Growd and substructure lithium niobate films

Abstract

Objective. Radio frequency magnetron sputtering (RFMS) is one of the most promising methods of synthesis of lithium niobate films. It is known that RFMS conditions (the composition and the pressure of the working gas and the power and the spatial inhomogeneity of plasma discharge) offer great opportunities for controlling the structure of complex composition films and their texture in particular. Currently, no publications have described research dedicated to the earliest stages of the growth of lithium niobate films during RFMS or on any substrates.

The aim of the work is to study the initial stages of the growth and the oriented crystallization of lithium niobate films in the process of RFMS and TA, PPT, and FTA on a Si substrate.

Methods and methodology. Lithium niobate films with a thickness of up to 1 μm were obtained by the method of  radio frequency magnetron sputtering (RFMS) on non-heated and heated (up to 550°C) substrates. The single-crystal LiNbO₃ plates of the (0001) orientation obtained by the Czochralski method were used as the target. RFMS was performed in Ar medium and Ar + O₂ mixtures (the fraction of O₂ content in the mixture ranged from 20 to 30 %) with a specific power of the high-frequency discharge of 15–30 W/cm². Plates of single-crystal Si (001) orientation and heterostructures of an amorphous SiO₂–Si – amorphous SiO₂ film were used as substrates. To study the initial growth stages of coatings using the TEM method, a carbon replica was deposited on the resulting films with a thickness of up to 100 nm (the duration of the RFMS process – 0.5–7 min) and then separated from the substrate (Si) together with the test coating in a mixture of H₂O+HNO₃+HF. Thermal annealing (TA) of samples in air was performed in situ in an ARL X’TRA Thermo Techno X-ray diffractometer chamber, an Anton Paar 1200N furnace. Pulsed photon treatment (PPT) was carried out on the upgraded installation UOLP-1M in air: at an energy density of E_i = 130 J/cm² and with E_i = 80 J/cm². The phase composition, substructure, and morphological features of the films were investigated by X-ray diffractometry (Bruker D2 Phaser, ARL X’TRA Thermo Techno), electron diffraction (EG-100M), and electron microscopy (ZEISS Libra 120, EMV-100BR); the analysis of the elemental composition was performed by electron Auger spectroscopy on an ESO-3 instrument with a DESA-100 analyzer.

Results. It is established that the initial stages of the growth of lithium niobate films during the HFMR process on a substrate Si heated to 550 °C (001) are characterized by island nucleation of crystallites and their subsequent coalescence. The research shows the possibility of controlling the texture of lithium niobate films in the process of RFMS under the conditions of exposure to plasma of an RF discharge by changing the composition of the working gas. The PPT effect is manifested in the crystallization of amorphous lithium niobate films that involves the formation of a single-phase nanocrystalline film of lithium niobate when treated in air in contrast to thermal treatment which results in the formation of a two-phase LN + LTN film.

Conclusion. The obtained results favour the use of lithium niobate films as a material for functional ferroelectric elements, for optoelectronics (optical waveguides, ring microresonators), acoustoelectronics (piezoelectric transducers in delay lines, filters), and semiconductor electronics (nonvolatile FRAM cell). This can significantly simplify the manufacturing technology of such elements and allow them to be introduced into the production of conventional CMOS structures while at the same time making some additions to the existing technology.

SOURCE OF FINANCING

The reported study was supported by the Russian Foundation for Basic Research (grant No. 18-33-00836).

CONFLICT OF INTEREST

The authors declare the absence of obvious and potential conflicts of interest related to the publication of this article.

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