«ABSTRACT Transparent conducting Al-doped ZnO (AZO) thin ﬁlms were deposited on soda-lime glass substrates by DC magnetron sputtering with a novel ...»
The Hall mobility (µ) of the ﬁlms increases monotonously with Ts while for the carrier concentration (N), there is discrepancy between these two values. At ﬁrst, N dropped to its lowest value of 1.00 × 1021 cm−3 at Ts = 200◦ C then increased gradually to its maximum value of 1.86 × 1021 cm−3 at Ts = 400◦ C. The effects of Ts on the electrical resistivity (ρ) of the AZO ﬁlms are, at least, twofold. Firstly, the crystallinity was effectively improved at higher Ts as shown in Fig. 1b whereas the crystal size of the ﬁlms increased considerably. Minami  had showed that owing to the improvement in the crystallinity, the µ increased with Ts up to 250◦ C for AZO ﬁlms deposited by RF magnetron sputtering. Secondary, the substrate heating is detrimental to the surface morphology as shown in Fig. 2. One of the main factors affecting ρ seems to be the roughness of the ﬁlm since higher surface roughness leads to a non uniform morphology of the ﬁlm . The resultant ﬁlms were embedded with more defects, which subsequently causes stronger boundary scattering and results in shrinkage of carrier lifetime. Therefore, the ﬁlm deposited at Ts = 200◦ C with largest RMS roughness has lowest carrier concentration. In contrast, a minimum resistivity of 4.62 × 10−4 Ω·cm was achieved for ﬁlm deposited at Ts = 400◦ C which has compact surface structure, good crystallinity, larger crystal grains and small RMS roughness, simultaneously.
3.3. Optical Properties Figure 7a shows the optical transmittance of the ﬁlms. The variations in transmittance are dependent on the ﬁlms thickness which is ascribed to the interference phenomena . It is noteworthy that the transmittance for the AZO ﬁlms in the NIR region improved considerably as compared to those using alloy targets by reactive magnetron sputtering [7, 26] and even slightly higher than those using Al-doped ZnO (1 wt.%) ceramic targets by RF sputtering . The average transmission as function of Ts was shown in the insert of Fig. 7a. It is obvious that both the average transmissions in the visible region (Tvis ) and those down to NIR region (Ttotal ) were enhanced consistently by substrate heating. Nevertheless, as Ts ≥ 300◦ C, the Tvis exceeded Ttotal. The absorption coefﬁcients (α) near the absorption edge of the ﬁlms were determined by  1 IO α = ln, (1) d I where d is the ﬁlm thickness; Io and I are the intensities of the incident and transmitted beams, respectively.
Being a direct-gap semiconductor, the optical band gap (Eopt ) of the ﬁlms can be described by the following
Transactions of the Canadian Society for Mechanical Engineering, Vol. 37, No. 3, 2013 Fig. 7. (a) Optical transmittance of AZO ﬁlms deposited at various Ts. Insert: the average transmittance of the deposited AZO ﬁlms; (b) plot of (αhv)2 versus photon energy for AZO ﬁlms deposited at various Ts. Insert: optical band gap as function of carrier concentration.
where N is the carrier concentration; h is the Plank constant; m∗ is the conduction band effective mass. A vc plot of Eopt versus N 2/3 gave a linear relationship as shown in the insert of Fig. 7. Besides, the Hall effect measurements indicate that the carrier concentrations in this study are ranged in ∼ 1021 cm−3 which are far higher than the Mott density (1018 cm−3 ) . Therefore, the degenerated electrons appear to enter the conduction band, causing a shift of the quasi-Fermi level of electrons in the conduction band.
4. CONCLUSIONS Transparent conducting Al-doped ZnO thin ﬁlms with 500 ± 10 nm in thickness had been deposited by DC magnetron sputtering using a novel sintered ceramic AZOY® target on soda-lime glass substrates. The as-deposited AZO ﬁlm has resistivity of 1.68 × 10−3 Ω·cm and average visible transmittance of 75.4%.
By employing substrate heating during deposition, both the electrical conductivity and the optical transparency of the ﬁlms had improved considerably. At Ts = 400◦ C, the resistivity of the ﬁlm has its minimum value of 4.62 × 10−4 Ω·cm under which the carrier concentration and mobility was 1.86 × 1021 cm−3 and
7.29 cm2 V−1 s−1, respectively. It is noteworthy that the transmittance in the NIR region was also improved considerably as compared to those employing traditional alloy targets by reactive magnetron sputtering and even slightly higher than those using ceramic targets (Al-doped ZnO, 1 wt.%) by RF sputtering. The optical 310 Transactions of the Canadian Society for Mechanical Engineering, Vol. 37, No. 3, 2013 band gaps lies between 3.23–3.56 eV, which are apparently wider than that of pristine ZnO owing to the Burstein–Moss effect.
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