A GRAPHENE PHOTODETECTOR

Abstract

A graphene photodetector includes a first graphene absorption layer connected to first and second metal electrodes, the first and second metal electrodes defining a channel on the first graphene layer operating as a plasmonic waveguide, a gate dielectric layer interposed between the first graphene layer and a second graphene layer. The second graphene layer used for electrical gating and includes first and second gate electrodes proximate to the first and the second metal electrodes, respectively. The photodetector also includes a photonic dielectric waveguide with a planarized cladding underneath the gate dielectric layer, the first and second gate electrodes remaining interposed therebetween. The distance between the first and the second metal electrodes, defining the width of the channel cross-section, is between 100 nm and 600 nm, and the distance between the first and second gate electrodes is at least 60% of the distance between the first and second metal electrodes.

Claims

1. A graphene photodetector comprising: a first graphene absorption layer (2) connected to a first metal electrode (3) at a first end (2a) of said first graphene layer (2) and to a second electrode (4) at a second end (2b) of the first graphene layer opposite to the first end (2a), said first and second metal electrodes (3, 4) being referred to as source and drain, respectively, said first and second metal electrodes (3, 4) defining on said first graphene layer (2) a channel (5) operating as a plasmonic waveguide, a gate dielectric layer (6) interposed between the first graphene layer (2) and a second graphene layer (7), said gate dielectric layer (6) being placed on the opposite side of the channel (5) with respect to the first graphene layer (2), said second graphene layer (7) being used for electrical gating and comprising first and second gate electrodes (8, 9) proximate to the first metal electrode (3) and the second metal electrode (4), respectively, said first and second gate electrodes (8, 9) being centered with respect to said channel (5), a photonic dielectric waveguide (10) with a planarized cladding (11) disposed underneath the gate dielectric layer (6), with the first and second gate electrodes (8, 9) remaining interposed between the gate dielectric layer (6) and the cladding (11), a distance between the first and the second metal electrodes (3, 4), defining the width of the channel cross-section, being comprised between 100 nm and 600 nm, a distance between the first and second gate electrodes (8, 9) being at least 60% of the distance between said first and second metal electrodes (3, 4).

2. The graphene photodetector according to claim 1, wherein the width of said channel (5) is between 250 nm and 450 nm.

3. The graphene photodetector according to claim 1, wherein a thickness of the first and second metal electrodes (3, 4), defining a height of the channel cross-section, is between 70 nm and 200 nm.

4. The graphene photodetector according to claim 3, wherein the thickness of the first and second metal electrodes (3, 4) is 100 nm.

5. The graphene photodetector according to claim 1, wherein a thickness of the gate dielectric layer (6) is comprised between 10 nm and 40 nm.

6. The graphene photodetector according to claim 1, wherein the thickness of the dielectric layer (6) is 20 nm.

7. The graphene photodetector according to claim 1, wherein the first and/or the second metal electrode (3, 4) are made of at least one of the following metals: Gold, Silver, Aluminum, Titanium nitride (TiN), or alloys thereof.

8. The graphene photodetector according to claim 1, wherein the distance (d.sub.1) between the first and the second metal electrode (3, 4), defining the width of the channel cross-section, is constant in the longitudinal extension (Y) of the channel (5).

9. The graphene photodetector according to claim 8, wherein the constant width of the channel cross section is between 250 nm and 450 nm.

10. The graphene photodetector according to claim 1, wherein the width of the channel (5) is periodically variable in the longitudinal extension (Y) of the channel, with sections having a minimum width (d.sub.1) alternating with sections having a maximum width (d.sub.1), and in which the width varies gradually between the minimum value and the maximum value, and vice versa, along said longitudinal direction.

11. The graphene photodetector according to claim 10, wherein the minimum width (d.sub.1) is between 100 nm and 250 nm and the maximum width (d.sub.1) is comprised between 450 nm and 600 nm.

12. The graphene photodetector according to claim 10, wherein the number of channel sections having the minimum width (d.sub.1) is between two and five.

13. The graphene photodetector according to claim 12, wherein in said channel, three sections having the minimum width (d.sub.1) are provided.

14. The graphene photodetector according to claim 10, wherein between two sections of minimum (d.sub.1) and maximum width (d.sub.1), adjacent to each other, the opposite surfaces of the channel (5) are angled at an angle () between 4 and 23 degrees, with respect to the longitudinal extension direction of the channel (5).

15. The graphene photodetector according to claim 1, wherein an optical mode of the dielectric waveguide has to be is quasi-Transverse-Electric (quasi-TE).

16. The graphene photodetector according to claim 1, wherein said channel (5) can be realized by using more than one graphene layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Further features and advantages of the invention will be made clearer by the detailed description hereinafter of some of its preferred embodiments illustrated, by way of non-limiting example, with reference to the accompanying drawings, in which:

[0044] FIGS. 1a to 1d are schematic views showing the cross sections of respective graphene photodetector embodiments according to the prior art,

[0045] FIG. 2 is a schematic cross section of a graphene photodetector realized according to the present invention,

[0046] FIGS. 3 and 4 are schematic top views of respective embodiments of the photodetector realized according to the invention,

[0047] FIG. 5 is a schematic top view, in enlarged scale, of a particular shown in FIG. 4,

[0048] FIG. 6 is a graph showing the ratio between the power absorbed by the active graphene layer and the power absorbed by the graphene gate electrodes shown versus the gate dielectric thickness, in the photodetector of the invention,

[0049] FIG. 7 is a schematic top view, in enlarged scale, showing the region of the gap between the metal electrodes, provided in the active graphene channel of the photodetector of the invention,

[0050] FIG. 8 is a graph showing the absorbed optical power density at a gold/graphene interface for selected gap widths, in the photodetector of the invention,

[0051] FIG. 9 is a graph showing the voltage responsivity in the photodetector of the invention as a function of the gap width,

[0052] FIG. 10 is a graph showing the power absorbed by metals and the power absorbed in the active graphene channel of the photodetector as respective functions of the metal electrode thickness,

[0053] FIG. 11 is a graph showing the optical absorption in the active graphene channel of the photodetector versus the distance between the dielectric layer and the dielectric waveguide,

[0054] FIG. 12 is a graph showing the optical absorption in the active graphene channel of the photodetector versus the thickness of the dielectric layer.

PREFERRED EMBODIMENTS OF THE INVENTION

[0055] With initial reference to FIG. 2, a graphene photodetector realized according to an embodiment of the present invention is globally indicated with 1. In FIG. 2 a schematic view of the cross section of the photodetector 1 is shown.

[0056] The photodetector 1 comprises a first graphene absorption layer 2 (having a planar configuration depicted with a dashed line) connected to a first metal electrode 3 at a first end 2a of the first graphene layer 2 and to a second metal electrode 4 at a second end 2b of the first graphene layer 2 opposite to the first end 2a. The first and second metal electrode 3, 4 are referred to as source and drain, respectively.

[0057] The contact between the graphene layer 2 and each of the metal electrode 3, 4 ensures the appropriate electrical connection to conduct and detect the photocurrent produced in the photodetector.

[0058] The first 3 and/or the second metal electrode 4 are preferably made of one or more of the following metals: Gold, Silver, Aluminum, Titanium nitride (TiN), or alloy thereof.

[0059] The first and second metal electrode 3, 4 further define on the first graphene layer 2 a channel 5 operating as a plasmonic waveguide, as clearly disclosed in the following.

[0060] The first and second metal electrode 3,4 are spaced apart and the distance between the first and the second metal electrode, indicated d.sub.1, defines the width of the channel cross-section.

[0061] The thickness of the first and second metal electrode, indicated t.sub.m, defines the height of the channel cross-section and is preferably comprised between 70 nm and 200 nm, and more preferably is 100 nm.

[0062] Preferably the distance d.sub.1 between the first and second metal electrode 3,4 is comprised between 100 nm and 600 nm, and more preferably is comprised between 250 nm and 450 nm.

[0063] The photodetector 1 further comprises a gate dielectric layer 6 interposed between the first graphene layer 2 and a second graphene layer 7 (also depicted with a dashed line), such a configuration realizing a capacitor, where the dielectric layer 6 is placed on the opposite side of the channel 5 with respect to the first graphene layer 2. Preferably the dielectric layer 6 is made of SiN or Al.sub.2O.sub.3.

[0064] The first and second graphene layer 2, 7 are preferably planar and parallel to each other, the distance between them being defined by the thickness of the dielectric layer 6, indicated t.sub.diel.

[0065] The second graphene layer 7 is used for electrical gating and comprises a first and a second gate electrode, indicated 8,9, which are located proximate to the first metal electrode 3 and the second metal electrode 4, respectively, in at least partial overlapping with the first graphene layer 2.

[0066] Preferably the first and second gate electrode 8,9 are spaced apart with a distance d.sub.2 and have a configuration centered with respect to the channel 5, as clearly shown in FIG. 2. The centered configuration means that the gating electrode 8,9 are arranged in a specularly symmetrical way with respect to a hypothetical median plane of symmetry of the cross section channel 5, identified in FIG. 2 with an axis indicated Z.

[0067] Preferably, the distance d.sub.2, between the gating electrodes 8,9 is at least 60% of the distance between said first and second metal electrode 3,4, and more preferably the distance d.sub.2 is comprised between 100 nm and 300 nm. In this range, more preferably the value of d.sub.2 is 150 nm.

[0068] As further disclosed below, the metal electrodes 3,4 on top of the first graphene layer 2, defining the active channel 5, are provided to either collect the photocurrent and to confine the light at the metal-graphene interface. Control of the electrostatic doping in the active channel, by changing the graphene chemical potential (applying an external voltage to the gating electrodes) is achieved by using a so called bottom split gate geometry obtained by the gating electrodes 8,9 of the second graphene layer 4.

[0069] The photodetector 1 further comprises a photonic dielectric waveguide 10 with a planarized cladding 11 disposed underneath the dielectric layer 6, with the first and second gating electrode 8, 9 remaining interposed between the dielectric layer 6 and the cladding 11. The waveguide 10 includes a core 12, preferably a silicon core, embedded in the cladding 11, preferably a SiO.sub.2 cladding.

[0070] The waveguide 10, preferably configured in a rectangular cross-section, is located centrally with respect to the active graphene channel 5. The dielectric spacer thickness between the waveguide 10 and the graphene gate electrodes is indicated by t.sub.clad. Preferably the waveguide 10 can have a rectangular cross section of 220 nm by 480 nm.

[0071] Referring to the top view of FIG. 3, the active channel 5 as well as the waveguide 10 are extended along a prevailing longitudinal direction, identified by the Y axis in the Figure. X indicates the direction perpendicular to the Y axis and directed parallel to the first graphene layer 2. The distance d.sub.1, defining the gap between the metal electrodes 3, 4 in the active graphene channel 5 is measured along the transverse direction X.

[0072] According to one embodiment of the invention, shown in the top view of FIG. 3, the distance d.sub.1 between the first and the second metal electrode 3, 4, defining the width of the active graphene channel 5, is constant along the longitudinal extension Y of the channel. This configuration is obtained by making the facing edges of the respective metal electrodes 3,4, parallel to each other, and spaced by the gap distance d.sub.1, for the prevailing longitudinal extension.

[0073] According to another embodiment of the invention, shown in the top view of FIG. 4, the distance d.sub.1, defining the width of the channel 5, may be periodically variable in the longitudinal extension of the channel, with channel sections having a minimum width, indicated d.sub.1, alternating with sections having a maximum width, indicated d.sub.1, and in which the width varies gradually between the minimum value d.sub.1 and the maximum value d.sub.1, and viceversa, along the longitudinal direction Y.

[0074] Preferably the minimum width d.sub.1 is comprised between 100 nm and 300 nm and the maximum width d.sub.1 is comprised between 450 nm and 600 nm. Preferably, the number of channel sections having the minimum width d.sub.1 may be comprised between two and five, and more preferably three channel sections having the minimum width d.sub.1 may be provided in the longitudinal extension of the channel.

[0075] In FIG. 5, the tapering configuration of the channel sections of FIG. 4 is shown in an enlarged scale. In the tapering configuration, the opposite surfaces of the channel are angled at an angle with respect to the longitudinal extension direction of the channel.

[0076] A small tapering angle (see FIG. 5) is desirable in order to efficiently convert the mode of the dielectric waveguide into the plasmonic mode of the detector structure. However, an excessively small tapering angle would lead to a long (in the propagation direction y) tapering section. This would be detrimental because losses in metal would increase with consequent reduction of responsivity. A preferred angle is defined in the range between 4 and 23. The distance between the gate electrodes and the dielectric waveguide (t.sub.clad, see FIG. 2) must be small enough to ensure good coupling between the dielectric waveguide and the detector stack. However, gate electrodes are electrically isolated. For this reason t.sub.clad0.

[0077] The thickness t.sub.diel of the gate dielectric layer is chosen small enough to maximize the optical absorption in the active graphene channel. However, the thickness t.sub.diel is preferably chosen to be at least 20 nm to prevent current leakage between the active channel and the gate electrodes.

[0078] The graphene-based photodetector of the claimed subject matter is proposed for exploiting the photo-conversion mechanisms (photovoltaic and photo-thermoelectric effect) occurring at the metal/graphene interface. Photovoltaic and photo-thermoelectric mechanism at the metal/graphene interface can be exploited to generate a photocurrent. Differently from the devices of the prior art, described with reference to FIGS. 1a to 1d, where a graphene homojunction is used, the photovoltaic effect is expected to give a relevant contribution, in addition to the photo-thermoelectric effect. The basic idea behind the claimed photodetector is to use a plasmonic waveguide to confine the optical field at the edges of the metal electrodes (source and drain) used to collect the photocurrent. This photodetector structure is designed to be integrated on top of the photonic dielectric waveguide 10 with the planarized cladding 11 and it is made up of a stack of the two graphene layers 2, 7 separated by the dielectric layer 6. The metal electrodes 3,4 on top of the first graphene layer 2 (active channel 5) are used to either collect the photocurrent and to confine the light at the metal/graphene interface. Control of the doping in the active channel 5 (first graphene layer 2) is achieved by using the bottom split gate geometry obtained by the second graphene layer 7.

[0079] The geometry of the photodetector is shown in FIGS. 2-4, where the device stack is integrated on top of the photonic waveguide 10 with the core 12 and the planarized cladding 11. The source and drain electrodes 3, 4 serve both as electrodes to collect the photocurrent and as plasmonic waveguide to confine the light at the metal/graphene interface.

[0080] In order to excite the plasmonic mode the optical mode of the dielectric waveguide has to be quasi-Transverse-Electric (quasi-TE).

[0081] The light from the dielectric waveguide 10 is coupled to the plasmonic mode of the Metal-Insulator-Metal (MIM) waveguide on top of the active graphene channel 5. Most of the optical power is absorbed at the graphene/metal interface at the edge of the metal contacts. Referring to FIG. 3, the width d.sub.1 of the MIM before metal absorption prevails on the graphene absorption is 300 nm.

[0082] In FIG. 4, in which the distance between metals varies periodically, regions where the MIM has a large width (d.sub.1 greater than 300 nm) and regions of small width (d.sub.1 less than 300 nm) are alternated.

[0083] As described above, with reference to the prior art solutions, a graphene layer interposed between the dielectric waveguide and the active graphene layer is detrimental because it would absorb a large amount of the optical power reducing the responsivity of the photodetector. In the proposed invention this problem is strongly mitigated. As a matter of fact, the use of a plasmonic waveguide enhances the electric field in the active graphene layer. Moreover, graphene optical absorption linearly scales with the number of layers. By using two graphene layers the active channel has a larger absorption with respect to gates.

[0084] In the graph of FIG. 6 the ratio between the power absorbed by the active graphene layer and the power absorbed by the graphene gate electrodes is shown as a function of the gate dielectric thickness (t.sub.diel). In this case the t.sub.clad is always 20 nm. The ratio of power absorbed by the active layer vs graphene gate electrodes spans from slightly more than 400% for a 20 nm thick layer to slightly less than 200% for a 80 nm thick gate dielectric. The graph has a monotonic decreasing trend, showing that the graphene gates absorb a significantly large part of the optical power when the thickness (t.sub.diel) of the gate dielectric is increased.

[0085] As to the graphene active channel, the major drawbacks of small gaps between the metal electrodes are the large absorption in metals and the non-trivial control of graphene chemical potential between the two metal electrodes. With small gaps, as it has been observed by the Applicant with a gap of 20 nm, the chemical potential in the gap is almost constant and does not vary from the left contact to the right contact. Since the gap region of the channel is the region where the largest part of the optical power is absorbed, if it not possible to control the chemical potential in the gap, it is not possible to maximize the PTE and PV photo-response. As a consequence of that, the voltage responsivity is poor. However, thanks to the field enhancement obtained in the gap, the embodiment with tapered sections has the advantage of increasing the amount of optical power absorbed in the active graphene channel. Two embodiments of the photodetectors can be compared: 1a realization of photodetector having a constant width of 300 nm and 2a photodetector with tapered sections having for instance a minimum width d.sub.1 equal to 250 nm and a maximum width d.sub.1 equal to 600 nm. In the realization constant width photodetector the absorbed optical power is less compared to the case of photodetector with periodically tapered width. Moreover, thanks to a minimum gap width (>100 nm), in the periodically tapered realizations the optical power is not confined only in the gap but a relevant part of the absorption occurs also in sections of the taper having larger width. FIG. 8 showing the optical power absorbed at the metal graphene interface as a function of the Y coordinate highlights this concept. In this system of coordinates Y=0 corresponds to the middle of the structure shown in FIG. 7. For tapers having minimum gap width of 20 nm, the optical power is absorbed almost entirely in the gap region. For devices having minimum gap width of 250 nm and of 70 nm the power is absorbed more uniformly along Y.

[0086] Optical power absorption in regions where the width is larger than 100 nm allows a more accurate control the chemical potential. This permits a better optimization of the PTE and PV effect and therefore the responsivity of the detector can be optimized.

[0087] For such reasons the solution with tapered sections and relatively large gap (>100 nm) represents the optimum design and the optimal range for d.sub.1 and d.sub.1 (see FIG. 4) is defined consequently.

[0088] In the graph of FIG. 9 the simulated voltage responsivity as a function of the gap width is shown.

[0089] FIG. 10 is a graph showing the power absorbed by metals and the power absorbed in the active channel in function of the metal electrode thickness t.sub.m, wherein t.sub.clad is 20 nm and t.sub.diel is also 20 nm. It is observed that the power absorption in the metals is reduced as the metal thickness is increased.

[0090] FIG. 11 is a graph showing the optical absorption in the active graphene channel versus the distance t.sub.clad, where the t.sub.diel is 20 nm and t.sub.m is 70 nm. The distance t.sub.clad must be chosen as thin as possible to maximize the optical absorption in the active graphene channel. It can be observed that for t.sub.clad=50 nm, the power absorbed in the active channel is reduced by 54% with respect to t.sub.clad=20 nm.

[0091] FIG. 12 is a graph showing the optical absorption in the active channel versus the thickness t.sub.diel of the dielectric layer, where t.sub.clad is 20 nm and t.sub.m is 70 nm. It can be observed that for t.sub.diel=50 nm, the power absorbed in the active channel is reduced by 60% with respect to t.sub.diel=20 nm.