Lect' 21 Epitaxy

1
Lect. 21 Epitaxy

• The term epitaxy (Greek epi "above" and taxis
  "in ordered manner") describes an ordered
  crystalline growth on a monocrystalline
  substrate.
• Epitaxial films may be grown from gaseous or
  liquid precursors. Because the substrate acts as
  a seed crystal, the deposited film takes on a
  lattice structure and orientation identical to
  those of the substrate.
• This is different from other thin-film deposition
  methods which deposit polycrystalline or
  amorphous films, even on single-crystal
  substrates.
• If a film is deposited on a substrate of the
  same composition, the process is called
  homoepitaxy otherwise it is called
  heteroepitaxy.
• Homoepitaxy is a kind of epitaxy performed with
  only one material. In homoepitaxy, a crystalline
  film is grown on a substrate or film of the same
  material. This technology is applied to growing a
  more purified film than the substrate and
  fabricating layers with different doping levels.
• Heteroepitaxy is a kind of epitaxy performed with
  materials that are different from each other. In
  heteroepitaxy, a crystalline film grows on a
  crystalline substrate or film of another
  material. This technology is often applied to
  growing crystalline films of materials of which
  single crystals cannot be obtained and to
  fabricating integrated crystalline layers of
  different materials. Examples include gallium
  nitride (GaN) on sapphire or aluminium gallium
  indium phosphide (AlGaInP) on gallium arsenide
  (GaAs).

2
Epitaxy
Metastable phase
Transient layer
_at_ t1gtto
_at_ to
Stable phase (crystal)
Nuclei of Crystal phase
Crystallizationinterface
At time to, the before the epitaxy takes place
At the nucleation stage where thefirst crystal
layer is being formed
_at_ t3gtt2
_at_ t2gtt1
Transient layer
Transient layer
Local homoepitaxial interface
Nuclei of Crystal phase
Grown epitaxial layer
Grown epitaxial layer
Crystallizationinterface
Crystallizationinterface
Early stage of epitaxial growth
Hetroepitaxial interface
Epitxy proceeds
3
Epitaxy

• The key processes of epitaxial growth are-
  Perpendicular mass transport from the bulk
  metastable phase material to the crystallization
  interface.- Lateral mass transport through
  lateral migration according to expitaxial order.
  - desorption from the crystallization area
  towards the bulk phase material.
• The lattice constant of the epitaxially grown
  layer needs to be close to the lattice constant
  of the substrate wafer. Otherwise the bonds can
  not stretch far enough and dislocations will
  result.
• Experimentally, it has proven that epitaxy growth
  occurs when the lattice misfit, defined as
  , is less than 15
• MethodsEpitaxial silicon is usually grown
  using- vapor-phase epitaxy (VPE), a
  modification of chemical vapor deposition. -
  Molecular-beam (MBE)- liquid-phase epitaxy (LPE)
  are also used, mainly for compound semiconductors.

af
dislocation
as
Dislocation due to the difference betweenthe
substrate (s) and film (f) lattice constants
Film Substrate
() Epitaxy Physical Principles and Technical
Implementation  By Marian A. Herman, Wolfgang
Richter, Helmut Sitter
4
Epitaxy

• The growth rate in Vapor phase deposition (VPE)
  epitaxy follows the same model as CVD.

Ref http//www2.ece.jhu.edu/faculty/andreou/495/2
007/LectureNotes/Handout7_Thin20Film20Deposition
.pdf
5
Epitaxy

• Silicon is most commonly deposited from silicon
  tetrachloride in hydrogen at approximately 1200
  C
• SiCl4(g) 2H2(g) ? Si(s) 4HCl(g)
• This reaction is reversible, and the growth rate
  depends strongly upon the proportion of the two
  source gases. Growth rates above 2 micrometres
  per minute produce polycrystalline silicon, and
  negative growth rates (etching) may occur if too
  much hydrogen chloride byproduct is present. In
  fact, hydrogen chloride may be added
  intentionally to etch the wafer. An additional
  etching reaction competes with the deposition
  reaction
• SiCl4(g) Si(s) ? 2SiCl2(g)
• Silicon VPE may also use silane, dichlorosilane,
  and trichlorosilane source gases. For instance,
  the silane reaction occurs at 650 C in this way
• SiH4 ? Si 2H2
• This reaction does not inadvertently etch the
  wafer, and takes place at lower temperatures than
  deposition from silicon tetrachloride. However,
  it will form a polycrystalline film unless
  tightly controlled, and it allows oxidizing
  species that leak into the reactor to contaminate
  the epitaxial layer with unwanted compounds such
  as silicon dioxide.
• VPE is sometimes classified by the chemistry of
  the source gases, such as hydride VPE and
  metalorganic VPE.

6
Lect. 22 Epitaxy

• Molecular beam epitaxy is a technique for
  epitaxial growth via the interaction of one or
  several molecular or atomic beams that occurs on
  a surface of a heated crystalline substrate.
• Molecular Beam Epitaxy takes place in high vacuum
  or ultra high vacuum (10-8 Pa).
• The most important aspect of MBE is the slow
  deposition rate (1 to 300 nm per minute), which
  allows the films to grow epitaxially.
• The slow deposition rates require proportionally
  better vacuum in order to achieve the same
  impurity levels as other deposition techniques.
• In solid-source MBE, ultra-pure elements such as
  gallium and arsenic are heated in separate
  quasi-knudsen effusion cells until they begin to
  slowly sublimate.

• A typical Knudsen cell contains a crucible (made
  of pyrolytic Boron Nitride, quartz, tungsten or
  graphite), heating filaments (often made of metal
  Tantalum), water cooling system, heat shields and
  orifice shutter.
• The gaseous elements then condense on the wafer,
  where they may react with each other. In the
  example of gallium and arsenic, single-crystal
  gallium arsenide is formed.
• The term "beam" simply means that evaporated
  atoms do not interact with each other or any
  other vacuum chamber gases until they reach the
  wafer, due to the long mean free paths of the
  beams.

Refhttp//www-opto.e-technik.uni-ulm.de/forschung
/jahresbericht/2002/ar2002_fr.pdf
7
Epitaxy

• the molecular beam condition that the mean free
  path of the particles
  should be larger than the geometrical size of
  the chamber is easily fulfilled if the total
  pressure does not exceed 10-5 Torr.

• Also, the condition for growing a sufficiently
  clean epitaxial layer must be satisfied, e.g.
  requiring for the monolayer deposition times of
  the beams tb and the background residual vapor
  tres the relation tres lt 10-5 tb.
• For a typical gallium flux J of 10-19 atoms/m2s
  and for a growth rate in the order of 1 ?m/h, the
  conclusion is that pres 10-11 Torr.
• Thus, UHV is the essential environment for MBE.
  Therefore, the rate of gas evolution from the
  materials in the chamber has to be as low as
  possible.
• So pyrolytic boron nitride (PBN) is chosen for
  the crucibles which gives low rate of gas
  evolution and chemical stability up to 1400 C.

• molybdenum and tantalum are widely used for the
  shutters, the heaters and other components, and
  only ultrapure materials are used as source.
• The operation time of a shutter of approximately
  0.1 s is normally much shorter than the time
  needed to grow one monolayer (typically 1 to 5 s)

8
Epitaxy

• Reflection high-energy electron diffraction
  (RHEED) is a technique used to characterize the
  surface of crystalline materials.
• RHEED systems gather information only from the
  surface layer of the sample, which distinguishes
  RHEED from other materials characterization
  methods that rely on diffraction of high-energy
  electrons.
• Transmission electron microscopy, another common
  electron diffraction method samples the bulk of
  the sample due to the geometry of the system.

• A RHEED system requires an electron source (gun),
  photoluminescent detector screen and a sample
  with a clean surface, although modern RHEED
  systems have additional parts to optimize the
  technique.
• The electron gun generates a beam of electrons
  which strike the sample at a very small angle
  relative to the sample surface.
• Incident electrons diffract from atoms at the
  surface of the sample, and a small fraction of
  the diffracted electrons interfere constructively
  at specific angles and form regular patterns on
  the detector.
• The electrons interfere according to the position
  of atoms on the sample surface, so the
  diffraction pattern at the detector is a function
  of the sample surface.

9
Epitaxy
y
k2
k1
k0
x
ko

• The oscillation of theRHEED signal
  exactlycorresponds to the timeneeded to grow a
  monolayer and thediffraction pattern on the
  RHEED window gives direct indicationover the
  state of the surface