- Email: [email protected]
Characterization of SnO2, In2O3, and ITO films prepared by thermal oxidation of DC-sputtered Sn, In and In–Sn films
ARTICLE IN PRESS
Vacuum 76 (2004) 177–180 www.elsevier.com/locate/vacuum
Characterization of SnO2, In2O3, and ITO films prepared by thermal oxidation of DC-sputtered Sn, In and In–Sn films J.J. Valenzuela-Ja´uregui, R. Quintero-Gonza´lez, J. Herna´ndez-Torres, A. Mendoza-Galva´n, R. Ramı´ rez-Bon Centro de Investigacio´n y de Estudios, Avanzados del IPN, Unidad Quere´taro, Apdo. Postal 1-798, 76001, Quere´taro, Qro., Me´xico
Abstract In this work, we deposited metal films of Sn, In and In–Sn on glass substrates at room temperature by means of the DC sputtering technique. Films of the corresponding metal oxides were obtained after thermal annealing the metal for 1 h in air at temperatures from 350 to 500 1C. We report here the properties of the three types of metal oxides obtained by this method as a function of the annealing temperature. The oxide films were studied by X-ray diffraction, transmission and reflection spectroscopy and by measurements of their sheet resistance between coplanar electrodes. The results show that materials with the properties of transparent conductive oxides (TCO) films can be obtained by this process. r 2004 Elsevier Ltd. All rights reserved. Keywords: Sputtering; Thermal oxidation; Metal layers; Transparent conductive oxides
1. Introduction Transparent conductive oxides (TCO) are materials with high optical transmission in the visible region, high reflection in the near-infrared region and very good electrical conductivity. These exceptional properties make TCO very important technological materials for applications to devices such as solar cells, flat panel displays, electrochromic devices, etc. [1–4]. Among the most Corresponding author. Fax: 52-442-4414939.
E-mail address: [email protected] (R. Ramı´ rez-Bon).
studied and widely applied TCO are SnO2, ZnO, In2O3 and In2O3:Sn (ITO). Several types of deposition techniques can be employed to prepare films of TCO including r.f. sputtering, DC sputtering, reactive evaporation, sol–gel, etc. An alternative process to obtain TCO films, which has been applied successfully to obtain films of several types of metal oxides [5–7], is the thermal oxidation of evaporated metal layers. This method takes advantage on the simplicity of the process to evaporate low melting point metal layers and their easy ways to oxidise.
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.07.008
ARTICLE IN PRESS J.J. Valenzuela-Ja´uregui et al. / Vacuum 76 (2004) 177–180
(110)
SnO2 Films (101)
2000
(211)
(200)
500 °C
450 °C 400 °C
1000 (2 0)
(220)
(211)
350 °C
(101)
In this work, we have applied the DC-magnetron sputtering technique to deposit Sn, In and In–Sn films in order to obtain the corresponding metal oxides by thermal oxidation in air at different temperatures. We studied and compared the oxidation process of three types of metal films as a function of the annealing temperature. We found that the transparency and sheet resistance of the films depend on the annealing temperature. In the three cases, we obtained polycrystalline TCO films with the best properties when the annealing process was carried out at 450 and 500 1C.
INTENSITY (A.U.)
178
Sn Film 0
2. Experimental
10
3. Results and discussion Figs. 1–3 shows the XRD patterns for the three types of as-deposited metal films and after
30 40 50 2 THETA (DEGREES)
60
70
(622)
5000
(440)
In2O3 Films (400)
(211)
6000
(222)
Fig. 1. X-ray diffraction patterns of the Sn metal film and films annealed at 350, 400, 450 and 500 1C.
500 °C
4000 450 °C
3000 400 °C
2000
350 °C
0 10
20
30 40 50 2 THETA (DEGREES)
In Film (103)
1000
(112)
(002)
(101)
INTENSITY (A.U.)
The metal films were deposited at room temperature on glass substrates in a conventional DC-magnetron sputtering system. The base pressure in the vacuum chamber was 1 10 6 Torr and the pressure of Argon was adjusted to 2.5 10 2 Torr during the deposition process. The power used in the growth process was 40 W and the target–substrate distance was 5 cm. The Sn and In films were obtained by sputtering from two inches Sn (99.9999%) and In (99.9999%) targets, respectively. The In (85 wt%)–Sn (15 wt%) films were deposited by sputtering from the In target with additional pieces of metallic Sn placed regularly onto its surface. The area covered by the Sn pieces was about 10% of the total target area. For the three cases, the metal films were deposited during 30 s. After deposition, the metal films were annealed in air at temperatures of 350, 400, 450 and 500 1C for 1 h. The metal and oxide films were studied by X-ray diffraction (XRD), transmission and reflection optical spectroscopy and by measurements of the sheet resistance between two coplanar silver paint electrodes. The In and Sn content in the In–Sn and ITO films was determined from energy dispersive X-ray analysis (EDAX) measurements.
20
60
70
Fig. 2. X-ray diffraction patterns of the In metal film and films annealed at 350, 400, 450 and 500 1C.
annealing at several temperatures. At the bottom of Fig. 1 it is seen that the pattern of the Sn film with diffraction peaks at about 30.6, 31.9, 43.8 and 44.81, correspond to diffraction signals produced by the (2 0), (1 0 1), (2 2 0) and (2 1 1) crystalline planes of the tetragonal structure of elemental Sn.
ARTICLE IN PRESS
500 °C
450 °C
1000
400 °C 350 °C
In-Sn Film 0 10
20
30 40 50 2 THETA (DEGREES)
60
70
Fig. 3. X-ray diffraction patterns of the In-Sn metal film and films annealed at 350, 400, 450 and 500 1C.
The annealing at 350 and 400 1C induces an amorphization of the structure of these films as shown by their XRD patterns. The film annealed at 450 1C has the crystalline structure of SnO2, its pattern displays diffraction peaks at about 26.5, 34.2, 37.9 and 51.71, which match with the peaks produced by the (1 1 0), (1 0 1), (2 0 0) and (2 1 1) planes of the cassiterite SnO2 crystalline phase. The film obtained annealing at 500 1C is also amorphous. On the other hand, in Fig. 2, the pattern of the In film displays diffraction peaks at about 33, 36, 54.5 and 63.21, corresponding to the reflections produced by the (1 0 1), (0 0 2), (1 1 2) and (1 0 3) planes of the tetragonal crystalline structure of elemental In. After thermal annealing the In metal film converts into In2O3 films as shown by the XRD patterns. The patterns of the oxide films exhibit diffraction peaks at about 21.5, 30.5, 35.5, 51 and 60.61, matching with the diffraction lines due to the (2 1 1), (2 2 2), (4 0 0) (4 4 0) and (6 2 2) planes of the cubic phase of In2O3. There are no diffraction signals of elemental indium in these patterns, indicating a total oxidation of the metal layer at all the annealing temperatures. In Fig. 3, the XRD pattern of In–Sn film displays a single diffraction peak at about 331 corresponding to the (1 0 1) planes of tetragonal In
179
crystalline phase. The In–Sn annealed films have the In2O3 structure as shown by their XRD patterns. It is observed that an increase in the intensity of diffraction peaks with annealing temperature indicates an improvement of the crystalline structure. The oxidation of In–Sn annealed films is also complete since there are no evidences of indium in their XRD patterns. Furthermore, since there are not evidences of crystalline phases related to some Sn compound, it can be concluded that Sn atoms are present in solution in the cubic bixbyte In2O3 structure. That is, the films obtained by thermal oxidation of In–Sn metal films are Sn-doped In2O3 (ITO) films. EDAX measurements were carried out on different points of the surface of ITO films obtained at 450 and 500 1C to determine their content of Sn atoms. The results show that the distribution of Sn atoms is uniform on the film surface with a concentration between 6 and 7 wt%, about the same value for both types of ITO films. Fig. 4 shows the transmission (T), reflection (R) and T+R optical spectra of SnO2, In2O3 and ITO films obtained by annealing the corresponding metal films at 450, 500 and 500 1C, respectively. For these cases of SnO2 and In2O3 films, the values of T in the visible region are between 75 and 85%.
100 TRANSMISSION, REFLECTION (%)
(622)
(440)
(400)
ITO Films
(101)
INTENSITY (A.U.)
2000
(211)
(222)
J.J. Valenzuela-Ja´uregui et al. / Vacuum 76 (2004) 177–180
90
T+R
80 70
T SnO2 450 °C In2O3 500 °C
60 50
ITO 40
500 °C
30 R
20 10 0 300
400 500 600 WAVELENGTH (nm)
700
800
Fig. 4. Transmission (T), Reflection (R) and T+R optical spectra of SnO2, In2O3 and ITO films obtained by annealing at 450, 500 and 500 1C, respectively.
ARTICLE IN PRESS J.J. Valenzuela-Ja´uregui et al. / Vacuum 76 (2004) 177–180
SHEET RESISTANCE (Ω/cm)
180
107
SnO2Films In2O3Films ITO Films
106 105 104
perature is increased from 350 to 500 1C. For all the annealing temperatures the sheet resistance of the ITO films is lower, indicating a larger concentration of free carriers in these films. This is another clear evidence of the incorporation of doping Sn atoms in the structure of ITO films. Considering the thickness of the oxide films the lowest values for electrical resistivity attained for the three types of oxide films were 5 10 2 O cm, 1.7 10 1 and 1.3 10 2 O cm for SnO2, In2O3 and ITO films respectively.
103 350 400 450 500 ANNEALING TEMPERATURE (°C) Fig. 5. Sheet resistance for the three types of metal oxides as a function of annealing temperature.
The T+R optical spectrum of the SnO2 film is lower than 100% indicating the loss of light energy, probably by light scattering. On the other hand, the T+R optical spectra of the In2O3 film is about 100% in most of the visible spectra region, indicating that this film is non-absorbing and the light scattering is negligible. For the case of ITO film, its optical transmission is about 90% in the visible spectra region and its T+R spectrum also displays values close to 100% in the visible region, however by comparing with the respective spectrum of the In2O3 film, it can be observed that the values of T+R for ITO films are slightly lower than those of In2O3 films in the long wavelength region. We attribute this difference between both types of films to free carrier absorption in the ITO films due to their larger free carrier concentration produced by the Sn doping. The sheet resistance of the oxide films decreases with annealing temperature as can be seen in Fig. 5. The sheet resistance of SnO2 films decreases one order of magnitude when the annealing temperature is increased from 350 to 450 1C, then it increases for the film annealed at 500 1C. For the case of In2O3 and ITO films their sheet resistance decreases in three orders and one order of magnitude, respectively, when the annealing tem-
4. Conclusions In this paper we have prepared SnO2, In2O3 and ITO films by thermal oxidation at several temperatures of sputtered Sn, In and In–Sn layers, respectively. We reported here the physical properties of the oxide films as a function of the annealing temperature. The results show that annealing at 450–500 1C produces films with characteristics of TCO, with optical transmission higher than 80% and electrical resistivity of the order of 10 1–10 2 O cm.
Acknowledgements We acknowledge the assistance of J. E. Urbina and M. A. Herna´ndez. This work was supported by CONACyT (Project No. 34514-U). References [1] [2] [3] [4]
Ginley DS, Bright C. Mater Res Soc Bull 2000;25:15–8. Granqvist CG, Hultaker A. Thin Solid Films 2002;411:1–5. Herrero J, Guillen C. Vacuum 2002;67:611–6. Park SK, Han JI, Kim WK, Kwak MG. Thin Solid Films 2001;379:49–55. [5] Manno D, Micocci G, Serra A, Di Giulio M, Tepore A. J Appl Phys 2000;88:6571–7. [6] Girtan M, Rusu GI, Rusu GG, Gurlui S. Appl Surf Sci 2000;162–163:492–8. [7] Sasi B, Gopchandran KG, Manj PK, Koshy P, PRao PP, Vaidyan VK. Vacuum 2003;68:149–54.
Vacuum 76 (2004) 177–180 www.elsevier.com/locate/vacuum
Characterization of SnO2, In2O3, and ITO films prepared by thermal oxidation of DC-sputtered Sn, In and In–Sn films J.J. Valenzuela-Ja´uregui, R. Quintero-Gonza´lez, J. Herna´ndez-Torres, A. Mendoza-Galva´n, R. Ramı´ rez-Bon Centro de Investigacio´n y de Estudios, Avanzados del IPN, Unidad Quere´taro, Apdo. Postal 1-798, 76001, Quere´taro, Qro., Me´xico
Abstract In this work, we deposited metal films of Sn, In and In–Sn on glass substrates at room temperature by means of the DC sputtering technique. Films of the corresponding metal oxides were obtained after thermal annealing the metal for 1 h in air at temperatures from 350 to 500 1C. We report here the properties of the three types of metal oxides obtained by this method as a function of the annealing temperature. The oxide films were studied by X-ray diffraction, transmission and reflection spectroscopy and by measurements of their sheet resistance between coplanar electrodes. The results show that materials with the properties of transparent conductive oxides (TCO) films can be obtained by this process. r 2004 Elsevier Ltd. All rights reserved. Keywords: Sputtering; Thermal oxidation; Metal layers; Transparent conductive oxides
1. Introduction Transparent conductive oxides (TCO) are materials with high optical transmission in the visible region, high reflection in the near-infrared region and very good electrical conductivity. These exceptional properties make TCO very important technological materials for applications to devices such as solar cells, flat panel displays, electrochromic devices, etc. [1–4]. Among the most Corresponding author. Fax: 52-442-4414939.
E-mail address: [email protected] (R. Ramı´ rez-Bon).
studied and widely applied TCO are SnO2, ZnO, In2O3 and In2O3:Sn (ITO). Several types of deposition techniques can be employed to prepare films of TCO including r.f. sputtering, DC sputtering, reactive evaporation, sol–gel, etc. An alternative process to obtain TCO films, which has been applied successfully to obtain films of several types of metal oxides [5–7], is the thermal oxidation of evaporated metal layers. This method takes advantage on the simplicity of the process to evaporate low melting point metal layers and their easy ways to oxidise.
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.07.008
ARTICLE IN PRESS J.J. Valenzuela-Ja´uregui et al. / Vacuum 76 (2004) 177–180
(110)
SnO2 Films (101)
2000
(211)
(200)
500 °C
450 °C 400 °C
1000 (2 0)
(220)
(211)
350 °C
(101)
In this work, we have applied the DC-magnetron sputtering technique to deposit Sn, In and In–Sn films in order to obtain the corresponding metal oxides by thermal oxidation in air at different temperatures. We studied and compared the oxidation process of three types of metal films as a function of the annealing temperature. We found that the transparency and sheet resistance of the films depend on the annealing temperature. In the three cases, we obtained polycrystalline TCO films with the best properties when the annealing process was carried out at 450 and 500 1C.
INTENSITY (A.U.)
178
Sn Film 0
2. Experimental
10
3. Results and discussion Figs. 1–3 shows the XRD patterns for the three types of as-deposited metal films and after
30 40 50 2 THETA (DEGREES)
60
70
(622)
5000
(440)
In2O3 Films (400)
(211)
6000
(222)
Fig. 1. X-ray diffraction patterns of the Sn metal film and films annealed at 350, 400, 450 and 500 1C.
500 °C
4000 450 °C
3000 400 °C
2000
350 °C
0 10
20
30 40 50 2 THETA (DEGREES)
In Film (103)
1000
(112)
(002)
(101)
INTENSITY (A.U.)
The metal films were deposited at room temperature on glass substrates in a conventional DC-magnetron sputtering system. The base pressure in the vacuum chamber was 1 10 6 Torr and the pressure of Argon was adjusted to 2.5 10 2 Torr during the deposition process. The power used in the growth process was 40 W and the target–substrate distance was 5 cm. The Sn and In films were obtained by sputtering from two inches Sn (99.9999%) and In (99.9999%) targets, respectively. The In (85 wt%)–Sn (15 wt%) films were deposited by sputtering from the In target with additional pieces of metallic Sn placed regularly onto its surface. The area covered by the Sn pieces was about 10% of the total target area. For the three cases, the metal films were deposited during 30 s. After deposition, the metal films were annealed in air at temperatures of 350, 400, 450 and 500 1C for 1 h. The metal and oxide films were studied by X-ray diffraction (XRD), transmission and reflection optical spectroscopy and by measurements of the sheet resistance between two coplanar silver paint electrodes. The In and Sn content in the In–Sn and ITO films was determined from energy dispersive X-ray analysis (EDAX) measurements.
20
60
70
Fig. 2. X-ray diffraction patterns of the In metal film and films annealed at 350, 400, 450 and 500 1C.
annealing at several temperatures. At the bottom of Fig. 1 it is seen that the pattern of the Sn film with diffraction peaks at about 30.6, 31.9, 43.8 and 44.81, correspond to diffraction signals produced by the (2 0), (1 0 1), (2 2 0) and (2 1 1) crystalline planes of the tetragonal structure of elemental Sn.
ARTICLE IN PRESS
500 °C
450 °C
1000
400 °C 350 °C
In-Sn Film 0 10
20
30 40 50 2 THETA (DEGREES)
60
70
Fig. 3. X-ray diffraction patterns of the In-Sn metal film and films annealed at 350, 400, 450 and 500 1C.
The annealing at 350 and 400 1C induces an amorphization of the structure of these films as shown by their XRD patterns. The film annealed at 450 1C has the crystalline structure of SnO2, its pattern displays diffraction peaks at about 26.5, 34.2, 37.9 and 51.71, which match with the peaks produced by the (1 1 0), (1 0 1), (2 0 0) and (2 1 1) planes of the cassiterite SnO2 crystalline phase. The film obtained annealing at 500 1C is also amorphous. On the other hand, in Fig. 2, the pattern of the In film displays diffraction peaks at about 33, 36, 54.5 and 63.21, corresponding to the reflections produced by the (1 0 1), (0 0 2), (1 1 2) and (1 0 3) planes of the tetragonal crystalline structure of elemental In. After thermal annealing the In metal film converts into In2O3 films as shown by the XRD patterns. The patterns of the oxide films exhibit diffraction peaks at about 21.5, 30.5, 35.5, 51 and 60.61, matching with the diffraction lines due to the (2 1 1), (2 2 2), (4 0 0) (4 4 0) and (6 2 2) planes of the cubic phase of In2O3. There are no diffraction signals of elemental indium in these patterns, indicating a total oxidation of the metal layer at all the annealing temperatures. In Fig. 3, the XRD pattern of In–Sn film displays a single diffraction peak at about 331 corresponding to the (1 0 1) planes of tetragonal In
179
crystalline phase. The In–Sn annealed films have the In2O3 structure as shown by their XRD patterns. It is observed that an increase in the intensity of diffraction peaks with annealing temperature indicates an improvement of the crystalline structure. The oxidation of In–Sn annealed films is also complete since there are no evidences of indium in their XRD patterns. Furthermore, since there are not evidences of crystalline phases related to some Sn compound, it can be concluded that Sn atoms are present in solution in the cubic bixbyte In2O3 structure. That is, the films obtained by thermal oxidation of In–Sn metal films are Sn-doped In2O3 (ITO) films. EDAX measurements were carried out on different points of the surface of ITO films obtained at 450 and 500 1C to determine their content of Sn atoms. The results show that the distribution of Sn atoms is uniform on the film surface with a concentration between 6 and 7 wt%, about the same value for both types of ITO films. Fig. 4 shows the transmission (T), reflection (R) and T+R optical spectra of SnO2, In2O3 and ITO films obtained by annealing the corresponding metal films at 450, 500 and 500 1C, respectively. For these cases of SnO2 and In2O3 films, the values of T in the visible region are between 75 and 85%.
100 TRANSMISSION, REFLECTION (%)
(622)
(440)
(400)
ITO Films
(101)
INTENSITY (A.U.)
2000
(211)
(222)
J.J. Valenzuela-Ja´uregui et al. / Vacuum 76 (2004) 177–180
90
T+R
80 70
T SnO2 450 °C In2O3 500 °C
60 50
ITO 40
500 °C
30 R
20 10 0 300
400 500 600 WAVELENGTH (nm)
700
800
Fig. 4. Transmission (T), Reflection (R) and T+R optical spectra of SnO2, In2O3 and ITO films obtained by annealing at 450, 500 and 500 1C, respectively.
ARTICLE IN PRESS J.J. Valenzuela-Ja´uregui et al. / Vacuum 76 (2004) 177–180
SHEET RESISTANCE (Ω/cm)
180
107
SnO2Films In2O3Films ITO Films
106 105 104
perature is increased from 350 to 500 1C. For all the annealing temperatures the sheet resistance of the ITO films is lower, indicating a larger concentration of free carriers in these films. This is another clear evidence of the incorporation of doping Sn atoms in the structure of ITO films. Considering the thickness of the oxide films the lowest values for electrical resistivity attained for the three types of oxide films were 5 10 2 O cm, 1.7 10 1 and 1.3 10 2 O cm for SnO2, In2O3 and ITO films respectively.
103 350 400 450 500 ANNEALING TEMPERATURE (°C) Fig. 5. Sheet resistance for the three types of metal oxides as a function of annealing temperature.
The T+R optical spectrum of the SnO2 film is lower than 100% indicating the loss of light energy, probably by light scattering. On the other hand, the T+R optical spectra of the In2O3 film is about 100% in most of the visible spectra region, indicating that this film is non-absorbing and the light scattering is negligible. For the case of ITO film, its optical transmission is about 90% in the visible spectra region and its T+R spectrum also displays values close to 100% in the visible region, however by comparing with the respective spectrum of the In2O3 film, it can be observed that the values of T+R for ITO films are slightly lower than those of In2O3 films in the long wavelength region. We attribute this difference between both types of films to free carrier absorption in the ITO films due to their larger free carrier concentration produced by the Sn doping. The sheet resistance of the oxide films decreases with annealing temperature as can be seen in Fig. 5. The sheet resistance of SnO2 films decreases one order of magnitude when the annealing temperature is increased from 350 to 450 1C, then it increases for the film annealed at 500 1C. For the case of In2O3 and ITO films their sheet resistance decreases in three orders and one order of magnitude, respectively, when the annealing tem-
4. Conclusions In this paper we have prepared SnO2, In2O3 and ITO films by thermal oxidation at several temperatures of sputtered Sn, In and In–Sn layers, respectively. We reported here the physical properties of the oxide films as a function of the annealing temperature. The results show that annealing at 450–500 1C produces films with characteristics of TCO, with optical transmission higher than 80% and electrical resistivity of the order of 10 1–10 2 O cm.
Acknowledgements We acknowledge the assistance of J. E. Urbina and M. A. Herna´ndez. This work was supported by CONACyT (Project No. 34514-U). References [1] [2] [3] [4]
Ginley DS, Bright C. Mater Res Soc Bull 2000;25:15–8. Granqvist CG, Hultaker A. Thin Solid Films 2002;411:1–5. Herrero J, Guillen C. Vacuum 2002;67:611–6. Park SK, Han JI, Kim WK, Kwak MG. Thin Solid Films 2001;379:49–55. [5] Manno D, Micocci G, Serra A, Di Giulio M, Tepore A. J Appl Phys 2000;88:6571–7. [6] Girtan M, Rusu GI, Rusu GG, Gurlui S. Appl Surf Sci 2000;162–163:492–8. [7] Sasi B, Gopchandran KG, Manj PK, Koshy P, PRao PP, Vaidyan VK. Vacuum 2003;68:149–54.