The Role of Oxygen Transfer in Oxide Heterostructures on Functional Properties

Abstract

achieved by interfacing two insulting oxides have attracted great attention in the past decade. [3][4][5][6][7][8][9][10] Different conduction mechanisms including polar catastrophe (interface), [5,10,11] strain, [12][13][14] intermixing (doping), [3,8,[15][16][17] and oxygen vacancy (defect) [18][19][20][21] have been proposed to explain such phenomenon. For example, carriers at the substrate-film interface (interface), bulk part of the film (film) and the surface layer of the substrate near the substratefilm interface (substrate surface layer) can all contribute to the effective conductivity of the samples. Polar catastrophe is a wellestablished model to explain the high conductivity at the interface between LaAlO 3 and SrTiO 3 (LAO/STO). [5,11,20] High mobility has also been reported when LaTiO 3 (LTO) and La 1-x Sr x TiO 3 (LSTO) are grown on SrTiO 3 (STO) [8,14,16,22] and KTaO 3 (KTO). [6] LTO is a polar perovskite like LAO, and as such, polar catastrophe mechanisms of this system are assumed to follow similarly to that of LAO/STO heterostructures. [5,11,20] However, unlike LAO, which is a band insulator, LTO is a Mott insulator that shows an insulator-metal transition when doped with Sr 2+ in polycrystalline bulk and epitaxial thin films. This allows the 3d 1 configuration to approach 3d 0 and may also result in the observed metallic behavior as reported in LSTO films on substrates such as (LaAlO 3 ) 0.3 (Sr 2 AlTaO 6 ) 0.7 (LSAT). [17,23] In addition, defects such as oxygen vacancy (OV) in the STO substrate are another factor which could affect the electric conductivity in STO-based heterostructures. The formation of OV in STO substrate is directly controlled by growth temperature and oxygen pressure as reported in different oxide/STO heterostructures. [5,15,18,20,21,[23][24][25][26][27][28] Enhanced conductivity is observed for these heterostructures grown (and STO single crystals annealed) in temperatures greater than 750 °C, at oxygen pressures lower than 10 -5 Torr or a combination of both. [29] For example, OVs can be introduced by reducing the STO substrate with pre-substrate annealing (PSA) at 750 °C and 10 -6 Torr. [28] Spinelli, et al. reported highly conductive STO crystals annealed at 10 -9 Torr and 700 °C. [29] Schneider et al. found substantial oxygen transfer from STO substrates to oxide films while studying the diffusion of isotope 18 O at the conditions of 750 °C and 1 × 10 -8 Torr. [19] Edmondson et al. observed STO substrate reduction at 10 -10 ~ 10 -7 Torr and 700 °C. [20] Therefore, the formation of OV near the STO surface or the diffusion of oxygen out A variety of mechanisms are reported to play critical roles in contributing to the high carrier/electron mobility in oxide/SrTiO 3 (STO) heterostructures. By using La 0.95 Sr 0.05 TiO 3 (LSTO) epitaxially grown on different single crystal substrates (such as STO, GdScO 3 , LaAlO 3 , (LaAlO 3 ) 0.3 (Sr 2 AlTaO 6 ) 0.7 , and CeO 2 buffered STO) as the model systems, the formation of a conducting substrate surface layer (CSSL) on STO substr...

Modification of Band Structure by Oxygen Filling in Epitaxial La0.88Sr0.12MnO3–x Thin Films

“…Oxygen vacancies in perovskite transition-metal oxides (TMOs) are often considered functional defects due to their involvement in electronic transport, indirect magnetic interactions, and ionic conduction. − Consequently, significant efforts have been dedicated to enhancing the processing conditions for the synthesis of TMO thin films. Some previous studies have focused on understanding the relationship between epitaxial strain and the formation of oxygen vacancies, as well as the dynamics of oxygen migration between substrates and oxide films. − In addition, several studies have explored the impact of oxygen partial pressure during the cooling process on the creation of oxygen vacancies. − In contrast to other TMOs, , it is known that oxygen vacancies are not readily formed in manganese-based oxides under conventional synthesis conditions, such as oxygen partial pressures ( P O 2 ) and temperatures. , In some cases, an excess of oxygen content can be problematic when attempting to create bulk-like manganite thin films. − While numerous studies have been conducted on the influence of oxygen vacancy concentration on electrical and magnetic properties, − fewer studies have focused on the modification of the band structure by oxygen vacancies. In this regard, it is crucial to address how to replenish oxygen vacancies and what effect the oxygen filling has on the band structure.…”