1. M.H.Nemati Sabanci University
Epitaxial Deposition
M.H.Nemati
Sabanci University

2. Outline Introduction Mechanism of epitaxial growth
Methods of epitaxial deposition
Applications of epitaxial layers

3. Epitaxial Growth
Deposition of a layer on a substrate which matches the crystalline order of the substrate
Homoepitaxy
Growth of a layer of the same material as the substrate
Si on Si
Heteroepitaxy
Growth of a layer of a different material than the substrate
GaAs on Si
Ordered, crystalline growth; NOT epitaxial
Epitaxial growth:

4. Motivation
Epitaxial growth is useful for applications that place stringent demands on a deposited layer:
High purity
Low defect density
Abrupt interfaces
Controlled doping profiles
High repeatability and uniformity
Safe, efficient operation
Can create clean, fresh surface for device fabrication

5. General Epitaxial Deposition Requirements
Surface preparation
Clean surface needed
Defects of surface duplicated in epitaxial layer
Hydrogen passivation of surface with water/HF
Surface mobility
High temperature required heated substrate
Epitaxial temperature exists, above which deposition is ordered
Species need to be able to move into correct crystallographic location
Relatively slow growth rates result
Ex. ~0.4 to 4 nm/min., SiGe on Si

6. General Scheme

7. Thermodynamics Specific thermodynamics varies by process
Chemical potentials
Driving force
Process involves High temperature process is mass transport controlled, not very sensitive to temperature changes
Close enough to equilibrium that chemical forces that drive growth are minimized to avoid creation of defects and allow for correct ordering
Sufficient energy and time for adsorbed species to reach their lowest energy state, duplicating the crystal lattice structure
Thermodynamic calculations allow the determination of solid composition based on growth temperature and source composition

8. Kinetics Growth rate controlled by kinetic considerations
Mass transport of reactants to surface
Reactions in liquid or gas
Reactions at surface
Physical processes on surface
Nature and motion of step growth
Controlling factor in ordering
Specific reactions depend greatly on method employed

9. Methods of epitaxial deposition
Vapor Phase Epitaxy
Liquid Phase Epitaxy
Molecular Beam Epitaxy

10. Vapor Phase Epitaxy Specific form of chemical vapor deposition (CVD)
Reactants introduced as gases
Material to be deposited bound to ligands
Ligands dissociate, allowing desired chemistry to reach surface
Some desorption, but most adsorbed atoms find proper crystallographic position
Example: Deposition of silicon
SiCl4(g) + 2H2(g) ↔ Si(s) + 4HCl(g),
SiCl4 introduced with hydrogen
Forms silicon and HCl gas
SiH4 breaks via thermal decomposition
Reversible and possible to do negative (etching)

11. Precursors for VPE
Must be sufficiently volatile to allow acceptable growth rates
Heating to desired T must result in pyrolysis
Less hazardous chemicals preferable
Arsine highly toxic; use t-butyl arsine instead
VPE techniques distinguished by precursors used

12. Liquid Phase Epitaxy
Reactants are dissolved in a molten solvent at high temperature
Substrate dipped into solution while the temperature is held constant
Example: SiGe on Si
Bismuth used as solvent
Temperature held at 800°C
High quality layer
Fast, inexpensive
Not ideal for large area layers or abrupt interfaces
Thermodynamic driving force relatively very low

13. Molecular Beam Epitaxy
Very promising technique
Beams created by evaporating solid source in UHV
Evaporated beam of particle travel through very high vaccum and then condense to shape the layer
Doping is possible to by adding impurity to source gas by(e.g arsine and phosphors)
Deposition rate is the most important aspect of MBE
Thickness of each layer can be controlled to that of a single atom
development of structures where the electrons can be confined in space, giving quantum wells or even quantum dots
Such layers are now a critical part of many modern semiconductor devices, including semiconductor lasers and light-emitting diodes.

14. Doping of Epitaxial Layers
Incorporate dopants during deposition(advantages)
Theoretically abrupt dopant distribution
Add impurities to gas during deposition
Arsine, phosphine, and diborane common
Low thermal budget results(disadvantages)
High T treatment results in diffusion of dopant into substrate
Can’t independently control dopant profile and dopant concentration

15. Applications Engineered wafers In CMOS structures
Clean, flat layer on top of less ideal Si substrate
On top of SOI structures
Ex.: Silicon on sapphire
Higher purity layer on lower quality substrate (SiC)
In CMOS structures
Layers of different doping
Ex. p- layer on top of p+ substrate to avoid latch-up

16. More applications Bipolar Transistor III-V Devices
Needed to produce buried layer
III-V Devices
Interface quality key
Heterojunction Bipolar Transistor
LED
Laser

17. Summary Deposition continues crystal structure
Creates clean, abrupt interfaces and high quality surfaces
High temperature, clean surface required
Vapor phase epitaxy a major method of deposition
Epitaxial layers used in highest quality wafers
Very important in III-V semiconductor production

18. References
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