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Enhancement of long-term stability of pentacene thin-film transistors encapsulated with transparent SnO2
Applied Surface Science 252 (2005) 1332–1338 www.elsevier.com/locate/apsusc
Enhancement of long-term stability of pentacene thin-film transistors encapsulated with transparent SnO2 Woo Jin Kim a, Won Hoe Koo a, Sung Jin Jo a, Chang Su Kim a, Hong Koo Baik a,*, Jiyoul Lee b, Seongil Im b a
Department of Metallurgical Engineering, Yonsei University, Seoul 120-749, Republic of Korea Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Republic of Korea
b
Received 18 January 2005; received in revised form 14 February 2005; accepted 16 February 2005 Available online 11 April 2005
Abstract The long-term stability of pentacene thin-film transistors (TFTs) encapsulated with a transparent SnO2 thin-film prepared by ion beam-assisted deposition (IBAD) was investigated. After encapsulation process, our organic thin-film transistors (OTFTs) showed somewhat degraded field-effect mobility of 0.5 cm2/(V s) that was initially 0.62 cm2/(V s), when a buffer layer of thermally evaporated 100 nm SnO2 film had been deposited prior to IBAD process. However, the mobility was surprisingly sustained up to 1 month and then gradually degraded down to 0.35 cm2/(V s) which was still three times higher than that of the OTFT without any encapsulation layer after 100 days in air ambient. The encapsulated OTFTs also exhibited superior on/off current ratio of over 105 to that of the unprotected devices (104) which was reduced from 106 before aging. Therefore, the enhanced long-term stability of our encapsulated OTFTs should be attributed to well protection of permeation of H2O and O2 into the devices by the IBAD SnO2 thin-film which could be used as an effective inorganic gas barrier for transparent organic electronic devices. # 2005 Elsevier B.V. All rights reserved. Keywords: OTFT; Ion beam-assisted deposition; Encapsulation; Lifetime; Pentacene
1. Introduction Organic thin-film transistors (OTFTs) have drawn much attention due to their great compatibility with plastic substrates available for a flexible display, smart * Corresponding author. Tel.: +82 2 2123 2838; fax: +82 2 312 5375. E-mail address: [email protected] (H.K. Baik).
cards and rf tags application over the last decades [1–4]. Organic semiconductors also make it possible to fabricate electronic devices at room temperature through simple methods without using complicated equipments [5]. Among organic semiconductors considered as active materials in organic thin-film transistors, pentacene was widely used for its mobility as high as that of a-Si or in some cases even exceeding that value [6,7]. However, when the OTFTs are
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.02.100
W.J. Kim et al. / Applied Surface Science 252 (2005) 1332–1338
operated in air ambient, they tend to easily degrade in the similar way that the lifetime of organic lightemitting diodes (OLEDs) is limited [8]. In order to enhance the stability of devices, some encapsulation layers capable of protecting H2O or O2 in the air from penetrating through the organic layers have been employed. SiOx, AlOx, AlOxNy and SiOxNy are common amorphous oxides used as inorganic gas barrier [9–11]. But, during the deposition process of the above-mentioned oxides, some serious damages might be induced on the organic layers, which would be attributed to energetic ions, X-rays and electron beam [12,13]. Encapsulation with an organic material, such as a vacuum deposited parylene was reported to have an excellent conformal coating and successfully protect the organic active layer from solvents in patterning process [14]. For the sake of more reliable operation of the devices, it is quite desirable for an organic layer to be used along with an inorganic oxide, because an organic material itself could not provide sufficient capability of blocking permeation of H2O or O2. Among the transparent amorphous oxides, which can perform function of gas barrier deposited by simple thermal evaporation, SiOx and SnO2 would be taken into account. It was previously reported by our group that SnO2 was more desirable one due to its high polarizability resulting in effectively suppressing permeation of water vapor by strong chemical interaction, particularly when the film could be made to have much denser microstructures [15]. Therefore, the SnO2 thin-film prepared by ion beam-assisted deposition (IBAD) is a promising candidate for transparent thin-film encapsulation adaptable for top-emitting organic light-emitting diodes (TEOLEDs) since it has relatively high transmittance (85% over 450 nm) as well as quite low water vapor transmission rate (WVTR) of below 103 g/(m2 day). In order to prevent possible damages induced from energetic ions during IBAD process, it is also desirable that a proper buffer layer should be employed [16]. In this paper, we report on the investigation of the long-term stability of our OTFTs in terms of electrical properties associated with materials characterization by X-ray photoemission spectroscopy (XPS) when the TFT was encapsulated with the IBAD SnO2 thin-film having a buffer layer of same kind of material on top of the device.
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2. Experiment Our OTFTs with the pentacene thin-film as an active layer were fabricated on SiO2/p+-Si substrate. The thickness of thermally grown SiO2 was 100 nm. Prior to the deposition of pentacene, the surface of SiO2 (to be used as a gate dielectric) was ultrasonically cleaned with trichloroethylene, acetone, isopropylalcohol and deionized water sequentially. Our thermal evaporation chamber was evacuated to a base pressure of 1 107 Torr. We then deposited the pentacene film through a shadow mask by heating the effusion cell containing pure pentacene powder (99.8%, ˚ /s at room Aldrich Chem. Co.) at a rate of 0.5 A temperature (RT). The resulting film thickness of the pentacene film was adjusted to 50 nm as measured with a quartz crystal monitor. The source and drain contacts were patterned by a second shadow mask, through which Au was thermally evaporated to the thickness of about 100 nm. The channel length, defined as the spacing between the source and drain pads, was 100 mm and the channel width was 1000 mm. SnO2 (to be used as a buffer layer) was
Fig. 1. (a) Schematic drawing of IBAD process and (b) photographic plan view (length, L = 100 mm and width, W = 1000 mm) and cross-section of the device structure.
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then deposited on top of the device by thermal evaporation through a third shadow mask to the film thickness of about 100 nm. Finally, another 80 nm SnO2 thin-film (to be used as an encapsulation layer) was consecutively deposited without breaking vacuum by IBAD process shown in Fig. 1(a). The end-hall ion gun was used to increase SnO2 ad-atom mobility. Ar+ ion energy and current density were 150 eV and 45 mA/cm2, and the deposition rate was 1 nm/s. In all cases of ion beam irradiation conditions, the incident angle of the ion beam was 458. Fig. 1(b) shows photographic plan view and schematic cross sectional view of our pentacene OTFT with encapsulation.
3. Results The I–V measurements of our OTFTs were performed with an HP 4145B parameter analyzer in the dark at RT. Fig. 2(a) displays the output characteristics of our OTFT without encapsulation and one can see the typical drain current–voltage (ID– VD) curves of the FET devices. The drain current in the saturation regime at a gate bias of 40 V was around 120 mA and the leakage current measured between the gate and source electrode (IG) was below 2 nA. It is then clear that the measured drain current could be used for evaluation of the electrical performance of our OTFTs. The curves in Fig. 2(b) are representing the drain current of the OTFT without encapsulation (open circle) and with encapsulation (close circle) when the gate bias of VG is 40 V. According to Fig. 2 (b), the saturation current of the unprotected OTFT is decreased to 35 mA from 120 mA after 100 days in air ambient, whereas the encapsulated device showing a relatively low initial current of 100 mA maintained considerable current level of 60 mA. The initial degradation of the OTFT with encapsulation might be attributed to some possible damages induced on the pentacene active layer during the encapsulation process in which small portion of the energetic ions could penetrate through the thermally evaporated buffer layer [16]. pffiffiffiffiffiffiffiffiffi The plots of ID versus VG and log10 ( ID) versus VG with and without encapsulation are presented in Fig. 3(b and c), respectively. The fieldeffect mobility of holes in the pentacene pffiffiffiffiffiffiffiffiffi layer was obtained from the slope in VG versus ID plot in the
Fig. 2. (a) The output characteristics of the reference OTFT without any encapsulation and (b) degradation of the drain current in the saturation regime when VG is 40 V after 100 days in air.
saturation regime where the source–drain voltage of VD = 40 V was applied. The initial field-effect mobility of the encapsulated and unprotected OTFT was calculated to be 0.5 and 0.62 cm2/(V s), respectively. As expected from serious degradation of saturation current of the OTFT without encapsulation in Fig. 2(b), the mobility of the device decreased sharply from 0.62 to 0.10 cm2/(V s) after 100 days. On/off current ratio and sub-threshold slope also showed a similar degradation trend according to the log10 ( ID) versus VG plots. The on/off current ratio decreased down to 104 from initial value of 106 with a very inferior sub-threshold slope of 7.2 V/dec. In the mean while, the encapsulated device maintained an on/off current ratio of 105 even after 100 days with similar sub-threshold slope p toffiffiffiffiffiffiffiffi theffi initial value. It is also interesting to note in the ID versus VG plot that the threshold voltage (VT) changes toward positive
W.J. Kim et al. / Applied Surface Science 252 (2005) 1332–1338
gate bias showing early turn-on upon degree of degradation. Although the initial VT values of both devices were almost same as approx. 2.5 V, the unprotected device displayed about 7 V as shown in Fig. 3(a), while the same-period-aged but encapsulated one showed only 0.1 V of VT after 100 days in air. Since the pentacene channel/SiO2 interfaces should be in the same condition for all OTFTs, we suspected if water molecules exist in the channel layer. Horowitz demonstrates that the surface of gate dielectrics in contact with organic channel layer has so many defects that it would strongly aggravate the charge transport between the source and drain electrode [17].
pffiffiffiffiffiffiffiffiffi Fig. 3. The plots of ID vs. VG and log10 ( ID) vs. VG. for the OTFTs without encapsulation (a) and with encapsulation (b).
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Hence, we should take into account the organic molecular layer beyond the first monolayer in the channel region to find out what would be responsible for the total charge transport. The water molecules in air are known to be adsorbed on pentacene surface or even penetrate into the channel layer during aging period [18]. Then, they could be one of the main leakage sources or active deep level traps when the OTFT is particularly under a high electric field of depletion state [19]. The property-endurance limit or aging effect of our OTFTs in terms of field-effect mobility and on/off current ratio upon time are shown in Fig. 4(a and b), respectively. The encapsulated TFT shows the best performance even after a long time exposure to air. The initial degradation of mobility from 0.62 to 0.5 cm2/(V s) is the expected behavior in line with
Fig. 4. The property-endurance limit or aging effect with time of our OTFTs in terms of field-effect mobility (a) and on/off current ratio (b).
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reduction of the drain current of the encapsulated OTFT in Fig. 2(b). However, the SnO2 film was deposited by using IBAD on top of the device without any buffer layer, the device was found to seriously degrade due to direct penetration of energetic ions into the organic active layer [16]. In order to avoid deterioration of the device during IBAD process, it is strongly required to employ a proper buffer layer of which is thermally evaporated SnO2 in this study. Unfortunately, thermally evaporated SnO2 film had quite open structure resulting in a loose microstructure and this would also be known from the fact that, when being used alone, it could not effectively suppress H2O permeation into the device as indicated in Fig. 4(a). Therefore, the buffer layer itself could not play an important role in blocking permeation of water vapor and it is the IBAD SnO2 layer that performs this function.
4. Discussion It was once reported that the main reason for degradation of the OTFTs with time was adsorption and penetration of H2O into the active layer of a pentacene film [18]. Fig. 5 represents O 1s peaks that we obtained from the both surfaces of aged and unaged pentacene films as they were scanned by X-ray photoemission spectroscopy. All the peak energies were calibrated with C 1s peak. Since OH molecular groups in pentacene are thought to be involved during
Fig. 5. XPS spectra for O 1s peaks obtained from the pentacene thin-films before and after exposed to air for 100 days.
pentacene film deposition, the O 1s peaks from the OH groups in aged and un-aged pentacene should be the same each other in intensity. It is then clear that the 100-day-aged pentacene film has much higher density of H2O than the un-aged film. Total amounts of H2O and OH group inside of the pentacene film could be estimated from corresponding O 1s peaks and calculated to be 1.7 and 5% for the un-aged and aged one, respectively. It is thus, considered that the water molecules are crucial clues for the observed property-degradation. With application of encapsulation to the OTFTs, the long-term stability in terms of the electrical properties was tremendously enhanced. This should be attributed to well protection of the devices from water vapor or oxygen in air due to our IBAD SnO2 film. Fig. 6 displays XPS O 1s spectrum of the IBAD SnO2 film comprised of two main peaks which were originated from the OH groups at around 531.6 eV and the O 1s peak corresponding to the Sn4+ at around 530.3 V while the inset in Fig. 6 represents the peak from Sn 3d3/2 (495.3 eV) to 3d5/2 (486.9 eV). It was clearly shown that the SnO2 film had O 1s peak from OH group inside of the film, which is a result of higher polarization and refractive index of the SnO2 film [20]. The H2O has a large dipole moment and lone-pair electrons, thus is a good donor. Molecular adsorption occurs by acid/base interaction between water vapor and the oxides [21]. Therefore, the continuous transmission of water vapor is restrained by the interaction between the formed OH groups and water vapor. However, although the thermally evaporated SnO2 film has the high refractive index of about 1.9 and considerable amount of OH groups inside of film, it is obvious that this layer could not prevent permeation of H2O effectively as shown in Fig. 4(a). Here, we should consider the influence of the film density on the water vapor permeation, because if the pore size and density are too large and high, the permeating water vapor would be expected to penetrate the films without interaction through the central pore space away from the surface OH groups within pores [15]. Consequently, when encapsulated with the IBAD SnO2, the device displays much enhanced performance in terms of long-term stability due to its quite low WVTR, which should come from strong chemical interaction between penetrating polar water vapor and hydroxyl elements inside of the film
W.J. Kim et al. / Applied Surface Science 252 (2005) 1332–1338
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Fig. 6. XPS spectra for O 1s peaks and Sn 3d2/3 and 3d2/5 spectra (inset) from the SnO2 thin-film prepared by IBAD.
in conjunction with a dense microstructure by IBAD process. However, even the encapsulated devices finally began to degrade its field-effect mobility with time after 1 month. This might be attributed to our pentacene films with a very rough surface of rms 9.3 nm as measured by atomic force microscope (AFM) in the channel region shown in Fig. 7(a). Since the IBAD SnO2 thin-film with a buffer layer was deposited on the rough pentacene surface, the encapsulation layer was also found to be rough following underlying layer. Fig. 7(b) shows the top surface of the encapsulation layer prepared by IBAD
and according to a section analysis of AFM profile, the vertical distance between marks was found to reach as high as 50 nm where the thickness of the IBAD SnO2 layer was 80 nm. Therefore, in addition to a dense microstructure, the barrier film should have a flat surface because vapor permeation should be mainly dominated by diffusion via the shortest path in the film. For this reason, a thin-film encapsulation consisted of multi-layer was more desirable to enhance the barrier properties and some polymer smoothing layers could be used to make a surface flat on which the inorganic barrier film will be deposited. Hence, a further study would be required to make our
Fig. 7. AFM images of 50 nm pentacene thin-film (a) and 80 nm IBAD SnO2 encapsulation layer on top of 100 nm SnO2 buffer layer (b).
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encapsulation meet the requirements needed for stable operation of a real device in air ambient.
5. Conclusion We have fabricated pentacene TFTs with the transparent SnO2 encapsulation layer prepared by IBAD to enhance long-term stability of the devices. Although the encapsulated OTFTs showed somewhat degraded field-effect mobility of 0.5 cm2/(V s), the devices exhibited much enhanced electrical performances sustaining the mobility up to 1 month. On/off current ratio also prevails in the case of the encapsulated devices showing around 105 that is just one order of magnitude lower than that of its initial value of 106, while the unprotected OTFT shows on/ off current ratio of 104 after 100 days. The IBAD SnO2 was found to effectively prevent H2O or O2 from permeating into the devices by means of densification of microstructures through IBAD process and chemical interaction between water vapor and SnO2 which resulted in quite low WVTR value accounting for enhancement of the long-term stability of the OTFTs. It is thus, concluded that a transparent SnO2 thin-film prepared by IBAD is a promising material that could be used as an inorganic gas barrier for transparent organic electronic devices with an appropriate buffer layer including some smoothing layers.
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[2] A.R. Brown, C.P. Jarret, D.M. de Leeuw, M. Matters, Synth. Met. 88 (1997) 37. [3] P.F. Baude, D.A. Ender, M.A. Haase, T.W. Kelly, D.V. Muyres, S.D. Theiss, Appl. Phys. Lett. 82 (2003) 3964. [4] C.D. Sheraw, L. Zhou, J.R. Huang, D.J. Gundlach, T.N. Jackson, M.G. Kane, I.G. Hill, M.S. Hammond, J. Campi, B.K. Greening, J. Francl, J. West, Appl. Phys. Lett. 80 (2002) 1088. [5] F. Garnier, R. Hajlaoui, A. Yassar, P. Strivastava, Science 265 (1994) 1684. [6] P.V. Necliudov, M.S. Shur, D.J. Gundlach, T.N. Jackson, J. Appl. Phys. 88 (2000) 6594. [7] H. Klauk, D.J. Gundlach, J.A. Nichols, T.N. Jackson, IEEE Trans. Electron. Devices 46 (1999) 1258. [8] H. Aziz, Z. Popovic, C. Tripp, N.-X. Hu, A.-M. Hor, G. Xu, Appl. Phys. Lett. 72 (1998) 2642. [9] M. Vogt, R. Hauptmann, Surf. Coat. Technol. 676 (1995) 74. [10] A.G. Erlat, B.M. Henry, J.J. Ingram, D.B. Mountain, A. McGuigan, R.P. Howson, C.R.M. Grovenor, G.A.D. Briggs, Y. Tsukahara, Thin Solid Films 388 (2001) 78. [11] A.G. Erlat, B.M. Henry, J.J. Ingram, C.R.M. Grovenor, G.A.D. Briggs, R.J. Chater, Y. Tsukahara, J. Phys. Chem. B 883 (2004) 108. [12] V. Bulovic, P. Tian, P. Burrows, M.R. Gokhale, S.R. Forres, M.E. Thompson, Appl. Phys. Lett. 70 (1997) 2954. [13] L.S. Hung, C.W. Tang, M.G. Manson, P. Raychudhuri, J. Madathil, Appl. Phys. Lett. 74 (2001) 544. [14] I. Kymissis, C.D. Dimitrakopoulos, S. Purushothaman, J. Vac. Sci. Technol. B 20 (3) (2002) 956. [15] W.H. Koo, S.M. Jeong, S.H. Choi, S.M. Lee, S.J. Lee, H.K. Baik, J. Phys. Chem. B 108 (2004) 18884. [16] S.M. Jeong, W.H. Koo, S.H. Choi, S.J. Jo, S.J. Lee, K.M. Song, H.K. Baik, Appl. Phys. Lett. 85 (2004) 1051. [17] G. Horowitz, J. Mater. Res. 19 (7) (2004) 1946. [18] Y. Qiu, Y. Hu, G. Dong, L. Wang, J. Xie, Y. Ma, Appl. Phys. Lett. 83 (2003) 1644. [19] J. Lee, K.B. Kim, J.H. Kim, S.I. Im, D.Y. Jung, Appl. Phys. Lett. 82 (2003) 4169. [20] V. Dimitrov, T. Komastsu, J. Solid State Chem. 100 (2003) 163. [21] V.E. Henrich, P.A. Cox, The Surface Science of Metal Oxides, Cambridge University Press, New York, 1996.
Enhancement of long-term stability of pentacene thin-film transistors encapsulated with transparent SnO2 Woo Jin Kim a, Won Hoe Koo a, Sung Jin Jo a, Chang Su Kim a, Hong Koo Baik a,*, Jiyoul Lee b, Seongil Im b a
Department of Metallurgical Engineering, Yonsei University, Seoul 120-749, Republic of Korea Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Republic of Korea
b
Received 18 January 2005; received in revised form 14 February 2005; accepted 16 February 2005 Available online 11 April 2005
Abstract The long-term stability of pentacene thin-film transistors (TFTs) encapsulated with a transparent SnO2 thin-film prepared by ion beam-assisted deposition (IBAD) was investigated. After encapsulation process, our organic thin-film transistors (OTFTs) showed somewhat degraded field-effect mobility of 0.5 cm2/(V s) that was initially 0.62 cm2/(V s), when a buffer layer of thermally evaporated 100 nm SnO2 film had been deposited prior to IBAD process. However, the mobility was surprisingly sustained up to 1 month and then gradually degraded down to 0.35 cm2/(V s) which was still three times higher than that of the OTFT without any encapsulation layer after 100 days in air ambient. The encapsulated OTFTs also exhibited superior on/off current ratio of over 105 to that of the unprotected devices (104) which was reduced from 106 before aging. Therefore, the enhanced long-term stability of our encapsulated OTFTs should be attributed to well protection of permeation of H2O and O2 into the devices by the IBAD SnO2 thin-film which could be used as an effective inorganic gas barrier for transparent organic electronic devices. # 2005 Elsevier B.V. All rights reserved. Keywords: OTFT; Ion beam-assisted deposition; Encapsulation; Lifetime; Pentacene
1. Introduction Organic thin-film transistors (OTFTs) have drawn much attention due to their great compatibility with plastic substrates available for a flexible display, smart * Corresponding author. Tel.: +82 2 2123 2838; fax: +82 2 312 5375. E-mail address: [email protected] (H.K. Baik).
cards and rf tags application over the last decades [1–4]. Organic semiconductors also make it possible to fabricate electronic devices at room temperature through simple methods without using complicated equipments [5]. Among organic semiconductors considered as active materials in organic thin-film transistors, pentacene was widely used for its mobility as high as that of a-Si or in some cases even exceeding that value [6,7]. However, when the OTFTs are
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.02.100
W.J. Kim et al. / Applied Surface Science 252 (2005) 1332–1338
operated in air ambient, they tend to easily degrade in the similar way that the lifetime of organic lightemitting diodes (OLEDs) is limited [8]. In order to enhance the stability of devices, some encapsulation layers capable of protecting H2O or O2 in the air from penetrating through the organic layers have been employed. SiOx, AlOx, AlOxNy and SiOxNy are common amorphous oxides used as inorganic gas barrier [9–11]. But, during the deposition process of the above-mentioned oxides, some serious damages might be induced on the organic layers, which would be attributed to energetic ions, X-rays and electron beam [12,13]. Encapsulation with an organic material, such as a vacuum deposited parylene was reported to have an excellent conformal coating and successfully protect the organic active layer from solvents in patterning process [14]. For the sake of more reliable operation of the devices, it is quite desirable for an organic layer to be used along with an inorganic oxide, because an organic material itself could not provide sufficient capability of blocking permeation of H2O or O2. Among the transparent amorphous oxides, which can perform function of gas barrier deposited by simple thermal evaporation, SiOx and SnO2 would be taken into account. It was previously reported by our group that SnO2 was more desirable one due to its high polarizability resulting in effectively suppressing permeation of water vapor by strong chemical interaction, particularly when the film could be made to have much denser microstructures [15]. Therefore, the SnO2 thin-film prepared by ion beam-assisted deposition (IBAD) is a promising candidate for transparent thin-film encapsulation adaptable for top-emitting organic light-emitting diodes (TEOLEDs) since it has relatively high transmittance (85% over 450 nm) as well as quite low water vapor transmission rate (WVTR) of below 103 g/(m2 day). In order to prevent possible damages induced from energetic ions during IBAD process, it is also desirable that a proper buffer layer should be employed [16]. In this paper, we report on the investigation of the long-term stability of our OTFTs in terms of electrical properties associated with materials characterization by X-ray photoemission spectroscopy (XPS) when the TFT was encapsulated with the IBAD SnO2 thin-film having a buffer layer of same kind of material on top of the device.
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2. Experiment Our OTFTs with the pentacene thin-film as an active layer were fabricated on SiO2/p+-Si substrate. The thickness of thermally grown SiO2 was 100 nm. Prior to the deposition of pentacene, the surface of SiO2 (to be used as a gate dielectric) was ultrasonically cleaned with trichloroethylene, acetone, isopropylalcohol and deionized water sequentially. Our thermal evaporation chamber was evacuated to a base pressure of 1 107 Torr. We then deposited the pentacene film through a shadow mask by heating the effusion cell containing pure pentacene powder (99.8%, ˚ /s at room Aldrich Chem. Co.) at a rate of 0.5 A temperature (RT). The resulting film thickness of the pentacene film was adjusted to 50 nm as measured with a quartz crystal monitor. The source and drain contacts were patterned by a second shadow mask, through which Au was thermally evaporated to the thickness of about 100 nm. The channel length, defined as the spacing between the source and drain pads, was 100 mm and the channel width was 1000 mm. SnO2 (to be used as a buffer layer) was
Fig. 1. (a) Schematic drawing of IBAD process and (b) photographic plan view (length, L = 100 mm and width, W = 1000 mm) and cross-section of the device structure.
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then deposited on top of the device by thermal evaporation through a third shadow mask to the film thickness of about 100 nm. Finally, another 80 nm SnO2 thin-film (to be used as an encapsulation layer) was consecutively deposited without breaking vacuum by IBAD process shown in Fig. 1(a). The end-hall ion gun was used to increase SnO2 ad-atom mobility. Ar+ ion energy and current density were 150 eV and 45 mA/cm2, and the deposition rate was 1 nm/s. In all cases of ion beam irradiation conditions, the incident angle of the ion beam was 458. Fig. 1(b) shows photographic plan view and schematic cross sectional view of our pentacene OTFT with encapsulation.
3. Results The I–V measurements of our OTFTs were performed with an HP 4145B parameter analyzer in the dark at RT. Fig. 2(a) displays the output characteristics of our OTFT without encapsulation and one can see the typical drain current–voltage (ID– VD) curves of the FET devices. The drain current in the saturation regime at a gate bias of 40 V was around 120 mA and the leakage current measured between the gate and source electrode (IG) was below 2 nA. It is then clear that the measured drain current could be used for evaluation of the electrical performance of our OTFTs. The curves in Fig. 2(b) are representing the drain current of the OTFT without encapsulation (open circle) and with encapsulation (close circle) when the gate bias of VG is 40 V. According to Fig. 2 (b), the saturation current of the unprotected OTFT is decreased to 35 mA from 120 mA after 100 days in air ambient, whereas the encapsulated device showing a relatively low initial current of 100 mA maintained considerable current level of 60 mA. The initial degradation of the OTFT with encapsulation might be attributed to some possible damages induced on the pentacene active layer during the encapsulation process in which small portion of the energetic ions could penetrate through the thermally evaporated buffer layer [16]. pffiffiffiffiffiffiffiffiffi The plots of ID versus VG and log10 ( ID) versus VG with and without encapsulation are presented in Fig. 3(b and c), respectively. The fieldeffect mobility of holes in the pentacene pffiffiffiffiffiffiffiffiffi layer was obtained from the slope in VG versus ID plot in the
Fig. 2. (a) The output characteristics of the reference OTFT without any encapsulation and (b) degradation of the drain current in the saturation regime when VG is 40 V after 100 days in air.
saturation regime where the source–drain voltage of VD = 40 V was applied. The initial field-effect mobility of the encapsulated and unprotected OTFT was calculated to be 0.5 and 0.62 cm2/(V s), respectively. As expected from serious degradation of saturation current of the OTFT without encapsulation in Fig. 2(b), the mobility of the device decreased sharply from 0.62 to 0.10 cm2/(V s) after 100 days. On/off current ratio and sub-threshold slope also showed a similar degradation trend according to the log10 ( ID) versus VG plots. The on/off current ratio decreased down to 104 from initial value of 106 with a very inferior sub-threshold slope of 7.2 V/dec. In the mean while, the encapsulated device maintained an on/off current ratio of 105 even after 100 days with similar sub-threshold slope p toffiffiffiffiffiffiffiffi theffi initial value. It is also interesting to note in the ID versus VG plot that the threshold voltage (VT) changes toward positive
W.J. Kim et al. / Applied Surface Science 252 (2005) 1332–1338
gate bias showing early turn-on upon degree of degradation. Although the initial VT values of both devices were almost same as approx. 2.5 V, the unprotected device displayed about 7 V as shown in Fig. 3(a), while the same-period-aged but encapsulated one showed only 0.1 V of VT after 100 days in air. Since the pentacene channel/SiO2 interfaces should be in the same condition for all OTFTs, we suspected if water molecules exist in the channel layer. Horowitz demonstrates that the surface of gate dielectrics in contact with organic channel layer has so many defects that it would strongly aggravate the charge transport between the source and drain electrode [17].
pffiffiffiffiffiffiffiffiffi Fig. 3. The plots of ID vs. VG and log10 ( ID) vs. VG. for the OTFTs without encapsulation (a) and with encapsulation (b).
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Hence, we should take into account the organic molecular layer beyond the first monolayer in the channel region to find out what would be responsible for the total charge transport. The water molecules in air are known to be adsorbed on pentacene surface or even penetrate into the channel layer during aging period [18]. Then, they could be one of the main leakage sources or active deep level traps when the OTFT is particularly under a high electric field of depletion state [19]. The property-endurance limit or aging effect of our OTFTs in terms of field-effect mobility and on/off current ratio upon time are shown in Fig. 4(a and b), respectively. The encapsulated TFT shows the best performance even after a long time exposure to air. The initial degradation of mobility from 0.62 to 0.5 cm2/(V s) is the expected behavior in line with
Fig. 4. The property-endurance limit or aging effect with time of our OTFTs in terms of field-effect mobility (a) and on/off current ratio (b).
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reduction of the drain current of the encapsulated OTFT in Fig. 2(b). However, the SnO2 film was deposited by using IBAD on top of the device without any buffer layer, the device was found to seriously degrade due to direct penetration of energetic ions into the organic active layer [16]. In order to avoid deterioration of the device during IBAD process, it is strongly required to employ a proper buffer layer of which is thermally evaporated SnO2 in this study. Unfortunately, thermally evaporated SnO2 film had quite open structure resulting in a loose microstructure and this would also be known from the fact that, when being used alone, it could not effectively suppress H2O permeation into the device as indicated in Fig. 4(a). Therefore, the buffer layer itself could not play an important role in blocking permeation of water vapor and it is the IBAD SnO2 layer that performs this function.
4. Discussion It was once reported that the main reason for degradation of the OTFTs with time was adsorption and penetration of H2O into the active layer of a pentacene film [18]. Fig. 5 represents O 1s peaks that we obtained from the both surfaces of aged and unaged pentacene films as they were scanned by X-ray photoemission spectroscopy. All the peak energies were calibrated with C 1s peak. Since OH molecular groups in pentacene are thought to be involved during
Fig. 5. XPS spectra for O 1s peaks obtained from the pentacene thin-films before and after exposed to air for 100 days.
pentacene film deposition, the O 1s peaks from the OH groups in aged and un-aged pentacene should be the same each other in intensity. It is then clear that the 100-day-aged pentacene film has much higher density of H2O than the un-aged film. Total amounts of H2O and OH group inside of the pentacene film could be estimated from corresponding O 1s peaks and calculated to be 1.7 and 5% for the un-aged and aged one, respectively. It is thus, considered that the water molecules are crucial clues for the observed property-degradation. With application of encapsulation to the OTFTs, the long-term stability in terms of the electrical properties was tremendously enhanced. This should be attributed to well protection of the devices from water vapor or oxygen in air due to our IBAD SnO2 film. Fig. 6 displays XPS O 1s spectrum of the IBAD SnO2 film comprised of two main peaks which were originated from the OH groups at around 531.6 eV and the O 1s peak corresponding to the Sn4+ at around 530.3 V while the inset in Fig. 6 represents the peak from Sn 3d3/2 (495.3 eV) to 3d5/2 (486.9 eV). It was clearly shown that the SnO2 film had O 1s peak from OH group inside of the film, which is a result of higher polarization and refractive index of the SnO2 film [20]. The H2O has a large dipole moment and lone-pair electrons, thus is a good donor. Molecular adsorption occurs by acid/base interaction between water vapor and the oxides [21]. Therefore, the continuous transmission of water vapor is restrained by the interaction between the formed OH groups and water vapor. However, although the thermally evaporated SnO2 film has the high refractive index of about 1.9 and considerable amount of OH groups inside of film, it is obvious that this layer could not prevent permeation of H2O effectively as shown in Fig. 4(a). Here, we should consider the influence of the film density on the water vapor permeation, because if the pore size and density are too large and high, the permeating water vapor would be expected to penetrate the films without interaction through the central pore space away from the surface OH groups within pores [15]. Consequently, when encapsulated with the IBAD SnO2, the device displays much enhanced performance in terms of long-term stability due to its quite low WVTR, which should come from strong chemical interaction between penetrating polar water vapor and hydroxyl elements inside of the film
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Fig. 6. XPS spectra for O 1s peaks and Sn 3d2/3 and 3d2/5 spectra (inset) from the SnO2 thin-film prepared by IBAD.
in conjunction with a dense microstructure by IBAD process. However, even the encapsulated devices finally began to degrade its field-effect mobility with time after 1 month. This might be attributed to our pentacene films with a very rough surface of rms 9.3 nm as measured by atomic force microscope (AFM) in the channel region shown in Fig. 7(a). Since the IBAD SnO2 thin-film with a buffer layer was deposited on the rough pentacene surface, the encapsulation layer was also found to be rough following underlying layer. Fig. 7(b) shows the top surface of the encapsulation layer prepared by IBAD
and according to a section analysis of AFM profile, the vertical distance between marks was found to reach as high as 50 nm where the thickness of the IBAD SnO2 layer was 80 nm. Therefore, in addition to a dense microstructure, the barrier film should have a flat surface because vapor permeation should be mainly dominated by diffusion via the shortest path in the film. For this reason, a thin-film encapsulation consisted of multi-layer was more desirable to enhance the barrier properties and some polymer smoothing layers could be used to make a surface flat on which the inorganic barrier film will be deposited. Hence, a further study would be required to make our
Fig. 7. AFM images of 50 nm pentacene thin-film (a) and 80 nm IBAD SnO2 encapsulation layer on top of 100 nm SnO2 buffer layer (b).
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encapsulation meet the requirements needed for stable operation of a real device in air ambient.
5. Conclusion We have fabricated pentacene TFTs with the transparent SnO2 encapsulation layer prepared by IBAD to enhance long-term stability of the devices. Although the encapsulated OTFTs showed somewhat degraded field-effect mobility of 0.5 cm2/(V s), the devices exhibited much enhanced electrical performances sustaining the mobility up to 1 month. On/off current ratio also prevails in the case of the encapsulated devices showing around 105 that is just one order of magnitude lower than that of its initial value of 106, while the unprotected OTFT shows on/ off current ratio of 104 after 100 days. The IBAD SnO2 was found to effectively prevent H2O or O2 from permeating into the devices by means of densification of microstructures through IBAD process and chemical interaction between water vapor and SnO2 which resulted in quite low WVTR value accounting for enhancement of the long-term stability of the OTFTs. It is thus, concluded that a transparent SnO2 thin-film prepared by IBAD is a promising material that could be used as an inorganic gas barrier for transparent organic electronic devices with an appropriate buffer layer including some smoothing layers.
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