System having light sensor with enhanced sensitivity including a multiplication layer for generating additional electrons

The optical device includes a waveguide and a light sensor on a base. The light sensor includes a ridge extending from slab regions positioned on opposing sides of the ridge. The ridge includes a multiplication layer and an absorption layer. The absorption layer is positioned to receive at least a portion of the light signal from the waveguide. Additionally, the absorption layer generates a hole and electron pair in response to receiving a photon of the light signal. The multiplication layer is positioned to receive the electron generated in the absorption layer and to generate additional electrons in response to receiving the electron.

RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No. 12/380,016, filed on Feb. 19, 2009, entitled “Optical Device Having Light Sensor Employing Horizontal Electrical Field”, and incorporated herein in its entirety.

FIELD

The present invention relates to optical devices and more particularly to devices having a light sensor.

BACKGROUND

The use of optical and/or optoelectronic devices is increasing in communications applications. These devices can include light sensors that receive light signals from a waveguide. These light sensors often employ a light-absorbing material that absorbs the received light signals. During operation of the light sensor, an electrical field is applied across the light-absorbing material. When the light-absorbing material absorbs a light signal, an electrical current flows through the light-absorbing material. As a result, the level of electrical current through the light-absorbing material indicates the intensity of light signals being received by the light-absorbing material.

The waveguides on optical and/or optoelectronic devices are often made of silicon. Because silicon does not absorb the light signals having the wavelengths that are used in communications applications, silicon is often not effective for use as the light-absorbing medium in the light sensors for communications application. In contrast, germanium is a material that can absorb these light signals and is accordingly often used as the light-absorbing medium in the light sensors for communications application.

These light sensors have been able to achieve adequate speeds when the waveguides have a cross-section with sub-micron dimensions. However, these light sensors are associated with undesirably high optical loss when used with waveguides having these dimensions. Further, the waveguides used in many communications applications employ larger waveguides. When these light sensors are used with larger waveguides, they generally lose speed and become associated with undesirable levels of dark current. Further, these light sensors can have an undesirably low sensitivity at low light levels.

For the above reasons, there is a need for improved light sensors.

SUMMARY

The optical device includes a waveguide and a light sensor on a base. The light sensor includes a ridge extending from slab regions positioned on opposing sides of the ridge. The ridge includes a multiplication layer and an absorption layer. The absorption layer is positioned to receive at least a portion of the light signal from the waveguide. Additionally, the absorption layer generates a hole and electron pair in response to receiving a photon of the light signal. The multiplication layer is positioned to receive the electron generated in the absorption layer and to generate additional electrons in response to receiving the electron.

The device can also include field sources that serve as sources of an electrical field in the ridge. The field sources can be configured such that the electrical field is substantially parallel to the base. For instance, in some instances, the ridge includes lateral sides between a top and the slab regions. The field sources can each contact one of the lateral sides and the lateral sides that are contacted by the field sources can be on opposing sides of the light-absorbing medium. As a result, the resulting electrical field can travel between the lateral sides of the ridge.

DESCRIPTION

The optical device includes a light-transmitting medium on a base. The device also includes a waveguide configured to guide a light signal through the light-transmitting medium. The optical device also includes a light sensor. The light sensor includes a ridge extending from slab regions positioned on opposing sides of the ridge. The ridge includes a multiplication layer and an absorption layer. The absorption layer is positioned to receive at least a portion of the light signal from the waveguide. Additionally, the absorption layer generates a hole and electron pair in response to receiving a photon of the light signal. The multiplication layer is positioned to receive the electron generated in the absorption layer and to generate additional electrons in response to receiving the electron. This ability of the light sensor to generate multiple electrons in response to the receipt of a single photon enhances the sensitivity of the light sensor.

Additionally, the light sensor includes field sources for generating an electrical field in the ridge during the operation of the light sensor. As will become evident below, this electrical field may interact with an interface between the light-transmitting medium and the light-absorbing medium in the light sensor. The interaction between the electrical field and this interface is a source of dark current in the light sensor. The field sources can be positioned such that the electrical field is substantially parallel to the base and accordingly also substantially parallel to this interface. Forming the electrical field parallel to this interface can reduce dark current in the light sensor. As a result, the light sensor can be associated with both reduced dark current and increased sensitivity.

Additionally, the width of the waveguide can be tapered before the light signal enters the light-absorbing medium. As a result, the light-absorbing medium can have a width that is smaller than the width of the waveguide. The reduced width increases the speed of the light sensor. Accordingly, even when used with waveguide sizes that are common in communications applications, the light sensor can have desirable levels of speed and dark current while also having the reduced optical loss associated with light sensors built on larger waveguides.

FIG. 1AthroughFIG. 1Dillustrate an optical device having a light sensor configured to receive light signals from a waveguide.FIG. 1Ais a perspective view of the device.FIG. 1Bis a cross-section of the light sensor. For instance,FIG. 1Bis a cross-section of the device shown inFIG. 1Ataken along the line labeled B.FIG. 1Cis a cross-section of the waveguide. For instance,FIG. 1Cis a cross-section of the device shown inFIG. 1Ataken along the line labeled C.FIG. 1Dis a cross-section of the optical device shown inFIG. 1Ctaken along the line labeled C and extending parallel to the longitudinal axis of the waveguide.

The device is within the class of optical devices known as planar optical devices. These devices typically include one or more waveguides immobilized relative to a substrate or a base. The direction of propagation of light signals along the waveguides is generally parallel to a plane of the device. Examples of the plane of the device include the top side of the base, the bottom side of the base, the top side of the substrate, and/or the bottom side of the substrate.

The illustrated device includes lateral sides10(or edges) extending from a top side12to a bottom side14. The propagation direction of light signals along the length of the waveguides on a planar optical device generally extends through the lateral sides10of the device. The top side12and the bottom side14of the device are non-lateral sides.

The device includes one or more waveguides16that carry light signals to and/or from optical components17. Examples of optical components17that can be included on the device include, but are not limited to, one or more components selected from a group consisting of facets through which light signals can enter and/or exit a waveguide, entry/exit ports through which light signals can enter and/or exit a waveguide from above or below the device, multiplexers for combining multiple light signals onto a single waveguide, demultiplexers for separating multiple light signals such that different light signals are received on different waveguides, optical couplers, optical switches, lasers that act a source of a light signal, amplifiers for amplifying the intensity of a light signal, attenuators for attenuating the intensity of a light signal, modulators for modulating a signal onto a light signal, light sensors that convert an light signal to an electrical signal, and vias that provide an optical pathway for a light signal traveling through the device from the bottom side14of the device to the top side12of the device. Additionally, the device can optionally, include electrical components. For instance, the device can include electrical connections for applying a potential or current to a waveguide and/or for controlling other components on the optical device.

The waveguide16is defined in a light-transmitting medium18positioned on a base20. For instance, the waveguide16is partially defined by a ridge22extending upward from slab regions of the light-transmitting medium18. In some instances, the top of the slab regions are defined by the bottom of trenches24extending partially into the light-transmitting medium18or through the light-transmitting medium18. Suitable light-transmitting media18include, but are not limited to, silicon, polymers, silica, SiN, GaAs, InP and LiNbO3. One or more cladding layers are optionally positioned on the light-transmitting medium18. The one or more cladding layers can serve as a cladding for the waveguide16and/or for the device. When the light-transmitting medium18is silicon, suitable cladding layers include, but are not limited to, silicon, polymers, silica, SiN, GaAs, InP and LiNbO3.

The portion of the base20adjacent to the light-transmitting medium18is configured to reflect light signals from the waveguide16back into the waveguide16in order to constrain light signals in the waveguide16. For instance, the portion of the base20adjacent to the light-transmitting medium18can be an optical insulator27with a lower index of refraction than the light-transmitting medium18. The drop in the index of refraction can cause reflection of a light signal from the light-transmitting medium18back into the light-transmitting medium18. The base20can include the optical insulator27positioned on a substrate28. As will become evident below, the substrate28can be configured to transmit light signals. For instance, the substrate28can be constructed of a light-transmitting medium18that is different from the light-transmitting medium18or the same as the light-transmitting medium18. In one example, the device is constructed on a silicon-on-insulator wafer. A silicon-on-insulator wafer includes a silicon layer that serves as the light-transmitting medium18. The silicon-on-insulator wafer also includes a layer of silica positioned on a silicon substrate. The layer of silica can serving as the optical insulator27and the silicon substrate can serve as the substrate28.

The optical device also includes a light sensor29configured to receive a light signal guided by the one or more waveguides16. The light sensor29is configured to convert the light signal to an electrical signal. Accordingly, the light signal can be employed to detect receipt of light signals. For instance, the light sensor29can be employed to measure the intensity of a light signal and/or power of a light signal. AlthoughFIG. 1Aillustrates a waveguide16carrying the light signal between the one or more components and the light sensor29, the device can be constructed such that the waveguide16carries the light signal directly from an optical fiber to the light sensor29.

The light sensor29includes a ridge22extending from slab regions positioned on opposing sides of the ridge22. The tops of the slab regions are defined by the bottom of trenches24on opposing sides of the ridge22. The ridge22includes an absorption layer30. For instance,FIG. 1Bshows a light-absorbing medium32that absorbs light signals serving as the absorption layer30. Suitable light-absorbing media include media that upon being exposed to an electrical field, produce an electron and hole pair in response to receiving a photon. Examples of light-absorbing media32that are suitable for detection of light signals in at the wavelengths commonly employed in communications applications includes, but are not limited to, germanium, silicon germanium, silicon germanium quantum well, GaAs, and InP. Germanium is suitable for detection of light signals having wavelengths in a range of 1300 nm to 1600 nm.

The absorption layer is positioned to receive at least a portion of a light signal traveling along the waveguide16. As is evident fromFIG. 1A, there is an interface between a facet of the light-absorbing medium32and a facet of the light-transmitting medium18. The interface can have an angle that is non-perpendicular relative to the direction of propagation of light signals through the waveguide16at the interface. In some instances, the interface is substantially perpendicular relative to the base20while being non-perpendicular relative to the direction of propagation. The non-perpendicularity of the interface reduces the effects of back reflection. Suitable angles for the interface relative to the direction of propagation include but are not limited to, angles between 80° and 89°, and angles between 80° and 85°.

The absorption layer30can be positioned on a seed portion34of the light-transmitting medium18. In particular, the light-absorbing medium32of the light sensor29can be positioned on a seed portion34of the light-transmitting medium18. The seed portion34of the light-transmitting medium18is positioned on the base20. In particular, the seed portion34of the light-transmitting medium18contacts the insulator27. The seed portion34of the light-transmitting medium18can be continuous with the light-transmitting medium18included in the waveguide16or spaced apart from the waveguide16. When the light signal enters the light sensor, a portion of the light signal can enter the seed portion34of the light-transmitting medium18and another portion of the light signal enters the light-absorbing medium32. Accordingly, the light-absorbing medium32can receive only a portion of the light signal. In some instances, the light sensor can be configured such that the light-absorbing medium32receives the entire light signal.

As will become evident below, during the fabrication of the device, the seed portion34of the light-transmitting medium18can be used to grow the light-absorbing medium32. For instance, when the light-transmitting medium18is silicon and the light-absorbing medium32is germanium, the germanium can be grown on the silicon. As a result, the use of the light-transmitting medium18in both the waveguides16and as a seed layer for growth of the light-absorbing medium32can simplify the process for fabricating the device.

The light sensor also includes a charge layer35between a portion of multiplication layer36and the absorption layer30. At least a portion of the multiplication layer36is positioned such that the absorption layer30is not located between the portion of the multiplication layer36and the base20. For instance, the portion of the multiplication layer36can contact the base20. In some instances, the multiplication layer36is positioned such that none of the absorption layer30is between the base20and the multiplication layer36. As a result, the multiplication layer36and the absorption layer30can be positioned adjacent to one another on the base20. Further, the multiplication layer36and the absorption layer30can be positioned adjacent to one another such that a line that is parallel to the top and/or bottom of the base20extends through both the multiplication layer36and the absorption layer30.

Although the multiplication layer36is shown as a single layer of material, the multiplication layer36can include multiple layers of material. Suitable materials for the multiplication layer36include, but are not limited to, materials that upon being exposed to an electrical field and receiving an electron can excite additional electrons. Examples include, but are not limited to, semiconductor materials including crystalline semiconductors such as silicon. As a result, in some instances, the light-transmitting medium18and the multiplication layer36can be the same material. InFIG. 7B, the light-transmitting medium18and the multiplication layer36are shown as the same material.

The multiplication layer36can include a doped region37that serves as the charge layer35. The multiplication layer36can include also include an undoped region38positioned such that the doped region37of the multiplication layer36is between the undoped region38of the multiplication layer36and the absorption layer30. The doped region37can be an N-type doped region or a P-type doped region. In one example, the multiplication layer36is a layer of silicon that includes a region doped with a p-type dopant and the doped region37is in contact with the absorption layer30as shown inFIG. 1B.

The light-absorbing medium32or the absorption layer30can include a first doped region40that serves as a field source for the electrical field to be formed in the ridge22. For instance,FIG. 1Billustrates the light-absorbing medium32including a first doped region40that serves as a field source for the electrical field to be formed in the ridge22. The first doped region40can be continuous and unbroken and can be included both the ridge22and in the slab region as is evident fromFIG. 1B. In particular, the first doped region40can be included both in a lateral side of the ridge22and in the slab region. The light-absorbing medium32or the absorption layer30can also include an undoped region between the multiplication layer36and the first doped region40.

As is evident inFIG. 1B, the portion of the slab region that includes the first doped region40can also include or consist of the light-absorbing medium32. As a result, the first doped region40can be formed in a single continuous medium. As an example the first doped region40can be formed in germanium that is included both in the ridge22and in the slab region. As is evident fromFIG. 1B, the first doped region40can extend up to the top side of the light-absorbing medium32. The first doped regions40can be an N-type doped region or a P-type doped region.

The multiplication layer36can include a second doped region41that serves as a field source for the electrical field to be formed in the ridge22. The second doped region41can be continuous and unbroken and can be included both the ridge22and in the slab region as is evident fromFIG. 1B. In particular, the second doped region41can be included both in a lateral side of the ridge22and in the slab region. As is evident inFIG. 1B, the portion of the slab region that includes the second doped region41can also include or consist of the same material as the multiplication layer36. As a result, the second doped region41can be formed in a single continuous medium. As an example the second doped region41can be formed in silicon that is included both in the ridge22and in the slab region. As is evident fromFIG. 1B, the second doped region41can extend up to the top side of the light-absorbing medium32. The second doped regions41can be an N-type doped region or a P-type doped region.

An N-type doped region can include an N-type dopant. A P-type doped region can include a P-type dopant. Suitable dopants for N-type regions include, but are not limited to, phosphorus and/or arsenic. Suitable dopants for P-type regions include, but are not limited to, boron. The first doped region40and the second doped region41can be doped so as to be electrically conducting. A suitable concentration for the P-type dopant in a P-type doped region that serves as the first doped region40or the second doped region41includes, but is not limited to, concentrations greater than 1×1015cm−3, 1×1017cm−3, or 1×1019cm−3, and/or less than 1×1017cm−3, 1×1019cm−3, or 1×1021cm−3. A suitable concentration for the N-type dopant in an N-type doped region that serves as the first doped region40or the second doped region41includes, but is not limited to, concentrations greater than 1×1015cm−3, 1×1017cm−3, or 1×1019cm−3, and/or less than 1×1017cm−3, 1×1019cm−3, or 1×1021cm−3.

As noted above, a region of the multiplication layer36can be an N-type doped region or a P-type doped region that serves as the charge layer35. Suitable dopants for N-type regions include, but are not limited to, phosphorus and/or arsenic. Suitable dopants for P-type regions include, but are not limited to, boron. Since the doped region37serves as the charge layer, that doped region37can have a lower concentration of dopant than the first doped region40and/or the second doped region41. For instance, a suitable concentration for the P-type dopant in a doped region37that serves as the charge layer35includes, but is not limited to, concentrations greater than 1×1015cm−3, 1×1016cm−3, or 1×1017cm−3, and/or less than 1×1019cm−3, 1×1020cm−3, or 1×1021cm−3. A suitable concentration for the N-type dopant in a doped region37that serves as the charge layer35includes, but is not limited to, concentrations greater than 1×1015cm−3, 1×1016cm−3, or 1×1017cm−3, and/or less than 1×1019cm−3, 1×1020cm−3, or 1×1021cm−3.

In one example, the multiplication layer36includes or consists of silicon, the light-absorbing material includes or consists of silicon, the first doped region40is an p-type region with a dopant concentration of about 1×1020cm−3, the second doped region41is an n-type region with a dopant concentration of about 1×1020cm−3, and the doped region37that serves as the charge layer is a p-type region with a dopant concentration of about 1×1017cm−3.

The first doped region40and the second doped region41are each in contact with an electrical conductor44such as a metal. Accordingly, the first doped region40provides electrical communication between one of the electrical conductors44and the light-absorbing medium32. In particular, the first doped region40provides electrical communication between an electrical conductor44and the light-absorbing medium32included in a lateral side of the ridge22. The second doped region41provides electrical communication between one of the electrical conductors44and the multiplication layer36. In particular, the second doped region41provides electrical communication between one of the electrical conductors44and the portion of the multiplication layer36at the lateral side of the ridge22.

During operation of the light sensor, electronics (not shown) in electrical communication with the electrical contacts are used to apply a reverse bias between the first doped region40and the second doped region41. When the first doped region40is a p-type region, the second doped region41is an n-type region, and the doped region37that serves as the charge layer is a p-type region, a positive charge develops at the charge layer35. As a result, there is an increased electrical field at the charge layer35. When a photon is absorbed in the undoped region of the absorption layer, a hole and electron pair are generated. The electron is pulled toward the positive charge at the charge layer35. The increased electrical field at the charge layer causes excites the electron and causes the electron to accelerate. The electron can accelerate to the extent that interaction of the electron with the lattice structure of the multiplication layer36excites additional hole and electron pairs. In turn, these electrons may excite further hole and electron pairs. In this way, a single photon results in the creation of multiple electrons. These electrons provide electrical current through the light sensor. The current level can be detected and/or measured by the electronics in order to determine the presence and/or intensity of the light signal. As a result, the creation of these additional electrons from a single photon increases the sensitivity of the light sensor.

The level of doping in the charge layer can affect the operation of the light sensor. For instance, the level of doing in the charge layer can be selected to cause a high level of electric field in the multiplication layer in order to achieve a high gain in the multiplication layer while also providing an electric field in the absorption layer that is low enough to reduce avalanche gain the absorption layer. The low gain in the absorption region can reduce free carriers that can absorb light without generating the electrical current that indicates the presence of light.

The light sensor can be configured to apply an electric field to the light-absorbing medium32that is substantially parallel to the base20. For instance, the light-absorbing medium32can include lateral sides35that connect a bottom side36and a top side37. The bottom side is located between the top side and the base20. In some instances, the lateral sides are substantially perpendicular relative to the base20.

As noted above, the light sensor is suitable for use with waveguide dimensions that are suitable for use in communications applications. Accordingly, a suitable height for the waveguide16(labeled h inFIG. 1C) includes, but is not limited to, heights greater than 1 μm, 2 μm, and 3 μm. A suitable width for the waveguide16(labeled w inFIG. 1C) includes, but is not limited to, widths greater than 0.5 μm, 2 μm, and 3 μm. Suitable waveguide dimension ratios (width of the waveguide16:height of the waveguide16) include, but are not limited to, ratios greater than 0.15:1, 0.5:1, and 1:1 and/or less that 0.25:1, 1:1, and 2:1. A suitable thickness for the slab regions adjacent to the waveguide includes, but is not limited to, a thickness greater than 0.1 μm, 0.5 μm, or 1 μm and/or less than 1.5 μm, 2 μm, or 3 μm.

In the light sensor, a suitable height for the ridge22(labeled H inFIG. 1B) includes, but is not limited to, heights greater than 0.5 μm, 1 μm, or 2 μm and/or less than 3.5 μm, 4 μm, or 5 μm. A suitable height for the light-absorbing medium32(labeled h inFIG. 1B) includes, but is not limited to, heights greater than 0.5 μm, 1 μm, or 2 μm and/or less than 3.5 μm, 4 μm, or 5 μm. As is evident inFIG. 1B, the slab region that includes the light-absorbing medium32can have a thickness that is different from the thickness of the slab region that excludes the light-absorbing medium32. A suitable thickness for the slab region that includes the light-absorbing medium32includes, but is not limited to, a thickness greater than 0.1 μm, 0.5 μm, or 1 μm and/or less than 1.5 μm, 2 μm, or 3 μm. A suitable thickness for the slab region that excludes the light-absorbing medium32includes, but is not limited to, a thickness greater than 0.1 μm, 0.5 μm, or 1 μm and/or less than 1.5 μm, 2 μm, or 3 μm.

The width of the light-absorbing medium32included in the ridge22of the light sensor can affect the performance of the light sensor. For instance, increasing the width of the light-absorbing medium32can increase the portion of the light-absorbing medium32that receives the light signals from the waveguide16and can accordingly increase the efficiency of the light sensor. However, increasing this width can reduce the speed of the light sensor by increasing the distance that the electrons generated in the light-absorbing medium32travel through the light-absorbing medium32. Similarly, the width of the multiplication region can slow the light sensor. As a result, it is desirable for the width of the multiplication region to be less than the width of the light-absorbing region. A suitable width ratio (width of the light-absorbing medium32:width of the multiplication layer36) includes widths ratios greater than 0.1:1, 0.5:1, or 1:1 and/or less than 0.12:1, 1.5:1, or 2:1. A suitable width for the light-absorbing medium32includes widths greater than 0.1 μm, 0.5 μm, or 1 μm and/or less than 1.5 μm, 2 μm, or 4 μm. A suitable width for the multiplication layer36includes widths greater than 0.1 μm, 0.2 μm, or 0.5 μm and/or less than 1 μm, 2 μm, or 3 μm.

In one example of the light sensor, the height for the ridge22is 3 μm, the height for the light-absorbing medium32is 3 μm, the slab region that includes the light-absorbing medium32is 1 μm, and the thickness of the slab region that excludes the light-absorbing medium32is 1 μm. In this example, the width of the multiplication region is 0.5 μm and the width of the light-absorbing region is 1.5 μm.

Rather than using first doped region40and the second doped region41as the field sources, electrical conductors44such as metal can be used as the field sources. For instance, the first doped region40and the second doped region41need not be formed and electrical conductors can be formed over the locations of the first doped region40and the second doped region41. The electrical conductors can then serve as the field sources.

FIG. 2is a topview of an optical device where the waveguide16includes a taper48. The taper48can be a horizontal taper and need not include a vertical taper although a vertical taper is an option. The taper48is positioned before the light sensor. For instance, the horizontal taper occurs in the light-transmitting medium18rather than in the light-absorbing medium32. The taper48allows the light-absorbing medium32to have a narrower width than the waveguide16. The reduced width of the light-absorbing medium32increases the speed of the light sensor. The optical component preferably excludes additional components between the taper and light sensor although other components may be present.

The waveguide can be aligned with the absorption layer30. For instance,FIG. 2shows the ridge22of the waveguide aligned with the absorption layer without any portion of the ridge being aligned with multiplication layer36. This alignment can reduce the entry of light signals into the multiplication layer36while providing an efficient absorption of the light signal in the absorption layer.

The optical device can be constructed using fabrication technologies that are employed in the fabrication of integrated circuits, optoelectronic circuits, and/or optical devices. For instance, the ridge22for the waveguide16and/or the seed portion34can be formed in the light-transmitting medium18using etching technologies on a silicon-on-insulator wafer. Horizontal tapers can be readily formed using masking and etching technologies. Suitable methods for forming vertical tapers are disclosed in U.S. patent application Ser. No. 10/345,709, filed on Jan. 15, 2003, entitled “Controlled Selectivity Etch for Use with Optical Component Fabrication,” and incorporated herein in its entirety.

FIG. 3AthroughFIG. 8illustrate a method of generating an optical device constructed according toFIG. 1AthroughFIG. 1C. The method is illustrated using a silicon-on-insulator wafer or chip as the starting precursor for the optical device. However, the method can be adapted to platforms other than the silicon-on-insulator platform.

FIG. 3AthroughFIG. 3Cillustrate a first mask50formed on the silicon-on-insulator wafer or chip to provide a device precursor.FIG. 3Ais a topview of the device precursor.FIG. 3Bis a cross-section of the device precursor shown inFIG. 3Ataken along the line labeled B.FIG. 3Cis a cross-section of the device precursor shown inFIG. 3Ataken along the line labeled C. The first mask50leaves exposed a region of the device precursor where a sensor cavity is to be formed while the remainder of the illustrated portion of the device precursor is protected. The sensor cavity is the region of the device precursor where the light-absorbing medium32and the slab region that includes the light-absorbing medium32are to be formed. A first etch is then performed so as to form the sensor cavity. The first etch yields the device precursor ofFIG. 3AthroughFIG. 3C. The first etch is performed such that the seed portion34of the light-transmitting medium18remains on the base20. Accordingly, the first etch can optionally be terminated before the base20is reached.

A suitable first mask50includes, but is not limited to, a hard mask such as a silica mask. A suitable first etch includes, but is not limited to, a dry etch.

As shown inFIG. 4AthroughFIG. 4C, the charge layer35is formed in the sensor cavity52ofFIG. 3AthroughFIG. 3C.FIG. 4Ais a topview of the device precursor.FIG. 4Bis a cross-section of the device precursor shown inFIG. 4Ataken along the line labeled B.FIG. 4Cis a cross-section of the device precursor shown inFIG. 4Ataken along the line labeled C. The charge layer35can be formed by forming the doped region37in a vertical wall of the light-transmitting region. The doped region37can be generated by forming a doping mask on the device precursor so the locations of the doped region37remains exposed and the remainder of the illustrated portion of the device precursor is protected. High angle dopant implant processes can be employed to form the doped region37. The doping mask can then be removed.

As shown inFIG. 5AthroughFIG. 5C, the light-absorbing medium32is formed in the sensor cavity ofFIG. 4AthroughFIG. 4C.FIG. 5Ais a topview of the device precursor.FIG. 5Bis a cross-section of the device precursor shown inFIG. 5Ataken along the line labeled B.FIG. 5Cis a cross-section of the device precursor shown inFIG. 5Ataken along the line labeled C. When the light-transmitting medium18is silicon and the light-absorbing medium32is germanium, the germanium can be grown on the seed portion34of the silicon. After formation of the light light-absorbing medium32, the device precursor can be planarized to provide the device precursor ofFIG. 5AthroughFIG. 5C. An example of a suitable planarization method is a chemical-mechanical polishing (CMP) process.

After planarization of the device precursor, a second mask can be formed on the device precursor as shown inFIG. 5AthroughFIG. 5C. The second mask54is formed such that the regions where the slab regions are to be formed remain exposed while protecting the remainder of the illustrated portion of the device precursor. A suitable second mask54includes a hard mask such as a silica mask.

A second etch is performed on the device precursor ofFIG. 5AthroughFIG. 5Cto provide the device precursor ofFIG. 6AthroughFIG. 6C.FIG. 6Ais a topview of the device precursor.FIG. 6Bis a cross-section of the device precursor shown inFIG. 6Ataken along the line labeled B.FIG. 6Cis a cross-section of the device precursor shown inFIG. 6Ataken along the line labeled C. The second etch is stopped when the slab region that excludes the light-absorbing medium32is formed to the desired thickness. Since the second etch etches the light-transmitting medium18and the light-absorbing medium32concurrently, the second etch may etch the light-transmitting medium18and the light-absorbing medium32to different depths. As a result,FIG. 7B(and alsoFIG. 2) illustrates the slab region that includes the light-absorbing medium32being formed to a different thickness than the slab region that excludes the light-absorbing medium32. In one example, the second etch is selected to etch the light-transmitting medium18more quickly than the light-absorbing medium32. As a result, the slab region that includes the light-absorbing medium32is illustrated as being thicker than the slab region that excludes the light-absorbing medium32.

The first doped region40and the second doped region41can be formed in the device precursor ofFIG. 6AthroughFIG. 6Cto provide the device precursor ofFIG. 7AthroughFIG. 7C.FIG. 7Ais a topview of the device precursor.FIG. 7Bis a cross-section of the device precursor shown inFIG. 7Ataken along the line labeled B.FIG. 7Cis a cross-section of the device precursor shown inFIG. 7Ataken along the line labeled C. The first doped regions40can be generated by forming a doping mask on the device precursor so the location of the first doped regions40remain exposed and the remainder of the illustrated portion of the device precursor is protected. High angle dopant implant processes can be employed to form the first doped regions40. The doping mask can then be removed. The same sequence can then be employed to form the second doped region41. The second doped region41can be formed before the first doped region40or the first doped region40can be formed before the second doped region41.

A first cladding56, metal conductors44, and a second cladding58can optionally be formed on the device precursor ofFIG. 7AthroughFIG. 7Cto provide the device precursor ofFIG. 8. The first cladding can be formed such that the portion of the first doped region40and the second doped region41that is to be contacted by the electrical conductors44remain exposed and the remainder of the illustrated portion of the device precursor are protected by the first cladding. A suitable first cladding includes, but is not limited to, PECVD deposited silica that is subsequently patterned using photolithography. The electrical conductors44can be formed on the resulting device precursor. The electrical conductors44can be formed so each electrical conductor44extends from the first doped region40or the second doped region41, out of the trench24, and over the light-transmitting medium18. Suitable electrical conductors44include metals such as titanium and aluminum. The metals can be deposited by sputtering and patterned by photolithography. A second cladding can optionally be formed on the resulting device precursor. The second cladding can be patterned such that the second cladding defines contact pads on the electrical conductors44. Since the contact pads can be positioned outside of the trenches24, the location of the contact pads is not illustrated inFIG. 8. A suitable second cladding includes, but is not limited to, PECVD deposited SiN that is subsequently patterned using photolithography.

The device can be used in conjunction with electronics that are in electrical communication with the portion of the electrical conductors44that act as contact pads. The electronics can apply electrical energy to the contact pads so as to form a reverse bias across the light sensor.