Aykutlu Dana Ashot Markosyan Riccardo Bassiri Edgard Bonilla Brian Lantz Martin Fejer LIGO Group Stanford University CA 94305 The Need for a Conductive Coating Charging is believed to degrade ID: 1032262
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1. Ultralow Absorption Conductive Coatings for Charge Dissipation in LIGO mirrorsAykutlu Dana, Ashot Markosyan, Riccardo Bassiri, Edgard Bonilla, Brian Lantz, Martin FejerLIGO Group, Stanford University, CA 94305
2. The Need for a Conductive CoatingCharging is believed to degrade the SNR through electrostatic coupling of the test mass to the chamber.The charging rates are estimated to be on the order of 10-6 C/day. Ionic species in the vacuum system may be a source of charging.Extraction of charges: UV Radiation for photoelectron generationDirected low-energy electron and ion beams UV excitationAu coatingPhotoelectronsNoncontact Discharge of the mirror canbe possible by photoelectronemission from the side(or an ion gun)e-ZnO coatinge-
3. OutlineRequirements and specifications for conductive coatingsContributions to optical absorption at 1064nmAtomic Layer Deposition of doped ZnO filmsAbsorption reduction due to interferenceOngoing work and Future directions
4. Requirements for conductive coatings for LIGO mirrorsOptical absorption < 1ppm @ 1064 nmOptical scattering < 1ppm @ 1064 nmSheet resistance < 100 TOhms/sqSurface Roughness < 0.3 nm RMSLong-term stability in vacuum under intense laser illuminationResistant to cleaning procedures (i.e. First-Contact)
5. 5 nm thick ZnO film Doping levels of Ne=1013, 1014 and 1015 cm-3.Typical Mobilities ~ 1-100 cm2/VsFree Carrier AbsorptionSheet resistivityFree carrier absorption at 1064 nm Drude model: Dielectric function
6. Saha, Dipto & Das, Amit & Ajimsha, Rohini & Misra, Pankaj & Kukreja, L.. (2013). Effect of disorder on carrier transport in ZnO thin films grown by atomic layer deposition at different temperatures. Journal of Applied Physics. 114. 10.1063/1.4815941. Absorption at 1064 nm: Bulk defects Energy levels of various defectsIn ZnOOxygen vacancy densities are higher at high deposition temperatures in ALDLin, B., Fu, Z., & Jia, Y. (2001). Green luminescent center in undoped zinc oxide films deposited on silicon substrates. Applied physics letters, 79(7), 943-945.Defect luminescence band
7. Resistance measurementsThe resistance of ultrathin AZO films is greatly affected by the atmosphereWe report the final stabilized value of resistance in N2 atmosphereAir reduces the resistance, possibly due to water film and changes in surface potentialAbsorption measurements are done in AirCapping AZO films will stabilize sensitivity to atmospheric compositionN2 purgeDepletion regionFermi LevelSiO2Al:ZnOSurface statesDetermine pinning
8. Effect of Air annealing on optical absorptionReduction in absorption:Possibly due to change in mobility and change in trap density0.8 ppm
9. Optical absorption in the presence of interferenceAbsorption can be tuned by placing the absorber in different locationsPeak field, High absorptionZero field, Low absorption
10. The conductive layer modifies mirror reflection and lossOptical absorption in the presence of interferenceTransfer Matrix Calculations
11. Effect of absorbing layer on mirror bandwidth (i.e. R>0.999999)Thinner films allow greater native film absorption for a given desired bandwidthwavelength
12. Effect of TILT on absorption (11.3 GOhm/sq resistivity, 1330ppm)15 nm thick ALD AZO deposited at 177oCFused silica substrate
13. Effect of TILT on absorption @ 1064nm(15nm thick, 580 GOhm/sq resistivity, 90ppm AZO film)Achieved after annealing ALD AZO films at 330oC, 2 Hrs in Air0.3 ppm absorptionR=580 GOhm/sq(HR mirror contribution of 0.65 ppm subtracted)
14. Magnetron Sputtered Films2% Al:ZnO target from Kurt-Lesker50W RF power, oxygen rich atmosphere (50 sccm Ar/2 sccm O2), 5mTorr pressureAbsorption and resistivity can be controlled by annealing12 nm thickness for 45 min deposition durationAs DepositedAbsorption and resistivityAnnealed at 450 C in AirAbsorption and resistivityAnnealed at 500 C in AirAbsorption and resistivity0.85ppm , 1.5 TeraOhm/sq6ppm , 0.18TeraOhm/sq0.1 ppm, 12 TeraOhm/sqAnnealing up to 475C decreases resistivity, and at 500C resistance suddenly increasesThis is attributed to formation of Al2O3 nanoclusters and resulting reduction in doping and mobility
15. Magnetron Sputtered AZO on HR3224HR CleanhalfHR+AZOhalfHR+AZOhalfHR CleanhalfMagnetron Sputtered12 nm thick film0.5 ppm 1.5TOhm/sq in Air, 300 TOhm/sq in N2, As Deposited, on Fused silicaOn the HR mirror absorption is negligible ( < 0.05ppm)Center of sample
16. Magnetron Sputtered AZO on HR3224HR CleanhalfHR+AZOhalfHR+AZOhalfHR Cleanhalf0.2ppmMagnetron Sputtered12 nm thick film0.5 ppm 300 TeraOhm/sqAs Deposited, on Fused silicaAnnealed at 170C in N26ppm 6.3 TeraOhm/sq in N2on Fused silicaAnnealed at 170C in N2Center of sample
17. Summary of results5nm Thick ALD AZOs with c.a. 10 TeraOhm/sq resistance, 0.7ppm absorption on Fused Silica substratesMagnetron sputtered films are promising6 Tohm/sq films with <0.1ppm absorption, 12nm thick 2% Al:ZnoSub-ppm absorption demonstrated with ALD AZO films on HR mirrors15nm thick, 580 GOhm/sq and 0.3 ppm absorption at 1064 nmTotal absorption including HR mirror is 0.95 ppmSub-ppm absorption demonstrated with Magnetron AZO films on HR mirrors12nm thick, 6.3 TOhm/sq and 0.2 ppm absorption at 1064 nmTotal absorption including HR mirror is 0.8 ppm12nm thick, 300 TOhm/sq and <0.05 ppm absorption at 1064 nmTotal absorption including HR mirror is 0.55 ppm
18. Future DirectionsScattering measurements (Caltech) in progressLong-term stability Test (Vacuum exposure with intense 1064nm beam)Capping with Al2O3 Address Further Questions raised : Phase modulation at 1064 nm due to Sub-bandgap Photoconductivity at 532 nm Depolarization due to ZnO birefringence?Resistive loss of the film, coupling to mechanical loss?Piezoelectric effects?THANK YOU! Questions?
19. Additional Slides
20. ATOMIC LAYER DEPOSITIONAllows DIGITAL DOPING: Precise control of conductivity
21. Doping with ALDAtomic Layer Deposition allows precise digital dopingDoping cycleZnO deposition
22. Optimization of pulse durations result in improved film stability
23. Effect of thickness on sheet resistanceDebye lengthBand Bending modifies sheet conductivityControl of film conductivity is complicated and depends on cap layersResistivityThicknessDepletion regionFermi LevelSiO2Al:ZnOSurface statesDetermine pinning
24. Effect of UV illumination on absorption at 1064nm10 nm Al-ZnO2.5 nm Ti-ZnOUV illumination causes strong persistent photoconductivity and reduction of sheet resistance
25. Surface Roughness of ALD AZO filmsFirst Contact peels offNanoscale domains from Unannealed filmsProblem solved by annealing at 500C
26. Sub-Bandgap Photoconductivity (SBGPC)532 nm is present in the LIGO detectorThe absorption of films at 532 nm is measured as 60ppmThere is photoconductivity at 532 nmStrength of photoconduction depends on film preparation procedureAmplitude modulation of 532 nm may cause phase modulation at 1064 nmPhotoconductivity at 532 nm
27. Phase modulation at 1064 nm due to SBGPCSub-Bandgap PhotoconductivityAt 532nm 30mW/cm2Measured on Fused Silica Corning 7979 1 GOhm resistance change should result in a phase change of 10-10 rad around 3 GOhm (mobility is assumed as 10 cm2/VsPhase change of <10-13 rad is required not to increase the noise floor of LIGOMax AM noise <0.01% at 532 nm, 30 mW/cm2
28. Photothermal Common Path Interferometry (PCI) setupAbsorption signal vs z-scan