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A simple methyl-terminated (-CH3) surface passivation approach has been employed to enhance the performance of the bilayer graphene/Si nanohole array (BLG/SiNH array) Schottky junction based self-powered near infrared photodetector (SPNIRPD). The as-fabricated SPNIRPD exhibits high sensitivity to light at near infrared region at zero bias voltage. The Ilight/Idark ratio measured is 1.43 × 107, which is more than an order of magnitude improvement compared with the sample without passivation (~6.4 × 105). Its corresponding responsivity and detectivity are 0.328 AW−1 and 6.03 × 1013 cmHz1/2W−1, respectively. The demonstrated results have confirmed the high-performance SPNIRPD compared with the photo-detectors of similar type and its great potential application in future optoelectronic devices.
© 2015 Optical Society of America
1. Introduction
Due to the popularity of high power crystal and fiber laser operating at near infrared region and light sources [1–3], the research works related to near infrared light detector have been very important for photonic community. Self-powered near infrared photodetectors (SPNIRPDs) have advantages of battery free, high performance and simple structure with low operational cost and therefore it have a wide range of applications for remote sensing, night version, military industry, laser pulse characterization and medical imaging [4]. Silicon, one of the popular semiconductors due to its low cost of materials and abundance (>90% in the Earth's crust), provides a versatile platform for photodetection [5]. Compared with their thin film form and bulk counterparts based photodetectors, Si nanostructure arrays (such as nanowire or nanohole array structure, etc.), is a novel design which has advantages of large interfacial area facilitate fast charge transport, and thus further enhancing sensitivity by shortening the travel paths of minority carriers for the charge collection [6, 7]. In order to further boost the device performance e.g. on/off ratio, respond speed, responsivity, surface passivation of the sample was modified with methyl-termination. Therefore, charge recombination velocity at the Si nanoarray surface reduces remarkably, leading to a substantial improvement in device performance. Compared with the Si nanowire array, the Si nanohole structure (SiNH array) proposed provides even larger contact surface area for higher stability and better light harvesting.
Graphene with high mobility, conductivity, flexibility, and high light transparency has stimulated enormous scientific interest [8]. It is an ideal material for transparent electrode, light detection [9, 10] and other laser photonic applications [11–14]. Despite being the most popular choice for transparent conductors in displays and solar cells, Indium Titanium Oxide, ITO is expensive, rare, brittle and degrading performance over time due to indium diffusion [15]. Therefore it is expected that graphene has good potential to replace ITO as flexible transparent conductors. By introducing graphene into the semiconductor nanostructures (CdSe nanobelt [15], GaAs nanocone [16]), the light harvesting of the devices can be further enhanced by forming excellent Schottky contact with substrate and low transmission optical losses. On the other hand, the SiNH array substrate will provide large contact area for the graphene, which is an important for the formation of effective Schottky junction. Additionally graphene can serve as better barrier for protection, and simplify the device structure to facilitate light absorption as well as carrier separation/transport. Bi-layer graphene (BLG) has attracted a great deal of attention from the research community due to its possibility to change bandgap energy under applied voltage [17]. It has optical transmission comparable with the monolayer graphene but with lower sheet resistance and its conductivity less sensitive to structure defects, grain boundaries and cracks etc. produced during the transfer and fabrication process. Therefore, it has a good balance between electrical conductivity and optical transmission. These properties can benefit the overall performance enhancement of the produced photo-detector devices. Therefore, in this work, the bi-layer graphene was selected to serves as a transparent electrode as well as an active layer for electron-hole separation and holes transport [18].
For the above reasons, this study has successfully demonstrated high performance SPNIRPD that combines bi-layers graphene and Si nanostructure array and surface passivation medication. In such device, sunlight could penetrate the bi-layer graphene film with low optical losses and reach the Schottky junction, and the electron-hole pairs excited in Si are separated by the built-in electric field and producing photocurrent. The nanohole structure used within the design provides large contact surface area for higher stability and better light harvesting. In this research, a simple surface passivation approach was employed to further enhance the bilayer graphene/SiNH array Schottky junction SPNIRPD. The optimized device shows that the responsitivity, detectivity, and Ilight/Idark ratio achieved are 0.328 AW−1, 6.03 × 1013 cmHz1/2W−1 and 1.43 × 107, respectively. In general, the performance of the proposed design is better than other graphene based self-powered photo-detectors as shown in Table 1.
Table 1. Summary of the devices performance of the present NIRPD and other photodetectors with similar device structures
2. Experiment and results
Figure 1(a) depicts the schematic illustration of a developed graphene/SiNH array photodetector. The SiNH array was prepared in electrolyte solution via electrochemical etching [19]. The bi-layer graphene was grown on Cu foils by CVD method [20]. The fabricated bi-layer graphene film was then spin-coated with 5 wt. % polymethylmethacrylate (PMMA) in chlorobenzene, and then the underlying Cu foil were removed in Marble’s reagent solution [21]. The bi-layer graphene film was rinsed in deionized water to remove the remaining ions. BLG/SiNH array Schottky junction photodetector was fabricate by using 5/50 nm Ti/Au electrode, which served as the electrical contact for bi-layer graphene. Then the PMMA-supported bi-layer graphene films were directly transferred onto the top of SiNH array. Then the residual PMMA on graphene film was removed by using acetone. Indium-gallium (In-Ga) alloy was then pasted on the rear side of Si subatrate to achieve ohmic contact [19]. The SEM images of the as-prepared SiNH array are shown in Fig. 1(b), from which one could observe well-aligned holes array with depths about 5-8 μm and diameters in the range of 0.8-1.1 μm. The efficient light harvesting of the SiNH array could be attributed to their distinct array structures as discussed before and shown in Fig. 1(b). Figure 1(c) shows the reflectance of SiNH array is much lower than the planar Si and goes up after -CH3 modification. The higher reflectance of CH3-SiNH array related to the SiNH array is due to the morphology change of the SiNH array during the modification process [22]. As shown in the inset of Fig. 1 (c), the Raman spectrum obtained from the bilayer film is mainly composed of two obvious peaks, i.e., 2D band peak at ~2668 cm−1 and G band peak at ~1576 cm−1 with the intensity ratio of I2D: IG ≈1.09. The D peak at ~1398 cm −1 is extremely weak, indicating a very low percentage of defects [20, 23].
Fig. 1 (a) Schematic illustration of the BLG/SiNH array Schottky junction SPNIRPD. (b) Typical cross-sectional view SEM image of the as-prepared SiNH array. Inset shows the corresponding plane view SEM image of nano array. Scale bars in the inset are 8 μm. (d) Re〉ection spectra of planar Si, SiNH array, and CH3-SiNH array. Inset shows Raman spectrum of the BLG.
Figure 2(a) shows the current-voltage (I-V) curves of both devices in the dark, from which one can see that the two devices exhibit obvious rectification behavior. From inset of Fig. 2(a), the obtained rectification ratios for SiNH array and CH3-SiNH array based PDs are 25 and 1.05 × 103 at ± 0.5 V respectively. The comparatively higher rectification ratio can be ascribed to the reduced leakage current and suppressed carrier recombination in the CH3-SiNH array based junction [22]. The I-V curves of BLG/CH3-SiNH array Schottky diode in the dark with respect to various temperatures (90 to 300 K) were measured and presented in Fig. 2(b). According to the thermionic emission (TE) theory, the current across the BLG/CH3-SiNH array Schottky junction at forward bias voltage can be described by Eq. (1) [24]
where, I0 is the reverse saturation current, n is the ideality factor, A is the Schottky contact area, A* is the effective Richardson-constant (112 Acm−2K−2 for n-Si) [24], ΦB is the Schottky barrier height, and I0 can be obtained by the extrapolation the semi-log I-V curves at different temperatures as shown in Fig. 2(c). Then the values of ΦB for the Schottky junction was calculated by Eq. (1), and presented in Fig. 2(d). It is found that ΦB increases from 0.26 to 0.82 eV with the increase in temperature from 90 to 300 K. We noted that this result has deviated from conventional Schottky theory prediction in which ΦB should decrease with the temperature rise. However, similar results have been often obtained from other graphene /semiconductor Schottky barrier diodes, and the literally inhomogeneous barrier height was suggested to be an important reason for this deviation [25].Fig. 2 (a) I-V curves of the BLG/SiNH array Schottky junction with and without methyl-terminated at ambient condition in the dark. Inset shows the same I-V characteristics curves of the both devices but with y-axis in log scale. (b) I-V characteristic curves of BLG/CH3-SiNH array Schottky junction at temperature ranging from 90 to 300 K. (c) I0 at varied temperature is deduced by extrapolating the I-V curves to the vertical axis at V = 0 V. (d) Dependence of barrier height ΦB on temperature.
As shown in Fig. 3(a), the photovoltaic performance of CH3-SiNH array based PD upon NIR light irradiation (λ = 850 nm, 5 mWcm−2) is significantly improved after surface passivation. It has short circuit density (Jsc) of 1.64 mA/cm2, and open circuit voltage (Voc) of 0.26 V. Notably, the device after -CH3 modification exhibits pronounced photovoltaic characteristics relative to that without passivation. We believed that the effective suppression of the recombination activities at SiNH array surface, due to reduced density of surface dangling bonds and defects after -CH3 modification, is responsible for the improved device photovoltaic performance [19, 22]. As illustrated in Fig. 3(b), both devices can be reversibly switched between low- and high-resistivity states when the NIR light are switched on and off repeatedly at zero bias voltage.
Fig. 3 (a) The photovoltaic characteristics of the both devices. (b) The photoresponse of the both devices.
Figure 4(a) shows response of BLG/CH3-SiNH array PD with respect to light modulation frequency of 10 Hz and 2000 Hz. The photoresponse (relative balance [Imax–Imin/Imax]) only slightly reduced by:16% of original value at a high modulation frequency of 2000 Hz, in sharp contrast to the device without surface passivation (the relative balance decay decreases:60%). It is about one third when compared with that of the device without surface passivation as shown in Fig. 4(b). Figure 4(c) shows the response/recovery time (τr/τf) under 2000 Hz switching frequency are 22/56 μs, being much faster than that of BLG/SiNH array without surface passivation (τr/τf = 71/162 μs) and other similar PDs as shown in Table 1. In addition to rapid photoresponse, excellent spectral responsivity is also one of the advantages of the fabricated devices. The spectral responsivity (in unit of AW−1) of BLG/SiNH array, and BLG/CH3-SiNH array are plotted in Fig. 5(b). Its spectral response to light illumination has a wide wavelength range from 400 to 1150 nm. Both spectral response profiles are similar shape with responsivity peak around 850 nm.
Fig. 4 (a) Response of the Schottky junction device to the pulsed photoirradiation at frequencies of 10 Hz and 2000 Hz. (b) The relative balance versus frequency of the pulsed NIR light. (c) A single normalized cycle measured at 2000 Hz to estimate both rise time (τr) and fall time (τf) of the SPNIRPD with and without methyl-terminated.
Fig. 5 (a) Energy band diagram of SPNIRPD before and after surface passivation under light illumination. (b) Responsivity of the Schottky junction under illumination of the different wavelength light.
3. Performance comparison
To evaluate the performance of the proposed design with the other reported graphene based devices, the responsivity (R) and detectivity (D*) calculated by Eq. (2) and (3) [16], τr/τf and Ilight/Idark were listed and compared in Table 1.
where Ip, Popt, η, h, c, λ, A, q, Id and G are photocurrent, incident light power, quantum efficiency, Planck constant, speed of light, light wavelength, active area, the unit of elementary charge, dark current and photoconductive gain, respectively. Here, for simplification we assume η = 1, and R is approximated to be 0.328 AW−1 at zero bias, A is equal to 0.04 cm2 and D* is calculated to be 6.03 × 1013 cmHz1/2W−1. Figure 3(b) shows that the photocurrent increases from 3.0 to 33 μA with the decrease in dark current from 4.7 to 2.3 pA after passivation, yielding an Ilight/Idark > 107, which is two order of magnitude higher than that without passivation (~6.4 × 105). Such an increase in Ilight/Idark ratio can be attributed to the surface coating of methylation layer, which provides a convenient way to effectively suppress the surface charge recombination velocity by reducing the density of dangling bonds and defects states at the SiNH array surface [26]. As a result, more electrons and holes are collected by the bottom In/Ga alloy and top Ti/Au electrodes, respectively, leading to promoted current under light illumination. Meanwhile, the insulating methyl group acting as blocking layer decreases the leakage current at zero bias, which is also called the dark current. The fast response and recover time (τr/τf = 22/56 μs) can be ascribed to enhance built-in electric field formed by the Schottky junction and effective surface passivation. It is noted that surface modification can considerably alter the surface electron affinity of SiNH array, for instance, the offset of surface electron affinity for the CH3-SiNH array could be as high as + 0.35 eV as shown in Fig. 5(a) [27, 28]. Therefore, the built-in electric field was greatly strengthened in the CH3-SiNH array device, as compared with the SiNH array device. On the other hand, the surface energy band bending caused by the -CH3 modification can facilitate the separation of photo-generated electron-hole pairs in the radial direction of SiNH array; the holes will accumulate on the NH inner surface, while the electrons will accumulate inside the Si to prevent the carriers recombination.4. Conclusion
In summary, we have demonstrated a high-performance NIRPD based on graphene and SiNH array Schottky junction with -CH3 passivation layer. Electrical analysis reveals the NIRPD with -CH3 exhibits high sensitivity to 850 nm illumination at zero bias voltage, the Ilight/Idark ratio can reach as high as 107, the highest value ever reported for the similar type of PDs. In addition, responsivity and detectivity of the NIRPD with -CH3 were calculated to be 0.328 AW−1 and 6.03 × 1013 cmHz1/2W−1, respectively, higher than that of the NIRPD without -CH3 and other NIRPDs. Moreover, further analysis shows that the NIRPD with -CH3 can work in a wide range of switching frequencies with high response speed (τr = 22 μs, τf = 56 μs). It is expected that this simple, high-performance BLG/SiNH array Schottky junction with a -CH3 interface passivation will have great potential for the future NIR detection or sensing applications.
Acknowledgments
This work is financially supported by the Research Grants Council (RGC) of Hong Kong (GRF 526511/PolyU B-Q26E) and PolyU internal research grants G-YL06, G-YN06.)
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