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The photoemissive effect of the ITO–Cs thin film
Applied Surface Science 221 (2004) 110–113
The photoemissive effect of the ITO–Cs thin film Zhao Shouzhen*, Xie Baosen, Liang Cuiguo Department of Electron Science and Technology, Nankai University, Tianjin 300071, PR China Received 21 October 2002; received in revised form 16 April 2003; accepted 26 June 2003
Abstract In the experiment, we found that when activated by cesium, the indium tin oxide (ITO) thin film would have a photoemissive effect under the radiation of visible light. This paper reports the experiment procedures and the testing results of the photoemissive characteristics of the ITO–Cs thin film. Some phenomena in the experiment are also analyzed in the paper. # 2003 Published by Elsevier B.V. PACS: 73.40; 79.60 Keywords: ITO transparent thin film; Cesium atom; Photoemissive effect
1. Introduction The indium tin oxide (ITO) thin film is a transparent semiconductor film. Sometimes the In2O3–SnO2 thin film is also called ITO thin film. Compared with other transparent thin films, the ITO thin film has excellent chemical and thermal stability and the ITO thin film is very easy to be etched into various shapes using photoetching methods. The ITO thin films are often used as transparent electrodes in vidicons, electroluminescent displays, plasma display screens, and liquid crystal displays [1–4]. According to some reports [5,6], the ITO thin film can also be used in electrostatic shielding, touch switches, etc. Because its energy gap is about 4.3–4.8 eV [7–10], ITO thin films do not have photoemissive effects under the radiation of visible light. Our experiment showed that ITO–Cs
*
Corresponding author. Tel.: þ1-651-283-3647; fax: þ1-651-224-2057. E-mail address: [email protected] (Z. Shouzhen). 0169-4332/$ – see front matter # 2003 Published by Elsevier B.V. doi:10.1016/S0169-4332(03)00863-8
thin films, however, do have such effects under the radiation of visible light.
2. The structure of the ITO–Cs thin film tube The glass tube for studying the photoemissive effect of the ITO thin film is shown in Fig. 1. The top of the tube is an aluminum film. The bottom of the tube is an ITO thin film. The ITO (In2O3–SnO2) thin film is deposited on a glass plate using the magnetron sputtering method. Its transparency is about 85%. The distance between the ITO thin film and the aluminum film electrode is about 4 cm. The ITO thin film is etched into a square area of 2 cm 2 cm. Then the ITO thin film glass plate and the aluminum film electrode are sealed to the glass tube using epoxy resins. After the epoxy resins become solid in the air, connect the tube and the glass holder containing the mixture powders of Zr and CsCr2O3 to the vacuum system, as shown in Fig. 2.
Z. Shouzhen et al. / Applied Surface Science 221 (2004) 110–113
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Fig. 1. The structure of the tube.
The tube is vacuumized by being heated in an electric oven at a constant temperature of 170 8C for half an hour in order to remove the air from the glass surface of the tube and the glass holder. After that, the glass holder containing the mixtures is heated by a high-frequency furnace. When the temperature reaches 700 8C, the mixtures will go through a chemical reaction and produce pure cesium. The cesium is in a liquid state under typical indoor temperatures. When the air pressure in the vacuum system reaches about 106 Torr, the tube and the pure cesium is heated by an electric oven so the cesium will be turned into a gaseous state and the ITO thin film will have a better absorption of the cesium. Meanwhile, a voltage of about 150 V is introduced to the ITO thin film and the aluminum film electrode. With the temperature increasing, the pure cesium in the gaseous state diffuses into the tube.
Fig. 2. The structure of the whole system.
The light source is an incandescent bulb. When we turn it on, a photocurrent can be detected. As Cs entering the tube, the photocurrent value increases. But the value will decrease when more and more Cs enters the tube. When the photocurrent is about to reach the maximum value, cut off the tube from the vacuum system by heating and sealing the glass vacuum channel of the tube.
3. Testing the photoemissive effect of the ITO–Cs thin film We start to do testing on the ITO–Cs thin film after we take the tube off the vacuum system. The results are recorded in Fig. 3. When measuring the
Fig. 3. The current curves of the ITO–Cs thin film after the tube is removed from the vacuum system. (A) Photocurrent, (B) dark current.
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ITO–Cs thin film would be. We will conduct further testing on this in future experiments.
4. Results analysis
Fig. 4. The appliance measuring the ITO–Cs thin film photosensitivity.
photocurrent, we keep the distance between the light source and the ITO–Cs thin film, the power of the light source, and the voltage between the electrodes unchanged. The photocurrent values and the dark current values are shown in Fig. 3. The photoelectric characteristics are measured in the experiment. The measuring appliance is shown in Fig. 4. An A-type incandescent lamp, a tungsten lamp operating at a filament color temperature of 2859 K, is used as the light source. The filter disk is composed of nine filters with different wavelengths ranging from 400 to 650 nm. The photocurrent is measured by a micro-current amplifier. Using the data collected, we calculate the relative photosensitivity of the ITO–Cs thin film. The relative photosensitivity curve is shown in Fig. 5. As we see from the curve, the relative photosensitivity increases when the wavelength decreases. Because we do not have filters with shorter wavelengths available, we are unable to measure the photocurrent of the ITO–Cs thin film under light source wavelengths below 400 nm. Therefore, at current stage, we do not know what the maximum value of the relative photosensitivity of the
Fig. 5. The relative photosensitivity curve of the ITO–Cs thin film.
The photocurrent before the tube is removed from the vacuum system is larger than that after the tube is removed from the vacuum system. This is because more cesium enters the tube when we remove the tube by heating the vacuuming channel. The extra cesium makes the photocurrent decrease even more. As indicated in the caption of Fig. 3, we did not include the data of this stage in Fig. 3. The intensity of the photocurrent is very sensitive to the amount of cesium. The Fig. 3 shows that the photocurrent value continuously increases as time passes by during the beginning stage. After a certain time, the photocurrent value reaches its maximum. Then it starts to decrease slowly. We believe the reasons of the changes are: first, we introduced extra Cs into the tube in the first place and secondly, the glass tube has slow deflation. In the experiment, we did not vacuum the glass tube by heating it for a long time (above 1 hr) under high temperatures (above 400 8C). This is because the epoxy resins are unable to stand temperatures above 180 8C for a long time. Because of the slow deflation, more and more oxygen enters the tube. A reaction then takes place between oxygen and cesium, both in a gaseous state, to form Cs2O in the solid state. Therefore, the extra pure Cs introduced into the tube is gradually reduced, which causes the photocurrent value to increase. After a certain time, about 600 hr as shown in Fig. 3, the photocurrent reaches the maximum. However, with the constant slow deflation, the reaction continues and the amount of cesium keeps dropping, which causes the insufficiency of Cs. Therefore, eventually the photocurrent value decreases. After the transparent ITO thin film is activated by cesium, its color changes to dark brown. When we allow the air into the tube, the dark brown color of the ITO–Cs thin film immediately disappears. The reason for this might be because Cs that covers the thin film reacts with O2 in the air and forms transparent Cs2O. This indicates that the dark brown color might be caused by the ITO thin film adsorbing cesium.
Z. Shouzhen et al. / Applied Surface Science 221 (2004) 110–113
At the wavelength between 0.4 and 2 mm, the transparency of the ITO thin film is above 85%. That is, the ITO thin film does not absorb photoenergy between 0.4 and 2 mm. So the ITO thin film does not have photoemissive effect at this range. But the ITO–Cs thin film does have photoemissive effect at this wavelength range. We believe that cesium has played an important role in causing the photoemissive effect of the ITO–Cs thin film. If the epoxy resins are replaced by low-melting point glasses so the tube can be heated longer and above 400 8C, there will be less slow deflation with the tube. Meanwhile, if we can introduce a right amount of volume of Cs needed, the photocurrent will not decrease until after an even longer period of time. Then the stability and the conversion efficiency of the ITO–Cs thin film photocurrent will be improved remarkably.
5. Conclusion In summary, the ITO–Cs thin film can have photoemissive effects under the radiation of visible lights. The photocurrent is stable and it can sustain for about 1000 hr. The thin film can be easily etched into different shapes and the cesium activation process is easy to launch. Therefore, we believe that the ITO–Cs thin film can play an important role in making flat
113
display devices and devices with large photo-cathodes. Further study is being done on how to increase the efficiency of the photoelectric transformation and how to better understand the photoemissive characteristics observed in the experiment. Also, more data will be collected to build a model for the photoemissive effect.
References [1] Y. Han, D. Kim, J.-S. Cho, et al., Solar Energy Mater. Solar Cell 65 (2001) 211–218. [2] A.K. Kulkarni, K.H. Schulz, T.-S. Lim, et al., Thin Solid film 308 (1997) 1–7. [3] J.E. Costellamo, Handbook of Display Technology (M), Academic press, New York, 1992. [4] Li Yanrong, Hui Zhengzhong, Introduction to the Electric Materials, Tsinghua University Publishing Co., China, 2001. [5] S. Ishibashi, Y. Higuchi, Y. Ota, et al., J. Vac. Sci. Technol. A 8 (1990) 1399–1405. [6] A.K. Kulkarni, K.H. Schulz, T.-S. Lim, et al., Thin Solid Film 345 (1999) 273–277. [7] S.A. Knickerbocker, A.K. Kulkarni, J. Vac. Sci. Technol. A 14 (1996) 757–6761. [8] A.K. Kulkarni, S.A. Knickerbocker, J. Vac. Sci. Technol. A 14 (1996) 1709–1713. [9] M. Bender, W. Seelig, C. Daube, et al., Thin Solid Film 326 (1998) 72–77. [10] V. Texeira, H.N. Cui, L.J. Meng, et al., Thin Solid Film 420 (2002) 70–75.
The photoemissive effect of the ITO–Cs thin film Zhao Shouzhen*, Xie Baosen, Liang Cuiguo Department of Electron Science and Technology, Nankai University, Tianjin 300071, PR China Received 21 October 2002; received in revised form 16 April 2003; accepted 26 June 2003
Abstract In the experiment, we found that when activated by cesium, the indium tin oxide (ITO) thin film would have a photoemissive effect under the radiation of visible light. This paper reports the experiment procedures and the testing results of the photoemissive characteristics of the ITO–Cs thin film. Some phenomena in the experiment are also analyzed in the paper. # 2003 Published by Elsevier B.V. PACS: 73.40; 79.60 Keywords: ITO transparent thin film; Cesium atom; Photoemissive effect
1. Introduction The indium tin oxide (ITO) thin film is a transparent semiconductor film. Sometimes the In2O3–SnO2 thin film is also called ITO thin film. Compared with other transparent thin films, the ITO thin film has excellent chemical and thermal stability and the ITO thin film is very easy to be etched into various shapes using photoetching methods. The ITO thin films are often used as transparent electrodes in vidicons, electroluminescent displays, plasma display screens, and liquid crystal displays [1–4]. According to some reports [5,6], the ITO thin film can also be used in electrostatic shielding, touch switches, etc. Because its energy gap is about 4.3–4.8 eV [7–10], ITO thin films do not have photoemissive effects under the radiation of visible light. Our experiment showed that ITO–Cs
*
Corresponding author. Tel.: þ1-651-283-3647; fax: þ1-651-224-2057. E-mail address: [email protected] (Z. Shouzhen). 0169-4332/$ – see front matter # 2003 Published by Elsevier B.V. doi:10.1016/S0169-4332(03)00863-8
thin films, however, do have such effects under the radiation of visible light.
2. The structure of the ITO–Cs thin film tube The glass tube for studying the photoemissive effect of the ITO thin film is shown in Fig. 1. The top of the tube is an aluminum film. The bottom of the tube is an ITO thin film. The ITO (In2O3–SnO2) thin film is deposited on a glass plate using the magnetron sputtering method. Its transparency is about 85%. The distance between the ITO thin film and the aluminum film electrode is about 4 cm. The ITO thin film is etched into a square area of 2 cm 2 cm. Then the ITO thin film glass plate and the aluminum film electrode are sealed to the glass tube using epoxy resins. After the epoxy resins become solid in the air, connect the tube and the glass holder containing the mixture powders of Zr and CsCr2O3 to the vacuum system, as shown in Fig. 2.
Z. Shouzhen et al. / Applied Surface Science 221 (2004) 110–113
111
Fig. 1. The structure of the tube.
The tube is vacuumized by being heated in an electric oven at a constant temperature of 170 8C for half an hour in order to remove the air from the glass surface of the tube and the glass holder. After that, the glass holder containing the mixtures is heated by a high-frequency furnace. When the temperature reaches 700 8C, the mixtures will go through a chemical reaction and produce pure cesium. The cesium is in a liquid state under typical indoor temperatures. When the air pressure in the vacuum system reaches about 106 Torr, the tube and the pure cesium is heated by an electric oven so the cesium will be turned into a gaseous state and the ITO thin film will have a better absorption of the cesium. Meanwhile, a voltage of about 150 V is introduced to the ITO thin film and the aluminum film electrode. With the temperature increasing, the pure cesium in the gaseous state diffuses into the tube.
Fig. 2. The structure of the whole system.
The light source is an incandescent bulb. When we turn it on, a photocurrent can be detected. As Cs entering the tube, the photocurrent value increases. But the value will decrease when more and more Cs enters the tube. When the photocurrent is about to reach the maximum value, cut off the tube from the vacuum system by heating and sealing the glass vacuum channel of the tube.
3. Testing the photoemissive effect of the ITO–Cs thin film We start to do testing on the ITO–Cs thin film after we take the tube off the vacuum system. The results are recorded in Fig. 3. When measuring the
Fig. 3. The current curves of the ITO–Cs thin film after the tube is removed from the vacuum system. (A) Photocurrent, (B) dark current.
112
Z. Shouzhen et al. / Applied Surface Science 221 (2004) 110–113
ITO–Cs thin film would be. We will conduct further testing on this in future experiments.
4. Results analysis
Fig. 4. The appliance measuring the ITO–Cs thin film photosensitivity.
photocurrent, we keep the distance between the light source and the ITO–Cs thin film, the power of the light source, and the voltage between the electrodes unchanged. The photocurrent values and the dark current values are shown in Fig. 3. The photoelectric characteristics are measured in the experiment. The measuring appliance is shown in Fig. 4. An A-type incandescent lamp, a tungsten lamp operating at a filament color temperature of 2859 K, is used as the light source. The filter disk is composed of nine filters with different wavelengths ranging from 400 to 650 nm. The photocurrent is measured by a micro-current amplifier. Using the data collected, we calculate the relative photosensitivity of the ITO–Cs thin film. The relative photosensitivity curve is shown in Fig. 5. As we see from the curve, the relative photosensitivity increases when the wavelength decreases. Because we do not have filters with shorter wavelengths available, we are unable to measure the photocurrent of the ITO–Cs thin film under light source wavelengths below 400 nm. Therefore, at current stage, we do not know what the maximum value of the relative photosensitivity of the
Fig. 5. The relative photosensitivity curve of the ITO–Cs thin film.
The photocurrent before the tube is removed from the vacuum system is larger than that after the tube is removed from the vacuum system. This is because more cesium enters the tube when we remove the tube by heating the vacuuming channel. The extra cesium makes the photocurrent decrease even more. As indicated in the caption of Fig. 3, we did not include the data of this stage in Fig. 3. The intensity of the photocurrent is very sensitive to the amount of cesium. The Fig. 3 shows that the photocurrent value continuously increases as time passes by during the beginning stage. After a certain time, the photocurrent value reaches its maximum. Then it starts to decrease slowly. We believe the reasons of the changes are: first, we introduced extra Cs into the tube in the first place and secondly, the glass tube has slow deflation. In the experiment, we did not vacuum the glass tube by heating it for a long time (above 1 hr) under high temperatures (above 400 8C). This is because the epoxy resins are unable to stand temperatures above 180 8C for a long time. Because of the slow deflation, more and more oxygen enters the tube. A reaction then takes place between oxygen and cesium, both in a gaseous state, to form Cs2O in the solid state. Therefore, the extra pure Cs introduced into the tube is gradually reduced, which causes the photocurrent value to increase. After a certain time, about 600 hr as shown in Fig. 3, the photocurrent reaches the maximum. However, with the constant slow deflation, the reaction continues and the amount of cesium keeps dropping, which causes the insufficiency of Cs. Therefore, eventually the photocurrent value decreases. After the transparent ITO thin film is activated by cesium, its color changes to dark brown. When we allow the air into the tube, the dark brown color of the ITO–Cs thin film immediately disappears. The reason for this might be because Cs that covers the thin film reacts with O2 in the air and forms transparent Cs2O. This indicates that the dark brown color might be caused by the ITO thin film adsorbing cesium.
Z. Shouzhen et al. / Applied Surface Science 221 (2004) 110–113
At the wavelength between 0.4 and 2 mm, the transparency of the ITO thin film is above 85%. That is, the ITO thin film does not absorb photoenergy between 0.4 and 2 mm. So the ITO thin film does not have photoemissive effect at this range. But the ITO–Cs thin film does have photoemissive effect at this wavelength range. We believe that cesium has played an important role in causing the photoemissive effect of the ITO–Cs thin film. If the epoxy resins are replaced by low-melting point glasses so the tube can be heated longer and above 400 8C, there will be less slow deflation with the tube. Meanwhile, if we can introduce a right amount of volume of Cs needed, the photocurrent will not decrease until after an even longer period of time. Then the stability and the conversion efficiency of the ITO–Cs thin film photocurrent will be improved remarkably.
5. Conclusion In summary, the ITO–Cs thin film can have photoemissive effects under the radiation of visible lights. The photocurrent is stable and it can sustain for about 1000 hr. The thin film can be easily etched into different shapes and the cesium activation process is easy to launch. Therefore, we believe that the ITO–Cs thin film can play an important role in making flat
113
display devices and devices with large photo-cathodes. Further study is being done on how to increase the efficiency of the photoelectric transformation and how to better understand the photoemissive characteristics observed in the experiment. Also, more data will be collected to build a model for the photoemissive effect.
References [1] Y. Han, D. Kim, J.-S. Cho, et al., Solar Energy Mater. Solar Cell 65 (2001) 211–218. [2] A.K. Kulkarni, K.H. Schulz, T.-S. Lim, et al., Thin Solid film 308 (1997) 1–7. [3] J.E. Costellamo, Handbook of Display Technology (M), Academic press, New York, 1992. [4] Li Yanrong, Hui Zhengzhong, Introduction to the Electric Materials, Tsinghua University Publishing Co., China, 2001. [5] S. Ishibashi, Y. Higuchi, Y. Ota, et al., J. Vac. Sci. Technol. A 8 (1990) 1399–1405. [6] A.K. Kulkarni, K.H. Schulz, T.-S. Lim, et al., Thin Solid Film 345 (1999) 273–277. [7] S.A. Knickerbocker, A.K. Kulkarni, J. Vac. Sci. Technol. A 14 (1996) 757–6761. [8] A.K. Kulkarni, S.A. Knickerbocker, J. Vac. Sci. Technol. A 14 (1996) 1709–1713. [9] M. Bender, W. Seelig, C. Daube, et al., Thin Solid Film 326 (1998) 72–77. [10] V. Texeira, H.N. Cui, L.J. Meng, et al., Thin Solid Film 420 (2002) 70–75.