Epitaxial Crystallisation of Doped Amorphous Silicon
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EPITAXIAL CRYSTALLISATION OF DOPED AMORPHOUS SILICON R.G. ELLIMAN**, S.T. JOHNSON*, K.T. SHORT* AND J.S. WILLIAMS* *Microelectronics Technology Centre, RMIT, Melbourne 3000 Australia; **Joint Appointment with CSIRO, Division of Chemical Physics, Clayton 3168 Australia.
ABSTRACT
This paper outlines a model to account for the influence of doping and electronic processes on the solid phase epitaxial regrowth rate of ion implanted (100) silicon. In addition we present data which illustrates good quality epitaxial crystallisation of silicon at 0 400 C induced by He+ ion irradiation. We tentatively suggest that electronic energy-loss processes may be responsible for this behaviour.
INTRODUCTION Amorphous silicon layers produced in single crystal substrates by ion implantation are metastable and recrystallise epitaxially from the crystalamorphous interface when annealed at temperatures of-500 0 C 1l]. For uncontaminated layers; i.e. those amorphised by implantation with silicon ions, the kinetics of regrowth have been extensively investigated[l-3]. Indeed, the substrate orientation dependenceSl~is reasonably well understood in terms of structural models of the amorphous-crystalline interface, such as that proposed by Spaepen and Turnbull £4,5). The regrowth of layers containing impurities is, however, less well understoodt0,6,7] . In particular, it has been observed that the presence of electrically active impurities can enhance the epitaxial growth rate by more than an order of magnitude Q,6-83 Furthermore, if equal concentrations of both n-type and p-type dopants are introduced into silicon by implantation, a compensating effect is observed in which the epitaxial growth rate is found to return to that of undoped silicon[D,8,a. These results clearly indicate the importance of electronic processes in determining the recrystallisation kinetics. In this paper we discuss a phenomenological model to explain the role of electronic doping in enhancing the rate of epitaxial crystallisation. In addition, we present new data on helium ion-beam-induced epitaxy of amorphous silicon at temper0 atures as low as 400 C. These intriguing results suggest further experiments which can be devised to probe details of solid phase recrystallisation processes in semiconductors. REGROWTH MODEL The Spaepen and Turnbull structural model of the crystal-amorphous interface during recrystallisation is based on minimum free energy and bonding argumentst5). They suggest that during regrowth the crystal-amorphous interface will facet to maximise the exposure of minimum free energy (111) planes and conclude that the interface consists of (111) terraces, bounded by< 110> ledges. Crystal growth is envisaged to proceed along these ledges, initiated at appropriate nucleation sites. The number of ledges, and hence nucleation sites, is determined by the angle the interface plane subtends to the (111) plane. From geometric considerations, the ledge concentration, and hence the regrowth rate, is expected to be approximately proportional to sin ( 0), where C i
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