Existence of electronic excitation enhanced crystallization in GeSb amorphous thin films upon ultrashort laser pulse irradiation.

The study of the interaction of ultrashort laser pulses with solids has raised some important questions concerning the nature of the transformations occurring at the material surface in the presence of very strong levels of electronic excitation. The existence of electronic excitation induced phase transitions has been already demonstrated in several materials (Si [1], GaAs [2–4], graphite [5]). In the case of crystalline semiconductors, excitation with pulses shorter than the longitudinal optical phonon emission time [6] may lead to a structural instability of the lattice, leading to the formation of a metastable transient phase with (semi)metallic character. It is not clear, however, whether the presence of such transient strong electronic excitation and/or transient phases may influence the final state or structure induced since, once the energy of the excited carriers has been transferred to the lattice, the structural transformation path is thermal in nature. Different nonthermal mechanisms which could lead to enhanced crystallization and to crystal damage recovery upon pulsed laser irradiation were proposed during the early 1980s [7,8]. More recently, both photonic effects and electron-hole plasma induced network softening have been invoked to explain structural changes induced in amorphous Si and Ge upon ns and ps laser pulse irradiation [9–11]. High Sb-content GeSb amorphous and crystalline thin films have been shown to both crystallize and amorphize upon irradiation with subnanosecond laser pulses. Both processes show optical contrast [12] making this material a potential candidate for applications in ultrashort laser pulse driven phase change optical recording [13,14]. These characteristics also make this material interesting for the study of possible high electronic excitation induced and enhanced crystallization processes. The aim of this work is to elucidate whether the high electronic excitation, induced by irradiation with subnanosecond laser pulses, can induce or enhance the crystallization process of the amorphous phase. Such an enhancement process should lead to appreciable changes in the minimum energy density required for crystallization as the laser pulse duration changes, according to the differences in the