ISSN 1998-0124 CN 11-5974/O4

2021, 14(6): 1967–1972 https://doi.org/10.1007/s12274-020-2866-x

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Efficient photovoltaic effect in graphene/h-BN/silicon heterostructure self-powered photodetector Ui Yeon Won1, Boo Heung Lee1, Young Rae Kim1,2, Won Tae Kang1,2, Ilmin Lee1, Ji Eun Kim1, Young Hee Lee2, and Woo Jong Yu1 ()

1 Department of Electrical and Computer Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea 2 Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Suwon 16419, Republic of Korea © Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020 Received: 11 February 2020 / Revised: 20 April 2020 / Accepted: 9 May 2020

ABSTRACT Graphene (Gr)/Si-based optoelectronic devices have attracted a lot of academic attention due to the simpler fabrication processes, low costs, and higher performance of their two-dimensional (2D)/three-dimensional (3D) hybrid interfaces in Schottky junction that promotes electron–hole separation. However, due to the built-in potential of Gr/Si as a photodetector, the Iph /Idark ratio is often hindered near zero-bias at relatively low illumination intensity. This is a major drawback in self-powered photodetectors. In this study, we have demonstrated a self-powered van der Waals heterostructure photodetector in the visible range using a Gr/hexagonal boron nitride (h-BN)/Si structure and clarified that the thin h-BN insertion can engineer asymmetric carrier transport and avoid interlayer coupling at the interface. The dark current was able to be suppressed by inserting an h-BN insulator layer, while maintaining the photocurrent with minimal decrease at near zero-bias. As a result, the normalized photocurrent-to-dark ratio (NPDR) is improved more than 104 times. Also, both Iph/Idark ratio and detectivity, increase by more than 104 times at −0.03 V drain voltage. The proposed Gr/h-BN/Si heterostructure is able to contribute to the introduction of next-generation photodetectors and photovoltaic devices based on graphene or silicon.

KEYWORDS graphene, hexagonal boron nitride, van der Waals heterostructure, self-powered

1 Introduction Since the discovery of graphene in 2004, two-dimensional (2D) layered materials such as graphene, hexagonal boron nitride (h-BN) and their heterostructures have attracted tremendous academic attention due to their unique properties and its atomically thin nature [1–5]. In particular, graphene is a promising as a transparent electrode material in optoelectronic applications due to its high electrical conductivity and optical transmittance. h-BN is a wide-bandgap insulator with a small lattice mismatch of 1.5% to graphene layer, making it a perfect candidate as the substrate for graphene interface [6]. As graphene forms a Schottky barrier junction in semiconductors, numerous researches on photodetector and solar cell based on graphene and bulk n-type Si heterostructure have been reported [7–14]. Today, most photodetectors use an external voltage source to promote the separation of generated electron–hole pairs. This method of using an external voltage source, however, requires bulkier packaging and more complex circuit design that results in a larger device. Consequently, a self-powered photodetector that can operate without external power source has drawn the attention of the research community [15–17]. Since self-powered photodetectors typically rely on the photovoltaic effect in a semiconductor junction, the detection of photons at zero-bias determines the performance of the device. The detectivity of photodetectors is one of the most important parameters in photodetector devices. It corresponds to

the ability of a photodetector to detect weak signals especially in dark environment. Assuming the dark current is dominated by the shot noise for estimation, the photo detectivity (D*) can be characterized by following equation [18]

( )*

dark

Jones2R AD

qI= (1)

where R is the photo responsivity, A is the active area, q is the unit electron charge, and Idark. is the dark current, respectively. To enhance device performance, dark current should be minimized while maintaining high responsivity. It is often observed however, that Iph/Idark ratio (Iph is the photocurrent) is hindered near zero-bias at relatively low illumination intensity due to low photovoltage (< ~ 0.3 V) in graphene (Gr)/Si photodetector [19–22].

Here, we present a self-powered photodetector with vertically stacked Gr/h-BN/Si van der Waals heterostructure to enhance sensitivity of photodetector by suppressing dark current. The dark current was able to be suppressed by inserting an h-BN insulator layer, while maintaining the photocurrent with minimal decrease at near zero-bias. We proposed the mechanisms of carrier flowing in an illuminated environment for reverse and forward current regions.

2 Results and discussion An illustration of the Gr/h-BN/Si van der Waals photodetector

Address correspondence to micco21@skku.edu

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(the fabrication steps is shown in Fig. S1 in the Electronic Supplementary Material (ESM) and its cross-sectional image of our device can be found in Fig. 1(a) and Fig. S2(a) in the ESM, respectively. In this proposed method, a thin h-BN layer is sandwiched between the graphene and Si layer (Fig. S2(c) in the ESM). The thickness of h-BN is measured to be about 5 nm and was characterized by atomic force microscopy (AFM) as shown in Fig. S2(d) in the ESM. Figure 1(b) shows the images of fabricated Gr/Si (left side) and Gr/h-BN/Si (right side), respectively (see the Methods section for details). G, D peak of monolayer graphene and E2g peak of h-BN were confirmed with Raman spectroscopy (Figs. S2(e) and S2(f) in the ESM). To study the impact of h-BN layer to the Gr/Si interface, J–V characteristics of both Gr/Si and Gr/h-BN/Si devices were measured from −0.5 to 0.5 V range in logarithmic scale (Fig. 1(c)). This experiment was performed in a high vacuum environment (~ 10−6 Torr) at room temperature with the source measure unit (SMU) limitation current level of less than 10−13 A. The inset presents the linear scale of the J–V curve for each device. Both Gr/Si (black dots) and Gr/h-BN/Si (red dots) exhibited the typical rectifying Schottky diode behavior with the formation of a Schottky barrier at the interface [23], whereby graphene and silicon acted as anode and cathode, respectively. When threshold voltage is increased from 0.29 to 0.72 V with the insertion of h-BN however, significant reduction of dark current is observed. The dark current of a Schottky diode can be expressed as [24],

n 0 [exp( / ) 1]J J qV nkT= - (2)

( )effn bn2tn n3

4π ( ) exp exp exp 1m q q qVJ kT x δkT nkTh

æ öé æ ö ù÷ ÷ç ç= - - - -ê ú÷ ÷ç ç÷ ÷ç çè ø è øê úë û

(3) where meffn is the effective mass of electron in the silicon, xn is the average barrier height, δ .is the thickness of insulating layer, ϕbn is the Schottky barrier height, n is the ideality factor and V is the applied voltage. Equation (3) indicates that the diode current can be minimized by the tunneling probability term

nexp( )x δ- which matches our experimental measurements. Ideality factor, n was also derived from the J–V curve. For voltage greater than a few kBT (e.g., > 0.1 V), the “−1” term in Eq. (2) above can be ignored [25]. By applying logarithm to both sides, Eq. (2) can be expressed as

( ) ( )n O Bln ln ( / )J J q nk T V= + (4)

The value of q/nkBT can be derived from the slope of the natural logarithm of the diode current versus the applied VDS. The extracted ideality factor, n. from the slope as shown in Fig. S3 in the ESM, saw an improvement from 1.98 to 1.58 when a thin h-BN layer was inserted. This improvement is attributed to the suppression of strong interlayer coupling between graphene

and silicon interface [25]. As the h-BN layer has small lattice mismatch with monolayer graphene and no charge traps due to its smooth surface [9], this reduces the number of interface trap states, thus making the device operate closer to an ideal one.

To further explore the role of the additional h-BN layer, J–V characteristics under light illumination of Gr/Si and 5 nm of Gr/h-BN/Si were measured. To measure photocurrent, 725 nm wavelength and 400 μW power of the laser was used. It should be noted that the typical absorption coefficient (α) of Si in visible light ranges between 8.50 × 102–2.55 × 104 (cm−1) which corresponds to an absorption depth (1/α) of roughly 400 nm–11 μm [26]. The electron–hole pairs generated by graphene were also neglected because the electron–hole pairs in the graphene have a much shorter lifetime (ps) and high transparency due to their atomically thin nature. Furthermore, the high transparency of up to 97% of monolayer of Gr/h-BN stack was demonstrated in Ref. [27] (see also Fig. S4 in the ESM). As a result, most of the incident light will be absorbed by the bottom silicon. J–V characteristics under light illumination also suggest that as h-BN layer is inserted (Fig. 2(b)), the photocurrent is decreased more than two orders along with the dark current when compared to the control sample (Fig. 2(a)). Meanwhile, with 20 nm of h-BN (Fig. S5(f) in the ESM), both dark current and photocurrent were highly suppressed to the measurement limit (less than ~ 10−13A) resulting from the lower tunneling probability through the thick h-BN layer. However, it should be noted that the photocurrent of the Gr/Si device is depressed near-zero VDS (VDS stands applied voltage through drain and source electrode) whereas the photovoltaic effect is strengthened in Gr/h-BN/Si device whereby the measured open circuit VDS was increased from 0.065 to 0.362 V. This is often observed when an insulator is placed between the Schottky junction interface [16, 28, 29].

The schematic band diagrams of Gr/Si (Figs. 2(c) and 2(d)) and Gr/h-BN/Si (Figs. 2(e) and 2(f)) photodetectors at equilibrium state (or low forward bias) with and without illumination are shown in Figs. 2(c)–2(f) (see also Fig. S5 in the ESM for revers bias condition). In the dark condition for Gr/Si, Schottky junction with corresponding band bending forms at the interface. Major charrier, electron transport dominates electrical characteristics resulting in diode behavior. The hole carrier concentration in the valence band of Si is significantly increased by light illumination (Fig. 2(d)) and photoinduced electron–hole pairs are separated by built-in band bending from the Si depletion layer. However, as there is a small built-in potential at the junction, the energy difference between graphene (~ 4.6 eV) and silicon (~ 4.32 eV) (the calculation of Si fermi-level in Section S1 in the ESM) fermi-level cannot effectively screen the interlayer coupling of the photoinduced electron–hole pairs at the graphene/Si interface. Consequently, the photoinduced electrons in the conduction band of Si will still diffuse to the graphene side and recombine

Figure 1 Device structure and electrical behaviour. (a) Illustration of Gr/h-BN/Si photodetector. (b) Optical Microscope image of Gr/Si photodetector and Gr/h-BN/Si photodetector. Scale bar 20 µm. (c) Comparison of Jdark–VDS characteristics of Gr/Si (black dots) and Gr/h-BN/Si (red dots) in dark illumination condition. Inset shows linear comparison of Jdark–VDS for each device.

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with the hole, resulting in the suppression of the photocurrent near zero-bias. This means that the graphene acts as the recombination center and the generated holes drifted to graphene side cannot contribute to increasing photocurrents.

As additional thin h-BN layer is inserted, due to negative electron affinity of h-BN [29–32], more than 4.05 eV of high electron tunneling barrier is formed at conduction band while less than 0.63 eV of small hole tunneling barrier is generated. These tunneling barrier restrains the diffusion of photoinduced electrons to the graphene side while photoinduced holes can easily tunnel through the small barrier with the help of internal electric field under low forward bias near zero-bias (Fig. 2(e))

[28, 33, 34]. As a result, most of the photoinduced holes can contribute to the photocurrent and under reverse bias (Fig. 2(f)).

I–V characteristics of a 5 nm thick h-BN device was also measured under illumination intensities of 120 μW–9 mW with a red laser (725 nm) (Fig. 3(a)). The current was further increased by several orders of magnitude as the device was exposed to higher illumination due to enhanced electron–hole pair generation in Si. We calculated the Iph/Idark ratio under different illumination intensities (Fig. 3(b)). The Iph/Idark ratio shows sharp increases near zero-bias due to the suppression of dark current and high photocurrent induced by enhanced

 Figure 2 Optical properties of Gr/Si and Gr/h-BN/Si photodetectors. The log plots of the JDS–VDS characteristics of Gr/Si (a) and Gr/h-BN/Si (b) with a 5 nm insulator in the dark (black dots) and under light illumination (red dots). The insets show the corresponding linear plots. Illumination was with400 μW light with a wavelength of 725 nm. Schematic energy band diagram of without/with light illumination under equilibrium state (or small forward bias) of (c) and (d) Gr/Si photodetector and (e) and (f) Gr/h-BN/Si photodetector. Blue dots in the Si conduction band and red dots in the Si valence bandare electron and hole carriers, respectively. Empty dots represent the movement of each carriers. The size of the arrows corresponds to the amount of transported carrier.

 Figure 3 Optoelectronic properties of Gr/h-BN/Si photodetector. (a) I−V characteristics of the device in dark and various illumination intensities of725 nm laser. Thickness of h-BN layer in this device is 5 nm. (b) Iph/Idark ratio with different illumination intensities extracted from (a). (c) Photovoltage(at IDS = 0 A) and photocurrent (at VDS = 0 V) with respect to laser power. Red (blue) dots indicate photovoltage (photocurrent). Color plot of Iph/Idark ratio (d)and detectivity (e) at various VDS from −1 to 1 V and illumination photon energies from 1.53−1.94 eV with 600 μW of laser power. (f) The comparison of detectivity between Gr/h-BN/Si and Gr/Si photodetector at various laser power at fixed −0.03 V and 725 nm.

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photovoltaic effect. As a result, the maximum Iph/Idark ratio of 107 is achieved with 9 mW laser illumination. Figure 3(c) shows that the photovoltage of the Gr/h-BN/Si photodetector increases from 0.33 to 0.45 V when the light intensity changes from 180 μW to 5.7 mW. Figure 3(c) also shows the dependence of photocurrent on light intensity at zero-bias voltage. The curve can be fitted with a power law, IP ~ Pθ, where θ is the response of the photocurrent to light intensity. The curve fit gives an approximate straight line with θ = 1.5, implying that the photocurrent is linearly proportional to the incident optical power. Next, we explored the spectral response of the device. The spectral response of Iph/Idark ratio and photodetectivity are explored in Figs. 3(d) and 3(e) with a fixed laser power of 600 μW. As mentioned above, both Iph/Idark ratio and detectivity show higher values near zero-bias across the measured visible light ranges. This is due to the relatively low dark current while the photocurrent still maintains its value near zero-bias. As the dark current is low enough, this compensates the suppressed photocurrent caused by the thin insulator. The spectral response of Gr/h-BN/Si device regarding responsivity is shown in Fig. S6 in the ESM. The peak responsivity of the device is about 110 mA/W at 1.71 eV (725 nm). As predicted, since the optical response of our Gr/h-BN/Si device is highly dependent on Si light absorption, the spectral response Gr/h-BN/Si device follows typical Si spectral response. In shorter wavelength, each photon has a larger energy than the bandgap energy of silicon, and hence the ratio of photons to power is reduced. Any energy above the bandgap energy is transferred to phonon energy. However, in longer wavelength, responsivity is suppressed due to its relatively low absorption coefficient and short diffusion length. This spectral response is comparable to other graphene-based silicon photodetectors [35–40]. In order to confirm the improvement of device characteristics according to the reduction of dark current, we compared the detectivity, responsivity, and normalized photocurrent-to-dark ratio (NPDR) of Gr/h-BN/Si and Gr/Si devices under −0.03 V, 725 nm laser (Fig. 3(f) and Fig. S7 in the ESM) [41]. The photo detectivity was found to be as high as 2.83 × 1010 Jones at 9 mW which is an increase of more than four orders of magnitude. We confirmed that regardless of wavelength, these measurements have a much higher value when h-BN is present between graphene and silicon. We also performed the stability test on Gr/h-BN/Si device under −0.03 V, 725 nm laser (Fig. S8 in the ESM)

Moreover, we investigated the response speed of the Gr/ h-BN/Si photodetector by using an optical chopper to generate the pulsed light (Figs. 4(a) and 4(b)) without the external bias. The variation of photovoltage is monitored with an oscilloscope to estimate the response speed. We performed pulse response with different frequencies ranging from 20–1,000 Hz (Fig. S9 in the ESM) to investigate the device performance as a function of frequency. We normalized our data (Vmax – Vmin)/Vmax .to calculate the rise time (tr). and fall time (tf). which resulted to 0.91 and 1.08 ms, respectively. The response speed is limited by the trapped photocarrier lifetime in the tunnel barrier.

3 Conclusion In conclusion, we have demonstrated a highly sensitive van der Waals heterostructure photodetector using a Gr/h-BN/Si structure. The dark current was able to be suppressed by inserting an h-BN insulator layer, while also maintaining the photocurrent with minimal decrease at near zero-bias. Figure 5 compares the Gr/Si photodetector photovoltage with given power density [18, 20, 22, 42, 43]. Consequently, we were able to improve both Iph/Idark ratio and detectivity, achieving a maximum

 Figure 4 Response time in the Gr/h-BN/Si photodetector. (a) Time-resolved photoresponse of the 5 nm h-BN device under 725 nm laser irradiation without external bias. The laser is mechanically closed and opened using a chopper with 100 Hz frequency. (b) Rise time (left panel) and fall time (right panel) of the device. Rise time (fall time) is measured from 10%−90% (from 90%−10%) of the maximum photovoltage after light is switched on (off).

 Figure 5 Comparison of record photovoltage based on Gr/Si photodetectors. Reported photovoltage of Gr/Si junction based photodetectors are shown as function of incident power density of laser.

value of 107 and 2.83 × 1010 Jones, respectively, representing a more than 104 times improvement both parameters at −0.03 V drain voltage. The proposed Gr/h-BN/Si heterostructure photodetector is able to contribute to the introduction of next- generation photodetectors and photovoltaic devices based on graphene.

4 Experimental

4.1  Material synthesis 

The monolayer island graphene was synthesized on a 100 um thick copper foil using the chemical vapor deposition (CVD) method. A copper foil was annealed in a furnace at atmosphere (ATM) to recrystallize its formation and the surface was then cleaned with H2 gas in the low pressure chemical vapor deposition (LPCVD). Subsequently, the furnace was heated to 1,030 °C with 1,000 sccm Ar gas and 200 sccm H2 gas for more than 3 h. The annealed copper foil was polished with FeCl3 to remove CuO which would interfere with the graphene growth. Then, the sample was placed on the ceramic plate in the middle of a one zone furnace. Gr growth was done in a vacuum

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environment. For one hour, the furnace temperature was increased to 1,030 °C with 1,000 sccm Ar gas and 200 sccm H2 gas and kept in that condition for 2 h. Next, Ar and H2 gas was lowered to 500 and 60 sccm, respectively and 0.7 sccm of CH4 gas was inserted as the carbon source. This condition was maintained for 90 min to allow for sufficient graphene growth. Finally, the supply of H2 and CH4 gas was turned off and the graphene on Cu foil was retrieved when the temperature of the furnace had dropped to 60 °C.

4.2  Device fabrication 

To fabricate the Gr/h-BN/Si photodetector, Si/SiO2 substrate (SiO2: 65 nm thick, P doping concentration: 5 × 1015 cm−3), which is an n-type silicon, was patterned using e-beam lithography and then etched with buffered oxide etch (BOE) diluted in deionized (DI) water for 2 min. Mechanically exfoliated h-BN and CVD grown monolayer graphene (hexagonal island) were then transferred onto etched Si/SiO2 substrate using the dry transfer method in sequence. This transfer process involved the spin-coating of a double-layer polymer stack of poly (vinyl alcohol) (PVA) (step 1: 1,000 rpm for 5 s; step 2: 4,000 rpm for 35 s) and poly (methyl methacrylate) (PMMA) (step 1: 1,000 rpm for 5 s; step 2: 4,000 rpm for 35 s). Next, the sample was placed on a hot plate (120 °C for 1 min) to increase the adhesion between the substrate and polymer layer. The bottom layer (PVA) was dissolved in DI water so that the PMMA layer above was floated to the surface of water with the target flake. Then, the reversed film was carefully placed on the designated spot using an optical microscope. After the transfer, the residual PMMA layer was removed with acetone. To avoid the effects of residual silicon oxide, exposed Si was soaked in BOE for 2 s prior to transfer. Gr/Si photodetector was fabricated using the same procedure without the transfer of the h-BN layer as shown in Fig. 1(b). Monolayer graphene was confirmed with Raman spectroscopy (Fig. S2(e) in the ESM) where G, 2D peak represents standard monolayer graphene characteristics, including negligible D peak, symmetric narrow 2D peak (FWHM of 26.84 cm−1 and large I2D/IG ratio (> 2) exhibit high crystal quality CVD growth in Cu foil).

4.3  Characterizations and measurements 

An atomic force microscope (AFM; SPA-400, SEIKO) was used to record the morphological images using the tapping mode. Raman and photoluminescence were measured using the Witec system (532 nm wavelength). Photocurrent measurements were conducted with a probe station and source/measure units (Keithley 4200 and Agilent B2900A) combined with 628, 725 and 811 nm lasers (RGB photonics Lambda Beam). An oscilloscope was used (Keysight infiniiVision 1000 X-series) for pulsed response while an UV/VIS Absorption spectrometer (V-670) was used to measure transmittance.

Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2018R1A2B2008069), R&D program of MOTIE/KEIT (No. 10064078), and Multi-Ministry Collaborative R&D Program through the National Research Foundation of Korea, funded by KNPA, MSIT, MOTIE, ME, and NFA (No. 2017M3D9A1073539). This work was supported under the framework of international cooperation program managed by the National Research Foundation of Korea (No. 2018K2A9A2A06017491). Y. H. L. acknowledges this work was supported from the Institute for Basic Science (No. IBS-R011-D1).

Electronic Supplementary Material: Supplementary material (scheme of the experimental procedure, AFM images, Raman spectra, SEM images, UV–Vis absorption spectra of CVD grown Monolayer graphene, photo responsivity, and frequency measurement data) is available in the online version of this article at https://doi.org/10.1007/s12274-020-2866-x.

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