Abstract:
A photodetector which uses single or multi-layer graphene as the photon detecting layer is disclosed. Multiple embodiments are disclosed with different configurations of electrodes. In addition, a photodetector array comprising multiple photodetecting elements is disclosed for applications such as imaging and monitoring.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is a photodetector which uses single or multi-layer graphene as the photon detecting layer. 
       BACKGROUND 
       [0002]    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. 
         [0003]    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 
       [0004]    The present invention provides a photodetecting device in which single or multiple layers of graphene are the photoconducting layer. 
         [0005]    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. 
         [0006]    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. 
         [0007]    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 
         [0008]      FIG. 1  shows a simple FET implementation of a graphene-based photodetector. 
           [0009]      FIG. 2  is a graph showing the internal potential generated across the channel of the graphene-based photodetector. 
           [0010]      FIG. 3  shows a split-gated FET implementation of a graphene-based photodetector. 
           [0011]      FIG. 4  is a graph showing the internal potential generated across the channel of the split-gated graphene-based photodetector. 
           [0012]      FIG. 5  shows a split-gated, graphene-based FET implementation of a graphene-based photodetector with integrated waveguide structure. 
           [0013]      FIG. 6  shows a graphene-based photodetector with metallic interdigitated fingers for enhancing the effective photo-detection area. 
           [0014]      FIG. 7  shows a top view of a graphene-based photodetector array. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    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 practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein. 
         [0016]    The present invention is the use of single or multi-layer graphene as the photodetecting layer in a photodetector. Because of graphene&#39;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-field 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 made with 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 made with 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. 
         [0017]    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 can be SiO2 or any dielectric material. Drain  6  and source  8  contacts are patterned on the gate oxide layer  12 . Between the drain  6  and source  8  contacts is a channel layer of graphene  14  that may be one or more layers thick. 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. 
         [0018]    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 VG, good detection efficiency can be obtained because a graphene p-n junction is formed at the source (or drain)—graphene interface. 
         [0019]      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. 
         [0020]    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. Source and drain regions  30  and  32 , respectively are deposited on single or few (2 to 5) layers of graphene  38 . A 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 . 
         [0021]    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 . 
         [0022]      FIG. 5  shows the split gate photodetection device as seen in  FIG. 3 , but with the addition of an optical waveguide  50 . 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  36 . 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. 
         [0023]      FIG. 6  shows another implementation of a graphene based photodetector. Interdigitated fingers  60 - 67  define multiple source regions and drain regions, which are patterned on top of graphene layer  70 . This implementation provides for a very sensitive photodetector because the effective photo-detection area can be greatly enhanced. Different source and drain metal 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 block the light absorption in source/graphene or drain/graphene contact to enhance the photocurrent generation. 
         [0024]    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 as 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. 
         [0025]    It is to be understood that the provided illustrative examples are by no means exhaustive of the many possible uses for my disclosure. 
         [0026]    From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this disclosure and, without departing from the spirit and scope thereof, can make various changes and modifications of the disclosure to adapt it to various usages and conditions. 
         [0027]    It is to be understood that the present disclosure is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims: