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)