MOCVDBasics & Applications

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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

MOCVDOMVPEOMCVDMOVPE

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; 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

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 onthe substrates (wafers)

Source https://en.wikipedia.org/wiki/Metalorganic_vapour_phase_epitaxy

Step for MOCVD processStep 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 + 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!

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

withhG = 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

The deposition rate /growth rate of film is proportional to v is

Limiting cases:hG >> kS : Reaction Limited Growth

kS >> hG : Transport Limited GrowthSG

G

kh

cv

11

SG

G

kh

cJJv

1121

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 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).

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 LED

Solar Cells source how MOCVD works by

AIXTRON

Referance

1. https://en.wikipedia.org/wiki/Metalorganic_vapour_phase_epitaxy , 26/5/2013.

2. Gerald B, Organometallic Vapor-Phase Epitaxy: Theory and Practice  

3. AIXTRON, how MOCVD works, Deposition Technology for Beginners

4. Hugh O. Pierson, HANDBOOK OF CHEMICAL VAPOR DEPOSITION(CVD) Principles, Technology, and Applications Second Edition, NOYES PUBLICATIONS Park Ridge, New Jersey, U.S.A.