Some Ion-Beam Modification Issues: Ion-Induced Amorphisation and Crystallisation of Silicon

摘要

This paper reviews the crystalline to amorphous and amorphous to crystalline phase transformations which can be induced in silicon by energetic ion irradiation. An overview of ion disordering and amorphisation is treated first. At temperatures or irradiation conditions under which the defects generated by the ion bombardment are relatively stable, disorder builds up with ion dose until complete amorphisation occurs. At elevated temperatures, the disordering and amorphisation processes can be considerably more complex. In this regime, dynamic annealing can occur during irradiation, whereby defects can annihilate and cluster to form defect bands. If the temperature is not too high, amorphisation can be nucleated with increasing dose at such defect bands but also at surfaces and interfaces, often well away from the maximum in the (nuclear) energy deposition distribution. Such nucleation-limited amorphisation is difficult to model, particularly as the critical dose for amorphisation depends in a complex way on irradiation temperature, ion mass, ion energy and ion flux. Once an amorphous layer forms in this regime, it can extend with increasing dose in a layer-by-layer manner. Again, there is no accepted model for this process. At higher irradiation temperatures, crystallisation of pre-existing amorphous layers can be induced. This ion beam induced epitaxial crystallisation (IBIEC) process occurs at temperatures well below that at which normal thermal epitaxial crystallisation takes place. This paper then gives an overview of the experiments and observations that have been made to study the IBIEC phenomenon. Studies of the dependence of the growth rate on irradiation temperature, ion dose, ion mass and ion flux again point to a complex process, but it is clear that the crystallisation is induced by ion displacements at or close to the amorphous-crystalline interface. Irradiations under ion channeling conditions, coupled with simulations of displacement distributions, have been used to probe the mechanism in more detail. Although it is now possible to establish that ion-induced defect generation precisely at the amorphous-crystalline interface is responsible for IBIEC, modelling of the process is again difficult. Such difficulties result from complex temperature, ion mass and flux dependencies, whereby the density of the collision cascade and inter-cascade effects appear to play dominant roles. Although much is known about both ion-induced amorphisation and crystallisation processes, the observed dependencies over a broad temperature range cannot as yet be quantitatively modelled.