MOCVD

VT 2004/5; Micro and Nano-Processing Technologies

Ch.14 : Epitaxial Growth

Micro and Nano- Processing Technologies

EPITAXIAL GROWTH

Mahdad [email protected]

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CONTROLOFThicknessCompositionThroughput (growth rate)UniformitiyDoping concentrationJunctions(X-tal) Quality, defectsReproducibility

BYGrowth temperatureGas flow rate- FlowmeterSource temperatureFlux cutoff – valves/shuttersX-tal orientationGeometry/ EnvironmentSource/Gas purityPrecleaning

Epitaxy is a well-controlled phase transitionwhich leads to a (single crystalline) solid.



Epitaxy techniques - the spectrum of options

Chemical Vapour Deposition (CVD)• Atmospheric pressure – APCVD• Low pressure CVD – LPCVD• Plasma enhanced CVD – PECVDVapor phase Epitaxy (hydrodynamic flow)• Hydride• Chlorine Metallo-Organic CVD (MOCVD)Liquid Phase Epitaxy

Molecular Beam Epitaxy (MBE) (ballistic flow)• Solid source• Gas source• Chemical beam• Organo-metallic source



LPE VPE MBE

GaAs/AlGaAs 750-825oC

- 600-700oC

InP+related 600-660oC

500-650oC

480-560oC




Chemical Vapour Deposition (CVD)• Atmospheric pressure – APCVD• Low pressure CVD – LPCVD• Plasma enhanced CVD – PECVDVapor phase Epitaxy (hydrodynamic flow)• Hydride• ChlorineMetallo-Organic CVD (MOCVD)Liquid Phase Epitaxy




CVD Applications

• Thin insulating filmsoxides, silicon nitride

• Polysilicon (gates/conductors)• Epitaxial silicon (single crystal on wafer)• Silicide materials• III-V compounds



Chemical Vapour Deposition (CVD)

Chemical reactions (Gas phase reactions) in a reactor to create a thin film layer at surface



CVD Important film parameters

• Stoichiometery: exact composition of film• Physical parameters: hardness, optical density• Electrical parameters:

dielectric constant, breakdown voltage• Purity of film: lack of contamination• Thickness and uniformity• Conformality and step coverage• pin hole and particle free• Adhesion



Four main CVD Reactions• Pyrolysis: heat driven break down• Reduction: usually react with Hydrogen• Oxidation: react with oxygen to form oxides• Nitradation: create nitrides with nitrogen compounds



Major CVD Processes• Reactants diffuse to surface• Chemical reaction(s) at surface• Film reformed at surface• Products desorbed and diffuse from surface

CARRIER GAS+ UNREACTED REACTANTS+ PRODUCTS

SUBSTRATE

TRANSFER OFREACTANTS TOSURFACE ADSORPTION

OF REACTANTS DESORPTION OF PRODUCTS

TRANSFER OFPRODUCTS TOMAIN FLOW

CARRIER GAS+ REACTANTS

SURFACE DIFFUSIONSURFACE REACTION

Sequence of events in thermal CVD or VPE reactor













TYPE OF CONTROL RATE LIMITING STEPS

MASS TRANSFERTYPE 1

Inpu Rate LimitedEquilibrium Process

Input rate of reactants to epitaxial growth region

MASS TRANSFERTYPE 2

Diffusion ControlledMass Transfer Limited

Transfer of reactants or products between maingas streaqm and sbstrate

surface by physicalprocesses such as

diffusion or convection

Kinetics ControlSurface Limited

Chemically Controlled

Adsorption of reactantsDesorption of products

Surface reactionSite incorporation

After D.W. Shaw, Mechanisms in Vapour Phase epitaxy of Semiconducors, Crystal Growth Vol1, Ed. C.H.L. Goodman, Plenum press, NY, 174



EXPERIMENTALPARAMETERS

MASS TRANSPORTTYPE 1

(Eqm. process)

MASS TRANSPORTTYPE 2

(Diffusion control)

KINETICS(Chemical Control, Surface Limited)

Growth temperature Dependence predicted by thermodynamics

Slight dependence due to concentration gradient

changes

Rate increasesexponentially with

increasing temperature

Total flow rate (constantpartial pressure and tube

diameter)

Rate increases with total flow rate due to increasein total mass input rate

Rate increases with flowrate increase Independent

Gas stream velocity(Constant mass input rate

and partial pressure)Independent

Rate increases with increasing veocity Independent

Crystalloraphicorientation

Independent Independent Dependent

Geometric orientaion of substrate

Independent Dependent Independent

Surface area Total amount depositedindependent of surface

area

Dependent on apparent or geometrical surface area

Dependent on true or actual surface area



Atmospheric & Low Pressure CVD

Atmospheric Pressure Low pressure

Cold wallHorizontalVerticalPancake

Hot wallPhotochemicalVPEMOCVD

Hot wallPlasma enhancedVertical isothermal

Overview of CVD systems

VPE: Vapour Phase Epitaxy (Si single crystal)MOCVD: Metalo-organic CVD (III-V compounds)











Plasma Enhanced CVD (PECVD)










Vapour Phase Epitaxy (VPE)A process used for depositing thin films on monocrystallinesubstrates using gases.



VAPOUR PHASE EPITAXY of III-V materials

Hydride VAPOUR PHASE EPITAXY(HVPE) for depositing GaInAsP

III-sources: InCl and GaClV-sources: PH3 and AsH3 (hydrides of phosphorus and arsenic)

Chloride VAPOUR PHASE EPITAXY (Cl-VPE) for depositing GaInAsP

III-sources: InCl and GaClV-sources: PCl3 and AsCl3 (chlorides of phosphorus and arsenic)

EXAMPLE: InP can be deposited using phosphine and indium chloride gases:

InCl (g) + PH3 (g) = InP (s) + HCl (g) + H2 (g)



Hydride VPE

Advantages:+ Purity+ High growth rate+ Throughput+ Versatile for selective growth

Disadvantages:- No Al or Sb alloys- Complex process/reactions- Difficult to control thin layer- Hazardous gases








Liquid Phase Epitaxy (LPE)

Liquid phase epitaxy (LPE) is the solidification from a liquid phase of aCrystalline layer onto a parent substrate such that the crystallinity of the substrate is maintained in the grown layer.

Used for initial demonstration and production of most optoelectronic devicesNear-equilibrium technique

Advantages:+ Simple, inexpensive equipment+ High utilization efficiency of precursor materials+ High purity material+ In-situ etching possible

Disadvantages:- Difficult to control thin layerthickness- Composition problematic- Poor uniformity- Poor morphology



LPE Growth Procedure

GaAs LPE introducted by H. Nelson (1963)

1) Heat to T>Tsat(saturation temperature)2) Lower T and introduction of seed crystal (Tipper, Dipper, Slider …)3) Withdraw of the sample from the melt

Growth rate will depend on:- Composition of solution- Degree of supersaturation- Cooling rate- Contact period- Subsrate orientation





Requirements in epitaxial growth

• Improved crystallinity• Reduced defects• Higher purity

• Precise control of thickness• Precise control alloy composition

• “Lattice matched” compounds• Abrupt or graded interfaces• Ability to engineer unique device structures

• Nanostructures• Superlattices• Strained layers

• New materials: Quarternaries• AlGaInAs (MBE), InPAsSb (MOCVD)



How accurate is the control needed?

• Source temperature: < 0.2 deg • In/Ga ratio control in InGaAs < 1 % to avoid dislocations

• 1.5 mm InGaAs/InGaAsP QW (l=±10 nm)Control of QW thickness < 6 %Control of Comp. InGaAs < 0.5 %

• Bragg mirrors thickness control < 1 % mm








Metal Organic Vapor Phase Epitaxy (MOVPE)

• Growth in “reactor”• Pressure 10s-100s of torr• Metal organic group III source material

• Trimethyl Gallium Ga(CH3)3• Trimethyl Indium In(CH3)3• MO vapor transported by H2 carrier gas

• Hydride group V source gas• Arsine AsH3• Phosphine PH3

• Thermal cracking at growth surface



MOCVD- Surface Chemistry



Example: OMCVD growth of GaN and related materials

Special problems for GaNand related materials.• No suitable substrate• Difficult to obtain p-typeepilayer

Ga(CH3)3+NH3 GaN+3 CH4



MOCVD- Gas Handling System



MOCVD Summary

• Growth rates 2-100 micron/hr• high throughput

• P-type doping• Zn (Diethyl Zinc), high diffusivity• C (CCl4, CBr4), amphoteric

• Complex growth kinetics• Delicate interaction between injected gasses, temperatures

• High background pressure• Parasitic incorporation• Intermixing of atoms at interfaces








Molecular Beam Epitaxy (MBE)Cho, JVST 8, s31 (1971)

• Homo-epitaxy and heteroepitaxy of semiconductor and compounds• Growth rate: 1µm/hr• Growth in high vacuum chamber

• Ultimate vacuum < 10e-10 torr• Pressure during growth < 10e-6 torr

• Elemental source material• High purity Ga, In, As (99.9999%)• Sources individually evaporated in high temperature cells

• In situ monitoring, calibration• Probing of surface structure during growth• Real time feedback of growth rate



Molecular Beam Epitaxy (MBE)

Variations• SSMBE solid evaporation sources• GSMBE metals + group V hydrides• MOMBE metalorganics + conventiona grop Vs• CBE all gas sources



MBE Growth Mechanism

- Absorption to the surface,- Surface migration - Incorporation into the crystal lattice - Thermal desorption



Molecular Beam Epitaxy



MBE- EPI930 System



MBE- In Situ Surface Analysis

• High energy (10-30 keV) electron beam• Reflection High Energy Electron Diffraction (RHEED)• Shallow angle of incidence• Beam reconstruction on phosphor screen



MBE- In Situ Growth Rate Feedback

Monitoring RHEED image intensity versus timeprovides layer-by-layer growth rate feedback



MBE- Summary

• Ultra high vacuum, high purity layers• No chemical byproducts created at growth surface• High uniformity (< 1% deviation)• Growth rates 0.1-10 micron/hr.• High control of composition• In situ monitoring and feedback• Mature production technology

Low P large mean free path beam nature of flux (molecular flow)Mechanical shutters fast switching abrupt interface

Low T limited interdiffusion



Gas Source MBE

• Combines advantages of MBE with gas sourcedelivery of group V atoms (as used in MOCVD)

• PH3, AsH3 used for group V sources• Thermally cracked at injector into P2, As2 and H2• P2, As2 dimers arrive at growth surface along with Ga, In• MBE surface kinetics maintained



Gas Source MBE

• Advantages of GSMBE• More abrupt junctions than in SSMBE• PH3 a more mature method for phosphorus MBE growth• Improved dynamic range of switching state

• As, P molecules travel around shutter in solid source MBE• Increased control of As/P ratio by adjustment of gas flow• Can replenish group V source material without breaking

vacuum

• Disadvantages• Requires gas handling system• Requires extra vacuum pumping to remove hydrogen• Arsine and Phosphine highly toxic



Comparison of epitaxial growth techniques



III-V Compound Semiconductors



Limits to Strained Layers: Critical Thickness







MOCVD

Documents

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nanoprocessing technologies

epitaxial growth region

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surface products

surface film

crystal growth vol1