33 Epi2 00

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33 Epi2 00

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

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