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Ultraviolet detecting properties of ZnO-based thin film transistors
Thin Solid Films 469–470 (2004) 75 – 79 www.elsevier.com/locate/tsf
Ultraviolet detecting properties of ZnO-based thin film transistors H.S. Bae, Seongil Im* Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Korea Available online 13 September 2004
Abstract ZnO-based thin film transistors (TFTs) have been fabricated on a SiO2/p-Si substrate by rf magnetron sputtering at room temperature and they exhibited a saturation current level of a few AA under a gate bias of 40V, electron mobility of less than 0.05 cm2/V s, and on/off current ratio of ~105 in the dark. Illuminated by ultraviolet (UV, k=340 nm) light with an optical power density of 1.26 mW/cm2, our TFT displayed a high photocurrent gain of more than 50 AA at the same gate bias of 40 V and it also showed that the photocurrent decreases with lowering the UV intensity. In a channel depletion state with a gate bias of 40 V, the photo-detecting sensitivity becomes much higher than in the accumulation state with the gate bias of 40 V. D 2004 Elsevier B.V. All rights reserved. Keywords: Zinc oxide; Photoconductivity; Thin film transistor; Photo-response
1. Introduction ZnO-based thin film transistors (TFTs) have been reported, attracting much attention because of their potential for replacing armorphous Si that has been widely used as a channel layer in conventional TFTs [1–3]. ZnO films with its large bandgap of 3.3 eV can also be employed to fabricate transparent TFTs [3]. The field effect electron mobility of ZnO-based TFTs reported in the literature is in the range of 0.01–3 cm2/V s and their on/off current ratio ranges from 102 to 107 [1–3]. However, realizing a stable ZnO-based TFTs with both decent mobility and on/off ratio still requires much more effort to search for the appropriate fabrication conditions. ZnO is also known to have a capacity to detect ultraviolet photons (UV) [4]. Park et al. [5] and Jeong et al. [6] already reported on the fabrication and electrical properties of n-ZnO/p-Si photodiodes that are able to detect visible and UV photons, respectively. Therefore, it is quite natural to see if ZnO-based TFTs have capabilities to detect UV and possibly visible photons. In the present paper, we report on the fabrication of ZnO-
* Corresponding author. Tel.: +82 2 2123 2842; fax: +82 2 392 1592. E-mail address: [email protected] (S. Im). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.06.196
based TFTs on SiO2/p-Si substrates and particularly focus on their photo-detection capabilities.
2. Experimental Our ZnO TFTs were fabricated on SiO2/p-Si substrates, as shown in the schematic cross-sectional and plan views of our TFT structure in Fig. 1(a) and (b). The width/length (W/ L) ratio of our TFTs was about 6 and the channel length (L) was 90 Am as indicated. Prior to the deposition of n-ZnO layers, 200-nm-thick SiO2 layers were thermally grown on p-type Si (100) substrates (7–25 Vd cm, doping ~1015 cm 3). The 100-nm-thick undoped ZnO films were subsequently deposited for 1 h using a undoped ZnO target (99.99%, Kurt J. Lesker) in a rf magnetron sputtering system in the 10 mTorr Ar/O2 (6:1) gas mixture at room temperature (RT). In our sputtering system, the target– substrate distance was 10 cm and rf power was fixed at 100 W. The film thickness was measured by 2 MeV 4He2+ Rutherford backscattering spectrometry. A shadow mask was used to pattern the ZnO layers. Then, Al source/drain electrode pads were subsequently deposited on the ZnO channel through the second mask by thermal evaporation. Since the as-deposited ZnO film is usually quite insulating,
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H.S. Bae, S. Im / Thin Solid Films 469–470 (2004) 75–79
known to be a shallow donor located in some interstitial sites within ZnO crystals [7,8]. All electrical and photoelectric characterizations were carried out with a semiconductor parameter analyzer (model HP 4155C, Agilent Technologies) at RT. For I–V measurements on TFTs, large ohmic contacts for gate electrodes were made on the backside of p-Si by applying indium (In) paste. Light illumination was performed with a light source (Oriel Optical System) which employed a 500 W Hg(Xe) arc lamp and a monochromator covering the range of 250–670 nm. The illuminated beam cross sections on our TFTs were identical to be 1 cm2 in all cases as indicated in Fig. 1(b). The optical power density of the monochromatic light was measured by a UV-enhanced Si detector.
3. Results and discussion
Fig. 1. Schematic (a) cross section and (b) plan views of our ZnO-based TFT exposed to photons. Beam cross section was 1 cm2 for all cases.
we treated the film using rapid thermal annealing (RTA) at 350 8C for 1 min in order to improve the conductivity of the ZnO layer and also to reduce the contact resistance of source/drain pads (Al). The RTA treatment was mostly performed in a forming gas (H2/N2=1:10) ambient to induce hydrogen doping into our ZnO films because H atom is well
Fig. 2(a) displays the drain current–drain voltage (I D– V D) curves obtained from our ZnO-based TFTs. The saturation current was about 1.7 AA under a gate bias of 40 V. (The mobility and on/off current ratio of our TFTs were about 0.05 cm2/V s and ~105, respectively. We estimated the field effect electron mobility using the slope of the mobility plot, i.e. the square root of saturation current (IDSAT) vs. gate bias (V G) in Fig. 4 and the threshold voltage of our TFT was then around 10 V.) Although the dark saturation current was only 1.7 AA, the current behavior totally changed under UV illumination (wavelength 340 nm) with photon energy over 3.6 eV and optical power density of 1.26 mW/cm2. As shown in Fig. 2(b), our ZnO-based TFT shows that its maximum drain current (at V G=40 V) increases up to about 50 AA under UV illumination, which is about an
Fig. 2. (a) I D–V D curves obtained from our ZnO-based TFT in the dark and (b) under UV illumination (340 nm, 1.26 mW/cm2).
H.S. Bae, S. Im / Thin Solid Films 469–470 (2004) 75–79
77
Fig. 3. I D–V D curves obtained under UV illumination (340 nm) with varied optical powers (V G=0V).
order of magnitude higher than the maximum dark saturation current. Hence the observed change is solely due to photoelectric effect in the ZnO channel. In particular, it is also interesting to compare the behavior of our TFT at V G=0V in the dark and under UV illumination. In the dark, our TFTs exhibit drain current of less than 50 nA under a drain bias of 25 V (50 nA was too small to be identified in Fig. 2(a)) while the same device exhibits drain current of more than 7.5 AA under the same drain bias under UV illumination. Similar photo-response was found under the same UV illumination (340 nm) with varied optical powers when the ZnO-TFT was under a gate bias of 0 V, as shown in Fig. 3. The amount of photocurrent gradually increases
with the power but in the state of 0 V gate bias the photo-response may have some limit to show the full potential of detector sensitivity. We thus characterized the spectral photo-response properties of our ZnO-TFT in more detail way that employs log10 I D–V G curves obtained under a drain-saturation regime (drain bias, V DS=30V). Fig. 4 exhibits the log10 I D–gate bias (V G) curves obtained from our TFT that was illuminated by photons (normalized power density 0.58 mW/cm2) with ultraviolet (UV, 254 nm), blue (B, 442 nm), green (G, 540 nm), and red (R, 650 nm) wavelengths. The photo-response was clearer in the depletion state near a gate bias of 40 V than in the accumulation state near 40 V, as shown in the figure. It is
Fig. 4. Spectral responses observed in the log10 I D–V G curves. The state of ZnO channel depletion exhibits much higher photoelectric effects than that of accumulation. The electron mobility and on/off current ratio of ~0.05 cm2/V s and ~105 were also estimated, respectively, in the dark.
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Fig. 5. Detailed spectral response as the ratio of I photo/I dark was obtained from log10 I D–V G curves (V DS=30 V, V G= 40 V).
because the initial dark depletion state draws almost no charge carriers (or no background signals) in the ZnO channel whereas the same channel readily introduces many electrons under the gate bias for the dark accumulation state (even before the light-induced carriers are generated). The largest photoelectric effect results from UV illumination. Blue and green photons also produced quite high photogains. However, red photons were ineffective, achieving a similar level of drain current compared to the dark current level. Our ZnO channel layer deposited by rf sputtering at room temperature is likely to contain a high density of point defects that may mainly consist of natural mid-gap states (deep acceptor) induced by Zn vacancies (single-charged) or O interstitials in ZnO polycrystals are known to exist near 2.3 eV below the conduction band edge of the ZnO. [9,10] Hence it is not surprising that only the photons with energy higher than ~2.3 eV have exhibited good photoelectric effects although another but a minor kind of deep defects was previously reported to be located at 0.78 eV as a deep donor (double-charged Zn vancancy) [11]. In Fig. 5, the detailed spectral responses of ZnO-TFT were plotted as photocurrent/dark current (I photo/I dark) ratios after their log10 I D–V G curves were measured under a gate bias of 40V for depletion state. As expected, the red illumination of 650 nm resulted in little photoelectric ratio among all the wavelength range of illumination and the illumination ranging from green (540 nm) to UV (254 nm) displayed high photo-responsivity of more than 105 that is almost the same value as the on/off ratio of our ZnO-TFT. The highest responsivity value was obtained from UV (254 nm) and green light near 500 nm. This means that the photoresponse mechanism probably includes the absorption occurring through the band-to-band and the band-to-main deep level transitions for the UV and the green lights, respectively.
4. Conclusions We have fabricated ZnO-based TFTs on SiO2/p-Si substrates with rf magnetron sputtering at room temperature. Our ZnO-TFTs exhibited a maximum saturation current of about 1.7 AA and an on/off current ratio of ~105 in the dark. Illuminated by UV and visible photons with energy higher than 2.3 eV, our TFTs display high photocurrent of more than 50 AA because the relevant photo-detection is associated with the band gap of ZnO and its deep level defects located near ~2.3 eV below the conduction band edge. According to our study with varying gate voltage, a much higher photoelectric ratio is achieved when we employ the depletion state of the ZnO TFT channel rather than the accumulation state. It is concluded that the ZnO-based TFTs can be promising UV photo-detecting device.
Acknowledgements Authors gratefully acknowledge the financial supports from KISTEP (M20204250033-02A0903-00440) and the Basic Research Program of KOSEF (R01-1999-000-000300), and in part from the BK 21 Program.
References [1] P.F. Carcia, R.S. McLean, M.H. Reilly, G. Nunes Jr., Appl. Phys. Lett. 82 (2003) 1117. [2] S. Masuda, K. Kitamura, Y. Okumura, S. Miyatake, H. Tabata, T. Kawai, J. Appl. Phys. 93 (2003) 1624. [3] R.L. Hoffman, B.J. Norris, J.F. Wager, Appl. Phys. Lett. 82 (2003) 733. [4] H. Fabricius, T. Skettrup, P. Bisgaard, Appl. Opt. 25 (1986) 2764.
H.S. Bae, S. Im / Thin Solid Films 469–470 (2004) 75–79 [5] C.H. Park, I.S. Jeong, J.H. Kim, S. Im, Appl. Phys. Lett. 83 (2003) 2946. [6] I.-S. Jeong, J.H. Kim, S. Im, Appl. Phys. Lett. 83 (2003) 2946. [7] C.G. Van de Walle, Phys. Rev. Lett. 85 (2000) 1012. [8] C ¸ . Kilic¸, A. Zunger, Appl. Phys. Lett. 81 (2002) 73.
79
[9] S. Im, B.J. Jin, S. Yi, Appl. Phys. Lett. 87 (2000) 4558. [10] J.Y. Lee, Y.S. Choi, J.H. Kim, M.O. Park, S. Im, Thin Solid Films 403–404 (2002) 553. [11] W. Gfpel, J. Vac. Sci. Technol. 16 (1979) 1229.
Ultraviolet detecting properties of ZnO-based thin film transistors H.S. Bae, Seongil Im* Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Korea Available online 13 September 2004
Abstract ZnO-based thin film transistors (TFTs) have been fabricated on a SiO2/p-Si substrate by rf magnetron sputtering at room temperature and they exhibited a saturation current level of a few AA under a gate bias of 40V, electron mobility of less than 0.05 cm2/V s, and on/off current ratio of ~105 in the dark. Illuminated by ultraviolet (UV, k=340 nm) light with an optical power density of 1.26 mW/cm2, our TFT displayed a high photocurrent gain of more than 50 AA at the same gate bias of 40 V and it also showed that the photocurrent decreases with lowering the UV intensity. In a channel depletion state with a gate bias of 40 V, the photo-detecting sensitivity becomes much higher than in the accumulation state with the gate bias of 40 V. D 2004 Elsevier B.V. All rights reserved. Keywords: Zinc oxide; Photoconductivity; Thin film transistor; Photo-response
1. Introduction ZnO-based thin film transistors (TFTs) have been reported, attracting much attention because of their potential for replacing armorphous Si that has been widely used as a channel layer in conventional TFTs [1–3]. ZnO films with its large bandgap of 3.3 eV can also be employed to fabricate transparent TFTs [3]. The field effect electron mobility of ZnO-based TFTs reported in the literature is in the range of 0.01–3 cm2/V s and their on/off current ratio ranges from 102 to 107 [1–3]. However, realizing a stable ZnO-based TFTs with both decent mobility and on/off ratio still requires much more effort to search for the appropriate fabrication conditions. ZnO is also known to have a capacity to detect ultraviolet photons (UV) [4]. Park et al. [5] and Jeong et al. [6] already reported on the fabrication and electrical properties of n-ZnO/p-Si photodiodes that are able to detect visible and UV photons, respectively. Therefore, it is quite natural to see if ZnO-based TFTs have capabilities to detect UV and possibly visible photons. In the present paper, we report on the fabrication of ZnO-
* Corresponding author. Tel.: +82 2 2123 2842; fax: +82 2 392 1592. E-mail address: [email protected] (S. Im). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.06.196
based TFTs on SiO2/p-Si substrates and particularly focus on their photo-detection capabilities.
2. Experimental Our ZnO TFTs were fabricated on SiO2/p-Si substrates, as shown in the schematic cross-sectional and plan views of our TFT structure in Fig. 1(a) and (b). The width/length (W/ L) ratio of our TFTs was about 6 and the channel length (L) was 90 Am as indicated. Prior to the deposition of n-ZnO layers, 200-nm-thick SiO2 layers were thermally grown on p-type Si (100) substrates (7–25 Vd cm, doping ~1015 cm 3). The 100-nm-thick undoped ZnO films were subsequently deposited for 1 h using a undoped ZnO target (99.99%, Kurt J. Lesker) in a rf magnetron sputtering system in the 10 mTorr Ar/O2 (6:1) gas mixture at room temperature (RT). In our sputtering system, the target– substrate distance was 10 cm and rf power was fixed at 100 W. The film thickness was measured by 2 MeV 4He2+ Rutherford backscattering spectrometry. A shadow mask was used to pattern the ZnO layers. Then, Al source/drain electrode pads were subsequently deposited on the ZnO channel through the second mask by thermal evaporation. Since the as-deposited ZnO film is usually quite insulating,
76
H.S. Bae, S. Im / Thin Solid Films 469–470 (2004) 75–79
known to be a shallow donor located in some interstitial sites within ZnO crystals [7,8]. All electrical and photoelectric characterizations were carried out with a semiconductor parameter analyzer (model HP 4155C, Agilent Technologies) at RT. For I–V measurements on TFTs, large ohmic contacts for gate electrodes were made on the backside of p-Si by applying indium (In) paste. Light illumination was performed with a light source (Oriel Optical System) which employed a 500 W Hg(Xe) arc lamp and a monochromator covering the range of 250–670 nm. The illuminated beam cross sections on our TFTs were identical to be 1 cm2 in all cases as indicated in Fig. 1(b). The optical power density of the monochromatic light was measured by a UV-enhanced Si detector.
3. Results and discussion
Fig. 1. Schematic (a) cross section and (b) plan views of our ZnO-based TFT exposed to photons. Beam cross section was 1 cm2 for all cases.
we treated the film using rapid thermal annealing (RTA) at 350 8C for 1 min in order to improve the conductivity of the ZnO layer and also to reduce the contact resistance of source/drain pads (Al). The RTA treatment was mostly performed in a forming gas (H2/N2=1:10) ambient to induce hydrogen doping into our ZnO films because H atom is well
Fig. 2(a) displays the drain current–drain voltage (I D– V D) curves obtained from our ZnO-based TFTs. The saturation current was about 1.7 AA under a gate bias of 40 V. (The mobility and on/off current ratio of our TFTs were about 0.05 cm2/V s and ~105, respectively. We estimated the field effect electron mobility using the slope of the mobility plot, i.e. the square root of saturation current (IDSAT) vs. gate bias (V G) in Fig. 4 and the threshold voltage of our TFT was then around 10 V.) Although the dark saturation current was only 1.7 AA, the current behavior totally changed under UV illumination (wavelength 340 nm) with photon energy over 3.6 eV and optical power density of 1.26 mW/cm2. As shown in Fig. 2(b), our ZnO-based TFT shows that its maximum drain current (at V G=40 V) increases up to about 50 AA under UV illumination, which is about an
Fig. 2. (a) I D–V D curves obtained from our ZnO-based TFT in the dark and (b) under UV illumination (340 nm, 1.26 mW/cm2).
H.S. Bae, S. Im / Thin Solid Films 469–470 (2004) 75–79
77
Fig. 3. I D–V D curves obtained under UV illumination (340 nm) with varied optical powers (V G=0V).
order of magnitude higher than the maximum dark saturation current. Hence the observed change is solely due to photoelectric effect in the ZnO channel. In particular, it is also interesting to compare the behavior of our TFT at V G=0V in the dark and under UV illumination. In the dark, our TFTs exhibit drain current of less than 50 nA under a drain bias of 25 V (50 nA was too small to be identified in Fig. 2(a)) while the same device exhibits drain current of more than 7.5 AA under the same drain bias under UV illumination. Similar photo-response was found under the same UV illumination (340 nm) with varied optical powers when the ZnO-TFT was under a gate bias of 0 V, as shown in Fig. 3. The amount of photocurrent gradually increases
with the power but in the state of 0 V gate bias the photo-response may have some limit to show the full potential of detector sensitivity. We thus characterized the spectral photo-response properties of our ZnO-TFT in more detail way that employs log10 I D–V G curves obtained under a drain-saturation regime (drain bias, V DS=30V). Fig. 4 exhibits the log10 I D–gate bias (V G) curves obtained from our TFT that was illuminated by photons (normalized power density 0.58 mW/cm2) with ultraviolet (UV, 254 nm), blue (B, 442 nm), green (G, 540 nm), and red (R, 650 nm) wavelengths. The photo-response was clearer in the depletion state near a gate bias of 40 V than in the accumulation state near 40 V, as shown in the figure. It is
Fig. 4. Spectral responses observed in the log10 I D–V G curves. The state of ZnO channel depletion exhibits much higher photoelectric effects than that of accumulation. The electron mobility and on/off current ratio of ~0.05 cm2/V s and ~105 were also estimated, respectively, in the dark.
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Fig. 5. Detailed spectral response as the ratio of I photo/I dark was obtained from log10 I D–V G curves (V DS=30 V, V G= 40 V).
because the initial dark depletion state draws almost no charge carriers (or no background signals) in the ZnO channel whereas the same channel readily introduces many electrons under the gate bias for the dark accumulation state (even before the light-induced carriers are generated). The largest photoelectric effect results from UV illumination. Blue and green photons also produced quite high photogains. However, red photons were ineffective, achieving a similar level of drain current compared to the dark current level. Our ZnO channel layer deposited by rf sputtering at room temperature is likely to contain a high density of point defects that may mainly consist of natural mid-gap states (deep acceptor) induced by Zn vacancies (single-charged) or O interstitials in ZnO polycrystals are known to exist near 2.3 eV below the conduction band edge of the ZnO. [9,10] Hence it is not surprising that only the photons with energy higher than ~2.3 eV have exhibited good photoelectric effects although another but a minor kind of deep defects was previously reported to be located at 0.78 eV as a deep donor (double-charged Zn vancancy) [11]. In Fig. 5, the detailed spectral responses of ZnO-TFT were plotted as photocurrent/dark current (I photo/I dark) ratios after their log10 I D–V G curves were measured under a gate bias of 40V for depletion state. As expected, the red illumination of 650 nm resulted in little photoelectric ratio among all the wavelength range of illumination and the illumination ranging from green (540 nm) to UV (254 nm) displayed high photo-responsivity of more than 105 that is almost the same value as the on/off ratio of our ZnO-TFT. The highest responsivity value was obtained from UV (254 nm) and green light near 500 nm. This means that the photoresponse mechanism probably includes the absorption occurring through the band-to-band and the band-to-main deep level transitions for the UV and the green lights, respectively.
4. Conclusions We have fabricated ZnO-based TFTs on SiO2/p-Si substrates with rf magnetron sputtering at room temperature. Our ZnO-TFTs exhibited a maximum saturation current of about 1.7 AA and an on/off current ratio of ~105 in the dark. Illuminated by UV and visible photons with energy higher than 2.3 eV, our TFTs display high photocurrent of more than 50 AA because the relevant photo-detection is associated with the band gap of ZnO and its deep level defects located near ~2.3 eV below the conduction band edge. According to our study with varying gate voltage, a much higher photoelectric ratio is achieved when we employ the depletion state of the ZnO TFT channel rather than the accumulation state. It is concluded that the ZnO-based TFTs can be promising UV photo-detecting device.
Acknowledgements Authors gratefully acknowledge the financial supports from KISTEP (M20204250033-02A0903-00440) and the Basic Research Program of KOSEF (R01-1999-000-000300), and in part from the BK 21 Program.
References [1] P.F. Carcia, R.S. McLean, M.H. Reilly, G. Nunes Jr., Appl. Phys. Lett. 82 (2003) 1117. [2] S. Masuda, K. Kitamura, Y. Okumura, S. Miyatake, H. Tabata, T. Kawai, J. Appl. Phys. 93 (2003) 1624. [3] R.L. Hoffman, B.J. Norris, J.F. Wager, Appl. Phys. Lett. 82 (2003) 733. [4] H. Fabricius, T. Skettrup, P. Bisgaard, Appl. Opt. 25 (1986) 2764.
H.S. Bae, S. Im / Thin Solid Films 469–470 (2004) 75–79 [5] C.H. Park, I.S. Jeong, J.H. Kim, S. Im, Appl. Phys. Lett. 83 (2003) 2946. [6] I.-S. Jeong, J.H. Kim, S. Im, Appl. Phys. Lett. 83 (2003) 2946. [7] C.G. Van de Walle, Phys. Rev. Lett. 85 (2000) 1012. [8] C ¸ . Kilic¸, A. Zunger, Appl. Phys. Lett. 81 (2002) 73.
79
[9] S. Im, B.J. Jin, S. Yi, Appl. Phys. Lett. 87 (2000) 4558. [10] J.Y. Lee, Y.S. Choi, J.H. Kim, M.O. Park, S. Im, Thin Solid Films 403–404 (2002) 553. [11] W. Gfpel, J. Vac. Sci. Technol. 16 (1979) 1229.