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

PHOTOEPITAXY Making atomically perfect thin films under milder and more controlled conditions. Mullin and Tunnicliffe 1984 Et 2 Te + Hg (pool) + H 2 (h  , 200 o C)  HgTe + 2C 2 H 6 MOCVD preparation requires 500 o C using Me 2 Te + Me 2 Hg Advantages of photo-epitaxy

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

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  1. 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

  2. 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

  3. 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  [HHgH•••Et2Te] (4-center TS)  HHgTeEt + C2H6 • HHgTeEt  HgTe + C2H6 (reductive elimination)

  4. 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

  5. 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)  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

  6. 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

  7. 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

  8. 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.

  9. CZOCHRALSKI 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

  10. 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)

  11. 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

  12. 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

  13. 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

  14. SAND TO SILICON CHIPS

  15. SAND TO SILICON CHIP

  16. PATTERNING Si WAFERS FOR CHIP MANUFACTURING THE BILLION DOLLAR MICROFABRICATION WAY

  17. Single crystal LiNbO3 electrooptical switch Ferroelectric perovskite in tetragonal form below Tc Ti channel diffused into LiNbO3 as Ti(4+): LiTixNbO3 aTi(4+) > a(Nb5+) > a(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

  18. 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

  19. T Temperature gradient STOCKBARGER fixed temperature gradient - moving crystal Tm Crystallization of melt on seed as crucible gradually displaced through temperature gradient from hotter to cooler end melt crystal Distance T BRIDGEMAN changing temperature gradient - static crystal T1 Tm T2 T3 Furnace gradually cooled and crystallization begins on seed at cooler end of crucible Distance BRIDGMAN AND STOCKBARGER METHODS

  20. 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

  21. 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

  22. T Temperature profile Tm Pulling direction Distance Crystal or powder Crystal growing from seed Localized melt region - impurities concentrated in melt ZONE MELTING CRYSTAL GROWTH AND PURIFICATION OF SOLIDS

  23. 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

  24. 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

  25. O2 + powdered precursor(s) O2 + H2 Fusion flame Liquid drops of molten precursor(s) Growing crystal Support for growing crystal VERNEUIL FUSION FLAME METHOD

  26. 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

  27. 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

  28. HYDROTHERMAL CRYSTAL GROWTH

  29. 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

  30. HYDROTHERMAL AUTOCLAVE Growth region Crystal seeds Separating baffle Dissolving region Source nutrient

  31. 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

  32. 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

  33. 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

  34. 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

  35. 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

  36. QUARTZ CRYSTALS GROW IN HYDROTHERMAL AUTOCLAVE SiO2 powder nutrient dissolving region 400°C T2 Baffle allows passage of minerlized species to quartz seed crystal NaOH/H2O mineralizer 360°C T1 SiO2 seed

  37. a-AllPO4 powder T1 Baffle H3PO4/H2O mineralizer T2 a-AlPO4 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

  38. SiO2 powder nutrient at hot end T2 Emerald - Cr(3+) doped beryl seed crystal at cool center of hydrothermal synthesis - crystal growth autoclave T1 T2 Al2O3/Cr2O3/BeO powder nutrients at hot end NH4Cl or HCl mineralizer EMERALD CRYSTALS GROW IN HYDROTHERMAL AUTOCLAVE

  39. 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

  40. Metal Powder T2 Baffle 10MHI/I2 mineralizer T1 Metalseed 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

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