190 Original scientific paper  MIDEM Society Effect of a new methacrylic monomer on diode parameters of Ag/p-Si Schottky contact Necati Basman1, Rukiye Uzun1, Ramazan Ozcakır2, Ibrahim Erol2, Guven Cankaya3, Orhan Uzun4 1Electrical and Electronics Engineering, University of Bulent Ecevit, Zonguldak, Turkey 2Department of Chemistry, University of Afyon Kocatepe, Afyonkarahisar, Turkey 3Department of Materials Engineering University of Yildirim Beyazit, Ankara, Turkey 4Department of Metallurgical and Materials Engineering University of Bulent Ecevit, Zonguldak, Turkey Abstract: 1-[4-(prop-2-yn-1-yloxy)phenyl]ethanone-O-methacryloyloxime (POEMO) is a new methacrylic monomer with side chain alkyne. In this study, Ag/POEMO/p-Si Schottky metal-interlayer-semiconductor (MIS) diode was fabricated and its diode parameters were investigated. Using the forward bias current-voltage (I-V) characteristic, the ideality factor and barrier height of the MIS structure were found as 2.81 and 0.70 eV, respectively. The barrier height value of 0.70 eV obtained for Ag/POEMO/p-Si MIS diode was higher than the value of 0.64 of conventional Ag/p-Si Schottky diode. Cheung-Cheung and Norde methods were also used to extract ideality factor, barrier height and series resistance values, and the obtained results were compared. Keywords: Schottky diode; electrical characterization; methacrylic monomer. Vpliv novega metakrilnega monomera na diodne parameter Ag/p-Si Schottky kontakta Izvleček: 1-[4-(prop-2-yn-1-yloxy)phenyl]ethanone-O-methacryloyloxime (POEMO) je nov metakrilni monomer s stransko alkilno verigo. V tem delu smo izdelali in analizirali lastnosti Ag/POEMO/p-Si Schottky-jeve MIS (kovina-vmesna plast-polprevodnik) diode. Idealni faktor in višina bariere diode pri priključeni prevodni napetosti je 2.81 in 0.70 eV. Dobljena višina bariere v dani strukturi je višja od 0.64 eV pri klasični Schottky-jevi diodi. Za natančno določitev idealnega faktorja, višine bariere in serijske upornosti smo uporabili Cheung-Cheung in Norde metodi. Rezultate obeh metod smo medsebojno primerjali. Ključne besede: Schottky-jeva diode; električna karakterizacija; metakrični monomer * Corresponding Author’s e-mail: nbasman@gmail.com Journal of Microelectronics, Electronic Components and Materials Vol. 46, No. 4(2016), 190 – 196 1 Introduction Organic electronic has drawn significant attention due to interesting optical, electrical, photoelectric, and magnetic properties of organic materials in the solid state. The advantages of thin-film formation, light weight, large area and mechanical flexibility provided by organic materials are other reasons of this increas- ing interest [1]. Furthermore, organic chemistry can tai- lor the materials properties according to the demand. Electronic devices based on organic materials have found a wide variety of applications including; light emitting diode, organic field effect transistor, Schottky diode, photovoltaic and solar cells [2, 3]. The metal/semiconductor (MS) contacts have crucial importance in electronic devices. However, many of these contacts are not fabricated as barely MS contacts; they are fabricated as metal-interlayer-semiconductor (MIS) contacts [4]. MIS structures are fabricated by cov- ering a semiconductor substrate with an organic/inor- ganic layer on which a metal electrode is deposited. In an MS contact, the characteristic quantity is the Schott- ky barrier height which measures the energy distance between the Fermi level and the edge of respective majority carrier band of the semiconductor at the inter- face. The barrier height of an MS contact can be modi- fied by inserting an interlayer between the metal and 191 the semiconductor. Therefore, numerous studies have been carried out to implement barrier height modifica- tion using organic/inorganic interlayer. In these stud- ies, either increasing or decreasing of the barrier height have been reported [5-9]. The ability of acrylic monomers in copolymerizing to produce variety of structures make it possible to pro- duce the desired properties for a wide range of applica- tions. Therefore, methacrylate polymers are one of the most important commercial polymers [10]. 1-[4-(prop- 2-yn-1-yloxy)phenyl]ethanone-O-methacryloyloxime (POEMO) is a new functional methacrylic monomer with side chain alkynes [11]. It is thought that investi- gation of further application areas for this new meth- acrylate monomer bearing an important group such as alkynes would be useful. For example, using the mono- mer as an interlayer at the interfaces in the MS contact may be interesting. The aim of this study was to investigate diode param- eters of a new diode fabricated using a new functional methacrylic monomer, i.e. Ag/POEMO/p-Si MIS diode. The current-voltage (I-V) measurement was carried out to obtain barrier height, ideality factor and series resistance of the device using I-V, Cheung-Cheung and Norde methods. 2 Experimental The synthesis of 1-[4- (prop-2-yn-1-yloxy) phenyl] et- hanone-Omethacryloyloxime (POEMO) monomer is shown in Fig. 1. Detailed description of synthesizing method can be found elsewhere [11-12]. Figure 1: The synthesis of POEMO monomer Ag/POEMO/p-Si MIS diode was prepared using a one side polished p-type Si (100) wafer. The wafer was chemically cleaned using the RCA cleaning procedure (i.e., a 10 min boil in NH3+H2O2+6H2O followed by a 10 min boil in HCl+H2O2+6H2O). Low resistivity ohmic con- tact to p-type Si substrate was made by Al metal, fol- lowed by a temperature treatment at 570 °C for 3 min in N2 atmosphere. The native oxide on the front surface of substrate was removed in HF:H2O (1:10) solution and finally the wafer was rinsed in de-ionised water for 30 min. Subsequently, POEMO was coated onto front surface of clean silicon substrate directly by dropping POEMO-acetone solution and waited for the evapora- tion of the solvent at room temperature. The contacting metal dot was formed by silver paste with a diameter of about 1.0 mm (diode area=7.85 10−3 cm2). Ag/POEMO/ p-Si MIS diode is thus obtained. The current-voltage (I-V) measurements of MIS diode were carried out by a Keithley 6487 picoammeter/voltage source at room temperature. 3 Results and discussion The non-linear I-V characteristic of a typical diode be- havior is described by the thermionic emission theory as follows [13, 14]: 0 exp 1 exp qV qVI I nkT kT  −   = −         (1) For bias levels larger than 3kT/q, Eq. (1) can be ex- pressed as: 0 exp qVI I nkT  =    (2) Here V, q, n, k and T and represent the applied voltage, the electron charge, the ideality factor, Boltzmann’s constant and the absolute temperature in Kelvin, re- spectively. I0 is the reverse saturation current which can be derived from the straight-line intercept of lnI by ex- trapolation at zero voltage and is given by: * 2 0 exp b qI AA T kT φ− =    (3) where A is the effective diode area and A* is the effec- tive Richardson constant of 32Acm-2k-2 for p-Si [14-16]. fb is the effective barrier height. Once I0 is obtained, then effective barrier height can be calculated as fol- lows: * 2 0 lnb kT AA T q I φ   =    (4) The ideality factor or the emission coefficient (n) is typi- cally used to measure how the practical diodes deviate from the ideal thermionic emission model or to take into account the contributions of other current trans- port mechanisms [17]. This parameter can be calculat- ed from the slope of linear region of semi-logarithmic I-V plot. Using Eq. (2), ideality factor can be obtained as follows: N. Basman et al; Informacije Midem, Vol. 46, No. 4(2016), 190 – 196 192 ( ) q Vn kT lnI  ∂=  ∂  (5) Fig. 2 illustrates the experimental I-V characteristic of Ag/POEMO/p-Si MIS diode at room temperature. Well- known characteristic features of rectifying contacts are the weak voltage dependence of reverse-bias current and the exponential increase of the forward-bias cur- rent [4, 18, 19]. It is obvious that the device exhibits a good rectification behavior. The downward curvature (non-linear region) in the semi-log I-V characteristic at high forward bias values is arisen from the series resis- tance (RS) associated with the contact wires or the bulk resistance of the organic interlayer and the inorganic semiconductor. Figure 2: The experimental current-voltage character- istic of the Ag/POEMO/p-Si MIS diode at room temper- ature. On the basis of TE theory, the experimental values of the effective barrier height and the ideality factor were determined from the intercept and the slope of the linear portion of the forward-bias I-V plot, respective- ly. The obtained values of effective barrier height and ideality factor were 0.70 eV and 2.81, respectively. For an ideal diode, ideality factor should be close to unity; but for reel diodes it is usually higher than one as we obtained [20-22]. Ideality factor which is greater than unity shows the deviation from thermionic emission theory. In the literature this case is assigned to the in- terface states, as well as fabrication-induced defects at the surface [18, 20-24]. In addition; interface state density distribution, quantum-mechanical tunneling, image-force lowering, the lateral distribution of barrier height inhomogeneities, the leakage current, series and shunt resistance are also proposed to explain the deviation [13, 18, 23, 25, 26]. Among these, the series resistance (RS) is an important and influential param- eter on the electrical characteristics of Schottky barrier diodes. Therefore, determination of series resistance (RS) value deserves attention for understanding the mechanism of diodes. The barrier height value of 0.70 eV obtained for the Ag/POEMO/p-Si device is higher than that achieved with conventional MS contact of Ag/p-Si whose barrier height value was 0.62 [27]. By means of the POEMO in- terlayer; a physical barrier is formed between the metal and the inorganic substrate, preventing the metal from directly contacting the Si surface. The POEMO organic interlayer affects the space charge region of the inor- ganic substrate [18, 28]. Thus, the change in barrier height can be explained by an interface dipole induced by the organic layer [4]. In the literature, many studies have been performed to modify the barrier height of Schottky barrier diodes by forming an interfacial layer between the metal and the semiconductor using the organic thin films [9, 18, 29, 30]. Here, we showed that POEMO could also be used to modify barrier height of Ag/p-Si diode. As the series resistance can be negligible at low voltage ranges of a forward bias region, the variation of current with voltage shows linearity. However for higher volt- ages, the current is deviated considerably from linearity by the series resistance, as can be seen in Fig. 2. Cheung and Cheung proposed a method for the determining the series resistance (RS) from the high voltage range of an I-V characteristic of a diode [31]. According to Cheung and Cheung, the forward bias current-voltage characteristic of a Schottky barrier diode with a series resistance is given by: ( ) 0 Sq V IRI I exp nkT  − =     (6) Here IRs denotes the voltage drop across series resis- tance of the diode. Using Eq. (6), the electrical param- eters viz. series resistance, ideality factor, and barrier height can be determined from the following equa- tions: ( ) S dV kTIR n dln I q   = −    (7) ( ) * 2 kT IH I V n ln q AA T    = −       (8) ( ) S bH I IR nφ= + (9) N. Basman et al; Informacije Midem, Vol. 46, No. 4(2016), 190 – 196 193 The plot of dV/d(lnI) vs. I gives a straight line for the data of downward curvature region (Fig. 3a). According to Eq. (7), this plot gives Rs as the slope and nkT/q as y-axis intercept. Hereby, n and Rs were calculated as 2.33 and 2.24 kΩ, respectively. The plot of H(I) function is given in Fig. 3b. The slope of H(I)-I plot is equal to series resis- tance (Rs), whereas the intercept of the y-axis gives nfb Accordingly, using the value of ideality factor obtained from Eq. (7), the barrier height (fb) and series resistance values were calculated as 0.74 eV and 2.49 kΩ, respec- tively. The series resistance values determined by two different equations of Cheungs’ are close to each other. Figure 3: Plot of Cheung’s fuctions; dV/d(InI) vs. I (a), H(I) vs. I (b) On the other hand, there are differences between the ideality factors and barrier heights obtained from I-V and Cheungs’ methods. These differences are attrib- uted to the differences of extraction region in the for- ward bias of I-V plot. In the I-V method, linear region is used for calculation, while Cheungs’ functions are re- lated to the nonlinear region of lnI-V plot in which the interfacial layer thickness between the metal and the semiconductor, the interface states and bulk series re- sistance are effective [19, 32, 33]. The barrier heights and series resistance of the Schott- ky diodes can also be calculated using modified Norde method [18, 34]. The following function has been de- fined in the modified Norde method: ( ) ( )* 2 I VV kTF V ln q AA Tγ   = −    (10) Where γ is the first integer (dimensionless) greater than n. I(V) is current obtained from the I-V curve. Fig. 4 shows the change of F(V) versus V for Ag/POEMO/p-Si diode. After determining the minimum value of F(V) by employing the data in Fig. 4, barrier height and series resistance of the diode can be determined by the fol- lowing equations: ( ) 00b V kTF V qφ γ= + − (11) ( )s kTR n qI γ= − (12) In Eq. (11), F(V0) is the minimum point of F(V) and V0 is the corresponding voltage. From the F(V)-V plot and Eqs. (11) and (12), the barrier height and series resis- tance were found to be 0.72 eV and 3.78 kΩ, respec- tively. The diode parameters, which were obtained via different methods, were summarized in Table 1. As can be seen in the table, there are differences in val- ues of series resistance obtained from Cheng-Cheung and Norde methods. These differences were originated from the fact that the full-voltage range of forward bias ln(I) – V data is used in Norde method, whereas only the a. b. Table 1: Diode parameters of Ag/POEMO/p-Si MIS Schottky diode calculated from I-V, Cheung-Cheung and Norde methods Diode parameters I-V Method Cheung-Cheung Method Norde Method d(lnI)-V Dv/d(lnI)-I H(I)-I F(V)-V Ideality Factor ( n ) 2.81 2.33 - - Barrier Height (fb eV) 0.70 - 0.74 0.72 Series Resistance (Rs kΩ) - 2.24 2.50 3.78 N. Basman et al; Informacije Midem, Vol. 46, No. 4(2016), 190 – 196 194 high-voltage region (viz. non-linear part of the plot) of the forward bias ln(I) – V data is used in Cheung’s meth- od [35, 36]. High series resistance value is accepted as current-limiting factor for the diodes. Gullu et al. attrib- uted the high series resistance to space-charge injec- tion into POEMO thin layer at higher forward bias volt- age. As the tunneling process is especially important for a thin interfacial layer, it is assumed that tunneling starts to control the current flow [18, 37]. 4 Conclusions In this study, we fabricated an Ag/POEMO/p-Si Schottky Barrier diode and investigated its diode parameters by I-V measurement. The ideality factor, series resistance and barrier height values were calculated by different methods and were compared. Based on the results the following conclusions could be deduced; - The Ag/POEMO/p-Si Schottky Barrier MIS diode showed good rectifying behavior indicating PO- EMO organic layer could be used as an interlayer. - The forward current-voltage characteristics indi- cated a nonlinear behavior because of the series resistance, as verified by the n value was larger than unity. - We observed that the fb value of 0.70 eV obtained for the Ag/POEMO/p-Si device was different than the BH value of the conventional Ag/p-Si contact. This case could be attributed to the POEMO or- ganic interlayer which modifies effective Schott- ky barrier by affecting the space charge region of the inorganic substrate. - The differences between barrier heights values obtained from different methods were attributed to presence of series resistance, interface states and the voltage drop across the interfacial layer 5 References 1. 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