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 (E exc (T)) and line widths (Г exc (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 E exc (T) and Г exc (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 hν p ≈ 18 meV. In the 300 -450 K range the energy of the phonons involved in the exciton-phonon interaction is hν p ≈ 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.

Articles you may be interested inImpacts of Si-doping and resultant cation vacancy formation on the luminescence dynamics for the near-bandedge emission of Al0.6Ga0.4N films grown on AlN templates by metalorganic vapor phase epitaxyWe investigated the nonlinear and saturable absorption characteristics of very thin amorphous undoped GaSe, Ge ͑0.01 at. %͒, and Sn ͑0.5 at. %͒ doped GaSe films by pump-probe and open aperture Z-scan techniques. Linear absorption measurements indicate blueshift in energy with increasing film thickness. Thinner films exhibit saturable absorption while thicker films exhibit nonlinear absorption for 4 ns and 65 ps pulse durations. The films exhibit competing effects between nonlinear and saturable absorption. Saturable absorption behavior weakens while nonlinear absorption appears with increasing film thickness. In addition to that, saturable absorption behavior is very sensitive to doping. Doping causes absorption behaviors to appear in thinner films compared to undoped films. These behaviors are attributed to increasing localized defect states with increasing film thickness and doping. To derive the transmission in open aperture Z-scan data, a theoretical model incorporating one photon, two photon, and free carrier absorptions and their saturations were considered. The experimental curves were fitted to the theory of open aperture Gaussian beam Z-scan based on the Adomian decomposition method. Nonlinear absorption coefficients and saturation intensity thresholds were extracted from the fitting of the experimental results for both pulse durations used for the experiments. Saturation intensity threshold values increased with increasing film thickness and doping. The lowest saturation intensity threshold for undoped GaSe film was found to be 1.9ϫ 10 −3 GW/ cm 2 for 45 nm film thickness and increased about two orders of magnitude for 74 nm film thickness.

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