Electronic defect states at the LaAlO 3 /SrTiO 3 heterointerface revealed by O K-edge X-ray absorption spectroscopy

• Published in: Physical chemistry chemical physics : PCCP Vol. 18; no. 20; pp. 13844 - 13851
• Main Authors: Palina, Natalia, Annadi, Anil, Asmara, Teguh Citra, Diao, Caozheng, Yu, Xiaojiang, Breese, Mark B. H., Venkatesan, T., Ariando, Ariando, Rusydi, Andrivo
• Format: Journal Article
• Language: English
• Published: 2016
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/C6CP00028B

Interfaces of two dissimilar complex oxides exhibit exotic physical properties that are absent in their parent compounds. Of particular interest is insulating LaAlO 3 films on an insulating SrTiO 3 substrate, where transport measurements have shown a metal–insulator transition as a function of LaAlO 3 thickness. Their origin has become the subject of intense research, yet a unifying consensus remains elusive. Here, we report evidence for the electronic reconstruction in both insulating and conducting LaAlO 3 /SrTiO 3 heterointerfaces revealed by O K-edge X-ray absorption spectroscopy. For the insulating samples, the O K-edge XAS spectrum exhibits features characteristic of electronically active point defects identified as noninteger valence states of Ti. For conducting samples, a new shape-resonance at ∼540.5 eV, characteristic of molecular-like oxygen (empty O-2p band), is observed. This implies that the concentration of electronic defects has increased in proportion with LaAlO 3 thickness. For larger defect concentrations, the electronic defect states are no longer localized at the Ti orbitals and exhibit pronounced O 2p–O 2p character. Our results demonstrate that, above a critical thickness, the delocalization of O 2p electronic states can be linked to the presence of oxygen vacancies and is responsible for the enhancement of conductivity at the oxide heterointerfaces.