Observation of superconducting phase transition in InN

InN superconductivity is very special among III-V semiconductors, because other III-V semiconductor (like GaAs, GaN, InP, InAs etc.) usually lacks strong covalent bonding and seldom shows superconductivity at low-temperature. In this paper, via current-voltage(I-V) measurement, we probe the superconducting phase transitions in InN. The possible connection with those chemical-unstable phase separated inclusions, like metallic indium or In2O3, was removed by HCl acid etching. It finds InN samples can show different phase transition behaviors. The vortex-glass (VG) to liquid transition, which is typical in type-II superconductors, is observed in the sample with large InN grain size. In contrast, the small grain-sized sample's superconducting properties are sensitive to acid etching, shows a transition into a non-zero resistance state at the limit of temperature approaches zero. Our work suggests that the grain size and inter-grain coupling may be two key factors for realizing InN superconductivity. InN superconductivity can become robust and chemical stable if the grain size and inter-grain coupling both are large enough.