Nanocrystals in crystalline silicon: Void formation and hollow particles

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S. Honda, C.W. White, R.A. Zuhr, and L.A. Boatner Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received 5 April 2001; accepted 2 July 2001)

Nanophase precipitates of CdS formed in amorphous SiO2 by ion implantation and thermal processing have recently been found to exhibit a “hollow-particle” or “shell-like” microstructure. The present investigations show that this hollow-particle microstructure can be reproduced for a variety of materials other than CdS, and these results provide new insight into the mechanisms responsible for the formation of hollow precipitates embedded in solid hosts. Various elemental metal nanocrystals were formed in (100)-oriented crystalline Si hosts by ion implantation coupled with thermal treatments in which the annealing parameters were varied to investigate the “hollow-particle” formation conditions. The results indicate that depending on the melting points and vapor pressure of the precipitates or on the initial state of the host material, several processes acting either independently or in concert can lead to hollow precipitate formation. First, the implantation of materials having a high vapor pressure, either at the implant temperature or when heated during annealing, can lead to the formation of cavities in the crystalline host. Hollow precipitates can then form by a partial filling and coating of the cavity walls by the implanted species in a diffusion-based gettering/ripening process. Internal void formation can also occur or be enhanced by volume contraction during cooling if the particle solidifies from a liquid phase.

I. INTRODUCTION

Ion implantation and thermal processing represent a versatile approach to materials modification that has recently been applied to the formation of either elemental or compound nanocrystalline precipitates in a wide range of host materials (e.g., see Refs. 1–3 for numerous recent references on the formation of nanocrystals by ion implantation). In this technique, high-fluence ion implantation produces a supersaturation of one or more implanted species in the near-surface region of a crystalline or amorphous solid. The implanted ions are then precipitated out of the host by a thermal treatment that is performed either during or subsequent to the implantation process. Nanophase particles formed in this way frequently exhibit electronic properties that differ from those of the corresponding bulk material, and in the case of both metal and semiconducting nanocrystals, these properties have led to the identification of a number of potential applications.4–6 The intrinsic microstructural properties of nanocrystalline precipitates formed by ion implantation/thermal treatment as well as the structural relationships between the precipitates and the solid host are frequently novel or 2670

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J. Mater. Res., Vol. 16, No. 9, Sep 2001 Downloaded: 30 Mar 2015

unusual.7–9 For example, we have found that CdS nanoparticles formed in amorphous SiO2 by this approach often contain a central “light-c