Abstract:
An angle cavity resonant photodetector assembly ( 8 ), which uses multiple reflections of light within a photodetector ( 14 ) to convert input light into an electrical signal. The photodetector ( 14 ) has a combination of generally planar semiconductor layers including semiconductor active layers ( 20 ) where light is converted into an electrical output. The photodetector ( 14 ) is positioned relative to a waveguide ( 10 ), where the waveguide ( 10 ) has a waveguide active layer ( 22 ) located between a pair of waveguide cladding layers ( 24 ) and ( 26 ) and includes a first end ( 28 ) for receiving light and a second end ( 30 ) for transmitting the light to the photodetector ( 14 ). The photodetector ( 14 ) has a first reflector ( 12 ) and second reflector ( 16 ) that provides for multiple reflections across the semiconductor active layers ( 20 ). In another embodiment, the waveguide ( 60 ) is positioned on one side of a cavity ( 58 ) and the photodetector ( 64 ) is positioned at an opposite end of the cavity ( 58 ) such that the light from the waveguide ( 60 ) travels across the cavity ( 58 ). The photodectetors ( 64 ) is angled relative to the propagation direction of the light. The photodetector includes the first reflector ( 62 ) and the second reflector ( 68 ), which causes the light to pass through different areas of the photodetector active layers ( 72 ).

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     This invention relates generally to semiconductor photodetectors and, more specifically, to an angle cavity resonant semiconductor photodetector that is able to generate an electrical output for a specific range of light using a waveguide and multiple reflectors to create a resonance of light within the photodetector. 
     2. Discussion of the Related Art 
     High frequency, wide bandwidth photodetectors, such as PIN photodiodes, that are used in a variety of systems for the transfer of light as a primary means of transferring information are known in the art. These systems are especially needed for high-speed communication systems, such as automatic teller machines, computer network systems, and multimedia applications. 
     Photodetectors are used to convert optical energy into electrical energy. A photodiode is typically used for high-speed applications. In high-speed applications, the speed and the responsivity of the photodetector are critical. Although fiber optic cable can transmit at speeds of greater than 100 GHz, current technology photodetectors are limited to 45-60 GHz bandwidths. With the current explosion of multimedia technologies and applications, such as the Internet, the telecommunications industry will require higher bandwidth systems such as optical systems with high speed photodiodes. 
     In the typical photodiode, an active semiconductor material generates an electrical current by the photogenerated electrons within the active material. Responsivity and speed are two variables that are often used to determine the performance of photodetectors. Responsivity is the measure of the effectiveness of a device in converting incident light to an output current. Speed is the measure of how quickly an output of the device changes in response to a change in the input to the device. For a photodiode to be effective in high-speed communication applications, it must have both a high responsivity and a high speed. Current high speed photodiodes typically have a responsivity of 0.2-0.4 amps/watt and a top end speed of 45-60 GHz. To increase the responsivity of a photodiode, the thickness of the active area is often increased so as to increase the quantum efficiency, thus creating more output current. This creates a problem, however, because a thicker active area increases the transit time, which decreases the speed. 
     Current high speed photodiode design must incorporate a tradeoff between quantum efficiency and bandwidth. 
     Most communication applications that involve photodiodes also require an optical coupling device for guiding the light to the photodiode active area. Since the requirements of the optical coupling device are to deliver the incident light to a relatively small area, typically there are a minimum number of components and materials that are required to carry out this task. Due to the difference of materials and the number of optical components that are used in the optical coupling device, there tends to be a high optical loss in the coupling device that degrades the overall performance of the photodiode. 
     State of the art optical communication systems have carriers of very high frequency that require the use of high-speed, high-responsivity photodetectors. As the demand for more information increases, so will the demand that communication systems be able to transmit more information, which will in turn require high-speed, high-responsivity photodetectors. The known photodetectors for high frequency applications are limited by having a low responsivity and a limited high-end frequency response. It has been recognized that the effectiveness of a communications system could be increased by providing a photodetector that employs multiple reflections between a waveguide and reflectors to produce a high responsivity and high-speed photodetector. 
     It is an object of the present invention to provide a resonant photodetector that provides for an increased responsivity and speed, as well as providing other improvements, over the known photodetectors, to improve the performance of the communication process. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, an angle cavity resonant photodetector assembly is disclosed that utilizes multiple reflections within a photodetector to convert an optical signal to a corresponding electrical signal output. The angle cavity resonant photodetector assembly includes a plurality of semiconductor layers that combine to define a waveguide, a photodetector, and supporting structure and circuitry. 
     The waveguide provides a path to direct light to the photodetector from a light source. The waveguide includes a first end that is positioned to receive light from the light source, and reduce the number of optical components required to couple the light to the photodetector. The waveguide further includes cladding layers that refract the light propagating through the waveguide towards the photodetector to limit the amount of light that escapes from the waveguide. 
     The photodetector includes a plurality of semiconductor layers aligned at an angle relative to the propagation direction of the light traveling through the waveguide. The photodetector semiconductor layers include first and second reflectors opposite each other for providing multiple reflections of the light propagating through the photodetector active area. This use of multiple reflections within the photodetector increases the quantum efficiency and allows for a smaller active area to be used in the photodetector while retaining or increasing the efficiency of operation. 
     Additional objects, advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of one embodiment of the angle cavity resonant photodetector assembly of the present invention; 
     FIG. 2 is a sectional view of another embodiment of the angle cavity resonant photodetector assembly of the present invention; 
     FIGS. 3 a - 3   f  illustrates the steps in the fabrication of one embodiment of the angle cavity resonant photodetector assembly of the invention; and 
     FIGS. 4 a - 4   f  illustrates the steps in the fabrication of another embodiment of the angle cavity resonant photodetector assembly of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following discussion of the preferred embodiments of the present invention directed to a angle cavity resonant photodetector assembly is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. For example, the description of the photodetector of the invention will be described for a PIN photodiode. However, the angle cavity resonant photodetector assembly of the invention may have application for other types of photodetectors and associated circuitry. 
     FIG. 1 shows a cross-sectional view of an angle cavity resonant photodetector assembly  8 , according to the invention. The resonant photodetector assembly  8  includes a plurality of semiconductor layers fabricated to define a waveguide  10 , a photodetector  14 , and a substrate wafer  18 . A light beam  15  is collected by the waveguide  10  from a light source (not shown) and is directed to the photodetector  14  where it is absorbed, as will be discussed below. According to the invention, the photodetector  14  is positioned at an angle relative to the waveguide  10  and the propagation direction of the light beam  15  through the waveguide  10 . The light absorbed by the photodetector  14  is converted to an electrical output that is conveyed to various associated circuitry (not shown) depending on the particular application. 
     The waveguide  10  includes a waveguide active layer  22  surrounded by an upper waveguide cladding layer  24  and a lower waveguide cladding layer  26 . A first end  28  of the waveguide  10  receives the light beam  15  to be detected. The light beam  15  is directed down the waveguide active layer  22  by reflections off of the waveguide cladding layers  24  and  26  to a second end  30  of the waveguide  10 . The second end  30  of the waveguide  10  is positioned adjacent to the photodetector  14  so that the light beam  15  is directed from the waveguide active layer  22  into the photodetector  14 . 
     The photodetector  14  includes first and second reflector layers  12  and  16 , an n contact semiconductor layer  32 , an n contact metal  34 , a plurality of semiconductor active layers  20 , a p contact layer  36 , and a p contact metal  38 . The reflector layers  12  and  16  define the outside layers of the photodetector  14 , where the active layers  20  are positioned between the reflector layers  12  and  16 . The second end  30  of the waveguide  10  is positioned adjacent to the second reflector layer  16 , and the reflector layer  12  is positioned on an opposite side of the active layers  20  from the reflector layer  16 . The light beam  15  propagating down the waveguide  10  passes through the second reflector layer  16  and enters the semiconductor active layers  20 . The semiconductor active layers  20  absorb the light passing through the layers  20  to convert the absorbed light into an electrical signal. The light that is not absorbed by the semiconductor active layers  20  is reflected off of the first reflector layer  12 , and is directed back through the semiconductor active layers  20 . The unabsorbed light continues to be reflected between the first reflector layer  12  and the second reflector layer  16  at different locations through the active layers  20  until it is eventually absorbed by the semiconductor active layers  20 . As shown, the angled configuration of the photodetector  14  relative to the propagation direction of the light beam  15  through the waveguide  10  causes the light beam  15  to be reflected downwards off of the reflector layer  12  at each reflection point in this embodiment. Of course, the orientation of the photodetector  14  relative to the waveguide  10  can be different in different embodiments that would change the reflection direction of the beam  15 . The angled orientation of the photodetector causes the reflected beam  15  to travel through different portions of the active layers  20 . 
     Due to the maximum utilization of the light received from the waveguide  10  by the angle cavity resonant photodetector assembly  8 , a much smaller semiconductor active region is required to convert the light into an electrical signal than a conventional photodetector known in the art. Having a smaller active area increases the speed of the photodetector  14 . Having opposing reflectors  12 ,  16  causes the light to pass through the absorbing layers  20  multiple times. This increases the responsivity compared to photodetectors in the known art. In the present embodiment of the invention, the responsiveness of the photodetector  14  is above 1 amp/watt compared to the typical response of 0.2-0.4 amp/watt for a photodetector in the known art. Additionally, a greater bandwidth of light can be absorbed by the photodetector  14 . The photodetector  14  can convert light with a bandwidth greater than 100 GHz. Typical photodetectors known in the art can only convert light at bandwidths up to 45-60 GHz. 
     In the present embodiment of the invention, the substrate layer  18  is made of semi-insulated InP, the waveguide active layer  22  is composed of InGaAs and both the waveguide cladding layers  24  and  26  are composed of InP. However, as will be appreciated by those skilled in the art, other semiconductor materials that are suitable for light propagation can be used. To allow the light beam  15  to enter the photodetector  14  with minimal losses, the composition of the waveguide active layer  22  is made such that the index of refraction n of the waveguide active layer  22  matches the index of refraction n of the second reflector layer  16 . More specifically, using Afromowitz&#39;s equation, known to those skilled in the art, the index of refraction n of the waveguide active layer  22  is determined from terms, which are dependent on the composition of the waveguide active layer  22 . That is: 
     
       
           n   2 =1+E d /E o +E d E 2 /E o   3 +ηE 4 /πl n [(2E o   2 −E g   2 −E 2 )/(E g   2 −E 2 )]  (1) 
       
     
     In this equation, E o  is the alloy photon energy; E d  is the dispersive energy of the photons in the alloy; η is a mathematical term; E g  is the band gap energy; and E is the energy of light. Any suitable technique known in the art can, however, be used to match the active layer  22  to the reflector layer  16 . In the preferred embodiment of the invention, the layer  22  is In(x)Ga(1−x)As(y)P(1−y), where the value of x is 0.53 and the value of y is 1, thus yielding a composition of In(0.53)Ga(0.47)As for light with wavelength from 1.2 to 1.55 micrometers. 
     In one embodiment, the photodetector  14  is a PIN type photodiode, but other types of photodetectors may be used within the scope of the present invention. The semiconductor active layers  20  include two intrinsic InGaAs layers  42  and  46 . The first reflector layer  12  has a highly reflective gold mirror coating. The second reflector layer  16  is a semiconductor reflector, such as a Bragg reflector, and includes a series of alternating layers of n doped GaP and GalnP layers. To achieve maximum reflectivity, the reflector layer  16  has thirty-six alternating layers, although a different number of layers or composition may be used. The p contact layer  36  is a heavily p doped InGaAs layer and the n contact layer  32  is a heavily n doped InP layer. The p contact  38  is positioned on the gold mirror coating of the first reflector layer  12 . The n contact  34  is positioned on the uncovered portion of the n contact layer  32 . 
     FIG. 2 shows another embodiment of an angle cavity resonant photodetector assembly  54 , according to the present invention, that has similarities to the assembly  8  discussed above. The assembly  54  includes a waveguide having a waveguide active layer  56  positioned on one side of a cavity  58  and a photodetector  64  positioned on an opposite side of the cavity  58 . The photodetector  64  is the same as the photodetector  14  discussed above. A cavity end  60  of the waveguide active layer  56  transmits a light beam  70  across the cavity  58  that is received by the photodetector  64 . As discussed above for the photodetector  14 , the photodetector  64  includes a first semiconductor reflector layer  62 , a plurality of semiconductor active layers  72  and a second semiconductor reflector layer  68 . The reflector layers  62  and  68  are positioned on opposite sides of the active layers  72 , as shown. The reflector layer  62  is index of refraction matched to the air within the cavity  58  as much as possible, so that the light beam  70  enters the photodetector  64  through the reflector layer  62 , as shown, with minimal losses. The light that enters the photodetector  64  is absorbed by the active layers  72 . The light that is not absorbed by the active layers  72  is reflected off of the reflector layer  68  to give the unabsorbed light another chance to be absorbed by the active layers  72 . Multiple reflections off of the reflector layer  62  and  68  provide multiple chances for the light beam  70  to be absorbed by the active layers  72 , as shown. In this embodiment, the light beam  70  is reflected upwards off of the reflector layer  68  because of the orientation of the waveguide active layer  56  to the photodetector  64 . Since the reflector layer  62  is not positioned against the waveguide, additional sources of light may be directed to the photodetector  64 . This allows the photodetector  64  to receive light from multiple sources and to detect multiple wavelengths. 
     The formation of the angle cavity resonant photodetector assembly  8  of the present embodiment of the invention follows conventional techniques and methods, some of which are illustrated in FIGS. 3 a - 3   f  according to one embodiment. Referring to FIG. 3 a , the waveguide  10  is formed by the formation of the substrate wafer  18 , the lower waveguide cladding layer  26  of undoped InP with a thickness about 2 μm, the waveguide active layer  22  of undoped InGaAsP with a thickness about 4 μm, and the upper waveguide cladding layer  24  of undoped InP with a thickness about 3 μm. These layers are grown on the semiconductor supporting structure by conventional epitaxial growth processes, such as molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD). Referring to FIG. 3 b , the waveguide fabrication is performed by etching an angled cavity for subsequent photodetector crystal growth structure. Referring to FIG. 3 c , a layer of silicon dioxide or silicon nitride is deposited, for surface passivation, on the angle cavity resonant photodetector assembly  8  with the exception of the area reserved for the photodetector  14 . Referring to FIG. 3 d , a selective regrowth for the photodetector  14  is performed on the waveguide  10 . 
     The selective regrowth of the photodetector  14  includes forming the semiconductor reflector layer  16  having a number of alternating n-doped GaP and GaInP layers where the number and thickness of the layers are selected to provide the desired reflectivity; forming the n contact layer  32  of heavily doped InGaAs with a thickness of about 150 nm; forming the semiconductor active layers  20 , including forming the two InGaAs layers  42  and  46  with about 30 nm thickness and spaced about 150 nm apart. Referring to FIG. 3 e , an edge trim etch is performed on the photodetector  14  to open the n contact layer  32  for the n contact  34  connection. Referring to FIG. 3 f , the p contact  38  and the n contact  34  are formed with the p contact  38  consisting of Ti—Pt—Au and the n contact  34  consisting of Ni—AuGe—Ni—Au. A gold coating is applied to the p contact layer  36  on the photodetector  14 . The completely formed angle cavity resonant photodetector assembly  8  wafer is then cut as needed and tested to specification. 
     The fabrication procedure used for the embodiment of the invention as illustrated in FIGS. 3 a - 3   f  is the same fabrication procedure as used on the other embodiment of the invention as illustrated in FIGS. 4 a - 4   f  with the exception that, referring to FIG. 4 f , the semiconductor reflector layer  62  is applied to the p contact layer  74  on the photodetector  64  instead of the gold coating being applied to the p contact layer  74 . 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.