In-Gap States of HfO₂ Nanoislands Driven by Crystal Nucleation: Implications for Resistive Random-Access Memory Devices

Abstract

Envisioned extremely scaled, high-performance memory devices request to conduct the step from thin semiconductor films to nanoscale structures and the use of promising high-k materials such as hafnium oxide (HfO 2 ). HfO 2 is well suited for use in resistive random-access memory (ReRAM) devices based on the valence change mechanism. Here, we provide a decidedly scaled system, namely, HfO 2 nanoislands that are grown by van der Waals epitaxy on highly oriented pyrolytic graphite (HOPG). The electronic and structural properties of these wellseparated, crystalline HfO 2 nanoislands are investigated by scanning probe methods as well as ab initio methods. The topography reveals homogeneously formed HfO 2 nanoislands with areas down to 7 nm 2 and a thickness of one unit cell. They exhibit several acceptor-and donor-like in-gap states in addition to the bulk band gap, implying bulk properties. X-ray photoelectron spectroscopy indicates hafnium carbide formation as one possible origin for defect states. Going further to the crystal nucleation of HfO 2 , nanocrystals with a diameter of 2.7−4.5 Å are identified next to carbon vacancies in the topmost HOPG layer, indicating that carbon is incorporated into the islands at early nucleation stages. A precise description of these nuclei is accomplished by the simulation of small Hf m O n (:C) clusters (m = 3 to 10; n = 3 to 22) with and without carbon incorporation using ab initio methods. The comparison of the theoretically determined lowest-energy clusters and electronic states with the experimental results allows us to identify the structure of the most relevant HfO 2 sub-nanometer crystals formed during the first nucleation steps and the nature of the in-gap states found at the surfaces of HfO 2 nanoislands. That way, a model system is derived that consists of distinct structural units, related to surface states or defect states, respectively, that will promote the tailoring of in-gap states of smallest HfO 2 structures and thus the scalability of memory devices.

The MoS2-Graphene-Sapphire Heterostructure: Influence of Substrate Properties on the MoS2 Band Structure

“…The valence band maximum (VBM) and the conduction band minimum (CBM) were determined from curves of the logarithm of the differential conductance (log[(dI/dV)/nS]) versus the bias voltage by fitting lines to the flanks of the curves on both sides. 34,35,56 The resulting quasiparticle bandgap (CBM − VBM) is larger for ML than for BL MoS 2 in accordance with the literature. 17 To rule out artifacts, like tip induced band bending, STS measurements were performed with decreasing tip−surface distance on different MoS 2 nanosheets (Figure 4b).…”

“…To further elucidate this point, the MoS 2 /G bond /S heterostructure displaying surface areas with increased and decreased charge densities (Figure a) has been examined by STS with a short tip–surface distance to record possible local in-gap electronic features. , Inspecting the STS curves obtained at random positions on the surface, we can clearly identify the Dirac point of SLG (marked with a red bar in Figure c) and a state which has been attributed to the G bond /S interface interactions previously (marked with an orange bar, for details see Supporting Information, Figure S6) . Most interestingly, an additional in-gap state is recorded at −0.91 ± 0.05 V for MoS 2 /G bond /S only on bright surface areas (Figure c, marked by a green bar).…”

“…In this connection, we focus especially on the influence of local variations, that is, the specific topography and the different electronic properties of the two substrate regions, G free /S and G bond /S. The results are gained with well-established STM and STS methods revealing electronic properties at well-defined positions on the 2D material surface. ,− These insights are additionally supported by scanning electron microscopy (SEM), Raman spectroscopy, and atomic force microscopy (AFM).…”

Heterogeneous Integration of Graphene and HfO2 Memristors

“…On the other hand, the vacancies could result from the incorporation of C into the HfO 2 lattice, which stabilizes the nucleation of the thin film, as shown recently for the growth of HfO 2 nanoislands on highly ordered pyrolitic graphite (HOPG). [36] Our results indicate that using the selected growth conditions, the defect concentration in graphene can be limited. This enabled us to use these graphene layers as bottom electrode in metalinsulator-metal (MIM) device structures.…”

“…On the other hand, the vacancies could result from the incorporation of C into the HfO 2 lattice, which stabilizes the nucleation of the thin film, as shown recently for the growth of HfO 2 nanoislands on highly ordered pyrolitic graphite (HOPG). [ 36 ]…”