33 Epi2 00
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33 Epi2 00 33 Epi2 00 Presentation Transcript

  • Epitaxial Growth (Campbell, Chapter 14)
    • defects in epitaxial growth
    • thermodynamics and kinetics
    • silicon epitaxy
  • Structural defects in epitaxial growth
    • Although the goal of epitaxial growth is to produce defect-free single crystal layers, structural and electrical defects can still form
    • Dislocations are almost always bad (electron-hole recombination)
      • misfit dislocations occur when lattice-mismatched growth is performed beyond the pseudomorphic limit
      • threading dislocations propagate into the growing layer and can “kill” device performance
    • Point defects are often observed:
      • equilibrium concentration:
      • stoichiometric defects in binary compounds
        • N-vacancies in GaN due to inhibited nitrogen incorporation
        • anti-site defects: As Ga in GaAs  “EL2”
    • Stacking faults are also commonly observed
  • Stacking faults in silicon formation of a stacking fault detail of a stacking fault
    • Stacking faults are errors in the stacking of atomic planes and can occur only when the succeeding layers are different
    • In face-centered cubic or diamond crystal structures, faults form when there is a “mistake” in the … ABCABC … stacking sequence
    a b c c a b a c b a c a b c a b c a
  • Stacking faults in silicon
  • Lattice-mismatched growth
    • one- or two-dimensional arrays of misfit dislocations can form at the interface between the strained layer and the substrate
    • because a dislocation line can never terminate within a crystal, threading dislocations connect a misfit segment
  • Misfit dislocations in GaAs/Si
  • Antiphase disorder
    • Antiphase disorder (or “antiphase domains”, APD’s) occur due to the symmetry in some unit cells (GaAs -- zincblende)
    • Particularly severe when doing heteroepitaxial growth of one unit cell type on another (GaAs-on-Si)
    antiphase domain
  • Thermodynamics of epitaxial growth
    • The influence of thermodynamics on epitaxial growth is well illustrated in the reduction of silicon tetrachloride:
    • SiCl 4 (gas) + 2 H 2 (gas)  Si (solid) + 4 HCl (gas) T~1250°C
      • Forward reaction : SiCl 4 is reduced to solid Si with HCl as a reaction by-product
      • Reverse reaction : Solid silicon is etched by HCl
    • The rate of the forward reaction is
    • The rate of the reverse reaction is
    • At equilibrium, the forward and backward rates are equal, so the equilibrium constant K SiCl 4 is unity:
  • Thermodynamics of epitaxial growth (2)
    • SiCl 4 (gas) + 2 H 2 (gas)  Si (solid) + 4 HCl (gas) T~1250°C
    • Depending on the partial pressures of the various gases present, silicon may be etched or deposited
    • Important experimentally-adjustable parameters include:
      • mole fractions of the gas species (reactants and products)
      • chlorine/hydrogen ratio in the feed gas
  • Kinetics of epitaxial growth
    • Consider again the reduction of silicon tetrachloride:
    • SiCl 4 (gas) + 2 H 2 (gas)  Si (solid) + 4 HCl (gas)
    • If the system is linear (i.e. fluxes are linearly related to driving forces) and the reduction follows a single reaction, then
    • The reaction at the surface is described by:
    • At steady state F 2 = F 1 leading to
    growth rate  F 2  c s  h g is the gas-phase mass transfer coefficient k s is the surface reaction rate coefficient
  • Kinetics of epitaxial growth (2)
    • The steady-state flux can be converted to a growth rate:
    number density of atoms (Si: 5  10 22 cm -3 ) R [=] cm sec -1 ( i.e. a growth velocity or growth rate ); R is obviously sensitive to h g , k s and c g Temperature dependence of growth growth rates of silicon from various chlorosilanes lower right: surface reaction limited upper left : mass transfer limited
    • Since epitaxial growth requires the incorporation of atoms or molecules at specific sites, the surface reaction rate may be influenced by the competition between species for those sites
    • There are two general mechanisms for adsorption onto a surface
      • physisorption (weak, low temperature, not applicable to CVD)
      • chemisorption (strong, high temperature, important to CVD)
    • The fractional coverage  follows the Langmuir adsorption isotherm
    Kinetics of epitaxial growth (3)  0 1  P low P --   P high P --  1 note: and the growth rate =  k s
  • Vapor phase epitaxy -- silicon
    • The obvious choice for Si VPE is the pyrolysis of siliane:
    • SiH 4 (gas)  Si (solid) + 2 H 2 (gas) T~1000°C
    • No chlorine means no etching, but there are problems…
      • Gas-phase nucleation of silicon particles
      • Much lower deposition rate than tetrachloride process
      • Silane is much more expensive, unstable, difficult to handle
    • Near-atmospheric pressure Si epi reactors typically use SiCl 4 or SiCl 2 H 2
    • Relatively high growth rates (~0.1  m/min)
    • High temperatures (>1150°C) achieved with graphite susceptor and RF heating
  • A generic Si AP-VPE system
  • Dopant incorporation in Si epitaxy
    • Dopants can be added to the gas stream to control the electrical characteristics of the epitaxial layer
    • For silicon:
      • n -type dopants: AsH 3 , PH 3
      • p -type dopant: B 2 H 6
    • Typically introduced in a very dilute form
    • n -type doping may significantly lower the epitaxial growth rate
      • hydrides adsorb strongly on active surface reaction sites
      • decompose slowly compared with SiH 4
    • p-type doping may significantly increase the epitaxial growth rate
      • p -type layer may help hydrogen to desorb, thus opening more sites for SiH 4 reduction
    all are gases at room temperature
  • Ultra-high vacuum chemical vapor deposition (UHV-CVD)
    • UHV-CVD growth of silicon is performed in systems with a base pressure of better than 10 -9 torr
    • Growth takes place at a pressure ~10 -3 torr using SiH 4 or GeH 4
    UHV-CVD can produce high quality epitaxial films at low temperatures (down to 450°C!!)
    • less interdiffusion
    • sharper interfaces
    • better alloy growth (SiGe)