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Metastability of the boron-vacancy complex (C center) in silicon: A hybrid functional study Cecil Ouma and Walter Meyer Department of Physics, University of Pretoria. Outline. Background Defects and metastable defects B-V Centre Experimental DLTS Observed properties of the B-V centre

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  1. Metastability of the boron-vacancy complex (C center) in silicon: A hybrid functional study Cecil Ouma and Walter MeyerDepartment of Physics, University of Pretoria

  2. Outline • Background • Defects and metastable defects • B-V Centre • Experimental • DLTS • Observed properties of the B-V centre • Computational aspects • Formation energies • Transition levels • Comparison with experiment • Conclusions

  3. Defects in semiconductors • The electronics industry is ever expanding and so is the research in device design in applications • Defects in semiconductors play an essential role • Required for doping • Side effect of fabrication –> detrimental -> limit and remove • Beneficial properties -> understand, use and control -> Model!

  4. Defects in semiconductors • Defects may occur either as point defects or defect complexes Substitutional impurity Self interstitial A fundamental understanding of defect properties is important in device engineering & applications Interstitial impurity Vacancy • Defects can be beneficial or detrimental in a semiconductors -> Need to understand!

  5. Defects in semiconductors • Defects may either be: • Stable: A defect which has a single fixed atomic configuration for a given charge state and their properties do not depend on the history of the sample. • Metastable (Bistable) : A defect that, in at least one charge state, has two stable configurations. Stable defects have been and are extensively Metastable defects provide an opportunity to test a variety of aspects of the capabilities of simulation techniques

  6. Dn Dn Dn-1 Defects in semiconductors Dn-1 • stable vs metastable c) Metastable defect in one charge state Dn a) Ordinary defect Total (electronic+elastic) energy Dn Dn-1 Dn-1 d) Metastable defect in both charge states a) Large lattice relaxation defect Defect configuration coordinate

  7. Boron-vacancy complex • Watkins 1976: Tentatively associated the Si-G10 EPR spectrum to the Bs-V complex in silicon • Sprenger et al. 1987: Tentatively associated the Si-G10 EPR spectrum to the Bs-V complex in silicon (ENDOR) • Londos 1992, Bains et al. 1985, Zangenberg et al. 2005 identify the DLTS peaks associated with the B-V centre and observe metastability

  8. Properties of the B-V complex

  9. Experimental background: DLTS

  10. Experimental observations Zangenberg et al. Appl. Phys. A 2005 DLTS after annealing at 215 K under • Zero bias • Reverse bias • Zero bias (again) Stable configurations Configuration A: Zero bias Configuration B: Reverse bias

  11. Computational background DFT with LDA and GGA functionals has a number of successes, but: • Band gaps of semiconductors are significantly under-estimated. • E.g. Ge is a metal. • Kohn-Sham states do not represent individual electron wave functions • Very unreliable in predicting DLTS levels. DFT with hybrid potentials correctly predict band gaps. Calculation of formation energies according to Zhang & Northrup. Calculate thermodynamic transition levels from Fermi-level dependence.

  12. Computational details MedeA-VASP package • 64 atom supercell • K-mesh: 2✕2✕2 MP • Ecut = 500 eV • Functionals: HSE06 • Formation energy calculated according to Zangh & Northrup

  13. Results: Calculated formation energies(with Fermi level at valence band)

  14. Results: Formation energies as a function of Fermi level.

  15. Results: Theoretical predictions and comparison with experiment Zero bias: Charge state: q=+1 Stable configuration: C2 High temperature Peaks Two peaks observable Reverse bias: Charge state: q=-1 Stable configuration: C1 Low temperature Peaks Only one peak observable Configuration B  C1 Configuration A  C2

  16. Results: Comparison between DLTS energy levels and calculated transition levels

  17. Conclusions • DFT with hybrid functionals may successfully be used to model the electronic properties of the metastable B-V complex in silicon. • The thermodynamic charge transition levels obtained were consistent with previous experimental observations. • There was correct qualitative prediction of the observed changes in the DLTS spectrum due to the metastability of the defect complex.

  18. AHSANTESANA!!!!!

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