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36
2.GrowthofABO
3andBO
2Crystals
tionofthermalinsulationandadditionalheatingzonesaswellaschanges
inconstructionofburners,ledtobetterqualityofcurrentlygrownSrTiO
3
samples.CommerciallyavailableSrTiO
3:Nb(100)singlecrystalswerein-
vestigated[12].InhomogeneousdistributionofTi
3+stateswasconfirmed.
However,itwasshownthatinSrTiO
3singlecrystalscontaining1.4at%
Nb,usedasconductivesubstratesforepitaxialgrowthofthinfilms,Nb
distributionisuniformeveninnanoandatomicscale.AlthoughnoNb
segregationonexistingdislocationswasobserved,itwasfoundthatdis-
locationsandgrainboundariesmaytrapelectronsinSrTiO
3dopedwith
donorsandlocallyreduceelectronicconductivityinthisway.Despitethe
improvedqualityofavailableVerneuilsinglecrystals,theycontainmuch
moredislocationsthansinglecrystalsgrownbyothermethods.
Czochralskitechnique,whencrystallizationofcongruentlymeltingox-
idecompoundsisconsidered,isinmostcasesthemethodofchoice.How-
ever,suchinstabilitiesasfootformation,non-uniformdiameterofgrowing
crystals,despiteusingofsophisticatedmethodsofdiametercontrol,and
disconnectionofgrowingcrystalfromthemeltarecommondimculties
observedduringCzochralskicrystallizationofTiO
2[13].PullingSrTiO
3
singlecrystalsfromthemeltsufersduetosimilarinstabilitiesleadingto
subsequentspiraling.Verylowthermalconductivityofstrontiumtitanate
(thermalconductivity<2.2W/(mK)attemperatures>1700OCandal-
mosttotalopaquenesstoinfraredradiationclosetothemeltingpointlead
toapoorheattransportthroughthegrowingcrystal.Asaresultofthe
thermalandopticalpropertiesofthematerial,theheatingpowerhastobe
continuouslyloweredinsuchanabruptwaythatstableCzochralskicrys-
tallizationisimpossibletobecarriedout.Nevertheless,suitableconditions
forthegrowthofshortsinglecrystals(upto15mmlong)werefoundfor
theCzochralskimethod.However,thestructuralqualityoftheobtained
crystalswaspoor[14].Cold-crucible(skulltechnique)CzochralskiSrTiO
3
singlecrystalsgrowthalsowasperformedbutthecrystalsobtainedhad
poorqualityandhighdensityofetchpits[15].
Edge-definedFilm-FedGrowth(EFG)method,duetodiferentmass
andheattransportconditionsenablesthecrystallizationofbetterquali-
tysinglecrystalsthantheCzochralskimethod.Thegrowthiscarried
outfromoxidemeltshavingstoichiometriccomposition,likeinthecase
ofCzochralskigrowth.Moltenoxide,duetocapillaryforceisdistributed
onthesurfaceofaniridiumorplatinumdiethatallowsshapingofgrowing
singlecrystal.TiO
2ribbonsinglecrystal,witha(110)surfaceand2mm×
20mm×80mmindimensions,wassuccessfullygrownbyEFGmethod.
Theribboncrystalswerepulledatabout5mm/hinanAratmosphere.