The Vacancy‐Induced Electronic Structure of the SrTiO_3−δ Surface

Abstract

interface was shown to be tunable by an electric-field effect, [10,11] leading to the discovery of gate-tunable exotic properties such as superconductivity, [11][12][13][14] magnetism, [15] and Rashba interaction, [16][17][18] which can be exploited in novel electronic and spintronic devices. [19] The intriguing conductivity arising between LAO and STO-both of which are wide band gap insulators with band gaps of 5.6 and 3.2 eV, respectively-was initially explained by the polar catastrophe model. [4,20] According to this model, a charge of 0.5 e − per square STO lattice parameter is transferred from the LAO surface to the STO side of the interface to prevent the electrical potential from diverging due to the polar stacking of LaO + and AlO 2 − layers. Figure 1a illustrates this behavior, in which a downward bending of STO bands near the interface creates a narrow potential well that accommodates and confines the transferred charge at the interface, forming the 2DEG. However, numerous reports have proposed alternative explanations for the origin of the 2DEG, including charge doping by oxygen vacancies and cation intermixing. [21][22][23][24][25][26] Interestingly, angle-resolved photoemission spectroscopy (ARPES) studies showed that a similar 2DEG can be stabilized on the bare surface of STO (001) cleaved under ultrahigh vacuum (UHV) conditions following ultraviolet (UV)-light irradiation. [27][28][29] The 2DEG in this case is an electron accumulation layer that screens the positively charged oxygen vacancy defects ( O 2 V + or O V ⋅⋅ in Kröger-Vink notation) formed near the surface. The positively charged oxygen vacancies donate charge (e − ) while also inducing downward band bending at the STO surface, providing a confining potential well for the doped electrons. This is illustrated schematically in Figure 1b and also in Figure 1c, where the 2DEG is shown to form from Ti 3d t 2gderived quantized states (subbands).Recent studies have employed a variety of techniques to generate a 2DEG on the STO surface, also based on oxygen vacancy formation, including in situ sputtering, [30] vacuum annealing, [31,32] and metal deposition. [33] However, the quantitative relationship between the surface and bulk electronic structures remains unclear in some of these studies, especially given that n-type conducting (Nb-doped) STO substrates are often used to permit ARPES measurements. [30][31][32] In these The emergence of a 2D electron gas (2DEG) on the (001) surface of oxygendeficient strontium titanate (SrTiO 3−δ ) is investigated. Using in situ soft X-ray spectroscopy and effective mass modeling, a series of quantitative band diagrams are developed to describe the evolution of near-surface and bulk carrier concentrations, downward band bending, and Fermi level along a lateral gradient of oxygen vacancies formed on SrTiO 3−δ by direct-current resistive heating under ultrahigh vacuum conditions. Electrons are accumulated over a 3 nm region near the surface, confined within a potential well with saturated 300 meV downward band be...

The Electronic Properties of Extended Defects in SrTiO3—A Case Study of a Real Bicrystal Boundary

“…Crystals 2020, 10, x FOR PEER REVIEW 7 of 19 of the sample to ambient conditions. It has been shown previously that the thermally reduced SrTiO3 surface is highly conductive under in situ conditions [18] but that the majority of the surface becomes passivated upon exposure to oxygen at room temperature, thus becoming insulating again [16]. Only a few conducting spots were found to remain highly conductive.…”

Effect of Annealing Temperature for Ni/AlOx/Pt RRAM Devices Fabricated with Solution-Based Dielectric

“…The formation and rupture mechanism of CF is confirmed to be associated with the characteristics of the RS layer in filamentary RRAM devices with the dependency on film thickness, measurement temperature and deposition temperature [64,65,66,67]. In this work, the device performance is found to be dependent on annealing temperature of the dielectric layer and the best performance is observed in the device with a dielectric layer annealed at 250 °C.…”

“…Figure 6a–c show XPS spectra of O 1s core levels for the AlO x thin films annealed at 225 °C, 250 °C and 275 °C. The O 1s CL spectrum can be de-convoluted into two sub-peaks with binding energies located at 531.1 eV (O 1 ) and 532.2 eV (O 2 ) [40,64,65,66,67]. The O 1 and O 2 peaks are associated with the metal-oxygen bonds (O 1 ) and hydroxyl group (O 2 ), respectively [5,66,67].…”

Polarity-field driven conductivity in SrTiO3/LaAlO3 : A hybrid functional study

“…Chemical potential of oxygen µO(p, T ), the black dots correspond to growth conditions extracted from the literature [5,16,47,69,84,[88][89][90][91]. The two blue diamonds correspond to the growth conditions from Reference 49, for p = 3×10 −5 mbar and p = 2×10 −3 mbar at T = 1153 K. The green pentagons correspond to annealing conditions reported in References 16 and 92, while the red diamonds correspond to the annealing conditions of bare STO films in ultra-high vacuum [93,94] with a reported metallic surface.…”