mo cvd material growth

7/27/2019 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

mo cvd material growth

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