Memory Effects in Nanolaminates of Hafnium and Iron Oxide Films Structured by Atomic Layer Deposition

Abstract

HfO₂ and Fe₂O₃ thin films and laminated stacks were grown by atomic layer deposition at 350 °C from hafnium tetrachloride, ferrocene, and ozone. Nonlinear, saturating, and hysteretic magnetization was recorded in the films. Magnetization was expectedly dominated by increasing the content of Fe₂O₃. However, coercive force could also be enhanced by the choice of appropriate ratios of HfO₂ and Fe₂O₃ in nanolaminated structures. Saturation magnetization was observed in the measurement temperature range of 5-350 K, decreasing towards higher temperatures and increasing with the films' thicknesses and crystal growth. Coercive force tended to increase with a decrease in the thickness of crystallized layers. The films containing insulating HfO₂ layers grown alternately with magnetic Fe₂O₃ exhibited abilities to both switch resistively and magnetize at room temperature. Resistive switching was unipolar in all the oxides mounted between Ti and TiN electrodes.

Keywords: atomic layer deposition; ferromagnetism; hafnium oxide; iron oxide; multilayers; nanolaminates; resistive switching.

Figure 1

Contents of elements (a) and residual impurities (b) measured by XRF, constituting the HfO₂-Fe₂O₃ films, expressed in atomic% against relative amount of HfO₂ deposition cycles. The elements are indicated in the legends. Polynomial lines are guides for the eye.

Figure 2

X-ray reflectivity results for selected stacks of Fe₂O₃ and HfO₂ layers grown on Si, denoted by the labels revealing the amounts of ALD cycles used for the deposition of constituent layers. The thicknesses of constituent layers as the results of the curve fittings are also given by labels. The curves with fitting results are presented for the four-layer laminate grown using equal amounts of cycles for both constituent oxides (a), the three-layer laminate grown using equal amounts of cycles for both constituent oxides (b), the double-layer consisting of relatively thicker Fe₂O₃ and thinner HfO₂ films (c), and the four-layer laminate containing Fe₂O₃ layers relatively thicker compared to the HfO₂ component (d).

Figure 3

Bright field STEM images of HfO₂-Fe₂O₃-HfO₂ nanolaminate grown using 100 ALD cycles for each constituent layer, taken under different magnifications (a,b), and an image of the interface between silicon substrate and the first HfO₂ layer in the same laminate (c).

Figure 4

Elemental mapping for iron (a), hafnium (b), and oxygen (c) in the HfO₂-Fe₂O₃-HfO₂ nanolaminate grown using 100 ALD cycles for each constituent layer.

Figure 5

Grazing incidence X-ray diffraction patterns of Fe₂O_3-, HfO_2-, and HfO₂-Fe₂O₃-laminated films. The growth cycle sequences are denoted by labels. The reflections supplied with Miller indexes are assigned as those belonging to either monoclinic (M, ICDD PDF-2 card no 43-1017) or tetragonal (T, card 01-078-5756) HfO₂, whereby reflections from rhombohedral hematite Fe₂O₃ are denoted by R (card 01-1053).

Figure 6

Magnetization-field curves from reference HfO₂ and Fe₂O₃ films measured at 5 K (a) and at 300 K (b), as compared to the curves from HfO-Fe₂O₃-laminated structures measured at 5 K (c) and 300 K (d). The films were grown on SiO₂/Si substrates using cycle sequences represented by labels.

Figure 7

Magnetic moment (a) and coercivity (b) against relative content of hafnium, expressed by Hf/(Hf + Fe) cation ratio, in HfO₂-Fe₂O₃ nanolaminates. For the cycle ratios and sequences applied for the growth of the samples with the corresponding atomic ratios, see Table 1.

Figure 8

Magnetization-temperature curves measured in zero-field-cooling (ZFC) and field-cooling (FC) mode from representative structures consisting of two HfO₂-Fe₂O₃ double layers (a) and one HfO₂-Fe₂O₃-HfO₂ triple layer (b). The films were grown on diamagnetic SiO₂/Si substrates using cycle sequences represented by labels.

Figure 9

Permittivity versus measurement frequency dispersion test results for reference HfO₂ films as well as nanolaminate stacks grown using amounts of ALD cycles and to the thicknesses indicated by the labels. The capacitor electrode areas were 0.002 mm².

Figure 10

Current-voltage characteristics demonstrating unipolar switching in TiN/HfO₂/Ti devices containing HfO₂ films grown to thicknesses of 24 (a) and 54 nm (b) using 200 and 500 ALD cycles, respectively.

Figure 11

Current-voltage characteristics demonstrating unipolar switching in TiN/HfO₂-Fe₂O₃/Ti devices containing (a) periodically laminated media grown using relatively large amounts of HfO₂ deposition cycles, (b) periodically laminated media grown using the same cycle numbers for HfO₂ and Fe₂O₃, and (c) three-layer stack with Fe₂O₃ layer grown in between HfO₂ films. The deposition cycle sequences are indicated by labels.

Figure 12

Current values measured after each RESET (HRS points) and SET (LRS points) processes at 0.1 V in TiN/HfO₂-Fe₂O₃/Ti devices containing (a) 500 and (b) 200 ALD cycles of HfO₂, (c) three-layer stack with Fe₂O₃ layer grown in between HfO₂ films, and (d) periodically laminated media grown using the same cycle numbers for HfO₂ and Fe₂O₃. The deposition cycle sequences are indicated by labels.