PZT Based High-speed Electro-Optic Modulator- Material Development, Fabrication, and Photonic Device Characterization

Abstract

Electro-optic (EO) modulator is a device that converts the electrical signal into an equivalent optical signal, which finds application in fields such as modulating the power of a laser beam, high-speed digital data recording, and optical communications. A pure phase modulation in an EO modulator can be achieved using materials with zero optical absorption and the presence of Pockel’s effect. Perovskites such as PZT, LNO, and BTO with wide bandgap and high Pockel’s coefficient are suitable to implement high-frequency pure-phase modulation. Among perovskites, thin-film lead zirconium titanate (PZT) has been used as a standard piezo-electric material in micro-electro-mechanical systems. Interestingly, it has one of the highest electro-optic coefficients that can be exploited to make high-speed on-chip light modulators as well as for other applications such as acousto-optic modulators and opto-MEMS. The fundamental challenge in direct integration of PZT on Si is the lattice mismatch that leads to the development of cracks in the film. The second issue is to use an appropriate method of deposition that makes the integration wafer scalable as well as a better control over the thickness with a fast deposition rate. These considerations motivated us to use RF deposition method for depositing PZT on Si with c-band transparent buffer layer insertion for photonic application. The optimization was carried out for MgO, TiO2 and Pt buffer layer followed by optimization of PZT deposition on the buffer layer. This work is the first-time demonstration of an EO modulator using sputtered-PZT loaded Si micro-ring resonator with an EO response of 14 pm/V. Despite being the first demonstration on sputtered PZT, the EO modulator has scope of improvement in the film quality and poling strategy used. The quality of deposited PZT was identified as the next step in improving the response of the EO modulator. The inherent drawback of the PZT loaded on Si photonic device pertaining to low EO overlap was circumvented by proposing a new architecture of modulator with Si photonic device on PZT as the bottom layer. Highly oriented PZT film was deposited on MgO (002) substrate by reducing the deposition RF power. The proposed architecture was fabricated and the realised Si MZI modulator on PZT had animproved EO response of 71 pm/V. The performance could further be improved by reducing the fabrication errors. With a co-planar poling strategy, the device performance is heavily dependent on the epitaxial nature of the film with the constraint of using only the in-plane direction of the Pockel’s tensor. To effectively use all components of the Pockel’s tensor, there was a need to make the poling process flexible so that the poling can be done not just in a co-planar direction but in a direction that corresponds to the maximum Pockel’s coefficient direction. To achieve this, we used remote Pt-buffered PZT growth to enable poling in any direction. This 3D poling mechanism was possible due to the availability of the bottom electrode for probing as well as the interdigitated electrode design that enabled us to extract the maximum possible EO effect from an epitaxial or a poly-crystalline film. It enabled engineering the electric field to match the direction of maximum with the maximum Pockel’s coefficient tensor direction. Using this poling mechanism, we achieve an EO response of 304 pm/V and 12 GHz high-speed bandwidth with an extrapolated bandwidth of 45 GHz. The VπL of 0.3 V-cm is measured. Other improvements that can be made to the EO modulator are by engineering the strain effects between the PZT and Pt layers. We observe a sudden peaking of the EO tensor at a strain in film related to the rumpling factor and phonon-softening modes leading to enhanced EO tensor values. Along with EO modulators, demonstrations of Accousto-optic (AO) modulator were done using the optimized PZT. Performance matching the simulated modulator was shown using Si MZI AOmodulator. Apart from PZT, we explored the possibility of a GaN platform for photonic application with a reported coupling efficiency of 5 dB/coupler loss. It was the first demonstration of an efficient coupler along with polarization-insensitive characteristics. We have demonstrated waveguides, micro-ring, and MZI on the GaN platform. The work opens the avenue to exploit GaN for electro-absorptive modulators and for non-linear photonic applications.