Van der Waals materials with the surface photovoltaic effect (SPV) enable photodetection over a tunable wavelength range with low power consumption, which benefits layer-dependent band gaps. However, sensitive SPV in the infrared region, especially in a quasisteady illumination condition, is still elusive in layered semiconductors. Here, using angle-resolved photoemission spectroscopy, we report a sensitive SPV in quasiequilibrium in ${\text{NbSi}}_{0.5}{\text{Te}}_2$, with photoresponsivity up to $3\times {10}^6\mathrm{V}/\left(\mathrm{W}{\mathrm{cm}}^{-2}\right)$ at low-intensity low-temperature conditions (LILT). The sensitive SPV is further confirmed by observing the Dember effect, where the photogenerated carrier density is high enough and diffusion currents suppress SPV. Temperature-dependent measurements indicate that intrinsic carriers freezing at low temperature leads to the ultrahigh photoresponse, while a small amount of photon-generated carriers in quasiequilibrium dominate the system. Our work not only provides a promising layered semiconductor for infrared optoelectronic devices with strong infrared SPV at LILT, which has application potential in fields such as quantum information and deep-space exploration, but also paves a way to enhance the light-matter interaction effect by freezing bulk carriers.

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• Electrical properties
• Electronic structure
• Optoelectronics
• 2-dimensional systems
• Layered semiconductors
• Surfaces
• Transition metal dichalcogenides
• Angle-resolved photoemission spectroscopy