Sub-micron auger electron spectroscopy characterization of lithium niobate ferroelectric domains and their fabrication

Abstract

Ferroelectrics are a novel class of materials with a built-in electric polarization state. Like ferromagnetic materials, by applying a strong external field, the direction of ferroelectric polarization can be switched, or 'poled'. Poling also switches the sign of the nonlinear coefficient, which determines the strength of a material's nonlinear optical interactions. By controlling the ferroelectric poled domain structures, the switching sign of the nonlinear coefficient can keep interacting optical waves in-phase, limiting deleterious material dispersion during nonlinear optical interactions. Lithium niobate (LiNbO 3) is one such ferroelectric crystal, prominently used in nonlinear optics. Periodically poled lithium niobate (PPLN) domain structures can produce the phase-matching conditions described above in a process called quasi-phase matching, creating powerful nonlinear optical devices. The applications of these devices are numerous, yet they have not reached their full potential due to the limitations of fabricating and characterizing nano-scale patterned domain structures. We first explored nano-fabrication of electrodes as a precursor to nano-scale poling. Periodic grating electrodes with 600 nm periods were fabricated using an innovative combined photolithography and electron beam lithography (EBL) liftoff method to create HV poling contact electrodes. A 10 kV bulk poling system was built and preliminary poling tests in three distinct poling configurations were performed on magnesium-doped lithium niobate (MgLN). We then adapted Auger electron spectroscopy (AES) as a new method to address the unique challenge of characterizing ferroelectric domains. In our initial AES characterization method, polar ferroelectric domains (+/-Z directions) in MgLN were differentiated from one another by the Auger O-KLL peak energy, with the -Z domains having higher peak energy due to the lower surface potential. We then discovered that +/-Z domains in PPLN can be differentiated with nano-scale resolution by the O-KLL peak amplitude, which is larger for -Z domains. The principle of this AES peak amplitude separation method was applied to mapping to achieve full imaging of PPLN's +/-Z domains with fields of view spanning from 7.5-200 microns. We ultimately demonstrate AES mapping as a new lithium niobate domain imaging method that is non-destructive, non-contact, unambiguous, with nano-scale resolution down to 67 nm.