Exciton photoconductivity in Ge-doped GaSe crystals

Spectral photoconductivity in Ge‐doped GaSe crystals was investigated as a function of temperature. It is found that when the crystal is doped with small concentrations of Ge atoms (0.01 at%), the photoconductivity is carried out by the ionization of the excitons. In these crystals, exciton photoconductivity is found to have an Urbach tail similar to the tail observed in exciton absorption. Investigation of the temperature dependence of the peak positions (Eexc(T)) and line widths (Γexc(T)) of the photoconductivity spectra indicates that the exciton ionization is due to the exciton–phonon and exciton–impurity interactions. From the analyses of the Eexc(T) and Γexc(T) data, it is found that at low temperatures (T < 50 K) the exciton–phonon interaction takes place via rigid‐layer phonon modes while in the 50–300 K temperature range it takes place via phonons with hνp ≈ 18 meV. In the 300–450 K range the energy of the phonons involved in the exciton–phonon interaction is hνp ≈ 36 meV. The anisotropy observed in the exciton photoconductivity taken parallel and perpendicular to the layers of doped GaSe crystals diminishes at T = 300 K and the peak positions of the photoconductivity in both directions coincide with the peak position of the free exciton peak (n = 1) in the absorption spectrum. Another important result found is that in these layered GaSe crystals the exciton states continue to exist at high temperatures, up to 450 K. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)