Large Switchable Circular Photogalvanic Effect in the Narrow-Band-Gap InSe/In2Se3 Ferroelectric Heterojunction

The photogalvanic effect in ferroelectric (FE) materials attracts considerable interest in solar cells, polarization-sensitive photodetection, and nonvolatile memory applications. However, narrow-band-gap FE materials that cover the full solar spectrum are currently rare, and there is still room to enhance the light-polarization sensitivity and to improve the switching ratio for FE optoelectronic devices. Here, we investigate the photogalvanic effect induced by elliptically polarized light in the narrow-band-gap ($<1.6$ eV) $\mathrm{In}\mathrm{Se}$/${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ FE heterojunction (HJ) stacked with the $\mathrm{In}\mathrm{Se}$ monolayer and $\ensuremath{\alpha}$-${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ FE monolayer. The dependence of the photocurrent on the polarization angle, $\ensuremath{\phi}$, and on the incidence angle, $\ensuremath{\alpha}$, is formulated within the nonequilibrium Green's function formalism, with which the photocurrent is then calculated by using first-principles methods. We find that both the linear photogalvanic effect (LPGE) and circular photogalvanic effect (CPGE) are generated from the infrared to the visible range. The CPGE is only induced at oblique incidence due to ${C}_{3v}$ symmetry of the HJ and can be ${10}^{3}$ times larger than the LPGE. The largest photocurrent is achieved when light is irradiated along the zigzag direction, which is dominated by the CPGE. The photocurrent can further be tuned by switching between upward and downward FE polarization, leading to a large switching ratio of 563. The photocurrent also shows an excellent light-polarization sensitivity at normal and oblique incidence, with an impressive extinction ratio of 410. These results shed light on the CPGE of FE materials for applications in low-dimensional FE optoelectronics.