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Temperature dependent electrical characteristics of an organic–inorganic heterojunction obtained from a novel organometal Mn complex
ARTICLE IN PRESS Physica B 405 (2010) 2329–2333
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Temperature dependent electrical characteristics of an organic–inorganic heterojunction obtained from a novel organometal Mn complex Y.S. Ocak a,, M.A. Ebeoglu b, G. Topal c, T. Kılıc- og˘lu d,e,n ˘ a
Department of Science, Faculty of Education, University of Dicle, Diyarbakır, Turkey Department of Electrical and Electronics Engineering, Faculty of Engineering, University of Dumlupinar, Kutahya, Turkey c Department of Chemistry, Faculty of Education, University of Dicle, Diyarbakır, Turkey d Department of Physics, Faculty of Art and Science, University of Dicle, Diyarbakır, Turkey e Department of Physics, Faculty of Art and Science, University of Batman, Batman, Turkey b
a r t i c l e in fo
abstract
Article history: Received 30 October 2009 Received in revised form 25 January 2010 Accepted 16 February 2010
This study includes synthesizing a Mn hexaamide (MnHA) organometal compound (C27H21N9O6MnCl2) (1/2H2O), fabrication of MnHA/n–Si organic–inorganic heterojunction and analysis of conduction mechanism of the device over the room temperature. After synthesizing the molecule, the structure of the compound was determined using spectroscopic methods. The Sn/MnHA/n–Si structure was constructed by forming a thin MnHA layer on n–Si inorganic semiconductor and evaporating Sn metal on organic complex. The structure has shown good rectifying behavior and obeys the thermionic emission theory. The current–voltage (I–V) characteristics of the diode have been measured at temperatures ranging from 300 to 380 K at 10 K intervals to determine the temperature dependent electrical characteristics of the device. & 2010 Elsevier B.V. All rights reserved.
Keywords: Ideality factor Barrier height Series resistance Organometal complex Macrocyclic hexaamide
1. Introduction Organometal compounds play an important role in recent studies related to electronic and optoelectronic devices. There are a number of studies both to synthesize this kind of molecules [1] and to understand their electrical and optoelectrical properties [2,3]. They have been used in the fabrication of devices such as solar cells [4–7], light emitting diodes [4] and Schottky diodes [8–12]. They have been chosen in order to benefit from organic and inorganic properties in a single compound. Namely, O’Regan and Gratzel [5] were reported a low cost, high efficiency dyesensitized solar cell using a Ru (II) complex. A number of solar cells and Schottky diodes have been fabricated using phthalocyanine complexes [6,8,10]. Recently, Akkılıc- et al [11,12] have shown the possibility of Schottky diodes formation by new synthesized Cu (II) complexes. So, both the synthesis of new organometal complexes and the use of them in the fabrication of devices are of great interest. Control over the electronic properties of semiconductors and metals have been the subject of many electronic and optoelectronic studies [13]. Many attempts have been executed to form Corresponding author.
E-mail addresses: [email protected] (Y.S. Ocak), [email protected], [email protected] (T. Kılıc-og˘lu). 0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.02.039
rectifying contacts or to realize a modification and the continuous control of the barrier height using organic or organometal compounds as an interfacial layer at metal–semiconductor (MS) devices [11–15]. It is well known that most of the electronic and optoelectronic devices work over the room temperature because of warming up. In addition, I–V characteristics of rectifying contacts measured only at room temperature does not give detailed information about their conduction mechanisms [16]. Therefore, analysis of the temperature dependent I–V characteristics of these structures over room temperature may have importance. The aim of the study is to synthesize a new Mn hexaamide (MnHA) organometal compound with C27H21N9O6MnCl2 1/2H2O formula in order to use it in the fabrication of the semiconductor devices and determine the current transport properties of the diode over room temperature. For this purpose, a MnHA compound has been synthesized and the compound has been identified using 1H NMR, 13C NMR, IR, UV–visible and LC–MS spectroscopic methods. Then, the compound has been used in the fabrication of Sn/MnHA/n–Si structure by forming its thin film on the inorganic semiconductor n–Si. It has been seen that the structure has shown the rectifying behavior. The temperature dependent I–V measurements between 300 and 380 K at 10 K interval have been taken to determine the current transport properties of the device.
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2.1. Synthesis of the Mn complex of 1,3,5,10,12,14,19,21,23-nonaaza-2,4:11,13:20,22-tribenzo-cycloheptaeicosane-6,9,15,18,24,27,hexaon-cis-7,16,25-trien (C27H21N9O6MnCl2) (1/2H2O) (MnHA)
. All used solvents and chemicals were analytical reagent grade and MnCl2 4H2O was used as analytical chemical. Before the reaction, Maleic acid and 2,6-diaminopyridine were purified. The reaction was carried out in a 1:2:2 molar ratio (MnCl2 4H2O:Maleic acid:2,6-diaminopyridine). A weighed amount of MnCl2 4H2O (1.91 g, 9.7 mmol) was dissolved in methanol (25 ml) at 0 1C in 100 ml two necked round-bottomed flask equipped with a condenser and a dropping funnel and the flask was placed in a magnetic stirrer in an ice-bath. 2,6-diaminopyridine solution in methanol (25 ml) was added drop wise to the stirred solution of MnCl2 4H2O. This was followed by the addition of maleic acid in methanol (25 ml). The reaction mixture was stirred continuously for 16–20 h. The resultant walnut green crystalline solid product was filtered, washed several times with methanol and dried in air. The compounds were recrystallized in benzene:methanole (1:1) and dried in vacuum. Mp 176–177 1C. The molecular structure of MnHA is given in Fig. 1. 2.2. Physical measurements of MnHA The UV–visible spectrum of the MnHA compound in methanol solution was recorded by Perkin-Elmer l-35 UV-spectrophotometer. The UV-spectrum is presented in Fig. 2. The figure shows the absorption of MnHA compound as an arbitrary unit vs. wavelength plot. The IR-spectrum was taken in the 4000– 400 cm 1 range using the KBr pellet technique with Mattson 1000 spectrophotometer. 1H NMR measurement was recorded by Bruker AC 400 MHz spectrometer in CDCl3. 13C NMR measurement was recorded at 200 MHz. The molecular weight of the complex was determined by using Agilent 1100 MSD spectrometer. The results of spectral analysis are given as follows: Electronic spectra of this complex gives two absorption bands in the regions 390–425 nm (lmax =413.42 nm) and 430–450 nm (lmax = 435.22 nm).
O HN
NH O N
Cl
O
O NH
Mn
HN
Cl
N
N
HN
.1/2H2O
NH O
O
Fig. 1. Suggested molecular structure of Mn hexaamide (C27H21N9O6MnCl2) (1/2H2O).
A (a.u.)
2. Experimental procedures
1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 400
500
600
(nm) Fig. 2. UV–visible spectroscopy (absorption vs. wavelength) of MnHA organometal complex.
Characteristics of IR bonds (cm 1): 3443, 3404, 3330, 3208, 3096, 3050, 2858, 1683, 1661, 1579, 1498, 1456, 1359, 1319, 1186, 1073, 1022, 975, 867, 746 and 560. The IR spectra of the complex derived from 2,6-diaminopyridine do not show any change in the pyridine ring which confirms that the nitrogen in the ring do not participate in the coordination. The suggested structure of the complex is dodecahedron geometry. 1 H NMR spectra (in DMSO) d(ppm): 5.88 (d,6H,J=8 Hz), 6.09 (s,6H), 7.08 (bs,6H), 7.47 (t,3H,J= 8 Hz) 13 C NMR spectra (in DMSO) d(ppm): 95.49, 136.23, 145.13, 152.86, 168.10 MS: the mass spectrum of the complex shows the molecular ion peaks at m/z 701 [C27H21N9O6MnCl2 1/2H2O] + . 2.3. Fabrication of MnHA/n–Si organic–inorganic heterojunction An n–Si wafer with (1 0 0) orientation and 1–10 O cm resistivity has been obtained from manufacture to fabricate a Sn/MnHA/ n–Si/Au structure. Firstly, to degrease the wafer, it was immersed in boiling 3-choloroethylene and rinsed in acetone and isopropanol by ultrasonic vibration for 5 min. It was, then, etched by solution of H2O/HF (10:1). Preceding each step, the wafer was rinsed in deionized water. After cleaning procedures, the wafer was dried under N2 and inserted into the vacuum chamber. To form ohmic contact, 250 nm Au was sputtered to the unpolished side of the wafer, and it was put in N2 atmosphere at 450 1C for 15 min. The native oxide layer on n–Si wafer was removed in H2O/ HF (10:1) solution and rinsed in deionized water, and it was dried under N2 atmosphere again. An organic MnHA layer on n–Si was formed by dipping the substrate in MnHA solution of 1 10– 3 mol L–1 in methanol. Then, Sn was evaporated through a shadow mask on MnHA layer in the vacuum system at 3 10–6 Torr in order to form a Schottky contact. The diameter of circular diodes was 1 mm. The current–voltage measurements of the diode have been carried out between 300 and 380 K at 10 K interval to determine the characteristic parameters of the diode by using Keithley 617 sourcemeter. The data were recorded using a computer via a GPIB card.
3. Results and discussion The I–V measurements of the MnHA/n–Si structure between 300 and 380 K with 10 K intervals are shown in Fig. 3. As it is seen from the figure temperature has strong effects on the I–V characteristics of the device. When a MS or a metal/interlayer/ semiconductor (MIS) device has been examined, the thermionic emission theory (TET) is generally taken into account. When TET
ARTICLE IN PRESS Y.S. Ocak et al. / Physica B 405 (2010) 2329–2333
1.00E-2
2.20
1.00E-3
2.10
2331
dV/dlnI -I 2.00 Ideality factor
Current (A)
1.00E-4
1.00E-5 380K 1.00E-6
1.90 lnI-V 1.80
1.00E-7
1.70 1.00E-8
300K
1.60 300
1.00E-9 -1.00
-0.50
0.00 Voltage (V)
0.50
320
1.00
340 Temperature (K)
360
380
Fig. 4. Temperature dependence of the ideality factor values obtained from I–V current and Cheung functions for MnHA/n–Si structure.
Fig. 3. Experimental ln I–V curves of MnHA/n–Si structure between 300 and 380 K.
is considered, the dimensionless ideality factor (n) which shows how closely the diode follows the ideal diode equation can be determined from the slope of the linear region of the semi-log forward bias I–V characteristics using [17]: q dV kT d lnðIÞ
ð1Þ
where q is the electron charge, k is the Boltzmann constant and T is the absolute temperature. The n value was calculated as 1.99 at 300 K. For an ideal diode, the ideality factor value is expected to be unity. The 1.99 ideality factor at 300 K implies the deviation from an ideal diode characteristic. This deviation can be attributed secondary mechanisms at the interface [18], series resistance of the structure and the existence of organic complex layer between Sn metal and Si [19]. Fig. 4 presents the fluctuation of n with temperatures over the room temperature. The n values obtained using Fig. 3 by the help of Eq. (1) decreases from 1.99 to 1.67 by increasing the temperature between 300 and 380 K. Previous studies related to MS or MIS structures have verified this result. For example, Karatas- et al [20] have shown the decrease of n from 2.024 to 1.108 between 150 and 400 K for Sn/p–Si Schottky diode. In addition, the decrease of n from 14.32 to 2.72 has been reported for Sn/rhodamine-101/p–Si Schottky structure ranging from 80 to 400 K [19]. In addition, the barrier height (fb) value of a structure can be calculated using I0 saturation current value determined from the intercept of ln I–V plot on I axis using 2 kT AA T ð2Þ fb ¼ ln I0 q where A is the diode area, A* is the Richardson constant equals to 110 A cm–2 K–2 for n–Si [21]. Fig. 5 shows the changes of fb values with temperature. The experimental values fb of Sn/MnHA/n–Si device range from 0.79 eV (300 K) to 0.99 eV (380 K). Akkılıc- et al. [22] reported the barrier height of Sn/n–Si Schottky diode as 0.63 eV after statistical analysis of 20 dots. The obtained fb value for Sn/MnHA/n–Si at 300 K is remarkably higher than the conventional Sn/n–Si Schottky diode. So it can be said that the
H(I)-I
1.00 Barrier Height (eV)
n¼
1.10
lnI-V 0.90
0.80
0.70 300
320
340 Temperature (K)
360
380
Fig. 5. Temperature dependence of the barrier height values obtained from I–V current and Cheung functions for MnHA/n–Si structure.
MnHA compound has strong effect on barrier height of the device. Since current transport across the MS interface is a temperature activated process, electrons at low temperatures are able to surmount the lower barriers and therefore, the current transport will be dominated by current flow through the patches of lower Schottky barrier height and a larger ideality factor. As the temperature increases, more and more electrons have sufficient energy to surmount the higher barrier [16,23–26]. Furthermore, one of the important electrical parameters of MS and MIS diodes is the series resistance (Rs), because the deflection from linearity at high voltages is shown the dependence on bulk
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resistivity. Cheung’s functions [27] have been used to obtain a precise series resistance Rs, n, and fb for the device. The Cheung’s method is achieved by using dV kT ¼ IRs þ n ð3Þ dðln IÞ q ð4Þ
and H(I) can be written as HðIÞ ¼ IRs þ nfb
ð5Þ
Fig. 6 shows the plots of dV/d ln I–I and H(I)–I obtained by using the Cheung functions only at 380 K, respectively. The Eq. (4) gives straight lines for the data in the downward-curvature region of forward bias I–V characteristics at all temperature between 300 and 380 K. The Rs and n values for the device from the slopes and y-axis intercepts of plots of dV/d ln I–I, respectively. The obtained n values with respect to temperature are presented in Fig. 4. The figure shows that the ideality factors obtained from both I–V method and Cheung’s method are in good agreement with each other and both of them decreases while temperature increases. However, the n values obtained from I–V plots, especially at lower temperatures, are lower than the values obtained from dV/d ln I–I plots. The difference between the values of the ideality factors can be attributed to the fact that the n values calculated from I–V plots are only under the effect of the interfacial properties and the ones obtained from dV/d ln I–I plots are under the effect of both the interfacial properties and the series resistance [17,28]. As a function of temperature, the experimental fb values were also obtained from H(I)–I plots using Eq. (5) from 0.75 eV (300 K) to 1.08 eV (380 K). The results are in reasonable agreement with the ones obtained from ln I–V curves. Furthermore, as shown in Fig. 7, the series resistance vs. temperature plots from Eqs. (3 and 4) are in good agreement with each other. Both of them decreases with the decrease in temperature. The Rs values were calculated in the range from 385 O (300 K) to 156 O (380 K) and 544 O to 138 O using dV/d ln I– I and H(I)–I plots, respectively. The close results of Rs are shown the consistency of the Cheung’s method. Aydogan et al [29] have ˘ shown that the Rs values of polyaniline/p–Si/Al structure obtained from dV/d lnI–I and H(I)–I plots decrease from 420 to 101 O and 483 to 108 O, respectively, between 77 and 300 K. In addition, 0.50
1.80
500
Series resistance (Ω)
kT I HðIÞ ¼ Vn ln q AA T 2
600
H(I)-I
400
300 dV/d lnI -I
200
100 300
320
340
360
380
Temperature (K) Fig. 7. Temperature dependence of the series resistance values obtained from I–V current for MnHA/n–Si structure.
S- . Karatas- et al. [20] have reported the decrease of Rs values of Sn/p–Si from 66 to 34 O and from 77 to 51 O in the temperature range 150–400 K. The increase of Rs with the fall of temperature can be attributed to the increase of n and lack of free carrier concentration at low temperatures [30].
4. Conclusion A Sn/organometal compound/n–Si/Au structure was fabricated by forming a thin film of a novel Mn hexaamide (MnHA) compound with C27H21N9O6MnCl2 1/2H2O molecular formula on n–Si. It was seen that the structure has a rectifying behavior. The characteristic parameters such as ideality factor, barrier height and series resistance were calculated over room temperatures between 300 and 380 K. It was observed that the barrier height value increased and the ideality factor and series resistance values decreased while the temperature increased.
0.40
References
0.30 1.40
H (I) (V)
dV/d lnI (V)
1.60
0.20
1.20 0.10
0.00 0.00E+000 4.00E-004
8.00E-004 Current (A)
1.20E-003
1.00 1.60E-003
Fig. 6. Experimental plots of dV/d lnI–I and H(I)–I obtained from Eqs. 3 and 4 for MnHA/n-Si structure at 380 K.
[1] A. Chaudry, N. Bansal, A. Gajraj, R.V. Singh, Journal of Inorganic Biochemistry 96 (2003) 393. [2] K.R. Rajesh, C.S. Menon, European Physical Journal B 47 (2005) 171. [3] F. Yakuphanoglu, F. Dagdelen, Y. Aydogdu, A. Aydogdu, M. S- ekerci, Materials ˘ ˘ ˘ ˘ Letters 57 (2003) 3330. [4] W.Y. Wong, Journal of Organometallic Chemistry 694 (2009) 2644. [5] B. O’Regan, M. Gratzel, Nature 353 (1991) 737. [6] R. Koeppe, N.S. Sariciftci, P.A. Troshin, R.N. Lyubovskaya, Applied Physics Letters 87 (2005) 244102. [7] M.S. Aziz, Solid-State Electronics 52 (2008) 1145. [8] F. Yakuphanoglu, Solar Energy Materials & Solar Cells 91 (2007) 1182. ˘ [9] C.Y. Kwong, A.B. Djurisic, P.C. Chui, L.S.M. Lam, W.K. Chan, Applied Physics A 77 (2003) 555. [10] F. Yakuphanoglu, M. Kandaz, B.F. S- enkal, Thin Solid Films 516 (2008) 8793. ˘ [11] K. Akkılıc- , Y.S. Ocak, S. Ilhan, T. Kılıc-oglu, Synthetic Metals 158 (2008) 969. ˘ [12] K. Akkılıc- , Y.S. Ocak, T. Kılıc- og˘lu, S. _Ilhan, H. Temel, Current Applied Physics 10 (2010) 337. ¨ . Gull ¨ u, ¨ N. Yıldırım, A. Tur ¨ ut, ¨ Journal of Electronic Materials 38 [13] M. C - akar, O (2009) 1995. [14] T. Kılıc- oglu, M.E. Aydın, G. Topal, M.A. Ebeoglu, H. Saygılı, Synthetic Metals ˘ ˘ 157 (2007) 540.
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Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Temperature dependent electrical characteristics of an organic–inorganic heterojunction obtained from a novel organometal Mn complex Y.S. Ocak a,, M.A. Ebeoglu b, G. Topal c, T. Kılıc- og˘lu d,e,n ˘ a
Department of Science, Faculty of Education, University of Dicle, Diyarbakır, Turkey Department of Electrical and Electronics Engineering, Faculty of Engineering, University of Dumlupinar, Kutahya, Turkey c Department of Chemistry, Faculty of Education, University of Dicle, Diyarbakır, Turkey d Department of Physics, Faculty of Art and Science, University of Dicle, Diyarbakır, Turkey e Department of Physics, Faculty of Art and Science, University of Batman, Batman, Turkey b
a r t i c l e in fo
abstract
Article history: Received 30 October 2009 Received in revised form 25 January 2010 Accepted 16 February 2010
This study includes synthesizing a Mn hexaamide (MnHA) organometal compound (C27H21N9O6MnCl2) (1/2H2O), fabrication of MnHA/n–Si organic–inorganic heterojunction and analysis of conduction mechanism of the device over the room temperature. After synthesizing the molecule, the structure of the compound was determined using spectroscopic methods. The Sn/MnHA/n–Si structure was constructed by forming a thin MnHA layer on n–Si inorganic semiconductor and evaporating Sn metal on organic complex. The structure has shown good rectifying behavior and obeys the thermionic emission theory. The current–voltage (I–V) characteristics of the diode have been measured at temperatures ranging from 300 to 380 K at 10 K intervals to determine the temperature dependent electrical characteristics of the device. & 2010 Elsevier B.V. All rights reserved.
Keywords: Ideality factor Barrier height Series resistance Organometal complex Macrocyclic hexaamide
1. Introduction Organometal compounds play an important role in recent studies related to electronic and optoelectronic devices. There are a number of studies both to synthesize this kind of molecules [1] and to understand their electrical and optoelectrical properties [2,3]. They have been used in the fabrication of devices such as solar cells [4–7], light emitting diodes [4] and Schottky diodes [8–12]. They have been chosen in order to benefit from organic and inorganic properties in a single compound. Namely, O’Regan and Gratzel [5] were reported a low cost, high efficiency dyesensitized solar cell using a Ru (II) complex. A number of solar cells and Schottky diodes have been fabricated using phthalocyanine complexes [6,8,10]. Recently, Akkılıc- et al [11,12] have shown the possibility of Schottky diodes formation by new synthesized Cu (II) complexes. So, both the synthesis of new organometal complexes and the use of them in the fabrication of devices are of great interest. Control over the electronic properties of semiconductors and metals have been the subject of many electronic and optoelectronic studies [13]. Many attempts have been executed to form Corresponding author.
E-mail addresses: [email protected] (Y.S. Ocak), [email protected], [email protected] (T. Kılıc-og˘lu). 0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.02.039
rectifying contacts or to realize a modification and the continuous control of the barrier height using organic or organometal compounds as an interfacial layer at metal–semiconductor (MS) devices [11–15]. It is well known that most of the electronic and optoelectronic devices work over the room temperature because of warming up. In addition, I–V characteristics of rectifying contacts measured only at room temperature does not give detailed information about their conduction mechanisms [16]. Therefore, analysis of the temperature dependent I–V characteristics of these structures over room temperature may have importance. The aim of the study is to synthesize a new Mn hexaamide (MnHA) organometal compound with C27H21N9O6MnCl2 1/2H2O formula in order to use it in the fabrication of the semiconductor devices and determine the current transport properties of the diode over room temperature. For this purpose, a MnHA compound has been synthesized and the compound has been identified using 1H NMR, 13C NMR, IR, UV–visible and LC–MS spectroscopic methods. Then, the compound has been used in the fabrication of Sn/MnHA/n–Si structure by forming its thin film on the inorganic semiconductor n–Si. It has been seen that the structure has shown the rectifying behavior. The temperature dependent I–V measurements between 300 and 380 K at 10 K interval have been taken to determine the current transport properties of the device.
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2.1. Synthesis of the Mn complex of 1,3,5,10,12,14,19,21,23-nonaaza-2,4:11,13:20,22-tribenzo-cycloheptaeicosane-6,9,15,18,24,27,hexaon-cis-7,16,25-trien (C27H21N9O6MnCl2) (1/2H2O) (MnHA)
. All used solvents and chemicals were analytical reagent grade and MnCl2 4H2O was used as analytical chemical. Before the reaction, Maleic acid and 2,6-diaminopyridine were purified. The reaction was carried out in a 1:2:2 molar ratio (MnCl2 4H2O:Maleic acid:2,6-diaminopyridine). A weighed amount of MnCl2 4H2O (1.91 g, 9.7 mmol) was dissolved in methanol (25 ml) at 0 1C in 100 ml two necked round-bottomed flask equipped with a condenser and a dropping funnel and the flask was placed in a magnetic stirrer in an ice-bath. 2,6-diaminopyridine solution in methanol (25 ml) was added drop wise to the stirred solution of MnCl2 4H2O. This was followed by the addition of maleic acid in methanol (25 ml). The reaction mixture was stirred continuously for 16–20 h. The resultant walnut green crystalline solid product was filtered, washed several times with methanol and dried in air. The compounds were recrystallized in benzene:methanole (1:1) and dried in vacuum. Mp 176–177 1C. The molecular structure of MnHA is given in Fig. 1. 2.2. Physical measurements of MnHA The UV–visible spectrum of the MnHA compound in methanol solution was recorded by Perkin-Elmer l-35 UV-spectrophotometer. The UV-spectrum is presented in Fig. 2. The figure shows the absorption of MnHA compound as an arbitrary unit vs. wavelength plot. The IR-spectrum was taken in the 4000– 400 cm 1 range using the KBr pellet technique with Mattson 1000 spectrophotometer. 1H NMR measurement was recorded by Bruker AC 400 MHz spectrometer in CDCl3. 13C NMR measurement was recorded at 200 MHz. The molecular weight of the complex was determined by using Agilent 1100 MSD spectrometer. The results of spectral analysis are given as follows: Electronic spectra of this complex gives two absorption bands in the regions 390–425 nm (lmax =413.42 nm) and 430–450 nm (lmax = 435.22 nm).
O HN
NH O N
Cl
O
O NH
Mn
HN
Cl
N
N
HN
.1/2H2O
NH O
O
Fig. 1. Suggested molecular structure of Mn hexaamide (C27H21N9O6MnCl2) (1/2H2O).
A (a.u.)
2. Experimental procedures
1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 400
500
600
(nm) Fig. 2. UV–visible spectroscopy (absorption vs. wavelength) of MnHA organometal complex.
Characteristics of IR bonds (cm 1): 3443, 3404, 3330, 3208, 3096, 3050, 2858, 1683, 1661, 1579, 1498, 1456, 1359, 1319, 1186, 1073, 1022, 975, 867, 746 and 560. The IR spectra of the complex derived from 2,6-diaminopyridine do not show any change in the pyridine ring which confirms that the nitrogen in the ring do not participate in the coordination. The suggested structure of the complex is dodecahedron geometry. 1 H NMR spectra (in DMSO) d(ppm): 5.88 (d,6H,J=8 Hz), 6.09 (s,6H), 7.08 (bs,6H), 7.47 (t,3H,J= 8 Hz) 13 C NMR spectra (in DMSO) d(ppm): 95.49, 136.23, 145.13, 152.86, 168.10 MS: the mass spectrum of the complex shows the molecular ion peaks at m/z 701 [C27H21N9O6MnCl2 1/2H2O] + . 2.3. Fabrication of MnHA/n–Si organic–inorganic heterojunction An n–Si wafer with (1 0 0) orientation and 1–10 O cm resistivity has been obtained from manufacture to fabricate a Sn/MnHA/ n–Si/Au structure. Firstly, to degrease the wafer, it was immersed in boiling 3-choloroethylene and rinsed in acetone and isopropanol by ultrasonic vibration for 5 min. It was, then, etched by solution of H2O/HF (10:1). Preceding each step, the wafer was rinsed in deionized water. After cleaning procedures, the wafer was dried under N2 and inserted into the vacuum chamber. To form ohmic contact, 250 nm Au was sputtered to the unpolished side of the wafer, and it was put in N2 atmosphere at 450 1C for 15 min. The native oxide layer on n–Si wafer was removed in H2O/ HF (10:1) solution and rinsed in deionized water, and it was dried under N2 atmosphere again. An organic MnHA layer on n–Si was formed by dipping the substrate in MnHA solution of 1 10– 3 mol L–1 in methanol. Then, Sn was evaporated through a shadow mask on MnHA layer in the vacuum system at 3 10–6 Torr in order to form a Schottky contact. The diameter of circular diodes was 1 mm. The current–voltage measurements of the diode have been carried out between 300 and 380 K at 10 K interval to determine the characteristic parameters of the diode by using Keithley 617 sourcemeter. The data were recorded using a computer via a GPIB card.
3. Results and discussion The I–V measurements of the MnHA/n–Si structure between 300 and 380 K with 10 K intervals are shown in Fig. 3. As it is seen from the figure temperature has strong effects on the I–V characteristics of the device. When a MS or a metal/interlayer/ semiconductor (MIS) device has been examined, the thermionic emission theory (TET) is generally taken into account. When TET
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1.00E-2
2.20
1.00E-3
2.10
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dV/dlnI -I 2.00 Ideality factor
Current (A)
1.00E-4
1.00E-5 380K 1.00E-6
1.90 lnI-V 1.80
1.00E-7
1.70 1.00E-8
300K
1.60 300
1.00E-9 -1.00
-0.50
0.00 Voltage (V)
0.50
320
1.00
340 Temperature (K)
360
380
Fig. 4. Temperature dependence of the ideality factor values obtained from I–V current and Cheung functions for MnHA/n–Si structure.
Fig. 3. Experimental ln I–V curves of MnHA/n–Si structure between 300 and 380 K.
is considered, the dimensionless ideality factor (n) which shows how closely the diode follows the ideal diode equation can be determined from the slope of the linear region of the semi-log forward bias I–V characteristics using [17]: q dV kT d lnðIÞ
ð1Þ
where q is the electron charge, k is the Boltzmann constant and T is the absolute temperature. The n value was calculated as 1.99 at 300 K. For an ideal diode, the ideality factor value is expected to be unity. The 1.99 ideality factor at 300 K implies the deviation from an ideal diode characteristic. This deviation can be attributed secondary mechanisms at the interface [18], series resistance of the structure and the existence of organic complex layer between Sn metal and Si [19]. Fig. 4 presents the fluctuation of n with temperatures over the room temperature. The n values obtained using Fig. 3 by the help of Eq. (1) decreases from 1.99 to 1.67 by increasing the temperature between 300 and 380 K. Previous studies related to MS or MIS structures have verified this result. For example, Karatas- et al [20] have shown the decrease of n from 2.024 to 1.108 between 150 and 400 K for Sn/p–Si Schottky diode. In addition, the decrease of n from 14.32 to 2.72 has been reported for Sn/rhodamine-101/p–Si Schottky structure ranging from 80 to 400 K [19]. In addition, the barrier height (fb) value of a structure can be calculated using I0 saturation current value determined from the intercept of ln I–V plot on I axis using 2 kT AA T ð2Þ fb ¼ ln I0 q where A is the diode area, A* is the Richardson constant equals to 110 A cm–2 K–2 for n–Si [21]. Fig. 5 shows the changes of fb values with temperature. The experimental values fb of Sn/MnHA/n–Si device range from 0.79 eV (300 K) to 0.99 eV (380 K). Akkılıc- et al. [22] reported the barrier height of Sn/n–Si Schottky diode as 0.63 eV after statistical analysis of 20 dots. The obtained fb value for Sn/MnHA/n–Si at 300 K is remarkably higher than the conventional Sn/n–Si Schottky diode. So it can be said that the
H(I)-I
1.00 Barrier Height (eV)
n¼
1.10
lnI-V 0.90
0.80
0.70 300
320
340 Temperature (K)
360
380
Fig. 5. Temperature dependence of the barrier height values obtained from I–V current and Cheung functions for MnHA/n–Si structure.
MnHA compound has strong effect on barrier height of the device. Since current transport across the MS interface is a temperature activated process, electrons at low temperatures are able to surmount the lower barriers and therefore, the current transport will be dominated by current flow through the patches of lower Schottky barrier height and a larger ideality factor. As the temperature increases, more and more electrons have sufficient energy to surmount the higher barrier [16,23–26]. Furthermore, one of the important electrical parameters of MS and MIS diodes is the series resistance (Rs), because the deflection from linearity at high voltages is shown the dependence on bulk
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resistivity. Cheung’s functions [27] have been used to obtain a precise series resistance Rs, n, and fb for the device. The Cheung’s method is achieved by using dV kT ¼ IRs þ n ð3Þ dðln IÞ q ð4Þ
and H(I) can be written as HðIÞ ¼ IRs þ nfb
ð5Þ
Fig. 6 shows the plots of dV/d ln I–I and H(I)–I obtained by using the Cheung functions only at 380 K, respectively. The Eq. (4) gives straight lines for the data in the downward-curvature region of forward bias I–V characteristics at all temperature between 300 and 380 K. The Rs and n values for the device from the slopes and y-axis intercepts of plots of dV/d ln I–I, respectively. The obtained n values with respect to temperature are presented in Fig. 4. The figure shows that the ideality factors obtained from both I–V method and Cheung’s method are in good agreement with each other and both of them decreases while temperature increases. However, the n values obtained from I–V plots, especially at lower temperatures, are lower than the values obtained from dV/d ln I–I plots. The difference between the values of the ideality factors can be attributed to the fact that the n values calculated from I–V plots are only under the effect of the interfacial properties and the ones obtained from dV/d ln I–I plots are under the effect of both the interfacial properties and the series resistance [17,28]. As a function of temperature, the experimental fb values were also obtained from H(I)–I plots using Eq. (5) from 0.75 eV (300 K) to 1.08 eV (380 K). The results are in reasonable agreement with the ones obtained from ln I–V curves. Furthermore, as shown in Fig. 7, the series resistance vs. temperature plots from Eqs. (3 and 4) are in good agreement with each other. Both of them decreases with the decrease in temperature. The Rs values were calculated in the range from 385 O (300 K) to 156 O (380 K) and 544 O to 138 O using dV/d ln I– I and H(I)–I plots, respectively. The close results of Rs are shown the consistency of the Cheung’s method. Aydogan et al [29] have ˘ shown that the Rs values of polyaniline/p–Si/Al structure obtained from dV/d lnI–I and H(I)–I plots decrease from 420 to 101 O and 483 to 108 O, respectively, between 77 and 300 K. In addition, 0.50
1.80
500
Series resistance (Ω)
kT I HðIÞ ¼ Vn ln q AA T 2
600
H(I)-I
400
300 dV/d lnI -I
200
100 300
320
340
360
380
Temperature (K) Fig. 7. Temperature dependence of the series resistance values obtained from I–V current for MnHA/n–Si structure.
S- . Karatas- et al. [20] have reported the decrease of Rs values of Sn/p–Si from 66 to 34 O and from 77 to 51 O in the temperature range 150–400 K. The increase of Rs with the fall of temperature can be attributed to the increase of n and lack of free carrier concentration at low temperatures [30].
4. Conclusion A Sn/organometal compound/n–Si/Au structure was fabricated by forming a thin film of a novel Mn hexaamide (MnHA) compound with C27H21N9O6MnCl2 1/2H2O molecular formula on n–Si. It was seen that the structure has a rectifying behavior. The characteristic parameters such as ideality factor, barrier height and series resistance were calculated over room temperatures between 300 and 380 K. It was observed that the barrier height value increased and the ideality factor and series resistance values decreased while the temperature increased.
0.40
References
0.30 1.40
H (I) (V)
dV/d lnI (V)
1.60
0.20
1.20 0.10
0.00 0.00E+000 4.00E-004
8.00E-004 Current (A)
1.20E-003
1.00 1.60E-003
Fig. 6. Experimental plots of dV/d lnI–I and H(I)–I obtained from Eqs. 3 and 4 for MnHA/n-Si structure at 380 K.
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