Tin dioxide (SnO2) is a kind of n-type wide band gapsemiconductor material with Eg=3.6–4.0 eV [1–4]. Due to itslow resistivity, high optical transparency, thermal and chemicalstability, and low cost, SnO2 is extensively used in manyapplications such as transparent electrodes, liquid crystaldisplays (LCD), gas sensors, solar cells, etc. [5–11]. Untilnow, most of the transparent conducting oxide (TCO) films aren-type, such as indium tin oxide (ITO) [12–14], fluorine dopedtin oxide (FTO) [15,16], aluminum doped zinc oxide (AZO)[17,18]. Recently, great interests have been paid to the researchof p-type TCO films. Kawazoe et al. reported p-typeconductivity from copper aluminium oxide (CuAlO2), but thehole concentration was low (1017 cm−3) and the transmittancewas only about 50% in the visible region [19]. Recently, a fewpapers reported on p-type conducting from indium–tin oxidefilms [20,21]. In these papers, Ji et al. prepared p-type
transparent conducting indium tin oxide films (ITO) by sol–gel method, and found that the optimal In /Sn ratio for p-typeconducting films was about 0.2, but the results were not so
Fig. 1. XRD spectra of the films after thermal oxidation at a=450 °C, b=500 °C,c=550 °C, d=600 °C, e=650 °C, f=700 °C, g=750 °C.
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satisfying as expected because of the poor film quality and thinthickness as limited by sol–gel method.
In order to improve the quality of the p-type TIO films,structural and electrical properties of the p-type conducting tin–indium oxide films prepared by thermal oxidation of InSn alloyfilms were studied, and improved crystal quality and electricalproperties than the p-type ITO films reported by Ji et al. wereachieved [20].
2. Experimental
The metallic InSn alloy films were deposited on quartz glassby DC magnetron sputtering. The target of ϕ=75 mm wasfabricated by melting tin (purity 99.999%) and indium (purity99.999%, In / Sn atomic ratio 0.2) together followed bysubsequent machining. Before deposition the substrates werecleaned ultrasonically in acetone, rinsed with deionized water
Fig. 2. SEM micrographs of the TIO films prepared at (a)=450 °C,
and dried by nitrogen blow. The base pressure of the depositionsystem was 1×10−3 Pa, during sputtering, the pressure in-creased to 1 Pa as the argon gas (purity 99.999%) was admittedinto the chamber, and no oxygen was supplied. The sputteringwas carried out at a cathode voltage of 240 V and an ion beamcurrent of 100 mA at the target–substrate distance of 100 mm.The substrate was not heated intentionally during deposition,and no temperature rise of the substrate during the 5-mindeposition was observed. After deposition, the alloy films wereoxidized in air between 400 and 750 °C to form the stable TIOoxide films.
The structural characteristics were examined with a highresolution X-ray diffractometer of Bede plc D1 system that useda CuKα radiation (λ=1.54056 Å). The surface morphology wastaken with a SIRION-100 field emission scanning electronmicroscope (SEM) of FEI Company. The thickness of the filmwas measured from the cross-sectional view of the SEM image.
(b)=500 °C, (c)=550 °C, (d)=600 °C, (e)=650 °C, (f)=700 °C.
Fig. 3. SEM image of the cross-section for the TIO thin film oxidized at 600 °C.
Fig. 5. Influence of the thermal oxidation temperature on the mobility.
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The electrical properties were measured by a HL5500PC Halleffect measurement system of Bio-Rad.
3. Results and discussion
3.1. Structural properties
Fig. 1 shows the XRD spectra of the TIO films formed by thermaloxidation in air at temperatures between 450 and 750 °C. It is foundthat the TIO films are in orthorhombic structure, which is different frommost SnO2 films that are in rutile structure [1–4]. As seen in Fig. 1, thefilms grew with (111) preferred orientation after lower temperatureoxidation, but at higher oxidation temperature, the intensity of the otherpeaks increased, and the orientation index was reduced. Similarphenomenon is found in preparation of high quality ZnO films [22].
Fig. 2 presents the SEM micrographs of the TIO films prepared attemperatures between 450 and 700 °C. All the TIO oxide films arecomposed of uniformly distributed particles of similar size.
Fig. 3 shows SEM cross-section for the TIO thin film oxidized at600 °C on quartz substrate. As seen in Fig. 3, the interface between thinfilm and substrate is clearly seen, which indicates that the film growswell on the quartz substrate. The thickness of the TIO film measured isabout 160 nm.
Fig. 4. Influence of the thermal oxidation temperature on the carrierconcentration.
3.2. Electrical properties
The carrier concentration, Hall mobility and resistivity of the filmswere measured at R.T. using Hall effect measurement. Figs. 4, 5 and 6show the influence of the oxidation temperature on the electricalproperties of the TIO films. As seen in Fig. 4, the conducting type andcarrier concentration of the TIO films depend on the oxidationtemperature with a critical point of about 575 °C. For the TIO filmsoxidized at temperature lower than this point, the TIO film is n-typeconducting, while for the films oxidized at temperature higher than thispoint, the TIO films show p-type conducting.
The above phenomena could be explained as follows: for TIO filmsoxidized at lower oxidation temperature, indium atoms do not haveenough energy to enter into the substitution sites of the tin ions, soindium atoms in the film do not behave as acceptors; instead, theycould even become donors if they occupy the interstitial sites [20,23].In addition, metal oxides usually contain high-density intrinsic pointdefects (oxygen vacancies and tin interstitials) which act as donors, sothe films oxidized at lower temperature show n-type conducting. As theoxidation temperature increases, more and more indium atoms enterinto the substitution sites of Sn, which act as acceptors, and when there
Fig. 6. Influence of the thermal oxidation temperature on the resistivity.
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are more holes than the intrinsic electrons, the films become p-typeconducting. It is found that the hole concentration reaches maximum of9.61×1018 cm−3 after oxidation at 600 °C. When the temperature ishigher than 600 °C, hole concentration declines, which is a result ofmore donor defects generated at higher oxidation temperature as pointdefects tend to reduce the Gibbs free energy. Obviously, the oxidationtemperature is a critical parameter to get p-type conducting films by thethermal oxidation of InSn alloy films.
In addition, as shown in Figs. 5 and 6, it can also be seen that theHall mobility for p-type conducting TIO films which are of high holeconcentration is very low, the reason for the decline of the Hall mobilityis the increase in the ionized impurities, i.e., ionized acceptors. Whenthe oxidation temperature is higher, the hole concentration decreases,i.e., the number of the ionized acceptor decreases, and the scattering ofcarrier is weakened, which results in the increase in the Hall mobility.The phenomenon is similar to the results of p-type SnO2 and p-typeZnO films as reported both by Ji et al. [20,24].
4. Conclusions
P-type conducting indium-doped SnO2 films were preparedby thermal oxidation of metallic InSn alloy films in air. XRDresults showed that the films were in orthorhombic structure.Hall effect measurement results showed that the oxidationtemperature was a critical parameter to get p-type conductingTIO films. It was found that 600 °C was the optimum oxidationtemperature to get p-type films with highest hole concentra-tions. The mobility of the carrier increased as the holeconcentration decreased. The highest hole concentration ofthe film with lowest resistivity achieved in this paper was9.61×1018 cm−3 (12.4 Ω cm).
Acknowledgements
This work was supported by the following foundations:Chinese Natural Science Foundation (no. 60576063), the PhDProjects of the Chinese Education Ministry (no. 20050335036),and the Analysis and Measurement Foundation of ZhejiangProvince (no. 03103).
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