Metode Pertumbuhan Kristal.pdf

8/10/2019 Metode Pertumbuhan Kristal.pdf

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Physics of Advanced Materials Winter School 2008 1

Growth of Bulk Single Crystal

and its Application to SiC

written by Zhenyu Liu

University of Cambridge, U.K., [email protected]

and Alexandros StavrinadisUniversity of Oxford, U.K., [email protected]

based on the lecture of Prof. Didier ChaussendeLaboratoire des Matriaux et du Gnie Physique (LMGP), France

Abstract

Silicon Carbide (SiC) used to serve as an abrasive due to its hardness. Nowadaysit has become more and more important in the semiconductor industry field

because of its novel electrical properties. In this paper the different growingmethods of single crystal materials were discussed. Particular emphasize was puton the growing parameters and their effect on the quality of single crystal siliconcarbide.

1. Introduction

1.1 Single Crystal

A single crystal, also called monocrystal, is a crystalline solid in which the crystallattice of the entire sample is continuous and unbroken to the edges of thesample, with no grain boundaries.

Grain boundaries have a lot of significant effects on the mechanical, physical andelectrical properties of materials. Therefore, single crystals are demanded inmany fields, such as microelectronics and optoelectronics, as well as structuraland high temperature materials.

Applications of single crystal materials are broad. Silicon single crystals andrelated materials have a large market in integrated circuits industry. Monocrystalsof sapphire are highly demanded in laser devices. For metallic materials, turbineblades can be made of single crystals of superalloys, which can achieve novelmechanical properties.

1.2 Growth MethodsA number of methods to grow single crystal has been developed and employed.

1.2.1 Flame fusion Method


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Figure 1. A sketch of an early Verneuil furnace

Figure 2. A simplified diagram of the Verneuil process

The raw materials are added to the top chamber of the furnace. Oxygen andhydrogen are blown into the cabin for combustion, where a high temperature(higher than 2000oC) is achieved. Liquid droplets of materials form single crystalat the tip. This method could provide a high growing speed. The quality of thecrystal produced, however, is limited by the irregular temperature distributionand cooling velocity. [1,2]

1.2.2 Czochralski Method

In the Czochralski method, a single crystal is pulled from the melt. The steps ofCzochralski method are illustrated in Fig. 3. [3] This method has had nearly onehundred years history, whereas currently is still the most widely used method tofabricate single crystal materials, especially large semiconductor and metallicmaterials. It can produce very high quality crystals.


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Figure 3. Czochralski method process

1.2.3 Bridgman-Stockbarger Method

In this method, the central chamber is turning as well as pushed down, from thehigh temperature region towards the low temperature region. The solid liquidinterface is moved along the charge.

Figure 4. Bridgman-Stockbarger Method

1.2.4 Floating Zone Method

Floating zone crystal growth is a method developed from Bridgman-StockbargerMethod. It is most broadly utilized in growing cylindrical boules of very high purity

Silicon single crystal. Its main advantage is the absence of crucible which is oneof the sources of contamination in the other methods. It is arguably one of themost enabling techniques developed in the information era, to allow integratedcircuits to be mass produced on smaller scales.


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The limitation of this method lies in the choice of appropriate solvent. For eachparticular crystal that is demanded, there should be some certain solvent, eitherwater, or molten salt or metals, to provide a stable crystallization.

1.2.6 Hydrothermal Method

Hydrothermal synthesis can be defined as a method of synthesis of single crystalswhich depends on the solubility of minerals in hot water under high pressure. [4]The crystal growth is performed in an apparatus consisting of a steel pressurevessel called autoclave, in which a nutrient is supplied along with water. Agradient of temperature is maintained at the opposite ends of the growthchamber so that the hotter end dissolves the nutrient and the cooler end causesseeds to take additional growth.

Figure 8. Hydrothermal Method

1.2.7

Sublimation Method

When materials cannot be grown from the liquid phase, the sublimation methodcould be a good alternative. It uses solid in a powder state as a source. Generallythe crystal quality is more difficult to control than when the growth is done fromthe liquid phase.

Figure 9. Sublimation method


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1.3 Basics in Growth Processes

A crystal growth process is an intricate mixture of phenomena which have to beperfectly controlled. Heat transfers are an example of those phenomena.

1.3.1 Three Heat Transfer Ways

Conduction is achieved by interaction between molecules without anymovement of the molecules. Basically it dominates the transfer of heat betweentwo solids in contact. It however also exists in fluids, due to the contact andtemperature difference, but with small effect compare to convection.

Convection is different from convection in that the fluid is free to flow, making themolecules possible to move and interact. In principal it is due to the differentdensity of the same liquid at different temperatures.

Radiation is universal for solids, which could emit as well as absorb photons,carrying energy and transporting between objects.

1.3.2 Overview of a growth process

Figure 10. Overview of a growth process

2.Bulk Growth of Silicon Carbide

2.1 Basics of Growing SiC

Cellsgrains

Segregation- doping-constituant

SizestwinGrain

Dislo

Residual

Fractur

ImpuritiesInclusions,

droplets

Fabrication

THERMICS HYDRODYNAMIC

TRANSPORT

SOLIDMECHANIC

CAPILLARITYINTERFACES

Gradients

GradientsSol

Growth

ChemicalreactionsPollution

Fluid

Convection- Solute

MarangoniConvectionHydrostatic

pressure

Free

surfaces

nucleatio

Homogeneo

Heterogeneo

WettingAdhesio

Strai

Elasticit

Plasticit

ation

Dendritesinclusion


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Figure 11. Possible ways of SiC single crystal growing

Several ways have been attempted to grow bulk silicon carbide. After more than

50 years developing, it has been widely recognized that powder sublimation andgaseous precursors cracking would be the most effective ways.

Figure 12. SiC single crystal structures evolution

Figure 13. Different SiC structures

Solid statereaction

Silica reductionwith carbon

Gaseousprecursors

cracking (CVD)

Powdersublimation

Crystallizationfrom solution

PolycrystalAnd/or

small crystals


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This process of growing is like a black-box process. Several key parameters canbe controlled. Firstly the structure of the crucible is very important since it iswhere the reaction takes place, which could directly affect the temperaturedistribution. RF power plays an important role because it supplies the energy ofthe phase transformation. The source and the seed are demanded for a highquality single crystal material. Some outside conditions, the pressure, for instance,

also can affect the whole growing process.

To judge the single crystal materials grown, there are two main parameters thatare the size and its quality. There are three essential approaches to improve thesize and quality of the single crystals. Modeling and simulation is an economicway to test the growing conditions. Characterization is the most effective way tounderstand the relationship between the growing condition and the crystal quality,in order to improve the processing. Experimentation enables a directmanagement of the growing process.

2.2 Process Related Structural defects

Understanding the nature of structural defects in bulk SiC grown by industryoriented processes and preventing their formation are crucial prerequisites for thetechnological use of this material. Although, the relative literature reveals a largevariety of different point, mesocopic and macroscopic defects, a SiC grower canprioritize the following types of structural anomalies as the ones to first eliminate:a) inclusions b) seed induced macrodefects c) micropipes and dislocations d)lowangle grain boundaries.

Figure 14: Macrodefects at the boule-seed interface related to the sticking qualityof the latter

The term inclusions can describe any nanoscopic or mesoscopic region inside thecrystal of any different chemical or structural composition than the rest of the

materials. Common inclusions are Si or C rich regions. These impurities the sizeof which can be several microns can be formed by the supercooling of Si or C richvapor on the surface of the as grown crystals surface. The temperature gradientinside the growth crucible is a parameter determining to some extend theformation of these inclusions according to the Si-C phase diagram. Other commontypes of inclusions are voids and SiC polytypes.

The quality of the seed used in the sublimation growth of SiC may also inducesevere macroscopic defects in the grown SiC. A common kind of substrateinduced structural faults is macrodefects at the boule-seed interface. Such defectsmay arise by topological anomalies on the surface of the seed which can result tobad sticking of the seed with the grown material (Figure 14). Internal stressesand any other kind of structural deformations of the seed can also propagate atthe near interface volume of the grown crystal resulting to macrodefects.


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SiC crystals also suffer from a variety of dislocations like screw dislocations.These dislocations are energetically expensive and thus unlikely to be producedby plastic deformations. Instead it is believed that they arise from the nucleationstep of the crystal on the seed and their presence is strongly depended on thegrowth mode followed. Another kind of dislocations is the basal plane dislocations(BPD). It is actually believed that the latter are associated with the plastic

deformation of the boule which happens via the buckling and motion of basalplane dislocations (Burgers vector along it). For these dislocations the primaryslip system in H-SiC is a/3(0001). Finally there are the threading edgedislocations (TED). A typical density of TED in SiC wafers 104-105 cm-2. Theircreation is associated with the seed use in the growth and not the growth process.A characteristic of them is that they interact with each other through their strainfield because of their mobility. At high temperature they align themselves incharacteristic arrays and domain walls.

A more complicate category of defects is the micropipes. They are 1D voidsextending along the growth axis of the boule and it is believed that theirformation can be partly understood using the famous Frank model which predictsthe existence of open core dislocation in crystals with large lattice parameters [2].Franks model also predicts a relationship between the open core diameter andthe Burgers vector length which has been validates for SiC. The association ofscrew dislocations and micropipes is clearly indicated when spiral growth arounda micropipes. However Franks model can not be adapted in the case of SiC on itsown: the Burgers vectors of the micropipes (superscrew dislocations) in SiC arevery large (>2c) indicating that micropipe are formed by the merging of morethan one screw dislocations with the same sign. Franks model does not predicthow neighboring parallel dislocation can overcome their repulsive models andmerge and various models have been suggested to explain this phenomenon.

Figure 15. Experimental relation between micropipes burgers vectors and micropipes coreradii [7].

Some of them assign an important role to inclusions. According to [8] the elastic

energy of neighboring dislocation with the same Burger vector can be eliminatedwhen these dislocations meet an inclusion such as polytype or S or C richinclusion. Then minimize their energy by merge forming a superscrew dislocation


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as schematically shown in Figure 15. According to [9] the motion of voids canalso result to the formation of micropipes. According to this model the motion ofvoids cause a recrystallization of the boule and an associated redistribution ofdefects. The dislocation are then pushed together by the growth front and arerelocated at the last solidifies part of the void bottom. This model explains theobservation that micropipes formed at high temperatures appear in groups.

Another model which describes the presence of paired micropipes is described in[10]. This model takes into account the fact that the front of a cantileverovergrowth on the surface of an as grown crystal can flex up or down. In thiscase, when the opposite fronts meet together, a pair of micropipes appears.Despite the applicability of the above models the formation of micropipes has notyet fully understood [11]. Nevertheless a significant improvement on theelimination of micropipe formation on bulk has been made over the last decadee.g. The analysis of defect-etched 4HSiC wafers from low micropipe densityboules has revealed areas up to 25 mm in diameter that are entirely free ofmicropipes [7].

2.3 Alternative Growth processes

Despite the wide applicability of the sublimation process for the growth of bulkSiC wafers it presents some intrinsic and largely unavoidable drawbacks e.g. Itoffers little control over the batch process, limited control of the crystallinity ofthe grown material, limited control over doping and homogeneity of the material,it is an intrinsically unsteady process due to changes of the source seed distance,the temperature field evolution and source impoverishment.

Consequently, some more modern vapor deposition techniques are nowconsidered as potential alternative routes to industrial manufacture of SiC waferssuch as high temperature chemical vapor deposition (HTCVD) and halide chemicalvapor deposition (HCVD). In these processes the growth reaction is fed by

gaseous precursors. HTCVD synthesis of SiC can be briefly described in thefollowing manner: In a vertical growth geometry Si and C precursor gases are fedupwoards through a heating zone to the seed crystal holder. At hightemperatures the reaction of the precursor gases results to the formation of SixCyclusters. These clusters are subsequently sublimed at the same temperature asthey are created and their supersaturation on the surface of a seed results to thegrowth of SiC. It is thus understood that in contrast to the sublimation process, inHTCVD the source material is made inside the growth apparatus. This offerscontinuous feeding of source material with good control over the Si/C ratio. Thisallows for growth of long ingots. An additional advantage of HTCVD is therelatively economical availability of high purity precursor gases [12].

An even more sophisticated technique for the synthesis of SiC is the continuous

feed physical vapor transport (CF-PVT) [2]. In this method the source SiCmaterial is made with a CVD method but it is fed inside porous graphite layersand subsequently transported to the seed by a sublimation-condensationmechanism. This method thus combines the advantages of HTCVD and thematurity of sublimation techniques. The use of precursor gases allows forconstant feed with well controllable source material. In addition, adjustment overthe flow rate of the precursor material offers precise control over thesupersaturation close to the seed surface, keeping temperature and thermalenvironment constant [12]. The deposition rate of this method is largelydetermined by the temperature used in the CVD step and the flow rate of the SiCprecursor which commonly is tetramethylsilane (TMS). Regarding temperature ithas been shown that for T from 1990-1925 the growth is limited by the

sublimation step between the foam top and the graphite lid, for T from 1950-1975 C all the SiC transferred to the upper foam is directly used and transferredto the lid, and for T from 2000C and over the growth is limited by the feeding


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step, i.e. the CVD step. By adjusting temperature and TMS flow rate one canachieve precise control of the supersaturation in the reaction chamber andmaximum growth rate, stable over lengthy growth experiments [12]. The aboveoptimization of the CF-PVT was possible through observation of the masstransport in the reaction chamber with in-situ x-ray imaging. This allowedresearchers to study the mass transport efficiency as a function of the

temperature and gas flow rate and find the optimum working parameters [12].

2.4 The 3C-SiC polytype

The latest developments on the new techniques for the growth of bulk SiC havebeen proved to be fruitful on the synthesis of the 3C- SiC phase. This isparticularly important considering the difficulties associated with the synthesis ofthe 3C phase: conventionally it requires conditions far from equilibrium, like highSi/C ratio in the feedstock (see Figure 13), and low temperatures because at hightemperatures a 3c to 6H solid state transition occurs (Figure 12). In additionthere is lack of 3C seeds. For the above reasons controlled growth of 3C-SiC viathe Lely method is not possible as the used temperatures in this technique aretoo high.

Today, a solution to the above problem has come from the CF-PVT growthtechnique. CF-PVT can yield polycrystalline 3C-SiC at supersaturation of theprecursor feedstock, even at conditions close to equilibrium. Thus at growth ratesRg>1.7mm/h stabilized 3C ingots can be grown even at high temperatures>2200 oC. The morphology of the 3C crystallites can be controlled by varying thetemperature and the saturation of the precursor [7].

The quality of the synthesized 3C-SiC via the CF-PVT has been proved to be good.The crystallites are well faceted and show very good quality of growth sectors.When the 3C is grown on a VLS 3C-SiC substrate complete elimination of DPBs(Figure 16), significant reduction of MTLs and SFs density of 104cm-1 can be

achieved. Finally a degradation of the epilayer as the distance from the seedincrease must be noted [2]

Figure 16. Presence of DPBs in 3C-SiC in relationship to the substrate [2].

Directly on 6H-SiC On VLS 3C-SiC

DPB


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3.Conclusions

SiC is a technologically important material with a growing scientific communityand industry around it. The need for single phase and defect free bulk SiC wafersrequires constant coupling of growth, characterization and modeling investigationof this material. The polytipsm of SiC, the intrinsic drawbacks of the widely useLely growth method and the variety of the parameters determining the yield ofbulk synthesis techniques makes bulk SiC synthesis a hard task. Today the 6Hand 4H polytypes are the most mature by sublimation or HTCVD processes. Thistask required thousand experiments coupled with simulations over severaldecades. Nevertheless, despite the many years of research on the synthesis ofbulk SiC many milestones are yet to be reached. A classic example is thesynthesis of the 3C-SiC polytype. Recent developments on the synthesis of thisphase via the CF-PVT are more than a technological breakthrough: they contrastprevious believes that 3C-SiC can be grown only at low temperatures and farfrom equilibrium. It is therefore obvious that fundamental phenomena like the3C-6H solid state transition are far from being well understood.

4. References

[1] K. Nassau et al., "Reconstructed" or "Geneva" ruby, Journal of CrystalGrowth, Vol. 5, Iss. 5, October 1969, Pages 338-344.

[2] D. C. Harris et al., A peek into the history of sapphire crystal growth,Proceeding of SPIE, Vol. 5078, September 2003, Pages 1-11.

[3] J. Hrknen et al., Radiation hardness of Czochralski silicon, FloatZone silicon and oxygenated Float Zone silicon studied by lowenergy protons, Nuclear Instruments and Methods in Physics ResearchSection A: Accelerators, Spectrometers, Detectors and Associated

Equipment,Volume 518, Issues 1-2, 1 February 2004, Pages 346-348

[4] Laudise, R.A. et al.. Hydrothermal Synthesis of Crystals C&ENSeptember 28: 30-43

[5] A. R Powel, L.B. Rowland, SiC Materials- Progress Status and potentialRoad Blocks, Proceedings of the IEEE, vol 90, no 6, pp- 942, June 2002

[6] J. Giocondi, G. S. Rohrer, M. Skowronski, V. Balakrishna, G. Augustine, H. M.Hobgood and R. H. Hopkins, An atomic Force microscopy study ofsuper-dislocation/micropipe complexes on the 6H-SiC(0001) growth

surface, J. Cryst. Growth, 181, 6, (1997) pp- 351-362

[7] T. A. Kuhr, E. K. Sanchez, , M. Skowronski, W. M. Vetter, M. Dudley,

Hexagonal voids and the formation of micropipes during SiCsublimation growth, J. Appl. Physics, 89,2001, p. 4625

[8] M. Dudley, X.R. Huang, , W. Huang, A. Powell, S. Wang, P. Neudeck, Themechanism of micropipe nucleation at inclusions in silicon carbide,Appl. Phys. Lett, 75, 1999, p. 784

[9] X. Ma, Super screw diclocations in silicon carbide: Dissociation,agreegation and formation, J. Appl. Physics, 99, 2006, p. 63513

[10] M. Soueidan, G. Ferro, B. Nsouli, M. Roumie, E. Polychroniadis, M. Kazan, S.Juillaguet, D. Chaussende, N. Habka, J. Stoemenos, J. Camassel, Y. Monteil.Characterization of a 3C-SiC Single Domain Grown on 6H-SiC(0001)by a Vapor-Liquid-Solid Mechanism.Crystal Growth & Design 6 (2006)

2598-2602.


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[11] D. Chaussende, F. Mercier, A. Boulle, F. Conchon, M. Soueidan, G. Ferro, A.Mantzari, A. Andreadou, E.K. Polychroniadis, C. Balloud, S. Juillaguet, J.Camassel, M. Pons. Prospects for 3C-SiC Bulk Crystal Growth.Proceedings of the E-MRS 2007 Spring meeting, 28 May - 1 June, 2007,Strasbourg, France, p. 1123-1127.

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Metode Pertumbuhan Kristal.pdf