Photothermal ionization spectroscopy of mercury vacancies in HgCdTe epitaxial films

The terahertz photoconductivity of narrow-gap Hg$_{1-x}$Cd$_x$Te alloys that originates from shallow double acceptors formed by mercury vacancies is investigated. The discrete spectrum of these acceptors and the matrix elements of the optical transitions from the ground to the excited states are calculated within a model taking into account the contribution from the conduction band and a model central-cell potential. Excited states with low ionization energies that are formed mainly by the states of light holes and are characterized by large matrix elements of optical transitions connecting them to the ground state are found. It is shown that the observed lines in the photoconductivity spectra are caused by transitions to these states for both neutral and singly ionized acceptors, rather than transitions to the valence-band continuum states.