Growth of BST films by MBE were carried out under a variety of oxidizing conditions and substrate temperatures. Typical oxygen pressures ranged from 0 Torr, with the substrate feeding atomic oxygen into the growing film, up to 1E-6 Torr with the oxygen either being introduced as an ambient oxygen background or as activated plasma. Films were characterized in-house by x-ray photoemission spectroscopy (XPS), depth resolved cathodoluminescence spectroscopy, and x-ray diffraction while the fabrication of interdigitated capacitor (IDC) structures and subsequent dielectric measurements we performed at Naval Research Laboratories. To facilitate XPS measurements of pristine sample surfaces, an ultra-high vacuum transfer line was designed and built to transfer samples from the MBE growth chamber into the XPS analysis chamber without exposing the sample to air.
Depth resolved cathodoluminescence spectroscopy (DRCLS) measurements reveal a strong dependence of the material’s electrically active defects on the growth parameters and chemical compositions. In particular, we see a large increase in the 2.55 eV and 2.95 eV emission intensity from the STO substrate for films grown with low oxygen pressures. It is known that the 2.55 eV and 2.95 eV emissions are related to oxygen vacancies, and it is shown that these defects are generated deep within the substrate by out diffusion of oxygen into the substrate. As we increase the oxygen pressure, we see an increase in the intensity of the 2.1 eV and 2.3 eV emission intensities, and we can understand this as the depopulation of a state within the bandgap of the material at 0.6 eV above the valence band allowing transitions from the oxygen vacancy and conduction band into this new state respectively. We then go on to show how an excess of Sr in reduced strontium titanate (STO) films can behave as an acceptor, depopulating this state and again leading to an increase in the 2.1 eV and 2.3 eV emissions.
The utility of these defects is displayed by an observed strong positive correlation between the 2.3 eV emission and the Q of the material. By reducing carriers in the material we can reduce the dielectric loss and this is evident from our DRCLS spectra. We also see a strong dependence on the 1.9 eV emission intensity with the Ba/Sr which in turn depends on the oxygen background pressure (P(O2)) and shows a strong negative correlation with the tunability of the material Again, these results highlight the dependence of dielectric properties on the material defects generated during growth and the effectiveness of DRCLS to probe these defects. Control of material defects through MBE opens an avenue for the engineering of materials through defect management at the atomic level.