Temperature Dependent Current Transport Mechanism of Photopolymer Based Al/NOA60/p-Si MPS Device

A photopolymer based Al/Norland Optical Adhesive 60 (NOA60)/p-Si MPS (metal-polymer-semiconductor) device was fabricated by a combination of vacuum evaporation and smear technique. The current transport properties of the device were investigated by using the forward bias current–voltage (I-V) characteristic in the temperature range of 80–300 K. The cross-sectional structure of polymer/semiconductor was revealed by the scanning electron microscope (SEM) image and it was seen that the NOA60 photopolymer was tidily coated on the p-Si surface. According to the I-V measurements at room temperature, the MPS device exhibits a good rectification ratio of 8140 at ± 1 V. The temperature-dependent I-V measurements (I-V-T) were analyzed based on the thermionic emission (TE) theory and an abnormal increase in zero-bias barrier height (BH) and a decrease in ideality factor (n) was observed with increasing temperature. Additionally, two different linear regions with distinct values from the theoretical value of the Richardson constant (A*) were observed in the conventional Richardson plot. Such deviations from the ideal TE theory have been attributed to the effect of BH inhomogeneities. Gaussian distribution (GD) of the BH model has applied the I-V-T results and the double GD BH with mean values of 0.75 ± 0.08 eV (80–140 K) and 1.02 ± 0.11 eV (140–300 K) were calculated. Moreover, the A* value of 31.4 A/cm2K2 was calculated close to the known value of p-Si from the modified Richardson plot. Thus, it has been concluded that the current transport of the Al/NOA60/p-Si MPS device can be explained by TE with a double GD BH model for a wide temperature region.


Introduction
Radiation curing is an important technology, especially for coatings [1]. In contrast to conventional methods of coating, radiation curing does not require any evaporation and solvent [2]. UV polymerization is currently the most used technique for radiation curing [3]. In this method, a light-sensitive molecule named a photoinitiator is used. The final coating film is a combination of monomer and oligomers and reflects their properties [4]. Photopolymers have a widespread application area such as 3D printing, medical, and photoresist technologies with advantages of fast curing speed, absence of volatile solvents, flexibility, etc. [5].
Metal-semiconductor (MS) junctions or Schottky barrier (SB) diodes, which are among the best-known semiconductor-based devices, have found many applications since their discovery and continued to be one of the popular research topics [6]. One of the most interesting aspects of SB devices, which act as both rectifier and ohmic contacts in electronic devices, is that the oxide (MOS) or insulating layers (MIS) between the metal-semiconductor can change the device properties and gain new functional features [7][8][9][10][11][12][13][14][15]. Especially in recent years, MPS devices, in which polymer and similar organic materials are used as an insulating layer, have attracted attention due to their production advantages and cost reduction [16][17][18][19][20][21][22].
Although photopolymers have the potential to be a suitable interface material for MPS devices with the abovementioned properties, there are no MPS device studies, except for one example using NOA61 photopolymer material, recently published by the authors [23]. The UV curable NOA60 material of Norland Optics, which is preferred in applications related to optical elements due to its high light transmission and colorless [24], has been successfully used in applications such as a filter for imaging [25], microlens arrays [26,27], polymer nanoimprinting [28], WLED [29] and biochip [30].
As with all semiconductor-based devices, the determination of the effective current transport mechanism for MPSs is important in terms of demonstrating and improving the operating performance of the device under different conditions. One of the most common methods used for this purpose is current-voltage measurements of the device at different temperatures. In this way, the effects of different processes, such as thermally activated mechanisms and tunneling, on charge transport within the device can be determined. Although all interface materials, whether polymer, insulator, or oxide, used between the metal-semiconductor junction have an improvement effect on the device by the passivation of the dangling bonds, the other effects such as interface states, series resistance and inhomogeneities of the barrier can still affect the performance of the device [31].
The main motivation for this study is the possibility of simplifying the production methods and reducing the cost by using the optically transparent coating and interface material from the same polymer material for an MPS device. For this purpose, a p-Si-based MPS device was produced by using the UV-curable NOA60 photopolymer material of Norland Optics fabricated with the help of the vacuum evaporation and smear technique. According to the measurements at room temperature, the device exhibits good rectification behavior. Considering the temperature-dependent measurements in the range of 80-300 K, it has been shown that thermionic emission is the effective current conduction mechanism for T > 100 K in the device, and the Gaussian Distribution barrier heights between the metal-semiconductor affects this mechanism.

Experimental
MPS devices where NOA60 was employed as an interface were fabricated on p-Si (100) wafers. The pieces of 20 × 20 mm dimensions were cut from boron-doped silicon wafers and subjected to mechanical cleaning and then chemically degreased with the RCA method. This process included boiling in NH 4 OH + H 2 O and HCl + H 2 O 2 with deionized water for 10 min, drying in N 2 environment and cleaning in HF: H 2 O (1:1) solution for 30 s to remove unwanted contaminations and possible oxide layer on the surface. High purity aluminum was thermally evaporated with help of tungsten filament onto the backside of the Si wafer to create the ohmic contacts under the pressure of 10 -7 Torr. NOA60 liquid photopolymer purchased from Norland company was coated on the polished surfaces of p-Si wafers to form a homogeneous layer. Then, the photopolymerization was performed by a UV light (the UVGL-58) at a wavelength (λ) of 365 nm. The sample was cured by 10 mW/cm 2 irradiation intensity for 60 min. Al/ NOA60/p-Si device was fabricated by the evaporation of Al contacts in a dotted pattern employing a 1 mm diameter holed mask. The production steps of the Al/NOA60/p-Si MPS device are shown in Fig. 1. The cross-sectional structure of the coated thin photopolymer organic layer on the p-Si was investigated with the help of a JSM-7600F Schottky Field Emission and LEO 440 SEM. The I-V characteristics of the fabricated MPS device were measured in the temperature range of 80-300 K with the aid of a temperature-controlled ARS-DMX-1SS high-performance closed-cycle cryostat and Keithley 4200 SCS parameter analyzer under the dark. The temperature of the sample was monitored by a GaAlAs sensor and a Lakeshore 330 autotuning temperature controller with a sensitivity better than ± 0.1 K.

SEM Analysis
The cross-sectional structure of the coated thin photopolymer organic layer on the p-Si was investigated with the help of an SEM and is shown in Fig. 2a. A focused image of the photopolymer-silicon interface obtained with a different SEM device is added also in Fig. 2b. According to the SEM images, the NOA60 photopolymer was tidily coated to the semiconductor surface. The thickness of the interfacial layer was calculated as about 1.50 μm from the SEM image. Due to the low viscosity (300 cps at 25 °C) of NOA 60, an interface layer of less thickness could be obtained as a result of the coating process compared to other photopolymers in similar series.

T Dependent I-V Analysis
The current I flowing across an MIS or MPS device under bias V is usually modeled with the help of thermionic emission theory [32][33][34]: According to this theory, the I is caused by thermionic emission of the majority carriers across the Schottky barrier. (1) The current and the barrier height, Φ b0 are linked by the reverse saturation current expression given below [32][33][34].
where V is the forward-bias voltage, q is the electron charge, R S series resistance, k is the Boltzmann constant, T is the absolute temperature, A is the contact area, A * is the effective Richardson constant of 32 A/cm 2 K 2 for p-Si. Starting from Eq. 1, ideality factor n can be written as follows in regions where the lnI-V change is linear [32][33][34]: The values of Φ b0 can be extracted by using the relation, Figure 3 shows the lnI-V characteristic measured at room temperature (300 K) for the Al/NOA60/p-Si device. The rectifier behavior of the device is quite pronounced and the rectification ratio (RR) for ± 1 V was calculated as 8140.
In Fig. 3 temperature-dependent forward bias lnI-V curves are given for the fabricated MPS device in the temperature range of 80-300 K. As can be seen in the graph, there is a linear region at intermediate forward bias voltages. For higher bias voltages the curves deviate from linearity by The cross-section SEM images of the p-Si/NOA60 interface the effect of series resistance and polymer interfacial layer. It is also observed that the linear region shifts to lower voltage values with increasing temperature. Similar characteristics have been reported in previous studies for MIS and MPS devices with different interface layers [34][35][36] (Fig. 4). In order to determine the possible current transport mechanism for the Al/NOA60/p-Si device, the parameters I 0 , Φ b0, and n were calculated for the linear region of InI-V by using Eqs. (1)(2)(3)(4) at the different temperatures and the results are presented in Table 1.
As can be seen in Table 1, the calculated parameters are strongly dependent on the temperature, and the values of I 0 and Φ b0 increase with increasing temperature, while the values of n decrease. The temperature dependence of n and Φ b0 are graphically shown in Fig. 5. Values of n greater than 1, which is far from the ideal ( n=1), are generally attributed to the voltage drop on the polymer interface material, the distribution of states, and the inhomogeneity of the barrier at the polymer-semiconductor material interface [35,[37][38][39][40].
To explore in more detail, the temperature dependence of n, the graph of n-1000/T was plotted and shown in Fig. 6. According to Fig. 6, n takes values between 2.0 and 2.2 for the high-temperature region (240-300 K) and it is increasing rapidly with temperature below 240 K. It is seen that n for these two regions varies linearly with the inverse of the temperature. This linear change is known as the T 0 anomaly, and n is given by the expression [41,42],  where n 0 and T 0 are constants. While n 0 and T 0 were calculated as 1.35 and 206 for the high temperature region (T > 250 K), they were found as 1.85 and 74 for the low temperature region (T < 250 K). Such high values of n and its linear variation with temperature have also been reported in previous studies and explained by the inhomogeneities of the barrier height [43,44]. The measured temperature-dependent values of I 0 can be used for the calculation of Φ b0 . These values reflect the effects of image barrier lowering and other minority carrier effects [32,45]. According to Eq. (2), the plot of lnI 0 /T 2 vs 1/T is called as Richardson plot and gives a straight line. If Eq. 2 is rewritten, the intercept (at 1/T = 0) and slope of the Richardson plot give A* and Φ b0 , respectively. However, for real MS, MIS, and MPS devices the I-V curves deviate from ideal TE theory, especially for low temperatures. Some researchers have proposed modified Richardson plots to account for this deviation. The best-known of these models were summarized in the recent study by Wong et al. [45]. In Fig. 7 the ideal lnI 0 / T 2 vs 1000/T and modified by the Hackam and Harrop [46], lnI 0 /T 2 vs 1000/nT Richardson plots of Al/NOA60/p-Si MPS device has shown. While lnI 0 /T 2 −1000/T plot has two different linear regions as 200-300 K and 80-180 K, lnI 0 /T 2 varies linearly with 1000/nT entire temperature region. According to the ideal Richardson plot, the calculated A* and Φ b0 values are 1.77 × 10 -5 A/cm 2 K 2 , 0.42 eV and 2.86 × 10 -13 A/cm 2 K 2 , 0.12 eV for regions 200-300 K and 80-180 K, respectively. These values were obtained as 3.42 × 10 -4 A/cm 2 K 2 , 1.06 eV for the modified Richardson plot. The extracted A* values are much lower than the known value 32

qΦ b0 kT
A/cm 2 K 2 of the theoretical value of the Richardson constant for p-Si. The discrepancy between the experimental and theoretical A* values shows that these models are not adequate for calculating the Φ b0 of the MPS device. Such deviations reported in previous studies show temperature dependency of Φ b0 and have been associated with spatial inhomogeneities of barrier height [47]. Namely, there are low and high barrier regions at the metal-semiconductor interface and the current prefer the surmount the low barrier regions [34,48]. Based on the theoretical lateral inhomogeneities approach by Tung, a linear correlation between n and Φ b0 was demonstrated in the study of Schmitsdorf et al. [42,49]. In the Φ b0 -n plot of the Al/NOA61/p-Si MPS device shown in Fig. 8, three different linear regions depending on the temperature are observed. In the first region (80-120 K) These results indicate that lateral inhomogeneities are effective in the MPS device and the dominant current transport mechanism starts to deviate from the ideal TE model with decreasing temperature [34,50]. For MPS devices that behave differently from the ideal TE theory, the I-V curves can still be described by Eq. (1) if the barrier height and ideality factor parameters are replaced by their apparent counterparts (n ap and Φ ap ). According to the Gaussian Distribution (GD) model proposed by Werner and Guttler [51,52], the measured value of apparent barrier height value is given by where Φ ap is the apparent barrier height, Φ b0 is the mean Schottky barrier height and, σ S is the standard deviation of the Gaussian distribution. The temperature dependence of σ S is usually weak and can be ignored. The apparent value of n has been expressed as where n ap is the apparent ideality factor, ρ 2 and ρ 3 are voltage coefficients. These voltage coefficients depend on the temperature and they quantify the deformation of the barrier height by the applied voltage. The plots of Φ ap -q/2kT and 1/n-q/2kT depicted in Fig. 9 show the existence of barrier heights with GD for the NOA60 based MPS device. The presence of three different linear regions for both graphs indicates that there are effective triple GD BHs in the MPS device. From slope and the intercept of the Φ ap -q/2kT graphs, the BH values for these three regions were calculated as 0.75 ± 0.08 eV (80-140 K), 1.02 ± 0.11 eV (140-220 K), and 1.00 ± 0.11 eV (220-300 K) with their σ S values. In the previous studies on MIS and MOS devices, the double GD of BH was frequently reported and some of them are triple. Based on the fact that the BH values for the 2nd and 3rd regions are very close to each other and the sigma values are the same, it can be said that there is mainly the double GD BH for the MPS device. Likewise, ρ 2 and ρ 3 values were obtained by using slope and intercept values of linear regions in the 1/n-q/2kT plots. All calculated values are summarized in Table 2. The results show that the ideality factor expresses the voltage deformation of the GD BH. The parameter ρ 3 is associated with the width of the region. Accordingly, region II (140-220 K) with 1.02 eV BH is wider than region I. The existence of the third region at high temperatures is more pronounced. This also shows that the applied voltage becomes more effective with increasing temperature. As a consequence, both temperature and voltage change the GD of the BH and affect the I-V characteristic of the MPS device.
The new form of Eq. (6) can be written, considering GD of BH and using Eq. (7) The modified Richardson plot, ln(I 0 /T 2 )−q 2 σ S 2 /2k 2 T 2 vs 1000/T of the MPS device has been shown in Fig. 10. As can be seen from Fig. 10, there are two straight lines. The first temperature region where linear variation is observed is quite large and ranges from 120 K to room temperature (300 K). From the slope and intercept of this linear plot mean barrier height and Richardson constant were calculated as 1.02 eV and 31.4 A/cm 2 K 2 , respectively. The obtained A* value of the GD BH model is very close to the known value of 32 A/cm 2 K 2 for p-Si compared to other models. Moreover, the calculated Φ b value is also in good agreement with those obtained in Figs. 7 and 9. According to the results, the TE with double GD BH model emerges as the most suitable current conduction mechanism explaining the I-V-T characteristics for the MPS device for a wide temperature region (120-300 K). A* and Φ b values calculated from  Inhomogeneities of BH can be caused by poor interface quality, non-uniformity of surface states and dislocations, polymer layer thickness, some phase changes with temperature, and the energy band alignment of lowest molecular orbital of polymer with respect to the conduction band minimum of semiconductor [36,47,49,53,54]. These BH inhomogeneities have important effects on the current transport mechanism of the device. Besides, the photopolymerization is not a homogeneous process. As the photopolymer film is exposed to UV light, highly absorbed by the reactive organic monomers. According to the Lambert-Beer law, the light intensity decreases with distance from the illuminated region of the film through absorption. It gives rise to a monomer concentration gradient. The monomer molecules migrate from the bulk region towards the illuminated surface and accumulate near the illuminated surface [55]. Therefore, it may affect the polymer-semiconductor junction region.

Conclusion
An NOA 60 photopolymer-based MPS device was fabricated and investigated the temperature-dependent I-V characteristics. It exhibits a high rectification ratio of 8140. According to the SEM image of the NOA60/p-Si interface, the photopolymer homogeneously was coated onto the semiconductor surface with about 1.5 µm thickness thanks to its low viscosity. The temperature-dependent measurements show that I-V plots have a linear behavior for the low and moderate forward bias region and it deviates at the higher bias values due to the effect of the series resistance. The analysis of the region changing linearly reveals that with increasing temperature, zero bias BH increases and n decreases. It has been shown that the main reason for this behavior, which is incompatible with the TE current transport model, may be BH inhomogeneities and the GD BH approach is successful in explaining this deviation. The double GD BH with mean values of 0.75 ± 0.08 eV (80-140 K) and 1.02 ± 0.11 eV (140-300 K) were calculated from apparent Φ ap and n ap expressions. Furthermore, the modified Richardson plot showed that the constant A* was calculated as 31.4 A/cm 2 K 2 , which is the very closest value to that of the p-Si material. As a result, the TE current transport mechanism with double GD BH is a suitable model to explain T-dependent I-V characteristics of the photopolymer-based Al/NOA60/p-Si MPS device.