Free-electron laser studies of energy transfer mechanisms in semiconductors doped with transition series ions

Shallow levels determine electrical and optical properties of semiconductors. Mid-infrared radiation from a free-electron laser can be used for an effective ionization of shallow impurities, leading to a variety of effects. In contrast to thermal ionization, the optically induced ionization process can be tuned to a particular level by adjusting the wavelength. In this way, different impurity and defect levels can be selectively addressed. The short-pulsed output of the free-electron laser allows the experiments to be performed in a manner, which utilizes its unique characteristics. In this contribution, we show how two-color spectroscopy with a free-electron laser can be used to unravel energy transfer between different centers in semiconductor matrices. In particular, energy storage at shallow centers in silicon and mid-infrared-induced Auger recombination process of long-living optically active centers will be discussed. Specific examples for rare earth- and transition metal-doped silicon and rare earth-doped III–V semiconductors will be presented.