Probing The Carrier Dynamics around The Band Gap of Amorphous Silicon Telluride

Amorphous chalcogenide alloys are key materials for data storage and energy scavenging applications due to their large non-linearities in optical and electrical properties. Silicon telluride (SiTe) is an example of these amorphous alloys because it undergoes a solid-state phase transformation upon the application of a sufficiently large electric field, commonly referred to as threshold switching. Here, we investigate the impact of this phase transformation on electrical properties by monitoring carrier lifetimes at and around the transformation. We implement an ultrafast near-infrared pump-probe spectroscopy technique that allows for direct monitoring of electronic and vibrational energy carrier lifetimes in amorphous silicon telluride. We confirm the ultrafast findings with the help of density functional theory calculations and ellipsometry. From the DFT calculation we find the optical bandgap of the film is 0.695+/- 0.15 eV and the mobility gap is 1.12 +/- 0.05 eV. We probe the reflectivity response of this alloy after pump heating in the energy range of 0.5 to 2.07 eV. From ultrafast technique we find that SiTe demonstrates the direct valence mobility edge to conduction mobility edge transition at 1.24 eV. It is slightly larger than the reported optical band gaps. Besides, we identify a hole trap state at 1.08eV which is near the valence band edge, indicating that the flow of electrons in SiTe is possible under the threshold voltage. The electrons of the low energies relaxes faster than the free electrons of bandgap energy, confirming the presence of trap states in SiTe and its conductive nature under its threshold voltage. The presence of localized states at the band edge is expected in the electronic structure of amorphous materials and is consistent with previous studies of Si−Te materials. These findings open the way of tuning the property of this chalcogenide alloy, which will lead to building new electronic devices with higher efficiency and better performance.