Epitaxial Growth(Campbell, Chapter 14)
• defects in epitaxial growth• thermodynamics and kinetics• silicon epitaxy
Structural defects in epitaxial growth
• Although the goal of epitaxial growth is to produce defect-free single crystal layers, structural and electrical defects can still form
• Dislocations are almost always bad (electron-hole recombination)– misfit dislocations occur when lattice-mismatched growth is
performed beyond the pseudomorphic limit– threading dislocations propagate into the growing layer and
can “kill” device performance• Point defects are often observed:
– equilibrium concentration: – stoichiometric defects in binary compounds
N-vacancies in GaN due to inhibited nitrogen incorporation anti-site defects: AsGa in GaAs “EL2”
• Stacking faults are also commonly observed
kTGnn fv exp
Stacking faults in silicon
formation of a stacking fault detail of a stacking fault
ab
c
cab
a
cb
acabcabca
• Stacking faults are errors in the stacking of atomic planes and can occur only when the succeeding layers are different
• In face-centered cubic or diamond crystal structures, faults form when there is a “mistake” in the …ABCABC… stacking sequence
Stacking faults in silicon
Lattice-mismatched growth
• one- or two-dimensional arrays of misfit dislocations can form at the interface between the strained layer and the substrate
• because a dislocation line can never terminate within a crystal, threading dislocations connect a misfit segment
Misfit dislocations in GaAs/Si
Antiphase disorder
• Antiphase disorder (or “antiphase domains”, APD’s) occur due to the symmetry in some unit cells (GaAs -- zincblende)
• Particularly severe when doing heteroepitaxial growth of one unit cell type on another (GaAs-on-Si)
antiphase domain
Thermodynamics of epitaxial growth
• The influence of thermodynamics on epitaxial growth is well illustrated in the reduction of silicon tetrachloride:SiCl4(gas) + 2 H2(gas) Si (solid) + 4 HCl (gas) T~1250°C Forward reaction: SiCl4 is reduced to solid Si with HCl as
a reaction by-product Reverse reaction: Solid silicon is etched by HCl
• The rate of the forward reaction is• The rate of the reverse reaction is • At equilibrium, the forward and backward rates are equal, so the
equilibrium constant KSiCl4 is unity:
224 HSiClff PPkr
4HClrr Pkr
2
4
24
4
HSiCl
HCl
r
fSiCl PP
P
k
kK
Thermodynamics of epitaxial growth (2)
SiCl4(gas) + 2 H2(gas) Si (solid) + 4 HCl (gas) T~1250°C
• Depending on the partial pressures of the various gases present, silicon may be etched or deposited
• Important experimentally-adjustable parameters include:– mole fractions of the gas species
(reactants and products)– chlorine/hydrogen ratio in the
feed gas
Kinetics of epitaxial growth
• Consider again the reduction of silicon tetrachloride:SiCl4(gas) + 2 H2(gas) Si (solid) + 4 HCl (gas)
• If the system is linear (i.e. fluxes are linearly related to driving forces) and the reduction follows a single reaction, then
sgg1 cchF
• The reaction at the surface is described by:
ss2 ckF
• At steady state F2 = F1 leading to
gsg
sgs c
kh
khc
growth rate F2 cs g
sg
sg ckh
kh
hg is the gas-phase mass transfer coefficient
ks is the surface reaction rate coefficient
N
c
kh
khR g
sg
sg
• The steady-state flux can be converted to a growth rate:
number density of atoms(Si: 51022 cm-3)
R [=] cm sec-1 (i.e. a growth velocity or growth rate); R is obviously sensitive to hg, ks and cg
Kinetics of epitaxial growth (2)
Temperature dependence of growth growth rates of silicon from various chlorosilanes
lower right: surface reaction limitedupper left: mass transfer limited
• Since epitaxial growth requires the incorporation of atoms or molecules at specific sites, the surface reaction rate may be influenced by the competition between species for those sites
• There are two general mechanisms for adsorption onto a surface – physisorption (weak, low temperature, not applicable to CVD)– chemisorption (strong, high temperature, important to CVD)
• The fractional coverage follows the Langmuir adsorption isotherm
Kinetics of epitaxial growth (3)
0
1
P
PP
1
low P -- P high P -- 1MkT
2
1note:
and the growth rate = ks
Vapor phase epitaxy -- silicon
• The obvious choice for Si VPE is the pyrolysis of siliane:SiH4 (gas) Si (solid) + 2 H2 (gas) T~1000°C
• No chlorine means no etching, but there are problems…– Gas-phase nucleation of silicon particles– Much lower deposition rate than tetrachloride process– Silane is much more expensive, unstable, difficult to handle
• Near-atmospheric pressure Si epi reactors typically use SiCl4 or SiCl2H2
• Relatively high growth rates (~0.1 m/min)• High temperatures (>1150°C) achieved with graphite susceptor
and RF heating
A generic Si AP-VPE system
Dopant incorporation in Si epitaxy
• Dopants can be added to the gas stream to control the electrical characteristics of the epitaxial layer
• For silicon:– n-type dopants: AsH3, PH3
– p-type dopant: B2H6
• Typically introduced in a very dilute form• n-type doping may significantly lower the epitaxial growth rate
– hydrides adsorb strongly on active surface reaction sites– decompose slowly compared with SiH4
• p-type doping may significantly increase the epitaxial growth rate– p-type layer may help hydrogen to desorb, thus opening more
sites for SiH4 reduction
all are gases at room temperature
Ultra-high vacuum chemical vapor deposition (UHV-CVD)
• UHV-CVD growth of silicon is performed in systems with a base pressure of better than 10-9 torr
• Growth takes place at a pressure ~10-3 torr using SiH4 or GeH4
UHV-CVD can produce high quality epitaxial films at low temperatures (down to 450°C!!)
less interdiffusion sharper interfaces better alloy growth
(SiGe)