Deposited by Sputtering of Sn and SnO 2

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1 Journal of the Korean Ceramic Society Vol. 49, No. 5, pp. 448~453, Comparative Study of Nitrogen Incorporated SnO 2 Deposited by Sputtering of Sn and SnO 2 Targets Youngrae Kim and Sarah Eunkyung Kim*, Microsystem Packaging Center, Seoul Technopark, Seoul Korea *Graduate School of NID Fusion Technology, Seoul National University of Science and Technology, Seoul Korea (Received August 10, 2012; Revised September 20, 2012; Accepted September 27, 2012) ABSTRACT Nitrogen-incorporated SnO 2 thin films were deposited by rf magnetron sputtering. Comparative structural, electrical and optical studies of thin films deposited by sputtering of the Sn metallic target and sputtering of the SnO 2 ceramic target were conducted. The SnO 2 thin films deposited by sputtering of the Sn metallic target had a higher electrical conductivity due to a higher carrier concentration than those by sputtering of the SnO 2 ceramic target. Structurally the SnO 2 thin films deposited by sputtering of the SnO 2 ceramic target had a better crystallinity and a larger grain size. This study confirmed that there were distinct and clear differences in electrical, structural, and optical characteristics between SnO 2 thin films deposited by reactive sputtering of the Sn metallic target and by direct sputtering of the SnO 2 ceramic target. Key words : Thin films, Tin compounds, Electrical conductivity, Semiconductors T 1. Introduction in oxide thin films have been widely studied for a long time because they are highly transparent in the visible range and electrically conductive. They have been well developed and extensively used in many applications such as transparent electrodes in flat panel displays, solar cells, gas sensors, heat reflecting films, and optoelectronics. 1-6) Tin oxide thin films have in general n-type conductivity due to the existence of intrinsic defects such as oxygen deficiency and tin interstitials. However, for optoelectronic or transparent device applications, a highly purified and stable p- type tin oxide with high conductivity and mobility is required. 7) P-type tin oxide can be achieved by either fabricating p-type SnO or doping in SnO 2 using elements with a lower valence cation, such as antimony, indium, or nitrogen, as the acceptor impurity. 8-10) Among many fabrication methods, sputtering is widely used to fabricate tin oxide thin films because the sputtering technique has many advantages, such as a relatively high deposition rate, production-compatibility, thickness uniformity, and scalability to large areas. 11) Moreover, the sputtering method is a convenient way to control the properties of thin films by changing the sputtering conditions, such as the oxygen partial pressure, total pressure, power, temperature, and so on. One of the important sputtering parameters typically overlooked is the type of sputtering target. There Corresponding author : Sarah Eunkyung Kim eunkyung@seoultech.ac.kr Tel : Fax : are two types of sputtering target: a Sn metallic target and a SnO 2 ceramic target. Compared to the direct sputtering of a ceramic target, the reactive sputtering of a metallic target is in general very useful because different properties and chemical compositions of thin films can be produced by controlling the reactive gas partial pressures. 12) In addition, the deposition rate of reactive sputtering, which affects the grain size and internal stress of thin films, is generally higher than that of direct sputtering of a ceramic target. 11,13) However, the direct sputtering of a ceramic target can provide better control of the stoichiometry of thin films than the reactive sputtering of a metallic target. In the present work, nitrogen-incorporated SnO 2 thin films were fabricated by sputtering of two different targets: a Sn metallic target and a SnO 2 ceramic target. The effects of the metallic target and the ceramic target on the properties of SnO 2 thin films were evaluated through structural, optical, and electrical analysis.in addition, the effects of nitrogen incorporation in SnO 2 thin films deposited by the two different sputtering targets were investigated. Emphasis was given to the effect of the sputtering target on the electrical conductivity of the nitrogen-incorporated SnO 2 thin films. 2. Experimental Procedure Both a Sn metallic target and a SnO 2 ceramic target were used during the fabrication of SnO 2 thin films and a borosilicate glass was used as a substrate. The sputtering atmosphere was an Ar/O 2 /N 2 gas mixture and the nitrogen flow content was varied from 10 to 40% of the N 2 /total gas ratio. Instead of the actual gas partial pressure during the deposi- 448

2 September 2012 Comparative Study of Nitrogen Incorporated SnO 2 Deposited by Sputtering of Sn and SnO 2 Targets 449 Table 1 Gas Flow Conditions of the Fabricated SnO 2 Thin Films Gas Content (%) = individual gas flow / total gas flow Sn Target (99.999%) SnO 2 Target (99.99%) Sample # Ar (%) O 2 (%) N 2 (%) Sample # Ar (%) O 2 (%) N 2 (%) Sn SnO Sn SnO Sn SnO Sn SnO Sn SnO tion, the nitrogen flow content of the sputtering gas in terms of the percent of the total gas flow rate (70 sccm) was reported here. The gas flow conditions are listed in Table 1. The substrate temperature was 100 C and the pressure was 7 mtorr and the power was 200 W. The average film thickness was kept at approximately 2000 Å. The microstructure of the SnO 2 thin films was determined by an X-ray diffraction (XRD), a scanning electron microscope (SEM), and a transmission electron microscope (TEM). An X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical states of the elements. The electrical conductivity, carrier mobility and concentration were measured using the Hall effect measurements, and the optical transmittance was measured by a UV/Vis spectrometer. 3. Results and Discussion Fig. 1 shows the X-ray diffraction patterns of the SnO 2 thin films deposited by sputtering of (a) a Sn metallic target and (b) a SnO 2 ceramic target. There were no diffraction peaks indicating tin (JCPDS # , major peak from (200)) or tin nitride (JCPDS # , major peak from (111)), in either case. This suggests that N atoms were in substitution oxygen sites forming SnO 2 thin films. In the case of the Sn metallic target, the degree of crystallinity of the SnO 2 thin films without nitrogen incorporation was very poor, but it improved with an increasing nitrogen flow content. As the nitrogen flow content increased, the corresponding d-spacing from the (101) peak was prolonged from Å to Å because it is supposed that the radius of an N 3- ion (1.71 Å) is larger than that of an O 2- ion (1.32 Å). 8) In the case of the SnO 2 ceramic target, the degree of crystallinity was good for the SnO 2 thin films without nitrogen incorporation and further improved with an increasing nitrogen flow content. Similar to the Sn metallic target case, the corresponding d-spacing was also broadened from Å to Å indicating the substitution of nitrogen atoms to oxygen sites. In addition, the microstructural changes of the SnO 2 thin films were observed through SEM and TEM images as shown in Fig. 2. The SnO 2 thin films deposited by sputtering of a Sn metallic target had a weak columnar grain growth, an average grain size of around 7.1 nm, and a relatively smooth surface. On the other hand, the SnO 2 thin films deposited by sputtering of the SnO 2 ceramic target had a clear columnar grain growth, an average grain size of Fig. 1. XRD patterns of the SnO 2 thin films: (a) Sn target and (b) SnO 2 target. around 15.5 nm, and a rough surface. These differences exist because direct sputtering of the SnO 2 ceramic target can provide a larger grain size and better control of the stoichilmetry than reactive sputtering of the Sn metallic target. An XPS analysis of the SnO 2 thin films was carried out and Sn 3d, O 1s and N 1s peaks are shown in Figs. 3, 4, and 5, respectively. The Sn 3d peak shown in Fig. 3 confirms that the fabricated thin films were clearly SnO 2 thin films because Sn 3d peaks represented the Sn 4+ chemical state.furthermore, as the nitrogen flow content increased, both Sn 3d 3/2 and Sn 3d 5/2 binding energies had almost no shift indicating that there was no change in the oxidation

3 450 Journal of the Korean Ceramic Society - Youngrae Kim and Sarah Eunkyung Kim Vol. 49, No. 5 Fig. 2. SEM and TEM images of the SnO2 thin films: (a) Sn target and (b) SnO2 target. state of the Sn in SnO2 thin films. In both target cases, judging from a slope shoulder at a higher binding energy side as shown in Fig. 4, the chemisorbed O was clearly observed at the surface of the thin films. Since the O 1s peak was slightly shifted toward a lower binding energy with an increasing nitrogen flow content, a decrease in O2- ions binding with Sn atoms, which is an increase in the oxygen deficiency, probably occurred. As shown in Fig. 5(a) the nitrogen incorporation in SnO2 thin films deposited by sputtering of a Sn metallic target showed that the N 1s peak was from ev to ev as the nitrogen flow content increased from 0% to 40%. This indicates that tin nitride (~397 ev), which is known as a donor, might have been produced.8,14) However, the nitrogen incorporation in SnO2 thin films deposited by sputtering of a SnO2 ceramic target was not noticeably produced even with an increasing nitrogen flow content, as shown in Fig. 5(b). Although a small amount of nitrogen was incorporated, a somewhat blurred and hazy peak of N 1s was found at around 400 ev, which is different from the films deposited by sputtering of a Sn metallic target. This indicates that substitutional nitrogen forming tin oxynitride (~400 ev), which is known as an acceptor, might have been produced.8,9,14) The defects produced by nitrogen incorporation was electrically opposite between the SnO2 thin films deposited by sputtering of a Sn metallic target and by sputtering of a SnO2 ceramic target. The electrical properties of the SnO2 thin films are summarized in Table 2. The carrier concentration was similar for the SnO2 thin films without nitrogen incorporation regardless of the target type, but as the nitrogen flow content increased, the SnO2 thin films deposited by sputtering of a Sn metallic target had one-order higher carrier concentration than those by sputtering of the SnO2 ceramic target. This might be due to the nature of the reactive sputtering method, which creates more defects in a lattice. However,

4 September 2012 Comparative Study of Nitrogen Incorporated SnO 2 Deposited by Sputtering of Sn and SnO 2 Targets 451 Fig. 3. Sn 3d core level XPS spectra of the SnO 2 thin films: (a) Sn target and (b) SnO 2 target. the mobility of the thin films deposited by sputtering of the SnO 2 target was higher than that by sputtering of the Sn metallic target because of a microstructure difference such as a large grain size. Overall the SnO 2 thin films deposited by sputtering of the Sn target had the highest electrical conductivity, as high as 134 Ω 1 cm 1, due to a high carrier concentration. Finally, the optical transmittance was measured and is shown in Fig. 6. The optical energy band gaps for the thin films deposited by sputtering of the SnO 2 ceramic target had very little change, and all of them were higher than the thin films deposited by sputtering of the Sn metallic target. These results strongly suggest that SnO 2 thin films deposited by sputtering of the SnO 2 ceramic target undergo very little change in carrier concentration and have better stoichiometry than SnO 2 thin films deposited by sputtering of a Sn metallic target. This is in good agreement with both the structural and electrical results explained above. The transmission spectra also indicated that the defect levels created in the band gap decreased as the nitrogen flow content increased. 4. Conclusions Fig. 4. O 1s core level XPS spectra of the SnO 2 thin films: (a) Sn target, 6) and (b) SnO 2 target. A comparative study of SnO 2 thin films deposited by sputtering of a Sn metallic target and a SnO 2 ceramic target was performed. Our investigation revealed that there were distinct and clear differences in electrical, structural, and optical characteristics between the SnO 2 thin films deposited by reactive sputtering of a Sn metallic target and those deposited by direct sputtering of a SnO 2 ceramic target. The SnO 2 thin films deposited by sputtering of the Sn metallic target had a higher electrical conductivity due to a higher carrier concentration. The SnO 2 thin films deposited by sputtering of a SnO 2 target had relatively low electrical conductivity due to a lack of carrier concentration, even though good crystallinity and a larger grain size helped a high mobility. It is evident that nitrogen could more effectively incorporate into the SnO 2 thin films by sputtering of a Sn target because of the reactive sputtering process, and it produced the electrical defect sites such as oxygen deficiency in thin films. The lack of nitrogen incorporation in the SnO 2 thin films deposited by sputtering of the SnO 2 target was probably due to the direct sputtering of a ceramic target which is difficult to react with sputtering gas such as nitrogen.

5 452 Journal of the Korean Ceramic Society - Youngrae Kim and Sarah Eunkyung Kim Vol. 49, No. 5 Table 2 Electrical Properties of the SnO2 Thin Films: (a) Sn Target and (b) SnO2 Target Target Type N2 flow content Conductivity (ohm cm) 1 Sn target SnO2 target Mobility (cm2/vs) Sn target SnO2 target Carrier Concentration (cm 3) Sn target 19 SnO2 target N2 = 0% x x1019 N2 = 10% x x1019 N2 = 20% x x x x1018 N2 = 30% x10 N2 = 40% x1019 Fig. 6. Optical transmittance of the SnO2 thin films: (a) Sn target,6) and (b) SnO2 target. Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Grant No ). REFERENCES Fig. 5. N 1s core level XPS spectra of the SnO2 thin films: (a) Sn target6) and (b) SnO2 target. 1. W. Guo, L. Fu, Y. Zhang, K. Zhang, L. Y. Liang, Z. M. Liu, and H. T. Cao, Microstructure, Optical, and Electrical Properties of p-type SnO Thin Films, Appl. Phys. Lett., :1-3 (2010). 2. S. Wang, Z. Wang, X. Liu, and L. Zhang, Dc and Ac Res-

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