p-Si Schottky diode

Journal of Alloys and Compounds 577 (2013) 30–36

Contents lists available at SciVerse ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom

Electrical properties of Au/perylene-monoimide/p-Si Schottky diode Ö.F. Yüksel a,⇑, N. Tug˘luog˘lu b, B. Gülveren a, H. Sß afak a, M. Kusß c,d a

Department of Physics, Faculty of Science, Selçuk University, Campus, 42075 Konya, Turkey Department of Research and Development, Sarayköy Nuclear Research and Training Center, 06983 Saray, Ankara, Turkey c Department of Chemical Engineering, Selçuk University, Campus, 42075 Konya, Turkey d Advanced Technology Research and Application Center, Selçuk University, Campus, 42075 Konya, Turkey b

a r t i c l e

i n f o

Article history: Received 20 November 2012 Received in revised form 19 April 2013 Accepted 22 April 2013 Available online 30 April 2013 Keywords: Organic compounds Electronic materials Semiconductors Thin films Schottky diode

a b s t r a c t In this work, we have fabricated an Au/perylene-monoimide (PMI)/p-Si Schottky barrier diode. We have investigated how electrical and interface characteristics like current–voltage characteristics (I–V), ideality factor (n), barrier height (UB) and series resistance (Rs) of diode change with temperature over a wide range of 100–300 K. Detailed analysis on the electrical properties of structure is performed by assuming the standard thermionic emission (TE) model. Possible mechanisms such as image force lowering, generation–recombination processes and interface states which cause deviations of n values from the unity have been discussed. Cheung–Cheung method is also employed to analysis the current–voltage characteristics and a good agreement is observed between the results. It is shown that the electronic properties of Schottky diode are very sensitive to the modification of perylene-monoimide (PMI) interlayer organic material and also to the temperature. The ideality factor was found to decrease and the barrier height to increase with increasing temperature. The temperature dependence of barrier height shows that the Schottky barrier height is inhomogeneous in nature at the interface. Such inhomogeneous behavior was explained on the basis of thermionic emission mechanism by assuming the existence of a Gaussian distribution of barrier heights. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction The metal–semiconductor (MS) contacts are still considered to be promising structures for many technological applications in the fields of electronics and optoelectronics due to their flexibility in control the electronic properties. Structures which are fabricated by inserting an organic material between the metal and semiconductor are particularly interesting. The reasons of this ever-mounting interest may be the diverse nature of their properties and also their performance in designing various classes of semiconductor device such as chemical sensors [1,2], light emitting diodes [3–5], thin film transistors [6] and solar cells [7] as compared to their inorganic counterparts. I–V characteristics of Schottky diodes [8,9] give detailed information about the charge transport process and the nature of barrier formed in the metal–semiconductor interface. Forming an organic layer on the semiconductor substrate modifies the electronic properties of MS contacts. Schottky barrier is manipulated by the energy difference between the work function of metal and the electron affinity of organic material. The chemical or structural characteristics of the organic material play crucial role in the device performance. A great number of works have been performed considering the electronic properties of organic semiconductors ⇑ Corresponding author. Tel.: +90 332 2232590; fax: +90 332 2412499. E-mail address: [email protected] (Ö.F. Yüksel). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.04.157

based on the Schottky diodes. For example, the electronic properties of GaAs Schottky diode by poly(3-4-ethylenedioxithiophene)block-poly (ethylene glycol) organic layer have been analyzed by Aydin et al. [10]. They have discussed the electrical characteristics of Au/PEDOT/n-GaAs/In contact, where the metal/semiconductor interface is modified with a thin layer of poly(3,4-ethylenedioxithiophene)-block-poly(ethylene glycol), 1 wt.% disp. in propylene carbonate (PEDOT) film. Farag et al. [11] have investigated the stability and possibility of organic-on-inorganic semiconductor contact barrier diode for the use in barrier modification of Si MS diodes. PTCDA is used as an interlayer for the modification of Ag/ n-GaAs(1 0 0) Schottky contacts by Kampen et al. [12]. Temperature is particularly important factor that influences the basic characteristic properties of a Schottky contact. For example, temperature dependence of electronic parameters of organic schottky diode based on fluorescein sodium salt has been studied by Yahia et al. [13] Hudait et al. [14] have studied the current transport characteristics of Au/n-GaAs Schottky diodes on Ge in the temperature range of 80–300 K. Lin [15] has investigated the temperature-dependent electrical properties of the n-type GaN Schottky diodes. They showed that the tunneling behavior is responsible for decreasing the barrier height and increasing the ideality factor with temperature in frame of thermionic emission model. The effect of temperature on a Schottky diode characteristics have been examined by P´erez et al. [16].

Ö.F. Yüksel et al. / Journal of Alloys and Compounds 577 (2013) 30–36

High mechanical flexibility, high current density, versatility of chemical structure and low fabrication costs facilitated by the techniques used for preparing the materials are main advantages of fabricating the microelectronic device with organic polymers. The performance of the device can be changed by selecting compatible organic materials and developing an effective route for their integration. Among the different organic compounds, perylene and its derivatives have finding more and more application such in photovoltaic solar cells [16–22], laser dyes [23,24], organic light-emiting field-effect transistor [25], organic light emitting diodes [26–28] and electro photography and future device applications [29–34]. A detailed study has been performed by Yüksel et al. [29] for Au/perylene-monoimide/n-Si device and the current–voltage characteristic has been determined using the thermionic emission theory over a wide temperature range of 75–300 K. In this work, we perform a detailed analysis on the characteristic properties including the current–voltage (I–V) and capacitance– voltage (C–V) variation. The ideality factor, barrier height and series resistance of Au/PMI/p-Si diode structure have been determined. We have also discussed that how the parameters such as temperature change the device performance. The temperature dependent barrier characteristics of diode is discussed in the frame of thermionic emission theory with a Gaussian distribution of the barrier heights due to the barrier height inhomogeneities prevailing at the organic–inorganic semiconductor interface. The results are compared with those calculated by Cheung method [35]. Physical explanations for the deviation of ideal diode behavior have been given by incorporating some mechanisms such as image force lowering, generation–recombination processes and the interface states [36,38]. We have also discussed how the type of Si wafer (n or p type) affects the performance of Schottky device with PMI interface. 2. Experimental procedures In preparation of diode, we have used a p-type Si wafer with thickness 380 lm with (1 0 0) orientation and having dimensions of 1.5 cm  1.5 cm. (i) Before making contacts, the wafer was chemically cleaned using the Radio Corporation of America (RCA) cleaning procedure; to remove the native oxide layer, the wafer was dipped 10 min in NH3 + H2O2 + 6H2O then 10 min in HCl + H2O2 + 6H2O. Finally, it was immersed in diluted HF for 60 s. The wafer was rinsed in de-ionized water of resistivity 18.2 M X cm with ultrasonic cleaning in each step. Finally, the sample was dried by exposing the surfaces to high-purity nitrogen. (ii) An aluminum (99.999 purity) back contact with a thickness of 1500 Å was prepared using the thermal evaporation at a pressure of 5  106 Torr. A thermal annealing process has been performed to obtain a low resistivity back contact, at a pressure of 5  106 Torr at 500  106 for 3 min. (iii) Front surface of samples were coated with an organic PMI (Fig. 1a) film by spin coating with 1200 rpm for 60 s. (iv) By using a physical mask with 2 mm diameter, Au (99.99 purity) contacts were made on the PMI film by the thermal evaporation at 5  106 Torr, with a thickness of 1500 Å. The diode contact area was measured as 3.14 mm2. The schematic representation of the device is shown in Fig. 1b. The current– voltage (I–V) measurements are performed by a Keithley 2410 Source Meter at a wide temperature range of 100–300 K using an ARS Closed Cycle Cryostat Model DE202 AI and a Lake Shore model 331 temperature controller. The capacitance– voltage (C–V) measurement was performed at 700 kHz frequency using an HP 4192A LF Impedance Analyzer at room temperature in dark at a test signal of 50 m Vrms.

3. Results and discussion 3.1. Current–voltage characteristics of Au/PMI/p-Si diode For Au/PMI/p-Si diode, the current–voltage (I–V) characteristics can be analyzed in the frame of thermionic emission model (TE) using the following relation [39,40];

31

(a)

(b) Fig. 1. (a) Chemical representation of PMI, and (b) Cross sectional view of Au/PMI/ p-Si Schottky diode.

h qV i I ¼ I0 eðnkT Þ  1

ð1Þ

where I0 is the reverse saturation current given by

 qU  B0 I0 ¼ AA T 2 e  kT

ð2Þ 

A is the diode area, A is the Richardson constant, T is absolute temperature, k is the Boltzmann constant, n is the ideality factor and UB0 is the effective barrier height at zero bias. From Eq. (2), the experimental values of UB0 have been determined from the intercepts of forward bias ln I vs. V curve at each temperature. One can also evaluate the effective barrier height (UB0) values at zero bias by

UB0

kT AA T 2 ¼ ln q I0

! ð3Þ

The ideality factor (n) values have been calculated from the slope of the ln I vs. V curve as,

n¼

q dV kT dðlnðIÞÞ

ð4Þ

In Fig. 2, the temperature dependence of ln I vs. V between 100 and 300 K, for the device is shown. It is remarkable that diode displays weak voltage dependence of reverse bias which means that it has good rectification behavior for both low and high temperatures. Fig. 3 depicts the values of UB0 (indicated by open circles) and n (indicated by closed triangles) as a function of temperature. The experimental value of n is increased, while the values of UB0 is decreased with a decrease in temperature, as can be seen in Fig. 3. As temperature increases, the number of electrons which surmounted the high barrier height will increase which in turn enhance the dominant barrier height. Although this behavior contradicts with the ideal diode behavior (i.e. the zero barrier height increases as temperature decreases), it can be analyzed according to the Werner–Güttler model [14]. The experimental values of UB0 and n range from 0.219 eV and 7.42 (at 100 K) to 0.567 eV and 3.53 (at 300 K), respectively. The variation of UB0 and n with temperature is presented in Table 1. The ideality factor value for room temper-

Ö.F. Yüksel et al. / Journal of Alloys and Compounds 577 (2013) 30–36

10 -2

In Fig. 4, the variations of ideality factor, n with inverse temperature are shown. In fact, n = 1 for an ideal diode. However, it is observed that, n values differ from unity for all temperatures. A high ideality factor is observed at low temperatures. As temperature increases, n values decrease exponentially. While there is a sharp decrease in ideality values at low temperatures, it becomes very slow from 175 K to 300 K. Reasonable physical explanation for this behavior can be provided by taking into account T0 and tunneling effects [14,36–38]. Generation–recombination mechanism [14,36–38] can also be considered as the cause of high ideality factor values and the variation of them with temperature. The increase in ideality factor with decreasing temperature is known as T0 effect [32]. As shown in Fig. 4, n was found to be inversely proportional to temperature as:

Au/PMI/p-Si

10 -3

Current (A)

10 -4 10 -5 10 -6 10 -7

100 K 125 K 150 K 175 K 200 K 225 K 250 K 275 K 300 K

10 -8 10 -9 10 -10 -1.0

-0.5

0.0

0.5

nðTÞ ¼ n0 þ

1.0

Voltage (V) Fig. 2. Current–voltage characteristics of Au/PMI/p-Si.

0.6

8

0.5

7

0.4

6

0.3

5

0.2

4

0.1

3

0.0

0

100

200

300

Ideaity Factor

Barrier Height (eV)

Au/PMI/p-Si

2 400

Temperature (K) Fig. 3. Temperature dependence of ideality factor and barrier height of Au/PMI/pSi.

ature (n = 3.53) are found greater than those reported for Au/p-Si Schottky diode structures by Yeganeh et al [41], as 1.006–1.41, depending on the Au contact thickness. On the other hand, at room temperature, the value of barrier height (0.567 eV) is smaller than those reported at same study (0.623–0.791 eV). Table 1 Temperature dependent values of diode parameters determined by different methods for an Au/PMI/p-Si Schottky diode. T (K)

100 125 150 175 200 225 250 275 300

I–V

Y(I)–I

T0 T

ð5Þ

where T0 and n0 are constants which were calculated to be 549 K and 1.61, respectively. Fig. 5 depicts the variation of ln (I0/T2) against 1000/T or 1000/nT according to Eq. (2). The dependence of ln (I0/T2) vs. 1000/T is found to be non-linear in the temperature interval considered; however, the dependence of ln (I0/T2) vs. 1000/nT gives a straight line. The non-linearity of the conventional ln (I0/T2) vs. 1000/T is caused by the temperature dependence of the barrier height and the ideality factor. Activation energy and Richardson constant A values were determined from the slope and the intercept at ordinate of the linear region of the ln (I0/T2) vs. 1000/T or 1000/nT plot as 0.07138 eV and 1.319  107 A/cm2 K2 or 0.66 eV and 1.276  105 A/cm2 K2, respectively. The A values are much lower than the theoretical value of 32 A/cm2 K2 for p-Si [39]. This deviation in Richardson plots may be due to the inhomogeneous barrier and potential fluctuations at the metal/semiconductor interface; that is, the current through the contact will flow preferably through the lower barriers [42–45]. Schmitsdorf et al. [46,47] have used Tung’s theoretical approach and they obtained a linear correlation between the experimental ideality factors and the zero-bias Schottky barrier heights. Fig. 6 shows a plot of the experimental barrier height versus the ideality factor. The straight line in Fig. 6 is the least squares fit to the experimental data. As can be seen from Fig. 6, there is a linear relationship between the experimental effective barrier heights and the ideality factors of the Schottky contact that was explained by lateral inhomogeneities of the barrier heights in the Schottky diodes [46,47]. The extrapolation of the experimental barrier

8 Au/PMI/p-Si

Ideality Factor

32

6

y = 549.08x+1.6137

4

H(I)–I

n

/B0 (eV)

n

Rs (X)

/B (eV)

Rs (X)

2

7.42 5.75 5.47 4.65 4.36 4.21 4.08 3.81 3.53

0.219 0.272 0.314 0.363 0.406 0.447 0.486 0.525 0.567

7.38 5.79 5.46 4.69 4.35 4.18 4.06 3.84 3.53

466.0 351.7 300.7 274.8 251.0 235.3 218.3 205.9 188.2

0.217 0.268 0.310 0.358 0.401 0.440 0.478 0.517 0.560

466.6 351.9 299.9 274.9 252.4 232.3 218.9 205.9 189.0

0 0.002

0.004

0.006 -1

0.008

0.010

0.012

-1

T (K ) Fig. 4. Plot of ideality factor vs. T V.

1

of Au/PMI/p-Si Schottky diode obtained from I–

33

Ö.F. Yüksel et al. / Journal of Alloys and Compounds 577 (2013) 30–36

Cheng and Cheung [35] have also defined a new function H(I) as,

-21 Au/PMI/p-Si

HðIÞ ¼ V 

ln(I 0 /T 2 ) (A/K 2 )

-22

103/T

-23

-24 103/nT

-25

-26

0

2

4

6

3

3

10

8

12

-1

10 /T or 10 /nT (K ) Fig. 5. Richardson plots of ln (I0/T2) against 1000/T or 1000/nT of Au/PMI/p-Si Schottky diode.

Au/PMI/p-Si

Barrier Height (eV)

0.8

0.6

0.2

0.0 2

4

6

8

10

Ideality Factor Fig. 6. Zero-bias apparent barrier height vs. ideality factor of a typical Au/PMI/p-Si Schottky diode at different temperatures.

  1 ei es ðnðVÞ  1Þ  q d WD

ð10Þ

where d is the thickness of the organic interfacial layer, WD is the width of the space charge region, ei and es are the permittivities of perylene-monoimide and silicon, respectively. d has been calculated as 8.5 nm from the corrected C–V measurement at 1 MHz frequency using the formula Corg = eie0A/d where Corg is the capacitance of the interfacial layer (Corg = 10.4 nF), WD is calculated from the corrected 1/C2–V characteristic at 700 kHz as 237.87 nm, ei = 3.2e0 is taken from optical measurements [50] and es = 11.8e0 [39]. The energy of the interface states Ess with respect to the top of the valance band at the surface of p-type inorganic semiconductor is introduced by [34,48,49]:

Ess  Ev ¼ qðUe  VÞ

heights versus ideality factor plot to n = 1 gives a homogeneous barrier height of approximately 0.731 eV. Series resistance is also one of the key parameters that determine the electrical properties of the diode system. An alternate approach developed by Cheung and Cheung [35] can also be applied to calculate the Schottky diode parameters like ideality factor n, effective barrier height UB, series resistance Rs. In this approach, Eq. (1) can be rewritten by including Rs as,

ð11Þ

0.4

Au/PMI/p-Si

0.3

h qðVIRs Þ i I ¼ I0 eð nkT Þ



nkT I ln q AA T 2



Y(I) (V)

ð6Þ

where IRs is the voltage drop across the series resistance of diode. Substituting I0 in Eq. (6), the applied voltage can be given as,

V ¼ IRs þ nUB þ

0.2 100 K 125 K 150 K 175 K 200 K 225 K 250 K 275 K 300 K

ð7Þ 0.1

Eq. (7) can also be reorganized to obtain a straight line equation defining

YðIÞ ¼

ð9Þ

in order to express the effective barrier height UB and series resistance Rs. Figs. 7 and 8 show the variations of Y(I) and H(I) functions with I, respectively. Experimental results are in good agreement with Eqs. (8) and (9), i.e. there is a linear relation between I and Y(I) and H(I) functions. It is noticeable that increase of the temperature shifts the calculated values upward as given in both figures. Temperature dependence of Rs values is analyzed in Fig. 9. It is clear from the figure that Rs values decrease with the temperature which is consistent with the basic features of MS systems. Bond bending and de-trapping mechanisms may assist to obtain the physical explanations for this behavior. A closer look at the Table 1 can help to investigate the reliability of both techniques. The values which were calculated by two different techniques are in a good agreement with each other. The effective barrier height UB, and ideality factor, n are smaller, while series resistance Rs values are higher than those calculated for Au/ PMI/n-Si [29]. From our results, it can be said that the type of Si (n or p type) is particularly important factor that influences the electronic properties of a system. In Schottky diodes, for the cases which interface states are in equilibrium with the semiconductor, the interface states density Nss is given by [34,48,49]

Nss ¼ 0.4

  nkT I ¼ nUB þ IRs ln q AA T 2

dV ¼ aI þ b dðlnðIÞÞ

ð8Þ

Here, the slope is a = Rs and the intercept on the current axis is b ¼ nkT , respectively. q

0.0 0.0000

0.0002

0.0004

0.0006

0.0008

Current (A) Fig. 7. Y(I) vs I characteristics of Au/PMI/p-Si.

0.0010

34

Ö.F. Yüksel et al. / Journal of Alloys and Compounds 577 (2013) 30–36

4.2x10 12

Au/PMI/p-Si

Au/PMI/p-Si 2.2

4.0x10 12 3.8x10 12

H(I) (V)

2.0

N ss

3.6x10 12 3.4x10 12

100 K 125 K 150 K 175 K 200 K 225 K 250 K 275 K 300 K

1.8

1.6

0.0002

0.0004

0.0006

0.0008

3.2x10 12 3.0x10 12 2.8x10 12 0.44

0.0010

0.45

0.46

0.47

Current (A)

0.48

0.49

0.50

0.51

E ss -E v

Fig. 8. H(I) vs I characteristics of Au/PMI/p-Si.

Fig. 10. The dependence of Nss without taking into account Rs as a function of Ess  Ev .

500 Au/PMI/p-Si

a zero-bias standard deviation rs0. Their relation in a simple form, proposed by Chand and Kumar [52] can be given as,

 B0 ðT ¼ 0Þ  Uap ¼ U

qr2s0 2kT

ð12Þ



 1 qn 1 ¼c nap 2kT

300

200

100 50

100

150

200

250

300

350

Temperature (K) Fig. 9. Variation of Rs with temperature of Au/PMI/p-Si.

1.0 Au/PMI/p-Si

0.2

0.8 0.0 0.6 y = -0.0088x+0.6908

-0.2

0.4 -0.4

0.2

(n -1 -1)

where Ev is the valance band edge. The values of Nss obtained as a function of applied voltage were converted to a function of (Ess  Ev) using Eq. (11). The interface state density vs. energy distribution curve of the Au/PMI/p-Si Schottky structure at room temperature (300 K) is given in Fig. 10. From this figure, in the range from 0.514–Ev to 0.451–Ev eV, the values of the Nss was found to vary from 3.00  1012 eV1 cm2 to 4.12  1012 eV1 cm2, respectively. It is also clear from Fig. 10 that the Nss values decrease with increasing energy and the exponential growth of the interface state density from midgap towards to the top of the valence band of the p-Si. This confirms that the density of interface states changes with applied bias and each of applied biases corresponds to a position inside the silicon gap. These changes have been attributed to the decrease in recombination center and the existence of an interfacial layer between perylene-monoimide layer and p-Si semiconductor [49].

ð13Þ

where c and n quantify the voltage deformation of the barrier height distribution. Fig. 11 shows the experimental plot of Uap (indicated by open circles) and (n1 ap  1) (indicated by closed circles) vs.  B0 and rs0 have been found, respectively, from (2kT)1. Values of U the intercept and the slope of straight line portion of the Uap vs. 1/2kT plot using Eq. (12). The values obtained were  B0 ¼ 0:691 eV and rso = 0.0938 eV. By comparing U  B0 and rs0 U parameters, it is seen that the standard deviation is  13.5% of the mean barrier height. The standard deviation is a measure of the barrier homogeneity. The lower value of rs0 corresponds to a more homogeneous barrier height. Fitting ideality factor nap in Fig. 11 is a straight line that gives voltage coefficients c and n from the intercept and slope of the plot as c = 0.6558 and n = 0.0037.

Barrier Height (eV)

R S (Ω)

400

-0.6

0.0

-0.8

-0.2 y = -0.0037x-0.6558

3.2. The analysis of barrier height inhomogeneities The Schottky barrier height inhomogeneity of the Au/PMI/n-Si Schottky diode was considered by Gaussian distribution of the barrier heights [42–47,51,52]. Werner and Guttler [42] have proposed that the abnormal behavior can be explained by assuming a Gauss B0 (mean) and ian distribution of SBH with a mean barrier height U

-1.0

-0.4 0

20

40 -1

(2kT) (eV)

60

80

-1

Fig. 11. Zero-bias apparent barrier height and ideality factor vs 1/2kT curves of a Au/PMI/p-Si Schottky diode according to Gaussian distribution of the barrier heights.

35

Ö.F. Yüksel et al. / Journal of Alloys and Compounds 577 (2013) 30–36

ln

I0

T2



1 qrs0 2 qUB0  ¼ lnðAA Þ  2 kT 2kT

ð14Þ

12x10-9 Au/PMI/p-Si 10x10-9

3x10 18 8x10 -9

C -2 (F -2 )



4x10 18

2x10 18

6x10 -9

C (F)

This deviation of the experimental Richardson constant than the theoretical A value of p-Si in Richardson plots may be due to the inhomogeneous barrier and potential fluctuations at the metal/semiconductor interface; that is, the current through the contact will flow preferably through the lower barriers [42–47]. The ln (I0/T2) vs. 1000/T plot has demonstrated a nonlinearity behavior at low temperatures in Fig. 5. To explain these behavior, the combination of Eqs. (2) and (12) can be rewritten as

4x10 -9

Fig. 12 gives the best linear fitting for the modified Richardson plot of ln (I0/T2)  (1/2)(qrs0/kT)2 vs. 1000/T for the value of rs0 = 0.0938 eV. This plot should give a straight line with the slope directly yielding the zero-bias mean barrier height UB0 as 0.713 eV in 100–300 K and the intercept at the ordinate gives A as 155.42 A/ cm2 K2 in 100–300 K, respectively. Richardson constant value of A is close to the theoretical value (32 A/cm2 K2).

1x10 18 2x10 -9

0

-3

-2

-1

0

1

0

Voltage (V) Fig. 13. Variation of 1/C2–V and C–V of Au/PMI/p-Si.

3.3. Capacitance–voltage characteristics of Au/PMI/p-Si diode For MS diodes, the capacitance measurement is one of the most important methods in determining the rectifying effect of contact interfaces. C–V measurements can be made by superimposing an ac voltage on a dc bias. In this way, incremental charges of one sign are induced on the metal surface and charges of opposite sign in the semiconductor. To allow the charge to move in and out of the interface states in response to an applied signal, the measurement is performed at sufficiently high frequencies. For Schottky diodes, the capacitance–voltage relationship can be given as;

1 C

2

¼

!

2

es qNa A

2

V bi 

kT V q

 ð15Þ

where Na is the carrier concentration, es is the permittivity of semiconductor (es = 11.8), Vbi is the built in potential which can be expressed as,

V bi ¼ V 0 þ

kT q

ð16Þ

Then the barrier height can be given by,

UB ¼ V 0 þ V p þ

kT q

ð17Þ

Here Vp is the potential difference between the Fermi level and top of the valance band in the neutral region of p-Si and calculated by

Vp ¼

  kT NV ln q Na

ð18Þ

where NV is the density states in the valance band edge and it’s value is NV = 1.04  1019 cm3 for p-Si at room temperature. Fig. 13 shows the reverse-bias 1/C2–V plot obtained from C–V data of the Au/PMI/p-Si Schottky diode at 700 kHz. The 1/C2–V plot of Au/PMI/p-Si Schottky diode are linear, which indicates the formation of a Schottky junction [48,49,51]. The variation of C2 with V is linear whose slope can be used to calculate Na as 1.32  1016 cm3. From the linearity of the curve, one can also say that there is a uniform distribution of the carriers and a constant density in the interface state. The intercept of the line with V axis also gives V0 value (0.546 V) and hence Vp, Vbi and UB values for Au/PMI/p-Si Schottky diode can be obtained as 0.124, 0.572 and 0.672 eV, respectively. The value for barrier height found from the C–V measurement is smaller than those reported for Au/p-Si Schottky diodes in [41]. As can be noticed that, UB value calculated by using Eq. (16) is slightly higher than the one obtained from I–V method. This can be due to the image force barrier lowering effect. Also the interfacial oxide layer decomposition, non-uniformity of the interfacial layer thickness and distribution of charges may cause the discrepancy between the barrier height values obtained by I–V and C–V measurements. 4. Conclusion

2

Fig. 12. Modified Richardson ln ðI0 =T 2 Þ  ðq2 r2s0 =2k T 2 Þ vs 103/T plot for the Au/ PMI/p-Si Schottky diode according to Gaussian distribution of the barrier heights.

In this work, electrical properties of Au/PMI/p-Si Schottky diode have been investigated by I–V measurements on the basis of thermionic emission theory. Useful information has been obtained by the parameters like ideality factor, barrier height, serial resistance, Rs as a function of temperature. C–V characteristics of the diode are also analyzed at room temperature. It is observed that, the electrical parameters strongly depend on the organic layer between the metal–semiconductor contacts. Increase in the temperature causes an exponential decrease in the ideality factor. It is clearly seen that the values decrease more rapidly at low temperatures. However, temperature has a lower effect on the determination of ideality factor values at high temperatures. It can be also estimated that the values become closer to its ideal

36

Ö.F. Yüksel et al. / Journal of Alloys and Compounds 577 (2013) 30–36

value at very high temperatures. In contrast to ideality factor, it is observed that the potential barrier height values increase with the temperature and the variation of potential barrier height with temperature is linear and sensitively temperature dependent. Effect of temperature on the serial resistance values have been illustrated in the Fig. 7. To check accuracy of the results, Cheung–Cheung method is also employed to calculate the electrical properties. It can be said that the values are good agreement with those obtained from I–V method. The inhomogeneities can be described by the Gaussian distribution of the barrier heights with a mean barrier height of  B0 ¼ 0:691 eV and a standard deviation of rso = 0.0938 eV. U Our results show that organic interlayer is a particularly important factor that influences the electronic properties of a diode system, depending on the temperature and the type of Si wafer. Acknowledgement This work is supported by Selçuk University BAP office with the research Project number 11401115. References [1] G. Harsâanyi, Sensor Rev. 20 (2000) 98. [2] M. Ben Ali, R. Ben Châabanne, F. Vocanson, C. Dridi, N. Jaffrezic-Renault, R.C. Dridi, R. Lamartine, Thin Solid Films 495 (2006) 368. [3] J.H. Burroughes, D.D.C. Braley, A.R. Brown, R.N. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Nature 347 (1990) 539. [4] D. Troadec, G. Veriot, R. Anthony, A. Moliton, Synth. Met. 124 (2001) 49. [5] L.S. Hung, C.H. Chen, Mater. Sci. Eng. 39 (2002) 143. [6] R. Ben Chaabane, A. Ltaief, C. Dridi, H. rahmouni, A. Bouzizi, H. Ben Ouada, Thin Solid Films 427 (2003) 371. [7] C. Brabec, V. Dyakonov, J. Parisi, N.S. Saricifci, Organic Photovoltaics: Concepts and Realization, Springer, Berlin, 2003. [8] H. Safak, M. Sahin, Ö.F. Yüksel, Solid-State Electron. 46 (2002) 49. [9] K. Sato, Y. Yasumura, J. Appl. Phys. 58 (2009) 3655. [10] M.E. Aydın, M. Soylu, F. Yakuphanooglu, W.A. Farooq, Microelectron. Eng. 88 (2011) 867. [11] A.A.M. Farag, B. Gunduz, F. Yakuphanoglu, W.A. Farooq, Synth. Met. 160 (2010) 2559. [12] T.U. Kampen, S. Park, D.R.T. Zahn, Appl. Surf. Sci. 190 (2002) 461. [13] I.S. Yahia, A.A.M. Farag, F. Yakophanoglu, W.A. Farooq, Synth. Met. 161 (2011) 881.

[14] M.K. Hudait, P. Venkateswarlu, S.B. Krupanidhi, Solid State Electron. 45 (2001) 133. [15] Y.-J. Lin, Thin Solid Films 519 (2010) 829. [16] R. P´erez, Nicholas J. Pinto, Alan T. Johnson Jr., Synth. Met. 157 (2007) 231. _ Org. Electron. 9 [17] M. Kusß, Ö. Haklı, C. Zafer, C. Varlıklı, S. Demic, S. Özçelik, S. Içli, (2008) 757. [18] T. Edvinson, C. Li, N. Pschirer, J. Schöneboom, F. Eickemeyer, E. Sens, G. Boschloo, A. Hagfeldt, J. Phys. S. Chem. Lett. 111 (2007) 15137. [19] M.E. Williams, R.W. Murray, Chem. Mater. 10 (1998) 3603. [20] G. Tamizhmani, J.P. Dodelet, Chem. Mater. 3 (1991) 1046. [21] W. Lu, J.P. Gao, Z.Y. Wang, Y. Qi, G.G. Sacripamte, J.D. Duff, P.R. Sundararajan, Macromolecules 32 (1999) 8880. [22] R.A. Cormier, B.A. Gregg, J. Phys. Chem. B 101 (1997) 11004. [23] M. Sadrai, G.R. Bird, Opt. Commun. 51 (1984) 62. [24] G. Qian, Y. Yang, Z. Wang, C. Yang, Z. Yang, M. Wang, Chem. Phys. Lett. 368 (2003) 555. [25] H.-S. Seo, M.-J. An, Y. Zhang, J.-H. Choi, J. Phys. Chem. C 114 (2010) 6141. [26] P. Pösch, M. Thelakkat, H.-W. Schmidt, Synth. Met. 102 (1999) 1110. [27] B.A. Gregg, J. Phys. chem. 100 (1996) 852. [28] R.A. Carmier, B.A. Gregg, J. Phys. Chem. B 101 (1997) 11004. [29] Ö.F. Yuksel, M. Kusß, N. Sßimsßir, H. Sß afak, M. Sßahin, E. Yenel, J. Appl. Phys. 110 (2011) 024507. [30] H. Sßafak, M. Sßahin, Ö.F. Yüksel, Solid-State Electron. 46 (2002) 49. [31] Ö.F. Yüksel, Phys. B 404 (2009) 1993. [32] N. Tug˘luog˘lu, S. Karadeniz, M. Sßahin, H. Sßafak, Appl. Surf. Sci. 233 (2004) 320. [33] N. Tug˘luog˘lu, S. Karadeniz, M. S ß ahin, H. Sßafak, Semicond. Sci. Technol. 19 (2004) 1092. [34] S. Sönmezog˘lu, S. Sßenkul, R. Tasß, G. Çankaya, M. Can, Thin Solid Films 518 (2010) 4375. [35] K. Cheung, N.W. Cheung, Appl. Phys. Lett. 49 (1986) 85. [36] R.T. Tung, J.P. Sullivan, F. Schrey, Mater. Sci. Eng. B 14 (1992) 266. [37] C.R. Crowell, G.I. Roberts, J. Appl. Phys. 91 (2002) 245. [38] H.C. Card, E.H. Rhoderick, J. Phys. D 4 (1971) 1589. [39] S.M. Sze, K.K. Ng, Physics of Semiconductor Devices, John Wiley, Hoboken, 2007. [40] F. Yakuphanog˘lu, Phys. B 388 (2007) 226. [41] M.A. Yeganeh, Sh. Rahmatollahpur, R. Sadighi-Bonabi, R. Mamedov, Phys. B 405 (2010) 3253. [42] J.H. Werner, H.H. Güttler, J. Appl. Phys. 69 (1991) 1522; H.H. Güttler, J.H. Werner, Appl. Phys. Lett. 56 (1990) 1113. [43] S. Chand, J. Kumar, J. Appl. Phys. 80 (1996) 288; S. Chand, J. Kumar, Appl. Phys. A 63 (1996) 171. [44] A. Gümüsß, A. Türüt, N. Yalçın, J. Appl. Phys. 91 (2002) 245. [45] C. Cosßkun, S. Aydogan, H. Efeoglu, Semicond. Sci. Technol. 19 (2004) 242. [46] R.F. Schmitsdorf, T.U. Kampen, W. Mönch, Surf. Sci. 324 (1995) 249. [47] W. Mönch, J. Vac. Sci. Technol. B 17 (1999) 1867. [48] Ö. Güllü, T. Kılıçog˘lu, A. Türüt, J. Phys. Chem. Solids 71 (2010) 351. _ A.R. Deniz, A. Türüt, Microelectron. Eng. 87 (2010) [49] S ß . Aydog˘an, Ü. Incekara, 2525. [50] Z. Nalçacıgil, MS Thesis, Selçuk University, Konya, Turkey, 2011. [51] A.A.M. Farag, I.S. Yahia, Synth. Met. 161 (2011) 32. [52] S. Chand, J. Kumar, J. Appl. Phys. 82 (1997) 5005.