M.H.Nemati Sabanci University Epitaxial Deposition M.H.Nemati Sabanci University

Outline Introduction Mechanism of epitaxial growth Methods of epitaxial deposition Applications 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 Epitaxy Liquid Phase Epitaxy Molecular 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 In CMOS structures 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 III-V Devices Needed to produce buried layer III-V Devices Interface quality key Heterojunction Bipolar Transistor LED Laser http://www.search.com/reference/Bipolar_junction_transistor http://www.veeco.com/library/elements/images/hbt.jpg

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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