TECHNICAL FIELD

The invention relates to a graphene optoelectronic sensor including one or more optical resonators for detecting discrete light wavelength bands primarily within the infrared wavelength bands. The invention involves the novel use of a graphene layer as a light absorbent.

BACKGROUND

In a large range of applications, imaging has become a popular method for obtaining data. Certain applications require an extraordinary quality to be able to correctly identify and distinguish individual objects in an image. To this end, much effort has been made in the use of graphene in optoelectronic sensors. Graphene with its single layer of honeycomb carbon lattice and single-pass light absorption at or below 2.5% has proven potentially very useable in the field of optoelectronics.

Currently known studies and attempts at obtaining a useable photo sensor involving graphene include the use of graphene as a resonator in itself. An example of this is described in WO1508373A1 which discloses a graphene sheet as a resonator layer supported at two points. The sensor includes a detection circuit responsive to piezo-resistive changes in the resonator graphene sheet. A problem with the use of graphene as a resonator in itself is however the low single-pass light absorption at or below 2.5% of graphene, which results in a photo sensor with relatively low internal quantum efficiency. Experiments have been made to improve the internal quantum efficiency by adding dots of various metals on the graphene 2D honeycomb lattice, but with only limited improvements. Another example of graphene used in a photo sensor is described in US 2015/0357504 A1 . Here, a graphene based transistor-type sensor is used as the active sensor device. A delicate structure of multiple very thin nanoribbons of graphene is grown on a substrate layer. The frequency sensitivity is here based on the dimensions of the various nanoribbons. This solution is very complicated to manufacture with acceptable quality yield in large scale production and is consequently quite expensive to manufacture.

SUMMARY OF THE INVENTION In order to alleviate the problems mentioned above, the object of the present invention is to provide a practically useable graphene optoelectronic sensor which is well suited for large scale production at low manufacturing costs, using graphene in a novel way as a light absorbent and not as a resonator by itself as in known technology. Due to its unique composition, the inventive graphene optoelectronic sensor reaches a high external quantum efficiency of between 65- 70% which is a substantial improvement over known graphene optoelectronic sensors with an external quantum efficiency below 10-15%. The unique inventive topography further enables the detection of more discrete wavelength bands on a sensor based on a single chip than what has so far been obtainable with previously known technology. Hence, the invention offers a graphene optoelectronic sensor including one or more optical resonators for detecting discrete light wavelength bands. The graphene optoelectronic sensor is especially characterized in:

- that a graphene light absorbent layer is directly applied on a first face of an optical resonator cavity layer, said optical resonator cavity layer consisting of a material having a refractive index at or above 2, and

- that a wavelength-selective light scattering unit is located on or adjacent to an opposite second face of each optical resonator cavity layer. In an advantageous embodiment, read-out terminals extend from the graphene light absorbent layer to a complimentary metal-oxide semiconductor (CMOS) readout circuit. A substrate layer is applied to the second face of each resonator cavity layer. This substrate layer consists of a material with a refractive index at or below 1 .5, such as for example silicon dioxide (SiO2). In a well-functioning embodiment of the invention, the optical resonator cavity layer consists of silicon (Si).

Advantageously, the optical resonators are arranged for detecting a selection of infrared wavelength bands.

In an embodiment well suited for large-scale production, the graphene optoelectronic sensor is directly applied to a complementary metal-oxide semiconductor (CMOS) read-out circuit.

In an embodiment suited for detecting a broad range of wavelength bands, multiple light scattering units, each adapted for unique infrared bands, are superimposed on each other with dielectric interstitial walls.

In another embodiment of the invention, multiple light scattering units are arranged in an array. The shape of the light scattering unit or units may be different and could for example be designed in the shape of a cross having cross bars having a rectangular shape or shaped as a modified cross having cross bars being more narrow in its central portions so as to form a "Maltese cross" shape. The dimensions of the cross are adapted for reflecting a predefined band of wavelengths. Other possible shapes of the net light scattering unit are for example a circular plate or being shaped of tangled up oblong bars forming a square net configuration.

In a multi-colour graphene optoelectronic sensor version, the optical resonators for detecting different wavelength bands exhibit mutually different proportions of thickness between the optical resonator cavity layer and the substrate layer. The total thickness of both the substrate layer and the optical resonator cavity layer are the same for all optical resonators of the graphene optoelectronic sensor.

In still a further embodiment, the first face of the resonator cavity layer also includes a metal layer and one or two semiconductor layers located on top of the CMOS read out circuit. These layers are sandwiched in different order on top of each other so as to form different absorber stack configurations. Hence, in this embodiment the graphene light absorbent layer at the first face of the resonator cavity layer forms part of an absorber stack also comprising a metal layer and one or several semiconductor layer.

According to one embodiment, the graphene layer is located on top, i.e. furthest away from the CMOS read out circuit and a semiconductor layer sandwiched between the graphene layer and a metal base layer being closest to the CMOS readout circuit. In this embodiment, the metal layer helps to have a better reflection and avoids light to penetrate into the readout circuit while the semiconductor can enhance absorption. The semiconductor layer and metal layer are preferably designed such that there is an isolating gap between these layers and the read out terminals while the graphene layer may be in contact with the read out terminals. This embodiment may thus improve certain features of the optoelectronic sensor as explained above but since graphene does not have a bandgap so the first and the original design will have a high dark current.

To avoid dark currents it is possible to have a vertical device where graphene and other semiconductors can be put on top of each other. In these absorber stack configurations a barrier will be formed on the interface which block dark currents. The backside metal is used to have a better reflection and avoids light to penetrate into the readout circuit. The respective layers of semiconductors and metals on respective side of the graphene layer in the absorber stack configuration are designed to be connected to a respective read out terminal only and isolated, e.g. by an air gap, to avoid contact between the other read-out terminal. The absorber stack could be designed to include further metal and/or semiconductor layers. It is also possible to completely exclude either the metal layer or the semiconductor layer and only include one or several layers of semiconductors or metal. This may be advantageous from production process even though the absorber stack may lose some of the desired properties achieved when using both a metal layer and a semiconductor layer.

The invention also includes a method for manufacturing a graphene optoelectronic sensor as described above. The method is especially characterized in the step of growing the graphene light absorbent layer directly onto a complimentary metal- oxide semiconductor (CMOS) read-out circuit or possibly on an additional metal or semiconductor layer in an absorber stack configuration.

An alternative manufacturing method according to the invention involves the step of growing the substrate layer of the optical resonators directly onto a complimentary metal-oxide semiconductor (CMOS) read-out circuit.

The invention provides many advantages over previously known technology. For example, the optoelectronic sensor according to the invention is CMOS- compatible which makes it possible to grow the sensor directly on top of a readout circuit. Furthermore, the use of a single graphene light absorbent layer for several optical resonators on a common single chip enables a high quality yield in large scale production. Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples. shows a simplified schematic cross-sectional view of a graphene optoelectronic sensor according to the present invention. The shown embodiment includes multiple light scattering units, superimposed on each other with interstitial dielectric separation walls. shows an exemplifying shape of a light scattering unit in the form of a cross configuration. shows a further exemplifying shape of a light scattering unit in the form of a modified cross configuration. shows an exemplifying shape of a light scattering unit in the form of a circular plate configuration shows yet another exemplifying shape of a light scattering unit in the form of a square fence configuration. shows a simplified schematic cross-sectional view of multicolour graphene optoelectronic sensor according to a further exemplifying embodiment of the invention, having multiple optical resonators positioned side-by-side on a complimentary metal-oxide semiconductor (CMOS) read-out circuit. shows a graph of infrared target wavelength bands for the photo sensor. shows a simplified schematic cross-sectional view of an

alternative embodiment of the invention wherein the

substrate layer is grown directly onto a complimentary metal-oxide semiconductor (CMOS) read-out circuit. shows an array of light scattering units according to a favourable embodiment of the invention. shows a partial schematic view cross-sectional view of a photo sensor according to the invention, in which the light scattering unit is positioned within the substrate layer at a small distance from the optical resonator cavity layer.

Fig. 10 finally shows a partial schematic view cross-sectional view of a photo sensor according to the invention, in which the light scattering unit is positioned within the substrate layer directly facing the optical resonator layer.

Fig. 1 1 shows a detailed view of the first face of the resonator cavity layer including graphene light absorbent layer.

Fig. 12 shows different absorber stack configurations in the first face of the resonator cavity layer including graphene and semiconductors DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

The invention will now be described with reference to embodiments of the invention and with reference to the appended drawings. With initial reference to Fig. 1 , this figure shows a simplified schematic cross-sectional view of a graphene optoelectronic sensor 1 according to an exemplifying embodiment of the present invention. The graphene optoelectronic sensor 1 includes an optical resonator 2 for detecting discrete light wavelength bands. Although the shown embodiment only has one single optical resonator 2, alternative embodiments with multiple optical resonators 2 will be described later in the description. A graphene light absorbent layer 3 is directly applied on a first face 4 of an optical resonator cavity layer 5. The optical resonator cavity layer 5 consists of a material having a refractive index at or above 2, such as for example silicon (Si). Germanium (Ge) may also be used in the optical resonator cavity layer 5, although it has a slightly narrower bandgap. In order to operate at shorter wavelengths the optical resonator can be made from Titanium Oxide (ΤΊΟ2). As further shown in Fig. 1 , a wavelength-selective light scattering unit 6 is located on an opposite second face 7 of each resonator cavity layer 5. The light-scattering unit 6 may alternatively be placed adjacent to an opposite second face 7 of each resonator cavity layer 5, which will be further described with reference to Fig 9 later in this description. Read-out terminals 8 extend from the graphene light absorbent layer 5 to a complimentary metal-oxide semiconductor (CMOS) readout circuit 9. The read-out terminals 8 are suitably made of a highly electrically conductive metal, such as aluminium or gold. In the shown embodiment, the graphene optoelectronic sensor 1 is directly applied to the complementary metal- oxide semiconductor (CMOS) read-out circuit 9. The invention also includes a method for manufacturing a graphene optoelectronic sensor 1 by the step of growing the graphene light absorbent layer 3 directly onto the read-out circuit 9. This is conveniently achieved by means of chemical vapour deposition, often abbreviated CVD. In alternative, not shown embodiments, the graphene optoelectronic sensor 1 may instead be separate from the read-out circuit 9 as long as it is connected to the read-out circuit 9 via the read-out terminals 8.

A substrate layer 10 is applied to the second face 7 of each resonator cavity layer 5. This substrate layer 10 consists of a material with a refractive index at or below 1 .5, such as for example silicon dioxide (SiO2) or glass.

The embodiment shown in Fig. 1 is especially adapted for detecting a broad range of wavelength bands and is therefore provided with two light scattering units 6 that are superimposed or stacked on each other with dielectric interstitial walls 1 1 . Each light scattering unit is adapted for unique infrared bands by unique geometrical configurations. In alternative embodiments of the invention there may be more than two light scattering units 6 in a sandwiched compound of multiple light scattering units 6. The light scattering elements 6 may conveniently be positioned slightly rotated in relation to each other so as to expose a maximum light scattering surface to incident light.

As is further shown in Fig. 1 , incident light is schematically illustrated by means of incident light arrows 12. In a practical embodiment of the invention, an anti- reflective coating layer 13 may be applied on the substrate layer 10. The anti- reflective coating layer 13 may for example be made of aluminium oxide or aluminium nitride in a manner known per se. The embodiment shown in Fig. 1 may be referred to as a "backward feeding topology" -version of the graphene optoelectronic sensor 1 , as opposed to a "forward feeding topology" -version later to be described with reference to Fig. 7. This is because the incident light, as illustrated by means of the incident light arrows 12, enters the resonator cavity layer 5 via the second face 7 and not via the first face 4 where the graphene light absorbent layer is located. Hence, the light is reflected by the graphene light absorbent layer 3 and the light scattering unit 6 receives the reflected light - as indicated by the light arrows 16 in a reversed or "backwards direction". The "backward feeding topology" -version of the graphene optoelectronic sensor 1 as shown in Fig. 1 is favourable in that this topology eliminates the need for longer read-out terminals 8, which saves expensive material cost.

In both the forward and backward feeding topology versions, the optical resonator cavity layer 5 forces the incident light to bounce inside the resonator cavity provided by the optical resonator cavity layer 5 and every time the trapped light impinges the graphene light absorbent layer 3, 2.3% of its power is absorbed by the graphene. It does not matter on which surface of the optical resonator cavity layer 5 the graphene is applied. In both embodiments, the silicon (or a material with similar refractive index) in the optical resonator cavity layer 5 constitutes the actual resonator and not the graphene itself as in previously known optoelectronic sensors based on graphene. Hence the invention introduces the use of graphene as a light absorbent layer 3 which - when applied directly to an optical resonator layer 5 as described, results in a high external quantum efficiency of between 65- 70% which is a substantial improvement over known graphene optoelectronic sensors with an external quantum efficiency below 10-15%. In Fig. 2a, a light scattering unit 6 is shaped in an exemplifying shape of a cross. The dimensions of the cross are adapted for reflecting a predefined band of wavelengths. Correspondingly, in Fig. 2b, a light scattering unit 6 is shaped in an exemplifying shape of a modified cross or "Maltese cross" configuration where the distal portions 14 are broader than the central portions 15 as shown. Another exemplifying embodiment is shown in Fig. 3, wherein a light scattering unit 6 is shaped in the form of a circular plate configuration. Another exemplifying embodiment is shown in Fig. 4, wherein a light scattering unit 6 is shaped in the form of a square fence configuration. Fig. 5 illustrates a multicolour graphene optoelectronic sensor 1 according to a further exemplifying embodiment of the invention. This multicolour graphene optoelectronic sensor 1 is provided with multiple optical resonators 2 positioned side by side on a complimentary metal-oxide semiconductor (CMOS) read-out circuit 9 as shown in the figure. In the shown embodiment, an exemplifying number of three optical resonators 2 exhibit mutually different proportions of thickness between the optical resonator cavity layer 5 and the substrate layer 10. However, the total thickness, t, of both the substrate layer 10 and the optical resonator cavity layer 5 remains the same for all three optical resonators 2 as shown in the figure and typically measures about 1 -2 pm. According to the invention, more optical resonators 2 than the three shown in Fig. 5 may be arranged on a single CMOS read-out circuit 9. A practical embodiment may for example include between four to six optical resonators 2 positioned side-by-side, for detecting four to six colours or desired wavelength bands. Separation walls 17 are located between the optical resonators 2. The separation walls 17 are preferably made of the same material as the substrate material, which in the shown example is silicon (Si). The graphene light absorbent layer 3 is etched in the manufacturing process so that gaps 18 are formed between neighbouring optical resonators 2, enabling individual wavelength detection read-out from each optical resonator 2. Just like in the previously described embodiment, this embodiment may have an anti-reflective coating layer 13 applied on the substrate layer 10.

Fig. 6 shows a graph of a selection of infrared target wavelength bands that may be detected by the graphene optoelectronic sensor 2. The wavelength, λ, in pm on the horizontal axis is here plotted against the transmittance, Tr, in % on the vertical axis.

An alternative embodiment of the graphene optoelectronic sensor 1 according to the invention is shown in Fig. 7. This embodiment may be referred to as a "forward feeding topology" -version of the graphene optoelectronic sensor 1 , as opposed to the "backward feeding topology" -version described initially with reference to Fig. 1 . Indeed, the embodiments shown in Figs. 5 and 7 are also of the "backward feeding topology" -version. Thus, in the "forward feeding topology" version shown in Fig. 7, the incident light, as illustrated by means of the incident light arrows 12, enters the resonator cavity layer 5 via the first face 4 where the graphene light absorbent layer is located. Hence, the light is here reflected by the light scattering unit 6 directly from a "forward direction". In this embodiment the graphene optoelectronic sensor 1 has been manufactured by means of a method wherein the substrate layer 10 is grown directly onto the complimentary metal-oxide semiconductor (CMOS) read-out circuit 9. The resonator layer 5 is then applied on the substrate layer 10 and is then topped by the graphene light absorbent layer 3 and an anti-reflective coating layer 13, facing the incident light as illustrated by the incident light arrows 12. In this alternative embodiment, the read-out terminals 8 are longer than in the previous embodiments as they have to reach the now more distant graphene light absorbent layer 3. The read-out terminals are separated by separation walls 17 preferably made of the same material as in the substrate layer 10, like for example silicon oxide (S1O2).

Fig. 8 shows an alternative arrangement of multiple light scattering units 6. Here, the light scattering units 6 are arranged in an array 19. Fig. 9 shows a partial schematic view cross-sectional view of a graphene optoelectronic sensor 1 , in which the light scattering unit 6 is embedded within the substrate layer 10 at a small distance, d, from the optical resonator cavity layer 5. Preferably, the distance, d, may be anywhere on the range of 0 to 1/10 of a desired detection wavelength, whereas the height, H, of the resonator layer 5 may typically be ¼ of said desired wavelength. Fig. 10 finally shows a similar embodiment wherein the light scattering unit 6 is positioned within the substrate layer 10 but flush with the second face 7 of the resonator layer 5. Hence, in this embodiment, the distance, d, illustrated in Fig. 9 is zero.

Fig. 1 1 shows a detailed view of the first face of the resonator cavity layer 4 consisting of a graphene light absorbent layer 3. The graphene light absorbent layer 3 is located on and in contact with the CMOS read-out circuit extending between and in contact with the read-out terminals 8 in order to absorb the incident light arrows passing through the wavelength-selective light scattering unit 6. However, even though the first face of the resonator cavity layer 4 may only comprise a graphene layer 3 could it also comprise a metal layer 20 and a first semiconductor layer 21 a, possibly also a second semiconductor layer 21 b, in order to form an absorber stack 22 a-c as shown in Figs. 12A-C.

In Fig. 12A comprises the absorber stack 22a the graphene light absorbent layer 3 on top of the first semiconductor layer 21 a which is sandwiched between the graphene light absorbent layer and the metal layer 20. The graphene light absorbent layer 3 stretches all the way between the read-out terminals 8 while the first semiconductor layer 21 a and the metal layer 20 are spaced apart from the read-out terminals 8.

In Fig. 12B comprises the absorber stack 22b a graphene light absorbent layer 3 which is sandwiched between the first semiconductor layer 21 a on top and the metal layer 20 beneath. In this embodiment, the graphene light absorbent layer 3 and the metal layer 20 are in contact with the first one of the read-out terminals 8a and spaced apart from the other read out terminal 8b while the first semiconductor layer 21 a on top of the graphene light absorbent layer 3 is spaced apart from the first one of the read-out terminals 8a and spaced apart from the other one 8b. Fig. 12C differs from figure 12B in that it comprises a further second semiconductor layer 21 b in the absorber stack 22c sandwiched between the first semiconductor layer 21 a on top and the graphene light absorbent layer 3 beneath. In this embodiment, the graphene light absorbent layer 3 and the metal layer 20 are in contact with the first one of the read-out terminals 8a and spaced apart from the other read out terminal 8b while the first semiconductor layer 21 a on top of the graphene light absorbent layer 3 is spaced apart from the first one of the read-out terminals 8a and spaced apart from the other one 8b.

In these configurations of the absorber stacks 22 a-c, the metal layer can be one selected from Ag, Au, Al and Cu. Semiconductor 1 may be a typical semiconductor such as Si, Ge and CuO and semiconductor 2 could be a wide- bandgap semiconductor such as ΤΊ02, NiO, Cr203 and Cu20. The metal layer helps to have a better reflection and avoids light to penetrate into the readout circuit while the semiconductor can enhance absorption.

The embodiments in figures 12B respectively 12C have a design and configuration in which the graphene light absorbent layer 3 in the absorber stack 22b respectively 22c forms a barrier which block dark currents. It is to be understood that the present invention is not limited to the exemplifying embodiments described above and illustrated in the drawings and a skilled person will recognize that many changes and modifications may be made within the scope of the appended claims. LIST OF REFERENCE NUMERALS USED IN THE DESCRIPTION AND IN THE ACCOMPANYING DRAWINGS:

1 . Graphene optoelectronic sensor

2. Optical resonator

3. Graphene light absorbent layer

4. First face of resonator cavity layer

5. Optical resonator cavity layer

6. Wavelength-selective light scattering unit

7. Opposite second face of resonator cavity layer

8. Read-out terminals

9. CMOS Read-out circuit

10. Substrate layer

1 1 . Dielectric interstitial walls between superimposed light scattering units

12. Incident light arrows

13. Anti-reflective coating layer

14. Distal portions of light scattering unit

15. Central portions of light scattering units

16. Reflected light arrows

17. Separation walls between multiple optical sensors

18. Etched gaps in the graphene light absorbent layer between optical resonators

19. Array of light scattering units

20. Metal layer

21 . Semiconductor layer

22. Absorber stack

t: Total thickness of both the substrate layer and optical resonator cavity layer d: Distance between light scattering unit and optical resonator cavity layer if the light scattering unit is embedded in the substrate layer.

H: Height of resonator layer

λ: Wavelength of incident light in pm

Tr: Transmittance in %