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Towards low-loss waveguides in SOI and Ge-on-SOI for mid-IR sensingTo cite this article: Usman Younis et al 2018 J. Phys. Commun. 2 045029

 

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J. Phys. Commun. 2 (2018) 045029 https://doi.org/10.1088/2399-6528/aaba24

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Towards low-loss waveguides in SOI and Ge-on-SOI for mid-IRsensing

UsmanYounis1,2,6 , Xianshu Luo3, BoweiDong1, LiHuang1, SudheerKVanga4, Andy Eu-Jin Lim3,PatrickGuo-Qiang Lo3, Chengkuo Lee1 , AndrewABettiol5 andKah-WeeAng1

1 Department of Electrical &Computer Engineering, NationalUniversity of Singapore, 4 EngineeringDrive 3, Singapore 1175832 Department of Electrical Engineering, Information TechnologyUniversity, Arfa Software Technology Park, Lahore, Pakistan 540003 Institute ofMicroelectronics, A*STAR (Agency for Science, Technology andResearch), 2 FusionopolisWay,#08-02 Innovis Tower,

Singapore 1386344 Department of Chemistry, University ofWisconsin, 1101University Ave,Madison,WI 53706,United States of America5 Department of Physics, National University of Singapore, 2 ScienceDrive 3, Singapore 1175426 The author towhomany correspondence should be addressed.

E-mail: usman.younis@itu.edu.pk and kahwee.ang@nus.edu.sg

Keywords: silicon on insulator technology, Germanium,waveguide components, infraredmeasurements,monolithic integrated circuits

AbstractSilicon-on-insulator is an attractive choice for developingmid-infrared photonic integrated circuits.It benefits frommature fabrication technologies and integrationwith on-chip electronics.We reportthe development of SOI channel and ribwaveguides formid-infraredwavelengths centered at 3.7 μm.Propagation loss of∼1.44 dB/cm and∼1.2 dB/cmhas beenmeasured for TE andTMpolarizations inchannel waveguides, respectively. Similarly, propagation loss of∼1.39 dB/cm and∼2.82 dB/cmhasbeenmeasured for TE andTMpolarized light in ribwaveguides. The propagation loss is consistentwith themeasurements obtained using a different characterization setup and for the samewaveguidestructures on a different chip. Given the tightly confined single-mode in our 400 nm thick Si core, thispropagation loss is among the lowest losses reported in literature.We also report the development ofGe-on-SOI stripwaveguides formid-infraredwavelengths centered at 3.7 μm.Minimumpropagationloss of∼8 dB/cmhas beenmeasuredwhich commensurate with that required for high powermid-infrared sensing. Ge-on-SOIwaveguides provide an opportunity to realizemonolithically integratedcircuit with on-chip light source and photodetector.

1. Introduction

Mid-infrared sensing has been an attractive area of research sincemost of themolecules find their vibrationalresonances in 2–20 μmpart of the electromagnetic spectrum. Startingwith lead-salt lasers, nonlinear opticalfrequency conversion sources, and quantum cascade lasers (QCLs), manymajor breakthroughs have beenreported over the decades formid-IR sensing applications [1]. In recent times we are now seeing its revival insilicon-on-insulator (SOI) based photonic integrated circuits (PICs). There has always been an industrial need todevelopmid-IR sensing PICs to trace gases which are hazardous for theworking environment. Secondly,mid-IRsensing can be employed to trace pollutants in the closed environment such as hospitals and large communalspaces. Other applications which can be realized usingmid-IR sensing PICs are security, forensics, clinicalanalysis, and foodmonitoring.

SOI is undoubtedly themost popularmaterial for the development of near-infrared optical devices [2–5],and in particular, for the realization of low-loss conventional optical elements including filters, splitters/combiners, tapers, directional couplers, resonators,multiplexers, and grating couplers [6]. These elements arenow readily available as constituent components for the development of PICs in all the leading foundries [7].Although,most of the developments in SOI photonics have concentrated largely on telecommunications,however, there is a recent surge inmaking use of SOI formid-IR applications. It was noted previously that thelow-loss transmission for SOIwaveguides is limited up to 3.6 μm [8], however, the optical elements

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demonstrated for thewavelengths up to 3.8 μmhave proven that SOI is still amajor contender for opticalinterconnects inmid-IR PICs [9]. In addition to this, the suspended SOIwaveguides can extend the transmissionwell into long-wave IR [8].

SOIwaveguides have been reportedwith various Si layer thicknesses in literature for 3.7–3.8 μmwavelengths. A 2 μmthick Si core ribwaveguide achievingminimum loss of 1.5 dB/cmhas been reported in[10]. Silicon corewith thickness of 400 nmhas been employed in SOI stripwaveguide to achieve propagationloss of 3.1 dB/cm in [11], and ribwaveguide to achieve propagation loss of 1.46 dB/cm in [12]. Finally, SOI stripwaveguide formed in 500 nm thick Si core has been reported to achieve propagation loss of 1.28 dB/cm in [9],which is the lowest value reported formid-IRwaveguides. It should be noted that these propagation losses havebeen reported only for TE polarization.

A significant challenge however is the integration of on-chip pump source in SOI basedmid-IR PICs.Although hybrid integration of near-IR sources in SOI platform is now amature technology, there have been fewdemonstrations to extend this integration intomid-IR [13].More recently, we have seen integration ofQCL onSi-on-N-on-insulator (SONOI) [14]. This opens up the option of developingmid-IR sensors for wavelengthsbeyond 3 μmandup to the available wavelength range ofQCLs. Itmeans that the detection of trace gases such asmethane (CH4)which finds its absorption peak near 3.5 μmcan be realized.We can develop SOI based PICs todetect pollutants such as formaldehyde (H2CO). Integration of tunable on-chip sources would help us achievegas imaging systems andmid-IR spectroscopy, however, it is a challenge realizing this usingQCLswhoseemission is defined at thewafer growth stage. Hybrid integration of III-V nonlinear optical frequency converterswith Si can be one of theways forward to achieve tunable on-chipmid-IR pump sources, wheremonolithicintegration of nonlinear optical frequency conversion has been previously reported for near-IRwavelengths[15]. In these sources, the signal and idler wavelengths are controlled by the phasematching period of nonlinearwaveguide and can be extended to producemid-IRwavelengths. Finally, it is encouraging to note that thefoundry scale efforts have started to develop low-lossmid-IR optical elements in SOI for sensingapplications [16].

Germanium is another contender to realize low-loss PIC formid-IR sensing applications due to itstransparency up to 15 μm. It has been employed to demonstrate various low-lossmid-IR optical elements on Sisubstrate [17]. Additionally, germanium-tin (GeSn), an alloy ofGe, has gained significant popularity as on-chipmid-IR source inGe-on-Si [18]. GeSn has also beenwidely reported for photodetection, and a relatively higherconcentration of 9%–10%Snhas been used to demonstrate photodetector and photoconductor for 2.2–2.4 μmwavelengths in [19] and [20], respectively. AwaveguideGeSn photodetector has been reported in [21]. Thereforeit is feasible to realize amonolithically integratedGeSn sensor forwavelengths near 2.2 μm in order to detectgreenhouse gases such aCO2. Finally, it has been reported that Ge-on-SOI provides better thermal stability andelectrical isolation thanGe-on-Si [22–24]. Therefore it is preferred to achievemonolithic integration ofGeSnmid-IR sensors inGe-on-SOI platform.

Low-loss Ge-on-Simid-IRwaveguides have been consistently reported previously, such as in [17, 25–27].Secondly, Ge-rich Si1−xGexwaveguides grown using low energy plasma enhanced chemical vapor deposition(LEPECVD) have demonstrated low-loss for wavelengths up to 8.5 μm [28, 29].Mid-IRwaveguides have alsobeen demonstrated inGe-on-insulator (GeOI) [30], andGe-on-SOI [22]. However, the achieved propagationloss inGeOI andGe-on-SOI is still higher than the reported values of 3–4 dB/cmor less inGe-on-Si andGe-richSi1−xGexwaveguides.

In this paperwe present ourwork on the development of low-lossmid-IRwaveguides in SOI andGe-on-SOI. Thework reported here complements the development ofmid-IR PICs for sensing applications. For SOIwaveguides, two structures including channel and ribwaveguides have been fabricated forwavelengths near3.7 μm. Secondly, Ge-on-SOIwaveguides have been developed formid-IRwavelengths centered at 3.7 μm.Stripwaveguides inGe core of thicknesses 0.85 μmand 2 μmhave been developed.

2. SOImid-IRwaveguides

In this work, Si channel waveguide is designedwith a dimension of 400 nm in height and 1.2 μm inwidth toensure singlemode operation at thewavelength of 3.7 μm. For ribwaveguide, the height andwidth is keptconsistent to the channel waveguide, whereas, the etch depth is 240 nm. Figures 1(a) and (b) show the simulatedTE00mode profile for channel and ribwaveguides, respectively. It is observed that the designedwaveguides areindeed single-mode for 3.7 μmexcitation.

Thewaveguide fabrication starts with commercially available 8-inch SOIwaferwith 220 nm thick Sioverlayer and 3 μmthick buried oxide layer. Firstly, blanket Si is epitaxially grown to achieve Si overlayer ofthickness 400 nm. SiO2 is then deposited as a hardmask forwaveguide definition. The device patterning has beendone using deep ultra-violet photolithography followed by single-step reactive ion etching forwaveguide

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definition. Plasma enhanced chemical vapour deposition SiO2 of thickness 3 μm is deposited as top cladding.Finally, deep trench is formed for end-fire coupling. In order to achieve efficient coupling into thewaveguides,200 μm long inverted nanotapers have been employed at each end of thewaveguides. The scanning-electronmicrographs (SEMs) of the fabricated ribwaveguide and nanotaper are given infigures 2(a) and (b), respectively.

Cut-backmethod has been employed tomeasure the propagation loss in our SOIwaveguides. The lightsource is a continuouswave tunablemid-IR quantum cascade laser fromPranalytica, Inc. The light set at3.682 μmhas been launched into thewaveguides using ZnSemid-IR objective. Amid-IRwave plate andpolarization beam cube has been used tomanipulate the polarization at the input. The output light is collimatedusing ZnSemid-IR objective and detected at PbSe photodetector. An iris is used to retain the excitedwaveguidemode at the photodetector. In addition to this, we have used amid-IR InSb camera fromXenics to precisely alignthewaveguides. Finally, themeasurements have been recorded using lock-in amplifier coupledwithmechanicalchopper. The schematic of characterization setup is given infigure 3.

The normalized transmission of waveguides of various lengths has been plotted infigures 4(a) and (b) forchannel and ribwaveguides, respectively. All the transmissionmeasurements have been performed by settingthe sample temperature at 300 K. The propagation loss calculated for channel waveguides is 1.44±0.03 and1.2±0.04 dB/cm for TE andTMpolarized light, respectively. This value is consistent with the propagation lossof 1.28±0.65 dB/cm at 3.8 μmreported for 500 nm thick Si core stripwaveguide in [9]. However, it should benoted that Si core in our case is 400 nm thickwhich results in higher confinement of thewaveguidemode.Wewould also like to highlight that themeasured loss of 1.44±0.03 dB/cm for TE polarization in this work ismoderately consistent with the loss of 2.65±0.08 dB/cmmeasured using the setup inwhich ZrF4fiber hasbeen used to excite the samewaveguide structures on a different chip [16, 31]. This demonstrates the process andperformance repeatability using differentmeasurement setups.

Figure 1.The simulated TE00mode profile for (a) channel waveguide, and (b) ribwaveguide. A commercial software has been used toperformmode-solving.

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Figure 2. SEMmicrographs of (a) ribwaveguide, and (b) inverted nanotaper employed at each end of thewaveguide.

Figure 3. Schematic of the optical characterization setup.

Figure 4.The normalized transmission of (a) channel waveguides, and (b) ribwaveguidesmeasured at room temperature. The dottedlines are a linearfit to the data in order to calculate the propagation loss.

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The propagation loss calculated for ribwaveguides usingmeasured data is 1.39±0.07 and 2.82±0.14 dB/cm for TE andTMpolarized light, respectively. The propagation loss in our ribwaveguides is consistent with thevalue of 1.46±0.2 dB/cm at 3.77 μmreported for 400 nm thick Si core and 220 nmdeep etched ribwaveguidein [12]. Again themeasured propagation loss of 1.39±0.07 dB/cm for TE polarization is consistent with theloss of 1.75±0.22 dB/cm reported for the samewaveguide structures on a different chip [16, 31].

The low propagation loss for TE andTMpolarizations in our case is complemented by the buried nature ofSOIwaveguides where SiO2 cladding, deposited using plasma enhanced chemical vapor deposition (PECVD),has enabled to circumvent effective surface and sidewall roughness. These results place ourmeasuredpropagation losses among the lowest reported for SOIwaveguides for 3.7–3.8 μmwavelengths.

3.Ge-on-SOImid-IRwaveguides

As stated in section 1, Ge-on-SOI is an attractive choice to realize amonolithically integratedmid-IR sensor. Itbenefits from an on-chip source and photodetector in the formof epitaxially grown direct band-gapGeSn.Secondly, it achieves better thermal and electrical isolation thanGe-on-Simaterial system. Therefore, we reportourwork on the development of low-loss Ge-on-SOI passivewaveguides which provide optical routing insuch PICs.

Mode-solving using a commercial software has been performed to determine the singlemode condition forGe-on-SOI stripwaveguides. The calculated ηeff for thefirst two guidedmodes is shown infigures 5(a) and (b),for 0.85 μmand 2 μmthickGe cores, respectively. The small index difference between the first two guidedmodes TE00 andTE01 indicatesmulti-mode excitation beyond 6 μmand 3 μmwidths for 0.85 μmand 2 μmthickGe cores, respectively. However, it has been observed inmeasurements that the onset ofmulti-modeexcitation is pushed to an increasedwidth in fabricatedGe-on-SOIwaveguides due to their relatively largepropagation loss, whichwill discussed later.

Stripwaveguides have been developed inGe-on-SOI, for which the starting SOI substrate has an overlayingSi layer of thickness 220 nmand a buried oxide of thickness 2 μm.TheGe growth process has been reported in[32].We have prepared two samples of Ge corewith thicknesses 0.85 μmand 2 μmin order to assess the lossvariation. Thewaveguide definition process includes patterning using direct laserwriting and etching inSF6/C4F8 chemistry using SiO2 hardmask. The samples arefinally laser diced to the length of∼5 mm for opticalmeasurements. The inset infigure 6 shows the SEM image of a fabricatedwaveguide in 2 μmthickGe core.

Fabry–Perotmethod has been employed tomeasure the propagation loss inwhich the cavity oscillationshave been recorded by varying sample temperature. The characterization setup is the same as describedpreviously in section 2, and given infigure 3. The calculated propagation loss using thesemeasurements isplotted infigure 6. Themeasurements show a decreasing trend in the propagation loss with an increase inwaveguidewidth. This can be explained due to the reduction inmode overlapwithwaveguide sidewalls [32].However, an abrupt increase in the propagation loss inwider waveguides, i.e., width beyond 7 μm in 0.85 μmthickGe core andwidth beyond 3.5 μmin 2 μmthickGe core, is attributed to themulti-mode excitation. This

Figure 5.The calculated effective refractive index ηeff for thefirst two guidedmodes TE00 andTE01 forGe core thicknesses (a)0.85 μm, and (b) 2 μm.The small index difference indicates that the onset ofmulti-mode excitation is near 6 μmwidth for 0.85 μmthickGe core, and 3 μmwidth for 2 μmthickGe core.

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width is slightly larger than the calculated onset of themulti-mode excitation as shown infigures 5(a) and (b) andis attributed to a relatively large propagation loss.

The lowest propagation loss achieved is∼8 dB/cm in ourGe-on-SOIwaveguides which is an improvementover 14 dB/cmpropagation loss achieved inGeOIwaveguides [30]. Secondly, the propagation loss achieved inour case is comparable to∼7 dB/cm reported earlier in [22] forGe-on-SOIwaveguides. Dislocation defectswhich developed duringGe growth on SOI have been identified as themain reason for a relatively largepropagation loss in our case, and it has been confirmed by performing transmission electronmicroscopy (TEM),as shown infigure 7.

Mid-IRwaveguides and devices developed inGe-on-Si have been reported to achieve superior performancedue to the techniques employed to significantly suppress the growth defects which occur atGe/Si interface. Onesuchmethod is to anneal the substrate at 850 °C [25]. Althoughwe have reported that post-growth rapid thermalannealing inGe-on-SOI has been able to improve the propagation loss up to 3.5 dB/cm [33], however achievingthe loss performance comparable with that of Ge-on-Si is still a challenge.

Having said that, the propagation loss of∼8 dB/cmmakes ourGe-on-SOIwaveguides useful for high powermid-IR sensing applications due to their large footprint. Ge-on-SOI has been used previously to demonstratehigh responsivitymonolithically integrated near-IR photodetector inwhichGe acts as an absorption layer and Siprovides waveguide core [34]. Similarly, a proposedmonolithically integratedGe-on-SOImid-IR sensorbenefits from low-loss propagation provided byGe core and the optical absorption in epitaxially grownGeSn

Figure 6.Propagation loss ofGe-on-SOIwaveguides. (dotted lines are a linear fit to the data).

Figure 7.The cross-sectional view of transmission electronmicrograph of Ge-on-SOIwaveguide. Dislocation defects, which occurduringmaterial growth, have been identified as themain cause of propagation loss.

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layer at the top, as shown infigure 8. GeSnwith 9%–10%Sn concentration is able to achieve photodetection in2.2–2.4 μmwavelengths [19, 20]. The large refractive index ofGeSn provides vertical coupling of light fromwaveguide core into the active layer. A possibility also exists to realize on-chip light source in the formofGeSninjection laser.

4. Conclusions

To conclude, we have developed singlemodewaveguides in SOI andGe-on-SOI formid-IRwavelengths. Thepropagation loss of 1.44 dB/cm and 1.2 dB/cmhas beenmeasured near 3.7 μmusing cut-backmethod for TEandTMpolarizations in channel waveguides, respectively. Similarly, propagation loss of 1.39 dB/cm and2.82 dB/cmhas beenmeasured for TE andTMpolarizations in ribwaveguides. These values are comparablewith the lowest propagation loss reported for SOIwaveguides in 3.7–3.8 μmwavelengths [9, 12]. However, wehave employed a thinner Si core to achieve tightly confined singlemode in channel and ribwaveguides, thereforeour loss performance could be considered as a relative improvement. Secondly, we have achieved aminimumloss of∼8 dB/cm in ourGe-on-SOIwaveguides. This performance is comparable with the propagation loss of∼7 dB/cm reported for Ge-on-SOIwaveguides [22], and a definite improvement over the propagation loss of14 dB/cm reported forGeOIwaveguides [30]. This demonstrates a feasibility to employGe-on-SOI in order toachievemonolithically integratedmid-IR sensors inwhich active regions can be realized inGeSn grownepitaxially onGe core.

Acknowledgments

This research is supported by theMinistry of EducationAcRFTier-1 funding (R-263-000-B39-112), NationalResearch FoundationCompetitive Research Programs (NRF-CRP15-2015-01 andNRF-CRP15-2015-02), andby theNational Research Foundation, PrimeMinister’sOffice, Singapore under itsmedium sized centreprogram.

ORCID iDs

UsmanYounis https://orcid.org/0000-0002-5538-2320Chengkuo Lee https://orcid.org/0000-0002-8886-3649

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