Experimental and theoretical work on the ionization of deep impurity
centres in the alternating terahertz field of high-intensity far-infrared laser
radiation, with photon energies tens of times lower than the impurity
ionization energy, is reviewed. It is shown that impurity ionization is due to
phonon-assisted tunnelling which proceeds at high electric field strengths into
direct tunnelling without involving phonons. In the quasi-static regime of
low frequencies the tunnelling probability is independent of frequency.
Carrier emission is accomplished by defect tunnelling in configuration
space and electron tunnelling through the potential well formed by the
attractive force of the impurity and the externally applied electric field. The
dependence of the ionization probability on the electric field strength permits
one to determine defect tunnelling times, the structure of the adiabatic
potentials of the defect, and the Huang–Rhys parameters of electron–phonon
interaction.
Raising the frequency leads to an enhancement of the tunnelling ionization and
the tunnelling probability becomes frequency dependent. The transition from the
frequency-independent quasi-static limit to frequency-dependent tunnelling is
determined by the tunnelling time which is, in the case of phonon-assisted
tunnelling, controlled by the temperature. This transition to the high-frequency
limit represents the boundary between semiclassical physics, where the radiation
field has a classical amplitude, and full quantum mechanics where the
radiation field is quantized and impurity ionization is caused by multiphoton
processes. In both the quasi-static and the high-frequency limits, the
application of an external magnetic field perpendicular to the electric field
reduces the ionization probability when the cyclotron frequency becomes
larger than the reciprocal tunnelling time and also shifts the boundary
between the quasi-static and the frequency-dependent limits to higher
frequencies.
At low intensities, ionization of charged impurities may also occur through the
Poole–Frenkel effect by thermal excitation over the potential well formed by the
Coulomb potential and the applied electric field. Poole–Frenkel ionization
precedes the range of phonon-assisted tunnelling on the electric field scale and
enhances the ionization probability at low electric field strengths. Applying
far-infrared lasers as sources of a terahertz electric field, the Poole–Frenkel effect
can clearly be observed, allowing one to reach a conclusion regarding the charge of
deep impurities.