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

Among the novel materials for electronic applications and novel device concepts beyond classical Si-based CMOS technology, SrTiO₃ 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 SrTiO₃ is still under debate. Infrared spectroscopy offers the possibility to characterize structural and electronic properties of SrTiO₃ in operando, but is hampered by the diffraction-limited resolution. To overcome this limitation and obtain nanoscale IR spectra of donor-doped Sr_1-xLa_xTiO₃ ceramics, scattering-type scanning near-field optical microscopy is applied. By exploiting plasmon–phonon coupling, the local electronic properties of doped SrTiO₃ 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 cm² V⁻¹s⁻¹, N = 7.1 × 10¹⁹ cm⁻³, and ε_∞ = 7.7) and local defects (µ≈ 5.4 cm² V⁻¹s⁻¹, N = 1.3 × 10²⁰ cm⁻³, and ε_∞ = 8.8) are found. In future, subsurface quantification of defects and free charge carriers at interfaces and filaments in SrTiO₃ can be envisioned.