Abstract

A detailed understanding of hot electron dynamics in materials can be obtained through the use of ultrafast optical probing techniques. Optical methods uniquely provide a direct means for performing state-resolved electron dynamics, In recent years, ultrafast optical methods have been used intensively to study hot electron dynamics in direct gap semiconductors and in metals. Among the goals is to determine electron-electron and electron-phonon coupling tor highly energetic, nonequilibrium electrons in the conduction band (cb). In contrast, there have been very few such studies of silicon because conventional optical pump-probe schemes are difficult to interpret for indirect gap materials. This is unfortunate since silicon is one of the most important of all solid state materials, with hot electron dynamics its key property to be exploited in new electron devices. Here we report the first direct subpicosecond (150 fs) time-and energy-resolved measurement of electron dynamics in optically excited silicon. This is accomplished in a laser pump-probe format by measuring the photoelectron spectrum generated by an ultrashort UV probe pulse. By this method, we have directly obtained the time-evolution of the photo-excited electron distribution function (bulk and interface) in Si with 150 fs resolution. These measurements would not be possible by the conventional reflectivity/ transmission or luminescence schemes.

© 1994 Optical Society of America