Position-sensitive detectors (PSDs) based on the lateral photo effect have been widely used in diverse applications including optical engineering, aerospace, and military fields. With increasing demands for long-working-distance, low-energy-consumption, and weak-signal-sensing systems, the poor responsivity of conventional silicon-based PSDs has become a bottleneck limiting their applications. Herein, we propose a high-performance passive PSD based on a graphene–Si heterostructure. The graphene is adapted as a photon-absorbing and charge-separation layer working together with Si as a junction, while the high mobility provides promising ultra-long carrier diffusion length and facilitates a large active area of the device. A PSD with a working area of is demonstrated to present excellent position sensitivity to weak light at the nanowatt level (much better than the limit of microwatts of Si P-I-N PSDs). More importantly, it shows very fast response and low degree of nonlinearity of , and extends the operating wavelength to the near-infrared region (1319 and 1550 nm). This work therefore provides a new strategy for high-performance and broadband PSDs.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The precise optical measurement of position, distance, displacement, angle, and other relevant physical variables are commonly achieved by position-sensitive detector (PSD) using the lateral photo effect [1–9]. Silicon (Si) P-N or P-I-N junctions [2–6] are the most commonly used structures for current PSD. Photoexcited electron–hole (e–h) pairs are separated by a built-in electric field at the junction, and the carriers diffuse in the surface layer and are collected by two terminal electrodes. The current/voltage difference detected by the two electrodes is then used to determine the position of light. The key factors that affect the performance of the PSD are the efficiency of photon absorption and carrier separation as well as the diffusion length of the carriers, which are usually improved by applying an electrical bias to the Si P-I-N junction . Nevertheless, the minimum detection power (, μW) of an Si-based PSD  is still depressing and has become the bottleneck limiting its applications. For instance, in laser guiding systems, the effective range of the seeker directly relies on the minimum detection power of the PSD components. PSDs with various architectures have been reported in past decades, such as Ti/Si amorphous superlattices , metal–Si or metal––Si [8–10], and GaAs/AlGaAs junctions . These devices are appealing for good linearity [8,9] and fast response, but they possess relatively lower photoresponsivity and are not suitable for weak signal detection. Moreover, the operating wavelength of the above-mentioned PSDs are generally in the visible region , while PSDs that can work at infrared (IR) wavelengths are normally based on thermopile detectors  and have the drawbacks of slow response speed and requiring a cooling system.
In this work, we present a broadband and high-performance PSD based on the graphene–Si heterojunction. It should be noted that the graphene–Si heterostructure has been used to fabricate solar cells  or photodetectors [14,15]. However, previous work has not focused on the position-sensitive function of the structure. Here, the graphene–Si junction is used to separate the photoexcited carriers, and then the holes in graphene would diffuse laterally (lateral photo effect) and be collected by the metal electrodes. The high-mobility [16,17] graphene behaves as a photon-absorbing and charge-separation layer working together with Si as a junction . More importantly, it also serves as a carrier extraction and transportation layer to ensure the ultra-long diffusion distance of the carriers and the large working area of the device. The graphene-based PSD is a passive device, has a power detection limit as low as , has nonlinearity of at power of , and has a large working area of . It also extends the operating wavelength of Si-based PSDs to the near-IR region (up to 1550 nm).
2. PRINCIPLES AND CHARACTERISTICS
A. Structure and Principle of the PSD
Figure 1(a) shows the schematic diagram of the graphene–Si hybrid PSDs. It is constructed by placing a large-area chemical vapor deposition (CVD) grown monolayer graphene onto a lightly -doped Si () substrate. Two pairs of Ni (5 nm)/Au (50 nm) electrodes are deposited on graphene for signal collection. The Raman spectrum of graphene is shown in Fig. S1 (in Supplement 1), which suggests the monolayer thickness and high quality of the graphene sample . As shown in Fig. 1(b), the pinning effect  due to the surface states result in the band bending of the Si surface layer (depletion layer) and a built-in electric field with the direction from bulk Si to surface. Under illumination, electron–hole pairs generated in the depletion layer of Si will be separated by the built-in electric field. As a result, the electrons sweep into the bulk Si and the holes accumulate at the surface, which gives rise to a lateral potential gradient between the illuminated and nonilluminated zones [1–10]. The photo-induced holes will diffuse, and the position of light could be determined by the difference in the photovoltage (or numbers of carriers) between the two electrodes. However, the defects, impurities, and the surface states in Si would restrict the diffusion of holes, leading to a very short diffusion length. On the other hand, by integrating graphene with Si, a Schottky junction is formed with a built-in electric field from bulk Si to graphene at the graphene–Si interface. Figure 1(c) shows the schematic structure and energy band diagram [13–15] of the graphene–Si heterojunction. Under illumination, the electron-holes generated in the depletion layer (the width is very high for lightly doped Si) of Si will be separated by the electric field. As a result, the holes enter the graphene and electrons enter the bulk Si . The separated carriers will weaken the built-in electric field across the graphene–Si junction, while the built-in electric field at the nonilluminated area will not change. This will cause a lateral potential gradient (electric field) between the illuminated and nonilluminated areas and the field drive the holes towards the electrodes [Fig. 1(c)], which is the so-called lateral photo effect [1–10]. Due to the high mobility [16,17] of graphene (Fig. S2 in Supplement 1), the diffusion length of the holes in graphene is very long, and the efficiency of signal collection at the electrodes could be greatly improved. In addition, graphene can also absorb light and produce photo-generated carriers, with electrons entering Si and holes remaining in the graphene, which will extend the operating wavelength of the device due to broadband absorption [20,21].
A PSD with operating area of was prepared in this work, and 100-nm aluminum was deposited on the back of Si acting as common-grounded electrode. Figure 1(d) shows the I-V characteristics of the graphene–Si junction working in the photodiode mode . The graphene–Si diode has good current rectification with applied bias in the dark, which suggests the presence of a built-in electric field. The negative short circuit current under illumination means that the current is flowing from Si to graphene, which is consistent with the hole injection into graphene [14,15]. Meanwhile, the number of holes entering graphene increases with the increase of light intensity. The carriers accumulated at the Si surface [Fig. 1(b)] or entering the graphene [Fig. 1(c)] will diffuse laterally, and the numbers of carriers reaching the electrodes and are different, which are determined by the distance between the light spot and electrodes. The photovoltage difference, , can then be used to detect the position of light. Figure 1(e) displays the position dependence of the photovoltage difference between the two electrodes with or without graphene on an Si surface. Here, “0 mm” represents the center of the device, while “ and 4 mm” represent the positions of the two electrodes. The output photovoltage difference on pure Si [inset of Fig. 1(e)] drops rapidly to zero at away from the electrode, suggesting that the diffusion length of carriers at the Si surface is very small. On the other hand, the carriers can diffuse very long in graphene [Fig. 1(c)] due to its high mobility and lack of surface-defect states. In addition, the native oxide (1–2 nm) on the surface of the bulk Si was widely reported; it allows hole tunneling and suppresses interface recombination in Si-based heterojunctions [22–24]. This is another reason that the carriers can diffuse that long in graphene. The photovoltage of or drops by only in the distance of 8 mm, as shown in Fig. S3a (in Supplement 1). When the position of light is close to the center of the device, the voltage difference is close to zero due to the isotropic diffusion of carriers, whereas the difference is great when the light is close to one of the electrodes. Furthermore, the almost linear dependence between the voltage difference and light position ensures that the device is competent in precisely identifying the light position. According to the diffusion length of carriers in graphene (Fig. S3b in Supplement 1), the operating area of the device could be more than .
B. Position-Sensitive Characteristics of the PSD
Figure 2(a) shows the photo-switching characteristics of the device with a laser (532 nm) focused on graphene at different positions under zero bias. To avoid the error caused by the damage of graphene (wrinkles, broken holes produced during the transfer procedure), the ratio between the difference and the sum of the output photovoltages of the two electrodes was employed to display the position-sensitive characteristics, which can effectively improve the linearity of the measurement (Fig. S4 in Supplement 1). Figure 2(b) shows the position dependence of the photovoltage ratios at direction with different laser spot sizes from 5 to 800 μm. The exactly same linear characteristics suggest that the laser spot size does not affect the position sensitivity of our device, which is in good accordance with the characteristic of “independent of the incident light shape” of PSDs. Indeed, the output signal of a PSD is only determined by the gravity center position of light. The similar linear dependence of signals in both directions ( direction and direction) implies that the diffusion of holes in graphene is isotropic, which is promising for imaging or other practical applications. The nonlinearity () is an important parameter of PSDs, which characterizes the position-detection error and is usually expressed as [8,9]6]. In our PSD, nonlinearity of is obtained under incident power of . Figure 2(c) shows the position-sensitive characteristics at different light powers. This demonstrates the characteristic of “independent of the incident light power” of our device, which is another characteristic of PSD. The nonlinearity of the device increases gradually with decreasing light power due to the decrease of photovoltage for weak signals. The spatial resolution (noise-equivalent position accuracy) of the PSDs can also be deduced and is shown in Fig. S5 (in Supplement 1), which shows a resolution of and for and light power, respectively. This is better than the value of a conventional PSD based on an Si P-I-N junction ( and for and light, respectively) or structure . Although the output signal of each electrode ( and ) is related to the light position, the sum of photovoltages () from the two electrodes keeps almost constant (Fig. S6 in Supplement 1). Figure 2(d) displays the sum of photovoltages () with the increase of light power, which shows almost linear dependence. This demonstrates that our PSD could also be used for the detection of incident light power, in addition to the positions. More results on control devices and discussion on the mechanism can be found in Fig. S7–9 in Supplement 1.
C. High Performance of the PSD
In order to explore the capability of our device to detect weak light signals, position-sensitive characteristics of the PSD at different light powers are carried out. Figure 3(a) displays the power dependence of the photovoltage difference. Evidently, the values increase linearly with power from a dozen nW to almost 100 μW. The linear region is very broad compared to the operating power range of Si-based PSDs from several μW to a few hundred μW. The phenomenon of saturation at high power can be interpreted by the balance between the internal built-in electric field and the accumulation of photoexcited carriers. The power detection limit of our PSD is , with its linear position-sensitive characteristic shown in Fig. 3(b). This demonstrates that our PSD can be used for weak signal detection down to the nanowatt (nWs) level, which is very promising in applications. Figure 3(c) shows the transient response of the device by using an acoustic optical modulator with frequency of 10 kHz to switch the light (633 nm, ). The rise () and fall () time (light located at 10 μm away from the electrode ) are and , respectively, where the curves are fitted using a single exponential function. The dependence of rise time as a function of light position is shown in Fig. 3(d). The increases of response time with distance could be understood as the increase of transit time of the holes in graphene with increasing distance between light and the electrode. Despite the increase, the response time of a few μs is still fast enough for various applications. The fast response is attributed to fast separation of photoexcited carriers at graphene–Si junctions and the high mobility of graphene for carrier transportation.
D. Characteristics of the PSD for Infrared Light
Infrared PSDs are especially important for military applications; for example, laser guiding systems. However, the photosensitive material of the conventional Si–based PSD is only Si, which limits the operating wavelength in the range of 300–1100 nm. The introduction of graphene in our PSD will extend the operating wavelength due to the broadband absorption caused by the special zero bandgap structure of graphene [20,21]. Figure 4(a) depicts the photoresponse of the device to a 1319-nm laser. A stable and fast response implies the capability of the device to reach the near-infrared. Figure 4(b) demonstrates the position-sensitive characteristics of the PSD to infrared light under different powers, which are linear and powerindependent. However, the minimum operating power of 1319 nm infrared is , due to the weak light absorption of graphene . The output difference, , of the PSD as a function of power is shown in Fig. 4(c). It shows good linearity and suggests the broad operating power range of our PSD for near-infrared light. The position-sensitive characteristic of the PSD to 1550-nm infrared light (4 mW) was also carried out. The similar ultrafast and stable response and the linear dependence are shown in Fig. 4(d), but with a minimum operating power of megawatt (mW) level. The weak photo response to 1550-nm light might be related to the energy barrier between graphene and Si.
E. Application of the PSD
To prove the capability of position detection of a graphene–Si-based PSD, an device was prepared and encapsulated, as shown in Fig. 5(a). When the laser beam (633 nm, ) moves along the trajectory of a square shape within the operating area [Fig. 5(a)], the real-time position of the light spot can be obtained through the output of the two pairs ( and ) of electrodes. Figure 5(b) exhibits the experimentally extracted trajectory of the light spot, which agrees well with the programmed pattern (white dashed square). The experimental errors of the two-dimensional PSD are larger than that of one-dimensional measurement [Fig. 2(b)], which is probably due to the influence of the electrodes and lack of system calibration. The adoption of pillow-shaped electrodes can effectively reduce the error for achieving high-precision detection. It should be emphasized that our PSD is a passive device, which means there is no power consumption during the measurement, while conventional Si-P-I-N-based PSDs would require a bias voltage. This advantage could be promising for portable and integrated devices.
In conclusion, we present a high-performance passive PSD based on the graphene–Si hybrid structure. The PSD characterizes excellent position sensitivity to weak light at the nWs level. More importantly, it shows very fast response speed, low degree of nonlinearity of , and extends the operating wavelength to the near-IR. The characteristic of our PSD is also independent on the size and power of the light spot, and can be used for detecting the power of incident light besides its position. This work therefore provides a new opportunity for PSDs with ultrahigh sensitivity and broadband response.
Fabrication of graphene–Si-based PSDs: Monolayer graphene film grown on copper foil catalyst surface was transferred onto lightly -doped substrate . Two pairs of Ni (5 nm)/Au (50 nm) electrodes (, and , ) were deposited on graphene for signal collection by using mask and thermal evaporation (TPRE-Z20-IV). In addition, 100-nm Al was deposited on the bottom of the Si substrate acting as common-grounded electrode.
Device characterization: The electrical characteristics were measured using a Keithley 2612 analyzer. The photovoltage were measured using focused laser beams (spot size ) with wavelengths of , , , and . In the spot-size-dependent experiment, various spot sizes were employed by controlling the focus of the objective lens. The position-sensitive characteristics were measured by moving the device with a two-dimensional motorized stage. In the response-time measurement, light () was modulated with an acoustic optical modulator (R21080-1DS) at frequency of 10 kHz. A digital storage oscilloscope (Tektronix TDS 1012, 100 MHz/1GS/s) was used to measure the transient response of the photocurrent. In the demonstration of two-dimensional PSD, the two-dimensional motorized stage and two Keithley 2612 analyzers were controlled by the Labview software simultaneously to obtain the real-time position of the laser. All the measurements were performed in air at room temperature.
National Key Research and Development Program of China (2017YFA0205700); National Natural Science Foundation of China (NSFC) (61774034, 61376104, 61422503).
See Supplement 1 for supporting content.
1. S. Arimoto, H. Yamamoto, H. Ohno, and H. Hasegawa, “Hydrogenated amorphous silicon position sensitive detector,” J. Appl. Phys. 57, 4778–4782 (1985). [CrossRef]
2. G. Lucovsky, “Photoeffects in nonuniformly irradiated p-n junctions,” J. Appl. Phys. 31, 1088–1095 (1960). [CrossRef]
3. E. Fortunato, G. Lavareda, M. Vieira, and R. Martins, “Thin film position sensitive detector based on amorphous silicon p-i–n diode,” Rev. Sci. Instrum. 65, 3784–3786 (1994). [CrossRef]
4. R. Martins and E. Fortunato, “Lateral photoeffect in large area one-dimensional thin-film position-sensitive detectors based in a-Si:H P-I-N devices,” Rev. Sci. Instrum. 66, 2927–2934 (1995). [CrossRef]
5. M. Vieira, “Speed photodetectors based on amorphous and microcrystalline silicon p-i–n devices,” Appl. Phys. Lett. 70, 220–222 (1997). [CrossRef]
6. E. Fortunato, G. Lavareda, R. Martins, F. Soares, and L. Fernandes, “Large-area 1D thin-film position-sensitive detector with high detection resolution,” Sens. Actuators A 51, 135–142 (1996). [CrossRef]
7. B. F. Levine, R. H. Willens, C. G. Bethea, and D. Brasen, “Lateral photoeffect in thin amorphous superlattice films of Si and Ti grown on a Si substrate,” Appl. Phys. Lett. 49, 1537–1539 (1986). [CrossRef]
8. S. Q. Xiao, H. Wang, Z. C. Zhao, Y. Z. Gu, Y. X. Xia, and Z. H. Wang, “The co-film-thickness dependent lateral photoeffect in Co-SiO2-Si metal-oxide- semiconductor structures,” Opt. Express 16, 3798–3806 (2008). [CrossRef]
9. J. Henry and J. Livingstone, “Optimizing the wavelength response in one-dimensional p-Si Schottky barrier optical PSDs,” Phys. Status Solidi A 208, 1718–1725 (2011). [CrossRef]
10. C. Q. Yu, H. Wang, and Y. X. Xia, “Enhanced lateral photovoltaic effect in an improved oxide-metal-semiconductor structure of TiO2/Ti/Si,” Appl. Phys. Lett. 95, 263506 (2009). [CrossRef]
11. P. F. Fonteint, P. Hendrikst, J. H. Woltert, A. Kucernakg, R. Peat, and D. E. Williams, “Differential measurements of the lateral photoeffect in GaAs/AlGaAs heterostructures,” Semicond. Sci. Technol. 4, 837–840 (1989). [CrossRef]
12. C. G. Mattsson, G. Thungstrom, H. Rodjegard, K. Bertilsson, H. E. Nilsson, and H. Martin, “Experimental evaluation of a thermopile detector with SU-8 membrane in a carbon dioxide meter setup,” IEEE Sens. J. 9, 1633–1638 (2009). [CrossRef]
13. X. Li, H. Zhu, K. Wang, A. Cao, J. Wei, C. Li, Y. Jia, Z. Li, X. Li, and D. Wu, “Graphene-on-silicon Schottky junction solar cells,” Adv. Mater. 22, 2743–2748 (2010). [CrossRef]
14. X. Li, M. Zhu, M. Du, Z. Lv, L. Zhang, Y. Li, Y. Yang, T. Yang, X. Li, K. Wang, H. Zhu, and Y. Fang, “High detectivity graphene-silicon heterojunction photodetector,” Small 12, 595–601 (2016). [CrossRef]
15. X. An, F. Liu, Y. J. Jung, and S. Kar, “Tunable graphene-Silicon heterojunctions for ultrasensitive photodetection,” Nano Lett. 13, 909–916 (2013). [CrossRef]
16. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, “Electric field effect in atomically thin carbon films,” Science 306, 666–669 (2004). [CrossRef]
17. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos, and A. A. Firsov, “Two-dimensional gas of massless Dirac fermions in graphene,” Nature 438, 197–200 (2005). [CrossRef]
18. Z. H. Ni, L. A. Ponomarenko, R. R. Nair, R. Yang, S. Anissimova, I. V. Grigorieva, F. Schedin, P. Blake, Z. X. Shen, E. H. Hill, K. S. Novoselov, and A. K. Geim, “On resonant scatterers as a factor limiting carrier mobility in graphene,” Nano Lett. 10, 3868–3872 (2010). [CrossRef]
19. M. Akatsuka and K. Sueoka, “Pinning effect of punched-out dislocations in carbon-, nitrogen- or boron-doped silicon wafers,” Jpn. J. Appl. Phys. 40, 1240–1241 (2001). [CrossRef]
20. R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, and A. K. Geim, “Fine structure constant defines visual transparency of graphene,” Science 320, 1308 (2008). [CrossRef]
21. K. F. Mak, L. Ju, F. Wang, and T. F. Heinz, “Optical spectroscopy of graphene: from the far infrared to the ultraviolet,” Solid State Commun. 152, 1341–1349 (2012). [CrossRef]
22. S. Q. Xiao, H. Wang, Z. C. Zhao, Y. Z. Gu, Y. X. Xia, and Z. H. Wang, “Lateral photovoltaic effect and magnetoresistance observed in Co-SiO2–Si metal-oxide–semiconductor structures,” J. Phys. D 40, 6926–6929 (2007). [CrossRef]
23. Y. Song, X. Li, C. Mackin, X. Zhang, W. Fang, T. Palacios, H. Zhu, and J. Kong, “Role of interfacial oxide in high-efficiency graphene-silicon Schottky barrier solar cells,” Nano Lett. 15, 2104–2110 (2015). [CrossRef]
24. Y. Jia, A. Cao, F. Kang, P. Li, X. Gui, L. Zhang, E. Shi, J. Wei, K. Wang, H. Zhu, and D. Wu, “Strong and reversible modulation of carbon nanotube-silicon heterojunction solar cells by an interfacial oxide layer,” Phys. Chem. Chem. Phys. 14, 8391–8396 (2012). [CrossRef]
25. L. Chi, P. Zhu, H. Wang, X. Huang, and X. Li, “A high sensitivity position-sensitive detector based on Au-SiO2-Si structure,” J. Opt. 13, 015601 (2011). [CrossRef]
26. X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, and E. Tutuc, “Large-area synthesis of high-quality and uniform graphene films on copper foils,” Science 324, 1312–1314 (2009). [CrossRef]