Regulation of absorption coefficients between multi-photon and free-carrier in PbTe films by deposition temperature

Abstract

We investigate the effects of deposition temperature on a crystal structure and electrical and optical properties of PbTe thin films sputtered on BaF2 (111). We observe that with the increase in deposition temperature, the grain size increases, and when it reaches 300 °C, the grain size reaches the maximum, and the preferred orientation begins to change. At 400 °C, the lattice mismatch rate decreases from 4.2% to 3.6% due to lattice contraction caused by reevaporation, and the conduction type changes from p-type dominated by a mismatch strain defect to n-type dominated by a Te vacancy. These changes lead to the interplay of various absorption mechanisms. We find that, in addition to the overall absorption coefficient curve significantly changing with deposition temperature, more importantly, the contribution of various internal absorption mechanisms to the below bandgap absorption spectrum does no longer synchronize. Instead, the contribution of an acoustic phonon to free-carrier absorption (FCA) decreases, while the relative contribution of optical phonon, impurity, two-photon absorption (2PA), and three-photon absorption (3PA) increases. This regulation effect reaches its maximum at 300 °C, which of various absorption mechanisms at 300 °C are 6.3, 11.6, 4.4, and 14.7 times higher than that at 20 °C corresponding to an optical phonon, impurity, 2PA, and 3PA processes, respectively. These results indicate that it should be possible to regulate the FCA, 2PA, and 3PA processes by changing the deposition temperature, thus making them suitable for applications in optoelectronic devices.