Journalof CrystalGrowth 104 (1990)157—164 157North-Holland

VOID FORMATION UPON ANNEALING OF SHAPED SAPPHIRE CRYSTALS

V.A. BORODIN, A.M. IONOV andT.N. YALOVETS

Institute of SolidStatePhysics,Academyof Sciencesof the USSR,Chernogolovka,MoscowDistrict 142432, USSR

Received14 November1989; manuscriptreceivedin final form 19 December1989

Shapedsapphirecrystalsweregrown from the melt by theStepanov(EFG) technique.High-temperaturevacuumannealingofcrystalsgrown in a reducingatmosphere,performedby employinggraphiteelementsin theheaterunit of thecrystallizationchamber,led to theformation of voids of two different types: (1) well-facetedvoids whichare theresultof vacancycoagulationand(2) poresof irregular form which decoratethe subgrainboundaries.The formation of thesedefectsmay be preventedby suppressingtheinteractionbetweenthealuminamelt and carbon.

1. Introduction

While growing shapedsapphirecrystals fromthe melt, we must deal with the problemsassoci-atedwith void formation. It hasbeenfound thatvoids of six different types appearin the crystals,dependingon the growth conditions and subse-quent thermal treatment.Voids of four differenttypesare formeddirectly in theprocessof crystalgrowth as a result of: (a) gaseousimpurity segre-gation and void formation at the crystallizationfront; (b) entrapmentof gas bubblesgrowing onthe die surface; (c) limited melt supply to the Fig. 2. SEM micrographof the transversesectionof vacancy

void.

~Ppcrystallizationzone; and (d) using a starting raw

materialwith an increasedcontentof certain im-purities.

High-temperaturevacuumannealingof crystalsgrown in a reducing atmosphere,performedbyemployinggraphiteelementsin the heaterunit ofthe crystallizationchamber,leadsto the formationof voids of two more types. First, in the crystalbulk, thereappearspecifically shapedvoids (ide-ally facetednegativecrystals,primarily in theformof hexagonalprisms, up to 200—300 A in heightand20 ~tm in extent in the basalplane; seefigs. 1

Fig. 1. Photomicrographof vacancyvoids in the(0001) plane and 2). Thesevoids result from oxygen vacancy

of sapphire. coagulation[1]. The optical transmissionspectra

0022-0248/90/$03.50© 1990 — Elsevier SciencePublishersB.V. (North-Holland)

158 V.A. Borodin et al / Voidformation upon annealingof shapedsapphirecrystals

A large number of mass-produced,as-growncrystals (5—15 mm diametersapphiretubes) wasfound to contain second-phaseinclusionsmeasur-

,-,/,, ing from fractionsof a micron to severalmicronsin extent. The inclusions themselvesdid not

~ i ~ / t noticeably affect the optical quality of the as-- / 260 grown crystals. However, a subsequenthigh tern-

2/ peraturevacuumannealled to an increasein the

/ I concentrationand thegrowth in the sizes of these~~ / 230 inclusionsup to a few microns,which dramatically/ increasedthe light scatteringof the sample [21.

The densityof the inclusionswasthe highestalong204 grain boundaries.Unlike the inclusion-free sam-

I ples, the inclusion-containingcrystalsreadily frac-200 220 240 260 280 300 tured along the decoratedboundaries.The sam-

X (nm) ples also failed during testsfor dielectricstrength

Fig. 3. Optical transmissionspectraof sapphire: 1, Verneuil- at voltagesof 20—25 kV/mm, with the electricalgrownsapphire;2, shapedsapphire,with no carbonusedin the dischargechannelalwaystraversingalongthe grainheatingunit; 3, shapedsapphire,with carbonusedin heating boundary[31.

unit; 4, specimen3 afterannealing.Investigationsof samplesby scanningelectron

microscopy (X-ray mapping images)and Augerof thesecrystals(fig. 3) revealedahigh concentra- spectroscopyrevealedthe presenceof carbon ontion of oxygenvacancies(to bemoreexact,F- and cleavageplanespassingalong the decoratedgrainF ~-centers which are oxygen vacancieshaving boundaries[3]. Undoubtedly,the graphitecompo-trappedtwo or oneelectrons,respectively),giving nentsof the growth chamberthermalunit are therise to the 204, 230 and 260 nm absorptionbands carbon source, and the basic way to minimizein the crystalsbeforeannealing.Subsequenthigh- their effect on the quality of the crystals is totemperaturevacuumannealingresultsin anabrupt decreasethe water vapor and oxygen contents,decreaseof the vacancyconcentration,along with which actively interact with the graphite, in thethe facetedvoid formation.It turns out, however, argonatmosphereof thechamber[4,5].that the volume of the voids formedupon anneal- Despite the fact that some of the chemicaling is several times greater than that calculated elementsforming the inclusionswere determinedfrom thevacancyconcentration.The disagreement in the annealedcrystals,attemptsto identify themay be explainedby the fact that not all individ- crystalline structureof the inclusions by X-rayual vacanciesavailable in the crystal form elec- phasemethodswere unsuccessful.According totronic color centers,and, in addition, thereexist the known phase diagram of the A1201—A14C3additional sources of vacancies, such as small system [5], there is no region of carbon solidvoids, which dissolveupon annealingand do not solutionin aluminumoxide; anincreaseof carbonmanifestthemselvesin the spectrabefore anneal- concentration gives rise to oxycarbidesAl 404Cing. and A12OC, and aluminumcarbideA14C3 in the

The heattreatmentof sapphirecrystals grown solidphase.It hasalso beenfound that aeutecticin a reducingatmosphereinducesformation of a exists in this systemwith the melting temperaturesecondtype of void. Theseare inclusionsof irreg- of 18400C [6]. This might suggest [4,5] thatular shape,which turn out to be poresprimarily carbon-containinginclusionsin sapphiretubesarelocated along grain boundaries; that is, the oxycarbidesAl 404C and Al 20C or, less likely,boundariesbecomedecorated.The main purpose aluminum carbide A14C3. However, an Auger-of the presentwork was to investigate this phe- electronspectroscopystudy of cleavageplanesofnomenon. annealedsampleshasshownthat the carbonline-

V.A. Borodin et al / Voidformation upon annealingofshapedsapphirecrystals 159

shape correspondsto graphite or to adsorbed aoxygen compounds of carbon rather than tooxycarbides.This castsdoubt on the oxycarbide •nature of the precipitatesobservablein crystals •after annealing.The compositionof the inclusions .. ~.

was not determinedbefore the anneal, because ~ • ~ — - -

therewaspracticallyno decorationandthecrystals -~ ‘~

could not becleavedacrossthe grain boundaries. -~ ~ •~~.~$jvTherefore,the questionconcerningthe nature . • ~! ~

of the carbon-containingsecondphase,which oc- ;~‘~

cursin crystalsafter annealing,and that concern- . - eing the compositionof initial precipitatesbefore • ~.• * ‘ . 20 pmannealing remain unanswered.Earlier, we had *

studied the composition of gasesevolving upon .. ~• ..•~. •e,cleaving a shapedsapphirecrystal with carbon ~ .. ~ -.~-

containing inclusions and found that the gases .•• —..,~ . — t, ~‘.. ~ •.

contain 02, CO, C02, Ar, H20 and N2, with the • . . (•~~~• ç,

nitrogen content being much higher than thecarbon monoxide content [7]. These results, atfirst glance, disagree with those of refs. [2—5],accordingto which one shouldexpect thatcarbon ~oxidesratherthannitrogenwill be predominantinthe massspectra.We have, therefore,carriedoutadditional studiesand attemptedto find a con- ~sistent explanationof the available experimental 20 pmfacts. •

Fig. 4. Inclusions in sapphire tubesgrown in chamberswithgraphitesusceptorsbefore(a)andafter(b) annealing.

2. Experimental technique and results

Sapphiretubeswith an outerdiameterof 9 mm of scanning electron microscopy (SEM) andandwall thicknessesof 0.8—0.9mm weregrown in Auger-electronspectroscopy.an argon atmosphereby the techniquedescribed SEM observationhas shown that the cleavagein ref. [5] using a device comprising a graphite surfacealong the decoratedgrain boundaryhasasusceptor.As a result of a deliberatedeparture complicatedrelief structure.Fig. 5a shows a partfrom the technologicalregimefor growing crystals of the cleavagesurface; the contrast signal re-(e.g., ref. [1]), we obtained sapphiretubes with corded along one scanningline (fig. 5b) char-carbon-containing second-phaseinclusions (fig. acterizesthe surfacerelief. Figs. 6a and 6b show4a)anda developedgrain boundarystructure.The mirror-symmetrycleavagesalonga decoratedgrainsampleswere annealedfor 4 h at 1850°C in a boundary. Figs. 5 and 6 demonstratethat, aftervacuum of 5 X iO~ Torr in a furnace with a annealing, the boundary is decoratedby pores,tungstenresistiveheater.The annealingled to a andthat the relationshipbetweenthe areasof thelargeincreasein the concentrationandsizeof the internal surfacesof pores and thoseof cleavageinclusions(fig. 4b). Ringsof 0.5—1.2mm thickness planesof the crystalmatrix dependson the degreewere cut from the annealedsamplesand divided of decoration.into 3 to 4 segments.The segmentswerecleaved The cleavagesurface was studied by Auger-along the inclusion-decoratedgrain boundaries, electron spectroscopyusing the electron spec-and the cleavagesurfaceswere studiedby means trometer “Escalab5.” The sampleswere cleaved

160 V.A. Borodin et al / Voidformation upon annealingof shapedsapphirecrystals

~27~

2l5c

1~ ~ % ~° ~Ar ___

AlI I I I________ _____ 0 200 400 600 800________________________________________________ E (eV)

j Fig. 7. Typical Auger-electronspectraof cleavagesurfacesalonga decoratedgrainboundary:(a) spectrumof anas-grownsample;(b) spectrumafter argon-ion etchingof an as-grownsample; (c) spectrumof an a~d(2 years) sample without

Fig. 5. (a) SFM image of the cleavagealonga decoratedgrain by a specialdevicedirectly in the analyticalcham-boundary;(b) surfacereliefalongonescanningline. ber of the spectrometerin an ultra-high vacuum

(10~to 10~0 Torr). In order to minimize theeffect of surfacechargingunder the action of theelectronbeam, a thin silver or indium layer wassputteredon the samples before cleaving them.After cleaving along the grain boundaries,the

~ unsputteredcleavage surfaces were studied bymeansof a defocussedbeamof 200—300 ~tm di-

ameter.The primarybeamenergy was3 keV, thebeamcurrentwas0.1—0.31~A,and themodulation

energywas4 eV.Investigationsof samplesof identical size re-

vealed the following common features: sampleswith a high concentrationof inclusions cleavedeasily along the grain boundariesand had highcarboncontentwith respectto oxygen; and sam-

10 pm ples with low concentrationsof inclusionshad a.1 smallercarboncontent.Fig. 7 showstypical Auger

Fig. 6. Mirror-symmetrical cleavagesurfaces(a and b) of spectraof the cleavagesurfacesin sampleswith asapphirealonga decoratedgrainboundary. high concentrationof inclusions. Along with the

V.A. Borodinet al / Voidformation upon annealingof shapedsapphirecrystals 161

principal oxygen (510 eV) and aluminum (53 eV the crystals grown from the carbon-contaminatedand 68 eV) lines, one can observe an intense melt. The pores are mainly formed along graincarbon line (272 eV), whose form, judging from boundariesand weaken them. This explains thethe data of ref. [8], correspondsnot to carbide easycleavageand the electrical breakdownalongcompoundsbut to either graphite or adsorbed the decoratedboundaries.The absence(or rathercarbon—oxygencompounds. small concentration)of suchporesbeforeanneal-

Thespectrumin fig. 7awasrecordedonemonth ing, their appearanceas a result of annealing,andafter the growth and annealingof the sapphire the mass-spectralanalysis data, which show atube sample.In the low-energyregion, one can somewhathigher concentrationof CO in sapphireobserveone peak of aluminum (68 eV), while tubes with carbon-containinginclusions, are allanotherpeakat 53 eV may also be present.It indicationsthat annealingof crystalsgrown fromshould, however,be noted that the 68 eV Auger the carbon-contaminatedmelt is followed by reac-peakof aluminumis predominantin sampleswith tionsinvolving aluminumoxide reductionand thea high content of inclusions and a high carbon formation of carbonmonoxide.content.In sampleswith low carboncontent, the The reduction of aluminum oxide has been53 eV peak,correspondingto aluminumin Al203, consideredby otherresearchers,but the results areis morepronounced.Apparently, the 68 eV Auger contradictory.Thecomplexityof the analysisarisespeak correspondsto partially reduced (to sub- from the fact that the reduction is progressingoxides)aluminum oxide. Argon-ion etchingof the along with the formation of intermediatecorn-surfaceexaminedin fig. 7a has shown that, at a pounds, i.e., suboxides,oxycarbides,and alumi-depthof 50—100 A, the carbonconcentrationde- num carbides, and the reaction kinetics arecreasesdrastically, and, in the low-energyregion governedby factors suchas temperature,pressure,of the spectrum, the 53 eV peak of aluminum, concentrationof the components,conditions ofcorresponding to the transition in the normal transportof the reactantsto the interactionzone,aluminumoxide, begins to dominate(seefig. 7b). and conditionsof removal of the reaction prod-A 215 eV line is due to argonimplanted into the ucts.Onemay assumethat, upon annealingof thesamplesurfaceas a result of the bombardment, carbon-contaminatedsapphire,the following reac-

Finally, some samples cut from the already tions should takeplace:analyzed (and annealed)sapphire tubes were Al ~ + Al ~ C —~ Al 0 + COstudiedagain two years later. It turned out that 2 3 4 4 2 3x

the spectrum (fig. 7c) from a freshly cleaved (ref. [9]), (1)surfaceof such an aged samplechanged funda- 2 A12O3+ 3 C = A14O4C + 2 COmentallyfrom the original spectrumin fig. 7a.Thecarbon concentration on the surface decreased (ref. [10]), (2)sharply,and,in addition to the 68 eV peakwhich 3 Al 203 + C = 2 Al 304 + COarisesfrom partly reducedaluminum oxide, thereappearedan intensivepeakof aluminum at 53 eV, (ref. [11]), (3)characteristicof normalA12O3. A1203+ 2 C = A120 + 2 CO

(ref. [12]). (4)

For the reactionin eq. (1) to occur,it is necessary3. Discussion that aluminum tetraoxycarbide(Al 404C) should

form at the crystal growth stage(in accordanceThe data obtained from electron microscopy with the phasediagram).For the restof the reac-

andoptical transmissionmicroscopystudiesof the tions to takeplace, it is necessarythat free carboncleavagesurfacesunambiguouslyindicatethat the shouldbe presentin the grown crystal. Therearecarbon-containinginclusions in annealedcrystals two nonconflicting possibilities: namely, the ex-are a specifickind of poresformedupon annealof istenceof carbonoversaturatedsolid solution in

162 V.A. Borodin et at / Void formation upon annealingofshapedsapphirecrystals

aluminumoxide or carbonenrichmentof the grain aluminum on the cleavagesurface(68 eV line inboundaries during the crystal growth process. fig. 7a) may be attributed to both the con-According to ref. [5], carbon is insoluble in solid densation of suboxides and the interaction ofaluminumoxide. However, it canbe assumedthat aluminum oxide with carbon. Argon-ion etchingweak solubility of carbondoesexist, sincecarbon of the cleavagesurfacemade it possible to esti-was found even on cleavageplanesof optically mate (from the drastic decreaseof the carbontransparentsapphiretubeswhich do not contain peakin the courseof etching andfrom the changeinclusionsvisible in an optical microscope.Unlike of the relationshipbetweenthe 53 eV and 68 eVthe inclusion-containing tubes, these samples lines) the condensedlayer thicknessto be 50—100cleavedaway from the grainboundaries. A (fig. 7b). A comparisonof figs. 7a and7c shows

During crystalgrowth,a grain boundaryemerg- that a two-year aging leadsto a considerablede-ing at the crystallizationfront is an effective site creaseof the carbonpeak and the appearanceoffor entrapmentof gaseousimpurities. With carbon the additional 53 eV line of aluminum. This canpresentin the melt, the principal impurities are probably be explainedby desorptionand diffu-A120, A1O, CO and CO2 [11]. Aluminum sub- sion of gaseouscarbon oxides from the sampleoxides are strongreducingagents.Therefore,one (the sample thickness is 0.5—1.2 mm) into thecannotexcludethe possibility that carbonoxide is atmosphere.The diffusion proceedseasily via thereducedto carbonalreadyin the solid phase[11], pore-softenedgrain boundariesemergingon thewhich may lead to carbon-enrichmentof the samplesurface.The appearanceof the 53 eV line,boundary. Inasmuchas Auger spectroscopydid pertaining to aluminum in aluminum oxide, maynot reveal the presenceof oxycarbidesin the an- be explainedby a gradual oxidation of the re-nealed samples,the occurrenceof the reaction in duced aluminum oxide and condensedsuboxideseq. (2) seemsunlikely, to the stoichiometriccomposition.

Theavailabledatado not disallowthe possibil- Next, we considerthe fact that, during anneal-ity of formationof an A1304 (AlO .A12O3) spinel ing of the crystals grown from carbon-con-accordingto the reaction in eq. (3), that is, the taminated melts, two processesare taking placeproductof aluminumoxide reduction.Possible,as simultaneously,i.e., the formation of poreswhichwell, is the occurrenceof thereactionin eq.(4) for decoratethe grain boundaries,and the formationwhich the chief featureis that both reactionprod- of voids, due to vacancycoagulation[1], faceteducts are gasesat elevatedtemperatures.However, by f0001 }, {1010} and/or f1120} planes.with decreasingtemperatures(for example,upon It hasbeenshown that an excessiveconcentra-

cooling of the grown or annealedcrystal), the tion of oxygen vacanciesis formed in sapphirereaction in eq. (4) is reversed[11]. In a growing crystals upon growing them in a reducingatmo-crystal, the interaction zonesof aluminum sub- sphere [11. However, the vacancyconcentrationoxides and carbonoxides may be not only grain calculatedfrom the absorptionspectraturnedoutboundaries,but also gaseousinclusionstrappedby to be severaltimes lower than the concentrationthe crystallizationfront during the crystalgrowth. which could provide,on condensation,the experi-In this respect,the assumptionsin ref. [12] appear mentally observabletotal volumeof vacancyvoids.to be correct, concerningthe existenceof a con- We may thereforeassertthat additionalsourcesofdensate including aluminum suboxides and re- oxygenvacanciesare the reducingreactionsin eqs.ducedcarbonon the porewalls in sapphire. (1), (3) and (4) occurring in crystalsupon anneal-

The form of the Auger-spectracarbonline (fig. ing.7) doesnot answerthe questionwhether reduced It shouldbe notedthat the presenceof oxygencarbonor its adsorbedoxides are presentin the vacanciesand their disappearanceduring anneal-condensate.Taking into accountthe above-men- ing was found to occur in practically all crystalstioned possibility of the reactionsin eqs. (1), (3) grown in chamberswith graphitesusceptors,in-and (4), one may assumethat both components eluding rather perfect samples which exhibitedare present. The presenceof partly reduced neither decoratedboundariesnor vacancy voids

V.A. Borodinet at / Voidformation upon annealingofshapedsapphirecrystals 163

after annealing.This indicatesa significantcontri- ble and die are fabricated (and which readilybution of annealing-generatedvacanciesto the dissolvesnitrogen),and,possibly,the atmosphericformationof vacancyvoidswhich are presentonly nitrogen.in crystalswith decoratedgrain boundaries.

We feel that our results are applicable toanotherproblem as well. Ref. [14] hasreported a 4. Conclusionsmethodof reversibledecorationof dislocationsinVerneuil-grown ruby and sapphirecrystals. The Annealing of shapedsapphirecrystals growndislocationswere decoratedas a result of anneal- from a carbon-contaminatedmelt gives rise to twoing in a furnacewith graphiteheatersin an argon typesof voids. The grainboundariesare decoratedatmosphereor in a low vacuum, that is in a CO by voids formed as a result of the evolution ofatmosphere.The decoration disappearedupon carbon oxide gases which are the product ofvacuum annealing in a furnace with tungsten aluminum oxide reductionby carbon.Thesereac-heaters.As for the decorationmechanism,it re- tions generateoxygen vacanciesin addition tomainedunclear; a possiblemechanismproposed those formed during crystalgrowth in the reduc-in ref. [14] was an alteration of the impurity ing atmosphere.The vacancy generationoccursvalencestateupon annealingin a reducingatmo- simultaneouslywith the processof vacancycon-sphere. densation, followed by the formation of flat,

We believe it is possibleto provide another faceted voids (200—300 A thick) in the (0001)explanationto the phenomenonof reversiblede- planeof sapphire.The formationof thesedefectscorationof corundum.Annealingof thin crystal- upon annealingof shapedsapphirecrystals mayline platelets(i.e., sampleswith a large specific be preventedby suppressingthe interaction be-surface) in a reducingatmosphereleads to the tweenaluminum oxide and carbon.surfacereductionof sapphireby carbonoxidesor,in other words, to the generation of oxygenvacancies. The presenceof excessive,nonequi- Acknowledgementslibrium concentrationsof vacancies and highvacancymobility leadsto condensationof vacan- The authors wish to expresstheir warm grati-cies into smallpores, in general,andmorespecifi- tude to V.A. Tatarchenkofor his attentionto thiscally into pores along dislocationlines. Vacuum work and the helpful discussionof the results.annealing,naturally, results in the dissipationofthe decoratinginclusions. It shouldbe noted thatwe also observed,in severalsamples,the decora- Referencestion of dislocationsalong with the formation ofvacancyvoids, but their formationconditionswere [1] TN. Yalovets and V.A. Borodin, Bull. Acad. Sci. USSR,

not identical Ser.Inorg. Mater. 24 (1988) 946.[2] V.A. Borodin, TA. Steriopolo, V.A. Tatarchenkoand

The results obtainedin this work also explain TN. Yalovets,Bull. Acad. Sci. USSR, Phys.Ser 47(1983)

the specific featuresof the compositionof gaseous 368.inclusions which were earlier investigated by a [3] V.A. Borodin, AM. Ionov, V.A. Tatarchenkoand TN.mass-spectrometrymethod [7]. It is clear that the Yalovets,Crystal Res.Technol. 19 (1984) K90.

absenceof high CO concentrationsin the mass [4] V.A. Borodin, TA. Steriopolo, V.A. Tatarchenko,LI.Chernyshova,TN. Yalovets andAM. Ionov,Bull. Acad.

spectrais due to the fact that CO (in annealed Sci. USSR, Ser.lnorg. Mater. 21(1985)1352.

crystals)residesin poresin the adsorbedstate.A [5] V.A. Borodin, TA. Steriopolo, V.A. Tatarchenkoand

higherconcentrationof nitrogenin the spectra,as TN. Yalovets, in: Growth of Crystals, Vol. 15 (Nauka,

comparedwith CO, is explainedby its higher level Moscow, 1986)(in Russian).[6] L.M. Foster, G. Long and MS. Hunter J. Am. Ceramof inertnesswith respectto aluminum oxide (i.e., Soc 39 (1956) 1 ,

low sticking factor). The nitrogensourceis, most [7] TN. Yalovets,V.A. Borodin et al., Bull. Acad.Sci. USSR,probably, the molybdenumfrom which the cruci- Ser. Inorg. Mater. 21(1985) 262.

164 V.A. Borodinet al / Void formation upon annealingof shapedsapphirecrystals

[8] A. Ioshi, L. Davis and P. Palmberg, in: Methods of 111] G.V. Samsonov,Ed., Interaction Between Carbon andSurfaceAnalysis, Ed. A.W. Czanderna(Elsevier,Amster- RefractoryMetals (Metallurgia,Moscow. 1977) (in Rus-dam, 1975). sian).

[9] V.G. Yelutin, Yu. A. Pavlov, V.P. Polyakov and S.V. [12] Kh. S. Bagdasarovet a!., Soviet Phys.-Cryst. 32 (1987)Sheboldayev,in: InteractionbetweenMetal Oxides and 467.Carbon(Metallurgia,Moscow, 1976) (in Russian). [13] AG. Vodopyanov,G.N. KozhevnikovandR.G.Zakharov,

[10] G.N. Kozhevnikovand AG. Vodopyanov,in: Silicon and Bull. Acad. Sci. USSR, Ser. Metals4 (1978) 12.Aluminum Suboxides in Electrometallurgy (Nauka, [14] MO. Kliya andMA. Chernyshova,Soviet Phys.-Cryst.11Moscow, 1977)(in Russian). (1966) 656.