Al hybrid heterojunction diode

Vacuum 151 (2018) 96e107

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Structural and dielectric properties of Au/perylene-66/p-Si/Al hybrid heterojunction diode M.M. Shehata*, M.O. Abdel-Hamed, K. Abdelhady Physics Department, Faculty of Science, Minia University, Minia, 61519, Egypt

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 January 2018 Received in revised form 8 February 2018 Accepted 9 February 2018 Available online 13 February 2018

Perylene-66 thin films were efficiently prepared at room temperature utilizing thermal evaporation technique (TET). The XRD and FT-IR of the powder and the as-deposited film were examined. A hybrid (organic/inorganic) heterojunction device based on perylene-66 growth onto p-type silicon wafer was characterized by the impedance spectroscopy (IS) method in the frequency range from 100 Hz to 2 MHz and the temperature range from 303 K to 383 K in dark. The ColeeCole plots show 2 relaxation mechanisms for all temperatures. The maximum barrier height WM value is 0.26 eV. The AC conductivity is found to comply Jonscher's universal power law, and the correlated barrier hopping was observed to be the predominant conduction mechanism for the charge carrier transport. © 2018 Published by Elsevier Ltd.

1. Introduction Semiconducting organic materials have been getting huge consideration because of their significant electrical, optoelectronic and processing properties for design and manufacture of novel class of electronic devices such as organic solar cells, organic lightemitting diodes, organic field-effect transistors, organic electrophotographic, photoreceptors and electro-chromic devices [1]. Organic materials like perylene subsidiaries were at that point reached out to the phase of dynamic applications. As they have effective electron tolerating and quick electron transporting properties as well as they have a significant chemical, thermal stability and non-toxicity [2]. They are considered as transition materials, which links the gap between narrow band molecular solids and delocalized wide band semiconductors [3]. This remarkable position has roused significant enthusiasm for their fundamental optical and electronic properties [3]. This materials were investigated in a certain molecular electronic diodes such as organic sensors and organic solar cells [4,5]. Impedance spectroscopy (IS) plays a vital role in analysis, examination of the electrical properties and the relaxation behaviors in electronic devices [6,7]. It is an intense method that is generally used to examine the interface charge transport phenomena and recombination through the device [8]. IS also allows the

* Corresponding author. E-mail address: [email protected] (M.M. Shehata). https://doi.org/10.1016/j.vacuum.2018.02.014 0042-207X/© 2018 Published by Elsevier Ltd.

examination of intrinsic materials parameters such as the frequency dependence of the real and the imaginary parts of impedance and the internal structure of the device [9]. This tool has been applied for dye sensitized solar cells and organic solar cells [10,11]. A purely sinusoidal voltage with different frequencies is applied to the device under test then the phase shift, the amplitude and the current are measured. The ratio between the applied voltage and the resultant current is the impedance of the studied device. Therefore, the frequency response of the system, including the energy storage (capacitors) and dissipation parameters (resistors) were recorded. Fabrication, electrical and photovoltaic characteristics of Perylene-66 based diodes, Al/p-Si/Perylene-66/Au, were investigated [12]. The main objective of this work is to achieve a better understanding about the dielectric properties of Al/p-Si/perylene-66/Au heterojunction, crystallinity and chemical structure of the as prepared films using IS, XRD and FTIR techniques respectively. The real 0 ðZ Þ and the imaginary ðZ 00 Þ parts of the impedance, Cole-Cole diagrams, equivalent circuit, relaxation mechanisms, activation en0 00 0 ergies, and dielectric constant ðε Þ /loss ðε Þ, real ðM Þ/imaginary 00 ðM Þ parts of electric modulus, conduction mechanisms and the DC electrical conductivity were calculated and interpreted on the basis of the impedance theories. The excess minority carrier lifetime, diffusion coefficient and the global mobility at each interfaces were also reported and discussed in order to provide a new insight for such structure using this technique.

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2. Experimental details Perylene-66 (Dibenzthiopheno-peryleneN,N0 edicyclohexylimide) with dye content of 40% was obtained from Sigma-Aldrich organization and utilized with no further cleansing. The molecular structure of perylene-66 was shown in Fig. 1. Scheme 1, shows the experimental details for the fabrications and the measurements of the Al/p-Si/Perylene-66/Au heterostructure, and the characterizations of powder and the asdeposited perylene-66 thin films. The time of etching is 30 s which is sufficient to remove the contamination layer and the native oxide on p-Si wafer surface, more than 30 s will lead to corrosion and deformation of the wafer surface. After etching the substrate was washed by deionized water of resistivity 18.2 MU cm, finally dried using filtered air. Thin films of perylene-66 onto a highly cleaned microscopy glass coated (for XRD characterization) by a high vacuum coating unit (Edwards Co. model E 306A, England). This was performed under a pressure of 106 Torr, that was assisted by liquid nitrogen (acting as a trap for the released vapor) during the thermal evaporation process. The coating unit is provided with a quartz crystal monitor (INFICON SQM-160 rate/thickness monitor) for estimating film thickness and the deposition rate. It was constant through the evaporation of perylene-66 powder and equals 5 Å/s. The substrate was held at 20 cm from the evaporation heating source in order to keep the substrate at room temperature during the evaporation process. The FT-IR estimations for the thin film was turned out for the perylene66 film on KBr single crystal substrate.

3. Results and discussion 3.1. Structural investigations of perylene-66 (powder e asdeposited film) The X-Ray powder diffraction (XRD) of Perylene-66, Dibenzthiopheno-perylene-N, N0 edicyclohexylimide is shown Fig. 2. It was made for the first time in a 2q range from 5 to 100 and its spectrum is exhibited in the inset of Fig. 3. The XRD pattern was fitted by Fityk program [13]. It is found that, the pattern has multi diffraction peaks with various intensities, showing that the powder of Peryelene-66 has a polycrystalline nature. The unit cell parameters were scanned using the CRYSFIR computer program [14,15]. The powder structure highly matches with monoclinic structure of a space group P2. The details of the crystal data are given in Table 1.

Scheme 1. Characterization, fabrication and measurement of Al/p-Si/Perylene-66/Au heterostructure.

Fig. 1. Molecular structure of Perylene-66 compound.

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Fig. 2. X-ray diffraction pattern of Perylene-66 in the powder form.

Fig. 4. Infrared spectra of Perylene-66 (a) for the powder, (b) as-deposited film of thickness 440 nm.

Fig. 3. X-ray diffraction pattern of the as-deposited Perylene-66 thin film of thickness 440 nm. Inset: Fityk fitting of perylene-66 powder.

The Miller indices, hkl, and lattice spacing, d (hkl), for each diffraction line were indexed by CHEKCELL program [14,15] as shown in Fig. 2. XRD pattern of the as-deposited Perylene-66 film is

shown in Fig. 3, which indicates a partially crystallized embedded in the amorphous background of the studied film. The chemical structure of the powder and the as-prepared film was explored by FT-IR spectroscopy. Fig. 4a and b, illustrates the FTIR spectra of the powder and the as-deposited perylene-66 film in the range (400-4000 cm1), respectively. It can be observed that, no difference between the as-prepared thin film and the powder. This means that, TET is a proper method to deposit undissociated and stoichiometric perylene-66 thin films. The transmittance bands and

Table 1 Powder crystal data of Dibenzthiopheno-perylene -N,N0 -dicyclohexylimide Dye content 40% (Peryelene-66). Crystal data Product name Empirical formula Molecular weight Crystal color/shape Crystal system Space group Unit cell Z/Volume (Å3) Calculated density (g/cm3) Estimated atom. Volume (Å3) Nb. of non-hydrogen atom Estimated volume of molecule (Å3) Total atomic volume (Å3) Total atomic volume/cell volume

Dibenzthiopheno-perylene -N,N0 -dicyclohexy limide Dye content 40% (Peryelene-66) C48H34N2O4S2 766.93 Blackish violet monoclinic P2 a ¼ 20.2460 Å, b ¼ 18.8100 Å,c ¼ 15.1780 Å, a ¼ 90.000 , b ¼ 99.880 , g ¼ 90.000 . 6/5694.5 1.34 18 56 1008 6048 1.06

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their assignments are recorded in Table 2, the broad band was observed at 3448 cm1 is corresponding to O-H stretching vibration. The hydroxyl group O-H in the spectrum shows that Perylene66 (as-deposited film and powder) absorbed water from the atmosphere because it is not a part of its structure. This results agree with [16,17].

3.2. Characterization of Al/p-Si/perylene-66/Au heterojunction in dark at different temperatures 3.2.1. Complex impedance analysis In order to draw a true explanation about the electrical performance of the Al/p-Si/perylene-66/Au heterojunction, the IS was studied in a frequency range (100 Hz to 2 MHz) at different tem0 peratures. This technique assists to separate both Z (resistor) and 00 Z (reactance) parts of the complex impedance spectroscopy Z  which is given by Refs. [8,9]: 0

00

Z  ðuÞ ¼ Z ðuÞ þ iZ ðuÞ

(1) 0

00

Figs. 5 and 6 show the frequency dependent of Z and Z of the complex impedance at different temperatures for the studied Al/pSi/perylene-66/Au heterojunction. From Fig. 5, it is clear that the 0 scale of Z was decreased with increasing the angular frequency. Its high value at lower frequencies was due to the total polarization which was caused by space charge, dipoles and electrons, especially 0 at p-Si/Perylene-66 (barrier) interface. At lower frequencies, the Z values are reduced with increasing the temperature which suggests a decrease in the barrier height at p-Si/Perylene-66 interface. Also, a negative temperature coefficient of resistance (NTCR) nature characterizes the studied Al/p-Si/perylene-66/Au heterojunction 0 [8]. At lower frequencies, Z of impedance is corresponding to shunt (recombination/leakage) resistance of the investigated device. At 0 higher frequencies, the Z values approach to resistance called series resistance and it is independent on the frequencies [8]. The loss 00 spectrum Z was characterized by some significant features in the 00 pattern such as, the reduce in the Z without any peaks at temperature (313 K) in the investigated frequency range. As the

0

Fig. 5. Frequency dependence of Z at different temperatures of the Al/p-Si/perylene66/Au heterostructure.

temperature was elevated ð > 313KÞ; the peaks in the loss spectrum were appeared. This peaks are due to the capacitive nature at p-Si/ Perylene-66 interface (space charge region) and they are occurring at certain characteristic frequency fmax which was shifted toward higher frequency value with increasing temperature. A significant broadening of the peaks recommends the presence of a temperature dependent electrical relaxation phenomenon in the measured Al/p-Si/perylene-66/Au heterojunction with increasing tempera0 00 ture. The merging of both Z and Z values at higher frequencies may be a sign of the buildup of space charge in the device [8,9,18]. The relaxation time ðtÞ can be determined using the relation [9]:

umax ¼ 2pfmax t ¼ 1

The variation of t with temperature is shown in Fig. 7. As seen in Fig. 7, The value of t decreases with increasing temperature; as the

Table 2 IR spectral data of perylene-66 in the powder form and in the as-deposited film. Wavenumber cm1

Assignments

Powder

As-deposited Film

439

442 566 637 732 807 856 1024 1120 1146 1179 1232 1296 1337 1401 1463 1504 1590

699 734 808 861 1022 1151 1186 1234 1299 1337 1387 1466 1504 1591 1661 1734 1768 2925 3059 3440

1729 1761 2854 2924 3448

(2)

C-C þ C-N þ C-S (out of plane bending vibration) C-C þ C-N þ C-S (in plane bending vibration) C-H (out of plane bending vibration)

C¼H (in plane bending vibration)

C¼C stretching vibration

C¼O stretching vibration C-H symmetrical stretching vibration C-H asymmetrical stretching vibration C-H asymmetrical stretching vibration (aromatic) O¼H stretching vibration

100

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00

Fig. 6. Frequency dependence of Z at different temperatures of the Al/p-Si/perylene66/Au heterostructure.

temperature increases more electrical charges (electron hole pair in p-Si substrate/exciton in Perylene-66 film) were thermally excited so that the probability of recombination occurrence increases and, consequently, the relaxation time become shorter as reported in Ref. [8]. Also, the dissipated thermal energy may contribute to form dipoles that can follow the applied alternating field [19]. The variation of relaxation time can be described by the Arrhenius equation [20]:




t ¼ to exp

DEt kB T

 (3)

where t0 is a constant characteristic relaxation time and it represents the time of a single oscillation of a dipole in the potential well, DEt is the free energy of activation for dipole relaxation and t represents the average or the most probable value of the spread of the relaxation time. The plot of lnðtÞ verses 1000/T for the studied device is shown in the inset of Fig. 7. The value of DEt and t0 were calculated from the slope and the intercept of the linear fit and found to be 0:39±0:02eV and  1:29 ns, respectively.

3.2.2. Cole-Cole diagrams and equivalent circuit Fig. 8 displays the Nyquist impedance spectra of the Al/p-Si/ perylene-66/Au heterojunction in the range of temperature (303 K-383 K) and their corresponding fits (solid line-blue color). Two semicircles for all spectra can be seen, which confirms the presence of two relaxation mechanisms at all temperatures. The semicircles size diminished with the increase of temperature which propose temperature reliant on relaxation mechanism and change of electrical conductivity of the investigated device. This Cole-Cole plot of the investigated device can be modeled by an electrical equivalent circuit and the schematic diagram of Al/p-Si/perylene66/Au heterojunction as shown in Fig. 9a and b, respectively. The equivalent circuit is a combination of resistance (R1, R2 and R3) and capacitance (C1 and constant phase element CPE). Series resistance R1 is related to the resistance of contacts and electrode system (grid on top, Au, and whole area bottom, Al). The large semicircle in the low frequency range describes the charge transfer/recombination processes at perylene-66/p-Si (barrier) interface (R3, CPE) [8,21]. R3 represents the shunt resistance which was directly proportional to the semicircle diameter and inversely proportional to the recombination rate. CPE is attributed to the junction capacitance or the space charge region capacitance (all electrons and holes are swept of this region by the existed electric field so it is depleted of any mobile charges and contain immobile positive and negative ions) of the investigated heterojunction. Whereas the charge transfer/ recombination processes at Al/p-Si (conducting) interface (R2,C1) was represented by the small semicircle in the high frequency range [8,21]. R2 is corresponding to the transport resistance and C1 is associated with the defects and the traps at this interface. The using of constant phase element CPE (imperfect capacitor), rather than ordinary capacitor in the equivalent electrical circuit, gives a good fit to the experimental data at p-Si/Perylene-66 interface. The agreement between the fitted curves and the experimental data is found to be satisfactory. This is may be due to the inhomogeneous nature of the organic active layer with p-Si substrate. The equivalent circuit elements are determined by fitting the experimental data using the EIS-Spectrum Analyzer software [22] as listed in Table 3a. The temperature dependent nature of R2 at Al/p-Si interface and R3 at p-Si/Perylene-66 interface can be expressed as:


DEa2 R2 ¼  R02 exp kT


and

DEa3 kT

 (4)

where R02 and R03 are the pre-exponential terms and DEa2   and DEa3 are the activation energies at the two interfaces. The variation of lnðR2 Þ versus 1000/T at Al/p-Si interface is illustrated in Fig. 10. Two linear curves are observed in this plot. The values of DEa2 are 0.33eV (333 K-383 K) and 0.10eV (303 K-333 K), respectively. The dependence of lnðR3 Þ versus 1000/T at p-Si/Perylene-66 interface is represented in the inset of Fig. 10. The value of DEa3 are 0.42eV (333 K-383 K). For n < 1, the real capacity C2 can be calculated according to a suggestion of Hsu [23,24]:

. 1n 1 C2 ¼ R3n : P n   ¼ ðR3 PÞ1=n Rn3

Fig. 7. Temperature dependence of the relaxation time of the Al/p-Si/perylene-66/Au heterostructure. Inset: ln(t) versus 1000/T.

R3 ¼  R03 exp

(5)

where P and n are the pre-exponential factor and the exponent (with a value between 0  n  1 ) of the constant phase element, respectively. The exponent n describes the degree of the capacitive nature of the component. The depletion region width  W at Perylene-66/p-Si interface can be written as a function of the junction capacitance C2 as following [25]:

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Fig. 8. Cole-Cole plots at different temperatures of the Al/p-Si/Perylene-66/Au heterostructure.

101

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Fig. 10. Ln (R2) versus 1000/T at Al/p-Si interface. Inset: Ln (R3) versus 1000/T at p-Si/ Perylene-66 interface.

. D1 ¼ d2 t1

Fig. 9. (a) Al/p-Si/Perylene-66/Au hybrid heterojunction device. (b) Equivalent circuit model of the Al/p-Si/Perylene-66/Au heterostructure.

W ¼ εεo A=C2

(6)

where A is the common cross sectional grid area (Au-electrode) in m2, ε ¼ 4:48 is the relative permittivity of the perylene-66 thin film (calculated from the optical measurements) [17], and  εo is the permittivity of free space (εo ¼ 8:854  1012 F=m). The excess minority carrier life time of the charge carriers before recombination at two interfaces (Al/p-Si and p-Si/perylene-66 interfaces) can be obtained from the relationships [8,21]:

t1 ¼ R2 C1 and t2 ¼ R3 C2

(7)

where t1 and t2 are the carrier life times at Al/p-Si interface and pSi/perylene-66 interface, respectively. The diffusion coefficients D1;2 and the global mobilities m1;2 at two interfaces, (D1 and m1 at Al/p-Si interface-D2  and m2 at p-Si/perylene-66 interface), can be calculated using the following relations [21,25]:

and

. D2 ¼ d2 t2

(8)

m1 ¼ qD1 =KB T and m2 ¼ qD2 =KB T

(9)

where d is the interfacial organic layer thickness, q is the electronic charge and T is the absolute temperature. The values of C2 ;  W; t1  ,t2  , D1 , D2 m1 and  m2 are calculated at different temperatures and mentioned in Table 3b. The charge mobility shows temperature dependence; with higher temperatures corresponding to higher mobilities. Table 3b, indicates a very little change in the depletion width with temperature. This suggests that the junction (barrier at p-Si/Perylene-66 interface) is almost constant with changing the temperature. The charge mobilities are directly proportional to the temperature while the charge carrier lifetimes are inversely proportional to it. At low temperatures, the mobility is low this leads to a decrease in the rate at which carriers can physically meet for recombination. On the contrary, a higher mobility increases the probability of finding the opposite charge carrier for recombination [26]. The dependence of the electrical charge carrier mobility m1 at Al/p-Si interface and m2 at p-Si/Perylene-66 interface have been analyzed according to the Arrhenius behavior [26]:




m1 ¼ m01 exp

DEm1 kT

 and

m2 ¼ m02 exp


DEm2 kT

(10)

where m01 and m02 are the mobility pre-factors and DEm1 and DEm2

Table 3a Fitting parameters of the equivalent circuit elements of Al/p-Si/Perylene-66/Au hybrid heterojunction diode at different temperatures. T(K)

303 313 323 333 343 353 363 373 383

R1 (U) (±0:02)

77.60 74.01 70.09 66.19 48.69 39.14 31.00 24.61 20.06

R2(U) (±0:02)

7420.70 6749.50 6155.70 5261.20 3817.30 2794.10 2079.30 1547.90 1205.40

R3(U) (±0:02)

48756 38095 19836 10452 7088 4685 3239 2279 1783

C1  109 F (±0:01)

9.363 9.484 10.176 10.676 10.774 10.647 10.425 10.102 9.763

CPE (±0:01) P1  107 F sn1

n

6.242 6.103 4.799 2.480 3.751 4.009 4.273 4.563 4.357

0.697 0.704 0.743 0.852 0.812 0.812 0.813 0.819 0.841

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Table 3b Calculated data of Al/p-Si/Perylene-66/Au hybrid heterojunction diode at different temperatures. T(K)

C2  108 F

t1  106 ðsÞ

t2  103 ðsÞ

D1  109 m2 =s

D2  1011 m2 =s

m1  107 m2 =V:s

m2  109 m2 =V :s

W  108 m

303 313 323 333 343 353 363 373 383

13.68 12.55 9.59 8.81 9.50 9.37 9.40 10.00 11.25

69.48 64.01 62.64 56.17 41.13 29.75 21.68 15.64 11.77

6.67 4.78 1.90 0.92 0.67 0.44 0.30 0.23 0.20

2.92 3.16 3.23 3.61 4.92 6.81 9.34 12.95 17.21

3.04 4.24 10.64 21.98 30.06 46.12 66.51 88.85 100.93

1.12 1.17 1.16 1.26 1.66 2.24 2.98 4.03 5.21

1.16 1.57 3.82 7.65 10.16 15.15 21.24 27.62 30.55

2.90 3.16 4.14 4.50 4.17 4.23 4.21 3.97 3.53

are the activation energies at two interfaces. The variation of lnðm1 Þ as a function of 1000/T at Al/p-Si interface is illustrated in Fig. 11. Two linear plots are observed in this plot. The values of DEm1 and m01 are 0.33eV (333 K-383 K), 0.03eV (303 K-333 K) and  7:31  103 m2 =V:s, 3:49  107 m2 =V:s, respectively. The dependence of lnðm2 Þ versus 1000/T at p-Si/Perylene-66 interface is represented in the inset of Fig. 11. The values of DEm2 and m02 are 0.37eV and 2:39  103 m2 =V:s (333 K-383 K), respectively.

3.2.3. Electrical conductivity studies The frequency-dependent total electrical conductivity stot of the Al/p-Si/perylene-66/Au heterojunction at various temperatures are shown in Fig. 12. It was obtained from the following equation [27]:

stot ¼ εo uε2 ¼ sdc þ sac

(11)

Jonscher's power law, the obtained values of the exponent s is smaller than unity and decreased from 0.4 to 0.1 with the increase of temperature. This behavior of s with temperature has been observed for different types of thin films [31]. In region (II), the conductivity shows a linear relationship with the frequency. The calculated values of s in this region were found to be in the range from 0.25 to 0.19. The s value and its variation with temperature reveal that the correlated barrier hopping model (CBHM) may be the conduction mechanism in these two regions. In the high frequencies region (III), the conductivity is temperature independent and increases linearly as the frequency increases with the s value is higher than unity. The well-localized hopping and/or reorientational motion may be the conduction mechanism in this frequency region [31e33]. As shown in Fig. 7a, at low frequencies (and T > 313 K), at p-Si/Perylene-66 interface, the electrical conductivity becomes almost constant therefore the DC electrical conductivity can be obtained from the extrapolation of the experimental data of stot to the zero frequency. It was found that, The DC electrical conductivity obeys the Arrhenius relation [34,35].

where,  ε2 the dielectric loss, sdc is the DC conductivity corresponding to zero frequencies, and sac is the AC conductivity. As the frequency increases, the sac increases because the polarization decreases. The increase in sac also leads to an increase in the eddy current in the device [28], which can be attributed to the gradual reduction in the series resistance of the investigated device [29]. The frequency dependence of stot is divided into three linear regions (I, II and III) with different slopes, which is corresponding to low, intermediate and high frequencies, respectively. Such dependence was described by Jonscher's power law [30] sac ¼ Aus , where A is a temperature-dependent constant, u is the angular frequency and s is the frequency exponent with 0 < s < 1. In the first low frequency region (I), the conductivity increases linearly as the frequency increases. By fitting the curves in this region according to

where  so  is the pre-exponential factor, k is Boltzmann constant and DE is the activation energy of this process. Fig. 13, shows the variation of DC electrical conductivity from  stot as a function of the reciprocal temperature. The activation energy and the pre-factor were obtained from the slope and the intercept of Arrhenius plot. The values of DE and  so  were 0.37 eV and  9:12  104 U1  cm1 , respectively.

Fig. 11. Ln (m1) versus 1000/T at Al/p-Si interface. Inset: Ln (m2) versus 1000/T at p-Si/ Perylene-66 interface.

Fig. 12. Frequency dependence of stot at different temperatures of the Al/p-Si/ Perylene-66/Au heterostructure.

sdc ¼ so  exp


DE kT

(12)

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Fig. 13. Temperature dependence of sdc of the Al/p-Si/Perylene-66/Au heterostructure.

3.2.4. Dielectric properties 0 00 In this study, the dielectric properties such as ε and ε were investigated using IS technique. The complex permittivity  ε can be defined in the following form [36]: 0

00

ε ðuÞ ¼ ε ðuÞ þ iε ðuÞ 0

ε ðuÞ ¼ Cd=εo A

and

(13) 00

0

ε ðuÞ ¼ ε ðuÞtanðdÞ

(14)

where C is the measured capacitance, d is the interfacial organic layer thickness, εo ¼ 8:854  1012 F=m, is the permittivity of free space, A is the common area (Au electrode) and u is the angular frequency of the applied electric field. The capacitance nature depends on the polarizability of the material. Figs. 14 and 15 show the frequency dependence of the real and the imaginary components of the dielectric function of the Al/p-Si/Perylene-66/Au heterojunction at different temperatures, respectively. From Fig. 14, it is 0 observed that, at low frequencies ε shows strong frequency and temperature dependence (barrier dominant) while, at high frequencies, a small variation of ε0 can be noticed (resistor dominant). 0 ε decreases by increasing frequency of the applied field (C ¼ I=2pfVÞ at constant temperature while, it increases by

00

Fig. 15. Frequency dependence of ε at different temperatures of the Al/p-Si/Perylene00 66/Au heterostructure. Inset: ln (ε ) versus ln (u) at p-Si/Perylene-66 interface.

increasing temperature at constant frequency. The decrease of the dielectric constant with frequency can be ascribed to the contribution of different types of polarizability, deformational (electrons or ions) and relaxation (orientational or interfacial) [37]. The ε0 data gives a significant information about the energy distribution of the 0 interface states of the device. In the ideal case, ε is frequency independent, but this device shows a dispersion with frequency due to the existence of the interface states at the interfacial (organic) layer with the semiconductor interface [38,39]. The decrease of the capacitance values in the intermediate frequency, Al/p-Si interface, region means that small part of the interface states can only follow the field change. Owing to the increasing frequency, the dipoles will 0 no more be able to follow the field. As a result, a decrease of ε at higher frequency approaches a small value resulting from the 0 interfacial polarization [8]. The increase of ε with temperature can be interpreted as following, the temperature facilitates the alignment of dipoles along the applied field direction and in this way expanding the dipole effective length which, thusly, should come about into increment in the dielectric constant values [35]. From Fig. 15 it is observed that, ε00 follows the inverse dependence on frequency but increases with the rise of temperature especially at low (p-Si/Perylene-66 interface) and intermediate (Al/ 00 p-Si interface) frequencies. The high values of ε at low and moderate frequencies are originated from different types of losses such as dipole losses, the migration of ions, the contribution of ion jump, conduction loss of ions migration and ion polarization loss. The ion vibrations may be the only source of dielectric loss at high fre00 quency in turn to a small value of ε [40]. The rise of temperature leads to an increase of the electrical conduction losses which leads to an increase in the dielectric loss, ε00 [40]. ε00 at a particular frequency in the temperature range is represented by the power law with angular frequency as [35,40]: 00

ε ¼  Bum  

0

Fig. 14. Frequency dependence of ε at different temperatures of the Al/p-Si/Perylene66/Au heterostructure.

(15)

where B is a temperature dependent constant, u is the angular frequency of the applied AC field and m is the exponent parameter. The values of m can be obtained by fitting the curves in the inset of Fig. 15 according to equation (15). The examined values of m for the sample under investigation are analogous to those obtained for different materials [8,40]. The exponent m is related to the temperature T and the maximum barrier height, WM according to the relation [8,40]:

M.M. Shehata et al. / Vacuum 151 (2018) 96e107

4k T m¼ B WM

105

(16)

The values of m are negative and linearly decrease as the temperature increase as demonstrated in Fig. 16. The calculated value of WM along with equation (16) is 0:26±0:01 eV. The WM is equal to the difference between the work functions of each semiconductor material p-Si and Perylene-66 and can be affected by the applied voltage, doping concentration and temperature. 3.2.5. Electric modulus formalism The analysis of complex electric modulus M  as a function of frequency allows to detect the presence of relaxation process on this device. The complex permittivity ε* data was transformed into the M * formalism using the following relations [36]:

M  ðuÞ ¼ 1=ε ðuÞ ¼ M0 þ iM00 ¼

ε0 ε0 þ i 02 00 00 ε02 þ ε 2 ε þε 2

(17)

The variation of M 0 and M00 with frequency at different temperature of the Al/p-Si/Perylene-66/Au heterojunction were shown in Figs. 17 and 18, respectively. It was observed that, at low frequency, M 0 values tend to zero, which confirms the negligible or the absence of electrode polarization phenomena [41]. The M 0 dispersion increases continuously with increasing frequency up to 0.64 MHz after that, they merged and decreased for all temperatures. Such behavior is observed in Ref. [8]. As was seen, the variation of M00 with frequency shows the relaxation peaks which were moving toward higher frequency as the temperature increase. The asymmetric broadening of the M 00 peaks confirm the spread of the relaxation time at Al/p-Si interface. The relaxation time can be calculated by equation (2). The dependence of relaxation time with temperature is illustrated in Fig. 19. This variation can be described by Arrhenius relation as in equation (3). The plot of lnðtÞ versus 1000=T for the studied device was shown in the inset of Fig. 19. Two linear regions were observed with DE and  to  values in this plot. The activation energy and the pre-factor were obtained from the slope and the intercept of Arrhenius plot, respectively. The calculated values of DE and  to  were 0.37eV (333 Ke383 K), 0.11eV (303 Ke333 K) and  1:42  1010  s, 9:64  107  s , respectively. For 00 Debye type relaxation, the peaks obtained from Z and M00 versus 00 frequency should be coincide [42]. Fig. 20 shows the Z and M00 versus frequency combined plots of the Al/p-Si/Perylene-66/Au 00 heterojunction at 343 K as an example. It is observed that Z plot

0

Fig. 17. Frequency dependence of M at different temperatures of Al/p-Si/Perylene-66/ Au heterostructure.

Fig. 18. Frequency dependence of M Perylene-66/Au heterostructure.

00

at different temperatures of the Al/p-Si/

Fig. 19. Temperature dependence of the relaxation time of the Al/p-Si/perylene-66/Au heterostructure. Inset: ln(t) versus 1000/T. Fig. 16. Temperature dependence of the experimental mean value of exponent m of the Al/p-Si/Perylene-66/Au heterostructure.

106

M.M. Shehata et al. / Vacuum 151 (2018) 96e107

00

00

Fig. 20. Frequency dependence of Z and M at 343 K of the Al/p-Si/Perylene-66/Au heterostructure.

has a peak at lower frequencies (p-Si/Perylene-66 interface) and a shoulder at intermediate frequencies which may be due to (Al/p-Si interface). M 00 plot shows peak at Al/p-Si interface. It is observed 00 that, the Z and M00 shoulder and peak do not overlap, respectively, which is an indication of the wide distribution of relaxation times and non-debye type relaxation process that characterize the investigated device. The longer, in the range (103 sÞ, relaxation time from Z 00 and the excess minority carrier t2 confirm the barrier at p-Si/Perylene-66 interface and suggest more exciton separation at this interface by the existed electric field in the depletion region. This leads to faster charge transport processes and enhance the performance of the studied device. The shorter, in the range (106 sÞ , relaxation time from M 00 and the excess minority carrier t1 confirm the conduction at Al/p-Si interface and gives an indication of defects and traps existed in this interface. These results are in agreement with the results reported in Ref. [8]. 4. Conclusion Thermally evaporated of undissociated and stoichiometric Perylene-66 organic films were found to be partially crystallized with monoclinic structure. The temperature has strong influence on the impedance spectroscopy and the conductivity of the Al/p-Si/ Perylene-66/Au diode especially at lower (p-Si/Perylene-66 interface) and intermediate (Al/p-Si interface) frequencies. The real and the imaginary parts are frequency dependent. They show NTCR type at lower frequencies and merge at higher frequencies. The relaxation times and the excess minority carrier life times at two interfaces are decreased with increasing temperature. The difference between the work functions of each semiconductor material p-Si and Perylene-66 film were estimated and found to be 0.26 eV. It was found that, The relaxation time, resistances, mobilities and DC conductivity obey the Arrhenius law with mean activation energies 0.34 eV at Al/p-Si interface and 0.39 eV at p-Si/Perylene-66 interface in the temperature range (323 K-383 K). The non-Debye behavior describes the investigated device. References [1] O. Ostroverkhova, Organic optoelectronic materials: mechanisms and applications, Chem. Rev. 116 (22) (2016) 13279e13412. [2] S. Benning, H.S. Kitzerow, H. Bock, M.F. Achard, Fluorescent columnar liquid crystalline 3, 4, 9, 10-tetra-(n-alkoxycarbonyl)-perylenes, Liq. Cryst. 27 (7) (2000) 901e906. [3] M.M. El-Nahass, A.M. Hassanien, N.M. Khusayfan, Optical characterizations of thermally evaporated perylene-66 (dye content 40%) thin films, Solid State

Commun. 154 (2013) 51e55. [4] Yanke Che, Xiaomei Yang, Guillin Liu, Chun Yu, Hongwei Ji, Jincai Zuo, Ling Zang, Ultrathin n-type organic nanoribbons with high photoconductivity and application in optoelectronic vapor sensing of explosives, J. Am. Chem. Soc. 132 (16) (2010) 5743e5750. [5] C. Li, M. Liu, N.G. Pschirer, M. Baumgarten, K. Müllen, Polyphenylene-based materials for organic photovoltaics, Chem. Rev. 110 (11) (2010) 6817e6855. [6] P. Kumar, A.K. Sharma, B.P. Singh, T.P. Sinha, N.K. Singh, Dielectric relaxation in complex perovskite oxide Sr (Gd0. 5Nb0. 5) O3, Mater. Sci. Appl. 3 (06) (2012) 369. [7] J.R. Macdonald, E. Barsoukov, Impedance spectroscopy: theory, experiment, and applications, History 1 (8) (2005). [8] M.M. Shehata, T.G. Abdel-Malik, K. Abdelhady, AC impedance spectroscopy on Al/p-Si/ZnTPyP/Au heterojunction for hybrid solar cell applications, J. Alloy. Comp. 736 (2018) 225e235. [9] I.S. Yahia, S. AlFaify, M.M. Abutalib, S. Chusnutdinow, T. Wojtowicz, G. Karczewski, S.M. El-Bashir, n-(CdMgTe/CdTe)/(p-(CdTe/ZnCdTe/ZnTe)/pGaAs) heterostructure diode for photosensor applications, Appl. Phys. A 122 (5) (2016) 491. [10] Q. Wang, J.E. Moser, M. Gr€ atzel, Electrochemical impedance spectroscopic analysis of dye-sensitized solar cells, J. Phys. Chem. B 109 (31) (2005) 14945e14953. [11] Lin Ke, Nan Zhang, Edna Low, Rui Wang, Zhi Ming Kam, Xizu Wang, Liu Bin, Jie Zhang, Organic photovoltaic annealing process analysis using impedance spectroscopy, Sol. Energy 144 (2017) 367e375. [12] M.M. El-Nahass, A.A.M. Farag, N.M. Khosifan, E.F.M. El-Zaidia, Fabrication, electrical and photovoltaic characteristics of perylene-66 based diodes (comparative study), Synth. Met. 209 (2015) 74e81. [13] M. Wojdyr, Fityk: a general-purpose peak fitting program, J. Appl. Crystallogr. 43 (5) (2010) 1126. [14] R. Shirley, The CRYSFIRE System for Automatic Powder Indexing: User's Manual, vol. 41, The Lattice Press, 2000, pp. 931e932. [15] M.M. El-Nahhas, H. Abdel-Khalek, E. Salem, Structural and optical properties of nanocrystalline 3, 4, 9, 10ePerylene-Tetracarboxylic-Diimide thin film, Adv. Condens. Matter Phys. 2012 (2012). [16] M.A.M. El-Mansy, M.M. El-Nahass, On the spectroscopic analyses of Perylene66, Spectrochim. Acta A Mol. Biomol. Spectrosc. 130 (2014) 568e573. [17] M.M. El-Nahass, A.M. Hassanien, Gamma radiation-induced changes on the optical properties of dibenzthiopheno-perylene-N, N0 -dicyclohexylimide thin films, Radiat. Phys. Chem. 97 (2014) 178e183. [18] P. Kumar, A.K. Sharma, B.P. Singh, T.P. Sinha, N.K. Singh, Dielectric relaxation in complex perovskite oxide Sr (Gd0. 5Nb0. 5) O3, Mater. Sci. Appl. 3 (06) (2012) 369. [19] I.S. Yahia, M. Fadel, G. Sakr, S. Shenouda, F. Yakuphanoglu, W. Farooq, Impedance spectroscopy of nanostructure p-ZnGa2Se4/n-Si heterojunction diode, Acta Phys. Pol., A 120 (3) (2011) 563e566. [20] J.H. Ahn, J.U. Lee, T.W. Kim, Impedance characteristics of ITO/Alq 3/Al organic light-emitting diodes depending on temperature, Curr. Appl. Phys. 7 (5) (2007) 509e512. [21] D. Wang, J. Zhu, L. Ding, P. Gao, X. Pan, J. Sheng, J. Ye, Interface electric properties of Si/organic hybrid solar cells using impedance spectroscopy analysis, Jpn. J. Appl. Phys. 55 (5) (2016), 056601. [22] A.S. Bondarenko, G.A. Ragoisha, in: A.L. Pomerantsev (Ed.), Progress in Chemometrics Research, Nova Science Publishers, New York, 2005, pp. 89e102 the program is available online at: http://www.abc.chemistry.bsu.by/vi/ analyser/. [23] C.H. Hsu, F. Mansfeld, Concerning the conversion of the constant phase element parameter Y0 into a capacitance, Corrosion 57 (9) (2001) 747e748. [24] M.S. Abouzari, F. Berkemeier, G. Schmitz, D. Wilmer, On the physical interpretation of constant phase elements, Solid State Ionics 180 (14) (2009) 922e927. [25] B.G. Streetman, S.K. Banerjee, Solid State Electronic Devices, Prentice-Hall, 2005. [26] C. Vijila, S.P. Singh, E. Williams, P. Sonar, A. Pivrikas, B. Philippa, J. Hobley, Relation between charge Carrier mobility and lifetime in organic photovoltaics, J. Appl. Phys. 114 (18) (2013), 184503. [27] B. Barıs¸, Frequency dependent dielectric properties in Schottky diodes based on rubrene organic semiconductor, Phys. E Low-dimens. Syst. Nanostruct. 54 (2013) 171e176. lu, Frequency and voltage dependence of [28] S.A. Yeris¸kin, M. Balbas¸ı, A. Tatarog dielectric properties, complex electric modulus, and electrical conductivity in Au/7% graphene doped-PVA/n-Si (MPS) structures, J. Appl. Polym. Sci. 33 (133) (2016) n-a. _ Uslu, Frequency and voltage [29] S. Demirezen, A. Kaya, S.A. Yeris¸kin, M. Balbas¸ı, I. dependent profile of dielectric properties, electric modulus and ac electrical conductivity in the PrBaCoO nanofiber capacitors, Results Phys. 6 (2016) 180e185. [30] Z.T. Al-Dhhan, C.A. Hogarth, Electrical conduction and observation of local defects in thin sandwich structures of Cu-SiO/CeO2-Cu, Int. J. Electron. 63 (5) (1987) 707e722. [31] A.M. Nawar, H.A. El-Khalek, M.M. El-Nahass, Dielectric and electric modulus studies on Ni (II) tetraphenyl porphyrin thin films, Org. Opto-Elect 1 (1) (2015) 25e38. [32] A.A. Atta, AC conductivity and dielectric measurements of bulk magnesium phthalocyanine (MgPc), J. Alloy. Comp. 480 (2) (2009) 564e567.

M.M. Shehata et al. / Vacuum 151 (2018) 96e107 [33] A.S. Ala’eddin, P. Poopalan, Correlation between the chemical composition and the conduction mechanism of barium strontium titanate thin films, J. Alloy. Comp. 509 (26) (2011) 7210e7215. [34] N.F. Mott, E.A. Davis, Electronic Processes in Non-crystalline Solids, Clarendon, Oxford, 1979, p. 465. [35] Y.A. El-Gendy, I.S. Yahia, F. Yakuphanoglu, Investigation of nanocrystalline CdS/Si diode using complex impedance spectroscopy, Mater. Res. Bull. 47 (11) (2012) 3397e3402. [36] M.M. El-Nahass, A.A.M. Farag, F.S.H. Abu-Samaha, E. Elesh, Temperature and frequency dependencies of AC and dielectric characterizations of copper tetraphenyl porphyrin thin films, Vacuum 99 (2014) 153e159. [37] M.M. El-Nahass, A.A. Atta, E.F.M. El-Zaidia, A.A.M. Farag, A.H. Ammar, Electrical conductivity and dielectric measurements of CoMTPP, Mater. Chem. Phys. 143 (2) (2014) 490e494. [38] S. Kochowski, B. Paszkiewicz, R. Paszkiewicz, Some effects of (NH 4) 2 S x

[39]

[40]

[41]

[42]

107

treatment of n-GaAs surface on electrical characteristics of metal-SiO 2eGaAs structures, Vacuum 57 (2) (2000) 157e162. lu, M. Sag lam, E. Ayyildiz, A. Türüt, On the forward bias excess B. Bati, Ç. Nuhog capacitance at intimate and MIS Schottky barrier diodes with perfect or imperfect ohmic back contact, Phys. Scripta 61 (2) (2000) 209. M.M. El-Nahass, A.A. Attia, G.F. Salem, H.A.M. Ali, M.I. Ismail, Dielectric and impedance spectral characteristics of bulk ZnIn 2 Se 4, Phys. B Condens. Matter 434 (2014) 89e94. Y.J. Wong, J. Hassan, M. Hashim, Dielectric properties, impedance analysis and modulus behavior of CaTiO 3 ceramic prepared by solid state reaction, J. Alloy. Comp. 571 (2013) 138e144. R. Martínez, A. Kumar, R. Palai, J.F. Scott, R.S. Katiyar, Impedance spectroscopy analysis of Ba0. 7Sr03TiO3/La0. 7Sr0. 3MnO3 heterostructure, J. Phys. D Appl. Phys. 44 (10) (2011), 105302.