The effect of film thickness, Se/S ratio and annealing temperature on the nonlinear absorption behaviors in amorphous GaSexS1−x (0 ≤ x≤ 1) thin films

The amorphous GaSexS1−x (0≤x≤1) semiconductor thin films were prepared with different thickness by using vacuum evaporation technique. The nonlinear absorption properties of the films were studied by Z-scan and ultrafast pump-probe techniques. Linear absorption measurements showed blue shift in energy as the film thickness increased. This can be attributed quantum confinement effect due to particle size decreasing with increasing film thickness. Pure GaSe and GaSexS1-x films exhibit saturable absorption behavior with 65 ps pulse duration for thinner films. In addition, only pure GaSe thinner films exhibit saturable absorption behavior for 4 ns pulses. The life time of the localized defect states was measured as 8 ns for GaSe and 3 ns for the others. In an attempt to investigate the effect of the number of localized defect states on the saturable absorber behavior we have deposited films with various thicknesses and annealed at various temperatures. Thinner films exhibit saturable absorption while thicker films exhibit nonlinear absorption incorporating one photon absorption, two photon absorption, and free carrier absorption for 65 ps pulse duration. It was found that increasing annealing temperature leads to reduction of the localized defect states. Saturation intensity thresholds increase with increasing film thickness, Se/S ratio and decreasing annealing temperature.