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Optical properties of MoO3 thin films for electrochromic windows
Solid State Ionics 113–115 (1998) 421–423
Optical properties of MoO 3 thin films for electrochromic windows Muhammad Yahaya*, M.M. Salleh, Ibrahim A. Talib Physics Department, Universiti Kebangsaan Malaysia, 43600 Bangi, Malaysia
Abstract Molybdenum oxide thin films have been used in microbatteries and other ionic devices. This paper reports the preparation of MoO 3 by electron beam evaporation technique. The films were deposited onto glass substrates at temperatures in the range of 100–2508C. The films were characterized by studying their structure, electrical and optical properties. The films formed at 1008C are amorphous with conductivity of about 2.5 3 10 25 V21 cm 21 . The absorption spectrum of MoO 3 doped with LiO 2 varies with dopant which suggests that MoO 3 is suitable for electrochromic films. The effect of deposition temperature on the optical and electrical properties of the films are discussed. 1998 Published by Elsevier Science B.V. All rights reserved. Keywords: Molybdenum oxide; Evaporation; Thin film; Slectrochromic; Conductivity
1. Introduction The transition metal oxide thin films have been used in electrochromic devices, solid state microbatteries and display panels. The materials in this category that have attracted most of research interest are MoO 3 , WO 3 and IrO x . Tungsten oxide is the most extensively studied electrochromic films. Molybdenum oxide films shows some common properties with tungsten oxide. Most of the oxides are generally prepared by thermal evaporation [1] and sputtering [2]. In the present work an attempt has been made in the preparation of MoO 3 thin films by electron beam evaporation at 100–2508C. The films were characterized by studying their structure, electrical and optical properties. MoO 3 in thin films form has an orthorhombic structure [3] with a 5 0.3628 nm, b 5 0.13855 nm and c 5 0.36964 nm. *Corresponding author.
MoO 3 doped with Li 2 O has been used as electrochromic films [4–6].
2. Experimental MoO 3 films were prepared by electron beam evaporation under a partial pressure of about 10 25 torr on glass substrates at various temperatures. The starting materials were molybdenum trioxide MoO 3 powder (with purity of 99.95%) in pellete form of about 13 mm diameter. The deposited films were then annealed at 3808C for 3 hours. The thickness of the films were in the range of 500–700 nm. The optical measurement was made over the wavelength range of 300 nm to 1000 nm using a Shimadzu double beam (UV-10) spectrophotometer. The electrical resistivity of the film was measured at room temperature by four probe technique using a Keitley 619 electrometer.
0167-2738 / 98 / $ – see front matter 1998 Published by Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00306-3
422
M. Yahaya et al. / Solid State Ionics 113 – 115 (1998) 421 – 423
3. Results and discussion The films deposited at room temperature were transparent and light blue in colour. The X-ray diffraction pattern shows that the films are amorphous. Most of the evaporated species are Mo 3 O 9 , Mo 4 O 12 and Mo 5 O 15 [7].We were unable to differentiate those species.
3.1. Optical properties The transmission curve in the wavelength range 300–1200 nm of the samples at various substrate temperature is shown in Fig. 1. As can be seen the absorption edge was observed at 400 nm and shifted towards higher wavelength with increasing deposition temperature.The absorption coefficient a can be determined from the expression [8] (1 2 R)2 (1 1 K 2 /n 2 ) T 5 ]]]]]]] exp(a d) 2 R 2 exp(22a d) Where n1iK is the complex refractive index, T is the transmittance, R is the reflectivity and d is the film thickness. If K 2 is much less than n 2 and exp(2a d) is much greater than R, then
Fig. 1. The optical transmission spectrum of MoO 3 films.
T 5 (1 2 R)2 exp(2a d) and lnT 5 2ln(1 2 R) 2 a d hence 1 a 5 ] ln h(1 2 R) /T j d For our purpose, assuming the reflectivity is only about 2% and the thickness of the film is approximately 500 nm, a plot of a 2 vs. E is shown in Fig. 2. The optical band gap is obtained from the plot of a 2 vs. E curve which is assumed of the form a 5 A(hy 2Eg )1 / 2 , where Eg is the optical band gap. At 1758C the optical band gap is 3.4 eV. From the similar plot, Eg for films deposited at 1008C is 3.7 eV, and at 2508C, Eg 53.0 eV, Eg decreases as the substrate temperature increases. Films doped with Li 2 O are also prepared using electron beam evaporation technique [5].The optical absorption is shown in Fig. 3. It appears that the absorption increases as the dopant is increased from 2 to 8%. This suggests that MoO 3 is suitable for electrochromic films.
Fig. 2. The absorption coefficient vs energy at 1758C.
M. Yahaya et al. / Solid State Ionics 113 – 115 (1998) 421 – 423
423
The films are amorphous. The conductivity and transmission of the films depend on the substrate temperature. The conductivity was increased as the substrate temperature increased but when the sample was annealed in air the conductivity was decreased and almost independent of deposition temperature. The measurement suggests that the optical band gap is 3.7 eV at 1008C and appears to decrease with deposition temperature. Films doped with Li 2 O shows a significant change in the absorption.
Acknowledgements
Fig. 3. The absorption of molybdenum oxide doped with (a) 2%, (b) 4%, (c) 6% and (d) 8% Li 2 O. Table 1 Electrical conductivity of MoO 3 Substrate temperature (8C)
Conductivity (V21 cm 21 ) Before annealing
100 175 250
25
2.5310 5.7310 25 9.5310 25
After annealing 6.6310 26 7.7310 26 8.8310 26
One of the authors (MY) is thankful to ASSSI for providing Takahashi Fellowship. We wish to thank Mr.and Mrs Yusof Musa for many helpful discussions. We also wish to thank Chemistry and Geological Departments, UKM for helping in the measurements. Part of the work has been carried out with the support of the Ministry of Science, Technology and Environment, Malaysia and Universiti Kebangsaan Malaysia under the IRPA -09-02-02-00 12 grant.
References
3.2. Electrical properties The conductivity of the film depends markedly on the substrate temperature. The lowest conductivity obtained was 2.5310 25 (Vcm)21 and increase to 9.5310 25 (Vcm)21 at 2508C. Table 1 shows the relation between conductivity and deposition temperature. The increase in conductivity with deposition temperature is due to the formation of oxygen ions vacancies which lies close to the valence band. It is interesting to note that when the films were annealed in air the conductivity decreased and were almost independent of deposition temperature.
4. Conclusion MoO 3 thin films were prepared by electron beam evaporation in the temperature range of 100–2508C.
[1] M.S. Wittingham, Proc. Solid State Chem. 12 (1978) 41. [2] S.K. Deb, Proc. Roy. Soc. London Ser. A 304 (1968) 211. [3] Parker Sybil, P., 1982. Encyclopedia of Chemistry. McGraw Hill, New York. [4] K. Kuwabara, M. Ohno, K. Sugiyana, Solid-state Ionics, Diffusion and Reaction 4 (1991) 319. [5] Bakush, M., Salleh, M.M., Yahaya, M., Talib, I.A., 1993. In: Isa, M.J., Fatah, A., Mat, A. (Eds.), Proc. Solid State Sc. Tech. Malaysian Solid State Sc. Tech. Soc. Pub. Kuala Lumpur, pp. 96–98. [6] Sabhapathy, V.K., Hussain, O.Md., Reddy, P.S., Uthanna, S., Srinivasulu Naidu, B., Jayarama, P., 1992. In: Chowdari, B.V.R., Chandra, S., Shri Singh, Srivastava, P.C., Proceeding of the 3rd Asian Conference on Solid-state lonics, Material and Applications. World Scientific Publishing, Singapore, pp. 635–639. [7] D.L. Neirk, J.C. Fagerli, M.L. Smith, D. Mosman, T.C. De Vore, J. Mol. Struct. 244 (1991) 165. [8] Pankove, J., 1971. Optical Process in Semiconductor. Dover Publication, New York.
Optical properties of MoO 3 thin films for electrochromic windows Muhammad Yahaya*, M.M. Salleh, Ibrahim A. Talib Physics Department, Universiti Kebangsaan Malaysia, 43600 Bangi, Malaysia
Abstract Molybdenum oxide thin films have been used in microbatteries and other ionic devices. This paper reports the preparation of MoO 3 by electron beam evaporation technique. The films were deposited onto glass substrates at temperatures in the range of 100–2508C. The films were characterized by studying their structure, electrical and optical properties. The films formed at 1008C are amorphous with conductivity of about 2.5 3 10 25 V21 cm 21 . The absorption spectrum of MoO 3 doped with LiO 2 varies with dopant which suggests that MoO 3 is suitable for electrochromic films. The effect of deposition temperature on the optical and electrical properties of the films are discussed. 1998 Published by Elsevier Science B.V. All rights reserved. Keywords: Molybdenum oxide; Evaporation; Thin film; Slectrochromic; Conductivity
1. Introduction The transition metal oxide thin films have been used in electrochromic devices, solid state microbatteries and display panels. The materials in this category that have attracted most of research interest are MoO 3 , WO 3 and IrO x . Tungsten oxide is the most extensively studied electrochromic films. Molybdenum oxide films shows some common properties with tungsten oxide. Most of the oxides are generally prepared by thermal evaporation [1] and sputtering [2]. In the present work an attempt has been made in the preparation of MoO 3 thin films by electron beam evaporation at 100–2508C. The films were characterized by studying their structure, electrical and optical properties. MoO 3 in thin films form has an orthorhombic structure [3] with a 5 0.3628 nm, b 5 0.13855 nm and c 5 0.36964 nm. *Corresponding author.
MoO 3 doped with Li 2 O has been used as electrochromic films [4–6].
2. Experimental MoO 3 films were prepared by electron beam evaporation under a partial pressure of about 10 25 torr on glass substrates at various temperatures. The starting materials were molybdenum trioxide MoO 3 powder (with purity of 99.95%) in pellete form of about 13 mm diameter. The deposited films were then annealed at 3808C for 3 hours. The thickness of the films were in the range of 500–700 nm. The optical measurement was made over the wavelength range of 300 nm to 1000 nm using a Shimadzu double beam (UV-10) spectrophotometer. The electrical resistivity of the film was measured at room temperature by four probe technique using a Keitley 619 electrometer.
0167-2738 / 98 / $ – see front matter 1998 Published by Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00306-3
422
M. Yahaya et al. / Solid State Ionics 113 – 115 (1998) 421 – 423
3. Results and discussion The films deposited at room temperature were transparent and light blue in colour. The X-ray diffraction pattern shows that the films are amorphous. Most of the evaporated species are Mo 3 O 9 , Mo 4 O 12 and Mo 5 O 15 [7].We were unable to differentiate those species.
3.1. Optical properties The transmission curve in the wavelength range 300–1200 nm of the samples at various substrate temperature is shown in Fig. 1. As can be seen the absorption edge was observed at 400 nm and shifted towards higher wavelength with increasing deposition temperature.The absorption coefficient a can be determined from the expression [8] (1 2 R)2 (1 1 K 2 /n 2 ) T 5 ]]]]]]] exp(a d) 2 R 2 exp(22a d) Where n1iK is the complex refractive index, T is the transmittance, R is the reflectivity and d is the film thickness. If K 2 is much less than n 2 and exp(2a d) is much greater than R, then
Fig. 1. The optical transmission spectrum of MoO 3 films.
T 5 (1 2 R)2 exp(2a d) and lnT 5 2ln(1 2 R) 2 a d hence 1 a 5 ] ln h(1 2 R) /T j d For our purpose, assuming the reflectivity is only about 2% and the thickness of the film is approximately 500 nm, a plot of a 2 vs. E is shown in Fig. 2. The optical band gap is obtained from the plot of a 2 vs. E curve which is assumed of the form a 5 A(hy 2Eg )1 / 2 , where Eg is the optical band gap. At 1758C the optical band gap is 3.4 eV. From the similar plot, Eg for films deposited at 1008C is 3.7 eV, and at 2508C, Eg 53.0 eV, Eg decreases as the substrate temperature increases. Films doped with Li 2 O are also prepared using electron beam evaporation technique [5].The optical absorption is shown in Fig. 3. It appears that the absorption increases as the dopant is increased from 2 to 8%. This suggests that MoO 3 is suitable for electrochromic films.
Fig. 2. The absorption coefficient vs energy at 1758C.
M. Yahaya et al. / Solid State Ionics 113 – 115 (1998) 421 – 423
423
The films are amorphous. The conductivity and transmission of the films depend on the substrate temperature. The conductivity was increased as the substrate temperature increased but when the sample was annealed in air the conductivity was decreased and almost independent of deposition temperature. The measurement suggests that the optical band gap is 3.7 eV at 1008C and appears to decrease with deposition temperature. Films doped with Li 2 O shows a significant change in the absorption.
Acknowledgements
Fig. 3. The absorption of molybdenum oxide doped with (a) 2%, (b) 4%, (c) 6% and (d) 8% Li 2 O. Table 1 Electrical conductivity of MoO 3 Substrate temperature (8C)
Conductivity (V21 cm 21 ) Before annealing
100 175 250
25
2.5310 5.7310 25 9.5310 25
After annealing 6.6310 26 7.7310 26 8.8310 26
One of the authors (MY) is thankful to ASSSI for providing Takahashi Fellowship. We wish to thank Mr.and Mrs Yusof Musa for many helpful discussions. We also wish to thank Chemistry and Geological Departments, UKM for helping in the measurements. Part of the work has been carried out with the support of the Ministry of Science, Technology and Environment, Malaysia and Universiti Kebangsaan Malaysia under the IRPA -09-02-02-00 12 grant.
References
3.2. Electrical properties The conductivity of the film depends markedly on the substrate temperature. The lowest conductivity obtained was 2.5310 25 (Vcm)21 and increase to 9.5310 25 (Vcm)21 at 2508C. Table 1 shows the relation between conductivity and deposition temperature. The increase in conductivity with deposition temperature is due to the formation of oxygen ions vacancies which lies close to the valence band. It is interesting to note that when the films were annealed in air the conductivity decreased and were almost independent of deposition temperature.
4. Conclusion MoO 3 thin films were prepared by electron beam evaporation in the temperature range of 100–2508C.
[1] M.S. Wittingham, Proc. Solid State Chem. 12 (1978) 41. [2] S.K. Deb, Proc. Roy. Soc. London Ser. A 304 (1968) 211. [3] Parker Sybil, P., 1982. Encyclopedia of Chemistry. McGraw Hill, New York. [4] K. Kuwabara, M. Ohno, K. Sugiyana, Solid-state Ionics, Diffusion and Reaction 4 (1991) 319. [5] Bakush, M., Salleh, M.M., Yahaya, M., Talib, I.A., 1993. In: Isa, M.J., Fatah, A., Mat, A. (Eds.), Proc. Solid State Sc. Tech. Malaysian Solid State Sc. Tech. Soc. Pub. Kuala Lumpur, pp. 96–98. [6] Sabhapathy, V.K., Hussain, O.Md., Reddy, P.S., Uthanna, S., Srinivasulu Naidu, B., Jayarama, P., 1992. In: Chowdari, B.V.R., Chandra, S., Shri Singh, Srivastava, P.C., Proceeding of the 3rd Asian Conference on Solid-state lonics, Material and Applications. World Scientific Publishing, Singapore, pp. 635–639. [7] D.L. Neirk, J.C. Fagerli, M.L. Smith, D. Mosman, T.C. De Vore, J. Mol. Struct. 244 (1991) 165. [8] Pankove, J., 1971. Optical Process in Semiconductor. Dover Publication, New York.