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V A.Zhikharev, F R.Batyrshin Zavoisky Physical Technical Institute, Kazan, 420029, RUSSIAN FEDERATION

INTRODUCTION The swelling of metals under heavy particles irradiation is an old problem of radiation physics The void creation in metals is usually explained by coagulation processes in oversaturated "solution" of vacancies generated by irradiation [1]. Oversaturation of vacancy "solution" is provided in metals by the presence of free moving dislocations which effectively absorb the interstitial component of Frenkei pairs [2]. In semiconductors such kind of sinks for interstitials is practically absent and the swelling is expected to be strongly suppressed. However a lot of experimental results on radiation induced swelling in semiconducting materials have been reported [3]. At room and higher temperatures the diffusive path of point defects often exceeds the depth of its generation. So the surface of irradiated sample can be an effective sink for the defects. This is most important for thin semiconducting films. The boundary conditions can sufficiently change the concentrations of different components of Frenkel pairs close to the sample surface and, thus, change the efficiency of the void formation. In this paper the role of boundary conditions in the growth of voids in thin films is being investigated,

THE MODEL The distribution of point defects produced by ion irradiation is supposed to be a function on z ( z-axis is perpendicular to irradiated surface) and homogeneous in (x,y) plane. Let Cv(t,z) and ci(t,z) stand for the concentrations of vacancies and interstitials, correspondingly. The diffusion of defects along z-axis is to change the concentrations in keeping with the equations: dCv,i(t,z) / dt = DV d2 CVji(t,z)/dx 2 + Gv,i - Svi - A

(1)

where DV,1 - the diffusion coefficients. The terms Sv,i,Gv,i and A describe the absorption of vacancies (interstitials) by voids, generation of defects and recombination, correspondingly. The diffiusion of defects in (x,y) plane reveals itself in the influence on S,G and A terms of the equations (1). The generation function GV,i is modeled in the present paper by Gaussian distribution centered at zo=0.7 Rp ,where Rp is the mean projectile path of ions in the material. Recombination term A can be written as follows [4]1 A(CN',Ci) = ct (DN+Di) Cv(t,z) Ci(t,z) Q (t)

155 Mat. Res. Soc. Symp. Proc. Vol. 389 0 1995 Materials Research Society

(2)

where ot is a recombination constant, Q(t) is a factor that takes into account the fact that recombination occurs only in the void-free volume of the sample. This factor will be determined below. The function Sv,i which describe the absorption of vacancies and iterstitials by voids was investigated for spatial homogeneous system in the paper [51 In this paper because of strong zdependence of defect generation we will divide the sample volume into the layers of homogeneity (LH) with a thickness of LO within which the concentration Cmvoid(t) of voids and mean radius RT(t) can be determined for the given moment ( m is a number of LH