Performance improvement of a tunnel junction memristor with amorphous insulator film

This study theoretically demonstrated the oxygen vacancy (VO2+)-based modulation of a tunneling junction memristor (TJM) with a high and tunable tunneling electroresistance (TER) ratio. The tunneling barrier height and width are modulated by the VO2+-related dipoles, and the ON and OFF-state of the device are achieved by the accumulation of VO2+ and negative charges near the semiconductor electrode, respectively. Furthemore, the TER ratio of TJMs can be tuned by varying the density of the ion dipoles (Ndipole), thicknesses of ferroelectric-like film (TFE) and SiO2 (Tox), doping concentration (Nd) of the semiconductor electrode, and the workfunction of the top electrode (TE). An optimized TER ratio can be achieved with high oxygen vacancy density, relatively thick TFE, thin Tox, small Nd, and moderate TE workfunction.


Intoduction
Novel two-terminal memory types mainly include Phase Change Random Access Memory (PCRAM), Magnetic Random Access Memory (MRAM), and Resistive Random Access Memory (RRAM). Specitically, the resistance switching of PCRAM is realized by altering the phase change materials between crystalline and amorphous states. The switching ratio is up to 10 5 , but the crystallization temperature of the material is high, and large power consumption is required due to the current-drive switching [1]. The resistance of MRAM is realized by modulating the spin direction of the free layer, showing excellent endurance characteristics. Its switching ratio is, however, too small (usually only several times) [2]. The RRAM is realized by forming and fracturing a conductive filament in peroxides, its processes are unstable and result in large variations between the RRAM devices [3]. In addition, studies identified another two-terminal memory-the charge trapping memory, whose switching between high and low capacitance states is based on charge trapping [4,5]. Besides, the two-terminal tunneling junction memristor (TJM) is another kind of promising device candidate in nextgeneration memories, owing to its attractive advantages, such as high density data storage, fast access speed, and low power consumption [6,7]. Among various nonvolatile TJMs, the ferroelectric tunneling junction (FTJ) exploiting the polarization-dependent tunneling electroresistance (TER) effect has specifically attracted much attention [8,9]. An FTJ consists of a thin ferroelectric barrier sandwiched between two electrodes with different work functions and screening lengths,its barrier profile can be modulated by polarization switching. Enormous research efforts have been devoted to improving the TER ratio in TJMs based on the HfO 2 -based polycrystalline ferroelectrics [10] and the traditional perovskite materials [11][12][13][14]. However, the the TER ratio of the current HfO 2 -based TJMs is not more than 100 [15]. One practical feasible way is to decrease the OFF-state tunneling current by employing an amorphous tunneling barrier [16]. Utilizing modulated tunneling barrier width technique, the BTO TJM has exhibited a TER ratio above 10 6 [17], but which suffers from the incompatibilities during the Si CMOS process.
Some amorphous films, including Al 2 O 3 , ZrO 2 , HfO 2 , and La 2 O 3 were also reported to have ferroelectric-like properties providing the non-volatile electrical performance in metal/insulator/metal (semiconductor) capacitors and field-effect transistors [18][19][20][21][22]. The underlying mechanism is supposed to be the movement of oxygen vacancy (V O 2+ ) dipoles or mobile ions under applied voltages, which is different from the ferroelectric properties of HfO x -based films induced by the orthorhombic phase. The previous experiments have shown that amorphous film based mobile-ionic TJM achieved a decent TER ratio enabled by the modulated tunneling barrier width by mobile ions [16].
Therefore, based on the above background, this work theoretically characterized the TJMs modulated by V O 2+related dipoles under the external electric fieldand via TCAD simulation. Physical mechanism underlying performance improvement in the devices and its dependence on dipole density (N dipole ), thicknesses of ferroelectric-like film (T FE ) and SiO 2 (T ox ), doping concentration (N d ) of semiconductor electrode, and workfunction of top electrode (TE) are investigated. Figure 1a shows the three-dimensional schematic of the proposed TJM consisting of a top metal electrode, ferroelectriclike insulator film, and bottom Si electrode. A SiO 2 interfacial layer forms between oxide insulator film and Si. The thicknesses of the insulator film and SiO 2 interfacial layer are 3 and 0.5 nm, respectively. Figure 1b and c show the underlying mechanism for the TER effect in TJM. The V O 2+ and negative charges forming the electric dipoles, can be modulated with the applied voltage providing a tunable tunneling barrier width in the device [16]. The 2D numerical simulation is carried out by sentaurus TCAD tool, realizing a dynamic nonlocal tunneling algorithm which is used to describe the tunneling at the interfaces and junctions. The complete coupling method of the quantum modified current continuity equation and Poisson equation is adopted for calculating the energy band alignment. The quantum model (density gradient quantum correction model) which considers the quantization of carriers near the tunneling junction is mainly used to accurately calculate the carrier density and distribution. The method for introducing quantization effects to the classical model is to introduce a potential-like quantity Λ n , where Λ n is calculated through the density gradient quantum correction model by Eq. (3) in Fig. 2. Once obtained the band alignment, the tunneling current is calculated using the nonlocal tunneling model based on the Wentzel-Kramers-Brillouin (WKB) approximation, and the nonlocal path trap-assisted tunneling (TAT) is also considered. Figure 2 shows the simulation framework, detailed formulations, parameters, and boundary conditions. Some other models, such as the high field velocity saturation model, doping-dependent mobility model, and Shockley Read Hall generation recombination model. The doping-dependent mobility model used in the simulation was proposed in [25]. The high-field saturation model was from a Canali model [26]. In addition, the distributions of V O 2+ and negative charges at ON-and OFF-states are assumed to be a Gaussian type, as shown in Fig. 3a and b, respectively. Since studies previously reported that while that V O 2+ was formed during the deposition of oxide films, it was also generated due to the scavenging effect with the formation of the TaON interfacial layer [27,28]. The negative charges can be related to the metal ion vacancy and, eg. Zr vacancy for ZrO 2 film, and Al vacancy for Al 2 O 3 film [29,30], and the negative charges are mobile [31][32][33]. The underlying mechanism for the P-V loops is the switching of dipoles formed by the V O 2+ and negative charges, which is a long range P switching during the electric field cycling [34]. Figure 4a shows the measured P-V curves for the TaN/ZrO 2 /SiO 2 /Si capacitor, the P r values are 0.57, 0.82 and 1.29 μC/ cm 2 for the voltage of 1.5, 2 and 2.5 V, respectively. Figure 4b shows the measured and simulated J-V read curves of TJM, in which the measured curves at ON-state and OFF-state are read after the pulses of + 1.5 V/1 μs and − 1.5 V/1 μs, respectively. The simulated results were based on a N diople of 3.6 × 10 12 cm −2 , which is determined by the polarization 0.57 μC/ cm 2 under 1.5 V. Obviously, the experimental results were consistent with the simulated ones.

The resistive switch enabled by modulating dipoles in the insulator
To get a deep insight into how the V O 2+ -related dipoles modulate the tunneling barrier height and width of the amorphous TJMs, we plot energy band diagrams and electron concentration distributions at a zero-external voltage of TJM at ON-state and OFF-state. As shown in Fig. 1b and c, the ON-state and OFF-state of the device are enabled by the accumulation of V O 2+ and negative charges, respectively, near the amorphous oxide/SiO 2 interface. Figure 5a and b show the energy band diagrams at the ON-state and OFF-states, respectively, and the insets show the zoomed-in view of the conduction band at the insulator/SiO 2 interface. Notably, the electrostatic potential profile for tunneling of amorphous layer induced by the accumulation of V O 2+ near the amorphous oxide/SiO 2 interface is lower than that induced by the negative charges. Figure 5c and d show the electron concentration distributions at the ON-and OFF-states, respectively. We observed that the V O 2+ -related dipoles affected the distribution of carriers in the semiconductor electrode, directly determining the lateral tunneling path of TJM. Furthermore, the accumulation of positive/negative charges near the amorphous oxide/ SiO 2 interfaces tugs/pushes the electrons toward/away from the TE decreasing/increasing the tunneling path. Moreover, the concentration of the V O 2+ -related dipoles increased, the effect of lateral tunneling path modulation became better. Figure 6a plots the simulated current density (J) versus the read voltage (V read ) for the TJMs with the different N diople , demonstrating the J is effectively determined by the direction and concentration of the dipoles. Figure 6b shows the

Research
Discover Nano (2023) 18:20 | https://doi.org/10.1186/s11671-023-03800-0 extracted ON-state current density (J ON ), OFF-state current density(J OFF ), and the TER ratio versus N dipole at a V read of 0.1 V, the N dipole varies from 0 to 7.5 × 10 12 cm −2 , with a step of 0.25 × 10 12 cm −2 . We also observed the J ON increases and J OFF decreases as N dipole increased,ncluding an exponential increase in the TER ratio of the TJMs with N dipole . The TER ratio of the device with a N dipole of 7.5 × 10 12 cm −2 is 10 6 times higher compared to the TJM with a N dipole of 0.5 × 10 12 cm −2 . It is also seen that J OFF descends at a faster rate compared to the variation of J ON , showing that the dipole has a greater influence on the OFF-state, corresponding with the results in the above-mentioned energy band diagram results. Studies have previously reported that the density of V O 2+ -related dipoles can be optimized by changing the deposition condition of the insulator film [35][36][37]. Experiments have demonstrated that the post-deposition annealing of the device structure can induce a scavenging effect, giving rise to the formation of oxygen vacancies, i.e. positive ions, in the insulator [38]. It has also been proposed that although TJM is more sensitive to temperature at more oxygen vacancies, allowing for the temperature-dependent mobility of oxygen vacancies and phonon-assisted detrapping process [39,40], lower temperatures can inhibit the accumulation of oxygen vacancies at the FE/SiO 2 interfaces.

Performance dependence on T FE and T ox
Since the resistive mechanism could be realized by switching the distribution of V O 2+ -related dipoles, we also investigated the effects of the T FE and T ox on the performance of the TJM devices to confirm this hypothesis. Figure 7a shows the J ON and J OFF for TJM with different T FE values of 2-6 nm. The open symbols represent J ON and the solid ones represent J OFF . Here, N d and N dipole values of the devices are fixed at 5 × 10 17 cm −3 and 2.5 × 10 12 cm −2 , respectively, and the TE workfunction is 4.3 eV. Figure 7b extracts the J ON and J OFF and TER ratio versus T FE .Obviously, while J was degraded with an increase in T FE , the TER ratio increased as T FE increased, proposed to be caused by the faster decrease of J OFF due to the more enhanced modulation effect of the tunneling barrier width at the OFF-state. To get a deep insight, the energy band diagrams and electron concentration distributions at a zero-external voltage of TJM at ON-state and OFF-state are plotted in Fig. 8a and b. The barrier height decreases and increases at ON-state and OFF-state with the increase of T FE , respectively, which is the reason for the increased TER ratio. In Fig. 8b, the number of electrons accumulated and exhausted at ON-and OFFstates increases as the increase of T FE , making the tunneling barrier width at the two states wider. Moreover, compared with the ON-state, the barrier width at OFF-state changed more with an increase in T FE , explaining the increased TER ratio. Therefore, we establish that a thicker T FE improved the TER ratio by promoting the modulation effect of tunneling barrier, which is due to the higher voltage distribution on the ferroelectric layer. Figure 9 shows the effect of T ox on the performance of the TJM. Figure 9a shows the J ON and J OFF of the TJM at different T ox of 0, 0.25, 0.5, 0.75, and 1 nm. Similarly, N d and N dipole values of the devices are fixed at 5 × 10 17 cm −3 and 2.5 × 10 12 cm −2 , respectively, and the TE workfunction is 4.3 eV. Obviously, at a fixed V read of 0.1 V, both J ON and J OFF decreased with an increasing T ox , which is also shown in Fig. 9b. This was caused by the slower decrease of J OFF due to the more reduced modulation effect of the tunneling barrier width at the OFF-state. The energy band diagrams and electron concentration distributions at a zero-external voltage of TJM at ON-state and OFF-state are plotted in Fig. 10a and b to get a deep insight.

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The barrier heights are almost constant and the tunneling widths in the SiO 2 increase with an increase in T ox at both the ON-and OFF-states, which is the reason for the decreased J. Furthermore, the tunneling widths in Si are constant at the ON-state, but narrow with an increase in T ox at the OFF-state, indicating that the modulation effect on semiconductor depletion is weakened as the T ox increases, blocking the J OFF decrease, and thus decreasing the TER ratio.
Performance dependence on N d Figure 11 shows the effect of T FE on the performance of the TJM. Figure 11a shows  Fig. 11b. Similarly, we extract the TER ratio of the devices at a V read of 0.1 V, the TER ratio decreases with an increase in N d . As the value of N d increases to ~ 10 20 cm −3 , the TER ratio is less than 10, proposed to be due to the reduced modulation effect of tunneling barrier height and width with the thin depletion layer and the short screen length of the heavily doped Si. Figure 12a and b show the energy band diagrams of the TJMs for different N d for heavily and lightly doped substrates, respectively. Figure 12c and d show the n e profiles of the TJMs with various N d for heavily and lightly doped substrates, respectively. The modulation of the tunneling barrier width is pronouncedly reduced with increased

Impact of TE workfunction on device performance
Finally, we also investigated the dependence of the amorphous TJM device performance on the TE workfunction.
Here, N dipole is fixed at 2.5 × 10 12 cm −2 and N d is 5 × 10 17 cm −3 . Figure 13a shows the J ON and J OFF versus read voltage curves for the TJMs with different TE workfunctions corresponding to Ti (4.3 eV), TaN (4.8 eV), TiN (5.1 eV), and Ni (5.3 eV). At a fixed V read , both J ON and J OFF decrease as the TE workfunction increases, which is mainly due to the raised tunneling barrier height and widened tunneling barrier, as shown in Fig. 9. Simiarly, we extract the TER ratios of the devices at V read = 0.1 V, the TER ratio first increases and then decreases with an increase in the TE workfunction. If the TE workfunction is too small/large, large quantities of negative/positive charges will accumulate at the interface of SiO 2 /Si, leading to a deep accumulation/depletion state of the semiconductor. However, it is difficult for V O 2+ -related dipoles with limited densities to modulate the deep accumulation/ depletion of the semiconductor, as shown in the electrostatic potential profiles for Ti and Ni in Fig. 13a. Therefore, we propose the requirenment of a moderate TE workfunction to achieve a high TER ratio. Investigations prove TiN to be the best choice for this model and this method is the easiest among the three optimizations in the experiment.
Furthermore, as shown in Fig. 14a and b, the influence of the dipoles on tunneling barrier height increase firstly and then decreases as the workfunction increases, and that on tunneling barrier width weakens with the increase of workfunction. Investigations also revealed that the varying trend of the TER ratio with work function is consistent with that of barrier height, indicating that the effect of TE workfunction on TJM is mainly affected by the tunneling barrier height and modulated by V O 2+ -related dipoles.

Conclusions
The TJMs modulated by the V O 2+ -related dipoles under the external electric field are theoretically characterized by the numerical TCAD simulation. The physical mechanism underlying the performance improvement in the devices and its dependence on N dipole , N d of the semiconductor electrode, and TE workfunction are investigated by comparing the J versus V read curves, energy band diagrams, and n e profile. It is demonstrated that an optimized TER ratio can be achieved with small N d , thick T FE , thin T ox , high oxygen vacancies density, and moderate TE workfunction.