DIRECT-ENERGY PROCESSES AND PHASE-TRANSITIONS IN SILICON

The phase transition of amorphous to single-crystal silicon has been investigated not only by conventional heating in a furnace but under direct-energy processes like pulsed-laser and ion beam irradiation. The first method allows the experimental determination of the free-energy-temperature diagram for amorphous, liquid and crystalline silicon. Due to the very fast heating and cooling the amorphous-to-liquid transition can be investigated in both directions. Ion beam irradiation induces either a layer-by-layer amorphization or crystallization by the movement of the initial alpha-Si/c-Si interface according to the substrate temperature. The two processes are governed by different types of defects created by the beam in the amorphous and in the crystalline side of the interface. The existence of a native-oxide layer at the interface between single crystal and deposited layer retards the ion beam crystallization until oxygen atoms are dispersed by beam mixing in the matrix. A recent alternative way of crystallizing deposited layers is by short high-temperature anneals obtained by incoherent-light irradiations. In this case the rupture of the native-oxide layer is achieved by the agglomeration of oxide into beads, thus allowing the realignment. This technique appears to be particularly promising for several technological applications.