Experimental evidence of a very thin superconducting layer in epitaxial indium nitride

Abstract

Indium nitride is one of the very few semiconductors which is known to have a superconducting phase at temperatures of T-c > 1 K. Superconductivity occurs in a window of carrier densities of approximately 10(18)-10(20)cm(3). This is a very low density when compared to other superconductors (i.e. metals, alloys, high T-c oxides) and thus raises interesting fundamental questions as well as technological possibilities. In this paper we address one key question about the dimensionality of the superconducting state of InN by using angle dependent critical field measurements. Our samples were grown by two different growth techniques (chemical vapour deposition and plasma-assisted molecular beam epitaxy) on c-oriented sapphire, with and without a GaN buffer layer. In both cases we find that for film thicknesses much larger than the coherence length d >> xi the angle dependence of the critical field (down to T < 280 mK) with respect to the c-axis continues to be clearly two-dimensional, demonstrating a characteristic cusp when the angle crosses 90 degrees with respect to the c-axis. This indicates that the superconducting electrons are most likely confined to a layer much thinner than the thickness of the InN film. Further we find the magnitude of the gap to be 2 Delta(0)/k(B)T(c) = 3.6, very close to the BCS prediction.