FIELD OF THE INVENTION The present invention relates to the use of single or multi-layer graphene as the photon detecting layer in a photodetector.

BACKGROUND Photodetectors are used to detect or sense light or other electromagnetic energy. Currently available photodetectors are generally used in a variety of practical applications such as wired and wireless communications, sensing, monitoring, scientific instrumentations, national security, and so on. Many optical photodetectors use semiconductor materials as the photodetection material system. Semiconductor materials, however, have a band gap, and therefore only photons having energy greater than the band gap can be detected, potentially leaving photons undetected. In addition, the intrinsic bandwidth of semiconductor based photodetectors is limited by the carrier transit time in the photodetection region. Both of these limitations results in a less than optimal photodetector.

BRIEF SUMMARY

Various aspects of the present invention are defined in the appended claims to which reference should now be made.

The present invention provides a photodetecting device in which single or multiple layers of graphene are the photoconducting layer. According to one aspect of the invention, a substrate is provided upon which a gate oxide layer deposited. A channel layer of graphene is then deposited on the gate oxide layer and source and drain contact regions are patterned on the graphene layer. According to another aspect, split gates may be provided on top of the source and drain regions. Also, multiple source and drain regions can be provided.

According to another aspect, multiple photodetection elements can be provided along with conventional signal processing readout circuitry to create a photodetecting array.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 shows a simple FET implementation of a graphene-based photodetector.

Figure 2 is a graph showing the internal potential generated across the channel of the graphene-based photodetector.

Figure 3 shows a split-gated FET implementation of a graphene-based photodetector.

Figure 4 is a graph showing the internal potential generated across the channel of the split- gated graphene-based photodetector.

Figure 5 shows a split-gated, graphene-based FET implementation of a graphene-based photodetector with integrated waveguide structure.

Figure 6 shows a graphene-based photodetector with metallic interdigitated fingers for enhancing the effective photo-detection area. Figure 7 shows a top view of a graphene-based photodetector array.

DETAILED DESCRIPTION

The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well- known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. Conventional processes well known in semiconductor processing can be utilized in fabricating the structures described herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practised and to further enable those of skill in the art to practise the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.

Embodiments of the present invention use single or multi-layer graphene as the

photodetecting layer in a photodetector. Because of graphene's unique property of being a zero or very small band gap material, photons at any wavelength (or any energy level) can be absorbed. Hence, graphene can be used as a universal material for a wide range of photonic applications at wavelengths ranging at least from ultraviolet to far- infrared. The carrier transport velocity in graphene under high E-fϊeld can approach Fermi velocity 10 ⁶ meter/second, which is 10 to 100 times larger than the carrier transport velocity in conventional semiconductors. This can lead to photodetectors with much higher bandwidth.

Devices employing the invention can work without direct external biases between the photocurrent generation paths, which naturally leads to zero dark current and may enable many applications in imaging, remote sensing and monitoring where low dark current is essential. Devices employing the invention can also work with direct external biases between the photocurrent generation path, which usually leads to better efficiency but with some dark current.

Referring to Fig. 1, a first embodiment of a photodetector made in accordance with this invention has a basic design similar to a conventional field effect transistor (FET) where the photodetection is realized by applying a gate bias to create a graphene p-n junction in order to maximize the detection efficiency. Backgate 10 is comprised of silicon back gate (either heavily doped or lightly doped). On top of the backgate 10, a gate oxide layer 12 is deposited as an insulating layer. Gate oxide layer 12 can be SiO ₂ or any dielectric material. On top of the gate oxide a layer of graphene 14 that may be one or more layers thick is provided. Graphene layer 14 can be doped or un-doped. The graphene layer can be created by a number of processes including mechanical exfoliation, chemical deposition, or growth. Next, drain 6 and source 8 contacts are patterned on the graphene/gate oxide layer 12, the graphene then forming a channel between the drain 6 and source 8 contacts. The graphene and the source and drain contacts should overlap by at least lOOnm so as to create good metal-graphene junctions. Gate bias V _G is applied to field-dope the graphene in the middle of channel layer 14. The doping of the graphene close to and underneath the source and drain is dominated by the contacts instead of the back gate. With proper selection of the gate bias V _G , good detection efficiency can be obtained because a graphene p-n junction is formed at the source (or drain)-graphene interface.

Fig. 2 shows the internal potential generated across the width d of the channel layer 14 of Fig. 1. As can be readily seen, the areas of maximum photodetection are in close proximity to the source 8 and drain 6 regions at which a graphene p-n junction is formed and the E- field is maximized in this area, leading to effective separation of photo-generated carriers.

With reference to Fig. 3, another implementation of the photodetector made in accordance with the present invention uses a split gated structure so that a high e-field photodetection region can be created. On top of substrate 34 and gate oxide layer 12 source and drain regions 30 and 32, respectively are deposited on single or few (2 to 5) layers of graphene 38. A transparent gate dielectric layer (can be high-K or low-K) 40 is deposited on top of the graphene 38, the source 30, and drain 32 regions. Gates 42 and 44 are then patterned on top of the dielectric layer 40. In figure 3 the gate dielectric layer 40 is shown partially cut away for explanatory purposes. This layer may cover the whole of the graphene channel as well as source and drain regions, apart from locations where electrical connections are required.

In operation, a gate bias is applied to gates 42 and 44, respectively, to create a sensitive photo-detection region in channel 38 in which a considerable E-field is produced. The magnitude of the gate bias is a function of the thickness of the top gate dielectric 40. Fig. 4 graphically illustrates the potential generated by the top gates 42 and 44 between the source 30 and drain 32 regions of the device in Fig. 3. Fig.5 shows the split gate photodetection device as seen in Fig. 3, but with the addition of an optical waveguide 50 which underlies the graphene. This device operates the same way as the photodetection device of Fig. 3, but is more efficient due to the properties of the waveguide 50, which captures photons impacting the device over a larger area and channels them to the graphene layer 38. Waveguide 50 can be fabricated from silicon, silicon nitride, silicon oxy-nitride or any low-loss material, using typical semiconductor processes such as material growth, wafer bonding, wet or dry etches, and so on.

Fig. 6 shows a modification which may be made to the embodiments previously described. Interdigitated fingers defining multiple source regions (64-67) and drain regions (60-63) are patterned on top of graphene layer 38. This implementation provides for a very sensitive photodetector because the effective photo-detection area can be greatly enhanced. Different source and drain metals with different work functions can be used in order to produce an internal potential at zero source-drain bias for photo-carrier separation. Applying a source- drain bias can further enhance the photo-detection efficiency. However in this case, a dark current will be introduced into the operation. Moreover, if the same metal is used for source and drain contact, a shadow mask can be used to block the light absorption in

source/graphene or drain/graphene contact to enhance the photocurrent generation. Photodetector arrays are very useful in applications such as imaging (in very broad wavelength range at least from far infrared to visible) and monitoring. The graphene-based photodetector of the present invention can also be fabricated in such an array 70, as shown in Fig. 7, using standard semiconductor processes. The addition of conventional signal processing circuitry 72 can provide a readout from all of the photodetecting elements 74 in the detector array, each element 74 of which may comprise a photodetector according to any of the previously described embodiments.

It will be understood by those skilled in the art that, although the present invention has been described in relation to the preceding example embodiments, the invention is not limited thereto and that there are many possible variations and modifications which fall within the scope of the invention. The scope of the present disclosure includes any novel feature or combination of features disclosed herein. The applicant hereby gives notice that new claims may be formulated to such features or combination of features during prosecution of this application or of any such further applications derived therefrom. In particular, with reference to the appended claims, features from dependent claims may be combined with those of the independent claims and features from respective independent claims may be combined in any appropriate manner and not merely in the specific combinations enumerated in the claims.

For the avoidance of doubt, the term "comprising", as used herein throughout the description and claims is not to be construed as meaning "consisting only of.