Pulsed laser-induced structures on semiconductor surfaces

Abstract

The laser-induced spatially periodic structures are produced of the surfaces of silicon samples with various surface conditions by the use of 10 psec mode-locked pulses and 20 ns Q-switched pulses of a ruby laser. The inspection of radial struture variations shows that the periodic structure formation is a phenomenon realted to the surface melting. Tte dominant structures can be classifed into three types according to the orientation and the spatial period of the induced structure. For ppolarized laser pulses, the dominant types are the $\Delta_-$ and $\Delta_+$ types which are orthogonal to the plane of incidence. For s-polarizd laser pulses, the $\Delta_0$ type is dominatly produced with an orientation parallel to the plane of incidence. Backward diffraction pattern from the periodic structure indicates that the peridic structure generally satisfies the selection rule imposed by the momentum matching condition of the scattered surface fields. The presence of the scattering centers on the surface influences the resulted periodic structures. Near the linear scattering centers, the periodic sturctures are formed parallel to the scattering centers. But aruond the isotropic point scattering centers, the structures take the form of confocal ellipses. These surface features are caused by the fact that the scattering centers have dimensions large compared to those of the microscopic surface roughnesses, and consequently play dominant role in the light scattering on the surface. Previously unappreciated deeiptic ripple patterns are found and explained as the trails of the capillary wave excited on the molten Si surface due to the radially non-uniform deposition of energy. Dynamic aspects of the periodic structure development are studied by measureing the post-irradiation diffraction efficiecy and time-resolved diffractionon on a shot-to-shot basis. The depth of the periodic sturcture is estimated from the measured post-irradiation diffraction efficiencies. Under the m...