Enhanced Optical and Electrical Properties of ITO / Ag / AZO Transparent Conductors for Photoelectric Applications

The enhancement of the optical and electrical properties of TCO films was investigated by depositing different layers of AZO (100 nm), Ag (5 nm)/AZO (95 nm), and ITO (45 nm)/Ag (5 nm)/AZO (50 nm) upon n-Si substrate at room temperature by magnetron sputtering method. The ITO/Ag/AZO device efficiently improved the electrical and optical properties with the low sheet resistance of 2.847Ω/sq. and an increase in the rectification ratio of 455.60% when compared with AZO and Ag/AZO devices. The combination of ITO/Ag/AZO provided the optimum results in all the electrical and optical properties. These results showed that within the optimized thickness range of 100 nm, compared to AZO and Ag/AZO, ITO/Ag/AZO device showed the improvement for both optical and electrical properties at room temperature.


Introduction
The rapid growth of photovoltaic (PV) devices dominates the industry today due to its meritorious effect on the environment by harvesting solar energy from the sun.This includes superior efficiency, competitive cost to produce electricity, and long-term reliability [1,2].Recently, significant work has been put forward to produce low-cost and highly efficient photoelectric devices using thin film technology imbibing the heterojunction concept.
The heterojunction devices are the devices in which the interface is between two solid-state materials having different structural and optical properties.One such use of the heterojunction devices is the implementation of transparent conducting oxide (TCO) layers in the Si-based devices, and it is one of the important developments in the ongoing progress of photovoltaic research [3].
TCO materials have attracted much attention for its significance in optical and electrical applications from the displays to the thin film solar cell technology and are used as transparent conductive electrodes in virtually every thin film solar cell design and single crystal cells.These coatings can reduce energy loss by as much as 30% [4].Thus, these qualities make TCO a promising material which can improve the efficiency of the photoelectric devices.Among all the TCOs, indium tin oxide (ITO) and aluminum zinc oxide (AZO) are widely used in commercial applications such as displays, solar cells, and photodiodes.ITO is known for its excellent electrical conductivity and AZO for its improved optical transparency and thermal stability [5][6][7][8].Hence, to achieve both properties of enhanced transparency and good conductivity in a single device, various combinations of layered TCOs were developed.Kim et al. reported that the bilayered ITO/AZO exhibited higher optical transparency and electrical conductivity than those of single-layer ITO and AZO films.
Hence, it was understood that overall functionalities of bilayer TCO structures were better than single-layer TCOs.However, TCO layers offer relatively high resistivity and low transmittance at room temperature, and the charge carrier generation and separation process still lags behind which lead to the reduction of photoresponse and rectification properties in the device [9].In this aspect, insertion of a thin metal layer such as Ag, Au, and Ni in combination with TCOs has shown to be an effective candidate for the reduction of sheet resistivity.When the light is incident on these inserted metals in between the TCO materials, the free electrons available in the metal layer speed up the light transportation from the top to the bottom of the device.Also, the metal layer helps to thrust up the separated charge carriers swiftly to the top electrode [10].
Several combinations of TCO/metal/TCO structures were analyzed and reported by various researchers.Yun et al. reported that the combination of ITO/Ni/AZO enhanced the electrical conductivity, mobility, and the carrier concentration [11].Other combinational structures having different metal layers such as ITO/Au/ITO [12], TCO/Ag/ TCO [13][14][15], AZO/Mo/AZO [16], and AZO/Au/AZO [17] were also reported.Each combination has its own individuality in enhancing overall performance of the device.Chua et al. reported that the multilayer structure of ZnO/Ag/ZnO having high transmittance of 90% at 580 nm and low sheet resistance of 3 Ω/sq.was fabricated by the Magnetron Sputtering [18].Sahu and Huang reported the multilayer electrodes of ITO/Ag/ITO achieving a low sheet resistance below 6 Ω/sq.and an optical transparency above 90% with overall thickness value of 100 nm at room temperature conditions by sputtering [19].These reports indicate that the enhancement of the Ag inserted heterojunction structures for PV devices.Besides, Ag film is chosen to improve the electrical properties of the electrodes further because Ag has the lowest resistivity of all metals for bulk materials under normal conditions [20][21][22][23].
In this work, we have presented the improved electrical and optical properties of ITO/Ag/AZO-trilayered photodetectors.A thin Ag layer is inserted between the top ITO and bottom AZO layers.The design was carried out by depositing the three layers, step by step.The optical and electrical properties of ITO/Ag/AZO sample were compared with AZO and Ag/AZO layers within the overall conditional thickness of 100 nm.Here, the interdependence between the electrical and optical properties of single-layer and multilayers has been investigated as a function of the film thickness and deposition time with the objective of developing highperformance transparent conductive electrodes by sputtering technique at room temperature.This study provides a new design for improved quality of TCO thin films in optoelectronic applications at a low cost.

Experimental Procedure
Prior to the growth of ITO/Ag/AZO, n-type silicon (Si) substrates were chemically cleaned by the ultrasonication method to avoid any contamination on the surface.AZO film was deposited on n-Si substrate by a sputtering system (SNTEK, Korea) at an RF power of 300 W.Then, a thin Ag layer was deposited over AZO layer by applying DC power of 50 W to the Ag (99.99%) target.Following this, a DC power source of 300 W was applied to an ITO target to form a layer upon the Ag film.Sputtering process has been performed at room temperature in an Argon atmosphere that was adjusted during the layer (ITO/Ag/AZO) deposition with varying the sputtering time for each layer.Aluminum (Al) grid was introduced on the top ITO layer, and the Al back contact was formed with Si as Ohmic electrodes.Both the prepared samples were annealed at 300 °C by rapid thermal annealing process for 10 minutes to increase the crystallinity of the sample device.
A field emission scanning electron microscope (FE-SEM, FEI Sirion) was used to observe the cross-sectional and top view images of the prepared samples.The crystal structures and preferential orientation of the TCO and Ag films were analyzed by X-ray diffraction (XRD, Rigaku, D/Max 2500) with Cu K-α radiation in θ-2θ scan mode.Optical transmittance studies were observed for the samples deposited on glass substrate with the UV spectrophotometer (Scinco, Neosys-2000).The electrical and optical properties were achieved using a probe station with the measuring instruments (Keithley 2440).

Results and Discussion
Three different samples of AZO (100 nm)/n-Si, Ag (5 nm)/ AZO (95 nm)/n-Si, and ITO (45 nm)/Ag (5 nm)/AZO (50 nm)/n-Si upon n-Si substrate are prepared which are schematically presented in Figure 1(a).The three devices were fabricated such that the optimum thickness was fixed for all the three devices, that is, overall 100 nm for each device.The thickness of each material measured at 10 points by FE-SEM.The thickness error range is shown in Figure 1(b) and Table 1.
The surface morphology of the three samples was examined using FE-SEM analysis.Figures 2(a Ag layer revealed the lowering of sheet resistance ( [13,16] and [24]).In the top view of the prepared samples as seen in Figures 2(d), 2(e), and 2(f), a uniform distribution of grains on the surface was observed.The grain size in the sample S3 (Figure 2(f)) was increased when compared to the S1 and S2 samples (Figures 2(d) and 2(e)).
The combination of metal and substrate is determined with indentation response.The relationship is given as follows: where σ y is the yield stress and E is the modulus.The grain size is dependent to the yield strength, and this relationship is well known by Hall-Petch relationship.This is given as follows: where d is the grain size and σ 0 is the intrinsic yield strength [25][26][27].The Ag layer is applied to the transparent electrode by seed layer in this research.We can control the grain size by changing the Ag layer thickness, and which is seed layer to improve the properties of the transparent electrode by depositing ITO on Ag seed layer.
The grain size was measured by XRD analysis, and it is shown in Figure 3.By specifying the size of individual grains as symbol D, the size distribution of many crystallites can be approximated by the distribution function g (D).For a given sample, the particle size distribution function g (D) is generally quite different from the thermal length distribution function p (L). Converting p (L) to g (D) requires knowledge of the shape of each crystal since the distribution of the column lengths of a given grain is related to the geometric boundaries of the grain.The general relation between the two distributions is given as follows: where N is the normalization factor and f(L 0 , D 0 ) dL is the number of columns in the single crystal of size D 0 , perpendicular to the reflecting surface, with a length between L 0 and L 0 + dL.If D 0 is too small to accommodate the columns which assumed length is L 0 , f(L 0 , D 0 )dL = 0.As you can see below the length distribution, p (L) is contained in Bragg peak profile's Fourier coefficient to be measured in the diffraction experiment.The Bragg peak profile is expressed as a Fourier series.It is given as follows: where L is the conjugate variable to (s − s 0 ) and A(L) and B(L) are the cosine and sine Fourier coefficients.s is a function of the diffraction angle θ and the X-ray wavelength λ. s is expressed as (2sin θ)/λ.The average grain size (D) area and the volume-weighted average grain size (D) vol for a normal distribution are given as follows: The width of g (D) is gained from the ratio of (D) vol to (D) area [28][29][30].
The optical quality of the AZO, Ag/AZO, and ITO/Ag/ AZO thin films based on its transparency was examined with the aid of UV-visible spectrophotometer, as shown in Figure 4.The transmittance spectra of AZO, Ag/AZO, and ITO/Ag/AZO with a reference glass sample are depicted in Figure 4 as a function of wavelength.The optical transmittance of the glass substrate is about 85% over the visible range of wavelength.The average value of transmittance for AZO, Ag/AZO, and ITO/Ag/AZO samples are 79.21%,31.37%, and 40.96%, respectively.Accordingly, the Ag/AZO bilayer shows a lower transmittance characteristic as compared to the AZO single layer and ITO/Ag/AZO trilayer.This is mainly due to the Ag layer as Ag is known for its light scattering phenomena especially in the wavelength range beyond 450 nm.These transmittance characteristics can be compared with the results reported by Sahu and Huang [19], Boscarino et al. [13], Jung et al. [14], and Ren et al. [15] for the trilayered photodetectors.The sheet resistance measured using the probe method for the AZO, Ag/AZO, and ITO/Ag/ AZO samples was 23.65 Ω/sq., 13.962 Ω/sq., and 1.106 Ω/sq., respectively.The increased carrier concentration is the reason for highly reduced sheet resistance observed in Aginserted sample.The figure of merit (Φ TC ) is an important factor which evaluates the overall performance of TCO materials and is given [32] as follows: where T is the transmittance and R s is the sheet resistance.5 International Journal of Photoenergy compared to them at Ag/AZO/n-Si device.The reflectance profile of AZO, Ag/AZO, and ITO/Ag/AZO samples was shown in Figure 4.The enhanced absorption and reduced reflection are the two most important factors for highly efficient photo devices [33].The average reflectance of prepared samples of AZO, Ag/AZO, and ITO/Ag/AZO was 4.26%, 25.11%, and 15.69% (400 nm-700 nm), respectively.
The fabricated AZO/n-Si, Ag/AZO/n-Si, and ITO/Ag/ AZO/n-Si heterojunction photodetectors were characterized to examine the diode performance under the dark as shown in Figure 5.The samples were deposited with the aluminum (Al) grid on the top surface.Al was also sputtered on the Si substrate as a back electrode.For a photovoltaic device performance, it is important to form a    7 International Journal of Photoenergy quality junction; otherwise, the photovoltaic effect will be faded away by recombination in advance of collecting the photo-generated carriers [7].The heterojunction AZO/n-Si gave a clear rectifying current flow with the rectifying ratio of 6.49 which was higher than Ag/AZO/n-Si with a rectifying ratio of 4.97.But in the third sample, ITO/Ag/AZO/n-Si showed a high rectifying ratio of about 113.38 indicating an improvement in the quality of the junction device.The electrical and optical studies are described in Table 2.
Each material work function of ITO, AZO, and Ag is about 4.7 eV, 4.26 eV, and 4.3 eV.The band diagram of ITO/Ag/AZO/n-Si is shown in Figure 6.ITO/Ag/AZO transparent electrode makes the electron accelerate and that makes result that the current increase at forward bias.Also, it reduces the saturation current by inhibiting the recombination of electron and hole [34,35].As a result, Schottky curve is shown clearly at dark I-V characteristics and rectifying ratio is improved.
The photoresponse studies of the AZO, Ag/AZO, and ITO/ Ag/AZO devices are observed as shown in Figures 7(a), 7(b), 7(c), and 7(d) for different wavelengths such as 380 nm, 600 nm, 900 nm, and 1100 nm, respectively.At the wavelengths 380 nm, 600 nm, and 900 nm, the photoresponse of ITO/Ag/AZO was prominent when compared to AZO and Ag/AZO devices, respectively.At wavelengths of 380 nm (Figure 7(a)), 600 nm (Figure 7(b)), and 900 nm (Figure 7(c)), the photoresponses of ITO/Ag/AZO were prominent than those of the AZO and Ag/AZO layers.But at a longer wavelength of 1100 nm (Figure 7(d)), the photoresponsive behavior of ITO/Ag/AZO-, Ag/AZO-, and AZO-layered devices was comparatively very low.In particular, the photoresponse of ITO/Ag/AZO was considerably lower than that of the Ag/AZO and AZO devices at high wavelength region.Since, it is a characteristic feature of ITO layer that excites free carriers without creating electron hole pairs at longer wavelength of 1100 nm, resulting in poor photoresponse [36].But at wavelengths of 350 nm and 600 nm, the photoresponse of ITO/Ag/AZO is much better than that of the AZO-and Ag/ AZO-layered devices.These two wavelengths are essential for most of the photoelectric applications [32].It is noteworthy to mention that the ITO/Ag/AZO device exhibited the high photoresponse at the wavelength of 900 nm which is also essential for Si-based solar cells and photodetectors [36,37].When AZO, Ag/AZO, ITO/Ag/AZO were used as transparent electrodes, the ITO/Ag/AZO transparent electrode reduced the sheet resistance and minimized the reduction of the transmittance.As a result, it can be confirmed that the performance index (FOM) of the transparent electrode is improved.This means that the light penetrates through the transparent electrode and is sufficiently absorbed at the junction of the device and the electron transfer becomes easy due to the low sheet resistance.As a result, the photocurrent increases and it shows a remarkable improvement in the photoresponse.

Conclusion
In summary, the optical and electrical properties of Agembedded TCO layers were systematically analyzed with multilayered devices within the optimized thickness range of 100 nm, that is, AZO (100 nm)/n-Si, Ag (5 nm)/AZO (95 nm)/n-Si, and ITO (45 nm)/Ag (5 nm)/AZO (50 nm)/n-Si.The optical transmittance and electrical conductivity are the two important features for any transparent conductive electrode designs.The ITO/Ag/AZO sample revealed the better optical and electrical properties than AZO and Ag/AZO samples.When compared with AZO and Ag/AZO layers deposited upon n-Si substrate at room temperature, the Aginserted ITO/Ag/AZO-trilayered sample exhibited better results in both electrical and optical studies.Among all the three samples, Ag-inserted ITO/Ag/AZO sample showed an improvement in optoelectronic properties especially in the photoresponse studies.The ITO/Ag/AZO layer had an enhanced rectification ratio with a value of 113.38, and its sheet resistance value was reduced to 1.106 Ω/sq.Thus, the layers AZO and Ag/AZO, respectively, deposited upon n-Si substrate were further improved satisfactorily for the electrical and optical properties by inserting Ag layer between the ITO and AZO layers.This result provides the advantageous features of ITO/Ag/AZO device for better performance in the photoelectric industry.
), 2(b), 2(c), 2(d), 2(e), and 2(f) show the cross-sectional and top view images of samples AZO (S1), Ag/AZO (S2), and ITO/Ag/AZO (S3).In the cross-sectional images of the sample S1 (Figure 2(a)), the continuous growth of 100 nm thick AZO layer was observed whereas in the sample S2 (Figure 2(b)), a thin layer of Ag is formed.In the sample S3 (Figure 2(c)), the insertion of Ag layer embedded between the ITO and AZO layers was seen.The layer thickness and metal insertion play an important role to determine the decrease in the sheet resistance.Wu and Chu, Boscarino et al., and Kim et al. reported that a thin

Figure 3 :
Figure 3: XRD patterns of the single AZO film, Ag/AZO film, and ITO/Ag/AZO film deposited on n-Si.

Table 1 :
The thickness and error range of AZO, Ag, and ITO.
Although Ag layer cause high reflectance, it can reduce sheet resistance and by depositing ITO on Ag layer, reflectance can be decreased, because the light to reflect at Ag layer is transmitted across the device again due to light scattering phenomenon.We can confirm that the transmittance and reflectance are improved at ITO/Ag/AZO/n-Si device

Table 2 :
The characterized electrical and optical parameters of AZO, Ag/AZO, and ITO/Ag/AZO devices.