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Supported by the BMBF, contract 05 KK7TS1

ISOLDE Workshop and Users Meeting 2009. Uncovering New Diffusion Phenomena in Compound Semiconductors. Th. Wichert 1 , J.Kronenberg 1 , K. Johnston 1 , J. Cieslak 1 , M. Deicher 1 , F. Wagner 1 , H. Wolf 1 , and The ISOLDE collaboration 2.

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Supported by the BMBF, contract 05 KK7TS1

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  1. ISOLDE Workshop and Users Meeting 2009 Uncovering New Diffusion Phenomena in Compound Semiconductors Th. Wichert1, J.Kronenberg1, K. Johnston1, J. Cieslak1, M. Deicher1, F. Wagner1, H. Wolf1, and The ISOLDE collaboration2 1Technische Physik, Universität des Saarlandes, D-66123 Saarbrücken, Germany 2CERN, CH-1211 Geneva 23, Switzerland thw@tech-phys.uni-sb.de Supported by the BMBF, contract 05 KK7TS1

  2. concentrationof dopants Nobel Prize in Chemistry 1943 "for his work on the use of isotopes as tracers in the study of chemical processes" one-sidedsource George de Hevesy monotonously decreasing profile Diffusion in Solids 2

  3. Jx = - D dC/dx dC dx Fick‘s First Law

  4. 550K (30 min) 828K (60 min) vacuum Cd atmosphere Diffusion of 111Ag in CdTe implanted profile • no loss of activity • diffusion towards increasing Ag concentration 4

  5. Jx = - D dC/dx dC dx Fick‘s First Law violated by the present data ? „uphill“

  6. Table of Content • Radiotracer Diffusion in Binary Semiconductors - Semiconductors and uphill diffusion - Uphill diffusion and self-interstitials - Self-interstitials and metal layers • On-line Diffusion Chamber Following the set-up by the Stuttgart group (T1/2 > 10 min):

  7. 60 keV Implantation a 550 … 900 K thermal treatment CdxZn1-xTe Ø = 6 mm d = 800 µm CdTe Used isotopes : 111Ag, 67Cu, 24Na, 43K, 56Mn,59Mn(59Fe), 65Ni, 61Mn(61Co), 101Cd(101Pd), 191Hg(191Pt), 193Hg(193Au) CdTe:111Ag Tdiff = 570 K tdiff = 30 min DAg = 7.6·10-9 cm2/s 1...30 µm Radiotracer Technique 111Ag+ 30nm

  8. Experimental Results Te-saturated CdTe: tdiff = 60 min, external Cd pressure CdTe t = 0: d-layer at x = 0 Uphill-diffusion profile symmetry: dopant in equilibrium with host crystal profile shape: result of intrinsic defects 8 8

  9. Cd Cd DCCd DCTe DCCd Control of Intrinsic Defects T-DC phase diagram (sketch) CdTe Initial CdTe: DC = DCTe external Cd pressure DC: deviation from stoichiometry 9 9

  10. intrinsic extrinsic donors acceptors Te sublattice: „perfect“ Defects Involved Cd sublattice … and all defects can be charged => Deviation from stoichiometry 10 10

  11. C : concentration (1 species) usually µ : chemical potentials (several species) in general Fick‘s First Law revised Particle Flux J 11 11

  12. Fick‘s First Law revised driven by the chemical potentials Changein the concentrations of Ag atoms intrinsic defects charge carriers 12 12

  13. Modelling of Experimental Results • interaction of dopants with intrinsic defects • charge states of extrinsic and intrinsic defects • profile of DC leads toformation of p/n junctions • drift in internal electric field H. Wolf et al., Physica B 308-310 (2001) 963 H. Wolf et al., Physica B, 340 - 342 (2003) 275 - 279 H. Wolf et al., Phys. Rev. Lett. 94 (2005) 125901. H. Wolf et al., Defect and Diffusion Forum 237-240 (2005) 491. F. Wagner et al., Proc. of the 1st Int. Conf. on Diffusion in Solids and Liquids (DSL 2005) H. Wolf et al., J. Electr. Mater. 35 (2006) 1350. F. Wagner et al., Physica B: Condensed Matter 401-402 (2007) 286-290 H. Wolf et al., Diffusion Fundamentals 8 (2008) 3.1-3.8 F. Wagner, Dissertation, Universität des Saarlandes (2008) H. Wolf et al., Defect and Diffusion Forum Vols. 289-292 (2009) 587-592 13

  14. ) -3 0,1 -- ++ ++ C] (cm Cd V Cd Cd i i Δ 0,0 [ eV p n n F µ -0,1 -0,2 16 2x10 16 1x10 0 16 -1x10 16 -2x10 16 -3x10 16 -4x10 Cdi Cdi ) 13 -3 10 concentration (cm 12 10 11 10 0 200 400 600 800 depth (µm) Profile Simulation • [ΔC] = [Cdi] - [VCd] • CdTe is Te-rich [ΔC] < 0 • [ΔC] determines µF • Cd-atmosphere: Ag - diffusion • [Ag] = [Agi+] highly mobile • Agi+ behaves like h+ • Ag profile reflects µF and [ΔC] [ΔCini] 828 K (60 min) 111Ag → CdTe

  15. Experiment and Simulation DC > 0 n-type DC > 0 n-type DC < 0 p-type D(Cdi) = 1.210-6 cm2/s information about:  diffusion coefficients  formation energies  ionizations energies 15

  16. Experiment and Simulation DC > 0 n-type DC > 0 n-type DC < 0 p-type D(Cdi) shouldnot depend onthe dopant! D(Cdi) = 1.210-6 cm2/s D(Cdi) = 7.010-7 cm2/s D(Cdi) = 4.010-7 cm2/s D(Cdi) = 1.510-7 cm2/s 16

  17. 104 Ag in Different Semiconductors CdTe Cd0.96Zn0.04Te ZnTe 828K, 60 min 828K, 60 min 928K, 24 h Cd atmosphere Cd atmosphere Zn atmosphere comparable behavior in different II-VI semiconductors 17

  18. Uphill Diffusion Profiles in Cd1-xZnxTe n p peak Matrix Isotope 18

  19. Box shaped profiles in Cd0.97Zn0.03Te n p box Matrix Isotope 19

  20. 111Ag in CdTe plus coppermetal layer ΔCCd ΔCTe Influence of Metal Layers Te-saturated (∆Cini = ∆C Te) 20

  21. 111Ag in CdTe plus coppermetal layer ΔCCd ΔCTe Influence of Metal Layers Te-saturated (∆Cini = ∆C Te) 21

  22. 111Ag in CdTe Influence of Metal Layers Cd-saturated (∆Cini = ∆CCd ) 22

  23. 111Ag in CdTe ( 580 K ) Influence of Metal Layers Te-saturated (∆Cini = ∆C Te)

  24. 111Ag in CdTe ( 580 K ) Influence of Metal Layers Te-saturated (∆Cini = ∆C Te)

  25. 111Ag in CdTe ( 580 K ) Influence of Metal Layers Te-saturated (∆Cini = ∆C Te)

  26. 111Ag in CdTe ( 580 K ) Influence of Metal Layers Te-saturated (∆Cini = ∆C Te)

  27. evaporatedmetal ΔCCd ΔCTe Uphill Diffusion Injection of Cd Interstitials by external Cd vapour pressure(symmetric Ag profile) metal layer evaporated on surface (asymmetric Ag profile) Cdi Cdi ΔCCd ΔCTe ΔCCd

  28. evaporatedmetal ΔCCd ΔCTe ΔCCd Uphill Diffusion external Cd vapour pressure(symmetric Ag profile) at 550 K at 800 K Cdi Cdi ΔCCd ΔCTe ΔCCd

  29. On-line Diffusion Chamber Catcher, Tape station, Detector Sputter-source • Catcher and tape-station • Germanium-detector + “PET” • Sample depot • Remote control (radioprotection) Manipulator Manipulator Furnace • Open beamline • Depot • Implantation • Annealing • Sputtering Beamport Depot 29

  30. On-line Diffusion Chamber Tape station Detector Sputter-source Detector Furnace Manipulator Beamport 30

  31. n p peak w/o layer Cu layer Summary 1. Diffusion experiments in binary semiconductors Importance of radioactive isotopes at ISOLDE : elements, implantation, purity. 2. Observation and understanding of unusual diffusion profiles General formulation of Fick‘s law : uphill diffusion, quantitativ understanding, metal layers. 3. On-line diffusion chamber for short-lived isotopes Access to well-needed light elements.

  32. 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 for financial support Thanks to … … you for your attention

  33. n p peak w/o layer Cu layer Summary 1. Diffusion experiments in binary semiconductors Importance of radioactive isotopes at ISOLDE : elements, implantation, purity. 2. Observation and understanding of unusual diffusion profiles General formulation of Fick‘s law : uphill diffusion, quantitativ understanding, metal layers. 3. On-line diffusion chamber for short-lived isotopes Access to well-needed light elements.

  34. Future : Short Lived Isotopes On-line diffusion chamber Following the set-up by the Stuttgart group (T1/2 > 10 min): • Diffusion of C, N and O in semiconductors • Self-diffusion in Al and in Al alloys • Diffusion of C, N, and O in grain boundaries of nanomaterials 36

  35. model predicts: profile analysis taking into accountdissolution of precipitates: quantitative descriptionof Dx(tdiff) effective D(Cdi) depends on ● density and size● dissolution rate of precipitates dissolution of precipitates ... ... taken into account used samples: different with respectto precipitates 37

  36. charge states of defects: Cdi0, Cdi+, Cdi2+ VCd0, VCd-, VCd2- Agi0, Agi+ AgCd0, AgCd- e-, h+ } donor acceptor donor acceptor intrinsic (6) } extrinsic (4) free carriers (2) defects taken into account defect concentration: C0: number of available lattice sites Defect interactions in local equilibrium: Three chemical potentials Three corresponding concentration quantities 38 38

  37. diffusion equations µA := µ(Agi0) unchanged upon defect reactions µD := µ(Cdi0) µq := µ(e-) AgCd is regarded as not mobile 39 39

  38. profile analysis charge density andstoichiometry deviation incorporation sitesof Ag dopant chemical potentials Ag dopant inequilibrium withhost crystal pn-transitions formedin front of penetratingCdi defects Ag dopant mostly incorporated as Agi+ essentially the same results for Cu, Au, and Na dopant 40

  39. 41 41

  40. 42 42

  41. Cu-Te alloy Cu-layer CdTe Cu-layer CdTe 550 K Cd-layer acting as strong source for intrinsic defects (Cdi) Cu and Au layer on surface 2-3 ML Cd at the interface would be sufficient ! 43 43

  42. Radioactive Tracers 600 isotopes, 68 elements 600 isotopes, 68 elements Plasma ion source Surface ionization (+/-) Laser ionization t1/2 > 2 d 1 h < t1/2 < 2 d current t1/2 < 1 h future/current 44

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