Ion implantation of Carbon and Silicon into Ge 2 Sb 2 Te 5 : Ion Profiles and Post Crystallization Redistribution
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Ion implantation of Carbon and Silicon into Ge2Sb2Te5: Ion Profiles and Post Crystallization Redistribution Guy M. Cohen, Simone Raoux, Marinus Hopstaken, and Siegfried Maurer IBM T.J. Watson Research Center, Yorktown Heights, New York 10598 ABSTRACT Ion implantation of Ge2Sb2Te5 (GST) enables localized doping of the film by using conventional lithography. Although the doped region dimensions and the doping concentration profile are defined by the opening in the mask and the ion energy, longitudinal and lateral straggling of implanted ions leads to a spread in the ions final location. Additionally, a thermal treatment such as one that induces a phase transition may lead to redistribution of the implanted dopants and further increase the spread. In this work we demonstrate doping of GST by ion implantation. Using Secondary Ion Mass Spectrometry (SIMS) we studied the as-implanted doping profiles obtain by ion implantation of carbon and silicon into GST. We also investigated by SIMS the dopant redistribution following a recrystallization annealing. The as-implanted ion profiles were found to be in fair agreement with TRIM simulation. The dopants profiles show little change after a crystallization annealing at 200°C for silicon doping and at 350°C for carbon doping. INTRODUCTION Doping of GST is used to modify the film properties such as the crystallization temperature. Typically, the dopant is added to the film during deposition so the resulting film is uniformly doped. Examples of commonly used dopants are nitrogen [1,2], silicon [3] and more recently carbon [4,5]. In this work we demonstrated successful GST doping by ion implantation. Since ion implantation enables doping in selective regions of the film, for example by the use of a lithographically defined mask, we also investigated the dopant depth profile in as-implanted films and in films following a re-crystallization anneal. Figure 1a illustrates an ideal, selectively doped, region in a GST film. The region is uniformly doped and its edges are perfectly aligned to the mask edge. In practice, as illustrated in Figure 1b, due to longitudinal and lateral straggling of the implanted ions the dopant concentration as a function of the depth will diverge from a box profile and ions will also spread laterally beyond the mask edge. Additional thermal treatment such as for crystallization annealing would lead to further spread of the dopant due to diffusion as shown by Figure 1c. This work provides a first account to doping of GST by carbon and silicon ion implantation and provides preliminary data related to dopant redistribution in GST. ion implantation
uniformly doped region
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Figure 1: The various processes that can lead to spreading of doped region defined by a mask. (a) Ideal dopant profile, (b) realistic dopant profile due to longitudinal and lateral straggling, (c) dopant profile further broadened by thermal treatment.
EXPERIMENT GST films in the range of 20 nm to 60 nm were deposited by magnetron sputtering from a compound
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