  The recent discovery of superconductivity in infinite-layer nickelate films
has aroused great interest since it provides a new platform to explore the
mechanism of high-temperature superconductivity. However, superconductivity
only appears in the thin film form and synthesizing superconducting nickelate
films is extremely challenging, limiting the in-depth studies on this compound.
Here, we explore the critical parameters in the growth of high quality
nickelate films using molecular beam epitaxy (MBE). We found that stoichiometry
is crucial in optimizing the crystalline structure and realizing
superconductivity in nickelate films. In precursor NdNiO3 films, optimal
stoichiometry of cations yields the most compact lattice while
off-stoichiometry of cations causes obvious lattice expansion, influencing the
subsequent topotactic reduction and the emergence of superconductivity in
infinite-layer nickelates. Surprisingly, in-situ reflection high energy
electron diffraction (RHEED) indicates that some impurity phases always appear
once Sr ions are doped into NdNiO3 although the X-ray diffraction (XRD) data
are of high quality. While these impurity phases do not seem to suppress the
superconductivity, their impacts on the electronic and magnetic structure
deserve further studies. Our work demonstrates and highlights the significance
of cation stoichiometry in superconducting nickelate family.
