Intrinsic defects and mid-gap states in quasi-one-dimensional indium telluride

Recently, intriguing physical properties have been unraveled in anisotropic semiconductors in which the in-plane electronic band structure anisotropy often originates from the low crystallographic symmetry. The atomic chain is the ultimate limit in material downscaling for electronics-a frontier for establishing an entirely new field of one-dimensional quantum materials. Electronic and structural properties of chain-like InTe are essential for a better understanding of device applications such as thermoelectrics. Here, we use scanning tunneling microscopy/scanning tunneling spectroscopy (STS) measurements and density functional theory (DFT) calculations to image the in-plane structural anisotropy directly in tetragonal InTe. As results, we report the direct observation of one-dimensional In1+ chains in InTe. We demonstrate that InTe exhibits a bandgap of about 0.40 +/- 0.02 eV located at the M point of the Brillouin zone. Additionally, line defects are observed in our sample and were attributed to In1+ chain vacancy along the c-axis-a general feature in many other TlSe-like compounds. Our STS and DFT results prove that the presence of In1+ induces a localized gap state, located near the valence band maximum. This acceptor state is responsible for the high intrinsic p-type doping of InTe that we also confirm using angle-resolved photoemission spectroscopy.