1. Introduction to Thin Film Growth and
Molecular Beam Epitaxy
Oleg Maksimov
maksimov@netzero.net

2. Slides outline
Survey of physical vapor deposition techniques
Pulsed laser deposition
Sputtering
Molecular beam epitaxy
RHEED
Oxide Growth
TiO2 - anatase
SrTiO3 or [(TiO2)m/(SrO)n], with m = n
Novel layered complex oxides [(TiO2)m/(SrO)n], with m ≠ n

3. Survey of vacuum deposition techniques
Physical Vapor Deposition Chemical Vapor Deposition
Pulsed Laser Deposition Metal-organic
Sputtering Atomic layer
Molecular Beam Epitaxy Etc…
Uses thermodynamical / Uses chemical processes to produce
mechanical processes to produce thin film.
thin film.
The substrate is exposed to more
The source material is placed in volatile precursors, which react
an energetic environment, so its and/or decompose on the substrate
particles can escape and surface.
condense on the substrate.

4. Pulsed laser deposition
•A high-power pulsed laser is
focused on the target. The target
is ablated to form a plume of
atoms, molecules, and
particulates directed towards the
substrate.
•The advantages of PLD are the
high deposition rates and
possibility to produce multi
component thin films with
preserved composition under
the high partial oxygen
pressure.
complex oxides
•The challenges include
minimizing particulate
formation and obtaining
uniform wafer coverage.

5. Sputtering
metals
•The sputtering target is bombarded with gaseous ions under high voltage
acceleration. As the ions collide with the target, atoms of the target material are
ejected against the substrate, where they condense.
•The advantage of sputtering is that a wide variety of materials can be sputtered
in a reactive atmosphere.
•The disadvantages are the absence of in-situ monitoring tools, poor control of
the charged plasma, and re-sputtering from the substrate.

6. Molecular beam epitaxy (MBE)
The advantages of MBE
•Growth is preformed in UHV
environment minimizing impurity
incorporation;
•In-situ growth monitoring is
possible;
•Each material is vaporized
independently from its own
effusion cell;
•Multiple sources are used to
grow alloy films and hetero
compound
structures;
semiconductors
•Deposition is controlled at sub-
monolayer level.
Extremely flexible technique
since growth parameters are
varied independently.
Invented in late 1960’s at Bell Laboratories by J. R. Arthur and A. Y. Cho.

7. Disadvantages of MBE
The disadvantages of MBE Effect of Base Pressure
Pressure Mean Free Path
• Growth is performed under low
oxygen partial pressure; (Torr) (m)
• Very low deposition rates: 1 µm
1 7 x 10-5
– 100 nm per hour are used;
• High equipment cost and long 10-3 7 x 10-2
set up time;
• Extreme flexibility 10-4 0.7
(uncontrolled flexibility = 10-5 7
chaos!)
• The other meanings of MBE: 10-6 70
Many Boring Evenings 10-7 700
Mostly Broken Equipment
Mega-Buck Evaporation 10-9 70 x 103
Make-Belief Experiments source – substrate distance ~ 0.3 m

8. MBE growth system

9. Types of MBE
Solid-Source MBE (SS-MBE)
Group-III and -V molecular beams for III-V
semiconductors (InxGa1-xAs);
Group-II and -VI molecular beams for II-VI
semiconductors (HgxCd1-xTe);
Other for IV-VI semiconductors, Heusler
alloys, silicides, metals…
Plasma-assisted MBE (PA-MBE)
Group-III molecular beams and nitrogen
plasma source for nitrides (AlxGa1-xN);
Oxygen plasma or atomic oxygen source for
oxides(MgxZn1-xO, TiO2);
Reactive-MBE (R-MBE)
Group-III molecular beams and ammonia
gas injector for nitrides (AlxGa1-xN);
Ozone gas injector for oxides;

10. Effusion cells
Heating System Radiation heating, tantalum wires with PBN insulators
Thermal insulation Shield made out of refractory metal and water cooling coil
100 °C ...1000 °C low temperature cells
Temperature range 800 °C ...1400 °C high temperature cells
up to 2000 °C based on custom design
Temperature stability <= 0.1 K depending on the PID controller

11. Single and dual filament cells

12. Types of crucibles
- do not decompose, react with the charge material, or outgas impurities under
operating conditions;
- made of Ta, Mo, BeO, graphite, and pyrolytical boron nitride.
Cylindrical crucible offers good charge
material capacity and long term flux stability.
However, uniformity of the deposited film is
reduced.
Conical crucible offers excellent uniformity
in the expense of charge material capacity. The
long-term flux stability is poor and geometry
permits large shutter flux transients.

13. Beam flux monitoring
Z-travel
Bayard-Alpert ionization gauge
or quartz crystal monitor

14. Epitaxial growth
Atoms / molecules arriving to the substrate surface may undergo:
• absorption to the surface,
depend on
• surface migration and dissociation,
substrate
• incorporation into the crystal lattice,
temperature
• thermal desorption.
Therefore, epitaxial growth is ensured by:
• very low rate of impinging atoms,
• long migration path on the surface,
• high possibility of the subsequent surface reactions.

15. Growth modes in epitaxy
Columnar Step-Flow
The mode by which epitaxial film grows depends on:
•the interface energy,
•the lattice mismatch between substrate and film,
•the growth temperature,
•the flux of the incoming atoms.
The process can be complicated by surface segregation and alloying.

16. Frank-van der Merwe growth mode
Columnar Step-Flow
- Low interface energy and small lattice mismatch are
necessary.
- Low rate of incoming atoms and long migration path also
promote layer-by-layer growth.
-(AlxGa1-xAs/GaAs, ZnSe/GaAs, TiO2/LaAlO3, BaO/SrTiO3).

17. Volmer-Weber growth mode
Columnar Step-Flow
- Island growth is possible in the hetero epitaxial systems with
high interface energy and large lattice mismatch (Al/Ge).

18. Stranski-Krastanov growth mode
Columnar Step-Flow
- Layer + island growth is possible in the systems with low
interface energy and large lattice mismatch (InAs/GaAs,
CdSe/ZnSe, SrO/LaAlO3).
- High rate of incoming atoms and short migration path also
promote layer + island growth.

19. Columnar growth mode
Columnar Step-Flow
-Columnar growth occurs in the case of extremely low surface
mobility of incoming atoms and growth anisotropy –
preferential growth direction (GaN/Si or GaN/GaAs).
- Film has a fiber structure. Columns have well defined
boundaries and facets.

20. MBE-grown GaN on GaAs (TEM)
On-zone-axis bright-field image showing the High-resolution image collected near the GaN
GaN/GaAs. The film has a columnar structure. film surface along GaN [11-20] zone axis,
Insert is a SAD pattern collected from the top part showing two neighboring columns. The boundary
of the film. between columns appears amorphous.

21. Step-flow growth mode
Columnar Step-Flow
- To promote step-flow growth substrate is slightly mis-oriented
(∼ 10 - 20 ) from a low-index plane. Annealing (H2/Ar, O2)
results in a high density of well-oriented terraces (steps) of
monatomic height (SiC, MgO). Arriving atoms migrate to the
step boundaries that are preferential binding sites.

22. Surface of SiC (0001)
AFM image of a commercial (0001)
Photograph of the hydrogen
6H-SiC wafer. The surface exhibits
etcher assembly.
randomly oriented scratches induced
by the vendor’s mechanical polish.

23. Hydrogen etching of SiC (0001)
AFM image of the same (0001) 6H-SiC wafer after hydrogen
etching at 1650°C, 650 Torr, 10% H2 in 90% Ar at ~1100 sccm
flow for 1 hour.

24. In-situ growth monitoring
Reflective high energy electron diffraction (RHEED)
RHEED is sensitive to surface structures and reconstructions
and is used to:
1. Observe removal of contaminants from the substrate surface -
surface reconstruction;
2. Calibrate growth rates – RHEED intensity oscillations;
3. Estimate the substrate temperature - surface reconstruction;
4. Determine the stoichiometry - surface reconstruction;
5. Analyze surface morphology – RHEED pattern;
6. Study growth kinetics – RHEED intensity oscillations.

25. RHEED geometry
A high energy (~10 - 30
keV) electron beam is
directed to the sample
surface at a grazing
angle (~1- 30). The
diffracted beam is
detected by fluorescence
on the phosphorus
screen.
Surface unit cell size - distance between streaks / spots;
Atomically flat surface – diffraction streaks;
Rougher surface – transmission spots;
Layer-by-layer growth mode - intensity oscillations.

26. Interpretation of RHEED patterns
(1) Diffraction pattern
from nearly ideal smooth
surface;
(2) Diffraction pattern
from smooth surface with
1 2 a high density of atomic
steps;
(3) Transmission
diffraction through 3D
3 4 clusters;
(4) Diffraction from
polycrystalline or
textured surface.

27. RHEED intensity oscillations
Different stages of layer-by-layer growth by nucleation of 2D islands
and the corresponding intensity of the diffracted RHEED beam.
- Direct measure of growth rates in MBE since oscillation frequency
corresponds to the monolayer growth rate.
- Magnitude of the RHEED oscillations damps because as the growth
progresses, islands nucleate before the previous layer is finished.