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–terahertz-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.

Figure 1

(a) Schematic configuration of THz-pump–THz-probe spectroscopy. L: lens; BS: beam splitter; PM: off-axis parabolic mirror; WG: wire-grid polarizer; EOC: electro-optic crystal; BD: balanced detector; QWP: quarter wave plate; and WP: Wollaston prism. The configuration of the setup for THz generation is the same as in Ref. 15. The inset shows that the THz pulse is split into two pulses using partial reflection between WG1 and WG2. The pulse intensity is tuned by the rotation of WG3. The distances between wire grids are 30 mm (WG1 and WG2), 50 mm (WG2 and WG3), and 40 mm (WG3 and WG4), respectively.

Figure 2

(a) THz field temporal profiles and (b) power spectra with and without (reference) the sample at 15 K.

Figure 3

Real part of the dielectric constant and conductivity of Ge:Ga at 15 K for different THz electric fields: (a), (b) without pump excitation; (c), (d) $E_{\text{THz-pump}}=3.6\mathrm{kV}/\mathrm{cm}$; (e), (f) $E_{\text{THz-pump}}=13.5\mathrm{kV}/\mathrm{cm}$. Dashed lines show fitting curves with the Drude-Lorentz model described by Eq. (1). The inset shows the primary excitation paths between the acceptor levels.

Figure 4

Bound hole density after the THz-pump pulse irradiation at 15 K. The symbols show experimental values deduced by fitting the optical constant (Fig. 3). The solid line is the theoretical calculation assuming the direct field-assisted tunneling process described by Eqs. (2, 3, 4). The inset shows the data measured at $T$ $=$ 10 and 20 K.