Imaging oxygen vacancies in EuO$_{1-x\, }$using Scanning Tunneling Microscopy and Spectroscopy

EuO$_{1-x}$, a ferromagnetic semiconductor, attracts plenty of attention due to its unique magnetic properties, such as the possible existence of the magnetic polarons and skyrmions. Here we present a scanning tunneling microscopy and spectroscopy study of the EuO$_{1-x}$ thin film made by pulsed laser deposition. After an ultra-high vacuum sputtering followed by a 300 $^{o}$C annealing, the topography and d$I$/d$V$ mapping of the EuO$_{1-x}$ were successfully measured. Interestingly, in d$I$/d$V$ mapping, around 5 nm sizes dot-like features with higher d$I$/d$V$ contrast were observed. The observed dot-like features were sparsely and uniformly distributed over the measured surfaces. After further annealing at 500 $^{o}$C for another 30 minutes, the number of the dot-like features in d$I$/d$V$ mapping increased to overlap each other, indicating the dot-like features are most likely related to the oxygen vacancies created by vacuum annealing. The d$I$/d$V$ spectra inside and outside of the dot-like features showed similar bandgap but distinct spectral weight in both valence and conduction bands. This work paves the road for further STM study on this intriguing material.