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Nanostructural characterisation of SnO2 thin films prepared by reactive r.f. magnetron sputtering of tin
Thin Solid Films 405 (2002) 270–275
Nanostructural characterisation of SnO2 thin films prepared by reactive r.f. magnetron sputtering of tin M.A. Gubbins1, V. Casey*, S.B. Newcomb Materials and Surface Science Institute, University of Limerick, Limerick, Ireland Received 30 May 2001; received in revised form 5 October 2001
Abstract Tin oxide thin films have been deposited on oxidised silicon substrates using a reactive r.f. magnetron sputter process with a tin target in a mixed oxygenyargon gas environment. Process parameters such as oxygen composition, substrate temperature and r.f. power have been varied and the resulting films characterised structurally using X-ray diffraction, atomic force microscopy and transmission electron microscopy (TEM). TEM has demonstrated that the films are composed of an amorphous matrix as well as columnar grains of SnO2 which are interspersed with pores. The deposition method is shown to be a useful way of fabricating nanocrystalline particles of tin oxide within an amorphous matrix and the results are briefly discussed in relation to their significance for the fabrication of tin oxide quantum dots and nanoclusters. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Tin oxide; Transmission electron microscopy (TEM); Atomic force microscopy (AFM); Nanostructures
1. Introduction Tin oxide thin films are technologically important materials and find application in areas such as electroluminescent displays w1x, heat reflectors w2x, mechanical surface coatings w3x and sensors w4x. Recently, it has been shown that the sensitivity of thin film tin oxide gas sensors increases as the particle size is reduced with nanometer sized particles yielding improved response and recovery times w5x. Sensor sensitivity and dynamic characteristics may be increased further by the presence of pores in the films, which provide channels for gas transport that increases the resistivity response and helps gas evacuation during recovery w6x. Fabrication routes for tin oxide nanoparticles and nanoclusters are thus of increasing interest, particularly if such routes can be optimised so that particle parameters such as size and composition can be controlled. In general, film properties are strongly influenced by the choice of deposition method and an ever increasing range of techniques is * Corresponding author. Tel.: q353-61-202290; fax: q353-61202423. E-mail address: [email protected] (V. Casey). 1 Now at: Seagate, 1 Disk Drive, Springtown Industrial Estate, Londonderry BT48 OBF, Northern Ireland.
being used to deposit SnO2 thin films. Some of the more common deposition techniques include dip coating w7x, reactive thermal evaporation w8x, CVD w9x, r.f. magnetron sputtering w10x, r.f. reactive sputtering w11x, spray pyrolysis w12x, plasma polymerisation w13x and glow discharge decomposition of tin compounds w14x. Sputter techniques using modern vacuum technology allow tight control over critical process parameters and this contributes greatly to the reproducibility of the films. For this reason, sputtering is used extensively in the fabrication of functional electronic thin films in the semiconductor industry. More generally, any fabrication route which facilitates the control of particle properties such as size and composition on a nanometer scale is significant because of the novel electronic and optical properties expected for nanocluster and quantum dot structures. In this paper, we describe an r.f. reactive magnetron sputter deposition process which has been used to prepare films containing semiconducting SnO2 nanoparticles embedded in an amorphous insulating matrix. The effects of process parameters such as substrate temperature and oxygen pressure on film structural properties and particle size are described.
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 7 2 8 - X
M.A. Gubbins et al. / Thin Solid Films 405 (2002) 270–275
2. Experimental Tin oxide thin films were deposited by reactive r.f. magnetron sputtering using a Leybold Lab500 system. The magnetron cathode (ION’X-3) with a water-cooled copper backing plate accommodates the 76 mm diameter tin target and is positioned at an angle of 458 to the rotating substrate holder normal and at a distance of 12 cm from the substrate. The substrate holder also incorporates an in-built heaterytemperature control which allows substrate heating to temperatures within the range of 20–600 8C. Oxidised silicon wafers were used as substrates. The chamber was evacuated to a pressure of 1.0 mPa and the substrate heater was allowed to reach the set point temperature before the introduction of argon. Flow rates were adjusted in order to attain an argon pressure of 0.53–0.60 Pa which was found to be the optimum pressure for plasma ignition. The target surface was cleaned by sputtering for 5 min in argon at an r.f. power of 60 W. The argon and oxygen flow rates were adjusted in order to give the desired gas composition while keeping the total pressure at 0.71 Pa. A rotation speed of 1.8 rev.ymin was used for the substrate holder and shutter control was used to set the deposition time. The substrate was allowed to cool to less than 100 8C before venting the chamber and removing the sample. Tin oxide thin films were deposited at two power levels, 40 and 80 W. Four substrate temperatures (330, 400, 440 and 480 8C) and three gas compositions (10, 20 and 30% oxygen in argon) were used in this study. Deposition times of 20 min were used for films deposited at 40 W, and 10 min for films deposited at 80 W. X-Ray diffraction (XRD) of the tin oxide films was carried out using a Philips X-Pert diffractometer operated in the u–2u mode over angles in the range of 20– 608. A Topometrix Explorer Atomic Force Microscope (AFM) was also used to elucidate the structure of the films. It was operated in the non-contact mode using a scanner with a maximum range in the xyy plane of 2.3 mm and a maximum z-range of 0.8 mm. Image reproducibility was verified by recording topography-forward, -reverse and internal-sensor images. Images were processed using SPMlab first-order levelling and shading in order to aid interpretation. Samples for transmission electron microscopy (TEM) were prepared using focused ion beam (FIB) thinning. Small rectangular bars measuring some 3=1=0.05 mm were milled in an FEI 200 workstation and the electron transparent regions were examined in a JEOL 2000FX operated at 200 keV. 3. Results and discussion We begin by describing the data obtained using XRD and AFM and then outline the TEM microstructural observations.
271
Table 1 XRD and TEM diffraction data for a tin oxide film deposited at low oxygen in argon composition (10%) and low power (40 W) XRD
TEM
Bulk SnO2
2u
Counts d-Spacing Ring no. d-Spacing d-Spacing hkl (nm) (nm) (nm)
26.6
100
0.335
75 21 57
0.264 0.237 0.176
33.921 37.98 51.826
1 2 3 4 5
0.3289 0.2838 0.2539 0.2316 0.1703
0.335
110
0.264 0.237 0.176
101 200 211
3.1. X-Ray diffraction Table 1 indicates that peaks characteristic of the tetragonal SnO2 phase were identified in a thin film sample deposited at low oxygen percentage composition (10%) and low power (40 W) conditions. Such peaks were not, however, seen for films deposited at the same oxygen pressure but a higher power (80 W) and this is indicative of the fact that the increased sputter rate of tin at 80 W leads to the formation of the metallic phase andyor SnO rather than SnO2. More critically, there is clear evidence that at 40 W the low oxygen composition is sufficient to oxidise the tin although changes in the deposition temperature (330–480 8C) were found not to lead to any structural modification of the oxide. Comparisons were made for thin films deposited at oxygen in argon compositions of 20 and 30%. XRD indicated that crystalline SnO2 was present for all combinations of temperature (330–480 8C) and power (40 and 80 W) and in this way there were few, if any, differences between the 40 W film deposited at an oxygen composition of 10%. Peak broadening was seen in XRD spectra taken from thin films deposited at 80 W and this indicates that a relatively fast deposition rate promotes the formation of a fine grain sized oxide. The grain size of the oxide was found to be in the range of 6–29 nm. Larger grain sizes tended to be seen in samples deposited at low power although no relationship was apparent between the grain size of the oxide and the temperature at which it was deposited. 3.2. Atomic force microscopy AFM images and the corresponding particle size histograms for films deposited at 20% oxygen in argon composition, 80 W and temperatures of 330, 400, 440 and 480 8C are shown in Fig. 1. The AFM images of all samples displayed a granular structure with an overall grain size ranging from 8 to 54 nm, which is small relative to films obtained using other deposition methods but not uncommon for sputtered films. There would appear to be a discrepancy between the XRD and AFM grain sizes. As we will see from the TEM data below,
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Fig. 1. AFM images of the surfaces of oxides deposited at a 20% oxygen in argon composition, 80 W and temperatures of (a) 330 8C, (b) 400 8C, (c) 440 8C and (d) 480 8C. The grain size distribution histograms for each temperature are also shown.
the film contains dagger-like crystallites with the ‘thick end’ of the crystallite at the surface and the ‘thin end’ within the film. The AFM grain sizes are averages over the surface of the film (relatively large crystallites) whereas the XRD averages over the entire thickness of the film (large and small crystallites) and so provides a lower estimate of grain size. The AFM data provides clear evidence for the way in which the grain size of
the oxide increases with substrate temperature and this is in apparent contrast to the X-ray analysis where no such trend was observed. The surfaces of the films were found to be reasonably smooth and exhibited surface roughness values which varied from 0.4 to 1.0 nm. This is in marked contrast to the much higher roughness (50– 100 nm) seen in films of similar thickness deposited by MOCVD w15x. No relationship was found between
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tion of this oxide, Table 1, demonstrating that there is a good correlation between the TEM and XRD data described. The origin of the weak reflections seen at a d-spacing of 0.2838 nm was not, however, determined, but is likely to originate from the capping layer used to protect the surface during ion beam milling of the sample. The microstructure of the layer deposited at an oxygen in argon composition of 10% was examined in further detail and a number of interesting features were observed. The thin film deposit was found to be banded, as demonstrated by the pair of under- and over-focus bright field images shown in Fig. 3a,b, respectively. Here a series of ‘layers’ of alternating light and dark contrast can be observed which were found to extend across the full thickness of the deposit. Such localised changes in contrast are indicative of a modulation in the Sn content of the film, the low contrast ‘sub-layers’ being of apparently lower Sn content than those exhibiting relatively strong absorption contrast. Given the way in which the target lies at an inclined angle to the plane of the substrate, the variation in Sn content is likely to originate from rotation of the substrate during deposition of the thin film. This view is supported by
Fig. 2. A tin oxide film deposited at 40 W, 440 8C and a 10% oxygen in argon composition showing (a) a low magnification bright field image of the layer (A, tin oxide; B, SiO2; C, Si; D, capping layer) and (b) a diffraction pattern taken from the deposit (ring numbers relate to Table 1).
surface roughness and deposition temperature whilst increases in the oxygen in argon composition to 30% gave surface roughness values which tended to the lower end of the range (0.4–0.6 nm) for both power levels. 3.3. Transmission electron microscopy The microstructures of the thin film oxide deposits have been examined using TEM and comparisons made as a function of the different oxygen in argon composition (10, 20 and 30%) used for films fabricated at 40 W and a substrate temperature at 440 8C. Fig. 2a shows a cross-sectional low magnification micrograph of a typical region of the thin film formed at the low oxygen composition (10%). Here the fully continuous deposit, which has a thickness of some 330 nm, has been marked at A and the underlying SiO2 and Si substrate at B and C, respectively. An electron diffraction pattern taken from the deposit is shown in Fig. 2b and Table 1 gives the measured d-spacings. Comparisons can be made with the tabulated d-spacings for the tetragonal SnO2 phase and this provides further evidence for the forma-
Fig. 3. Higher magnification (a) under-focus and (b) over-focus bright field micrographs of the tin oxide film deposited at 40 W, 440 8C and 10% oxygen in argon (X, crystallite region; Y, amorphous region; P, pore).
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at the highest oxygen composition (30%). Here the thickness of the deposit is as low as 50 nm but as before tapered columnar crystallites can be seen to penetrate downward from the surface into an otherwise amorphous matrix. Crystallite diameters vary from 3 to 10 nm and vertical pores are also evident but the compositional modulation observed in the sample deposited at 10% oxygen composition was not evident. 4. Discussion and conclusions
Fig. 4. A dark field image of the tin oxide film deposited at 40 W, 440 8C and 10% oxygen in argon composition showing the presence of amorphous (A) and crystalline (C) oxide phases.
the closeness of the length of time per rotation of the substrate (33 s) and the estimated time for the deposition of each pair of sub-layers within the deposit (40 s). The latter was determined from a knowledge of the total deposition time (1200 s), the overall film thickness (330 nm) and the average thickness of a pair of sub-layers (11 nm). The oxide deposit was also examined in dark field, as typified by the image shown in Fig. 4. Here the critical point is the way in which the thin film was found to contain both crystalline and amorphous SnO2 phases, the latter being confirmed by the objective aperture size dependent nature of the speckle contrast w16x seen in regions such as that marked at A. Comparison with the diffraction pattern shown earlier in Fig. 2b indicates the presence of the diffuse halo marked at A and this provides further evidence for the formation of the amorphous phase. Figs. 3 and 4 not only indicate the way in which approximately the lower third of the deposit contains only the amorphous oxide but also that the thin film clearly contains a number of crystallites. Such grains exhibit a tendency to have a high aspect ratio common in tin oxide thin films w15,17x, whilst there is an apparent relationship between the presence of crystallites at the surface of the deposit and local increases in surface roughness. Compare, for example, the local areas marked at X and Y in Fig. 3a. The defocus images are also of interest for the way in which they show that the crystalline parts of the deposit are interspersed with pores. The visibility of the pores has been enhanced using Fresnel contrast as a function of the defocus conditions in Fig. 3 and examples of the vertically aligned pores have been marked at P in Fig. 3a. TEM examination of samples deposited at the higher oxygen in argon composition of 20 and 30% displayed similar characteristics to the sample described above. Fig. 5 shows an example of a thin film layer deposited
The tin oxide films deposited using the method described have been shown to contain both crystalline and amorphous SnO2. The crystallites tended to be columnar in nature and were interspersed with fine pores. The banding composition evident in the films produced at the low oxygen composition (10%) is likely to originate from the fact that the cathode is not oxidised at this low oxygen pressure and so deposition occurs primarily from a metallic tin target. The compositional variation across the thin film originates from the effective variation of the target to substrate distance due to the rotation of the substrate as explained above. At higher oxygen pressures, the target is oxidised and the deposition occurs from a tin oxide surface, as evidenced by a marked reduction in the sputter yield at higher oxygen pressures. The properties of nanostructured materials are determined by the interplay of three main parameters: domain size; composition; and interfaces. The sputter process tends to produce small grain sizes and is thus inherently suited to the production of nanocrystalline thin films. The morphological structure of the films includes significant interface regions between the nanocrystallites. However, the semiconducting nature of the polycrystalline SnO2 phase may give rise to a dominance of the interfacial regions in determining the electronic properties of the overall film. Grain surface depletion layers may extend through a large portion of the grains and in such a case carriers will be confined to a volume that
Fig. 5. A bright field TEM micrograph of the tin oxide film deposited at 40 W, 440 8C and an oxygen in argon composition of 30% (A, SnO2; B, SiO2; C, Si).
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is significantly less than the geometric size of the film. This in turn may lead to quantum size effects in the electrical transport properties of the film. The existence of an amorphous (non-conducting) matrix between the columnar grains is interesting as it offers the possibility of generating SnO2 quantum dot and nanocluster structures via the deposition route described. Quantum dots need to meet several conditions in order to be useful for room temperature operation. In addition to the obvious maximum size restriction, the size distribution must be uniform since the energy bandy level structure varies with dot size. Defect densities should be low as localised defects produce variations in the electrical and optical properties between dots. It is also desirable to know the shape of the dots in order to calculate the energy band structure. The deposition technique used here offers some promise for the production of semiconducting quantum dots: a significant energy offset between the energy structure of the grains and the amorphous matrix is expected; grain sizes and shapes are relatively narrowly defined and the grains are structurally stable. However, the approach suffers from some of the short-comings common with other quantum dot fabrication routes such as colloidal suspensions. Grain surfaces and interfaces will be dominated by electronic defects so making it difficult to reproduce the exact band structure whilst there is significant connectivity, at least physically, between the nanoparticles. Acknowledgments The authors would like to acknowledge the Irish Higher Education Authority (HEA) for a grant under its Capital Equipment Scheme which was used to purchase the Lab500 system. One of the authors (Mark A.
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Gubbins) would like to acknowledge both Enterprise Ireland and the UL Physics Department for partial support for this work. References w1x J. Molloy, P. Maguire, S.J. Laverty, J.A. McLaughlin, J. Electrochem. Soc. 142 (1995) 4285. w2x M. Mwamburi, E. Wackelgard, ¨ A. Roosb, Thin Solid Films ˚ 374 (2000) 1. w3x H.L. Hartnagel, A.L. Dawar, A.K. Jain, C. Jagadish, Semiconducting Transparent Thin Films, IOP Publishing Limited, 1995, p. 322. w4x K. Ihokura, J. Watson, The Stannic Oxide Gas Sensor, CRC Press Incorporated, 1994. w5x F. Lua, Y. Liua, M. Donga, X. Wang, Sens. Actuators B 66 (2000) 225. w6x F. Edelman, H. Hanna, S. Seifrieda, C. Alofa, H. Hochea, A. Balogha, P. Wernerb, K. Zakrzewskac, M. Radeckac, P. Pasierbc, A. Chackc, V. Mikhelashvilid, G. Eisensteind, Mater. Sci. Eng., B 69y70 (2000) 386. w7x H. Dislich, E. Hussmann, Thin Solid Films 77 (1981) 129. w8x V. Casey, M.I. Stephenson, J. Phys. D: Appl. Phys. 23 (1990) 1212. w9x R. Lalauze, P. Breuil, C. Pijolat, Sens. Actuators B 3 (1991) 175. w10x H.S. Park, H.W. Shin, D.H. Yun, H.K. Hong, C.H. Kwon, K. Lee, S.T. Kim, Sens. Actuators B 24y25 (1995) 478. w11x K. Steiner, G. Sulz, E. Neske, E. Wagner, Sens. Actuators B 26y27 (1995) 64. w12x E. Shanthi, A. Banerjee, K.L. Chopra, Thin Solid Films 88 (1982) 93. w13x N. Inagaki, Y. Hashimoto, J. Polym. Sci.: Polym. Lett. 24 (1986) 447. w14x G.S. Devi, S. Manorama, V.J. Rao, Sens. Actuators B 28 (1995) 31. w15x S. Woo Lee, P. Ping Tsai, H. Chen, Sens. Actuators B 41 (1997) 55. w16x W.M. Stobbs, in: P.H. Gaskell (Ed.), The Structure of NonCrystalline Materials, Taylor and Francis, London, 1973, p. 253. w17x G.S. Park, G.M. Yang, Thin Solid Films 365 (2000) 7.
Nanostructural characterisation of SnO2 thin films prepared by reactive r.f. magnetron sputtering of tin M.A. Gubbins1, V. Casey*, S.B. Newcomb Materials and Surface Science Institute, University of Limerick, Limerick, Ireland Received 30 May 2001; received in revised form 5 October 2001
Abstract Tin oxide thin films have been deposited on oxidised silicon substrates using a reactive r.f. magnetron sputter process with a tin target in a mixed oxygenyargon gas environment. Process parameters such as oxygen composition, substrate temperature and r.f. power have been varied and the resulting films characterised structurally using X-ray diffraction, atomic force microscopy and transmission electron microscopy (TEM). TEM has demonstrated that the films are composed of an amorphous matrix as well as columnar grains of SnO2 which are interspersed with pores. The deposition method is shown to be a useful way of fabricating nanocrystalline particles of tin oxide within an amorphous matrix and the results are briefly discussed in relation to their significance for the fabrication of tin oxide quantum dots and nanoclusters. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Tin oxide; Transmission electron microscopy (TEM); Atomic force microscopy (AFM); Nanostructures
1. Introduction Tin oxide thin films are technologically important materials and find application in areas such as electroluminescent displays w1x, heat reflectors w2x, mechanical surface coatings w3x and sensors w4x. Recently, it has been shown that the sensitivity of thin film tin oxide gas sensors increases as the particle size is reduced with nanometer sized particles yielding improved response and recovery times w5x. Sensor sensitivity and dynamic characteristics may be increased further by the presence of pores in the films, which provide channels for gas transport that increases the resistivity response and helps gas evacuation during recovery w6x. Fabrication routes for tin oxide nanoparticles and nanoclusters are thus of increasing interest, particularly if such routes can be optimised so that particle parameters such as size and composition can be controlled. In general, film properties are strongly influenced by the choice of deposition method and an ever increasing range of techniques is * Corresponding author. Tel.: q353-61-202290; fax: q353-61202423. E-mail address: [email protected] (V. Casey). 1 Now at: Seagate, 1 Disk Drive, Springtown Industrial Estate, Londonderry BT48 OBF, Northern Ireland.
being used to deposit SnO2 thin films. Some of the more common deposition techniques include dip coating w7x, reactive thermal evaporation w8x, CVD w9x, r.f. magnetron sputtering w10x, r.f. reactive sputtering w11x, spray pyrolysis w12x, plasma polymerisation w13x and glow discharge decomposition of tin compounds w14x. Sputter techniques using modern vacuum technology allow tight control over critical process parameters and this contributes greatly to the reproducibility of the films. For this reason, sputtering is used extensively in the fabrication of functional electronic thin films in the semiconductor industry. More generally, any fabrication route which facilitates the control of particle properties such as size and composition on a nanometer scale is significant because of the novel electronic and optical properties expected for nanocluster and quantum dot structures. In this paper, we describe an r.f. reactive magnetron sputter deposition process which has been used to prepare films containing semiconducting SnO2 nanoparticles embedded in an amorphous insulating matrix. The effects of process parameters such as substrate temperature and oxygen pressure on film structural properties and particle size are described.
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 7 2 8 - X
M.A. Gubbins et al. / Thin Solid Films 405 (2002) 270–275
2. Experimental Tin oxide thin films were deposited by reactive r.f. magnetron sputtering using a Leybold Lab500 system. The magnetron cathode (ION’X-3) with a water-cooled copper backing plate accommodates the 76 mm diameter tin target and is positioned at an angle of 458 to the rotating substrate holder normal and at a distance of 12 cm from the substrate. The substrate holder also incorporates an in-built heaterytemperature control which allows substrate heating to temperatures within the range of 20–600 8C. Oxidised silicon wafers were used as substrates. The chamber was evacuated to a pressure of 1.0 mPa and the substrate heater was allowed to reach the set point temperature before the introduction of argon. Flow rates were adjusted in order to attain an argon pressure of 0.53–0.60 Pa which was found to be the optimum pressure for plasma ignition. The target surface was cleaned by sputtering for 5 min in argon at an r.f. power of 60 W. The argon and oxygen flow rates were adjusted in order to give the desired gas composition while keeping the total pressure at 0.71 Pa. A rotation speed of 1.8 rev.ymin was used for the substrate holder and shutter control was used to set the deposition time. The substrate was allowed to cool to less than 100 8C before venting the chamber and removing the sample. Tin oxide thin films were deposited at two power levels, 40 and 80 W. Four substrate temperatures (330, 400, 440 and 480 8C) and three gas compositions (10, 20 and 30% oxygen in argon) were used in this study. Deposition times of 20 min were used for films deposited at 40 W, and 10 min for films deposited at 80 W. X-Ray diffraction (XRD) of the tin oxide films was carried out using a Philips X-Pert diffractometer operated in the u–2u mode over angles in the range of 20– 608. A Topometrix Explorer Atomic Force Microscope (AFM) was also used to elucidate the structure of the films. It was operated in the non-contact mode using a scanner with a maximum range in the xyy plane of 2.3 mm and a maximum z-range of 0.8 mm. Image reproducibility was verified by recording topography-forward, -reverse and internal-sensor images. Images were processed using SPMlab first-order levelling and shading in order to aid interpretation. Samples for transmission electron microscopy (TEM) were prepared using focused ion beam (FIB) thinning. Small rectangular bars measuring some 3=1=0.05 mm were milled in an FEI 200 workstation and the electron transparent regions were examined in a JEOL 2000FX operated at 200 keV. 3. Results and discussion We begin by describing the data obtained using XRD and AFM and then outline the TEM microstructural observations.
271
Table 1 XRD and TEM diffraction data for a tin oxide film deposited at low oxygen in argon composition (10%) and low power (40 W) XRD
TEM
Bulk SnO2
2u
Counts d-Spacing Ring no. d-Spacing d-Spacing hkl (nm) (nm) (nm)
26.6
100
0.335
75 21 57
0.264 0.237 0.176
33.921 37.98 51.826
1 2 3 4 5
0.3289 0.2838 0.2539 0.2316 0.1703
0.335
110
0.264 0.237 0.176
101 200 211
3.1. X-Ray diffraction Table 1 indicates that peaks characteristic of the tetragonal SnO2 phase were identified in a thin film sample deposited at low oxygen percentage composition (10%) and low power (40 W) conditions. Such peaks were not, however, seen for films deposited at the same oxygen pressure but a higher power (80 W) and this is indicative of the fact that the increased sputter rate of tin at 80 W leads to the formation of the metallic phase andyor SnO rather than SnO2. More critically, there is clear evidence that at 40 W the low oxygen composition is sufficient to oxidise the tin although changes in the deposition temperature (330–480 8C) were found not to lead to any structural modification of the oxide. Comparisons were made for thin films deposited at oxygen in argon compositions of 20 and 30%. XRD indicated that crystalline SnO2 was present for all combinations of temperature (330–480 8C) and power (40 and 80 W) and in this way there were few, if any, differences between the 40 W film deposited at an oxygen composition of 10%. Peak broadening was seen in XRD spectra taken from thin films deposited at 80 W and this indicates that a relatively fast deposition rate promotes the formation of a fine grain sized oxide. The grain size of the oxide was found to be in the range of 6–29 nm. Larger grain sizes tended to be seen in samples deposited at low power although no relationship was apparent between the grain size of the oxide and the temperature at which it was deposited. 3.2. Atomic force microscopy AFM images and the corresponding particle size histograms for films deposited at 20% oxygen in argon composition, 80 W and temperatures of 330, 400, 440 and 480 8C are shown in Fig. 1. The AFM images of all samples displayed a granular structure with an overall grain size ranging from 8 to 54 nm, which is small relative to films obtained using other deposition methods but not uncommon for sputtered films. There would appear to be a discrepancy between the XRD and AFM grain sizes. As we will see from the TEM data below,
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Fig. 1. AFM images of the surfaces of oxides deposited at a 20% oxygen in argon composition, 80 W and temperatures of (a) 330 8C, (b) 400 8C, (c) 440 8C and (d) 480 8C. The grain size distribution histograms for each temperature are also shown.
the film contains dagger-like crystallites with the ‘thick end’ of the crystallite at the surface and the ‘thin end’ within the film. The AFM grain sizes are averages over the surface of the film (relatively large crystallites) whereas the XRD averages over the entire thickness of the film (large and small crystallites) and so provides a lower estimate of grain size. The AFM data provides clear evidence for the way in which the grain size of
the oxide increases with substrate temperature and this is in apparent contrast to the X-ray analysis where no such trend was observed. The surfaces of the films were found to be reasonably smooth and exhibited surface roughness values which varied from 0.4 to 1.0 nm. This is in marked contrast to the much higher roughness (50– 100 nm) seen in films of similar thickness deposited by MOCVD w15x. No relationship was found between
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273
tion of this oxide, Table 1, demonstrating that there is a good correlation between the TEM and XRD data described. The origin of the weak reflections seen at a d-spacing of 0.2838 nm was not, however, determined, but is likely to originate from the capping layer used to protect the surface during ion beam milling of the sample. The microstructure of the layer deposited at an oxygen in argon composition of 10% was examined in further detail and a number of interesting features were observed. The thin film deposit was found to be banded, as demonstrated by the pair of under- and over-focus bright field images shown in Fig. 3a,b, respectively. Here a series of ‘layers’ of alternating light and dark contrast can be observed which were found to extend across the full thickness of the deposit. Such localised changes in contrast are indicative of a modulation in the Sn content of the film, the low contrast ‘sub-layers’ being of apparently lower Sn content than those exhibiting relatively strong absorption contrast. Given the way in which the target lies at an inclined angle to the plane of the substrate, the variation in Sn content is likely to originate from rotation of the substrate during deposition of the thin film. This view is supported by
Fig. 2. A tin oxide film deposited at 40 W, 440 8C and a 10% oxygen in argon composition showing (a) a low magnification bright field image of the layer (A, tin oxide; B, SiO2; C, Si; D, capping layer) and (b) a diffraction pattern taken from the deposit (ring numbers relate to Table 1).
surface roughness and deposition temperature whilst increases in the oxygen in argon composition to 30% gave surface roughness values which tended to the lower end of the range (0.4–0.6 nm) for both power levels. 3.3. Transmission electron microscopy The microstructures of the thin film oxide deposits have been examined using TEM and comparisons made as a function of the different oxygen in argon composition (10, 20 and 30%) used for films fabricated at 40 W and a substrate temperature at 440 8C. Fig. 2a shows a cross-sectional low magnification micrograph of a typical region of the thin film formed at the low oxygen composition (10%). Here the fully continuous deposit, which has a thickness of some 330 nm, has been marked at A and the underlying SiO2 and Si substrate at B and C, respectively. An electron diffraction pattern taken from the deposit is shown in Fig. 2b and Table 1 gives the measured d-spacings. Comparisons can be made with the tabulated d-spacings for the tetragonal SnO2 phase and this provides further evidence for the forma-
Fig. 3. Higher magnification (a) under-focus and (b) over-focus bright field micrographs of the tin oxide film deposited at 40 W, 440 8C and 10% oxygen in argon (X, crystallite region; Y, amorphous region; P, pore).
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at the highest oxygen composition (30%). Here the thickness of the deposit is as low as 50 nm but as before tapered columnar crystallites can be seen to penetrate downward from the surface into an otherwise amorphous matrix. Crystallite diameters vary from 3 to 10 nm and vertical pores are also evident but the compositional modulation observed in the sample deposited at 10% oxygen composition was not evident. 4. Discussion and conclusions
Fig. 4. A dark field image of the tin oxide film deposited at 40 W, 440 8C and 10% oxygen in argon composition showing the presence of amorphous (A) and crystalline (C) oxide phases.
the closeness of the length of time per rotation of the substrate (33 s) and the estimated time for the deposition of each pair of sub-layers within the deposit (40 s). The latter was determined from a knowledge of the total deposition time (1200 s), the overall film thickness (330 nm) and the average thickness of a pair of sub-layers (11 nm). The oxide deposit was also examined in dark field, as typified by the image shown in Fig. 4. Here the critical point is the way in which the thin film was found to contain both crystalline and amorphous SnO2 phases, the latter being confirmed by the objective aperture size dependent nature of the speckle contrast w16x seen in regions such as that marked at A. Comparison with the diffraction pattern shown earlier in Fig. 2b indicates the presence of the diffuse halo marked at A and this provides further evidence for the formation of the amorphous phase. Figs. 3 and 4 not only indicate the way in which approximately the lower third of the deposit contains only the amorphous oxide but also that the thin film clearly contains a number of crystallites. Such grains exhibit a tendency to have a high aspect ratio common in tin oxide thin films w15,17x, whilst there is an apparent relationship between the presence of crystallites at the surface of the deposit and local increases in surface roughness. Compare, for example, the local areas marked at X and Y in Fig. 3a. The defocus images are also of interest for the way in which they show that the crystalline parts of the deposit are interspersed with pores. The visibility of the pores has been enhanced using Fresnel contrast as a function of the defocus conditions in Fig. 3 and examples of the vertically aligned pores have been marked at P in Fig. 3a. TEM examination of samples deposited at the higher oxygen in argon composition of 20 and 30% displayed similar characteristics to the sample described above. Fig. 5 shows an example of a thin film layer deposited
The tin oxide films deposited using the method described have been shown to contain both crystalline and amorphous SnO2. The crystallites tended to be columnar in nature and were interspersed with fine pores. The banding composition evident in the films produced at the low oxygen composition (10%) is likely to originate from the fact that the cathode is not oxidised at this low oxygen pressure and so deposition occurs primarily from a metallic tin target. The compositional variation across the thin film originates from the effective variation of the target to substrate distance due to the rotation of the substrate as explained above. At higher oxygen pressures, the target is oxidised and the deposition occurs from a tin oxide surface, as evidenced by a marked reduction in the sputter yield at higher oxygen pressures. The properties of nanostructured materials are determined by the interplay of three main parameters: domain size; composition; and interfaces. The sputter process tends to produce small grain sizes and is thus inherently suited to the production of nanocrystalline thin films. The morphological structure of the films includes significant interface regions between the nanocrystallites. However, the semiconducting nature of the polycrystalline SnO2 phase may give rise to a dominance of the interfacial regions in determining the electronic properties of the overall film. Grain surface depletion layers may extend through a large portion of the grains and in such a case carriers will be confined to a volume that
Fig. 5. A bright field TEM micrograph of the tin oxide film deposited at 40 W, 440 8C and an oxygen in argon composition of 30% (A, SnO2; B, SiO2; C, Si).
M.A. Gubbins et al. / Thin Solid Films 405 (2002) 270–275
is significantly less than the geometric size of the film. This in turn may lead to quantum size effects in the electrical transport properties of the film. The existence of an amorphous (non-conducting) matrix between the columnar grains is interesting as it offers the possibility of generating SnO2 quantum dot and nanocluster structures via the deposition route described. Quantum dots need to meet several conditions in order to be useful for room temperature operation. In addition to the obvious maximum size restriction, the size distribution must be uniform since the energy bandy level structure varies with dot size. Defect densities should be low as localised defects produce variations in the electrical and optical properties between dots. It is also desirable to know the shape of the dots in order to calculate the energy band structure. The deposition technique used here offers some promise for the production of semiconducting quantum dots: a significant energy offset between the energy structure of the grains and the amorphous matrix is expected; grain sizes and shapes are relatively narrowly defined and the grains are structurally stable. However, the approach suffers from some of the short-comings common with other quantum dot fabrication routes such as colloidal suspensions. Grain surfaces and interfaces will be dominated by electronic defects so making it difficult to reproduce the exact band structure whilst there is significant connectivity, at least physically, between the nanoparticles. Acknowledgments The authors would like to acknowledge the Irish Higher Education Authority (HEA) for a grant under its Capital Equipment Scheme which was used to purchase the Lab500 system. One of the authors (Mark A.
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Gubbins) would like to acknowledge both Enterprise Ireland and the UL Physics Department for partial support for this work. References w1x J. Molloy, P. Maguire, S.J. Laverty, J.A. McLaughlin, J. Electrochem. Soc. 142 (1995) 4285. w2x M. Mwamburi, E. Wackelgard, ¨ A. Roosb, Thin Solid Films ˚ 374 (2000) 1. w3x H.L. Hartnagel, A.L. Dawar, A.K. Jain, C. Jagadish, Semiconducting Transparent Thin Films, IOP Publishing Limited, 1995, p. 322. w4x K. Ihokura, J. Watson, The Stannic Oxide Gas Sensor, CRC Press Incorporated, 1994. w5x F. Lua, Y. Liua, M. Donga, X. Wang, Sens. Actuators B 66 (2000) 225. w6x F. Edelman, H. Hanna, S. Seifrieda, C. Alofa, H. Hochea, A. Balogha, P. Wernerb, K. Zakrzewskac, M. Radeckac, P. Pasierbc, A. Chackc, V. Mikhelashvilid, G. Eisensteind, Mater. Sci. Eng., B 69y70 (2000) 386. w7x H. Dislich, E. Hussmann, Thin Solid Films 77 (1981) 129. w8x V. Casey, M.I. Stephenson, J. Phys. D: Appl. Phys. 23 (1990) 1212. w9x R. Lalauze, P. Breuil, C. Pijolat, Sens. Actuators B 3 (1991) 175. w10x H.S. Park, H.W. Shin, D.H. Yun, H.K. Hong, C.H. Kwon, K. Lee, S.T. Kim, Sens. Actuators B 24y25 (1995) 478. w11x K. Steiner, G. Sulz, E. Neske, E. Wagner, Sens. Actuators B 26y27 (1995) 64. w12x E. Shanthi, A. Banerjee, K.L. Chopra, Thin Solid Films 88 (1982) 93. w13x N. Inagaki, Y. Hashimoto, J. Polym. Sci.: Polym. Lett. 24 (1986) 447. w14x G.S. Devi, S. Manorama, V.J. Rao, Sens. Actuators B 28 (1995) 31. w15x S. Woo Lee, P. Ping Tsai, H. Chen, Sens. Actuators B 41 (1997) 55. w16x W.M. Stobbs, in: P.H. Gaskell (Ed.), The Structure of NonCrystalline Materials, Taylor and Francis, London, 1973, p. 253. w17x G.S. Park, G.M. Yang, Thin Solid Films 365 (2000) 7.