PHOTOEPITAXY

Making 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

• 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

PHOTOEPITAXY

Making atomically perfect thin films under milder and more controlled conditions

MERCURY PHOTOSENSITIZATION IDEA FOR PHOTOEPIXIAL FORMATION OF HgTe

• Hg(6s2), 1S/(H2) (weakly 6s- bonded VDWs GS) (h) Hg*(6s16p1), 1P/(H2) (strongly 6p- bonded ES) HHgH (insertion transient - mercury dihydride)

• HHgH + Et2Te [HHgH•••Et2Te] (4-center TS) HHgTeEt + C2H6

• HHgTeEt HgTe + C2H6 (reductive elimination)

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

• 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) GaAs…Brn(ad)

• GaAs…Brn(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

EXTENSIONS OF PHOTOLYTIC DEPOSITION METHODS, LASER WRITING AND LASER ETCHING

GROWTH OF SINGLE CRYSTALS: VAPOR, LIQUID, SOLID PHASE CRYSTALLIZATION

Useful for property measurements and fabrication of devices

High P crystal pulling equipment - art or science?

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

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.

Pulling direction of seed on rod

Heater

CZOCHRALSKI

Crucible

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

Melt just above mp

Growing crystal

Crystal seed

Counterclockwise rotation of melt and crystal being pulled from melt, helps unifomity of temperature and homogeneity of crystal growth

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)

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

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., Ga1+xAs and GaAs1+x 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

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

SAND TO SILICON CHIPS

SAND TO SILICON CHIP

PATTERNING Si WAFERS FOR CHIP MANUFACTURING THE BILLION

DOLLAR MICROFABRICATION WAY

Single crystal LiNbO3 electrooptical switch

Ferroelectric perovskite in tetragonal form below Tc

Ti channel diffused into LiNbO3 as Ti(4+): LiTixNbO3

Ti(4+) > (Nb5+) > (Li+) so higher RI channel

Light coupled from external optical fiber to LiTixNbO3 higher RI channel surrounded by lower RI LiNbO3 causes waveguiding of light in channel by TIR

Light waveguides along LiTixNbO3 channel - voltage off

Voltage on - E-field between LiTixNbO3 channels causes polarizability of LiNbO3 region between channels to increase and light in LiTixNbO3 channel no longer confined and switches to other LiTixNbO3 channel

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

BRIDGMAN AND STOCKBARGER METHODS

STOCKBARGER fixed temperature gradient - moving crystal

BRIDGEMAN changing temperature gradient - static crystal

T

T Distance

Distance

Crystallization of melt on seed as crucible gradually displaced through temperature gradient from hotter to cooler end

melt crystal

Furnace gradually cooled and crystallization begins on seed at cooler end of crucible

Tm

Tm

T1

T2

T3

Temperature gradient

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

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

T

Distance

Crystal or powder

Localized melt region - impurities concentrated in melt

Crystal growing from seed

Temperature profile

Pulling direction

Tm

ZONE MELTING CRYSTAL GROWTH AND PURIFICATION OF SOLIDS

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

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 Cr2

6+/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

O2 + powdered precursor(s)

O2 + H2

Fusion flame

Liquid drops of molten precursor(s)

Growing crystal

Support for growing crystal

VERNEUIL FUSION FLAME METHOD

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

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

HYDROTHERMAL CRYSTAL GROWTH

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

HYDROTHERMAL AUTOCLAVE

Growth region

Dissolving region

Crystal seeds

Separating baffle

Source nutrient

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

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

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

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 >374oC

• 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

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

QUARTZ CRYSTALS GROW IN HYDROTHERMAL AUTOCLAVE

400°C T2

360°C T1

SiO2 powder nutrient dissolving region

Baffle allows passage of minerlized species to quartz seed crystal

NaOH/H2O mineralizer

SiO2 seed

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 > T1

• H3PO4/H2O mineralizer

• AlPO4 seed crystal at hot end

T1

T2

AllPO4 powder

Baffle

H3PO4/H2O mineralizer

AlPO4 seed

EMERALD CRYSTALS GROW IN HYDROTHERMAL AUTOCLAVE

T2

T1

T2

SiO2 powder nutrient at hot end

Al2O3/Cr2O3/BeO powder nutrients at hot end

Emerald - Cr(3+) doped beryl seed crystal at cool center of hydrothermal synthesis - crystal growth autoclave

NH4Cl or HCl mineralizer

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

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

T2

T1

Metal Powder

Baffle

10MHI/I2 mineralizer

Metalseed