Transcript Epitaxy growth

Epitaxial Deposition
M.H.Nemati
Sabanci University
Outline
Introduction
 Mechanism of epitaxial growth
 Methods of epitaxial deposition
 Applications of epitaxial layers

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:
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
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
General Scheme
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
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
Methods of epitaxial deposition
Vapor Phase Epitaxy
 Liquid Phase Epitaxy
 Molecular Beam Epitaxy

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)



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
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
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.

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
Applications

Engineered wafers





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
More applications

Bipolar Transistor

http://www.search.com/reference/Bipolar_junction_transistor

III-V Devices




http://www.veeco.com/library/elements/images/hbt.jpg
Needed to produce
buried layer
Interface quality key
Heterojunction Bipolar
Transistor
LED
Laser
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
References












P. O. Hansson, J. H. Werner, L. Tapfer, L. P. Tilly, and E. Bauser, Journal of Applied
Physics, 68 (5), 2158-2163 (1990).
G. B. Stringfellow, Journal of Crystal Growth, 115, 1-11 (1991).
S. M. Gates, Journal of Physical Chemistry, 96, 10439-10443 (1992).
C. Chatillon and J. Emery, Journal of Crystal Growth, 129, 312-320 (1993).
M. A. Herman, Thin Solid Films, 267, 1-14 (1995).
D. L. Harame et al, IEEE Transactions on Electron Devices, 42 (3), 455-468 (1995).
G. H. Gilmer, H. Huang, and C. Roland, Computational Materials Science, 12, 354-380
(1998).
B. Ferrand, B. Chambaz, and M. Couchaud, Optical Materials, 11, 101-114 (1999).
R. C. Cammarata, K. Sieradzki, and F. Spaepen, Journal of Applied Physics, 87 (3),
1227-1234 (2000).
R. C. Jaeger, Introduction to Microelectronic Fabrication, 141-148 (2002).
R. C. Cammarata and K. Sieradzki, Journal of Applied Mechanics, 69, 415-418 (2002).
A. N. Larsen, Materials Science in Semiconductor Processing, 9, 454-459 (2006).