Camphor-Based CVD Bilayer Graphene/Si Heterostructures for Self-Powered and Broadband Photodetection

This work demonstrates a self-powered and broadband photodetector using a heterojunction formed by camphor-based chemical vaper deposition (CVD) bilayer graphene on p-Si substrates. Here, graphene/p-Si heterostructures and graphene layers serve as ultra-shallow junctions for UV absorption and zero bandgap junction materials (<Si bandgap (1.1 eV)) for long-wave near-infrared (LWNIR) absorption, respectively. According to the Raman spectra and large-area (16 × 16 μm2) Raman mapping, a low-defect, >95% coverage bilayer and high-uniformity graphene were successfully obtained by camphor-based CVD processes. Furthermore, the carrier mobility of the camphor-based CVD bilayer graphene at room temperature is 1.8 × 103 cm2/V·s. Due to the incorporation of camphor-based CVD graphene, the graphene/p-Si Schottky junctions show a good rectification property (rectification ratio of ~110 at ± 2 V) and good performance as a self-powered (under zero bias) photodetector from UV to LWNIR. The photocurrent to dark current ratio (PDCR) value is up to 230 at 0 V under white light illumination, and the detectivity (D*) is 8 × 1012 cmHz1/2/W at 560 nm. Furthermore, the photodetector (PD) response/decay time (i.e., rise/fall time) is ~118/120 μs. These results support the camphor-based CVD bilayer graphene/Si Schottky PDs for use in self-powered and ultra-broadband light detection in the future.


Introduction
Si-based photodetectors (PDs) are widely used in pollution analyzers, biological research, combustion flame monitoring, and optical communication due to their compatibility with the silicon integrated circuit (IC) industry [1][2][3]. Until now, Si-based PDs with different structures (e.g., Schottky photodiodes, p-n photodiodes, metal-semiconductor-metal (MSM) photodiodes, phototransistors, and p-i-n photodiodes) have been intensively studied and developed [4][5][6]. Among them, Si Schottky photodiodes provide superior radiation resistance and high-speed operation and can be easily integrated with photoelectronic and microelectromechanical systems for photon detection [7,8]. Furthermore, the self-powered operation mode (at zero bias) of Schottky photodiodes (at room temperature), the graphene grown by camphor-based CVD exhibits few defects, >95% coverage bilayer structures, a high-uniformity, and a high carrier mobility (1.8 × 10 3 cm 2 /V·s). The rectification ratio of the camphor-based CVD bilayer graphene/Si Schottky PD is~110 at ±2 V. The camphor-based CVD bilayer graphene/Si Schottky PDs in this study show ultra-broadband (from UV to LWNIR) photoresponsivity, a high photocurrent-to-dark-current ratio (up to 230), and a high detectivity (D*) (8 × 10 12 cmHz 1/2 /W at 560 nm) under zero bias. Moreover, the camphor-based CVD bilayer graphene/Si Schottky PDs also exhibits a fast photoresponse property (rise/fall time~118/120 µs). This research could provide a new means of low-cost production of the camphor-based CVD bilayer graphene/Si Schottky PDs and extend the applications of Si-based devices.

Device Fabrication and Methods
A copper (100) substrate (thickness: 0.3 mm, the Nilaco corporation) was cleaned by acetone for 3 min in the ultrasonic cleaner. After that, the copper substrate was blow dried by N 2 gas and then placed on the middle of in the reactor (a 1.2 m-length quartz tube with a 1-inch diameter). For the synthesis of graphene, the copper substrate was annealed in Ar (with flow rate: 200 sccm) and H 2 (with flow rate: 100 sccm) in ambient conditions under 760 torr at 950 • C for 80 min and 15 min, respectively. Then, 1.2 mg of camphor powder (Yu Li Hang biochemical industrial co.) was heated at 125 • C, and its vapors were sent into the reactor by carrier gas (Ar/H 2 flow rate: 98/8 sccm) under 760 torr for 10 min. As shown in Figure 1, the schematic diagram is our atmospheric pressure chemical vapor deposition (APCVD) system. After the process of growing graphene on copper substrate, the graphene layers were transferred to p-type Si (111) substrates (resistivity of 1-30 Ω cm), with the top SiO 2 film being partially removed (etched by buffered oxide etchant (BOE) (HF:NH 4 F = 1:5)) and cleaned in acetone and isopropanol prior to use. Figure 2 is the schematic of the graphene transfer and PD fabrication process flow.
Micromachines 2020, 11, x FOR PEER REVIEW 3 of 13 CVD bilayer graphene/Si Schottky PDs in this study show ultra-broadband (from UV to LWNIR) photoresponsivity, a high photocurrent-to-dark-current ratio (up to 230), and a high detectivity (D*) (8 × 10 12 cmHz 1/2 /W at 560 nm) under zero bias. Moreover, the camphor-based CVD bilayer graphene/Si Schottky PDs also exhibits a fast photoresponse property (rise/fall time ~118/120 μs). This research could provide a new means of low-cost production of the camphor-based CVD bilayer graphene/Si Schottky PDs and extend the applications of Si-based devices.

Device Fabrication and Methods
A copper (100) substrate (thickness: 0.3 mm, the Nilaco corporation) was cleaned by acetone for 3 min in the ultrasonic cleaner. After that, the copper substrate was blow dried by N2 gas and then placed on the middle of in the reactor (a 1.2 m-length quartz tube with a 1-inch diameter). For the synthesis of graphene, the copper substrate was annealed in Ar (with flow rate: 200 sccm) and H2 (with flow rate: 100 sccm) in ambient conditions under 760 torr at 950 °C for 80 min and 15 min, respectively. Then, 1.2 mg of camphor powder (Yu Li Hang biochemical industrial co.) was heated at 125 °C, and its vapors were sent into the reactor by carrier gas (Ar/H2 flow rate: 98/8 sccm) under 760 torr for 10 min. As shown in Figure 1, the schematic diagram is our atmospheric pressure chemical vapor deposition (APCVD) system. After the process of growing graphene on copper substrate, the graphene layers were transferred to p-type Si (111) substrates (resistivity of 1-30 Ω cm), with the top SiO2 film being partially removed (etched by buffered oxide etchant (BOE) (HF:NH4F = 1:5)) and cleaned in acetone and isopropanol prior to use. Figure 2 is the schematic of the graphene transfer and PD fabrication process flow.   The morphology of the graphene/Si heterostructure was analyzed using a field emission scanning electron microscope (FESEM). The characteristics and dimensions of the graphene layers were confirmed by micro Raman spectroscopy (the resolution~1.4 cm −1 ) with 532 nm laser excitation. The Hall mobility of the camphor-based CVD graphene was measured with an Ecopia HMS-5000 (Ecopia corporation, Anyang, South Korea) at room temperature.
The Ohmic contacts of the camphor-based CVD bilayer graphene/Si heterojunction (active region: 9 mm × 8 mm) were deposited with Ti (20 nm)/Au (80 nm) using the thermal evaporator. Photocurrent was generated under the illumination of solar simulator (LSH 150, Taiwan Fiber Optics, Inc., Taipei, Taiwan). The spectral responsivity of the camphor-based CVD bilayer graphene/Si PDs was measured with the QEX10-Quantum Efficiency Measurement System (PV Measurements, Inc., Boulder, CO, USA-under zero bias. The I-V characteristics of the fabricated PDs were measured by an Agilent

Measurement Results and Discussion
The FESEM image of the morphology of graphene/Si heterostructure is shown in Figure 3a. There are no obvious cracks and holes in the graphene area after optimizing the transfer processes. Furthermore, some wrinkles of graphene are also observed, induced by the solution interface and the defects of the Cu substrate during the transfer processes [35]. In order to realize the quality and the layer number of the camphor-based CVD graphene in this study, the Raman spectra are shown in Figure 3b; the peak positions for the D band, G band, and 2D band in the graphene are 1350, 1600, and 2700 cm −1 , respectively [36,37]. The intensity ratio of the 2D and G peaks (i.e., I 2D /I G ) and the full width at half maximum (FWHM) of the 2D peak can be used to further determine the graphene layer number (i.e., I 2D /I G > 1.3 and FWHM < 30 cm −1 representing single-layer graphene and I 2D /I G < 0.7 and FWHM > 70 cm −1 representing three or more layers of graphene) [36][37][38][39][40]. The calculated I 2D /I G ratio value and FWHM of the 2D peak are~1 and 45 cm -1 , respectively, corresponding to bilayer graphene. However, the weak D peak indicates that the few defects in the graphene layers could be induced during high-temperature CVD processes. Two-dimensional Raman mapping is used to investigate the uniformity of graphene across the surface. In Figure 3c, the Raman mapping for the camphor-based CVD graphene shows the variation in the I 2D /I G ratio at each sampling point (each sampling step: 400 nm) over a 16 µm × 16 µm region, indicating the presence of uniform bilayer graphene with a high quality and high coverage throughout the surface [36,37]. For semiconductor materials, carrier mobility is a critical parameter for determining the performance of optoelectronic devices. Therefore, the carrier mobility (1.8 × 10 3 cm 2 /V·s) of the camphor-based CVD bilayer graphene was obtained from the room temperature Hall measurement. Compared to the conventional CVD graphene (mobility: 100~1000 cm 2 /V·s) [41,42], the camphor-based CVD graphene shows a higher mobility. However, the biggest issue of CVD graphene is that its carrier mobility is much lower than that of graphene obtained from the exfoliation of graphite.
Micromachines 2020, 11, x FOR PEER REVIEW 5 of 13 extracted further by fitting the rectification I-V curve using the above equation. One should note that ideal factor value is >1, indicating that the inhomogeneity (resulting from surface roughness or defects) in the graphene/p-Si Schottky junction area [46].  Figure 4c shows the I-V characteristics of the camphor-based CVD bilayer graphene/p-Si Schottky PDs measured in the dark and under white light illumination of different power densities. At zero bias, the photocurrent to dark current ratio (PDCR = (Ip − Id)/Id, where Id is the dark current and Ip is the photocurrent) [47] of the camphor-based CVD graphene/p-Si Schottky PDs is 13.25 and 230 under white light illumination, with 6.25 and 112 mW/cm −2 , respectively. From the PDCR values, the camphor-based CVD bilayer graphene/p-Si Schottky PDs are still available even under zero bias, mainly owing to the photovoltaic properties of Schottky junctions [48,49]. To demonstrate the feasibility of camphor-based CVD bilayer graphene/p-Si Schottky PDs in practical uses, a spectral responsivity analysis was performed under zero bias from UV to LWNIR (300 to 1250 nm), as shown in Figure 4d. The responsivity is 0.09 A/W at 300 nm (UV region), 0.41 A/W at 560 nm (visible light region), and 0.04 A/W at 1250 nm (LWNIR region), indicating that the ultra-broadband detection of the camphor-based CVD bilayer graphene/p-Si Schottky PDs is owing to the effective UV light absorption of the ultra-shallow graphene/p-Si Schottky junction and the LWNIR absorption (>Si cutoff wavelength) of graphene (zero bandgap). As compared with commercial Si PN and PIN PDs [20], the depletion region of the ultra-shallow junction will be formed near the top surface of the camphorbased CVD bilayer graphene/p-Si Schottky PDs, as shown in Figure 5. This would lead to the direct In the schematic of the camphor-based CVD bilayer graphene/p-Si Schottky PDs, as shown in Figure 4a, the graphene/p-Si junction area (i.e., active region) is~9 mm × 8 mm and the thickness of the metal contact and SiO 2 are 100 nm. Figure 4b shows the I-V and fitting curves of the camphor-based CVD bilayer graphene/p-Si Schottky PDs measured with an Agilent (B1500 A) source meter in the dark. The reverse dark current and a rectification ratio of the camphor-based CVD bilayer graphene/p-Si Schottky PD are~20 nA (@−2 V bias) and~110 at ±2 V bias, respectively, indicating the good rectification property of the camphor-based CVD graphene/p-Si Schottky junction even with a bilayer-graphene thickness as low as~0.7 nm [43]. Furthermore, the rectification I-V characteristics can be clarified by using I = AA* T 2 exp[−qΦ B /k B T] [exp(qV/nk B T)−1] based on thermionic emission theory [44,45], where A is the effective graphene/p-Si junction area (0.72 cm 2 ), A* is the Richardson constant (≈32 A cm −2 K −2 for p-Si) [46], T is the temperature (300 K), q is the electron charge (1.6 × 10 −19 C), qΦ B is the Schottky barrier high (SBH) of the graphene/p-Si Schottky junction, n is the ideal factor(~4.5), and k B is the Boltzmann constant. Therefore, the SBH (~0.81 eV) in this study could be extracted further by fitting the rectification I-V curve using the above equation. One should note that ideal factor value is >1, indicating that the inhomogeneity (resulting from surface roughness or defects) in the graphene/p-Si Schottky junction area [46]. absorption of UV light therein, without a long penetration distance. The depletion region width is evaluated to be ~0.18 μm using W = [2ε0εr (Φbi + V)/eNd] 1/2 [44], where the free space permittivity is ε0 = 8.85 × 10 −14 F/cm, the relative permittivity of Si is εr = 11.8, the built-in potential is Φbi = ~0.62 V (Appendix A), the reverse-bias voltage is V = 0 V, and the doping concentration is Nd = ~2 × 10 16 cm −3 . Thus, the photocarriers could be effectively induced by UV light due to a limited UV penetration depth (~20 nm) within the depletion region formed [20].    Figure 4c shows the I-V characteristics of the camphor-based CVD bilayer graphene/p-Si Schottky PDs measured in the dark and under white light illumination of different power densities. At zero bias, the photocurrent to dark current ratio (PDCR = (I p − I d )/I d , where I d is the dark current and I p is the photocurrent) [47] of the camphor-based CVD graphene/p-Si Schottky PDs is 13.25 and 230 under white light illumination, with 6.25 and 112 mW/cm −2 , respectively. From the PDCR values, the camphor-based CVD bilayer graphene/p-Si Schottky PDs are still available even under zero bias, mainly owing to the photovoltaic properties of Schottky junctions [48,49]. To demonstrate the feasibility of camphor-based CVD bilayer graphene/p-Si Schottky PDs in practical uses, a spectral responsivity analysis was performed under zero bias from UV to LWNIR (300 to 1250 nm), as shown in Figure 4d. The responsivity is 0.09 A/W at 300 nm (UV region), 0.41 A/W at 560 nm (visible light region), and 0.04 A/W at 1250 nm (LWNIR region), indicating that the ultra-broadband detection of the camphor-based CVD bilayer graphene/p-Si Schottky PDs is owing to the effective UV light absorption of the ultra-shallow graphene/p-Si Schottky junction and the LWNIR absorption (>Si cut-off wavelength) of graphene (zero bandgap). As compared with commercial Si PN and PIN PDs [20], the depletion region of the ultra-shallow junction will be formed near the top surface of the camphor-based CVD bilayer graphene/p-Si Schottky PDs, as shown in Figure 5. This would lead to the direct absorption of UV light therein, without a long penetration distance. The depletion region width is evaluated to bẽ 0.18 µm using W = [2ε 0 ε r (Φ bi + V)/eN d ] 1/2 [44], where the free space permittivity is ε 0 = 8.85 × 10 −14 F/cm, the relative permittivity of Si is ε r = 11.8, the built-in potential is Φ bi =~0.62 V (Appendix A), the reverse-bias voltage is V = 0 V, and the doping concentration is N d =~2 × 10 16 cm −3 . Thus, the photocarriers could be effectively induced by UV light due to a limited UV penetration depth (~20 nm) within the depletion region formed [20].   Note that the responsivity obtained here is competitive with that obtained from the conventional CVD graphene/Si Schottky PDs (0.51 A/W at 532 nm) [49] and much higher than that obtained from the pristine graphene PDs (1 × 10 −3 A/W) [47], graphene oxide/Si Schottky PDs (~63 mA/W at 445 nm), [50] and graphene/insulator/Si (GIS) PDs (~20 mA/W at 1200 nm) [8]. Furthermore, the detectivity (D*) is an important parameter used to analyze the PD performance [51,52]. Under zero bias, the detectivity (D*) of the camphor-based CVD graphene/p-Si Schottky PDs at 560 nm in this study is 8 × 10 12 cmHz 1/2 /W by D* = (f 1/2 )/P = R/(2eJ d ) 1/2 , where P is the incident optical power, R is the responsivity, e is 1.6 × 10 −19 C, J d is the dark current density, and f is the frequency bandwidth of the PD [47,51]. Compared with graphene oxide/Si Schottky PDs (~1.2 × 10 12 cm Hz 1/2 /W) [50] and the conventional CVD graphene/Si PDs (~10 10 cm Hz 1/2 /W) [53], the detectivity of the camphor-based CVD graphene/p-Si Schottky PDs exhibits a better performance.
As shown in Figure 6, the band structures based on Anderson's rule are used to clarify the physics behind the above-observed I-V characteristics of camphor-based CVD graphene/p-Si Schottky PDs [46,[54][55][56][57]. Figure 6a is the band structure of the camphor-based CVD graphene/p-Si heterostructure at thermal equilibrium under zero bias. As the reverse bias applied to the camphor-based CVD graphene/p-Si PDs (i.e., graphene is at a positive voltage compared to p-Si), the graphene Fermi level (E Fg ) is lower than the Si Fermi energy (E FSi ), leading to the SBH (qΦ B ) lowering, as shown in Figure 6b. In the beginning, the hole injection will be inhibited by the Si bandgap, and then it increases gradually with increasing reverse bias because of the SBH decrease. While under forward bias as shown in Figure 6c, the E Fg is higher than E FSi (i.e., graphene is at a negative voltage compared to p-Si), resulting in the increase in SBH. Moreover, the barrier height (qΦ bi ) of the hole from p-Si to graphene will be reduced gradually with an increasing forward bias, leading to a higher dark current. Therefore, the dark current under a low reverse bias (from 0 V to −2 V) is much lower than under a low forward bias (form 0 V to +2 V), as shown in Figure 4b,c. It is noteworthy that the SBH tunable behavior of the camphor-based CVD graphene/p-Si heterostructure is consistent with the numerous study results of the conventional CVD bilayer graphene/Si heterostructures reported previously [58][59][60][61][62][63].
To highlight the stable and high-speed photoresponse properties of the camphor-based CVD bilayer graphene/p-Si Schottky PDs, the time-resolved measurements were performed under a 3 V bias, as shown in Figure 7a. The current increased to a high value (i.e., ON state) and then decreased to a low value (i.e., OFF state) with switching on and off the light, respectively; the stable and reversible photoresponse was also observed even with a small PDCR value (~4) under 3 V bias. Note that the relatively low PDCR value is possibly due to the increase in the dark current under forward bias. Furthermore, in order to quantize the PD operation speed, the rise/fall time of PDs can be revealed by the transition between the light ON and OFF states. As shown in Figure 7b, the rise time (the time difference from 10% to 90% of the saturation photocurrent after switching on the light) and the fall time (the time difference from 90% to 10% of the saturation photocurrent after switching off the light) of the camphor-based CVD bilayer graphene/p-Si Schottky PDs can be revealed to be 118 and 120 µs, respectively [64][65][66][67]. Compared with conventional CVD graphene/Si PDs [49,53], conventional CVD graphene/ZnO PDs [68], and graphene oxide/Si Schottky PDs [50], the camphor-based CVD bilayer graphene/p-Si PDs show a much faster operation speed, resulting from the higher carrier mobility of camphor-based CVD graphene than the conventional CVD graphene and better graphene/p-Si junction quality.

Conclusions
In summary, a self-powered and broadband PD using a camphor-based CVD bilayer graphene/p-Si heterojunction was fabricated. Here, graphene layers could unite with p-Si to form ultra-shallow Schottky junctions for absorbing UV light effectively and, at the same time, graphene layers with zero bandgap could also provide a higher LWNIR absorption than Si. Raman spectra analyses, large-area (16 × 16 µm 2 ) Raman mapping images, and Hall measurements could reveal that our camphor-based CVD graphene has low defects, >95% coverage bilayer structures, a high-uniformity, and a high carrier mobility (1.8 × 10 3 cm 2 /V·s). By employing camphor-based CVD bilayer graphene/p-Si Schottky junctions, our PDs also show a high rectification ratio value (~110 at ±2 V) and high photo-sensing performances (such as an ultra-broadband (from UV to LWNIR) photoresponsivity (under zero bias), a high PDCR value (up to 230 at 0 V), and high detectivity (D*) (8 × 10 12 cmHz 1/2 /W at 560 nm)). In addition, the operation speed of PD was revealed by the rise time and fall time as fast as 118 and 120 µs, respectively. This work demonstrates that the low-cost camphor-based CVD bilayer graphene incorporating p-Si holds promise for practical applications in the new generation of self-powered and ultra-broadband photon detection.