p-Si semiconductor contacts

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Electrical characteristics of organic/inorganic Pt(II) complex/p-Si semiconductor contacts A. Gencer Imer a,n, C. Temirci a,nn, M. Gülcan b, M. Sö nmez c a b c

Department of Physics, Faculty of Science, University of Yüzüncü Yıl, 65080 Van, Turkey Department of Chemistry, Faculty of Science, University of Yüzüncü Yıl, 65080 Van, Turkey Department of Chemistry, Faculty of Science and Arts, University of Gaziantep, 27310 Gaziantep, Turkey

a r t i c l e i n f o

Keywords: Contact Organic Rectifier Schottky Diode

abstract We produced Pt(II) complexes using the bidentate ligand N-aminopyrimidine-2-thione (APTH). The optical transmission of thin Pt-APTH films was measured. The optical bandgap of the material was 2.58 eV. With the expectation that it might have semiconductor properties and that the Pt-APTH complex might exhibit rectifier behavior when brought into appropriate contact with a semiconductor, we fabricated Pt-APTH/p-Si contacts by direct addition of a solution of Pt-APTH to the front side of p-Si wafers. Forward bias current–voltage measurements revealed satisfactory rectifying behavior for the Pt-APTH/p-Si contacts, with a mean rectification ratio of 4.40
102 and a mean barrier height of 0.765 eV. Cheung and Norde functions were used to obtain and verify some electrical characteristics of the contacts. The results obtained from both methods are compared and discussed. & 2014 Elsevier Ltd. All rights reserved.

1. Introduction Organic materials have a wide variety of applications in modern electronic technology owing to their structural flexibility, low production costs, ease of synthesis, and suitability for large-area device production [1–5]. The electronic and optoelectronic properties of organic/inorganic semiconductor devices have been widely studied in Schottky diodes, photovoltaic cells, and photodiodes [2–24]. The interface layer and states in such devices have a dominant influence in determining the electrical characteristics, mainly the barrier height and ideality factor [6–24]. Many studies have focused on modifying the barrier height using an organic thin film on an inorganic semiconductor [6–24]. The first study on barrier height enhancement using a polycrystalline aromatic organic compound on p- and n

Corresponding author. Tel.: þ90 432 225 7840. Corresponding author. Tel.: þ90 432 225 1725. E-mail addresses: [email protected] (A. Gencer Imer), [email protected] (C. Temirci). nn

n-type Si was by Forrest et al. [10,11]. They determined that the Schottky barrier height and ideality factor for diodes fabricated using a nonpolymeric organic thin film were 0.77 eV and 1.8, and 0.76 eV and 1.7 for Au and Ti contacts, respectively. Campbell et al. reported that the organic thin film at an organic/inorganic semiconductor interface could control a dipole layer and thus decrease or increase the effective barrier height [25]. Many attempts have been made to define the factors that affect parameters obtained from current–voltage characteristics for these junction types [26–29]. The electrical parameters of organic/inorganic semiconductor diodes may depend on the chain length of the organic moiety [27]. The charge transport properties of these contacts can also be affected by the thickness of the organic thin film on the inorganic semiconductor [28]. The current–voltage mechanism of organic/ inorganic junctions may also be influenced by oxygen in the air during fabrication [29]. The fabrication and electrical characterization of organic/ inorganic semiconductor contacts based on organic compounds such as tetra-amide-I/p-Si [17], orange G/n-Si [13],

http://dx.doi.org/10.1016/j.mssp.2014.03.035 1369-8001/& 2014 Elsevier Ltd. All rights reserved.

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pyronine/p-Si [30], and methyl violet/p-Si [5] have attracted much attention because of their chemical stability and excellent electrical properties [2]. These studies showed that structures with an organic thin film exhibit rectifying behavior, with an ideality factor and barrier height greater than those for conventional metal/semiconductor contacts. We previously studied the rectifying and ohmic behavior of metal/organic diodes comprising the bidentate ligand N-aminopyrimidine-2-thione (APTH) in complex with Cu(II) and Co(II) [31,32]. The fundamental electronic parameters of these contacts were determined from current–voltage measurements. In the present study, we investigated the electrical characteristics of Pt(II) in complex with APTH (Pt-APTH), and of Pt-APTH/p-Si contacts formed as an organic/inorganic semiconductor rectifier heterojunction via current–voltage (I–V) measurements in the dark at room temperature. Our novel Pt-APTH complex exhibits rectifier behavior and could be used in the fabrication of organic-based electrical devices.

formed on the p-Si substrate, the native oxide on the front side was removed with HF:H2O (1:10) solution and the substrate was rinsed in deionized water for 30 s and dried under N2. The organic layer was directly formed by addition of a solution of Pt-APTH complex in dimethylformamide (0.2 mg/mL) on the front surface of the p-Si substrate and the solvent was allowed to evaporate at room temperature for 24 h. After a Pt-APTH film of  500 nm had formed, Au was evaporated on the top using a tungsten heater through a shadow mask in an Edwards auto 306 vacuum coating system at  10  5 Torr. Contacts with an area of 7.85
10  3 cm2 are denoted D1, D2, D3, D4, D5, D6, D7, D8, and Dwtc (dot without a top contact). The Dwtc diode was constructed to verify that the rectifying behavior occurs at the Pt-APTH/p-Si interface. I–V measurements were carried out on the contacts using a Keithley 6487 picoammeter/ voltage source in the dark at room temperature. Optical measurements were performed using a Shimadzu UV-2450 spectrophotometer in the range 200–900 nm.

2. Experimental 2.1. Synthesis of the Pt-APTH complex

3. Results and discussion

The APTH ligand was synthesized in a two-step processes as previously described [33,34]. The Pt-APTH complex was prepared according to the following procedure. A solution of [PtCl4]2  was made by boiling PtCl2 (0.5 mmol) in concentrated HCl (5 mL). After cooling and dilution with distilled water (15 mL), a hot solution of N-APTH (1 mmol, 0.307 g) in chloroform/methanol (70 mL, 1:1 v/v) was added and the mixture was refluxed for 30 h. The precipitated compound was removed by filtration, washed with diethyl ether and then cold methanol/water, and dried under vacuum in a desiccator. The yield was 225 mg (49%), m.p. 216 1C. Anal. calcd. for C34H26N6O3PtS2 (825.12 g/mol): C, 49.45%; H, 3.17%; N, 10.18%; S, 7.77%. Found: C, 50.13%; H, 3.15%; N, 10.33%; S, 8.17%. Selected IR data, ν (cm  1): 3423 ν(NH), 3058 (C–H pyrimidine), 1664 ν (C ¼O), 1597 (C ¼N pyrimidine) 1145, 742 ν(C ¼S), 510–520 (M–O), 430–440 (M–N). UV-Vis [λ (nm), ε (M  1 cm  1)]: 271, 285, 290, 294, 351, 373, 384, 393, 585. μeff, Dia, Λo (S cm2 mol  1) 14.4. API-ES: m/z 826.8 [192Pt (N-APT)2 þ1]. The molecular structure of the Pt-APTH complex is shown in Fig. 1.

3.1. Optical characterization of Pt-APTH films

2.2. Contact fabrication Wafers of p-type Si with (100) orientation and resistivity of 5–10 Ω cm were used to fabricate Pt(II) complex/p-Si contacts. Wafers were first chemically cleaned using the standard RCA procedure (10-min boil in NH4 þH2O2 þ6H2O followed by 10-min boil in HClþH2O2 þ6H2O) and etched with dilute HF:H2O (1:10) for 30 s to remove the native oxide layer on the surface. Before each step, wafers were rinsed in deionized water with resistivity of 18 MΩ. They were then dried under N2 atmosphere and immediately inserted into a vacuum system for metallization. The back contact was formed by evaporation of Al on the back side of the cleaned p-Si substrate, with subsequent annealing at 580 1C for 3 min in N2. Before the organic thin layer was

Optical transmission measurements were performed in the range 200–900 nm for Pt-APTH films. The dependence of the absorption coefficient α on photon energy was derived from the transmission spectra according to the relation [35] α¼

1 Io ln ; d I

ð1Þ

where d is the film thickness and Io and I are the intensity of incident and transmitted light, respectively. Fig. 2 shows α as a function of the incident photon energy for a Pt-APTH thin film on glass. The absorption edge corresponding to the bandgap energy is sharp for single crystals. By contrast, the absorption coefficient for Pt-APTH exhibits a relatively slow decrease with decreasing incident photon energy (Fig. 2). This relatively slow decrease may be attributable to optical scattering due to defects and grain boundaries. It could also arise from intrinsic absorption in regions of the thin Pt-APTH film with highly concentrated defect states. The optical bandgap energy (Eg) of the Pt-APTH film can be determined from its optical absorption edge. The optical absorption coefficient α as a function of incident photon energy hν can be expressed as [35] αðhνÞ ¼ Aðhν  Eg Þn ;

ð2Þ

where A is a proportionality constant and n¼ 1/2 for direct transitions [35]. The bandgap energy can be estimated from a plot of (α(hν))2 versus hν, as represented in Fig. 3. We calculated a value of Eg ¼ 2.58 eV by extrapolation of the linear portion of the plot to the energy axis. This value corresponds to the energy difference between the highest occupied molecular orbital and lowest unoccupied molecular orbital of Pt-APTH.

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Fig. 1. Structure proposed for the Pt-APTH complex.

Fig. 2. Variation in absorption coefficient as a function of energy for a Pt-APTH film on glass.

Fig. 4. Experimental data for forward and reverse bias current versus voltage for the Pt-APTH/p-Si contacts.

Fig. 3. Dependence of (α(hν))2 on photon energy for a Pt-APTH film on glass.

3.2. Current–voltage properties Current–voltage measurements were carried out to investigate the electrical properties of the Pt-APTH/p-Si contacts.

The I–V characteristics in Fig. 4 exhibit rectifying behavior: while the reverse current shows weak voltage dependence, the forward current increases exponentially with the voltage. The value of the forbidden energy gap for the Pt-APTH film estimated from optical transmission measurements indicates that the complex can show semiconductor behavior. The rectifier behavior of the Pt-APTH/p-Si contacts also supports the notion that the Pt-APTH complex is an n-type semiconductor. In previous studies, N-APTH in complex with Co(II) and Cu(II) showed n-type semiconductor behavior [31,32]. As an additional control, we fabricated Pt-APTH/n-Si contacts (data not shown) but we did not observe any remarkable value for the barrier height. Consideration of polarization in the current–voltage measurements also indicated that the Pt-APTH complex shows n-type semiconductor behavior. The interface characteristics of any junction have a dominant effect on its properties. During production, two situations can occur: the junction shows either rectifier or ohmic behavior. The type of contact is mainly related to

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Table 1 Electrical characteristics of Pt-APTH/p-Si heterojunction rectifying contacts according to I–V characteristics, Cheung functions, and Norde functions. Sample

D1 D2 D3 D4 D5 D6 D7 D8 Dwtc

I–V characteristics

[dV/d(ln I)]–I plot

H(I)–I plot

Norde functions

n

Φb (eV)

n

Rs (Ω)

Φb (eV)

Rs (Ω)

Vmin (V)

F(Vmin) (V)

Φb (eV)

Rs (kΩ)

4.069 3.516 4.555 2.044 3.480 4.757 5.518 2.106 2.094

0.774 0.757 0.777 0.734 0.778 0.738 0.740 0.728 0.861

4.849 4.918 2.396 2.032 3.948 5.212 5.199 4.989 1.861

994 1028 1660 949 1009 1285 1829 588 1067

0.789 0.759 0.780 0.780 0.772 0.767 0.764 0.749 0.883

945 1109 1704 993 1067 1318 1832 958 1089

0.692 0.344 0.626 0.298 0.452 0.535 0.469 0.300 0.582

0.719 0.715 0.734 0.671 0.733 0.686 0.702 0.668 0.763

0.832 0.775 0.833 0.744 0.820 0.767 0.754 0.742 0.931

3008 3125 2875 3087 3375 2095 3117 2655 2930

production conditions such as the evaporation temperature and chemical cleaning process [32]. The rectifying behavior observed in Fig. 4 means that simple thermionic emission theory can be used to determine characteristic parameters of the device. According to thermionic emission theory [36], the forward bias current of a device is due to a thermionic emission current and can be expressed as     qðV IRs Þ qðV  IRs Þ I ¼ I o exp 1  exp  ; ð3Þ nkT kT where

  qΦ I o ¼ ARn T 2 exp  b kT

ð4Þ

is the saturation current, q is the electronic charge, V is the applied voltage, Rs is the series resistance, k is the Boltzmann constant, and T is ambient temperature (in K), A is the effective diode area (7.85
10  3 cm2), Φb is the zero-bias barrier height, and Rn is the Richardson constant, which is 32 A cm  2 K  2 for p-Si [37]. n is the ideality factor, which is a measure of the conformity of a diode to pure thermionic emission. It is determined from the slope of the straight-line region of the forward-bias lnI–V characteristics according to n¼

q dV : kT dðln I Þ

ð5Þ

In the case of an ideal diode, n must equal 1 or at least be close to 1 for a rectifier diode [38]. However, experimental results for n generally deviate from the theory. This deviation can be attributed to effects such as a non-ideal interface structure, inhomogeneity of the interface layer, and the presence of an oxide layer at the interface [32]. The values of Φb and n for Pt-APTH/p-Si heterojunctions were calculated from the y-axis intercept and slope of the linear region of Fig. 4 according to Eqs. (4) and (5). The results are listed in Table 1. The full correct barrier height cannot be calculated for a heterojunction, but the results give us an idea about the rectifier behavior. The mean values obtained for the Pt-APTH/p-Si contacts were n ¼3.571 and Φb ¼0.765 eV. In our previous study we determined values of n¼ 1.39 and Φb ¼0.75 eV for Al/CuAPTH contacts [31]. Thus, the mean Φb value for the diodes in the present study are consistent with our previous results [31]. However, the ideality factor in the present

Fig. 5. Experimental heterojunction dots.

dV/d(ln I)

versus

I

plots

for

Pt-APTH/p-Si

study is greater than in the previous study. The n values determined for the contacts are greater than 1, indicating that the contacts show non-ideal behavior that can probably be attributed to their interfaces [31]. The electrical parameters are significantly affected by even small changes at the interface. The Pt-APTH film coated on the p-Si wafer has a granular appearance, and thus the surface of the film cannot be completely uniform, which is not desirable for a rectifier junction. In addition, interfacial chemical reactions are inevitable, which is a significant negative factor for rectifier contacts. Despite these potential drawbacks, the electrical characteristics determined for our Pt-APTH/p-Si rectifier contacts are quite satisfactory. The effect of Rs can be monitored in terms of the downward concave curvature of a forward-bias I–V plot at sufficiently high voltage [5]. Different methods can be used to determine the value of Rs for any contact [13], among which the Cheung functions are an important approach, according to which [39] dV nkT ¼ þIRs dðln I Þ q

ð6Þ

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and 

   nkT I ln ; HðIÞ ¼ V  n 2 q AR T

ð7Þ

where HðIÞ ¼ IRs þ nΦb :

ð8Þ

As is evident from Fig. 5, Eq. (6) should give a straight line for forward-bias region of the I–V graph, and thus a plot of dV/d(ln I) versus I is linear. Rs and n were calculated from the slope and y-axis intercept of the plot, respectively. The dV/d(ln I)–I plot for the Pt(II) complex/p-Si device is shown in Fig. 5. The mean value of n ¼3.57 obtained from the forward-bias lnI–V curves are in good agreement with n ¼3.93 from the dV/d(ln I)–I plot (Table 1). The ideality factor, which indicates the quality of a diode, is equal to 1 for an ideal diode. Smaller values of n are more desirable for a diode. The mean values of 3.57 and 3.93 indicate that the fabricated samples have organic/ insulator/inorganic semiconductor rectifying contact properties rather than organic/inorganic semiconductor rectifying intimate contact. This deviation in the characteristics of the contacts could be caused by chemical reaction between p-Si and P-APTH leading to diffusion of charge carriers in the interface [31,32]. To inhibit these reactions between contact components, a contamination layer could be formed at the interface [32]. As shown in Fig. 6, plots of H(I) versus I give straight lines in accordance with Eq. (8). The slope of these lines gives a second determination of Rs to check the consistency of the Cheung method. Using the value of n obtained from Eq. (6), a value for Φb is obtained from the y-axis intercept of the H(I)–I curve in Fig. 6. The mean value obtained is 0.783 eV, in good agreement with the current– voltage result. The mean Rs values of 1156 and 1224 Ω obtained from the slope of dV/d(ln I)–I and H(I)–I plots, respectively, are also in good agreement. The very high

Fig. 6. Experimental H(I) versus I plots for different Pt-APTH/p-Si heterojunction dots.

5

mean Rs could be related to space charge injection into the Pt-APTH film at sufficiently high forward bias voltage [13]. Results for n, Φb, and Rs obtained from the Cheung functions are listed in Table 1. These results indicate that the junctions have reasonable characteristics that are comparable to those of metal/inorganic semiconductor rectifying contacts. Rectification ratios of 2.17
102, 2.03
102, 1.25
102, 1.47
103, 4.65
102, 1.21
102, 6.11
102, 1.02
102, and 6.46
102 were obtained for samples D1, D2, D3, D4, D5, D6, D7, D8, and Dwtc, respectively, at 70.8 V. The mean rectification ratio was 4.40
102, which indicates that the Pt-APTH/p-Si contacts have good rectifying properties. Another method was developed by Norde to determine the barrier height and series resistance for contacts [40]. The Norde function is   V kT IðVÞ ln FðVÞ ¼  ; ð9Þ γ q ARn T 2 where γ is an integer greater than n, V is the applied voltage, and I(V) is the current obtained from I–V measurements. Fig. 7 shows plots of F(V) versus V for the Pt-APTH/ p-Si rectifier contacts. Using the data in Fig. 7, we can determine the barrier height for each diode as Φb ¼ FðV min Þ þ

V min kT  ; q γ

ð10Þ

where F(Vmin) is the value of F(V) corresponding to the minimum voltage. After determining F(Vmin) for each sample using data from Fig. 7, values for Φb can be obtained from Eq. (10). Values of Rs can be obtained using the Norde method according to Rs ¼

kTðγ nÞ : qðI min Þ

ð11Þ

Table 1 shows good agreement for Φb obtained from forward-bias ln I–V data, Cheung functions, and Norde

Fig. 7. F(V) versus V plots for Pt-APTH/p-Si rectifier contacts in the case of eV 4 3kT, where thermionic emission dominates.

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functions. The Rs results for the Pt-APTH/p-Si contacts vary between the Norde method and the Cheung functions. In fact, the Norde function is not appropriate for determining Rs for high n values [41]. Rs varies as a function of the applied voltage. Bending of the current–voltage curve at higher voltage under a forward bias is usually attributed to Rs. A voltage range is used when determining Rs via the Cheung functions, so in a sense the average Rs is obtained. In contrast, a specific voltage is used in determining Rs according to the Norde function. Thus, the Rs values obtained may not exactly match those determined via the Cheung functions. 4. Conclusions We produced a novel organic Pt-APTH complex and fabricated organic/inorganic semiconductor Pt-APTH/p-Si heterojunction rectifier contacts. The Pt-APTH complex was characterized as an n-type semiconductor. Electrical parameters of the Pt-APTH/p-Si contacts were determined using current–voltage data and Cheung and Norde functions. The results reveal that the Pt-APTH/p-Si contacts show good rectifier characteristics, with a mean rectification ratio of 4.40
102 and a mean barrier height of 0.765 eV. This novel Pt-APTH complex with rectifier properties can be applied as an organic semiconductor in the fabrication of electrical devices. Acknowledgement We would like to thank the Yüzüncü Yıl University Scientific Research Management Office (BAPB) for their support. References [1] J.M. Shaw, P.F. Seidler, IBM J. Res. Dev. 45 (2001) 3–9. [2] Ş. Erten, S. İçli, Inorg. Chim. Acta 361 (2008) 595–600. [3] P. Stallinga, H.L. Gomes, M. Murgia, K. Mullen, Org. Electron. 3 (2002) 43–51. [4] M.M. El-Nahass, H.M. Zeyada, K.F. Abd-El-Rahman, A.A.A. Darwish, Energy Mater. Sol. Cells 91 (2007) 1120–1126. [5] Ş. Aydoğan, M. Sağlam, A. Turut, Microelectron. Eng. 85 (2008) 278–283.

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