introduction to thin film growth and molecular beam epitaxy
TRANSCRIPT
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
Survey of vacuum deposition techniques
Physical Vapor Deposition
Pulsed Laser Deposition
Sputtering
Molecular Beam Epitaxy
Uses thermodynamical / mechanical processes to produce
thin film.
The source material is placed in an energetic environment, so its
particles can escape and condense on the substrate.
Chemical Vapor Deposition
Metal-organic
Atomic layer
Etc…
Uses chemical processes to produce thin film.
The substrate is exposed to more volatile precursors, which react
and/or decompose on the substrate surface.
Pulsed laser deposition
•A high-power pulsed laser is focused on the target. The target is ablated to form a plume of atoms, molecules, andparticulates 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.
•The challenges include minimizing particulate formation and obtaining uniform wafer coverage.
complex oxides
Sputtering
•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.
metals
Molecular beam epitaxy (MBE)
Invented in late 1960’s at Bell Laboratories by J. R. Arthur and A. Y. Cho.
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 structures;•Deposition is controlled at sub-monolayer level.
Extremely flexible technique since growth parameters are
varied independently.
compound semiconductors
Disadvantages of MBE
The disadvantages of MBE
• Growth is performed under low oxygen partial pressure;
• Very low deposition rates: 1 µm – 100 nm per hour are used;
• High equipment cost and long set up time;
• Extreme flexibility (uncontrolled flexibility = chaos!)
• The other meanings of MBE:Many Boring Evenings Mostly Broken EquipmentMega-Buck EvaporationMake-Belief Experiments
Effect of Base Pressure
Pressure Mean Free Path
(Torr) (m)
1 7 x 10-5
10-3 7 x 10-2
10-4 0.7
10-5 7
10-6 70
10-7 700
10-9 70 x 103
source – substrate distance ~ 0.3 m
MBE growth system
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;
Effusion cells
Heating SystemThermal insulation
Radiation heating, tantalum wires with PBN insulatorsShield made out of refractory metal and water cooling coil
Temperature range100 °C ...1000 °C low temperature cells800 °C ...1400 °C high temperature cellsup to 2000 °C based on custom design
Temperature stability <= 0.1 K depending on the PID controller
Single and dual filament cells
Types of crucibles
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.
Cylindrical crucible offers good charge material capacity and long term flux stability. However, uniformity of the deposited film is reduced.
- do not decompose, react with the charge material, or outgas impurities under operating conditions;
- made of Ta, Mo, BeO, graphite, and pyrolytical boron nitride.
Bayard-Alpert ionization gauge or quartz crystal monitor
Z-travel
Beam flux monitoring
Epitaxial growth
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.
depend on substrate
temperature
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.
Columnar Step-Flow
Frank-van der Merwe growth mode
- 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).
Columnar Step-Flow
Volmer-Weber growth mode
- Island growth is possible in the hetero epitaxial systems with high interface energy and large lattice mismatch (Al/Ge).
Columnar Step-Flow
Stranski-Krastanov growth mode
- 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.
Columnar Step-Flow
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.
Columnar Step-Flow
MBE-grown GaN on GaAs (TEM)
On-zone-axis bright-field image showing the GaN/GaAs. The film has a columnar structure.
Insert is a SAD pattern collected from the top part of the film.
High-resolution image collected near the GaN film surface along GaN [11-20] zone axis,
showing two neighboring columns. The boundary 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.
Columnar Step-Flow
Surface of SiC (0001)
AFM image of a commercial (0001) 6H-SiC wafer. The surface exhibits
randomly oriented scratches induced by the vendor’s mechanical polish.
Photograph of the hydrogen etcher assembly.
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 - 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.
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.
(1) Diffraction pattern from nearly ideal smooth surface;
(2) Diffraction pattern from smooth surface with a high density of atomic steps;
(3) Transmission diffraction through 3D clusters;
(4) Diffraction from polycrystalline or textured surface.
1 2
3 4
Interpretation of RHEED patterns
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.