  We propose a two-dimensional crystal which possesses low indirect band gaps
of 0.55 eV (monolayer) and 0.43 eV (bilayer) and high carrier mobilities
similar to those of phosphorene: GeP3. GeP3 has a stable three-dimensional
layered bulk counterpart which is metallic and is known from experiment since
1970. It has a small cleavage energy, which suggests exfoliation of bulk
material as viable means for the preparation of mono- and few-layer materials.
The material shows strong interlayer quantum confinement effects, resulting in
a band gap reduction from mono- to bilayer, and then to a semiconductor-metal
transition between bi- and triple layer. Under biaxial strain, the indirect
band gap can be turned into a direct one. Pronounced light absorption in the
spectral range from ~600 to 1400 nm is predicted for monolayer and bilayer and
promises applications in photovoltaics.
