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Physics of Thin Films

PES 449 / PHYS 549


Molecular Beam Epitaxy

 Ohring Chapter 7


Molecular Beam Epitaxy

  • evaporation at very low deposition rates
  • typically in ultra-high vacuum
  • very well controlled
  • grow films with good crystal structure
  • expensive
  • often use multiple sources to grow alloy films
  • deposition rate is so low that substrate temperature does not need to be as high


Epitaxy

epitaxy = growth of film with a crystallographic relationship between film and substrate
homoepitaxy (autoepitaxy, isoepitaxy) = film and substrate are same material

heteroepitaxy = film and substrate are different materials

structures

  • matched
    • common in homoepitaxy, sometimes in heteroepitaxy
    • matched epitaxy
  • strained (pseudomorphy)
    • film grows with structure different from bulk
    • strained epitaxial film
    • not stable
      • at some thickness film will convert to bulk structure
    • example: Co is hcp
      • can deposit as fcc up to one micron thick
    • example: strained layer superlattices
    • can make materials with unusual properties
  • relaxed
    • form edge dislocations
    • relaxed epitaxial film

    strained vs. relaxed depends on minimizing energy of system

    • strain energy vs. dislocation energy


Surface and Interface Crystallography

surfaces - not always the same as the bulk
  • vertical changes
    • outer layers may move in or out from bulk positions
  • lateral changes
    • surface may reconstruct
      • atoms move laterally on surface

interfaces

use Miller indices to specify planes and or directions

example: NiO on Ni: NiO(100)||Ni(111)

lattice misfit f = [nao(substrate) - mao(film)] / ao(film)

ao = bulk lattice constant (function of temperature !!)

n, m = integers

f > 0 = film in tension

f < 0 = film in compression


Parameters

  • substrate temperature
    • critical epitaxial temperature TC
      • depends on deposition rate and materials
      • typical values 100 - 500 C
      • T > TC = perfect epitaxial growth
      • T < TC = polycrytalline growth
  • deposition rate
    • lower rate improves epitaxy
      • if system is clean
    • for good epitaxy, typically want equation for dep rate

    Example: Ge on Ge

    Ge on Ge graph

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