Pb(Zr,Ti)O ₃-GaN heterostructures for RF MEMS applications

Ferroelectric (Pb(Zr _0.3Ti _0.7)O ₃ or PZT(30/70))/GaN/Sapphire heterostructures were fabricated by the sol-gel process. The structure and composition of PZT, and its dielectric and ferroelectric properties in Pt/PZT/Ru or Pt/PZT/Pt, and Pt/PZT/GaN configurations were characterized to assess their feasibility as RF MEMS devices for eventual insertion in RF communication systems. The X-ray diffraction and pole figure analysis confirmed the stoichiometric, phase-pure PZT with (111) out-of-plane relationship on (0001) GaN. The secondary ion mass spectrometry (SIMS) depth profile indicated a stable interface with insignificant inter-diffusion of the Pb, Zr, or Ti elements into the GaN. The properties of PZT in Pt/PZT/Ru/GaN or MFM configuration were high capacitance density (C/A = 1.25 μF/cm ²) and polarization (30 μC/cm ²). In contrast, PZT in Pt/PZT/GaN or MFS configuration exhibited lower capacitance density (C/A = 0.35 μF/cm ²) and asymmetrical hysteresis loops (polarization ∼4 μC/cm ²). From the calculated spatial distribution of the electric field, E(x), and potential, V(x), which stem from all the charge densities within the entire MFS system, one may eventually design and fabricate PZT/GaN heterostructures in terms of controllable parameters (e.g., PZT phase and orientation, donor and acceptor ionization energies, and thickness and screening lengths of PZT and GaN), such that the depolarization field (E _depol) is minimized and the polarization is maximized in PZT.