Surface Stability of SrNbO$_{3+δ}$ Grown by Hybrid Molecular Beam Epitaxy

Abstract: 4d transition metal oxides have emerged as promising materials for numerous applications including high mobility electronics. SrNbO${3}$ is one such candidate material, serving as a good donor material in interfacial oxide systems and exhibiting high electron mobility in ultrathin films. However, its synthesis is challenging due to the metastable nature of the d$^{1}$ Nb$^{4+}$ cation and the limitations in the delivery of refractory Nb. To date, films have been grown primarily by pulsed laser deposition (PLD), but development of a means to grow and stabilize the material via molecular beam epitaxy (MBE) would enable studies of interfacial phenomena and multilayer structures that may be challenging by PLD. To that end, SrNbO${3}$ thin films were grown using hybrid MBE for the first time using a tris(diethalamido)(tert-butylimido) niobium precursor for Nb and an elemental Sr source on GdScO${3}$ substrates. Varying thicknesses of insulating SrHfO${3}$ capping layers were deposited using a hafnium tert-butoxide precursor for Hf on top of SrNbO${3}$ films to preserve the metastable surface. Grown films were transferred in vacuo for X-ray photoelectron spectroscopy to quantify elemental composition, density of states at the Fermi energy, and Nb oxidation state. Ex situ studies by X-ray absorption near edge spectra illustrates the SrHfO${3}$ capping plays an important role in preserving the Nb 4d$^{1}$ metastable charge state in atmospheric conditions.