Epitaxial Deposition

1
Epitaxial Deposition

• M.H.Nemati
• Sabanci University

2
Outline

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

3
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
4
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

5
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

6
General Scheme
7
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

8
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

9
Methods of epitaxial deposition

• Vapor Phase Epitaxy
• Liquid Phase Epitaxy
• Molecular Beam Epitaxy

10
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)

11
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

12
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 800C
• High quality layer
• Fast, inexpensive
• Not ideal for large area layers or abrupt
  interfaces
• Thermodynamic driving force relatively very low

13
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.

14
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
• Cant independently control dopant profile and
  dopant concentration

15
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

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More applications

• Bipolar Transistor
• Needed to produce buried layer
• III-V Devices
• Interface quality key
• Heterojunction Bipolar Transistor
• LED
• Laser

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ransistor
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pg
17
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

18
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