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n-Si heterojunction diode under dark and illumination
ARTICLE IN PRESS
Physica B 388 (2007) 226–229 www.elsevier.com/locate/physb
The current–voltage characteristics of FSS/n-Si heterojunction diode under dark and illumination Fahrettin Yakuphanoglu Department of Physics, Faculty of Arts and Sciences, Firat University, 23119, Elazig, Turkey Received 27 January 2006; received in revised form 22 May 2006; accepted 29 May 2006
Abstract The current–voltage characteristics of the organic/inorganic fluorescein sodium salt FSS/n-Si heterojunction diode under dark and illumination have been investigated. The diode parameters such as barrier height and ideality factor were determined. The ideality factor n value at different temperatures was found to be in range from 6.30 to 4.56. These values suggest that the FSS/n-Si heterojunction diode is a non-ideal contact. A space charge limited-conduction (SCLC) mechanism, where current increases superlinearly, i.e, I / V m42 , takes place in the diode. The photocurrent properties of the diode under various illuminations have been investigated. The photocurrent in the reverse direction is strongly increased by photo-illumination. r 2006 Elsevier B.V. All rights reserved. PACS: 73.40.c; 85.30.Fg Keywords: Heterojunction diode; Space charge limited conduction; Photocurrent
1. Introduction Organic semiconductors have obtained considerable interest in the fields of electronic and photonic devices due to a wide range of applications as low-cost, large-area and disposable or throwaway electronics on thin and flexible substrates. In particular, organic photodiodes have been improved to a level that is sufficient for many applications [1–7]. The electronic and optoelectronic devices require new functional materials with special optical and electrical properties. Such properties are determined by a complex combination of many physical factors as well as chemical nature of starting material. Organic semiconducting layers are one of such class of prospective thin layer materials. Their low-cost production is very simple using recent high vacuum evaporation and spin coating technology. Organic semiconductors can be functionalized to obtain the specific optical and electrical properties [8–12]. Hence, organic semiconductors have been used to fabricate inorganic/organic p–n diode and organic photovoltaic devices [13,14]. E-mail address: [email protected]. 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.05.430
On the other hand, there are various models such as Schottky, Poole–Frenkel and space charge limited-conduction (SCLC) mechanisms to explain charge transport mechanisms in various materials [15]. The electronic properties of semiconductor materials are strongly affected by the presence of carrier trapping centers in forbidden band gap. The space charge limited-current technique is used to explain electrical properties of semiconductor and insulator materials, and in this model, current shows a power-law dependence on applied voltage, I / V m [16]. In previous study, we investigate the electrical conductivity and optical properties of fluorescein sodium salt (FSS), which is a p-type organic semiconductor [17]. The aim of present study is to obtain the new organic heterojunction diode and explain current–voltage (I–V) characteristics under dark and illumination. 2. Experimental n-type Si (1 0 0) with a doping concentration of 4–7 1014 cm3 is used as n-type semiconductor with resistivity of 6.5–10.5 O-cm. In order to remove the native oxide on surface on n-Si, the substrate was etched by HF
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227
Scheme 1. The molecular structure of semiconductor fluorescein sodium salt. Fig. 2. The dark I–V curves of the FSS/n-Si diode at different temperatures.
Fig. 1. Schematic structure of the FSS/n-Si diode.
and then was rinsed in deionized water using an ultrasonic bath for 10–15 min and finally was chemically cleaned according to method based on successive baths of methanol and acetone. The chemical structure of the FSS was given in Scheme 1 [17]. The solution of the FSS was prepared in methanol. The solution of the FSS was homogenized for 1 h by mixing with rotation before the deposition. Then, the film of the FSS was prepared by casting the solution on n-Si with subsequent drying [18]. The standard geometry of the FSS/n-Si diode is the sandwich, as shown in Fig. 1. The I–V characteristics of the diode under dark and illumination were measured using a KEITHLEY 2400 sourcemeter. The light source consisting of a halogen lamp was used for I–V measurements under illumination. The I–V characteristics in the dark were also measured at different temperatures, i.e. from 298 to 343 K. 3. Results and discussion
region between n-Si and FSS semiconductors. From these measurements, information about the junction properties such as ideality factor, barrier height and the reverse saturation current of the FSS/n-Si is provided. The FSS/nSi device indicates organic/inorganic heterojunction behavior. This junction behaves like a Schottky contact or a metal/semiconductor contact at low voltages [19]. The I–V characteristic of the diode can be analyzed using the ideal diode [19], qV qV I ¼ I 0 exp 1 exp , (1) nkT nkT where n is the ideality factor, V the voltage drop across the rectifying barrier and I0 the saturation current. The saturation current according to thermionic emission mechanism can be expressed as [20] qfB 2 I 0 ¼ A A T exp , (2) kT where A is the Richardson constant, A the contact area, fB the barrier height and k the Boltzmann constant. The forward I–V curves at different temperatures are shown in Fig. 3 and I0 value was determined from Fig. 3. The temperature dependence of the I0 is shown in Fig. 4. The fB and A values were calculated from Fig. 4 and were found to be 0.11 eV and 3.42 109 A/cm2 K2, respectively. As seen in Fig. 3, at low voltages, the current increases exponentially. Thus, the Eq. (1) is simplified as qV , (3) I ¼ I 0 exp nkT
3.1. The dark I–V characteristics of the FSS/n-Si diode
where n is the ideality factor which can be written as
The I–V curves of the FSS/n-Si diode at different temperatures are shown in Fig. 2. The diode exhibits a rectifying effect. The current increases with increasing temperature. This suggests that the diode has a negative resistance temperature coefficient. At low voltages, there is an exponential increase in the forward current with applied voltage. This indicates the formation of the depletion
n¼
q dV . kT d ln I
(4)
The n values at different temperatures were determined and were found to be in range from 6.30 to 4.56. The n value decreases with increasing temperature and the FSS/nSi diode has large n values. Deviation of n from unity may be attributed to either recombination of electrons and holes
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F. Yakuphanoglu / Physica B 388 (2007) 226–229
Fig. 5. The photo-I–V curves of the diode under different illuminations of the FSS/n-Si diode.
Fig. 3. The forward current curves of the FSS/n-Si diode at different temperatures.
light is from FSS side. The photocurrent increases with reverse bias voltage and the photocurrent effect was the most intense under illumination of 1550 lux. Photogenerated carriers are the holes and electrons. The increase in the photocurrent is due to the drift velocity of photogenerated electrons and holes in FSS. The photocurrent in the reverse direction is strongly increased by photoillumination. This behavior yields useful information on the electron–hole pairs, which were effectively generated in the junction by incident photons. The photocurrent is higher than the dark current at the same reverse bias. This suggests that the light generates carrier-contributing photocurrent due to the production of electron–hole pairs as a result of the light absorption. The generation of photoelectrons is due to electron transfer from n-Si into FSS through the potential barrier at the interface. This is a result of a difference in electron affinities between the two semiconductors. The generated electrons are swept towards the Si along the potential barrier at the interface due to the influence of the electric field applied, whereas, holes are accelerated towards the FSS.
Fig. 4. ln(I0/T2) vs. 1000/T plot of the FSS/n-Si diode.
in the depletion region, and/or the increase of the diffusion current due to increasing the applied voltage [21]. The obtained n values suggest that the FSS/n-Si heterojunction diode is a non-ideal contact. The current increases exponentially when the applied voltage is applied gradually in the positive direction. But, at large positive voltages, the current is deviated from the exponential trend. This results from a voltage drop across a series resistance associated with the neutral region of semiconductors [21]. 3.2. The effect of illumination on I–V characteristics of the FSS/n-Si diode The-I–V curves of the diode under different illuminations are shown in Fig. 5. The direction of the incidence of
3.3. The space charge limited-current mechanism of the FSS/n-Si diode At higher voltages (0:65pV p2), a space charge injection can proceed through the FSS/n-Si heterojunction from the n-Si to the organic layer, and in this case, the current is called the space charge limited current (SCLC) [16]. The detail information about transport mechanism of the diode can be provided by current–voltage characteristics. The I–V curves at different temperatures are shown in Fig. 6. The current changes in the form of I / V m . The exponent m values were calculated from the slope of the Fig. 6. At lower voltages, the slope, m of ln I–ln V plots is approximately unity, while at higher voltages, the slope changes between 3.13 and 2.80.
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4. Conclusions The current–voltage characteristics of the FSS/n-Si under dark and illumination have been investigated. The obtained results suggest that the FSS/n-Si heterojunction diode is a non-ideal contact. The ideality factor n value at different temperatures was found to be in range from 6.30 to 4.56. The space charge limited-conduction (SCLC) mechanism in FSS/n-Si diode takes place. The photocurrent in the reverse direction is strongly increased by photo-illumination. Acknowledgement Fig. 6. The SCLC curves of the FSS/n-Si diode at different temperatures.
The m values indicate that at high-voltage region, the carrier transport may be dominated by a SCLC [16], where current increases superlinearly, i.e, I / V m42 suggesting that the traps are exponentially distributed. The m value decreases with increasing temperature. This confirms a SCLC model controlled by an exponential distribution of traps. The power-law dependence between current and voltage is characterized by space charge limited currents. Therefore, the current is expressed as [16] J ¼ e m NV
o eP0 kT t
l
V lþ1 d 2lþ1
,
(5)
where e is the dielectric constant of the semiconductor, eo the permitivity in the free space, e the electronic charge, m the mobility of carrier charges, Nv the effective density of states in valence band edge, d the thickness of the sample, l (l ¼ m 1) the parameter given by l ¼ T t =T and Tt the a characteristic temperature of the exponential distribution of the traps [16]. The total density of trap states is given by N t ¼ P0 kT t .
(6)
P0 is the trap density per unit energy range at the valence band. The exponential trapping distribution is given by [16]
This work was partially supported by The Management Unit of Scientific Research Projects of Firat University (FU¨BAP) under Project 1230. The authors are grateful to The Management Unit of Scientific Research Projects of Firat University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
[16]
PðEÞ ¼ P0 expðE=kT t Þ
(7)
where E is the energy above the valence band edge. The Tt values were calculated using the obtained m values and were found to be in range from 635.6 to 618.4 K. The obtained Tt values decrease with increasing temperature. This suggests that the total trap density decreases as the temperature increases.
[17] [18] [19] [20] [21]
C.W. Tang, Appl. Phys. Lett. 48 (1986) 183. H. Hoppe, N.S. Sariciftci, J. Mater. Res. 19 (2004) 1924. J. Rostalski, D. Meissner, Sol. Energy Mater. Sol. Cells 63 (2000) 37. D. Gebeyehu, B. Maennig, J. Drechsel, K. Leo, M. Pfeiffer, Sol. Energy Mater. Sol. Cells 79 (2003) 81. D. Wo¨hrle, et al., J. Mater. Chem. 5 (1995) 1819. P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93 (2003) 3693. P. Schilinsky, C. Waldauf, J. Hauch, C.J. Brabec, Thin Solid Films 451 (2004) 105. D. Whorle, D. Meissner, Adv. Mater. 2 (1991) 129. J.B. Whitlock, P. Panayotatos, G.D. Sharma, G.R. Birds, Opt. Eng. 32 (1993) 1921. S. Karg, M. Meier, W. Riess, J. Appl. Phys. 82 (1997) 1951. H. Bottcher, J. Prakt. Chem. 334 (1992) 14. J. Simon, J.J. Andre, Molecular Semiconductors, Springer, Berlin, 1985. S. Gunster, S. Siebentritt, D. Meissner, Mol. Cryst. Liq. Cryst. 230 (1993) 351. K. Takahashi, S.I. Nakatani, T. Matsuda, H. Nambu, T. Kormura, K. Murata, Chem. Lett. 11 (1994) 2001. K.C. Kao, W. Hwang, Electrical Transport in Solids with Particular Reference to Organic Semiconductors, Pergamon Pres Ltd., Headington Hill Hall, Oxford, England, 1981. M.A. Lampert, P. Mark, Current Injection in Solids, Academic Press, New York, London, 1970. F. Yakuphanoglu, M. Sekerci, E. Evin, Physica B, 382 (2006) 21. F. Yakuphanoglu, M. Arslan, Opt. Mater. 27 (2004) 29. S.M. Sze, Physics of Semiconductor Devices, second ed., Wiley, New York, 1981. A. Bar-Lev, Semiconductors and electronic Devices, second ed., Prentice Hall International, Englewood Cliffs, NJ, 1984. E.H. Rhoderick, Metal Semiconductor Contacts, Oxford University Press, Oxford, 1978.
Physica B 388 (2007) 226–229 www.elsevier.com/locate/physb
The current–voltage characteristics of FSS/n-Si heterojunction diode under dark and illumination Fahrettin Yakuphanoglu Department of Physics, Faculty of Arts and Sciences, Firat University, 23119, Elazig, Turkey Received 27 January 2006; received in revised form 22 May 2006; accepted 29 May 2006
Abstract The current–voltage characteristics of the organic/inorganic fluorescein sodium salt FSS/n-Si heterojunction diode under dark and illumination have been investigated. The diode parameters such as barrier height and ideality factor were determined. The ideality factor n value at different temperatures was found to be in range from 6.30 to 4.56. These values suggest that the FSS/n-Si heterojunction diode is a non-ideal contact. A space charge limited-conduction (SCLC) mechanism, where current increases superlinearly, i.e, I / V m42 , takes place in the diode. The photocurrent properties of the diode under various illuminations have been investigated. The photocurrent in the reverse direction is strongly increased by photo-illumination. r 2006 Elsevier B.V. All rights reserved. PACS: 73.40.c; 85.30.Fg Keywords: Heterojunction diode; Space charge limited conduction; Photocurrent
1. Introduction Organic semiconductors have obtained considerable interest in the fields of electronic and photonic devices due to a wide range of applications as low-cost, large-area and disposable or throwaway electronics on thin and flexible substrates. In particular, organic photodiodes have been improved to a level that is sufficient for many applications [1–7]. The electronic and optoelectronic devices require new functional materials with special optical and electrical properties. Such properties are determined by a complex combination of many physical factors as well as chemical nature of starting material. Organic semiconducting layers are one of such class of prospective thin layer materials. Their low-cost production is very simple using recent high vacuum evaporation and spin coating technology. Organic semiconductors can be functionalized to obtain the specific optical and electrical properties [8–12]. Hence, organic semiconductors have been used to fabricate inorganic/organic p–n diode and organic photovoltaic devices [13,14]. E-mail address: [email protected]. 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.05.430
On the other hand, there are various models such as Schottky, Poole–Frenkel and space charge limited-conduction (SCLC) mechanisms to explain charge transport mechanisms in various materials [15]. The electronic properties of semiconductor materials are strongly affected by the presence of carrier trapping centers in forbidden band gap. The space charge limited-current technique is used to explain electrical properties of semiconductor and insulator materials, and in this model, current shows a power-law dependence on applied voltage, I / V m [16]. In previous study, we investigate the electrical conductivity and optical properties of fluorescein sodium salt (FSS), which is a p-type organic semiconductor [17]. The aim of present study is to obtain the new organic heterojunction diode and explain current–voltage (I–V) characteristics under dark and illumination. 2. Experimental n-type Si (1 0 0) with a doping concentration of 4–7 1014 cm3 is used as n-type semiconductor with resistivity of 6.5–10.5 O-cm. In order to remove the native oxide on surface on n-Si, the substrate was etched by HF
ARTICLE IN PRESS F. Yakuphanoglu / Physica B 388 (2007) 226–229
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Scheme 1. The molecular structure of semiconductor fluorescein sodium salt. Fig. 2. The dark I–V curves of the FSS/n-Si diode at different temperatures.
Fig. 1. Schematic structure of the FSS/n-Si diode.
and then was rinsed in deionized water using an ultrasonic bath for 10–15 min and finally was chemically cleaned according to method based on successive baths of methanol and acetone. The chemical structure of the FSS was given in Scheme 1 [17]. The solution of the FSS was prepared in methanol. The solution of the FSS was homogenized for 1 h by mixing with rotation before the deposition. Then, the film of the FSS was prepared by casting the solution on n-Si with subsequent drying [18]. The standard geometry of the FSS/n-Si diode is the sandwich, as shown in Fig. 1. The I–V characteristics of the diode under dark and illumination were measured using a KEITHLEY 2400 sourcemeter. The light source consisting of a halogen lamp was used for I–V measurements under illumination. The I–V characteristics in the dark were also measured at different temperatures, i.e. from 298 to 343 K. 3. Results and discussion
region between n-Si and FSS semiconductors. From these measurements, information about the junction properties such as ideality factor, barrier height and the reverse saturation current of the FSS/n-Si is provided. The FSS/nSi device indicates organic/inorganic heterojunction behavior. This junction behaves like a Schottky contact or a metal/semiconductor contact at low voltages [19]. The I–V characteristic of the diode can be analyzed using the ideal diode [19], qV qV I ¼ I 0 exp 1 exp , (1) nkT nkT where n is the ideality factor, V the voltage drop across the rectifying barrier and I0 the saturation current. The saturation current according to thermionic emission mechanism can be expressed as [20] qfB 2 I 0 ¼ A A T exp , (2) kT where A is the Richardson constant, A the contact area, fB the barrier height and k the Boltzmann constant. The forward I–V curves at different temperatures are shown in Fig. 3 and I0 value was determined from Fig. 3. The temperature dependence of the I0 is shown in Fig. 4. The fB and A values were calculated from Fig. 4 and were found to be 0.11 eV and 3.42 109 A/cm2 K2, respectively. As seen in Fig. 3, at low voltages, the current increases exponentially. Thus, the Eq. (1) is simplified as qV , (3) I ¼ I 0 exp nkT
3.1. The dark I–V characteristics of the FSS/n-Si diode
where n is the ideality factor which can be written as
The I–V curves of the FSS/n-Si diode at different temperatures are shown in Fig. 2. The diode exhibits a rectifying effect. The current increases with increasing temperature. This suggests that the diode has a negative resistance temperature coefficient. At low voltages, there is an exponential increase in the forward current with applied voltage. This indicates the formation of the depletion
n¼
q dV . kT d ln I
(4)
The n values at different temperatures were determined and were found to be in range from 6.30 to 4.56. The n value decreases with increasing temperature and the FSS/nSi diode has large n values. Deviation of n from unity may be attributed to either recombination of electrons and holes
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Fig. 5. The photo-I–V curves of the diode under different illuminations of the FSS/n-Si diode.
Fig. 3. The forward current curves of the FSS/n-Si diode at different temperatures.
light is from FSS side. The photocurrent increases with reverse bias voltage and the photocurrent effect was the most intense under illumination of 1550 lux. Photogenerated carriers are the holes and electrons. The increase in the photocurrent is due to the drift velocity of photogenerated electrons and holes in FSS. The photocurrent in the reverse direction is strongly increased by photoillumination. This behavior yields useful information on the electron–hole pairs, which were effectively generated in the junction by incident photons. The photocurrent is higher than the dark current at the same reverse bias. This suggests that the light generates carrier-contributing photocurrent due to the production of electron–hole pairs as a result of the light absorption. The generation of photoelectrons is due to electron transfer from n-Si into FSS through the potential barrier at the interface. This is a result of a difference in electron affinities between the two semiconductors. The generated electrons are swept towards the Si along the potential barrier at the interface due to the influence of the electric field applied, whereas, holes are accelerated towards the FSS.
Fig. 4. ln(I0/T2) vs. 1000/T plot of the FSS/n-Si diode.
in the depletion region, and/or the increase of the diffusion current due to increasing the applied voltage [21]. The obtained n values suggest that the FSS/n-Si heterojunction diode is a non-ideal contact. The current increases exponentially when the applied voltage is applied gradually in the positive direction. But, at large positive voltages, the current is deviated from the exponential trend. This results from a voltage drop across a series resistance associated with the neutral region of semiconductors [21]. 3.2. The effect of illumination on I–V characteristics of the FSS/n-Si diode The-I–V curves of the diode under different illuminations are shown in Fig. 5. The direction of the incidence of
3.3. The space charge limited-current mechanism of the FSS/n-Si diode At higher voltages (0:65pV p2), a space charge injection can proceed through the FSS/n-Si heterojunction from the n-Si to the organic layer, and in this case, the current is called the space charge limited current (SCLC) [16]. The detail information about transport mechanism of the diode can be provided by current–voltage characteristics. The I–V curves at different temperatures are shown in Fig. 6. The current changes in the form of I / V m . The exponent m values were calculated from the slope of the Fig. 6. At lower voltages, the slope, m of ln I–ln V plots is approximately unity, while at higher voltages, the slope changes between 3.13 and 2.80.
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229
4. Conclusions The current–voltage characteristics of the FSS/n-Si under dark and illumination have been investigated. The obtained results suggest that the FSS/n-Si heterojunction diode is a non-ideal contact. The ideality factor n value at different temperatures was found to be in range from 6.30 to 4.56. The space charge limited-conduction (SCLC) mechanism in FSS/n-Si diode takes place. The photocurrent in the reverse direction is strongly increased by photo-illumination. Acknowledgement Fig. 6. The SCLC curves of the FSS/n-Si diode at different temperatures.
The m values indicate that at high-voltage region, the carrier transport may be dominated by a SCLC [16], where current increases superlinearly, i.e, I / V m42 suggesting that the traps are exponentially distributed. The m value decreases with increasing temperature. This confirms a SCLC model controlled by an exponential distribution of traps. The power-law dependence between current and voltage is characterized by space charge limited currents. Therefore, the current is expressed as [16] J ¼ e m NV
o eP0 kT t
l
V lþ1 d 2lþ1
,
(5)
where e is the dielectric constant of the semiconductor, eo the permitivity in the free space, e the electronic charge, m the mobility of carrier charges, Nv the effective density of states in valence band edge, d the thickness of the sample, l (l ¼ m 1) the parameter given by l ¼ T t =T and Tt the a characteristic temperature of the exponential distribution of the traps [16]. The total density of trap states is given by N t ¼ P0 kT t .
(6)
P0 is the trap density per unit energy range at the valence band. The exponential trapping distribution is given by [16]
This work was partially supported by The Management Unit of Scientific Research Projects of Firat University (FU¨BAP) under Project 1230. The authors are grateful to The Management Unit of Scientific Research Projects of Firat University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
[16]
PðEÞ ¼ P0 expðE=kT t Þ
(7)
where E is the energy above the valence band edge. The Tt values were calculated using the obtained m values and were found to be in range from 635.6 to 618.4 K. The obtained Tt values decrease with increasing temperature. This suggests that the total trap density decreases as the temperature increases.
[17] [18] [19] [20] [21]
C.W. Tang, Appl. Phys. Lett. 48 (1986) 183. H. Hoppe, N.S. Sariciftci, J. Mater. Res. 19 (2004) 1924. J. Rostalski, D. Meissner, Sol. Energy Mater. Sol. Cells 63 (2000) 37. D. Gebeyehu, B. Maennig, J. Drechsel, K. Leo, M. Pfeiffer, Sol. Energy Mater. Sol. Cells 79 (2003) 81. D. Wo¨hrle, et al., J. Mater. Chem. 5 (1995) 1819. P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93 (2003) 3693. P. Schilinsky, C. Waldauf, J. Hauch, C.J. Brabec, Thin Solid Films 451 (2004) 105. D. Whorle, D. Meissner, Adv. Mater. 2 (1991) 129. J.B. Whitlock, P. Panayotatos, G.D. Sharma, G.R. Birds, Opt. Eng. 32 (1993) 1921. S. Karg, M. Meier, W. Riess, J. Appl. Phys. 82 (1997) 1951. H. Bottcher, J. Prakt. Chem. 334 (1992) 14. J. Simon, J.J. Andre, Molecular Semiconductors, Springer, Berlin, 1985. S. Gunster, S. Siebentritt, D. Meissner, Mol. Cryst. Liq. Cryst. 230 (1993) 351. K. Takahashi, S.I. Nakatani, T. Matsuda, H. Nambu, T. Kormura, K. Murata, Chem. Lett. 11 (1994) 2001. K.C. Kao, W. Hwang, Electrical Transport in Solids with Particular Reference to Organic Semiconductors, Pergamon Pres Ltd., Headington Hill Hall, Oxford, England, 1981. M.A. Lampert, P. Mark, Current Injection in Solids, Academic Press, New York, London, 1970. F. Yakuphanoglu, M. Sekerci, E. Evin, Physica B, 382 (2006) 21. F. Yakuphanoglu, M. Arslan, Opt. Mater. 27 (2004) 29. S.M. Sze, Physics of Semiconductor Devices, second ed., Wiley, New York, 1981. A. Bar-Lev, Semiconductors and electronic Devices, second ed., Prentice Hall International, Englewood Cliffs, NJ, 1984. E.H. Rhoderick, Metal Semiconductor Contacts, Oxford University Press, Oxford, 1978.