Abstract:
A planar, waveguide-based silicon Schottky barrier photodetector includes a third terminal in the form of a field plate to improve the responsivity of the detector. Preferably, a silicide used for the detection region is formed during a processing step where other silicide contact regions are being formed. The field plate is preferably formed as part of the first or second layer of CMOS metallization and is controlled by an applied voltage to modify the electric field in the vicinity of the detector&#39;s silicide layer. By modifying the electric field, the responsivity of the device is “tuned” so as to adjust the momentum of “hot” carriers (electrons or holes, depending on the conductivity of the silicon) with respect to the Schottky barrier of the device. The applied potential functions to align with the direction of momentum of the “hot” carriers in the preferred direction “normal” to the silicon-silicide interface, allowing for an increased number to move over the Schottky barrier and add to the generated photocurrent.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/339,855, filed Mar. 10, 2010 and herein incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a silicon-based Schottky barrier infrared (IR) optical detector and, more particularly, to a planar, waveguide-based IR optical detector including an additional field plate to improve the responsivity of the detector. 
     BACKGROUND OF THE INVENTION 
     Semiconductor devices using metal-semiconductor barriers (referred to as Schottky barriers) instead of p-n junctions have been developed to convert incident light into electrical energy. Silicon is often used as the semiconductor material in Schottky barrier photodetectors operating in the IR portion of the electromagnetic energy spectrum. In its most conventional form, a silicon-based Schottky barrier photodiode consists of a thin metallic film (such as a silicide film) disposed on a silicon layer. Incident light is applied perpendicular to (i.e., “normal to”) this structure, passing through the relatively thin metallic film, where the thin film absorbs only a portion of the light, thus resulting in extremely low external quantum efficiency levels. As a result, conventional “normal incidence” photodetectors require a relatively large active detection area in order to collect a sufficient amount of optical energy to function properly. However, as the detection area increases, the dark current (unwanted noise signal) increases as well. Moreover, while relatively simple in structure, such normal incidence detectors typically require cooling, again associated with a relatively high dark current value. 
     Improvements in optical absorption and quantum efficiency in silicon-based Schottky barrier photodetectors have been the source of much investigation over the years. In one case, the optical absorption has been improved by inducing a surface plasmon mode at the metal-semiconductor interface, as disclosed in U.S. Pat. No. 5,685,919 issued to K. Saito et al. on Nov. 11, 1997. In this arrangement, a semicylindrical lens is disposed over the metallic layer and used to re-orient the incoming light from normal incidence to an angle associated with creating the surface plasmon layer. U.S. Pat. No. 4,857,973, issued to A. C. Yang et al. on Aug. 15, 1989 discloses an alternative Schottky barrier photodetector arrangement, where the photodetector is monolithically integrated with a single crystal silicon rib waveguide and positioned to absorb the “tail” of the optical signal as it passes along the rib waveguide underneath a silicide layer. While an improvement in absorption efficiency may be achieved with the Yang et al. structure, significant losses remain in terms of scattering losses along the sidewalls of the rib waveguide structure inasmuch as the rib is created by partially removing portions of a relatively thick silicon layer. Moreover, significant difficulties remain in terms of controlling the dimensions (particularly the height), as well as the smoothness, of such a rib waveguide structure. Indeed, the implementation of a “rib” structure (particularly with sub-micron dimensions) is extremely difficult with CMOS-based conventional processing technologies. Further, the non-planar geometry of the Yang et al. structure is not considered as a preferred arrangement from a manufacturing point of view, particularly in terms of the reliability and robustness of the design. 
     An exemplary prior art silicon-based photodetector that is compatible with conventional CMOS processing is described in U.S. Pat. No. 7,358,585, issued to V. Patel et al. on Apr. 15, 2008 and assigned to the assignee of this application. In the Patel et al. structure, a silicide layer (or other appropriate metallic layer) is disposed over a planar silicon waveguide layer formed as a sub-micron thick surface layer of a “silicon-on-insulator” (SOT) structure (this sub-micron surface waveguide layer often referred to in the art as the “SOT layer”). Ohmic contacts are applied to both the planar SOT layer of the SOT structure and the silicide layer. An optical signal propagating laterally along the optical waveguide within the planar SOT layer will thus pass under the silicide layer, where the “tail” of the optical energy will intercept the silicide and be converted into electrical energy. Since the arrangement of Patel et al. is based on implementing a silicide detector on a planar silicon surface and does not require the formation of a single crystal silicon rib waveguide, significant improvements in efficiency over the structure of Yang et al. can be realized, while also being compatible with conventional planar CMOS processing technologies. 
     While considered to be an advance over existing devices, the planar, waveguide-based structure of Patel et al. has been found to be somewhat limited in its responsivity, associated with the inherent properties of the silicide material itself. Inasmuch as silicide detectors were initially designed for use in power monitoring applications, responsivity and processing simplicity were not the concerns that they are today. 
     Thus, a need remains for a high speed silicon-based detector that remains compatible with standard CMOS processing, yet provides the responsivity required for use as a power monitor or feedback detector in high speed systems. 
     SUMMARY OF THE INVENTION 
     The need remaining in the prior art is addressed by the present invention, which relates to a silicon-based Schottky barrier infrared (IR) optical detector and, more particularly, to a planar, waveguide-based IR optical detector including an additional field plate to improve the responsivity of the detector. 
     In accordance with the present invention, a separate conductive region (i.e., “field plate”) is disposed over, and separated from the silicide layer forming the detector itself. Preferably, a conventional metal layer being used for other purposes (for example, the first layer of the standard metal conductor fabrication process) in a multi-layer metallization step in a CMOS fabrication process is used to create the field plate without needing to change any steps in the process. Moreover, it is preferred that the silicide layer used to form the detector itself is fabricated during the same processing step (i.e., defined as part of the same photolithography step) as other contact regions in the device. 
     A voltage is applied to the field plate to create an electric field in the vicinity of the detector&#39;s silicide layer. By creating the electric field, the responsivity of the device may be “tuned” so as to adjust the direction and momentum of “hot” carriers (electrons or holes, depending on the conductivity of the silicon) with respect to the Schottky barrier/silicon layer interface. The applied electric field functions to change the direction of movement of the “hot” carriers to align with the preferred direction “normal” to the silicon-silicide interface, allowing for an increased number of carriers to move over the Schottky barrier and add to the generated photocurrent. That is, the inclusion of a biased field plate provides the directional influence required to adjust the direction of the “hot” carriers so as to assist in their injection over the Schottky barrier. 
     In one embodiment, the actual voltage applied to the field plate is adjusted to modify the directional influence applied to the momentum of the carriers, where the adjustments may also be such that an “opposite” directional influence is applied (which decreases the generated photocurrent). The ability to “tune” the responsivity with an applied voltage is useful in matching the performance of the photodetector to other components in the system. 
     It has also been discovered that the thickness of the silicide used to form the photodetector plays a relatively small role in device performance, where a silicide layer of thickness up to 500 Å can be used within significant degradation in performance. Thus, it is been found that the detector silicide can be formed during the conventional fabrication process used to form other silicide regions (such as contact regions) and may have the same thickness and microstructure as these other silicide regions and still provide adequate performance as a power monitor detector. This approach is considered to significantly reduce the overall processing complexity of the detector fabrication process, since the same photolithography step and processing steps are used to form the detector silicide layer and the other contact regions. 
     Other and further embodiments and features of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings, where like numerals represent like parts in several views: 
         FIG. 1  is a band diagram associated with the operation of a prior art, normal incidence, Schottky barrier photodetector; 
         FIG. 2  is a cut-away side view of an exemplary prior art SOI-based silicon photodetector; 
         FIG. 3  is a diagram of the solid angle associated with the escape of hot electrons in moment space; 
         FIG. 4  is a cut-away side view of an exemplary SOI-based Schottky barrier photodetector of the present invention, incorporating a field plate over the silicide layer; 
         FIG. 5  is a plot of the change in responsivity of the inventive detector, illustrating the ability to “tune” the responsivity as a function of voltage applied to the field plate. 
     
    
    
     DETAILED DESCRIPTION 
     As described above, the present invention is directed to a silicon-based Schottky barrier detector including an additional electrode (i.e., “field plate”) to apply an electric field across the silicide region of the detector so as to increase the movement of “hot” carriers (either electrons or holes, depending on the conductivity of the silicon; hereinafter described primarily as “hot” electrons) over the Schottky barrier and improve the responsivity of the device. The presence of the electric field will preferentially direct “hot” electrons in the manner required to cross the Schottky barrier (i.e., in a direction normal to the silicide-silicon interface) and thus increase the number of carriers that actually cross the barrier and contribute to the photocurrent. Advantageously, the fabrication process and materials of the inventive detector remain compatible with conventional, planar CMOS process technology associated with the semiconductor industry. 
     In accordance with the present invention, a Schottky barrier junction is formed on the sub-micron thick, planar silicon waveguiding surface layer (“SOT layer”) of an SOI structure. This Schottky barrier junction is formed in such a way that the optical signal propagating laterally along a waveguide within the planar SOI layer (traveling in the direction parallel to the junction) is continuously absorbed into the overlying silicide layer as it passes thereunder. As a result of this geometry, even a relatively thin silicide layer (e.g., a few monolayers thick) will absorb a large portion of the signal over a distance of a few microns. 
     The field plate layer (preferably fabricated as a metal region of a conventional CMOS multi-layer metallization layer) is positioned above the detector silicide layer and is controlled by an applied voltage to create an electric field across the detector silicide layer. Creating this electric field has been found to modify the direction of the “hot” electrons, resulting in a higher or lower (depending upon the polarity of the applied field) number of “hot” electrons crossing the Schottky barrier and generating a larger (or smaller) photocurrent or responsivity. As will be described in detail below, the responsivity of the detector can be increased or decreased (in general, “tuned”), depending on the polarity of the voltage applied to the field plate layer with respect to the voltage potential between the anode and cathode terminals of the photodetector itself. 
     The basic operation of a conventional, prior art “normal incidence” Schottky-barrier infrared photodetector is first described so as to form a basis for distinguishing the subject matter of the present invention. Referring to  FIG. 1 , an energy band diagram is illustrated to describe the operation of such a conventional Schottky-barrier photodetector. The incoming infrared radiation is incident in a normal incidence manner on a silicide layer and results in the excitation of a photocurrent across the Schottky barrier (defined as having a barrier height φ ms ) by internal photoemission into an underlying silicon layer. In the arrangement of  FIG. 1 , it is presumed that the silicon layer comprises an n-type silicon material. Since silicon itself is transparent to IR radiation with a photon energy less than the band gap of silicon (1.12 eV), the silicon layer does not contribute to the photocurrent by absorption of infrared photons (through the creation of electron-hole pairs). Particularly, the infrared photons are absorbed into the silicide layer by transferring their energy and momentum to the free carriers. Indeed, the “hot” electrons (or “hot” holes when using a p-type layer) that have sufficient energy, as well as the requisite directional properties, will go over the Schottky barrier and be injected into the silicon layer, leaving a net positive (negative) charge on the silicide electrode. 
     In order to improve the performance of the Schottky barrier IR detector to detect radiation in the 1.10-1.65 μm band of interest for optical communications, the previously-cited Patel et al. arrangement proposed the use of a lateral incidence photodetector in place of the conventional normal incidence photodetector structure, the lateral incidence photodetector formed as an integral part of an SOI structure including a sub-micron surface layer waveguide. 
       FIG. 2  contains a side view of an exemplary Schottky barrier IR photodetector arrangement  10  formed in accordance with the Patel et al. design. Photodetector  10  comprises a conventional “silicon-on-insulator” (SOI) structure, including a silicon substrate  12 , insulator layer  14  (usually of SiO 2 ) and a planar silicon surface layer  16  (also referred to as SOI layer  16 ), where SOI layer  16  is preferably formed to have a thickness of less than one micron and is used to support the propagation of an optical mode therealong, as shown in  FIG. 2 . It is to be understood that SOI layer  16  may comprise either a planar single crystal silicon or a planar crystalline silicon (or Si—Ge) layer that has been processed to be in a “strained” lattice state (the strained silicon layer exhibiting a higher mobility rate for the carriers by reducing the mean free path length L). 
     In order to form the metal-semiconductor interface required for a Schottky barrier, a silicide strip  18  is disposed along a portion of top surface  20  of planar SOI layer  16 . In practice, any one of the silicides that forms an appropriate Schottky barrier for the wavelength to be detected may be used in the formation of this structure, since the ability to form a silicide on a silicon surface is generally understood in the planar CMOS processing industry. Accordingly, silicides based on cobalt, nickel, molybdenum, tantalum, tungsten and titanium are the most desired silicide layers for telecommunications application (and are also compatible with CMOS processes). 
     Silicide strip  18  may be formed as a single crystal (which is possible with some silicides), or as a polycrystalline material. For a polycrystalline silicide strip, scattering from grain boundaries plays a role (in association with the strip thickness) in determination of the “gain” factor of the detector. In this case, the processing conditions can be controlled, using well-known means, to optimize the grain formation in the silicide. 
     A first electrical contact  22  is made to silicide strip  18  so as to form a first electrode of photodetector  10 . A second electrical contact  24  is made directly to planar SOI layer  16  along its top surface  20 , where first and second contacts  22 ,  24  are illustrated in  FIG. 2 . Thus, as an optical beam propagates along planar SOI layer  16 , the “hot” electrons injected from silicide strip  18  will result in the generation of a photocurrent between first electrical contact  22  and second electrical contact  24 , where a measurement of this photocurrent can then be used as an indication of the optical power of the propagating lightwave signal. 
     As a result of utilizing this waveguide-based structure for the IR photodetector, it is possible to implement a device that is capable of operating at (or even above) room temperature (which is problematic with the prior art normal incidence detectors). Responsivity in general is a measure of quantum efficiency of the device measured by determining the ratio of generated photocurrent to incoming optical power. More particularly, responsivity R can be defined by the following relation: 
               R   =     C   ⁢           ⁢   1   *       [     1   -     (       λ   *     ϕ   ms       1.24     )       ]     2         ,         
where C1 is a known constant, λ is the free space wavelength of the propagating signal and φ ms  is the energy of the Schottky barrier.
 
     This limitation can be understood with reference to  FIG. 3 , which is a depiction of the solid angle required for the escape of “hot” electrons into momentum space. The quantities of energy and moment are both illustrated in  FIG. 3 . While a large number of “hot” electrons are created in the first instance upon incidence of photons on the silicide, only a fraction of the “hot” electrons with energy greater than φ ms  and momentum in a direction nearly normal (perpendicular) to the silicide-silicon interface can cross the Schottky barrier. The diagram in  FIG. 3  illustrates vectors associated with both the energy E 1/2  and momentum φ ms   1/2  of the “hot” electrons, where the shaded region denotes the small portion of these electrons in the “nearly normal” region that actually contribute to the photocurrent by having their momentum in a direction nearly normal to the interface. As shown, this “nearly normal” region is associated with a small range of angular distribution around the normal direction. 
     The responsivity of a photodetector, when defined as the ratio of generated photocurrents/input optical signal power, is clearly associated with the number of hot electrons meeting this “nearly normal” angular directional criterion (defined by the quantity 2πr 2 (1−cos θ)), shown as the shaded region of  FIG. 3 . Merely increasing the energy imparted to these carriers is not sufficient to increase the generated photocurrent (and hence responsivity of the detector), unless the direction of momentum is also proper; that is, along a direction nearly normal to the interface between the silicon and silicide. In accordance with the present invention, therefore, an external electric field is applied to the silicide region of the detector, using the field plate configuration, to influence the direction of momentum of the “hot” electrons. 
       FIG. 4  illustrates an exemplary silicon-based photodetector  30  formed in accordance with the present invention that addresses these limitations of the Patel et al. device. The elements that correspond to the prior art Patel et al. structure carry the same reference numerals. As shown in  FIG. 4 , photodetector  30  further comprises a field plate  32  disposed above and separated from silicide strip  18 . A dielectric region  34  separates field plate  32  from silicide strip  18 . Advantageously, field plate  32  is easily incorporated into the standard CMOS process used for the fabrication of opto-electronic devices, where field plate  32  can be incorporated into the processing steps associated with the fabrication of the standard “metal 1” or “metal 2” layers. Moreover, as mentioned above, it has been discovered that silicide strip  18  may itself be fabricated during a conventional set of process steps without the need to “modify” the process to create an ultra-thin silicide strip. This additional realization further simplifies the overall device fabrication process. 
     In accordance with the present invention, a voltage V F  is applied to field plate  32 , which will induce an electric field across silicide strip  18 . The presence of the electric field will, as discussed above, influence the direction of the momentum of the “hot” electrons and allow for a greater number of “hot” electrons to escape over the Schottky barrier and increase the generated photocurrent. The applied field across silicide strip  18  will preferentially shift the direction of momentum of these “hot” electrons in the direction normal to the silicon-silicide interface, the desired direction for movement of the carries over the barrier. 
     It is to be noted that the vertical separation S between field plate  32  and silicide strip  18  influences the responsivity of the device. In particular, the closer field plate  32  is to silicide strip  18 , the less applied voltage is needed to create the same strength electric field. However, if field plate  32  is positioned too close to the waveguiding structure, it will absorb a portion of the propagating signal, reducing the sensitivity of photodetector  30 . As mentioned above, a preferred metal layer to be used as field plate  32  is the “metal 1” layer of a standard CMOS process; however, a “metal 2” layer can also be used. The spacing between silicide strip  18  and field plate  32  is preferably on the order of 0.2-2.0 μm, with a preferred minimum spacing on the order of about 0.4 μm. 
       FIG. 5  is a graph of the generated photocurrent as a function of bias voltage (V C −V A ) for three different values of V F . In plot A, V F &lt;V A ; in plot B, V F =V A ; and in plot C, V F &gt;V A . Plot B is associated with the prior art arrangement, where the voltage applied to field plate  32  does not create an electric field along silicide strip  18 . Referring to plot A, if voltage V F  is selected to be less than the voltage present at anode  24  (the contact to SOI layer  16 ), the created electric field along silicide strip  18  will “add” to the existing field and thus adjust the moment of additional “hot” electrons to be in a direction essentially normal to the material interface. These additional “hot” electrons will then escape over the Schottky barrier and add to the created photocurrent. 
     As shown in  FIG. 5 , by adjusting the value of V F , it is possible to “tune” the influence of the applied electric field on the movement of “hot” electrons over the barrier. In fact, it is possible to apply a voltage to field plate  32  that actually subtracts from the nominal value and reduces the responsivity of photodetector  30 . Decreasing the responsivity may be desired in situations where there is a need to “match” the responsivity to other system components. 
     In general, by virtue of the application of a tunable voltage to field plate  32 , it is possible to tune the responsivity of the photodetector of the present invention. The tuning may be performed initially upon fabrication to provide a desired responsivity value, or may exist in the finished device as a ‘tunable’ element that is capable of being adjusted over time in various installations. 
     While the inventive photodetector has been described in an embodiment where “hot” electrons are the carriers responsible for the generation of the photocurrent, it is to be understood that “hot” holes may be the carriers creating the photocurrent in an arrangement where p-doped silicon is utilized (and the polarity of the voltage applied to the field plate adjusted accordingly). 
     In light of all of the above, therefore, the present invention is intended to be limited only by the scope of the claims appended hereto.