A poly(phenoxy-imine)/p-silicon rectifying device was fabricated and the current-voltage characteristics of the device were examined as a function of temperature in the 40–300 K range. The temperature dependence of the main parameters, namely, the barrier height (Φb), ideality factor (η), reverse current (I0) and series resistance (Rs), were investigated. It was seen that the Φb and the I0 values of the device increased with increasing temperature, while the η and the Rs values decreased. The temperature dependences of the Φb and the η were interpreted by the assumption of a Gaussian distribution of the barrier heights arising from barrier inhomogeneities that prevailed at the interface of the poly(phenoxyimine)/p-silicon. From ln(I0/T2) vs. 1/ηT plot, the values of the activation energy (Ea) and Richardson constant (A*) were calculated as 0.324 eV and 2.84×10-7 A cm-2K-2, respectively. The experimental value of the Rs from the forward current-voltage plots decreased with an increase in the temperature.
Conducting polymers are particularly attractive materials for the electronic industry. The conducting polymers are different from the inorganic conductors, since they can also act as a semiconductor. These properties can be explained by the fact that they behave as-quasi-one-dimensional systems due to their relatively intermolecular and strong intramolecular electronics interactions. For inorganic semiconductors, the doping process refers to the replacement of atoms in the crystal lattice. However, for conjugated polymers, it refers to oxidation or reduction of the polymer backbone. A polaron is generated as a result of oxidation. This polaron describes the lattice distortion which extends over several monomer units in the backbone.
Among the conjugated polymers, those with extended π-systems involving alternating C=C and C-C bonds are predominant. This is the case for many prototypical conjugated polymers such as polyacetylene, poly(p-phenylene) and poly(p-phenylene-vinylene). Since the C=N group is isoelectronic with the C=C group, the incorporation of the nitrogen atoms into the conjugated system leads to another class of conjugated polymers related to poly(p-phenylene vinylene)s, namely poly(Schiff bases) and polyketimines (polyketanils), which have imine and ketimine groups in the main chain or in pendant groups, respectively (1). Conjugated poly(phenoxy-imine)s can be considered as simple derivatives of the corresponding poly(phenoxy-ketimine)s in which an alkyl (R) group of the azomethine group is replaced by the hydrogen atom. In addition, conjugated poly(phenoxy-imine)s are composed of alternating single and double bonds between covalently bound carbon atoms, leading to one unpaired electron (the p electron) per carbon atom. This electronic delocalization provides the pathway for charge transport along the polymer main chain and pendant imine groups. When a conjugated polymer and an inorganic semiconductor are brought into intimate contact, electrons flow from the higher Fermi-level material to the lower one. So far, poly(phenoxy-imine)s have been synthesized by the oxidative polycondensation method, and their properties have been extensively investigated, but surprisingly, electronic device applications of a poly(phenoxy-imine) have not been studied sufficiently.
For conducting polymers there are numerous processing methods and for their integration into various devices and applications. In the semiconductor device applications e.g. Schottky diodes (2), heterojunctions (3), light-emitting diodes (LEDs), field effect transistors (FET), photodiodes (4) and photovoltaic cells, conjugated polymers have recently become major focal points of interest. During the last few years, many works have been performed to study the different aspects of conductive polymer/inorganic semiconductor interface formation using various deposition techniques. As the commercialization potential of these type of devices is considered to be high, the main advantage lies in the possibility of their low-cost fabrication.
Fabrication of solid state heterojunction or Schottky devices using conducting polymers is one of the most important applications: these include metal-insulator (low conductive polymer)-semiconductor (MIS) diodes and p-n junction diodes. In the present study, the contacts between a poly(phenoxy-imine), i.e. poly(3-((2-phenylhydrazono)methyl)phenol) hereafter poly(3-PHMP), and p-type Si exhibited good rectifying behavior and the poly(3-PHMP)-Si interfaces are considered to be Schottky contacts because the polymer does not support charge depletion and acts as a metal when in contact with the inorganic Si.
The ideal metal-semiconductor theory applies when both materials are perfectly pure and there is no interaction between the two materials. The charge transfer characteristics across the polymer-semiconductor interface are largely responsible for the electrical properties of the rectifying junction. The junction characteristics of the rectifying device are described by the response of the interface of device materials and the nature of the barrier also provides important information. The many physical properties of the rectifying device can be explained by the assumption of a homogeneous barrier. However, recently the consequences of the possible existence of inhomogeneity in the barriers have been frequently studied.
Our study mainly covers the deposition of poly(3-PHMP) film on a silicon wafer to obtain a rectifying junction and to investigate effects of the temperature on the rectifying device.
Poly(3-((2-phenylhydrazono)methyl)phenol) (poly(3-PHMP)) was provided by Kahramanmaras Sutcu Imam University, Faculty of Science and Arts, Department of Chemistry, Polymer Research Laboratory. The synthesis of poly(3-PHMP) was given in Ref. (5). Briefly, solutions of phenylhydrazine (1.08 g, 10 mmol) in ethanol (2 ml) and m-hydroxybenzaldehyde (1.22 g, 10 mmol) in ethanol (3 ml) were mixed and stirred for ~30 min at room temperature. The precipitated product was filtered and washed with cold ethanol. 3-PHMP was purified by recrystallization in ethanol to provide 2.06 g of the compound as a pale yellow-white solid. The purity was confirmed by silica plates and melting point determination. Thereafter, the reaction flask was charged under stirring with 7 ml of distilled water, 0.14 g (2.5 mmol) of KOH, and 0.53 g (2.5 mmol) 3-PHMP and was heated to the required temperature. Then the oxidant (NaOCl) was added. After reaction completion, the mixture was neutralized with HCl (1.0 m). The precipitate was separated from inorganic salts and unreacted 3-PHMP. The polymeric product (Figure 1A), dark brown powder, was dried in an oven at 105°C.
The Si wafer was cleaned using the Standard RCA-1 and RCA-2 silicon wafer cleaning process. Ohmic contacts were performed on the back side of the Si wafer by evaporating Al and the wafer annealed at 580°C for 3 min in a N2 atmosphere. The poly(3-PHMP) polymer film was grown on the Si substrate by simple drop coating. Figure 1B shows SEM image of as-prepared polymer film on the p-Si. SEM image have been obtained with a Zeiss EVO40 set. The image clearly shows the thorough deposition of the polymer over the p-Si.
Then Au contacts were deposited on top of the poly(3-PHMP) polymer film by vacuum evaporation at pressures of the order of 10-5 Torr. The thickness of the polymer film was determined from capacitance-voltage characteristics as 180 nm using a Hewlett Packard 4192 A (Hewlett Packard, Rohnert Park, CA, USA) (50 Hz–13 MHz) LF impedence analyzer. In the study, about 20 films/devices fabricated and only one of them was investigated. The I–V measurements of the poly(3-PHMP)/p-Si device were carried out with a Keithley 487 Picoammeter/Voltage Source in the dark. The measurements were performed in the temperature range 40–300 K. Poly(phenoxy-imine) has been supplied from Faculty of Science and Arts, Department of Chemistry, Kahramanmaraş Sütçü İmam University, Kahramanmaraş, Turkey. All measurements have been performed at Atatürk University, Faculty of Science, Department of Physics, in Erzurum.
Our study mainly covers the deposition of poly(3-PHMP) film on a silicon wafer to obtain a rectifying junction and to investigate effects of the temperature on the rectifying device. Although a known behavior has been obtained with temperature, response of the device to temperature is very regular due to the poly(3-PHMP) film. Hence, we thought that poly(3-PHMP) film may be useful material in applications of the temperature sensors.
Results and discussion
The current of the junction device can be affected, to some extent, by the charge, injection properties of the polymer/inorganic material interfaces and/or, the energy barriers for the injection of the charge carriers into the polymer material.
The forward and reverse bias current-voltage for the poly(3-PHMP)-Si device is shown in Figure 1. As can be seen in the figure, the experimental data showed particularly good rectifying characteristics and the reverse currents were almost independent of bias. However, the current was limited by the series resistance of the device for higher bias. For the actual diodes, the reverse current should be independent of the magnitude of the applied reverse bias voltage. However, our experimental data revealed that the experimental reverse currents depended on the bias as monotonically.
The forward bias (V) current of the Schottky diode is given by;
where I0 is the saturation current and indicates that the current approaches an asymptote and becomes independent of the voltage V. It is given by:
Here A* is the effective Richardson constant and depends on the carrier effective mass m*:
The Φb given in the Eq. 2 is the barrier height and calculated from Eq. 4:
The quality of a diode can be assessed by its ideality factor η (or n), calculated using the diode equation. For non-ideal diodes, the current-voltage relation, Eq. 1, changed as:
Thus, when η is 1, the diodes are ideal. According to Eqs. , ,  and  the ideality factor describes the voltage dependence of the barrier height and it can be determined from the slope of the linear part of a ln(I/(1-exp(-qV/kT)) versus V plot using Eq. 6:
Schottky barrier heights and the diode ideality factors of the device varied in the range of 0.64–0.10 eV and 1.90–8.56, respectively, in measured temperature range. For 300 K, the ideality factors have been found as 2.38, 3.83 and 4.0 in Refs. (6–8), respectively. The apparent barrier height and the ideality factor were found to be strongly temperature dependent, as shown in Figure 2. In the figure, the barrier height values increased and the ideality factor values decreased when the temperature was increased. These observations are attributed to the inhomogeneity barrier height concept because the observations were not supported by thermionic emission theory. According to the barrier inhomogeneity model, the interface is composed on a distribution of the regions with different barriers instead of one uniform barrier height. The reason of the barrier inhomogeneity is atomic inhomogeneity at the interface too. Hence, the interface of the poly(3-PHMP)-Si diode is composed of a distribution of regions with low and high barrier height. In the inhomogeneous barrier diodes, the experimental barrier height lowering and the ideality factor increasing with decreasing temperature are not compatible with the thermionic emission theory of homogeneous junction devices (9). Furthermore, the increase in the barrier height can be explained by the lateral distribution of the barrier height when the temperature increased.
Figure 3 shows the reverse current versus temperature. It is observed that the reverse current increased with the increasing temperature above 120 K due to the increasing of tunneling of the carriers toward the higher temperature.
Figure 4 depicts a plot of η vs barrier height (Φb) each temperature for the poly(3-PHMP)-Si Schottky diode. The plot exhibited two linear regions. The first one was from 0.1 to 0.22 eV and the other was above 0.25 eV up to 0.64 eV. This variation can be explained by lateral inhomogeneities of the barrier height (10, 11). In the barrier inhomogeneity model, interface states have noticeable effect on barrier. The rectifying contacts may be sensitive to the lateral inhomogeneous distributions of the barrier height when the inhomogeneities exhibit lowers barrier heights. The extrapolation of a straight line in Figure 4 to η=1 gives homogenous barrier heights of approximately 0.306 eV for low temperatures (below 80 K) and 1.32 eV for high temperatures.
The apparent barrier height is given by Eq. 7 (12):
where Φ̅bo refers to the mean (average) barrier height (BH) and σ0 is the standard deviation at zero bias and it is the measure of barrier height inhomogeneity. According to Eq. 7 a plot of Φb versus q/2kT gives a linear behavior and can be used to determine the standard deviation at zero bias and the mean barrier height (BH). The temperature dependence of standard deviation is usually small and can be neglected. In this model the observed variation of ideality factor with temperature is given by Eq. .
The experimental Φb versus q/2kT and (η-1-1) versus q/2kT plots are shown in Figure 5. These plots each had two linear regions. The Schottky barrier may or does not have a single barrier height in most experimental researches, but it has spatial distribution and is experimentally measured, which the barrier height distribution does show more or less Gaussian distribution. One reason for that may be the fabrication method namely; the experimental conditions may affect the nature of the device. So, the poly(3-PHMP)-Si Schottky diode depicted double Gaussian distributions with Φ̅b(T=0)=0.96 eV and σ0=0.131 eV in the 40–80 K and Φ̅b(T=0)=0.38 eV and σ0=0.044 eV in the 100–300 K.
where and are proportionality coefficients of the bias dependence of the mean barrier height, where σΦ is the standard deviation of barrier height. Similarly, the ideality factor exhibited a linear variation depending on the temperature with two regions. For the ideality factor, the fit of the figure gave ρ2=-0.373 V and ρ3=-0.0068 V in the 17–45 (1/eV) and ρ2=-0.48 V, ρ3=-0.0032 V in the 45–150 (1/eV) range, respectively.
The decrease in the ideality factor with increasing temperature was also shown in Figure 6. As can be seen in Figure 6, the change in the η with temperature has been found to change linearly with inverse temperature as follows:
where η0 and T0 are constants and they were found to be about 0.76 and 296 K, respectively. The analysis of Eq. 9 indicates that the ideality factor is affected by the interface states in the presence of the poly(phenoxy-imine) film between Au an p-Si. The main density of states between metal and semiconductor without any interlayer are due to the dangling bonds which may occur on surface semiconductor. Namely these are states which are in equilibrium with the semiconductor. In addition to these surface states, there may additional states called metal induced gap states (MIGS) due to the effects of the contact metal. An increase of the density of interface states (DOS) results in an increase for the ideality factor. After inserting a poly(phenoxy-imine) film between Au and p-Si there will be new DOS and it is possible to additional increase of the barrier height and these may behave as traps. Our results confirm the density of states results in the poly(phenoxy-imine) film.
The temperature-dependent ideality factor was proposed to be included in the expression of I0 by Hackam and Harrop as follows: (13)
In this instance, the modified Richardson plot or the Arrhenius plot should be ln(I0/T2) versus 1/ηT plot and this plot is shown in Figure 7. The plot was found to be linear in the measured temperature range. In the ln(I0/T2) versus 1/ηT plot the values of activation energy Ea and Richardson constant A* were determined as 0.324 eV and 2.84×10-7 A cm-2K-2, respectively. The value of A* is much lower than the known value of p-Si.
In an ideal diode, the current becomes independent of the reverse bias voltage. However, the experimental reverse currents of the poly(3-PHMP)-Si device vary with reverse bias voltages, as seen in Figure 8.
An important parameter of Schottky diode characteristics is series resistance. In practice, the effect of series resistance is often observed for Si Schottky diodes at high forward biases. The series resistance of conventional Schottky diodes contains two parts namely, the resistance in the substrate and the resistance in the epitaxial layer. If the series resistance is large, it limits the forward current at high bias and the I–V characteristic becomes linear rather than exponential. The series resistance of rectifier can be extracted from the forward biased diode for high voltages as a function of temperature. Norde’s technique (14) was used to calculate the Schottky parameters such as the barrier height, and the series resistance. For this purpose, a (V) function is plotted versus voltage and this function is given as follows:
Figure 9 shows the F(V)-V plots of the poly(3-PHMP)-Si device. Using Norde’s function the barrier height and the series resistance values are determined by Eqs.  and :
where F(V0) is the minimum of F(V) and is the minimum voltage corresponds to F(V0).
The temperature dependence of the barrier height and the series resistance obtained from Norde model of the poly(3-PHMP)-Si device are shown in Figures 10 and 11. The variation of the barrier height with temperature was linear like that in Figure 2. It is clearly seen in Figure 12 that the Rs values are strongly dependent on the temperature, which it decreases with the increasing temperature. This variation can be attributed to non-ideal diode characteristics of the poly(3-PHMP)-Si device and non-pure thermionic emission (TE) mechanism due to the low temperature effects.
The saturation current, the barrier height, the ideality factor and the series resistance values were determined and it was found that all parameters were strongly temperature-dependent. It is found that the barrier height of the junction device increases while the corresponding ideality factor (η) decreasing with an increase in the temperature. The temperature-dependence of the barrier height of poly(3-PHMP)-Si device showed a double Gaussian distributions with Φ̅b(T=0)=0.38 eV and σ0=0.044 eV in the 100–300 K and Φ̅b(T=0)=0.96 eV and σ0=0.131 eV in the 40–80 K. The values of the series resistance (Rs) determined from Norde’s method were strongly the temperature-dependent.
The financial support by “Atatürk University Scientific Research Project Council (Project No: 2013/312)” is gratefully acknowledged.
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