h G

Report
MOCVD
Basics & Applications
Sisay
Outline
 Introduction
 Advantages/Disadvantages
 Basic transport and growth mechanisms
 Application
What is MOCVD?
 MOCVD stands for Metal-Organic Chemical Vapour Deposition.
 MOCVD is a technique that used to grow/deposit thin solid films,
usually semiconductors, on solid substrates (wafers)using organo
metallic compounds as sources.
 The films grown by MOCVD are mainly used for the fabrication of
electronic and optoelectronic devices.
 The electronic and optoelectronic devices produced by MOCVD are
used in cell phones , optical communication, optical storage (CD,
DVD), traffic lights, bill boards (LEDs), lighting and solar cells.
 Using MOCVD we can build up many layers, each of a precisely
controlled thickness, to create a material which has specific optical and
electrical properties.
Overview of Epitaxy Techniques
Technique
Strengths
Weaknesses
LPE (liguid
phase epitaxy)
Simple, High purity
Scale economies Inflexible,
Non-uniformity
HVPE
( hydride vapor
phase epitaxy)
Well developed Large scale
No Al alloys Complex
process/reactor control difficult,
Hazardous sources
MBE
Simple process, Uniform,
Abrupt interface In-situ monitoring
As/P alloy difficult, Expensive ,
Low throughput
MOCVD
OMVPE
OMCVD
MOVPE
Most flexible, Large scale production
Abrupt interface Simple reactor, High
purity, selective in situ monitoring
Expensive sources Most parameters
to control Accurately Hazardous
precursors
Why MOCVD?
 High grown layers quality
 Faster growth rate than MBE, can be a few microns per hour; multiwafer capability easily achievable
 Doping uniformity/reproducibility
 High throughput and no ultra high vacuum needed (compared to
MBE),
 Economically advantageous.
 Highest flexibility, Different materials can be grown in the same
system.
 Precision in deposition thickness and possible sharp interfaces
growth –thus, it is very suitable for hetero-structures, e.g., multi
quantum wells (MQW)
 Higher temperature growth; growth process is thermodynamically
favorable
Disadvantages
 Many materials that we wish to deposit have very low vapour
pressures and thus are difficult to transport via gases
 Not abruptable process as MBE due to gas flow issues
 Human Hazard ,that is, Toxic and corrosive gases are to be
handled
 high temperatures
 complex processes
 Carbon contamination and unintentional Hydrogen
incorporation are sometimes a problem
Schematics
Basic transport and growth mechanisms
Deposition process takes place on
the substrates (wafers)
Source https://en.wikipedia.org/wiki/Metalorganic_vapour_phase_epitaxy
Step for MOCVD process
Step 1. The atoms that we would like to be in our crystal are combined with a
complex organic gas molecules and passed over a heated semiconductor
substrate.
Ga(CH3)3
+ AsH3 
(Trimethal gallium gas)
(Arsene gas)
Step 2. The heat break up the molecules and deposite the desired atoms on the
surface layer by layer, e.g., Ga and As atoms on the substrate surface.
3CH4 +
(Methane gas)
GaAs
(on the substrate
Step 3. The atoms bond to the substrate surface and a new crystalline layer is
grown, in this case GaAs,
 The reaction occurs in the chamber (reactor)
 Arsene gas is highly toxic & highly flammable! Trimethal gallium gas is
highly toxic!! Methane gas is highly explosive!
Kinematics reaction
 J1: molecular flux from the gas phase to the substrate surface,
J2: consumption flux of GaAs corresponding to the surface reaction:
J1 ≈ hG (CG – CS)
J2 ≈ kSCS
with
hG = Gas phase mass transport coefficient,
CG = gas-phase concentration,
CS = Concentration on surface
kS = Surface reaction rate
Kinematics reaction
 In Steady-state conditions:
J1=J2
That is
v  J1  J 2 
cG
1 1

hG kS
The deposition rate /growth rate of film is proportional to v is
v
Limiting cases:
hG >> kS : Reaction Limited Growth
kS >> hG : Transport Limited Growth
cG
1
1

hG k S
Reaction limited growth
 Small kS
 Growth controlled by processes on surface adsorption
• decomposition
• Surface reaction
• chemical reaction
• desorption of products
 kS kS is highly temperature dependent (increases with T)
 Common limit at lower temperatures
 Often preferred, slow but epitaxial growth
 Temperature and reactant choices are important
Mass Transport Limited Growth
 Small hG





Growth controlled by transfer to substrate
hG is not very temperature dependent
Common limit at higher temperatures
Non-uniform film growth
Gas dynamics and reactor design are important
Material source should be




sufficiently volatile
high enough partial pressure to get good growth rates
stable at room temperature
produce desired element on substrate with easily removable byproducts
Growth of III-V semiconductors:
 Group III: generally metalorganic molecules (trimethyl- or
triethyl- species)
 Group V: generally toxic hydrides (AsH3; PH3 flammable as
well); alternative: alkyls (TBAs, TBP).
Desirable properties of precursors:
•
•
•
•
•
•
•
•
Low toxicity
Liquid at room temperature
Suitable vapor pressure at room temperature
Low carbon contamination in grown layer(avoid
CH3radicals), however, for some applications C doping
is desired
No parasitic reactions with other sources
Good long term stability (should not decompose in
bubbler)
Pyrolysistemperature should match growth
temperature
Inexpensive for industrial mass production
Carrier gas should be
 “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
Application
…Application
Laser diode:
Transistors
Solar Cells
LED
source how MOCVD works by
AIXTRON
Referance
1.
2.
3.
4.
https://en.wikipedia.org/wiki/Metalorganic_vapour_phase_epita
xy , 26/5/2013.
Gerald B, Organometallic Vapor-Phase Epitaxy: Theory and
Practice
AIXTRON, how MOCVD works, Deposition Technology for
Beginners
Hugh O. Pierson, HANDBOOK OF CHEMICAL VAPOR
DEPOSITION(CVD) Principles, Technology, and Applications
Second Edition, NOYES PUBLICATIONS Park Ridge, New Jersey,
U.S.A.

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