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Highly stable transparent and conducting gallium-doped zinc oxide thin films for photovoltaic applications. E. Fortunato , L.Raniero,L.Silva,A.Gonc-alves,A.Pimentel,P.Barquinha,H.A´guas, L.Pereira, G. Gonc-alves,I.Ferreira,E.Elangovan,R.Martins
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Highly stable transparent and conducting gallium-doped zinc oxide thin films for photovoltaic applications E. Fortunato , L.Raniero,L.Silva,A.Gonc-alves,A.Pimentel,P.Barquinha,H.A´guas, L.Pereira, G. Gonc-alves,I.Ferreira,E.Elangovan,R.Martins CENIMAT/I3N,MaterialsScienceDepartment,FCT-UNLandCEMOP-UNINOVA,CampusdeCaparica,2829-516Caparica,Portugal 教授:林克默博士 學生:董祐成 日期:99/09/13
Outline • 1. Introduction • 2. Experimental • 3. Results and discussion • 4. Conclusions
Introduction • Transparent conducting oxide (TCO) with an optical transmission exceeding 80% in the visible region (500–650nm) and a resistivity less than 10-3 Ωcm have been widely used inavariety of applications for more than a half-century. • However , most of these techniques require moderate substrate temperatures to obtain low resistivity . Among the available techniques, RF magnetron sputtering presents several advantages including the production of highly transparent and conducting gallium zinc oxide (GZO) without heating the substrate.
Experimental(1) • The GZO films were deposited initially on soda lime glass substrates by RF (13.56MHz) magnetron sputtering using a 5cm planar ceramic oxide target consisting ZnO (98wt%):Ga2O3 (2 wt%) from Super Conductor Materials, Inc. with a purity of 99.99%. • The sputtering was carried out at RT with an argon deposition pressure of 0.15Pa. The substrate–target distance (10cm) and the RF power (175W) were maintained constant for all depositions. The typical growth rate for these deposition conditions is 30 nm/min.
Experimental(2) • The deposition conditions optimized from the GZO films deposited on glass substrates were later used to deposit the GZO films on polyethylene naphthalate (PEN) substrates. The PEN substrates possess high stiffness, low thermal shrinkage, and high chemical resistance. • The film thickness was measured using a surface profilometer (Dektak 3D from Sloan Tech.). The surface morphology was analyzed using a field effect scanning electron microscope (FE-SEM,S-1400 Hitachi) . The electrical resistivity (ρ), free carrierconcentration (N) and Hall mobility (μ) were inferred by the four point probe method and Hall effect measurements in van der Pauw geometry (Biorad HL5500) at a constant magnetic field of 0.5T.
Experimental(3) • X-ray diffraction measurements were performed using Cu-Ka radiation (Rigaku DMAX III-Cdiffractometer) in Bragg–Brentanogeometry(θ/2θcoupled). • The optical transmittance measurements were per-formed with a Shimadzu UV/VIS 3100 PC double beam spectro-photometer in the wavelength range from 300 to 2500 nm.
Results and discussion Dependence of the electrical resistivity (ρ), carrierconcentration (N) and Hall mobility (μ) as a function of film thickness of the GZO films deposited at room temperature (inthisstudy).
A maximum bulk resistivity (ρ) of ~ 5.7×10-4Ωcm obtained for the 100 nm thick films was decreased with the increasing film thickness to reach a minimum of ~ 2.8×10-4Ωcm at 1100nm. • We have observed a continuous increase on the mobility (μ) as the thickness increases (from 11 to 18 cm2/Vs) .This behavior can be attributed to a reduction in the ionized impurity scattering and or an increase in the crystallite size. • However , we notice that ρ tends to saturate for thicknesses above 500 nm . Similar results have been obtained for Al-doped ZnO.
XRD patterns obtained from the GZO films deposited with different thicknesses (inset shows the pattern from the thickest sample).
For all the films , only the ZnO (002) peak at 2θ = 34.31° is observed revealing that the films are polycrystalline with a hexagonal structure and a preferred orientation along the c-axis perpendicular to the substrate. • As the thickness increases the peak intensity corresponding to the plane (002) increases significantly , whereas the peak width decreases.
Dependence of Hall mobility of the GZO films with the crystallite size (data obtained through Scherrer’s formula).
A linear dependence between the mobility and the crystallite size was obtained through the equation , μ =k+0.5983dc, where k is a constant and dc is crystallitesize. • This is consistent with the previous results since the mobility is mainly dependent on the grain boundary scattering and lattice defects, which decrease with the increase of the crystallite size. • This also suggests that the only way to decrease the resistivity is by increasing the mobility .
Surface SEM micrographs (with 40° tilt angle) of GZO films deposited at room temperature, with different thicknesses
Shows two typical SEM micrographs of GZO films with a low (110nm) and high (1110nm) film thicknesses respectively,with an apparent viewing angle of 40 ° • The surface roughness is increased with the increasing film thickness, suggesting an enhancement of the grain size as already confirmed by the electrical and X-ray diffraction measurements..
Optical transmittance obtained from the GZO films as a function of film thickness.
shows the optical transmittance vs. wavelength in the visible and near-infrared region obtained from the films with thick- nesses ranging from 110 to 1100 nm. • The near-infrared transmittance decreases as the film thickness increases, whereas the average transmittance at the visible range is obtained as 80% and 90% from the low and high thickness films, respectively. These changes in the optical properties are consistent with the changes observed in the electrical, structural and morphological properties.
Conclusions • The set of data achieved shows that highly conducting and transparent GZO films can be deposited by RF magnetron sputtering at room temperature. • The data show that the crystalline structure , surface morphology and the electro-optical properties are dependent on the film thickness.Overall,the produced films present a resistivity close to 2.8×10-4Ωcm, Hall mobility 18 cm2/Vs and transmittance ＞80%. • Further work is under way to increase the Hall mobility without heating the substrate , either during deposition or after deposition , to be compatible with the emergent plastic electronic industry.
