We investigate the effects of an ultrathin yttrium interlayer at the interface of lanthanum aluminate and silicon with thermal annealing. Addition of the yttrium interlayer resulted in a smaller increase in the valence band offset, a reduction in the interface trap density, and a close to four-order reduction in leakage current after 800 degrees C thermal annealing. The valence band offsets are measured with X-ray photoelectron spectroscopy and compared with those obtained from electrical measurements. The good correlation of the two measurements indicates a change in the valence band offset caused by interface dipoles. The performance improvements are attributed to a reduction in lanthanum interdiffusion and the formation of an yttrium-rich silicate interfacial layer, which minimize leakage current flow and reduce hydrogen desorption in interface trap formation. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3494150] All rights reserved.