Electric field ionization of gallium acceptors in germanium induced by single-cycle terahertz pulses

The electric field ionization of gallium acceptors in germanium was studied by using terahertz-pump-hterahertz-probe spectroscopy. As the pump electric field increases, the distinct absorptions due to acceptor transitions centered at 2.0 and 2.2 THz decrease, and simultaneously, a free carrier response emerges in the lower frequency region. These behaviors clearly show that the terahertz-pump pulse ionizes neutral acceptors. The pump electric field dependence of the released hole density is reproduced by a model assuming direct field-assisted tunneling of acceptors.