PHOTOEPITAXY Making atomically perfect thin films under milder and more controlled conditions

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PHOTOEPITAXYMaking atomically perfect thin
films under milder and more controlled conditions

• Mullin and Tunnicliffe 1984
• Et2Te Hg (pool) H2 (h?, 200oC) ? HgTe 2C2H6
• MOCVD preparation requires 500oC using Me2Te
  Me2Hg
• Advantages of photo-epitaxy
• Lower temperature operation, multi-layer
  formation, less damage of layers - ternaries
  HgxCd1-xTe, n- and p-doping, Te and Hg/Cd rich,
  diodes, IR detectors, multi-layers, quantum size
  effect devices HgxCd1-xTe-HgTe-HgxCd1-xTe

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PHOTOEPITAXYMaking atomically perfect thin
films under milder and more controlled conditions

• Lower interlayer diffusion, easy to fabricate
• Abrupt boundaries, less defects, strain,
  irregularities at interfaces
• Note that H2 gas window in apparatus prevents
  deposition of HgTe on observation port
• CdTe can be deposited onto GaAs at 200-250oC even
  with a 14 lattice mismatch
• GaAs is susceptible to damage under MOCVD
  conditions 650-750oC

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MERCURY PHOTOSENSITIZATION IDEA FOR PHOTOEPIXIAL
FORMATION OF HgTe

• Hg(6s2), 1S/(H2) (weakly 6s-s bonded VDWs GS)
  (h?) ? Hg(6s16p1), 1P/(H2) (strongly 6p-p
  bonded ES) ? HHgH (insertion transient - mercury
  dihydride)
• HHgH Et2Te ? HHgHEt2Te (4-center TS) ?
  HHgTeEt C2H6
• HHgTeEt ? HgTe C2H6 (reductive
  elimination)

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EXTENSIONS OF PHOTOLYTIC DEPOSITION METHODS,
LASER WRITING AND LASER ETCHING

• Laser writing
• Substrate GaAs
• Me3Al or Me2Zn adsorbed layer or gas phase
• Focussed UV laser on film
• Photodissociation of organometallic precursor
• Me3Al or Me2Zn ? Al or Zn C2H6
• Creates sub-micron lines of Al or Zn

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EXTENSIONS OF PHOTOLYTIC DEPOSITION METHODS,
LASER WRITING AND LASER ETCHING

• Laser photoetching
• GaAs substrate, gaseous or adsorbed layer of
  CH3Br
• Focussed UV laser, creates reactive Br atoms
• CH3Br(g) (h?) ? CH3(g) Br(g)
• Br(g) GaAs(s) ? GaAsBrn(ad)
• GaAsBrn(ad) ? GaBrn(g) AsBrn(g)
• Adsorbed reactive Br erode surface regions
  irradiated with laser, vaporization of volatile
  GaBrn and AsBrm from surface, creates sub-micron
  etched line

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High P crystal pulling equipment - art or science?
GROWTH OF SINGLE CRYSTALS VAPOR, LIQUID, SOLID
PHASE CRYSTALLIZATION Useful for property
measurements and fabrication of devices
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GROWTH OF SINGLE CRYSTALS MICRONS TO METERS

• Vapor, liquid, solid phase crystallization
  techniques
• Single crystals vital for meaningful property
  measurements of materials
• Single crystals allow measurement of anisotropic
  phenomena in crystals with symmetry lower than
  cubic (isotropic)
• Single crystals important for fabrication of
  devices, like silicon chips, yttrium aluminum
  garnet and beta-beryllium borate for doubling and
  tripling the frequency of CW or pulsed laser
  light, quartz crystal oscillators for mass
  monitors

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LET'S GROW CRYSTALS

• Key point to remember when learning how to be a
  crystal grower (incidentally, an exceptionally
  rare profession and extraordinarily well paid)
• Many different techniques exist, hence one must
  think very carefully as to which method is the
  most appropriate for the material under
  consideration, size of crystal desired, stability
  in air, morphology or crystal habit required,
  doping, defects, impurities and so forth
• So let's proceed to look at some case histories.

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Pulling direction of seed on rod
Crystal seed
Inert atmosphere under pressure prevents material
loss and unwanted reactions Layer of molten
oxide like B2O3 prevents preferential
volatilization of one component - precise
stoichiometry control
Growing crystal
Heater
Melt just above mp
Counterclockwise rotation of melt and crystal
being pulled from melt, helps unifomity of
temperature and homogeneity of crystal growth
Crucible
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CZOCHRALSKI METHOD

• Interesting crystal pulling technique (but can
  you pronounce and spell the name!)
• Single crystal growth from the melt precursor(s)
• Crystal seed of material to be grown placed in
  contact with surface of melt
• Temperature of melt held just above melting
  point, highest viscosity, lowest vapor pressure
• Seed gradually pulled out of the melt (not with
  your hands of course, special crystal pulling
  equipment is used)

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CZOCHRALSKI METHOD

• Seed gradually pulled out of the melt (not with
  your hands of course, special crystal pulling
  equipment is used)
• Melt solidifies on surface of seed
• Melt and seed usually rotated counterclockwise
  with respect to each other to maintain constant
  temperature and to facilitate uniformity of the
  melt during crystal growth, produces higher
  quality crystals, less defects
• Inert atmosphere, often under pressure around
  growing crystal and melt to prevent any materials
  loss

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GROWING BIMETALLIC SINGLE CRYSTALS LIKE GaAs
REQUIRES A MODIFICATION OF THE CZOCHRALSKI METHOD

• Layer of molten inert oxide like B2O3 spread on
  top of the molten feed material to prevent
  preferential volatilization of the more volatile
  component of the bimetal melt
• Critical for maintaining precise stoichiometry,
  e.g., Ga1xAs and GaAs1x when made rich in Ga
  and As, become p- and n-doped!!!
• The Czochralski crystal pulling technique
  invaluable for growing many large single crystals
  as a rod, to be cut into wafers and polished for
  various applications
• Utility of some single crystals made by
  Czochralski listed below

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EXAMPLES OF CZOCHRALSKI GROWN SCs -
SOLIDIFICATION OF STOICHIOMETRIC MELT

• LiNbO3 - NLO material - perovskite - temperature
  dependent tetragonal-cubic -ferroelectric -
  paraelectric phase transition at Curie T -
  refractive index control - electrooptical switch
• SrTiO3 - perovskite substrate - epitaxial growth
  of high Tc defect perovskite YBa2Cu3O7
  superconducting films - fabrication of SQUIDS
• GaAlInP - quaternary alloy semiconductor - near
  IR diode lasers
• GaAs wafers - laser diodes, Lincoln log photonic
  crystal switch
• NdxY3-xAl5O12 - near IR slab lasers - 1.06
  microns
• Si - microelectronic chips, Ge - semiconductor
  high electron mobility faster electronics than Si

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SAND TO SILICON CHIPS
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SAND TO SILICON CHIP
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PATTERNING Si WAFERS FOR CHIP MANUFACTURING THE
BILLION DOLLAR MICROFABRICATION WAY
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BRIDGMAN AND STOCKBARGER METHODS

• Stockbarger method is based on a crystal growing
  from the melt, involves the relative displacement
  of melt and a temperature gradient furnace, fixed
  gradient and a moving melt/crystal
• Bridgman method is again based on crystal growth
  from a melt, but now a temperature gradient
  furnace is gradually lowered and crystallization
  begins at the cooler end, fixed crystal and
  changing temperature gradient
• Both methods are founded on the controlled
  solidification of a stoichiometric melt of the
  material to be crystallized

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BRIDGMAN AND STOCKBARGER METHODS
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BRIDGMAN AND STOCKBARGER METHODS

• Stockbarger and Bridgman methods both involve
  controlled solidification of a stoichiometric
  melt of the material to be crystallized
• Enables oriented solidification
• Melt passes through a temperature gradient
• Crystallization occurs at the cooler end
• Both methods benefit from seed crystals and
  controlled atmospheres

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ZONE MELTING CRYSTAL GROWTH AND PURIFICATION OF
SOLIDS

• Method related to the Stockbarger technique -
  thermal profile furnace employed - material
  contained in a boat
• Only a small region of the charge is melted at
  any one time - initially part of the melt is in
  contact with the seed
• Boat containing sample pulled at a controlled
  velocity through the thermal profile furnace
• Zone of material melted, hence the name of the
  method - oriented solidification of crystal
  occurs on the seed - simultaneously more of the
  charge melts

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ZONE MELTING CRYSTAL GROWTH AND PURIFICATION OF
SOLIDS
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ZONE MELTING CRYSTAL GROWTH AND PURIFICATION OF
SOLIDS

• Partitioning of impurities occurs between melt
  and crystal
• This is the basis of the zone refining methods
  for purifying solids
• Impurities concentrate in liquid more than the
  solid phase where structure-energy constraints of
  crystal sites more severe, impurities swept out
  of crystal by moving the liquid zone
• Used for purifying materials like W, Si, Ge, Au,
  Pt to ppb level of impurities, often required for
  device applications

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VERNEUIL FUSION FLAME METHOD

• 1904 first recorded use of the method, useful for
  growing crystals of extremely high melting metal
  oxides, examples include
• Ruby red from Cr3/Al2O3 powder, sapphire blue
  from Cr26/Al2O3 powder, luminescent host CaO
  powder
• Starting material fine powder, passed through
  O2/H2 flame or plasma torch
• Melting of the powder occurs in the flame, molten
  droplets fall onto the surface of a seed or
  growing crystal, leads to controlled crystal
  growth

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VERNEUIL FUSION FLAME METHOD
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RUBY - CRYSTAL PRESSURE SENSOR?

• Cr(3) determines Oh monatomic Cr(3) or
  diatomic Oh (Cr(3)-O-Cr(3)) sites in Al2O3
  corundum lattice
• t2g to eg d-d electronic transition red shifts
  with concentration - red to blue color of ruby
  and sapphire
• t26 to eg transitions sensitive to Cr-O distance
  - pressure decreases these distances and
  increases CF splitting causing blue shifts
  proportional to pressure - hence senses pressure

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CRYSTAL GROWING METHODS COCHRALSKI, BRIDGMAN,
STOCKBARGER, ZONE MELTING, VERNEUIL

• All methods have the advantage of rapid growth
  rates of large crystals required for many
  advanced device applications
• BUT the crystal quality obtained by all of these
  techniques must be checked for inhomogeneities in
  surface and bulk composition and structure,
  gradients, domains, mosaicity, impurities,
  point-line-planar defects, twins, grain
  boundaries
• THINK how you might go about checking this if you
  were confronted with a 12"x12"x12" crystal -
  useful methods include confocal optical
  microscope, polarization optical microscope
  birefringence, Raman microscope, spatially
  resolved XRD, TEM, ED, EDX, AFM

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HYDROTHERMAL CRYSTAL GROWTH
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HYDROTHERMAL SYNTHESIS AND GROWTH OF SINGLE
CRYSTALS

• Basic methodology, water medium and high
  temperature growth, above normal boiling point,
  water acts as a pressure transmitting agent
• Water functions as solublizing phase, often
  mineralizing agent added to enable transport of
  reactants and crystal growth, speeds up chemical
  reactions between solids
• Useful technique for the synthesis and crystal
  growth of phases that are unstable in a high
  temperature preparation in the absence of water

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HYDROTHERMAL AUTOCLAVE
Growth region
Crystal seeds
Separating baffle
Dissolving region
Source nutrient
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HYDROTHERMAL SYNTHESIS AND GROWTH OF SINGLE
CRYSTALS

• Temperature gradient reactor - dissolution of
  reactants at one end - transport with help of
  mineralizer to seed at the other end -
  crystallization at the other end
• Because some materials have negative solubility
  coefficients, crystals can grow at the hotter end
  in a temperature gradient hydrothermal reactor,
  counterintuitive
• Good example is alpha-AlPO4 known as Berlinite,
  important for its high piezoelectric coefficient
  - larger than alpha-quartz with which it is
  isoelectronic - and use as a high frequency
  oscillator

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HYDROTHERMAL GROWTH OF QUARTZ SINGLE CRYSTALS

• Water medium - Nutrients 400oC - Seed 360oC
• Pressure 1.7 Kbar - Mineralizer 1M NaOH
• Uses of single crystal quartz radar, sonar,
  piezoelectric transducers, mass monitors
• Annual global production hundreds of tons of
  quartz crystals, amazing

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HYDROTHERMAL METHODS SUITABLE FOR GROWING MANY
TYPES OF SINGLE CRYSTALS

• Ruby Cr2O3/Al2O3 ? Cr3/Al2O3 and sapphire
  Cr26/Al2O3
• Chromium dioxide Cr2O3 CrO3 ? 3CrO2
• Yttrium aluminum garnet 3Y2O3 5Al2O3 ?
  Y3Al5O12
• Corundum alpha-Al2O3
• Zeolites Al2O3.3H2O Na2SiO3.9H2O NaOH/ H2O ?
  Na12(AlO2)12(SiO2)12.27H2O
• Emerald 6SiO2 Al(Cr)2O3 3BeO ?
  Be3Al(Cr)2Si6O18
• Berlinite alpha-AlPO4
• Metals Au, Ag, Pt, Co, Ni, Tl, As

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ROLE OF THE MINERALIZER IN HYDROTHERMAL SYNTHESIS
AND CRYSTAL GROWTH

• Consider growth of quartz crystals - control of
  crystal growth rate, through mineralizer,
  temperature pressure
• Solubility of quartz in water is important
• SiO2 2H2O ? Si(OH)4
• Solubility about 0.3 wt even at supercritical
  temperatures gt374oC
• A mineralizer is a complexing agent (not too
  stable) for the reactants/precursors, which have
  to be solublized (not too much) and transported
  to the growing crystal

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ROLE OF THE MINERALIZER IN HYDROTHERMAL SYNTHESIS
AND CRYSTAL GROWTH

• NaOH mineralizer, dissolving reaction, 1.3-2.0
  KBar
• 3SiO2 6OH- ? Si3O96- 3H2O
• Na2CO3 mineralizer, dissolving reaction, 0.7-1.3
  KBar
• SiO2 2OH- ? SiO32- H2O
• CO32- H2O ? HCO3- OH-
• NaOH creates growth rates about 2x greater than
  with Na2CO3 because of different concentrations
  of hydroxide mineralizer

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QUARTZ CRYSTALS GROW IN HYDROTHERMAL AUTOCLAVE
SiO2 powder nutrient dissolving region
400C T2
Baffle allows passage of minerlized species to
quartz seed crystal
NaOH/H2O mineralizer
360C T1
SiO2 seed
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EXAMPLES OF HYDROTHERMAL CRYSTAL GROWTH AND
MINERALIZERS

• Berlinite alpha-AlPO4 - larger piezoelectric
  coefficient than quartz
• Powdered AlPO4 cool end of reactor, negative
  solubility coefficient T2 gt T1
• H3PO4/H2O mineralizer
• AlPO4 seed crystal at hot end

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EMERALD CRYSTALS GROW IN HYDROTHERMAL AUTOCLAVE
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EXAMPLES OF HYDROTHERMAL CRYSTAL GROWTH AND
MINERALIZERS

• Emeralds Be3Al(Cr)2Si6O18 Beryl contains
  Si6O1812- six rings
• SiO2 powder at hot end 600oC
• NH4Cl or HCl/H2O mineralizer, 0.7-1.4 Kbar, cool
  central region for seed, 500oC
• Al2O3/BeO/Cr3 dopant powder mixture at other hot
  end 600oC
• 6SiO2 Al(Cr)2O3 3BeO ? Be3Al(Cr)2Si6O18

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EXAMPLES OF HYDROTHERMAL CRYSTAL GROWTH AND
MINERALIZERS

• Metal crystals - Metal powder at hot end 500oC
• Mineralizer 10M HI/I2 - Metal seed at cool end
  480oC
• Dissolving reaction transports Au to the seed
  crystal
• Au 3/2I2 I- ? AuI4-
• Metal crystals grown include
• Au, Ag, Pt, Co, Ni, Tl, As at 480-500oC