Effect of Cationic Interface Defects on Band Alignment and Contact Resistance in Metal/Oxide Heterojunctions

Abstract

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.

Complex impedance spectra of polymer‐derived SiC annealed at ultrahigh temperature

“…The Nyquist plot and the corresponding fitting of the complex impedance for the PDC‐SiC annealed at different temperatures are shown in Figure 5. During the fitting, all samples were found to be offset along the Z′ axis, which can be a finite series resistance (R c ) originated from the contact leads of the experimental setup 24 . The Nyquist plot for samples at 1300°C and 1500°C shows a typical semicircle (Figure 5A), indicating that the charge transport mechanism can be represented by a capacitor and a resistor in parallel, and the contribution of the impedance from free carbon is dominant 12 .…”

Space charge governs the kinetics of metal exsolution

“…The magnitude of the shift in binding energy furthermore depends on the ambient reducing and oxidizing conditions and was shown to be related to the presence of space charge regions at the surface. [44][45][46] The formation of surface space charge regions is related to the inherent defect chemistry and the respective redox response of the material under different oxygen chemical potential. 44,[47][48][49] Notably, the time-scale of the retarded exsolution response after oxidizing pre-annealing is very similar to the dynamic response of the space charge layer, detected for Nb-doped SrTiO3 by electrical measurements.…”

Enhanced photovoltaic performance via polarization in Bi/Co co‐doped KN‐based ferroelectric ceramics

“…Additionally, the grains have a smaller electrical conductivity in the 0.98KN-0.02BC sample than the grain boundaries, which is possibly caused by the stronger bonding of the grains of electrons and ions, and defects diffuse more easily in the grain boundaries. 54,55 The complex impedance spectrum of the 0.98KN-0.02BC sample at 500 • C with various DC biases is shown in Figure 8D. With increasing voltage, the impedance radius slightly observed, indicating an increase in conductivity due to the applied DC bias.…”

“…Accordingly, the charge transport in the 0.98KN–0.02BC ceramics is double‐ionized oxygen vacancies for the grains and single‐ionized oxygen vacancies for the grain boundaries, respectively. Additionally, the grains have a smaller electrical conductivity in the 0.98KN–0.02BC sample than the grain boundaries, which is possibly caused by the stronger bonding of the grains of electrons and ions, and defects diffuse more easily in the grain boundaries 54,55 …”