Clustering Equilibrium and Deactivation Kinetics in As doped Si
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Clustering equilibrium and deactivation kinetics in As doped Si Dario Nobili, and Sandro Solmi, CNR-LAMEL Institute, Via Gobetti 101-40129 Bologna, Italy, Jenta Shao, Shanghai Institute of Optics and Fine Mechanics, Shanghai, China, Marco Merli INFM, University of Ferrara, Ferrara Italy
ABSTRACT Clustering equilibrium was studied on silicon on insulator specimens uniformly doped with As at concentrations CAs up to 7.6 x 1020 cm-3. Values of the carrier density n* after equilibration at 700, 800 and 900°C are reported. It is shown that both the concentration and the temperature dependence of n* can be accurately simulated by a simple cluster model detailed in the Appendix. The analysis of the clustering kinetics of these compositions was performed at temperatures in the range 550 to 800°C. It is found that at 800°C the kinetics accurately complies with the rate equation: -dn/ dt = A{exp[-(E-αn)/kT] – (n0- n)/( n0- n*) exp[-(E-αn*)/kT] which is the one reported in Ref.[14] complemented by the second term on the right to account for the declustering process. Deviations leading to rates lower than predicted by the above equation are observed by annealing at lower temperatures. The dependence of this phenomenon on composition and temperature is reported.
INTRODUCTION Clustering of the dissolved As atoms is accepted as the phenomenon responsible of the electrically inactive unprecipitated dopant in Silicon. This aggregation process presents an unusual amplitude, in fact the limiting concentration CSat [1] of dissolved As in equilibrium with its precipitates, the monoclinic SiAs, exceeds by one order of magnitude the limiting value ne of the electrically active dopant [2,3]. Different cluster models were proposed which hypothesize the formation of aggregates, electrically inactive at room temperature, consisting of a fixed number of dopant atoms in specific lattice configurations which can also include vacancies or even interstitials [4-11]. The number of As atoms in the clusters range from 2 to 4, following the different models. Theoretical calculations indicate that different types of clusters can coexist in equilibrium [8-11], a conclusion which is supported by recent experiments on reverse annealing phenomena [12]. A mass action equilibrium is assumed to exist between the clustered and the ionized As. Quantitative data on this equilibrium are still poor, although it is known [1,2,13] that in heavily doped specimens the carrier density depends only on temperature and is insensitive to the excess dopant in the limits of sensitivity of the techniques. To define a functional relation between the ionized As and the dopant concentration CAs, additional results of the carrier density n* after thermal equilibration at constant temperatures, for a wide range of CAs values, are needed. We determined accurate values of the carrier concentration n*, after equilibration at 700, 800 and 900ºC, of specimens uniformly doped with As in a wide range of concentrations. These figures show a pronounced saturation of n* with increasing CA
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