Cohesive strength of nanocrystalline ZnO:Ga thin films deposited at room temperature
© Samantilleke et al; licensee Springer. 2011
Received: 5 November 2010
Accepted: 7 April 2011
Published: 7 April 2011
In this study, transparent conducting nanocrystalline ZnO:Ga (GZO) films were deposited by dc magnetron sputtering at room temperature on polymers (and glass for comparison). Electrical resistivities of 8.8 × 10-4 and 2.2 × 10-3 Ω cm were obtained for films deposited on glass and polymers, respectively. The crack onset strain (COS) and the cohesive strength of the coatings were investigated by means of tensile testing. The COS is similar for different GZO coatings and occurs for nominal strains approx. 1%. The cohesive strength of coatings, which was evaluated from the initial part of the crack density evolution, was found to be between 1.3 and 1.4 GPa. For these calculations, a Young's modulus of 112 GPa was used, evaluated by nanoindentation.
Doped ZnO thin films are widely used as transparent electrodes in optoelectronic and electro-optic devices such as solar cells and flat panel displays [1–3], because of their unique properties, specifically low electrical resistivity and high transmittance in the visible spectral region . These properties are obtained using substrate temperatures higher than 200°C, but growing interest in flexible substrates has led to the use of polymeric alternatives, which require the deposition of films at low temperature . Furthermore, the deposition on polymeric substrates decreases the quality of the film properties ; therefore, the pursuit toward an understanding of the structural, electromechanical and electro-optical properties of nanocrystalline (nc) thin films is crucial for device applications.
ZnO:Ga (GZO) thin films were deposited by dc-magnetron sputtering on glass and polyethylene naphthalate (PEN) substrates, under an Ar atmosphere with a base pressure of 2 × 10-4 Pa, from a GZO target (zinc oxide/gallium oxide, 95.5/4.5 wt.%) of 2" diameter. A target current density of 0.6 mA/cm2 was applied, and a deposition rate of 21 nm/min was obtained. No bias was applied to the substrate holder during the depositions, which took place at room temperature. The working pressure (P w) was varied from 0.41 to 0.86 Pa, with the target-to-substrate distance kept at a constant 8 cm. The crystallinity and crystal orientation was studied using a Bruker AXS Discover D8 (Madison, USA) for X-ray diffraction (XRD). Glass substrates were used to avoid the presence of polymer substrate peaks. The electrical resistivity, carrier concentration and Hall mobility of the coatings on glass substrates were all measured using Van der Pauw geometry under a magnetic field of 1 Tesla. The electromechanical tests were carried out on 10 × 40 mm2 samples using a computer-controlled tensile testing machine (Minimat, Polymer Labs, Loughborough, UK), which was mounted on an optical microscope stage (Nikon Optiphot-100, Tokyo, Japan). One of the grips of the instrument was displaced at a constant speed of 0.2 mm/min. The applied load and stage displacement values were recorded at 1-s intervals. Crack development was recorded through a CCD camera connected to the microscope, with the evolution of the crack density obtained by the subsequent video analysis. The thickness of the polymer substrates was measured using a Fischer Dualscope MP0R instrument (Sindelfingen, Germany).
Results and discussion
where β is the measured FWHM, θ is the Bragg angle of the peak, λ is the X-ray wavelength (1.5418 Å), ε is the effective particle size and τ is the effective strain. The average particle size, calculated from the plot cos θ versus sin θ shown in Figure 1b, was 8.7 nm. The particle size (D v) calculated from Scherrer's formula (D v = 0.94λ/(β cos θ)), was 8.9 nm, which is very close to that calculated from Equation 1 ). The presence of strain in the ZnO crystal lattice, caused indirectly by P w, can be expected to exert significant influence on the mechanical properties of the nc-GZO thin film.
Assuming that the residual stresses were negligible, in the initial stage of fragmentation, the average fragment length was related to the stress acting in the coating. The average fragment length (ℓ) is ℓ 0(σ/β)-α, where a normalizing factor (ℓ 0) of 1 μm was chosen. In addition, σ is the axial stress acting in the coating, and α and β are the Weibull shape and scale parameters, respectively. These parameters were derived from a plot of ln(ℓ) versus ln(σ), shown in Figure 4b, using the initial part of the crack density evolution of the PEN/GZO coatings, displayed in Figure 4a.
where Γ is the gamma function, ℓ c = (3/2)ℓ sat is the critical length and ℓ sat is the experimental mean fragment length at saturation, which is also the inverse of the crack density at saturation . As shown in Figure 4a, the GZO coatings prepared at P w of 0.53 and 0.86 Pa revealed mean fragment lengths at saturation of 3.11 and 1.94 μm, respectively.
where σi is the internal stress and ε i = σ i (1 - νc)/E c, the internal strain, with E c and νc being the Young's modulus and Poisson ratio, respectively, of the coating. Young's modulus of GZO was measured by nanoindentation at 113 and 112 GPa from samples prepared at 0.60 and 0.86 Pa, respectively. Young's modulus of the PEN substrate was determined from tensile testing (4.23 GPa). The cohesive strength of the coatings, which was evaluated from the initial part of the crack density evolution, was found to be between 1.3 and 1.4 GPa. The crack onset strains (COScor) occurs for nominal strains of 1.1 and 1.0%, respectively. The COS and cohesive strength of GZO are relatively similar to those reported in the literature for other polycrystalline conducting films .
The material, opto-electrical properties, COS, the coating cohesive strength, as well as the influence of mechanical strain on the electrical properties of nc GZO thin films were investigated. The estimated average crystalline size of nc-GZO films was approx. 8.7 nm, and the bandgap shifted from 3.73 eV (0.41 Pa) to 3.48 eV (0.86 Pa), where the low resistivity (approx. 10-4 Ω cm) and the high electron density (>1020 cm-3) explain the dominating scattering process as the ionized impurity scattering. The COS is similar for different GZO coatings and occurs for nominal strains approx. 1%. The cohesive strength of coatings, which was evaluated from the initial part of the crack density evolution, was found to be between 1.3 and 1.4 GPa, while the Young's modulus was evaluated by nanoindentation.
crack onset strains
full-width at half-maximum
The authors acknowledge the receipt of funding from the Portuguese Foundation for Science and Technology (FCT) Grant PTDC/CTM/69316/2006, INL project 156: SIMBIO, NANO/NMed-SD/0156/2007 and the CIENCIA 2007 programme.
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