Thin Film Growth

1
Thin Film Growth

• Lecture 12
• G.J. Mankey
• gmankey_at_mint.ua.edu

2
Factors Influencing Film Growth

• Surface energy.
• Interfacial energy.
• Substrate temperature.
• Interface crystallography.
• Type of deposition source.
• Background gas.
• Surfactants.

3
Surface Energy

• Bauer (1980s) described film growth in terms of
  surface energy with the relation

• It states that for layer by layer growth to
  occur, the surface energy of the growing film
  must be less than or equal to the surface energy
  of the substrate minus the interfacial energy of
  the film and substrate.
• The equation is based on the assumption that the
  substrate temperature is high enough to achieve
  thermodynamic equilibrium.

4
Equilibrium Growth Modes

• Layer-by layer or Frank-van der Merwe--the film
  grows with only the surface layer having
  fractional coverage at any instant i.e. the
  "interface width" at the film vacuum interface is
  one atomic layer.
• Island or Volmer-Weber--the film does not "wet"
  the substrate since the Bauer condition is not
  met. The "interface width" of the growing
  surface increases with time.
• Layerisland or Stranski-Kastranov--After wetting
  the substrate, the film grows in an island mode
  and the interface width increases with time.

5
Metastable Growth Modes

• Diffusion-limited--the incoming atoms "stick
  where they land" and the film growth is
  characterized by a interface width which
  increases as da where a is the power law exponent
  (1/4 for perfect diffusion-limited growth).
• Downward funneling--The impinging atoms maintain
  a component of vertical momentum and tend to fill
  in the lower areas of the interface, so the
  interface width saturates at 3-7 atomic layers.

6
Interfacial Energy

• Some materials have a negative enthalpy of mixing
  with one another--they are miscible.
• These materials tend to mix strongly at the film
  substrate interface to give a film-substrate
  "interface width" that may extend over many
  atomic layers.
• One example is transition metals on Si which tend
  to form silicides with varying stoichiometry over
  the vertical interface width.

7
Immiscible Materials

• An immiscible film-substrate combination will
  generally form an atomically-sharp interface,
  however there are some examples are in the
  literature where the epitaxial strain at the
  interface will change the situation.
• If the Bauer condition is satisfied, layer or
  layerisland growth will be observed.
• When the film has a high surface energy compared
  to the substrate, a layer of substrate material
  may segregate to the surface with the film
  sandwiched in the middle.

8
Substrate Temperature

• The substrate temperature and flux are the
  primary factors that determine the resulting film
  morphology.
• Low substrate temperatures or high flux rates (gt1
  nm/sec) result in metastable thermodynamic
  phases.
• Interface mixing and substrate segregation may be
  suppressed for low temperature growth.
• Details depend on materials properties such as
  melting points, epitaxial mismatch and enthalpy
  of mixing.
• Atomically smooth films are usually the exception
  rather than the rule.

9
Interface Crystallography

• If the growing film crystal structure has a
  specific relation to the substrate
  crystallography the film is epitaxial which means
  the film and substrate have an in-plane
  orientation relationship.
• If the film has the same crystal structure as
  well as the same lattice constant then the film
  is pseudomorphic to the substrate.
• If only an out of plane relationship is
  maintained then the film is highly textured with
  most of the surface comprised of the same
  crystallographic plane and rotated domains in the
  plane of the film.

10
Deposition Sources

• Evaporators--the incoming atoms have kinetic
  energies of the order of less than 1 eV--the
  atoms have a small amount of energy to dissipate
  on the growing surface.
• Sputter sources--the incoming atoms may have
  energies as high as several hundred eV--the
  incoming atoms are likely to mix strongly at the
  interface with the film since their energies are
  sufficient to break bonds in most solids.
• Pulsed laser deposition--kinetic energies similar
  to evaporators with the incoming atoms often in
  highly excited electronic states or multiply
  ionized states.

11
Background Gas

• In poor vacuum (gt10-7 mbar) the gas is mostly
  water vapor. The water adsorbed on the surface
  may increase the mobility of surface atoms and
  the oxygen may be incorporated in the growing
  film.
• Intermediate vacuum (10-9 mbar lt P lt 10-7 mbar)
  the presence of CO and CO2 on the surface may
  dominate the growth process--Look for C to be
  incorporated in the film.
• The gas may help to stabilize metastable
  phases--see vast literature of Fe/Cu(100) system.

12
Surfactants

• A surfactant bonds more strongly to the film
  material than the substrate, so the Volmer-Weber
  growth mode may be suppressed.
• Surfactants are characterized by a "floating out"
  efficiency or capability of the surfactant atoms
  to remain on the surface without being
  incorporated in the film.
• Surfactants may also significantly alter surface
  morphology to enable the formation of atomically
  smooth films.
• It is difficult to remove surfactants after they
  have done their job.