Bandgap Control of the Oxygen‐Vacancy‐Induced Two‐Dimensional Electron Gas in SrTiO₃

Abstract

STO is central to modern oxide electronics since it serves as the main workhorse for complex functional oxide heterostructure fabrications. After the two-dimensional electron gas (2DEG) at the interface between STO and LAO had been unveiled, [7] a large number of exotic properties of the 2DEG were revealed such as a critical thickness for the appearance of conductivity, [8] Kondo effect, [9] interface superconductivity, [10] an electrically tunable ground state, [11,12] electronic phase separation [5] and recently discovered high-temperature superconductor-like gap behavior. [13] In addition, the practicability of monolithically integrated oxide electronics based on the LAO/STO interface system has been demonstrated. [14] As the research on the LAO/STO interface system is going on, the properties of STO itself have become a center of attention. For example, oxygen vacancies in STO have been a longstanding issue of debate in understanding the emergence of novel electronic and magnetic phases in the 2DEG. Additionally, a fundamental issue related to STO remaining up to now is that although STO single crystals are easy to make metallic with oxygen vacancies, [15,16] it is rather difficult to make an insulating STO thin film completely metallic via oxygen vacancies. Instead, oxygen-deficient STO films are typically semiconducting at low temperatures. [17][18][19][20] Besides, the 2DEG at the LAO/STO interface fabricated on STO films [21][22][23][24] is more localized at low temperatures than that fabricated on STO single crystals; [7] it was found that the insertion of a STO film layer degrades the LAO/STO interface significantly; [25,26] the fully metallic state of the LAO/STO interface based on STO films is achieved when STO films are deposited at a very high temperature of 1100 °C. [27] These perhaps indicate the presence of point defects/disorder in the STO films deposited at the typical temperature range of 600-800 °C.A 2DEG in STO-based heterostructures can be created via oxygen vacancies (2DEG-V) when the overlayer contains elements such as Al which have a strong affinity for oxygen. [28][29][30] Hence the 2DEG-V can be easy to realize by depositing an amorphous, nonpolar LAO (aLAO) film onto STO at room temperature. In this work, our focus is on the 2DEG-V at the interface between amorphous LAO films and STO films, and its relation to the bandgap of STO films, which was found to significantly increase due to the defective nature of STO thin films.In this work, we fabricated 2DEG-V heterostructures by depositing aLAO films on STO films that were pre-deposited on LAO single-crystal substrates. STO films of 150 nm thickness were deposited from a single-crystal STO target on (100)-oriented LAO singlecrystal substrates, using pulsed laser deposition (KrF laser λ = 248 nm) in 10 -2 Torr oxygen partial pressure at various temperatures ranging from 30 to 750 °C. After deposition, the STO films were cooled to room temperature in the deposition oxygen pressure. Amorphous 25-nm-thick LAO films were subsequently deposit...

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