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We report high-harmonic generation up to the ninth order from boron-doped Si at room temperature, pumped by intense THz pulses. For comparison with the experiment, we also performed FDTD simulations coupled to a full-band Monte Carlo device model. The simulations predict a significantly stronger flattening of the conversion efficiency with pump power than that observed in the experiments, which points to less pronounced scattering at high fields than expected.

High-harmonic generation (HHG) is a phenomenon resulting from the strongly nonlinear, non-perturbative interaction between intense electromagnetic pulses and matter. It has been intensively investigated ever since its first observation with optically excited noble-gas atoms in the late 1980s [1] . The HHG effect is not only widely applied in table-top, coherent X-ray sources but is also the key for the generation of intense attosecond pulses [2] . Gases and atom clusters are the most intensively employed materials for HHG. Recently, HHG has also been observed in bulk crystals [3] , which has triggered a surge of research interest in HHG from solids [4] .

Minority carrier traps in Czochralski-grown (Cz) silicon crystals doped with either boron, aluminum, gallium, or indium impurity atoms have been investigated by means of deep-level transient spectroscopy and other junction-related techniques. The experimental data have suggested that minority carrier trapping effects in Cz-Si samples doped with different acceptor impurities are associated with complexes incorporating a substitutional group-III impurity atom and two oxygen atoms, which are found to be negative-U defects with close locations of E(-/+) occupancy level at about Eu + 0.32 eV. We have determined the energy barriers and frequency factors for the reversible transformations of the complexes between deep donor and shallow acceptor states. These parameters are discussed in relation to light-induced degradation behavior of solar cells on p-type Cz-Si crystals.

Experimental investigations of the relaxation times in germanium doped by shallow arsenic donors at low temperatures have been performed with use of pump-probe technique. The dedicated setup utilizes terahertz radiation from the NovoFEL free electron laser. Relaxation times of several excited states have been measured. The values of measured lifetimes fall into nanosecond range. Experimental results are compared with theoretical calculations which take into account acoustic phonon assisted relaxation.