At uniform excitation of semiconductors by laser radiation with pre-threshold power, locally melted regions are formed on irradiated surfaces. This is induced by thermo diffusive instability of a distribution of uniformly generated electron-hole plasma. The shapes of locally melted regions give rise to a great variety of interesting surface patterns. A mathematical model of the surface dynamics, when the instability of the melt front arises along a chosen wave vector, is proposed. The results of computer simulation of interface dynamics of solitary melted region are compared with experimental data.