Temperature dependent studies on radio frequency sputtered Al doped ZnO thin film (original) (raw)
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E3S Web of Conferences, 2017
Transparent Conductive Oxides (TCOs) characterized by high visible transmittance and low electrical resistivity play an important role in photovoltaic technology. Aluminum doped zinc oxide (AZO) is one of the TCOs that can find its application in thin film solar cells (CIGS or CdTe PV technology) as well as in other microelectronic applications. In this paper some optical and electrical properties of ZnO:Al thin films deposited by RF magnetron sputtering method have been investigated. AZO layers have been deposited on the soda lime glass substrates with use of variable technological parameters such as pressure in the deposition chamber, power applied and temperature during the process. The composition of AZO films has been investigated by EDS method. Thickness and refraction index of the deposited layers in dependence on certain technological parameters of sputtering process have been determined by spectroscopic ellipsometry. The measurements of transmittance and sheet resistance were also performed.
Keywords: Al–F co-doped ZnO Transparent conducting oxide (TCO) Thin film magnetron sputtering Substrate temperature a b s t r a c t ZnO is a wide bandgap semiconductor that has many potential applications such as solar cells, thin film transistors, light emitting diodes, and gas/biological sensors. In this study, a composite ceramic ZnO target containing 1 wt% Al 2 O 3 and 1.5 wt% ZnF 2 was prepared and used to deposit transparent conducting Al and F co-doped zinc oxide (AFZO) thin films on glass substrates by radio frequency magnetron sputtering. The effect of substrate temperatures ranging from room temperature (RT) to 200 • C on structural, morphological , electrical, chemical, and optical properties of the deposited thin films were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), Hall effect measurement, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and UV–vis spectrophotometer. The XRD results showed that all the AFZO thin films had a (0 0 2) diffrac-tion peak, indicating a typical wurtzite structure with a preferential orientation of the c-axis perpendicular to the substrate. The FE-SEM and AFM analyses indicated that the crystallinity and grain size of the films were enhanced while the surface roughness decreased as the substrate temperature increased. Results of Hall effect measurement showed that Al and F co-doping decreased the resistivity more effectively than single-doping (either Al or F doping) in ZnO thin films. The resistivity of the AFZO thin films decreased from 5.48 × 10 −4 to 2.88 × 10 −4-cm as the substrate temperature increased from RT to 200 • C due to the increased carrier concentration and Hall mobility. The optical transmittances of all the AFZO thin films were over 92% in the wavelength range of 400–800 nm regardless of substrate temperature. The blue-shift of absorption edge accompanied the rise of the optical band gap, which conformed to the Burstein-Moss effect. The developed AFZO thin films are suitable as transparent conducting electrodes for various optoelectronic applications.
Applied Surface Science, 2012
A simple but scalable approach to the production of surface-textured Al-doped ZnO(AZO) films for lowcost transparent electrode applications in thin-film solar cells is introduced in this study by combining pulsed dc magnetron sputtering (PDMS) with wet etching in sequence. First, structural, electrical, and optical properties of the AZO films prepared by a PDMS were investigated as functions of deposition temperature to obtain transparent electrode films that can be used as indium-free alternative to ITO electrodes. Increase in the deposition temperature to 230 • C accompanied the improvement in crystalline quality and doping efficiency, which enabled the lowest electrical resistivity of 4.16 × 10 −4 cm with the carrier concentration of 1.65 × 10 21 cm −3 and Hall mobility of 11.3 cm 2 /V s. The wet etching of the films in a diluted HCl solution resulted in surface roughening via the formation of crater-like structures without significant degradation in the electrical properties, which is responsible for the enhanced light scattering capability required for anti-reflective electrodes in thin film solar cells.
In this study, a qualitative relationship between the surface and bulk electronic states for Al-doped ZnO (AZO) thin films (thickness<260 nm) is established. To this end, AZO films were deposited on soda lime glass substrates by varying substrate temperature (Ts) from 303K to 673K in RF magnetron sputtering. All these AZO films are found to have grown in ZnO hexagonal wurtzite structure with strong (002) orientation of the crystallites and with an average transmittance of 84%–91% in the visible range. Room temperature scanning tunneling spectroscopy measurements reveal semiconducting behavior for the films deposited at Ts373K and semi-metallic behavior for those deposited at Ts>373 K. Further, these films show two modes of electron tunneling, (a) direct tunneling at lower bias voltage and (b) FN tunneling at higher bias voltage, with transition voltage (Vtrans) shifting towards lower bias voltage (and thereby reducing the barrier height (U)) with increasing Ts. This is attributed to additional (local) density of states near the Fermi level of these AZO films because of higher carrier concentration (ne) at increased Ts. Thus, qualitatively, the behavior in both the local surface electronic states and bulk state electronic properties for these deposited AZO films are found to follow similar trends with increasing Ts. The variation in local barrier heights (indicative of the local surface electronic structures) across the AZO film surface is found to be smaller for the films deposited at Ts373 K, where semiconducting behavior is observed and wider for the semi-metallic AZO films deposited at higher Ts>373 K, indicating a larger inhomogeneity of local surface electronic properties at higher bulk carrier concentration.
In thin film solar cell technologies, room temperature deposition of highly conductive Al-doped ZnO thin films as transparent conductive oxide is one of the most important factors that prevents the distortion of the bottom layers. In this work, we prepared Al-doped ZnO targets at different heat-treatment pressures including 10 3 mbar (air), 10 −2 mbar (low-vacuum) and 10 −5 mbar (high-vacuum). The results indicated that by performing the heat-treatment in vacuum under a pressure of 10 −2 mbar (low-vacuum heat-treatment), the electrical resistivity of the AZO target reached the lowest value compared to the other heat-treatment conditions. Subsequently, the AZO thin films with a thickness of 1150 nm were deposited on soda-lime glass substrates by RF magnetron sputtering at room temperature. The XRD patterns of all the as-deposited AZO thin films clearly showed that the thin films had a wurtzite structure with a preferred orientation along the c-axis. Furthermore, the Hall effect measurement results confirmed that the AZO thin film prepared from the low-vacuum heat-treated target had the excellent electrical properties in comparison with the other ones. The optical band gap of the as-deposited AZO thin films was about 3.15 eV and the minimum value of the electrical resistivity was measured about 1.8 × 10 −4 Ω cm.
Effect of substrates and post-deposition annealing on rf-sputtered Al-doped ZnO (AZO) thin films
Journal of Materials Science: Materials in Electronics, 2019
Al-doped zinc oxide (AZO) thin films were deposited on glass, quartz, and indium tin oxide (ITO) substrates using rf magnetron sputtering. The influence of the substrate material and post-deposition annealing (300-700 °C) in air on the structural, optical and electrical properties were studied. All as-deposited and annealed films were investigated using X-ray diffraction (XRD), grazing incidence XRD, field effect scanning electron microscope, Raman spectroscopy, UV-visible spectroscopy, Fourier transform infrared spectroscopy (FTIR), and four-point probe measurements. The AZO films were crystalline and preferentially oriented along the (002) diffraction plane. The average crystallite size decreased with annealing temperature and ranged from 19.6 to 30.3 nm. The AZO films were non-porous, dense, and continuous with columnar growth. The prominent Raman peaks showed anomalous doped ZnO modes. The deconvoluted Raman spectra showed the presence of A 1 (LO) and A 1 (TO) ZnO modes. FTIR revealed the Al-O and Zn-O stretching vibrations in the films. AZO films had high optical transmittance (61 to 78%) at visible wavelengths and an average band gap of 3.27 ± 0.04 eV, which is suitable for optoelectronics applications. The resistivity (4.5 × 10 −4 to 9 × 10 −4 Ω cm) and high figure of merit value indicates that AZO thin films may be suitable transparent conductive oxides.
Magnetron sputtered Al-ZnO thin films for photovoltaic applications
The optimization process of the RF magnetron sputtered Aldoped ZnO (AZO) thin films was carried out by studying its structural, optical, electrical, and morphological properties at different RF power and different working pressures for its use as a front-contact for the copper indium diselenide (CIS) based thin film solar cell. The structural study suggests that the preferred orientation of grains along the ( 002) plane having a hexagonal structure of the grains. The optical and electrical properties suggest that the films show an average transmission of 85 % and a resistivity of the order of 10 -4 Wcm. The morphology analysis suggests the formation of packed grains having a homogeneous surface.
Journal of Physics: Conference Series, 2013
High quality Aluminium-doped Zinc Oxide (AZO) films have been obtained by suitably controlling the magnetron sputtering parameters and the substrate temperature. The Xray diffraction studies showed that a transition of orientation from (002) plane to (103) plane, versus substrate temperature. The surface morphology characterized by scanning electron microscopy and atomic force microscopy exhibited a dense and compact structure at higher temperature. For 200 nm thick AZO films deposited at temperature 530°C, using a ZnO target with an Al 2 O 3 content of 3 wt%, the lowest electrical resistivity is 6.810 -4 .cm and transmittance is over 85% in the visible spectral region. The conductivity improvement of AZO films was closely related to the crystallanity characterized by the (103) orientation and the densely packed structure.
Surface & Coatings Technology, 2017
Thin Al-doped ZnO (AZO) films were deposited by magnetron sputtering on a non-heated quartz substrate. As-deposited samples have a nanocrystalline structure, a high transparency in the visible part of the spectrum, but a relatively low conductivity. After deposition, the films were isochronally annealed for one hour in hydrogen atmosphere at 200, 300 or 400 °C. The influence of such treatment on the structural properties was analysed by GIXRD and correlated with UV-Vis, photoluminescence and impedance measurements. The structural investigation demonstrated that the heat treatment reduces the strain in the material, the volume of the crystal lattice decreases and the crystal size grows. By measuring the optical properties it was shown that heating increases the optical gap and gradually reduces the number of point defects, mostly related to interstitial atoms. As a result of this process, the conductivity at room temperature increased more than 9 orders of magnitude due to an enhancement of the mobility and the concentration of free carriers. The activation energy for defect annihilation was estimated to be about 1 eV and corresponds to the diffusion of interstitial atoms with the annihilation of vacancies. The concentration of free carriers increases due to the activation of the dopants that act as shallow donors.
Materials, 2020
In this study, high transparent thin films were prepared by radio frequency (RF) magnetron sputtering from a conventional solid state target based on ZnO:MgO:Al2O3 (10:2 wt %) material. The films were deposited on glass and silicon substrates at the different working pressures of 0.21, 0.61, 0.83 and 1 Pa, 300 °C and 250 W of power. X-ray diffraction patterns (XRD), atomic force microscopy (AFM), UV-vis absorption and Hall effect measurements were used to evaluate the structural, optical, morphological and electrical properties of thin films as a function of the working pressure. The optical properties of the films, such as the refractive index, the extinction coefficient and the band gap energy were systematically studied. The optical band gap of thin films was estimated from the calculated absorption coefficient. That parameter, ranged from 3.921 to 3.655 eV, was hardly influenced by the working pressure. On the other hand, the lowest resistivity of 8.8 × 10−2 Ω cm−1 was achieved ...