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«1. Introduction. Nucleation and growth processes arise in a variety of natural and technological applications (cf. [12] and the references therein), ...»

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We shall set S(y; x, t) = 0 if (y, 0) ∈ C(x, t), since in this case a nucleation at y will / never create a grain covering x at time t.

The Hopf-Lax formulas used to derive the causal cone offer the possibility to interpret maximal nucleation times from a geometric optics point of view (cf. [5]).

Note that S(y; x, t) corresponds to the time, when a front starting at x at time t and travelling (in negative time direction) with the Hamiltonian H(x, t, p) := G(x, t)|p| arrives at y. It is well-known (cf. [42]) that (fixing x and t), the arrival time σ = t − S is a (positive) viscosity solution of the eikonal equation

–  –  –

Example. In [41], the growth of a grain with a time-homogeneous growth rate G(x, t) ≡ G2 (x) has been considered for linear functions G2 (x) = ax + b and spatial dimension d = 2 ( a ∈ R2, b ∈ R+ ). Without restriction of generality one can assume that a = (a1, 0), and by solution of a problem in the calculus of variation, analogous to [41] the causal cone is obtained as

–  –  –

the region union of the random grains, which is now a random closed set (RACS).

The well known theory of Choquet-Matheron [33] shows that it is possible to assign a unique probability law associated with a random closed set Ξ ⊂ Rd on the measurable space (F, σF ) of the family of closed sets in Rd endowed with the σalgebra generated by the hit-or-miss topology, by assigning its hitting function H Ξ.

The hitting function of Ξ is defined as HΞ : K ∈ K → P(Ξ ∩ K = ∅).

More precisely, we define a random closed set Ξ as a random object Ξ : (Φ, A, P) → (F, σF ).

Moreover, we denote by K the family of compact sets in Rd.

In our case, using the above analysis of the growth process, it is possible to show [17] that a unique probability measure PΘ can be associated with the germgrain process Θ = Θ(t), t ∈ R+. From now on, in a canonical sense, we shall denote by P this probability measure, and by E (respectively V), expectations (respectively variances) with respect to this probability, whenever they exist as finite values.

3.1. Stochastic Geometric Measures. In the following we discuss the quantitative description of the geometric process Θ, which can be obtained in terms of mean densities of volumes, surfaces, edges, and vertices (at the respective Hausdorff dimensions), based on the analysis in [14, 43]. Let Θ(t) be a d−dimensional random closed set having boundary of Hausdorff dimension d − 1, with integer d.

The mean local volume density and mean local surface density, respectively, of the random closed Θ(t) at point x are defined by E[Hd (Θ(t) ∩ Br (x))] ρ(x, t) := lim (3.2) Hd (Br (x)) r→0

–  –  –

. for any B ∈ BRd.

3.2. The Volume Density. It seems obvious that ρ will be influenced by the nature of the nucleation and growth processes. However, the probability that the point x is not covered by time t may be expressed in terms of the probability that no nucleation event occurs inside the causal cone (see also [9] ),

–  –  –

For small free volume density ρ∗, the first order terms on the right-hand side dominate and thus, ρ(x, t) ≈ ρ∗ (x, t). For increasing ρ∗, there is an obvious saturation effect yielding ρ(x, t) ρ∗ (x, t). This relation can also be express as an evolution equation

for the (volume) density:

–  –  –

which widely is known as (Johnson-Mehl-) Avrami-Kolmogorov formula (cf. [18, 23, 27, 38, 46, 50] and the references therein).

4.1. Differential Equations in Spatial Dimension One. For nucleation and growth processes in spatial dimension one (d = 1) one can easily show by a comparison result for ordinary differential equations that

–  –  –

This shows that E(x, t; s) ⊂ B(x; R+ (x, t, s)), where B(x; R) denotes the ball around x with radius R. By similar reasoning we can deduce the inclusion E(x, t; s) ⊃ B(x; R− (x, t, s)) with

–  –  –

Note that the constant α0 3 T is a bound for the number of nucleations in a microscale cell in the time interval [0, T ], which is usually small or of order one.

5. Variance and Mesoscale Averaging. In the following we discuss the variance of the random variable χ(x, t) := IΞ(t) (x), taking the values 0 or 1 only, and the variance of a local averaging on a mesoscale, which yields error estimates for corresponding mesoscale quantities.

5.1. Variance of the Volume Density. The volume density ρ(x, t) is the expected value of the random variable χ(x, t). Hence, χ(x, t)2 = χ(x, t) and we obtain for the local variance

–  –  –

Clearly, with such a pessimistic estimate the variance is not decreased, since we have not introduced information about the possible dependence or independence of the random variables χ(x, t) at different locations. This will be done in the following section within a local averaging procedure.

5.2. Error Estimates for Mesoscale Averaging. In the following we shall consider local averages of the volume density at the mesoscale. We assume that λ = 3N with ≥ G0 T. We perform the local averaging over a cell

–  –  –





and therefore we have to expect that the variance grows in time like T d.

6. Extensions. In the following we consider some possible extensions of the mesoscale averaging to further situations of interest. We shall not develop a detailed theory for these cases, but only outline the major analogies and differences to the growth considered above.

6.1. Anisotropic Growth. In anisotropic growth, which appears for many materials with an underlying crystal structure such as metals or semiconductors, the form of the normal velocity G in the growth model (1.1) has to be changed to

–  –  –

We shall assume that H(x, t,.) is a convex function for all (x, t). In this case, the level set formulation of the growth model is given by Ω(t) = {φ(., t) ≤ 0} for φ being the viscosity solution of the Hamilton-Jacobi equation

–  –  –

The causal cone can be defined in the same way as in the isotropic growth situation above, and using (6.1) one can also derive an analogous Hopf-Lax representation of the causal cone as

–  –  –

The basic ideas and results of mesosale averaging such as the Avrami-Kolmogorov formula remain unchanged in the anisotropic setting, the only difference is the slightly more complicated computation of the causal cone.

6.2. Polycrystalline Growth. A challenging example in modern semiconductor processing is the growth of polycrystalline structures on amorphous substrates (cf. e.g. [39]). In these processes, a crystalline material is deposited on an amorphous substrate and crystals nucleate randomly. Since the material is crystalline, each nuclei has a special orientation, which is a random variable in the nucleation process.

Hence, nucleation should be modeled as a Poisson process in D × R+ × S d with a rate α = α(x, t, ν) for ν ∈ S d.

The initial orientation of the nuclei determines the subsequent anisotropic growth of the crystal, i.e., the level set formulation of the growth of the j-th grain becomes Ωj (t) = {φj (., t) ≤ 0}

–  –  –

which is the polycrystalline equivalent of the classical Avrami-Kolmogorov formula (4.2).

Acknowledgements. Stimulating discussions on the subject of the paper are acknowledged to Alessandra Micheletti (University Milano) and on polycrystalline growth to Hajdin Ceric (Technical University Vienna) and Peter Smereka (University Michigan).

REFERENCES

[1] I.Athanasopoulos, G.Makrakis, J.F.Rodrigues, eds., Free Boundary Problems: Theory and Applications (Chapman & Hall/CRC, Boca Raton, 1999).

[2] M.Avrami, Kinetics of phase change I-III, J.Chem.Phys. 7 (1939), 1103-1112; 8 (1940), 212G.Barles, H.M.Soner, P.E.Souganidis, Front propagation and phase-field theory, SIAM J. Cont.

Optim. 31 (1993), 439-469.

[4] P.Bennema, G.H.Gilmer, Kinetics of crystal growth, in [28], 263-327.

[5] H.A.Buchdahl, An Introduction to Hamiltonian Optics (Cambridge University Press, 1970).

[6] M.Burger, Growth fronts of first-order Hamilton-Jacobi equations, SFB Report 02-8 (University Linz, 2002).

[7] M.Burger, V. Capasso, Mathematical modelling and simulation of non-isothermal crystallization of polymers, Math. Models and Meth. in Appl. Sciences 11 (2001), 1029-1054.

[8] M.Burger, V.Capasso, G.Eder, Modelling crystallization of polymers in temperature fields, ZAMM 82 (2002), 51-63.

[9] M.Burger, V.Capasso, C.Salani, Modelling multi-dimensional crystallization of polymers in interaction with heat transfer, Nonlinear Analysis B, Real World Applications 3 (2002), 139-160.

[10] R.E.Caflisch, D.G.Meyer, A reduced order model for epitaxial growth, in: S.Y.Cheng, C.W.Shu, T.Tang, Recent Advances in Scientific Computing and Partial Differential Equations (AMS, Providence, 2003).

[11] J.W.Cahn, The time cone method for nucleation and growth kinetics on a finite domain, in:

Proceedings of an MRS Symposium on Thermodynamics and Kinetics of Phase Transformations, MRS Symposium Proceedings 398 (1996), 425-438.

[12] V. Capasso, On the stochastic geometry of growth, in: T.Sekimura et. al., eds., Morphogenesis and Pattern Formation in Biological Systems (Springer,Tokyo, 2003), 45-58.

[13] V.Capasso, D.Bakstein, An Introduction to Continuous-Time Stochastic Processes (Birkh¨user, a New York, 2005).

[14] V.Capasso, A.Micheletti, Local spherical contact distribution function and local mean densities for inhomogeneous random sets, Stochastics Stoch. Rep. 71(2000), 51-67.

[15] V.Capasso, A.Micheletti, Stochastic geometry of spatially structured birth-and-growth processes. Application to crystallization processes, in: E. Merzbach, ed., Spatial Stochastic Processes, Lecture Notes in Mathematics - CIME Subseries (Springer, Berlin, Heidelberg, 2003), 1-40.

[16] V.Capasso, A.Micheletti, On the hazard function for inhomogeneous birth-and-growth processes Preprint (University Milano, 2004).

[17] V.Capasso, C.Salani, Stochastic birth-and-growth processes modelling crystallization of polymers in a spatially heterogenous temperature field, Nonlinear Analysis, Real World Applications 1 (2000), 485-498.

[18] J.W.Christian, The Theory of Transformations in Metals and Alloys (Pergamon, Oxford, 1981).

[19] P.Colli, C.Verdi, A.Visintin, eds., Free Boundary Problems: Theory and Applications (Birkh¨user, Basel, 2003).

a [20] F.Da Lio, Remarks on the strong maximum principle for viscosity solutions to fully nonlinear parabolic equations, Comm. Pure Appl. Anal. 3 (2004), 395-415.

[21] M.Delfour, J.P.Zolesio, Shapes and Geometries (SIAM, Philadelphia, 2001).

[22] G.Eder, Crystallization kinetic equations incorporating surface and bulk nucleation, ZAMM 76 (1996), S4, 489-492.

[23] G.Eder, Fundamentals of structure formation in crystallizing polymers, in K.Hatada, T.Kitayama, O.Vogl, eds., Macromolecular Design of polymeric Materials (M.Dekker, New York, 1997), 761-782.

[24] L.C.Evans, Partial Differential Equations, Graduate Studies in Mathematics 19 (AMS, Providence, RI, 1998).

[25] H.Federer, Geometric Measure Theory (Springer, Berlin, Heidelberg, New York, 1969).

[26] F.Gibou, C.Ratsch, R.E.Caflisch, Capture numbers and scaling laws in epitaxial growth, Phys.

Rev. B 67 2003, 155403.

[27] L.Granasya, T.B¨rzs¨nyi, T.Pusztai, Crystal nucleation and growth in binary phase-field theoo ory, J. Cryst. Growth 237-239 (2002), 1813-1817.

[28] P.Hartman, Crystal Growth: An Introduction (North Holland Publishers, Amsterdam, 1973).

[29] J.Herrick, S.Jun, J.Bechhoefer, A.Bensimon, Kinetic model of DNA replication in eukaryotic organisms, J. Mol. Biol. 320 (2002), 741-750.

[30] D.Hug, G.Last, W.Weil, A local Steiner-type formula for general closed sets and applications, Math. Z. 246 (2004), 237-272.

[31] W.Johnson, R.Mehl, Reaction kinetics in processes of nucleation and growth,Trans. AIME 135 (1939), 416-442.

[32] A.N.Kolmogorov, On the statistical theory of the crystallization of metals, Bull.Acad.

Sci.USSR, Math.Ser. 1 (1937), 355-359.



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