GROWN AND SUBSTRUCTURE OF LITHIUM NIOBATE FILMS

Abstract

Objective. Radio frequency magnetron sputtering (RFMS) is one of the most promising methods of synthesis of lithium niobate fi lms. 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 fi lms and their texture in particular. Currently, no publications have described research dedicated to the earliest stages of the growth of lithium niobate fi lms 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 fi lms in the process of RFMS and TA, PPT, and FTA on a Si substrate. Methods and methodology. Lithium niobate fi lms 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 specifi c power of the highfrequency discharge of 15–30 W/cm2. Plates of single-crystal Si (001) orientation and heterostructures of an amorphous SiO2–Si – amorphous SiO2 fi lm were used as substrates. To study the initial growth stages of coatings using the TEM method, a carbon replica was deposited on the resulting fi lms 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 theupgraded installation UOLP-1M in air: at an energy density of Ei = 130 J/cm2 and with Ei = 80 J/cm2. The phase composition, substructure, and morphological features of the fi lms 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 fi lms 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 fi lms 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 fi lms that involves the formation of a single-phase nanocrystalline fi lm of lithium niobate when treated in air in contrast to thermal treatment which results in the formation of a two-phase LN + LTN fi lm. Conclusion. The obtained results favour the use of lithium niobate fi lms as a material for functional ferroelectric elements, for optoelectronics (optical waveguides, ring microresonators), acoustoelectronics (piezoelectric transducers in delay lines, fi lters), and semiconductor electronics (nonvolatile FRAM cell). This can signifi cantly 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

Keywords

• lithium niobate
• thin fi lms
• radiofrequency magnetron sputtering
• grown
• structure
• thermal treatment
• pulsed photon treatment.