1. Thin film technology, early stage growth

2. This lecture Vapor from source to vicinity of substrate
Vapor condensation on substrate, adsorption and reevaporation
Nucleation and coalescence
Epitaxy
Strained layer epitaxy

3. Vapor pressure & impingement rate &
Entropy term Enthalpy term
See next slide for data on entropy and enthalpy
Impingement rate of deposited specie;
Must be >> residual gas impingement rate

5. Incident flux on substrate
jΩ= z *δA *cos θ/π
ji = JΩcosβ /R2
Flux from source
Flux on substrate
δA is orifice area of the source
Θ is the source opening angle
β is the angle of sample relative to source
R is distance between source and substrate

6. Magnesium deposition rate
Magnesium, 900 K, P=2.47 Pa;
Impingement rate z = 440 Å-2/s
jΩ= 440 Å-2/s * cos 0o * 1 cm2 = 1.4*1018 s-1
At 10 cm distance from evaporation source:
(β=0 gives maximum flux)
ji = JΩcosβ /R2 = 1.4*1018 s-1 * cos 0o / (10 cm)2
ji =1.4 Å-2/s

7. Condensation flux
Substrate is also a source of atoms, but the re-evaporation flux is small because substrate temperature is low.
Incident flux from source is higher because it is at higher temperature than subsrate
Incident flux must be greater than the re-evaporation flux.

8. Bathroom analogy You start showering
Water vapor is generated, but the bathroom air can accomodate some water vapor
After some time, saturation is reached and water phase is formed
Water starts condensing on cool surfaces, leading to mirror fogging
But if you keep mirror warm, that will prevent fogging
A clean mirror stays free of condensate longer, dirt acts as nucleation site

9. Supersaturation
At high temperature pressure has to be high before supersatursation takes place.
Before vapor pressure exceeds critical value pe, rate of growth is zero.

10. Surface processes condensation re-evaporation binding at kink
binding at defect
adsorption
nucleation
surface diffusion

11. Bonds at surface
Chemisorption: strong bonds, ca. 1 eV Physisorption, ca. 0.1 eV
Desorption probability ~ Desorption probability ~

12. Surface tension γ
If the two materials are very different, wetting is difficult.
Nanolaminates or superlattices need a matching pair of materials that can wet each other.

13. Fate of a nucleus

14. Wetting & contact angle θ
sl = solid-liquid; lg = liquid-gas; sg= solid-gas interfaces

15. Nucleus formation
ΔEnucleus = 2πraγ -πr2aΔEcondensation

16. Kettle analogy
Homogenous nucleation is rare, because it is energetically unfvourable, and because perfectly flat surfaces are rare.
In a kettle, water does not boil uniformly thru the volume of water.
Boiling starts at scratches at kettle bottom.
Condensation is a phase transition just like boiling.

17. Terrace-kink model
Surface kinks and other defects are favoured attachment sites.
Silicon wafer is intentionally miscut (by e.g. 5o) and misaligned to introduce kinks.

18. Island coalescence

19. Nucleus density
T1 low
T2 medium
T3 medium
T4 high

20. Island ripening

21. Island growth and coalescence
Cu on NaCl

22. Plasma pre-treatment
H2 plasma treatment breaks Si-N, Si-Si, and Si-O bonds and ensures many nucleation sites  no amorphous layer

23. Growth modes layer-by-layer island growth columnar growth
(Frank-van der Merwe) (Volmer-Weber)
Stranski-Krastanov = mixture of FVM and VW

24. Thin film structure

25. Auger analysis (AES)
In island growth mode some substrate is still visible after many layers have been deposited, and surface sensitive techniques like Auger or XPS get signal from substrate.
In layer-by-layer growth mode substrate signal rapidly fades.

26. Which growth mode ?
Film substrate potential W = Ufs/Uff is a ratio of film-substrate and film-film atom interaction potentials.
W >>1  layer-by-layer growth
W >  Stranski-Krastanov
W ~ 1  island growth

27. Surface chemistry Certain bonds are very strong, and likely to form.
Ti-O for example, or
Au-S (called thiol-bond).
This is achieved by coating a monomolecular layer containing sulphur end group.

28. Epitaxial films
Polycrystalline film on single crystalline substrate is the normal case.
Single crystal film (epitaxial layer) on single crystal subsrate is a special case.

29. Etching of surface SiO2 (s) + H2 (g) <==> SiO (g) + H2O (g)
Cleaning
Deposition ()
Etching ()
SiCl4 (g) + 2 H2 (g) <==> Si (s) + 4 HCl (g)

30. Lattice mismatch
In homoepitaxy film and substrate lattices are almost identical
(not fully, because dopants change lattice constants slightly)
In heteroepitaxy lattice mismatch has to be small enough:
η = (af – as)/as where a is lattice constant
Higher the mismatch η , the larger the film-substrate potential W must be

31. Epitaxy Heteroepitaxy: CdTe on GaAs. Heteroepitaxy:
Stacking faults are arranged at regular intervals at the interface and epilayer is perfect
Heteroepitaxy:
Aluminum on spinel

32. Epitaxy considerations
Thermal mismatch:
CTE differences cause strains
Diffusion:
high T produces surface diffusion which helps epitaxy, but too high T produces diffusion of film atoms into substrate
Usable temperature range e.g. 0.35T – 0.4T
Defects:
Misfit dislocations can propagate into film
Impurities (from the gas phase):
Can act as faux nucleation sites, prevent surface diffusion, generate defects, cause oxidation (oxygen, water vapor)
Surface impurites (not removed by cleaning):
Native oxides completely prevent epi, and must be in situ removed

33. Strained & relaxed Misfit dislocations generated at interface
Only so far...

34. Strained & relaxed (2) Relaxed, with defects Strained,
with limited epi layer thickness

35. Epi on epi
If Y2O3 grows epitaxially on silicon, then silicon grows epitaxially on Y2O3 .

36. Structure vs. temperature
Theory
Experiment

37. Major factors affecting film structure
Deposition rate
Vacuum level
Substrate temperature
Surface structure and chemistry