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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

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epitaxial deposition

Epitaxial Deposition


Sabanci University

  • Introduction
  • Mechanism of epitaxial growth
  • Methods of epitaxial deposition
  • Applications of epitaxial layers
epitaxial growth
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:

  • 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
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
  • 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
  • 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
Methods of epitaxial deposition
  • Vapor Phase Epitaxy
  • Liquid Phase Epitaxy
  • Molecular Beam Epitaxy
vapor phase 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
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

Liquid Phase Epitaxy
molecular beam epitaxy
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
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
  • 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
More applications
  • Bipolar Transistor
    • Needed to produce buried layer
  • III-V Devices
    • Interface quality key
    • Heterojunction Bipolar Transistor
    • LED
    • Laser



  • 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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