Nanostructures Stacked on Hafnium Oxide Films Interfacing Graphene and Silicon Oxide Layers as Resistive Switching Media

Abstract

SiO₂ films were grown to thicknesses below 15 nm by ozone-assisted atomic layer deposition. The graphene was a chemical vapor deposited on copper foil and transferred wet-chemically to the SiO₂ films. On the top of the graphene layer, either continuous HfO₂ or SiO₂ films were grown by plasma-assisted atomic layer deposition or by electron beam evaporation, respectively. Micro-Raman spectroscopy confirmed the integrity of the graphene after the deposition processes of both the HfO₂ and SiO₂. Stacked nanostructures with graphene layers intermediating the SiO₂ and either the SiO₂ or HfO₂ insulator layers were devised as the resistive switching media between the top Ti and bottom TiN electrodes. The behavior of the devices was studied comparatively with and without graphene interlayers. The switching processes were attained in the devices supplied with graphene interlayers, whereas in the media consisting of the SiO₂-HfO₂ double layers only, the switching effect was not observed. In addition, the endurance characteristics were improved after the insertion of graphene between the wide band gap dielectric layers. Pre-annealing the Si/TiN/SiO₂ substrates before transferring the graphene further improved the performance.

Keywords: atomic layer deposition; graphene; hafnium oxide; resistive switching; silicon oxide; stacked nanostructures.

Figure 1

Graphic representation of the stacked nanostructures combined into one figure with and without graphene interface layers devised for resistive switching (left panel) and bird’s-eye-view through an optical microscope of the Ti electrode matrix fabricated on the topmost oxide layer by maskless photolithography (right panel).

Figure 2

Bird’s-eye-view of SEM images of the surfaces of the graphene layer, as deposited on Cu foil (a), graphene transferred to ALD-grown SiO₂ film (b), ALD-grown HfO₂ film on graphene (c), electron beam evaporated SiO₂ film on graphene (d), and cross-sectional image (the dark field, obtained in STEM mode of SEM) of a thin lamella prepared from the SiO₂/Graphene/HfO₂ (Sample 3 in Table 1) stack structure (e). Note that graphene is not visible in this image at this resolution. The oxide layers in the image have been false-colored to make the distinction between them “easier”.

Figure 3

Typical Raman spectra of graphene-based stacked nanostructures. All spectra are normalized according to the G–band intensities. The positions and widths of the main bands (G and 2D) are indicated as labels in the spectra. The order of layers in stacked structures is also denoted by labels.

Figure 4

Measured and modeled X-ray reflectivity curves from SiO₂ film, as grown by ALD (a), ALD-grown SiO₂ annealed at 1000 °C (b), ALD-grown HfO₂ (c), and EBE-grown SiO₂ (d) on Si/TiN substrates. For the description of samples, see Table 1.

Figure 5

The survey (a) and elemental (b,c) XPS scans from the surfaces of non-annealed (as-deposited) and annealed SiO₂ thin films. Positions of bands characteristic of oxygen (b) and silicon-containing compounds or elemental Si (c) are denoted by labels.

Figure 6

Current–voltage characteristics measured in resistive switching regime on TiN/SiO₂/HfO₂/Ti (Sample 2) (a) and TiN/SiO₂/graphene/HfO₂/Ti (Sample 1) (b) cell structures (Table 1) containing as-deposited SiO₂ and HfO₂ dielectric films. The diameter of the top titanium electrode was 250 μm. LRS and HRS denote the low- and high-resistance states, respectively.

Figure 7

Current–voltage (a) and endurance (b) characteristics measured in a resistive switching regime on the TiN/SiO₂/HfO₂/Ti cell structure built on the TiN/SiO₂ base stack annealed at 1000 °C (Sample 4 in Table 1). The low- and high-resistivity states, denoted by LRS and HRS, respectively, were recorded at reading voltages of 0.2 V.

Figure 8

Resistively switching current–voltage curves measured from the TiN/SiO₂/graphene/(EBE)SiO₂/Ti devices (Sample 5 in Table 1).

Figure 9

Current–voltage (a), endurance (b), and retention (c) characteristics measured in resistive switching regime on the TiN/SiO₂/graphene/HfO₂/Ti (Sample 3 in Table 1) cell structure built on the TiN/SiO₂ base stack annealed at 1000 °C. The low- and high-resistivity states, denoted by LRS and HRS, respectively, were recorded at voltages of 0.2 V.