Introduction to thin film growth and molecular beam epitaxy
Introduction to Thin Film Growth and
Molecular Beam Epitaxy
Survey of physical vapor deposition techniques
Pulsed laser deposition
Molecular beam epitaxy
TiO2 - anatase
SrTiO3 or [(TiO2)m/(SrO)n], with m = n
Novel layered complex oxides [(TiO2)m/(SrO)n], with m ≠ n
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.
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.
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
•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
•The challenges include
formation and obtaining
uniform wafer coverage.
•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.
Molecular beam epitaxy (MBE)
The advantages of MBE
•Growth is preformed in UHV
environment minimizing impurity
•In-situ growth monitoring is
•Each material is vaporized
independently from its own
•Multiple sources are used to
grow alloy films and hetero
•Deposition is controlled at sub-
Extremely flexible technique
since growth parameters are
Invented in late 1960’s at Bell Laboratories by J. R. Arthur and A. Y. Cho.
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
• 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
Types of MBE
Solid-Source MBE (SS-MBE)
Group-III and -V molecular beams for III-V
Group-II and -VI molecular beams for II-VI
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
Group-III molecular beams and ammonia
gas injector for nitrides (AlxGa1-xN);
Ozone gas injector for oxides;
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
Types of crucibles
- do not decompose, react with the charge material, or outgas impurities under
- 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
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.
Atoms / molecules arriving to the substrate surface may undergo:
• absorption to the surface,
• surface migration and dissociation,
• incorporation into the crystal lattice,
• 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.
Growth modes in epitaxy
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.
Frank-van der Merwe growth mode
- Low interface energy and small lattice mismatch are
- Low rate of incoming atoms and long migration path also
promote layer-by-layer growth.
-(AlxGa1-xAs/GaAs, ZnSe/GaAs, TiO2/LaAlO3, BaO/SrTiO3).
Volmer-Weber growth mode
- Island growth is possible in the hetero epitaxial systems with
high interface energy and large lattice mismatch (Al/Ge).
Stranski-Krastanov growth mode
- Layer + island growth is possible in the systems with low
interface energy and large lattice mismatch (InAs/GaAs,
- High rate of incoming atoms and short migration path also
promote layer + island growth.
Columnar growth mode
-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.
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.
Step-flow growth mode
- 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.
Surface of SiC (0001)
AFM image of a commercial (0001)
Photograph of the hydrogen
6H-SiC wafer. The surface exhibits
randomly oriented scratches induced
by the vendor’s mechanical polish.
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.
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 -
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.
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
Surface unit cell size - distance between streaks / spots;
Atomically flat surface – diffraction streaks;
Rougher surface – transmission spots;
Layer-by-layer growth mode - intensity oscillations.
Interpretation of RHEED patterns
(1) Diffraction pattern
from nearly ideal smooth
(2) Diffraction pattern
from smooth surface with
1 2 a high density of atomic
diffraction through 3D
3 4 clusters;
(4) Diffraction from
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.
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