Nanospectroscopy of Infrared Phonon Resonance Enables Local Quantification of Electronic Properties in Doped SrTiO₃ Ceramics

Abstract

Abstract Among the novel materials for electronic applications and novel device concepts beyond classical Si‐based CMOS technology, SrTiO 3 represents a prototype role model for functional oxide materials: It enables resistive switching, but can also form a 2D electron gas at its interface and thus enables tunable transistors. However, the interplay between charge carriers and defects in SrTiO 3 is still under debate. Infrared spectroscopy offers the possibility to characterize structural and electronic properties of SrTiO 3 in operando, but is hampered by the diffraction‐limited resolution. To overcome this limitation and obtain nanoscale IR spectra of donor‐doped Sr 1‐x La x TiO 3 ceramics, scattering‐type scanning near‐field optical microscopy is applied. By exploiting plasmon–phonon coupling, the local electronic properties of doped SrTiO 3 are quantified from a detailed spectroscopic analysis in the spectral range of the near‐field ‘phonon resonance’. Single crystal‐like mobility, an increase in charge carrier density N and an increase in ε ∞ at grain boundaries ( µ ≈ 5.7 cm 2 V −1 s −1 , N = 7.1 × 10 19 cm −3 , and ε ∞ = 7.7) and local defects ( µ ≈ 5.4 cm 2 V −1 s −1 , N = 1.3 × 10 20 cm −3 , and ε ∞ = 8.8) are found. In future, subsurface quantification of defects and free charge carriers at interfaces and filaments in SrTiO 3 can be envisioned.