n-Si metal-insulator-semiconductor (MIS) capacitor

Journal Pre-proof Impedance spectroscopy of Au/TiO2/n-Si metal-insulator-semiconductor (MIS) capacitor

A.Büyükbaş Uluşan, A. Tataroğlu PII:

S0921-4526(19)30825-7

DOI:

https://doi.org/10.1016/j.physb.2019.411945

Reference:

PHYSB 411945

To appear in:

Physica B: Physics of Condensed Matter

Received Date:

30 September 2019

Accepted Date:

09 December 2019

Please cite this article as: A.Büyükbaş Uluşan, A. Tataroğlu, Impedance spectroscopy of Au/TiO2/nSi metal-insulator-semiconductor (MIS) capacitor, Physica B: Physics of Condensed Matter (2019), https://doi.org/10.1016/j.physb.2019.411945

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Journal Pre-proof Impedance spectroscopy of Au/TiO2/n-Si metal-insulator-semiconductor (MIS) capacitor A. Büyükbaş Uluşan*, A. Tataroğlu Department of Physics, Faculty of Science, Gazi University, Ankara, Turkey Corresponding author: [email protected] (A. Buyukbas Ulusan) Abstract In this study, the electrical properties of Au/TiO2/n-Si metal-insulator-semiconductor (MIS) capacitor were investigated by impedance spectroscopy (IS) technique. Impedance measurements were performed in the frequency range of 10 Hz-1 MHz for various bias voltages. The Cole-Cole plots show a single dielectric relaxation. The equivalent circuit was estimated from the shape of the Cole-Cole plots. The equivalent circuit of the MIS capacitor consists of a parallel resistor (Rp) and capacitor (Cp) in series with a resistor (Rs). It is observed that the Cole-Cole plots indicate a semicircle. The parameters of the equivalent circuit were determined by fitting the impedance measurement data. While the Rp value decreases with increasing the bias voltage, the Cp and Rs value are almost independent of the bias voltage. From the variation of log(Rp) with log(V), the dominant conduction mechanism of the MIS capacitor was determined as space-charge limited current (SCLC) mechanism. Keywords: MIS-Capacitor device; Titanium oxide; Impedance spectroscopy; Cole-Cole plot

1. Introduction The impedance spectroscopy (IS) is used to analyze electrical characteristics of solid materials such as oxides, ceramics, polymers and glasses. The IS is an effective method to study the frequency dependence of impedance of materials [1-3]. Also, impedance spectroscopy measures impedance and to evaluate the dielectric properties of a material. The dc electrical response and frequency dependent behavior of materials can be described with real and imaginary parts of the complex impedance. Impedance data is analyzed by equivalent circuit model. The simplest equivalent circuit is a parallel resistor-capacitor circuit (RC element). Moreover, the equivalent circuit for the MIS device can be designed as a parallel resistor Rp and capacitor Cp network connected with a series resistance Rs [1-7]. Titanium oxide (TiO2) being a transition metal oxide has attracted much attention because of its optical and electrical properties such as high refractive index, resistivity and dielectric constant [8-12]. However, the energy band gap TiO2 is not sufficient for reducing leakage current. Depending on the crystalline phase and purity, the bandgap of TiO2 changes between 3.0 and 3.5 eV. TiO2 is used in many electronic and optoelectronic device applications such as transistors, capacitors, solar cells, and sensors. It is also used for optical and anti-reflection coatings. TiO2 films can be formed by different techniques such as chemical vapor deposition 1

Journal Pre-proof [13], sol-gel [14], evaporation [15], and DC/RF magnetron sputtering [16,17]. Among them, the magnetron sputtering technique is one of the most promising techniques. TiO2 contains three different crystalline phases called as anatase, rutile and brookite. Anatase and rutile are the most common phases, and they can synthesize as thin film. Brookite is rarely utilized, and is a minority product of most synthesis. Thermally, rutile phase is the most stable, but anatase and brookite phase are metastable. Also, anatase and brookite phase can transform to the rutile phase upon heating. The anatase form occurs at temperatures below 350 °C. At temperatures above 600 °C, the anatase experience a phase transition and convert into the rutile phase[1821]. The aim of the present work is to analyze the results of the electrical impedance measurements. The frequency-dependent impedance measurements of the MIS capacitor modeled with the equivalent circuit was studied in the range of 10 Hz to 1 MHz. From the equivalent circuit analysis, electrical parameters and conduction mechanism of the MIS capacitor were determined. 2. Experimental details The Au/TiO2/n-Si (MIS) capacitor was fabricated on n-type Si wafer. In our previous study [22], more information on fabrication processes was given. Radio frequency (RF) magnetron sputtering was applied to deposit TiO2 thin film on Si substrate. The impedance measurements of the MIS capacitor were carried out by using a Solartron SI1260 Impedance/Gain-Phase analyzer and Solartron 1296 Dielectric Interface. Impedance measurements were performed in the frequency range of 10 Hz-1 MHz for various bias voltages at room temperature. 3. Results and discussion Impedance spectroscopy is a useful technique for probing the electrical and dielectric properties of semiconductor devices. Also, it is used to measure the impedance of a device in dependence of the frequency of alternating current (ac) and to determine the transport mechanism. The equivalent circuits of the semiconductor devices can be built with the help of impedance spectroscopy. Therefore, the MIS capacitor was modeled by an equivalent circuit designed as a single parallel resistor Rp and capacitor Cp network with series resistor Rs [2327]. Equivalent circuit defining the impedance spectroscopy is shown in Fig. 1.

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Fig. 1. Equivalent circuit consisting a series resistance and a pair of parallel resistance and parallel capacitance. In proposed equivalent circuit model, the Rp, Cp and Rs represent the bulk resistance, bulk capacitance and contact resistance, respectively. The parameters of the equivalent circuit is calculated by fitting the measured impedance data. The impedance (Z) of a semiconductor device represented by parallel RC circuit is given as follows, 1

1

1

(1)

𝑍 = 𝑌 = 𝐺 + 𝑖𝜔𝐶 = (1 𝑅) + 𝑖𝜔𝐶

where Y, G, C, R and ω(=2πf) are the admittance, conductance, capacitance, resistance, and angular frequency of the ac excitation, respectively. The complex impedance (Z*) of the equivalent circuit given in Fig. 1 is expressed as follows [23-29], 𝑅𝑝

𝑍 ∗ = 𝑅𝑠 + 1 + 𝑖𝜔𝑅𝑝𝐶𝑝 = 𝑍΄ +𝑖𝑍΄΄ = 𝑅𝑒𝑍 + 𝑖𝐼𝑚𝑍

(2)

where Z' and Z" are the real and imaginary part of complex impedance, respectively. The Z' and Z" are given in the following equation,

[

𝑅𝑒𝑍 = 𝑍΄ = 𝑅𝑠 + ΄΄

𝐼𝑚𝑍 = 𝑍 =

[

𝑅𝑝

]

(3)

1 + (𝜔𝑅𝑝𝐶𝑝)2

𝜔𝑅2𝑝𝐶𝑝

]

(4)

1 + (𝜔𝑅𝑝𝐶𝑝)2

The plot of real part versus imaginary part of the complex impedance is known as Cole-Cole plot and its shape is a semicircle. The semicircle corresponds to a certain relaxation process and a single relaxation time. The semicircle is expressed by the following equation, 2

𝑅𝑝 2

[𝑅𝑒𝑍 ― (𝑅 + )] + ( ―𝐼𝑚𝑍) = ( ) 𝑅𝑝

𝑠

2

2

(5)

2

This equation defines a circle centered at (Rs+Rp/2,0) with radius Rp/2.

3

Journal Pre-proof The impedance measurements were carried out in the frequency range from 10 Hz to 1 MHz at various dc bias voltages. The variation of Z' and Z" with frequency at various bias voltages is shown in Fig. 2 (a) and (b). As seen in Fig. 2(a), the Z' value for each dc voltage remains constant up to a certain frequency, and then decreases with increase in frequency. The behavior of Z′ at low frequencies is due to the fact that the space charge polarization is dominant. At higher frequencies, Z′ value is to become independent of frequency. Also, from Fig. 2(a), it follows rather that the value of Z' decreases with increasing bias voltage up to about 10 kHz, then it increases up to about 100 kHz and after then it tends to be very small constant value for all bias voltages. Moreover, it is clear that the Z′ plots exhibit a single relaxation process.

Fig. 2. (a) Z'-Log f and (b) Z"-Log f plots at various bias voltages. 4

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As seen in Fig. 2(b), the Z" value for each dc voltage increases up to a certain frequency with the increase in frequency and then reaches a maximum peak. After this peak, its value decreases. The behavior of Z" with frequency indicates the single relaxation process in the device. The peak position shifts towards the higher frequency with increasing applied dc bias voltage. This shift results from the decrease in the bulk resistance with dc bias voltage. Thus, the relaxation time (τ=1/ωmax=1/2πfmax) of the MIS capacitor was determined from the position of the maximum peak. The obtained values of relaxation time and fmax for each applied bias voltage are given in Table 1. The value of τ decreases with increasing bias voltage. This result is attributed to the injection of charge carriers into the capacitor. Table 1. The obtained values of fmax and τ for each applied bias voltage. Voltage (V) 1 1.5 2 2.5 3 3.5 4

fmax (kHz) 1.92 2.93 4.47 5.84 7.94 10.40 12.10

τ (Sec) 82.90 54.32 35.61 27.25 20.05 15.30 13.15

To determine impedance characteristics, the real part (Z') versus imaginary part (Z") of the complex impedance (Z*) is plotted. This plot known as Cole-Cole plot or Nyquist plot should be a semicircle indicating Debye type relaxation [28-32]. The impedance data are presented in the Cole-Cole plots. Fig. 3 shows the Cole-Cole plots of the capacitor at various dc bias voltages. These plots indicate a single semicircle decreasing in size with increasing bias voltage.

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Fig. 3. Cole-Cole plots. At high frequencies, the minimum Z' value represents the existence of a series resistance Rs to the capacitor Cp. However, at low frequencies, the maximum Z' value corresponds to the summation of the series resistance Rs and the parallel resistance Rp to the capacitor Cp. The series resistance (Rs), bulk resistance (Rp) and bulk capacitance (Cp) values obtained by fitting the single semicircles for each bias voltage are shown in Table 2. The value of Rp decreases rapidly with increase in voltage. However, it is found that the value of Cp and Rs almost don’t change by bias voltage. This behavior of Cp with voltage is exactly the result expected for the space charge limited current (SCLC) mechanism. The decrease in the Rp with applied bias, the shift in the peak given in Fig. 2(b) causes. In addition, this decrease is attributed to the increase in the number of injected carrier in to the capacitor [6,25,30-37]. Table 2. The Rs, Rp and Cp values obtained from the fit of the experimental data for each dc bias voltage. Voltage (V) 1 1.5 2 2.5 3 3.5 4

Rs (Ω) 25.77 22.59 21.19 19.92 18.88 17.91 16.95

Rp (Ω) 28649 19896 14773 11632 9440 7967 6684 6

Cp (nF) 2.98 2.82 2.59 2.36 2.18 2.07 2.01

Journal Pre-proof Space charge limited current (SCLC) suggests that current density (J) is given by JVm+1. Besides, from SCLC, the parallel resistance (Rp) is given by RpV-m [27-32]. For SCLC with exponential trap distribution and for trap free SCLC are m>1 and m=1, respectively. The exponent m value is obtained from the slope of log(RP) versus log(V). Fig. 4 demonstrates the plot of log(RP) vs. log(V). The value of m was found to be 1.05 for the capacitor. It is clear that the Rp decreases linearly. This linear behavior indicates the conduction in the capacitor follows SCLC conduction with an exponential trap distribution.

Fig. 4. The log(RP) versus log(V) plot. 4. Conclusion The impedance measurements of the Au/TiO2/n-Si (MIS) capacitor were performed in the frequency range of 10 Hz-1 MHz for various dc bias voltages. The prepared MIS capacitor was modeled by a single parallel RpCp circuit with a resistance Rs. The relaxation times of the capacitor were determined from the position of the maximum peak of Z"-Log (f) plots. The real part (Z') versus imaginary part (Z") of the complex impedance was plotted. This plot known as Cole-Cole plot shown a semicircle. The parameters such as Rp, Cp and Rs of the equivalent circuit is calculated by fitting the measured impedance data. The Cp and Rs value don’t almost change with the applied voltage. However, the Rp value decreases with increasing the dc voltage. Also, the conduction mechanism of the capacitor was investigated by using impedance spectroscopy. The conduction mechanism was determined from the variation of log(Rp) with log(V) and was found to be the SCLC conduction. The obtain results show that the MIS capacitor can be modeled by the proposed equivalent circuit. References [1] E. Barsoukov, J.R. Macdonald, Impedance Spectroscopy Theory, Experiment, and Applications, 2nd Ed., Wiley, New Jersey, 2005. [2] V.F. Lvovich, Impedance Spectroscopy: Applications to Electrochemical and Dielectric Phenomena, Wiley, New Jersey, 2012. [3] R.A. Gerhardt, “Impedance Spectroscopy and Mobility Spectra,” Chapter in Encyclopedia of 7

Journal Pre-proof Condensed Matter Physics, Elsevier Press, 350-363, 2005. [4] A. Büyükbaş, A. Tataroğlu, M. Balbaşı, J. Nanoelectron. Optoelectron. 9 (2014) 515-519. [5] K.F. Albertin, M.A. Valle, I. Pereyra, J. Integrated Circuits Systems 2 (2007) 89-93. [6] G. Chauhan, R. Srivastava, P. Tyagi, A. Kumar, P.C. Srivastava, M.N. Kamalasanan, Synthetic Metals 160 (2010) 1422-1426. [7] R. Tripathi, A. Kumar, C. Bharti, T.P. Sinha, Curr. Appl. Phys. 676 (2010) 676-681. [8] B.R. Weinberger, R.B. Garber, Appl. Phys. Lett. 66 (1995) 2409-2411. [9] Y. Ju, M. Wang, Y. Wang, S. Wang, C. Fu, Adv. Condens. Matter Pyhs. 2013 (2013) 365475. [10] H. Tanrıkulu, A. Tataroğlu, E.E. Tanrıkulu, A. Büyükbaş Uluşan, Indian J. Pure Appl. Phys, 56 (2018)142-148. [11] A. Djeghlouf, D. Hamri, A. Teffahi, A. Saidane, F.S. Al Mashary, M. Al Huwayz, M. Henini, I. Orak, A.Y. Alyamani, J. Alloys Compd. 775 (2019) 202-213. [12] S.-Chang Shei, Adv. Mater. Sci. Eng. 2013 (2013) 545076. [13] T. Maruyama, S. Arai, Sol. Energy Mater. Sol. Cells 26 (1992) 323-329. [14] M. Kumar, M. Kumar, D. Kumar, Microelectron. Eng. 87 (2010) 447-450. [15] P. Lobl, M. Huppertz, D. Mergel, Thin Solid Films 251 (1994) 72-79. [16] A. Bengi, U. Aydemir, S. Altındal, Y. Ozen, S. Ozcelik, J. Alloys Compd. 505 (2010) 628-633. [17] D. Yoo, I. Kim, S. Kim, C.H. Hahn, C. Lee, S. Cho, Appl. Surf. Sci. 253 (2007) 3888-3892. [18] H. Long, A. Chen, G. Yang, Y. Li, P. Lu, Thin Solid Films 517 (2009) 5601-5604. [19] S.-H. Song, X. Wang, P. Xiao, Mater. Sci. Eng. B 94 (2002) 40-47. [20] D.A.H. Hanaor, C.C. Sorrell, J. Mater. Sci. 46 (2011) 855-874. [21] M. Ishfaq, M.R. Khan, A. Ali, S. Bhardwaj, C. Cepek, A.S. Bhatti, Mater. Sci. Semicond. Process. 63 (2017) 107-114. [22] A. Buyukbas-Ulusan, A. Tataroglu, Silicon 10 (2018) 2071-2077. [23] L. Bi-Xin, C. Jiang-Shan, Z. Yong-Biao, M. Dong-Ge, Chin. Phys. Lett. 28 (2011) 057201. [24] Y. Liu, A. Liu, Z. Hu, W. Liu, F. Qiao, J. Phys. Chem. Solids 73 (2012) 626-629. [25] S.H. Kim, S.C. Lim, J.H. Lee, T. Zyung, Curr. Appl. Phys. 5 (2005) 35-37. [26] V.V. Brus, Semicond. 46 (2012) 1012-1015. [27] A. Buyukbas‑Uluşan, S. Altındal Yerişkin, A. Tataroğlu, M. Balbaşı, Y. Azizian Kalandaragh, J. Mater. Sci. Mater. Electron. 29 (2018) 8234-8243. [28] S. Jeon, S. Park, Microelectron. Eng. 88 (2011) 872-876. [29] S. Maity, D. Bhattacharya, S.K. Ray, J. Phys. D: Appl. Phys. 44 (2011) 095403. [30] M. Bouzitoun, C. Dridi, R.B. Chaabane, H.B. Ouada, H. Gam, M. Majdoub, Sci. Techn. Adv. Mater. 7 (2006) 772-779 [31] H. Trad, A. Rouis J. Davenas, M. Majdoub, Eur. Phys. J. Appl. Phys. 68 (2014) 10103. [32] H. Hrichi, M. Benzarti-Ghedira, N. Jaballah, R.B. Chaabane, M. Majdoub, H.B. Ouada, Sensors Transducers 27 (2014) 209-216. [33] S.K. Barik, R.N.P. Choudhary, A.K. Singh, Adv. Mat. Lett. 2 (2011) 419-424. [34] C.-Chia Chen, B.-Chao Huang, M.-Shiang Lin, Y.-Jui Lu, T.-Yi Cho, C.-Hao Chang, K.-Cheng Tien, S.-Hao Liu, T.-Hui Ke, C.-Chih Wu, Organic Electronics 11 (2010) 1901-1908. [35] M. Braik, C. Dridi, M. Ben Al, A. Ali, M.N. Abbas, A. Errachid, Synthetic Metals 209 (2015) 135-142. [36] B. Park, O. Eun Kwon, S. Hong Yun, H. Goo Jeon, Y. Ho Huh, J. Mater. Chem. C 2 (2014) 8614-8621. [37] C.K. Suman, J. Yun, S. Kim, S.-Doo Lee, C. Lee, Curr. Appl. Phys. 9 (2009) 978-984.

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Author’s name A. Büyükbaş Uluşan A. Tataroğlu

Affiliation Gazi University Gazi University

Journal Pre-proof Author Contributions Dr. A. Buyukbas-Ulusan conducted the experimental measurements and data analysis parts of the manuscript. The theoretical analysis and manuscript preparation were handled together by both Dr. A. Buyukbas-Ulusan and Prof. Dr. A. Tataroglu.