Research presentation for the materials science & engineering internship done at Massachusetts Institute of Technology for the photonics area

1. Germanium Technology for the
Mid – Long Wave Infrared
Melvin J. Núñez Santiago, Eveline Postelnicu, Kazumi Wada,
Jurgen Michel, Samuel Serna, L.C. Kimerling and Anuradha Agarwal
Department of Materials Science & Engineering, Materials Research Laboratory
Massachusetts Institute of Technology, Cambridge, MA 02139
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2. sponsor
logo
emat@mit
Germanium Technology for the Mid – Long
Wave Infrared
 Design Ge on Si waveguides for sensors
 Process low loss amorphous Ge
 Test Ge waveguide transmission for Mid-LWIR
Objectives
Graphic
Strategy/Approach
 For device design: Lumerical for thickness and
width information for single moded TE and TM
 For materials characterization: XRD, SEM,
Waveguide transmission loss
Implications/Key Findings
Melvin J. Núñez Santiago
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 Optimization of design
 Process verification
 Device characterization

3. Motivation
Why Amorphous Germanium?
• It is CMOS compatible
• Has transmission losses lower than 2 dB/cm over the 2-14 µm wavelength range
• Low processing temperature
• Ideal for chemical sensing
Why 3.3 – 10 µm?
• Gases and other heavy organic molecules are present
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4. Introduction for waveguides sources of loss
• Etching process
• Slanted sidewalls and roughness
• Light scattering
• Absorption
• Coupling
• Waveguide design
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5. Design
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• Design optimization
• MODE analysis
• Bending simulation

6. Device Design (Ridge Structure)
Si
Ge
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Refractive index at 3.3 microns wavelength
-Si ~ 3.4335
Refractive index at 10 microns wavelength
-Si ~ 3.4215
Specifications of Si in Lumerical
Ridge Thickness
Channel Thickness
Ridge Width

7. 3.3µm ridge width optimization
x
0.25µ
m
0.75m
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Ge 0.5µm
0.5µm
Opt.

8. 3.3µm channel thickness optimization
Y max
Y min
50.6u
m
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Ge Opt.
Adds to 1µ
0.850µm

9. MODE analysis for 3.3µm wavelength
TM MODE
Width 0.85 µm
Ridge Thickness 0.858 µm
Channel Thickness 0.142µm
Results (no bending) Eff Index dB/cm loss
MODE 1 3.55611 2.4723
MODE 2 3.478683 2.1613
MODE 3 3.421995 0.031124
MODE 4 3.421528 0.0061998
• 100nm ± etching precision considered
• TM Mode is better due to less side wall roughness 9
TE MODE

10. Confinement of the MODE and bending results
• dB/cm losses of difference between no bending and bending analysis ~ 0.13 dB/cm
• Bending of 200µm radius is acceptable because of no significant add loss
Bending results
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Confinement of the TM Mode Values %
Ge 81.1029
Si 15.6621
Air 3.235
Confinement of the TE Mode Values %
Ge 67.4848
Si 30.513
Air 2.0022
Bend 90-degress at 200µm Eff Index dB/cm loss
MODE 1 3.561496 2.5059
MODE 2 3.48151 2.262
MODE 3 3.38165 0.071462
MODE 4 3.382528 0.037486

11. 10µm (2µm thick) ridge width optimization
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Ge
1µm
1µm
Opt.

12. 10µm (2µm thick) channel thickness optimization
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Ge Opt.
Adds to 2µ
4µm

13. TE MODE TM MODE
MODE analysis for 10µm wavelength(2µm thick)
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ridge width 4 µm
ridge thickness 1.25 µm
channel thickness 0.75 µm
Results (no bending) Eff Index dB/cm loss
MODE 1 3.564946 1.5688
MODE 2 3.484325 1.1742
MODE 3 3.374354 0.082051
MODE 4 3.367325 0.099504
• TE is better due to more mode confinement and less side wall roughness
• TM is a bigger mode, thus needs more thickness to prevent showed effect

14. Confinement of the MODE and bending results
• dB/cm losses of difference between no bending and bending analysis ~ 0.11 dB/cm
• Bending of 200µm radius is acceptable because of no significant add loss
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Confinement of the TE Mode Values %
Ge 73.0971
Si 22.4119
Air 4.491
Bend 90-degress at 200µm Eff Index dB/cm loss
MODE 1 3.632957 1.6529
MODE 2 3.543748 1.2932
MODE 3 3.386915 0.60377
MODE 4 3.371086 0.30156
Confinement of the TM Mode Values %
Ge 61.3194
Si 36.8221
Air 1.8585

15. 10µm (3µm thick) ridge width optimization
Ge
1.5µm
1.5µm
Opt.

16. 10µm channel thickness optimization
Ge Opt.
Adds to 3µ
3.5µm

17. TE MODE TM MODE
Mode analysis for 10µm wavelength (3µm thick)
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ridge width 3.5 µm
ridge thickness 2.25 µm
channel thickness 0.75 µm
Results (no bending) Eff Index dB/cm loss
MODE 1 3.616139 1.7912
MODE 2 3.612848 1.731
MODE 3 3.381009 0.12959
MODE 4 3.37654 0.055701
• TE and TM have similar confinement and less side wall roughness compared to previous 2µ Ge thick
• TM MODE is shifted and more confined due to thickness increment

18. Confinement of the MODE and bending results
• dB/cm losses of difference between no bending and bending analysis ~ 0.12 dB/cm
• Bending of 200µm radius is acceptable because of no significant add loss
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Confinement of the TE Mode Values %
Ge 86.4176
Si 12.0075
Air 1.5749
Bend 90-degress at 200µm Eff Index dB/cm loss
MODE 1 3.694297 1.8767
MODE 2 3.690517 1.8122
MODE 3 3.391877 0.4312
MODE 4 3.382186 0.52998
Confinement of the TE Mode Values %
Ge 86.18145
Si 12.1315
Air 1.68705

19. XRD Analysis
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20. Materials Characterization
XRD spectrum of α-Ge XRD spectrum of x-Ge
• α-Ge has broad peaks as expected
• There is a crystalline peak due to silicon
• One clearly defined peak on crystalline
• Peak is shifted due to the out of plane orientation
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21. SEM Measurement
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22. Materials characterization
Cross section of α-Ge
920nm of α-Ge
• Etching is sloped out and not straight
• Etching provided ridge structure
• Thickness measurement is close to designed 1 micron
• Lithography process verified for 10-micron wavelength 22
Top down view of α-Ge bending

23. Materials characterization
Ebeam dose test of FOx-16 α-Ge
• Best dosage range is from 3,000-5,500 µC
• Holes imperfections in the dose is due to expired resist 23

24. Transmission loss measurement
Mid-IR
camera
Sample
Microscope
Lens
Laser
Mirrors
Chopper
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25. Transmission loss measurement
Sample placement Alignment waveguide mode x - Ge
• x-Ge:
- Transmission was seen through alignment waveguide (10 micron wide from x- Ge sample from LETI)
-No single mode transmission was measured from x- Ge sample from LETI
-x-Ge is more likely to avoid processing issues due to higher epitaxial material
• α- Ge
-No mode was seen in α- Ge waveguides (processing challenges) 25

26. Impact
Design:
• We optimized the design of a Ge on FZ-Si ridge waveguide for 3.3 and 10-micron
single mode operation.
Process
• Verification was conducted using SEM to determine issues with sidewall
roughness and etching profile that may lead to no transmission in amorphous Ge
material.
• Try a new etching recipe and a new resist
Device characterization
• X ray diffraction was utilized to verify the amorphous nature of the Ge.
• Measurement technology led to improved understanding of coupling
requirements
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27. References
1. D. K. Sparacin, “Process and design techniques for low loss integrated
silicon photonics,” thesis, 2006.
2. R. W. Millar, K. Gallacher, U. Griskeviciute, L. Baldassarre, M. Sorel, M.
Ortolani, and D. J. Paul, “Low loss germanium-on-silicon waveguides for
integrated mid-infrared photonics,” Silicon Photonics XIV, 2019.
3. K. Gallacher, R. Millar, U. Griškevičiūte, L. Baldassarre, M. Sorel, M.
Ortolani, and D. J. Paul, “Low loss Ge-on-Si waveguides operating in the 8–
14 µm atmospheric transmission window,” Optics Express, vol. 26, no. 20,
p. 25667, 2018.
4. N. S. Patel, C. Monmeyran, A. Agarwal, and L. C. Kimerling, “Point defect
states in Sb-doped germanium,” Journal of Applied Physics, vol. 118, no.
15, p. 155702, 2015.
5. C. Monmeyran, “Point defect engineering in germanium,” thesis, 2017.
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28. Acknowledgement
Thanks to my principal investigator Dr. Anuradha Agarwal
and graduate student Eveline Postelnicu for their time,
effort and guidance through this entire learning experience
This work was supported by the MRL Research
Experience for Undergraduates Program, as part of
the MRSEC Program of the National Science
Foundation under grant number DMR-14-19807.
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