Spectroellipsometric study of lead magnesium niobate-lead titanate (PMN-PT) thin films

Abstract

Spectroscopic ellipsometry (SE) has been utilized to non-destructively characterize optical properties of (1-x)Pb(Mg1/3Nb2/3)O3-(x)PbTiO3 [(l-x)PMN-(x)PT] thin films as potential materials for integrated optical devices, such as optical switches and waveguide modulators. Using this technique, the refractive indices, depth profiles and surface roughness of PMN-PT thin films were determined. The PMN-PT films were epitaxially grown on MgO(001) and Si(001) single crystal substrates by pulsed laser deposition method. They were structurally characterized by X-ray diffractometer using CuKa radiation. In order to check the results obtained by SE, the depth profiles and surface roughness of PMN-PT films were examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). It is found that the depth profiles and surface roughness of PMN-PT film obtained by SE are similar to those obtained by SEM and AFM. The effects of composition of PMN-PT (x= 0.1, 0.3, 0.35 and 0.4) and deposition temperature (630oC to 710oC) on the refractive indices of PMN-PT films were revealed. For simplicity, the PMN-PT films were grown on MgO single crystal. The optical bandgap energies of these films were also obtained by transmittance measurement. The refractive index and extinction coefficient of PMN-PT are described by Sellmeier and Cauchy extinction model respectively. It is observed that the refractive index of PMN-PT film increases with increasing PbTiO3 content and decreases with increasing deposition temperature. The 0.65PMN-0.35PT film grown at 670oC has better crystallinity and lower extinction coefficient. The energy bandgap of PMN-PT film is composition-dependent and decreases with increasing PbTiO3 content. Because of the small change of deposition temperature ranging from 630oC to 710oC, the energy bandgaps of 0.65PMN-0.35PT films with different deposition temperatures show no significant change. For developing practical integrated optical devices, waveguide structure of PMN-PT/MgO/TiN were fabricated on Si substrate. The refractive index and thickness of each layer were measured by SE. Our results show that the refractive indices of PMN-PT films grown on Si substrate are less than that grown on MgO substrate. The refractive indices of 0.65PMN-0.35PT films on Si and MgO substrate at 635 nm are 2.502 and 2.585 respectively. To compare the refractive indices in thin film form and single crystal form, the refractive indices of single crystals with mechanical polishing were studied by SE. It is noticed that the optical data is strongly influenced by the surface preparation condition. In order to obtain the optical properties of 0.65PMN-0.35PT single crystal, the optical model of the crystal is described by using a semi-infinite bulk substrate with one layer of roughness on the top. Our results show that the refractive index of single crystal is larger than that of the films grown on MgO and Si single crystal with the same composition. The refractive index of 0.65PMN-0.35PT single crystal is 2.603 at 635 nm.