Abstract:
In a method for adjusting the sensitivity of a photodetector, the bandgap of the photodetection material is adjusted by inducing strain in the photodetection material. Such adjustments can be made in situ and continuously, in a reproducible and repeatable manner. In embodiments of the method, the photodetection material is graphene, carbon nanotubes or graphene nanoribbon. The use of graphene permits a dynamically-adjustable sensitivity over a dynamic range of radiation having wavelengths of 1.38 microns or less, up to at least 60 microns. In an adjustable photodetector, a graphene layer is suspended over a silicon substrate by a layer of an insulating material. Adjusting the voltage across the graphene layer and the silicon substrate induces strain in the graphene layer by electrostatic attraction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 61/422,399 filed on Dec. 13, 2011, which is incorporated by reference herein in its entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    Not applicable. 
       FIELD OF THE INVENTION 
       [0003]    The present invention relates to photoelectronic devices, and more particularly to bandwidth tuning of photodetectors. 
       BACKGROUND OF THE INVENTION 
       [0004]    Photodetectors comprise layers of photosensitive materials that can detect incident light of certain photon energies (which may also be expressed as wavelengths) related to the bandgap of the material. A bandgap is defined as the gap, expressed as an energy, between the lowest point of the conduction band and the highest point of the valence band of the material&#39;s electron energy dispersion relation (E-k) diagram, an example of which is shown as  FIG. 1 . Absorption of photons having at least the bandgap energy excites electrons from the valence band energy to the conduction band energy, thereby producing a photocurrent which can then be measured. 
         [0005]    In existing photodetector technology, various materials having different bandgaps are used to detect different spectral ranges of incident electromagnetic radiation. However, the bandgaps, and thus the detectable range of photon energies, are fixed for the materials that are used in the device. This presents a profound limitation of the usefulness of the photodetector device. 
       SUMMARY OF THE INVENTION 
       [0006]    In a method according to the present invention, the sensitivity of a photodetector is adjusted by inducing strain in the photodetection material, thereby altering its bandgap. In embodiments of the invention, the photodetection material may be graphene layers, carbon nanotubes or graphene nanoribbons. The use of graphene as a photodetection material permits a dynamically adjustable sensitivity to incident photons. In an embodiment of the method, strain is induced in the graphene layer by an electrostatic actuator. 
         [0007]    In a photodetector according to the present invention, a photodetection material is suspended over an electrically-conductive substrate by a layer of insulating material. An opening in the insulating layer exposes the graphene to the substrate. A voltage is applied across the graphene layer and the substrate. Adjusting the voltage varies the strain induced in the graphene layer, changing the bandgap of the graphene and, thus, the sensitivity of the photodetector to photons of different energies. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    For a better understanding of the present invention, reference is made to the following detailed description of an exemplary embodiment considered in conjunction with the accompanying drawings, in which: 
           [0009]      FIG. 1  is an exemplary electron energy dispersion relation (E-k) diagram; 
           [0010]      FIG. 2A  is a schematic diagram of an adjustable bandgap-tunable photodetector according to an embodiment of the present invention, with an unstrained layer of graphene as the photodetection material; 
           [0011]      FIG. 2B  is an exemplary E-k diagram for the graphene layer of  FIG. 2A ; 
           [0012]      FIG. 3A  is a schematic diagram of the photodetector of  FIG. 2A  wherein the graphene layer is strained; 
           [0013]      FIG. 3B  is an exemplary E-k diagram for the graphene layer of  FIG. 3A ; 
           [0014]      FIG. 4A  is a schematic diagram of the photodetector of  FIG. 2A  wherein the graphene layer is strained to a greater degree than is represented in  FIG. 3A ; 
           [0015]      FIG. 4B  is an exemplary E-k diagram for the graphene layer of  FIG. 4A ; 
           [0016]      FIG. 5  is a graph showing the spectral ranges of conventional photodetection materials and the spectral range of graphene; 
           [0017]      FIGS. 6-10  are schematic diagrams illustrating a sequence of steps in fabricating an adjustable bandgap-tunable photodetector of the same general type as the photodetector of  FIG. 2A ; 
           [0018]      FIG. 11  is a photograph of a photodetection circuit prepared according to the principles of the present invention; and 
           [0019]      FIG. 12  is a plot of photocurrents generated by the photodetection circuit of  FIG. 11  under three different degrees of induced strain. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    The present invention provides methods and devices for active in-situ tuning of the bandgap of a photodetector device by inducing strain in the photodetection material. It has particular applicability to materials such as graphene, carbon nanotubes (CNT) or graphene nanoribbons (GNR), but may also be applied to other photodetection materials having electron valence bands that may be altered by inducing strain in the material. The spectral sensitivity of a photodetector that uses such materials can be continuously modulated to detect a wide range of photon energies or wavelengths (A) of the incident light while the device is in operation. The modulation method is hereinafter referred to as “active bandgap tuning” (ABT). Strain induction in graphene, CNT, GNR or other materials can be achieved actively using conventional MEMS actuation techniques, including and not limited to electrostatic actuation, pneumatic actuation or thermal actuation. 
         [0021]    The ABT technique can be advantageously used with graphene as the photodetection material, as discussed hereinbelow with regard to the exemplary embodiments.  FIGS. 2A ,  3 A and  4 A show a schematic representation of an electrostatically-actuated graphene-based ABT photodetector  10  with different sized bandgaps. The photodetector  10  includes a graphene layer  12  supported on an electrically-insulated material, such as silicon oxide layer  14  on an electrically-conductive substrate, such as silicon substrate  16 . The graphene layer  12  is suspended over a trench  18  in the silicon oxide layer  14 . A source electrode  20  and a drain electrode  22  are in electrical contact with the graphene layer  12 . Strain is applied electrostatically to the graphene layer  12  by applying an actuation voltage (V ACT ) (not shown) across the silicon substrate  16  and the drain electrode  22 . A source-drain voltage (V SD ) (not shown) is applied across the source and drain electrodes  20 ,  22 . 
         [0022]      FIGS. 2B ,  3 B and  4 B are electron energy dispersion relation (E-k) diagrams related to  FIGS. 2A ,  3 A and  4 A, respectively. In its unstrained state ( FIGS. 2A and 2B ), graphene exhibits a “zero” bandgap energy band structure at room temperature (i.e., at roughly 300 K), and generates a photocurrent I ph1  at any wavelength λ 1  of incident light. As V ACT  is increased, the graphene layer  12  is strained and the bandgap opens ( FIGS. 3A and 3B ). A photocurrent I ph2  is generated at a wavelength λ 2  having an energy that is equal to or greater than the bandgap energy of the strained graphene layer  12 . As V ACT  is further increased, the graphene layer  12  is further strained and the bandgap opens more, such that incident light at a wavelength λ 3 , having an energy that is greater than that of wavelength λ 2 , is needed to generate a photocurrent I ph3  ( FIGS. 4A and 4B ). 
         [0023]    Upon induction of strain, the bandgap of graphene can be opened up to at least 0.9 eV, although larger bandgaps are theoretically possible. Graphene also exhibits photodetection capability in the infrared (“IR”) spectral range. Thus, the ABT technique combines bandgap tunability of graphene from 0 to at least 0.9 eV (A 1.38 μm) with its photodetection capabilities to detect IR wavelengths from less than 1.38 μm to a high upper limit as the bandgap approaches “zero.” However, 14 μm may be the practical upper limit considering the limitations of the IR transmission band of atmosphere at sea level. Further, a graphene-based ABT photodetector would be operable at common environmental temperatures and at cryogenic temperatures.  FIG. 5  is a graph comparing the theoretical spectral range of graphene at 300 K with the spectral ranges and operating temperatures of conventional photodetection materials (e.g., Ge, InGaAs, InSb, CdTe, HgCdTe, PbS and PbSe). 
         [0024]    The exemplary embodiment of a graphene-based ABT photodetector  10  is electrostatically actuated. By controlling V ACT , the amount of strain and bandgap opening in the graphene layer  12  can be precisely controlled, thereby controlling the photoresponse of the photodetector  10 . Strain induction in materials such as graphene, CNT or GNR can also be precisely implemented by other well-established MEMS technology, such as pneumatic, piezoelectric or magnetic actuation, or by various mechanical structures such as, but not limited to, membrane, cantilever and/or fixed beam structures. The induction of strain in the graphene layer is repeatable and reproducible. 
         [0025]    In an embodiment of the present invention, an ABT photodetector of the same general type of photodetector  10 , may be prepared as illustrated sequentially in  FIGS. 6-10  and described hereinbelow. This exemplary method uses a graphene layer as a photodetection material. The method may be readily adapted by those having ordinary skill in the relevant arts to use CNT or GNR, or other available photodetection materials whose bandgaps may be controlled by inducing strain in the materials. Suitable photodetection materials are presently available from commercial sources. 
         [0026]    Referring to  FIG. 6 , a silicon oxide layer  24  is formed on a silicon substrate  26  using any of a number of well-known methods. A suitable thickness for the silicon oxide layer is 300 nm. The exposed surface  28  of the silicon oxide layer  24  is then cleaned, for example, by sequentially washing it with acetone, isopropyl alcohol (IPA), and de-ionized (DI) water. 
         [0027]    Referring to  FIG. 7 , a trench  30  is etched into the silicon oxide layer  24  so as to expose a surface  32  of the silicon substrate  26 . In some embodiments, it may be preferred to etch a hole pattern rather than a trench. Etching may be performed using photoresist methods. 
         [0028]    Referring to  FIG. 8 , a graphene layer  34  is transferred onto the surface  28  of the silicon oxide layer  24  such that it is suspended over the trench  30  and away from the exposed surface  32  of the silicon substrate  26 . 
         [0029]    Referring to  FIG. 9 , electrodes  36 ,  38  are defined on the graphene layer  34  to collect photo-excited carriers during the operation of the photodetector. In operation, one electrode  36  will serve as a source electrode  36  and the other electrode  38  will serve as a drain electrode  38 . Methods of defining metal electrodes, such as photoresist masking and e-beam evaporation, are well-known, but the range of suitable electrodes need not be limited to those types. 
         [0030]    Referring to  FIG. 10 , an electrical voltage V SD  is connected across the source and drain electrodes  36 ,  38  to create a circuit with the photodetection material (e.g., graphene layer  34 ). An actuation voltage V ACT  is connected across the drain electrode  38  and the silicon substrate  26 . 
         [0031]    In a proof-of-concept experiment, a photodetection circuit  40 , seen in  FIG. 11 , was prepared by transferring a graphene layer  42  onto a flexible polyimide (DuPont™ Kapton®) substrate  44 , and metal electrodes  46 ,  48  were defined on the graphene layer  42  and substrate  44 . The graphene layer  42  is not visible in  FIG. 11  because of its natural transparency, but its location is indicated by the reference arrow. The circuit  40  was then mounted on a stage (not shown) and fixed in position with an active area of the graphene layer  42  at the edge of the circuit  40  that was over the stage, and a portion of the circuit  40  opposite the active area of the graphene layer  42  extending off of the stage in a cantilevered fashion. A precise micro Z-stage with a probe needle was positioned in contact with the cantilevered portion of the circuit  40 . The height of the Z-stage was adjusted precisely by turns of its screw to move the cantilevered portion of the circuit  40  up or down, thus flexing the circuit and inducing strain on the graphene layer  42 . The circuit was then electrically connected to a Keithley® source meter (Keithly Instruments, Inc., Cleveland, Ohio) and lock-in amplifier across the electrodes  46 ,  48  to measure photocurrent. The graphene layer  42  was then irradiated with radiation having a wavelength of 532 nm from a green laser, while the height of the Z-stage was adjusted step-wise to move the cantilevered portion of the circuit  40  downward, flexing the circuit  40 . The photocurrent generated by the circuit was measured at each step as the source voltage was varied.  FIG. 12  is a plot of the measured photocurrents against voltage for each of three degrees of flexion. It can be seen that the photocurrent/voltage relationships are different for each degree of flexion. This demonstrates that the photoelectric response of the graphene layer  42  to incident light of a fixed frequency changes as the strain on the graphene layer  42  is varied. The decrease in photocurrent at higher degrees of strain is believed to be caused by an increase in the bandgap of the graphene layer  42 , since fewer charge carriers would be excited from the valence band to the conduction band. 
         [0032]    ABT photodetectors have numerous potential applications. Several examples of such applications are summarized hereinbelow. 
         [0033]    Tunable IR imaging: Currently-available IR imaging technologies provide IR sensing in a mid-wave window (about 1 to 5 μm) (MWIR) by using photodetection materials such as PbS and PbSe, and in a long-wave window (about 10 to 12 μm) (LWIR) using mercury cadmium telluride (MCT). Thus, there is still a range of IR greater than 12 μm which cannot be sensed readily using current technology. The use of graphene-based ABT photodetectors can extend the range of detectable wavelengths beyond 12 μm up to the far IR range. Although, as discussed above, there may be a practical upper limit of 14 μm because of the transmission band of atmosphere at sea level, imaging applications may be realized outside of the atmosphere. This extended ability for IR imaging can be used in immediate applications such as, but not limited to, deep space exploration, satellite imaging and surveillance, etc. 
         [0034]    Tunable phototransistors for data storage: ABT photodetectors can be used to adjust the characteristics of phototransistors by tuning the sensitivity of the phototransistor to a certain incident wavelength. It can be used in optical data storage or optical data reading devices. Thus, the ABT technique can be used in novel future applications related to tunable phototransistors. 
         [0035]    Optical Wheatstone bridge: An optical Wheatstone bridge with tunable photosensitive arms can be utilized not only for selective imaging, but also to perform selective signal detection. In an exemplary application of an optical Wheatstone bridge, one arm of the bridge includes a photodetector having a fixed spectral range and the opposite arm includes an ABT photodetector that is continuously adjusted to analyze the spectrum of a light source. 
         [0036]    Tunable solar energy harvesting devices: Using the ABT technique, a photoactive layer of graphene can be made sensitive to a desired wavelength of light, such as wavelengths in the IR range. This creates an opportunity to harvest energy from a wider spectrum of solar or other environmental radiation than is harvested using currently-available photocells. The ABT photocell can be tuned to take advantage of the energy spectra that are available under various environmental conditions (e.g., sunny skies vs. overcast skies; or changes from natural to artificial light). 
         [0037]    It will be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the scope of the invention as defined in the appended claims.