RESEARCH

ARTIC

LE

Copyright © 2012 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol. 12, 1–9, 2012

Preparation of Insulating SiO2 Thin Filmsby the Sol–Gel Process

Tung-Li Hsieh, Ann-Kuo Chu, and Wen-Yao Huang∗

Department of Photonics, Institute of Electro-Optical Engineering, National Sun Yat-Sen University,Kaohsiung 80424, Taiwan, R.O.C.

Organic dispersion agents can effectively decrease the surface roughness of films. Here, filmscontaining organic dispersion agents are used to produce metal-insulator-metal structures. It wasfound that addition of Triton caused films to become denser, and thicker, and the leak current ofdevices to decrease by 10 times compared with that without Triton because of its uniform dispersionin the films. The resulting films were used as the insulator layer of thin film transistors containinga semiconductor layer of evaporated pentacene. The interface between the insulator and semi-conductor layers was found to affect the arrangement of pentacene, and O2 plasma was used toimprove the interface activity to increase the order of the pentacene molecules and enhance thecarrier mobility of the devices.

Keywords: TEOS, Sol–Gel, Spin-Coating.

1. INTRODUCTION

Thin film transistors (TFTs) are generally manufacturedby physical or chemical vapor deposition. Because thesemethods are complicated, they often lead to process issuesand high cost. Wet process technology is a relativelysimple and inexpensive alternative. TFTs can be dividedinto four important components: substrate, insulator layer,semiconductor layer and electrode. The development ofthe sol–gel method began in 1846 when Ebelmen1 foundthat alcohol and SiCl4 can form metal alkoxides in asol–gel state under atmospheric conditions, although thesignificance of this finding was not immediately recog-nized. In the 1930s, Geffcken and Berger2 found that metalalkoxides can be used to prepare thin films. The devel-opment of the sol–gel method in the 1970s was initiatedby Schroeder et al.3�4 In addition, Levene and Thomas5

and Dislich6 controlled the rate of hydrolysis and conden-sation of siloxane compounds to prepare different glasses.Recently, numerous scientists have adjusted the param-eters of the sol–gel method to prepare nanoscale prod-ucts with a wide range of applications.7–19 In the initialstate of a sol–gel reaction, hydrolysis and condensationoccur in a solution containing inorganic or organic com-pounds to form a colloid containing nanoparticles. Underdifferent pH and temperature conditions, these colloids

∗Author to whom correspondence should be addressed.

Fig. 1. Diagram showing the structure of the device.

Fig. 2. Effect of TEOS concentration on film thickness.

J. Nanosci. Nanotechnol. 2012, Vol. 12, No. xx 1533-4880/2012/12/001/009 doi:10.1166/jnn.2012.6913 1

RESEARCH

ARTIC

LE

Preparation of Insulating SiO2 Thin Films by the Sol–Gel Process Hsieh et al.

Fig. 3. SEM images of SiO2 thin films formed using a TEOS concentration of (a) 10 wt%, (b) 20 wt%, (c) 30 wt%, and (d) 40 wt%.

(a) (b)

(c) (d)

Fig. 4. AFM images of films containing different concentrations of TEOS (a) 10 wt%, (b) 20 wt%, (c) 30 wt%, and (d) 40 wt%.

2 J. Nanosci. Nanotechnol. 12, 1–9, 2012

RESEARCH

ARTIC

LE

Hsieh et al. Preparation of Insulating SiO2 Thin Films by the Sol–Gel Process

generate a silicon net structure or increase in size. When ahybrid material is prepared using sol–gel technology,20–31

it forms through hydrolysis and condensation of metallicalkoxide compounds. Hydrolysis involves the dissolutionof the precursor Si(OR)4 in acid or alkali aqueous solu-tion. The alkoxy group reacts with an OH group of water,turning the precursor into Si(OH)n(OR)4−n. Depending oncatalyst type, hydrolysis can be divided into electrophilicand nucleophilic substitution for acidic and alkali cata-lysts, respectively. Next, the metallic alcoholate formedby hydrolysis is involved in a dealcoholization reaction.The dealcoholized alcoholic and residual hydroxyl groupthen condense, or the alkoxy group of the metal alcoho-late will condense with these alcoholic groups. Finally,metal-oxide–metal bonding is formed. The suspended mat-ter generated by the system in the initial stage of the reac-tion is called a colloid, which contains particulate withdiameters in the range of 1–1000 nm. Because the particlesare extremely small, the influence of gravity on them canthen be neglected. The interactions between colloid parti-cles are only Van der waals forces and Brownian motion.In this work, the experimental focus is the insulator layer,which is prepared by the sol–gel method. Different condi-tions are used to grow different thin films, and their sur-face characteristics are analyzed. The thin films are thenused to prepare TFTs. The characteristics, critical voltage,carrier mobility and on/off current ratio of the TFTs aremeasured. In addition, the main factors limiting device per-formance and methods to overcome these factors will bediscussed.

2. EXPERIMENTAL DETAILS

2.1. Preparation of Sol–Gels

Hydrochloric acid (0.14 g) was added to deionized water(21.6 g) in a conical flask so that the pH of the solutionwas about 2. Isopropyl alcohol (IPA, 18 g) was addedand then the solution was stirred magnetically for 30 min.TEOS was added and then the solution was stirredmagnetically for 2.5 h. The top of the flask was sealedwith glue and left undisturbed for one week.32

2.2. Preparation of Insulator Layer ThinFilm and Device

The indium tin oxide (ITO) glass substrate was firstcleaned sequentially with neutral detergent, deionizedwater, acetone and IPA in an ultrasonic bath for 15 minuteseach. The ITO glass was heated for 5 minutes at 100 �C ona hot plate to remove residual IPA. Heat-resistant tape withan area of 0�6× 2 cm was placed on the ITO glass sub-strate to facilitate measurements. A glass pipette was usedto add SiO2 solution dropwise onto the ITO glass substrate.An SiO2 film was formed by spin-coating at 500 rpm for

10 seconds and then 4000 rpm for 20 seconds. The edgesof the substrate were cleaned with IPA, and then the filmwas dried at 80 �C on at hot plate for 10 minutes. Organicsemiconductor was deposited on the insulator layer by vac-uum deposition under a pressure of 2×10−5 Torr and rateof 0.4–1 Å/s. A shadow mask was fixed on the devicewith heat-resistant tape. Aluminum33 was deposited on thedevice as an electrode at a rate of 1 Å/sec up to a thick-ness of 300 Å, 3 Å/sec for 300–1000 Å, and 5–10 Å/secfor 1000–2000 Å. The channel width and length of thedevice were 30 and 300 �m, respectively (Fig. 1). Thedevice was characterized using a semiconductor parameteranalyzer (HP 4155).

3. RESULTS AND DISCUSSION

3.1. Film Formation Characteristics and ElectricalProperties of the Thin Film

The optimal concentration of TEOS for film formation wasdetermined (Fig. 2). SEM images (Fig. 3) revealed thatsolutions containing 20 and 30 wt% TEOS formed theflattest films. When the amount of TEOS is decreased34

to 10 wt%, the centrifugal force is larger than the adhe-sion force during film formation, so the film is not totallyadsorbed on the ITO substrate. Hence, after the solvent isevaporated, defects such as cracks are formed. This can beclearly seen in AFM images (Fig. 4). In addition, the sur-face roughness of the film increases if the concentration ofTEOS is too low (Fig. 5). For a solution containing 40 wt%TEOS, the amount of reacted SiO2 molecules is larger,so the polymers formed are also larger.35 This results inclusters in the films, which are observed as white particlesin Figure 2(d). XRD was used to confirm that the thin filmswere composed of SiO2 (Fig. 6). The XRD diffractionpeak of ITO, which was used as a substrate, is presented

Fig. 5. Surface roughness of films containing different concentrationsof TEOS.

J. Nanosci. Nanotechnol. 12, 1–9, 2012 3

RESEARCH

ARTIC

LE

Preparation of Insulating SiO2 Thin Films by the Sol–Gel Process Hsieh et al.

(a) (b)

(c) (d)

(e)

Fig. 6. XRD patterns of films containing different concentrations of TEOS (a) 0 wt% (ITO substrate), (b) 10 wt%, (c) 20 wt%, (d) 30 wt%, and(e) 40 wt%.

in Figure 6(a). The XRD diffraction peak of ITO was sub-tracted from those of films formed from solutions contain-ing different concentrations of TEOS. For the peak at 2� of21.3�, the lattice plane distance (d-spacing) was calculatedto be 4.17 Šusing the Bragg diffraction formula (n� =2d sin ��. The diffraction peak of an SiO2 thin film is typ-ically about 21�, proving that the films formed on the ITOsubstrates are SiO2. The thin films were transformed into

devices with a MIM structure, and then the leak currentwas measured (Fig. 7). The thin film containing 20 wt%TEOS had the lowest leak current of those examined. Todecrease the surface roughness of the thin films, Triton wasadded to the SiO2 solutions as a dispersion agent.36 Tritonis a mixture containing polyethylene glycol, which has ahigh viscosity and can be used as a surfactant.37 It canalso protect the surface of thin films from moisture and

4 J. Nanosci. Nanotechnol. 12, 1–9, 2012

RESEARCH

ARTIC

LE

Hsieh et al. Preparation of Insulating SiO2 Thin Films by the Sol–Gel Process

Fig. 7. Comparison of leak currents from devices containing differentconcentrations of TEOS effect of adding a dispersion agent to the SiO2

solution.

oxygen in the air. The thickness of films prepared fromsolutions containing different concentrations of SiO2 and0.1 g Triton are compared in Figure 8. Triton increasedthe viscosity of the solution, which also caused the filmthickness to increase. SEM images of the films contain-ing Triton (Fig. 9) show the films are flatter and con-tain fewer defects than those formed without Triton. Thesefindings were further confirmed by AFM measurements(Fig. 10). The surface roughness of thin films contain-ing different concentrations of TEOS was reduced to therange of 0.1–0.8 nm (Fig. 11). The films were convertedinto devices with a MIM structure and their leak currentswere measured (Fig. 12). The leak current decreased byabout 10–100 times after adding Triton. Therefore, addi-tion of Triton causes the SiO2 molecules to form an evendistribution in the solution. The increased viscosity of thesolution induced by Triton causes the film to form as aflatter, denser layer, which significantly reduces the leakcurrent.

Fig. 8. Effect of adding Triton to films containing different concentra-tions of TEOS on film thickness.

Fig. 9. SEM images of films containing Triton and different concentra-tions of TEOS (a) 10 wt%, (b) 20 wt%, (c) 30 wt%, and (d) 40 wt%.

J. Nanosci. Nanotechnol. 12, 1–9, 2012 5

RESEARCH

ARTIC

LE

Preparation of Insulating SiO2 Thin Films by the Sol–Gel Process Hsieh et al.

(a) (b)

(c) (d)

Fig. 10. AFM images of films containing Triton and different concentration of TEOS (a) 10 wt%, (b) 20 wt%, (c) 30 wt%, and (d) 40 wt%.

3.2. Electrical Properties of TFTs

A semiconductor parameter analyzer was used to measurethe relationship between drain current (ID� (see Eq. (1)below) and drain–source voltage (VDS� for TFTs at dif-ferent gate–source voltage (VGS�.

38 The carrier mobility(field-effect mobility) was calculated using the saturationzone formula:

I1/2D =

[(W

2L

)CI�

]1/2

�VGS−VT � (1)

Fig. 11. Surface roughness of films containing Triton and different con-centrations of TEOS.

where ID is the drain current, � is the carrier mobility,W is the channel width, L is the channel length, CI isthe insulator capacitance per unit area, VT is the thresh-old voltage, and VGS is the gate electrode–drain electrodevoltage. The thin films with low current leakage (thosecontaining Triton) were converted into devices to measuretheir electrical properties (Fig. 13). The devices containedpentacene with a thickness of 50 nm as a semiconductorlayer, which was evaporated at a rate is 0.4 Å/s. Figure 13shows that the devices do not display semiconductor char-acteristics, indicating that the pentacene molecules weredisordered. The silica-based insulator layer was not surface

Fig. 12. Comparison of the leak currents of films containing Triton anddifferent concentrations of TEOS.

6 J. Nanosci. Nanotechnol. 12, 1–9, 2012

RESEARCH

ARTIC

LE

Hsieh et al. Preparation of Insulating SiO2 Thin Films by the Sol–Gel Process

(a) (b)

(c) (d)

Fig. 13. ID −VDS curves for devices containing Triton and different concentrations of TEOS (a) 10 wt%, (b) 20 wt%, (c) 30 wt%, and (d) 40 wt%.

treated, so the pentacene molecules may have been dis-ordered during thermal evaporation. Another reason forthe disordered pentacene molecules could be the deposi-tion rate. When the deposition rate is lower than 1 Å/s,molecules will be disordered and anisotropic, but whenthe deposition rate is about 1 Å/s, they will adopt an ori-ented arrangement. When the molecules are disordered,the layer will contain a large number of lattice boundaries,which reduces carrier mobility. Semiconductor character-istics are not observed when the carrier mobility of a layeris very small. O2 plasma was used to increase the activ-ity of pentacene to improve its dispersion characteristics

(a) (b)

Fig. 14. Use of O2 plasma to increase the activity of pentacene (a) without and (b) with O2 plasma treatment.

and enhance the uniformity of its films.39 The deposi-tion rate was also controlled at 1 Å/s so that the pen-tacene molecules would form an ordered arrangement.The AFM images in Figure 14 show that the interfacewas improved under such conditions. Pentacene moleculeswere arranged in an oblique manner, and the lattice sizewas increased from 100 to 150 nm. Because of the increaseof crystal volume, the influence of defects is reduced,and the carrier mobility of pentacene is then effectivelyenhanced (Fig. 15). The characteristics of devices mea-sured after activation of the interface by O2 plasma arealso summarized in Table I.

J. Nanosci. Nanotechnol. 12, 1–9, 2012 7

RESEARCH

ARTIC

LE

Preparation of Insulating SiO2 Thin Films by the Sol–Gel Process Hsieh et al.

(a) (b)

(c) (d)

Fig. 15. ID−VDS curves for devices containing different concentrations of TEOS after interface activation by O2 plasma (a) 10 wt%, (b) 20 wt%,(c) 30 wt%, and (d) 40 wt%.

Table I. Characteristics of devices containing different concentrationsof TEOS after treatment with O2 plasma.

Concentration 10 wt% 20 wt% 30 wt% 40 wt%

VT −13�9 −8�9 −10�1 −14�1On/off ratio 4�5×102 2�5×104 2�8×103 3�9×102

Mobility (cm2/vS) 0.348 0.218 0.253 0.353

4. CONCLUSIONS

To reduce the number of defects in thin films, Triton wasadded as a dispersion agent to the sol–gels used to formthin films. This allowed a very dense layer to be formed,which reduced the surface roughness of thin films, andalso their leak current to around 10−7–10−9. Devices con-taining these thin films with pentacene as a semiconduc-tor layer were fabricated. To enhance the performance ofthese devices, O2 plasma was used to improve the inter-face between the insulator and semiconductor layers, andthe deposition rate was increased to form an ordered layerof pentacene molecules and achieve good semiconduc-tor characteristics. Because the crystal size in the semi-conductor layer affects carrier mobility, thermal treatmentcould be used to increase crystal size. Addition of a self-assembled layer between the insulator and semiconductor

layers might also improve device performance throughdifferent arrangements of molecules.

References and Notes

1. J. Ebelmen, J. Ann. 57, 331 (1846).2. W. Geffcken and E. Berger, German Patent 736, 411 (1939).3. H. Schroeder, Phys. Thin Film 5, 87 (1969).4. J. L. Woodhead, Silicate Ind. 37, 191 (1972).5. L. Levene and I. M. Thomas, U.S. Patent 640, 093 (1972).6. H. Dislich, Angewandt Chemie 10, 363 (1971).7. G. S. Sur and J. E. Mark, J. Eur. Polym. 21 (1051).8. J. E. Mark and G. S. Sur, Polym. Bull. 14, 325 (1985).9. S. J. Clarson and J. E. Mark, Polym. Commun. 28, 249 (1987).

10. Y. Imai, Adv. Polymer Sci. 140, 1 (1999).11. J. A. He, L. Samuelson, L. Li, J. Kumar, and S. K. Tripathy, Adv.

Materials 11, 435 (1999).12. M. Guglielmi, G. Brusatin, G. Facchin, and M. Gleria, Appl.

Organometallic Chem. 13, 339 (1999).13. T. Yoshihiro, M. Akinori, K. Kenzo, and U. Kenji, J. Lumin. 87, 767

(2000).14. M. Opallo and J. Kukulka, Electrochem. Commun. 2, 394 (2000).15. M. Shinya, N. Hiroyoshi, K. Yoshihiko, M. Kimihiro, and I. Hiroshi,

J. Organometallic Chemistry 611, 40 (2000).16. S. H. Jang, M. G. Han, and S. S. Im, Synth. Met. 110, 17 (2000).17. T. C. Chang, Y. T. Wang, Y. S. Hong, H. B. Chen, and J. C. Yang,

Polymer Degradation and Stability 69, 317 (2000).18. D. Kai and B. Regina, Catalysis Commun. 1 (2000).

8 J. Nanosci. Nanotechnol. 12, 1–9, 2012

RESEARCH

ARTIC

LE

Hsieh et al. Preparation of Insulating SiO2 Thin Films by the Sol–Gel Process

19. W. C. Jae and I. S. Kyung, Polymer 42, 727 (2001).20. H. K. Schmidt, Mater. Res. Soc. Symp. Proc. 32, 327 (1984).21. H. K. Schmidt, Mater. Res. Soc. Symp. Proc. 180, 961 (1990).22. U. Schubert, H. Sing, and N. Lorenz, A. Chem. Mater. 7, 2010 (1995).23. D. A. Loy and K. J. Shea, Chem. Rev. 95, 1431 (1995).24. P. Judenstein and C. Sanchez J. Mater. Chem. 6, 511 (1996).25. K. J. Shea, D. A. Loy, and O.W. Webster, Chem. Mater. 1, 574 (1989).26. K. M. Choi and K. J. Shea, J. Am. Chem. Soc. 116, 9052 (1994).27. K. M. Choi and K. J. Shea, J. Phys. Chem. 98, 3207 (1994).28. K. M. Choi, J. C. Hemminger, and K. J. J. Shea, Phys. Chem.

99, 4720 (1995).29. H. W. Oviatt, K. J. Shea, S. Kalluri, Y. Q. Shi, W. H. Steier, and

L. R. Dalton, Chem. Mater. 7, 493 (1995).30. H. W. Oviatt, Jr, K. J. Shea, and J. H. Small, Chem. Mater. 5, 943

(1993).

31. D. A. Loy, G. M. Jamison, B. M. Baugher, S. A. Myers, R. A.Assink, and K. J. Shea, Chem. Mater. 8, 656 (1996).

32. C. J. Brinker and G. W. Scherer, Academic Press, New York (1990).33. E. Yengel, L. Wang, M. Ozkan, and C. S. Ozkan, Proc. of SPIE

7411, 74110B (2009).34. J. T. Lue, Encyclopedia of Nanosci. Nanotechnol. X, 1 (2007).35. R. Ciriminna, M. Sciortino, G. Alonzo, A. de Schrijver, and

M. Pagliaro, Chem. Rev. (2010).36. J. Hilding, E. A. Grulke, Z. G. Zhang, and F. Lockwood, J. Disp.

Sci. Technol. 24, 1 (2003).37. R. Zhang and P. Somasundaran, Adv. Colloid Interface Sci. 123–126,

213 (2006).38. K. Park, J. Y. Choi, H. J. Lee, J. Y. Kwon, and H. Kim, Japanese

J. Appl. Phys. 50 (2011).39. M. W. Lee and C. K. Song, Japanese J. Appl. Phys. 42 (2003).

Received: xx Xxxx xxxx. Accepted: xx Xxxx xxxx.

J. Nanosci. Nanotechnol. 12, 1–9, 2012 9