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A power-free graphene mid-infrared photodetector array

Graphene plasmons can resonantly enhance the incident light absorption and offer a potential for tunable spectral selectivity for mid-infrared (MIR) detection. High-performance tunable graphene plasmonic devices are, however, typically based on electrode-patterned graphene, which requires high power input and are technologically challenging in compact assembly. Here we demonstrate a tunable MIR photodetector array operating at zero input bias voltage. Our devices consist of integrating monolayer graphene with periodically"type-printed"ferroelectric superdomain. The spatial graphene carrier density patterns can be readily modulated by artificially defined ferroelectric superdomain with periodic nanoscale-wide stripe domains, achieving nonuniform pattering of conductivity and subsequently enabling graphene plasmons excitation and confinement for a selective transmission resonance in MIR regime. A high photoresponsivity of ~30 mA W-1 at room temperature is achieved in our device. We also observe that our device array features a tunable detection performance with spectral selectivity from 7.2 to 8.5 {\mu}m by directly reconfiguring the periodicity of ferroelectric superdomain. Our strategy could lead to the development of smart fabrication of on-chip MIR photodetector array for application of tunable spectral systems with low-energy consumption.


4
Conceptual design and fabrication of device Fig. 1a schematically illustrates the fabrication process and the structure of the photodetector designed in this work (see Methods for detail). Here we used a conductive atomic force microscope (AFM) probe (Bruker), with platinum-coating and tip radius ~30 nm, to construct ferroelectric superdomain, that is, the epitaxial ferroelectric BFO thin film was periodically switched to an adjacent upward and downward striped domain array (step 1). Of cause, large-scale switching of ferroelectric superdomain has also been experimentally demonstrated in our previous work using water-printing technique [22]. These above ferroelectric polarization switching combined with the advanced micro-/nano-lithography techniques can help achieve desired ferroelectric polarization patterns using a large-scale type-printing method (ref. [22]). Fig. 1b, c shows the crystal structure of BFO thin film using an annular bright-field (ABF) high-resolution scanning transmission electron microscopy (STEM). We can see iron atoms (yellow spheres) dramatically shift in upward or downward domains in the microscopy, respectively, indicating the direction of ferroelectric polarization had been reversed.
The phase difference of adjacent domains is 180° (Fig. 1d), which is characterized by a piezoelectric force microscope (PFM). The period of polarized strips (P, sum of adjacent upward and downward domain width) was controlled from 10 μm to 200 nm in the same BFO film (Fig. S1). The upward domain width was kept equal with the downward domain in each period, that is, the width of ferroelectric domain stripes (W) ranges from 5 μm to 100 nm.
Then, we transferred a CVD-grown SLG sheet onto the BFO film (step 2) using the wet transfer method [23]. After this, to assess the graphene doping level induced by the electrostatic effect of remanent polarization in ferroelectric domains, we used a non-contact monitoring technique of Raman spectroscopy, where the Raman G-band frequency shifts can be used as a spatially resolved probe for the detection of graphene carrier-density [24]. Fig. 1e maps the Raman G-band frequencies, revealing that the periodic stripes of the lower and higher frequencies directly relate to up-/downpolarized domains, respectively.
To facilitate photocurrent collection and reduce contact resistance, the source/drain electrodes were deposited on the graphene sheet (step 3). In general, the device presents a zero overall photocurrent if source/drain electrodes use the same metal. In this case, we employ an asymmetric metal-doped scheme to break that symmetric built-in electric field profile in the graphene channel, following the earlier report of metal-graphene-metal graphene operating at zero bias between source 5 and drain [2]. In this case, we deposited 20-nm-thick palladium (Pd) and titanium (Ti) layers on the graphene sheet, finally we covered an 80-nm-thick gold (Au) layer to prevent contact electrodes oxidation. A close-up scanning electron microscopy (SEM) image focusing on the active region of the as-fabricated device is given in Fig. 1f (also see an overall top-view optical microscope image for the device in Fig. S2). To exactly recognize the active region for fabrication and measurements, the BFO film was etched to a fixed rectangle region before polarization using the reactive ions etching method. The graphene sheet, highlighted with an orange color, shown in Fig. 1f (corresponding to the original SEM image in Fig. S3), covers only the fixed rectangle area and the excessive graphene was etched using oxygen plasma. The contact electrodes (white color) were deposited on the graphene sheet and the overlapped area is 1 μm × 10 μm.  Corresponding to a period of 1000 nm in Raman mapping, the peak positions of Raman G-band for 6 graphene on upward and downward domains were 1586 and 1597 cm −1 (Fig. 2b), respectively. The periodic Raman frequency shifts from lower to higher range, indicating that the graphene owned a sharp transition from lower to higher carrier concentration at the edges of up-/down-polarized domains [25,26]. For graphene, the formula of F = ' E   (ref. [24]) was used to estimate the doping level through the POG shifts (   ) of graphene, where EF is the Fermi energy and '

Spatially controlling of graphene carrier density
 is the integral constant (as given in Supplementary Section S1). The actual carrier concentration (n) can be derived from the Fermi energy following FF = En (ref. [ More quantitative information about the Raman G-band frequency is plotted in Fig. 2c. The average data were simultaneously collected during the process of Raman mapping measurements. As above-mentioned, the graphene doping level is estimated as ~3×10 12 cm − and ~1.2×10 13 cm − on up and down-polarized domains, respectively. The doping levels in graphene on up-/downpolarized domains are comparable to +121 meV (near the Dirac point) and −448 meV (typical pdoped), respectively, in conventional top-gated graphene transistor [25]. The periodic patterns for graphene carrier modulation offer a novel approach to create multiple, parallel p-i junctions on the graphene sheet without applied gate voltage or tailoring graphene. The schematic principle diagram 7 is shown in Fig. 2d. Iron atoms (yellow spheres) were dramatically deviated in upward or downward domains by appling voltage. This nonvolatile and recoverable feature provides a key function of our device. This reversible feature in ferroelectric domains, easily switched using electric or large-area water printable technologies [22,28], ranges from micro-to nano-scales and thus provides a novel and feasible route to control graphene carrier density with a precise spatial resolution. Combined with the advances in switching ferroelectric domains, this proof of concept for rewritable graphene carrier patterns at nanoscale to create desired periodic arrays, such as micro-/nano-striped periodicities demonstrated here, could potentially be used to confine the intrinsic graphene surface plasmons for enhancing the photodetection in the infrared region.

Tuning of graphene plasmons by ferroelectric superdomain
Then we focus on the optical performance of graphene on various patterned BFO ferroelectric domains. We employ an AFM probe to guide the incident light to specific micro-regions, as schematically illustrated in Fig. 3a. The schematic of this transmission measurement is shown in the inset of Fig. 3b, where T and T0 represent the transmission of BFO film epitaxial growth on STO substrate with and without graphene, respectively. The transmission extinction in this architecture, 1-T/T0, can be expressed by the following equation (1) which is directly related to the dynamical conductivity σ(ω) of graphene; Z0, ω, and ns represent the vacuum impedance, the frequency of the light, and the refractive index substrate, respectively.
The measured extinction spectra of the graphene/BFO hybrid structures are presented in Fig. 3b.
The gray squares, orange diamonds, and wine triangles present the measured 1−T/T0 of graphene on pristine BFO film (unpolarized), single ferroelectric domain with upward and downward polarization, respectively. The solid lines correspond to the fitted results following equation (1), and the dynamical conductivity of graphene is fitted using the Drude model [27]. The extinction spectra (1−T/T0) for graphene on periodically poled BFO with domain period (P) ranging from 10 μm to 200 nm are shown in Fig. 3c, d. In these cases, the length (L) and width (W both upward and downward) of the striped ferroelectric domain is 10 μm and P/2, respectively. We observe three prominent features in these periodically doping graphene stripes. Firstly, the extinction intensity increases significantly when P is below 1 μm. Secondly, it occurs a remarkably selective resonant peak with P from 1 μm to 200 nm. Thirdly, the resonant peaks shift from low frequency (1184 cm −1 ) to high frequency (1392 cm −1 ) with decreasing domain period. According to the Raman 9 G-band frequencies (Fig. 2b), we fit the spectra with solid lines employing the graphene at the same doping level in each patterned carrier, that is 3 × 10 12 cm − and 1.2 × 10 13 cm − on upward and downward domains, respectively. The detail fitting procedures are presented in Supplementary Section S2, according to equation (1).
To realize the enhanced transmission extinction and the frequency selectivity beyond broadband wavelengths response, we introduce a plasmonic resonance model in a graphene/ferroelectric hybrid structure without lithographically patterning graphene onto BFO film arranged in a periodically polarized domains array, as shown in Fig. 3a. The spatial carriers in graphene driven by ferroelectric polarization and modulated by applying voltage have been reported [26,29]. However, few investigations focus on directly tuning the graphene plasmons using rewritable ferroelectric domains at subwavelength scales without gating voltage. In this work, we preliminarily investigate the coupling effect of graphene plasmons and incident wavelengths using the finite element method.

Tunable mid-infrared photodetector array
To measure the photocurrent of our device, the incident signals generated by an automated wavelength tuning laser resource were added on the SLG sheet at 90° (in vertical direction) with a zero-bias voltage on source/drain (VSD). Due to rewritable merits for controlling graphene carrier patterns, it gives a feasible method to modulate the photodetection by varying the ferroelectric domain period. The fabricated devices are presented in Fig. 4a and Supplementary Fig. S2. We first study the photocurrent information in the graphene/ferroelectric structures, as mapping in Fig. 4b. All these photocurrents are obtained at the peak values by 12 points under each wavelength illumination. The highest photocurrents in each unit are therefore achieved near the resonant wavelengths when the domain periods are below 1 μm. In contrast, when we increase the size of the periodic domain from 1 μm to 5 μm or 10 μm, the selectivity of photoresponse in our device is yet shown in Fig. 4b, ranging from 8.8 to 6.6 μm due to the surface plasmons mismatched to the graphene carrier-density array.
Thus, it is concluded that the contribution for enhancing photocurrent can be ignored when the domain period exceeds 5 μm (P= 5, 10 μm), and the behavior is more likely as the single downward polarized ferroelectric domain doped graphene.
We further investigate the photoresponsivity, Rph = Iph/Pin, where Iph and Pin are the photocurrent and incident laser power, respectively [10]. The plots in Fig. 4c show the peak photoresponsivity  We now turn to the performance of the typical graphene MIR photodetectors, and the key merits are listed in Table 1. Compared with the earlier reported metal plasmon-enhanced graphene MIR 13 photodetectors, e.g. metal-graphene-graphene [2], the intrinsic plasmon in patterned graphene offers an incomparable advantage in the mid-infrared range: its surface plasmons could be used to resonantly enhance absorption [5] for tunable photodetection controlled with a grating effect, providing an appealing spectral selectivity with ultra-high tunability [10]. For our photodetector demonstrated here, it harvests an ultra-high enhanced photoresponsivity up to ~30 mA W −1 under 8.5 μm illumination. The obtained photoresponsivity is not the highest to record in graphene photodetectors, but is comparable to that of conventional graphene plasmonic devices for MIR detection based on nano-patterned photograting structures. This ultra-high responsivity could attribute to the effective plasmon excitations and perfect crystallinity in graphene sheet, and the latter can effectively reduce the light scattering compared with the nanopatterned graphene. Moreover, with the conveniently writing of graphene carrier-density patterns by switching ferroelectric domains at nanoscale, avoiding the complex nanofabrication in conventional graphene plasmonic devices, the fabricated device owns a remarkably selective response over broadband wavelengths in the midinfrared region.  (Fig. 1a, step 1). For the convenience of guiding incident light to the specific areas and fixing the active illuminating regions, the BFO film was patterned to an array with a fixed area in each unit using a reactive ion etching (RIE) method before polarization. The etched depth was controlled at ~10 nm by ion reactive time.
The single-layer graphene was transferred on BFO film using an improved wet method (Fig. 1a, step 2). Briefly, the polymethyl methacrylate (PMMA, Aladdin) solution (20 mg mL −1 ) was spin- 15 coated on CVD-grown monolayer graphene/copper foils (SixCarbon Technology Shenzhen) at 3000 rpm for 30 s and dried at 120 ℃ for 90 s in air. Ammonium persulfate (0.1 M, Aladdin) was used to etch copper substrate, and the PMMA/graphene film was washed by deionized water several times to remove the etchant residue. Then the pre-polarized BFO film was placed in water with a tilting angle underneath the PMMA/graphene film to support it. After drying in air, the PMMA was removed with an acetone bath at 50 ℃ and washed with ethanol. The monolayer graphene was also etched to the same area as the marked BFO using oxygen plasmon. Subsequently, the source and drain electrodes of Au/Ti and Au/Ni were deposited on the border of graphene to completely fabricate the device (Fig.   1a, step 3). The thicknesses of Au layer and both Ti and Pd layers are 80 and 20 nm, respectively.