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

strained (pseudomorphy)

film grows with structure different from bulk

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

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 = [na_o(substrate) - ma_o(film)] / a_o(film)

a_o = 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 T_C
  - depends on deposition rate and materials
  - typical values 100 - 500 C
  - T > T_C = perfect epitaxial growth
  - T < T_C = polycrytalline growth
deposition rate
- lower rate improves epitaxy
  - if system is clean
- for good epitaxy, typically want
Example: Ge on Ge

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