Electroresistance effect in MoS₂-Hf_0.5Zr_0.5O₂ heterojunctions

Two-dimensional (2D) materials exhibit diverse electrical, mechanical, optical, and thermal functionalities, which have attracted significant interest from both the fundamental and technological viewpoints.^1–4 A possibility of additional functionalities emerging from a combination of the 2D materials with ferroelectrics (FE) has ushered in a promising research direction, which has already shown a significant promise for device applications.^5–16 A typical example includes graphene/BaTiO₃/La_0.67Sr_0.33MnO₃ (graphene/BTO/LSMO) ferroelectric tunnel junctions (FTJs), where the downward polarization state (P_down) was found to be highly conducting with respect to the upward polarization (P_up) state, resulting in an enhanced tunneling electroresistance (TER) of around 6000.⁹ It has also been demonstrated that when one of the conducting metallic electrodes, such as Co or Pt,^17,18 is replaced by a doped semiconductor, the TER value increases by 2–3 orders of magnitude due to charge accumulation or depletion at the ferroelectric–semiconductor interface.^19,20 Such an enhanced TER of ∼10⁴ has been achieved by employing BTO-based FTJs with MoS₂, a 2D semiconducting transition metal dichalcogenide, as one of the electrodes¹⁰ with the P_down state showing a higher conductivity than the P_up state. An interplay between FE polarization and electronic properties of MoS₂ can result in unusual functionalities. For instance, optically induced changes of the electronic properties in MoS₂ in MoS₂/BTO/SrRuO₃ FTJs via creation of the electron–hole pair result in polarization reversal, enabling an optical electroresistance effect.⁵ Charge accumulation or depletion at the MoS₂/BTO interface can tune the conductivity of MoS₂, leading to an asymmetric switching of polarization in MoS₂/BTO/SrRuO₃ FTJs and a gradual change in TER magnitude.¹⁰ In MoS₂/Pb(Zr,Ti)O₃ (PZT)-based devices, a memristive behavior has been demonstrated based on the ferroelectric field effect and fine tunability of device resistance through writing/erasing of electrically poled nano-domains.¹³ The FE field-effect transistors employing MoS₂ have been shown to exhibit a negative capacitance effect.^21–23 In spite of these advantages, perovskite FE thin films have poor compatibility with complementary metal oxide semiconductor (CMOS) technology.²⁴ In this regard, ferroelectric hafnium oxide (HfO₂) thin films are promising candidates due to their high scalability and compatibility with CMOS.²⁵ Resistive switching phenomena have been demonstrated in HfO₂-based films with different dopants.^26–37 However, most reports on this topic involve HfO₂ films with metal or heavily doped semiconducting electrodes.

In this Letter, we report a sizable TER of ∼10³ at 0.8 V read bias in the MoS₂/Hf_0.5Zr_0.5O₂ (HZO)/W/Si heterojunctions detected by a combination of piezoresponse force microscopy (PFM)³⁸ and conductive atomic force microscopy (c-AFM). A high conductance (ON state) was observed for the polarization in Hf_0.5Zr_0.5O₂ (HZO) oriented toward MoS₂ (upward polarization), while a low conductance (OFF state) was obtained when the polarization was directed away from MoS₂ (downward polarization). The observed effect, which could be described by a Fowler–Nordheim tunneling (FNT) process, is a result of coupling between the HZO polarization and semiconducting properties of MoS₂, which leads to the modulation of the effective barrier width and height via reversible accumulation/depletion of electrons at the MoS₂/HZO interface.

High-quality 4.5-nm-thick HZO thin films were grown by atomic layer deposition on a Si substrate, using tungsten as a bottom electrode. Thin MoS₂ flakes were mechanically exfoliated from a single crystal using an adhesive tape³⁹ and were deposited on a bare HZO surface inside a nitrogen-filled glovebox. PFM imaging, local spectroscopic studies, and c-AFM measurements were performed by means of a commercial AFM system (MFP-3D, Asylum Research) using Pt/Ir-coated Si probes (PPP-EFM, NANOSENSORS). All PFM measurements were done in the resonant enhanced mode using a ∼350 kHz AC signal with ∼0.8 V drive amplitude. Macroscopic electrical characterization was carried out using a combination of a Keysight 33621A arbitrary waveform generator and a Tektronix TDS 3014B oscilloscope. All biases mentioned in this paper are given with respect to the top electrode.

Polarization switchability was initially tested by PFM on the bare HZO surface. Figure 1(a) shows that local PFM hysteresis loops in the as-grown films are shifted to the left, indicating a switchable polarization with a preference for the downward state. Next, to quantify the remanent polarization, the TiN top electrodes with Al capping layers were deposited on the HZO film producing an Al/TiN/HZO/W capacitor structure. After the wake-up procedure,⁴⁰ the macroscopic measurements using the positive-up-negative-down (PUND) technique yielded the remanent polarization (P_r) value of ∼13 μC/cm² [Fig. 1(b)], which is close to the P_r value of ∼17–20 μC/cm² reported in the literature.^41,42

After confirming the ferroelectric properties of the HZO films with metal electrodes, the switching behavior of the HZO film with a ∼4.8-nm-thick MoS₂ flake as a top electrode was tested by local PFM spectroscopic measurements. The butterfly-type PFM amplitude loop indicates the switchability of HZO underneath MoS₂ [Fig. 1(c)]. Similar to the as-grown HZO film with a bare surface [Fig. 1(a)], the PFM loops exhibit a horizontal shift toward the negative bias. Note that the PFM hysteresis loop in Fig. 1(c) is a representative loop out of multiple loops taken at multiple locations in each device. To enable macroscopic testing of the MoS₂/HZO/W heterojunction, a tungsten top electrode along with a buffer organic layer was deposited on MoS₂. The P–V loops obtained on such a device reveal a P_r of around 3 μC/cm² in the pristine state, confirming that the ferroelectricity was still preserved after transfer of MoS₂ onto the HZO film [Fig. 1(d)]. The lower P_r observed in this device might be due to the lower conductivity of the semiconducting MoS₂ electrodes compared to the metallic electrodes, resulting in incomplete switching of the polarization in the voltage range of the measurement.

PFM imaging of MoS₂/HZO/W in the pristine state reveals a uniform downward polarization state of the FE film [Figs. 2(a) and 2(b)]. After the application of a negative (−4.8 V, 1 s) pulse to MoS₂ via a conductive tip, the whole volume of HZO underneath the MoS₂ flake switched to the upward polarization, which is manifested by the 180° inversion in the PFM phase contrast [Fig. 2(d)]. The upward polarization state is stable against multiple consecutive scans. The PFM amplitude signal of that state is larger than that of MoS₂/HZO/W polarized downward [Fig. 2(c)]. Polarization of MoS₂/HZO/W can be switched by application of a positive (+4.8 V, 1 s) pulse but only partially [Figs. 2(e) and 2(f)]. This incomplete switching of polarization of the HZO film underneath MoS₂ is likely due to the change in the electronic properties of MoS₂ mediated by HZO polarization (see discussion below). Reduction in the conductivity of MoS₂ in the depletion mode prevents the applied voltage to drop entirely across the HZO film, thereby leading to an incomplete polarization switching. In addition, a possibility of charge injection/trapping at the MoS₂/HZO interface, which can lead to domain pinning,⁴³ cannot be ruled out.

To explore the resistive switching in the MoS₂/HZO/W heterojunctions, conductivity measurements for fully polarized up and down states were performed by acquiring local current–voltage (I–V) curves in the c-AFM mode. During the I–V measurements, a bipolar triangular waveform with the pulse amplitude of 0.8 V is applied to MoS₂/HZO/W and the current through the junction is collected from the bottom electrode and amplified. For the pristine MoS₂/HZO/W heterojunction in the downward polarization state, the I–V curves reveal negligible current flow, indicating a high resistance (OFF) state [Fig. 3(a), red curve]. On the other hand, in the MoS₂/HZO/W heterojunction polarized upward by application of the −4.8 V pulse, the current shows significant enhancement at a read bias above ±0.25 V [Fig. 3(a), black curve], indicating that the device was brought to a low resistance (ON) state. The semi-log I–V curve in Fig. 3(a) and the corresponding R_OFF/R_ON vs bias plot in Fig. 3(b) show an approximately three orders of magnitude current increase at a read bias of −0.8 V. That the enhanced TER was due to the semiconducting MoS₂ layer is confirmed by the lack of any resistive switching behavior in the Al/TiN/HZO/W device [Fig. 3(a), inset]. The TER ratio of ∼10³ observed in MoS₂/HZO/W is at least an order of magnitude larger than the TER values reported for HZO-based FTJs with metallic (Pt, TiN, and Au) or heavily doped Si semiconducting electrodes.^28,29,32 Numerous I–V plots were taken at multiple locations on the MoS₂ flakes for the upward and downward HZO polarization states and were found to bear reproducible results. Similar reproducibility was observed for multiple MoS₂/HZO/W heterojunctions. Moreover, I–V testing was done in the lower read bias range (±0.8 V) to avoid polarization switching in the HZO film underneath MoS₂ and minimize the effect of possible oxygen vacancy migration.³⁷ Incomplete switching of polarization may reduce the ultimate TER ratio of ∼10³ but, at the same time, opens a possibility of tuning the electroresistance effect by modulating the fraction of the switched polarization.¹⁰ 

Interestingly, the resistive switching of the MoS₂/HZO/W heterojunctions displays an opposite behavior in terms of the TER sign as compared to that observed in the MoS₂/BTO/SrRuO₃ FTJs,¹⁰ which exhibit the ON state for the downward polarization of BTO and the OFF state for the upward polarization. The opposite TER sign can be explained by assuming the n-type conductivity of MoS₂ on top of HZO, which is consistent with earlier reports.^44–46 The n-type conductivity in MoS₂ was suggested to be induced by interfacial oxygen vacancies at the MoS₂/HfO₂ interface, resulting in the creation of donor levels close to the conduction band edge of MoS₂.⁴⁴ Such oxygen vacancies have also been reported earlier in the HZO thin films^47,48 and attributed to the redox reactions at the top electrode/HZO interface during annealing.⁴⁷ Formation of oxygen vacancies across the film thickness cannot be ruled out either. We note that the exfoliation and deposition of the MoS₂ flakes in different gas environments (ambient conditions used for MoS₂/BTO fabrication in Ref. 10 and dry nitrogen conditions used in this study for MoS₂/HZO) could have also played a role in the opposite TER sign by affecting the composition of the adsorbate molecular layer at the MoS₂/HZO interface, which is known to have a strong effect on the FTJ properties.⁹ 

The underlying mechanism of the observed resistive switching effect can be explained by considering the accumulation or depletion of electrons (majority carriers in n-type MoS₂) at the MoS₂/HZO interface. When MoS₂/HZO is polarized downward, electrons at the MoS₂/HZO interface will be depleted and the top negative bound polarization charge of HZO at the MoS₂/HZO interface will be screened by positive impurity ions or defects at the interface [Fig. 3(c)]. An increase in the width of the depletion layer at the MoS₂/HZO interface will result in an increase in the effective barrier width, thus providing a larger resistance to the current flow through the heterojunction. In contrast, when the polarization points toward the MoS₂ (upward), the positive bound polarization charge will induce accumulation of electrons at the MoS₂/HZO interface accompanied by the decrease in the effective barrier width [Fig. 3(d)]. Synchrotron-based hard X-ray photoemission spectroscopy (HAXPES) studies have revealed a lower Schottky barrier height when polarization is upward as compared to the downward polarization state.^47,49 The net effect will be an increase in the current flow through the heterojunction, giving rise to an enhanced electroresistance ratio. Resistive switching could also be affected by band bending at the MoS₂/HZO interface. It has been shown earlier²⁰ that in SrRuO₃/BTO/n-SrTiO₃ FTJs, reversible metallization of the BTO barrier caused by band bending would result in a decrease in the effective barrier width by two-unit cells, producing a sizable TER. It is likely that such an effect might also occur at the MoS₂/HZO interface, and the polarization orientation corresponding to the ON state is consistent with the theoretical predictions. A lack of a parabolic shape in the conductance (dI/dV) vs voltage plots rules out direct tunneling through the device.⁵⁰ A current increase at higher biases is consistent with the Fowler–Nordheim tunneling (FNT) mechanism.^51,52 Reasonable fitting of the field dependence of current density by the FNT equation⁵¹ j_FN ∼ E² × exp(−B/E), (⁠$B=8π2mHZOϕ3/23he$⁠, where $ϕ$ is the barrier height, $mHZO$ is the effective electron mass in HZO, E is the electric field, $e$ is the electronic charge, and $h$ is Planck's constant) suggests that such a tunneling mechanism is highly plausible [Fig. 3(b), inset]. The value of the constant B was obtained from the fitting, which enabled us to determine the quantity $mHZOϕ3/2$⁠. Using $mHZO=0.1me,$⁵³ where $me$ is the free electron mass, the interfacial barrier height was found to be ∼0.64 eV. A similar barrier height of ∼0.7 eV at the top TiN/HZO interface with the polarization upward was measured in the TiN/HZO/TiN capacitor structures using in-operando HAXPES.⁴⁹ The FNT mechanism was also found to be responsible for conduction at high fields in Ti/Al₂O₃/HZO/p-Si heterojunctions³² although the authors of that study did not specify the barrier height in their devices. Additionally, oxygen vacancy movement in the HfO₂-based ferroelectric thin films under applied electric fields has been demonstrated earlier^26,37 and such movement could, in principle, contribute to the observed conductivity. However, given the absence of measurable current (within the resolution of our experimental setup) for the downward polarization state, a contribution of oxygen vacancy migration to the observed conductivity can be ruled out.

In conclusion, a sizeable TER ratio of ∼10³ was measured in the MoS₂/HZO/W heterojunctions. The observed electroresistance effect can be ascribed to the Fowler–Nordheim tunneling mechanism and polarization-induced modulation of the potential barrier due to accumulation/depletion of majority carriers at the MoS₂/HZO interface. This work opens up further avenues for exploiting functionalities emerging from combining technologically relevant HfO₂-based ferroelectrics with 2D semiconductors.

This work was supported by the National Science Foundation through the Nebraska Materials Research Science and Engineering Center (MRSEC, Grant No. DMR-1420645), Grant No. ECCS-1917635, and the Russian Science Foundation (Project No. 20-19-00370).