me_598_-_lecture_19_-_epi_growth_techniques.20110415.4da845fa8362c5.71732161

7/28/2019 me_598_-_lecture_19_-_epi_growth_techniques.20110415.4da845fa8362c5.71732161

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Epitaxial GrowthTechniques

598XL


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Crystal Growth

Advancement in crystal growth techniques is one of the majorfactors responsible for semiconductor technology momentum today. LEDs Lasers Detectors

Solar Cells HBT, HEMT, .

Thermodynamics Chemical reactions Phase diagrams

Figure of merit: material quality, uniformity and controllability Thickness Composition Interface Doping

Defects


3/20

Deposition Techniques

Epitaxial growth techniques: epi definition LPE HVPE MOCVD MBE

Deposition of dielectric films or polycrystalsemiconductors

PECVD Evaporation Sputtering

ALD


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Hot HCl (g) + Ga (l) ------> GaCl (g)

GaCl (g) + NH3 (g) -------> GaN (s) + HCl (g) + H2 (g)

Hydride gaseous sourceMetal liquid atomic source

Fast growth rate (> 100 um/h)Surface defects for non-nitride materials Oxford Instruments


5/20

MBE Inventor:

Bell Lab, Alfred Cho

UHV : 10-11 torr

Not thermodynamic equilibriumcondition Governed by the kinetics of the

surface process

Solid atomic source effusioncell-controlled vapor pressure

Shutters Substrate-heated GSMBE and MOMBE (CBE) Slower growth rate

Better thickness andheterojunction control

Lower growth temperature High cost


6/20

RHEED 5- 50 keV

Glancing angleincidence

Surface sensitive

ML

Rice U.

UHV

in situ monitoring


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MOCVD Nomenclature

MOCVD OMVPE MOVPE OMCVD AP-MOCVD LP-MOCVD

Most widely used epitaxial technique at commercial scale Major components:

Gas handling system Reactor chamber Heating system Exhaust system


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Vacuum and

Exhaust

system

Gas handle

system

ComputerControl

Reactor

MOCVD Growth System


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A MOVD growth system


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Gas Handling System

Sources: Metalorganics (MO) or alkyls: TMGa, TEGa, TMAl, TMIn, DMZn, etc. Bubbler Partial pressure of the MO in the bubbler:

Log[p(torr)]=B-A/T

Hydride: AsH3, PH3, NH3, Si2H6 Cylinder Gas cabinet

Carrier gas: H2 or N2 H2 purifier Valves, MFCs, tubes, pumps etc. Purity, cleanness, leak-tight


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MOCVD Gas Handling System

Aixtro


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MOCVD: gas handling system


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Metalorganic (MO) Compound

Vapor pressure: Determines the concentration of source

material in the reactor and the deposition rate. Too low: difficult to transport the source into

the deposition zone and to achieve reasonable

growth rates.

Too high: may raise safety concerns

Liquid vs solid MO sources: easier to control

the delivery from a liquid than from a solid.

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(C2H

5)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(C2H

5)3

TEIn 0.31 2815 8.94 -32

Zn(C2H

5)2

DEZn 8.53 2190 8.28 -28

Mg(C5H

5)2

Cp2Mg 0.05 3556 10.56 175

Log[p(torr)]=B-A/T


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The mole flow rate of MO sources andAlloy Composition

The flow rate:

F (mol/min)=p MO/p Bubbler*[flow rate (ml/min)]/22400(mol/ml)

Alloys composition:

e.g the Al% in AlxGa1-xAs can be estimated byx Al=FAl/(FAl+ F Ga)

IF the pyrolysis and incorporation efficiencies of

Al and Ga sources are the same.

Gas phase ratio vs solid phase ratio needs to be calibrated!


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Example: MOCVD growth of GaN and related materials


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Example: MOCVD growth of GaN and related materials

Georgia Tech


18/20

In situ Monitoring in MOCVD


19/20

Control Parameters Temperature

Pressure Growth rate

V/III ratio

Doping

Growth pulse

Substrate orientation Desorption condition

Black box?


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Comparison of Epitaxial

Techniques

Growth method time features limit

LPE

(Liquid phaseepitaxy)

1963 Growth from

supersaturatedsolution onto substrate

Limited substrate areas

and poor control over thegrowth of very thin

layers

HVPE

(Hydride vapor

phase epitaxy

1958 Use metal halide as

transport agents to

grow

No Al contained

compound,

thick layer

MBE

(Molecular Beam

Epitaxy)

1958

1967

Deposit epilayer at

ultrahigh vacuum

Slow and 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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