Ferroelectricity in yttrium-doped hafnium oxide

TLDR: In this article, structural and electrical evidence for a ferroelectric phase in yttrium doped hafnium oxide thin films is presented, based on X-ray diffraction.

Abstract: Structural and electrical evidence for a ferroelectric phase in yttrium doped hafnium oxide thin films is presented. A doping series ranging from 2.3 to 12.3 mol% YO1.5 in HfO2 was deposited by a thermal atomic layer deposition process. Grazing incidence X-ray diffraction of the 10 nm thick films revealed an orthorhombic phase close to the stability region of the cubic phase. The potential ferroelectricity of this orthorhombic phase was confirmed by polarization hysteresis measurements on titanium nitride based metal-insulator-metal capacitors. For 5.2 mol% YO1.5 admixture the remanent polarization peaked at 24 μC/cm2 with a coercive field of about 1.2 MV/cm. Considering the availability of conformal deposition processes and CMOS-compatibility, ferroelectric Y:HfO2 implies high scaling potential for future, ferroelectric memories.

Figures

FIG. 1. (Color online) (a) Fabrication flow of MIM capacitors based on atomic layer deposited Y:HfO2 dielectrics with post deposition anneal (PDA) or post metallization anneal (PMA) in N2 atmosphere. (b) TEM micrograph and HRTEM enlargement of the fully crystalline TiN/Y:HfO2/ TiN MIM stack on silicon treated with PMA.

FIG. 2. (Color online) RBS measurements on 10 nm thick films reveal a linear incorporation of YO1.5 into HfO2 with ALD cycle ratio of TEMAH and Y(MeCp)3 precursor. Structural drawings of the metal organic precursors are given.

FIG. 5. (Color online) P-V hysteresis loops acquired on 10 nm Y:HfO2 MIM capacitors treated with PMA (a) and PDA (b) processing clearly exhibit ferroelectric nature. For PDA treated samples a linear dielectric response is observed at low YO1.5 content. For high YO1.5 admixture to HfO2 a linear dielectric response is observed for both processing schemes.

FIG. 6. (Color online) Remanent polarization (a) and dielectric permittivity (b) maxima for PMA and PDA treated Y:HfO2 MIM capacitors are contrasted to the monoclinic phase fraction (c) evaluated from weighing the integral intensity of monoclinic against higher symmetry reflections.

FIG. 7. (Color online) GI-XRD diffractograms of Y:HfO2 samples reveal the coexistence of monoclinic and orthorhombic phase for low YO1.5 doping and complete stabilization of the cubic phase for higher doping content. When directly comparing PMA against PDA treated samples a higher monoclinic phase fraction for PDA samples is observed. Reference pattern are computed from ab initio calculations presented by Lowther et al.31 on HfO2 phase stability (cubic Fm3m, orthorhombic Pbc21, monoclinic P21=c). D-spacing was adjusted. For better visibility, reference patterns were scaled to the 2nd highest square root intensity (highest intensity for all patterns: 111 reflections). Stars indicate Pt reflections.

FIG. 4. (Color online) A characteristic transformation from sub-loop behavior to saturation with increasing P-V sweep voltage (a), as well as ferroelectric capacitance peaks in dual sweep C-V (b) are observed for TiN/10 nm 5.2 mol% Y:HfO2/TiN MIM capacitors.

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Q1. What are the contributions in "Ferroelectricity in yttrium-doped hafnium oxide" ?

In this paper, the effect of the top electrode interface on the hysteretic behavior of epitaxial ferroelectric Pb ( Zr, Ti ) O3 thin films with bottom SrRuO3 electrode was investigated.