Si heterojunction

Sensors and Actuators A 291 (2019) 87–92

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

High responsivity and high-speed 1.55 ␮m infrared photodetector from self-powered graphene/Si heterojunction Chunxiao Wang a,b , Yuan Dong a , Zhijian Lu a , Shirong Chen a,∗ , Kewei Xu a , Yuanming Ma a , Gaobin Xu a,∗ , Xiaoyun Zhao b , Yongqiang Yu a,∗ a School of Electrical Science and Applied Physics, Micro Electromechanical System Research Center of Engineering and Technology of Anhui Province, Hefei University of Technology, Hefei, Anhui, 230009, PR China b School of Physics and Electronic Engineering, Fuyang Normal University, Fuyang, Anhui, 236037, PR China

a r t i c l e

i n f o

Article history: Received 22 February 2019 Received in revised form 30 March 2019 Accepted 31 March 2019 Available online 2 April 2019 Keywords: Graphene Infrared photodetector Schottky junction Heterojunction

a b s t r a c t Graphene has shown great potentials for new-generation photodetectors in view of its outstanding optical and electrical properties, especially its ultra-broad range absorption. Most of graphene(Gr)/Si hybrid two-dimensional(2D)-three-dimensional(3D) photodetectors, which offer a perspective on future application in integrated optoelectronics, are still however enabled the excellent detection on visible light. Herein, we reported a self-powered Gr/Si Schottky heterojunciton with a high sensivity to communication light of 1.55 ␮m wavelenght by using graphene film as active area. The resultant photodetectors showed a high-speed response speed up to 5.0 ␮s, togther with a responsivity approaching 39.5 mAW−1 , which are comparable with previous graphene-based photodetectors and superior to previous Gr/Si heterojunction. The high-performance of the schottky heterojunction can be ascribed to featuring a built-in field facilitating to separate photocarriers. Combined our results with the methodology of devcie fabrication, can be utilized as pathway for large-area integration of 1.55 ␮m communication light photodetectors. © 2019 Elsevier B.V. All rights reserved.

1. Introduction In our modern life, photodetectors are widely used in optical communication, imaging, medical, military, aerospace and other fields [1–3]. Recently, the interest in near/mid-infrared detection was arose in low-dimensional nanomaterials, such as colloidal quantum dots [4], atomically thin noble metal dichalcogenide [5], and black phosphorene [6]. Graphene (Gr) is a promising two-dimensional (2-D) carbon nanometer material. Its outstanding optical and electronic properties attract many attention in the photodetection domain: (1) excellent electrical conductivity and good transmittance to the light. The transmittance to visible waveband reaches 97.7% and it can almost be regarded as transparent. Therefore, it can replace metal as transparent electrode [7–10] and improve the effective receiving area of the detector [11–13]. (2) The zero band gap structure of graphene makes it being capable of absorbing photons in all wavelength theoretically, indicating it shows a potenial in ultra-wideband optical detection [10,14–16]. (3) Due to carriers follow a special quantum tunneling effect and do not backscatter when encountering impuri-

∗ Corresponding authors. E-mail addresses: [email protected] (S. Chen), [email protected] (G. Xu), [email protected] (Y. Yu). https://doi.org/10.1016/j.sna.2019.03.054 0924-4247/© 2019 Elsevier B.V. All rights reserved.

ties, graphene has strong local conductivity [17,18] and extremely high carrier mobility [18,19]. Graphene shows promise for widespectrum, high-speed and fexible photodetectors. Importantly, the Gr-based photodetectors show great pential in telecommunication wavelengths, but were limited in responsitivity for future application. Recently, several structures based on graphene were constructed to study the 1.55 ␮m light photoresponse properties. Graphene-field effect transistor (FET) was firstly reported with responsivity of 0.5 mAW−1 [20]. A sysmmetric metal-graphenemetal photodetector was fabricated and showed a responsivity of 1.5 mAW−1 [21]. Due to the low absorptivity, zero band gap, lack of gain mechanism and picosecond scale carrier lifetime of graphene, most of Gr-based photodetectors show low responsivity at the level of several to tens of mAW-1 . Various methods have been applied to improve the efficiency of photoelectric detection. For example, the efficiency of graphene-based photodetectors can be increased up to 20 times by introducting a plasmonic nanostructures [22], and even the responsivity can be achieved up to 83 AW-1 in previous graphene-Si phototransistors by using Au nanoparticales array [23]. There are many other methods to enhance the interaction between the incident light and the graphene, such as introducing a nanohole (NH), quantum dots (QD) [10,24,25] and microcavities(MC) [26]. Recently, a horizontal pure graphene p-n junction was fabricated based on a complex ion implantation and chemical vapor deposition (CVD) techniques, and exhibited the highest

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Table 1 Comparison of device parameters for the Gr/Si heterojunction with previous graphene-based photodetectors. Device Structures/Active materials

Response wavelength (nm)

R (mAW−1 )

Gr/Si Gr/Si Gr/Si Gr/Si Gr phototransistor Gr-FET Metal/Gr/Metal Gr/Si Gr/MC Gr p-n junction Gr/interlayer/Gr Gr/ SiO2 / Si Al2 O3 /Gr/ Si Gr/SiQDs/Si Gr/SiNH arrays

1550 532 850 200-1100@850 1550 1550 1550 1550 830-930@865 532-1550 300-1000@900 410-950@890 200-400@400 400-1000@600 850

39.5 510 29 435 83000 0.5 6.1 ˜ 2.89.9

˜ responsivity (1.4–4.7 AW−1 ) for 1.55 ␮m light [27]. The abovementioned menthods are helpful to constuct Gr-based phtodetectors for telecommunication wavelengths. However, the complex structure and manufacturing process are unfavorable factors for practical application. Gr/Si heterojunction has recently attracted good attention for the development of high-performance photodetectors. On the one hand, the simple device structure is easy to construct on planar silicon and acts as a Schottky diode. The barriers of the junction depend on the Silicon concentration and is thereby controllable. On the other hand, Gr/Si heterojunction is compatible with the mature silicon-based platform, making it a good potential for large-scale integration into photodetectors networks and read-out circuits. In the past few years, numerous studies have been reported to construct Gr/Si heterojunction photodetectors, as shown in Table 1. A responsivity approaching hundreds of mAW−1 was achieved in visible light range and ultrahigh detectivity was also attained by introducing a thin interfacial oxide layer, indicating high devcie performance [15,28,29]. We can find that most of Gr/Si photodetectors have been repoted to operate at visible light illumination. Therefore, in these Gr/Si heterojunction photodetector, optical absorption takes place in sicilon side, while graphene only acts as a transparent schottky electrode to collect carriers. The photogenerated electron-hole pairs in silicon are subsequently separated by the built-in field in Gr/Si junction interface. If optical absorption takes place in graphene side, the Gr/Si heterojunction can also greatly reduce the dark current by tuning built-in field and thus increase the detectivity comparing with Gr-based 1.55 ␮m phototransistors or photoconductors. Therefore, a trade-off must be made between the efficiency and detectivity for application in 1.55 ␮m wavelength light detection of Gr-based photodetectors. Here, a Gr/Si heterojunction was constructed, in which a bilayer CVD-graphene was used as Schottky electrode and acted as the active region for 1.55 ␮m light as well. The electrical characteristics of the device exhibit a good rectification behavior. The resultant Gr/Si heterojunction showed a high sensivity to 1.55 ␮m wavelength light with a fast response speed up to 5.0 ␮s, a high current ON/OFF ratio up to 104 and a responsivity approaching 39.5 mAW−1 and a detectivity up to 3.11 × 1011 Jones, which are better than most of previous graphene-based photodetectors. This can pave a way for fabricate low-cost and easily integrated novel IR photodetectors. 2. Experimental details The device architecture of the Gr/Si heterojunction photodetector is schematically shown in Fig. 1a. The active bilayer graphene film (BLG) was grown on Cu foil by using chemical vapor deposition (CVD) method as previous reports. The as-synthesized BLG was

21 1400-4700 700 730 250 350 328

␶r /␶f (␮s) 5/8 130/135 93/110 1200/3000 0.6/60 – – – – 1.2/0.8 – 320/750 – – 22/56

D* (Jones)

Ref.

1011 – 1011 109 – – – – – 1012 1012 1013 1013 109 1013

Our work [25] [16] [18] [23] [20] [21] [30] [31] [27] [32] [29] [33] [34] [35]

characterized by a Raman (LabRam HR Evolution) and ultraviolet visible near infrared spectrophotometer (CARY 5000). In order to study the electrical properties of the BLG, BLG was then transferred to the prepared-cleaned SiO2 (300 nm)/Si and Ag electrodes were chosen as the source and drain contact electrode. The BLG-transfer properties were carried out using a semiconductor characterization system (Keithley 4200-SCS) at room temperature. The Gr/Si heterojunction was constructed as follow. Briefly, a photoresist window (diameter of 5 mm) was first defined on the pretreated SiO2 (80 nm)/n-type Si (1–10 cm−1 ) substrate by a mask, and then a BOE solution containing 5% HF was chosen to remove the SiO2 layer for 300 s. The BLG was then transferred to the top of the prepared Si substrate as the top electrode. An In/Ga paste electrode was formed on the back side of the substrate using as an ohmic contact for n-type Si. The electronic characteristics was performed by Keithley 4200-SCS at room temperature. The photoresponse properties were characterized under the illumination of an optical fiber coupled laser (1.55 ␮m). The response speed was recorded by a Tektronix TDS2022B digital oscilloscope under varied pulsed light illumination.

3. Results and discussion Fig. 1a shows the architecture of Gr/Si heterojunction, and the corresponding photograph as shown in Fig. 1b. In this structure, the BLG was used as the top electrode as Schottky contact for ntype Si and the active area for 1.55 ␮m light as well. Due to the graphene film acting as an active area for 1.55 ␮m light in this structure, the electrical and optical properties were further characterized. Raman spectroscopy is firstly employed to characterize the structural properties of graphene films, including number of layers. The Fig. 1c shows a typical Raman spectrum of the CVD-synthesized graphene film using laser excitation at 532 nm wavelength. Three distinctive peaks are observed, the G peak (1587.66 cm−1 ), 2D peak (2676.52 cm−1 ) and the D peak (1344.41 cm−1 ). The intensity ratio of 2D peak and G peak is about 1.42, indicating that the prepared graphene is double-layered. However, a D peak with 3% of magnitude of the 2D peak is observed, suggesting lower defects of the BLG film. The weak D peak indicates that the graphene has high crystal quality [36,37]. UV–vis-IR absorption spectrum of the BLG film was then measured, as shown in Fig. 1d. Notably, the BLG film shows a wideband absorption, and the absorption at 1.55 ␮m is larger than 1%. The electrical transfer characteristics of the BLG film is shown in Fig. 1e. The IDS -VG curve shows an ambipolar transfer property with the Dirac point at 38.5 V under ambient condition, which can be attributed to the absorption of water or defects [27]. Graphene thereby tends to be slightly p-doped and the Fermi level is slightly lower than the Dirac point. The graphene carrier mobil-

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Fig. 1. (a) Schematic illustration of the Gr/Si heterojunction. (b) Photograph of a Gr/Si heterojunction. (c) Raman spectra of the BLG with a 532 nm laser excitation. (d) UV-vis absorption spectrum of the BLG on glass. (e) The IDS -VG curve under VDS = 0.1 V. (f) Energy band diagram of the Gr/Si heterojunction at equilibrium.

ity can be further estimated to be about 1800 cm2 V−1 s−1 based on IDS -VG curve. Therefore, Fig. 1f shows the energy band diagram of the Gr/Si heterojunction at equilibrium. The photoresponse behavior of the Gr/Si heterojunction is further studied under 1.55 ␮m light illumination. Fig. 2a shows the current-voltage (I–V) curves of the device under dark and 1.55 ␮m light illumination, respectively. It can be clearly seen that the heterojunction shows pronounced rectification behavior and remarkable photoresponse. The rectification ratio is then estimated to be 240 at ±2 V. The ideality factor (n) of the device is determined to be 1.6 based on the following equation (Fig. 2b), 2

 q  

I = SAT exp −

nkT

 qV 

exp −

kB T



−1

Furthermore, the device performance parameters can be estimated based on this measurement condition. The linear dynamic range (LDR), responsivity (R) and specific detectivity (D* ), the important figures of merit for a photodetector, can be expressed as follows: LDR = 20 log R=

Ip P



D∗ = (1)

˜ cm2 ), A is the RichardWhere S is the effective junction area (0.196 son constant (120 A cm−2 k−2 ), q is the basic electric charge,  is the Schottky barrier height (SBH)of the Gr/Si heterojunction, kB is the Boltzmann constant, n is the ideal factor, T is the absolute temperature (300 K), and V is applied voltage. The SBH can be then calculated to be  ≈ 0.79 eV, which is large enough to drive the charge quickly. The above results unambiguously demonstrate the excellent rectifying behavior of the Gr/Si heterojunction. To evaluate the optoelectronic performance of Gr/Si heterojunction, the 1.55 ␮m light was used to illuminate the center of the device. Fig. 2c shows the time response of the heterojunction at zero bias and −1 V, respectively. The device can be effectively switched on and off under pulsed light illumination and has good repeatability and stability, yielding a high current ON/OFF ratios (ION /IOFF ) of 1.2 × 104 and 1.1 × 102 at zero and −1 V bias, respectively. The high ˜ -11 A) under ION /IOFF can be attributed to the low dark current (10 the zero bias. Meanwhile, the fast photoresponse can be sure that there is no obvious photo-thermal effect caused by Si substrate. On the other hand, the photocurrent of the pure Si photodetectors ˜ under 1.55 ␮m light illumination at zero bias, is in the level of nA indicating the photocurrent of the device was mainly attributed to graphene absorption. Therefore, the photoresponse characteristics prove that the Gr/Si heterojunction can operate in 1.55 ␮m light detection as a self-powered photodetector due to the photovoltaic behavior, indicating a promising potential in satisfying the requirement of Si-based photodetectors in 1.55 ␮m light detection.

I  p

Id

(2) (3)

S R 2qId

(4)

Where Ip is photocurrent, Id is dark current, and P is light power. Based on above Eq. (2), the LDR is estimated to be as high as 80 dB in self-powered operating mode, indicating that the device can work under a wide range of light power. The value of 40 dB was also achieved at bias of −1 V. These results are better than most Graphene-based photodetectors, and even comparable with Gr/Si Schottky diodes operating in visible light [15,16,25,31]. The R and D* are then calculated to be 1.3 mAW−1 and 3.11 × 1011 Jones based on Eq. (3) and (4), respectively. The responsivity and detectivity are higher than those of the previous Gr-based FET photodetector [20]. These results suggest that the Gr/Si heterojunction shows good signal to noise ratio (SNR) and relatively high photo-electric conversion efficiency to 1.55 ␮m light. Moreover, Fig. 2d plots current density-voltage (J–V) curves of the device under the dark and different 1.55 ␮m light intensity, indicating the strong dependence of the photocurrent with the light power. The fitting curve of photocurrent on incident light intensity (Fig. 2e) shows excellent linear dependence according to a power law of Ip = aP 0.95 at zero bias, where a present proportionality constant. The nonunity exponent of the law is attributed to the complex process of photocarriers generation and recombination [29]. The applied bias voltage dependent R curves were plotted in Fig. 2f, suggesting that the R can be effectively improved and reach to 39.5 mAW−1 at 2 V bias voltage under 0.14 mWcm-2 light illumination. Response speed, another key figure of merit for photodetectors, is critical to high-speed optical communication. A higher photoresponsivity is usually achieved in graphene-based photoconductors

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Fig. 2. (a) I–V curves of the Gr/Si heterojunction under dark and 1.55 ␮m light illumination (light intensity fixed at 0.73 mWcm−2 ), respectively. Inset shows the I–V curves in semi-logarithmic coordinates. (b) The experimental data and the fitting curve of the dark current versus voltage (c) Time-dependent photocurrent response of the device under 1.55 ␮m light illumination at bias of 0 and −1 V, respectively. (d) J–V characteristics of the device under various light intensity. (e) The dependence of photocurrent on light intensity. (f) Bias voltage dependent responsivity curves under different light intensity illumination.

Fig. 3. (a) Normalized response versus frequency (f) curve of the device. (b) Normalized time response characteristics of the device under pulsed light frequency of 100 Hz, 10 kHz and 50 kHz, respectively. (c) and (d) Magnified plot of one response cycle for 10 kHz and (d) for 50 kHz.

or phototransistors due to the high mobility of graphene. However, the response speed is limited to the level of seconds or milliseconds due to persistent photoconductivity (as shown in Table 1). In this study, a varied pulsed frequency (f) of the light was used to explore the response speed of the Gr/Si heterojunction. The device was fixed at zero bias and the photovoltage signals were recorded by an oscilloscope under pulsed light illumination. Fig. 3a shows the normalized response, from which the 3-dB bandwidth is esti-

mated to be 2.0 kHz, which indicates that the Gr/Si heterojunction can follow high pulsed frequency of the light. The detailed temporal response of the device under different pulsed light can be confirmed the high frequency response, as shown in Fig. 3b. It can be clearly seen that the device can follow better the pulse light of 100 Hz without clear signal degeneration. Interestingly, a distinguished ON/OFF states can be observed from the temporal response curves not only under 10 kHz but 50 kHz, demonstrating that the device has the

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Fig. 4. (a) Time response of the Gr/Si heterojunction under 1.55 ␮m light illumination at bias of (a) −2 V and (b) +2 V. Energy band diagram of the device under (c) reverse and (d) forward bias.

capability to follow a high pulse frequency light with good stability and reproducibility. Furthermore, the rise time (␶r )/fall time (␶f ) are determined to be 12 ␮s/40 ␮s and 5 ␮s/8 ␮s from a single normalized cycle under 10 kHz and 50 kHz (Fig. 3c and d), respectively. The response speed is comparable to Gr/Si visible photodetectors and higher than Gr-based photoconductors and phototransistors, especially in 1.55 ␮m infrared light detection [15,16,25,29,35]. Anyway, the response speed of the Gr/Si heterojunction is limited by slow diffusion time in bulk silicon substrate. The above 1.55 ␮m light response characterizations demonstrate the excellent device performance of Gr/Si heterojunction in terms of relatively high responsivity and high speed. We can also find that the device demonstrated a good 1.55 ␮m light response not only at zero bias and reverse bias but also especially at forward bias (Fig. 2a), which is different from conventional Gr/Si Schottky diode or Si-based p-n photodetectors. As for traditional junction photodetectors, the photoresponse is observed in the range of reverse bias voltage. There is not obvious photoresponse in the range of forward bias voltage due to the different diffusion/drift direction between the carriers and the photocarriers. A possible mechanism is proposed here to elucidate the device performance in details. Fig. 4a and b shows the time response of the device at bias of −2 V and +2 V, respectively, exhibiting a good sensitivity and reproducibility with larger photocurrent and relatively speed at +2 V than those at −2 V. Energy band diagram of the device was used to help us to understand these results, as shown in Fig. 4 c and d. In the case of 1.55 ␮m light response for the device, the photocarriers were mainly generated from top graphene absorption because there is an ultra-low absorption coefficient of Si substrate for 1.55 ␮m light. When a reverse bias was applied at graphene electrode, the built-in field between graphene and Si increased (Fig. 4c), leading the photoelectrons in graphene side easily transferring to Si side. The built-in field is helpful to separate photocarriers generated in graphene at reverse bias, enhancing the response speed. The high ION /IOFF ratio is attributed to be the excellent rectifying behavior of the device such as, as discussed in above section. When the

device was operated at forward bias, the built-in field decreased. The reduced SBH makes most carrier relatively easier to diffuse through the barrier and forms larger dark current. Under 1.55 ␮m light illumination, it is not effective for the photoelectrons generated in graphene to transfer Si side due to Femi level of the graphene going down, as illustrated in Fig. 4d. Therefore, most of photoelectrons were collected by Ag electron on graphene. In this case, the photocarriers and carriers show the same movement direction, leading the photocurrent being observed. The lower response speed is attributed to the prolonger photocarriers lifetime. The photoresponse behavior of the device operated on forward bias is similar with the graphene photoconductors, showing a slow response speed and larger photocurrent [38]. 4. Conclusion In summary, A Gr/Si heterojunction with excellent rectification behavior was constructed and then used as a self-powered photodetector. The 1.55 ␮m light photoresponse characteristics of the device are then systematically studied. The resultant heterojunction demonstrated a high ION /IOFF ratio of 1 × 104 at zero bias, a response speed up to 5.0 ␮s and a larger responsivity approaching 39.6 mAW−1 , which are comparable with previous graphene-based photodetectors. Our work would open a new avenue for the fabrication of Si compatible heterojunction for high-speed and low-cost 1.55 ␮m light photodetectors. Acknowledgements This work was supported by grants from the Nature Science Foundation of Anhui Province (No. J2014AKZR0059), the Fundamental Research Funds for the Central Universities (No. JZ2015HGXJ0182), the Key Project of Natural Science Foundation of the Anhui Higher Education Institutions (No. KJ2018A0341), and the projects of Fuyang Normal University (No. 2017JYXM43, 2018FSKJ13, 2018HXXM29).

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Biographies Chunxiao Wang received her MS Degree in science from Dalian University of Technology of China in 2008. She is currently a lecturer in School of Physics and Electronic engineering, Fuyang Normal University. Her main research interest is semiconductor photodetectors. Yuan Dong is currently pursuing for his MS Degree under the supervision of Prof. Yongqiang Yu in School of Electronic Science and Applied Physics, Micro Electromechanical System Research Center of Engineering and Technology of Anhui Province, Hefei University of Technology, majoring in micro/nano functional materials and devices. Zhijian Lu is currently working forward his MS Degree under the guidance of Prof. Yongqiang Yu in School of Electronic Science & Applied Physics, Micro Electromechanical System Research Center of Engineering and Technology of Anhui Province, Hefei University of Technology, majoring in micro-nano functional materials and devices. Shirong Chen received her MS Degree in detection technology and automation from Institute of Intelligent Machines, Chinese Academy of Sciences in 2006. She is currently a lecturer in School of Electrical Science and Applied Physics, Micro Electromechanical System Research Center of Engineering and Technology of Anhui Province, Hefei University of Technology. Her main research interest is microelectronic technology and microelectronic packaging. Kewei Xu received his MS Degree in micro/nano functional materials and devices in School of Electronic Science and Applied Physics in 2018, Hefei University of Technology. She is currently a faculty in an IC company. Yuanming Ma is currently pursuing for his Ph.D. Degree under the supervision of Prof. Gaobin Xu and Ying Huang in School of Electronic Science and Applied Physics, Micro Electromechanical System Research Center of Engineering and Technology of Anhui Province, Hefei University of Technology, majoring in flexible electronical devices and their applications. Gaobin Xu received his Ph.D. in Key Laboratory of MEMS of Ministry of Education, Southeast University. He is currently a professor in School of Electronic Science and Applied Physics, Hefei University of Technology. He is a chief in Micro Electromechanical System Research Center of Engineering and Technology of Anhui Province. His main research interest is in MEMS sensors’ design, manufacturing and packaging technique. Xiaoyun Zhao received his Ph.D in plasma physics from University of Science and Technology of China in 2017. He is currently an assistant professor in School of Physics and Electronic Engineering, Fuyang Normal University. His main research interest is the numerical simulation of the plasma boundary. Yongqiang Yu received his Ph. D. in material physics and chemistry from Hefei University of Technology in 2013. He is currently an associate Professor in School of Electronic Science and Applied Physics, Micro Electromechanical System Research Center of Engineering and Technology of Anhui Province, Hefei University of Technology. His main research interest is low-dimensional semiconductor and micro/nano Si-based photodetectors.