High-performance graphene photodetector by interfacial gating

Graphene based photo-detecting has received great attentions and the performance of such detector is stretching to both ends of high sensitivity and ultra-fast response. However, limited by the current photo-gating mechanism, the price for achieving ultra-high sensitivity is sacrificing the response time. Detecting weak signal within short response time is crucial especially in applications such as optical positioning, remote sensing, and biomedical imaging. In this work, we bridge the gap between ultra-fast response and ultra-high sensitivity by employing a graphene/SiO2/lightly-doped-Si architecture with revolutionary interfacial gating mechanism. Such device is capable to detect<1 nW signal (with responsivity of ~1000 A W-1) and the spectral response extends from visible to near-infrared. More importantly, the photoresponse time of our device has been pushed to ~400 ns. The current device structure does not need complicated fabrication process and is fully compatible with the silicon technology. This work will not only open up a route to graphene-based high performance optoelectronic devices, but also have great potential in ultra-fast weak signal detection.


INTRODUCTION
Graphene-based photodetectors have aroused considerable interest and various types of device configurations and mechanisms have been developed [1][2][3][4][5][6][7][8]. Current prototype devices have shown outstanding performance with individual functionalities aiming for different applications, that is, ultra-fast or ultra-sensitive detection. On the fast-detecting side, benefited from the high mobility and ultrafast carrier dynamics, intrinsic graphene based photodiode has shown photoresponse at a fs timescale [2]. On the ultra-sensitive side, by employing the photo-gating mechanism, hybrid graphene photoconductor has exhibited ultra-high gain up to 10 10 [8]. However, there is a huge gap between the two mechanisms, like two sides of a coin: a fs detection only has a responsivity of ~mA W -1 and a pW detection responses only in milliseconds to seconds timescale, while numerous applications such as optical positioning, remote sensing, biomedical imaging, desire both speed and sensitivity. The gap between the binary performances is limited by the current mechanisms employed. A fast detecting relies on the high carrier mobility of intrinsic graphene and suffers from its gapless nature and low efficiency of electron-hole pair disassociation. While the bottleneck of photo-gating is the slow charge transfer and/or charge trapping process in the time scale of ~ms, or even seconds [3,5,[7][8][9][10][11][12][13][14][15][16][17], which is indeed necessary for the charges in the channel to recirculate between source and drain, to give rise to ultra-high gain.
In this work, by adopting a new concept of interfacial gating effect from lightly-doped silicon(Si)/silicon dioxide (SiO 2 ) interface, we successfully bridge the gap between ultra-fast response and ultra-sensitivity of graphene based photodetector. Such device architecture separates the photoexcited electron-hole pairs by intrinsic self-built electric field at Si/SiO 2 interface, and in turn the accumulated charges at the interface would gate graphene and introduce high gain of photoresponse by taking advantage of the high mobility of graphene.
This charge transfer free strategy with fast accumulation of photoexcited carrier at the interface ensures the fast response of photocurrent at the graphene channel. Moreover, the current device structure does not need any complicated fabrication process and is fully compatible with the silicon technology.

A. Device fabrication
Monolayer graphene samples are mechanically exfoliated from highly oriented pyrolytic graphite, and deposited on lightly p-doped Si substrate that is terminated with 300 nm of SiO 2 .
Source and drain electrodes (5 nm Ni adhesion layer, followed by a 50 nm Au capping layer) are defined using electron beam lithography (FEI, FP2031/12 INSPECT F50) and deposited by thermal evaporation (TPRE-Z20-IV). More than ten devices are fabricated and all of them show very good photoresponse behaviour. In addition, other control devices on 300 nm SiO 2 /heavily-doped Si, and lightly-doped Si covered by SiO 2 with different thicknesses or 30 nm Al 2 O 3 are also fabricated. The Al 2 O 3 layer is deposited by using the atomic layer deposition (Sunaletmr-100).

B. Photoresponse measurement
Electrical and photoresponse characteristics of the devices are measured using a Keithley 2612 analyzer under dark and illuminated conditions. Light is switched on and off by using an optical chopper or acoustic optical modulator (R21080-1DS) at different frequencies. The light source is an Ar + laser with wavelength of 514 nm. A super continuum light source (SuperK Compact ns kHz) is employed to attain the spectral photocurrent response. In all the photocurrent measurements, the laser and super continuum light are focused on the sample with a 50x objective (NA= 0.5) and the spot size of light is ~1 μm, much smaller than the graphene channel length. In the power dependent experiment, optical attenuators are introduced to change the input power. A digital storage oscilloscope (Tektronix TDS 1012, 100 MHz/1GS/s) is used to measure the transient response of photocurrent.

RESULTS AND DISCUSSION
A. Characterization of graphene photodetector Figure 1a shows the schematic diagram and a representative optical microscopy image of our device. A lightly p-doped silicon wafer (1-10 Ω cm) and thermally-grown 300 nm thick SiO 2 layer are employed as the gate electrode and dielectric, respectively. We have compared the Si substrates with different doping concentrations and the above mentioned one provides the best device performance. The mechanically exfoliated monolayer graphene is characterized by optical contrast and Raman spectroscopy (see Supplement 1, Figure S1a) [18]. The G band and 2D band (with a full width at half maximum of 27.7 cm -1 ) locate at 1580.3 and 2675.2 cm -1 , respectively, and the ratio of I 2D /I G is ~2.3. Figure S1b shows a transfer characteristic of the device at an applied bias voltage V D = 10 mV measured in dark, suggests that graphene is slightly p-doped because of interactions with the substrates, and the absorbed water/oxygen molecules in air. The estimated holes mobility (μ) is ~5,000 cm 2 V -1 s -1 . All these features indicate the high quality of monolayer graphene and device.

B. Mechanism of photodetector by interfacial gating effect
The working principle of our graphene photodetector can be understood through the energy band diagram of oxide-silicon interface and its effect on graphene as shown in Figure   1b and 1c. The localized interface states such as positive charge states (qφ 0 ) with energies within the silicon bandgap exist at the oxide-silicon interface, induce a negative depletion layer (-) in silicon near the interface and the formation of built-in electric field (E) [19]. Due to the presence of built-in electric field, the photogenerated electron-hole pairs in lightly p-doped Si will be separated: the holes (red points) in the valence band of Si diffuse toward the bulk Si, while the electrons (blue points) accumulate at the SiO 2 /Si interface ( Figure 1b). This leads to the appearance of a negative voltage at the interface, which can be negligible in heavily doped silicon due to the very short lifetime of the photogenerated carriers [20]. As a result, the additional negative voltage could effectively gate the graphene channel through capacitive coupling, lowers the Fermi level (E f(Gr) ) to its new position (E′ f(Gr) ), as shown in Figure 1c.
Therefore, the increase of hole density and high positive photocurrents in graphene are achieved.

C. Photoresponse of graphene photodetectors
The photoresponse characteristics at V D = 1 V and zero gate voltage (V G = 0 V) are recorded with laser focused on the device at wavelength of 514 nm (spot size ~1 μm). Figure   2a shows This is because the accumulation of photogenerated electrons at SiO 2 /silicon interface will lead to a reversed electric field balance to the equilibrium built-in field. Correspondingly, less photo-induced electron/hole pairs will be separated as the net built-in field becomes weaker under higher illumination power. The responsivity of the device under different light power is calculated and shown in Figure 2c, which is defined as , where I ph and P are the photocurrent and incident light power, respectively. The device shows a remarkable responsivity up to ~1000 A W -1 at an incident light power of ~0.6 nW, which is among the highest values of previously reported monolayer graphene photodetectors [1,2,5,[21][22][23][24].
Based on this value of R, we also estimate external quantum efficiencies 2.42×10 5 % and a specific detectivity 1.1×10 10 Jones (1 Jones = 1 cm Hz 1/2 W -1 ) at V D = 1 V and V G = 0 V, where e is electron charge, h is Planck's constant, is frequency of light, A is the effective area of the device, Δf is the electrical bandwidth, R is the responsivity, and i n is the noise current (the dark current waveform of the device is shown in Supplement 1, Figure S2). Figure 2d shows the spectral photocurrent response of the device at ~0.05 μW light power from visible to near-infrared, which is obtained by a super continuum light source with a tunable filter. It can be seen that the excitation wavelength dependence of photocurrent is almost flat in visible regime and drops abruptly beyond ~1050 nm. This is a predictable outcome of the photoresponse mechanism proposed above, i.e. lightly-doped silicon is indeed the light absorbed medium, with photon response range from ~200-1100 nm. This also implies that by replacing silicon with other semiconductors, the photoresponse of device can be further extended to longer wavelength, e.g. HgCdTe with adjustable bandgap (from 0.7-25 μm) for mid-infrared photodetection [25]. we measure the photocurrents of the device at different gate voltages as plotted in Figure 3c (red circles), which are well consistent with the characteristic sigmoidal curve of photocurrent (blue line). It shows clearly that photoresponse can be reversed in sign and can even be switched off electrically by tuning the gate. Figure 3d shows the dependence of the photocurrent with different bias voltage V D under different light power. As expected, a linear dependence of the photocurrent is observed. The above results imply that the responsivity of our device can be effectively tuned, which is an attractive feature for developing tunable photodetectors for imaging applications, with responsivity adjustable to gate and bias voltages.
According to the photodetection mechanism described in Figure 1, the interfacial accumulated carriers will also diffuse in bulk Si in the lateral direction. To confirm this, we perform a spatial dependence of the photocurrent as a function of light position away from the graphene channel, as shown in Figure S4 (see Supplement 1). We find that the photocurrent still exists, even when the light spot is not on the graphene channel, whereas on the SiO 2 substrate, similar to photodetector based on graphene/Si junction [27].  and fall ( off ) time are calculated to be ~400 and ~760 ns, respectively, based on curve fits of the transients with an exponential function. Such an ultra-fast response speed is superior to other graphene-based photoconductors with photo-gating mechanism [3,[5][6][7][8][10][11][12][13][14][15][16][17]. More interestingly, the response time of our device increase very slowly with the decrease of light power, as shown in Figure 4b. On the other hand, although ultra-high responsivity has been achieved in graphene based hybrid structures and/or heterostructures, a significant increase of response time with the decrease of light power is commonly observed [24]. This behavior was also observed in phototransistors based on organic/inorganic composites before [28], and could be associated with the decay of transfer rate of electrons and/or holes from the light-absorbing materials to the conducting materials, especially in the case of weak light signal. The high speed response of our device is attributed to the fast separation of the electron-hole pairs assisted by the built-in electric field at the lightly doped silicon/SiO 2 interface. Specifically, the holes would be quickly driven into bulk Si before they recombine with the accumulated electrons, wherein there do not exist charge transfer process as common graphene based hybrid photodetectors do [3,[5][6][7][8][9][10][11][12][13][14][15][16][17]. With the aim to further investigate the high speed photodetection of the device, we also perform the time-dependence of photoresponse at a high modulation frequency of 0.5 MHz under different light power, as shown in Figure 4c and 4d. It is demonstrated that our device could resolve weak signals at nWs level under high frequency operation, which is promising for high speed weak signal detections. Experimental results from additional device on 300 nm SiO 2 /lightly-doped Si substrate are also shown in Figure S8 (see Supplement 1). The light wavelength is 514 nm.

CONCLUSION
In summary, by taking advantage of interfacial gating effect from lightly doped silicon/SiO 2 interface, we demonstrate a simple approach to graphene photodetection with high reponsivity and fast response. The proposed graphene photodetector exhibits high responsivity of ~1000 A W -1 for weak signal of <1 nW and a spectral response that extends from visible to near-infrared. More importantly, the photoresponse time of our device has been pushed to ~400 ns and degrades quite slowly with the decrease of light power, which is superior compared to other graphene based photo-gating devices. Moreover, in comparison with the previous graphene-based devices with top gated p-n junctions [29,30], integrated with optical structures (e.g., plasmonic architecture [22], optical cavity [23], and waveguide [31]) and hybrid with light-absorbing materials (e.g., 2D vdW crystals [4,7,10,14,15], QDs [3,8,11], nanowire/tube [6,9]), our device possesses the advantages of simple fabrication process and is fully compatible with the silicon technology. This work therefore not only opens up a route to graphene-based high performance optoelectronic devices, but also provides the potential to access an even wider spectral range by combing graphene with other oxide-semiconductor system.