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The melt growth of oxide and related single crystals
ofC~yst~6to~t~ ~l(1~7?)~13—~26 ~No~t~•l~ollü.n~ ?~lis~~ C~i~’
THE MELT GROWTH OF OXIDE AND RELATED SINGLE CRYSTALS B. COCKAYNE Royal Signals- and RadarEstablishment, St. Andrews Road, Great Malvern, Worcestershire WRJ4 3PS, England
This article reviews the principle changes which have occurred during the last three years in the techniques used to produce single crystals of oxide and similar single crystals from the melt. The technological demands created by device applications are emphasised and the current state of knowledge upon the major crystalline defects is discussed. The role of accurate phase diagram analysis, the importance now attached to the atmosphere in which a crystal grows and the relevance of looking at crystals with greater resolution are stressed particularly.
I. Introduction
Table 1 Application
An important consequence of the demonstration
of maser and laser action in oxide and similar transparent single crystals nearly two decades ago has been the development of a high temperature crystal growth technology for such materials. During the last decade,
Solid state lasers
this technology has been transformed from a smallscale research exercise into an industrial process as materials with improved properties, produced more ~t’nnnmj~I1ij than hitherto, have been demanded in
Substrates for magnetic mates-ials Surface acoustic
Insulating sub-
Examples of material used 3~,LiY 3~, Y 3Al5O12—Nd 05Er05—Tm Ho3~ La 3~ 2Be2O5—Nd CaLa 3~ 4(SiO4)3O—Nd Al 203 Gd3Ga5Oj2
LiNbO3
414
B. Cockayne /Melt growth of oxide and related single crystals
Table 2 Technique
Principal characteristics
Applications
Czochralski
Crystal rotation and lift Stationary crucible, rf or resistance heating Large melt volume Moderate temperature gradients Crystal unconfined Large crystal sections
Most oxides and fluorides ph few sulphides where liquid er sulation is applicable
Bridgman—Stockbarger
Crucible lowering No rotation Resistance or RF heating Large melt volume Low temperature gradients Crystal confined Large crystal sections
Predominant use is for halide~ plus a few oxides
Edge-defined film fed growth
Crystal lift No rotation Crucible stationary RF heating mainly Small melt volume but large melt reservoir High temperature gradients Crystal unconfined but shaped Small crystal sections
Mainly oxides plus LiF
Travelling zone
Moving melt, no crucible Crystal rotation optional Optical or immersed strip heating Small melt volume
Chiefly oxides
I , eOC~ylle/ ~e1t 8To~1hof o~i~e ~Te1~1edsi~gleeTy81~l8
2. Re~ie~ofteth~~es 2.1. Czochralski This process has continued to be the most adaptable to the problems associated with the growth of optically transparent crystals. It is employed over a wider span of materials and melting points than any other melt-growth method, ranging from mixed cornpounds such as nitrates (NaNO2, M.pt 271°C), through germanates (Pb5Ge3O1 1 M~~t 738°C),fluorides (LiYF4, M.pt 838°C),molybdates (PbMoO4, M.pt 1050°C), niobates (LiNbO3, M.pt 1260°C), tungstates (CaWO4, M.pt 1570°C), tantalates (LiTaO3, M.pt 1650°C), garnets (Y3A15012, M.pt 1970°C) and spinels (MgAl2O4, M.pt 2105°C), to single oxides such as sapphire (m.pt 2050°C) and yttria (M.pt 2410°C). It is the technique used cornmercially to produce all the major oxide device materials such as LiNbO3, LiTaO3, Gd3Ga5O12, Y3A15012 and A1203 in both pure and doped forms as well as more speculative materials such as YA1O3 and La2Be2O5. The size of crystals has increased markedly; 5 cm diameter X 30 cm long crystals of the device materials are now common-place and, in some instances, diameters up to 10 cm can be obtained. The weights of
~15
~ ~ frü~~~ of ~ff~ts i~1udiflg(i)
the reflection of a laser beam verficatly ~nc~clenfupon the melt surface which becomes displaced when intercepted by the meniscus [5], (ii) the signal from an infra-red television camera viewing the crystal directly [6], (iii) crystal weight [7,81and, (iv) crucible weight [9—13]. Weight sensing appears to be the technique most generally adopted but variations in approach are apparent. Both transducer-based mechanisms and linear variable differential transformers have been used to measure weight and a choice also exists between coupling the weighing cell directly to the crystal/crucible or through an amplifying beam mechanism. All these methods have proved satisfactory but crucible weighing seems to have been applied most generally. The inherent disadvantages of weighing the crucible, namely, levitation effects, decreased sensitivity due to a greater total mass and changes in weight during growth due to refractory degradation and evaporative loss from the melt, have to be offset against the difficulties in weighing the crystal which arise from the maintenance of a friction-free rotational and translational system and possible buoyancy effects due to convective flow in the gas ambient. Automatic diametercontrol has obvious economic attractions for large crystals produced routinely on a commercial basis but the reproducibil-
B. Cockayne /Melt growth of oxide and related single crystals
416 Table 3 Compound
CaWO
Time (see)
441 LiF 720
Compound Time (sec) times cibles The
taken for a
Ba2NaNb5O15 495
4
YA1O3
Gd3Ga5Oj2 480
450
LiEs-0 5Y0 5F4 810
rise or fall in temperature at the crucible wall to be 96% translated
ventional iridium crucibles. A significant parameter in any crystal growth systern, but particularly in the Czochralski process, is the temperature distribution within the total thermal environment. Detailed data upon the temperature distribution of particular systems are tedious to obtain, technically difficult to measure in some instances and are rarely quoted. A possible means of characterising a system and investigating changes therein is to use the data provided by an infra-red viewing system, as illustrated in fig. 1 [161. Thermograms of this type provide a qualitative picture of heat distribution over th’e exterior of crystal growth systems so that areas of excessive loss or non-uniform heat distribution can be identified. Further information can be gained by the examination of transparent model systems where the isotherm patterns, typified by the white lines in fig. 1, can be used to judge the external effect of making —
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—
.
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to the melt centre in 5 cm diameter cru-
applicability of this technique to oxides has been emphasised by the production of Y3A15012, Lu3A15O12 and garnets of mixed composition using molybdenum crucibles in a resistance-heated tungsten furnace [21]. A particular difficulty encountered in the crystallisation of fluorides is how to limit the evaporation of those materials which have a high vapour pressure at their melting point. A sealed crucible can be used if the material is compatible with silica or platinum but many fluorides adhere to metal crucibles, causing crystal cracking. A variation which uses traditional high purity graphite crucibles has been reported [22], in which the seal is formed by the evaporating fluóride itself. This method conserves the crucible, retains the non-wetting characteristics of carbon with fluorides, permits high temperature use and aids the volatiisation of impurities as the material does not ,
.
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Fig. 1. Thermograms of the afterheater in a model Czochralski system constructed out of silica and placed above a 3.8 cm diameter Pt crucible contained in MgO refractory and heated to 1000°C. (a) and (b) delineate the relative changes in the heat emission pattern and isotherms (white lines) obtained by inserting a silica baffle at AA. (c) shows typical isotherm patterns in the gas surrounding the afterheater; fluctuations in these patterns around the viewing port (not shown) and the hole through which the seed is inserted give some indication of the violent convective flow present in these regions. An AGA Thermovision was used to record
the thermograms [161.
shown that the sensitivity of interface position to thermal conditions increases as the heat transfer coefficients between crucible and surroundings decrease, as the diameter of the crucible decreases and as the thermal conductivity of the crucible material is lowered. For minimum sensitivity, the thermal con~-b,~’f4,i~f~, t-,f fhe t~ni.~jh1e shrnild be large for materials
[26,27]; this confirms the prediction [28] that convection within the capillary channel of the die is so curtailed that solutes segregating at the interface can only return to the crucible with difficulty, by diffusion against the fluid flow, hence the impurity concentration in the solid, C~,must approach that in the bulk liquid, Cj,, so that Cs/CL = keff =
418
B. Cockayne / Meltgrowth of oxide and related single crystals
2.4. Travelling zone techniques Vertically moving molten zones, with fusion produced by optical techniques, have been used for producing small research crystals of high melting point materials for many years but few significant advances have been recorded. The interest once shown in gas lasers as a heat source has largely disappeared. Resistance heated strips immersed within the molten zone have, however, stimulated greater interest. Some five years ago it was shown that a platinum strip, immersed in a CaCO3—Li2CO3 eutectic solvent zone, could be moved through a CaCO3 feed rod to yield single crystals of calcite [30,31]. Further improvements have been reported recently [32] which allow crystals up to 2.5 cm diameter x 3.0 cm long to be obtained. The previously troublesome problem of bubble entrapment has been eliminated by using crystals rather than sintered powder as the feed material and a homogeneous temperature distribution has been achieved over the total area of the strip by locally increasing the current density with selectively placed holes. 2.5. Crystaiisation within static systems Methods in which growth is induced within a static system by some form of heat exchange either at the
sions of 18 cm diameter X 13 cm thick and weighing 15 kg can be grown. Heat exchangers applied to the base of a crucible containing a melt have also been applied to Czochralski growth to stabiise the thermal flow characteristics and to aid diameter control by modifying the interface shape [36]. A modified Stober technique has been employed in the production of EuO crystals enriched with 1 by slowly cooling non-stoichiometric Eu1 170 melts contained in a sealed molybdenum crucible [37]. A further static melt technique, namely electric arc fusion, is still used for the commercial production of large polycrystals of alkaline-earth oxides [38], the grain size being sufficiently large for single crystals with centimetric dimensions to be selected. 2.6. The Vemeuil process This flame-fusion process is the traditional method for oxide crystallisation. It is widely employed for small sapphires and rubies and has been extended to the growth of materials such as MgAl2O4 [39], Y203—RF3 mixtures (R = lanthanide ion) [40], and CoA12O4. A number of useful stress analyses have been made of crystals grown in this process [39,41], establishing the relative importance of bulk, surface and localised stresses. However, the process is not
/
419
B. Cockayne Melt growth ofoxide and related single crystals erally recognised to be any which can absorb, reflect, refract or scatter magnetic, optical, acoustic and electrical energy either generated within or incident upon the material. Thus, colour centres, facets, striations, dislocation low-angle boundaries, twins, individual
selected which inhibited both dislocation and twin generation. Dislocations in garnet crystals offer some intriguing problems. They are normally absent from meltgrown Y3A15012 crystals [44], but have been ob-
dislocations with large strain fields, voids, cellular structures, precipitates, inclusions and more destructive defects such as cracks are all important defects which have to be eliminated or controlled. Most of these defects were recognised in the early solid state laser hosts typified by ruby, calcium tungstate and Y3A15012. Detailed information upon the nature, cause and control of many of these defects can be found in the Proceedings of ICCG 1 and 2. As the number of materials investigated has increased it has become apparent that a substantial number of these defects can occur in most transparent melt-grown crystals. Thus, much of the recent literature can be considered as a catalogue of the discovery of similar defects in an increasingly wide range of compounds. The high-level of activity in this field has, however, allowed progress to be made in both the understanding and relevance of particular defects as described below.
served when inclusions are present [45,46]. Dislocations are more readily generated in Gd3Ga5012 crystals where helical defects of an interstitial character up to 300 pm in diameter and several cms length have been seen by both optical and X-ray topographic techniques [47,48]. Some recent work [49] suggests that in the absence of dislocations caused by inclusions in YAG, a marked connection exists between dislocation formation and interface shape. Crystals of undoped YAG grown with an interface which is concave towards the melt show dislocation structures which are very similar to those in Gd3Ga5012. Y3A15012 used for lasers is normally grown with a convex interface whilst Gd3Ga5012 used for substrates requires a planar interface which can easily become concave, hence the concave interface in Y3A15012 had to be deliberately induced for these experiments. The similarity between these gamets confirms the general occurrence of particular defects throughout the spectrum of melt-grown gas-nets provided that comparable interface morphologies are induced. The mechanical properties and slip or twinning systems available for deformation have not been measured for many current refractory materials, particularly at high temperatures, so, interpretation of dislocation structures in terms of the stresses present during growth is largely qualitative. Even when information is available, difficulties can ensue because dislocations can be generated by a multiplicity of mech-
3.1. Dislocations Dislocation structures in oxides etc have been widely studied using chemical etching procedures to reveal dislocations as individual pits. Such procedures can be misleading as great care is needed to distinguish dislocation etch pits from etch pits caused by inclusion or surface damage. X-ray topography has provided significant information but a useful innovation has been the application of electron microscopy ‘
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B. Cockayne /Melt growth of oxide and related single crystals
3.2. Facets The observation that facets can form on the solid— liquid interface ofmelt-grown compounds is now well recognised but the strain associated with the facets in oxides is not so well understood. Most of the efforts attempting to understand the phenomenon have been concerned with garnets in comparative studies between Y3A15012 and Gd3Ga5012 where the mean lattice parameter of the faceted regions is slightly larger than that of the unfaceted regions (&z/a 1 .3 X I0~) [53]. Three principal sources of the strain have been proposed (a), changes in cation ratio between facet and matrix (b), impurity segregation and (c) oxygen segregation, with the latter considered to be most likely because the similarity in the magnitude of the strain in two separate materials implies a common mechanism [53]. In a recent review [54] it has been suggested that facet strain due to oxygen segregation is consistent with the entrapment of oxygen vacancies on the facets due to the high lateral growth rates involved in facet formation and the low diffusion constant of oxygen with respect to the cations present. This view is supported by luminescence studies which show that faceted regions contam higher concentrations of electron traps than the matrix [54]. Facets have also been seen in Stockbarger-grown garnets [21]. In this case, the low temperature gradients give an approximately planar solid—liquid interface except near to the crucible wall where the interface becomes slightly convex towards the melt and it is here that the facets occur. This distribution and form is very similar to that detected in Czochralski grown garnets under approximately olanar interfacial growth conditinns
mainly from the hot liquid from the crucible base and walls rising to the surface and displacing the cooler liquid at the melt surface. The convectional flow is complicated by crystal rotation and is very dependent upon factors such as melt depth and temperature gradient in addition to crystal and crucible dimensions and material parameters. Many useful reviews discussing these factors are available [55—58]. In only a very few cases have direct 1 : I relationships been established between striations and fluctuations in temperature within melts of high melting point materials, although qualitative and semi-quantitative relationships are more numerous. The nature of the segregation causing the compositional change and associated strain across a striation is often assumed rather than investigated. Some estimate of the strain has been made in garnets used for substrates [54] and a typical value is 5 X lO~but in most applications the level of strain due to striations has not constituted a sufficient problem thus far to prompt a very detailed study. Striations can be eliminated or reduced in a number of ways. In pure cornpounds, growth at the congruently melting composition is an obvious solution but with deliberately doped or slightly impure materials the lowest temperature gradients and smallest melt depths compatible with crystal diameter control and the avoidance of other defects must be employed in order to limit convection. It is worth noting that striations do occur in Stockbarger grown crystals [19] where the liquid is coolest at the crucible base and convectional instabilities associated with temperature differences should be absent. Such striations could be attributed to convection from other sources such as surface tension ~
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B. Cockayne /Melt growth of oxide and related single crystals
in a liquid—solid tr~nsition~nd the e~lut!ollof~s~s
3.4. Voids Voids (cavities) can form in most melt-grown compounds regardless of the basic compositional type i.e. oxide, fluoride etc. They were observed extensively in early Czochralski crystals of laser hosts such as calcium tungstate [63,64], Y3A15012 and alumina [65,66] and were associated with the segregation of dopant impurity, gaseous impurity or basic constituents. In a fully developed state, they can adopt the cellular morphology associated with constitutionally supercooled growth. Some voids have been presumed to form by the entrapment of impurity-rich liquid in a cell-boundary groove or solute trail which contracts on freezing to form a cavity (mechanism I). The entrapment of liquid due to thermal supercooling is a similar phenomenon but thermal supercooling is genes-ally only dominant at high growth rates as typified by those used in the EFG process. Other modes of void formation can be considered. For instance, the segregation of gaseous impurity directly (mechanism II), the entrapment of an escaping volatile impurity (mechanism III), the capture of gas bubbles displaced from the melt (mechanism IV), condensation of vacancies (mechanism V) and solidification with a limited supply of liquid akin to the piping effect observed in metal castings (mechanism VI). VI is only likely to apply to the terminal stages of Stockbarger solidification or to methods such as EFG if the liquid supply becomes limited. V is unlikely because of the low equilibrium vacancy concentration 4 atathe verymelting rapid point most materials, i0 to retain even coolingofprocedure would typically be necessary this concentration under which conditions most insulator single crystals would crack. A recent -
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which have been absorbed or trapped during melt preparation from component oxides. Similar criteria apply to related compounds such as fluorides. 3.5. Colour centres
Colour centres in oxide-based crystals constitute a complex problem and have been extensively studied in terms of their creation by ultra violet or ‘y-irradiation. The general character of the room temperature absorption which is symptomatic of colour centre formation is a broad featureless band extending approximately from 0.3 to 0.7 pm; this imparts a brown coloration to the crystals. The occurrence of colour centres and the extent and character of the absorption produced is well documented but substantially less is known about their cause. The problem is complex as some centres can be removed by annealing in oxidising atmospheres [69], others by annealing under reducing or vacuum conditions [70,71]. In LaA1O3, colour centres have been attributed to a positive ion vacancy—hole combination [72] due mainly to the presence of divalent impurities or vacancies on the trivalent rare-earth site. Generally, the impurity ion remains unidentified, although the importance of Fe in some Nd-doped YA1O3 single crystals has been recognised [73]. Frequently, it is the low-level of impurity in the single crystals which leads to the ambiguity because therelative preciseamounts level of of total impurities uncertain and the different impuis rities even less sure. In recent work [67] a different approach has been adopted, namely, to dope the crystals with specific impurities to see which increase and 1L
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B. Cockayne /Melt growth of oxide and related single crystals
the usual coloration couici be induced at the 1 ppm level. This amount could easily be transported from the refractory surrounds to the crucible but at such a low-level it could also be present in material which is 99 .9999% pure. 3.6. Cracking Cracking is a common problem in the growth of refractory single crystals which characteristically fail by brittle fracture when the stresses induced in the growing or cooling crystal exceed the yield stress of the material; very few oxide materials have an extensive plastic range. The stresses acting upon the crystal can be accentuated by factors such as poor seeding, defects within the crystal, surface condition and anisotropic properties of the material itself [71] but they arise primarily from the temperature gradients built into the crystal growing system by virtue of its size, shape or composition. In most instances, the appropriate temperature gradients during growth and the requisite crystal cooling rates are derived empirically. However, during Czochralski growth, one important environmental factor is often ignored, namely, convection in the gas which surrounds the crystal. The gas is convectionally unstable because, like the melt, it is heated from below but the mode of convection in the gas is more complicated due to the slot in the afterheater through which the operator views the crystal during seeding, growth and meltingoff procedures; this slot allows cooler gas to be drawn into the afterheating system in the region just above the melt—crystal interface. The general effect of this complex convecting system is to produce rapid fluctuations in temperature at any given point which under some conditions can reach amnlifiu1~snf Sfl°I’
The usefulness of a sapphire or similar transparent window is sometimes negated by gradual condensation or reaction of evaporating components upon the inner surface. In this instance, automatic diameter control by weight sensing can provide a solution as direct viewing of the melt surface is required only during seeding and the initial stages of growth, thus reducing substantially the time for which visibility is necessary. Temperature fluctuations in the gas ambient may also influence the wetting of liquid to solid in the EFG and Czochralski techniques, particularly with materials having a large meniscus height; this could be a significant factor in ultimate shape control but as yet its importance is not established.
4. Phase diagrams
An accurate knowledge of the phase relationships in the vicinity of melt-grown compounds is often a relevant factor in determining the optimum composition for avoiding defects such as striations and precipitates. Many oxide and fluoride materials have been assumed tactily to occur as congruently melting line compounds at discrete ratios of their component atoms, e.g. LiNbO3, LiTaO3, Gd3Ga5O12 and LiYF4. An increasing number of such compounds are now known to have congruently melting compositions which differ from the accepted stoichiometric value by several molar per cent and some compounds also possess a homogeneity range. Thus, the congruently melting composition of Gd3Ga5O12 is now accepted as Gd3 05Ga4 95012 and a homogeneity range of 4 .~—‘ôi
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B. Cockayne /Meltgrowth of oxide and related single crystals
false assumption that some of these materials are
Table 4
incongruently melting. Despite the advances made in phase diagram
Compound
knowledge,compounds it is nearly always that oxidewith and fluoride remainassumed stoichiometric respect to the gaseous ion i.e. 0 or Fm Deviations
Gd
from this condition are undoubtedly difficult to detect but this does not mean that the effect is absent. The information link between phase diagrams and crystal growth does not operate entirely in the forward direction. In some instances, the growth of crystals helps to confirm the existence of particular compounds and can aid refinement of the phase relationship data [78,79].
5. The role of atmosphere
The atmosphere in which crystals grow can either be inert or reactive. Oxygen containing atmospheres in oxide growth and HF or F2 containing atmospheres for fluorides can be considered as reactive, Oxides and fluorides can be produced successfully in both reactive and inert atmospheres but advantages are to be gained by a controlled level of reactivity, Thus, when small amounts of oxygen are present, oxide crystals tend to have a smooth surface provided that the melt is non-evaporative and the crucible is not oxidised [43]. Such crystals show less tendency to crack. Similarly, oxygen and hydroxyl contamination is avoided halideshalides grownwhich in reactive atmospheres [76,80]. inOrganic crack within the furnace to yield free halogens are an alternative way of producing a reactive atmosphere [81,82]. The concentration of reactive gas can be critical as in the case of Czochralski grown gallium garnets tnhai-p inrrenth,p nyvoen content
nroduces increasing
3Ga5O12 Y3Sc2Ga1~74Al1~26O12
423
Lattice spacing (A) 12.376 12.376
Magnetic susceptibility1) (jzm emu g +770 —2.5
6. Engineering of materials Oxide and fluorides have featured notably in the deliberate engineering of crystals to produce specific properties and a few examples are quoted here. Widespread substitutions within the gamet structure have been made [12,85,86] to provide substrates with appropriate lattice spacings for the epitaxial deposition of a variety of magnetic garnet layers used in bubble devices as the quest for high mobility small bubbles has continued. In resonator applications, substitutions within the Gd3Ga5O12 garnet have been made to provide a substrate for Y3Fe5O12 which has no residual magnetism as shown in table 4; other substitutions within the garnet lattice have also improved magneto-optical properties [87]. Partial substitutions within Pb5Ge3011 have provided a useful means of modifying the pyroelectric properties [88]. Appropriate substitutions can also be made to improve energy transfer from the exciting radiation31todoped the emitting LiYF ion in laser materials such as Ho 4 where sensitisation particular concentrations of 3+ and Tm3~havewith a marked effect upon transfer Er efficiency arid laser performance [89]. Localised changes at a surface by either in or out diffusion of substitutional ions have also been utilised to produce surface channel structures in materials such as LiNbO 3 and LiTaO3 which have potential in integrated optical devices [90].
B. Cockayne /Melt growth of oxide and related single crystals
424
ii.
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Fig. 2. (a) A scanning electron micrograph of one type of void structure found in Czochralski single crystals of CaWO 4. The dendrites projecting into the void are indicative of the presence of liquid at some stage. Structureless voids are also found in the same sample (bar marker = 25 pm). (b) A scanning electron micrograph of one type of void, showing no internal structure, found in Stockbarger single crystals of LiRF4. Voids containing structure are also found in the same sample (bar marker = 5 pm). (c) A transmission electron micrograph of a low-angle tilt boundary, composed entirely of edge-dislocations, observed in Czochralski single crystals of YA1O3 (bar marker = 0.75 pm). (d) A transmission electron micrograph of inversion boundaries in Czochralski single crystals of LiTaO3 confirming the presence of domains in unpoled material (bar marker = 0.4 pm). (e) An X-ray reflection topograph of a Czochralski single crystal of Gd3Ga5O12 yielding a wealth of detail upon interface shape, growth striations, dislocations and facets (bar marker = 2 mm).
nature and causes of crystalline defects. However, the
thicknesses which can still be handled and where
B. Cockayne I Melt growth of oxide and
that is lot nonually appar6nt usiliS optic~microscopy. Fig. 2c clearly shows the existence of simple tilt boundaries in YA1O3 using HVEM; such detail is often confused in etching studies by pits which form from defects other than dislocations. Fig. 2d establishes the occurrence of domains in unpoled LiTaO3 for which there is no definitive polar etchant. Fig. 2e serves to illustrate the wealth of detail which can be obtained about defect form and distribution in garnet crystals using X-ray topography. Another important technique is the determination of accurate lattice spacing data which can be used to study changes in strain within crystals [53] and to identify the presence of the changes in composition when the existence of a homogeneity range is suspected [74]. Many of these techniques have not yet revealed their full potential in the examination of oxide and related materials but such studies undoubtedly have a big part to play in improving the understanding of the nature of crystalline defects, in aiding the interpretalion of mechanisms by which defects can form and in the evolution of methods for defect control during crystal preparation.
8. Conclusions
relatedsingle crystals
425
[6] D,F, O’Kane, V. Sadagopan and E.A. Giess, J. Electrochem. Soc. 120 (1~7~ ~ [7] W. Bardsley, G.W. Green, C.H. Holiday and D.T.J. Hurle, J. Crystal Growth 16 (1972) 277. [8] W. Bardsley, B. Cockayne, G.W. Green, D.T.J. Hurle, G.C. Joyce, J.M. Roslington, P.J. Tufton and H.C.
Webber, J. Crystal Growth 24/25 (1974) 369. [9] A.E. Zinnes, B.E. Nevis and D.C. Brandle, J. Crystal 19 (1973) [10] Growth A.J. Valentino and187. C.D. Brandle, J. Crystal Growth 26 (1974) 1
[11] T.R. Kyle and G. Zydzik, Mater. Res. Bull. 8 (1973) 443. [12] D. Mateika, J. Herrnring, R. Rathe and Ch. Rusche, J. Growthand 30(1975) 311. J. Crystal Growth 21 [13] Crystal R.C. Reinert M.A. Yatsko, (1974) 283. [141 B. Cockayne, R.L. Nowill and J.G. Plant, unpublished work. [15] V.1. Aleksandrov, V.V. Osiko, A.M. Prokhorov and Tatarintsev, ECCG-1, Zurich, 1976. [16] V.M. B. Cockayne, D.W.Proc. Jones, M.J. Hail and B. Lent, unpublished work. [17] E. Weinberg and N.K. Svrinivasan, J. Crystal Growth 26
(1974) 210. [18] K. Recker, F. Wallrafen and S. Haussuhl, J. Crystal Growth 26 (1974) 97. [19] D.A. Jones, B. Cockayne, R.A. Clay and P.A. Forrester, J. Crystal Growth 30 (1975) 21. [20] K.B. Seiranian, P.P. Fedorov, L.S. Garashina, G.V. Molev, V.V. Karelin and B.P. Sobolev, J. Crystal Growth 26 (1974) 61.
A sound technological base now exists for single crystal growth from the melt for the insulating class of materials exemplified by oxides and fluorides. A general understanding of defect generation, characterisation and control is also available but the detailed atomic processes and precisely measured conditions which lead to the presence or absence of a particular defect are still often a matter for conjecture. It is the pursuit of this detailed knowledge in these materials which now presents a major challenge to crystal ~
~
[21] A.G. Petrosyan and Kh.S. Bagdasarov, J. Crystal Growth 34(1976)110. [22] D.A. Jones, J. Crystal Growth 34 (1976) 149. [23] C.E. Chang and W.R. Wilcox, J. Crystal Growth 21 (1974) 135. [24] 5. Sen and W.R. Wilcox, J. Crystal Growth 28 (1975) 36. [25] (a) H.E. Labelle, Jr. and A.I. Mlavsky, Mater. Res. Buli. 6 (1971) 571. (b)B. Chalmers, H.E. Labeile, Jr. and A.I. Mlavsky, J. Crystal Growth 13/14 (1972) 84.
(c) A.D. Morrison, R.W. Stormont and F.H. Cocks, J. Am. Ceram. Soc. 58 (1975) 41.
B. Cockayne / Melt growth of oxide and related single crystals
426
[33] D. Viechnicki and F. Schmid, J. Crystal Growth 11 (1971) 345. [34] D. Viechnicki, F. Schmid and J.W. McCauley, J. Appl. Phys. 43(1972)4508. [351D. Viechnicki and F. Schmid, J. Crystal Growth 26 (1974) 162. [361 B. Cockayne, J.G. Plant and A.W. Vere, J. Crystal Growth 15 (1972) 167. [37] K.G. Banaclough, G. Garton and P.J. Walker, J. Crystal Growth 35 (1976) 321. [38] MM. Abraham, C.T. Butler and Y. Chen, J. Chem. Phys. 55 (1971) 3752. [39] R. Falkenberg, J. Crystal Growth 13/14 (1972) 723. [40] M.M. Gritsenko, A.A. Popova, E.F. Smirnova nad L.I. Shokina, Soviet Phys.-Cryst. 21(1976)105. [41] A.Yu Malinin, V.5. Papkov, V.D. Chumak and A.P. Vidanov, Izv. Akad. Nauk SSSR (Ser. Fiz.) 37 (1973) 2367. [42] R.C. De Mattei, R.A. Huggins and R.S. Feigelson, J. Crystal Growth 34 (1976) 1. [43] B. Cockayne, B. Lent, J.S. Abell and P.M. Marquis, J. Mater. Sci. 10(1975)1874. [44] J. Basterfield, M.J. Prescott and B. Cockayne, J. Mater. Sci. 3 (1968) 33 [45] B. Hardiman, R. Bucksch, P. Korczak, Phil. Mag. 27 (1973) 777. [46] Kh.S. Bagdasarov, L.M. Dedukh, IA. Zhizheiko, A.M. Kekorkov and V.!. Nikitenko, Soviet Phys.-Cryst. 15 (1970) 278. [47] W.T. Stacy, J.A. Pistorius and MM. Janssen, J. Crystal Growth 22 (1974) 37 [48] K. Takagi, 1. Fukazawa and M. Ishll, J. Crystal Growth 36 (1976) 185. [49] B. Cockayne and J.G. Plant, unpublished work. [50] V.G. Govorkov, N.N. Voinova, Kh.S. Bagdasarov and E.A. Stepantsov, Soviet Phys.-Cryst. 20 (1976) 598. [51] A.W. Vere, J.G. Plant, B. Cockayne, KG. Barraclough and I.R. Harris, J. Mater. Sd. 4 (1969) 1075. [52] RB. Maciolek, T.L. Schuller and ST. Liu, J. Electron. Mater. 5 (1976) 415. [53] B. Cockayne, J.M. Roslington and A.W. Vere, J. Mater. Sci. 8 (1973) 382. [54] W.T. Stacy, J. Crystal Growth 24/25 (1974) 137. [551E. Jakeman and D.T.J. Hurle, Rev. Phys. Tech. 3 (1972) 3. rc~i T
fl
‘~-—..-~-
~
‘
[62] P.A.C. Whiffen, T.M. Bruton and J.C. Brice, J. Crystal Growth 32 (1976) 205. [631 K. Nassau and A.M. Broyer, J. Appl. Phys. 33 (1962) 3064. [64] B. Cockayne, D.S. Robertson and W. Bardsley, Brit. J. Appl.Phys. 15(1964)1165. [65] B. Cockayne, J. Crystal Growth 3/4 (1968) 60. [661 B. Cockayne, M. Chesswas and D.B. Gasson, J. Mater. Sd. 2 (1967) 7. [67] B. Cockayne, in: Crystal Growth and Materials, Eds. E. Kaldis and Hi. Scheel (North-Holland, Amsterdam, 1977) p. 705. [681 R. Oeder, A. Scharmann and D. Schwabe, J. Crystal Growth 36 (1976) 1. [69] J.B. Willis and M. Dixon, J. Crystal Growth 3/4 (1968) 236. [70] V.A. Antonov, PA. Arsenev, I.G. Linda and V.L. Farshtendiker, Phys. Status Solidi (a) 15 (1973) K63. [71] B. Cockayne, B. Lent, J.S. Abell and I.R. Harris. J. Mater. Sci. 8 (1973) 871. [72] H. Fay and C.D. Brandle, in: Crystal Growth, Ed. H.S. Peoser (Pergamon, Oxford, 1967) p. 51. [73] R.F. Belt, J.R. Latore and R. Uhrin, Appl. Phys. Letters 25 (1974) 218. [74] M. Allibert, C. Chatillon, 1. Mareshal and F. Lissalde, J. Crystal Growth 23(1974) 289. [751 L.O. Svaasand, M. Eriksrud, G. Nakken and A.P. Grande, i. Crystal Growth 22 (1974) 230. [76] R.C. Pastor, M. Robinson and W.M. Akutagawa, Mater. Res. Bull. 10 (1975) 501. [77] J.S. Abell, I.R. Harris, B. Cockayne and J.G. Plant, J. Mater. Sci. 11(1976)1807. [78] i. Eckstein, K. Recker and F. Walhafen, J. Crystal Growth 30 (1975) 276. [79] D.S. Robertson, N. Shaw and I.M. Young, J. Phys. D9 (1976) 1257. [80] R.C. Pastor and A.C. Pastor, Mater. Res. Bull. 10 (1975) 117. [811 MS. Orlov, Soviet. J. Opt. Technol. 42 (1975) 733. [82] R.C. Pastor and K. Arita, Mater. Res. Bull. 11(1976) 1037. [83] C.D. Brandle, D.C. Miller and J.W. Nielsen, 1. Crystal Growth 12(1972)195. [84] RH. Hopkins, R.G. Seidensticker and A.M. Stewart, J. Crystal Growth 33 (1976) 223.
THE MELT GROWTH OF OXIDE AND RELATED SINGLE CRYSTALS B. COCKAYNE Royal Signals- and RadarEstablishment, St. Andrews Road, Great Malvern, Worcestershire WRJ4 3PS, England
This article reviews the principle changes which have occurred during the last three years in the techniques used to produce single crystals of oxide and similar single crystals from the melt. The technological demands created by device applications are emphasised and the current state of knowledge upon the major crystalline defects is discussed. The role of accurate phase diagram analysis, the importance now attached to the atmosphere in which a crystal grows and the relevance of looking at crystals with greater resolution are stressed particularly.
I. Introduction
Table 1 Application
An important consequence of the demonstration
of maser and laser action in oxide and similar transparent single crystals nearly two decades ago has been the development of a high temperature crystal growth technology for such materials. During the last decade,
Solid state lasers
this technology has been transformed from a smallscale research exercise into an industrial process as materials with improved properties, produced more ~t’nnnmj~I1ij than hitherto, have been demanded in
Substrates for magnetic mates-ials Surface acoustic
Insulating sub-
Examples of material used 3~,LiY 3~, Y 3Al5O12—Nd 05Er05—Tm Ho3~ La 3~ 2Be2O5—Nd CaLa 3~ 4(SiO4)3O—Nd Al 203 Gd3Ga5Oj2
LiNbO3
414
B. Cockayne /Melt growth of oxide and related single crystals
Table 2 Technique
Principal characteristics
Applications
Czochralski
Crystal rotation and lift Stationary crucible, rf or resistance heating Large melt volume Moderate temperature gradients Crystal unconfined Large crystal sections
Most oxides and fluorides ph few sulphides where liquid er sulation is applicable
Bridgman—Stockbarger
Crucible lowering No rotation Resistance or RF heating Large melt volume Low temperature gradients Crystal confined Large crystal sections
Predominant use is for halide~ plus a few oxides
Edge-defined film fed growth
Crystal lift No rotation Crucible stationary RF heating mainly Small melt volume but large melt reservoir High temperature gradients Crystal unconfined but shaped Small crystal sections
Mainly oxides plus LiF
Travelling zone
Moving melt, no crucible Crystal rotation optional Optical or immersed strip heating Small melt volume
Chiefly oxides
I , eOC~ylle/ ~e1t 8To~1hof o~i~e ~Te1~1edsi~gleeTy81~l8
2. Re~ie~ofteth~~es 2.1. Czochralski This process has continued to be the most adaptable to the problems associated with the growth of optically transparent crystals. It is employed over a wider span of materials and melting points than any other melt-growth method, ranging from mixed cornpounds such as nitrates (NaNO2, M.pt 271°C), through germanates (Pb5Ge3O1 1 M~~t 738°C),fluorides (LiYF4, M.pt 838°C),molybdates (PbMoO4, M.pt 1050°C), niobates (LiNbO3, M.pt 1260°C), tungstates (CaWO4, M.pt 1570°C), tantalates (LiTaO3, M.pt 1650°C), garnets (Y3A15012, M.pt 1970°C) and spinels (MgAl2O4, M.pt 2105°C), to single oxides such as sapphire (m.pt 2050°C) and yttria (M.pt 2410°C). It is the technique used cornmercially to produce all the major oxide device materials such as LiNbO3, LiTaO3, Gd3Ga5O12, Y3A15012 and A1203 in both pure and doped forms as well as more speculative materials such as YA1O3 and La2Be2O5. The size of crystals has increased markedly; 5 cm diameter X 30 cm long crystals of the device materials are now common-place and, in some instances, diameters up to 10 cm can be obtained. The weights of
~15
~ ~ frü~~~ of ~ff~ts i~1udiflg(i)
the reflection of a laser beam verficatly ~nc~clenfupon the melt surface which becomes displaced when intercepted by the meniscus [5], (ii) the signal from an infra-red television camera viewing the crystal directly [6], (iii) crystal weight [7,81and, (iv) crucible weight [9—13]. Weight sensing appears to be the technique most generally adopted but variations in approach are apparent. Both transducer-based mechanisms and linear variable differential transformers have been used to measure weight and a choice also exists between coupling the weighing cell directly to the crystal/crucible or through an amplifying beam mechanism. All these methods have proved satisfactory but crucible weighing seems to have been applied most generally. The inherent disadvantages of weighing the crucible, namely, levitation effects, decreased sensitivity due to a greater total mass and changes in weight during growth due to refractory degradation and evaporative loss from the melt, have to be offset against the difficulties in weighing the crystal which arise from the maintenance of a friction-free rotational and translational system and possible buoyancy effects due to convective flow in the gas ambient. Automatic diametercontrol has obvious economic attractions for large crystals produced routinely on a commercial basis but the reproducibil-
B. Cockayne /Melt growth of oxide and related single crystals
416 Table 3 Compound
CaWO
Time (see)
441 LiF 720
Compound Time (sec) times cibles The
taken for a
Ba2NaNb5O15 495
4
YA1O3
Gd3Ga5Oj2 480
450
LiEs-0 5Y0 5F4 810
rise or fall in temperature at the crucible wall to be 96% translated
ventional iridium crucibles. A significant parameter in any crystal growth systern, but particularly in the Czochralski process, is the temperature distribution within the total thermal environment. Detailed data upon the temperature distribution of particular systems are tedious to obtain, technically difficult to measure in some instances and are rarely quoted. A possible means of characterising a system and investigating changes therein is to use the data provided by an infra-red viewing system, as illustrated in fig. 1 [161. Thermograms of this type provide a qualitative picture of heat distribution over th’e exterior of crystal growth systems so that areas of excessive loss or non-uniform heat distribution can be identified. Further information can be gained by the examination of transparent model systems where the isotherm patterns, typified by the white lines in fig. 1, can be used to judge the external effect of making —
—-‘-,
—
.
~-.
-
to the melt centre in 5 cm diameter cru-
applicability of this technique to oxides has been emphasised by the production of Y3A15012, Lu3A15O12 and garnets of mixed composition using molybdenum crucibles in a resistance-heated tungsten furnace [21]. A particular difficulty encountered in the crystallisation of fluorides is how to limit the evaporation of those materials which have a high vapour pressure at their melting point. A sealed crucible can be used if the material is compatible with silica or platinum but many fluorides adhere to metal crucibles, causing crystal cracking. A variation which uses traditional high purity graphite crucibles has been reported [22], in which the seal is formed by the evaporating fluóride itself. This method conserves the crucible, retains the non-wetting characteristics of carbon with fluorides, permits high temperature use and aids the volatiisation of impurities as the material does not ,
.
-
-
I , ~ /~1i~ ofo~i~ ~T~1~ ~1ll~1~ ~
411
I
~
I
Fig. 1. Thermograms of the afterheater in a model Czochralski system constructed out of silica and placed above a 3.8 cm diameter Pt crucible contained in MgO refractory and heated to 1000°C. (a) and (b) delineate the relative changes in the heat emission pattern and isotherms (white lines) obtained by inserting a silica baffle at AA. (c) shows typical isotherm patterns in the gas surrounding the afterheater; fluctuations in these patterns around the viewing port (not shown) and the hole through which the seed is inserted give some indication of the violent convective flow present in these regions. An AGA Thermovision was used to record
the thermograms [161.
shown that the sensitivity of interface position to thermal conditions increases as the heat transfer coefficients between crucible and surroundings decrease, as the diameter of the crucible decreases and as the thermal conductivity of the crucible material is lowered. For minimum sensitivity, the thermal con~-b,~’f4,i~f~, t-,f fhe t~ni.~jh1e shrnild be large for materials
[26,27]; this confirms the prediction [28] that convection within the capillary channel of the die is so curtailed that solutes segregating at the interface can only return to the crucible with difficulty, by diffusion against the fluid flow, hence the impurity concentration in the solid, C~,must approach that in the bulk liquid, Cj,, so that Cs/CL = keff =
418
B. Cockayne / Meltgrowth of oxide and related single crystals
2.4. Travelling zone techniques Vertically moving molten zones, with fusion produced by optical techniques, have been used for producing small research crystals of high melting point materials for many years but few significant advances have been recorded. The interest once shown in gas lasers as a heat source has largely disappeared. Resistance heated strips immersed within the molten zone have, however, stimulated greater interest. Some five years ago it was shown that a platinum strip, immersed in a CaCO3—Li2CO3 eutectic solvent zone, could be moved through a CaCO3 feed rod to yield single crystals of calcite [30,31]. Further improvements have been reported recently [32] which allow crystals up to 2.5 cm diameter x 3.0 cm long to be obtained. The previously troublesome problem of bubble entrapment has been eliminated by using crystals rather than sintered powder as the feed material and a homogeneous temperature distribution has been achieved over the total area of the strip by locally increasing the current density with selectively placed holes. 2.5. Crystaiisation within static systems Methods in which growth is induced within a static system by some form of heat exchange either at the
sions of 18 cm diameter X 13 cm thick and weighing 15 kg can be grown. Heat exchangers applied to the base of a crucible containing a melt have also been applied to Czochralski growth to stabiise the thermal flow characteristics and to aid diameter control by modifying the interface shape [36]. A modified Stober technique has been employed in the production of EuO crystals enriched with 1 by slowly cooling non-stoichiometric Eu1 170 melts contained in a sealed molybdenum crucible [37]. A further static melt technique, namely electric arc fusion, is still used for the commercial production of large polycrystals of alkaline-earth oxides [38], the grain size being sufficiently large for single crystals with centimetric dimensions to be selected. 2.6. The Vemeuil process This flame-fusion process is the traditional method for oxide crystallisation. It is widely employed for small sapphires and rubies and has been extended to the growth of materials such as MgAl2O4 [39], Y203—RF3 mixtures (R = lanthanide ion) [40], and CoA12O4. A number of useful stress analyses have been made of crystals grown in this process [39,41], establishing the relative importance of bulk, surface and localised stresses. However, the process is not
/
419
B. Cockayne Melt growth ofoxide and related single crystals erally recognised to be any which can absorb, reflect, refract or scatter magnetic, optical, acoustic and electrical energy either generated within or incident upon the material. Thus, colour centres, facets, striations, dislocation low-angle boundaries, twins, individual
selected which inhibited both dislocation and twin generation. Dislocations in garnet crystals offer some intriguing problems. They are normally absent from meltgrown Y3A15012 crystals [44], but have been ob-
dislocations with large strain fields, voids, cellular structures, precipitates, inclusions and more destructive defects such as cracks are all important defects which have to be eliminated or controlled. Most of these defects were recognised in the early solid state laser hosts typified by ruby, calcium tungstate and Y3A15012. Detailed information upon the nature, cause and control of many of these defects can be found in the Proceedings of ICCG 1 and 2. As the number of materials investigated has increased it has become apparent that a substantial number of these defects can occur in most transparent melt-grown crystals. Thus, much of the recent literature can be considered as a catalogue of the discovery of similar defects in an increasingly wide range of compounds. The high-level of activity in this field has, however, allowed progress to be made in both the understanding and relevance of particular defects as described below.
served when inclusions are present [45,46]. Dislocations are more readily generated in Gd3Ga5012 crystals where helical defects of an interstitial character up to 300 pm in diameter and several cms length have been seen by both optical and X-ray topographic techniques [47,48]. Some recent work [49] suggests that in the absence of dislocations caused by inclusions in YAG, a marked connection exists between dislocation formation and interface shape. Crystals of undoped YAG grown with an interface which is concave towards the melt show dislocation structures which are very similar to those in Gd3Ga5012. Y3A15012 used for lasers is normally grown with a convex interface whilst Gd3Ga5012 used for substrates requires a planar interface which can easily become concave, hence the concave interface in Y3A15012 had to be deliberately induced for these experiments. The similarity between these gamets confirms the general occurrence of particular defects throughout the spectrum of melt-grown gas-nets provided that comparable interface morphologies are induced. The mechanical properties and slip or twinning systems available for deformation have not been measured for many current refractory materials, particularly at high temperatures, so, interpretation of dislocation structures in terms of the stresses present during growth is largely qualitative. Even when information is available, difficulties can ensue because dislocations can be generated by a multiplicity of mech-
3.1. Dislocations Dislocation structures in oxides etc have been widely studied using chemical etching procedures to reveal dislocations as individual pits. Such procedures can be misleading as great care is needed to distinguish dislocation etch pits from etch pits caused by inclusion or surface damage. X-ray topography has provided significant information but a useful innovation has been the application of electron microscopy ‘
~-..
1~,-,.-
~
~
m’r(~(’nnv
h~shnwn
420
B. Cockayne /Melt growth of oxide and related single crystals
3.2. Facets The observation that facets can form on the solid— liquid interface ofmelt-grown compounds is now well recognised but the strain associated with the facets in oxides is not so well understood. Most of the efforts attempting to understand the phenomenon have been concerned with garnets in comparative studies between Y3A15012 and Gd3Ga5012 where the mean lattice parameter of the faceted regions is slightly larger than that of the unfaceted regions (&z/a 1 .3 X I0~) [53]. Three principal sources of the strain have been proposed (a), changes in cation ratio between facet and matrix (b), impurity segregation and (c) oxygen segregation, with the latter considered to be most likely because the similarity in the magnitude of the strain in two separate materials implies a common mechanism [53]. In a recent review [54] it has been suggested that facet strain due to oxygen segregation is consistent with the entrapment of oxygen vacancies on the facets due to the high lateral growth rates involved in facet formation and the low diffusion constant of oxygen with respect to the cations present. This view is supported by luminescence studies which show that faceted regions contam higher concentrations of electron traps than the matrix [54]. Facets have also been seen in Stockbarger-grown garnets [21]. In this case, the low temperature gradients give an approximately planar solid—liquid interface except near to the crucible wall where the interface becomes slightly convex towards the melt and it is here that the facets occur. This distribution and form is very similar to that detected in Czochralski grown garnets under approximately olanar interfacial growth conditinns
mainly from the hot liquid from the crucible base and walls rising to the surface and displacing the cooler liquid at the melt surface. The convectional flow is complicated by crystal rotation and is very dependent upon factors such as melt depth and temperature gradient in addition to crystal and crucible dimensions and material parameters. Many useful reviews discussing these factors are available [55—58]. In only a very few cases have direct 1 : I relationships been established between striations and fluctuations in temperature within melts of high melting point materials, although qualitative and semi-quantitative relationships are more numerous. The nature of the segregation causing the compositional change and associated strain across a striation is often assumed rather than investigated. Some estimate of the strain has been made in garnets used for substrates [54] and a typical value is 5 X lO~but in most applications the level of strain due to striations has not constituted a sufficient problem thus far to prompt a very detailed study. Striations can be eliminated or reduced in a number of ways. In pure cornpounds, growth at the congruently melting composition is an obvious solution but with deliberately doped or slightly impure materials the lowest temperature gradients and smallest melt depths compatible with crystal diameter control and the avoidance of other defects must be employed in order to limit convection. It is worth noting that striations do occur in Stockbarger grown crystals [19] where the liquid is coolest at the crucible base and convectional instabilities associated with temperature differences should be absent. Such striations could be attributed to convection from other sources such as surface tension ~
~
.~.
-
421
B. Cockayne /Melt growth of oxide and related single crystals
in a liquid—solid tr~nsition~nd the e~lut!ollof~s~s
3.4. Voids Voids (cavities) can form in most melt-grown compounds regardless of the basic compositional type i.e. oxide, fluoride etc. They were observed extensively in early Czochralski crystals of laser hosts such as calcium tungstate [63,64], Y3A15012 and alumina [65,66] and were associated with the segregation of dopant impurity, gaseous impurity or basic constituents. In a fully developed state, they can adopt the cellular morphology associated with constitutionally supercooled growth. Some voids have been presumed to form by the entrapment of impurity-rich liquid in a cell-boundary groove or solute trail which contracts on freezing to form a cavity (mechanism I). The entrapment of liquid due to thermal supercooling is a similar phenomenon but thermal supercooling is genes-ally only dominant at high growth rates as typified by those used in the EFG process. Other modes of void formation can be considered. For instance, the segregation of gaseous impurity directly (mechanism II), the entrapment of an escaping volatile impurity (mechanism III), the capture of gas bubbles displaced from the melt (mechanism IV), condensation of vacancies (mechanism V) and solidification with a limited supply of liquid akin to the piping effect observed in metal castings (mechanism VI). VI is only likely to apply to the terminal stages of Stockbarger solidification or to methods such as EFG if the liquid supply becomes limited. V is unlikely because of the low equilibrium vacancy concentration 4 atathe verymelting rapid point most materials, i0 to retain even coolingofprocedure would typically be necessary this concentration under which conditions most insulator single crystals would crack. A recent -
-----,--—‘~----
~
which have been absorbed or trapped during melt preparation from component oxides. Similar criteria apply to related compounds such as fluorides. 3.5. Colour centres
Colour centres in oxide-based crystals constitute a complex problem and have been extensively studied in terms of their creation by ultra violet or ‘y-irradiation. The general character of the room temperature absorption which is symptomatic of colour centre formation is a broad featureless band extending approximately from 0.3 to 0.7 pm; this imparts a brown coloration to the crystals. The occurrence of colour centres and the extent and character of the absorption produced is well documented but substantially less is known about their cause. The problem is complex as some centres can be removed by annealing in oxidising atmospheres [69], others by annealing under reducing or vacuum conditions [70,71]. In LaA1O3, colour centres have been attributed to a positive ion vacancy—hole combination [72] due mainly to the presence of divalent impurities or vacancies on the trivalent rare-earth site. Generally, the impurity ion remains unidentified, although the importance of Fe in some Nd-doped YA1O3 single crystals has been recognised [73]. Frequently, it is the low-level of impurity in the single crystals which leads to the ambiguity because therelative preciseamounts level of of total impurities uncertain and the different impuis rities even less sure. In recent work [67] a different approach has been adopted, namely, to dope the crystals with specific impurities to see which increase and 1L
~
~
~
~
T~
~
422
B. Cockayne /Melt growth of oxide and related single crystals
the usual coloration couici be induced at the 1 ppm level. This amount could easily be transported from the refractory surrounds to the crucible but at such a low-level it could also be present in material which is 99 .9999% pure. 3.6. Cracking Cracking is a common problem in the growth of refractory single crystals which characteristically fail by brittle fracture when the stresses induced in the growing or cooling crystal exceed the yield stress of the material; very few oxide materials have an extensive plastic range. The stresses acting upon the crystal can be accentuated by factors such as poor seeding, defects within the crystal, surface condition and anisotropic properties of the material itself [71] but they arise primarily from the temperature gradients built into the crystal growing system by virtue of its size, shape or composition. In most instances, the appropriate temperature gradients during growth and the requisite crystal cooling rates are derived empirically. However, during Czochralski growth, one important environmental factor is often ignored, namely, convection in the gas which surrounds the crystal. The gas is convectionally unstable because, like the melt, it is heated from below but the mode of convection in the gas is more complicated due to the slot in the afterheater through which the operator views the crystal during seeding, growth and meltingoff procedures; this slot allows cooler gas to be drawn into the afterheating system in the region just above the melt—crystal interface. The general effect of this complex convecting system is to produce rapid fluctuations in temperature at any given point which under some conditions can reach amnlifiu1~snf Sfl°I’
The usefulness of a sapphire or similar transparent window is sometimes negated by gradual condensation or reaction of evaporating components upon the inner surface. In this instance, automatic diameter control by weight sensing can provide a solution as direct viewing of the melt surface is required only during seeding and the initial stages of growth, thus reducing substantially the time for which visibility is necessary. Temperature fluctuations in the gas ambient may also influence the wetting of liquid to solid in the EFG and Czochralski techniques, particularly with materials having a large meniscus height; this could be a significant factor in ultimate shape control but as yet its importance is not established.
4. Phase diagrams
An accurate knowledge of the phase relationships in the vicinity of melt-grown compounds is often a relevant factor in determining the optimum composition for avoiding defects such as striations and precipitates. Many oxide and fluoride materials have been assumed tactily to occur as congruently melting line compounds at discrete ratios of their component atoms, e.g. LiNbO3, LiTaO3, Gd3Ga5O12 and LiYF4. An increasing number of such compounds are now known to have congruently melting compositions which differ from the accepted stoichiometric value by several molar per cent and some compounds also possess a homogeneity range. Thus, the congruently melting composition of Gd3Ga5O12 is now accepted as Gd3 05Ga4 95012 and a homogeneity range of 4 .~—‘ôi
r’.
f~
~
1
lfl1
-
B. Cockayne /Meltgrowth of oxide and related single crystals
false assumption that some of these materials are
Table 4
incongruently melting. Despite the advances made in phase diagram
Compound
knowledge,compounds it is nearly always that oxidewith and fluoride remainassumed stoichiometric respect to the gaseous ion i.e. 0 or Fm Deviations
Gd
from this condition are undoubtedly difficult to detect but this does not mean that the effect is absent. The information link between phase diagrams and crystal growth does not operate entirely in the forward direction. In some instances, the growth of crystals helps to confirm the existence of particular compounds and can aid refinement of the phase relationship data [78,79].
5. The role of atmosphere
The atmosphere in which crystals grow can either be inert or reactive. Oxygen containing atmospheres in oxide growth and HF or F2 containing atmospheres for fluorides can be considered as reactive, Oxides and fluorides can be produced successfully in both reactive and inert atmospheres but advantages are to be gained by a controlled level of reactivity, Thus, when small amounts of oxygen are present, oxide crystals tend to have a smooth surface provided that the melt is non-evaporative and the crucible is not oxidised [43]. Such crystals show less tendency to crack. Similarly, oxygen and hydroxyl contamination is avoided halideshalides grownwhich in reactive atmospheres [76,80]. inOrganic crack within the furnace to yield free halogens are an alternative way of producing a reactive atmosphere [81,82]. The concentration of reactive gas can be critical as in the case of Czochralski grown gallium garnets tnhai-p inrrenth,p nyvoen content
nroduces increasing
3Ga5O12 Y3Sc2Ga1~74Al1~26O12
423
Lattice spacing (A) 12.376 12.376
Magnetic susceptibility1) (jzm emu g +770 —2.5
6. Engineering of materials Oxide and fluorides have featured notably in the deliberate engineering of crystals to produce specific properties and a few examples are quoted here. Widespread substitutions within the gamet structure have been made [12,85,86] to provide substrates with appropriate lattice spacings for the epitaxial deposition of a variety of magnetic garnet layers used in bubble devices as the quest for high mobility small bubbles has continued. In resonator applications, substitutions within the Gd3Ga5O12 garnet have been made to provide a substrate for Y3Fe5O12 which has no residual magnetism as shown in table 4; other substitutions within the garnet lattice have also improved magneto-optical properties [87]. Partial substitutions within Pb5Ge3011 have provided a useful means of modifying the pyroelectric properties [88]. Appropriate substitutions can also be made to improve energy transfer from the exciting radiation31todoped the emitting LiYF ion in laser materials such as Ho 4 where sensitisation particular concentrations of 3+ and Tm3~havewith a marked effect upon transfer Er efficiency arid laser performance [89]. Localised changes at a surface by either in or out diffusion of substitutional ions have also been utilised to produce surface channel structures in materials such as LiNbO 3 and LiTaO3 which have potential in integrated optical devices [90].
B. Cockayne /Melt growth of oxide and related single crystals
424
ii.
~
is _
*1
~
Fig. 2. (a) A scanning electron micrograph of one type of void structure found in Czochralski single crystals of CaWO 4. The dendrites projecting into the void are indicative of the presence of liquid at some stage. Structureless voids are also found in the same sample (bar marker = 25 pm). (b) A scanning electron micrograph of one type of void, showing no internal structure, found in Stockbarger single crystals of LiRF4. Voids containing structure are also found in the same sample (bar marker = 5 pm). (c) A transmission electron micrograph of a low-angle tilt boundary, composed entirely of edge-dislocations, observed in Czochralski single crystals of YA1O3 (bar marker = 0.75 pm). (d) A transmission electron micrograph of inversion boundaries in Czochralski single crystals of LiTaO3 confirming the presence of domains in unpoled material (bar marker = 0.4 pm). (e) An X-ray reflection topograph of a Czochralski single crystal of Gd3Ga5O12 yielding a wealth of detail upon interface shape, growth striations, dislocations and facets (bar marker = 2 mm).
nature and causes of crystalline defects. However, the
thicknesses which can still be handled and where
B. Cockayne I Melt growth of oxide and
that is lot nonually appar6nt usiliS optic~microscopy. Fig. 2c clearly shows the existence of simple tilt boundaries in YA1O3 using HVEM; such detail is often confused in etching studies by pits which form from defects other than dislocations. Fig. 2d establishes the occurrence of domains in unpoled LiTaO3 for which there is no definitive polar etchant. Fig. 2e serves to illustrate the wealth of detail which can be obtained about defect form and distribution in garnet crystals using X-ray topography. Another important technique is the determination of accurate lattice spacing data which can be used to study changes in strain within crystals [53] and to identify the presence of the changes in composition when the existence of a homogeneity range is suspected [74]. Many of these techniques have not yet revealed their full potential in the examination of oxide and related materials but such studies undoubtedly have a big part to play in improving the understanding of the nature of crystalline defects, in aiding the interpretalion of mechanisms by which defects can form and in the evolution of methods for defect control during crystal preparation.
8. Conclusions
relatedsingle crystals
425
[6] D,F, O’Kane, V. Sadagopan and E.A. Giess, J. Electrochem. Soc. 120 (1~7~ ~ [7] W. Bardsley, G.W. Green, C.H. Holiday and D.T.J. Hurle, J. Crystal Growth 16 (1972) 277. [8] W. Bardsley, B. Cockayne, G.W. Green, D.T.J. Hurle, G.C. Joyce, J.M. Roslington, P.J. Tufton and H.C.
Webber, J. Crystal Growth 24/25 (1974) 369. [9] A.E. Zinnes, B.E. Nevis and D.C. Brandle, J. Crystal 19 (1973) [10] Growth A.J. Valentino and187. C.D. Brandle, J. Crystal Growth 26 (1974) 1
[11] T.R. Kyle and G. Zydzik, Mater. Res. Bull. 8 (1973) 443. [12] D. Mateika, J. Herrnring, R. Rathe and Ch. Rusche, J. Growthand 30(1975) 311. J. Crystal Growth 21 [13] Crystal R.C. Reinert M.A. Yatsko, (1974) 283. [141 B. Cockayne, R.L. Nowill and J.G. Plant, unpublished work. [15] V.1. Aleksandrov, V.V. Osiko, A.M. Prokhorov and Tatarintsev, ECCG-1, Zurich, 1976. [16] V.M. B. Cockayne, D.W.Proc. Jones, M.J. Hail and B. Lent, unpublished work. [17] E. Weinberg and N.K. Svrinivasan, J. Crystal Growth 26
(1974) 210. [18] K. Recker, F. Wallrafen and S. Haussuhl, J. Crystal Growth 26 (1974) 97. [19] D.A. Jones, B. Cockayne, R.A. Clay and P.A. Forrester, J. Crystal Growth 30 (1975) 21. [20] K.B. Seiranian, P.P. Fedorov, L.S. Garashina, G.V. Molev, V.V. Karelin and B.P. Sobolev, J. Crystal Growth 26 (1974) 61.
A sound technological base now exists for single crystal growth from the melt for the insulating class of materials exemplified by oxides and fluorides. A general understanding of defect generation, characterisation and control is also available but the detailed atomic processes and precisely measured conditions which lead to the presence or absence of a particular defect are still often a matter for conjecture. It is the pursuit of this detailed knowledge in these materials which now presents a major challenge to crystal ~
~
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(c) A.D. Morrison, R.W. Stormont and F.H. Cocks, J. Am. Ceram. Soc. 58 (1975) 41.
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426
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