Mo Cvd Material Growth
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Transcript of Mo Cvd Material Growth
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MOCVD technology and material growth
1.Introduction
2.The MOVD technique and growth system
3.Metalorganic compound
4.Gas phase and surface reaction
5.Materials Characterization6.MOCVD growth of GaN
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Most of the advances in semiconductor processing have centered on the ability to
decrease the physical dimensions of the electronic device structure.
Lateral dimension:
Photolithographic,Deposition
Etching techniques
Vertical dimension:
Epitaxial deposition
1.Introduction
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Compare of epitaxial methods
Growth method time features limit
LPE
(Liquid phase
epitaxy)
1963 Growth form
supersaturated
solution onto substrate
Limited substrate areas
and poor control over the
growth of very thin
layers
VPE(Vapor phase
epitaxy
1958 Use metal halide astransport agents to
grow
No Al containedcompound, thick layer
MBE
(Molecular Beam
Epitaxy)
1958
1967
Deposit epilayer at
ultrahigh vacuum
Hard to grow materials
with high vapor pressure
MOCVD
(Metal-Organic
Chemical Vapor
Deposition)
1968 Use metalorganic
compounds as the
sources
Some of the sources like
AsH3 are very toxic.
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Some about the name of MOCVD
In the reference, MOCVD also have some other names. Differentpeople prefer different name. All the names refer to the same growth
method.
MOCVD (Metalorganic chemical vapor deposition)
OMCVD(Organometallic CVD)
MOVPE (MO vapor phase epitaxy)
OMVPE
AP-MOCVD (Atmosphere MOCVD)
LP-MOCVD (Low pressure MOCVD)
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2. The MOVD growth system
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Vacuum and
Exhaust system
Gas handle
system
Computer
Control
Reactor
MOCVD Growth System
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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 oxygencontamination 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 ensurethe stable and repeat.
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Reactor-1
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Reactor-2
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Aixtron Model-2400 reactor
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Exhaust system
Pump and pressure controller
For low pressure growth, we use mechanic pump and
pressure controller to control the growth pressure. The pump
should be designed to handle large gas load.
waste gas treatment system
The treatment of exhaust gas is a matter of safety concern.
The MOCVD system for GaAs and InP use 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.
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3. Metalorganic compound
The vapor pressure of the MO source is an important consideration inMOCVD, 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. Further more, it is easier to control the delivery from aliquid than from a solid.
Vapor pressures of Metalorganic compounds are calculated in terms of the
expression
Log[p(torr)]=B-A/T
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Vapor pressure of most common MO compounds
Compound P at 298 K
(torr)
A B Melt point(oC)
(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(torr)]=B-A/T
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Calculate the mole flow rate of MO sources
When we read some reference, we often see people use mol/min to indicate the flow
rate. Normally, we use the formula to calculate it.
F (mol/min)=p MO/p Bubbler*[flow rate (ml/min)]/22400 (mol/ml)
We need to calculate the mole flow rate before we determine the growth condition. If
we want to grow alloys, we can use the mole flow rate to estimate the alloys
composition.
For example, if we grow AlGaN, we can estimate the Al concentration use the
following formula if we assume the efficiency of Al and Ga sources is the same.
xAl
=FAl
/(FAl
+ FGa
)
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The basic reaction describe GaN growth can simply write as
Ga(CH3)3+NH3 GaN+3CH4
The growth procedure as follows:
1. MO sources and hydrides inject to the reactor.
2. The sources are mixed inside the reactor and transfer to the deposition area
3. At the deposition area, high temperature result in the decomposition of
sources and other gas-phase reaction, forming the film precursors which
are useful for film growth and by-products.
4. The film precursors transport to the growth surface
5. The film precursors absorb on the growth surface
6. The film precursors diffuse to the growth site
7. At the surface, film atoms incorporate into the growing film through
surface reaction
8. The by-products of the surface reactions absorb from surface
9. The by-products transport to the main gas flow region away from the
deposition area towards the reactor exit
4. Gas phase and surface reaction
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reaction
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Physical characterization
X-ray diffraction (XDS)
Transmission electron microscopy (TEM)
Optical microscopy
Scanning electron microscopy (SEM)Atom force microscopy (AFM)
Secondary ion mass spectroscopy (SIMS)
Electrical Measurements
Van der Pauw Hall
Capacitance-voltage (C-V)
Optical measurements
Photoluminescence (PL)
5. Materials Characterization
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Two Step MOCVD Growth procedure
High temperature
treatment
Buffer
layer
Epilayer
Growth
TMGa
NH3
Temperature1150
o
C550oC
1050oC
Ga(CH3)3+NH3 GaN+CH4
6. MOCVD grow GaN and related materials
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Some basic problem related to GaN growth
MOCVD and other epitaxial techniques have developed more than 30 years, but
high quality GaN and related compound only available in recent years. There
are some special problems for GaN and related materials.
No suitable substrate
Difficult to obtain p-type epilayer