TiO2 thin-film transistors fabricated by spray pyrolysis

https://doi.org/10.1063/1.3330944

Wöbkenberg et al., Applied Physics Letters 96, 082116 (2010).

TiO2 thin-film transistors fabricated by spray pyrolysis Paul H. Wöbkenberg,1 Thilini Ishwara,1 Jenny, Nelson,1 Donal D. C. Bradley,1 Saif A. Haque,2 and Thomas D. Anthopoulos,1,a) 1)

Department of Physics, Blackett Laboratory Imperial College London, London SW7 2BW, UK

2)

Department of Chemistry Imperial College London, London SW7 2AZ, UK

Abstract We demonstrate electron transporting thin-film transistors based on TiO2 films deposited from solution by spray pyrolysis under ambient atmosphere. The field-effect electron mobility is found to depend strongly on the device architecture and the type of source and drain electrodes employed. For optimised transistors a maximum mobility value of 0.05 cm2/Vs is obtained. Furthermore, the TiO2 transistors show air-stable operating characteristics with a shelf life time of several months. This is the only report on electron transporting transistors based on thin-films of TiO2 deposited by spray pyrolysis. Such devices could be used for the study of charge carrier transport in TiO2 and other related materials.

a)

Author to whom correspondence should be addressed; Electronic mail: [email protected] -1-

https://doi.org/10.1063/1.3330944


In recent years metal oxide semiconductors (MOxS) have been the focus of intense research because of their potential in numerous technological applications including photocatalysis,1 solar cells,2 light-emitting diodes,3 and very recently thin-film transistors (TFTs).4 Titanium dioxide (TiO2) is a well known member of the MOxS family and one of the most promising materials for application in hybrid photovoltaics such as dye-sensitized solar cells (DSCs).2,5 Apart from its use in DSCs, TiO2 has also been explored in numerous other applications. For example, Haque et al.3 reported on polymer light-emitting diodes employing TiO2 as the electron injection electrode while Jameson et al.6 used single crystals of rutile TiO2 in twoterminal field-programmable rectification devices. Furthermore, Kim et al.7 utilised interlayers of TiOx, deposited from a soluble precursor, as optical spacers in bulk-heterojunction organic photovoltaics (OPVs) while Campbell et al.8 and Majewski et al.9 explored the high k properties of TiO2 for use as the gate dielectric in hole transporting (p-channel) thin-film transistors (TFTs). An additional attractive feature of TiO2 is its processability. For instance, it can be deposited employing solution-based techniques such as sol-gel10,11 and spray-pyrolysis (SP).12,13 Spray pyrolysis, in particular, appears most attractive because it is scalable and potentially a low-cost process suitable for the deposition of dense TiO2 layers onto large –area substrates. Despite its advantages however, use of spray pyrolysis for the fabrication of TFTs remains surprisingly limited.14,15 Here, we report the fabrication of electron-transporting (n-channel) transistors based on thin-films of TiO2 deposited by spray pyrolysis. TiO2 based transistors exhibit strong electron accumulation with maximum carrier mobility on the order 0.05 cm2/Vs. The latter is comparable to mobility values derived from single crystal rutile TiO2 based TFTs.4 Most importantly TiO2 transistors are found to be air-stable with the electron mobility increasing as a function of exposure time to atmospheric air. This is the only report of transistors based on a TiO2 semiconductor film deposited by spray pyrolysis. Such devices could one day be explored -2-

https://doi.org/10.1063/1.3330944


for fundamental studies such as the carrier-induced ferromagnetism observed recently in Codoped TiO216 as well as the newly demonstrated memory resistors (so-called memristors).17 Thin-film transistors were fabricated using heavily doped Si++ wafers as common gate electrode with a 200 nm thermally oxidized SiO2 layer acting as the gate dielectric. Using conventional photolithography, gold source and drain (S-D) electrodes were defined in a bottom-gate, bottom-contact (BG-BC) configuration. A 10 nm layer of titanium was used acting as an adhesion layer for the gold on SiO2. Bottom-gate, top-contact (BG-TC) transistors were prepared by thermal evaporation of the aluminium S-D contacts after deposition of the TiO2 onto the SiO2 dielectric. TiO2 was deposited from a precursor solution containing titanium (IV)iso-propoxide (Ti-iPr), 2,4, pentanedione (PD) in absolute ethanol with concentration of 5 vol.% at Ti-iPr:PD molar ratio of 1:2, using an air-brush and N2 as the carrier gas. Deposition was performed at a substrate temperature of 450˚C by a pulse solution feed. Freshly deposited films were subsequently heat treated for 30 minutes at 500˚C in ambient air in order to remove any residual un-reacted precursor. Deposition of TiO2 films directly at 500˚C was found to yield similar results. The devices were then transferred into a glove box filled with N2 for electrical characterisation using an HP 4156C semiconductor parameter analyser. UV-Vis absorption measurements were carried out on TiO2 thin-films in ambient atmosphere. TiO2 films deposited by spray pyrolysis at 450˚C are known to be of the anatase form, one of the three main forms of titanium dioxide.13 The optical properties of as-deposited TiO2 films were studied using optical absorption measurements (data not shown). From the onset of absorption the optical bandgap (EG) was approximated to 3.3 eV. This is in agreement with values reported in the literature.13 Due to the wide bandgap of TiO2, as-prepared films are highly transparent in the spectral region 400-750 nm with transmittance values ranging between 7090 %.

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Next we examined the suitability of spray pyrolysed TiO2 films for use in transistors. Initial tests were carried out in nitrogen atmosphere in the dark employing BG-BC transistors [Fig. 1(a) inset] using gold S-D electrodes. Representative sets of the output and transfer characteristics obtained from a TiO2 TFT (L = 5 μm, W = 50 cm) are shown in Fig. 1(a) and (b), respectively. The maximum electron mobility, measured in saturation, for this device was 310-3 cm2/Vs. The on/off current ratio and threshold voltage (VT) measured was 105 and VT ~ 33 V, respectively. The relatively large VT is attributed to the significant density of electron traps present at the SiO2/TiO2 interface. These traps are possibly formed during semiconductor deposition at the relatively high temperature of 450oC. A strongly injection-limited charge transport is observed as evident from the super-linear behaviour of ID at low VD [Fig. 1(b)]. This is attributed to the relatively large contact barrier for electrons (ϕAu = 1.3 eV) expected from the difference between the Fermi level of Au (EF(Au) = 5 eV) and the conduction band (CB) of TiO2 (~ 3.8 eV).2 In practice however, the resulting contact barrier could be influenced by extrinsic factors such as surface states or interface chemistry. The simplified energy diagram of the contact, neglecting band bending, is shown in Fig. 1(c). In an effort to minimise the injection barrier BG-TC TFTs employing low workfunction contact materials were fabricated. The inset in Fig. 2(a) shows the BG-TC device architecture employed. Aluminium, with EF(Al) = 4.1 eV, was found to provide an Ohmic-like contact with the CB of TiO2 (ϕAl = 0.3 eV, Fig. 1(c)). The room temperature transfer and output characteristics obtained from a TiO2 transistor with Al S-D electrodes (L = 60 μm, W = 1 mm) are shown in Fig. 2. From the transfer curves, in saturation, [Fig. 2(a)] a maximum electron mobility of 0.05 cm2/Vs and a VT ~ 32 V, are calculated. Both parameters were extracted directly from the ID1/2 vs. VG plot. Although the electron mobility in TiO2 is lower than in other transparent oxides,18,19 it compares favourably with mobility values extracted from TFTs based on single crystals of rutile TiO2.4

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The electron transport characteristics of TiO2 TFTs as a function of temperature were also investigated. Fig. 2(c) displays the electron mobility, measured in saturation, as a function of temperature. A thermally activated electron transport with an associated energy of EA ~ 90 meV is measured. Hence, charge transport in these films is most likely governed by a hopping-type conduction mechanism20 rather than conduction in fully delocalized energy bands. This result is in line with previous reports on TiO2 films deposited by spray pyrolysis under similar conditions, where polycrystalline features appeared in otherwise predominantly amorphous films for deposition temperatures above 400°C.12,13 The low EA is comparable with the value of 76 meV obtained from reduced TiO2 films prepared by sputtering.21 The latter is attributed to the high level of electron concentration (in Ref. 21 due to reductive doping and in the present study due to field-doping) present in both films. Interestingly, for temperatures >350 K the electron mobility exceeds 0.1 cm2/Vs. The latter is comparable to the electron mobility obtained from transistors based on high quality anatase TiO2 films produced using an elaborate oxygen modulated pulsed laser deposition method.23 From these results we can conclude that SP is a useful method for the deposition of high quality TiO2 films. The ambient stability of the TiO2 transistors has also been investigated by exposing them to ambient air at room temperature. Fig. 3 demonstrates the air-stable operation of TiO2 transistors over several days. It can be seen from Fig. 3(a) that the electron mobility hardly changes over a period of approximately six days although the current on/off ratio decreases slightly [Fig. 3(b)]. The latter is attributed to a modest increase of the off current. The reason for this current increase is subject to ongoing investigation. Overall it is noteworthy that the transistors do not appear to degrade during storage in ambient air, which is an important advantage over most electron transporting organic semiconductors. In summary, we have demonstrated electron transporting transistors based on TiO2 films deposited by spray pyrolysis. Electron mobility of up to 0.05 cm2/Vs has been achieved at room temperature. Importantly, TiO2 transistors exhibit air-stable electron transport for a prolonged -5-

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period of time. Apart from their potential use in transparent electronics, such oxide-based transistors could one day be used as testbed for fundamental studies of the physical properties of simple as well as multi-component oxide materials systems.

Acknowledgments We are grateful to the Engineering and Physical Sciences Research Council (EPSRC), Research Councils UK (RCUK) and the Lee-Lucas endowment for financial support.

Reference list: 1

Cui, H., Shen, H.-S., Gao, Y.-M., Dwight, K. & Wold, A. Mater. Res. Bull. 28, 195 (1993).

2

Grätzel, M. Nature 414, 338-344 (2001).

3

Haque, S. A., Koops, S., Tokmoldin, N., Durrant, J. R., Huang, J., Bradley, D. D. C. & Palomares, E. Adv. Mater. 19, 683-687 (2007).

4

Katayama, M., Ikesaka, S., Kuwano, J., Yamamoto, Y., Koinuma, H. & Matsumoto, Y. Appl. Phys. Lett. 89, 242103 (2006).

5

O'Regan, B. & Grätzel, M. Nature 353, 737-740 (1991).

6

Jameson, J. R., Fukuzumi, Y., Wang, Z., Griffin, P., Tsunoda, K., Meijer, G. I. & Nishi, Y. Appl. Phys. Lett. 91, 112101 (2007).

7

Kim, J. Y., Kim, S. H., Lee, H.-H., Lee, K., Ma, W., Gong, X. & Heeger, A. J. Adv. Mater. 18, 572-576 (2006).

8

Campbell, S. A., Gilmer, D. C., Xiao-Chuan, W., Ming-Ta, H., Hyeon-Seag, K., Gladfelter, W. L. & Jinhua, Y. Electron Devices, IEEE Transactions on 44, 104-109 (1997).

9

Majewski, L. A., Schroeder, R. & Grell, M. Adv. Funct. Mater. 15, 1017-1022 (2005).

10

Grätzel, M. J. Sol-Gel Sci. Technol. 22, 7-13 (2001).

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11


Vishwas, M., Sharma, S. K., Narasimha Rao, K., Mohan, S., Gowda, K. V. A. & Chakradhar, R. P. S. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 74, 839-842 (2009).

12

Kavan, L. & Grätzel, M. Electrochimica Acta 40, 643-652 (1995).

13

Conde-Gallardo, A., Guerrero, M., Castillo, N., Soto, A. B., Fragoso, R. & CabañasMoreno, J. G. Thin Solid Films 473, 68-73 (2005).

14

Adamopoulos, G., Bashir, A., Wöbkenberg, P. H., Bradley, D. D. C. & Anthopoulos, T. D. Appl. Phys. Lett. 95, 133507 (2009).

15

Aneeqa Bashir, P. H. W., Jeremy Smith, James M. Ball, George Adamopoulos, Donal D. C. Bradley, Thomas D. Anthopoulos,. Adv. Mater. 9999, NA (2009).

16

Toyosaki, H., Fukumura, T., Yamada, Y., Nakajima, K., Chikyow, T., Hasegawa, T., Koinuma, H. & Kawasaki, M. Nat Mater 3, 221-224 (2004).

17

Yang, J. J., Pickett, M. D., Li, X., OhlbergDouglas, A. A., Stewart, D. R. & Williams, R. S. Nat Nano 3, 429-433 (2008).

18

Nomura, K., Ohta, H., Takagi, A., Kamiya, T., Hirano, M. & Hosono, H. Nature 432, 488-492 (2004).

19

Chiang, H. Q., Wager, J. F., Hoffman, R. L., Jeong, J. & Keszler, D. A. Appl. Phys. Lett. 86, 013503 (2005).

20

H. Bässler, P. M. B., R. J. Perry,. Journal of Polymer Science Part B: Polymer Physics 32, 1677-1685 (1994).

21

Tang, H., Prasad, K., Sanjines, R., Schmid, P. E. & Levy, F. J. Appl. Phys. 75, 20422047 (1994).

22

Wagner, S., Gleskova, H., Cheng, I. C. & Wu, M. Thin Solid Films 430, 15-19 (2003).

23

Katayama, M., Ikesaka, S., Kuwano, J., Koinuma, H. & Matsumoto, Y. Appl. Phys. Lett. 92, 132107 (2008).

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Figure Captions

Figure 1. (a) Output and (b) transfer characteristics measured for a bottom gate – bottom contact TiO2 transistor employing gold S-D electrodes (L = 5 μm, W = 50 cm). Inset: the BG-BC transistor architecture used. (c) Schematic representation of the simplified energy band diagram of TiO2 and aluminium and gold electrodes.

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Figure 2. (a) Output and (b) transfer characteristics of bottom gate – top contact TiO2 transistor employing aluminium S-D electrodes (L = 60 μm, W = 1 mm). Inset: the BG-TC transistor architecture used. (c) Arrhenius plot of electron mobility, measured in saturation, versus inverse temperature. An activation energy of approximately 90 meV is calculated.

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Figure 3. (a) Electron field-effect mobility, measured in saturation, as a function of exposure time to ambient air. (b) Current on/off ratio as a function of exposure time to ambient air. All electrical measurements were performed in the dark.

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