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Annealing Experiments and Optical Characterization of Tellurium Dioxide TeO 2 (Final Report)

Annealing Experiments and Optical Characterization of Tellurium Dioxide TeO 2 (Final Report). Mat.E. 198B, Spring 2001 Instructor Dr. R. Chung Advisor Dr. S. Harsha by George Calvert (Dr. G. Foulon, and Dr. L. Gordon) Crystal Technology Inc. (CTI), Palo Alto. Contents. Back ground

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Annealing Experiments and Optical Characterization of Tellurium Dioxide TeO 2 (Final Report)

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  1. Annealing Experiments and Optical Characterization of Tellurium Dioxide TeO2(Final Report) Mat.E. 198B, Spring 2001 Instructor Dr. R. Chung Advisor Dr. S. Harsha by George Calvert (Dr. G. Foulon, and Dr. L. Gordon) Crystal Technology Inc. (CTI), Palo Alto

  2. Contents • Back ground • Objectives A. Transparency range for TeO2 B. Changes in the transparency (Absorptions) Optical absorptions 1. Chemically induced (point defects) 2. Thermally induced (excitons) • Possible absorption mechanisms • Experiments A. Chemistry, Diffusion, Thermodynamics B. High temperature spectroscopy • Conclusions (Proposed absorption mechanisms) • Further work

  3. What is TeO2 ? • TeO2 is an acousto-optic optical material growth since the 1960’s • Used in laser defectors and modulators • World market $20 to $30 million per/year • Motivation is to improve crystal quality by understanding absorption mechanisms CTI Standard Products

  4. TeO2 Transparency Range(350 nm to 6500 nm)Transmission (%) versus wave length (nm)

  5. Absorption Mechanisms in Crystals Color centers • Impurities (Cr, Fe, H) • Crystal defects (vacancies, interstitials) • Electron and hole centers • Irradiation induced centers Band edge color effects • Thermally induced electron hole centers (excitons)

  6. Quartz (SiO2) TeO2 Teinterstitials Tevacancies Te+2,Te+3 Ointerstitials Ovacancies O- Impurities H interstitials O-H bond

  7. O-H bond stretch absorption O-H

  8. TeO2 Annealing Experiments Vacuum(700°C) N2 (700°C) H2 (400°C) H2 (700°C) 4 hr.

  9. Diffusion in Hydrogenated TeO2 1 5 3 4 2 Diffusion species H+, H2 Te+2,Te+3, Teinter. ,Tevac. O-, Ointer. ,Ovac. As-grown #5 #4 #3 #4 #5 #2 #3 #1 #2 #1

  10. Diffusion Near Bubbles in TeO2 Diffusion species H+, H2 Te+2,Te+3,Teinter.Tevac. O- , , Ointer, ,Ovac. Absorption Mechanism H+ , H2 Te+2,Te+3,Teinter.Tevac. O- , , Ointer, ,Ovac. As-grown #3 #2 #1

  11. Annealing of Hydrogen Yellowed TeO2 Clear Oxygen Air Nitrogen (100%) Clear Absorption mechanism H+ , H2 Te+2 Te+3, Teinter. ,Tevac. O- , , Ointer, ,Ovac.

  12. Tellurium Diffusion in TeO2 Experiment No yellowing effect see Teinter. Tevac. Te+3 Te metal Empty hole 700°C, 5 hrs.

  13. Metal Reactivity in TeO2 Ellingham Diagram TeO2 Au Fe Al Pb Cu Gº = -33,400 cal/mol T = 1006°K

  14. Calculate (Po2) for TeO2 Te (s) + O2 (g)  TeO2 (s) Gºsystem = Gºoxide - Gºmetal - Gºoxygen G = Gº + RT ln Po2G Equib. = 0Po2 = exp (Gº/RT) (1) Gº = -33,400 cal/mol, T = 1006°K (2) Po2 = 2.3 x 10-8 atm 1. A.Cottrell Intro to Metallurgy 2. S. Otsuka Thermodynamics of O2 in TeO2, Electrochemical Measurement

  15. TeO2 Phase Diagram Bubbles seen in TeO2 are O2 (gas)

  16. TeO2 Thermal Band Edge Shift Excitons

  17. Optical Absorption Mechanisms

  18. Optical Absorption Mechanisms in TeO2 Point DefectsBand edge Effects O-H (N) Excitons (Y) Teinter.(N) Tevac. (~) Ointer.(N) H+, H2 (Y) Te+2, Te+3 (Y) Ovacancies (Y) Impurities (Y)

  19. Work Completed • Measured TeO2 transparency range (300 to 6500 nm) • Eliminated O-H bond stretch as an absorption mechanism in TeO2 • Identified a diffusion process in hydrogenated TeO2 • Identified bubbles in as-grown TeO2 to be O2 gas • Measured TeO2 band edge shift versus temperature • Proposed optical absorption mechanisms

  20. Possible Topics for Further work • Explore thermodynamics of TeO2 further (optimize crystal growth atmosphere) • Investigate thermal band shift (build a more sophisticated spectroscopy oven) • Refine a model for optical absorption mechanisms in TeO2

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