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Photo-Induced Doping in Graphene/Silicon Heterostructures Xiao-Juan Wang,†,‡ Liping Zou,† Dong Li,† Qichong Zhang,† Fengli Wang,† and Zengxing Zhang*,† †

Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, School of Physics Science and Engineering, Tongji University, Shanghai 200092, China ‡ School of Physics and Electronics, Henan University, Kaifeng 475004, China S Supporting Information *

ABSTRACT: Photoinduced doping in graphene and its related heterostructures has drawn much interest as one of the possible ways to control the electronic properties of graphene. In this paper, we report that graphene/silicon (Gr/Si) heterostructures are an effective configuration for photoinduced doping in graphene. Raman spectroscopy, electrical and photoelectrical measurements are used to characterize the photoinduced doping effect. The results demonstrate that the graphene in the Gr/Si heterostructure is p-doped by light irradiation, and the doping effect can be controlled by varying the irradiation time. For the electrical properties of the Gr/Si Schottky junction, the photoinduced doping effect reduces the barrier height and series resistance but enhances the ideality factor. For the photovoltaic properties, the doping strengthens the open-circuit voltage, short-circuit current, fill factor, and conversion efficiency. The work should be helpful on developing effective ways for graphene doping and in depth understanding and better use of Gr/Si Schottky junctions for electronics and optoelectronics.

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of graphene under light irradiation.18−20 These remarkable researches are stimulating the great interest to study the photoinduced doping effect in graphene and its related heterostructures. While graphene is stacked on silicon, the resulted graphene/ silicon (Gr/Si) heterostructures exhibit an obvious Schottky behavior.21,22 Owing to the high transparent conductive properties of the graphene, Gr/Si heterostructures demonstrate outstanding optoelectronic properties and have been developed for high-performance solar cells and photodetectors.23−27 In this paper, we reported that Gr/Si heterostructures could be used to effectively dope graphene under light irradiation. We used Raman spectroscopy, electrical and photoelectrical measurements to characterize the photoinduced doping effect in graphene. On the basis of these measurements, the possible doping mechanism was discussed. And the photoinduced doping effect on the photovoltaic properties of Gr/Si heterostructures was also studied.

INTRODUCTION Graphene, a single carbon atomic layer with a honeycomb structure, possesses unique properties of high carrier mobility, long phase coherence length, excellent thermal conductivity, strong mechanical behavior, and outstanding optical transmittance, which are combined to make it a promising material for future carbon-based nanoelectronics or optoelectronics.1−7 However, the intrinsic graphene is a zero band gap semiconductor. For further electronic or optoelectronic applications, it is necessary to open the band gap and control its charge carrier type and density.8 So far, various methods, including atomic chemical doping,9,10 molecular noncovalent modification,11 metal contact doping,12,13 and so on, have been employed to tailor the electronic properties of the graphene. Conventional atomic doping seems to be an effective way but it always induces defects and thus degrades the carrier mobility of the graphene. Therefore, it is necessary to develop an alternative way to control the doping and avoid the defects induced. Recently, several groups reported that light could be used to realize the charge doping in graphene.14−17 Since it is a noncontacting approach, the doping in this way could not induce additional defects and is very easy to be manipulated. Because of the interfacial charge trap sites in gate oxide, graphene on SiO2 substrates can be locally doped by light irradiation and has been developed for graphene p−n junctions.14,16 Meanwhile, it is reported that light can induce doping in graphene/boron nitride (Gr/BN) heterostructures because of the defects state in the bulk crystalline BN flakes, which can preserve the high carrier mobility of graphene.17 In addition, water or oxygen molecules in air also cause the doping © XXXX American Chemical Society

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EXPERIMENTAL METHODS Graphene Synthesis. Graphene films were synthesized with a chemical vapor deposition (CVD) method by using 25μm-thick copper (Cu) foils (Alfa Aesar, item No. 13382) as substrates in a horizontal quartz tube furnace.28 After the furnace was heated up to 1000 °C and evacuated to less than 100 mTorr, Cu foils were loaded into the center of the Received: September 30, 2014 Revised: December 21, 2014

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Figure 1. (a) Photograph and schematic plans of the produced Gr/n-Si heterostructures. (b) Typical Raman spectrum of the graphene on SiO2 in the Gr/n-Si heterostructures. (c) Typical Raman spectrum of the graphene on Si in the Gr/n-Si heterostructures. (d) Typical dark I−V characteristic curve of the Gr/n-Si heterostructures.

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RESULTS AND DISCUSSION Figure 1a shows typical photograph and schematic plans of the produced Gr/n-Si configurations. The details of the configuration fabrication process are described in the Experimental Section. Typically, the etched back side of the Si wafers is fully covered with Au/Ni film, which is treated by thermal annealing to form an Ohmic contact with the Si.30,31 The CVD-derived monolayer graphene is transferred on the top of the patterned wafers, which is in contact with the previously deposited Au/Ni electrodes around the silicon windows. As a result, the graphene and the etched silicon adhere to each other to form a heterostructure in an intimate van der Waals contact, and the top Au/Ni electrode provides a good electrical contact with the graphene.21 Figure 1b shows a typical Raman spectrum of the graphene transferred on SiO2 in the Gr/n-Si heterostructures. As shown in Figure 1b, the D band is nearly free, and the intensity ratio of the 2D and the G band is larger than 2, indicating that the graphene is a high-quality monolayer graphene.28,31−33 Figure 1c shows the Raman spectrum of the graphene on the etched silicon in the Gr/n-Si heterostructures. The 2D band is symmetric and much higher than the G band, exhibiting a similar feature to the graphene on SiO2 except for the appearance of the small D band. The appearance of the small D band should be due to strain or defects. The hydrophobic Si makes the graphene not easy be adhered on during the wet transfer process, which could possibly result in strain or defects in the graphene. The intensity at around 1430 cm−1 is from the underneath Si, indicating that the graphene is indeed on the Si.34 Figure 1d shows a typical current−voltage (I−V) characteristic curve of the Gr/n-Si heterostructures under dark condition. The I−V curves demonstrate that the Gr/n-Si heterostructures exhibit obvious rectifying characteristics, indicating that they are well-defined Schottky diodes with a rectifier ratio of 104−105. The photoinduced doping effect was carried out with a white light of a power of 1 mW. Figure 2a shows the dark I−V

temperature zone where they were annealed for 30 min under the protection of 100 SCCM H2 gas flow. Ten SCCM CH4 was then introduced in the furnace for another 30 min. After CH4 was shut down, the Cu foils were taken out and cooled down to room temperature rapidly. Graphene films were then synthesized on the both sides of the Cu foils. In order to get free-standing graphene, the obtained graphene/Cu foils were first spin-coated with poly methyl methacrylate (PMMA) on one side and baked. The other side was then cleaned by O2 plasma followed by etching Cu in an aqueous solution of 0.1 M (NH4)2S2O8 overnight.29 After the Cu was dissolved thoroughly, graphene/PMMA films were rinsed three times with deionized (DI) water for further use. Graphene/Silicon (Gr/Si) Heterostructures Fabrication. Si wafers used here are n-type with a resistivity of 1−10 Ω·cm and a crystal termination of (100), which are covered with 300 nm-thick thermal oxide film. The back side of silicon wafers was first etched by buffered oxide etchant (BOE) and then covered with Au/Ni film, which was annealed in vacuum at 600 °C for 1 min to form an Ohmic contact.30,31 A 0.1 cm2 area silicon square window in the center of the topside of the wafers was produced by a standard photolithography and wet etching method. It was then rapidly and totally covered with CVD-derived monolayer graphene that was in contact with the predeposited Au/Ni electrodes around the silicon window. Characterizations. Raman spectroscopy was characterized on a Horiba Jobin Yvon LabRAM HR Raman system by using a 514 nm laser. The electrical and photoelectrical properties of the produced Gr/n-Si heterostructures were characterized with a Keithley 4200-SCS semiconductor analyzer at room-temperature in atmosphere. The light irradiation source was a lightemitting diode (LED) lamp whose power can be modulated. We used an optical power meter of SGN-1 to measure the power of the light and calculated the incident power on the heterostructures according to their area. B

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Figure 2. (a) The dark I−V curves and (b) the barrier heights, the series resistances and the ideality factors of a same Gr/n-Si heterostructure with different light irradiation times from 0 to 120 min. The light power is 1 mW.

accumulate on the graphene boundary and the electrons to accumulate on the Si boundary. The accumulation of the charge carriers partially offsets the space charges that produce Schottky junction, leading to the reduction of the Schottky barrier height of ΦB. Moreover, the p-doping effect would lower the sheet resistance of the graphene film, resulting in the decrease in the RS of the heterostructures.4,36 This maybe increases the ideality factor n of the Gr/n-Si Schottky junctions after light irradiation. The holes accumulation on the surface of the graphene makes it p-doped. Raman spectroscopy is a powerful tool on the study of carbon based nanomaterials. The charge doping in graphene can be monitored by Raman spectroscopy. Figure 3b shows the in situ Raman spectra of the graphene on the Si in the Gr/n-Si junctions with different irradiation time from 0 to 70 min. For the Raman measurement, the excited laser with a wavelength of 514 nm is focused on the same position. After the junctions are irradiated for a certain time with the white light, the white light is shut off and the excited laser is switched on and the Raman spectrum is measured once. As shown in the Raman spectra, both of the G band and the 2D band are downshifted after light irradiation. And the variation is slowly enhanced with the irradiation time increasing. In comparison of the graphene without irradiation, 70 min irradiation makes the G band and the 2D band downshift 1.6 and 7.4 cm−1, respectively. The variation of the G band and the 2D band can be attributed to charge doping or strain effect.37,38 By considering their correlated variations, it is possible to distinguish the contributions from charge doping or strain effect. For the uniaxial strain, the average value of Δω2D/ΔωG is about 2.2. For the hole doping, the value is approximately 0.7.15,20 According to the reported vector decomposition method, the uniaxial strain downshifts the G band and the 2D band of 4.2 and 9.2 cm−1, respectively; and the hole doping upshifts the G band and the 2D band of 2.6 and 1.8 cm−1, respectively. Gr/n-Si Schottky junctions exhibit photovoltaic properties, which could be developed for solar cells or photodetectors. The photoinduced doping effect of the Gr/n-Si diodes on these properties was further studied, and the results are shown in Figure 4. Figure 4a shows the light I−V characteristic curves of the Gr/n-Si junction with different light irradiation time from 0 to 120 min. The results clearly demonstrate that the photovoltaic properties of the Gr/n-Si junction vary as the light irradiation time increases. Figures 4b and c show the variations of the photovoltaic properties, including open-circuit voltage (VOC), short-circuit current (ISC), fill factor (FF), and

characteristic curves of the same Gr/n-Si heterostructure with different light irradiation time from 0 to 120 min. The method to test the heterostructures is described in the following: to test the heterostructures with the light irradiation time of 10 min, we first irradiate the heterostructures for 10 min, and then shut off the white light and test the I−V characteristic curve under the dark condition; for 30 min, we irradiate the structure for another 30 − 10 = 20 min, and then shut off the white light and test the dark I−V curve and so forth. The results in Figure 2a indicate that the Gr/n-Si heterostructure preserves its rectifying behavior after light irradiation except that the forward current is continuously enhanced with the turn-on voltage continuously reduced as the irradiation time increases. The variations of the Schottky barrier height (ΦB), the ideality factor (n) and the series resistance (RS) (extracted from the forward I−V curve35) as functions of the irradiation time are depicted in Figure 2b, indicating that ΦB and RS are reduced but n is enhanced as the irradiation time prolonged. Figure 3a shows the energy diagram of the Gr/n-Si Schottky junctions. While graphene is in contact with n-Si, the difference

Figure 3. (a) Energy diagram of Gr/n-Si Schottky junctions. WGr is the work function of the graphene, χ is the electronic affinity of the silicon, and ΦB is the Schottky barrier height. (b) Raman spectra of the graphene at a same position on Si in the Gr/n-Si heterostructure with different light irradiation time from 0 to 70 min. The light power is 1 mW.

of their Fermi levels causes electrons transfer from the n-Si to the graphene, generating a built-in electric field with a Schottky barrier height (ΦB). Under light irradiation, the incident light can pass through the graphene transparent electrode and generate electron−hole pairs in n-Si. The built-in electric field separates the generated electron−hole pairs and drives the holes from the n-Si to the graphene. This should be the reason for the reduction of the Schottky barrier height (ΦB). The generated charge carriers’ separation causes the holes to C

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Figure 4. (a) The I−V curves, (b) the short-circuit currents and the open-circuit voltages, (c) the fill factors and the conversion efficiencies of the same Gr/n-Si heterostructure under the light illumination with different irradiation time from 0 to 120 min. The light power is 1 mW.

Table 1. Characteristics of the Same Gr/n-Si Heterostructure before and after Irradiation for 120 mina before irradiation after irradiation a

ΦB/eV

RS/kΩ

n

VOC/V

ISC/mA

FF/%

η/%

0.827 ± 0.005 0.792 ± 0.005

0.794 ± 0.055 0.455 ± 0.045

2.623 ± 0.115 3.454 ± 0.120

0.305 ± 0.001 0.330 ± 0.001

0.348 ± 0.010 0.521 ± 0.010

15.832 ± 0.103 17.383 ± 0.095

1.729 ± 0.087 3.970 ± 0.092

The light power is 1 mW.

Figure 5. (a) Time-dependent photocurrent response curve of a bare n-Si with the light switching on and off. (b) Time-dependent photocurrent response curve of the Gr/n-Si heterostructure with the light switching on and off. Here the photocurrent in Gr/n-Si is the short-circuit current of ISC with the applied voltage of 0. The light power is 1 mW.

The improvement of the VOC can be explained by the following equation. For Schottky junctions, I−V characteristic curves under light illumination can be expressed by

conversion efficiency (η). The images display that all of the parameters increase with increasing light irradiation time. The increasing rates are first fast but slowly slow down as the irradiation time prolongs, which is consistent with the variation of n. Table 1 shows the variations of the parameters of the Gr/n-Si Schottky junction before and after light irradiation for 120 min. The results exhibit that n increases from 2.63 to 3.74, ΦB decreases from 0.83 to 0.79 eV, and RS decreases from 0.80 kΩ to 0.46 kΩ. For the photovoltaic properties, VOC increases from 0.31 to 0.33 V, ISC is from 0.35 mA to 0.52 mA, and FF is from 15.9% to 17.4%, leading to the boost in conversion efficiency η from 1.72% to 3.00%, which is improved by about 75%. It should be noted here that LED white light with small power is used instead of the standard sunlight to avoid temperature variation of the devices. And to avoid the test influenced by unknown factors, the graphene and the silicon are not further processed. These solutions cause the measured conversion efficiency to be lower than those reported in the literatures.26,39,40 However, the performance improvement of the Gr/Si junctions is obvious by light irradiation due to the photoinduced doping effect. Our results demonstrate that Gr/ Si junctions can be effectively modulated by varying the light irradiation time.

⎡ ⎛ eV ⎞ ⎤ ⎟ − 1 − I I = IS⎢exp⎜ ⎥ L ⎣ ⎝ nkT ⎠ ⎦

(1)

where IS is the reverse saturation current, e is the electronic charge, n is the ideality factor, k is the Boltzmann constant, T is the absolute temperature, and IL is the current from the excess photoexcited carriers. VOC is defined as the bias current of 0. From eq 1, VOC can be deduced as VOC =

I ⎞ nkT ⎛ ln⎜1 + L ⎟ e IS ⎠ ⎝

(2)

Therefore, VOC is dependent on the ideality factor n, which is consistent with our experimental results. As shown in Figure 2b and 4b, their variations are almost similar. Photoelectrical characteristics further prove that the photoinduced doping effect in the configurations is mainly related to the Gr/n-Si Schottky structures. Bare n-Si often exhibits rapid response to the incident light. Under the light irradiation, the excited photocurrent increases fast and then slowly reaches saturation, as shown in Figure 5a. After the light switched off, D

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the photocurrent rapidly disappears. The results in Figure 5a indicate that they are repeatable, and the generated photocurrent is nearly similar. However, the Gr/n-Si heterostructures exhibit a different behavior, as shown in Figure 5b. Under the light irradiation, the photocurrent is first generated and then goes up to a certain value. However, it maintains the slowly increasing trend as the light irradiation time increases without any saturation. When the light is switched off, the photocurrent rapidly disappears. In the following cycles, the generated photocurrent is not as high as the initial value obtained in the first cycle. It goes up to a certain value at the end of the last cycle under the light irradiation and keeps on increasing with increasing irradiation time, which is obviously different from the photoresponse phenomenon in bare silicon. This observation indicates that the photoinduced doping effect mainly originates from the Gr/n-Si structure. It could be that the transferred holes are trapped by the graphene defects, leading to the photoinduced doping effect in Gr/n-Si structures lasting for a certain period if the structures are kept in a dark condition. It slowly disappears after several days, indicating that the Gr/n-Si heterostructures could be a promising configuration for photodoping graphene.

CONCLUSION In conclusion, we have studied the photoinduced doping effect in Gr/n-Si heterostructures by Raman spectroscopy, electrical and photoelectrical characterizations, and found that the white light can be used to effectively dope the graphene in the Gr/nSi heterostructures. The built-in electrical field in the Schottky junctions makes the photoexcited holes drift onto the graphene leading to it p-doped. As a result, the electrical and photoelectrical properties of the Gr/n-Si structures can be modulated by varying the light irradiation time. The photoinduced doping effect reduces the Schottky barrier height ΦB, but increases the ideality factor n, short-circuit current ISC, open-circuit voltage VOC, fill factor FF, and conversion efficiency η. 120 min light-irradiation can strengthen the conversion efficiency by about 75%. This kind of photoinduced doping effect can last for a certain period in dark condition, indicating that the Gr/n-Si heterostructures could be a promising configuration for graphene doping. The results of this work could be helpful on developing effective ways for graphene doping, and attaining in-depth understanding of the effective use of Gr/Si Schottky junctions for electronics and optoelectronics. ASSOCIATED CONTENT

S Supporting Information *

Results for the Gr/n-Si heterostructure under white light irradiation in nitrogen and under infrared light irradiation in air and additional Gr/n-Si heterostructures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by NSFC (11104204) and Shanghai Pujiang Program (12PJ1408900). E

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