High Broad‐Band Photoresponsivity of Mechanically Formed InSe–Graphene van der Waals Heterostructures

High broad-band photoresponsivity of mechanically formed InSe–graphene van der Waals heterostructures is achieved by exploiting the broad-band transparency of graphene, the direct bandgap of InSe, and the favorable band line up of InSe with graphene. The photoresponsivity exceeds that for other van der Waals heterostructures and the spectral response extends from the near-infrared to the visible spectrum. The development of van der Waals (vdW) heterostructures fabricated by mechanically stacking 2D crystals has led to the discovery of fundamental physical phenomena and to the realization of 2D functional devices ranging from sensitive photo­transistors to tunnel diodes.1-13 The electronic properties of these devices can be modified not only by careful selection of the materials within the stack, but also by adjusting the built-in strain and relative orientation of the component crystalline layers. Among the vdW crystals, the metal chalcogenide III–VI compound, InSe, represents an exfoliable and stable semiconductor that extends the library of vdW crystals. In its bulk form, InSe has a direct bandgap Eg = 1.26 eV at T = 300 K, which can be increased due to quantum confinement by reducing the number of atomic layers in the crystalline sheet.14-17 Recent reports of bendable photodetectors,18 large-scale image sensors,19 electroluminescence in p–n junctions,20 and field-effect transistors (FETs) with large current on/off ratios (≈108) and high carrier mobilities (μ = 0.1 m2 V−1 s−1) at room temperature21 have demonstrated the potential of InSe for future technologies. This burgeoning research field is still in its infancy and offers exciting opportunities for discoveries and the realization of functional devices that implement InSe in combination with other vdW crystals. Crucial to these future developments is the formation of good interfaces and Ohmic contacts to the optically active InSe layer. Devices currently used are mostly prepared from InSe with metal contacts.14, 20-22 On the other hand, the stability, flexibility, strength, high conductivity, and weak optical absorbance of graphene make this single atomic carbon layer a particularly attractive option for use as a transparent electrical contact. Furthermore, the work function of graphene can be adjusted by the electric field effect,23 an attractive feature for device operation and adjustable band alignment at an interface with a layered compound. Here, we report on van der Waals graphene/InSe heterostructures formed by mechanical contact due to attractive vdW forces at the graphene/InSe interface. We exploit the broad-band transparency of graphene and the favorable band line up of graphene with InSe to create high-performance photo­detectors. We demonstrate vertical and planar graphene–n-InSe–graphene heterostructures with a high photoresponsivity (up to ≈105 A W−1 at λ = 633 nm), not yet achieved in other 2D vdW crystals, and with a spectral response that extends from the near-infrared to the visible spectrum. The highest photoresponsivity is observed in device architectures where the InSe and graphene layers are vertically stacked and an optical window is created by overlapping the two graphene electrodes, thus enabling sensitive photodetection. These heterostructures provide innovative device architectures that enable access to fast electron speeds and high broad-band spectral response. Our InSe crystals were grown using the Bridgman method from a polycrystalline melt of In1.03Se0.97. The γ-polytype crystal structure of InSe was probed by X-ray diffraction (XRD) using a DRON-3 X-ray diffractometer in a monochromatic Cu–Kα radiation of wavelength λ = 1.5418 Å. The primitive unit cell contains three InSe layers each of which has a thickness of 8.320 Å and consists of four covalently bonded monoatomic sheets in the sequence Se-In-In-Se; along the c-axis, the primitive unit cell has a lattice constant of c = 24.961 Å and, within each a–b plane, atoms form hexagons with lattice parameter a = 4.002 Å. In its bulk form, InSe contains native donors due to In-interstitial atoms.24 From Hall effect measurements on bulk InSe at T = 300 K, we obtain an electron density n ≈ 1021 m−3, an electron mobility μ = 0.1 m2 V−1 s−1, and a Fermi energy EF ≈ 0.21 eV below the conduction band minimum. The InSe nanosheets were prepared from the as-grown crystals by mechanical exfoliation using adhesive tape and have thicknesses t that range from 20 to ≈100 nm, over which InSe retains a direct bandgap14-16 and a relatively high electron mobility (10−2–10−1 m2 V−1 s−1).21, 22 The exfoliated InSe flakes were then integrated into planar and vertical device structures incorporating graphene electrodes. We first focus on the planar devices: a graphene layer, grown on a copper substrate by low-pressure chemical vapor deposition (CVD), was transferred onto a SiO2/Si substrate and patterned by electron beam lithography (EBL) into two contacts prior to the transfer of an InSe nanoflake (see the Experimental Section for details of the device fabrication). Figure 1a illustrates the schematic layered structure and an optical image of a graphene–n-InSe–graphene planar device structure with Au-metal contacts on two graphene layers. The n-Si layer serves as a gate electrode and the graphene layers, g1 and g2, serve as source and drain to an InSe channel of length l ≈ 2 μm, width w ≈ 10 μm, and thickness t ≈ 30 nm. As shown in Figure 1b, the current through the InSe flake has a linear dependence on the bias voltage Vs applied between the two graphene electrodes. The I–Vs characteristics are symmetric relative to negative and positive values of Vs and, for each Vs, the current increases with increasing gate voltage Vg. Separate Au-contacts on each graphene layer enable us to assess their conductivity: they exhibit a linear dependence of the current on the bias voltage and a minimum conductance at around a gate voltage Vg ≈ 60 V, corresponding to p-type doping and hole concentration of p ≈ 5 × 1012 cm−2, typical for CVD graphene (inset of Figure 1b).25 The transport characteristics of the device are reproducible and stable; also, a fast response time is observed with a cut-off frequency of f ≈ 104 Hz. Figure 1c shows the characteristic temporal dependence of the current modulated at a frequency of f = 100 Hz, with rise (τr) and decay (τd) times of the current of τd ≈ τr 0, the Fermi level in graphene moves upward toward the Dirac point and more electrons can diffuse into InSe, thus decreasing the effective length l over which the bias Vs is dropped; in contrast, for Vg 0; correspondingly, the dark current decreases as Vg is made more negative, but the photocurrent increases due to an increased number of carriers photogenerated over a larger length l. From the change of the photocurrent (ca. ±20%) over the range of Vg from −60 to +60 V, we estimate that l changes by ca. ±20%. This dependence was not observed in the Au–InSe–Au planar devices where the dark current and photocurrent both decrease or increase with varying the gate voltage. Under an applied bias Vs, electrons and holes that are photoexcited in InSe are swept by the electric field in opposite directions and are extracted at the graphene electrodes to generate a photocurrent ΔI = etαP/hvτl/τt, where P is the incident power, α is the absorption coefficient of InSe at the photon energy hv, e is the electron charge, t is the thickness of the InSe layer, and τl/τt is the ratio of the minority carrier lifetime (τl) and transit time (τt) of electrons in InSe (for the derivation of ΔI, see Section SI, Supporting Information). Thus the photoresponsivity R of our device can be approximately described by R = ΔI/P = etα/hvτl/τt. Furthermore, we can express the external and internal quantum efficiencies as EQE = R(hv/e) = tατl/τt and IQE = τl/τt, respectively. These relations indicate that large values of R, EQE, and IQE can be achieved if the lifetime of the minority carriers (holes) is longer than the transit time of electrons. Figure 4a shows the measured photoresponsivity at Vs = 2 V, Vg = 0 V, and λ = 633 nm: R is strongly dependent on the optical power P and reaches a maximum value of R = 4 × 103 A W−1 at the lowest incident power P = 10 pW, which corresponds to τl/τt ≈ 3 × 105 for α = 106 m−1 at hv = 1.96 eV (λ = 633 nm) and t = 30 nm. From the measured values of R, we estimate a maximum external quantum efficiency EQE = Rhv/e ≈ 5 × 103 and a specific detectivity D* = R(A/2eI)1/2 ≈ 1010 m W−1 s–1/2, where A = 20 μm2 is the area of the device and I = 0.6 × 10−6 A is the dark current at Vs = 2 V and Vg = 0 V. As shown in Figure 2b, the photoresponse depends on the photon energy and we estimate that R and EQE decrease by a factor of ≈30 and 50, respectively, for hv decreasing from ≈2 to ≈1.3 eV. The decrease of R with increasing P is analogous to the P-dependence reported previously for the photoresponsivity of other graphene-based photodetectors.4 According to our model, the photoresponsivity is determined by the ratio τl/τt. Thus the decrease of R with increasing P suggests a decrease of τl and/or an increase of τt. The transit time of photoinduced carriers can be increased by the power due to enhanced scattering.28 Also, an increasing power can induce Auger recombination processes and increases the carrier recombination rate, thus reducing τl. Since this behavior is not observed in devices based on thick bulk InSe flakes, we infer that Auger-like carrier recombination on traps is enhanced in these 2D vdW crystals due to stronger Coulomb interactions. Finally, R decreases linearly with decreasing applied bias voltage Vs, as expected from the increase in the electron transit time τt = l2/μVs: for Vs = 2 V, l = 2 μm, and μ = 0.1 m2 V−1 s−1, we estimate a transit time τt ≈ 2 × 10−11 s and a carrier lifetime τl ≈ 5 × 10−6 s. Our values of R and EQE for the InSe/graphene photo­detector are significantly larger (by a factor of 104) than those measured in devices in which Au-contacts replace the two graphene electrodes on the InSe flake, see Figure 4a. We attribute the lower photoresponsivity in the Au–InSe–Au heterostructure to the presence of a Schottky barrier at the Au/n-type InSe interface (inset of Figure As shown in Figure this form the work function for = eV) is lower than that for (φInSe = thus in a Schottky contact at each interface of the heterostructure and in I–Vs (not A barrier also form due to a interface between InSe and the metal contact, as reported by Our that the graphene electrodes enable an of photogenerated carriers in Since graphene is optically high photoresponsivity can also be achieved in where the InSe and graphene layers are vertically stacked with the graphene layer as a broad-band optical These structures have the that the of the graphene electrodes, determined by the thickness t of the InSe flake, is to a more sensitive through a decrease of the carrier transit compared to the planar for the of the vertical are to the graphene contact layer. we have corresponding to devices architectures of A (Figure and (Figure and Figure Supporting which make use of and graphene grown by (see the Experimental Section and Section Supporting for details of the device fabrication). Figure shows optical of device the and layers an InSe nanoflake of thickness t = 27 nm. The of the optical window on the surface of the InSe is approximately 4 × 4 (see in Figure and corresponds to the region of the InSe flake optical a photocurrent. As shown in Figure the dependence of the dark current, on the applied bias Vs is This which is from that observed for the planar device (Figure from a contact at the InSe/graphene interface by surface defects or material the (see in the Experimental Devices of A and exhibit a stable and reproducible photoresponsivity (Figure with values of R of up to 105 A W−1 at λ = 633 and response to at P as low as over an area of the InSe flake of μm2 for device A (see photocurrent map in the inset of Figure From this value of R, we estimate an external quantum efficiency EQE = Rhv/e ≈ 105 and a specific detectivity D* = R(A/2eI)1/2 ≈ m W−1 s–1/2, where A = μm2 is the area of the device and I = × A is the dark current at Vs = 2 V and Vg = 0 V. Finally, we that a power of the form R = with n ≈ where is a a good to of R for both vertical (see line in Figure and planar the photoresponsivity measured in our planar and vertical devices is significantly larger than that reported for based on graphene and/or vdW 2D crystals at and laser which not values of 103 A W−1 (see by We attribute the enhanced photoresponsivity and detectivity of these devices to the favorable alignment of the bands at the graphene/InSe interface. In particular, our devices are based on InSe flakes that are nm) than other 2D vdW crystals, reported and InSe remains a semiconductor to thicknesses of a In contrast, metal such as have a only in the Also, we that a higher photoresponsivity A has achieved in based on graphene and the of such to be by a optical response due to the of photogenerated from the strongly In contrast, for our 2D the for the photoresponse not on a effect as in the of both electrons and holes at the graphene electrodes is by a low potential barrier at each InSe/graphene interface. These of our heterostructures enable relatively fast transit times for carriers and modulation of the dark and photocurrent at time In our demonstrate that mechanically formed of InSe and graphene have optical and electrical properties with potential for in Our innovative be to other material and device should be possible to band and potential by InSe with other layered semiconductors, and by the or of the component layers, which is to in other van der Waals 2D crystals. developments in which the InSe layer is by bandgap such as or and quantum This of vertical heterostructure enable fast electron are also attractive due to the potential to access an spectral range than that achieved in our current of the InSe flake and graphene were by atomic force microscopy in under The for the a laser (λ = 633 nm), an linear and an optical The laser beam was focused to a ≈ 1 using a and the were measured at low power < 0.1 to lattice the of the photocurrent (Figure 2b), from a was through a m of was modulated with a mechanical f = and focused onto the The photocurrent was measured using a time constant of t = 3 The measurements of the dark current and photocurrent the applied voltage and were made using a the temporal studies of the dark current (Figure we used a and a The response of the dark current to an was in the frequency range f = Hz. The temporal of the photocurrent (Figure was under constant bias voltage = V) and by a mechanically modulated laser with λ = 633 nm, P ≈ and frequency f in the range Hz. The photocurrent was measured using a and a was used as a voltage these the device was in with a 1 We measured the voltage across the which us to voltage with a low of The exfoliated InSe flakes were integrated into 2D planar structures incorporating or graphene contacts fabricated by In these grown on a copper was transferred to a SiO2/Si substrate layer thickness of 300 nm) by first with and on the surface of a to the The was then in a and in and then mechanically on the the by in in and in an at for The graphene was then patterned using electron beam lithography and in an and, of as described transferred to a where the InSe was exfoliated and transferred to the contact were formed to provide contacts to the graphene layers, in the structures shown in Figure of The of the vertical devices is more than for the planar devices and We have used two to as A and both described Devices of A (Figure were formed using exfoliated graphene to form structures to the vertical devices by In this the graphene electrode was formed by mechanically using a and onto a SiO2/Si substrate layer thickness of nm) which in and an A was then using a The InSe flake t = 27 nm) and graphene electrode were also formed by mechanical exfoliation using the but were onto for flakes of each material were and a implement to a in the around each the layer of each of the flakes was then using The flakes, along with their were then onto the surface of a of and was on a and the was to a to a of these were then used to each flake over the of the device the was into contact with the Finally, the was of the onto the substrate at and the was in an to the flake in contact nm) were formed to provide contacts to the two graphene layers. Devices of (Figure and Section Supporting were fabricated using large area CVD graphene grown on deposition of alignment on a SiO2/Si substrate layer thickness of 300 nm), graphene was transferred and as described InSe flakes were by exfoliation and a flake of thickness t was using optical microscopy = and in devices and The graphene layer was patterned and in an to a graphene which a to the graphene under the InSe contact nm) were then at each of the lower contact In the were also formed to provide contacts to the graphene which was in a (see layer formed by a negative using electron beam lithography was then this the of the InSe flake and the of the lower graphene contact close to the flake, to the formation of electrical between and lower graphene A window with an area of approximately 2 × 2 was formed in the layer on the surface of the InSe to the mechanical contact with the graphene layer. Finally, a layer of graphene was transferred as described in this the which was to the in was not but used as the layer in a of electron beam lithography in which the graphene contact was by in an The contact at to the lower to form a graphene/InSe/graphene vertical heterostructure and the contacts formed at an of the (see Figure Supporting Information). The of the characteristics of the devices A and is to the of at the graphene/InSe interface. This from layers used the the negative layer for in the InSe layer is in the transfer of the graphene contact. This work was supported by the and the of of and the As a to our and this by the materials are and be for but are not or from than should be to the The is not for the or of by the than should be to the corresponding for the