An ab initio study on resistance switching in hexagonal boron nitride

Abstract

Two-dimensional materials have been widely investigated to implement memristive devices for data storage or neuromorphic computing applications because of their ultra-scaled thicknesses and clean interfaces. For example, resistance switching in hexagonal boron nitride (h-BN) has been demonstrated. This mechanism is most of the time attributed to the movement of metallic ions. It has however also been reported when h-BN is contacted with two inert electrodes such as graphene or Pt. We suggest here that the switching mechanism of the latter devices, which has not yet been clearly established, relies on locals change of the electronic structure of h-BN as caused by atomic defects, e.g., multi-vacancies. This class of intrinsic h-BN defects can create electrically controllable interlayer bridges. We use a combination of hybrid density functional theory and the Non-equilibrium Green’s function formalism to show that a single interlayer bridge resulting from the presence of a trivacancy in a graphene/h-BN/graphene stack leads to a switching voltage of ~5 V and a high-to-low resistance ratio >100. Both values lie within the reported experimental range and thus confirm the likelihood that intrinsic defects play a key role in the resistance switching of h-BN in contact with inert electrodes.

Molecular Bridges Link Monolayers of Hexagonal Boron Nitride during Dielectric Breakdown

“…Our DFT study also showed many other types of defects that are stable and hence could be generated by electrical stress and contribute to the leakage current; a finding supported by our experimental observation that only ∼30% of the above mentioned CAFM I-V curves show electrical shorting of the h-BN layer behavior, with presumably the other 70% of the observations representing the effect of alternative defect configurations . A recent extension of the DFT approach demonstrated that resistive switching in h-BN for use in memristor devices may also be based on the reversible formation of such molecular bridge defects, at least for the case studied involving tri-vacancy defects . Importantly for our study, this work also showed that the electronic transmission function of the bridge-type defects is ∼1000× greater than that of pristine h-BN, indicating that a significant increase in charge transfer would occur through these bridge defects.…”

“…Such defects have been termed molecular bridges by Strand et al 31 and our DFT results reveal three bridge structures corresponding to the lowest formation energies among all structures considered in our study. As such, we postulate that the generation of such molecular bridge defects contribute significantly to the observed out-of-plane charge transfer and electrical shorting between layers, an assumption supported by the electronic transmission calculations of Ducry et al 32 The presence of such molecular bridges across the van der Waals gap between adjacent h-BN layers thus provides charge transport 'highways' that facilitate vertical current flow across the h-BN stack. We cannot fully confirm yet whether the three bridge structures are indeed the lowest energetically among all possible defect configurations and charge states, but other defect pairs tend to have much larger formation energy and hence while they may contribute to the charge transfer, these non-bridging defects are presumably smaller in number.…”

“…Significantly, a set of defects formed by molecular bridges between layers exhibits the lowest formation energy of all the defect configurations tested. These molecular bridge defects have been shown to greatly enhance the electron transmission in the out-of-plane direction . All of these observations strongly indicate that the generation of molecular bridges between layers under electrical stress plays an important role in the electron transport across the capacitor.…”

“…We have therefore undertaken DFT calculations to elucidate the possible defect configurations, which could contribute to the h-BN degradation in agreement with our experimental findings. The results from the previous DFT studies of defects in electrically stressed h-BN are also incorporated. , …”

“…The stress-induced increase in leakage current may arise from the generation of many different types of defects. However, DFT calculations , show that three defect pairs in particular created from adjacent boron and nitrogen monovacancies (labeled V B V N0 , V B V N1 , and V N V N0 ) have the lowest formation energy. Moreover, these three defects form molecular bridges between two adjacent layers, which effectively “electrically shorts” the two layers at the defect location, thereby resulting in a substantial increase in the out-of-plane leakage current.…”