Interface engineering for a stable chemical structure of oxidized-black phosphorus: Via self-reduction in AlO_x atomic layer deposition

We evaluated the change in the chemical structure between dielectrics (AlO_x and HfO_x) grown by atomic layer deposition (ALD) and oxidized black phosphorus (BP), as a function of air exposure time. Chemical and structural analyses of the oxidized phosphorus species (P_xO_y) were performed using atomic force microscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, first-principles density functional theory calculations, and the electrical characteristics of field-effect transistors (FETs). Based on the combined experiments and theoretical investigations, we clearly show that oxidized phosphorus species (P_xO_y, until exposed for 24 h) are significantly decreased (self-reduction) during the ALD of AlO_x. In particular, the field effect characteristics of a FET device based on Al₂O₃/AlO_x/oxidized BP improved significantly with enhanced electrical properties, a mobility of ∼253 cm² V^-1 s^-1 and an on-off ratio of ∼10⁵, compared to those of HfO₂/HfO_x/oxidized BP with a mobility of ∼97 cm² V^-1 s^-1 and an on-off ratio of ∼10³-10⁴. These distinct differences result from a significantly decreased interface trap density (D_it ∼ 10¹¹ cm^-2 eV^-1) and subthreshold gate swing (SS ∼ 270 mV dec^-1) in the BP device caused by the formation of stable energy states at the AlO_x/oxidized BP interface, even with BP oxidized by air exposure.