Epitaxial Deposition

M.H.Nemati

Sabanci University

Outline

IntroductionMechanism of epitaxial growthMethods of epitaxial depositionApplications of epitaxial layers

Epitaxial Growth Deposition of a layer on a

substrate which matches the crystalline order of the substrate

Homoepitaxy Growth of a layer of the

same material as the substrate

Si on Si Heteroepitaxy

Growth of a layer of a different material than the substrate

GaAs on Si

Ordered, crystalline growth; NOT epitaxial

Epitaxial growth:

Motivation

Epitaxial growth is useful for applications that place stringent demands on a deposited layer: High purity Low defect density Abrupt interfaces Controlled doping profiles High repeatability and uniformity Safe, efficient operation

Can create clean, fresh surface for device fabrication

General Epitaxial Deposition Requirements Surface preparation

Clean surface needed Defects of surface duplicated in epitaxial layer Hydrogen passivation of surface with water/HF

Surface mobility High temperature required heated substrate Epitaxial temperature exists, above which deposition is

ordered Species need to be able to move into correct

crystallographic location Relatively slow growth rates result

Ex. ~0.4 to 4 nm/min., SiGe on Si

General Scheme

Thermodynamics Specific thermodynamics varies by process

Chemical potentials Driving force

Process involves High temperature process is mass transport controlled, not very sensitive to temperature changes

Close enough to equilibrium that chemical forces that drive growth are minimized to avoid creation of defects and allow for correct ordering

Sufficient energy and time for adsorbed species to reach their lowest energy state, duplicating the crystal lattice structure

Thermodynamic calculations allow the determination of solid composition based on growth temperature and source composition

Kinetics

Growth rate controlled by kinetic considerations Mass transport of reactants to surface Reactions in liquid or gas Reactions at surface Physical processes on surface

Nature and motion of step growth Controlling factor in ordering

Specific reactions depend greatly on method employed

Methods of epitaxial deposition

Vapor Phase EpitaxyLiquid Phase EpitaxyMolecular Beam Epitaxy

Vapor Phase Epitaxy Specific form of chemical vapor deposition (CVD) Reactants introduced as gases Material to be deposited bound to ligands Ligands dissociate, allowing desired chemistry to

reach surface Some desorption, but most adsorbed atoms find

proper crystallographic position Example: Deposition of silicon

SiCl4(g) + 2H2(g) ↔ Si(s) + 4HCl(g), SiCl4 introduced with hydrogen Forms silicon and HCl gas SiH4 breaks via thermal decomposition Reversible and possible to do negative (etching)

Precursors for VPE

Must be sufficiently volatile to allow acceptable growth rates

Heating to desired T must result in pyrolysis Less hazardous chemicals preferable

Arsine highly toxic; use t-butyl arsine instead VPE techniques distinguished by precursors

used

Liquid Phase Epitaxy

Reactants are dissolved in a molten solvent at high temperature Substrate dipped into solution while the temperature is held

constant Example: SiGe on Si

Bismuth used as solvent Temperature held at 800°C

High quality layer Fast, inexpensive Not ideal for large area layers or abrupt interfaces Thermodynamic driving force relatively very low

Molecular Beam Epitaxy

Very promising technique Beams created by evaporating solid source in UHV Evaporated beam of particle travel through very high vaccum

and then condense to shape the layer Doping is possible to by adding impurity to source gas by(e.g

arsine and phosphors) Deposition rate is the most important aspect of MBE Thickness of each layer can be controlled to that of a single atom development of structures where the electrons can be confined in space,

giving quantum wells or even quantum dots Such layers are now a critical part of many modern semiconductor

devices, including semiconductor lasers and light-emitting diodes.

Doping of Epitaxial Layers

Incorporate dopants during deposition(advantages)

Theoretically abrupt dopant distribution Add impurities to gas during deposition Arsine, phosphine, and diborane common

Low thermal budget results(disadvantages)

High T treatment results in diffusion of dopant into substrate

Can’t independently control dopant profile and dopant concentration

Applications

Engineered wafers Clean, flat layer on top of

less ideal Si substrate On top of SOI structures Ex.: Silicon on sapphire Higher purity layer on lower

quality substrate (SiC) In CMOS structures

Layers of different doping Ex. p- layer on top of p+

substrate to avoid latch-up

More applications

Bipolar Transistor Needed to produce

buried layer

III-V Devices Interface quality key Heterojunction Bipolar

Transistor LED Laser

http://www.veeco.com/library/elements/images/hbt.jpg

http://www.search.com/reference/Bipolar_junction_transistor

Summary

Deposition continues crystal structure Creates clean, abrupt interfaces and high

quality surfaces High temperature, clean surface required Vapor phase epitaxy a major method of

deposition Epitaxial layers used in highest quality wafers Very important in III-V semiconductor

production

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