Growth and ferroelectric properties of yttrium-doped hafnium oxide/indium-tin oxide polycrystalline heterostructures with sharp and uniform interfaces

摘要

We report on the growth of polycrystalline yttrium-5%-doped HfO2 (YHO) films with sharp and uniform interfaces over a large area on indium-tin oxide (ITO) layers and their ferroelectric properties. On substrates of thermally oxidized silicon, YHO/ITO bilayers were deposited in amorphous form by means of pulsed-laser deposition and were subsequently crystalized by two-step post-deposition annealing (PDA) at low and high temperatures. The crystallized YHO/ITO heterostructures had ultra-flat surfaces: atomic-force-microscopy images exhibited a root-mean-square roughness of 0.15 nm over an area of 100 mu m(2). The roughness was much smaller than that of single-component polycrystalline films of ITO. X-ray diffraction and transmission electron microscopy (TEM) suggested the formation of YHO in higher symmetry phases including ferroelectric orthorhombic (o) YHO. TEM also confirmed crystallization of YHO layers in a pseudo-coherent manner from the YHO/ITO interfaces and resultant sharp and uniform YHO/ITO interfaces. These results are attributable to good lattice matching between the o-YHO layer and the ITO layer. It is suggested that this lattice matching was realized by the reduction of ITO during the crystallization in the two-step PDA process. The reduction might occur in the vicinity of the YHO/ITO interface via H2O incorporated in the amorphous bilayers. Pt/YHO/ITO junctions fabricated by using the two-step PDA method showed a polarization-electric field (P-E) hysteresis loop with remanent polarizations (P-r) of no less than 13 mu C/cm(2) at YHO thicknesses of 12 and 6 nm. These results indicate that the two-step PDA method can be used to fabricate high-quality YHO/ITO heterostructures in terms of surface and interface morphology, as well as ferroelectric properties. Published by AIP Publishing.