“…For all spectra, a rigid shift towards higher binding energy is visible for the sample with lower oxygen content. This can be explained by a relative shift of the conduction band towards the Fermi level 31 , 32 , which is also correlated with an increased donor dopant concentration 33 . Figure 4 ( a ) Ta 4f, O 1s and VB spectra for thin-films grown with a power density of 1.20 W/cm 2 and a relative O 2 flow rate of 25% (black line) and 10% (red line).…”
In this study, we investigated the influence of oxygen non-stoichiometry on the resistive switching performance of tantalum oxide based memristive devices. Thin-films of tantalum oxide were deposited with varying sputter power and oxygen partial pressure. The electroforming voltage was found to decrease with increasing power density or decreased oxygen partial pressure, while the endurance remained stable and the resistance window ROFF/RON was found to increase. In-depth XPS analysis connects these observations to a controllable oxygen sub-stoichiometry in the sputter-deposited films. Our analysis shows that the decrease of the forming voltage results from an increase in carrier density in the as-prepared thin-films, which is induced by the presence of oxygen vacancies.
“…For all spectra, a rigid shift towards higher binding energy is visible for the sample with lower oxygen content. This can be explained by a relative shift of the conduction band towards the Fermi level 31 , 32 , which is also correlated with an increased donor dopant concentration 33 . Figure 4 ( a ) Ta 4f, O 1s and VB spectra for thin-films grown with a power density of 1.20 W/cm 2 and a relative O 2 flow rate of 25% (black line) and 10% (red line).…”
In this study, we investigated the influence of oxygen non-stoichiometry on the resistive switching performance of tantalum oxide based memristive devices. Thin-films of tantalum oxide were deposited with varying sputter power and oxygen partial pressure. The electroforming voltage was found to decrease with increasing power density or decreased oxygen partial pressure, while the endurance remained stable and the resistance window ROFF/RON was found to increase. In-depth XPS analysis connects these observations to a controllable oxygen sub-stoichiometry in the sputter-deposited films. Our analysis shows that the decrease of the forming voltage results from an increase in carrier density in the as-prepared thin-films, which is induced by the presence of oxygen vacancies.
“…[3,15,31,32] Most studies across the literature therefore limit their discussion of ionic defects to oxygen vacancy defects, while other types of ionic defect species are often ignored. Thermodynamically, however, the defect structure of Nb:SrTiO 3 [26,33] and in particular of its surface [29,30,34] involves additional defect formation processes, which become particularly important at high dopant concentration and go beyond standard oxygen chemistry: Upon oxidation, the redoxchemistry of Nb:SrTiO 3 forces the formation of negatively charged strontium vacancies [27,30,34,35] as a natural result of Schottky disorder. [33,36,37] Cation vacancy defects are acceptortype defects, counter-compensating the extrinsic donor doping and lowering the electron concentration in the material.…”
mentioning
confidence: 99%
“…[33,36,37] Cation vacancy defects are acceptortype defects, counter-compensating the extrinsic donor doping and lowering the electron concentration in the material. [33,34,36,37] In the near-surface region, where ionic defect dynamics differ significantly from the bulk, the Schottky defect-equilibrium has been found to be active at temperatures as low as 670 K, [29,30] making it relevant for almost any thermal treatment of Nb:SrTiO 3 single crystals or thin films. For bare Nb:SrTiO 3 surfaces, the formation of cationic surface defects results in an insulating skin layer forming during oxidation [33,34,37] and leading to poor surface conductivity even in highly n-doped compounds.…”
mentioning
confidence: 99%
“…This process starts from the surface of SrTiO 3 and evolves over time via diffusion into the bulk. [29,30,34,52] The formation of Sr vacancy defects manifests itself by a shift of the Fermi-level into the band gap, as the sample is oxidized. [29] Figure 4 illustrates valence band (VB) spectra obtained on the bare surface of Nb:SrTiO 3 by in situ x-ray photoemission spectroscopy (XPS) at elevated temperature of 670 K and under applied oxygen atmosphere (near ambient pressure (NAP) XPS).…”
has therefore become one of the most important strides in oxide electronics, [6,7] but also in the design of epitaxial Li-ion batteries [8-11] and water splitting catalysts. [12-14] A prototype-and supposedly simple-case is the heterojunction of a transition metal oxide with a high work function metal such as Pt, forming Schottky-type transport barriers, which promote diode-like device characteristics, functional in electronic [1-4,15-21] and photonic concepts. [22-25] While Schottky-type transport barriers typically result from electronic charge transfer from the metal-oxide to the metal, complex oxides also offer a wide variety of mobile ionic defects. These provide additional ionic charges that affect the electronic charge screening within the Schottky contact. [17,26,27] The Pt/Nb:SrTiO 3 heterojunction is a prominent example, as its interfacial contact resistance results in a (fairly simple) metal-insulator-metal (MIM) structure, possessing rectifying I(V)characteristics, memristive switching behavior, [1,4,5,17,28] photo-catalytic activity [22] and sensor characteristics. [29,30] These properties are highly sensitive to the ionic constitution of the heterojunction's interface. To this end, the resistance switching behavior of these junctions can be attributed primarily to oxygen (vacancy) Heterojunctions between high-work-function metals and metal oxides typically lead to Schottky-type transport barriers resulting from charge transfer between the neighboring materials. These yield versatile electronic functionality exploited for current rectification, memristive behavior, or photocatalysis. Height, width, and shape of the interfacial transport barrier are strongly affected by charge screening via ionic defects, which are often extremely difficult to probe. The ionic nature of a variable contact resistance in heterojunctions between Nb-doped SrTiO 3 (Nb:SrTiO 3) and platinum is explored. A control of cationic vacancy defects at the interface is achieved by different annealing procedures in oxidizing and reducing conditions before establishing Pt/Nb:SrTiO 3 heterojunctions. Detailed analysis of electronic transport across the heterojunctions reveal significantly varied transport barriers resulting from the cationic defect structure at the interface. These findings are supported by conductive-tip atomic force microscopy and in situ photoemission spectroscopy showing diminished conductivity of the Nb:SrTiO 3 surface and the formation of an insulating surface skin layer after oxygenation. At high doping level, oxygen stoichiometry cannot explain the observed behavior. The increased transport barrier height is therefore linked to strontium vacancy defects. The tailored cation disorder yields access to the ionic control of electronic transport in functional oxide heterojunctions.
“…One such material interface that enables tunability of electronic properties, relevant for spin transport is that of complex oxides 16 . Although such material interfaces are commonly replete with oxygen vacancies and surface charge [17][18][19] , the tunability of several functional properties with temperature, electrical field, stress and strain has led to the unexpected emergence of new phenomena not encountered in other material systems.…”
We report on the temperature and electric field driven evolution of the magnetoresistance lineshape at an interface between Ni/AlOx and Nb-doped SrTiO3. This is manifested as a superposition of the Lorentzian lineshape due to spin accumulation and a parabolic background related to tunneling anisotropic magnetoresistance (TAMR). The characteristic Lorentzian line shape of the spin voltage is retrieved only at low temperatures and large positive applied bias. This is caused by the reduction of electric field at large positive applied bias which results in a simultaneous reduction of the background TAMR and a sharp enhancement in spin injection. Such mechanisms to tune magnetoresistance are uncommon in conventional semiconductors.Spin voltage measured at different semiconducting interfaces have been widely studied using different combination of materials as spin contacts and employing different measurement techniques. Such studies are commonly performed using the popular three terminal (3T) and four terminal non-local (NL) geometries 1-6 . In spite of the fact that both these electrical transport schemes fail to resolve outstanding issues related to the precise understanding, origin and magnitude of spin accumulation across semiconducting interfaces, these are more accentuated using the 3T geometry 7-11 . This is rooted in the inability of the 3T scheme to clearly ascertain the origin and magnitude of the spin voltage, possible considerations being spin accumulation in the semiconductor or localized states either in the tunneling barrier or at the semiconducting surface 12 . Earlier studies involving amorphous tunnel barriers showed that the tunneling conductance and spin polarization can be strongly influenced by the presence, concentration and type of impurities in the tunneling barrier via the formation of impurity mini bands and highly conducting multiresonant channels [13][14][15] . Attempts to mitigate such impurities by designing epitaxial barriers has also proved non-trivial in this context 4,11 . Additionally, the nature and type of impurities offer further challenges to validate proposed theories that seek to explain experimental observations using either spin injection (ferromagnet/tunnel barrier) or non-magnetic (metal/tunnel barrier) contacts 8,9 . Increasing the parameter space by using new transport schemes and/or choosing different materials will be an useful approach to understand the experimental findings related to the origin of spin accumulation across semiconducting interfaces. One such material interface that enables tunability of electronic properties, relevant for spin transport is that of complex oxides 16 . Although such material interfaces are commonly replete with oxygen vacancies and surface charge 17-19 , the tunability of several functional properties with temperature, electrical field, stress and strain has led to the unexpected emergence of new phenomena not encountered in other material systems.In this context SrTiO 3 (STO) is a relevant material. STO single crystals exhibit a large...
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