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

outline
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
defects in semiconductors
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!
defects in semiconductors1
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!
defects in semiconductors2
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

defects in semiconductors3

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

boron vacancy complex
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
experimental observations
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

computational background
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.

computational details
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
results theoretical predictions and comparison with experiment
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

conclusions
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.