A key goal in the development and optimization of common sensor technologies is the miniaturization of the devices. However, there are no piezoelectric films available for usage in miniaturized devices under extreme conditions, especially in oxidizing environments at high temperatures close or above 1000°C. This lack in thermal and chemical stability can be filled by piezoelectric oxide materials provided that their deposition on high-temperature stable substrates can be realized.
Langasite single crystals (La3Ga5SiO14, LGS) fulfill necessary requirements. In particular, they show piezoelectric behavior up to 1470°C (melting point) and are stable in oxidizing atmospheres. Thus, the challenge consists of the epitaxial growth of single crystalline thin films. Two different scenarios should be realized: a homo-epitaxial approach realizing a monolithic device (minimal thermal stress) by using doped LGS thin films as electrodes and conductive paths on an undoped LGS single crystal and a hetero-epitaxial approach that allows to use different substrates such as structured silicon, alumina or silicon carbide.
Epitaxial growth requires, beside a low lattice mismatch, an accurate stoichiometry and a high mobility of the film forming particles. Pulsed laser deposition (PLD) is used because of its well known stoichiometric transfer of the target composition to the substrate. Additionally, the substrates are heated up to temperatures from 400°C to 600°C to increase the mobility of the surface atoms and to enable crystallization of the thin films. Crystalline growth of LGS on Si substrates is realized with this setup at a substrate temperature of 600°C.
Ultra high vacuum and the high substrate temperatures lead to volatilization of Ga-sub-oxides (Ga2O) that are formed during the deposition process. This effect causes a large Ga deficit in the deposited films that results in off-stoichiometric films. In this work, two ways to suppress this behavior are realized: the deposition under oxygen atmosphere with a partial pressure up to 20 Pa to prevent the formation of sub-oxides and a massive increase of the Ga content in the target to counterbalance the Ga loss.