Abstract:
Avalanche photodiodes having special lateral doping concentration that reduces dark current without causing any loss of optical signals and method for the fabrication thereof are described. In one aspect, an avalanche photodiode comprises: a substrate, a first contact layer coupled to at least one metal contract of a first electrical polarity, an absorption layer, a doped electric control layer having a central region and a circumferential region surrounding the central region, a multiplication layer having a partially doped central region, and a second contract layer coupled to at least one metal contract of a second electrical polarity. Doping concentration in the central section is lower than that of the circumferential region. The absorption layer can be formed by selective epitaxial growth.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
       [0001]    This application claims priority to U.S. Patent Application Ser. No. 61/571,279, entitled “Ge/Si Avalanche Photodiode with an Undepleted Absorber for High Speed Optical Communication”, filed on Jun. 24, 2011, which is herein incorporated in its entirety by reference. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present disclosure relates to photosensitive devices. More particularly, the present disclosure relates to an avalanche photodiode. 
         [0004]    2. Description of Related Art 
         [0005]    An avalanche photodiode (APD) is a type of photosensitive semiconductor device in which light is converted to electricity due to the photoelectric effect coupled with electric current multiplication as a result of avalanche breakdown. APDs differ from conventional photodiodes in that incoming photons internally trigger a charge avalanche in APDs. Avalanche photodiodes are typically employed in laser rangefinder applications and long-range fiber optic telecommunication applications. 
         [0006]    One of the parameters that impact the applicability and usefulness of APDs is dark current, which is a type of relatively small electric current that flows through a photosensitive device, such as a photodiode, even when no photons are entering the photosensitive device. Dark current is one of the main sources of noise in photosensitive devices. Consequently, dark current is a limiting factor for Ge/Si APDs in high-speed optical communication applications. 
         [0007]    There is, therefore, a need for a novel and non-obvious design of APDs that reduces the effect of dark current to achieve high performance. 
       SUMMARY 
       [0008]    The present disclosure provides APDs having special lateral doping concentration that reduces the dark current without causing any loss of optical signals to achieve high device performance and methods of their fabrication. 
         [0009]    In one aspect, an avalanche photodiode may comprise a substrate and a multi-layer structure disposed on the substrate. The multi-layer structure may comprise: a first contact layer coupled to at least one metal contact of a first electrical polarity; an absorption layer on which the first contact layer is disposed, the absorption layer absorbing photons of an optical beam incident on the multi-layer structure; an electric field control layer on which the absorption layer is disposed; a multiplication layer on which the electric field control layer is disposed, the multiplication layer configured such that an avalanche breakdown occurs in the multiplication layer in response to the absorption layer absorbing the photons of the optical beam; and a second contact layer on which the multiplication layer is disposed, the second contact layer coupled to at least one metal contact of a second electrical polarity. The absorption layer may be made of a first material. The electric field control layer may be made of a second material and aids distribution of an electric field inside the multiplication layer. A central region of the electric field control layer may be doped with a first type of dopant at a first level of concentration and a circumferential region of the electric field control layer surrounding the central region is doped with the first type of dopant at a second level of concentration higher than the first level. 
         [0010]    In some embodiments, a portion of the central region of the multiplication layer may be doped with a second type of dopant. The second type of dopant may be arsenic, phosphorous, or other n-type dopants for Si. 
         [0011]    In some embodiments, one or more of a size, a doping concentration, and a thickness of the central region of the multiplication layer may be controlled so that an electric field in the central region is higher than that of regions of the multiplication layer that surround the central region. 
         [0012]    In some embodiments, one or more of a size, a doping concentration, and a thickness of the central region of the electric field control layer may be controlled so that electric field entering into the absorption layer above the circumferential region of the electric field control field is minimized. 
         [0013]    In some embodiments, the absorption layer may be made of Ge or other III-IV materials including InGaAsP and InGaAs. 
         [0014]    In some embodiments, the electric field control layer may be made of Si, SiGeC or other Si alloys. 
         [0015]    In some embodiments, the first type of dopant may be boron, BF 2  or other p-type dopants for Si. 
         [0016]    In some embodiments, the multiplication layer may be made of Si. 
         [0017]    In some embodiments, the substrate may comprise a Si substrate or a silicon-on-insulator (SOI) substrate. 
         [0018]    In another aspect, a method of making an avalanche photodiode, may comprise: forming a second contact layer on a substrate, the second contact layer coupled to at least one metal contact of a second electrical polarity; forming a multiplication layer on the second contact layer; forming an electric field control layer on the multiplication layer, the electric field control layer doped with a first type of dopant; forming an absorption layer on the electric field control layer; and forming a first contact layer on the absorption layer, the first contact layer coupled to at least one metal contact of a first electrical polarity. 
         [0019]    In some embodiments, the method may further comprise doping a portion of a central region of the multiplication layer with a second type of dopant. 
         [0020]    In some embodiments, the absorption layer may be formed by selective epitaxial growth. 
         [0021]    In some embodiments, the selective epitaxial growth may comprise causing the selective epitaxial growth by using molecular beam epitaxy, chemical vapor deposition, or vapor phase epitaxy. 
         [0022]    In yet another aspect, an avalanche photodiode may comprise a substrate and a multi-layer structure disposed on the substrate. The multi-layer structure may comprise: a first contact layer coupled to at least one metal contact of a first electrical polarity; an absorption layer on which the first contact layer is disposed, the absorption layer absorbing photons of an optical beam incident on the multi-layer structure; an electric field control layer on which the absorption layer is disposed; a multiplication layer on which the electric field control layer is disposed, the multiplication layer configured such that an avalanche breakdown occurs in the multiplication layer in response to the absorption layer absorbing the photons of the optical beam; and a second contact layer on which the multiplication layer is disposed, the second contact layer coupled to at least one metal contact of a second electrical polarity. The absorption layer may be made of a first material. The electric field control layer may be made of a second material and aids distribution of an electric field inside the multiplication layer. A central region of the electric field control layer may be doped with a first type of dopant at a first level of concentration and a circumferential region of the electric field control layer surrounding the central region is doped with the first type of dopant at a second level of concentration higher than the first level. A portion of a central region of the multiplication layer may be doped with a second type of dopant. 
         [0023]    In some embodiments, one or more of a size, a doping concentration, and a thickness of the central region of the electric field control layer may be controlled so that electric field entering into the absorption layer above the circumferential region of the electric field control field is minimized. 
         [0024]    In some embodiments, one or more of a size, a doping concentration, and a thickness of the central region of the multiplication layer may be controlled so that an electric field in the central region is higher than that of regions of the multiplication layer that surround the central region. 
         [0025]    In some embodiments, the second type of dopant may be arsenic, phosphorous, or other n-type dopants for Si. 
         [0026]    In some embodiments, the absorption layer may be made of Ge or other III-IV materials including InGaAsP and InGaAs. 
         [0027]    In some embodiments, the electric field control layer may be made of Si, SiGeC or other Si alloys. 
         [0028]    In some embodiments, the first type of dopant may be boron, BF 2  or other p-type dopants for Si. 
         [0029]    In some embodiments, the multiplication layer may be made of Si. 
         [0030]    In some embodiments, the substrate may comprise a Si substrate or a silicon-on-insulator (SOI) substrate. 
         [0031]    These and other features, aspects, and advantages of the present disclosure will be explained below with reference to the following figures. It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the present disclosure as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure. The drawings may not necessarily be in scale so as to better present certain features of the illustrated subject matter. 
           [0033]      FIG. 1  is a cross-sectional view of a conventional APD. 
           [0034]      FIG. 2A  is a cross-sectional view of an APD in accordance with some embodiments of the present disclosure. 
           [0035]      FIG. 2B  is a cross-sectional view of an APD in accordance with an exemplary embodiment of the present disclosure. 
           [0036]      FIG. 3A  is a cross-sectional view of an APD in accordance with some embodiments of the present disclosure. 
           [0037]      FIG. 3B  is a cross-sectional view of an APD in accordance with an exemplary embodiment of the present disclosure. 
           [0038]      FIG. 4  is a chart showing test results of a conventional APD and an APD in accordance with the present disclosure. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Overview 
       [0039]    The present disclosure provides APDs having lateral doping concentration that reduces the effect of dark current without causing any loss of optical signals to achieve high device performance and method for their fabrication.  FIGS. 1-3  are not drawn to scale and are provided to convey the concept of the various embodiments of the present disclosure. 
       Exemplary Embodiments 
       [0040]      FIG. 1  is a cross-sectional view of a conventional APD. Referring to  FIG. 1 , a typical APD  100  has a substrate  110  made of silicon (Si) and a multi-layer structure  120  disposed on the substrate  110 . The multi-layer structure  120  includes a first-type contact layer  130  coupled to at least one first-type metal contact  135 , an absorption layer  140  made of germanium (Ge) on which the first-type contract layer  130  is disposed, an electric field control layer  150  made of first-type Si on which the absorption layer  140  is disposed, a multiplication layer  160  made of Si on which the electric field control layer  150  is disposed, and a second-type contact layer  170  made of second-type Si on which the multiplication layer  160  is disposed. The APD  100  has at least one second-type metal contact  175  coupled to the n-type contact layer  170 . The APD  100  further has an anti-reflection coating  180  that covers the multi-layer structure  120 . 
         [0041]    Representative APDs of the present disclosure are schematically shown in cross-sectional views in  FIGS. 2A ,  2 B,  3 A and  3 B. 
         [0042]      FIG. 2A  is a cross-sectional view of an APD  200  in accordance with an embodiment of the present disclosure. Referring to  FIG. 2 , the APD  200  may comprise a substrate  210  and a multi-layer structure  220  disposed on the substrate  210 . The multi-layer structure  220  may comprise: a first-type contact layer  230  coupled to at least one first-type metal contact  235 , an absorption layer  240  made of Ge on which the first-type contract layer  230  is disposed, an electric field control layer  250  made of first-type Si on which the absorption layer  240  is disposed, a multiplication layer  260  made of Si on which the electric field control layer  250  is disposed, and a second-type contact layer  270  made of second-type Si on which the multiplication layer  260  is disposed. At least one second-type metal contact  275  is coupled to the contact layer  270 . The APD  200  may further comprise an anti-reflection coating  280  that covers the multi-layer structure  220 . 
         [0043]    In some embodiments, a central region  252  of the electric field control layer  250  is doped with a first type of dopant at a first level of concentration, and a circumferential region  254  of the electric field control layer  250  surrounding, or encircling, the central region  252  is doped with the first type of dopant at a second level of concentration that is higher than the first level of concentration. In other words, the circumferential region  254  may be seen as a “guard ring” encircling the central region  252  and having a higher concentration of the first type of dopant than that of the central region  252 . The first type of dopant may be, for example, boron, BF2 or other p-type dopants for Si. 
         [0044]    In some embodiments, a central region  262  of the multiplication layer  260  is doped with a second type of dopant. The second type of dopant may be, for example, arsenic, phosphorous, or other n-type dopants for Si. 
         [0045]    In some embodiments, the absorption layer  240  comprise Ge or other III-IV materials, such as InGaAsP or InGaAs, which have a large lattice mismatch with the substrate  210 . 
         [0046]    In some embodiments, one or more factors, such as the size, doping concentration and thickness of the guard ring, or the circumferential region  254  of the electric field control layer  250 , are controlled to prevent or minimize charge carriers, or electric field, from moving into the absorption layer  240 . 
         [0047]    In some embodiments, one or more factors, such as the size, doping concentration and thickness of the partially doped region, or the central region  262 , of multiplication layer  260  are controlled so that the electric field in the central region  262  is higher than the electric field in other regions of the multiplication layer  260 , such as those regions of the multiplication layer  260  that surround the central region  262 . Since the central region  262  of the multiplication layer  260  is the main path of photo-generated carriers, it is necessary to maintain a high electric field in the central region  262  for the avalanche process to occur. This design keeps the electric field low in regions of the multiplication layer  260  that surround the central region  262  to avoid excessive noise during the avalanche process. 
         [0048]    In some embodiments, the substrate  210  is a Si substrate or a silicon-on-insulator (SOI) substrate. 
         [0049]      FIG. 2B  illustrates an exemplary embodiment of the APD  200 . In this embodiment, the substrate  210  is a silicon-based substrate, the first-type contact layer  230  is a p-type contact layer coupled to at least one p-type metal contact  235 , the absorption layer  240  is a blanket Ge absorption layer, the electric field control layer  250  is a p-type Si layer, the multiplication layer  260  is a Si multiplication layer, and the second-type contact layer  270  is an n-type Si layer coupled to at least one n-type metal contact  275 . 
         [0050]      FIG. 3A  is a cross-sectional view of an APD  300  in accordance with an embodiment of the present disclosure. Referring to  FIG. 3 , the APD  300  may comprise a substrate  310  and a multi-layer structure  320  disposed on the substrate  310 . The multi-layer structure  320  may comprise: a first-type contact layer  330  coupled to at least one first-type metal contact  335 , an absorption layer  340  made of Ge on which the first-type contract layer  330  is disposed, an electric field control layer  350  made of first-type Si on which the absorption layer  340  is disposed, a multiplication layer  360  made of Si on which the electric field control layer  350  is disposed, and a second-type contact layer  370  made of second-type Si on which the multiplication layer  360  is disposed. At least one second-type metal contact  375  is coupled to the contact layer  370 . The APD  300  may further comprise an anti-reflection coating  380  that covers the multi-layer structure  320 . 
         [0051]    The basic structure of the APD  300  is similar to that of APD  200  except that the absorption layer  340  is disposed on the electric field control layer  350  by selective epitaxial growth (SEG) which may comprise, for example, causing the selective epitaxial growth by using molecular beam epitaxy, chemical vapor deposition, or vapor phase epitaxy. 
         [0052]    In some embodiments, a central region  352  of the electric field control layer  350  is doped with a first type of dopant at a first level of concentration, and a circumferential region  354  of the electric field control layer  350  surrounding, or encircling, the central region  352  is doped with the first type of dopant at a second level of concentration that is higher than the first level of concentration. In other words, the circumferential region  354  may be seen as a “guard ring” encircling the central region  352  and having a higher concentration of the first type of dopant than that of the central region  352 . The first type of dopant may be, for example, boron, BF 2  or other p-type dopants for Si. 
         [0053]    In some embodiments, a central region  362  of the multiplication layer  360  is doped with a second type of dopant. The second type of dopant may be, for example, arsenic, phosphorous, or other n-type dopants for Si. 
         [0054]    In some embodiments, the absorption layer  340  comprise Ge or other III-IV materials, such as InGaAsP or InGaAs, which have a large lattice mismatch with the substrate  310 . 
         [0055]    In some embodiments, one or more factors, such as the size, doping concentration and thickness of the guard ring, or the circumferential region  354  of the electric field control layer  350 , are controlled to prevent or minimize charge carriers, or electric field, from moving into the absorption layer  340 . 
         [0056]    In some embodiments, one or more factors, such as the size, doping concentration and thickness of the partially doped region, or the central region  362 , of multiplication layer  360  are controlled so that the electric field in the central region  362  is higher than the electric field in other regions of the multiplication layer  360 , such as those regions of the multiplication layer  360  that surround the central region  362 . 
         [0057]    In some embodiments, the substrate  310  is a Si substrate or an SOI substrate. 
         [0058]      FIG. 3B  illustrates an exemplary embodiment of the APD  300 . In this embodiment, the substrate  310  is a silicon-based substrate, the first-type contact layer  330  is a p-type contact layer coupled to at least one p-type metal contact  335 , the absorption layer  340  is an SEG Ge absorption layer, the electric field control layer  350  is a p-type Si layer, the multiplication layer  360  is a Si multiplication layer, and the second-type contact layer  370  is an n-type Si layer coupled to at least one n-type metal contact  375 . 
       Exemplary Test Results 
       [0059]    The higher doping concentration in the central region  262 ,  362  of the multiplication layer  260 ,  360  can be achieved by: (i) depositing a thin layer (&lt;200 nm) of Si on wafers (thinner than the multiplication layer  260 ,  360 ), and implanting with masks to result in the higher concentration in the central region  260 ,  360 , and then depositing Si to reach the thickness of the multiplication layer  260 ,  360 ; or (ii) directly implanting (with masks) on wafers to make a higher concentration in the central region of the wafer surface, and depositing Si layer to form the multiplication layer  260 ,  360 . The formation of the guard ring, or circumferential region  254 ,  354  in the electric field control layer  250 ,  350  can be achieved by two rounds of implantations. The first implantation is to form the electric filed control layer  250 ,  350  using a conventional method. The second implantation (with masks) is to increase the doping concentration at certain regions, namely the guard ring, or circumferential region  254 ,  354 . 
         [0060]      FIG. 4  is a chart showing test results of a conventional APD and an APD in accordance with the present disclosure, such as the APD  200  of  FIG. 2  or the APD  300  of  FIG. 3 . The vertical axis represents current in units of ampere and the horizontal axis represents voltage in units of voltage. Curve (a) in  FIG. 4  is the dark current of an APD in accordance with the present disclosure. Curve (b) in  FIG. 4  is the dark currant of a conventional APD. Curve (c) in  FIG. 4  is the photo current of an APD in accordance with the present disclosure. Curve (d) in  FIG. 4  is the photo current of a conventional APD.  FIG. 4  demonstrates that the presence of a guard ring, such as the central region  252  of the electric field control layer  250  in the APD  200  or the central region  252  of the electric field control layer  250  in the APD  300 , can effectively decrease device dark current. Meanwhile, it does not cause any loss of optical signals. As a result, device performance is greatly improved. 
       CONCLUSION 
       [0061]    Although some embodiments are disclosed above, they are not intended to limit the scope of the present disclosure. It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed embodiments of the present disclosure without departing from the scope or spirit of the present disclosure. In view of the foregoing, the scope of the present disclosure shall be defined by the following claims and their equivalents.