Effect of channel thickness on the field effect mobility ...

Journal of Alloys and Compounds 621 (2015) 189–193

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Effect of channel thickness on the field effect mobility of ZnO-TFT fabricated by sol gel process Yasemin Caglar a,⇑, Mujdat Caglar a, Saliha Ilican a, Seval Aksoy a, Fahrettin Yakuphanoglu b a b

Anadolu University, Faculty of Science, Department of Physics, 26470 Eskisehir, Turkey Firat University, Faculty of Arts and Sciences, Department of Physics, 23169 Elazig, Turkey

a r t i c l e

i n f o

Article history: Received 9 July 2014 Received in revised form 23 September 2014 Accepted 24 September 2014 Available online 2 October 2014 Keywords: ZnO-TFT Sol gel spin coating Field effect mobility

a b s t r a c t In this study, bottom-gate structured ZnO TFTs were fabricated on pSi/SiO2 substrate. ZnO active layers with different thickness were deposited by using sol gel spin coating method and the electrical performances of the obtained TFTs were investigated regarding the thickness of the channel layer. The effects of channel thickness on the structural and morphological properties of ZnO were examined. The mobility values of TFTs were significantly improved by increasing the thickness of active channel layer and the highest mobility of ZnO TFTs was obtained by 1.09 cm2/V s for 140 nm layer. ZnO TFTs showed a high off-current and this current increased as the channel thickness increased. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Amorphous or polycrystalline Si have been played an important role in the flat-panel display industry. But they have some disadvantages such as light sensitivity, light degradation, low mobility and opacity [1]. Recently, therefore, metal oxide semiconductors have been investigated as an alternative material for channel layer of thin films transistors (TFTs) due to their high carrier mobility, low processing temperature and better electrical stability than polycrystalline silicon [2,3]. Among the oxide semiconductors, ZnO has been recognized as one of the most promising semiconductor materials for producing next-generation thin film transistors (TFTs) because of having a wide band gap (3.37 eV) n-type semiconductor and high exciton binding energy (60 meV) at room temperature. Several methods have been attempted to prepare ZnO-based TFTs films: atomic layer deposition (ALD) [4], radio frequency (RF) sputtering [5–7], chemical bath method [8], spray pyrolysis [9], pulsed laser deposition [10] and sol–gel method [11–13]. Among these methods, the sol–gel method is one of the most commonly used methods for preparation of transparent and conducting oxides owing to its simplicity, safety, non-vacuum system of deposition. Other advantage of the sol–gel method is that it can be adapted easily for production of large-area films. The electrical parameters such as the on/off ratio, threshold voltage, the sub-threshold swing, and field effect mobility are very ⇑ Corresponding author. Tel: +90 222 3350580; fax: +90 222 3204910. E-mail addresses: [email protected], ycaglar@semiconductorslab. com (Y. Caglar). http://dx.doi.org/10.1016/j.jallcom.2014.09.190 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

important in the TFT performance. Therefore, many efforts have been undertaken to improve TFT performance. Most of these studies are related to reduce the crystalline defects and enhance the field-effect mobility. The performance of TFT depends on the properties of active layer, such as crystalline structure, conductivity, and electronic states at grain boundaries and at the semiconductor–insulator interface. So, the thickness of the active layer plays an important role in determining the ZnO TFT device performance. There are some reports on the possible effects of channel thickness on electrical properties ZnO TFT. In many of these reports, RF sputtering process relatively expensive was used. Chung et al. [7] deposited the ZnO layers with the thickness of 30–150 nm on bottom gate patterned Si substrate by RF sputtering at room temperature and the field effect mobility was obtained around 0.15 cm2/ V s. Huang et al. [14] fabricated ZnO TFTs with thickness varied from 20 and 100 nm by rf magnetron sputtering at room temperature and then annealed them at a high temperature of 950 °C. The obtained TFT exhibited the best performance with a field effect mobility of 27.5 cm2/V s, a threshold voltage of 2.4 V and an on/ off ratio of 7 103. Oh et al. [6] used rf magnetron sputtering for TFT fabrication and the highest field-effect mobility achieved was 0.1 cm2/V s. Basu et al. [15] reported that 45 nm and 70 nm ZnO TFTs produced by room-temperature RF magnetron sputtering exhibited good performance and the reported mobilities were 8.36 cm2/V s and 16.4 cm2/V s, respectively. Up to now, in the available literature, there is one report about the influence of channel layer thickness on the performance of ZnO TFT by simple low-cost spin coating method. In this report, Kim et al. [5] investigated the effect of film thickness on properties of Sn-doped ZnO

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Y. Caglar et al. / Journal of Alloys and Compounds 621 (2015) 189–193

(ZTO) thin film transistors (TFTs) and reported a high mobility reaching 17.02 cm2/V s depending on the Sn concentration. In this study, we fabricated the bottom-gate structured ZnO TFTs on p-type Si using sol gel spin coating method and investigated their electrical performances regarding the thickness of the channel layer. 2. Experimental details The p-Si/SiO2 (50 nm) (University wafer, USA) was used as a dielectric layer. The SiO2 surface was ultrasonically cleaned with acetone, ethanol, and deionized water. Then, the ZnO films were deposited by sol–gel spin coating method. The ZnO precursor solution was prepared from zinc acetate dihydrate (Zn(CH3COO)22H2O, ZnAc, 99.5% purity), monoethanolamine (C2H7NO, MEA, 98% purity), and 2-methoxyethanol (C3H8O2, 99.5% purity). The molar ratio of MEA to ZnAc was maintained at 1.0 and the concentration of zinc acetate dihydrate was 0.35 M. The solution was stirred at 40 °C for 2 h to yield a clear and homogeneous solution. The prepared solution was dropped onto p-Si/SiO2 substrate and rotated at 3000 rpm for 30 s using a spin coater. After the deposition, the films were dried at 300 °C for 10 min into a furnace to evaporate the solvent and remove organic residuals. This procedure was repeated three, four and five times. These films were named ZnO-TFT3, ZnO-TFT4 and ZnO-TFT5, respectively. The films were then inserted into a tube furnace and annealed in air at 850 °C for 2 h. Then, Al and Au source/drain and Ag gate electrodes were deposited by thermal evaporation method (Vaksis PVD Handy-MT/101T, Turkey). The source/drain electrodes with 200 nm thicknesses were made through the shadow mask. The schematic cross-section of the ZnO-TFT is shown in Fig. 1. XRD measurements were performed in air with an X-ray powder diffractometer (BRUKER D8 Advance). The diffractometer reflections of all the films were taken at room temperature. A sealed X-ray tube operated at 40 kV and 40 mA with CuKa radiation was used. All measurements were performed in reflection geometry as coupled h–2h scans (30° 6 2h 6 60°) at a divergent slit of 0.5 mm width. Surface morphologies were studied using a ZEISS Ultraplus model field emission scanning

electron microscopy (FESEM). Also, the thicknesses of the films were determined as 65 nm, 85 nm and 140 nm by FESEM. The electrical measurements were performed using a KEITHLEY 4200 SCS/CVU semiconductor characterization system and SIGNATONE Semi-automatic Probe Station.

3. Results and discussion The crystallinity and the preferred orientation of the ZnO films were analyzed by the XRD method. Fig. 2 shows some typical powder XRD patterns of all the films. XRD diffraction peaks belonging to (1 0 0), (0 0 2), and (1 0 1) planes were observed in all the ZnO films. Compared to powder diffraction data of zincite structure (JCPDS card file no: 36-1451, Zincite phase), the XRD patterns of all the samples exhibited the enhanced intensities for the peaks corresponding to (0 0 2) plane, indicating preferential orientation along the c-axis. The average crystallite sizes were calculated by using the well-known Scherrer equation and were found to be 47, 53, and 58 nm for ZnO-TFT3, ZnO-TFT4 and ZnO-TFT5, respectively. The surface morphology of the ZnO films was examined using FESEM. In set in Fig. 2 shows the top view FESEM images of the films. In these figures, a smooth, almost uniform and small-grained microstructure can be clearly seen. Although slightly porous areas are seen in the first two images, the numbers of these porous areas decrease in the ZnO-TFT5 film and grain boundaries can be seen more clearly, resulting denser film. Fig. 3 shows the source-to-drain current (IDS) curves as a function of the source-to-drain voltage (VDS) for different gate voltages (Vg) from 0 V to 10 V with respect to the thickness of the ZnO channel layers. All of the ZnO-TFTs exhibit the typical field effect

ZnO-TFT3

Al drain

Al Drain

electrode

electrode

ZnO SiO2 (50nm) p-type Si Bottom electrode (Ag)

ZnO-TFT4

(b)

ZnO-TFT5

(a) Fig. 1. Photography on the probe station (a) and schematic structure (b) of ZnO-TFT.

191

(100)

(101)

Intensity

(002)


ZnO-TFT3 30

35

40

45

50

55

60

50

55

60

50

55

60

(100)

(101)

Intensity

(002)

2θ (degree)

ZnO-TFT4 30

35

40

45

(101)

(100)

Intensity

(002)

2θ (degree)

ZnO-TFT5 30

35

40

45

2θ (degree) Fig. 2. XRD spectra and SEM images of the ZnO films.

transistor characteristic. Also they show a clear pinch-off and saturation. This means that ZnO channel can be depleted of free electrons. The drain current of the ZnO-TFTs increases with positive gate voltages. Thus it means that the ZnO-TFTs typically works in n-channel operational mode. As seen in Fig. 3, ZnO TFTs show a high off-current and this current increases as the channel thickness increases. This increase in the off-current with increasing the thickness has been reported in previous studies on ZnO [6,7]. The reason of this tendency may be the increase in the conductivity of channel layer depending on the increase in the thickness. That is, increasing the channel layer thickness leads to higher off-current. The leakage current reaches from a minimum of 7.04 109 A to 3.13 104 A. One of the most important parameters of the transistor is the field effect mobility, which indicates how easy the charge carriers can drift under the influence of electric field. Fig. 4 shows the IDS as

Fig. 3. Output characteristics of ZnO-TFTs at various gate voltages.

a function of the Vg at a fixed VDS. The saturation current can be determined by the following relation [9],

Ids ¼

WC i l V gs V th 2 2L

ð1Þ

where Ci is the insulator capacitance per unit area (Ci = 69 nF/cm2 for 50 nm SiO2), Vth is the threshold voltage and l is the field-effect mobility (channel mobility). The l and Vth values of the ZnO-TFTs were calculated by fitting straight lines into the plots of the square root of Ids1/2 versus Vg (Fig. 5), as given in Table 1. As seen in this table, mobility values are significantly improved by increasing

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Fig. 5. Plot of Ids versus Vg for the ZnO-TFTs. Fig. 4. Transfer characteristics of ZnO-TFTs.

active channel layer. The obtained mobility values are better than those previously reported ZnO TFTs which were fabricated by using vacuum systems such as RF Sputtering and ALD, whereas Ion/off ratio values are lower than reported [4,6,7,16]. The values of Vth are comparable to the reported previous works for ZnO TFTs [4,6,7]. The subthreshold swing (SS), defined by the maximum slope in the transfer curve expressed as a log scale for Vg < Vth, is calculated by

SS ¼

dV g dðlog IDS Þ

ð2Þ

The estimated SS values and the on-to-off current (Ion/off) ratio are given in Table 1. SS values are quite high and increase as the thickness increases. Because the effective interface trap state

Table 1 Transistor parameters of the ZnO-TFTs. TFT

W/L

l (cm2/V s)

Vth (V)

Ion/off

SS (V/dec)

ZnO-TFT3 ZnO-TFT4 ZnO-TFT5

20 12.2 41

5.61x103 4.54x101 1.09

12.50 4.91 12.50

88.01 11.44 2.50

13.2 14.3 23.5

density is related to the SS value, the increase in SS value implies that the interface trap state density increases with increasing the thickness [17]. Such a high value for SS of 24.1 V/dec was also reported by Kwon et al. [4]. Ion/off ratio is much lower than the expected value and decreases as the thickness increases. Actually,


the first value for Ion/off ratio may be acceptable but then it should be increased rapidly. Similar results for Ion/off ratio and SS value were reported by Kim et al. [5] for undoped ZnO film. Also the tendency of Ion/off ratio and SS value with increasing thickness are similar to the results reported by Kim et al. [5] for ZTO TFTs of 30 at.% Sn. In the FESEM images, a noticeable increase in grain size cannot be seen, but the grain boundaries becomes more apparent. According to the grain size values obtained from XRD results, the increase in mobility can be attributed to the increase in grain size. The large grains can lead to reduction in electron scattering centers and it may also play an important role in the increasing of mobility. Similar results were reported previously [5,7,15,18]. Thus, the increases of active layer thickness improve the electrical characteristics of ZnO-TFT device. 4. Conclusions ZnO TFTs were fabricated on p-Si/SiO2 substrate regarding the thickness of the channel layer. Sol gel spin coating method was used in the process of fabrication of ZnO active layers. From the XRD results a slight increase in the crystallite size obtained using Scherrer equation was observed. Although slightly porous areas were shown on the surface in the SEM images, the numbers of these porous areas decreased and grain boundaries appeared more clearly depending on the increase in thickness. Within the scope of electrical characterization of the TFTs some important parameters such as l, Vth, SS and Ion/off were investigated. The mobility values of TFTs were significantly improved by increasing the thickness of active channel layer and the highest mobility of ZnO TFTs was obtained by 1.09 cm2/V s for 140 nm layer. The obtained mobility values are better than those previously reported ZnO TFTs which were fabricated by using vacuum systems such as RF Sputtering and ALD, whereas Ion/off ratio values are lower than reported. These

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results suggest that TFTs produced with such a simple method provide higher performance than that of systems requiring a high vacuum. Acknowledgment This work was supported by Anadolu University Commission of Scientific Research Projects under Grant No. 1101F009. References [1] S. Masuda, K. Kitamura, Y. Okumura, S. Miyatake, H. Tabata, T. Kawai, J. Appl. Phys. 93 (2003) 1624. [2] Ü. Ozgür, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog˘an, V. Avrutin, S.-J. Cho, H. Morkoç, J. Appl. Phys. 98 (2005) 041301. [3] C.-Y. Tsay, H.-C. Cheng, Y.-T. Tung, W.-H. Tuan, C.-K. Lin, Thin Solid Films 517 (2008) 1032. [4] S. Kwon, S. Bang, S. Lee, S. Jeon, W. Jeong, H. Kim, S.C. Gong, H.J. Chang, H.-H. Park, H. Jeon, Semicond. Sci. Technol. 24 (2009) 035015 (6pp). [5] C.G. Kim, N.-H. Lee, Y.-K. Kwon, B. Kang, Thin Solid Films 544 (2013) 129–133. [6] B.-Y. Oh, M.-C. Jeong, M.-H. Ham, J.-M. Myoung, Semicond. Sci. Technol. 22 (2007) 608–612. [7] J.H. Chung, J.Y. Lee, H.S. Kim, N.W. Jang, J.H. Kim, Thin Solid Films 516 (2008) 5597–5601. [8] H.-C. Cheng, C.-F. Chen, C.-C. Lee, Thin Solid Films 498 (2006) 142–145. [9] M. Ortel, Y.S. Trostyanskaya, V. Wagner, Solid-State Electron. 86 (2013) 22–26. [10] M. Gupta, F.R. Chowdhury, D. Barlage, Y.Y. Tsui, Appl. Phys. A 110 (2013) 793– 798. [11] H.-C. You, Y.-H. Lin, Int. J. Electrochem. Sci. 7 (2012) 9085–9094. [12] C.-Y. Tsay, K.-S. Fan, S.-H. Chen, C.-H. Tsai, J. Alloy. Comp. 495 (2010) 126–130. [13] F. Yakuphanoglu, S. Mansouri, Microelectron. Reliab. 51 (2011) 2200–2204. [14] H.-Q. Huang, F.-J. Liu, J. Sun, J.-W. Zhao, Z.-F. Hu, Z.-J. Li, X.-Q. Zhang, J. Phys. Chem. Solids 72 (2011) 1393–1396. [15] S. Basu, P.K. Singh, C. Ghanshyam, P. Kapur, Y.-H. Wang, J. Electron. Mater. 41 (9) (2012) 2362–2368. [16] Y. Kawamura, M. Horita, Y. Ishikawa, Y. Uraoka, J. Display Technol. 9 (9) (2013) 694–698. [17] W.-Y. Chen, J.-S. Jeng, J.-S. Chen, J. Appl. Phys. 114 (2013) 103706. [18] M.G. Yun, S.H. Kim, C.H. Ahn, S.W. Cho, H.K. Cho, J. Phys. D: Appl. Phys. 46 (2013) 475106 (5pp).