Origin of strain in H+ ion implanted LiTaO3 single crystals

To enhance the performance of surface acoustic wave filters, industries are actively investigating the substitution of bulk lithium tantalate piezoelectric single-crystal substrates with thin films integrated onto silicon substrates. Ion implantation in these single crystals serves as the initial stage of the Smart-Cut process, widely recognized for enabling material transfer onto silicon. Identification of the physical phenomena behind ion implantation in piezoelectric single crystals remains a challenge due to the emergence of strain and, thus, local electric fields gradients. In this work, we address strain generated in LiTaO3 single crystals by H+ ion implantation in relation to the structural modifications and as a function of implanted fluence. We show that strain is not only proportional to the local H concentration, the case found in common target materials, such as Si crystals, but also proportional to the fluence with some non-linear coefficients depending on the modified elastic constants of the implanted material. We evidence that the damage redistribution is at the root of this intriguing phenomenon, which is probably linked to the emergence of gradual electric fields. Targeting a unified law for the evolution of strain with local H-concentration and H-fluence and generated damage, we provide the generalized form for fluence-normalized strain as a function of the H-to-damage concentration ratio, which is independent of fluence. Depending on the ratio range, four distinct evolutionary regimes describe the function. These are discussed in detail.