Transcript Slide 1

PART III: METALORGANIC
CHEMICAL VAPOR DEPOSITION
 Description of the MOCVD equipment
 Analysis of the MOCVD growth process
 Growth modes in MOCVD
Metalorgenic Chemical Vapor Deposition (MOCVD)
[Metalorganic Vapor Phase Epitaxy (MOVPE),
OMCVD, OMVPE]
 One of the premier techniques for epitaxial growth of thin layer
structures (semiconductors, oxides, superconductors)
 Introduced around 25 years ago as the most versatile technique for
growing semiconductor films.
 Wide application for devices such Lasers, LEDs, solar cells,
photodetectors, HBTs, FETs.
 Principle of operation: transport of precursor molecules (group-III
metalorganics + group-V hydrides or alkyls) by a carrier gas (H2, N2)
onto a heated substrate; surface chemical reactions.
 Complex transport phenomena and reactions, complicated models to
determine reactor designs,growth modes and rates.
 In-situ diagnostics less common than in MBE.
Description of the MOCVD equipment
• R. L. Moon and Y.-M. Houng, in Chemical vapor deposition Principles and applications, edited by M. L. Hitchman and K. F.
Jensen (Academic Press, London, 1993).
• G. B. Stringfellow, Organometallic vapor phase epitaxy: theory
and practice (Academic Press, Boston, 1989).
MOCVD Facility, horizontal reactor
Gas handling
• Research system (left): AIX 200
• 1X2” wafer capacity
Reactor
Glove box
• Production system (right):
AIX 2600
• Up to 5X10” wafer capacity
(AIX 3000)
Schematics of a MOCVD system
Carrier
gas
Material
sources
Gas
handling
system
In-situ
diagnostics
NO electron
beam probes!
Reflectance
Ellipsometry
RAS
Exhaust system
Safety system
Reactor
Gas handling system
 The function of gas handling system is mixing and metering of the
gas that will enter the reactor. Timing and composition of the gas
entering the reactor will determine the epilayer structure.
 Leak-tight of the gas panel is essential, because the oxygen
contamination will degrade the growing films’ properties.
 Fast switch of valve system is very important for thin film and abrupt
interface structure growth,
 Accurate control of flow rate, pressure and temperature can ensure
stability and repeatability.
Carrier gas
 “Inert” carrier gas constitutes about 90 % of the gas phase 
stringent purity requirements.
 H2 traditionally used, simple to purify by being passed through a
palladium foil heated to 400 °C. Problem: H2 is highly explosive in
contact with O2  high safety costs.
 Alternative precursor : N2: safer, recently with similar purity, more
effective in cracking precursor molecules (heavier).
 High flux  fast change of vapor phase composition. Regulation:
mass flow controller
 P ~ 5- 800 mbar
Mass flow controllers
Material sources
 Volatile precursor molecules transported by the carrier
gas
 Growth of III-V semiconductors:
 Group III: generally metalorganic molecules (trimethylor triethyl- species)
 Group V: generally toxic hydrides (AsH3; PH3
flammable as well); alternative: alkyls (TBAs, TBP).
Hidrides and dopants
 Form: gases from high pressure cylinders
 Mixed into the carrier gas line
 Flow control: valve + mass flow controller (MFC)
Bubblers
Metalorganics
 Liquid (or finely divided solid – TMIn) contained in a
stainess steel bubbler.
 Vapor pressure fixed by constant temperature in a thermal
bath; T ≈ -20oC ÷ 40oC; DT = ±1oC.
 Controlled H2 flow through the bubbler  saturated
H2, N2
stream; composition depends on H2 flow rate 
adjustment through MFC
To reactor
 P ressure controller (PC) to keep a fixed pressure in the
bubbler and throttles the resulting mixture of H2 and MO
down to the reactor pressure.
PC
MFC
Valve NC
Valve NO
Bubbler
Thermal
bath
To reactor
Metalorganic compounds
 Optimal thermal decomposition temperature between 300 and 500°C
 availability of transported reactant at the substrate surface.
 The vapor pressure of the MO source is an important consideration in
MOCVD, since it determines the concentration of source material in the
reactor and the deposition rate. Too low a vapor pressure makes it
difficult to transport the source into the deposition zone and to achieve
reasonable growth rates. Too high a vapor pressure may raise safety
concerns if the compound is toxic.
 Vapor pressures of Metalorganic compounds are calculated in terms of
the expression
Log(p)=B-A/T
Vapor pressure of most common MO compounds
Compound
P at 298 K
A
B
Melt point
(oC)
(torr)
(Al(CH3)3)2
TMAl
14.2
2780
10.48
15
Al(C2H5)3
TEAl
0.041
3625
10.78
-52.5
Ga(CH3)3
TMGa
238
1825
8.50
-15.8
Ga(C2H5)3
TEGa
4.79
2530
9.19
-82.5
In(CH3)3
TMIn
1.75
2830
9.74
88
In(C2H5)3
TEIn
0.31
2815
8.94
-32
Zn(C2H5)2
DEZn
8.53
2190
8.28
-28
Mg(C5H5)2
Cp2Mg
0.05
3556
10.56
175
Log(p)=B-A/T
Flow rate of MO sources
Ideal gas equation  MO flux QMO
•PMO(Tbub) = equilibrium vapor
pressure of the metalorganic
component
•Tbub = bubbler temperature
•QB = carrier gas flux at standard
atmosphere
•Pstandard = standard atmosphere
•PB = regulated bubbler pressure
piV  N i k BT

PMO Tbub m bar 
V
m l / min
Ni
t
m ol/ min 
N At
J 
 l 
4
k B    Tbub K   N A 

10
K 
 m ol

QMO m ol/ min 
PMO Tbub m bar  QB m ol/ min 
Pstandard m bar
PB m bar
 J 
8.314 104 
  Tbub K 
K
m
ol


(Rolf Engelhardt, Ph.D. Thesis, TU Berlin,
2000, http://edocs.tu-berlin.de/diss/2000/engelhardt_rolf.pdf)
Partial pressure of MO sources
PMO  reactor m bar 
Pstandard m bar
PB m bar
m bar
P
Qtot m ol/ min  standard
Preactor m bar
PMO Tbub m bar  QB m ol/ min 
• PMO-reactor = partial pressure of the metalorganic components in
the reactor
• PMO(Tbub) = equilibrium vapor pressure of the metalorganic
component
• QB = carrier gas flux
• Pstandard = standard atmosphere
• PB = regulated bubbler pressure
• Qtot = total gas flux
(Rolf Engelhardt, Ph.D. Thesis, TU Berlin,
2000, http://edocs.tu-berlin.de/diss/2000/engelhardt_rolf.pdf)
MOCVD reactors
 Different orientations and geometries.
 Most common:
 Horizontal reactors: gases inserted laterally with
respect to sample standing horizontally on a slowlyrotating (~60RPM) susceptor plate.
 Vertical reactors: gases enter from top, sample
mounted horizontally on a fast-rotating (~5001000RPM) susceptor plate.
Horizontal reactors
 Primary vendors: AIXTRON
(Germany).
 The substrate rests on a graphite
susceptor heated by RF induction or
by IR lamps.
 Quartz liner tube, generally
rectangular
 Gas flow is horizontal, parallel to the
sample.
 Rotation ~ 60RPM for uniformity by
H2 flux below the sample holder.
Horizontal reactors
 Advantages
 Common reactor  high experience.
 Uniform epitaxial growth provided the gas velocity is large enough, and
attention is paid to hydrodynamic flow.
 Small height above the wafer  the effect of natural convection is
minimized.
 Quite large gas velocity  very rapid changes in the gas phase
composition.
 Disadvantages
 Uniformity can either be achieved by very high gas flow, ( inefficient
deposition), or by implementing rotation, which is tricky in this type of
design.
 Throughput: difficult to scale this design up to accommodate large
volume production.
Planetary reactors
 Primary vendors: AIXTRON.
 Derived from horizontal reactor.
 Material: stainless steel
 Very widespread now for production, and
can
achieve
uniformities.
very
good
wafer
 Uniformity: rotation of the main disk +
individual satellites.
 Up to 5X10” wafer capacity (AIX 3000,
see photo)
Vertical reactors
 Primary vendors: Veeco (former Emcore
(USA)).
 Gas flow generally normal to the wafer.
 Temperature gradients  buoyancy
induced convection  high residence
time of the gases  degradation of
heterostructure compositional abruptness.
 Solution: rotation of susceptor at high
angular velocities (centrifugal “pumping
action” to suppress convection and obtain
more efficient use of precursors.
Simulated streamlines in a vertical spinning cylinder
reactor for MOCVD of GaAs from TMGa, AsH3, H2. Gases
enter at 600K through the top plane and react at the flat
top surface of the spinning inside cylinder. The rotation rate
is 1000rpm and the deposition surface temperature is 900K
(http://www.cs.sandia.gov/CRF/MPSalsa/ )
Vertical reactors






Features




Advantages
All stainless construction
MBE vacuum technology
Safety (no glass)
Electrical resistance heating
Gate valve, and antechamber for
minimizing O2/H2O contamination.
High precursor utilization efficiency
Scaling to very large wafers/ multiple wafers.
Multiple wafer capacity:
Up to 3 x 8", 5 x 6", 12 x 4", and 20 x 3"
 Disadvantages:
 Very high speed rotation, up to 1200 rpm.
 Possible memory effects.
Reflectance anisotropy spectroscopy
(Reflectance difference spectroscopy)
•
•
•
•
Linear polarized light source directed  on the sample.
Light is reflected from the sample.
The reflection is monochromatized and a spectrum is detected.
Only requirement for the system: transparent ambient and a window above
the sample.  easily fulfilled for MOVPE and MBE
• Bulk: isotropic signal
• Surface: reconstruction
 anisotropy in two 
directions (with square
lattices)
• RAS signal: normalized
change of polarization
along two  axes.
Markus Pristovsek, Ph.D. Thesis, TU Berlin,
2001, http://edocs.tuberlin.de/diss/2000/pristovsek_markus.pdf)
Reflectance anisotropy spectroscopy
(Reflectance difference spectroscopy)
A RAS spectrum can be used to identify a surface, by comparing it to
spectra measured on well-ordered reference surfaces with known
reconstruction (measured at the same time, e.g., by RHEED in MBE).
RAS spectra of a c(4x4) and a
ß2(2x4) reconstruction on a GaAs
(001) surface. Grey spectra are the
spectra of a 33%c(4x4) /66%ß2(2x4) and
66%c(4x4) /33%ß2(2x4).
(Markus Pristovsek, Ph.D. Thesis, TU Berlin,
2001, http://edocs.tuberlin.de/diss/2000/pristovsek_markus.pdf)
Exhaust system
 Pump and pressure controller
 Low pressure growth: mechanic pump and pressure controller 
control of growth pressure. The pump should be designed to handle
large gas load (rotary pump).
 Waste gas treatment system
 The treatment of exhaust gas is a matter of safety concern.
 GaAs and InP: toxic materials like AsH3 and PH3. The exhaust gases
still contain some not reacted AsH3 and PH3, Normally, the toxic gas
need to be removed by using chemical scrubber.
 For GaN system, it is not a problem.
AIXTOX system
Safety issues
 Concerns:
 Flammable gases (H2)
 Toxic gases (AsH3, PH3)
 Safety measures:
 Lab underpressurization.
 Design of hydrides cylinders.
 Extensive gas monitoring systems placed in different
locations, able to detect the presence of gas as small as parts
per billion.
 Alarms located in different parts of the buildings + beeper
calls to operators.
 Immediate shut down of the system to a failsafe condition in
case of leakages and other severe failures.
 Alternatives: use of alternate gases
 N2 carrier
 TBAs, TBP (toxic but liquid  low vaopr pressure)
Analysis of the MOCVD growth
process
MBE versus MOCVD growth rate
MBE
Tcell  Pv(T)
Ballistic
transport
Sticking
coefficient = 1
MOCVD
Flow rate
f (total flow F, total
pressure P, vapor pressure Pv)
r = r (F, P, Pv,
mass transport,
reaction kinetics)
Diffusive mass
transport
Chemical
reaction kinetics
r = r (T)
Growth steps in (MO)CVD
1.
Flow
reactant
(precursors)
reactor
3. At
theofsame
time:
chemicaltoreactions
tube,
either by:

homogeneous,
heterogeneous
 Mixingdeposit)
in gas handling
manifold,
(parasitic
 reduction
of
then concentration,
enter the reactorshift in alloy
reactant
 Separate until
the reactor
composition,
reduced
growth(no
rate,
premature
sideroughening.
reactions)
epitaxial
surface
4. (Partially
decomposed)
precursor
2.
In the reactor:
establishment
of gas
diffusion
to the surface
layers governing
transport
ofreaction
mass, to
energy
and
momentum:
entry effects
form
the
desired
material.
and possibly achievement
5. Simultaneous
desorptionofofsteadyreaction
state condition.
products
(hydrocarbons), surface
diffusion of material to lattice sites.
R. L. Moon and Y.-M. Houng, in Chemical vapor
deposition - Principles and applications, edited by M.
L. Hitchman and K. F. Jensen (Academic Press,
London, 1993).
Reactive-flow conservation equations
(Crosslight Procom User’s manual)
The state of the gas phase in a reactor can be completely described
by the continuum mass density r, the individual chemical species
number density ni, the momentum density rv, and the energy density
E. The basic partial differential conservation equations are:
total mass (continuity equation)
individual species (precursors,
intermediate species…)
momentum (Navier-Stokes equation)
energy (heat conduction equation)
Total energy density
Chemical production rate of species i
Number density of species i
Heat flux
Fluid velocity
Radiative heat flux
Number of chemical species present
Diffusion velocity of species i
Pressure tensor
Fluid mass density
Simplified model of (MO)CVD reaction kinetics
 Simplified deposition process of a film, starting from a molecule AB in
the gas phase
(L. Vescan, in Handbook of thin film process technology, edited by
D. A. Glocker and S. I. Shah (Institute of Physics Publishing, Bristol, 1995), p. B1.4:1)
AB(g)  A(s) + B(g)
 J1: molecular flux from the gas phase to the substrate surface,
J2: consumption flux of AB corresponding to the surface reaction:
J1 ≈ hG (CG – CS)
J2 ≈ kSCS
(~supersaturation)
with
hG = gas diffusion rate constant,
CG = gas-phase concentration of AB,
CS = surface concentration of AB,
kS = heterogeneous rate constant
J1
J2
Simplified model of (MO)CVD reaction kinetics
 Steady-state conditions:
CG
J1  J 2  J  1
kS  hG1
 Growth rate r = J0 (with 0 = unit volume of the crystal)
 r  mole fraction of the species AB in the gas phase, and determined by
the smaller of the rate constants hG, kS.
 Limiting cases:
r ≈ kS C G  0
r ≈ hG CG 0


surface kinetics control
mass transport control
Interpretation in terms of supersaturation
 Driving force: supersaturation (chemical potential difference between gas
phase and solid)  out-of-equilibrium process; equilibrium at the vaporsolid interface
 The relative importance of surface kinetics and mass transport can be
interpreted as a function of the chemical potential dependence on the
reaction coordinate. If most of the chemical potential drop occurs in the
boundary layer (red line), the growth is controlled by mass transport; if it
occurs at the interface (green line), the growth is kinetically limited
Chemical potential
Input gas
phase
Boundary
layer
Interface
Solid
Reaction
kinetics
Mass
transport
Reaction coordinate
R. L. Moon and Y.-M. Houng, in Chemical vapor
deposition - Principles and applications, edited by M.
L. Hitchman and K. F. Jensen (Academic Press,
London, 1993).
Mass transport
 Fundamental and very complex aspect in reactor design
 Factors influencing gas flow in a reactor:
 temperature
 concentration and momentum gradients
 gravity ( convection)
 homogeneous, heterogeneous chemical reactions ( parasitic nucleation)
 Simplified (2 regions) picture in a horizontal reactor:
 Upper region: turbulence or vorticity  good mixing and heat transfer
 Close to the susceptor: region of laminar flow (boundary or stagnant layer)
 molecular diffusional
transport to the hot
substrate, where the
transverse velocity is zero.
Mass transport
 Assuming a gas velocity U = U in the bulk gas
phase, and U = 0 at the growth surface 
calculation of boundary layer width (D. W. Kisker and T. F. Kuech, in Handbook of
crystal growth, edited by D. T. J. Hurle (Elsevier Science, Amsterdam, 1994), Vol. 3, p. 93)
d ~ (PU)-1/2, where P is the total reactor pressure.
 If the molecular transport in the boundary layer proceeds by diffusion alone,
the rate constant hG can be written as
hG 
D
d
where D ~ P-1 is the diffusion coefficient
  mass-transport-limited growth rate r ~ p AB
where CG ~ pAB = AB partial pressure
U
P
  growth rate is practically independent of the growth temperature, and
depends linearly on the species partial pressure.
Reaction kinetics
 Two kinds of thermally-activated reactions
 Reactions in the gas phase (homogeneous reactions)
 Reactions at the surface (heterogeneous reactions)
 Forward and reverse rates are characterized by rate constants that
can be expressed in an Arrhenius form:
k = A exp (-E/kBT),
where E is the activation energy for the process.
 Surface kinetics are poorly known processes, in which a number of
sub-processes can be identified. Among them:
 adsorption of reactant species,
 heterogeneous decomposition reactions,
 surface migration,
 incorporation and desorption of products.
Reaction kinetics
 In the most simplified picture, the chemistry of heterogeneous
reactions can be modeled by taking into account only adsorption and
desorption:
k ads
AB(g) +   A  + B(g)
k des
 where  is a vacant surface site,
A is an adsorbed state,
kads and kdes are the adsorption and desorption rate constants
 Assumptions: no interaction between absorbed species; equivalence
among all the adsorption sites.
G. B. Stringfellow, Organometallic vapor phase epitaxy: theory
and practice (Academic Press, Boston, 1989).
Reaction kinetics
 Steady state (adsorption rate = desorption rate):
 
kads
A*
Q


adsorption coefficient K 
kdes *AB( g ) 1  Q p AB
with Q = fraction of occupied lattice sites

Q assumes the form of a Langmuir isotherm: Q 
Kp AB
1  Kp AB
G. B. Stringfellow, Organometallic vapor phase epitaxy: theory
and practice (Academic Press, Boston, 1989).
Reaction kinetics
 MOCVD of binary compound semiconductors: two molecules AB and
CD are transported to the surface, and are adsorbed on cation and
anion sites, respectively.
 For this noncompetitive process, the growth rate of the bimolecular
reaction is proportional to the anion and cation surface coverages
(Langmuir-Hinshelwood isotherm):
r  k ABCD Q ABQCD 
k ABCD K AB p AB K CD pCD
1  K AB p AB 1  KCD pCD 
 III-V semiconductors:
tipically V/III ratio ~ 100  QAB << 1; QCD ≈ 1
  r  K’ pAB,
with K’ a typical rate constant for the process, temperaturedependent.
  growth rate depends only on temperature and on the group-III
precursor partial pressure, and not on the group-V one.
Reaction kinetics for GaAs
Overall reactions:
 TMGa + AsH3:
AsH3 + Ga (CH3)3 → GaAs + 3CH4↑
 TEGa + AsH3:
AsH3 + Ga (C2H5)3 → GaAs + 3C2H4↑ + 3H2 ↑
Lower activation energies for
decomposition for TEGa than
for TMGa  ~200K lower
temperature for 50%
decomposition.
(Markus Pristovsek, Ph.D. Thesis, TU Berlin,
2001, http://edocs.tuberlin.de/diss/2000/pristovsek_markus.pdf)
Reaction kinetics for GaAs
Overall reactions:
 TMGa + AsH3:
AsH3 + Ga (CH3)3 → GaAs + 3CH4↑
 TEGa + AsH3:
AsH3 + Ga (C2H5)3 → GaAs + 3C2H4↑ + 3H2 ↑
TMGa decomposition is strongly
enhanced at the onset of the
AsH3 de-composition.
This is most likely due to
hydrogen radicals produced by
AsH3 decomposition.
(Markus Pristovsek, Ph.D. Thesis, TU Berlin,
2001, http://edocs.tuberlin.de/diss/2000/pristovsek_markus.pdf)
Reaction kinetics for GaAs
Proposed mechanisms (TMGa +
AsH3):
Complex series of decomposition
steps in the gas phase and on the
surface, each with its own
characteristic reaction constant and
activation energy.
K. F. Jensen, Adv. Chem. Ser. 245, 397 (1995)
Growth modes in MOCVD
Growth mode: studies on GaAs from TMGa and AsH3
Effect of substrate temperature
 Studies for atmospheric pressure (AP = 105Pa = 1000mbar) and for low
pressure (LP = 104Pa = 100mbar), and different surface orientations.
 Three regimes:
 Low T: kinetically limited growth  strong T dependence, low P
dependence (r  K’ pTMGa), with K’ dependent on T.
 Mid T: mass transport-limited
growth  r does not depend
appreciably on T and surface
orientation, but increases with
decreasing P (r  pTMGa P-1/2 ).
 High T: increasingly low growth
rates, probably due to homogeneous
reactions in the gas phase, causing
a depletion of reactants, or surface
re-evaporation.
G. B. Stringfellow, Organometallic vapor phase epitaxy: theory
and practice (Academic Press, Boston, 1989).
Growth mode: studies on GaAs from TMGa and AsH3
Effect of reactor pressure
 Studies for T = 650°C and V/III ratio ≈ 100
 Two regimes:
 P > 100mbar, growth is limited by mass transport, and r ~ P-1/2
 After a transition region, at P < 20mbar, the growth rate becomes
independent on P, and growth becomes kinetically limited.
G. B. Stringfellow, Organometallic vapor phase epitaxy: theory
and practice (Academic Press, Boston, 1989).
Growth mode: studies on GaAs from TMGa and AsH3
Effect of TMGa partial pressure
 Studies for different T and substrate orientations
 Three regimes:
 T = 700oC: r  pTMGa at all TMGa pressures
and substrate orientations (mass transport
limited)
 T = 500oC: r saturates for high TMGa
pressures and depends on orientation
(kinetically limited). Evidence for
(orientation-dependent) incomplete AsH3
decomposition (with TMGa completely
pyrolized).
 T = 1000oC: decreased growth rate: gasphase reactions ( reduction of gas-phase
nutrients) and surface desorption (
orientation dependence)
R. L. Moon and Y.-M. Houng, in Chemical vapor deposition - Principles
and applications, edited by M. L. Hitchman and K. F. Jensen (Academic
Press, London, 1993).
500°C
700°C
1000°C