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Exotic diffusion phenomena in II-VI semiconductors

Exotic diffusion phenomena in II-VI semiconductors. H. Wolf, F. Wagner, J. Kronenberg, M. Deicher, and Th. Wichert. Technische Physik, Universität des Saarlandes, D-66123 Saarbrücken, Germany ISOLDE Workshop 14.02.2007. e-mail: h.wolf@mx.uni-saarland.de. Outline. Experimental results.

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Exotic diffusion phenomena in II-VI semiconductors

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  1. Exotic diffusion phenomenain II-VI semiconductors H. Wolf, F. Wagner, J. Kronenberg, M. Deicher, and Th. Wichert Technische Physik, Universität des Saarlandes, D-66123 Saarbrücken, Germany ISOLDE Workshop 14.02.2007 e-mail: h.wolf@mx.uni-saarland.de

  2. Outline Experimental results Exotic diffusion profiles of 111Ag in CdTe Quantitative description defect reactions drift-diffusion of Ag in CdTe simulation of Ag profiles New exotic diffusion profiles short-lived isotopes at ISOLDE used for diffusion experiments

  3. Au in Si solution for finite source: CdTe:111Ag Tdiff = 570 K tdiff = 30 min monotonously decreasing profile „simple“ diffusion process

  4. 111Ag Implantation 550 … 900 K CdTe Ø = 6 mm d = 800 µm thermal treatment 111Ag CdTe CdTe:111Ag Tdiff = 570 K tdiff = 30 min 1...30 µm experimental procedure

  5. Tdiff = 825 K tdiff = 60 min „exotic“ diffusion profiles in CdTe Shapes of diffusion profiles strongly depend on external vapor pressure H. Wolf, F. Wagner, Th. Wichert, and ISOLDE Collaboration, Phys. Rev. Lett.94, 125901, 2005

  6. 111Ag 111Ag CdTe (550 µm) CdTe (750 µm) 30 nm Cu 30 nm Au Tdiff = 550 K tdiff = 30 min Tdiff = 550 K tdiff = 120 min 111Ag 111Ag codiffusion experiments Cu (Au) layer pushes Ag to the reverse of the crystal

  7. ZnTe CdTe CdTe:In Cd pressure vacuum Zn pressure 111Ag 67Cu 111Ag Tdiff = 800 K tdiff = 60 min Tdiff = 900 K tdiff = 49 h Tdiff = 800 K tdiff = 60 min more „exotic“ diffusion profiles „exotic“ diffusion profiles are not restricted to Ag diffusion in CdTe „normal“ diffusion profile observed in CdTe:In

  8. Defects of consideration Defect interactions Quantities of conservation Drift of charged defects Quantitative description

  9. intrinsic extrinsic donors acceptors Te sublattice: „perfect“ Defects of consideration Cd sublattice

  10. dissociative kick-out annihilation Defect reactions

  11. Charge states: Formation energies: F(Agi), F(AgCd) F(Cdi), F(VCd) Energy levels: ED (donors) EA (acceptors) additional: (Chemical) potentials: mAg, mCd, mF (electrons) (holes) Defect concentrations

  12. No changes upon defect reactions: Total Ag concentration Deviation from stoichiometry Charge density Quantities of conservation

  13. Poisson equation: µF: electric potential : electric field Drift of charged defects: Drift of charged defects

  14. Flux of defect Y: Particle conservation: diffusion drift Drift-Diffusion equation: Quantitative description of Ag profiles to be solved simultaneously for CAg, DC, and r/e

  15. known from literature [1,2]: FCdi, FVCd, and energy levels of Cdi, VCd, Agi, AgCd boundary conditions: surface is in equilibrium with external Cd pressure conservation of total Ag content Free parameters: DFAg = FAgi – FAgCd DCinit D(Agi), D(AgCd), D(Cdi), D(VCd) DFAg Model parameters [1] R.Grill et al., Nuclear Instruments and Methods in Physics A 487 (2002) 40 [2] M.A. Berding, Phys. Rev. B 60 (1999) 8943

  16. F(Agi)-F(AgCd) < 1.5 eV DCinit = -3.3∙1016 cm-3 D(Agi) = 510-7 cm2/s D(AgCd) = 0 D(Cdi) = 110-6 cm2/s D(VCd) = 510-9 cm2/s Profile simulation Tdiff = 825 K tdiff = 60 min Excellent simulation of profiles with a common set of parameters

  17. At diffusion temperature: High mobility of Agi, and CdiLow mobility of AgCd, and VCd Preferred incorporation of Ag as Agi Strong variation of DC during diffusion (Te-excess Cd-excess) external source of intrinsic defects: PCd Requirements for „exotic“ diffusion Propagation of Ag is strongly supportet by Drift of charged defects in internal electric field(generated by inhomogeneous distribution of defects)

  18. 111Ag 111Ag CdTe (550 µm) CdTe (750 µm) 30 nm Cu 30 nm Au Tdiff = 550 K tdiff = 30 min Tdiff = 550 K tdiff = 120 min 111Ag 111Ag codiffusion experiments External source of intrinsic defects?

  19. Cu-Te alloy Cu-layer CdTe Cu-layer CdTe 550 K Cd-layer acting as strong source for intrinsic defects (Cdi) codiffusion experiments (1 ML Cd at the interface might be sufficient!)

  20. 111Ag, 67Cu, Investigated materials: CdTe, ZnTe New „exotic“ diffusion profiles

  21. 111Ag, 67Cu, 24Na, 43K, 56Mn 59Fe (59Mn), 65Ni Investigated materials: CdTe, ZnTe, CdZnTe, CdS New „exotic“ diffusion profiles

  22. CdTe:24Na CdTe:43K Tdiff = 750 K tdiff = 9 h Tdiff = 750 K tdiff = 10 h vacuum vacuum Alkali dopants in CdTe 43K: slow diffusion no „exotic“ diffusion profile 24Na: fast diffusion „exotic“ diffusionprofile

  23. CdS:24Na CdS:43K Tdiff = 850 K tdiff = 30 min Tdiff = 850 K tdiff = 60 min vacuum vacuum Alkali dopants in CdS 43K: slow diffusion no „exotic“ diffusion profile 24Na: fast diffusion „exotic“ diffusionprofile Diffusion of Na (K) in CdS much faster than in CdTe!

  24. CdZnTe:65Ni depletion of 65Ni at the surface curious dip at 60 µm sample inhomogeneity? Tdiff = 800 K tdiff = 60 min No diffusion profile detected for CdTe:59Fe, CdZnTe:56Mn: ( ) Cd pressure diffusion temperature too low ? diffusion time too short ? investigation at a smaller depth scale sputter chamber under construction! Magnetic dopants in Cd(Zn)Te

  25. Summary Quantitative understanding of exotic diffusion profiles observed for Ag and Cu in CdTe During diffusion: High mobility of Agi, and CdiPreferred incorporation of Ag as Agi Strong variation of DC Drift of charged interstitial defects in internal electric field External source of intrinsic defects: Vapor pressure (PCd)interface to metal layer Short-lived isotopes at ISOLDE: Exotic diffusion phenomena are not restricted to Ag and Cu in CdTe

  26. Neutral defects:(C0 = 1.48·1022 cm-3) Charged defects:(mi: intrinsic Fermi energy) donors Agi, Cdi acceptors AgCd, VCd Defect concentrations Formation energies: F(Agi), F(AgCd) F(Cdi), F(VCd) Energy levels: ED (donors) EA (acceptors) (gD, gA: degeneracy) Chemical potentials: mAg dopant concentration mCd concentration of intrinsic defects mF carrier concentration

  27. Incorporation sites of Ag Ag is present as Agi+ to a large extent Height of profile requires DF < 1.5 eV Profile depends only weakly on DFAg for DFAg < 1.5 eV

  28. Drift and diffusion strong compensation of drift and diffusion Ag profile reflects Fermi energy mF Agi+ behaves like a free carrier

  29. 111Ag diffusion in CdTe Tdiff = 550 K tdiff = 30 min depletion layer of about 100 mm un-etched crystal!

  30. Tdiff = 570 K tdiff = 30 min Vacuum Tdiff = 800 K tdiff = 60 min Cd pressure „common“ concentration profile symmetric, centred concentration profile depletion layers Systematc investigations pre-treatment: etching in bromine-methanol solution

  31. t = 15 min t = 60 min t = 240 min t = 420 min t = 600 min t = 900 min Time dependent measurement of diffusion Tdiff = 700 K

  32. Experiment NAg = 2.5∙1011 cm-2 NAg = 1016 cm-2 NAg = 2∙1017 cm-2 Ag Konzentration Tdiff = 800 K tdiff = 60 min

  33. 111Ag Flächenkonzentration: N = 2∙1017 cm-2 Tdiff = 800 K tdiff = 60 min Cd Druck Vakuum Te Druck Äußere Bedingungen Starker Einfluss von äußerem Cd / Te Druck auch bei hoher Ag Konzentration Zusammenhang mit intrinsischen Defekten (Stöchiometrieabweichung)

  34. 111Ag Cu 67Cu Ag Codiffusion of Ag and Cu in CdTe Tdiff = 550 K, tdiff = 30 min strongly enhanced diffusion of Ag by codiffusion of Cu no significant influence of Ag on the diffusion of Cu

  35. Codiffusion of Ag and Au in CdTe 111Ag 550 K (Vak.) Au 30 min 120 min

  36. tdiff = 2 h tdiff = 4 h tdiff = 49 h similar effects like in CdTe Agi diffusion slower than in CdTe chemical self diffusion significantly slower than in CdTe Ag diffusion in ZnTe Tdiff = 900 K

  37. B.O. Wartlick et al., Philosophical Magazine B75 (1997) 639 I. Lyobomirsky et al., J. Appl. Phys 81 (1997) 6684 M.A. Kovalets et al., Fizika i Khimiya Obrabotki Materialov 21 (1987) 125 I. Lyobomirsky et al., Journal of Electronic Materials 26 (1997) 97 Diffusion coefficients from literature Ag in CdTe irregular!

  38. EBIC on CdTe:In (2∙1016 cm-3) 1) CdTe:In 1) pn-transition after annealing under Cd pressure (TA = 773 K) 2) annealing tA = 16 h p-type (DC < 0) n-type (DC > 0) 3) tA = 57 h effects from sample boundary diffusion of Cdfrom all sides 1) E. Belas, R. Grill, A.L.Toth, P. Moravec, P. Horodysky, J. Franc, P. Höschel, H. Wolf, T. Wichert II-VI Workshop, 04.-08.10 2004, Chikago, to be published in IEEE Transactions on Nuclear Science

  39. Uni Münster: H. Mehrer N.A. Stolwijk H. Bracht A. Rodriguez Schachtrup Uni Prague: R. Grill E. Belas Uni Bonn: R. Vianden D. Eversheim CERN: ISOLDE Collaboration BMBF, DFG

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