1. MOCVD Basics & Applications
Sisay

2. Outline Introduction Basic transport and growth mechanisms Application
Advantages/Disadvantages
Basic transport and growth mechanisms
Application

3. 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.

4. Overview of Epitaxy Techniques
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
Organometallic Vapor-Phase Epitaxy: Theory and Practice  By Gerald B. Stringfellow

5. Why MOCVD? High grown layers quality
Faster growth rate than MBE, can be a few microns per hour; multi-wafer 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

6. 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

7. Schematics

8. Basic transport and growth mechanisms

9. Deposition process takes place on the substrates (wafers)
Source

10. 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) 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.
3CH GaAs
(Methane gas) (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!
By varying the composition of the gas we can change the properties of the crystal in the atomic level

11. Kinematics reaction hG = Gas phase mass transport coefficient,
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

12. Kinematics reaction
In Steady-state conditions:
J1=J2
That is
The deposition rate /growth rate of film is proportional to v is
Limiting cases:
hG >> kS : Reaction Limited Growth
kS >> hG : Transport Limited Growth

13. 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

14. 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

15. 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 by-products
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).

16. 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

17. 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

18. Application

19. …Application Laser diode: Transistors LED
Solar Cells source how MOCVD works by
AIXTRON
A laser device emits a narrow beam of concentrated light with a high density of energy and a very sharp color. In order to create laser light from a diode, it is very important to have ultra high crystal quality and atomic sharp layer interfaces. The advantage of laser diodes over other laser types is the high speed of light availability. They can be switched on and off easily and have a very small device footprint.

20. Referance
, 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.