Surface stability of NbN single-crystal films

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Abstract
The surfaces of NbN(100) and NbN(111) single-crystal films were analyzed as a function of annealing temperature in ultrahigh vacuum by low-energy electron diffraction and x-ray photoelectron spectroscopy. The NbN(100) surface changed from a (1×1) to a ((2)1/2×(2)1/2)45° structure at 900 °C accompanied by a loss of nitrogen. The NbN(111) surface, initially richer in nitrogen, transformed from a (1×1) to a (2×2) structure at 800 °C with an even greater loss of nitrogen from the surface. Increasing the carbon content of the films increased the temperatures at which reconstructions were observed. Structural models of the surfaces are presented that are consistent with these observations. At higher temperatures (up to 1100 °C), additional nitrogen was lost from films of both orientations. Measurements of superconducting transition temperatures and energy gaps indicated that the entire film was nitrogen deficient after annealing, with the greatest nitrogen loss at the surface.

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