22\ US B2. (*) Notice: Subject to any disclaimer, the term of this. (12) United States Patent Hui et al.

Transcription

1 III III 0 II III III US B2 lid II0I II 0I II (12) United States Patent Hui et al. (10) Patent o.: US 9,678,370 B2 (45) Date of Patent: Jun. 13, 2017 (54) CARRIER-DEPLETIO BASED SILICO WAVEGUIDE RESOAT CAVITY MODULATOR WITH ITEGRATED OPTICAL POWER MOITOR (71) Applicants:IMEC VZW, Leuven (BE); Universiteit Gent, Ghent (BE) (72) Inventors: Yu Hui, Hangzhou (C); Wi Bogaerts, Melle (BE) (73) Assignees: IMEC VZW, Leuven (BE); Universiteit Gent, Ghent (BE) (*) otice: Subject to any disclaier, the ter of this patent is extended or adjusted under 35 U.S.C. 154(b) by 81 days. (21) Appl. o.: 14/338,564 (22) Filed: Jul. 23, 2014 (65) Prior Publication Data US 2015/ Al ov. 5, 2015 (30) Foreign Application Priority Data Jul. 23, 2013 (EP) (51) Int. Cl. G02F 1/025 ( ) G02F 1/01 ( ) G02F 1/313 ( ) (52) U.S. Cl. CPC...G02F 1/025 ( ); G02F 1/0121 ( ); G02F 1/3132 ( ); GO2F 2201/58 ( ); G02F 2203/15 ( ) (58) Field of Classification Search one See application file for coplete search history. (56) References Cited U.S. PATET DOCUMETS 2002/ Al* 10/2002 Wagner...H01L31/ / / Al * 4/2003 Doash...G02B 6/ / / Al* 3/2012 Costa...H01L27/ /2 2014/ Al* 4/2014 Li...G01J1/ / Yu, Hui et al., "Using Carrier-Depletion Silicon Modulators for Optical Power Monitoring", Optics Letters, vol. 37, o. 22, ov. 15, 2012, pp Tu, Xiaoguang et al., "Fabrication of Low Loss and High Speed Silicon Optical Modulator Using Doping Copensation Method", Optics Express, vol. 19, o. 19, Sep. 12, 2011, pp * cited by exainer Priary Exainer Uyen Chau Le Assistant Exainer Chad Sith (74) Attorney, Agent, or Fir McDonnell Boehnen Hulbert & Berghoff LLP (57) ABSTRACT A carrier-depletion based silicon waveguide resonant cavity odulator includes a silicon waveguide based resonant cavity. The resonant cavity includes an optical odulation section and an optical power onitoring section. The optical power onitoring section includes an integrated lateral PI diode including a doping copensated I region having a high defect density and a low net free carrier concentration. The doping copensated I region ay be fored by perforing a P-type iplantation step and an -type iplantation step with overlapping ion iplantation windows. 9 Clais, 3 Drawing Sheets 22\ 21 B A' 10

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5 1 CARRIER-DEPLETIO BASED SILICO WAVEGUIDE RESOAT CAVITY MODULATOR WITH ITEGRATED OPTICAL POWER MOITOR CROSS-REFERECE TO RELATED APPLICATIO US 9,678,370 B2 This application clais priority to European Patent Application o filed on Jul. 23, 2013, the contents of 10 which are hereby incorporated by reference. FIELD The present disclosure is related to carrier-depletion based 15 silicon waveguide resonant cavity odulators having an integrated optical power onitor, and to ethods for fabricating such resonant cavity odulators with an integrated power onitor. A carrier-depletion based silicon waveguide resonant cay- 20 ity odulator of the present disclosure ay be integrated on a silicon photonics platfor without the need for additional process steps. BACKGROUD 25 An objective of silicon photonics is to integrate different functionalities, including wavelength ultiplexing and deultiplexing, routing, optical eission, odulation, and detection on a silicon based platfor, such as a silicon-on- 30 insulator (SOI) based platfor. A copact, power-efficient and high-speed integrated silicon odulator is a key building block of silicon photonics. The carrier-depletion odulator is becoing a preferred solution for electro-optic odulation in silicon, because of 35 its copatibility with copleentary etal-oxide seiconductor (CMOS) process technology, its processing siplicity, and its high operation speed. For exaple, a carrier-depletion based icro-ring odulator ay coprise a icro-ring resonator with an integrated 40 Pjunction. The icro-ring can be coupled to a neighboring waveguide. The resonant wavelength ay be odified by tuning the effective refractive index of the icro-ring waveguide. Tuning is obtained by reverse biasing the P junction integrated with the icro-ring. 45 Due to its resonant nature, the icro-ring odulator has the advantages of copact size, low power consuption, and low driving voltage. However, a drawback of this structure is that optical odulation occurs only within a narrow wavelength range near the resonance wavelength. A 50 typical optical bandwidth of effective odulation is less than 0.1 n. Further, a silicon waveguide is very sensitive to teperature variations. For exaple, a change in the abient teperature of 1 C. results in a shift of the resonance 55 wavelength of a silicon icro-ring odulator by about 0.1 n. This akes the silicon icro-ring odulator very sensitive to any teperature fluctuation. Therefore, a silicon icro-ring odulator generally needs to be integrated with a dynaic theral stabilization syste. 60 Such dynaic theral stabilization syste ay include a heater to change the teperature, a feedback circuit to control the heater power, and an optical power onitor that onitors the ring dynaics and that provides a feedback signal to the circuit controlling the heater power. 65 The 1.12 ev band gap of silicon akes it transparent in the teleco wavelength band around 1.55 ti. In order to 2 enable onitoring of the optical power inside a silicon icro-ring odulator, several solutions have been proposed, such as: integration of a Geraniu photodetector by eptiaxial growth; bonding of a Ill-V seiconductor based photodetector; or using a dedicated ion iplantation step (typically Si+ iplantation) to introduce lattice defects in the silicon waveguide in a predeterined area. These lattice defects ay lead to defect-state ediated sub-bandgap absorption and result in the generation of a photocurrent. While the observed effect is generally uch weaker than in case of direct absorption in a Ge or Ill-V photo-detector, it is an advantage of this approach that the functionality ay be ipleented relying exclusively on silicon carrier-depletion odulation without adding processing steps. A silicon icro-ring odulator wherein defect-state ediated absorption is used for integrated power onitoring has been described by Hui Yu et al. in "Using carrierdepletion silicon odulators for optical power onitoring", Optics Letters Vol. 37, o. 22, A icro-ring resonator with a lateral P junction is described, wherein the P junction is ebedded in an SOI waveguide by ion iplantation. After ion iplantation, a rapid theral annealing (RTA) ay be perfored at a teperature above 1000 C. to activate the dopants and to repair daage in the silicon lattice without causing uch dopant redistribution. Still, there ay reain soe residual crystal defects that can ediate sub-bandgap absorption. It is shown that, after being heated to 1075 C., a reverse-biased P diode still produces a substantial photocurrent, based on defect-ediated absorption in the ion-iplanted SOI waveguides. This photocurrent is used as a power onitor feedback signal. This eans that the widely utilized carrier-depletion odulator ay also be used for optical power onitoring without any additional processing. For exaple, a sall section of a P-junction-ebedded ring ay be used to onitor the optical power inside the ring. It is suggested that the responsivity of both the odulator and the onitor ay be enhanced by using an interdigitated P junction instead of a lateral P junction, to enlarge the overlap between the optical ode and the carrier-depletion region, or by reducing the RTA teperature to increase the crystal defect density. SUMMARY The present disclosure ais to provide a carrier-depletion based silicon waveguide resonant cavity odulator with an integrated optical power onitor based on defect-ediated sub-bandgap absorption, wherein the power onitor has a good responsivity, and wherein the power onitor ay be integrated without the need for additional processing steps and without introducing any additional absorption aterials. The present disclosure further ais to provide a ethod for fabricating such resonant cavity odulator with an integrated power onitor. The present disclosure is related to a carrier-depletion based silicon waveguide resonant cavity odulator that includes a silicon waveguide based resonant cavity. In an exaple, the resonant cavity has an optical odulation section and an optical power onitoring section. The optical odulation section ay include an integrated lateral P diode, and the optical power onitoring section ay include an integrated lateral PI diode coprising a doping copensated I region having a high defect density and a low net free carrier concentration. In the context of the present disclosure, the ter "PI diode" refers to a diode having a p-type seiconductor region (P) and an n-type seiconductor region (), with

6 3 therein between a seiconductor region (I) having a net doping concentration that is lower than the net P-type doping concentration of the P region and lower than the net -type doping concentration of the region. In ebodients of the present disclosure, the doping copensated region (I region) ay be an intrinsic region or a near intrinsic region, or it ay be a lowly doped p-type region or a lowly doped n-type region. In operation, when light propagates through the resonant cavity silicon waveguide, free photo-carriers ay be generated in the PI diode through defect ediated absorption, resulting in a photocurrent. The photocurrent is a easure for the optical power inside the resonant cavity and ay be used as a feedback signal to a circuit controlling a heater power. In a fabrication process of a carrier-depletion based silicon waveguide resonant cavity odulator of the present disclosure, the doping copensated region ay for exaple be fored by perforing a P-type iplantation step using a first iplantation window and by perforing an -type iplantation step using a second iplantation window, with overlapping first and second ion iplantation windows at the location where the doping copensated region is to be provided. The sae ion iplantation steps as used in the fabrication of the optical odulation section of the carrierdepletion based odulator ay be used to realize the in-waveguide optical power onitor. Therefore, a PI photodiode wherein photocarriers are generated through defect ediated absorption ay be obtained inside a waveguide based resonant cavity using the sae process flow as used for fabricating the carrier-depletion based odulator. In ebodients of the present disclosure, the waveguide resonant cavity ay for exaple coprise a icro-ring resonator or a Fabry-Pérot cavity, the present disclosure not being liited thereto. It is a potential advantage of devices and ethods of the present disclosure that there is no need for using non-silicon photoactive aterials, such as Ge or Ill-V seiconductor aterials, to enable optical power onitoring. It is a potential advantage of devices and ethods of the present disclosure that the doping copensated region ay have a high defect density, resulting in a good photocurrent (good responsivity) through defect ediated absorption, while having a low net free carrier concentration, thus avoiding or reducing undesired free carrier absorption losses. It is a potential advantage of devices and ethods of the present disclosure that there is no need for additional processing steps to realize the PI photodiode. The processing of the PI photodiode is fully copatible with the processing of the silicon carrier-depletion odulator. Therefore, the power onitor (PI diode) ay be integrated without additional cost. It is a potential advantage of ebodients of the present disclosure that the optical power onitor ay be provided directly inside the waveguide based resonant cavity odulator, thus leading to a reduced coplexity of the photonics circuit. Certain objects and advantages of various inventive aspects have been described hereinabove, Of course, it is to be understood that not necessarily all such objects or advantages ay be achieved in accordance with any particular ebodient of the disclosure. Thus, for exaple, those skilled in the art will recognize that the disclosure ay be ebodied or carried out in a anner that achieves or optiizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advan- US 9,678,370 B2 4 tages as ay be taught or suggested herein. Further, it is understood that this suary is erely an exaple and is not intended to liit the scope of the disclosure. The disclosure, both as to organization and ethod of operation, 5 together with features and advantages thereof, ay best be understood by reference to the following detailed description when read in conjunction with the accopanying drawings. 10 BRIEF DESCRIPTIO OF THE FIGURES FIG. 1 scheatically illustrates a icro-ring coprising an optical odulation section and an optical power onitoring section in accordance with an ebodient of the 15 present disclosure. FIG. 2 scheatically shows a cross section along line A-A' (FIG. 1) of the optical odulation section of a icroring according to an ebodient of the present disclosure. FIG. 3 scheatically shows a cross section along line 20 B-B' (FIG. 1) of the optical power onitoring section of a icro-ring according to an ebodient of the present disclosure. Any reference signs in the clais shall not be construed as liiting the scope of the present disclosure. 25 In the different drawings, the sae reference signs refer to the sae or analogous eleents. DETAILED DESCRIPTIO 30 In the following detailed description, nuerous specific details are set forth in order to provide a thorough understanding of the disclosure and how it ay be practiced in particular ebodients. However, it will be understood that the present disclosure ay be practiced without these spe- 35 cific details. In other instances, well-known ethods, procedures and techniques have not been described in detail, so as not to obscure the present disclosure. The present disclosure will be described with respect to particular ebodients and with reference to certain draw- 40 ings but the disclosure is not liited thereto but only by the clais. The drawings described are only scheatic and are non-liiting. In the drawings, the size of soe of the eleents ay be exaggerated and not drawn on scale for illustrative purposes. The diensions and the relative 45 diensions do not necessarily correspond to actual reductions to practice of the disclosure. Furtherore, the ters first, second, third and the like in the description and in the clais, are used for distinguishing between siilar eleents and not necessarily for describing 50 a sequential or chronological order. The ters are interchangeable under appropriate circustances and the ebodients of the disclosure can operate in other sequences than described or illustrated herein. Moreover, the ters top, botto, over, under and the like 55 in the description and the clais are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the ters so used are interchangeable under appropriate circustances and that the ebodients of the disclosure described herein are 60 capable of operation in other orientations than described or illustrated herein. The ter "coprising", used in the clais, should not be interpreted as being restricted to the eans listed thereafter; it does not exclude other eleents or steps. Rather, it should 65 generally be interpreted as specifying the presence of the stated features, integers, steps or coponents as referred to, but does not preclude the presence or addition of one or

7 US 9,678,370 B2 5 ore other features, integers, steps or coponents, or ing, e.g. Rapid Theral Annealing, for dopant activation. groups thereof. Thus, the scope of the expression "a device The doping concentration in the P-type region 33 and in the coprising eans A and B" should not be liited to devices -type region 34 ay for exaple be in the order of 1018 consisting only of coponents A and B. c 3, the present disclosure not being liited thereto. The present disclosure relates to a carrier-depletion based 5 In ebodients of the present disclosure, the location of silicon waveguide resonant cavity odulator with an integrated optical power onitor based on defect ediated tially the sae lateral position along the length of the optical the P junction in the strip portion 202 ay be at substan- carrier absorption, wherein the power onitor has an odulation section 21. In other ebodients of the present iproved responsivity as copared to prior art power onitors, and wherein the power onitor ay be integrated 10 portion 202 ay vary along the length of the optical odu- disclosure, the location of the P junction in the strip without the need for additional processing steps and without lation section 21: the P junction ay for exaple be introducing any additional absorption aterials. interdigitated. The present disclosure is further described for ebodients wherein the silicon waveguide resonant cavity odu- P-type contact 37 and an -type contact 38, to enable tuning As illustrated in FIG. 2, the icro-ring also contains a lator is a silicon icro-ring odulator. However, the present 15 of the effective refractive index in the optical odulation disclosure is not liited thereto. The silicon waveguide section 21 by depleting the P junction (reverse biasing) resonant cavity ay for exaple be a Fabry-Pérot cavity or inside the icro-ring waveguide 20. A heavily doped P any other suitable cavity known by a person skilled in the region 35 and a heavily doped region 36 are fored in art. the silicon layer at the location of respectively the P-type FIG. 1 scheatically illustrates a icro-ring odulator 20 contact 37 and the -type contact 38 to enable the foration according to an ebodient of the present disclosure (top of good ohic contacts. The Pttype region and the ttype view). The odulator coprises a first waveguide 10, which region ay for exaple be fored by ion iplantation ay for exaple be a straight waveguide as shown in FIG. followed by annealing, e.g. Rapid Theral Annealing, for 1, the present disclosure not being liited thereto, and a dopant activation. The doping concentration in the Pttype second waveguide 20 having a icro-ring shape, the second 25 region and in the ttype region ay for exaple be in the waveguide 20 being optically coupled to the first waveguide order of 1020 c 3, the present disclosure not being liited 10 by evanescent coupling. The icro-ring waveguide 20 thereto. The P-type contact 37 ay for exaple be electrically connected to the heavily doped P region 35 though the coprises an optical odulation section 21 and an optical power onitoring section 22. upper cladding layer 32 by eans of electrically conductive In the exaple shown in FIG. 1, the second waveguide plugs 39. Siilarly, the -type contact 38 ay for exaple has a ring shape. However, the present disclosure is not be electrically connected to the heavily doped + region 36 liited thereto and any other suitable (closed) shape known though the upper cladding layer 32 by eans of electrically to a person skilled in the art ay be used for second conductive plugs 39. waveguide 20, such as for exaple a racetrack shape or a Although in FIG. 2 the icro-ring waveguide shown is a rectangular shape with rounded corners. 35 rib waveguide, wherein an electrical path between the heavily doped P region 35 (p-type contact 37) and the P In ebodients of the present disclosure, the first waveguide 10 and the second waveguide 20 (e.g. icro-ring) ay junction in the strip portion 202 of the waveguide and for exaple have a silicon core layer and silicon oxide between the heavily doped region 36 (-type contact 38) cladding layers. For exaple, the waveguide core ay be and the P junction is established through the doped slab fored in the device layer of an SQl substrate. However, the 40 portions 201, the present disclosure is not liited thereto. present disclosure is not liited thereto and other suitable For exaple, the icro-ring waveguide ay be a strip aterial cobinations ay be used. waveguide, wherein an electrical path is established between In ebodients of the present disclosure, the icro-ring the contacts and a P junction in the strip waveguide by odulator is a carrier depletion-based odulator with an foring doped silicon bridges between the contact areas and integrated P junction. 45 the strip waveguide. Other suitable configurations ay be FIG. 2 shows a cross-section along line A-A' (optical used. odulation section 21) and FIG. 3 shows a cross-section In the optical power onitoring section 22, scheatically along line B-B' (optical power onitoring section 22) of the illustrated in FIG. 3, the rib waveguide coprises a P-type icro-ring waveguide 20 of FIG. 1. In the exaple illustrated in FIG. 2 and FIG. 3, the icro-ring waveguide is a 50 -type region 34 at a second lateral side of the strip portion, region 33 at a first lateral side of the strip portion 202, an rib waveguide, containing slab portions 201 (shallow etched and a doping copensated region 40 between the P-type portions) and a strip portion 202 (unetched portion) wherein region 33 and the -type region 34, the doping copensated light ay be confined. The core of the icro-ring waveguide region 40 being located in the strip portion 202. In this way 20 is stacked between a lower cladding layer 31 and an a lateral PI diode is fored, with the doping copensated, upper cladding layer 32. The lower cladding layer 31 ay 55 e.g. intrinsic, region located in the strip portion of the for exaple be a silicon oxide layer, such as an oxide layer icro-ring. Although in the exaple illustrated in FIG. 3, of a SQl substrate. The upper cladding layer 32 ay for the P-type region 33 and the -type region 34 both extend exaple be a deposited silicon oxide layer. into the strip portion of the icro-ring, the present disclosure In the optical odulation section 21 of the icro-ring is not liited thereto. (FIG. 2), the rib waveguide coprises an integrated P 60 In ebodients of the present disclosure, the doping junction fored by a P-type region 33 at a first lateral side copensated region 40 is a region with a high defect density. of the strip portion 202 and an -type region 34 at a second This high defect density ay for exaple be obtained by lateral side of the strip portion 202. The P-type region 33 and doping copensation. In ebodients of the present disclosure, the doping copensated region 40 ay for exaple the -type region 34 for a lateral P junction that is located in the strip portion 202 of the icro-ring waveguide 65 be fored as a result of the ion iplantation steps that are 20. The P-type region 33 and the -type region 34 ay for used for foring the P-type region 33 and the -type region exaple be fored by ion iplantation followed by anneal- 34 (both in the optical power onitoring section 22 and in

8 US 9,678,370 B2 7 the optical odulation section 21), by providing an overlap between both ion iplantation windows. In other ebodients, the doping copensated region 40 ay for exaple be fored as a result of the ion iplantation steps that are perfored for foring the Pttype region 35 and the 5 ttype region 36, by providing an overlap between both ion iplantation windows. In still other ebodients an overlap between ion iplantation windows ay be provided for both the P-type and -type iplantation and the Pttype and ttype iplantation to for the copensated region By providing an overlap between the P-type (Pttype) ion iplantation window and the -type (ttype) ion iplantation window in the optical power onitoring section 22, in the strip portion 202 of the waveguide, the -type dopants 15 and the P-type dopants ay at least partially, e.g. copletely, copensate each other and a doping copensated region 40, such as for exaple an intrinsic region, ay be fored. As a result, the free carrier concentration reains low in the doping copensated region 40, which eliinates 20 undesired free carrier absorption. On the other hand, the density of lattice defects doubles in this region 40 due to the repeated ion iplantations. The lattice defects result in defect states in the forbidden band of silicon. Through these states, photons at 1.55 t whose energy is below the gap of 25 the forbidden band ay excite electrons fro the valence band to the conduction band, which is well-known as defect ediated sub-bandgap absorption. Generally, the optical-toelectrical conversion quantu efficiency of the defect ediated absorption is proportional to the defect density. 30 It is a potential advantage of ebodiens of the present disclosure that only existing ion iplantation steps that are indispensable to ipleent a carrier-depletion based odulator ay be used, so no additional processing steps are required. 35 In operation, when light propagates through the icroring 20, part of the propagating light is absorbed by defect ediated absorption in the power onitoring section 22 of the icro-ring. This absorption gives rise to a photocurrent that is a easure for the optical power inside the icro-ring. 40 This photocurrent ay be used as a feedback signal towards a feedback control circuit that adjusts the power of an integrated heater, so as to dynaically control the operation wavelength of the ring odulator. The foregoing description details certain ebodients of 45 the disclosure. It will be appreciated, however, that no atter how detailed the foregoing appears in text, the disclosure ay be practiced in any ways. It should be noted that the use of particular terinology when describing certain features or aspects of the disclosure should not be taken to 50 iply that the terinology is being re-defined herein to be restricted to including any specific characteristics of the features or aspects of the disclosure with which that terinology is associated. While the above detailed description has shown, 55 described, and pointed out novel features of the disclosure as 8 applied to various ebodients, it will be understood that various oissions, substitutions, and changes in the for and details of the device or process illustrated ay be ade by those skilled in the technology without departing fro the disclosure. The invention claied is: 1. A carrier-depletion based silicon waveguide resonant cavity odulator coprising: a silicon waveguide based resonant cavity, wherein the resonant cavity includes an optical odulation section and an optical power onitoring section, wherein the optical power onitoring section includes an integrated lateral PI diode including a doping copensated I-region having a high defect density, wherein the doping copensated I-region is fored by perforing a P-type ion iplantation in the I-region and an -type ion iplantation in the I-region. 2. The carrier-depletion based silicon waveguide resonant cavity odulator according to clai 1, wherein the lateral PI diode coprises a P-doped region having a net P-type doping concentration, an -doped region having a net -type doping concentration, and the doping copensated I-region is between the P-doped region and the -doped region, wherein the I-region has a net doping concentration that is lower than the net P-type doping concentration and lower than the net -type concentration. 3. The carrier-depletion based silicon waveguide resonant cavity odulator according to clai 2, wherein the doping copensated region is an intrinsic region. 4. The carrier-depletion based silicon waveguide resonant cavity odulator according to clai 3, wherein the resonant cavity includes a icro-ring resonator. 5. The carrier-depletion based silicon waveguide resonant cavity odulator according to clai 3, wherein the resonant cavity coprises a Fabry-Pérot cavity. 6. The carrier-depletion based silicon waveguide resonant cavity odulator according to clai 4, wherein icro-ring resonator includes an etched portion and an unetched portion, and wherein P-doped region, the -doped region, and the I-region extend into the unetched portion. 7. The carrier-depletion based silicon waveguide resonant cavity odulator according to clai 4, wherein the optical odulation section includes an integrated P junction fored by a second P-doped region and a second -doped region. 8. The carrier-depletion based silicon waveguide resonant cavity odulator according to clai 7, wherein icro-ring resonator includes an etched portion and an unetched portion, and wherein the P junction fored by the second P-doped region and the second -doped region is disposed in the unetched portion. 9. The carrier-depletion based silicon waveguide resonant cavity odulator according to clai 8, wherein first P-doped region, the first -doped region, and the I-region extend into the unetched portion. * * * * *