Evanescent coupling of photodiode with optical waveguide

Embodiments described herein include an apparatus comprising a semiconductor-based photodiode disposed on a semiconductor layer, and an optical waveguide spaced apart from the semiconductor layer and evanescently coupled with a depletion region of the photodiode. The photodiode may be arranged as a vertical photodiode or a lateral photodiode.

TECHNICAL FIELD

Embodiments presented in this disclosure generally relate to semiconductor-based optical devices.

BACKGROUND

Silicon-on-insulator (SOI) optical devices may include waveguides, optical modulators, detectors, CMOS circuitry, metal leads for interfacing with external semiconductor chips, and the like. Although crystalline silicon is excellent at forming waveguides with submicron dimensions, silicon tends to be a poor material for both generating and absorbing light at wavelengths used for optical communication. While III-V semiconductors may be better suited as photodetectors, these materials tend to be expensive and their fabrication techniques tend to be less compatible with Si fabrication processes.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Overview

One embodiment presented in this disclosure is an apparatus comprising a semiconductor-based photodiode disposed on a semiconductor layer, and an optical waveguide spaced apart from the semiconductor layer and evanescently coupled with a depletion region of the photodiode.

Another embodiment in this disclosure is an apparatus comprising a lateral photodiode disposed on a semiconductor layer, and an optical waveguide spaced apart from the lateral photodiode and top-coupled with a depletion region of the lateral photodiode.

Another embodiment in this disclosure is an apparatus comprising a vertical photodiode disposed on a semiconductor layer, and an optical waveguide spaced apart from the vertical photodiode and side-coupled with a depletion region of the vertical photodiode.

Example Embodiments

Embodiments of the present disclosure describe a photonic device that includes a photodetector (e.g., a semiconductor-based photodiode) coupled to one or more optical waveguides. In some embodiments, the semiconductor-based photodiode is disposed on a semiconductor layer (such as a SOI layer), and the photodiode is formed of semiconductor material(s) different than that of the semiconductor layer. For example, the semiconductor layer may be formed of crystalline silicon (e.g., a SOI layer), and the photodiode is formed of germanium, indium gallium arsenide (InGaAs), and so forth. Unlike silicon, germanium is an efficient photodetector material at wavelengths of optical signals that are typically used for optical data communication. Moreover, because single crystal germanium and single crystal silicon have closely matched lattice constants, single crystal germanium can be grown on crystalline silicon. In one embodiment, germanium is coupled to at least one optical waveguide that is not formed using the SOI layer, to form an optical detector.

While an optical waveguide can extend directly beneath the germanium to transfer the optical signal propagating through the optical waveguide into the germanium, doing so results in most of the optical signal being absorbed at an entrance facet of the germanium. However, the entrance facet of the germanium may have defects which contribute to dark current appearing in the resulting electrical signal. Further, the electrical field that is used to convert the optical signal into the electrical signal may be relatively weak at the entrance facet of the germanium. The absorption of light in the entrance facet with low electric fields typically results in an intensity-dependent reduction of speed or bandwidth of the optical detector.

Instead of directly coupling the optical waveguide to the germanium, in some embodiments, the optical waveguide is spaced apart from the germanium and light is coupled evanescently from the optical waveguide to the germanium in a controlled manner. In some embodiments, the optical waveguide is either top-coupled or side-coupled to the germanium. By changing the shape of the optical waveguide, changing the distance between the optical waveguide and the germanium, using multiple coupled optical waveguides (which may have different dimensioning), the interaction of the optical mode with the specific regions of the optical detector can be engineered. Doing so enables light to be absorbed only in the desired region(s) of the germanium detector in a controlled manner (e.g., in the high field region of the optical detector). This method avoids or minimizes absorption in the low field region and hence mitigating the intensity-dependent loss of bandwidth. As a result, the photonic device may be capable of supporting greater bandwidths (e.g., 60 gigahertz (GHz) or greater) at higher intensities of the optical signal.

FIG. 1is a block diagram of an exemplary apparatus100, according to one or more embodiments. The apparatus100comprises an optical detector135that is optically coupled with an optical device120.

The optical detector135comprises a photodiode105that is disposed on a semiconductor layer130. In some embodiments, the semiconductor layer130comprises a SOI layer (e.g., a silicon layer arranged above an oxide layer), although other types of semiconductor material(s) are also contemplated.

In some embodiments, the photodiode105is disposed on a surface of the semiconductor layer130(e.g., the photodiode105directly contacts a top surface of the semiconductor layer130). In some embodiments, some or all of the photodiode105extends into a recess formed from a surface of the semiconductor layer130. In other embodiments, the photodiode105may be arranged above the semiconductor layer130.

Each of the photodiode105and the semiconductor layer130is formed of one or more semiconductor materials. In some embodiments, the semiconductor layer130comprises silicon, and the photodiode105comprises a germanium photodiode, although other combinations of semiconductor materials are also contemplated. In one example, the semiconductor layer130comprises silicon, and the photodiode105is formed partly from silicon of the semiconductor layer130and partly from germanium.

The photodiode105may be implemented as a p-n photodiode having differently-doped regions arranged in a contacting relationship, or as a p-i-n photodiode having an undoped intrinsic region arranged between the differently-doped regions. The photodiode105may be implemented as a lateral photodiode having the different regions (e.g., the differently-doped regions and/or the undoped intrinsic region) arranged laterally, or as a vertical photodiode having the different regions arranged vertically.

The apparatus100further comprises one or more optical waveguides115that are evanescently coupled with a depletion region110of the photodiode105. In some embodiments, the depletion region110(which, in some cases, may be formed at least in the intrinsic region of the photodiode105) extends along a first dimension, and at least a portion of the optical waveguide115extends parallel to the depletion region110along the first dimension.

In some embodiments, the optical waveguide115is formed of silicon nitride or silicon oxynitride, but other suitable materials are contemplated (e.g., having suitable propagation characteristics at wavelengths used for optical communication, compatible with complementary metal-oxide-semiconductor fabrication processes used to form the apparatus100, and so forth). The optical waveguide115may be positioned with a predefined spacing relative to the semiconductor layer130and/or to the photodiode105. For example, the optical waveguide115may be formed at a predefined height relative to the top surface of the semiconductor layer130, e.g., spaced apart from the semiconductor layer130, with the height being controlled by the thickness of one or more dielectric layers (e.g., oxide layers). The dimensioning of the optical waveguide115may be changed to control the interaction of the optical mode of the optical waveguide115with specific regions of the photodiode105. The optical waveguide115may also be composed of multiple layers of waveguide layers.

The optical device120is external to the optical detector135and may have any suitable implementation for communicating optical signals with the optical waveguide115. In some embodiments, the optical device120comprises an optical fiber. In other embodiments, the optical device120may comprise an optical waveguide formed in a same layer as the optical waveguide115, an optical waveguide formed in the semiconductor layer130, or an optical waveguide of photonic circuitry external to the optical detector135. The photonic circuitry may be formed in the same semiconductor layer130as the optical detector135, or in separate waveguiding layer(s). Active and/or passive optical alignment techniques, which are not discussed in detail here but are known to the person of ordinary skill in the art, may be used to optically align the optical device120and the optical waveguide115. The optical device120is configured to transmit an optical signal125that is propagated by the optical waveguide115and evanescently coupled into the depletion region110of the photodiode105.

FIG. 2is a diagram200of an exemplary optical detector having a vertical photodiode215, according to one or more embodiments. More specifically, the diagram200provides a cross-sectional view of the optical detector. The features illustrated in the diagram200may be used in conjunction with other embodiments, e.g., representing one possible implementation of the optical detector135ofFIG. 1.

In the diagram200, a block220of germanium is disposed on, and directly contacts, a SOI layer205(one example of the semiconductor layer130ofFIG. 1). Other suitable semiconductor materials are also contemplated for the block220and the SOI layer205. The block220may be formed using any suitable techniques, e.g., epitaxially growing germanium onto a top surface235of the SOI layer205in selected regions using selective area epitaxy, and in some cases selectively etching or polishing the germanium to control the dimensions of the block220. As shown, the SOI layer205has a height h1and a width w1, and the block220has a height h2and a width w2. In some embodiments, the height h1is between about 0.05 microns and 0.25 microns, and the width w1is between about 5 microns and 10 microns, although other ranges are also contemplated. In some embodiments, the height h2is between about 0.5 microns and 1.5 microns, and the width w2is between about 0.25 microns and 10 microns, although other ranges are also contemplated.

Within the vertical photodiode215, an electrical contact225is created by doping the top of the block220of germanium with a first conductivity type (e.g., n-type or p-type). The electrical contact225is shown as extending to a top surface of the block220, which is opposite a bottom surface of the block220that contacts the top surface235of the SOI layer205. The electrical contact225is inset from a lateral extent of the block220by a width w3, which in some embodiments may be between about 0.2 microns and 1.5 microns.

The electrical contact225is more heavily doped than the bulk of the block220. In some embodiments, the bulk of the block220is undoped and represents an intrinsic region of a p-i-n implementation of the photodiode105. Although the depletion region110is shown as being formed partly in the intrinsic region, in alternate embodiments the depletion region110may be formed around the p-n junction of a p-n photodiode (e.g., with no intrinsic region).

In other embodiments, the bulk of the block220is doped with the first conductivity type, and is more lightly doped than the electrical contact225. Further, in cases where the bulk of the block220is doped, the bulk of the block220may be doped with a single doping level or with multiple doping levels. For example, the dopant concentration may be increased in a step-wise or a substantially continuous manner as the distance from the depletion region110and/or the optical waveguide115increases.

An electrical contact210of the SOI layer205is doped with a second, different conductivity type that is different than the first conductivity type of the electrical contact225(e.g., p-type or n-type). The electrical contact210is shown as extending to the top surface235of the silicon substrate. For illustration, the electrical contact210is positioned laterally outward from the lateral extent of the block220. However, arranging the electrical contact210in any peripheral region surrounding the block220is also contemplated. The electrical contact210is more heavily doped than the bulk of the SOI layer205. In some embodiments, the bulk of the SOI layer205is more lightly doped than the electrical contact210. The bulk of the SOI layer205may be doped with a single doping level or with multiple doping levels. For example, the dopant concentration may be increased in a step-wise or a substantially continuous manner as the distance from the depletion region110and/or the optical waveguide115increases. In other embodiments, the bulk of the SOI layer205is undoped.

Thus, in one exemplary implementation, the vertical photodiode215is a p-i-n photodiode in which the SOI layer205(including the electrical contact210) is doped p-type, the bulk of the block220of germanium is an undoped intrinsic region, and the electrical contact225of the block220is doped n-type. However, for all the embodiments where dopant type(s) are specified, the dopant types may be reversed—e.g., the SOI layer205may be doped n-type while the electrical contact225is doped p-type.

In the diagram200, a silicon nitride waveguide230(one example of the optical waveguide115ofFIG. 1) is spaced apart from the block220. Other suitable materials are also contemplated for the optical waveguide115. The silicon nitride waveguide230has a width w4, is spaced apart from a lateral extent of the block220by a width w6, and has a height h3relative to the top surface235. In some embodiments, the width w4is between about 0.5 microns and 1.5 microns, although other ranges are contemplated. Further, the width w4may remain constant along the length of the silicon nitride waveguide230, or may be varied (e.g., tapered) along the length of the silicon nitride waveguide230. In some embodiments, the width w6is between about 0.5 microns and 2 microns, and the height h3is between about 0.2 microns and 0.8 microns, although other ranges are contemplated.

In some embodiments, some or all of the widths w2, w3, w4, w6and the heights h2, h3are controlled such that the silicon nitride waveguide230is evanescently coupled with the depletion region110of the vertical photodiode215.

In the diagram200, electrical contacts245-1,245-2are formed in a single layer250(e.g., a metal layer) above the vertical photodiode215. A via240-1electrically couples the electrical contact245-1to the electrical contact210, and a via240-2electrically couples the electrical contact245-2to the electrical contact225. In some embodiments, the electrical contacts245-1,245-2and/or the vias240-1,240-2are metallic or formed from silicide. The electrical contacts245-1,245-2and the vias240-1,240-2may have any suitable dimensions. For example, the via240-2may have a width w7near the electrical contact245-2, which in some embodiments may be between about 1 micron and 10 microns.

Because the presence of electrically conductive materials may have an undesired effect on the performance of the optical detector, the electrical contacts245-1,245-2and the vias240-1,240-2may be arranged away from the depletion region110and/or the silicon nitride waveguide230. For example, the via240-2may extend in a vertical direction away from the vertical photodiode215, and the via240-1may be positioned away from the lateral extent of the block220by a width w5. In some embodiments, the width w5is between 0.5 microns and 2 microns, although other ranges are also contemplated. Further, the via240-1is arranged on one side of the block220, and the silicon nitride waveguide230is arranged on an opposite side.

FIGS. 3 and 4are diagrams300,400illustrating exemplary implementations of an optical detector having a vertical photodiode coupled with an optical waveguide, according to one or more embodiments. More specifically, the diagrams300,400provide a top view of different implementations of the optical detector depicted in the diagram200ofFIG. 2.

In the diagram300, the block220extends with a length l1along a first dimension (which corresponds to into/out of the page in the diagram200). As a result, the depletion region110(which, in some cases, is included in an intrinsic region of the block220) of the vertical photodiode215also extends along the first dimension.

The electrical contact245-2extends parallel to the block220along the first dimension. At least a portion of a tapered optical waveguide305(representing one example of the silicon nitride waveguide230) extends with a length l2along the first dimension, parallel to the intrinsic region. The tapered optical waveguide305receives an optical signal310at the non-tapered end, which is evanescently coupled into the depletion region110. The tapered optical waveguide305tapers as it extends along the first dimension, which improves the coupling of the optical signal310into the depletion region110.

In the diagram300, a plurality of vias315(representing one example of the via240-1) are coupled with the electrical contact210. Generally, using the plurality of vias315tends to reduce a resistance of the vertical photodiode, which increases an RC bandwidth. The plurality of vias315are arranged in a line extending along the first dimension. A single via240-2is coupled with the electrical contact225.

In the diagram400, at least a portion of a curved optical waveguide405(representing another example of the silicon nitride waveguide230) extends along the first dimension, parallel to an extent of the vertical photodiode215(or an extent of the intrinsic region). As shown, the curved optical waveguide405receives the optical signal at a curved waveguide section410, and propagates the optical signal310through the curved waveguide section410to a straight or tapered waveguide section415. The waveguide section415extends with a length l3along the first dimension, parallel to an extent of the vertical photodiode215.

Using the tapered optical waveguide305or the curved optical waveguide405, the optical signal310may be coupled into the depletion region110away from the endface of the block220in a gradual manner. Generally, in situations when an optical waveguide is butt-coupled to a vertical photodiode215, a significant amount of an optical signal is absorbed in a region where the electric field is weak. Additionally, the light absorption by the germanium is not uniformly distributed at the edge of the optical detector, which generates a greater number of electron-hole pairs near the interface. The electric field generated by the excess photocarriers tends to screen the depletion region, which reduces the bandwidth of the optical detector especially when the intensity of the optical signal310is high.

By using the tapered optical waveguide305or the curved optical waveguide405to evanescently couple the optical signal310into the depletion region110, the optical detector may have a greater bandwidth. For example, the optical detector may support bandwidths of 60 GHz or greater at higher intensities of the optical signal, as the optical detector is less sensitive to intensity-dependent decreases in bandwidth.

FIGS. 5 and 6are diagrams500,600illustrating an exemplary implementation of an optical detector having a lateral photodiode505coupled with a tapered optical waveguide530, according to one or more embodiments. More specifically, the diagram500provides a top view of the optical detector, and the diagram600provides a cross-sectional view of the optical detector. The features illustrated in the diagrams500,600may be used in conjunction with other embodiments, e.g., representing one possible implementation of the optical detector135ofFIG. 1.

In the diagram500, the lateral photodiode505is disposed on (here, directly contacting) a top surface235of a SOI layer525(representing one example of the SOI layer205). The SOI layer525is disposed above an insulating layer615(e.g., an oxide layer). Electrical contacts605,610having relatively heavy doping are formed on opposing ends of the SOI layer525.

In some embodiments, the lateral photodiode505is formed of germanium, although other semiconductor materials are also contemplated. The lateral photodiode505comprises a first region510doped with a first conductivity type (as shown, p-type) and a second region515doped with a second, different conductivity type (as shown, n-type). An undoped intrinsic region520of the lateral photodiode505is arranged between the first region510and the second region515. Although the lateral photodiode505is illustrated as a p-i-n photodiode, in other embodiments the lateral photodiode505is a p-n photodiode (e.g., with the first region510and the second region515directly contacting each other).

In some embodiments, the SOI layer525has a doping profile similar to that of the lateral photodiode505. For example, the SOI layer525comprises a first region625arranged beneath the first region510of the lateral photodiode505and doped with the first conductivity type, and a second region635arranged beneath the second region515of the lateral photodiode505and doped with the second conductivity type. The electrical contact605is more heavily doped with the first conductivity type than the first region625, the electrical contact610is more heavily doped with the second conductivity type than the second region635. As shown, the SOI layer525further comprises an intrinsic region630arranged beneath the intrinsic region520of the lateral photodiode505. The doping profile of the SOI layer525may be increased in a step-wise or a substantially continuous manner.

In the lateral photodiode505, the depletion region110(as well as the intrinsic region520) extends along a first dimension (the vertical direction as shown inFIG. 5), and the tapered optical waveguide530extends parallel to the depletion region110in the first dimension. In this way, the tapered optical waveguide530is spaced apart from the lateral photodiode505and top-coupled with the depletion region110. The tapered optical waveguide530may be formed of any suitable material(s), such as silicon nitride. The tapered optical waveguide530receives the optical signal at a relatively wide portion of a tapered section535, and propagates the optical signal through a narrow portion of the tapered section535into a straight waveguide section540.

Beneficially, evanescently coupling the optical signal from the tapered optical waveguide530into the depletion region110can be controlled and light may absorbed along the length of the lateral photodiode505in a gradual manner. Further, the width (tapering) of the tapered optical waveguide530can be controlled to allow absorption of the optical signal by the lateral photodiode505to occur only in the depletion region110.

Further, in some embodiments, the optical detector further comprises a second optical waveguide620that is evanescently coupled with the tapered optical waveguide530. As shown in the diagram600, the tapered optical waveguide530is arranged above the depletion region110, and the second optical waveguide620is arranged above the tapered optical waveguide530. While the second optical waveguide620is depicted in conjunction with the lateral photodiode505, other implementations of the optical detector (e.g., using the vertical photodiode215ofFIGS. 2-4) may be modified to include multiple optical waveguides.

The second optical waveguide620may be formed of any suitable material(s), such as silicon nitride. The positioning and/or dimensioning of the tapered optical waveguide530and the second optical waveguide620may be controlled to further optimize placement of the optical mode in the depletion region110. In some cases, the width of the intrinsic region520may be independent of the width of the tapered optical waveguide530and/or the second optical waveguide620. A height of the tapered optical waveguide530relative to the top of the lateral photodiode505may be controlled to optimize placement of the optical mode.

FIG. 7is a diagram700illustrating an exemplary implementation of an optical detector having a lateral photodiode505extending into a recess705formed the SOI layer525, according to one or more embodiments. The features illustrated in the diagram700may be used in conjunction with other embodiments, e.g., representing one possible implementation of the optical detector135ofFIG. 1.

In the diagram700, a recess705is formed from the top surface235of the SOI layer525to a recessed surface710, which in some cases is planar and/or parallel to the top surface235. In some embodiments, the recess705is formed through a suitable silicon etching process. The lateral photodiode505is formed, e.g., by epitaxially growing germanium into the recess705. Thus, the first region510, the second region515, and the intrinsic region520of the lateral photodiode505extends into the recess705. In some cases, and as shown, part of the lateral photodiode505extends out of the recess705(e.g., to a height above the top surface235). In other cases, however, the lateral photodiode505may be disposed entirely within the recess705.

FIG. 8is a diagram800illustrating an exemplary implementation of an optical detector with an intrinsic region of a lateral photodiode805extending into a recess formed into a substrate, according to one or more embodiments. The features illustrated in the diagram800may be used in conjunction with other embodiments, e.g., representing one possible implementation of the optical detector135ofFIG. 1.

As with the diagram700, germanium may be deposited into the recess705. However, the germanium remains undoped in the diagram800. In this way, the first region625of the SOI layer525forms the p-type region of the lateral photodiode805, and the second region635of the SOI layer525forms the n-type region of the lateral photodiode805. The intrinsic region of the lateral photodiode805is formed by the intrinsic region520and the intrinsic region630of the SOI layer525.

FIG. 9is a method900of operating an exemplary apparatus, according to one or more embodiments. The method900may be performed in conjunction with other embodiments, e.g., using one of the implementations of the optical detector depicted inFIGS. 2-8.

The method900begins at block905, where an optical signal is received from an optical device. The optical device may be external to the optical detector. In some embodiments, the optical device comprises an optical fiber or an optical waveguide of photonic circuitry external to the optical detector. At block915, the optical signal is propagated via an optical waveguide of the optical detector. The optical waveguide of the optical detector is spaced apart from the semiconductor layer on which a photodiode of the optical detector is disposed.

At block925, the optical signal is evanescently coupled from the optical waveguide to a depletion region of a photodiode. In some embodiments, the optical waveguide is top-coupled or side-coupled with the photodiode. In some embodiments, the depletion region is formed partly in an intrinsic region of the photodiode. The method900ends following completion of block925.

FIG. 10is a method1000of constructing an exemplary apparatus, according to one or more embodiments. The method1000may be performed in conjunction with other embodiments, e.g., to form one of the implementations of the optical detector depicted inFIGS. 2-8.

The method1000begins at an optional block1005, where a recess is formed from a surface of a semiconductor layer. In some embodiments, the semiconductor layer comprises a silicon layer (e.g., a Sal layer) and the recess is formed using a suitable etching process.

At block1015, a photodiode is formed relative to the surface of the semiconductor layer. In some embodiments, the photodiode is formed at least partly in the recess. In other embodiments, the photodiode is formed on the surface. In cases where the semiconductor layer comprises a silicon layer, the photodiode may be formed using germanium epitaxially grown on the silicon layer.

At block1025, a first optical waveguide is formed that is spaced apart from the photodiode. The first optical waveguide is also spaced apart from the semiconductor layer. In some embodiments, the first optical waveguide is arranged above a lateral photodiode, such that the first optical waveguide is top-coupled with a depletion region of the lateral photodiode. In other embodiments, the first optical waveguide is arranged above a vertical photodiode, such that the first optical waveguide is side-coupled with a depletion region of the vertical photodiode. In some embodiments, the first optical waveguide is formed of silicon nitride.

At optional block1035, a second optical waveguide is formed that is spaced apart from the first optical waveguide. The positioning and/or dimensioning of the first optical waveguide and the second optical waveguide may be controlled to optimize a placement of the optical mode in the depletion region of the photodiode.

At block1045, vias are formed that couple to the photodiode. In some embodiments, the vias extend to electrical contacts formed in the semiconductor layer, which are more heavily doped than the rest of the semiconductor layer. The vias may be arranged away from the optical waveguide to reduce any optical losses that might be caused by the conductive material. For example, the optical waveguide may be disposed to one side of the photodiode, and the vias may be disposed on other, different sides of the photodiode. At block1055, electrical contacts are formed that are coupled to the vias. In some embodiments, the electrical contacts are formed in a metal layer. The method1000ends following completion of block1055.