Integrating AlN with GdN Thin Films in an in Situ CVD Process: Influence on the Oxidation and Crystallinity of GdN
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The application potential of rare earth nitride (REN) materials has been limited due to their high sensitivity to air and moisture leading to facile oxidation upon exposure to ambient conditions. For the growth of device quality films, physical vapor deposition methods, such as molecular beam epitaxy, have been established in the past. In this regard, aluminum nitride (AlN) has been employed as a capping layer to protect the functional gadolinium nitride (GdN) from interaction with the atmosphere. In addition, an AlN buffer was employed between a silicon substrate and GdN serving as a seeding layer for epitaxial growth. In pursuit to grow high-quality GdN thin films by chemical vapor deposition (CVD), this successful concept is transferred to an in situ CVD process. Thereby, AlN thin films are included step-wise in the stack starting with Si/GdN/AlN structures to realize long-term stability of the oxophilic GdN layer. As a second strategy, a Si/AlN/GdN/AlN stacked structure was grown, where the additional buffer layer serves as the seeding layer to promote crystalline GdN growth. In addition, chemical interaction between GdN and the Si substrate can be prevented by spatial segregation. The stacked structures grown for the first time with a continuous CVD process were subjected to a detailed investigation in terms of structure, morphology, and composition, revealing an improved GdN purity with respect to earlier grown CVD thin films. Employing thin AlN buffer layers, the crystallinity of the GdN films on Si(100) could additionally be significantly enhanced. Finally, the magnetic properties of the fabricated stacks were evaluated by performing superconducting quantum interference device measurements, both of the as-deposited films and after exposure to ambient conditions, suggesting superparamagnetism of ferromagnetic GdN grains. The consistency of the magnetic properties precludes oxidation of the REN material due to the amorphous AlN capping layer.