Defects-Induced Linear Dichroism in Anisotropic Layered SiP₂

Linear dichroism emerging from anisotropic band structures in semiconductors plays a significant role for optoelectronic applications. Lattice defects in low-dimensional anisotropic semiconductors can induce defect states within the electronic bandgap, resulting in linearly dichroic light absorption or emission. Here, the defect-induced dual-channel linear dichroism in layered silicon diphosphide is demonstrated, in which the intrinsic exciton peak (2.06 eV) hosts a linear polarization along the x direction of the lattice, while the polarization of the defect state emissions (1.6 and 1.4 eV) is perpendicular to the x direction. Combining photoluminescence measurements with the first-principles calculations, the lower-energy photoluminescence peaks are attributed to specific composite defects, such as the one composed of a phosphorus substitutional defect on a silicon site and a phosphorus vacancy. These defects can induce localized in-gap deep states near the valence band maximum with highly anisotropic wavefunction, contributing to the observed linear dichroism. Notably, as the sample thickness decreases, the defect-induced photoluminescence becomes weaker. This observation of dual-channel linear dichroism in silicon diphosphide provides a new material platform for light polarization modulation and optoelectronic devices.