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Impact ionization phenomenon in single-crystalline rutile TiO2
Applied Surface Science 244 (2005) 394–398 www.elsevier.com/locate/apsusc
Impact ionization phenomenon in single-crystalline rutile TiO2 H. Hashimoto*, T. Teraji, T. Ito Department of Electrical Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan Received 31 May 2004; accepted 14 September 2004 Available online 13 January 2005
Abstract Titanium oxide (TiO2) is known as a material suitable for photocatalysis, where photo-excited high-energy holes in TiO2 play an important role. We applied high electric fields for TiO2 to generate high-energy holes using an impact ionization phenomenon. In current–voltage (I–V) measurements, currents rose suddenly at a threshold voltage, indicating that an impact ionization phenomenon occurred in TiO2. The threshold voltage was controllable from 10 to 170 V by varying the metal electrode thickness. This means that an electric field enhancement effect was significant at the edge of the thin film electrode. At higher voltages, there was observed a relation I / V2, indicating that space-charge-limited currents dominated the currents flowing in the specimen. # 2004 Elsevier B.V. All rights reserved. PACS: 72.20.Jv; 72.20.Ht Keywords: TiO2; Impact ionization; Space-charge-limited current; Photocatalysis
1. Introduction Photo-exited high-energy holes are known to play an important role in photocatalysis effects of TiO2. As a matter of fact, irradiation of ultraviolet lights with wavelength (380 nm) shorter than its band-gap light is needed to generate high-energy holes for TiO2 photocatalysis. Such ultraviolet lights are contained * Corresponding author. Tel.: +81 6 6879 7723; fax: +81 6 6879 7704. E-mail address: [email protected] (H. Hashimoto).
only slightly (few % in energy) in the natural light. Thus, a number of investigations have been performed on the photocatalysis in the visible light region by employing suitable dopants in order to narrow down the band gap [1,2]. But, such a doping procedure often induces creations of defects and vacancies in TiO2 that can enhance recombination of photoexcited electron–hole pairs. One possible solution for increasing the photocatalysis effect would be to make p-type semiconducting TiO2. Unfortunately, it is, however, difficult to fabricate p-type TiO2. This may be because the valence band maximum of TiO2 is
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.09.148
H. Hashimoto et al. / Applied Surface Science 244 (2005) 394–398
located at a very deep energy below the vacuum level [3]. Recently, we have found, both theoretically and experimentally, that high-energy electrons can be created in diamond with the band gap of 5.5 eV by means of electric-field-induced impact ionization excitations [4]. Thus, we also expect that high-energy holes can be created similarly in intrinsic (i) TiO2 by applying electric fields higher than 105 V/cm. In this paper, we have fabricated microstructures on rutile TiO2 and have investigated electric properties of TiO2 under such high fields. Possibility of the catalysis effect without light irradiation will be discussed in relation to field-induced impact ionizations.
2. Experimental Bernoulli-method-grown single-crystalline rutile (1 1 0) substrates with mirror-polished surfaces were used. These were commercially available (from Shinko-sha Co. Ltd.). At first, all the substrates were chemically cleaned using ethanol, acetone, acids and ultrapure water. Metal–insulator–metal (M–I–M) structures, as typically shown in Fig. 1, were fabricated on these crystals using a standard photolithography technique. Titanium and platinum selected as electrodes were deposited using electron beam (EB) evaporation at 1 106 Pa or DCmagnetron sputtering (MS) method at 4 106 Pa.
395
If Ti was selected as the electrode metal, very thin Pt layers were deposited as a passivation material, where the interface between the metal Ti and substrate TiO2 was expected to be substantially strong because of an interfacial Ti–O reaction [5]. A crystal oscillator was used to monitor the thickness of these metal electrodes. The total thickness of the metal electrodes ˚ . Current– thus fabricated ranged from 100 to 400 A voltage (I–V) characteristics were measured in a vacuum chamber at a pressure of 1 104 Pa using high-voltage source-measure units (Keithley 237).
3. Results and discussion Typical I–V characteristics measured for three M– I–M specimens are shown in Fig. 2. In these cases, the main metal electrodes concerned were Ti thin layers while covered Pt ultra-thin layers worked as their passivation, as well. The total electrode thicknesses ˚ . In each case of the total were 100, 200 and 400 A metal thickness, currents increased rapidly above each threshold voltage. The observed features were as ˚ thick electrode, a follows. In the case of the 100 A rapid increase in the total current was observed at V > 10 V (the threshold voltage), indicating that impact ionization events occurred in that V region. The average electric field of 2 104 V/cm at 10 V between electrodes is, however, not so high as to onset such impact ionization events. Thus, a possible origin
Fig. 1. Optical microscope image of M–I–M structure fabricated on rutile TiO2 (I) with thin metal electrodes (M).
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H. Hashimoto et al. / Applied Surface Science 244 (2005) 394–398
˚. Fig. 2. Typical I–V characteristics taken from three M–I–M specimens with electrode thicknesses of 100, 200 and 400 A
for the rapid I increases is an electric field enhancement at the edge of the thin film electrodes. As shown in Fig. 2, the specimen with the thinner electrode had the lower threshold voltage. This is what we expected for the electrode thickness dependence of I–V characteristics, meaning that the field enhancement at the electrode edges plays an important role on the I– V characteristics. In other words, the threshold voltages can be controlled by changing the electrode thickness. At high voltages, where the rapid I increases were saturated, a relation that I / V2 was observed in all the cases, indicating that space-charge-limited currents dominated the currents flowing in the
specimen [6]. This phenomenon may, in turn, prevent the examined specimens from rapid electric breakdown. In order to investigate the mechanism of electron injections from the electrode to TiO2, possible differences in the I–V characteristic were investigated between different materials of the thin film electrodes with an identical thickness. As shown in Fig. 3, an M– ˚ thick Pt electrodes had a I–M specimen with 100 A higher threshold voltage, compared to the specimen with Ti/Pt bi-layered electrodes having the same total thickness of the electrodes. The observed difference should be related to the work function difference
Fig. 3. Difference in I–V characteristics between two M–I–M specimens with Pt single-layer and Ti/Pt bi-layer electrodes. Shift of the threshold voltage on the higher voltage side was observed for the former, compared with the latter.
H. Hashimoto et al. / Applied Surface Science 244 (2005) 394–398
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minimum of TiO2, EC, b, the field enhancement factor and Ae, the current injection area. When w = FTiEC = 1.0 eV, one can deduce bd and Aed from the gradient and vertical axis intercept of the plots near the onset of the rapid I increases for the ˚ electrode thickness, d. For the specimen with 100 A thick electrodes, we obtained b100 A˚ = 30 and ˚ Ae,100 A˚ = 5.7 1013 cm2, for the one with 200 A thick electrodes b200 A˚ = 6.8 and Ae,200 A˚ = 9.5 ˚ thick electrodes 1013 cm2, and for the one with 400 A 13 b400 A˚ = 2.8 and Ae,400 A˚ = 6.3 10 cm2. It is concluded from the present F–N analysis that the electric field enhancement roughly inversely depends on the metal electrode thickness while the emission areas involved in the electron injection, ranging from 6 to 10 1013 cm2 for all the specimens, were well localized. Fig. 4. Fowler–Nordheim plots for specimens with thicknesses of ˚ . The insert shows the model of band structure 100, 200 and 400 A with field enhancement.
between these two metals since the electron injection from the metal electrode to insulating TiO2 is expected to follow the Fowler–Nordheim (F–N) tunneling mechanism. As a matter of fact, the work function of Ti and Pt are FTi = 4.3 and FPt = 5.7 eV, respectively. This difference in the work function should affect the I–V properties and qualitatively explain the observed difference that the threshold voltage was larger in the Pt electrode case than in the Ti/Pt one. It is concluded from this result that the F–N tunneling mechanism is reasonably responsible for the electron injection mechanism, at least, at the onset of the rapid current increase phenomenon. Furthermore, a more detailed analysis has been performed. Fig. 4 shows F–N plots of the three specimens having the Ti/Pt electrodes with the total ˚ . The F–N relation thicknesses of 100, 200 and 400 A for I–V data may be described as follows [7]: ! 0:5 I 1:54 106 Ae b2 104:52’ ln 2 ¼ ln V ’d2
2:84 109 d’1:5 1 V b
where w means the energy difference between the Fermi level of the electrode and the conduction band
4. Conclusion Rapid increases were observed in current–voltage characteristics taken from M–I–M structures with single-crystalline TiO2 as the I layer. The threshold voltages obtained for the rapid increases were controlled by varying the metal electrode thickness. The average electric fields were substantially lower than those expected for occurrence of field-induced impact ionization events, indicating that the electric field was sufficiently enhanced at the electrode edges to induce the carrier amplification. The field enhancement factors estimated from F–N plots ranged from 3 to 30 for the ˚, decreasing electrode thicknesses from 400 to 100 A meaning that the electrode thickness can effectively change the field enhancement factor. It is also found that the electron injection mechanism near the onset of the rapid current increases was determined by the F–N tunneling mechanism. At voltages sufficiently higher than the threshold voltages, space-charge-limited currents governed the flowing currents, and prevented the specimens from possible rapid field breakdown.
References [1] M. Anpo, M. Takeuchi, J. Catal. 216 (2003) 505. [2] L. Miao, S. Tanemura, H. Watanabe, Y. Mori, K. Kaneko, S. Toh, J. Cryst. Growth 260 (2004) 118.
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H. Hashimoto et al. / Applied Surface Science 244 (2005) 394–398
[3] N. Daude, C. Gout, C. Jouanin, Phys. Rev. B 15 (1977) 3229. [4] T. Ito, T. Watanabe, M. Irie, J. Nakamura, T. Teraji, Diamond Relat. Mater. 12 (2003) 434.
[5] M.A. Henderson, Surf. Sci. 419 (1999) 174. [6] P.N. Murgatroyd, Thin Solid Films 17 (1973) 335. [7] N.S. Xu, in: R.V. Latham (Ed.), High Voltage Vacuum Insulation, Academic, London, 1995, p. 127.
Impact ionization phenomenon in single-crystalline rutile TiO2 H. Hashimoto*, T. Teraji, T. Ito Department of Electrical Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan Received 31 May 2004; accepted 14 September 2004 Available online 13 January 2005
Abstract Titanium oxide (TiO2) is known as a material suitable for photocatalysis, where photo-excited high-energy holes in TiO2 play an important role. We applied high electric fields for TiO2 to generate high-energy holes using an impact ionization phenomenon. In current–voltage (I–V) measurements, currents rose suddenly at a threshold voltage, indicating that an impact ionization phenomenon occurred in TiO2. The threshold voltage was controllable from 10 to 170 V by varying the metal electrode thickness. This means that an electric field enhancement effect was significant at the edge of the thin film electrode. At higher voltages, there was observed a relation I / V2, indicating that space-charge-limited currents dominated the currents flowing in the specimen. # 2004 Elsevier B.V. All rights reserved. PACS: 72.20.Jv; 72.20.Ht Keywords: TiO2; Impact ionization; Space-charge-limited current; Photocatalysis
1. Introduction Photo-exited high-energy holes are known to play an important role in photocatalysis effects of TiO2. As a matter of fact, irradiation of ultraviolet lights with wavelength (380 nm) shorter than its band-gap light is needed to generate high-energy holes for TiO2 photocatalysis. Such ultraviolet lights are contained * Corresponding author. Tel.: +81 6 6879 7723; fax: +81 6 6879 7704. E-mail address: [email protected] (H. Hashimoto).
only slightly (few % in energy) in the natural light. Thus, a number of investigations have been performed on the photocatalysis in the visible light region by employing suitable dopants in order to narrow down the band gap [1,2]. But, such a doping procedure often induces creations of defects and vacancies in TiO2 that can enhance recombination of photoexcited electron–hole pairs. One possible solution for increasing the photocatalysis effect would be to make p-type semiconducting TiO2. Unfortunately, it is, however, difficult to fabricate p-type TiO2. This may be because the valence band maximum of TiO2 is
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.09.148
H. Hashimoto et al. / Applied Surface Science 244 (2005) 394–398
located at a very deep energy below the vacuum level [3]. Recently, we have found, both theoretically and experimentally, that high-energy electrons can be created in diamond with the band gap of 5.5 eV by means of electric-field-induced impact ionization excitations [4]. Thus, we also expect that high-energy holes can be created similarly in intrinsic (i) TiO2 by applying electric fields higher than 105 V/cm. In this paper, we have fabricated microstructures on rutile TiO2 and have investigated electric properties of TiO2 under such high fields. Possibility of the catalysis effect without light irradiation will be discussed in relation to field-induced impact ionizations.
2. Experimental Bernoulli-method-grown single-crystalline rutile (1 1 0) substrates with mirror-polished surfaces were used. These were commercially available (from Shinko-sha Co. Ltd.). At first, all the substrates were chemically cleaned using ethanol, acetone, acids and ultrapure water. Metal–insulator–metal (M–I–M) structures, as typically shown in Fig. 1, were fabricated on these crystals using a standard photolithography technique. Titanium and platinum selected as electrodes were deposited using electron beam (EB) evaporation at 1 106 Pa or DCmagnetron sputtering (MS) method at 4 106 Pa.
395
If Ti was selected as the electrode metal, very thin Pt layers were deposited as a passivation material, where the interface between the metal Ti and substrate TiO2 was expected to be substantially strong because of an interfacial Ti–O reaction [5]. A crystal oscillator was used to monitor the thickness of these metal electrodes. The total thickness of the metal electrodes ˚ . Current– thus fabricated ranged from 100 to 400 A voltage (I–V) characteristics were measured in a vacuum chamber at a pressure of 1 104 Pa using high-voltage source-measure units (Keithley 237).
3. Results and discussion Typical I–V characteristics measured for three M– I–M specimens are shown in Fig. 2. In these cases, the main metal electrodes concerned were Ti thin layers while covered Pt ultra-thin layers worked as their passivation, as well. The total electrode thicknesses ˚ . In each case of the total were 100, 200 and 400 A metal thickness, currents increased rapidly above each threshold voltage. The observed features were as ˚ thick electrode, a follows. In the case of the 100 A rapid increase in the total current was observed at V > 10 V (the threshold voltage), indicating that impact ionization events occurred in that V region. The average electric field of 2 104 V/cm at 10 V between electrodes is, however, not so high as to onset such impact ionization events. Thus, a possible origin
Fig. 1. Optical microscope image of M–I–M structure fabricated on rutile TiO2 (I) with thin metal electrodes (M).
396
H. Hashimoto et al. / Applied Surface Science 244 (2005) 394–398
˚. Fig. 2. Typical I–V characteristics taken from three M–I–M specimens with electrode thicknesses of 100, 200 and 400 A
for the rapid I increases is an electric field enhancement at the edge of the thin film electrodes. As shown in Fig. 2, the specimen with the thinner electrode had the lower threshold voltage. This is what we expected for the electrode thickness dependence of I–V characteristics, meaning that the field enhancement at the electrode edges plays an important role on the I– V characteristics. In other words, the threshold voltages can be controlled by changing the electrode thickness. At high voltages, where the rapid I increases were saturated, a relation that I / V2 was observed in all the cases, indicating that space-charge-limited currents dominated the currents flowing in the
specimen [6]. This phenomenon may, in turn, prevent the examined specimens from rapid electric breakdown. In order to investigate the mechanism of electron injections from the electrode to TiO2, possible differences in the I–V characteristic were investigated between different materials of the thin film electrodes with an identical thickness. As shown in Fig. 3, an M– ˚ thick Pt electrodes had a I–M specimen with 100 A higher threshold voltage, compared to the specimen with Ti/Pt bi-layered electrodes having the same total thickness of the electrodes. The observed difference should be related to the work function difference
Fig. 3. Difference in I–V characteristics between two M–I–M specimens with Pt single-layer and Ti/Pt bi-layer electrodes. Shift of the threshold voltage on the higher voltage side was observed for the former, compared with the latter.
H. Hashimoto et al. / Applied Surface Science 244 (2005) 394–398
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minimum of TiO2, EC, b, the field enhancement factor and Ae, the current injection area. When w = FTiEC = 1.0 eV, one can deduce bd and Aed from the gradient and vertical axis intercept of the plots near the onset of the rapid I increases for the ˚ electrode thickness, d. For the specimen with 100 A thick electrodes, we obtained b100 A˚ = 30 and ˚ Ae,100 A˚ = 5.7 1013 cm2, for the one with 200 A thick electrodes b200 A˚ = 6.8 and Ae,200 A˚ = 9.5 ˚ thick electrodes 1013 cm2, and for the one with 400 A 13 b400 A˚ = 2.8 and Ae,400 A˚ = 6.3 10 cm2. It is concluded from the present F–N analysis that the electric field enhancement roughly inversely depends on the metal electrode thickness while the emission areas involved in the electron injection, ranging from 6 to 10 1013 cm2 for all the specimens, were well localized. Fig. 4. Fowler–Nordheim plots for specimens with thicknesses of ˚ . The insert shows the model of band structure 100, 200 and 400 A with field enhancement.
between these two metals since the electron injection from the metal electrode to insulating TiO2 is expected to follow the Fowler–Nordheim (F–N) tunneling mechanism. As a matter of fact, the work function of Ti and Pt are FTi = 4.3 and FPt = 5.7 eV, respectively. This difference in the work function should affect the I–V properties and qualitatively explain the observed difference that the threshold voltage was larger in the Pt electrode case than in the Ti/Pt one. It is concluded from this result that the F–N tunneling mechanism is reasonably responsible for the electron injection mechanism, at least, at the onset of the rapid current increase phenomenon. Furthermore, a more detailed analysis has been performed. Fig. 4 shows F–N plots of the three specimens having the Ti/Pt electrodes with the total ˚ . The F–N relation thicknesses of 100, 200 and 400 A for I–V data may be described as follows [7]: ! 0:5 I 1:54 106 Ae b2 104:52’ ln 2 ¼ ln V ’d2
2:84 109 d’1:5 1 V b
where w means the energy difference between the Fermi level of the electrode and the conduction band
4. Conclusion Rapid increases were observed in current–voltage characteristics taken from M–I–M structures with single-crystalline TiO2 as the I layer. The threshold voltages obtained for the rapid increases were controlled by varying the metal electrode thickness. The average electric fields were substantially lower than those expected for occurrence of field-induced impact ionization events, indicating that the electric field was sufficiently enhanced at the electrode edges to induce the carrier amplification. The field enhancement factors estimated from F–N plots ranged from 3 to 30 for the ˚, decreasing electrode thicknesses from 400 to 100 A meaning that the electrode thickness can effectively change the field enhancement factor. It is also found that the electron injection mechanism near the onset of the rapid current increases was determined by the F–N tunneling mechanism. At voltages sufficiently higher than the threshold voltages, space-charge-limited currents governed the flowing currents, and prevented the specimens from possible rapid field breakdown.
References [1] M. Anpo, M. Takeuchi, J. Catal. 216 (2003) 505. [2] L. Miao, S. Tanemura, H. Watanabe, Y. Mori, K. Kaneko, S. Toh, J. Cryst. Growth 260 (2004) 118.
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[3] N. Daude, C. Gout, C. Jouanin, Phys. Rev. B 15 (1977) 3229. [4] T. Ito, T. Watanabe, M. Irie, J. Nakamura, T. Teraji, Diamond Relat. Mater. 12 (2003) 434.
[5] M.A. Henderson, Surf. Sci. 419 (1999) 174. [6] P.N. Murgatroyd, Thin Solid Films 17 (1973) 335. [7] N.S. Xu, in: R.V. Latham (Ed.), High Voltage Vacuum Insulation, Academic, London, 1995, p. 127.