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Increased spectral sensitivity of Si photodetector by surface plasmon effect of Ag nanowires
Infrared Physics & Technology 76 (2016) 621–625
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Regular article
Increased spectral sensitivity of Si photodetector by surface plasmon effect of Ag nanowires Hong-Sik Kim a, Melvin David Kumar b, Hyunki Kim a, Joondong Kim a,⇑ a b
Photoelectric and Energy Device Application Lab (PEDAL) and Department of Electrical Engineering, Incheon National University, Incheon 406772, Republic of Korea Department of Physics, Aditanar College of Arts and Science, Tamil Nadu 628216, India
h i g h l i g h t s High-performing NIR photodetector was achieved by using AgNWs. Solution process was applied to coat AgNWs on Si. Surface plasmon effect significantly enhances the photoresponses. AgNW-embedding Si photodetector was effective to suppress reverse current.
a r t i c l e
i n f o
Article history: Received 5 August 2015 Revised 20 April 2016 Accepted 21 April 2016 Available online 22 April 2016 Keywords: AgNW ITO layer Si photodetector Surface plasmon
a b s t r a c t Highly-sensitive Si photodetectors were prepared by using Ag nanowires (AgNWs). A transparent indium-tin-oxide (ITO) coating was coated on a Si substrate followed by spin-coating of AgNWs-containing solution. AgNWs having average length of 5–20 lm with a diameter of about 40–60 nm were observed in FESEM images. The haze effect of AgNWs was totally avoided because of the optimum value of diameter. The transmittance of above 85% was shown by AgNWs over a broad spectral range due to surface plasmon resonance effect. The AgNW-coated device showed an excellent rectifying ratio of 288. Under light illumination, AgNWs-coated device exhibited a significant photoresponse ratio of 5373. This advanced feature of AgNWs-templated method would be applied in broadband wavelength photodetection devices. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction In the modern technology, optoelectronic devices such as sensors, solar cells, displays etc., demand for smart transparent electrodes to make an efficient way of interaction with incident light [1,2]. Especially in the case of optical sensors, as the communication of the device is limited to the selective range of incident wavelengths, they require highly conductive electrodes that should be transparent to wide spectral range [3,4]. There were many transparent conductive oxide electrodes (ITO, AZO, FTO, CuO) used in the optoelectronic devices for a couple of decade [5–7]. Among them, ITO was intensively used for demonstrating its better trade-off between electrical conductivity and optical transmittance [8,9]. However, the researchers and industrialists are strongly motivated to replace the ITO due to its high material costs [10,11], scarcity of indium [12] and high resistivity at room tem⇑ Corresponding author. E-mail address: [email protected] (J. Kim). http://dx.doi.org/10.1016/j.infrared.2016.04.026 1350-4495/Ó 2016 Elsevier B.V. All rights reserved.
perature [13]. Recently, the metal nanowires and carbon nanotubes have been potentially incorporated in the devices as transparent electrodes [1,2,14,15]. Specifically, the silver nanowire (AgNW) exhibited low sheet resistance with high optical transmittance at UV, Visible and NIR regions [16,17]. Jacab et al. [18] reported that the spectral sensitivity of silver nanowire (AgNW) is higher than that of gold nanowire. Besides that, AgNW can be easily coated in thin film form which causes surface texturing effect and localized plasmonic resonance effect [19,20]. These unique features ensured AgNW as a promising candidate in the flexible devices. However, the high resistance at nanowirenanowire contacts, easily oxidizing nature in open atmosphere and deprived bonding with substrates have limited the usage of AgNW in most of the commercial devices [21,22]. Zeng et al. [23] suggested that a transparent conductive material can be coated along with AgNWs in order to make electrical connection between the nanowires and reduce the oxidation effect. Hence, we have chosen a thin ITO layer in attached with AgNWs to deposit over an IR detector so as to enhance its sensitivity. The compatibility
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of AgNWs with ITO thin film is admirable because of the common inherent properties such as wide transmittance and highly conductive nature. The satisfactory combination of AgNW and ITO provides the strong and stable optical and electrical properties to the Si photodetector. In this work, the emitter doped p-type Si sensors were taken in to account for the realization of the benefits of AgNWs. One of the devices was deposited with an ITO layer of 200 nm thickness using DC sputtering whereas another device was coated with both ITO and AgNW films on the top surface. The structural, optical and electrical characteristics were carried out. The spin coated AgNW film exhibited the steady transmittance of above 85% over a wide spectral range. Due to surface plasmonic effect of AgNWs, the device interacted well with the incident wavelengths thereby increasing the sensitivity.
2. Experimental procedure A thin layer of ITO was deposited over a 4-in. emitter doped p-type Si wafer using DC magnetron sputtering system (SNTEK, Korea). During the deposition, DC power of 300 Watt was applied to a 4-in. ITO target (In2O2 containing 10 wt% SnO2) for 600 sec in the ambient condition to obtain 200 nm thick ITO layer. Further, AgNWs dispersed in Isopropyl Alcohol (IPA) solution was spin coated over the ITO surface. Followed by that, the prepared device was annealed at 150 °C for about 3 min on a hot plate in order to blend the AgNWs into the bottom ITO layer. In the second device, the AgNWs were not coated on the bare ITO layer. Aluminum (Al) paste was deposited on the back side of both the devices using DC sputtering method and annealed at 500 °C for 10 min to make the back electrodes. The prepared devices were characterized using a field emission scanning electron microscope (FESEM, FEI Sirion) to observe the surface of the AgNW coated device. Electrical characteristics such as reverse saturation current, ideality factor, rectification properties etc., were measured using Keithley electrometer 2400. The improved photo-responsivity due to surface plasmon
effect of AgNWs was measured by monochrometer system (McScience, K3100) mounted on a probe station.
3. Results and discussions FESEM images of the AgNW coated photodetector are shown in Fig. 1(a) and (b). AgNW meshes are equally spread over the surface with average length of about 5–20 lm. The diameter and density of AgNW (number of NWs/unit surface area) are the important parameters that play a key role in determining the electrical conductivity of the device [24,25]. In the present case, the concentration of the solution (AgNW + IPA) was maintained at 25 mg/ml so as to keep the optimum density required for a photodetector. The diameter of the observed AgNWs were about 40–60 nm as shown in Fig. 1(b). When the diameter exceeds the critical limit (more than 60 nm), it increases the concentration of AgNW by means of which a foggy appearance effect takes place on the surface. This is usually referred as haze effect of nanowires. The unclear appearance of the surface by increasing diameter is not suitable for displays and sensors as it fails to produce the sharp optical properties. However, the haze effect is appreciated in solar cells because of enhanced scattering effect of larger diameter NWs [24]. Then, the nanowire-nanowire junctions seen in Fig. 1(b) are clear and indicated that the oxidation process has been side-stepped. In general, when AgNWs are exposed to air, silver oxide layer will form on the surface and subsequently reduce the conductivity. This could be easily identified from the FESEM images of nanowire junctions [22]. These silver oxide layer can be removed using standard HCl solution [22]. However, in the present case, since AgNWs were indented onto the ITO layer through annealing process, the AgNWs were not subjected to oxidation process. This implies that the combination of ITO and AgNWs establishes a compatible electrical bonding between them which eliminates the formation of oxide layer. Since the efficiency of a photodetector is strongly reliant on the optical transmission and sheet resistance, they are considered as
Fig. 1. Surface morphology of AgNWs showing (a) the random nanowire networks spread over the surface of Si photodetector (b) the average diameter of about 40–60 nm. (c) Schematics of AgNW/ITO-coated Si photodetector and a photo image.
H.-S. Kim et al. / Infrared Physics & Technology 76 (2016) 621–625
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Fig. 2. Transmittance spectra of AgNWs and ITO films deposited on glass substrates.
the significant factors especially in the devices where the transparent conductor is used as a front electrode. The AgNWs dissolved in IPA was spin coated on the glass substrate for recording the transmittance spectrum. To observe the differences between the transmission spectra of ITO layer and AgNWs, a bare ITO coated glass substrate was also subjected to transmission analysis as shown in Fig. 2. The average transmittance of the AgNW and ITO films was measured as 85.45% and 75.21% respectively in the wavelength range of 300–1700 nm. It is clearly seen that the transmittance of ITO was drastically reduced at both UV and IR regions. Therefore, the usage of ITO material as transparent electrodes is not feasible in UV and IR based optoelectronic devices. A sharp absorption is raised around 350 nm in the transmission spectrum of AgNW as a result of localized surface plasmonic (LSP) effect [16,26]. Moreover, due to the sub-wavelength structure of AgNWs, the incident light gets diffracted all over the surface to propagate the plasmon polaritons. This causes broad transmittance over UV, visible and IR regions [27]. The plasmonic effects showed up by the AgNWs are helpful to enhance the light extraction efficiency and tailor the propagation direction. Though the AgNWs yielded better transmittance than the ITO film, the sheet resistance of ITO was measured as 28.63 X/sq which is slightly higher than 13.29 X/sq of AgNW. But the presented values of transmittance and sheet resistance in the present work are comparatively higher than some of the previously reported results [25,28]. Enlightening the physical structure of the presented device would make easy to understand the electrical and photoresponse characteristics. Here, an emitter doped p-type Si is the active part of device where the rectifying junction exists. To enhance the light extraction efficiency and sensitivity in the range of NIR wavelengths, the basic device was coated with a combination of ITO and AgNW films. The electrical properties such as rectifying capacity, leakage current density and built-in voltage of the junction can be investigated from the currents produced by the detector in both forward and reverse bias voltages under dark condition. The semi-log curves of I–V characteristics of both ITO coated and AgNW/ITO-coated detectors are shown in Fig. 3. The forward current (IF) produced by ITO coated device at 1 V is greater than that of AgNW/ITO-coated detector as shown in inset of Fig. 3. Decreasing forward current is mainly because of increasing series resistance. Typically, the AgNW network offers comparatively higher series and sheet resistances depending on its oxidizing nature. Another important thing to notify is that the IF value begins to rise around the bias voltage of 0.4 V in both the devices. This soaring location of bias voltage denotes the barrier height of the rectifying junction. Therefore, the presence of AgNW does not influence on the barrier height or built in potential of the device. The semi-log
Fig. 3. Semi-log I–V plots under dark condition for AgNW/ITO coated and ITO coated Si photodetectors graph.
curve of I–V plot exactly depicts the reverse saturation current (IRS) even if it is trivial. As shown in Fig. 3, the IRS which is directly associated with surface leakage current is found to be negligible as 8 lA in the case of AgNW-coated device, where as it is measured as 49 lA in ITO coated device. Hence it is understood that the AgNW coating nullifies the surface leakage current and thus increasing the rectification ratio of the junction [29]. Rectification ratio is calculated from the ratio of the forward biased current to the reverse biased current at a particular bias voltage. At 0.5 V, the calculated rectification ratios of both the AgNW/ITO coated and ITO coated devices were 288 and 79.7 respectively. The rectification ratio is the value which determines how speedily a junction collects and separates the photo-generated carriers. Therefore, the AgNWcoated Si photodetector improves the swiftness of the response to the incident wavelength. The ideality factor (n) which describes how near a fabricated device is to the ideal diode, was calculated for both devices using the following relation,
n¼
q @V kT @ðln IÞ
ð1Þ
where q, kT and I are the electron charge, thermal energy (eV) and current respectively. The ideality factors of the AgNW/ITO coated and bare ITO coated detectors were found to be 1.34 and 1.55 respectively. Hence, it is clarified that the transparent conductive AgNW network works well in extracting the incident light in such a way to transport a large number of photo-generated carriers from junction to electrode and vice-versa quickly. Photoresponses of the prepared devices were recorded by exposing the device to the targeted wavelengths as shown in Fig. 4(a–d) and summarized in Table 1. The corresponding wavelength light was periodically on and off and the respective currents were measured at zero voltage. Other than k = 400 nm, the AgNW-coated Si detector showed the better responses at all the rest of the wavelengths. The transmittance profiles of both the devices have intact reflected in the photo-responses. AgNW showed a sharp absorption in the transmission spectrum at low wavelength that reduced the photoresponse at 400 nm. And, it is seemed from Fig. 4(a–d) that both the prepared detectors produced more or less same current values at k = 600, 900 and 1100 nm. Yet, the off current value is almost zero for AgNW-coated Si detector whereas ITO coated Si detector showed the little off currents for all spectral range. This implies that the photo-generated carriers in the bare ITO coated device are not brought back to the equilibrium state even after the targeting wavelength is cut-off [30]. This causes worst consequences in the sensitivity of the device. Whereas, the
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Fig. 4. Photoresponses of the AgNW/ITO coated and ITO coated Si photodetectors at wavelengths of (a) 400 nm, (b) 600 nm, (c) 900 nm and (d) 1100 nm. (e) Photoresponse ratio of the prepared detectors.
Table 1 Photoresponses of AgNWs/ITO coated Si device and ITO coated Si device. k is the wavelength.
AgNWs/ITO coated Si device ITO coated Si device
k = 400 nm
k = 600 nm
k = 900 nm
k = 1100 nm
2584.65
2571.20
5373.36
143.86
1311.15
1282.80
865.48
27.49
AgNW-coated detector yielded the current when light is on and immediately becomes zero at off condition as a proof of its rapid response to the incident light. In order to reinforce the quick response of the device, the photoresponse ratio was calculated from the ratio of on and off current values and plotted as shown in Fig. 4 (e). Now it shows the better view of the photoresponses for different wavelengths. As expected, the AgNW-coated detector presented the dominated photoresponses at all the chosen wavelengths when compared to the ITO coated device. The response ratio is gradually increased from 400 nm and attained the maximum response ratio of 5373 at 900 nm and then decreased to 144 at 1100 nm. The reason for low response at 1100 nm is due to the inter band transitions of ITO layer which was deposited beneath the AgNW network [5]. Overall, the performance of the AgNW coated Si photodetector was satisfactory in broad wavelength region due to surface plasmon effect. Furthermore, the AgNW network made easy and efficient the interaction between the device and incident light. Further improvements may be achieved by metal-assisted plasmon-enhanced approaches [31–33]. 4. Conclusions Si photodetectors were fabricated with ITO and AgNW coatings to analyze the sensitivity of AgNW network. To avoid the oxidation process of AgNW in open atmosphere, the ITO layer was deposited
underneath. As a result of surface plasmonic effect, the AgNW exhibited the transmittance of above 85% over a wide range of wavelength region. It is identified from IRS value that there is no surface leakage current in AgNW coated Si detector. The highest rectification ratio of 288 was calculated for AgNW device thereby demonstrating the fast collection and separation efficiency of rectifying junction. As a consequence of surface plasmonic effect, the Si photodetector adopting AgNWs showed the better responses at visible and NIR wavelengths. It is suggested that the proposed detector with AgNW network would be best candidate for NIR based applications. Conflict of interest The authors declare that there is no conflict of interest regarding the publication of this paper. Acknowledgements The authors acknowledge the financial support of Business for Cooperative R&D between Industry, Academy, and Research Institute funded Korea Small and Medium Business Administration (C0297688), Basic Science Research Program through the National Research Foundation (NRF) of Korea by the Ministry of Education (NRF-2015R1D1A1A01059165) and the Korea Institute of Energy Technology Evaluation and Planning, in a grant funded by the Ministry of Knowledge Economy (KETEP-20133030011000). H.S. Kim and M.D. Kumar are equally contributed to this work. References [1] A.R. Rathmell, S.M. Bergin, Y.L. Hua, Z.Y. Li, B.J. Wiley, The growth mechanism of copper nanowires and their properties in flexible, transparent conducting films, Adv. Mater. 22 (2010) 3558–3563.
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Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Regular article
Increased spectral sensitivity of Si photodetector by surface plasmon effect of Ag nanowires Hong-Sik Kim a, Melvin David Kumar b, Hyunki Kim a, Joondong Kim a,⇑ a b
Photoelectric and Energy Device Application Lab (PEDAL) and Department of Electrical Engineering, Incheon National University, Incheon 406772, Republic of Korea Department of Physics, Aditanar College of Arts and Science, Tamil Nadu 628216, India
h i g h l i g h t s High-performing NIR photodetector was achieved by using AgNWs. Solution process was applied to coat AgNWs on Si. Surface plasmon effect significantly enhances the photoresponses. AgNW-embedding Si photodetector was effective to suppress reverse current.
a r t i c l e
i n f o
Article history: Received 5 August 2015 Revised 20 April 2016 Accepted 21 April 2016 Available online 22 April 2016 Keywords: AgNW ITO layer Si photodetector Surface plasmon
a b s t r a c t Highly-sensitive Si photodetectors were prepared by using Ag nanowires (AgNWs). A transparent indium-tin-oxide (ITO) coating was coated on a Si substrate followed by spin-coating of AgNWs-containing solution. AgNWs having average length of 5–20 lm with a diameter of about 40–60 nm were observed in FESEM images. The haze effect of AgNWs was totally avoided because of the optimum value of diameter. The transmittance of above 85% was shown by AgNWs over a broad spectral range due to surface plasmon resonance effect. The AgNW-coated device showed an excellent rectifying ratio of 288. Under light illumination, AgNWs-coated device exhibited a significant photoresponse ratio of 5373. This advanced feature of AgNWs-templated method would be applied in broadband wavelength photodetection devices. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction In the modern technology, optoelectronic devices such as sensors, solar cells, displays etc., demand for smart transparent electrodes to make an efficient way of interaction with incident light [1,2]. Especially in the case of optical sensors, as the communication of the device is limited to the selective range of incident wavelengths, they require highly conductive electrodes that should be transparent to wide spectral range [3,4]. There were many transparent conductive oxide electrodes (ITO, AZO, FTO, CuO) used in the optoelectronic devices for a couple of decade [5–7]. Among them, ITO was intensively used for demonstrating its better trade-off between electrical conductivity and optical transmittance [8,9]. However, the researchers and industrialists are strongly motivated to replace the ITO due to its high material costs [10,11], scarcity of indium [12] and high resistivity at room tem⇑ Corresponding author. E-mail address: [email protected] (J. Kim). http://dx.doi.org/10.1016/j.infrared.2016.04.026 1350-4495/Ó 2016 Elsevier B.V. All rights reserved.
perature [13]. Recently, the metal nanowires and carbon nanotubes have been potentially incorporated in the devices as transparent electrodes [1,2,14,15]. Specifically, the silver nanowire (AgNW) exhibited low sheet resistance with high optical transmittance at UV, Visible and NIR regions [16,17]. Jacab et al. [18] reported that the spectral sensitivity of silver nanowire (AgNW) is higher than that of gold nanowire. Besides that, AgNW can be easily coated in thin film form which causes surface texturing effect and localized plasmonic resonance effect [19,20]. These unique features ensured AgNW as a promising candidate in the flexible devices. However, the high resistance at nanowirenanowire contacts, easily oxidizing nature in open atmosphere and deprived bonding with substrates have limited the usage of AgNW in most of the commercial devices [21,22]. Zeng et al. [23] suggested that a transparent conductive material can be coated along with AgNWs in order to make electrical connection between the nanowires and reduce the oxidation effect. Hence, we have chosen a thin ITO layer in attached with AgNWs to deposit over an IR detector so as to enhance its sensitivity. The compatibility
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H.-S. Kim et al. / Infrared Physics & Technology 76 (2016) 621–625
of AgNWs with ITO thin film is admirable because of the common inherent properties such as wide transmittance and highly conductive nature. The satisfactory combination of AgNW and ITO provides the strong and stable optical and electrical properties to the Si photodetector. In this work, the emitter doped p-type Si sensors were taken in to account for the realization of the benefits of AgNWs. One of the devices was deposited with an ITO layer of 200 nm thickness using DC sputtering whereas another device was coated with both ITO and AgNW films on the top surface. The structural, optical and electrical characteristics were carried out. The spin coated AgNW film exhibited the steady transmittance of above 85% over a wide spectral range. Due to surface plasmonic effect of AgNWs, the device interacted well with the incident wavelengths thereby increasing the sensitivity.
2. Experimental procedure A thin layer of ITO was deposited over a 4-in. emitter doped p-type Si wafer using DC magnetron sputtering system (SNTEK, Korea). During the deposition, DC power of 300 Watt was applied to a 4-in. ITO target (In2O2 containing 10 wt% SnO2) for 600 sec in the ambient condition to obtain 200 nm thick ITO layer. Further, AgNWs dispersed in Isopropyl Alcohol (IPA) solution was spin coated over the ITO surface. Followed by that, the prepared device was annealed at 150 °C for about 3 min on a hot plate in order to blend the AgNWs into the bottom ITO layer. In the second device, the AgNWs were not coated on the bare ITO layer. Aluminum (Al) paste was deposited on the back side of both the devices using DC sputtering method and annealed at 500 °C for 10 min to make the back electrodes. The prepared devices were characterized using a field emission scanning electron microscope (FESEM, FEI Sirion) to observe the surface of the AgNW coated device. Electrical characteristics such as reverse saturation current, ideality factor, rectification properties etc., were measured using Keithley electrometer 2400. The improved photo-responsivity due to surface plasmon
effect of AgNWs was measured by monochrometer system (McScience, K3100) mounted on a probe station.
3. Results and discussions FESEM images of the AgNW coated photodetector are shown in Fig. 1(a) and (b). AgNW meshes are equally spread over the surface with average length of about 5–20 lm. The diameter and density of AgNW (number of NWs/unit surface area) are the important parameters that play a key role in determining the electrical conductivity of the device [24,25]. In the present case, the concentration of the solution (AgNW + IPA) was maintained at 25 mg/ml so as to keep the optimum density required for a photodetector. The diameter of the observed AgNWs were about 40–60 nm as shown in Fig. 1(b). When the diameter exceeds the critical limit (more than 60 nm), it increases the concentration of AgNW by means of which a foggy appearance effect takes place on the surface. This is usually referred as haze effect of nanowires. The unclear appearance of the surface by increasing diameter is not suitable for displays and sensors as it fails to produce the sharp optical properties. However, the haze effect is appreciated in solar cells because of enhanced scattering effect of larger diameter NWs [24]. Then, the nanowire-nanowire junctions seen in Fig. 1(b) are clear and indicated that the oxidation process has been side-stepped. In general, when AgNWs are exposed to air, silver oxide layer will form on the surface and subsequently reduce the conductivity. This could be easily identified from the FESEM images of nanowire junctions [22]. These silver oxide layer can be removed using standard HCl solution [22]. However, in the present case, since AgNWs were indented onto the ITO layer through annealing process, the AgNWs were not subjected to oxidation process. This implies that the combination of ITO and AgNWs establishes a compatible electrical bonding between them which eliminates the formation of oxide layer. Since the efficiency of a photodetector is strongly reliant on the optical transmission and sheet resistance, they are considered as
Fig. 1. Surface morphology of AgNWs showing (a) the random nanowire networks spread over the surface of Si photodetector (b) the average diameter of about 40–60 nm. (c) Schematics of AgNW/ITO-coated Si photodetector and a photo image.
H.-S. Kim et al. / Infrared Physics & Technology 76 (2016) 621–625
623
Fig. 2. Transmittance spectra of AgNWs and ITO films deposited on glass substrates.
the significant factors especially in the devices where the transparent conductor is used as a front electrode. The AgNWs dissolved in IPA was spin coated on the glass substrate for recording the transmittance spectrum. To observe the differences between the transmission spectra of ITO layer and AgNWs, a bare ITO coated glass substrate was also subjected to transmission analysis as shown in Fig. 2. The average transmittance of the AgNW and ITO films was measured as 85.45% and 75.21% respectively in the wavelength range of 300–1700 nm. It is clearly seen that the transmittance of ITO was drastically reduced at both UV and IR regions. Therefore, the usage of ITO material as transparent electrodes is not feasible in UV and IR based optoelectronic devices. A sharp absorption is raised around 350 nm in the transmission spectrum of AgNW as a result of localized surface plasmonic (LSP) effect [16,26]. Moreover, due to the sub-wavelength structure of AgNWs, the incident light gets diffracted all over the surface to propagate the plasmon polaritons. This causes broad transmittance over UV, visible and IR regions [27]. The plasmonic effects showed up by the AgNWs are helpful to enhance the light extraction efficiency and tailor the propagation direction. Though the AgNWs yielded better transmittance than the ITO film, the sheet resistance of ITO was measured as 28.63 X/sq which is slightly higher than 13.29 X/sq of AgNW. But the presented values of transmittance and sheet resistance in the present work are comparatively higher than some of the previously reported results [25,28]. Enlightening the physical structure of the presented device would make easy to understand the electrical and photoresponse characteristics. Here, an emitter doped p-type Si is the active part of device where the rectifying junction exists. To enhance the light extraction efficiency and sensitivity in the range of NIR wavelengths, the basic device was coated with a combination of ITO and AgNW films. The electrical properties such as rectifying capacity, leakage current density and built-in voltage of the junction can be investigated from the currents produced by the detector in both forward and reverse bias voltages under dark condition. The semi-log curves of I–V characteristics of both ITO coated and AgNW/ITO-coated detectors are shown in Fig. 3. The forward current (IF) produced by ITO coated device at 1 V is greater than that of AgNW/ITO-coated detector as shown in inset of Fig. 3. Decreasing forward current is mainly because of increasing series resistance. Typically, the AgNW network offers comparatively higher series and sheet resistances depending on its oxidizing nature. Another important thing to notify is that the IF value begins to rise around the bias voltage of 0.4 V in both the devices. This soaring location of bias voltage denotes the barrier height of the rectifying junction. Therefore, the presence of AgNW does not influence on the barrier height or built in potential of the device. The semi-log
Fig. 3. Semi-log I–V plots under dark condition for AgNW/ITO coated and ITO coated Si photodetectors graph.
curve of I–V plot exactly depicts the reverse saturation current (IRS) even if it is trivial. As shown in Fig. 3, the IRS which is directly associated with surface leakage current is found to be negligible as 8 lA in the case of AgNW-coated device, where as it is measured as 49 lA in ITO coated device. Hence it is understood that the AgNW coating nullifies the surface leakage current and thus increasing the rectification ratio of the junction [29]. Rectification ratio is calculated from the ratio of the forward biased current to the reverse biased current at a particular bias voltage. At 0.5 V, the calculated rectification ratios of both the AgNW/ITO coated and ITO coated devices were 288 and 79.7 respectively. The rectification ratio is the value which determines how speedily a junction collects and separates the photo-generated carriers. Therefore, the AgNWcoated Si photodetector improves the swiftness of the response to the incident wavelength. The ideality factor (n) which describes how near a fabricated device is to the ideal diode, was calculated for both devices using the following relation,
n¼
q @V kT @ðln IÞ
ð1Þ
where q, kT and I are the electron charge, thermal energy (eV) and current respectively. The ideality factors of the AgNW/ITO coated and bare ITO coated detectors were found to be 1.34 and 1.55 respectively. Hence, it is clarified that the transparent conductive AgNW network works well in extracting the incident light in such a way to transport a large number of photo-generated carriers from junction to electrode and vice-versa quickly. Photoresponses of the prepared devices were recorded by exposing the device to the targeted wavelengths as shown in Fig. 4(a–d) and summarized in Table 1. The corresponding wavelength light was periodically on and off and the respective currents were measured at zero voltage. Other than k = 400 nm, the AgNW-coated Si detector showed the better responses at all the rest of the wavelengths. The transmittance profiles of both the devices have intact reflected in the photo-responses. AgNW showed a sharp absorption in the transmission spectrum at low wavelength that reduced the photoresponse at 400 nm. And, it is seemed from Fig. 4(a–d) that both the prepared detectors produced more or less same current values at k = 600, 900 and 1100 nm. Yet, the off current value is almost zero for AgNW-coated Si detector whereas ITO coated Si detector showed the little off currents for all spectral range. This implies that the photo-generated carriers in the bare ITO coated device are not brought back to the equilibrium state even after the targeting wavelength is cut-off [30]. This causes worst consequences in the sensitivity of the device. Whereas, the
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Fig. 4. Photoresponses of the AgNW/ITO coated and ITO coated Si photodetectors at wavelengths of (a) 400 nm, (b) 600 nm, (c) 900 nm and (d) 1100 nm. (e) Photoresponse ratio of the prepared detectors.
Table 1 Photoresponses of AgNWs/ITO coated Si device and ITO coated Si device. k is the wavelength.
AgNWs/ITO coated Si device ITO coated Si device
k = 400 nm
k = 600 nm
k = 900 nm
k = 1100 nm
2584.65
2571.20
5373.36
143.86
1311.15
1282.80
865.48
27.49
AgNW-coated detector yielded the current when light is on and immediately becomes zero at off condition as a proof of its rapid response to the incident light. In order to reinforce the quick response of the device, the photoresponse ratio was calculated from the ratio of on and off current values and plotted as shown in Fig. 4 (e). Now it shows the better view of the photoresponses for different wavelengths. As expected, the AgNW-coated detector presented the dominated photoresponses at all the chosen wavelengths when compared to the ITO coated device. The response ratio is gradually increased from 400 nm and attained the maximum response ratio of 5373 at 900 nm and then decreased to 144 at 1100 nm. The reason for low response at 1100 nm is due to the inter band transitions of ITO layer which was deposited beneath the AgNW network [5]. Overall, the performance of the AgNW coated Si photodetector was satisfactory in broad wavelength region due to surface plasmon effect. Furthermore, the AgNW network made easy and efficient the interaction between the device and incident light. Further improvements may be achieved by metal-assisted plasmon-enhanced approaches [31–33]. 4. Conclusions Si photodetectors were fabricated with ITO and AgNW coatings to analyze the sensitivity of AgNW network. To avoid the oxidation process of AgNW in open atmosphere, the ITO layer was deposited
underneath. As a result of surface plasmonic effect, the AgNW exhibited the transmittance of above 85% over a wide range of wavelength region. It is identified from IRS value that there is no surface leakage current in AgNW coated Si detector. The highest rectification ratio of 288 was calculated for AgNW device thereby demonstrating the fast collection and separation efficiency of rectifying junction. As a consequence of surface plasmonic effect, the Si photodetector adopting AgNWs showed the better responses at visible and NIR wavelengths. It is suggested that the proposed detector with AgNW network would be best candidate for NIR based applications. Conflict of interest The authors declare that there is no conflict of interest regarding the publication of this paper. Acknowledgements The authors acknowledge the financial support of Business for Cooperative R&D between Industry, Academy, and Research Institute funded Korea Small and Medium Business Administration (C0297688), Basic Science Research Program through the National Research Foundation (NRF) of Korea by the Ministry of Education (NRF-2015R1D1A1A01059165) and the Korea Institute of Energy Technology Evaluation and Planning, in a grant funded by the Ministry of Knowledge Economy (KETEP-20133030011000). H.S. Kim and M.D. Kumar are equally contributed to this work. References [1] A.R. Rathmell, S.M. Bergin, Y.L. Hua, Z.Y. Li, B.J. Wiley, The growth mechanism of copper nanowires and their properties in flexible, transparent conducting films, Adv. Mater. 22 (2010) 3558–3563.
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