Improvement of the Electrical Performance of Ag/MEH-PPV/SiNWs Schottky Diode by the Insertion of a Thin Layer of MEH-PPV Polymer and Study of the Annealing Effect

Poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) thin layer was deposited on silicon nanowires (SiNWs) by electroless dipping method. SiNWs were obtained using Ag-assisted chemical etching process. Scanning Electron Microscopy (SEM) images reveal a vertical alignment of the SiNWs as well as the formation of MEH-PPV layer on their surfaces. The presence of MEH-PPV polymer on the SiNWs surface was confirmed by Energy-dispersive X-ray (EDX). Current–Voltage (I–V) measurements were performed for the electrical characterization of Ag/MEH-PPV/SiNWs diodes before and after annealing. The ideality factor (n), the barrier height (φb) and the series resistance (RS) are determined using the Cheung method. The diode parameters are strongly affected by the immersion duration in MEH-PPV solution as well as the annealing temperature. The rectification rate of the diodes was increased by MEH-PPV deposition. The annealing temperature has a great influence on the diode parameters by the thermal activation of carriers at Ag/MEH-PPV and MEH-PPV/SiNWs interfaces. I–V characteristics show an ohmic character for temperatures above 250 °C. The electrical parameters such as equivalent carrier concentration (ND) and built-in voltage (Vb) and other values of φb are calculated from Capacitance–Voltage (C–V) measurements.


Introduction
Due to their interesting optoelectronic properties and specific morphological form, silicon nanowires (SiNWs) are proving to be promising candidates for sensors, solar cells, diodes, and transistor compounds [1][2][3][4]. Organic electronics have recently experienced relentless interest [5,6]. In order to improve the performance of SiNWs and integrating it into the field of organic electronics, research efforts have focused on the passivation process by depositing layers acting as a barrier with the external environment [7,8]. Conjugated polymers are materials of great interest in photovoltaic, organic electronics and optoelectronic devices [9][10][11]. The formation of a heterostructure containing organic material (conjugated polymer) and inorganic semiconductor material (SiNWs) gives interesting electrical results [12,13]. The mixture of SiNWs and MEH-PPV polymer in a single hybrid layer has been adopted by researchers for applications in photovoltaic domain [14,15]. In this case, MEH-PPV presents an organic electron donor and the SiNWs layer is an inorganic electron acceptor. SiNWs in contact with MEH-PPV appear to be a promising material for the realization of high performance diodes. Indeed, MEH-PPV molecules offer a large contact surface with SiNWs while exploiting the high surface-to-volume ratio of the nanowires layer. Salem et al. [16] found an enhancement of the electrical and the luminescent properties of porous silicon covered by MEH-PPV polymer. The analyses indicate the possibility of exciton transfer from porous silicon to MEH-PPV. Also, Jlassi et al. [17] have investigated the electrical and the photoluminescence properties of porous silicon/MEH-PPV heterostructures. They reported that the MEH-PPV/PS nanocomposite shows a Richardson-Schottky junction with a relatively high ideality factor and series resistance Rs, attributed to interface states between the two materials. They found that the transport mechanism in the nanocomposite was assured by the Space Charge Limited Current (SCLC). Recently, Rahmani et al. [11] have studied the electrical properties of Ag/P3HT/SiNWs heterostructure. They investigated the effects of annealing temperatures and immersion durations of SiNWs layer in P3HT solution on the diode parameters. They found that the P3HT layer and the annealing temperature play important roles by changing the interface states and the trapping centers in Ag/P3HT/ SiNWs contacts. The fabrication and the characterization of organic Schottky diodes (OSD) based on polymer have been reported by some researchers [18][19][20]. They pointed out the importance of a thin organic intermediate layer between the metal and the semiconductor in the rectification behavior of the diode as well as its influence on the potential barrier and the ideality factor. It has been reported that annealing is one of the parameters that is considered in obtaining an optimum performance for Schottky diodes [21,22]. Since MEH-PPV is a single crystalline polymer, annealing temperature can easily affect its electronic properties. Azhar et al. have studied the effect of annealing temperature on the electrical properties of MEH-PPV thin film [23]. They reported that the highest conductivity of MEH-PPV layer is obtained for 50 °C due to the improvement of the hole mobility owing to crystallization of the polymer.
In this paper, our goal is the improvement of the Ag/ MEH-PPV/SiNWs diode performance by the insertion of a thin layer of MEH-PPV polymer for the development of OSD type diodes. The electrical optimization was made by varying the immersion time in MEH-PPV solution as well as the annealing temperature. Also, the aim of this work is to determine the electronic transport in MEHPPV/SiNWs junction.

Experimental
A p-type Si(100) wafer with 0.1-2 Ω cm resistivity and a thickness of 500 µm was used for the development of silicon nanowires (SiNWs). Substrates were cleaned with deionized water followed by rinsing with acetone during 30 min. The Metal Assisted Chemical Etching (MACE) method was adopted for obtaining SiNWs. Etching of silicon during 20 min was performed in aqueous solution containing HF (5 M) and AgNO 3 (0.05 M) with the same volume proportions. The samples were dipped in nitric acid for 5 min followed by rinsing in deionized water and dried at room temperature to remove the dendrites. Poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) powder with a regioregular percentage superior to 90% was dissolved in chloroform by taken a concentration of 0.2 mg/ ml. The solution was placed in ultrasonic bath during 30 min for a good solubility of the polymer. The MEH-PPV powder was purchased from Sigma-Aldrich. The deposition of MEH-PPV molecules on SiNWs surface was carried out by dip coating method without stirring of the solution to avoid any turbulence effect during the reaction. An ohmic contact with a thickness of 200 nm has been formed by coating the back side of the silicon wafer with aluminum using an evaporation system and subsequently annealed at 500 °C for 30 min. The electrical contacts were in the circular form with about 5 mm of diameter and 0.5 mm thick using silver conductive paste under curing temperature of 50 °C during 5 min. The rest of the surface was left uncoated. The Ag paste was purchased from Sigma-Aldrich. Capacitance-voltage (C-V) measurements were measured at 50 Hz with a Modulab XM MTS system. The sample is fixed on an electronic test plate and the current-voltage (I-V) measurements were carried out using a synchronic detection (SR830 -Lock-In Amplify) controlled by a Lab-View self-made program for an applied bias ranges from − 10.5 to 10.5 V. A detailed description accompanied by a schematic of the experimental setup of the I-V measurements are presented in our previous work [24]. I-V and C-V measurements have been made at room temperature under dark conditions. The forward bias condition corresponds to the case in which a negative voltage is applied to the SiNWs or MEH-PPV/SiNWs layer with respect to the ohmic contact in c-Si. The thermal annealing was performed in air by using a programmable furnace maintained at the desired annealing temperature. In order to stabilize the thermal conditions, the annealing was delayed by 15 min at each step. The microscopic images as well as the Energy-Dispersive X-ray (EDX) measurements were performed using Scanning Electron Microscopy (SEM) (JEOL JSM-5600 LV equipped with EDX spectrometer.

Results and Discussion
Scanning Electron Microscopy (SEM) combined with Energy Dispersive X-ray (EDX) analysis was used to investigate the surface morphology and the chemical composition of the MEH-PPV/SiNWs interface. We present in Fig. 1, the cross-section SEM images of SiNWs/MEH-PPV structure for immersion durations (D im ) of 150 min and 60 min and EDX spectra recorded in two different sites, denoted S1 and S2. S1 and S2 correspond to positions in the middle of SiNWs layer and in MEH-PPV/SiNWs interface, respectively. SEM images show the coverage of the SiNWs layer by the MEH-PPV polymer with thicknesses of 3.9 µm and 3 µm for immersion times of 150 min and 60 min, respectively. The thickness of the SiNWs layer is about 2.2 µm.
The EDX profile from different positions shows detection of Si, C, O and some impurities. The atomic percentages of each element in S1 and S2 are given in Table 1. The carbon element is mainly related to the presence of the MEH-PPV since the polymer molecule skeleton is essentially composed of a carbon chain. From the EDX of the SiNWs layer, we notice that the amounts of C are almost identical for the two samples. On the other hand, at the MEH-PPV/SiNWs interface, the percentage increasing of carbon is 7% due to the increasing of the immersion duration from 60 to 150 min (50% for 150 min and 43% for 60 min). We note also that the quantity of oxygen for 150 min is almost half of that  Table 1 The atomic percentage of Si, C and O in the two positions (S1 and S2) of MEH-PPV/SiNWs nanocomposites obtained after immersion durations of 60 min and 150 min Position S1 Position S2 found for 60 min whether for S1 or S2 regions. Therefore, the immersion duration favors more condensation of the polymer at the MEH-PPV/SiNWs interface; this generated changes in the electronic structure of these nanocomposites.
To investigate such possible changes, we have realized and studied the Ag/MEH-PPV/SiNWs heterostructure. In Fig. 2, the schematic cross section view of Ag/MEH-PPV/SiNWs junction in forward polarization is shown. The junction is formed by Si substrate with a thickness of 500 µm whose back face is covered with a 200 nm thick layer of aluminum.
On the front face of Si substrate, a layer of SiNWs with 2.2 µm thickness has been formed and on which a layer of MEH-PPV with a thickness of a few micrometers has been deposited. Figure 3 exhibits the current-voltage (I-V) characteristics in semi-Log scale of Ag/SiNWs and Ag/MEH-PPV/SiNWs heterostructure for different D im . We notice a linear region at low voltages and a curvature at high forward bias due to the series resistance (Rs) effect. Both Ag/MEH-PPV/SiNWs and Ag/SiNWs structures were fabricated in the same conditions. So, the changes found in the diode characteristics are mainly due to the presence of the polymeric layer. As can be seen in Fig. 3, the diodes which contain MEH-PPV polymer for 30 min, 60 min and 90 min show good rectification behaviors. The rectification rate (RR) strongly depends on D im . At 5 V, the highest RR is found for a D im of 30 min (RR is 125) while for the heterostructure without polymer, RR is equal to 2.9. For relatively medium durations the RR is high, while for relatively long durations this rate is very low and the diode becomes almost ohmic. Generally, the rectifying behavior of the Schottky diode can be described by thermionic emission-diffusion theory and/ or field emission theory in the case of heavily doped semiconductor [25]. Since the MEH-PPV polymer and SiNWs behave like lightly doped p-type materials. The electrical characteristics of the MEH-PPV/SiNWs junction were analyzed by adopting thermionic emission-diffusion theory. According to this theory, the I-V relation is expressed as: where q is electron charge, T is the absolute temperature, V is the applied voltage, k is the Boltzmann constant, I s is the saturation current and n is the ideality factor.
Due to the presence of native oxidation of the SiNWs surface, this equation is corrected by the serie resistance Rs. In this case, it is written: The Schottky diode parameters were determined using Cheung and Cheung method [26]. Cheung functions are deduced from Eq. (2) and they can be described by the following equations: A is the diode area, A* is the effective Richardson constant and φ b is the energy barrier.
The   Table 2. The barrier height can be calculated from the y-axis intercept of the H(I) plot. The values of φ b are also given in Table 2. The obtained results indicate that the MEH-PPV thin layer formed at the SiNWs surface has modified the Φ b of Ag/SiNWs Schottky diode. Some experimental studies have been made for the barrier height modification using the organic thin films as interfacial layer [20,27]. In most of these studies, the authors reported that the existence of organic thin films cause an increase in the barrier height value. The increase of Φ b can be attributed to the formation of a physical barrier between the silver and the SiNWs layer by the MEH-PPV polymer, preventing the metal from directly contacting the nanowires surface as well as the enlargement of the space charge area.
The reason for this is the adaptation of structurally different regions by the junction established between the MEH-PPV and SiNWs layers. In this case, not only the electron concentrations are modified, but the interface states are also changed. In fact, the interface states play an important role in the determination of the organic Schottky diode parameters.
Since the MEH-PPV layer has sufficiently thick, the equation between the density of the interface states density (Ns) and the ideality factor n can be expressed as follows [28,29]: where ε i is the dielectric permittivity of the interfacial layer (MEH-PPV layer) and δ is its thickness. ε i=4 ε 0 [30], ε 0 is the permittivity of free space. For immersion duration equal to 60 min, the calculated interface state density is 1.77 × 10 10 cm −2 eV −1 and it reduces to 1.52 × 10 10 cm −2 eV −1 for 150 min. This confirms our previous interpretations about the influence of the polymer layer on the interface states more particularly and on the diode parameters in general. In Fig. 5, the I-V curves of Ag/SiNWs and Ag/MEH-PPV/SiNWs in a double logarithmic scale are presented. This presentation allows us to determine the transport mechanism governing the two types of diode. For all structures, the variation is linear with change in slope by passing from low to high voltage. The experimentally m values of Ag/MEH-PPV/SiNWs diodes are given in Table 3. For low biases, the slope is around unity thus indicating an ohmic character of the diode whereas this slope increases for an immersion time of 90 min and approaches 2   for high biases. In this case, the dominant regime is the space charge limited conduction (SCLC). This regime that occurs when the equilibrium charge concentration is negligible compared to the injected charge and this forms a space charge region near the injection electrode. The ohmic behavior indicates that generation-recombination (G-R) centers dominate the conduction mechanism. These centers are generally defect levels positioned in the energy gap and they interact with both conduction and valence bands [30]. So, the MEH-PPV molecules lead to significant changes of the interface states and the trapping centers in Ag/MEH-PPV/SiNWs heterojunction. Figure 6 displays the capacitance-voltage (C-V) (a) and C 2 -V (b) plots of Ag/SiNWs and Ag/MEH-PPV/SiNWs Schottky diodes for different immersion durations recorded at 50 kHz. It is clearly that the C-V characteristics depend on the MEH-PPV layer. We notice that the capacitances of the different diodes decrease as the reverse voltage increases (Fig. 6a). This is mainly due to the width increasing of the space charge region for reverse-biased Schottky diode as well as to the presence of a high density of interface states [31,32]. The junction capacitance was analyzed in the reverse biased using the following relation: where ε s , ε 0 , N D , and V b are dielectric constant of silicon, permittivity of free space, equivalent carrier concentration and built-in voltage, respectively. Figure 6b shows a linear variation of C −2 with applied voltage. The interception of the straight line of C −2 -V curves with the voltage axis provides the V 0 value. The N D value is evaluated from the slope of the linear profile.
V p is the potential difference between the Fermi level and the top of valance band in the neutral region of SiNWs. V p can be calculated using the following relation: N v = 1.6 × 10 18 cm −3 is the effective density of states in Si valance band for 300 K [33].
The electrical parameters obtained from C 2 -V plots of Ag/MEH-PPV/SiNWs diodes are given in Table 4. The barrier heights deduced from C −2 -V are higher than that determined from Cheung method. The difference between the two values of φ b becomes small by increasing the immersion time. This behavior of Schottky barrier height obtained from two methods can be explained by the inhomogeneities that occur at metal/semiconductor interface [33,34]. This is due to structural defects, stacking faults, and non-uniformity of the interfacial organic layer at Ag/SiNWs interface [35][36][37]. From Table 4, we deduce that the presence of MEH-PPV leads to a decrease of N A and V d values which reach their minimums for an immersion time of 60 min. We proposed an   (Fig. 7). This diagram was built by referring to studies on similar heterostructures [13,38] and based on the results we made in the present work. The values of the potential barrier and the built in potential displayed in the diagram of Fig. 7 are those found for the Ag/MEH-PPV/SiNWs structure immersion duration of 30 min which presents the optimized time that gives the best quality of the diode in terms of ideality factor and series resistance. We carried out an annealing for the various diodes by varying the temperature from 50° to 250° with a step of 50°. I-V curves show good rectification behavior for diodes that contain MEH-PPV polymer obtained at low or medium immersion durations (30 min, 60 min, 90 min and 120 min) and for annealing below 200 °C. For high annealing temperature (AT), the I-V curves of the different structures show ohmic character. Figure 8 shows dV/dln(I) versus I plots obtained from forward bias I-V characteristics at different AT. The values of n and Rs obtained from the adjustment with the Cheung function (dV/dln(I)) are given in Table 5. From this table, we deduce that n and Rs changed with increasing the annealing temperature. For any AT, the values of n are between 3 and 4 for all structures except that obtained after immersion time of 120 min and annealed at 150 °C which shows an n value equal to 2.88. We also notice a decrease in ideality factor and then an increase were observed for annealing temperature above 150 °C. This behavior is seen for all immersion durations. Rs values vary between 228 Ω (found for SiNWs annealed at 200 °C) and 1978 Ω (found for d im equal to 120 min and an A.T. of 150 °C). We also note that Rs decreases with the annealing temperature. This result is similar to that found by Çatır et al. [31]. This decreasing and it is attributed to the reduction of undesirable impurities in the Ag/MEH-PPV interface by the annealing treatment. Figure 9 shows the H(I) plots and their adjustments by the Eq. (4). All curves are straight lines. The obtained values of the potential barrier (φ b ) for different annealing temperatures are given in Table 5. For all immersion durations, the barrier height decreases with increasing AT. The reduction in barrier height may be due to the thermal generation of carriers in the MEH-PPV layer; these carriers increase with increasing AT and neutralize immobile charges in the MEH-PPV/SiNWs interface [39]. The diode MEH-PPV/SiNWs will have an ohmic behavior for high annealing temperatures and the structure approaches an MIS structure. This is due to the strong thermal oxidation at high temperatures. These same findings are found in our previous work when we studied the annealing effect on Ag/P3HT/SiNWs type diodes [13]. The optimization of the elaboration parameters by the electrical study shows that the Ag/MEH-PPV/SiNWs Schottky diode obtained for immersion duration of 120 min and annealed at 150 °C exhibited better electrical properties in terms of ideality factor and barrier height. The obtained nanocomposites show characteristics as a candidate for producing organic/inorganic diodes and organic Schottky diodes (OSD).

Conclusion
The electrical properties of Ag/MEH-PPV/SiNWs diodes were investigated as a function of annealing temperature and immersion duration in the MEH-PPV solution. The I-V characteristics show the rectifying nature of the diode, with a rectification ratio reaching 125 for a 30 min immersion time. The diode parameters were obtained by adopting the Cheung method assuming the thermionic emission model of the Schottky Diode. Inserting a thin organic layer of MEH-PPV polymer between silver and SiNWs improved the performance of the Schottky diode. The annealing treatment is useful for minimizing interface states and contaminations that increase the quality of SiNWs/MEH-PPV-based diodes. The immersion duration of 120 min and annealing temperature of 150 °C are the optimized parameters. These parameters give the best performance of the diode. C-V measurements showed a   Funding The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

SiNWs
Data Availability Data will be made available on reasonable request.

Competing Interests
The authors declare no competing interests.