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semiconductor contact with organic interlayer
Thin Solid Films 597 (2015) 14–18
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Electrical and photoelectrical properties of Ag/n-type Si metal/ semiconductor contact with organic interlayer Enise Ozerden a, Yusuf Selim Ocak b,c, Ahmet Tombak a, Tahsin Kilicoglu d,⁎, Abdulmecit Turut e a
Department of Physics, Faculty of Art & Science, Batman University, 72100 Batman, Turkey Department of Science, Faculty of Education, Dicle University, 21280 Diyarbakir, Turkey Science and Technology Application and Research Center, Dicle University, 21280 Diyarbakir, Turkey d Department of Physics, Faculty of Science, Dicle University, 21280 Diyarbakir, Turkey e Department of Physics Engineering, Faculty of Science, Istanbul Medeniyet University, 34730 Istanbul, Turkey b c
a r t i c l e
i n f o
Article history: Received 27 March 2015 Received in revised form 14 October 2015 Accepted 7 November 2015 Available online 11 November 2015 Keywords: Organic interlayer Metal/organic layer/inorganic semiconductor structures Schottky barrier height Photoelectrical properties
a b s t r a c t Electrical and photoelectrical features of Metal/Organic Interlayer/Inorganic Semiconductor (MIS) Schottky device were investigated by using current–voltage (I–V) and capacitance–voltage (C–V) measurements at room temperature. For this purpose, 9,10-dihydrobenzo[a]pyrene-7(8H)-one (9,10-H2BaP) thin film was used as organic interlayer between Ag metal and n-Si semiconductor. Firstly, optical properties of the organic thin film were determined from optical absorption spectrum, and its optical band gap was found to be 3.73 eV. Then, the electrical parameters of the Ag/9,10-H2BaP/n-Si diode such as ideality factor (n), barrier height (Φb(I–V)), diffusion potential (Vd), barrier height (Φb(C–V)) and carrier concentration (Nd) were calculated from I–V and C–V characteristics at room temperature. The Φb values obtained from both measurements were compared with each other. Besides, the effect of light on I–V measurements of the structure was examined at illumination intensities ranging from 40 to 100 mW/cm2 with 20 mW/cm2 intervals using a solar simulator with AM1.5 filter. Light sensitivity, open circuit voltage (VOC) and short circuit current (ISC) parameters of Ag/9,10-H2BaP/n-Si structure were calculated from under light measurements. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Schottky contacts have taken a significant role in modern semiconductor device technology [1,2]. They have found a large number of electronic and optoelectronic implementations including field-effect transistors (FETs), solar cells and photodetectors [1,3,4]. The principal parameters of the Schottky barrier diodes (SBDs) are barrier height (BH) and ideality factor [5,6]. These fundamental parameters of the SBDs are strongly influenced by the existence of an interfacial layer between a metal and a semiconductor [1,7]. The interfacial layer may possess a strong effect on electrical and photoelectrical characteristics of the diode [1,8,9]. Organic semiconductors used as the interfacial layer in between the inorganic semiconductor and metal can change the characteristics of MS contacts [10–12]. The new electrical features of the structures could be promoted with the choice of appropriate organic semiconductor [13]. Due to their good stability and barrier height enhancement properties of organic semiconductor materials, they have been widely used in electronics and optoelectronics such as fabrication of the SBDs, organic light emitting diodes (OLEDs), field effect
⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (T. Kilicoglu).
http://dx.doi.org/10.1016/j.tsf.2015.11.013 0040-6090/© 2015 Elsevier B.V. All rights reserved.
transistors (FETs), photovoltaic devices (PVDs), solar cells and sensors [11,14–20]. 9,10-Dihydrobenzo[a]pyrene-7(8H)-one organic compound named 9,10-H2BaP was used to modify Ag/n-Si metal/semiconductor (MS) contact. This compound is a derivative of benzo[a]pyrene which is a five ring polycyclic aromatic hydrocarbon (PAH). Benzo[a]pyrene (BaP) is known to be mutagenic and highly effective carcinogenic. The molecular structure of the 9,10-H2BaP organic compound with the empirical formula C20H14O is shown in Fig. 1. Also, the 9,10-H2BaP compound is fallen within as organic structure such as OFET and OPV materials, dyes, ketones and carbonyl compounds. In this paper, our aim is to obtain an Ag/9,10-H2BaP/n-Si/Au-Sb structure, to determine the electrical and photoelectrical properties of obtained structure by using current–voltage (I–V) and capacitance–voltage (C–V) measurements at the room temperature and to investigate whether or not the Ag/9,10H2BaP/n-Si/Au–Sb structure shows a rectifying contact behavior. 2. Experimental procedure The sample was prepared using a polished n-type Si wafer received as the fabrication with (100) orientation and having 1–10 Ω·cm resistivity at the room temperature. The 9,10-dihydrobenzo[a]pyrene7(8H)-one organic compound was provided as ready by means of the Sigma-Aldrich. Firstly, the n-type Si wafer was chemically cleaned for
E. Ozerden et al. / Thin Solid Films 597 (2015) 14–18
15
at various frequencies. Optical absorption spectrum of the 9,10-H2BaP thin film on a quartz substrate was also obtained with Perkin Elmer Lambda 25 UV/Vis spectrophotometer in the range of 200 to 1100 nm wavelength. 3. Results and discussion
Fig. 1. The molecular structure of the 9,10-H2BaP molecule (molecular formula: C20H14O).
5 min in boiling trichloroethylene and cleaned by ultrasonic vibration in acetone and methanol to get rid of organic contaminations. The native oxide layer on the wafer surface was removed in a HF:H2O (1:10) solution for 30 s. Preceding each step, the n-Si wafer was washed with 18 MΩ deionized water and dried under N2 atmosphere. Then, Au–Sb (99%–1%) alloy was thermally evaporated on the unpolished side of nSi substrate. The obtained structure was annealed at 420 °C for 3 min in N2 atmosphere to make low resistivity ohmic contacts. The native oxide layer on the front of the substrate was removed in HF:H2O (1:10) solution for 30 s and was rinsed in deionized water and was dried under N2 atmosphere before forming organic layer on n-Si substrate. The 9,10-H2BaP organic layer is directly formed by adding a 9,10-H2BaP solution of 1 × 10− 3 mol/L in chloroform and methanol mixture on the front surface of the n-Si substrate and then waiting for the solvent to evaporate at the room temperature. The cross section image of 9,10-H2BaP/n-Si structure was obtained by FEI Quanta 250 FEG scanning electron microscopy (SEM). It was seen that the 9,10H2BaP thin film is very smooth and the thickness of the film is about 180 nm. The cross section of the 9,10-H2BaP thin film was given in Fig. 2. After the formation of the 9,10-H2BaP organic thin film, Schottky contact of structure was coated by evaporating with Ag dots with a diameter of about 1.5 mm. The evaporation process in a vacuum system of the structure was performed at 2 × 10−6 Torr. After all this process Ag/9,10-H2BaP/n-Si/Au–Sb diode structure was obtained. The I–V measurements of the device were carried out with a Keithley 2400 Sourcemeter in the dark and under a Newport Oriel 9600 solar simulator with AM1.5 filter at illumination intensities ranging from 40 to 100 mW/cm2 at the room temperature. The C–V characteristics of the structure were measured at the room temperature by using an Agilent Hewlett Packard (HP) 4294A Impedance Analyzer (40 Hz–110 MHz)
Fig. 2. The SEM image magnified ×120,000 times of the cross-section of the 9,10-H2BaP organic thin film formed on the n-Si inorganic semiconductor.
The optical absorption spectrum of the 9,10-H2BaP organic thin film on the quartz is shown in Fig. 3. The optical properties of the 9,10H2BaP organic thin film were analyzed by the following relationship [21,22] and the optical energy band gap (Eg) of the film was calculated from the relation: m αhν ¼ B hν−Eg
ð1Þ
where α is the absorption coefficient of the film, B is an energyindependent constant, h is the Planck's constant (h = 6.626 × 10−34 J·s), ν is the frequency of the radiation and m is a constant which determines the type of optical transitions and depends on the nature of the transition. The exponent m gets the values of 1/2, 2, 3/2 or 3 for direct allowed, indirect allowed, direct forbidden or indirect forbidden transitions, respectively. Fig. 4 indicates (Ahν)1/2 versus hν and (Ahν)2 versus hν (inset) plots of the 9,10-H2BaP organic thin film on the quartz according to Eq. (1). The non-direct and direct band gaps of the 9,10H2BaP organic compound were obtained as 3.73 and 5.37 eV, respectively, by extrapolating the linear portion of these graphs to intercept the photon energy axis which indicates that the non-direct allowed transition dominates in the 9,10-H2BaP organic thin film. We suggest that the 9,10-H2BaP organic thin film possesses 3.73 eV indirect optical band gap. Furthermore, the trap levels of the 9,10-H2BaP organic thin film were obtained as 1.55 and 1.35 eV from intercept with axis by extrapolating of the linear portions in the energy range of 1.13–6.20 eV of photon energy axis of the (Ahν)1/2–hν graph. The experimental semilogarithmic I–V graph obtained using I–V measurements of the Ag/9,10-H2BaP/n-Si organic–inorganic structure at ambient temperature in dark is displayed in Fig. 5. As can be also seen from the figure, the structure shows perfect rectifying behavior. The characteristic properties of the rectifying contact behavior explain as the weak voltage dependence of the reverse bias current and the
Fig. 3. The plot of absorption of the 9,10-H2BaP organic thin film on the quartz.
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E. Ozerden et al. / Thin Solid Films 597 (2015) 14–18
where I0 is the saturation current, e is the electronic charge, T is the absolute temperature, k is the Boltzmann's constant (k = 8.625 × 10−5 eV/K), V is the applied voltage and n is the dimensionless ideality factor which indicates how closely the ideal diode behavior and this value is 1 for the ideal diode. The n value of the diode is determined from the slope of the forward bias ln I–V plot of the diode by using Eq. [5]: n¼
e dV : kT dð ln IÞ
ð3Þ
The value of the n for Ag/9,10-H2BaP/n-Si diode has been calculated from Fig. 5 as 1.30 at room temperature. The n value of the structure is expected to be unity for the ideal one. The value of 1.30 for the ideality factor at room temperature means the deviation from the ideal diode property. High values of the n can be attributed to the effects of the series resistance of device, the existence of organic compound layer between Ag metal and n-Si semiconductor [24], the presence of the secondary mechanisms at the interface and fabrication-induced defects [25,26], a wide distribution of the low patches (or barrier inhomogeneities) and the bias voltage dependence of the SBH [5]. In addition, the saturation current in Eq. (2) can be written as:
1/2
Fig. 4. (Ahυ) the quartz.
2
−hν and (Ahυ) −hν (inset) plots of the 9,10-H2BaP organic thin film on
exponential increase in the forward bias current. Due to series resistance (Rs) of bulk resistance of the 9,10-H2BaP organic interlayer and Si inorganic semiconductor, a curvature downward region occurs at high current in the forward bias I–V plot [13]. Besides, the Rs which was given rise to the downward curvature at enough high bias region as it is seen from the Fig. 5 is more effective if compared with the interface states (Nss). It is well known that the Rs is influential only at high forward bias region while the Nss is important in the low and intermediate bias regions [23]. The thermionic emission (TE) theory can be used in the determination of parameters of the diode due to rectifying behavior. According to the theory, the net TE current expression is given as [5]: eV −1 I ¼ I 0 exp nkT
ð2Þ
Fig. 5. The forward and reverse bias current–voltage characteristic of the Ag/9,10-H2BaP/ n-Si MIS diode at room temperature in dark.
eΦ I0 ¼ SA T 2 exp − b kT
ð4Þ
where A∗ is the effective Richardson constant equals to 110 A/K2 cm2 for n-Si [27], S the effective surface area of the diode (S = 1.767 × 10−2 cm2) and Φb the barrier height of the diode and the Φb was calculated by using I0 value determined from the intercept of ln I–V plot on I axis by using the equation is given as follows: Φb ¼
kT SA T 2 ln I0 e
! ð5Þ
The barrier height value of the diode has been calculated as 0.67 eV by using Eq. (5). The obtained 0.67 eV value of Φb for Ag/9,10-H2BaP/nSi structure due to 9,10-H2BAP organic thin layer is clearly lower than that achieved with traditional Ag/n-Si MS contacts [28]. This difference can be ascribed to the presence of the 9,10-H2BaP organic interface layer at the MS interface modifying the effective BH by influencing the space charge region of the inorganic silicon substrate [3,29–32]. Thereby, it is known that the organic interlayer modifies the barrier height in the device and prevents direct contact of the metal from the Si surface [3, 29–32]. The 9,10-H2BaP organic interlayer seems to cause an important modification of the interface states even though the organic–inorganic interface becomes abrupt and unreactive [29–34]. The capacitance–voltage (C–V) characteristics can also significantly inform about the BH and the interface state charge concentration for the Schottky contacts. The junction capacitance dominates for the reversedbiased diodes, while the diffusion capacitance dominates in strongly forward-biased diodes [35]. The C–V measurements for Ag/9,10-H2BaP/ n-Si diode at different frequencies (50 kHz-5 MHz) are presented in Fig. 6. As shown in the figure, the capacitance of the structure decreases with increasing frequency. Therefore, this situation can be explained that the interface states have strong effects on electrical properties and the total capacitance of the Ag/9,10-H2BaP/n-Si diode. Thus, the capacitance of the structure strongly depends on applied bias voltage and frequency due to the presence of the interface states [7,36,37]. Besides, the increasing capacitance of the structure at low frequencies depends on the ability of the interface states to follow the applied ac signal while they cannot follow the ac signal at sufficiently high frequencies (f ≥ 1 MHz) because the carrier life time (τ) is more large according to the measured period at high frequencies [23,38]. It has been clearly shown in Fig. 6. For this reason, the capacitance of diode has a tendency to stabilize at higher frequencies. Also, the C−2–V plot can give information about the characteristic parameters of the structure such as BH,
E. Ozerden et al. / Thin Solid Films 597 (2015) 14–18
17
Fig. 6. The capacitance–voltage plots of the Ag/9,10-H2BaP/n-Si MIS diode at various frequencies.
Fig. 8. In I–V graphs for Ag/9,10-H2BaP/n-Si MIS device under various light intensities of 40, 60, 80 and 100 mW/cm2 and in dark.
diffusion potential and carrier concentration. In order to determine these parameters of the diode, the C−2–V graph is plotted in Fig. 7 at 500 kHz and at room temperature. The depletion region capacitance of the metal/n-Si diodes is written as follows [5,39]:
where Nc is the density of states in the conduction band (Nc = 2.8 × 1019 cm−3). The Φb(C–V) value of the device was calculated by using Vd0 and Nd values which can be obtained from C−2–V plot and was found to be 0.78 eV. This value which can be obtained from C–V data is higher than that obtained from I–V data. This difference between the obtained Φb(I–V) and Φb(C–V) values for Ag/9,10-H2BaP/n-Si diode can be explained by the different nature of the I–V and C–V measurements. It is explained that the capacitance of the diode at interface is insensitive to potential fluctuations and that the C–V method for defining of the Φb averages over the whole area and measurements [13]. The current I across the interface exponentially depends on the barrier height Φb and is extremely sensitive to the potential fluctuations at the interface which consist of low and high barrier areas [13,40,41]. The discrepancy between the barrier height values of the structure can be also attributed to the presence of the interface layer which exists between the metal and the semiconductor, the trap states in the semiconductor, barrier height inhomogeneity which is presence at M/S interface and spatial distribution of the BH which given rise to interface layer and inhomogeneous BH [13, 42,43]. The I–V measurements were also carried out to investigate the effect of light on the electrical characteristics of the Ag/9,10-H2BaP/n-Si diode under illumination intensity ranging from 40 to 100 mW/cm2 at 20 mW/cm2 intervals and in the dark. The obtained results are shown in Fig. 8. As seen from the figure, the structure has very strong light sensitivity and the reverse bias current of the diode increases with the increasing light intensity. This behavior of the Ag/9,10-H2BaP/n-Si structure is a conventional behavior of a photodiode [44]. Because diode shows photodiode feature rather than solar cell, there is a significant current increase at reverse bias but is not important change at forward bias. That is, the reverse bias current values for the Ag/9,10-H2BaP/ n-Si diode under illumination is much higher than that under dark
C −2 ¼
2ðV d0 þ V Þ εs eS2 N d
ð6Þ
where is εs is the dielectric constant of the semiconductor (εs = 11.9), Nd is the donor concentration, V is the applied reverse bias voltage and Vd0 is the diffusion potential at zero bias obtained from the intersection and slope of the linear reverse bias C−2–V graph. Thus, Vd0 and Nd values from C−2–V plot are calculated as 0.54 V and 2.49 × 1015 cm−3, respectively. The barrier height value of the structure can be determined from the equation: ΦbðC−V Þ ¼ V d0 þ
kT Nc ln Nd q
ð7Þ
Table 1 VOC and ISC values of the Ag/9,10-H2BaP/n-Si/Au–Sb diode under various light intensities and at −2 V. Light intensity (mW/cm2) Light sensitivity (Iillumination/Idark) ISC (μA) VOC (mV)
Fig. 7. The reverse bias C−2–V plot at 500 kHz for the Ag/9,10-H2BaP/n-Si MIS structure.
40 60 80 100
4.12 5.55 8.45 11.63
15.9 21.1 30.9 40.4
95 85 85 75
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E. Ozerden et al. / Thin Solid Films 597 (2015) 14–18
reverse bias current of the structure increased with the increasing light intensity. References
Fig. 9. The linear change of the ISC and light sensitivity parameters versus light intensity for the Ag/9,10-H2BaP/n-Si/Au–Sb device (at −2 V).
condition. Furthermore, the characteristic photovoltaic parameters of the structure such as light sensitivity, short circuit current or photocurrent (ISC) and open circuit voltage (VOC) values were measured and indicated in Table 1 for each light intensity. The changes in these parameters of the diode demonstrate that the light absorption at the interface gives rise to production of electron–hole pairs and forms a carriercontributing photocurrent [44,45]. It can be said that the Ag/9,10H2BaP/n-Si device has the photodiode characteristics due to this linear correlation between photoelectrical parameters with light intensity and the linear dependence of ISC and light sensitivity of the structure (at − 2 V) to light intensity in Fig. 9. The photovoltaic parameters of the organic-on-inorganic (OI) structures depend on the electrical and optical properties of the organic interfacial layer [46]. In conclusion, the organic–inorganic structure has been formed by using the 9,10-H2BaP organic thin film on n-Si wafer to investigate the electrical and photoelectrical characteristics of the Ag/9,10-H2BaP/nSi/Au–Sb diode from the I–V and C–V measurements at room temperature. The optical band gap of the organic thin film was also determined as 3.73 eV from optical absorption spectrum. It was seen that the structure from the obtained results exhibited perfect rectifying behavior and photodiode property. The ideality factor and barrier height values for Ag/9,10-H2BaP/n-Si diode were calculated as 1.30 and 0.67 eV, respectively, from the forward bias I–V characteristics at room temperature. In addition, photovoltaic characteristics of the structure determined from the I–V measurements under illumination intensity range from 40 to 100 mW/cm2 at 20 mW/cm2 intervals and in the dark. It was observed that the structure had a very strong light sensitivity and the
[1] T. Kılıçoğlu, M.E. Aydın, G. Topal, M.A. Ebeoğlu, H. Sayğılı, Synth. Met. 157 (2007) 540–545. [2] K. Akkılıç, M.E. Aydın, İ. Uzun, T. Kılıçoğlu, Synth. Met. 156 (2006) 958–962. [3] T. Kılıçoğlu, M.E. Aydın, Y.S. Ocak, Phys. B Condens. Matter 388 (2007) 244–248. [4] E. Ayyıldız, A. Türüt, S. Tüzemen, Solid State Commun. 110 (1999) 419–423. [5] E.H. Rhoderick, R.H. Williams, Clarendon Press, Oxford, 1988. [6] S. Chand, J. Kumar, J. Appl. Phys. 82 (1997) 5005–5010. [7] Y.S. Ocak, M.F. Genişel, T. Kılıçoğlu, Microelectron. Eng. 87 (2010) 2338–2342. [8] Ş. Karataş, A. Türüt, Phys. B Condens. Matter 381 (2006) 199–203. [9] S.J. Fonash, J. Appl. Phys. 54 (1983) 1966–1975. [10] K.R. Rajesh, C.S. Menon, J. Non-Cryst. Solids 353 (2007) 398–404. [11] R.K. Gupta, K. Ghosh, P.K. Kahol, Physica E: Low-dimensional Systems and Nanostructures 41 (2009) 1832–1834. [12] R.K. Gupta, K. Ghosh, P.K. Kahol, Microelectron. Eng. 87 (2010) 221–224. [13] Ö. Güllü, T. Kılıçoğlu, A. Türüt, J. Phys. Chem. Solids 71 (2010) 351–356. [14] S.A. Moiz, M.M. Ahmed, K.S. Karimov, ETRI J. 27 (2005) 319–325. [15] M. Zhu, T. Cui, K. Varahramyan, Microelectron. Eng. 75 (2004) 269–274. [16] Y. Shirota, J. Mater. Chem. 10 (2000) 1–25. [17] Ş. Aydoğan, M. Sağlam, A. Türüt, Vacuum 77 (2005) 269–274. [18] R. Singh, D.N. Srivastava, R.A. Singh, Synth. Met. 121 (2001) 1439–1440. [19] R.L. Van Meirhaeghe, W.H. Laflere, F. Cardon, J. Appl. Phys. 76 (1994) 403–406. [20] K. Akkılıç, Y.S. Ocak, T. Kılıçoğlu, S. İlhan, H. Temel, Curr. Appl. Phys. 10 (2010) 337–341. [21] O.A. Azim, M.M. Abdel-Aziz, I.S. Yahia, Appl. Surf. Sci. 255 (2009) 4829–4835. [22] K.R. Rajesh, C.S. Menon, The European Physical Journal B-Condensed Matter and Complex Systems 47 (2005) 171–176. [23] S. Alialy, H. Tecimer, H. Uslu, Ş. Altındal, Journal of Nanomedicine & Nanotechnology 4 (2013) 1–7. [24] Ş. Karataş, M. Çakar, Synth. Met. 159 (2009) 347–351. [25] W. Mönch, J. Vac. Sci. Technol. B 17 (1999) 1867–1876. [26] R.T. Tung, Phys. Rev. B 45 (1992) 13509–13523. [27] S.M. Sze, K.N. Kwok, in, Wiley, New York, 2007. [28] A. Keffous, M. Siad, A. Cheriet, N. Benrekaa, Y. Belkacem, H. Menari, W. Chergui, A. Dahmani, Appl. Surf. Sci. 236 (2004) 42–49. [29] T. Kampen, A. Schüller, D.R.T. Zahn, B. Biel, J. Ortega, R. Perez, F. Flores, Appl. Surf. Sci. 234 (2004) 341–348. [30] A.R.V. Roberts, D.A. Evans, Appl. Phys. Lett. 86 (2005) 072105-072101 - 072105072103. [31] T.U. Kampen, S. Park, D.R.T. Zahn, Appl. Surf. Sci. 190 (2002) 461–466. [32] A. Bolognesi, A. Di Carlo, P. Lugli, T. Kampen, D.R.T. Zahn, J. Phys. Condens. Matter 15 (2003) S2719–S2728. [33] M. Çakar, C. Temirci, A. Türüt, Synth. Met. 142 (2004) 177–180. [34] D.R.T. Zahn, T.U. Kampen, H. Mendez, Appl. Surf. Sci. 212-213 (2003) 423–427. [35] C.H. Chen, I. Shih, J. Mater. Sci. Mater. Electron. 17 (2006) 1047–1053. [36] M. Çakar, N. Yıldırım, Ş. Karataş, C. Temirci, A. Türüt, J. Appl. Phys. 100 (2006) 074505–074505-074506. [37] M. Çakar, C. Temirci, A. Türüt, ChemPhysChem 3 (2002) 701–704. [38] E. Özerden, M. Yıldız, Y.S. Ocak, A. Tombak, T. Kılıçoğlu, Mater. Sci. Semicond. Process. 28 (2014) 72–76. [39] S.M. Sze, in, John Wiley and Sons. Inc., New York, 1981. [40] A. Tombak, Y.S. Ocak, S. Asubay, T. Kılıçoğlu, F. Özkahraman, Mater. Sci. Semicond. Process. 24 (2014) 187–192. [41] H. Tecimer, A. Türüt, H. Uslu, Ş. Altındal, İ. Uslu, Sensors Actuators A Phys. 199 (2013) 194–201. [42] T. Kılıçoğlu, Y.S. Ocak, Microelectron. Eng. 88 (2011) 150–154. [43] S. Alialy, Ş. Altındal, E.E. Tanrıkulu, D.E. Yıldız, J. Appl. Phys. 116 (2014) 083709. [44] F. Yakuphanoğlu, Sol. Energy Mater. Sol. Cells 91 (2007) 1182–1186. [45] M.M. El-Nahass, H.M. Zeyada, K.F. Abd-El-Rahman, A.A.A. Darwish, Sol. Energy Mater. Sol. Cells 91 (2007) 1120–1126. [46] F. Yakuphanoğlu, Y.S. Ocak, T. Kılıçoğlu, W.A. Farooq, Microelectron. Eng. 88 (2011) 2951–2954.
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Electrical and photoelectrical properties of Ag/n-type Si metal/ semiconductor contact with organic interlayer Enise Ozerden a, Yusuf Selim Ocak b,c, Ahmet Tombak a, Tahsin Kilicoglu d,⁎, Abdulmecit Turut e a
Department of Physics, Faculty of Art & Science, Batman University, 72100 Batman, Turkey Department of Science, Faculty of Education, Dicle University, 21280 Diyarbakir, Turkey Science and Technology Application and Research Center, Dicle University, 21280 Diyarbakir, Turkey d Department of Physics, Faculty of Science, Dicle University, 21280 Diyarbakir, Turkey e Department of Physics Engineering, Faculty of Science, Istanbul Medeniyet University, 34730 Istanbul, Turkey b c
a r t i c l e
i n f o
Article history: Received 27 March 2015 Received in revised form 14 October 2015 Accepted 7 November 2015 Available online 11 November 2015 Keywords: Organic interlayer Metal/organic layer/inorganic semiconductor structures Schottky barrier height Photoelectrical properties
a b s t r a c t Electrical and photoelectrical features of Metal/Organic Interlayer/Inorganic Semiconductor (MIS) Schottky device were investigated by using current–voltage (I–V) and capacitance–voltage (C–V) measurements at room temperature. For this purpose, 9,10-dihydrobenzo[a]pyrene-7(8H)-one (9,10-H2BaP) thin film was used as organic interlayer between Ag metal and n-Si semiconductor. Firstly, optical properties of the organic thin film were determined from optical absorption spectrum, and its optical band gap was found to be 3.73 eV. Then, the electrical parameters of the Ag/9,10-H2BaP/n-Si diode such as ideality factor (n), barrier height (Φb(I–V)), diffusion potential (Vd), barrier height (Φb(C–V)) and carrier concentration (Nd) were calculated from I–V and C–V characteristics at room temperature. The Φb values obtained from both measurements were compared with each other. Besides, the effect of light on I–V measurements of the structure was examined at illumination intensities ranging from 40 to 100 mW/cm2 with 20 mW/cm2 intervals using a solar simulator with AM1.5 filter. Light sensitivity, open circuit voltage (VOC) and short circuit current (ISC) parameters of Ag/9,10-H2BaP/n-Si structure were calculated from under light measurements. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Schottky contacts have taken a significant role in modern semiconductor device technology [1,2]. They have found a large number of electronic and optoelectronic implementations including field-effect transistors (FETs), solar cells and photodetectors [1,3,4]. The principal parameters of the Schottky barrier diodes (SBDs) are barrier height (BH) and ideality factor [5,6]. These fundamental parameters of the SBDs are strongly influenced by the existence of an interfacial layer between a metal and a semiconductor [1,7]. The interfacial layer may possess a strong effect on electrical and photoelectrical characteristics of the diode [1,8,9]. Organic semiconductors used as the interfacial layer in between the inorganic semiconductor and metal can change the characteristics of MS contacts [10–12]. The new electrical features of the structures could be promoted with the choice of appropriate organic semiconductor [13]. Due to their good stability and barrier height enhancement properties of organic semiconductor materials, they have been widely used in electronics and optoelectronics such as fabrication of the SBDs, organic light emitting diodes (OLEDs), field effect
⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (T. Kilicoglu).
http://dx.doi.org/10.1016/j.tsf.2015.11.013 0040-6090/© 2015 Elsevier B.V. All rights reserved.
transistors (FETs), photovoltaic devices (PVDs), solar cells and sensors [11,14–20]. 9,10-Dihydrobenzo[a]pyrene-7(8H)-one organic compound named 9,10-H2BaP was used to modify Ag/n-Si metal/semiconductor (MS) contact. This compound is a derivative of benzo[a]pyrene which is a five ring polycyclic aromatic hydrocarbon (PAH). Benzo[a]pyrene (BaP) is known to be mutagenic and highly effective carcinogenic. The molecular structure of the 9,10-H2BaP organic compound with the empirical formula C20H14O is shown in Fig. 1. Also, the 9,10-H2BaP compound is fallen within as organic structure such as OFET and OPV materials, dyes, ketones and carbonyl compounds. In this paper, our aim is to obtain an Ag/9,10-H2BaP/n-Si/Au-Sb structure, to determine the electrical and photoelectrical properties of obtained structure by using current–voltage (I–V) and capacitance–voltage (C–V) measurements at the room temperature and to investigate whether or not the Ag/9,10H2BaP/n-Si/Au–Sb structure shows a rectifying contact behavior. 2. Experimental procedure The sample was prepared using a polished n-type Si wafer received as the fabrication with (100) orientation and having 1–10 Ω·cm resistivity at the room temperature. The 9,10-dihydrobenzo[a]pyrene7(8H)-one organic compound was provided as ready by means of the Sigma-Aldrich. Firstly, the n-type Si wafer was chemically cleaned for
E. Ozerden et al. / Thin Solid Films 597 (2015) 14–18
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at various frequencies. Optical absorption spectrum of the 9,10-H2BaP thin film on a quartz substrate was also obtained with Perkin Elmer Lambda 25 UV/Vis spectrophotometer in the range of 200 to 1100 nm wavelength. 3. Results and discussion
Fig. 1. The molecular structure of the 9,10-H2BaP molecule (molecular formula: C20H14O).
5 min in boiling trichloroethylene and cleaned by ultrasonic vibration in acetone and methanol to get rid of organic contaminations. The native oxide layer on the wafer surface was removed in a HF:H2O (1:10) solution for 30 s. Preceding each step, the n-Si wafer was washed with 18 MΩ deionized water and dried under N2 atmosphere. Then, Au–Sb (99%–1%) alloy was thermally evaporated on the unpolished side of nSi substrate. The obtained structure was annealed at 420 °C for 3 min in N2 atmosphere to make low resistivity ohmic contacts. The native oxide layer on the front of the substrate was removed in HF:H2O (1:10) solution for 30 s and was rinsed in deionized water and was dried under N2 atmosphere before forming organic layer on n-Si substrate. The 9,10-H2BaP organic layer is directly formed by adding a 9,10-H2BaP solution of 1 × 10− 3 mol/L in chloroform and methanol mixture on the front surface of the n-Si substrate and then waiting for the solvent to evaporate at the room temperature. The cross section image of 9,10-H2BaP/n-Si structure was obtained by FEI Quanta 250 FEG scanning electron microscopy (SEM). It was seen that the 9,10H2BaP thin film is very smooth and the thickness of the film is about 180 nm. The cross section of the 9,10-H2BaP thin film was given in Fig. 2. After the formation of the 9,10-H2BaP organic thin film, Schottky contact of structure was coated by evaporating with Ag dots with a diameter of about 1.5 mm. The evaporation process in a vacuum system of the structure was performed at 2 × 10−6 Torr. After all this process Ag/9,10-H2BaP/n-Si/Au–Sb diode structure was obtained. The I–V measurements of the device were carried out with a Keithley 2400 Sourcemeter in the dark and under a Newport Oriel 9600 solar simulator with AM1.5 filter at illumination intensities ranging from 40 to 100 mW/cm2 at the room temperature. The C–V characteristics of the structure were measured at the room temperature by using an Agilent Hewlett Packard (HP) 4294A Impedance Analyzer (40 Hz–110 MHz)
Fig. 2. The SEM image magnified ×120,000 times of the cross-section of the 9,10-H2BaP organic thin film formed on the n-Si inorganic semiconductor.
The optical absorption spectrum of the 9,10-H2BaP organic thin film on the quartz is shown in Fig. 3. The optical properties of the 9,10H2BaP organic thin film were analyzed by the following relationship [21,22] and the optical energy band gap (Eg) of the film was calculated from the relation: m αhν ¼ B hν−Eg
ð1Þ
where α is the absorption coefficient of the film, B is an energyindependent constant, h is the Planck's constant (h = 6.626 × 10−34 J·s), ν is the frequency of the radiation and m is a constant which determines the type of optical transitions and depends on the nature of the transition. The exponent m gets the values of 1/2, 2, 3/2 or 3 for direct allowed, indirect allowed, direct forbidden or indirect forbidden transitions, respectively. Fig. 4 indicates (Ahν)1/2 versus hν and (Ahν)2 versus hν (inset) plots of the 9,10-H2BaP organic thin film on the quartz according to Eq. (1). The non-direct and direct band gaps of the 9,10H2BaP organic compound were obtained as 3.73 and 5.37 eV, respectively, by extrapolating the linear portion of these graphs to intercept the photon energy axis which indicates that the non-direct allowed transition dominates in the 9,10-H2BaP organic thin film. We suggest that the 9,10-H2BaP organic thin film possesses 3.73 eV indirect optical band gap. Furthermore, the trap levels of the 9,10-H2BaP organic thin film were obtained as 1.55 and 1.35 eV from intercept with axis by extrapolating of the linear portions in the energy range of 1.13–6.20 eV of photon energy axis of the (Ahν)1/2–hν graph. The experimental semilogarithmic I–V graph obtained using I–V measurements of the Ag/9,10-H2BaP/n-Si organic–inorganic structure at ambient temperature in dark is displayed in Fig. 5. As can be also seen from the figure, the structure shows perfect rectifying behavior. The characteristic properties of the rectifying contact behavior explain as the weak voltage dependence of the reverse bias current and the
Fig. 3. The plot of absorption of the 9,10-H2BaP organic thin film on the quartz.
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E. Ozerden et al. / Thin Solid Films 597 (2015) 14–18
where I0 is the saturation current, e is the electronic charge, T is the absolute temperature, k is the Boltzmann's constant (k = 8.625 × 10−5 eV/K), V is the applied voltage and n is the dimensionless ideality factor which indicates how closely the ideal diode behavior and this value is 1 for the ideal diode. The n value of the diode is determined from the slope of the forward bias ln I–V plot of the diode by using Eq. [5]: n¼
e dV : kT dð ln IÞ
ð3Þ
The value of the n for Ag/9,10-H2BaP/n-Si diode has been calculated from Fig. 5 as 1.30 at room temperature. The n value of the structure is expected to be unity for the ideal one. The value of 1.30 for the ideality factor at room temperature means the deviation from the ideal diode property. High values of the n can be attributed to the effects of the series resistance of device, the existence of organic compound layer between Ag metal and n-Si semiconductor [24], the presence of the secondary mechanisms at the interface and fabrication-induced defects [25,26], a wide distribution of the low patches (or barrier inhomogeneities) and the bias voltage dependence of the SBH [5]. In addition, the saturation current in Eq. (2) can be written as:
1/2
Fig. 4. (Ahυ) the quartz.
2
−hν and (Ahυ) −hν (inset) plots of the 9,10-H2BaP organic thin film on
exponential increase in the forward bias current. Due to series resistance (Rs) of bulk resistance of the 9,10-H2BaP organic interlayer and Si inorganic semiconductor, a curvature downward region occurs at high current in the forward bias I–V plot [13]. Besides, the Rs which was given rise to the downward curvature at enough high bias region as it is seen from the Fig. 5 is more effective if compared with the interface states (Nss). It is well known that the Rs is influential only at high forward bias region while the Nss is important in the low and intermediate bias regions [23]. The thermionic emission (TE) theory can be used in the determination of parameters of the diode due to rectifying behavior. According to the theory, the net TE current expression is given as [5]: eV −1 I ¼ I 0 exp nkT
ð2Þ
Fig. 5. The forward and reverse bias current–voltage characteristic of the Ag/9,10-H2BaP/ n-Si MIS diode at room temperature in dark.
eΦ I0 ¼ SA T 2 exp − b kT
ð4Þ
where A∗ is the effective Richardson constant equals to 110 A/K2 cm2 for n-Si [27], S the effective surface area of the diode (S = 1.767 × 10−2 cm2) and Φb the barrier height of the diode and the Φb was calculated by using I0 value determined from the intercept of ln I–V plot on I axis by using the equation is given as follows: Φb ¼
kT SA T 2 ln I0 e
! ð5Þ
The barrier height value of the diode has been calculated as 0.67 eV by using Eq. (5). The obtained 0.67 eV value of Φb for Ag/9,10-H2BaP/nSi structure due to 9,10-H2BAP organic thin layer is clearly lower than that achieved with traditional Ag/n-Si MS contacts [28]. This difference can be ascribed to the presence of the 9,10-H2BaP organic interface layer at the MS interface modifying the effective BH by influencing the space charge region of the inorganic silicon substrate [3,29–32]. Thereby, it is known that the organic interlayer modifies the barrier height in the device and prevents direct contact of the metal from the Si surface [3, 29–32]. The 9,10-H2BaP organic interlayer seems to cause an important modification of the interface states even though the organic–inorganic interface becomes abrupt and unreactive [29–34]. The capacitance–voltage (C–V) characteristics can also significantly inform about the BH and the interface state charge concentration for the Schottky contacts. The junction capacitance dominates for the reversedbiased diodes, while the diffusion capacitance dominates in strongly forward-biased diodes [35]. The C–V measurements for Ag/9,10-H2BaP/ n-Si diode at different frequencies (50 kHz-5 MHz) are presented in Fig. 6. As shown in the figure, the capacitance of the structure decreases with increasing frequency. Therefore, this situation can be explained that the interface states have strong effects on electrical properties and the total capacitance of the Ag/9,10-H2BaP/n-Si diode. Thus, the capacitance of the structure strongly depends on applied bias voltage and frequency due to the presence of the interface states [7,36,37]. Besides, the increasing capacitance of the structure at low frequencies depends on the ability of the interface states to follow the applied ac signal while they cannot follow the ac signal at sufficiently high frequencies (f ≥ 1 MHz) because the carrier life time (τ) is more large according to the measured period at high frequencies [23,38]. It has been clearly shown in Fig. 6. For this reason, the capacitance of diode has a tendency to stabilize at higher frequencies. Also, the C−2–V plot can give information about the characteristic parameters of the structure such as BH,
E. Ozerden et al. / Thin Solid Films 597 (2015) 14–18
17
Fig. 6. The capacitance–voltage plots of the Ag/9,10-H2BaP/n-Si MIS diode at various frequencies.
Fig. 8. In I–V graphs for Ag/9,10-H2BaP/n-Si MIS device under various light intensities of 40, 60, 80 and 100 mW/cm2 and in dark.
diffusion potential and carrier concentration. In order to determine these parameters of the diode, the C−2–V graph is plotted in Fig. 7 at 500 kHz and at room temperature. The depletion region capacitance of the metal/n-Si diodes is written as follows [5,39]:
where Nc is the density of states in the conduction band (Nc = 2.8 × 1019 cm−3). The Φb(C–V) value of the device was calculated by using Vd0 and Nd values which can be obtained from C−2–V plot and was found to be 0.78 eV. This value which can be obtained from C–V data is higher than that obtained from I–V data. This difference between the obtained Φb(I–V) and Φb(C–V) values for Ag/9,10-H2BaP/n-Si diode can be explained by the different nature of the I–V and C–V measurements. It is explained that the capacitance of the diode at interface is insensitive to potential fluctuations and that the C–V method for defining of the Φb averages over the whole area and measurements [13]. The current I across the interface exponentially depends on the barrier height Φb and is extremely sensitive to the potential fluctuations at the interface which consist of low and high barrier areas [13,40,41]. The discrepancy between the barrier height values of the structure can be also attributed to the presence of the interface layer which exists between the metal and the semiconductor, the trap states in the semiconductor, barrier height inhomogeneity which is presence at M/S interface and spatial distribution of the BH which given rise to interface layer and inhomogeneous BH [13, 42,43]. The I–V measurements were also carried out to investigate the effect of light on the electrical characteristics of the Ag/9,10-H2BaP/n-Si diode under illumination intensity ranging from 40 to 100 mW/cm2 at 20 mW/cm2 intervals and in the dark. The obtained results are shown in Fig. 8. As seen from the figure, the structure has very strong light sensitivity and the reverse bias current of the diode increases with the increasing light intensity. This behavior of the Ag/9,10-H2BaP/n-Si structure is a conventional behavior of a photodiode [44]. Because diode shows photodiode feature rather than solar cell, there is a significant current increase at reverse bias but is not important change at forward bias. That is, the reverse bias current values for the Ag/9,10-H2BaP/ n-Si diode under illumination is much higher than that under dark
C −2 ¼
2ðV d0 þ V Þ εs eS2 N d
ð6Þ
where is εs is the dielectric constant of the semiconductor (εs = 11.9), Nd is the donor concentration, V is the applied reverse bias voltage and Vd0 is the diffusion potential at zero bias obtained from the intersection and slope of the linear reverse bias C−2–V graph. Thus, Vd0 and Nd values from C−2–V plot are calculated as 0.54 V and 2.49 × 1015 cm−3, respectively. The barrier height value of the structure can be determined from the equation: ΦbðC−V Þ ¼ V d0 þ
kT Nc ln Nd q
ð7Þ
Table 1 VOC and ISC values of the Ag/9,10-H2BaP/n-Si/Au–Sb diode under various light intensities and at −2 V. Light intensity (mW/cm2) Light sensitivity (Iillumination/Idark) ISC (μA) VOC (mV)
Fig. 7. The reverse bias C−2–V plot at 500 kHz for the Ag/9,10-H2BaP/n-Si MIS structure.
40 60 80 100
4.12 5.55 8.45 11.63
15.9 21.1 30.9 40.4
95 85 85 75
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reverse bias current of the structure increased with the increasing light intensity. References
Fig. 9. The linear change of the ISC and light sensitivity parameters versus light intensity for the Ag/9,10-H2BaP/n-Si/Au–Sb device (at −2 V).
condition. Furthermore, the characteristic photovoltaic parameters of the structure such as light sensitivity, short circuit current or photocurrent (ISC) and open circuit voltage (VOC) values were measured and indicated in Table 1 for each light intensity. The changes in these parameters of the diode demonstrate that the light absorption at the interface gives rise to production of electron–hole pairs and forms a carriercontributing photocurrent [44,45]. It can be said that the Ag/9,10H2BaP/n-Si device has the photodiode characteristics due to this linear correlation between photoelectrical parameters with light intensity and the linear dependence of ISC and light sensitivity of the structure (at − 2 V) to light intensity in Fig. 9. The photovoltaic parameters of the organic-on-inorganic (OI) structures depend on the electrical and optical properties of the organic interfacial layer [46]. In conclusion, the organic–inorganic structure has been formed by using the 9,10-H2BaP organic thin film on n-Si wafer to investigate the electrical and photoelectrical characteristics of the Ag/9,10-H2BaP/nSi/Au–Sb diode from the I–V and C–V measurements at room temperature. The optical band gap of the organic thin film was also determined as 3.73 eV from optical absorption spectrum. It was seen that the structure from the obtained results exhibited perfect rectifying behavior and photodiode property. The ideality factor and barrier height values for Ag/9,10-H2BaP/n-Si diode were calculated as 1.30 and 0.67 eV, respectively, from the forward bias I–V characteristics at room temperature. In addition, photovoltaic characteristics of the structure determined from the I–V measurements under illumination intensity range from 40 to 100 mW/cm2 at 20 mW/cm2 intervals and in the dark. It was observed that the structure had a very strong light sensitivity and the
[1] T. Kılıçoğlu, M.E. Aydın, G. Topal, M.A. Ebeoğlu, H. Sayğılı, Synth. Met. 157 (2007) 540–545. [2] K. Akkılıç, M.E. Aydın, İ. Uzun, T. Kılıçoğlu, Synth. Met. 156 (2006) 958–962. [3] T. Kılıçoğlu, M.E. Aydın, Y.S. Ocak, Phys. B Condens. Matter 388 (2007) 244–248. [4] E. Ayyıldız, A. Türüt, S. Tüzemen, Solid State Commun. 110 (1999) 419–423. [5] E.H. Rhoderick, R.H. Williams, Clarendon Press, Oxford, 1988. [6] S. Chand, J. Kumar, J. Appl. Phys. 82 (1997) 5005–5010. [7] Y.S. Ocak, M.F. Genişel, T. Kılıçoğlu, Microelectron. Eng. 87 (2010) 2338–2342. [8] Ş. Karataş, A. Türüt, Phys. B Condens. Matter 381 (2006) 199–203. [9] S.J. Fonash, J. Appl. Phys. 54 (1983) 1966–1975. [10] K.R. Rajesh, C.S. Menon, J. Non-Cryst. Solids 353 (2007) 398–404. [11] R.K. Gupta, K. Ghosh, P.K. Kahol, Physica E: Low-dimensional Systems and Nanostructures 41 (2009) 1832–1834. [12] R.K. Gupta, K. Ghosh, P.K. Kahol, Microelectron. Eng. 87 (2010) 221–224. [13] Ö. Güllü, T. Kılıçoğlu, A. Türüt, J. Phys. Chem. Solids 71 (2010) 351–356. [14] S.A. Moiz, M.M. Ahmed, K.S. Karimov, ETRI J. 27 (2005) 319–325. [15] M. Zhu, T. Cui, K. Varahramyan, Microelectron. Eng. 75 (2004) 269–274. [16] Y. Shirota, J. Mater. Chem. 10 (2000) 1–25. [17] Ş. Aydoğan, M. Sağlam, A. Türüt, Vacuum 77 (2005) 269–274. [18] R. Singh, D.N. Srivastava, R.A. Singh, Synth. Met. 121 (2001) 1439–1440. [19] R.L. Van Meirhaeghe, W.H. Laflere, F. Cardon, J. Appl. Phys. 76 (1994) 403–406. [20] K. Akkılıç, Y.S. Ocak, T. Kılıçoğlu, S. İlhan, H. Temel, Curr. Appl. Phys. 10 (2010) 337–341. [21] O.A. Azim, M.M. Abdel-Aziz, I.S. Yahia, Appl. Surf. Sci. 255 (2009) 4829–4835. [22] K.R. Rajesh, C.S. Menon, The European Physical Journal B-Condensed Matter and Complex Systems 47 (2005) 171–176. [23] S. Alialy, H. Tecimer, H. Uslu, Ş. Altındal, Journal of Nanomedicine & Nanotechnology 4 (2013) 1–7. [24] Ş. Karataş, M. Çakar, Synth. Met. 159 (2009) 347–351. [25] W. Mönch, J. Vac. Sci. Technol. B 17 (1999) 1867–1876. [26] R.T. Tung, Phys. Rev. B 45 (1992) 13509–13523. [27] S.M. Sze, K.N. Kwok, in, Wiley, New York, 2007. [28] A. Keffous, M. Siad, A. Cheriet, N. Benrekaa, Y. Belkacem, H. Menari, W. Chergui, A. Dahmani, Appl. Surf. Sci. 236 (2004) 42–49. [29] T. Kampen, A. Schüller, D.R.T. Zahn, B. Biel, J. Ortega, R. Perez, F. Flores, Appl. Surf. Sci. 234 (2004) 341–348. [30] A.R.V. Roberts, D.A. Evans, Appl. Phys. Lett. 86 (2005) 072105-072101 - 072105072103. [31] T.U. Kampen, S. Park, D.R.T. Zahn, Appl. Surf. Sci. 190 (2002) 461–466. [32] A. Bolognesi, A. Di Carlo, P. Lugli, T. Kampen, D.R.T. Zahn, J. Phys. Condens. Matter 15 (2003) S2719–S2728. [33] M. Çakar, C. Temirci, A. Türüt, Synth. Met. 142 (2004) 177–180. [34] D.R.T. Zahn, T.U. Kampen, H. Mendez, Appl. Surf. Sci. 212-213 (2003) 423–427. [35] C.H. Chen, I. Shih, J. Mater. Sci. Mater. Electron. 17 (2006) 1047–1053. [36] M. Çakar, N. Yıldırım, Ş. Karataş, C. Temirci, A. Türüt, J. Appl. Phys. 100 (2006) 074505–074505-074506. [37] M. Çakar, C. Temirci, A. Türüt, ChemPhysChem 3 (2002) 701–704. [38] E. Özerden, M. Yıldız, Y.S. Ocak, A. Tombak, T. Kılıçoğlu, Mater. Sci. Semicond. Process. 28 (2014) 72–76. [39] S.M. Sze, in, John Wiley and Sons. Inc., New York, 1981. [40] A. Tombak, Y.S. Ocak, S. Asubay, T. Kılıçoğlu, F. Özkahraman, Mater. Sci. Semicond. Process. 24 (2014) 187–192. [41] H. Tecimer, A. Türüt, H. Uslu, Ş. Altındal, İ. Uslu, Sensors Actuators A Phys. 199 (2013) 194–201. [42] T. Kılıçoğlu, Y.S. Ocak, Microelectron. Eng. 88 (2011) 150–154. [43] S. Alialy, Ş. Altındal, E.E. Tanrıkulu, D.E. Yıldız, J. Appl. Phys. 116 (2014) 083709. [44] F. Yakuphanoğlu, Sol. Energy Mater. Sol. Cells 91 (2007) 1182–1186. [45] M.M. El-Nahass, H.M. Zeyada, K.F. Abd-El-Rahman, A.A.A. Darwish, Sol. Energy Mater. Sol. Cells 91 (2007) 1120–1126. [46] F. Yakuphanoğlu, Y.S. Ocak, T. Kılıçoğlu, W.A. Farooq, Microelectron. Eng. 88 (2011) 2951–2954.