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Origin of unusual Ag diffusion profiles in CdTe

ISOLDE Workshop and Users meeting 2013. Origin of unusual Ag diffusion profiles in CdTe. J. Lehnert, M . Deicher, K. Johnston, Th . Wichert, H. Wolf Experimentalphysik , Universität des Saarlandes , 66123 Saarbrücken,Germany The ISOLDE Collaboration CERN, Geneva, Switzerland.

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Origin of unusual Ag diffusion profiles in CdTe

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  1. ISOLDE Workshop and Users meeting 2013 Origin of unusual Ag diffusion profiles in CdTe J. Lehnert, M. Deicher, K. Johnston, Th. Wichert, H. WolfExperimentalphysik, Universität des Saarlandes, 66123 Saarbrücken,Germany The ISOLDE CollaborationCERN, Geneva, Switzerland Support by BMBF, contract 05KK7TS2

  2. Outline • Diffusion • Uphill diffusion of Ag in CdTe • Influence of metal layers on Ag diffusion • Influence of oxidized crystal surfaces • Summary

  3. CdTe Radiotracer technique 60 keV Implantation a 550 … 900 K CdTe Ø = 6 mm d = 800 µm thermal treatment 111Ag+ 30 nm Mechanical sectioning • Resolution > 1 µm • T1/2 > 1 h 3

  4. Diffusion in solids 14 -3) 10 for 111 Ag CdTe 13 10 concentration (cm 12 10 11 10 0 200 400 600 800 depth (µm) Monotonously decreasing profiles 4

  5. Diffusion of 111Ag in CdTe ) 14 14 10 10 -3 T = 570 K T = 800 K diff diff t = 30 min t = 60 min 13 10 diff diff 13 10 Cdpressure Argon concentration (cm 12 10 12 10 11 10 11 10 0 200 400 0 200 400 600 800 depth (µm) depth (µm) Monotonously decreasing profile Uphill diffusion profile (H. Wolf et al., Phys. Rev. Lett. 94 (2005) 125901) 5

  6. Model • Deviation from stoichiometry • Initial state of the crystal Te rich • Highly mobile Cd interstitials • Highly mobile Ag dopant (interstitial) • Source of Cd interstitials during diffusion 14 10 13 10 DC < 0 12 10 11 10 0 200 400 600 800 DC > 0 DC < 0 DC > 0 depth (µm) Cd Cd DC > 0 DC < 0 DC > 0 • Ag dopants images the profile of the intrinsic defects

  7. Alternative sources for Cdi defects? • Metal layers • Oxidized crystal surfaces

  8. Influence of metal layers (Cu) Diffusion withCulayerat500 K Diffusion withoutCulayerat500 K withoutmetallayer 111Ag in CdTe withmetallayer 30 nmCulayer DC < 0 Metal layers can initialize uphill diffusion (H. Wolf et al., Appl. Phys. Lett. 100 (2012) 171915 )

  9. metal-Te alloy metal layer CdTe 550 K Cdisource Model metal layer CdTe metal layer CdTe CdTe ΔC > 0 ΔC < 0 Ag dopant images the profile of deviation of stoichiometry again?

  10. Can Ag image a previously prepared deviation of stoichiometry? Etching HCl 1st Annealing 550 K 30 min Metallayer Ar atmosphere ΔC > 0 ΔC < 0 ΔC < 0 Implantation 111Ag 2nd Annealing 550 K 60 min CdTe CdTe CdTe CdTe Concentration measurement Ar atmosphere ΔC > 0 ΔC < 0 ΔC > 0 ΔC < 0 Creating the deviation of stoichiometry before implantation of Ag

  11. Experimental result metal layer 550 K 60 min Argon CdTe CdTe ΔC > 0 ΔC < 0 • Cd source is created by Te-metal alloy • Concentration of the dopant images the profile of deviation from stoichiometry again • Can we create a Cd source at the surface without evaporated metals?

  12. Influence of the condition of the crystal surface as delivered etched (Br-methanol, KOH) 550 K 30 min Argon 570 K 30 min Argon • As delivered samples show uphill diffusion • Br-methanol, KOH etching suppresses uphill diffusion • Surface condition has significant effect on Ag diffusion

  13. Mechanism • Natural oxidation at room temperature and air 1) • TEM measurements: TeO2 at the surface 2) • No evidence of Cd at the surface CdTe CdTe Te oxide RT+air Cdi source • Cd source is created by natural oxidation process • Can we create a Cd source by oxidizing etching? 1) T. B. Wu, J. S. Chen, C. D. Chiang, Y. M. Pang, and S. J. Yang, J. Appl. Phys. 71, 5212 (1992) 2) F. A. Ponce, R. Sinclair, and R. H. Bube, Appl. Phys. Lett. 39, 951 (1981)

  14. Experimental idea Etching HCl/H2O2 Implantation etched CdTe 550 K 30 min Argon as delivered ΔC < 0 ΔC < 0 ΔC < 0 Annealing 550 K 30 min 550 K 30 min Argon Concentration measurement CdTe CdTe ΔC>0 ΔC<0 • Oxidation creates a Cdsource

  15. metal-Te alloy metal layer CdTe Summary - Uphill diffusion is initialized by a Cd source - Ag dopant images the profile of the deviation from stoichiometry - Cd source can be provided by: external Cd pressure evaporated metal layer natural and forced oxidation Te oxide CdTe Cd Cd DC >0 DC <0 DC >0 Cdisource Cdisource Outlook • Verifying of the results by diffusion experiments AND observation of the state of the surface (XPS or similar method)

  16. Thanksto Uni Freiburg: M. Fiederle A. Fauler Uni Bonn: R. Vianden Uni Prague: R. Grill E. Belas CERN: ISOLDE Collaboration BMBFforfinancialsupport

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