Negative series resistance and photo-response properties of Au/PPY-MWCNTs composite/TiO2/Al2O3/n-Si/Al photodiode

The paper addresses a novel approach concerning the appearance of negative series resistance (Rs) at high frequencies for both temperatures and voltages. Most of the previous studies have focused on the relationship between voltage and current (I - V) to determine the value of Rs using several methods. By measuring capacitance and conductance as a function of voltage, we were able to develop a systematic analysis of series resistance. At high frequencies of 2 × 107, 107 Hz, Rs has negative values however, at frequency 106 Hz it takes both positive and negative values, whilst from (105 − 10) Hz it has positive values. Here in this article, we synthesized Au/PPY-MWCNTs/TiO2/Al2O3/n-Si/Al structure which can be used in a variety of applications such as supercapacitors, and diodes. We investigated the electrical properties such as ideality factor (n), barrier height (фb), series resistance using several approaches such as conventional, Chueng, and Nord methods. The structure has shown rectification with a good response to daylight illumination. The structure response to daylight illumination indicates that photodiodes have the potential to be used as solar detectors.


Introduction
The conductance-voltage (G-V) and capacitance-voltage (C-V) characteristics are pivotal tools for exploring the electrical properties and conduction mechanisms of Metal-Semiconductors (MS), Metal-Oxide-Semiconductor MOS, Metal-Polymer-Semiconductor (MPS), and Metal-Insulator-Semiconductor (MIS), conducting polymers with multiwalled carbon nanotubes (MWCNTs) composites [1][2][3][4][5][6][7]. Due to the presence of an interfacial oxide layer, boundary states, and series resistance, nanotubes (MWCNTs) composites deposited on the oxide layer on silicon structures differ from those expected for their perfect case [8][9][10][11][12][13]. In the previous year's, the importance of silicon technology, the semiconductor/oxide (Si/SiO 2 ) interfacial boundary, and surface imperfections were extensively discussed [14,15], although, insufficient revisions have examined the G-V-T and C-V-T characteristics to define factors such as border state and series resistance of MOS, MS, and MIS Schottky diodes [16][17][18][19]. Trial results, particularly at room temperature, do not provide comprehensive data about the nature of the barrier formation or conduction mechanisms at the Metal -semiconductors (MS) or Metal-Oxide-semiconductors MOS Schottky diodes [20]. Consequently, the G -V and C-V characteristics of these device has been studied in a wide range of temperatures (80-400 K). There are several methods for calculating series resistance (Rs), the theoretical expression of Rs is quite un-cleared and has not been disclosed in the literature [21][22][23]. However, the method proposed by Nicollian and Goetzberger [24] for determining R s of any MOS structure is thought to be more accurate than the others. In addition to that, the NSS is asymmetrical with the semiconductor in MOS constructions with a sufficiently thick interfacial oxide layer, and they cannot work along with the metal. Here we synthesized a composite of Polypyrrole with multi-well carbon tubes which was then deposited on TiO 2 /Al 2 O 3 /n-Si structure. The original in this article is the series resistance (Rs) has negative values at high frequencies for all temperatures and voltages. Previously, several approaches have been used to study R s and its behavior depending on the relationship between voltage and current (I -V). Here we presented a complete study of series resistance by studying the capacitance and conductance with voltages and frequencies. At high-frequencies of 2×10 7 Hz and10 7 Hz, the series resistance (R s ) has negative values; however, at frequency 10 6 Hz, Rs has positive and negative values, whilst from (10 5 − 10) Hz, Rs takes positive values. The I-V charcuteries have been used to investigate the electrical properties of PPY-MWCNTs composite/TiO 2 /Al 2 O 3 /n-Si such as ideality factor (n), barrier height (ф b ), and series resistance (R s ) using several approaches such as conventional, Chueng, and Nord methods. Also, the structure has shown a good response to daylight illumination confirming its photodiode properties.

Materials and methods
The Au/PPy-MWCNT/TiO 2 /Al 2 O 3 /n-Si/Al structure was synthesized by mixing the two suspensions of polypyrrole (PPy) and MWCNTs, the resources of PPy and MWCNTs were purchased from Sigma Aldrich with a purity of 99.9%. A crystalline Silicon wafer was cleaned to eliminate all surface contaminations, then a thin layer of aluminum oxide Al 2 O 3 was deposited on the surface of the silicon wafer using a spin coater. likewise, in the same way, a thin film of titanium dioxide (TiO 2 ) dropped was deposited upon the aluminum oxide. Finally, drops of PPy-MWCNTs composites were deposited on the surface of the titanium dioxide film (TiO 2 ) to obtain PPy-MWCNTs/TiO 2 /Al 2 O 3 structure. To measure the electrical and dielectric properties of the structure, two electrodes from gold and aluminum were deposited on the upper and lower surface. , it is obvious that R s is robust with both voltage and temperature in the temperature range of (223-323K), which increases with temperature decrease. This variation in R s as a function of temperature is possible for semiconductors in the temperature range where no freezing performance of the carriers. We believe that the trap charges have enough energy to leak from the traps located at the metalsemiconductor interface.      dispersed for all voltages and have positive values at different temperatures as the following (223, 253, 273, 293, 323) K, (-35×10 4 to 7×10 5 , -22×10 5 to 5×10 5 , -25×10 5 to 45×10 5 , -37×10 5 to 30×10 5 , -22×10 4 to 45×10 4 ) Ω. Figures 5(a)-(e) shows the series resistance as a function of frequency at different temperatures and voltages of Au /PPy-MWCNT composite/TiO 2 /Al 2 O 3 /n-Si/Al. The series resistance merged at high frequencies as displayed in all figures and having negative values while at lnf=14 Hz, Rs have relaxation peaks with negative and positive values, on the other hand at lnf=13 Hz, R s curves dispersed at all temperatures. At a high temperature of 323 K and voltages of V=0 V, V=−1 V, the curve represents Rs at increases with temperature increase, it violates the rule and deviates from it as seen in figures (c, d). The Rs figures merged before and after relaxation as well as their values at various voltages are (2,1, 0, −1, −2 )V,(−75×10 3 to 15×10 4 , −35×10 4 to 7×10 5 , −37×10 4 to 55×10 4 , −25×10 4 to 2×105, −22×10 5 to 1×10 4 )Ω. Figures 6(a)-(e) displays the series resistance as a function of temperature at different frequencies and voltages of Au /PPy-MWCNT composite/TiO 2 /Al 2 O 3 /n-Si/Al. The R s at Voltage=−2 V takes negative values at high frequencies from (2×10 7 to 5×10 6 ) Hz, its values independent of temperatures, although at frequencies between (10 5 to 10) Hz, it has positive values, and decrease with temperatures increase as shown in figure 6(a). At Voltages equal (−1 V, 0 V), and high frequencies, Rs takes negative values, while at all other frequencies, it takes positive values. Rs has shown several behaviors with temperatures, the first one decreases with temperature increase, while the second behavior decreases with temperatures increase as seen in figures 6(b), (C). At Voltages equal (1 V, 2 V), Rs has the same behavior, it decreases with temperature increase as seen in figures (d, e). The difference in Rs with temperature may be attributed to the parameters that cause the ideality factor to increase and the absence of free carrier concentration at low temperatures [33].  Figures 8(a), (b) displays lnI versus lnV at different temperatures, two conduction mechanisms can be seen, the first of which is tunneling as seen in figure 8(b) where a negative resistance appeared. The second mechanism appeared as straight lines as seen in figure 8(a). The ideality factor (n) and barrier height (f b ) can be obtained from the slope and y-axis intercept of the I-V plots. Regarding to this theory, the current can be given by the following equation [34]:

Electrical properties
Anywhere, I o is the interrupt of the lined area of the lnI-V plot; V and q are the applied voltage and charge of the electron, respectively. k is Boltzmann's constant, n is ideality factor, T is the temperature. A is the diode area and A * is Richardson constant (A * =32 Acm −2 K −2 for p-type Si) and Ф b is the barrier height. From equations (1) and (2). The barrier height and ideality factor formalism can be arranged as given in equations (3), (4), in addition to that, these factors were determined using different methods such as conventional, Cheung, and Nord according to the following equations [22,35,36].
Where H(I) can be arranged as: Figure 5. A straight line is drawn from the I-V curves using equations (6) and (8), the linear part of dV/d(lnI) plot against I can be used to calculate the value of R s through the slope, and the interrupt of this plot gives nkT/q value. So, both n and R s values are determined. Similarly, the plot of H(I) against (I) concerning equation (8) provides also a straight line where y-intercept and slope give nФ b and R s respectively. Using Although the ideality factor in an ideal diode is one [37], the structure we synthesized has two oxide thin films sandwiched between PPy-MWCNTs composite and Si, which increases the ideality factor and series resistance as seen in table 1. This condition may be attributed to border state spreading or boundary inhomogeneity due to the border layer's inhomogeneity [38][39][40]. Figure 12 illustrates the I-V of the Au/MWCNT-Ppy composite /TiO 2 /Al 2 O 3 /n-Si/Al structurebased photodiode under dark at room temperature and under the illumination of intensity 100 mW cm −2 . The structure has shown a good response to the illumination confirming its photodiode properties that may be used in the photodetector industry [41,42]. The photo-response of the Au/ MWCNT-Ppy composite /TiO 2 /Al 2 O 3 /n-Si/Al structure has been explored by measuring the  photocurrent under daylight illumination of 100 mW cm −2 as displayed in figure 13. As the light is turned on, there is an unforeseen increase in current, which can be attributed to the increased thickness of the depletion area under reverse bias, which leads to a reduction in the recombination of the photogenerated electron-hole pair. As a result, increased electron population in upper-energy conditions contributes to rapidly increased photocurrent.

Conclusion
The work discussed a novel behivour of series resistance as it has negative values at high frequencies in a wide range of temperatures and voltages. Most of the prevoius studies have focused on studying the relationship between voltage and current (I -V) to assess the importance of R s in electronic devices by several methods. Here we presented a comprehensive study of series resistance by exploring the capacitance and conductance with voltages and frequencies. We synthesized Au/PPY MWCNTs/TiO 2 /Al 2 O 3 /n-Si/Al structure that has considerable sensitivity to light to explore their use in photodetectors or solar cells. We studied the electrical properties such as ideality factor (n), barrier height (ф b ), series resistance using several approaches such as conventional, Chueng, and Nord methods using I-V and C-V characteristics. The results showed that at highfrequency 2×10 7 , 10 7 Hz, the series resistance (Rs) has negative values however, at frequency 10 6 Hz it takes both positive and negative values, whilst from (10 5 -10) Hz, Rs has positive values. The structure has shown photodiode properties that enhance its use in photodetector and solar cell applications.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).