Rystal Structure And The Ferroelectric Property Of HfO₂Film

As representative high-k dielectrics, HfO2have been extensively implemented in microelectronics industry. Recently, doped-HfO2thin films prepared by a special technique were discovered to exhibit pronounced ferroelectricity. Due to its superior compatibility to silicon-CMOS integration technology, it is expected this novel ferroelectric material will lead to a significant breakthrough in development of integrated ferroelectric-semiconductor devices such as non-volatile ferroelectric memories, which can improve traditional ferroelectrics in electronic devices, including less elements, control easily in preparation technology, good stability with silicon and silicon dioxide. Since HfO2ferroelectric property was discovered as a kind of new ferroelectric materials, it is necessary to understand the origin of the atomic structure and the ferroelectric property so that HfO2colud be used widely.To address these issues, the HfO2phase structure and the ferroelectric properties by first principle method are studied and compared. Firstly, electronic structures of the HfO2monoclinic (P2]/c) phase, tetragonal (P42/nmc) phase, cubic phase (Fm3m), orthorhombic phase (Pbca), orthorhombic phase (Pnma), cubic phase (Pa3) and ferroelectric orthorhombic (Pca21) phase are studied and compared. And it has been found that the valence and conduction band structure of7HfO2phases mainly consist of O2s, O2p states and Hf5d state, and this states show different hybrization in seven phases. It means that all these phase structures are similar. Secondly, by translations of fluorite related structures, six known phase structures and ferroelectric phase structures are futher compared and studied, according to the atomic displacements on the basis of the structural transformations and relative total energies, the origin of the ferroelectric Pca21structure was found to transform from tetragonal P42/nmc structure or orthorhombic Pbca structure. Thirdly, the spontaneous polarization of HfO2ferroelectric phase structure has been studied by berry phase method and born effective charge method, respectively. It has been found that the spontaneous polarization22.07p.C/cm-is consistent with the previous results10-24μC/cm2,including ion contribution and electronic contribution which are51.78and29.71μC/cm2, respectively. In the ion contribution, polarization contributions are mianly from the oxygen ions, while the main contribution to the polarization of four oxygen ions is the second type ion. And the spontaneous polarization obtained from tetragonal phase structure as the reference structure is closer to the previous results then cubic phase. And the spontenous polarizations by born effective charge method were65.65μC/cm2(tetragonal P42/nmc structure as reference structure) and66.22μC/cm2(cubic Fm3m structure as reference structure) which are larger than the previous results while are close to each other. Lastly, by the comparison of the fluorite-related structures and the electronic structures of the tetragonal phase (paraelectric phase) and the ferroelectric phase, the ferroelectric origin of HfO2orthorhombic Pca21has been compared and studied. It has been found that in the vicinity of-16eV, both phases exist a peak in Hf5d state, O2p state and O2s state. And the peak intensity of Pca21phase is higher than that of P42/nmc phase, while the peaks of Hf5d state, O2p and O2s states are moving to low energy0.45eV. It has been shown that density peaks moving to low energies is related to the enhancement of Hf-O hybrization. So it can be reckoned that the spontaneous polarization of HfO2ferroelectric structure is from the atomic orbital hybridization in the tetrahedron.