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VERTICAL BRIDGMAN GROWTH OFVERTICAL BRIDGMAN GROWTH OFSAPPIRE –SEED CRYSTAL SHAPESAND SEEDING CHARACTERISTICSAND SEEDING CHARACTERISTICS
Navid Khoob2/5/2015
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2/5/2015
OUTLINE Motivation Introduction Traditional VB Traditional VB VB Furnace Scrutinizing the reusability a-plane & c-plane Easy crystal release Producible seeding with single crystal growth Heat Exchange Method
V i l B id M h d Vertical Bridgman Method Shape and Size of the seed crystal Aims LAGNs LAGNs Experimental Shape of seed Thin, Tapered and Full-diameter VB growth PROCESSES Experimental Sepecimens Result & Discussion Single Crystal Growth from Various Shapes of Seed Single Crystal Growth from Various Shapes of Seed Thin, Tapered and Full-diameter Summary Reference
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MOTIVATION
Methods of growing sapphire for the fabricationof GaN-based LED devices have recently beenattracting considerable attentionattracting considerable attention.
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INTRODUCTION
Well known techniques for growing large andhigh-quality sapphire crystals include theK l (KP) [1 2] h t h (HE) [2 4]Kyropoulos (KP) [1,2], heat exchange (HE) [2–4],edge-defined, film-fed growth (EFG) [2,5–7], andCzochralski (CZ) methods [2,8].
All these methods have been studied extensivelyd d i d il [2 9 13]and compared in detail [2,9–13].
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INTRODUCTION
We have investigated sapphire crystal growthusing the traditional vertical Bridgman (VB)method, in which the crystals are grown in acrucible with rotation and translation in a hotcrucible with rotation and translation in a hotzone with an appropriate temperaturedistribution.
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INTRO : VB FURNACE
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INTRODUCTION
We first demonstrated with our VB growth of c-axis, 3-in. diameter sapphire that grown crystalscould be easily and nondestructively releasedfrom a W crucible and that the crucibles could befrom a W crucible and that the crucibles could bereused many times [14].
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A,C AND R-PLANE
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INTRODUCTION
We also showed that the easy crystal release wasdue to a gap between the crucible inner wall andthe grown crystal formed by differential thermalthe grown crystal formed by differential thermalcontraction during cooling to room tempera-ture[14].
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INTRODUCTION
Our experimental demonstrations and theoreticalconfirmations of crucible reusability havereceived favorable attention for their practicalreceived favorable attention for their practicalvalue in the industrial production of largesapphire.
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INTRODUCTION
The reuse of crucibles has been a very importantconsideration in the HE and VB methods, inwhich the alumina melt is solidified in thewhich the alumina melt is solidified in thecrucible under crystal shape control.
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INTRODUCTION
However, we have not yet presented detailed information about the
Size Sh Shape
Seeding Process with precise temperature control Crystal growth process after seeding Crystal growth process after seeding.
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REPRODUCIBLE SEEDING WITH SINGLE-CRYSTAL GROWTH
It was very difficult when we could observeneither the seed crystal nor the growth interfaceduring the seeding process in eitherduring the seeding process in either
the HEor the VB method.
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HEAT EXCHANGE METHOD
In the HE method, a seed crystal placed at thebottom of the crucible is kept from melting by acooling flow of He [3].
This allows seeding that reproducibly yieldsg p y ysingle-crystal growth, with the seed crystalremaining unmelted.
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HEAT EXCHANGE METHOD
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VERTICAL BRIDGMAN
in the VB method, with no special technique to prevent the melting of the seed crystal, it is very important to implement precise temperature measurement and control in the vicinity of the measurement and control in the vicinity of the crucible bottom during seeding.
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VERTICAL BRIDGMAN
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SHAPE AND SIZE OF THE SEED CRYSTAL
It was well known that the difficulty of reproducible seeding for single-crystal growth can be strongly dependent on the shape and size of be strongly dependent on the shape and size of the seed crystal.
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AIMS
We first propose a technique for precise temperaturemeasurement at the crucible bottom in the VB growthfurnace and verify its utility in providing reproduciblefurnace and verify its utility in providing reproducibleseeding.
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AIMS
Then, we examine three representative kinds ofseed crystals: thin, tapered and full-diameter
d d t i i l t l hiseeds, used to grow c-axis single-crystal sapphirein the traditional VB growth technique.
We discuss their differences with respect toreproducible seeding for single-crystal growth,
d h i d li i i f l land the generation and elimination of low-anglegrain boundaries (LAGBs) [9] in their seedingprocesses.p
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LOW-ANGLE GRAIN BOUNDARIES (LAGBS)
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LOW-ANGLE GRAIN BOUNDARIES (LAGBS) A grain boundary is the interface between two grains, orcrystallite in a polycrystalline materialcrystallite, in a polycrystalline material.
They are defects that tend to decrease the electrical andythermal conductivity of the material.
Preferred sites for the onset of corrosion Preferred sites for the onset of corrosion
It is convenient to separate grain boundaries by the extent ofp g ythe misorientation between the two grains.
LAGBs are those with a misorientation less than about 15 LAGBs are those with a misorientation less than about 15degrees.
they are composed of an array of dislocation and theirproperties and structure are a function of the misorientation.
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Experimental23
VB FURNACE
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DESCRIPTION
The temperature distribution as measured withno crucible present is shown at the right side inth fithe figure.
The W crucible is mounted on a crucible shaft The W crucible is mounted on a crucible shaftthat can rotate and also translate vertically.
The graphite heater is powered by a radiofrequency coil positioned towards the outerperiphery of the carbon felt heat shieldperiphery of the carbon-felt heat shield.
An argon atmosphere is maintained in the An argon atmosphere is maintained in theairtight chamber at just over 100 kPa. 25
SHAPES OF SEEDS
The thin seed, the tapered seed and the full-diameter seed are shown in Fig. 2(a)–(c)
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VB GROWTH PROCESSES The VB growth processes with these seed crystals and
crucibles are as follows Charging process: the seed and raw materials are placed in the Charging process: the seed and raw materials are placed in the
crucible.
Melting process: the crucible is elevated into the high Melting process: the crucible is elevated into the hightemperature zone and all raw materials are melted.
Seeding process: a part of the seed is melted during the slow Seeding process: a part of the seed is melted during the slowelevation of the crucible into the high-temperature zone, where thetemperature gradient is about 10 °C/cm.
Growth process: the crucible is lowered at 3mm/h.
C li Cooling process: the crucible is cooled to room temperature asthe heating power is reduced to zero.
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EXPERIMENTAL SPECIMENS. Th l d b h id i The crystals grown were cut and both sides mirror-polished as experimental specimens.
Green-laser scattering to observe the profile of theseeding interface.
A crossed polarizer to evaluate the LAGBs.
X-ray topography to evaluate the LAGBs the LAGBs, the internal residual stress And And the dislocation distribution.
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Resultsand discussion
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TEMPERATURE MEASUREMENT ANDSEEDING PROCESS
It was very important to achieve satisfactoryy p yreproducibility in the seeding process, requiringprecise measurement and control of thetemperature near the crucible bottomtemperature near the crucible bottom.
This was so because the present VB methodprovides no mechanism to prevent the melting ofthe seed crystal, such as the He-gas cooling in theHE method [3 4]HE method [3,4].
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THERMOCOUPLE
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THERMOCOUPLE
We constructed a new thermocouple structure topgive precise and repeatable measurements intemperatures above 2000 °C.
The thermocouple is set between the crucibleshaft and the crucible (Fig. 3(a))shaft and the crucible (Fig. 3(a))
The thermocouple consists of two simple wires,one of 95%W–5%Re and the other 74%W–26%Re,and two W discs to which the wires are firmlyconnectedconnected.
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EQUIVALENT ELECTRIC CIRCUIT
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THERMOCOUPLE
The thermocouple measured thetemperature at the crucible bottom top±0.5 °C at about 2000 °C, allowing us toestablish the seeding process with highreproducibility in the very lowtemperature gradient of about 10°C/cm at temperatures near the melting°C/cm at temperatures near the meltingpoint of sapphire.
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REPRODUCIBILITY OF SEEDING PROCESS
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SEEDINGTEMPERATURE AND SEEDING INTERFACEPOSITION
The experimental results confirming thep greproducibility of seeding are shown in Table 1.
Attaining consistent reproducibility was mostdifficult for crystals grown using the tapered seedshown in Fig. 2(b).shown in Fig. 2(b).
In Table 1, the seeding temperature is the valuemeasured by the thermocouple and the seedinginterface position was defined as the distancefrom seed bottom to the center of the seedingfrom seed bottom to the center of the seedinginterface. 36
SEEDINGTEMPERATURE AND SEEDING INTERFACEPOSITIONPOSITION
The seeding temperature corresponded well tog p pthe seeding interface position as shown in Table1.
Four crystals with seeding temperatures of2012±1 °C showed seeding interface positions of20±1 mm, which may indicate acceptablereproducibility in their seeding processes.
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SEEDINGTEMPERATURE AND SEEDING INTERFACEPOSITION
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Single-crystal growth g y gfrom various shapes of
d seed 39
REPRESENTATIVE CRYSTALS GROWN
R t ti t l f h f th Representative crystals grown from each of thethree kinds of c-axis seeds and crucibles shown inFig. 2 are shown in Fig. 6.
Photographs of as-grown crystal ingots grown fromthin, tapered and full-diameter seeds are shown inthin, tapered and full diameter seeds are shown inFig. 6(a-1), (b-1) and (c-1).
Th di i t f fil d t t d b The seeding interface profiles detected by greenlaser scattering from regions indicated by yellowdashed lines
The crossed-polarizer images of the c-plane waferscut from crystal bodies at the red dashed linescut from crystal bodies at the red dashed lines
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REPRESENTATIVE CRYSTALS GROWN
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CRYSTAL GROWTH WITH THIN SEEDS
Reproducible seeding and single-crystal growthprocesses were established using seed crystals withdiameters greater than 20 mm in W crucibles.
It f d th t t l f thi d It was found that crystals grown from thin seedswith a diameter of 10 mm could not be releasedfrom the crucibles without breaking the crystalbody off of the thin-seed portion.
S ll Di t b f i ffi i t l Small Diametersbecause of insufficient clearancebetween the grown crystal and the inner cruciblewall at the thin-seed well
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CRYSTAL GROWTH WITH THIN SEEDS
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CRYSTALS GROWN WITH DIFFERENTSEEDING POSITIONS
X-ray topographic images of a-plane wafersy p g p g pparallel to the growth direction cut from threecrystals grown with different seeding positions.
LAGBs generated at the periphery of seedingportions are observed in all three crystals.portions are observed in all three crystals.
These LAGBs propagated into the crystal bodies.
Some LAGBs and residual stress are freshlyf d t th h ld ti i h t lformed at the shoulder portions in each crystal. 44
CRYSTALS GROWN WITH DIFFERENT SEEDINGPOSITIONS
these LAGBs were caused by large bubblesgenerated at the interface between the crucibleinner wall and the periphery of the grown crystal orinner wall and the periphery of the grown crystal, orsome sticking stress between the crucible inner walland the crystal surface.
On the other hand, the residual stress may begenerated by thermal stress at the shoulder portionsgenerated by thermal stress at the shoulder portionsduring cooling.
However, most of these LAGBs and residual stressesdisappear in the upper portions of the crystal bodies.
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CRYSTALS GROWN WITH DIFFERENT SEEDINGPOSITIONS
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DIFFERENT SHOULDER SHAPES
shows X-ray topographic images of a-plane waferscut from the other two crystals, grown with differenty , gshoulder shapes.
LAGBs inherited from the seed and residual stressare observed in both crystalsare observed in both crystals.
We conclude that both the LAGBs and the residualstress were not significantly affected by the shouldershape.
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DIFFERENT SHOULDER SHAPES
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X-RAY TOPOGRAPHIC IMAGES AND A CROSSED-POLARIZER IMAGE OF ANOTHER CRYSTALGROWN FROM A THIN SEED.
The LAGBs inherited from the seed portion are the circularThe LAGBs inherited from the seed portion are the circularLAGBs in Fig. 10(b).
The weak residual stress in Fig 10(b) may be related to that of The weak residual stress in Fig. 10(b) may be related to that ofthe shoulder portion, as shown in Fig. 10(a).
On the other hand, no defects are apparent in the crossed-polarizer image in Fig. 10(c).
the LAGBs and the residual stress observed in the X-raytopographic images in Fig. 10(a) and (b) are weak LAGBs andresidual stresses that are undetectable as defects by crossed-polarizer image. 49
X-RAY TOPOGRAPHIC IMAGES AND A CROSSED-POLARIZER IMAGE
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X-RAY TOPOGRAPHIC IMAGES AND A CROSSED-POLARIZER IMAGE OF ANOTHER CRYSTALGROWN FROM A THIN SEED.
We conclude that reproducible seeding andsingle-crystal growth are easily established withthi dthin seeds.
While slight LAGBs are generated at theperiphery of the seeding interface and inheritedin the full-diameter body, and these are notcompletely eliminated in our growth process.
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CRYSTAL GROWTH WITH TAPERED SEEDS
R d ibl di bli h d Reproducible seeding processes were establishedusing tapered seeds with an angle of less than 45°and length greater than twice their diameter.g g
It is considered that the LAGBs were caused bylarge bubbles generated at the interface betweenthe crucible inner wall and the periphery of thethe crucible inner wall and the periphery of thegrown crystal.
Large bubbles often formed, especially at thet d ibl i ll j t b th ditapered crucible inner wall just above the seedinginterface.
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THREE TYPICAL CRYSTALS GROWN FROM C-AXISTAPERED SEED
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CRYSTAL GROWTH WITH TAPERED SEEDS
We concluded from many investigations of VBy ggrowth using tapered seeds that obtainingreproducible seeding was more difficult than withthe other two seed types but that LAGB crystalthe other two seed types, but that LAGB crystalgrowth was relatively easy to achieve.
We attribute this to the combination of thetendency of crystal growth to favor orderlinessalong the tapered crucible wallalong the tapered crucible wall.
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CRYSTAL GROWTH WITH TAPERED SEEDS
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CRYSTAL GROWTH WITH FULL-DIAMETER SEED Reproducible seeding processes were established by Reproducible seeding processes were established by
using full-diameter seed with a length greater than about half its diameter.
Precise temperature measurement and control were also important, to keep the seeding interface position consistent within a very few millimeters. co s ste t w t a ve y ew ete s.
Fig. 13(a-1) and (b-1) show X-ray topographic images of a-plane wafers cut from regions that include the
di i t f f t l A d B seeding interface of crystals A and B. Fig. 13(a-2) and (b-2) show X-ray topographic images
of c-plane wafers cut from regions grown after seeding of c plane wafers cut from regions grown after seeding of crystals A and B.
Fig. 13(a-3) and (b-3) show crossed-polarizer images of c-l f f h f ( ) d (bplane wafers cut from near where wafers (a-2) and (b-
2) were cut. 56
CRYSTAL GROWTH WITH FULL-DIAMETER SEED
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DESCRIPTION OF LAST SLIDE’S FIGURE
A.1- a typical result of unsuccessful seeding with yp ga full-diameter seed.
Some sub-grain boundaries generated at the di i t f seeding interface .
we could not grow single crystals without grain boundaries with the seeding process shown in boundaries with the seeding process shown in Fig. 13(a-1).
B.1- more usual results, with successful seeding b f ll di d by full-diameter seed.
B.2- ring-shaped LAGBs at the periphery of the c-plane had a tilt angle of less than 0 5° c plane had a tilt angle of less than 0.5 .
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FIG. 14 IS A PHOTOGRAPH OF 3-, 4- AND 6-IN. DIAMETER CRYSTALS, GROWN FROM C-AXISFULL-DIAMETER SEEDS.
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Fig. 15 shows X-ray topographic images of a 3-in.g y p g p gdiameter crystal grown from c-axis full-diameterseed.X t hi i ( ) d (b) h X-ray topographic images (a) and (b) show an a-plane wafer cut parallel to the growth direction.
LAGBs generated at the periphery near theseeding interface can be seen in Fig. 15(a) andi h d LAGB l i ibl t thring-shaped LAGBs are also visible at the
periphery of the c-plane wafer, as shown in Fig.15(b).( )
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FIG. 15. X-RAY TOPOGRAPHIC IMAGES
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We found, however, that the thickness of thering-shaped LAGB at the periphery of the wafer
ld d t l th l illi tcould decrease to less than several millimeterswith improved seeding conditions, such as theshape of the seeding interface, seeding position,and the keeping time of the seeding process.
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FIG. 16. X-RAY TOPOGRAPHIC IMAGE (A) AND CROSSED-POLARIZER IMAGE (B)
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Summary Su a y 64
SUMMARY
The growth of c-axis sapphire by the traditionalVB method was examined by using thin, taperedand full-diameter seeds and W crucibles shapedto match the seed crystal. Reproducible seedingto match the seed crystal. Reproducible seedingprocesses were established by using precisetemperature measurement and control with thehelp of a newly developed thermocouplehelp of a newly developed thermocouple.
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