Stabilization of phase-pure rhombohedral HfZrO4 in pulsed laser deposited thin films

Abstract

Controlling the crystalline structure of Hafnium Zirconate and its epitaxial relationship to a semiconducting electrode has a high technological interest, as ferroelectric materials are key ingredients for emerging electronic devices. Using Pulsed Laser Deposition, a phase pure, ultrathin film of Hf0.5Zr0.5O2 is grown epitaxially on a GaN (0001) / Si (111) template. Since standard microscopy techniques do not allow to determine with certitude the crystalline structure of the film due to the weak scattering of oxygen, differentiated differential phase contrast (DPC) Scanning Transmission Electron Microscopy is used to allow the direct imaging of oxygen columns in the film. Combined with X-Rays diffraction analysis, the polar nature and rhombohedral R3 symmetry of the film are demonstrated.

Guidelines for the stabilization of a polar rhombohedral phase in epitaxial Hf0.5Zr0.5O2 thin films

“…Phase analysis by means of 2h scans around poles corresponding to a representative domain (Figure 8(c,d)) reveals 3:1 multiplicity, pertaining to an r-phase, on both these substrates. HAADF-STEM images reported by Lours et al [47] clearly show these domains and the coherent domain boundaries on the GaN-buffered Si substrate (Figure 8(e)). The HAADF-STEM image from just one domain (Figure 8 sectional lamella.…”

“…Next, we grew 6 nm thick HZO layers on substrates that, by virtue of their symmetry, impose (111) orientation to the film. In this pursuit, we used (0001) hexagonal substrates such as GaN buffered Si [47] with a 1 ¼a 2 ¼3.23 Å, a ¼ 120 and sapphire with a 1 ¼a 2 ¼3.46 Å, a ¼ 120 , both of which provide an initial compressive strain to the {111} plane of HZO (a 1 , a 2 $ 3.56-3.62 Å, a $ 120 depending on the polymorph). The {111} pole figures on both these substrates (Figure 8(a,b)) clearly show that these films are (111) oriented.…”

“…Remanent polarization (P r ) values as large as 34 mC/cm 2 were observed on these films with thickness of 5 nm, this being the highest P r reported in HfO 2 -ZrO 2 alloys. Later, pure r-phase was also reported on 6 nm HZO layers grown epitaxially on hexagonal GaN buffered Si substrates [47], whereas mixed m-and r-phases are reported on HZO layers epitaxially grown directly on Si (111) [48].…”

An Overview of Ferroelectric Hafnia and Epitaxial Growth

“…Further experiments confirmed that YHO films grown on (001) YSZ contain four types of orientations: o 1 , o 2 , o 3 , o 4 , whereas films grown on (100) ITO/(100) YSZ contain two types of orientations: o 5 and o 6 . [74] In addition, Yoong et al [56] studied the growth of epitaxial HZO thin films on [74] 700 0.013 9 --YHO/(001)ITO/(001)YSZ [74] YHO/(110)ITO/(110)YSZ [34] 700 0.013 15 16 Yes YHO/(ITO)/(111)YSZ [136] 700 0.013 14 10 -YHO/YSZ [50] 700 0.013 20 --HZO/(0001)GaN/(111)Si [76] 750 0.1 5.83 --YHO/ITO/YSZ [33,35,137] a) 0.013 10-115 25 No YHO/YSZ/(001)Si [77] 700 0.2 20 -HZO/TiN/(001)YSZ [138] 700 2.6*10 À8 15 7-30 -HZO/TiN/(011)YSZ [138] HZO/TiN/(111)YSZ [138] HZO/LSMO/(001)LAO [56] 550 0.13 10 20 Yes…”

Critical effect of the bottom electrode on the ferroelectricity of epitaxial Hf0.5Zr0.5O2thin films

“…5 Epitaxial films are being investigated now very actively. Several conducting or semiconducting substrates or templates, including Si wafers, 6 TiN, 7 GaN, 8 and indium tin oxide, 9 have been used to deposit epitaxial films of ferroelectric doped HfO 2 . In contrast, epitaxy of hafnia on perovskite-type oxide electrodes has only been reported on Nb-doped SrTiO 3 (Nb:STO) 10 and La 0.67 Sr 0.33 MnO 3 (LSMO).…”