Solid state synthesis - Georgia Institute of Technologyww2.chemistry.gatech.edu/class/6182/wilkinson/solid-state.pdf · Many solid state synthesis techniques rely on high temperatures

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Solid State Synthetic Methods

A. P. WilkinsonCHEM 6182, March 2001

Overview� High temperature direct reaction

� Limited by slow diffusion in solids� May be limited by phase diagram

� Control of oxidation state� CO/CO2 buffers, getering etc.

� Precursor methods for reducing path lengths� Stoichiometric precursors, sol-gel methods, co-precipitation

� Insertion, intercalation, ion exchange� Vapor phase transport� Fluxes� Electrochemical crystal growth� High pressure techniques� Thin film growth methods

� CVD, sputtering evaporation, sol-gel etc.� Large single crystal growth methods

The standard ceramic route

� The easiest way to make many solid state materials is direct reaction of their components at high temperatures

� PbO + TiO2 --(~900oC)---> PbTiO3

� Grind/mix powdered reactants, press into a pellet and heat

Problems with ‘heat and beat’

� Requires high temperature because reaction is diffusion limited– can be expensive– may give incomplete reaction– may give compositionally inhomogeneous products– there may be some loss of the reactants– there is little chance of getting kinetic control– may not get desired microstructure

Formation of MgAl2O4

� MgO + Al2O3 → MgAl2O4– Reaction only occurs at contact points between grains of MgO

and Al2O3

� Get nucleation near contact point and then growth of product– Growth requires diffusion of Mg2+ / Al3+ through the product – Very slow

Traditional solid state synthesis� Reaction is diffusion limited

– high temperatures to get diffusion» use absolute temperature that is > 2/3

of the MP of lowest melting reactant– high temperatures usually lead to

thermodynamically stable products

NiO Al2O3NiAl2O4

Ni2+

Al3+

NiO(s) + Al2O3(s) NiAl2O4(s) x = product layer thickness

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Nucleation� Nucleation of desired phase is a key step� For MgAl2O4 reaction, the reactants and products all

have structures based on close packed oxide– As the lattice constants of reactants and products are not

dissimilar you can get nucleation on surface of reactants� This leads to epitaxial growth – product orientation is

defined by substrate it is growing off

Self Propagating High Temperature Synthesis (SHS)

� Extreme exothermiticity of a reaction can be used to provide high temperatures needed for diffusion– Thermite Fe2O3 + Al → Al2O3 +Fe

� Has been used to make a number of useful materials including refractory ceramic parts that can be pressed and machined to final size– AlN+TiB2

– Si3N4 + SiC + TiN– Can produce functionally graded

materials. Have a composition and hence property gradient

Solid Flame propagation� Combustion front sweeps through powder mixture

Overcoming the diffusion barrier� Need intimate mixture of reactants� Can be obtained in several ways

– very small particle size reactants– find molecular precursor that has the needed elements in the

correct ratio» eg. Ba[TiO(C2O4)2] for BaTiO3

– Make a solution of needed metals and dry the solution out without demixing the components

» co-precipitate reactants in a solid solution salt� e.g. carbonates for Brownmillerite (Ca2Fe2O5)

» crystallize from gels prepared using sol-gel chemistry

Ferrites from stoichiometric precursors 1

�Can use series of oxalates MFe2(C2O4)3.6H2O precipitated from solution, M = Mn2+, Co2+, Ni2+, Zn2+

– MFe2(C2O4)3.6H2O + 2O2 → MFe2O4 + 6H2O + 6CO2

– Forms spinel at low temperature but does not give good control of stoichiometry because solubilities of metals are not the same

Ferrites from stoichiometric precursors 2

�Can use pyridinates M3Fe6(AcO)17O3OH.12Py precipitated from solution, M = Mg2+, Mn2+, Co2+, Ni2+

�Give ferrites with excellent control of stoichiometry

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Chromites from stoichiometric precursors

� Chromites MCr2O4 require very high temperature for direct preparation from oxides ( 1400 – 1700 °C)

� Decompose precursors such as – (NH4)2Mg(CrO4)2.6H2O– MnCr2O7.4Py– (NH4)2Mn(CrO4)2.2NH3

– NH4Fe(CrO4)2

� Get good control over stoichiomtery

What is sol-gel chemistry?

� Polymerize a solution of precursor molecules to form a sol or gel

Si(OEt)4(soln) + 2H2O ---->SiO2(gel) + 4EtOH

Steps in sol formation� Precursor initially undergoes hydrolysis

– Si(OEt)4 + H2O → Si(OEt)3OH + EtOH� Then get condensation

– Si(OEt)3OH + Si(OEt)3OH → (EtO)3SiOSi(OEt)3 + H2O– Or Si(OEt)3OH + Si(OEt)4 → (EtO)3SiOSi(OEt)3 + EtOH

� Both steps are often accelerated using an acid or base catalyst– Base catalysis leads to highly branched polymers, acid

catalysis does not» This effects properties of sol/gel

A PZT (Pb[Zr1-xTix]O3) gel

Sol-Gel PZT (Pb[Zr1-xTix]O3)�PZT is one of the best known perovksite

ferroelectrics– used in SPMs, sonar, memory devices, thermal

imaging.......�The sol-gel processing of PZT has been

extensively explored– thin film applications

�Typically, hydrolyze solution of Pb(OAc)2, Ti(iOPr)4, Zr(OBu)4 in 2-methoxyethanol

Sol-gel PbTiO3

� Mix Pb(OAc)2 and Ti(OR)4 in dry alcohol

� Add water– solution starts to become viscous and eventually

becomes an elastic gel

� This gel is a polymer with a - O -Ti(X2) - O -backbone and the lead is either incorporated into the polymer or trapped in the polymer network

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PbTiO3 sol-gel continued

� Take the wet gel (alcogel) and dry it– drying in air causes a collapse of the gel to give a

xerogel– drying using supercritical solvents can give aerogels

� Heat dry gel in air to burn off organics– usually gives amorphous (glassy) PbTiO3

– anneal the glass to form a crystalline product– crystallizes at low temperatures (<500oC for PT)

Advantages sol-gel chemistry

� Cheap way to make thin films and fibres compared to CVD but can be expensive relative to heat and beat

� Molecular mixing of precursors prior to hydrolysis helps get good homogeneity

� Can make high purity materials� Low firing temperatures

– metastable phases– reduced loss of volatile components

Compositional homogeneity� May not get perfectly homogeneous products even with sol-gel

chemistry

Alkoxide A

Alkoxide B

A hydrolyzes faster than B

A hydrolyzes at the same rate as B

Different hydrolysis rates can lead to compositional segregationAlso some components in solution may not be incorporated into polymer backbone

Nonhydrolytic sol-gel syntheses�The reactions forming polymers by the addition

of water to an alkoxide are hydrolytic process.�Can form similar polymer but get bridging

oxygen from ether or alkoxide– As there is no water this type of process is referred

to as nonhydrolytic» Not widely used but supposedly capable of forming very

homogeneous gels

Low temperature synthesis of ZrW2O8

� Decomposition into ZrO2 and WO3 occurs above 1050K– need method that gives atomic scale mixing of

zirconium and tungsten» decomposition of soluble precursors?

� used by Sleight, still needs high temperatures» alkoxide sol-gel?» non-hydrolytic sol-gel?» other route?

ZrW2O8 by non-hydrolytic sol-gel methods

� Non-hydrolytic routes are “advertised” as providing good compositional homogeneity– is this true?

� ZrX4 + 2WX6 + 8R2O --> “ZrW2O8” + 16RX– R should not be a primary alkyl group

� Zr(OR’)4 + 2WX6 + 4R2O -> “ZrW2O8” + 8RX + 4R’X ?

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Synthetic conditions� Use CHCl3 or CH2Cl2 as a solvent� Use iPr2O as an oxygen donor� Solubility of metal halides and alkoxides

can be a problem– use THF to enhance solubility of zirconium

alkoxide – CHCl3 gives better solubility of WCl6– Heat at 110oC for several days

Precipitate formation� Use of WCl6 always gave

a blue precipitate rather than a gel

Gel-formation� WCl6 does not give a gel� WBr5 gives a gel

Crystallization of “ZrW2O8” gels

750oC

600oC500oC

WCl6, Zr(iOPr)4, iPr2O, THF, CHCl3

Cubic

“Hexagonal” ZrW2O8

Cheap variations on sol-gel

� Aqueous solutions of metal salts can sometimes be made into a gel by the use of a complexing agent (say citrate) and a solvent like ethyleneglycol− This “Puchini” process is very commonly used

− the citrate and ethylene glycol not only bind the metals they form a polyester polymer

− Can be used to form films like alkoxide sol-gel method

− Metal salts like nitrates are much cheaper than alkoxides

Soft chemistry (Chimie Douce)

� Many solid state synthesis techniques rely on high temperatures and produce thermodynamic products

� Synthesis that use low temperatures to producemetastable (kinetic) products are often referred to as examples of ‘soft chemistry’.� Dehydration, Ion exchange, insertion, deintercalation etc.

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The synthesis of TiO2(B)

� 2KNO3 + 4TiO2 -(900oC)---> K2Ti4O9

� K2Ti4O9 + HNO3(aq) -(RT)--> H2Ti4O9.H2O

– this is an ion exchange process

� H2Ti4O9.H2O ---(500oC)---> TiO2(B) + 2H2O

– a topotactic dehydration

Hexagonal WO3� WO3 crystallizes at high T with a ReO3 structure� WO3•0.5H2O can be prepared by hydrothermal treatment of

tungstic acid (WO3•2H2O ) – from acidification of Na2WO4 soln.� Dehyration of the hemihydrate leads to an orthorhombic WO3 that

then transforms to hexagonal WO3

Pyrochlore WO3

� Obtain [(NH4)2O]xW2O6, x = 0.5 by hydrothermal treatment of (NH4)10W12O41•5H2O in acidic ethylene glycol

� Produce W2O6•xH2O by ion exchange of ammonium in acidic solution

� Dehydrate at 100 °C to get pyrochlore WO3

Ion exchange

� Many solid state materials can be ion exchanged under moderate conditions

– zeolites and α zirconium phosphates can be ion exchanged in water

– β aluminas and NaZr2(PO4)3 type materials can be ion exchanged in molten salts

� NaAl11O17 + AgNO3 --(300oC) --> AgAl11O17 + NaNO3

Insertion reactions

� Many inorganic solids have cavities can that can be subsequently filled by other ions. This process is referred to as insertion.

� Take WO3, coat with a little H2PtCl6 solution and then heat the material. This gives WO3 with Pt metal particles on the surface

� WO3(Pt) + H2 -----> HxWO3

– this is an example of hydrogen spillover

Intercalation� It is possible to modify many layered materials by

introducing species into the interlayer space� C + K → C8K� TiS2 + nBuLi → LixTiS2� ZrS2 + CoCp2 –(120ºC/tol)� (CoCp2)xZrS2

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Intercalation into graphite�Graphite will react with alkali metal and

halogens to form quite well defined compounds by intercalation

C8KGraphite

The intercalation of Li into TiS2

Intercalation of alkali metals into TaS2

� TaS2 + xNa → NaxTaS2 x = 0.4 – 0.7

The structure of TaS2

The electronic structure of TaS2Staging in intercalation compounds

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Vapor Phase Chemical Transport� Can overcome diffusion limitations on reaction by getting

one or preferably both reactants into the gas phase– Set up equilibrium between solid products/reactants and gas

phase species

Transport agent B reactswith solid A to form a gasthat decomposes in a cooler

or hotter region

For exothermic reactions deposit A at hotter end

For endothermic reactions deposit A at cooler end

Examples of Vapor Phase Transport� 2-MnP2 two layer stacking varient of MnP2

– Mn + 2P -(1at% I2, sealed tube)� MnP2» Hot end of tube at ~ 800K cool end 80 K lower, MnP2 formed at cooler end

of tube» Both components are transportable

� CaSn2O4– 2CaO + SnO2 –(900 °C traces CO or H2)-> CaSn2O4

» Reaction accelerated because SnO is volatile

� NiCr2O4– NiO + C2O3 –(1100 °C traces O2)-> NiCr2O4

» Reaction accelerated by the formation of volatile CrO3

� Nb5Si3– 11Nb + 3SiO2 –(1000 °C, trace H2)�Nb5Si3 + 6NbO

» Reaction does not occur in absence of H2, volatile SiO is formed

Examples of Vapor Phase Transport� If the reaction product is transportable get faster

reaction than if only one reactant is transportable– Also can easily get single crystals

� Al + S ---( traces I2 sealed SiO2 tube)-> Al2S3– Very slow reaction in absence of I2 due to formation on

Al2S3 layer on surface of Al» With I2 get rapid reaction due to formation of volatile S2(g) and AlI3(g)

» Large crystals are formed

Crystallization from melts

� Several different approaches − For many conducting reduced phases it is possible to

electrolyze a melt and form crystals of the desired reduced product on the electrode

− MOLTEN SALT ELECTROLYSIS− dissolve the compound of interest or appropriate

reactants in a molten salt and try to recrystallize the compound from the molten salt (flux)

− FLUX GROWTH− melt the composition of interest and cool it so that it

crystallizes

Phase diagram can give problems� Crystal growth of an

incongruently melting phase can be very difficult– Can not just go straight

from a stoichiometric melt» May need to use flux or

other method

� A4B can be grown directly from stoichiometric melt AB3can not

Advantages of Molten Salt Electrolysis� Can produce single crystals� Electrochemical control of growth� Isothermal process� Solutions may be purified by coulometry� Reproducible� Small changes in temperature do not normally

influence growth rate� Relatively short reaction times� Apparatus simple and inexpensive

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Disadvantages of Molten Salt Electrolysis

� Red-Ox potential may be unfavorable� Low current efficiencies� Difficulty of finding a suitable solvent restricts

use� Container and electrode attack� Products often sensitive to melt composition;

changes with time during electrolysis� High nucleation rates lead to twinning, inclusions,

agglomerations etc.

Oxides made by MSE� Metallic oxides

- NaxWO3, x = 0.85 – 0.3

� Quasi-low dimensional materials- A0.9Mo6O17 ( A = Li, K, Rb, Tl)- K0.30MoO3- La2Mo2O7- La5Mo4O16 (ferromagnetic semiconductor)

� Superconducting materials- YBa2Cu3O7 ( Eutectic, stab. Zirconia anode)- (K,Ba)BiO3 (NaOH-KOH saturated with water vapor)- LiTi2O4

Mo oxides with M-M bonds by MSE� Isolated or Quasi-isolated clusters

- La2Mo2O7 Digonal- La5Mo4O16 Digonal- ZnMo3O8 Triangular- Y4Mo18O32 Rhomboidal Mo4- LaMo2O5 Mo6 octahedra and Mo3 cluster- LaMo8O14 Bi-faced capped octahedron- SrMo5O8 Edge-shared bi-octahedron

� Compounds with extended metal-metal bonds- KMo4O6- NaxMo2O4- SrMo5O8

1 cm La2Mo2O7 crystal

CMR materials by MSE� La0.94Mn0.98O3 (Tc = 240 K)� Na0.12La0.86MnO3 (Tc = 310 K)� Sr0.34La0.66MnO3 (Tc = 370 K)� Sr0.12La0.84Mn0.99O3 (Tc = 325 K)

La0.94Mn0.98O3

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Electrocrystallization of Ba1-xKxBiO3(BKBO)

� The high Tc superconductor Bi0.6K0.4BiO3 can be crystallized by electrolyzing a melt of composition KOH:Ba(OH)2.8H2O:Bi2O352.4:0.54:1.00 under a water saturated Aratmosphere

� Melt can be used at 180oC. Bi2O3 never fully dissolves. Platinum electrodes are used and crystals grow on the anode.

Electrocrystallization of tungsten bronzes (NaxWO3)

� Melts of Na2WO4 and WO3 (750oC) can be electrolyzed to give large crystals of NaxWO3

� x can take values between 0.32 and 0.93

� Compounds with high values of x have a red-orange coloration (look similar to metallic copper) and are good electrical conductors

Flux growth

� Oxides like Bi2O3 and PbO have low melting points and may be used as solvents– PbTiO3 can be crystallized from PbO/PbF2

mixtures– need to pick a flux that is compatible with the

desired product� Alkali and alkaline earth metal hydroxides

and halides are also frequently used as fluxes

The PbO/PbF2 phase diagram

The NaOH / KOH pseudobinaryphase diagram Growing from wet KOH/NaOH

� Many superconducting copper oxides have been grown by this method (Stacy et al.)

� Synthesis of EuBa2Cu3O7-δ– take stoichiometric amounts of CuO, Eu2O3 and

Ba(OH)2.8H2O and dissolve in molten KOH/NaOH at 450oC

– gives clear blue solution– solution held at 450oC under flowing dry air– as water is lost the product crystallizes out

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Reactive halide fluxes� Growth from halide fluxes in the presence of trace

amounts of water can lead to interesting products– BaCl2 + 6Fe2O3 + H2O → BFe12O19 + 2HCl

» Grow large single xtals

� From CaCl2 flux grew CaFeO4, Ca3Al10O18, CaCrO4, CaSiO4 by adding Fe2O3, Al2O3, Cr2O3 and SiO2 to flux

� From BaCl2 flux grew BaFe12O19, BaWO4, BaSi2O5, BaPbO3, BaTi3O7 by addition of metal oxide to flux

Molten metal fluxes� Molten metals can sometimes be used as solvents.

However, metal should not form stable compounds with reactants– Mn + 2Si –(Cu liquid/1200 °C)-> MnSi2

» Heat materials in sealed ampul. Using a Cu solvent avoids Mn loss due to heating at high temps

– Ru + 2P –(Sn liquid)-> RuP2» 1:2:100 ratio of Ru, P and Sn sealed evacuated quartz tube. Heated

to 1200 °C and then slow cooled. Crystals recovered from Sn by washing with HCl

� Kaner et al., Mater. Res. Bull. 12, 1143 (1977)

Large single crystals from a melt�The preparation of large high quality single

crystals is a crucial stage in the manufacture of many technologically important devices– Silicon is grown for the semiconductor industry– LiNbO3 is melt grown for telecommunications

applications– YAG (Y3Ga5O12) is grown for laser applications

Single crystal silicon

The Czochralski Method The crystal growth procedure�Seed is brought into

contact with melt and pulled out slowly while rotating

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The LiNbO3 phase diagram Commercial LiNbO3 crystals

Gadolinium Gallium Garnet�Used as substrate for growth of devices

Stockbarger method�Move the crucible containing a seed and the

melt through a temperature gradient so that the melt crystallizes onto the seed crystal

Bridgman method�Adjust furnace so that temperature gradient

varies with time and the melt grows of a crystal in the crucible

Zone melting�Sweep a molten zone through the crucible in

such a way that the melt crystallize onto a seed– Method used for purifying existing crystals as

impurities tend to stay with the liquid

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Floating zone growth of silicon�Move polycrystalline ingot into hot zone a

seed formation of single crystal

Verneuil method

�Used for growing large crystals of high melting point solids– for example, Ruby Cr2O3

doped Al2O3

High pressure methods� Three types of high pressure reaction

– Hydro or solvo thermal. Solids are heated in a liquid medium and the reaction takes place by virtue of reactants dissolving in solvent at temperature / pressure

– High pressure solid / gas reactions. May arrange things so that solid is exposed to very high O2 or F2 pressures

» Facilitates prep of high oxidation state metals

– Solid – solid reactions involving compression of reactants in flexible/compressible container

» Favors formation of dense high coordination number phases

Piston Cylinder Press� Can achieve 50 kbar and 1800K

– Sample is placed in container (Pt, Au ..) and the container is embedded in a pyropholite block

» Pyropholite acts as a pressure transmitting medium– Squeeze sample by forcing WC piston into WC cylinder

Multianvil Press� Can achieve ~200 kbar and over 2000K

– Sample volume may be low for some designs

Belt design� Can achieve 150 kbar, 2300K

– Relatively large sample volume– Sample in Au/Pr container or for chalocogenides BN or MgO

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Synthetic diamond� Prepared as a powder on a vast scale for cutting

tools and other applications– Requires high pressure to make bulk diamond

The production of diamonds

Large synthetic diamonds Why use high pressures?� High pressure allows the preparation of new

compositions, new structures, unusual oxidation states– PbSnO3 does not form as a perovskite at ambient pressure,

but will at high pressure– CaFeO3 can only be prepared at high P. At ambient P

Brownmillerite (CaFeO2.5) forms– Superconducting oxygen excess La2CuO4+δ can be prepared

at high oxygen P– La2Pd2O7 can be prepared at high oxygen P. Normally only

get Pd2+ oxides» Oxygen P can be generated in-situ by decomposition of say KClO3.

However, beware! KCl may be incorporated into the product

Hydrothermal growth of quartz crystals� Large quartz crystals are needed as oscillators for

timing applications– Are grown from basic aqueous solution at high P/T due to

improved solubility of SiO2

Hydrothermally grown quartz

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Hydrothermal reactor designs� Depending on design may be useable to 10 kbar

Applicability and value of hydrothermal synthesis� Hydrothermal techniques can be used to

synthesize a wide variety of materials– zeolites and aluminophosphates

– optical materials like KTP (KTiOPO4)

– BaTiO3 (widely used ferroelectric)

� Synthesis can be carried out at low temperature (relative to direct reaction of solids)

� High quality samples can be made

Templated growth from solution

� Many hydrothermal synthesis of materials such as zeolites or mesoporous materials use ‘templates’ as part of the synthesis

� A molecule or ion is added to the synthesis mix to direct the formation of the solid– the solid grows around the template

Self assembled templates� In zeolitic materials the template is a single

molecule or ion� Self assembled aggregates of molecules or ions

can also serve as templates– Surfactants aggregate into a variety of structures

depending on conditions

Self assembled surfactant structures� In solution surfactants can self assemble to form

micelles, rods, sheets and 3D structures– All of these can in principle be used as templates– Rod like surfactant aggregates have provided some of

the most interesting structures� A lot of work has been done exploring the

formation of silicate structures using self assembled templates– Other inorganic oxides have also been examined

Materials containing rod like assemblies

Material is highly ordered and gives a diffraction pattern but thesilicate walls are not crystalline, they are glass like

MCM-41 silicate structures

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A typical MCM-41 synthesis� MCM-41 contains rod like surfactant aggregates. � MCM-41 materials can be made in many ways.� Aluminosilicate materials have been prepared as

follows� C16H33NMe3OH, catapal alumina, TMA silicate and

amorphous silica stirred in water� Heated in autoclave for 48 hours at 150 º C� Recover by filtration and remove template by heating in

nitrogen to 540 ºC for 1 hour and then heating in air for 6 hours� Surfactant decomposes by Hoffman elimination reaction

Pore size distribution� The pore size distribution in MCM-41 is usually

quite narrow as well ordered materials can be made, but it is not as tightly defined as that for a zeolite as MCM-41 is not a crystalline product

Growth of thin films� Can be done in many different ways

– Electrochemical deposition– Coat substrate with sol and heat– Decompose organometallic precursor over substrate– Flame pyrolysis– Evaporate material onto substrate– Sputter material of target onto substrate– Laser ablation – use laser to blow a plume of material of a

target and allow plume to impinge on substrate

Sputtering

�Produce ions from gas and accelerate ions into target so that they knock material off the surface of the target

Evaporation�Heat material so that it evaporates onto

substrate

Thin film CVD diamond�Can deposit diamond film from hydrogen rich

flame or arc containing C2H2 or CH4.– Conditions are far from equilibrium but growth is

kinetically favored– Can monitor film quality by Raman spectroscopy

» Often contains some sp2 carbon and may not be nice cubic diamond, but diamond like carbon

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Diamond growth�Hydrogen atoms in flame or arc remove H from

growing surface and provide clean C sites for further reaction with carbon containing species

CVD GaAs�Can make materials like GaAs in many

different ways– Flow mix of GaMe3 and AsH3 over heated

substrate– Flow over heated substrate

– Using single source precursor can get around some problems such as control of stoichiometry and prereaction leading to snow formation in vapor phase

GaAsGaAstBu

tBu

tBu

tBuMe Me

MeMe

CVD amorphous Si�Doped thin film silicon can be very useful

but simple CVD process produces amorphous material that can not be doped– Dangling bonds act as traps for doped in

electrons or holes– Preparation in presence of hydrogen ties up all

the dangling bonds and allows good control of doping

Band structure of amorphous Si

�Hydrogenation ties up electrons in orbitals between the bands