Transcript Epitaxial Deposition - Pennsylvania State University

Epitaxial Deposition
Daniel Lentz
EE 518
Penn State University
March 29, 2007
Instructor: Dr. J. Ruzyllo
Outline
Introduction
 Mechanism of epitaxial growth
 Methods of epitaxial deposition
 Properties of epitaxial layers
 Applications of epitaxial layers
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Epitaxial Growth
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Deposition of a layer on
a substrate which
matches the crystalline
order of the substrate
Homoepitaxy
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Growth of a layer of the
same material as the
substrate
Si on Si
Heteroepitaxy
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Growth of a layer of a
different material than
the substrate
GaAs on Si
Ordered,
crystalline
growth;
NOT
epitaxial
Epitaxial
growth:
Motivation
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Epitaxial growth is useful for applications that place
stringent demands on a deposited layer:
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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
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Surface preparation
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Clean surface needed
Defects of surface duplicated in epitaxial layer
Hydrogen passivation of surface with water/HF
Surface mobility
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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
Modified from http://www.acsu.buffalo.edu/~tjm/MOVPE-GaN-schematic.jpg
Thermodynamics
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Specific thermodynamics varies by process
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Chemical potentials
Driving force
High temperature process is mass transport controlled, not very
sensitive to temperature changes
Steady state
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
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Growth rate controlled by kinetic
considerations
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Mass transport of reactants to surface
Reactions in liquid or gas
Reactions at surface
Physical processes on surface
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Nature and motion of step growth
Controlling factor in ordering
Specific reactions depend greatly on method
employed
Kinetics Example
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Atoms can bond to flat surface,
steps, or kinks.
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As-rich GaAs surface
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Lowers energy
Causes kinks and steps on surface
Results in motion of steps on
surface
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http://www.bnl.gov/nsls2/sciOps/chemSci/growth.asp
As only forms two bonds to
underlying Ga
Very high energy
Reconstructs by forming As dimers
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On surface requires some critical
radius
Easier at steps
Easiest at kinks
If start with flat surface, create step
once first group has bonded
Growth continues in same way
Vapor Phase Epitaxy
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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
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SiCl4 introduced with hydrogen
Forms silicon and HCl gas
Alternatively, SiHCl3, SiH2Cl2
SiH4 breaks via thermal decomposition
Precursors for VPE
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Must be sufficiently volatile to allow
acceptable growth rates
Heating to desired T must result in pyrolysis
Less hazardous chemicals preferable
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Arsine highly toxic; use t-butyl arsine instead
VPE techniques distinguished by precursors
used
Varieties of VPE
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Chloride VPE
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Chlorides of group III and V elements
Hydride VPE
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Chlorides of group III element
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Hydrides of group V element
Organometallic VPE
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Group III hydrides desirable, but too unstable
Organometallic group III compound
Hydride or organometallic of group V element
Not quite that simple
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Combinations of ligands in order to optimize
deposition or improve compound stability
Ex. trimethylaminealane gives less carbon
contamination than trimethylalluminum
http://upload.wikimedia.org/wikipedia/en/thumb/e/e5/Trimethylaluminum.png/100px-Trimethylaluminum.png,
http://pubs.acs.org/cgi-bin/abstract.cgi/jpchax/1995/99/i01/f-pdf/f_j100001a033.pdf?sessid=6006l3
Other Methods
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Liquid Phase Epitaxy
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Reactants are dissolved in
a molten solvent at high
temperature
Substrate dipped into
solution while the
temperature is held
constant
Example: SiGe on Si
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Bismuth used as solvent
Temperature held at
800°C
High quality layer
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Fast, inexpensive
Not ideal for large area
layers or abrupt interfaces
Thermodynamic driving
force relatively very low
Molecular Beam Epitaxy
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Very promising technique
Elemental vapor phase
method
Beams created by
evaporating solid source in
UHV
Doping of Epitaxial Layers
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Incorporate dopants during deposition
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Theoretically abrupt dopant distribution
Add impurities to gas during deposition
Arsine, phosphine, and diborane common
Low thermal budget results
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High T treatment results in diffusion of dopant into
substrate
Reason abrupt distribution not perfect
Properties of Epitaxial Layer
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Crystallographic structure of film reproduces that of
substrate
Substrate defects reproduced in epi layer
Electrical parameters of epi layer independent of
substrate
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Dopant concentration of substrate cannot be reduced
Epitaxial layer with less dopant can be deposited
Epitaxial layer can be chemically purer than
substrate
Abrupt interfaces with appropriate methods
Applications
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Engineered wafers
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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
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Layers of different doping
Ex. p- layer on top of p+
substrate to avoid latch-up
More applications
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Bipolar Transistor
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http://www.search.com/reference/Bipolar_junction_transistor
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III-V Devices
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http://www.veeco.com/library/elements/images/hbt.jpg
Needed to produce
buried layer
Interface quality key
Heterojunction Bipolar
Transistor
LED
Laser
Summary
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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
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P. O. Hansson, J. H. Werner, L. Tapfer, L. P. Tilly, and E. Bauser, Journal of Applied
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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).
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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).