Title Investigation of memory effect in Au / ( Ti-Cu ) Ox-gradient thin film / TiAlV structure

The paper presents the results of the analysis of resistive switching properties observed in (Ti-Cu)-oxide thin film with gradient distribution of elements over the thin film thickness. Thin films were prepared using the multisource reactive magnetron co-sputtering process. Programmed profile of the pulse width modulation coefficient during sputtering of the Cu target allowed to obtain the designed gradient U-shape profile of Cu concentration in the deposited thin film. Electrical measurements of Au/(Ti-Cu)Ox/TiAlV structure showed the presence of nonpinched hysteresis loops in the voltage–current plane testifying the resistive switching behavior. Additionally, the initial forming process of conducting filaments has been observed as well. Optical, x-ray, and ultraviolet photoelectron spectroscopy measurements allowed to create the scheme of the bandgap alignment of the prepared thin films with respect to the Au and TiAlV electrical contacts. Detailed structure and elemental profile investigations allowed to conclude about the presence of conducting filaments of the observed resistive switching mechanism occurring in the prepared test structure. The obtained results showed that the prepared gradient (Ti-Cu)Ox thin film could be an interesting alternative to the conventional multilayer stack construction of resistive switching devices.

However, the most commonly used material as the active layer in resistive switching devices is TiO2-x (e.g., [36][37][38][39][40][41] ). In addition, a crucial role in the resistive switching mechanism plays the material used for metal electrodes. Usually such materials are used as Au, Ag, Ni, Ti, W, TiN, or ITO [2,22,42] . Some examples of resistive switching behavior were also found in structures based on nanowires [43] or nanotubes [26,43] , where the resistive switching device is characterized by the presence of a pinched or nonpinched hysteresis loop in the I-V characteristics in the DC plane.
In the present paper, the results of the analysis carried out for (Ti-Cu)-oxide semiconducting thin film placed between Au and Ti6Al4V metal electrodes have been presented. Contrary to the results presented in the literature, experimental electrical measurements performed for DC simulation showed bipolar resistive switching properties which were observed not for a conventional multilayer stack construction but for a single thin 3 film with the gradient distribution of the elements over the thickness of the structure. The (Ti-Cu)-oxide semiconducting thin film with U-shape like distribution profile of copper allowed to observe nonpinched hysteresis loops with the addition of the forming curve. The fabricated structures showed a hysteresis (memory) loop under both positive and negative bias conditions, and the conducting filament type of switching was indicated based on the performed investigations. The discussions on the possible explanation of the observed electrical behavior in the prepared structures were supported with investigations of surface properties performed using X-ray and UV photoelectron spectroscopy, and cross-sectional elemental analysis.

Experimental
The deposition system and the method for the preparation of gradient thin films with different profiles of element concentration have already been described in detail in [44][45][46][47][48] . The thin films were deposited in a reactive mode in the magnetron co-sputtering process, using two circular titanium targets (99.995%) and one circular copper target (99.995%). Targets were sputtered simultaneously in an oxygen atmosphere of 99.999% purity. The targets used in the process were 28.5 mm in diameter and their thickness was 3 mm [50] . The process uses magnetrons with an unbalanced magnetic field set in a confocal configuration vs. the substrate.
The target-substrate distance was 14 cm. Before the deposition process, the working chamber was pumped down to a pressure of 10 -3 Pa. Thin films were sputtered without additional intentional heating of the substrates during the process. Each magnetron was powered with a separate MSS2 power supply from Dora Power System [49] . The applied power supply allowed to obtain a maximum power of up to 2 kW in the unipolar pulsed DC mode. The power delivered to the magnetrons was controlled independently for each of the magnetrons [47] . DC pulses of the magnetron power supply consisted of groups of unipolar sinusoidal pulses with a frequency of 140 kHz, while the power supplied to the magnetrons was regulated by changing the width of the groups of these pulses (PWM method -pulse width modulation). To obtain a gradient distribution of elements as a function of the thickness of the deposited layers, magnetrons equipped with titanium targets were supplied with a constant coefficient pwmTi = 100% during the entire deposition process. On the contrary, the magnetron with the cooper target was powered by pulses with the pwmCu coefficient which value was changed from 60% to 10% and to 0% for the half of the time of deposition and then for the remaining half of the time of deposition, it was changed from 0% to 10% and to 60% (

Electrical properties
Resistivity of as-deposited thin film was determined to be equal 1⋅10 3 Ωcm. The type of electrical conductivity was determined by the Seebeck coefficient measurements performed in the range from 25 °C to 125 °C. The result of the thermoelectric voltage as a function of the difference of temperature measured between two opposite electrical contacts is shown in Figure   1. The Seebeck coefficient (+82.14 V) testified about p-type conductivity of the prepared gradient thin film. The p-type of electrical conduction is often reported for Cu2O (or CuO)based thin films, whereas TiO2 is naturally n-type semiconducting oxide. [50,51] In the presented case of the prepared mixed (Ti-Cu)Ox thin film, the obtained result testifies that holes were the majority charge carriers.

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The DC measurements of the current-to-voltage characteristic were conducted in a transverse Au/(Ti-Cu)Ox/Ti6Al4V configuration system (Figure 2). The structure was powered by forcing the constant current which was swept from (0 nA) to (60 nA), then (60 nA) -(0 nA) -(-60 nA) and again from (-60 nA) -(0 nA) -(60 nA). Depending on the direction of the applied current, the structure was either in high (HRS) or in low (LRS) resistance state at the same value of the forcing current. When the direction of the current was changed to the opposite polarity, the structure still "remembered" its high resistance state until, again, the direction of the current was changed and the structure returned to its low resistance state.
Measurements of I-V characteristics were performed in several tens of cycles.

. Surface properties
The oxidation state of the copper at the surface of (Ti0.48Cu0.52)Ox thin film was analyzed with the use of XPS Cu2p core level as shown in Figure 6. The Cu2p core level has split spinorbit components with BE of 19.8 eV and the intensity ratio of Cu2p1/2 and Cu2p3/2 of ca. 0.5.
The well-known fact is that it is possible to distinguish Cu oxidation states taking into consideration not only the position of Cu2p3/2 peak but also the satellite features that could be visible above the binding energy of this peak. According to Biesinger [52,53] , the shake-up satellite peaks are present for samples containing Cu 2+ species but are absent for samples containing only Cu 0 and Cu + species. Therefore, in the case of the measured thin film, the binding energy of Cu2p3/2 and the occurrence of the well-visible satellite peak at ca 940-945 eV indicates the presence of Cu 2+ species related to the CuO oxide. [52][53][54] The XPS spectrum of the Ti2p core level is presented in Figure 6b. The position of Ti2p doublet and the binding energy separation between Ti2p3/2 and Ti2p1/2 (marked in the figure as EW) equal to 5.8 eV testifies the +4 oxidation state of titanium present at the surface. The ratio of the area of the Ti2p3/2 and Ti2p1/2 peaks is equal to 2:1, confirming the presence of the stoichiometric TiO2 at the surface of the mixed oxide (Ti0.48Cu0.52)Ox thin film. Furthermore, the O1s spectrum (Figure 6c) was deconvoluted into three peaks related to the lattice oxygen (for TiO2 and CuO), hydroxyl radicals (OH -) and adsorbed water molecules (H2Oads).    (Figure 2). The measured nonpinched hysteresis shape of the I-V curves could be due to a wide (about 420 nm) area of the region rich in titanium oxide that results in a relatively high series resistance. We believe that in the discussed case, switching from a high to low resistance state connected with the formation of filament-type conduction paths allows electrical charge carriers to flow between two opposite contacts. Observed first cycle in I-V plane that differs from the rest of the results suggests the presence of the formation process of 15 the conducting paths, when for the first time the forcing current was applied to the investigated structure.

Conclusions
The paper presents the results of investigations of the memory effect observed in a thin film structure with gradient U-shape like profile of Cu distribution prepared using the magnetron co-sputtering process. Structural, elemental, and band-gap analyses allowed discussion on the reason for the presence of nonpinched hysteresis in I-V plane that resulted in the observed switching effect of memory. As it was concluded based on the performed investigations, repeatable resistive switching with high difference of 6•10 2 times between LRS and HRS was obtained due to the formation of conducting filaments over the prepared gradient thin film. Presented results proved that thin films with gradient element distribution profile can be an interesting alternative to the conventional multilayer stack configuration for the preparation of devices utilizing memory effects.