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Epitaxial Deposition. Daniel Lentz EE 518 Penn State University March 29, 2007 Instructor: Dr. J. Ruzyllo. Outline. Introduction Mechanism of epitaxial growth Methods of epitaxial deposition Properties of epitaxial layers Applications of epitaxial layers. Epitaxial Growth.

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epitaxial deposition

Epitaxial Deposition

Daniel Lentz

EE 518

Penn State University

March 29, 2007

Instructor: Dr. J. Ruzyllo

  • Introduction
  • Mechanism of epitaxial growth
  • Methods of epitaxial deposition
  • Properties of epitaxial layers
  • 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
general scheme
General Scheme

Modified from http://www.acsu.buffalo.edu/~tjm/MOVPE-GaN-schematic.jpg

  • Specific thermodynamics varies by process
    • Chemical potentials
    • Driving force
  • High temperature process is mass transport controlled, not very sensitive to temperature changes
  • Steady state
  • 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
kinetics example
Kinetics Example
  • Atoms can bond to flat surface, steps, or kinks.
    • On surface requires some critical radius
    • Easier at steps
    • Easiest at kinks
  • As-rich GaAs surface
    • As only forms two bonds to underlying Ga
    • Very high energy
  • Reconstructs by forming As dimers
    • Lowers energy
    • Causes kinks and steps on surface
  • Results in motion of steps on surface
    • If start with flat surface, create step once first group has bonded
    • Growth continues in same way


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 introduced with hydrogen
    • Forms silicon and HCl gas
    • Alternatively, SiHCl3, SiH2Cl2
    • SiH4 breaks via thermal decomposition
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
varieties of vpe
Varieties of VPE
  • Chloride VPE
    • Chlorides of group III and V elements
  • Hydride VPE
    • Chlorides of group III element
      • Group III hydrides desirable, but too unstable
    • Hydrides of group V element
  • Organometallic VPE
    • Organometallic group III compound
    • Hydride or organometallic of group V element
  • Not quite that simple
    • Combinations of ligands in order to optimize deposition or improve compound stability
    • Ex. trimethylaminealane gives less carbon contamination than trimethylalluminum



other methods
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

Elemental vapor phase method

Beams created by evaporating solid source in UHV

Other Methods
doping of epitaxial layers
Doping of Epitaxial Layers
  • Incorporate dopants during deposition
    • Theoretically abrupt dopant distribution
    • Add impurities to gas during deposition
    • Arsine, phosphine, and diborane common
  • Low thermal budget results
    • High T treatment results in diffusion of dopant into substrate
    • Reason abrupt distribution not perfect
properties of epitaxial layer
Properties of Epitaxial Layer
  • Crystallographic structure of film reproduces that of substrate
  • Substrate defects reproduced in epi layer
  • Electrical parameters of epi layer independent of substrate
    • Dopant concentration of substrate cannot be reduced
    • Epitaxial layer with less dopant can be deposited
  • Epitaxial layer can be chemically purer than substrate
  • Abrupt interfaces with appropriate methods
  • 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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