High-Speed, Self-Biased Broadband Photodetector-Based on a

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High-speed, self-biased broadband photodetector based on a solution-processed Ag nanowire/Si Schottky junction Mohit Kumar, Malkeshkumar Patel, Hong-Sik Kim, Joondong Kim, and Junsin Yi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14250 • Publication Date (Web): 19 Oct 2017 Downloaded from http://pubs.acs.org on October 21, 2017

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ACS Applied Materials & Interfaces

High-speed, self-biased broadband photodetector based on a solution-processed Ag nanowire/Si Schottky junction

Mohit Kumara,b, Malkeshkumar Patela,b, Hong-Sik Kima,b, Joondong Kima,b,* and Junsin Yi c,** a

Department of Electrical Engineering, Incheon National University, 119 Academy Rd. Yeonsu,

Incheon, 22012, Republic of Korea b

Photoelectric and Energy Device Application Lab (PEDAL), Multidisciplinary Core Institute

for Future Energies (MCIFE), Incheon National University, 119 Academy Rd. Yeonsu, Incheon, 22012, Republic of Korea c

College of Information and Communication Engineering, Sungkyunkwan University, Suwon

440746, Republic of Korea

ABSTRACT In this work, we demonstrate the effectiveness of Ag nanowires (AgNWs) to design a high-speed broadband photodetector. A simple AgNW solution was spin-coated on a Si substrate to form a Schottky junction. The junction properties were investigated using current-voltage characteristics and Mott-Schottky analysis. The present device had a remarkably fast response speed, e.g., rising time (τr = 784 ns) and fall time (τf = 92 µs), with good reproducibility over a wide range of switching frequencies (50 Hz – 50 kHz). Such a high performance was attributed to the strong electric field created at the AgNW/Si interface without an external electric field, enabling the efficient separation of photogenerated electron-hole pairs. The present study will open a new avenue to design future optoelectronics devices and energy devices including solar cells.

Keywords: Self-biased; High-speed; Broadband; Solution processes; Schottky junction; Energy devices. 1 ACS Paragon Plus Environment


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1. INTRODUCTION Understanding the interactions between photons and electrons is extremely important for the architecting of the advanced optoelectronic devices, such as light-emitting diodes, solar cells, and photodetectors.1 An essential feature for an efficient photoactive device is significant photogenerated carrier collection per incident photon, which is described by the quantum efficiency and time taken by the device to produce a photoresponse.1,2 Generally, due to rapidly developed integrated circuit technology and its excellent compatibility, single-crystalline Si is used to design photodetectors.3 However, Si-based p-n junction photodetectors still suffer from various difficulties, such as a lacking adjustability and low photoresponsivity.4 Most of these issues can be resolved by designing a metal/semiconductor Schottky junction with an appropriate material, which effectively produces a strong electrical field at the interface to collect photogenerated carriers.5 However, metal/semiconductor Schottky contacts possess many interfacial defects, which in turn not only give rise to a high series resistance but also trap photogenerated carriers, resulting in a low efficiency.6 In addition, the metal can strongly reflect solar radiation, as a matter of fact, make it undesired for the photoelectric devices.7 One advantage of a metal/semiconductor Schottky junction is its ability to be utilized for photoelectric devices if the metal layer can allow light penetration into the semiconductor, e.g., graphene8 or metal nanowires (NWs).9 On the other hand, a Schottky junction made by satisfactorily connecting metallic NWs not only provides a high optical transmittance over a broad wavelength range but also ensures a low resistance, resulting in fast photoresponses. Another merit of a NW-based Schottky photodetector is its ability to be utilized in a semiconductor without any extra doping process.10 Recently, metal NW networks connected on Si using a simple solution method were

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demonstrated to provide excellent electrical conductivity along with high optical transmittance.912

Networks with high surface-to-volume ratios of well-connected metal NWs has generated

enormous interest due to their excellent electrical conductivity.9 However, to the best of our knowledge, a photodetector made of Ag NWs (AgNWs) has yet to be explored to utilize their full potential of fast detection. In this work, a AgNW/p-Si photodetector was designed for zero-biased broadband highspeed detection. The device performance analysis revealed that the as-fabricated photodetector exhibited high sensitivity to a wide spectrum of light illumination and detected very fast optical signal with a frequency bandwidth as high as 15500 Hz. The rise and fall times were estimated to be 784 ns and 92 µs, respectively. In addition, the responsivity and detectivity of the device were calculated to be 182 mA W−1 and ~1013 Jones, respectively. The observed performance was attributed to the strong electric field generated at the AgNW/Si interface, enabling the device with efficient photodetection. The present study will be a benchmark to design future optoelectronics devices.

2. RESULTS AND DISCUSSION Figure 1(a) depicts a schematic diagram of the AgNWs/p-Si Schottky junction. Since AgNWs are used as the transparent conducting layer, the device should show a high transmittance along with a low resistance. The transmittance and resistance of the AgNWs were measured after coating them on a glass substrate under similar conditions as the device fabrication. Figure 1(b) shows the transmittance of the AgNW-coated glass substrate, depicting a high value over a broad wavelength range from 200 nm to 800 nm. The average transmittance of the AgNWs is greater than 90%. As an alternative, 200 nm-thick ITO can be used as a TC oxide



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(TCO); however, this not only shows relatively lower transmittance compared to the AgNWs but also absorbs most of the spectrum below 350 nm due to its band gap [Figure 1(b)].10 Furthermore, the resistance of the AgNWs and ITO were determined to be 15 Ω/□ and 22.5 Ω/□, respectively, which confirms that the nontoxic AgNWs are a better alternative to ITO. Figure 1(c) shows a planar-view FESEM image of the AgNW-coated Si substrate, with a magnified view depicted in Figure 1(d). Due to their distinct contrast, both Si and the AgNWs are easily visualized. Notably, the AgNWs are uniformly distributed over the entire substrate. The effective coverage area and average diameter of the AgNWs are ~50% and 38.5 nm, respectively. In addition, these AgNWs are well connected, which will facilitate carrier transport. The formation of a Schottky junction was confirmed by performing current-voltage (I-V) measurements under dark conditions [Figure 2(a)]. The I-V characteristics were highly nonlinear and asymmetric, confirming the formation of a potential barrier at the Si-AgNW interface.11 For better visualization, the I-V characteristics were also plotted on a semi-log scale [inset of Figure 2(a)]. The diode parameters, such as the saturation current density (Jo) and ideality factor (n), were calculated using the one-diode model equation for a photovoltaic device, which refers to the recombination rate and represents the heat generated due to free carrier flow through the junction.10,12

= ( − 1)

(1)

where J is the current density, q/kT is the thermal voltage, and V is the applied voltage. The J0 and n values of the device are 0.12 µA/cm2 and 1.06, respectively. The low saturation current density and n value close to 1 indicate that the as-prepared device possesses an excellent Schottky junction with minimum interfacial defects, offering promise for use in high-speed optical detection applications.13


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Thus, to analyze the photoresponse of the device, the changes in the I-V characteristics of the AgNW/Si junction were measured under illumination with a wavelength of λ = 850 nm at different intensities varying from 0.02 to 110 mW cm−2. Notably, the device responds well to light, and the I-V characteristics considerably change [Figure 2(b)]. In fact, under light illumination, the photovoltage linearly increases up to 50 mW cm-2 [Figure 2(c)]. On the other hand, the photocurrent under self-biased condition remains very low, particularly at intensities

High-Speed, Self-Biased Broadband Photodetector-Based on a

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