the film growing direction for the Zn_0.87Al_0.06Ni_0.07O and Zn_0.83Al_0.06Ni_0.11O films as well as the (100) orientation growth for the Zn_0.81Al_0.04Ni_0.15O film. The annealing results in a slight increase in the grain size. A weak Ni diffraction peak was detected for the annealed films with high Ni content. The annealing enhances the room temperature ferromagnetism of the films. A temperature dependence of magnetization confirms that the Curie temperature is above 400 K for the annealed films. The films magnetically annealed exhibit an anisotropic magnetization behavior. The annealed Zn_0.87Al_0.06Ni_0.07O film has the lowest resistivity (8.73 × 10⁻³ Ω cm), the highest free electron concentration (1.73 × 10²⁰ cm^− 3) and Hall mobility (4.16 cm²V^− 1 s^− 1). A temperature dependence of the resistivity from 50 K to 300 K reveals that the carrier transport mechanism is Mott's variable range hopping in the low temperature range and thermally activated band conduction in the high temperature range.     

Structural and optical properties of nitrogen doped ZnO films

Zinc oxide is getting an enormous attention due to its potential applications in a variety of fields such as optoelectronics, spintronics and sensors. The renewed interest in this wide band gap oxide semiconductor relies on its direct high energy gap (E_g ∼ 3.437 eV at low temperatures) and large exciton binding energy. However to reach the stage of device production the difficulty to produce in a reproducible way p-type doping must be overcome.In this study we discuss the structural and optical properties of ZnO films doped with nitrogen, a potential p-type dopant. The films were deposited by magnetron sputtering using different conditions and substrates. The composition and structural properties of the films were studied combining X-ray diffraction (XRD), Rutherford backscattering (RBS), and heavy ion elastic recoil detection analysis (HI-ERDA). The results show an improvement of the quality of the films deposited on sapphire with increasing radio-frequency (RF) power with a preferentially growth along the c-axis. The ERDA analysis reveals the presence of H in the films and a homogeneous composition over the entire thickness. The photoluminescence of annealed samples evidences an improvement on the optical quality as identified by the well structured near band edge recombination.     

High density plasma treatment of polyimide substrate to improve structural and electrical properties of Ga-doped ZnO films

We investigated the effects of a high density O₂ plasma treatment on the structural and electrical properties of sputter-deposited GZO films. The GZO films were deposited on polyimide substrate without substrate heating by RF magnetron sputtering from a ZnO target mixed with 5 wt.% Ga₂O₃. Prior to the GZO film growth, we treated a polyimide substrate with highly dense inductively coupled oxygen plasma. The optical transmittance of the GZO film, about 80%, was maintained regardless of the plasma pre-treatment. However, the resistivity of the film was strongly influenced by the plasma pre-treatment. The resistivity of the GZO film decreased from 1.02 × 10^− 2 Ω cm without an O₂ plasma pre-treatment to 1.89 × 10^− 3 Ω cm with an O₂ plasma pre-treatment.     

The properties of radio frequency sputtered transparent and conducting ZnO:F films on polyethylene naphthalate substrate

We report on the properties of ZnO:F films deposited by RF sputtering on polyethylene naphthalate (PEN) substrates and compared them with films deposited on glass. Detailed and systematic investigations of various properties of films were deposited on PEN substrates were carried out as functions of thickness and annealing ambient. The films were deposited at room temperature and annealed at 150 °C in either Ar or 7% H₂/Ar ambients. These films exhibited carrier concentrations between 2 × 10¹⁸/cm³ and 9.5 × 10¹⁹/cm³, mobility between 3 and 11 cm²/V-s, and resistivity between 10^− 1 and 10^− 2 Ω-cm. Hall mobility variation with concentration has been explained assuming ionized impurity and lattice scattering to be the dominant mechanisms. The transmission of the films varied from 68 to 80% with increasing thickness and the absorption edge was limited by the absorption of the PEN substrate. The mechanical flexibility of the films was measured in terms of its critical radius of bending which was determined from the onset of a sharp increase in electrical resistance. The critical radius varied between 6.5 and 17 mm for film thicknesses varying from 20 to 200 nm. The thickness dependence of critical strain and critical radius can be explained by Griffith defect theory.     

Effect of rf power on the properties of rf magnetron sputtered ZnO:Al thin films

Aluminum doped zinc oxide ZnO:Al thin films were prepared using rf magnetron sputtering. The preparation was performed at room temperature and low pressure with varying rf power between P = 50 W and P = 500 W. Structural, electrical and optical film properties were studied depending on rf power. Special attention was paid to correlations among film structure, sheet resistance and optical transmission. Films with largest crystallite size exhibited highest optical transmission, but not lowest electrical resistivity. An explanation for this finding was sought in terms of the amount of Al atoms incorporated in the films and the places they occupy, parameters which are in turn related to rf power.     

Structural, electrical and optical properties of ZnO:Al films deposited on flexible organic substrates for solar cell applications

Aluminum doped ZnO thin films (ZnO:Al) were deposited on glass and poly carbonate (PC) substrate by r.f. magnetron sputtering. In addition, the electrical, optical properties of the films prepared at various sputtering powers were investigated. The XRD measurements revealed that all of the obtained films were polycrystalline with the hexagonal structure and had a preferred orientation with the c-axis perpendicular to the substrate. The ZnO:Al films were increasingly dark gray colored as the sputter power increased, resulting in the loss of transmittance. High quality films with the resistivity as low as 9.7 × 10^− 4 Ω-cm and transmittance over 90% have been obtained by suitably controlling the r.f. power.     

Optical and ferromagnetic characteristics of Mn doped ZnO thin films grown by filtered cathodic vacuum arc technique

Mn doped ZnO (ZnO:Mn) thin films with ~ 10 at.% of Mn were grown on quartz substrates by filtered cathodic vacuum arc (FCVA) technique at low substrate temperature (≤ 200 °C). The influence of substrate temperature and oxygen flow rate on the optical, electrical and magnetic properties of the ZnO:Mn thin films was studied. Both room temperature ferromagnetism and ultraviolet photoluminescence were observed in all films. A maximum saturation moment of 2.9 × 10⁻²⁴ A m²/Mn can be achieved for the films grown in an optimum condition. This suggests that the fabrication of high-quality ZnO:Mn films by FCVA technique has the potential to realize efficient magneto-optic devices operating at ultraviolet regime.     

Changes in electrical and structural properties of indium oxide thin films through post-deposition annealing

The annealing effects on the electrical properties and microstructures of indium oxide (In₂O₃) thin films were investigated. The In₂O₃ thin films with the thickness of about 150 nm were annealed at various temperatures ranging from 100 to 600 °C in air after the sputtering deposition. It was found that the carrier density of the In₂O₃ thin films decreased with increasing in the annealing temperature and then started to increase at a certain temperature. This indicated that the reduction of the In₂O₃ thin films was promoted at high annealing temperature. The Hall mobility of the In₂O₃ thin films increased through the reduction; furthermore, the d-spacing of the In₂O₃ crystal lattice plane tended toward ideal value. It can be believed from these results that one of the principal electron scattering in the In₂O₃ thin films is attributed to excess oxygen atoms that expand the d-spacing.     

Fluorine-doped ZnO transparent conducting thin films prepared by radio frequency magnetron sputtering

Fluorine-doped ZnO transparent conducting thin films were prepared by radio frequency magnetron sputtering at 150 °C on glass substrate. Thermal annealing in vacuum was used to improve the optical and electrical properties of the films. X-ray patterns indicated that (002) preferential growth was observed. The grain size of F-doped ZnO thin films calculated from the full-width at half-maximum of the (002) diffraction lines is in the range of 18-24 nm. The average transmittance in visible region is over 90% for all specimens. The specimen annealed at 400 °C has the lowest resistivity of 1.86 × 10^− 3 Ω cm, the highest mobility of 8.9 cm² V^− 1 s^− 1, the highest carrier concentration of 3.78 × 10²⁰ cm^− 3, and the highest energy band gap of 3.40 eV. The resistivity of F-doped ZnO thin films increases gradually to 4.58 × 10^− 3 Ω cm after annealed at 400 °C for 4 h. The variation of the resistivity is slight.     

Phase transition and optical properties of Ge doped ZnO films synthesised by sputtering

Ge doped ZnO films were deposited on Si substrates by sputtering technique. With the increasing annealing temperature, the crystal quality of samples becomes gradually better and the phase transition can be observed at annealing temperature of 600°C. X-ray photoelectron spectroscopy results show the incorporation of Ge into the ZnO films with 14·81 at-%Ge content. Fourier transform infrared spectroscopy absorption spectra of samples annealed at above 600°C display vibration mode of ν (ZnO₄) and ν (GeO₄) in Zn₂GeO₄. The enhancement of ultraviolet emission intensity should be attributed to the yielded mass holes caused by Ge doping and the rising crystal quality. The sample annealed at 800°C displays the strongest blue emission due to the native defects in Zn₂GeO₄ films or/and surface defects.     

Thermoelectric properties of nanostructured Al-substituted ZnO thin films

ZnO:Al nano-polycrystalline thin films were deposited by radio-frequency magnetron sputtering on glass substrates. The analysis of the morphology reveals well-connected whiskers with a preferred c-axis orientation perpendicular to the substrate and a dense columnar grain structure. The as-deposited films exhibited a low electrical resistivity of 1 × 10^− 3 Ω cm. Annealing in air produces an increase of the resistivity by more than three orders of magnitude and an increase in the absolute value of the Seebeck coefficient proportional to the resistivity. Annealing of the as-deposited sample in reducing Ar/H₂ atmosphere leads to a decrease in both the resistivity and the absolute value of the Seebeck coefficient. The change in the electrical transport properties is caused by the absorption and desorption of oxygen. Both resistivity and Seebeck coefficient recover to their initial values during annealing of the air-treated sample in reducing Ar/H₂ atmosphere, indicating a reversible process. The analysis by transmission electron microscopy after annealing reveals a stable columnar grain structure with an increase of the grain size. The increase in grain size is larger when the sample is annealed in reducing rather than in oxidising atmosphere. In summary, the reducing Ar/H₂ atmosphere was found to be advantageous for the thermoelectric properties resulting in a maximum power factor of 0.3 mW/K²m at 800 K.     

Structural properties of low-temperature grown ZnO thin films determined by X-ray diffraction and X-ray absorption spectroscopy

The epitaxial growth of ZnO thin films on Al₂O₃ (0001) substrates have been achieved at a low-substrate temperature of 150 °C using a dc reactive sputtering technique. The structures and crystallographic orientations of ZnO films varying thicknesses on sapphire (0001) were investigated using X-ray diffraction (XRD). We used angle-dependent X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy to examine the variation of local structure. The XRD data showed that the crystallinity of the film is improved as the film thickness increases and the strain is fully released as the film thickness reached about 800 Å. The Zn K-edge XANES spectra of the ZnO films have a strong angle-dependent spectral feature resulting from the preferred c-axis orientation. The wurtzite structure of the ZnO films was explicitly shown by the XRD and EXAFS analysis. The carrier concentration, Hall mobility and resistivity of the 800 Å-thick ZnO film were 1.84 × 10¹⁹ cm^− 3, 24.62 cm²V^− 1s^− 1, and 1.38 × 10^− 2 Ω cm, respectively.     

Investigation on Mn doped ZnO thin films grown by RF magnetron sputtering

In this paper, we have investigated the Zn_1 − x Mn_xO (x = 0.05, 0.10 and 0.15) thin films grown by RF magnetron sputtering. The grown films on sapphire [Al₂O₃(0001)] substrates have been characterized using X-ray Diffraction (XRD), Photoluminescence (PL) and Vibrating Sample Magnetometer (VSM) in order to investigate the structural, optical and magnetic properties of the films respectively. It is observed from XRD that all the films are single crystalline with (002) preferential orientation along c-axis. PL spectra reveal that the addition of Mn marginally shifts the Near Band Edge (NBE) position towards the higher energy side. The magnetic measurements of the films using VSM clearly indicate the ferromagnetic nature.     

Structural, optical, and magnetic properties of Fe-doped ZnO films prepared by spray pyrolysis method

Zn_1−xFe_xO thin films with different Fe content were deposited on glass substrates at 450 °C by spray pyrolysis technique. The effect of doping on the structural and optical properties of ZnO films was investigated. X-ray diffraction has shown that the films are polycrystalline and textured with the c-axis of the wurtzite structure along the growth direction. Scanning electron microscopy has shown that the surface of the films are homogeneous. The magnetic measurements performed at 5 K using a SQUID magnetometer showed the co-existence of paramagnetic, antiferromagnetic and ferromagnetic contributions.     

Structural and optical properties of high quality ZnO thin film on Si with SiC buffer layer

ZnO thin films are grown on Si substrates with SiC buffer layer using ion plasma high frequency magnetron sputtering. These substrates are fabricated using a technique of solid phase epitaxy. With this technique SiC layer of thickness 20-200 nm had been grown on Si substrates consisting pores of sizes 0.5-5 μm at SiC and Si interface. Due to mismatching in lattice constants as well as thermal expansion coefficients, elastic stresses have been developed in ZnO film. Pores at the interface of SiC and Si are acting as the elastic stress reliever of the ZnO films making them strain free epitaxial. ZnO film grown on this especially fabricated Si substrate with SiC buffer layer exhibits excellent crystalline quality as characterized using X-ray diffraction. Surface topography of the film has been characterized using Atomic Force Microscopy as well as Scanning Electron Microscopy. Chemical compositions of the films have been analyzed using Energy Dispersive X-ray Spectroscopy. Optical properties of the films are investigated using Photoluminescence Spectroscopy which also shows good optical quality.     

Influence of vanadium concentration on the microstructure and magnetic properties of V-doped ZnO thin films

Vanadium doped ZnO thin films (Zn_1 − xV_xO, where x = 0.05 or x = 0.13) were grown on c-cut sapphire substrates using pulsed laser deposition technique. Their structure and magnetic properties were examined in relation to the doping concentration. All deposited films were highly oriented along the c-axis and exhibited ferromagnetic behavior with a Curie temperature up to 300 K. The crystal structure was found to be better for layers with lower vanadium concentration. The films had a porous fine-grained microstructure and a column-like character as the V concentration was reduced. A weak dependence of magnetization on temperature was observed. The saturation magnetization was found to be strongly dependent on the crystal structure, grain size and V-ion concentration.     

Synthesis,characterization and photoresponse study of undoped and transition metal (Co,Ni, Mn) doped ZnO thin films

The synthesis, characterization and photoresponse studies of undoped and transition metal doped zinc oxide thin films are carried out in this work, in prospect of visible light photo detection and sensor applications. The undoped and transition metal ions such as, Co, Ni and Mn doped ZnO films in this study were synthesized by chemical solution deposition, involving spin-coating. We have characterized the deposited films using X-ray diffraction, scanning electron microscopy, photoluminescence and UV–vis spectroscopy studies. The devices of the films for photoresponse study were fabricated by top Ag contacts on the film surface in metal–semiconductor–metal configuration. The current–voltage characteristics and switching measurements of these devices were studied under the illumination of an incandescent lamp. We found a high ON/OFF ratio of 8 and highest photocurrent density of 0.7 mA/cm² for Ni doped ZnO film.