Waveguide optoelectronic device

Abstract

A waveguide optoelectronic device comprising a rib waveguide region, and method of manufacturing a rib waveguide region, the rib waveguide region having: a base of a first material, and a ridge extending from the base, at least a portion of the ridge being formed from a chosen semiconductor material which is different from the material of the base wherein the silicon base includes a first slab region at a first side of the ridge and a second slab region at a second side of the ridge; and wherein: a first doped region extends along: the first slab region and along a first sidewall of the ridge, the first sidewall contacting the first slab region; and a second doped region extends along: the second slab region and along a second sidewall of the ridge, the second sidewall contacting the second slab region.

Claims

1. A waveguide optoelectronic device, comprising a rib waveguide region, the rib waveguide region having a silicon base and a ridge extending from the silicon base, wherein the silicon base includes a first slab region at a first side of the ridge and a second slab region at a second side of the ridge, wherein: a first doped region extends along: the first slab region and along a first sidewall of the ridge, the first sidewall contacting the first slab region; and a second doped region extends along: the second slab region and along a second sidewall of the ridge, the second sidewall contacting the second slab region, wherein the ridge comprises: a lower ridge portion in contact with and extending away from the silicon base, the silicon base and the lower ridge portion both including silicon; and an upper ridge portion in contact with and extending away from the lower ridge portion, the upper ridge portion including a semiconductor material that is different from silicon, and wherein: the first doped region which extends along the first sidewall includes a lower sidewall portion located at the lower ridge portion and an upper sidewall region located at the upper ridge portion; and the second doped region which extends along the second sidewall includes a lower sidewall portion located at the lower ridge portion and an upper sidewall region located at the upper ridge portion.

2. The waveguide optoelectronic device of claim 1, further comprising: a first electrical contact; and a second electrical contact, wherein the first electrical contact is in direct contact with the first slab region and the second electrical contact is in direct contact with the second slab region, and wherein the silicon base is composed of silicon.

3. The waveguide optoelectronic device of claim 2, wherein the waveguide optoelectronic device is: a waveguide electro absorption modulator (EAM) and the rib waveguide region is a rib waveguide modulation region; or a waveguide photodiode (PD).

4. The waveguide optoelectronic device of claim 2, wherein the semiconductor material is silicon germanium (SiGe), a metal alloy of silicon, a metal alloy of germanium, or a metal alloy of SiGe.

5. The waveguide optoelectronic device of claim 4, wherein the semiconductor material is GeSn or SiGeSn.

6. The waveguide optoelectronic device of claim 2, wherein: the first electrical contact is located on top of the first slab region; and the second electrical contact is located on top of the second slab region.

7. The waveguide optoelectronic device of claim 2, wherein the first doped region is n doped and the second doped region is p doped.

8. A method of manufacturing the waveguide optoelectronic device of claim 1, the method comprising the steps of: providing a layer of silicon; etching a cavity into the layer, the cavity having a base, a first cavity edge and a second cavity edge; implanting the base with a first dopant to create a first doped slab region; implanting the base with a second dopant to create a second doped slab region laterally spaced from the first doped slab region; growing the semiconductor material within the cavity; etching the semiconductor material to form a waveguide ridge which lies within the cavity and extends upwards from the base and overlies a portion of the first doped slab region and a portion of the second doped slab region, the waveguide ridge having the first sidewall which contacts the first doped slab region and the second sidewall which contacts the second doped slab region; implanting the first sidewall with the first dopant; implanting the second sidewall with the second dopant; and etching away the first cavity edge and the second cavity edge.

9. The method of claim 8, wherein the step of etching a cavity into the layer is a deep etching step and the etch has a depth of at least 2 μm.

10. The method of claim 8, further comprising the step of: creating the lower ridge portion composed of the silicon directly underneath the waveguide ridge by: etching the first doped slab region by a height which is less than an unetched height of the first doped slab region; and etching the second doped slab region by a height which is less than an unetched height of the second doped slab region.

11. The method of claim 8, wherein the semiconductor material is silicon germanium (SiGe), a metal alloy of silicon, a metal alloy of germanium, or a metal alloy of SiGe.

12. The method of claim 11, wherein the semiconductor material is GeSn or SiGeSn.

13. The waveguide optoelectronic device of claim 1, wherein each of the lower sidewall portion of the first doped region, the lower sidewall portion of the second doped region, the first slab region, and the second slab region has a respective dopant concentration that is higher than a dopant concentration of the upper sidewall region of the first doped region and higher than a dopant concentration of the upper sidewall region of the second doped region.

14. The waveguide optoelectronic device of claim 13, wherein each of the first slab region and the second slab region has a respective dopant concentration that is higher than a dopant concentration of the lower sidewall portion of the first doped region and higher than a dopant concentration of the lower sidewall portion of the second doped region.

15. The waveguide optoelectronic device of claim 1, wherein each of a distance by which the lower sidewall portion of the first doped region extends into the ridge and a distance by which the lower sidewall portion of the second doped region extends into the ridge is greater than a distance by which the upper sidewall region of the first doped region extends into the ridge and greater than a distance by which the upper sidewall region of the second doped region extends into the ridge.

16. The waveguide optoelectronic device of claim 1, further comprising: an input rib waveguide coupled to an input of the rib waveguide region to couple light into the rib waveguide region; and an output rib waveguide coupled to an output of the rib waveguide region to couple light out of the rib waveguide region.

17. The waveguide optoelectronic device of claim 16, wherein a height of the silicon base, a height of the lower sidewall portion of the first doped region, and a height of the lower sidewall portion of the second doped region are such that the mode center of the rib waveguide region is located at the same height above the silicon base as the mode center of the input rib waveguide, the output rib waveguide, or the input rib waveguide and the output rib waveguide.

18. The waveguide optoelectronic device of claim 1, further comprising: a silicon substrate below the silicon base; and a crystalline cladding layer between the silicon base and the silicon substrate.

19. The waveguide optoelectronic device of claim 18, further comprising a buried oxide layer, disposed on opposing horizontal sides of the crystalline cladding layer, wherein the crystalline cladding layer is a material which is different than the buried oxide layer.

20. The waveguide optoelectronic device of claim 1, wherein the lower ridge portion is composed of silicon and the upper ridge portion is composed of the semiconductor material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other features and advantages of the present invention will be appreciated and understood with reference to the specification, claims, and appended drawings wherein:

(2) FIG. 1 depicts a schematic diagram of a waveguide device in the form of a waveguide electro-absorption modulator (EAM) according to an embodiment of the present invention;

(3) FIG. 2A depicts a cross section along the line A-A′ in FIG. 1;

(4) FIG. 2B depicts a cross section along the line B-B′ in FIG. 1;

(5) FIG. 3A depicts the cross section of FIG. 2A with an optical mode shown propagating through the waveguide;

(6) FIG. 3B depicts the cross section of FIG. 2B with an optical mode shown propagating through the waveguide;

(7) FIG. 4A depicts a cavity etching step in a method of manufacturing a rib waveguide modulation region according to an embodiment of the present invention;

(8) FIG. 4B depicts an implantation step in the method of manufacturing a rib waveguide modulation region;

(9) FIG. 4C depicts a further implantation step in the method of manufacturing a rib waveguide modulation region;

(10) FIG. 4D depicts an annealing step in the method of manufacturing a rib waveguide modulation region;

(11) FIG. 4E depicts a SiGe growth step in the method of manufacturing a rib waveguide modulation region;

(12) FIG. 4F depicts a further etch step in the method of manufacturing a rib waveguide modulation region;

(13) FIG. 4G depicts a sidewall implantation step in the method of manufacturing a rib waveguide modulation region;

(14) FIG. 4H depicts a further sidewall implantation step in the method of manufacturing a rib waveguide modulation region;

(15) FIG. 4I depicts a further annealing step in the method of manufacturing a rib waveguide modulation region;

(16) FIG. 4J depicts a further etch step in the method of manufacturing a rib waveguide modulation region;

(17) FIG. 5 depicts an alternative embodiment of a waveguide electro-absorption modulator according to the present invention;

(18) FIG. 6 depicts a further alternative embodiment of a waveguide electro-absorption modulator according to the present invention, this embodiment including further grading within the dopant regions;

(19) FIG. 7a depicts a cross-section along the line A-A′ of a variant device;

(20) FIG. 7b depicts a cross-section along the line B-B′ of a variant device; and

(21) FIG. 8 depicts a cross-section along the line A-A′ of a further variant device.

DETAILED DESCRIPTION

(22) The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments of a waveguide optoelectronic device (EAM) and/or method of manufacturing a rib waveguide modulation region provided in accordance with the present invention and is not intended to represent the only forms in which the present invention may be constructed or utilized. The description sets forth the features of the present invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and structures may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention. As denoted elsewhere herein, like element numbers are intended to indicate like elements or features. A waveguide optoelectronic device 1 according to a first embodiment of the present invention is described below with reference to FIGS. 1, 2A, 2B, 3A and 3B.

(23) As shown in FIG. 1, the waveguide device 1 is suitable for coupling to standard optical input and output waveguides 2a, 2b such as silicon rib waveguides. Whilst this coupling could be achieved by direct coupling between input/output waveguides and the device, in the embodiment shown, coupling is achieved by way of an input taper 3a and an output taper 3b which allows for the waveguide device to be fabricated using smaller waveguide dimensions than those of the input/output waveguides 2a, 2b, thereby resulting in faster speeds of operation.

(24) FIG. 2A and FIG. 3A each show a cross section of the waveguide optoelectronic device taken at line A-A′ of FIG. 1, that is to say, transverse to the direction of the propagation of light along the waveguides. FIG. 2B and FIG. 3B each show a cross section taken at line B-B′ of FIG. 1, at the output at the taper portion 3b which couples to the output rib waveguide 2b. Again, this cross section is taken transverse to the direction of the propagation of light.

(25) The waveguide optoelectronic device 1 comprises a ridge modulation or photodetection region with a height h.sub.WG; the ridge modulation region being made up of a base 11 manufactured from a first waveguide material M.sub.1 and a ridge 12 manufactured from a second waveguide material M.sub.2 which is different from the first waveguide material.

(26) The base 11 includes a first slab region extending away from a first sidewall of the waveguide ridge in a first direction and a second slab region extending away from a second sidewall of the waveguide ridge in a second direction; the second direction being opposite from the first direction.

(27) The waveguide optoelectronic device includes a first doped region, the first doped region including a first doped slab region 13a and a first doped sidewall region extending along the first sidewall of the waveguide.

(28) In the embodiment shown in FIG. 2A and FIG. 3A, the ridge of the waveguide is formed from a lower ridge portion 12a and an upper ridge portion 12b. The lower ridge portion is in contact with and extends away from the base; the base and lower ridge portion both being formed from the first material M.sub.1. The upper ridge portion is made from the second material M.sub.2 located on top of the lower ridge portion in that it is in contact with and extends away from the lower ridge portion.

(29) The first doped sidewall region extends along the entire sidewall of the ridge including both the lower ridge portion 12a and the upper ridge portion 12b. The first doped sidewall region therefore comprises a first lower sidewall portion 13b which extends along the first sidewall at the lower ridge portion of the ridge; and a first upper sidewall portion 13c which extends along the sidewall at the upper ridge portion of the ridge.

(30) Similarly, at the second side of the rib waveguide, the waveguide optoelectronic device comprises a second doped slab region 14a and a second doped sidewall region extending along the second sidewall of the waveguide. The second doped sidewall is made up of a second lower sidewall portion 14b which extends along the second sidewall at the lower ridge portion of the ridge; and a second upper sidewall portion 14c which extends along the sidewall at the upper ridge portion of the ridge.

(31) The dopant concentration at the doped slab regions and the lower doped sidewall regions are higher than those of the upper doped sidewall regions. In the embodiment shown in FIGS. 2A and 3A, the first doped slab region and first lower sidewall doped region are n++ doped whilst the first upper sidewall is n doped; the n++ dopant region typically containing at least one-two orders of magnitude more dopant per cm.sup.3 as compared to the n doped region. The second doped slab region and second lower sidewall doped region are p++ doped whilst the second upper sidewall is p doped.

(32) In this embodiment of FIGS. 2A and 3A, the first material M.sub.1 is formed from silicon, Si and the second material M.sub.2 is silicon germanium (SiGe). However, as described above, it is envisaged that the structure of this embodiment could equally be applied to other suitable optical materials. Examples of suitable dopant concentrations for an M.sub.1/M.sub.2 structure of Si/SiGe are shown in Table 1 below.

(33) TABLE-US-00001 TABLE 1 Doping type Doping range [1/cm.sup.3] n 1e15-1e18 p 1e15-1e18 n++ 1e18-1e20 p++ 1e18-1e20

(34) As can be seen in FIG. 2A, the first doped slab region can be defined by a thickness d.sub.np1 by which it extends downwards into the slabs of the first material M.sub.1. The first lower sidewall portion 13b and second lower sidewall portion 14b each extend upwardly away from the slab by a height h.sub.3 which corresponds to the height of the lower portion of the ridge. These lower sidewall portions 13b, 14b extend into the ridge by respective distance d.sub.np2, d.sub.pp2, each of these respective distances being less than half the total cross sectional width of the lower ridge portion, such that an undoped region separates the n++ region from the p++ region thereby forming a p-i-n junction.

(35) An electrical contact (not shown) will be located at each of the slab regions in order to apply a bias across the junction which is formed by the doped regions. These electrical contacts will be located directly onto the slab (i.e. at the upper surface of the slab, on either side of the ridge). Typically the contacts may be equidistant from the respective sidewalls of the ridge.

(36) The first and second upper sidewall portions 13c, 14c extend into the upper ridge portion of the ridge by a distance d.sub.n, d.sub.p each of which is less than the respective distances d.sub.np2, d.sub.pp2, by which the lower sidewall portions 13b, 14b each extend into the lower portion 12a of the rib waveguide. Examples of typical measurements are given (in nm) in Table 2.

(37) TABLE-US-00002 TABLE 2 Geometry Tolerance h.sub.1 [nm] 100-800  h.sub.2 [nm] 100-400  h.sub.3 [nm]  0-400 d.sub.np1, d.sub.np2 [nm] 50-300 d.sub.pp1, d.sub.pp2 [nm] 50-300 d.sub.p [nm] 50-300 d.sub.n [nm] 50-300

(38) In this embodiment, the waveguide device takes the form of a waveguide electro-absorption modulator (EAM). However, it is envisaged that the device could instead take the form of another optoelectronic component such as a waveguide photodiode (PD).

(39) Referring in particular to FIG. 3A and FIG. 3B, the parameters of the device are chosen such that the optical mode within the rib waveguide of the device 1 matches up with the optical mode propagating through the input waveguide and any input coupler such as a taper. In particular, it is the height of the mode in the ridge of the device h.sub.mode1 relative to the base of the device which matches up to the height of the mode h.sub.mode2 above the base of the input waveguide or taper. Typically, the optoelectronic device, the input/output waveguides, and any coupler, will be located within the same plane (i.e. the bottom surface of the base of the optoelectronic device will be level with the bottom surface of the base of the input/output waveguides). For example, the optoelectronic device, input/output waveguides and couplers may all be fabricated upon a planar insulating layer such as a buried oxide (BOX) layer (not shown).

(40) Referring to FIGS. 4A to 4K, an example is described of a method of manufacturing a rib waveguide modulation region according to the present invention.

(41) Initially, a layer 401 of a first semiconductor material M.sub.1 is provided; the layer having an upper surface 401a and a lower surface (corresponding to the bottom surface of the base of the optoelectronic device) 401b. In some embodiments, this base layer of the initial semiconductor layer will be located upon an insulator layer such as a BOX layer. Typically, the first material will be silicon, but it is envisaged that the method described herein could be applied to other materials suitable for use with optoelectronic components such as metal alloys of silicon.

(42) The upper surface 401a of the initial layer of the first material is etched down to a given height (h.sub.2+h.sub.3) above the bottom of the layer 401b, the etching process therefore resulting in a cavity 402 located within the initial layer of the first material 401. The cavity formed by the etching process will have a base 402a; a first cavity edge 402b; and a second cavity edge 402c.

(43) Once the cavity 402 has been created, a photoresist 403 is deposited onto the first material M.sub.1 covering all but a portion of the base of the cavity, the uncovered portion of the base 402a extending from the first cavity edge 402b to less than half way across the total length of the base of the cavity. The base of the cavity will ultimately become the first and second slabs of the optoelectronic device.

(44) An implantation step is then carried out on the uncovered portion of the base of the cavity 402a to implant the uncovered portion with a first dopant, in this case an n type dopant to create a first slab doped portion 13a. In this case, the doped portion has a dopant concentration which may lie within the range of 1e18-1e20 cm.sup.−3. Typically, the dopant is applied vertically, i.e. at a direction which is parallel or substantially parallel to the edge of the cavity.

(45) Examples of a suitable n type dopants include: phosphorus and arsenic. An Example of a suitable p type dopant is boron.

(46) Once the implantation of the first slab doped portion is complete, the photoresist 403 is removed and the implantation process is repeated at the other side of the cavity to give rise to the second slab doped region as shown in FIG. 4C. Again, a photoresist 404 is deposited onto the first material M.sub.1; this time covering all but a second portion of the base of the cavity, the uncovered portion of the base 402a extending from the second cavity edge 402c to less than half way across the total length of the base of the cavity such that it does not contact the already implanted n doped region. An implantation step is carried out on the uncovered second portion of the base of the cavity 402a to implant the uncovered second portion with a second dopant, in this case a p type dopant, in order to create a second slab doped portion 14a. Again, the doped portion has a dopant concentration which may lie within the range of 1e18-1e20 cm.sup.−3. The doped portions extend by respective depths of d.sub.np1 and d.sub.pp1 into the base of the cavity (i.e. into the slabs of the finished device).

(47) A subsequent annealing step is carried out as shown in FIG. 4D.

(48) Following annealing, a second material M.sub.2 is grown inside the cavity, the second material being different from the first material. In this embodiment, the second material M.sub.2 is typically epitaxially grown Silicon Germanium (SiGe), although it is envisaged that other optically suitable materials could be used including: III-V materials and metal alloys of silicon, germanium or SiGe. The height by which the epitaxially grown layer M.sub.2 extends from the base of the cavity will form the height of the upper portion of the ridge.

(49) A further etch step to create the upper ridge portion 12b is then carried out, as shown in FIG. 4F, in which a region of the second material M.sub.2 is etched away above the first slab doped portion 13a and a region of the second material M.sub.2 above the second slab doped portion 14a. The etching extends along the entire depth of the cavity such that uncovered doped regions are left fully exposed either side of the remaining portion of the second material M.sub.2 which forms the upper ridge portion of the optoelectronic device. Note that each of the first and second slab doped portions actually extend laterally beyond the first and second side walls of the upper portion of the ridge such that the upper portion of the ridge overlays part of the first slab doped portion and also overlays part of the second doped portion.

(50) Once the upper ridge has been created, sidewall implantation steps are carried out to implant the first and second sidewalls 13c, 14c with n and p dopants respectively. Firstly, as shown in FIG. 4G, the first sidewall of the upper ridge 12b is doped by applying a photoresist to cover the second material M.sub.2 and the second doped slab region 14a before implanting the n-type dopants at the first sidewall 13c at an angle to the sidewall. The first cavity edge 402b can be used as a shield, and the angle of implantation therefore chosen such that the edge of the cavity shields the first doped slab portion, meaning that the n dopant is applied only to the first sidewall 13c and not to the first doped slab region. As depicted in FIG. 4H, the sidewall implantation steps are then repeated for the second sidewall 14c of the ridge. Specifically, a photoresist to cover the second material M.sub.2 and the first doped slab region 13a before implanting the p-type dopants at the sidewall at an angle to the sidewall is applied. The first cavity edge 402c can be used as a shield, and the angle of implantation therefore chosen such that the edge of the cavity shields the second doped slab portion, meaning that the p dopant is applied only to the second sidewall 14c and not to the second doped slab region.

(51) A further annealing step is carried out as depicted in FIG. 4I. Annealing (as shown in both FIG. 4D and FIG. 4I) may be performed at temperatures of 450-800° C. and for a typical duration of 30 minutes or less.

(52) FIG. 4J depicts a further etch step in which the first and second doped slab portions 13a, 14a are etched by a depth h.sub.2, thereby creating the lower ridge portion 12a. For some embodiments of the optoelectronic device such as that shown in FIG. 5, there is no lower ridge 12a, so this extra etch step is not carried out. The embodiment of FIG. 5 differs from that of FIG. 2A and FIG. 3A only in that the ridge 12 of the optoelectronic device is made entirely of the second material M.sub.2, so there is no lower ridge portion. Instead, the ridge is a single piece of the second material M.sub.2 which extends directly from the base of the device. Finally (not shown), the remainder of the cavity walls may be etched away to leave the final device. FIG. 6 depicts a further alternative embodiment of a waveguide electro-absorption modulator according to the present invention, this embodiment differing from that of FIG. 2A in that it includes further grading within the dopant regions. The graded dopant regions include a first intermediate doped region 16a having a dopant of the same type as the first slab portion but of a dopant concentration between that of the first doped slab portion and the first doped sidewall; and a second intermediate doped region 16b having a dopant of the same type as the first slab portion but of a dopant concentration between that of the second doped slab portion and that of the second doped sidewall. The first and second intermediate doped regions may be applied in the same method as the sidewall doped portions but using a steeper implantation angle (i.e. the angle of implantation angle makes a smaller angle with the sidewall than the angle of implantation used for the sidewall doping.

(53) FIGS. 7A and 7B depict a further alternative embodiment of a waveguide electro-absorption modulator according to the present invention, along the lines A-A′ and B-B′ respectively. This embodiment differs from that shown in FIG. 6 in that the device further includes a region 701 formed of a lower refractive index material M.sub.3 (for example, silicon oxide) as well as a further substrate material 702 (for example, silicon). This may be provided by etching a region of the region 701, so as to provide a cavity with width W.sub.e, and then to grow further substrate material into that cavity. The width W.sub.e in this example has a value from 0.5 μm to 20 μm. The region 701, as shown in FIG. 7A, can be characterized in having a gap therein which is below the active waveguide region. The region 701 has a height h.sub.b in this example from 0.2 μm to 4 μm. The substrate material 702 may have a thickness from 200 μm to 800 μm. The part of the device shown in FIG. 7B has a complete region 701 below the passive waveguide i.e. it has not been etched, and so is substantially continuous. This structure can improve mode matching from the passive waveguide shown in FIG. 7B to the active waveguide shown in FIG. 7A.

(54) FIG. 8 shows a further alternative embodiment of a waveguide electro-absorption modulator according to the present invention, along the line A-A′. This embodiment differs from that shown in FIG. 7a in that one upper sidewall portion 14c of the waveguide comprises a silicon region 901, having a width W.sub.b which is within the range 0.1 μm to 0.4 μm. The silicon upper sidewall portion 901 is either entirely or partially n or p doped, while the opposite upper sidewall portion 13c is formed of SiGe or Ge and contains dopants of an opposite polarity to the silicon upper sidewall portion. The addition of a silicon sidewall helps reduce the capacitance of the device, and can therefore increase the operational radio-frequency bandwidth.

(55) Although exemplary embodiments of a waveguide electro-absorption modulator and method of manufacturing a rib waveguide modulation region have been specifically described and illustrated herein, many modifications and variations will be apparent to those skilled in the art. Accordingly, it is to be understood that a waveguide electro-absorption modulator constructed according to principles of this invention may be embodied other than as specifically described herein. The invention is also defined in the following claims, and equivalents thereof