Annealing Experiments and Optical Characterization of Tellurium Dioxide TeO2 Final Report

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

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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

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What is TeO2 ?

• TeO2 is an acousto-optic optical material growth
  since the 1960s
• 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

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

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Quartz (SiO2) TeO2
Teinterstitials
Tevacancies Te2, Te3
Ointerstitials Ovacancies O-
Impurities H interstitials
O-H bond
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O-H bond stretch absorption
O-H
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TeO2 Annealing Experiments

• Vacuum(700C) N2 (700C) H2 (400C) H2
  (700C) 4 hr.

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Diffusion in Hydrogenated TeO2

• Diffusion species
• H, H2
• Te2,Te3, Teinter. , Tevac.
• O-, Ointer. , Ovac.

1
3
5
2
4
As-grown
5
4
3
4
5
2
3
1
2
1
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Diffusion Near Bubbles in TeO2

• Diffusion species
• H, H2
• Te2, Te3, Teinter.Tevac.
• O- , , Ointer, , Ovac.

Absorption Mechanism H , H2
Te2,Te3,Teinter.Tevac. O- , , Ointer, , Ovac.
As-grown
3
2
1
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Annealing of Hydrogen Yellowed TeO2

• Oxygen
• Air
• Nitrogen (100)

Clear
Clear
Absorption mechanism H , H2 Te2 Te3,
Teinter. , Tevac. O- , , Ointer, , Ovac.
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Tellurium Diffusion in TeO2 Experiment

• Te metal Empty hole
• 700C, 5 hrs.

No yellowing effect see Teinter.
Tevac. Te3
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Ellingham Diagram
Metal Reactivity in TeO2
TeO2
Au Fe Al
Pb Cu
?Gº -33,400 cal/mol T 1006K
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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 1006K (2) Po2
2.3 x 10-8 atm 1. A.Cottrell Intro to
Metallurgy 2. S. Otsuka Thermodynamics of O2
in TeO2, Electrochemical Measurement
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TeO2 Phase Diagram

• Bubbles seen in TeO2 are O2 (gas)

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TeO2 Thermal Band Edge Shift
Excitons
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Optical Absorption Mechanisms
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Optical Absorption Mechanisms in TeO2

• Point Defects Band edge Effects
• O-H (N) Excitons (Y)
• Teinter. (N)
• Tevac. ()
• Ointer. (N)
• H, H2 (Y)
• Te2, Te3 (Y)
• Ovacancies (Y)
• Impurities (Y)

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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

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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