Abstract:
Various particular embodiments include a method for forming a photodetector, including: forming a structure including a barrier layer disposed between a layer of doped silicon (Si) and a layer of germanium (Ge), the barrier layer including a crystallization window; and annealing the structure to convert, via the crystallization window, the Ge to a first composition of silicon germanium (SiGe) and the doped Si to a second composition of SiGe.

Description:
TECHNICAL FIELD 
     The subject matter disclosed herein relates to integrated circuits. More particularly, the subject matter relates to a germanium (Ge) photodetector with a silicon-on-insulator (SOI) doping source. 
     BACKGROUND 
     Germanium (Ge) photodetectors are often used in a silicon photonics platform for receiving light from optical fibers or other on-chip light sources and converting the light to an electrical current. To receive optical signals with high signal to noise ratio, the dark current of a Ge photodetector is required to be low (typically ˜1 μA). To this extent, reduction of dark current in Ge photodetectors is desirable. 
     SUMMARY 
     A first aspect is directed to a method for forming a photodetector, including: forming a structure including a barrier layer disposed between a layer of doped silicon (Si) and a layer of germanium (Ge), the barrier layer including a crystallization window; and annealing the structure to convert, via the crystallization window, the Ge to a first composition of silicon germanium (SiGe) and the doped Si to a second composition of SiGe. 
     A second aspect is directed to a semiconductor structure, comprising: a barrier layer separating a first layer of silicon germanium (SiGe) and a second layer of SiGe; and a crystallization window in the barrier layer, wherein the first layer of SiGe contacts the second layer of SiGe in the crystallization window of the barrier layer. 
     A third aspect is directed to a photodetector, including: a semiconductor layer including a silicon (Si) portion and an n-type silicon germanium (SiGe) portion; a SiGe layer including an alternating sequence of n+-type SiGe and p-type SiGe; a barrier layer separating the semiconductor layer from the SiGe layer; a crystallization window in the barrier layer, wherein the SiGe layer contacts the n-type SiGe portion of the semiconductor layer in the crystallization window of the barrier layer; and a plurality of metal contacts to the SiGe layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various embodiments of the invention. 
         FIG. 1  depicts a germanium (Ge) photodetector structure, according to embodiments. 
         FIGS. 2A and 2B  depict examples of dark current measurements for a known Ge photodetector ( FIG. 2A ) and a Ge photodetector structure disclosed herein ( FIG. 2B ). 
         FIGS. 3-7  depict a process for forming a Ge photodetector, according to embodiments. 
         FIGS. 8A and 8B  depict contact arrangements for use with a Ge photodetector, according to embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     As noted, the subject matter disclosed herein relates to integrated circuits. More particularly, the subject matter relates to a germanium (Ge) photodetector with a silicon-on-insulator (SOI) doping source. 
       FIG. 1  depicts a germanium (Ge) photodetector structure  10 , according to embodiments. The Ge photodetector structure  10  is formed on a substrate (e.g. a substrate of an integrated circuit). The substrate includes a silicon (Si) handle wafer  12 , a buried oxide (BOX) layer  14 , and a silicon-on-insulator (SOI) layer  16 . A portion of the SOI layer  16  has been converted to n-type silicon germanium (SiGe)  18 . In embodiments, the n-type SiGe  18  has a small percentage of Ge. For example, the n-type SiGe may comprise about 50% Si and about 50% Ge, with a range of 1% to 70% Ge. 
     A silicon nitride (Si 3 N 4 ) barrier layer  20  is disposed between the SOI layer  16 /n-type SiGe  18  and a photodiode layer  22  of alternating n+/p doped SiGe. Typical n-type SiGe  18  and n+/p photodiode layer  22  dopant concentrations of about 1e 13  to about 1e 16  at/cm 2  and typical dopants of boron, arsenic, and phosphorous may be used. In embodiments, the layer  22  of n+/p doped SiGe has a small percentage of Si. For example, the layer  22  of n+/p doped SiGe may comprise about 99% Ge and about 1% Si, with a range of about 50% Ge to about 99% Ge. A crystallization window  24 , which will be described in greater detail below, is formed in the Si 3 N 4  layer  20 , where typical width dimensions of the window range from about 0.2 μm to about 1.0 μm, where larger windows can result in a larger range of germanium concentrations in n+/p SiGe  22 . A plurality of metal contacts  26  are formed through a dielectric layer  28  and a Si 3 N 4  layer  30  to the layer  22  of n+/p doped SiGe. 
       FIGS. 2A and 2B  depict examples of dark current measurements for a known Ge photodetector ( FIG. 2A ) and a Ge photodetector structure  10  according to embodiments ( FIG. 2B ). Comparing  FIG. 2B  with  FIG. 2A , the Ge photodetector structure  10  may provide, for example, a factor of 10 reduction of dark current. 
     An illustrative process for forming the Ge photodetector  10  is depicted in  FIGS. 3-7 . The following description assumes that the reader is skilled in the art of semiconductor processing. To this extent, a detailed description of the semiconductor processing techniques used in the formation of the Ge photodetector  10  will not be provided herein. 
     As shown in  FIG. 3 , a portion  16 ′ of the SOI layer  16  is selectively implanted with an n-type dopant  32 . Typical n-type dopant concentrations of about 1e 13  to about 1e 16  at/cm 2  and typical dopants of arsenic and phosphorous may be used. A mask  34  and an oxide layer  36  used during the doping process are subsequently removed to provide the structure shown in  FIG. 4 . 
     A Si 3 N 4  layer barrier  20  is deposited over the SOI layer  16 / 16 ′ as depicted in  FIG. 5 . A portion of the Si 3 N 4  layer  20  located above the n-type doped portion  16 ′ of the SOI layer  16  is selectively etched to form a crystallization window  24 . A layer  38  of intrinsic Ge is then deposited over the Si 3 N 4  layer  20 . The Ge layer  38  fills the crystallization window  24 , contacting the surface of the n-type doped portion  16 ′ of the SOI layer  16  through the crystallization window  24 . A protective encapsulation layer  40 , comprising, for example, one or more layers of Si 3 N 4  and silicon dioxide (SiO 2 ), is deposited over the Ge layer  38 . 
     A rapid thermal anneal (RTA) is then performed, where typical temperatures of between about 900° C. and about 1100° C. may be used. During the RTA, as depicted in  FIG. 6 , the intrinsic Ge layer  38  ( FIG. 5 ) melts and some of the Ge diffuses through the crystallization window  24  into the n-type doped portion  16 ′ of the SOI layer  16  (arrow A) to form an n-type SiGe  18 . The n-type SiGe  18  thus formed (which has a lower melting temperature than Si) also melts during the RTA. In embodiments, the n-type SiGe  18  may comprise about 50% Si and about 50% Ge. 
     During the RTA, the Si and the n-type dopant of the n-type doped portion  16 ′ of the SOI layer  16  also diffuse through the crystallization window  24  (arrow B) intermixing with the melted Ge layer  38  to form an n-type SiGe layer  42 . In embodiments, the n-type SiGe layer  42  has a graded percentage of Si. For example the n-type SiGe layer  42  may start with a higher level of Si (about 1% to about 50%) and grades to a very small percentage of Si (&lt;1%) over several microns of length. 
     After completion of the RTA, both the n-type SiGe  18  and the n-type SiGe layer  42  cool and recrystallize. As depicted in  FIG. 7 , the n-type SiGe layer  42  may then be implanted with n-type and p-type dopants  43  using one or more mask structures  44  to form a photodetector layer  22  of n+/p doped SiGe. Metal contacts  26 , as shown in  FIG. 1 , may then be formed to the layer  22  of n+/p doped SiGe. As depicted in  FIGS. 8A and 8B , the metal contacts  26  may be provided, for example, using a lateral contact geometry ( FIG. 8A ) or interdigitated contact geometry ( FIG. 8B ) to provide n+/n/p or n+/p junction designs. In both geometries, there is the option of using or not using an intrinsic region. 
     The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a”, “an” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     When an element or layer is referred to as being “on”, “engaged to”, “connected to” or “coupled to” another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to”, “directly connected to” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Spatially relative terms, such as “inner,” “outer,” “beneath”, “below”, “lower”, “above”, “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the example term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The foregoing description of various aspects of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously, many modifications and variations are possible. Such modifications and variations that may be apparent to an individual in the art are included within the scope of the invention as defined by the accompanying claims. 
     The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.