KTaO3(001) preparation methods in vacuum: effects on surface stoichiometry, crystallography, and in-gap states

Abstract

KTaO3 single crystals with different orientations are used as substrates for the epitaxial growth of thin films and/or as hosts for two-dimensional electron gases. Due to the polar nature of the KTaO3(001) surface, one can expect difficulties and challenges to arise in its preparation. Maintaining good insulating characteristics without adding undesirable in-gap electronic states, obtaining good crystalline order up to the top surface layer, a sufficiently flat surface, and complete cleanliness of the surface (without water, C, or OH contaminants), are, in general, difficult conditions to accomplish simultaneously. Cleaving in vacuum is likely the best option for obtaining a clean surface. However, since KTaO3 is cubic and lacks a well-defined cleavage plane, this method is not suitable for sample growth or reproducible device fabrication. Here, we systematically evaluate the effect of typical preparation methods applied on the surfaces of KTaO3(001) single crystals. In particular, we used annealing in vacuum at different temperatures, light sputtering with Ar+ ions at a low energy (500 eV) followed by annealing, heavy Ar+ ion bombardment and annealing, and grazing Ar+ ion bombardment under continuous azimuthal rotation combined with both annealing in vacuum and in O2 atmosphere. Possible side effects after each treatment are evaluated by a combination of techniques, including low-energy ion scattering at forward angles, Auger electron spectroscopy, low-energy electron energy loss, x-ray photoelectron spectroscopy, low-energy electron diffraction, and time of flight-secondary ion mass spectrometry. Advantages and shortcomings of each preparation method are discussed in detail.