Appl. Phys. A 75, 397–399 (2002) / Digital Object Identifier (DOI) 10.1007/s003390100979
Applied Physics A Materials Science & Processing
Transparent conducting antimony-doped tin oxide films deposited on flexible substrates by r.f. magnetron-sputtering X. Hao1,∗ , J. Ma1 , D. Zhang1 , X. Xu2 , Y. Yang1 , H. Ma1 , S. Ai1 1 School of 2 State Key
Physics and Microelectronics, Shandong University, Jinan, Shandong, 250100, P.R. China Laboratory of Crystal Materials, Shandong University, Jinan, Shandong, 250100, P.R. China
Received: 20 April 2001/Accepted: 23 July 2001/Published online: 17 October 2001 – Springer-Verlag 2001
Abstract. Transparent conducting antimony-doped tin oxide (SnO2 :Sb) films were deposited on organic substrates by r.f. magnetron-sputtering. Polycrystalline films with a resistivity of ≈ 6.5 × 10−3 Ω cm, a carrier concentration of ≈ 1.2 × 1020 cm−3 and a Hall mobility of ≈ 9.7 cm2 v−1 s−1 were obtained. The average transmittance of these films reached 85% in the wavelength range of the visible spectrum.
1%–10% Sb in weight show optimal properties [4]. So 6% by weight of the target used in this study was Sb2 O3 . The sputtering gases with a mixture of Ar and O2 (both 99.999% in purity) were controlled via a crystal-controlled, high-frequency power source. The organic substrates were not intentionally heated.
PACS: 61.82.Fk; 73.61.-r; 81.15.Cd
2 Results and discussion
Transparent conducting thin films have been studied for many years because of their importance in various fields, such as solar cells and gas sensors [1–5]. Films deposited on organic substrates have many merits compared with films on hard substrates. They are used in flexible electro-optical devices, plastic liquid crystal displays, transparent electromagnetic shielding materials and unbreakable heat-reflecting mirrors. In recent years, both Sn-doped In2 O3 (ITO) [6–10] and Al-doped zinc oxide films [11, 12] prepared on organic substrates have been studied. However, to our knowledge, no report has been published on SnO2 films prepared on organic substrates. Compared with ITO and ZnO films, SnO2 have high stability in acidic and basic solutions and in oxidizing environments at high temperatures [13]. This advantage has significance in semiconductor devices. Here we describe the preparation and properties of SnO2 :Sb films deposited on flexible substrates by r.f. magnetron-sputtering.
The dependence of resistivity on r.f. power for the SnO2 :Sb films is shown in Fig. 1. With increasing r.f. power, the film resistivity initially decreases. At power = 100 W, it reaches its minimum, and then it increases. It can be considered that the organic substrate surface is damaged by the energetic particle bombardment when power > 100 W. Hence, we conducted further studies at r.f. power = 100 W. The oxygen content in sputtering gases also affects the resistivity of the films. With an increase in oxygen content, the resistivity de-
1 Experimental details The system for depositing and measuring films has been reported previously [11, 12]. The SnO2 :Sb films were deposited on polypropylene adipate (PPA) thin-film (100 µm thick) substrates. A target with a mixture of SnO2 (99.99% purity) and Sb2 O3 (99.99% purity) was employed for the source materials. It has been reported that antimony-doped SnO2 films with ∗ Corresponding
author. (Fax: +86-531/8565427, E-mail:
[email protected])
Fig. 1. Dependence of the resistivity on r.f. power for SnO2 :Sb films. (Deposition parameters: oxygen content, 10%; sputtering gas pressure, 1 Pa)
398
Fig. 2. Variation in the resistivity of SnO2 :Sb films with sputtering gas pressure and oxygen percentage in sputtering gases. Curve a: oxygen content = 10%; curve b: sputtering gas pressure = 1 Pa
creases when the oxygen content is below 10%. Above 10% the resistivity does not change significantly with increasing oxygen content. The resistivity of the films also depends on the sputtering gas pressure. It increases with increasing sputtering gas pressure, but not as obviously as the former. These results are exhibited in Fig. 2. The following parameters were found to be optimal and were used in film preparation: an r.f. power of 100 W, an oxygen content in sputtering gases of 10% and a gas pressure of 1 Pa. Under these conditions, the deposition rate was ≈ 7 nm min−1. Figure 3 shows the reciprocal temperature dependence of resistivity, carrier concentration and Hall mobility obtained from Hall measurements in the temperature range 23–300 K for a typical film deposited in optimal conditions. At room temperature, the resistivity ρ, the carrier concentration n and the Hall mobility µ of the sample are 6.5 × 10−3 Ω cm, 1.2 × 1020 cm−3 and 9.7 cm2 v−1 s−1 , respectively. In the temperature range below 140 K, both µ
Fig. 3. The resistivity, Hall mobility and carrier concentration as a function of reciprocal temperature for the typical SnO2 :Sb sample
and ρ remain invariant. However, if the temperature is higher than 140 K, an increase in µ and a decrease in ρ occur with increasing temperature. In the whole temperature range, n seems to maintain a constant value. According to the scattering mechanism of transparent conducting films suggested by Zhang et al. [14], for the investigated films, ionized impurity scattering seems dominant in the low-temperature range. In the high-temperature range, owing to small crystalline sizes, grain-boundary scattering becomes the dominant scattering mechanism. Figure 4 shows the X-ray diffraction spectrum for a SnO2 :Sb film on a PPA substrate (the influence of the organic substrate has been removed). The spectrum reveals that the deposited film was polycrystalline and retained the rutile structure. The transmittance spectrum as a function of wavelength in the range 300–800 nm for above (with 360 nm thickness) is shown in Fig. 5. The average transmittance can be as high as 85%. Table 1 shows the sheet resistance of SnO2 :Sb films with the same thickness (360 nm) deposited on glass and different
Fig. 4. X-Ray diffraction spectrum for a typical film deposited on PPA organic substrate
Fig. 5. Optical transmittance versus wavelength for a typical sample. Thickness: 360 nm
399 Table 1. Sheet resistance of 360-nm-thick samples deposited on different substrates in the same fabrication conditions Substrate
Sheet resistance (Ω m−2 )
Glass Polypropylene adipate (PPA) Polyimide (PI) Polyethylene glycol tephthalate (PET)
65 178 391 424
organic substrates in optimal conditions. Although the sheet resistance of the films on organic substrates is higher than that on the glass substrate, the films deposited on organic substrates are still useful in many fields.
3 Conclusions Good transparent conducting Sb-doped SnO2 films can be prepared on PPA substrates by r.f. magnetron-sputtering. These films deposited as polycrystalline material with a rutile structure, had a resistivity as low as 6.5 × 10−3 Ω cm and 85% transmittance in the visible region. The r.f. power, sputtering gas pressure and oxygen content in the sputtering gases were the main parameters affecting the resistivity of the obtained films.
Acknowledgements. This work was supported by the Foundation for University Key Teacher by the Ministry of Education, the National Natural Science Foundation of China (Nos. 69876025 and 60076006), and the Natural Science Foundation of Shandong Province.
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