Tuning of electrical properties and persistent photoconductivity of SnO2 thin films via La doping for optical memory applications

This work explored the potential of utilizing Lanthanum doped tin oxide Sn_1−xLa_xO₂ (x = 0.01 to 0.1) based Metal-Semiconductor-Metal Ohmic photoconductors for optical memory applications making use of the persistent photoconductivity (PPC) property. The structural, optical, and electrical properties of Sn_1−xLa_xO₂ thin films deposited on glass substrates using the spray pyrolysis method, with a focus on the impact of lanthanum concentration on the photoresponse characteristics was investigated. Raman spectroscopy confirmed the presence of oxygen vacancies and nanometric grain size in the films along with the typical Raman active modes of tin oxide. The Sn⁴⁺ and La³⁺ oxidation states in Sn_1−xLa_xO₂ as well as the contributions from lattice oxygen and oxygen vacancies were identified using XPS. Photoluminescence studies revealed emissions in the UV, violet, blue, and yellow regions, corresponding to tin interstitials, oxygen vacancies, and other defects, with intensity variations based on La concentration. All films exhibited n-type conductivity, with La content influencing both resistivity and carrier concentration. Photoconductivity measurements demonstrated enhanced photocurrent under UV illumination, with La doping affecting energy levels and defect states. The Sn_0.90La_0.10O₂ film possessed a photocurrent retention of nearly 64 % within a span of 10⁴ s, showing that higher concentration of La favoured the enhancement of retention of photocurrent for a comparatively longer duration. The significant persistent photoconductivity requires the conditions like optically active materials, a built-in electric field to separate electron-hole pairs, and defect states to trap carriers, which are all met by the prepared Sn_1-xLa_xO₂ photoconductor with higher La doping levels, confirming the suitability of these films for practical use as optical non-volatile memory elements.