Chemical state detection and charge transfer in complex oxide heterostructures via in situ Auger Electron Spectroscopy

TL;DR

This work establishes in situ Auger Electron Spectroscopy as a chemically sensitive, real-time probe of oxidation states during complex oxide thin-film growth. A parameter-free escape-depth model is developed to translate AES intensities into depth-resolved oxidation-state information for Mn- and V-based perovskites, enabling quantitative analysis of interfacial charge transfer. The study provides direct, depth-resolved evidence of Mn2+ interfacial character at the LaMnO3/SrTiO3 interface and characterizes vanadate behavior with depth, demonstrating the method's capacity to monitor and ultimately control chemical states during growth. The approach promises atomic-scale tunability of interfacial chemistry across oxide heterostructures and related systems, with broad implications for electronics, optics, and energy technologies.

Abstract

Understanding and controlling the chemical states both in the bulk and at the interfaces of complex oxide thin films is essential for engineering a wide range of electronic, optical, and magnetic functionalities, which arise through emergent phenomena such as two-dimensional electron gases, interfacial magnetism, and associated phase transitions. Here, we demonstrate the use of in situ Auger Electron Spectroscopy (AES) as a powerful tool for probing oxidation states and dynamic chemical processes during the growth of complex oxide heterostructures. By leveraging the chemical sensitivity of AES to subtle changes in valence electron populations, we show that this technique can distinguish distinct oxidation states in multivalent perovskite manganate and vanadate systems with high fidelity during deposition. Furthermore, we show evidence for dynamic chemical phenomena, specifically charge transfer processes at the polar-nonpolar LaMnO3/SrTiO3 interface. Our results establish in situ AES as a powerful diagnostic tool for monitoring and controlling interfacial chemistry during thin film growth, offering a pathway toward the atomic-scale engineering of chemical states in functional oxide heterostructures.

Figures (11)

• Figure 1: (a) Electron occupation in the valence band of metal and metal oxides as a function of oxidation states. Core shells of increasing binding energy are marked as C", C' and C. Auger transitions involving these core levels and the valence band are shown. For the metal oxides, the electronic charge lost to oxygen is shown as q' and q, wherein $q'<q$. N is the number of valence electrons or corresponds to the electron population in the valence band of a nascent metal. The valence electron population in the metal oxide after oxidation is (N-q') and (N-q) respectively. The difference in the shades of the valence bands at different oxidation states indicates this. Valence electron population decreases progressively as the metal is increasingly oxidized. (b) Qualitative variation of the corresponding Auger fine spectra of type CXX' for the metal and metal oxide scenarios shown in (a). C is a Core level and X and X' are either core or valence levels. The intensities of Auger transitions that involve valence electrons (CVV and CC'V) decreases as the metal is increasingly oxidized.
• Figure 2: (a) The ratio of intensities of the LMM and LVV peaks of Manganese in 100 UC LMO/ 100 UC CMO bilayers. The orange and green shaded regions represent CMO and LMO respectively. RHEED pattern of: (b) the SrTiO$_{3}$ (001) substrate before growth, (c) the film surface after the deposition of 100 UC of CMO, and (d) the film surface after subsequent deposition of 100 UC of LMO. High resolution out-of-plane thin film X-ray diffraction patterns of $\sim$ 20 nm thick films of (e) CMO and (f) LMO grown on STO (001) substrates. The CMO in (e) is completely relaxed while the LMO shown in (f) is strained.
• Figure 3: (a) The ratio of intensities of the LMM and LMV peaks of Vanadium in 100 UC SVO/ 100 UC LVO bilayers. The orange and green shaded regions represent SVO and LVO respectively. RHEED pattern of: (b) the SrTiO$_{3}$ (001) substrate before growth, (c) the film surface after the deposition of 100 UC of SVO, and (d) the film surface after subsequent deposition of 100 UC of LVO. (e) High resolution out-of-plane thin film X-ray diffraction patterns of $\sim$ 20 nm thick films of SVO and LVO grown on STO (001) substrates.
• Figure 4: (a) Time dependent evolution of the intensity of the specular spot during the growth of LMO and CMO layers indicating layer-by-layer growth. (b) The ratio of LMM and LVV intensity values for 20 UC CMO/ 20 UC LMO heterostructure grown on STO substrate. The Bulk +4,+3 and +2 oxidation state levels of Mn are marked for reference. Parameter free escape depth models with and without charge transfer in LMO/STO interface are plotted. The green and orange shaded regions represent LMO and CMO respectively. Oxidation states of Mn after the deposition of (c) 1 unit cell, (d) 2 unit cells - the advent of charge transfer and (e) $>$ 2 unit cells. The shades of the MnO$_6$ octahedra qualitatively represent the oxidation state in Mn, with it gradually increasing to +3 as the LMO layer gets thicker.
• Figure S1: (a) LMM fine spectra of Mn in CMO and LMO also showing the KLL type peaks of Oxygen (b) LMM fine spectra in V in SVO and LVO also showing the KLL type peaks of Oxygen