Resistive switching memory performance in oxide hetero-nanocrystals with well-controlled interfaces

ABSTRACT

For realization of new informative systems, the memristor working like synapse has drawn much attention. We developed isolated high-density Fe₃O₄ nanocrystals on Ge nuclei/Si with uniform and high resistive switching performance using low-temperature growth. The Fe₃O₄ nanocrystals on Ge nuclei had a well-controlled interface (Fe₃O₄/GeO_x/Ge) composed of high-crystallinity Fe₃O₄ and high-quality GeO_x layers. The nanocrystals showed uniform resistive switching characteristics (high switching probability of ~90%) and relatively high Off/On resistance ratio (~58). The high-quality interface enables electric field application to Fe₃O₄ and GeO_x near the interface, which leads to effective positively charged oxygen vacancy movement, resulting in high-performance resistive switching. Furthermore, we successfully observed memory effect in nanocrystals with well-controlled interface. The experimental confirmation of the memory effect existence even in ultrasmall nanocrystals is significant for realizing non-volatile nanocrystal memory leading to neuromorphic devices.

Graphical abstract

KEYWORDS:

• Memristor
• interface control
• nanocrystal
• iron oxide
• silicon
• germanium
• resistive switching characteristics

CLASSIFICATION:

• 212 Surface and interfaces

1. Introduction

New informative systems toward next-generation human society such as artificial intelligence and internet of things have been extensively studied. Now, the systems for huge, fast, and complex calculations are requiring neuromorphic devices [1]. A lot of researchers have enthusiastically paid attention to memristor working like synapse: i.e. memorizing an electric current history [2,3]. Resistance random access memory (ReRAM), which can realize memristor, has great advantages such as high-speed response, low power consumption, and good scalability [4–10].

The resistive switching characteristics were mainly observed in oxide-based materials such as TaO_x [11], TiO_x [12,13], and so on [14–16]. Fe₃O₄ with a higher Clark number (Fe is rank 4^th) is a more industrially compatible material in terms of element abundance, low material cost, and non-toxicity [17,18]. Polycrystalline Fe₃O₄ film showed resistive switching characteristics based on ionic movement [19,20]. However, the Off/On resistance ratio (Off/On ratio) is low (~10) because the leakage current passing through the grain boundaries may degrade the Off resistance. Therefore, forming high-quality Fe₃O₄ is a key for increasing Off/On ratio.

Nanostructuring approach can be one of the ways to form high-crystallinity materials. We have developed single-crystalline spherical nanocrystals (NCs), where there are almost no defects due to the elastic relaxation of the lattice mismatch strain between NCs and substrates [21]. Hence, the NCs without current leak paths likely show higher Off resistance, leading to higher Off/On ratio. Recently, we have developed core/shell epitaxial Fe₃O₄/Ge NCs by combining oblique deposition at high temperature with ultrathin SiO₂ technique [22–25] (left image in Figure 1(a)), where Ge oxides (GeO_x) were formed at the Fe₃O₄/Ge interface by exposure to oxygen atmosphere after epitaxial growth of Fe₃O₄ (at the final stage of Fe₃O₄ growth or when exposing to air). Its detail is described in our previous study [17]. The Fe₃O₄/Ge NCs exhibited high Off/On ratio of ~100 [26], but had large positional variability. To control the resistive switching characteristics well, understanding the mechanism is strongly required. So far, it has been revealed that the resistive switching is originated from the thickness change of high resistance layer (HRL) near Fe₃O₄/GeO_x/Ge interface caused by electric-field-induced oxygen vacancy movement (Figure 1(b)). However, the identity of HRL, a key of this nanoionics-based resistive switching, has not been clarified: in the interface layer, existence or non-existence of Fe-Ge mixing, and oxidation degree, etc. The mechanism unclearness related to poor resistive switching performance is ascribed to the unclear and uncontrolled Fe₃O₄/GeO_x/Ge interface.

Figure 1. (a) Schematic diagram of isolated Fe₃O₄/GeO_x/Ge NC memory (left image). The formation of high-crystallinity Fe₃O₄ and high-purity GeO_x at the interface between Fe₃O₄ and Ge (right image) aiming at high resistive switching performance. (b) Resistive switching mechanism in Fe₃O₄/GeO_x/Ge NCs that have some unclearness: high resistance layer (HRL) which is a key for nanoionics-based resistive switching

In this study, aiming at forming well-controlled Fe₃O₄/GeO_x/Ge interface in the NC system, we perform low-temperature growth of epitaxial Fe₃O₄ NCs without the interface reaction degrading the performance by depositing Fe on Ge nuclei at room temperature (RT) under a low-pressure oxygen atmosphere and subsequently post-annealing. Then, we obtain Fe₃O₄/GeO_x/Ge NCs, where Fe₃O₄ is highly crystalline and the high-quality GeO_x is formed between Fe₃O₄ and Ge. The post-annealed NCs showed uniform resistive switching characteristics (higher switching probability of ~90%) and higher Off/On ratio of ~58. It is found that HRL related to resistive switching mechanism is composed of Fe₃O₄ near the interface and GeO. Furthermore, we successfully observe the memory effect in the NCs with well-controlled interface. The experimental confirmation of the memory effect existence even in ultrasmall NCs is significant for realizing non-volatile NC memory leading to neuromorphic device.

2. Experimental details

Fe₃O₄ NCs on Ge nuclei were formed on Si substrates in the chamber equipped with Knudsen cell for Ge and an electron beam evaporator for Fe in the following way. As-doped Si(111) substrates with dimensions of 2 mm×7 mm×0.3 mm were introduced into ultrahigh vacuum chamber at a base pressure of 1 × 10⁻⁸ Pa. After degassing the substrates at 500°C for 6 h, Si(111) clean surfaces were obtained by flashing at 1250°C. The ultrathin SiO₂ films with the thickness of 0.3 nm were formed on Si(111) substrates by oxidizing the Si surfaces at 600°C for 10 min under an oxygen pressure of 2 × 10⁻⁴ Pa [17,21–26]. Epitaxial Ge nuclei with an areal density of ~10¹¹ cm⁻² were formed by depositing 25 MLs of Ge onto the ultrathin SiO₂ films at 600°C. 1–30 MLs of Fe were deposited on Ge nuclei at RT for coating Ge nucleus surfaces. Finally, Fe₃O₄ NCs were grown on Fe-coated Ge nuclei by deposition of 21 ML Fe on the Ge nuclei at RT under an oxygen pressure of 2 × 10⁻⁴ Pa. In this Fe₃O₄ growth by Fe deposition at low-pressure oxygen atmosphere, oxygen vacancies are likely generated in Fe₃O₄ [27,28]. For the above-mentioned Fe coating and Fe₃O₄ growth, the oblique Fe deposition was performed in the direction of out-of-plane incident angle of 25º, enabling the formation of isolated Fe₃O₄ NCs on Ge nuclei, which details are reported in our previous study [26]. For Fe and Ge deposition, Fe and Ge fluxes are 0.13 and 0.35 ML/min, respectively. To enhance their crystallinities, Fe₃O₄ NCs were post-annealed at 250-400°C for 30 min under an oxygen pressure of 2 × 10⁻⁴ Pa.

Reflection high energy electron diffraction (RHEED) observations were performed with 13 keV electron beam incident in the <112> _Si direction. The structures of Fe₃O₄/GeO_x/Ge NCs were observed by cross-sectional high-resolution transmission electron microscopy (HRTEM) with 200 keV electron beam incident in the direction of <10> _Si. GeO_x layers were likely formed at the Fe₃O₄/Ge interface at the final stage of Fe₃O₄ growth or after exposure to oxygen atmosphere. To determine the compositions of the aforementioned interfaces, we performed XPS measurements in the Fe2p and Ge3d regions.

Conductive-atomic force microscopy (C-AFM) measurements were carried out using a Pt-Ir-coated Si cantilever at RT in air. For absolute current (I_S)-sample bias voltage (V_S) measurement, the V_S was swept at the rising and falling speed of 0.4 V/s and the voltage step of 0.006 V in the following order: (1) 0 V → 2 V, (2) 2 V → 0 V, (3) 0 V → −2 V, and (4) −2 V → 0 V.

3. Results and discussions

3.1. Resistive switching characteristics in the NCs grown at RT

The morphologies of the NC samples grown at RT (as-grown NCs) with various Fe coating layers were measured by AFM. An AFM image of as-grown NCs with Fe coating layer of 30 MLs in Figure 2(a) indicates the formation of isolated NCs that have high areal density of ~10¹¹ cm⁻² and small size of ~20-40 nm. From RHEED observations, it was confirmed that Fe₃O₄ NCs were epitaxially grown on Ge nuclei/Si (Figure 2(b)), which detailed RHEED analyses are reported in our previous studies [17,26]. The analyses of 224_Fe3O4 spots in RHEED patterns were performed by fitting with Gaussian function. The full width at half maximum (FWHM) values of the 224_Fe3O4 fitted peaks in Fe₃O₄ NCs on Ge nuclei with Fe coating layers of 6, 15, and 30 MLs were 5.5, 4.7, and 3.9 nm⁻¹, respectively (Figure 2(d) and Supplemental Material 1), indicating that the crystallinity is enhanced with increasing the deposition amount of Fe coating layer. This trend is similar to the previously reported result in Fe₃O₄ NCs on Ge nuclei formed by Fe deposition at high temperature [29], where it was reported that the oxidation of Ge nucleus surface at the first stage of Fe₃O₄ growth was prevented by Fe coating, resulting in the higher crystallinity of Fe₃O₄. Therein, at the final stage of Fe₃O₄ growth or after exposure to oxygen atmosphere, the GeO_x was inserted at the Fe₃O₄/Ge interface [30]. Therefore, it is considered that as-grown NCs also have GeO_x at the Fe₃O₄/Ge interface.

Figure 2. (a) AFM image, (b) RHEED pattern, and (c) The I_S-V_S curve of as-grown NCs with the Fe coating layer of 30 MLs. The V_S was swept in the following order: (1) 0V→ 2V, (2) 2V → 0V, (3) 0V → −2V, and (4) −2V → 0V. (d) FWHM of 224_Fe3O4 (left axis) and P_switch (right axis) as a function of the deposition amount of Fe coating layer in as-grown NCs (the solid squares and circles, respectively). The open square and circle denote the FWHM of 224_Fe3O4 and P_switch in NCs annealed at 250°C, respectively. (e) Schematic of the Fe₃O₄ crystallinity effect on the resistive switching characteristics, where low- and high-crystallinity Fe₃O₄ are denoted as LC-Fe₃O₄ and HC-Fe₃O₄. Electric-field-applied region gets smaller in Fe₃O₄ with more oxygen vacancies (V_O⁺) because of large screening effect by high concentration carriers

The resistive switching characteristics in as-grown NCs with Fe coating layers of 6, 15, and 30 MLs were measured using C-AFM. The V_S was swept in the following order: (1) 0 V → 2 V, (2) 2 V → 0 V, (3) 0 V → −2 V, and (4) −2 V → 0 V. In each sample, the I_S-V_S curves were acquired 15–20 times on each NC and measured NC number was 6–8 for each sample (total I_S-V_S curves: 120 for each sample). Figure 2(c) is the I_S-V_S curve of as-grown NCs with Fe coating layer of 30 MLs. The NCs showed the bipolar-type resistive switching characteristics with high resistive state (HRS) and low resistive state (LRS). The resistive switching probability was estimated by defining as the switching observation number divided by measurement number (Supplemental Material 2). The average resistive switching probabilities in one sample (P_switch) were ~18, ~32, and ~43% in Fe₃O₄ NCs on Ge nuclei with Fe coating layers of 6, 15, and 30 MLs, respectively (Figure 2(d)). The Fe coating amount dependence of the crystallinity and the P_switch demonstrated that P_switch becomes higher as the crystallinity of Fe₃O₄ NCs is enhanced. In NCs with lower crystallinity, it can be considered that there are more defects (oxygen vacancies) in Fe₃O₄. The carrier concentration can be increased in Fe₃O₄ due to more oxygen vacancies, enhancing the screening effect of applied electric field in Fe₃O₄. As a result, the electric-field-induced oxygen vacancy movement becomes difficult to occur in Fe₃O₄, resulting in low P_switch in defective samples (Figure 2(e)). On the other hand, in NCs with higher crystallinity, the electric-field-induced oxygen vacancy movement likely occurs due to the small screening effect, resulting in higher P_switch (Figure 2(e)).

3.2. Composition and structural analyses of annealed NCs

To enhance the crystallinity of Fe₃O₄ NCs, as-grown NCs were post-annealed under an oxygen pressure of 2 × 10⁻⁴ Pa, where the two post-annealing temperatures were used: 250°C and 400°C. The analyses of 224_Fe3O4 spots in RHEED patterns were performed by fitting with Gaussian function. The FWHM values of the 224_Fe3O4 fitted peaks in the NCs annealed at 250°C and 400°C were 1.5 and 0.5 nm⁻¹, respectively, indicating that the crystallinity of Fe₃O₄ was enhanced as post-annealing temperature became higher (Figure 3(a)). The compositions of as-grown NCs and the NCs annealed at 250°C and 400°C were investigated by XPS measurements in the Fe2p region (Supplemental Material 3). Other studies have reported that FeO and γ-Fe₂O₃ have satellite peaks near 715 or 720 eV, respectively, while Fe₃O₄ has no satellite peaks [30]. The XPS spectra of as-grown NCs and the NCs annealed at 250°C and 400°C displayed no satellite peaks, indicating that all the samples are Fe₃O₄. We also measured the XPS spectra in the Ge3d region to obtain the compositional information near the Fe₃O₄/Ge interface (Supplemental Material 3). All the NCs had the peaks coming from oxidation states of Ge, proving that GeO_x was formed at the interface between Fe₃O₄ and Ge nuclei. In the sample post-annealed at higher temperature (NCs annealed at 400°C), remarkable Fe-Ge peaks were also observed. The XPS spectra in the Ge3d region were deconvolved with Gaussian function at the peak positions of Ge⁰ (29.3 eV), Ge⁺ (30.1 eV), Ge²⁺ (31.1 eV), Ge³⁺ (32.0 eV), Ge⁴⁺ (32.6 eV), and Fe-Ge (29.8 eV) [31,32]. The integrated intensity ratios of Fe-Ge peak to the sum of all the peaks (Fe-Ge mixing ratio) were almost zero for as-grown (~0.6%) and NCs annealed at 250°C (~0.3%), while the ratio was high (17.4%) for NCs annealed at 400°C (Figure 3(a)). In NCs annealed at 400°C, this high Fe-Ge mixing ratio implies that Fe-Ge mixing occurred at the region near the interface, namely in GeO_x interface layer, during high-temperature annealing process. This demonstrates that as-grown NCs and NCs annealed at 250°C without high-temperature annealing process have higher-quality GeO_x with almost no Fe-Ge mixing than NCs annealed at 400°C.

Figure 3. (a) FWHM estimated from 224_Fe3O4 peak in RHEED (left axis) and Fe-Ge mixing ratio measured from XPS spectra (right axis) in each sample (as-grown: as-grown NCs, 250: NCs annealed at 250°C, and 400: NCs annealed at 400°C). (b) Low-magnification HRTEM image of NCs annealed at 250°C. (c) Enlarged HRTEM image of the square region in (b). FFT patterns of (d) regions B and (e) A in (c). The region marked by the broken line denotes GeO_x. Theoretical FFT patterns of (f) Fe₃O₄ and (g) Ge

Figure 3(b) shows a cross-sectional HRTEM image of NCs annealed at 250°C with high-crystallinity Fe₃O₄ and high-quality GeO_x. It was confirmed that NCs with small size of ~30 nm were formed, which is consistent with the result of AFM observation. Figure 3(c) is an enlarged image of the square region in Figure 3(b). The crystal structures in the regions A and B in Figure 3(b) were analysed by fast Fourier transformation (FFT). As shown in Figure 3(d) and (e), the FFT patterns of regions B and A were consistent with the theoretical diffraction ones of Fe₃O₄ and Ge with the epitaxial relationship of (012)_Fe3O4//(111)_Ge (Figure 3(f) and (g)), respectively, demonstrating that Fe₃O₄ shell/Ge core NCs were epitaxially formed. In addition, the bright contrast region was observed between Fe₃O₄ shell and Ge core, as indicated by the arrow in the HRTEM image (Figure 3(c)), implying that amorphous GeO_x exists. Thus, HRTEM observations revealed the structures of NCs: Fe₃O₄/GeO_x/Ge NCs.

3.3. Resistive switching characteristics in annealed NCs

In C-AFM measurements, bipolar-type resistive switching characteristics were observed in as-grown NCs (Figure 2(c)) and NCs annealed at 250°C (Figure 4(a)), while the resistive switching seldom occurred in NCs annealed at 400°C (Figure 4(b)). By considering there is Fe-Ge mixing (low-quality GeO_x) only in NCs annealed at 400°C, it was found that the unobserved resistive switching characteristics in NCs annealed at 400°C were attributed to the existence of FeGe alloys in GeO_x although NCs annealed at 400°C have higher-crystallinity Fe₃O₄ than other NCs. To quantitatively investigate the nanostructure interface effect on the resistive switching characteristics, we measured P_switch and Off/On ratio, where the I_S-V_S curves were acquired 15–20 times on each NC and measured NC number was 6–8 for each sample (total I_S-V_S curves: 120 for each sample). The P_switch values were ~43, ~90, and ~3% in as-grown NCs, NCs annealed at 250°C and 400°C, respectively (Figure 4(c) and Supplemental Material 2). Note that NCs annealed at 250°C exhibited substantially higher P_switch (~90%) than those of the other NCs. Besides, the standard deviation of the resistive switching probabilities of individual NCs in the NC sample annealed at 250°C (Supplemental Material 2) was 0.09, which was extremely smaller than those of as-grown NCs (0.26) and previous Fe₃O₄ NCs formed at high temperature (0.3). This indicates that the resistive switching uniformity was improved by forming high-crystallinity Fe₃O₄ and high-quality GeO_x. The NCs annealed at 250°C exhibited higher Off/On ratio (~58) at a reading V_S (V_read) of ~ −0.5 V than those of the other NCs: ~0 for NCs annealed at 400°C and ~9 for as-grown NCs (Figure 4(c)). During the I_S-V_S measurement cycles, the HRS resistance gradually got larger, indicating that the NCs memorize the current history like memristor. From its higher P_switch and Off/On ratio, the NCs annealed at 250°C are most suited for NC memory among them. Compared with as-grown NCs, high P_switch and Off/On ratio in NCs annealed at 250°C can be attributed to the low concentration of oxygen vacancies; this relates to the higher Fe₃O₄ crystallinity, which can be explained by the same mechanism as for as-grown NCs (Figure 2(e)). Another remarkable fact is that the formation of FeGe alloys in GeO_x prevented the resistive switching (Figure 4(d)). In the NCs with low-quality GeO_x (including FeGe alloys), the electric field is not applied to GeO_x because FeGe alloys in GeO_x work as current leak paths. It should be noted that the resistive switching characteristics were affected by character of Fe₃O₄ and GeO_x, implying that HRL related to resistive switching is composed of both GeO_x and Fe₃O₄ near Fe₃O₄/GeO_x interface. For further enhancement of resistive switching performance, almost perfect stoichiometric Fe₃O₄ and high-quality GeO_x without current leak paths are key structures.

Figure 4. The I_S-V_S curves of (a) NCs annealed at 250°C and (b) at 400°C. (c) P_switch and Off/On ratio of all the NCs. The solid squares and circles denote the P_switch and the Off/On ratio, respectively. (d) Schematic of the GeO_x quality effect on the resistive switching characteristics

The resistive switching mechanism in this system can be discussed below. In the bipolar-type resistive switching, there are mainly two mechanisms such as filament- or interface-type. If the resistive switching mechanism in the Fe₃O₄ is filament-type, the resistive switching can be caused in Fe₃O₄ films without Ge. However, in our previous study, resistive switching was not observed in conventional pulse laser deposition Fe₃O₄ films on Si substrate without Ge [17]. The quality of the pulse laser deposition films, which is related to some defects (oxygen vacancy concentration, point defect species), is similar to that of the present Fe₃O₄/Ge NCs because of the same growth condition of low oxygen pressure (10⁻⁴ Pa). Therefore, the filament-type mechanism is ruled out in the present system. On the other hand, in terms of the deficiency, the quality of pulse laser deposition films and the present Fe₃O₄/Ge NCs can be quite different from the reported films formed by sputtering method exhibiting resistive switching [19,20].

In the case of interface-type resistive switching, there are several candidates of resistive switching interface: Pt-Ir cantilever/Fe₃O₄, Fe₃O₄/GeO_x, and GeO_x/Ge. The C-AFM observations in air might cause the anodic oxidation of NC surface under Pt-Ir cantilever, bringing the resistance change when applying negative V_S. In the same mechanism, Fe₃O₄ films without Ge must show resistive switching behavior. In our previous study, the pulse laser deposition Fe₃O₄ films without Ge did not occur. Therefore, the resistive switching by anodic oxidation is ruled out. Next, we discuss whether the resistive switching at the GeO_x/Ge interface can be caused or not. In our previous study [25], Ge nuclei covered with GeO_x, namely the sample without Fe₃O₄, did not exhibit resistive switching, indicating that the GeO_x/Ge is not a resistive switching interface in the present system. Therefore, the possible resistive switching interface is narrowed down to the Fe₃O₄/GeO_x.

We consider the HRL, where the electric field is mainly applied when applying V_S to NCs. For resistive switching, the electric field application to GeO_x is required because the resistive switching did not occur when the electric field cannot be applied to GeO_x due to the formation of FeGe alloys working as current leak paths. This indicates that GeO_x is a part of HRL. In addition, the resistance of Fe₃O₄ should be relatively high because the resistive switching performance becomes higher with enhancing Fe₃O₄ crystallinity. This indicates that the electric field should also be applied to Fe₃O₄ near Fe₃O₄/GeO_x interface. From these results, it is considered that HRL is composed of both GeO_x and Fe₃O₄ near Fe₃O₄/GeO_x interface. However, we believe that the main contribution of the HRL resistance comes from GeO_x because resistive switching seldom occurred at zero electric field in GeO_x in NCs annealed at 400°C.

As shown in Figure 5, when applying V_S to NCs, positively charged oxygen vacancies move in GeO_x and Fe₃O₄ near Fe₃O₄/GeO_x interface. When the x value of GeO_x gets larger (smaller) by this ionic movement, the volume of the high resistance GeO_x with large x becomes larger (smaller) [33] and the HRL thickness composed of high resistance GeO_x increases (decreases). On the other hand, as the effect of the electric field applied to Fe₃O₄ near Fe₃O₄/GeO_x interface, two possibilities are considered: (1) changing resistance states of Fe oxide layer near Fe₃O₄/GeO_x interface: LRS of Fe₃O₄ and HRS such as γ-Fe₂O₃ or FeGeO_x by the ionic movement, (2) supplying oxygen vacancies or oxygens from Fe₃O₄ region to GeO_x. By either function, the electric-field resistive change is enhanced. This deep comprehension about the resistive switching mechanism is presenting the material design guideline for improving various resistive switching characteristics.

Figure 5. Resistive switching mechanism in Fe₃O₄/GeO_x/Ge NCs

3.4. Direct observation of memory effect

Here, the memory effect in NCs annealed at 250°C was directly observed by performing current mapping with C-AFM. Figure 6(a) and (b) show AFM and current mapping images, respectively, before applying the V_S of +2 V. The current mapping images were observed at the V_read of −0.5 V, which applied voltage does not cause the resistive switching. White contrasts indicate the higher electric currents between cantilever and sample. Almost all NCs show white contrasts in current mapping images, which indicates that the initial states of NCs were LRS. We applied voltages to the samples by scanning the square region of Figure 6(b) at V_S of +2 V to cause the resistive switching. After applying the V_S, the white contrasts in the square region disappeared and most of the NC sites inside the square region became darker compared with other NCs outside the region (Figure 6(c) and (d)). Thus, it was directly observed that the resistance of NCs in the square region changed from LRS to HRS through the application of V_S of +2 V. AFM and a current mapping image were observed at the V_read of −0.5 V (Figure 6(e) and (f)) when the time of 1800 s passed after the resistive switching by scanning at V_S of +2 V. The results demonstrate that the contrast in square region was kept darker, indicating that the resistance in NCs has been memorized since the resistive switching (LRS to HRS). We also confirmed the resistance retention of more than 7200 s in such C-AFM method (Figure 6(g) and (h)). The direct observation of the memory effect in NCs demonstrates that NCs annealed at 250°C have a high potentiality as a non-volatile memory. Unlike the film memory material, the observation of the memory effect in ultrasmall NCs has never been reported in ReRAM. The experimental confirmation of the memory effect existence even in ultrasmall NCs is significant for realizing non-volatile NC memory leading to neuromorphic device.

Figure 6. Memory effect of NCs annealed at 250°C (a, c, e, g) AFM and (b, d, f, h) current mapping images of the NCs at the V_S of −0.5V: (a, b) before applying V_S of +2V, (c, d) just after applying V_S of +2V, (e, f) 1800 s later since the resistive switching, and (g, h) 7200 s later since the resistive switching, respectively. The current mapping image when applying V_S of +2V is shown in the inset

4. Conclusions

We developed high-density and isolated Fe₃O₄/GeO_x/Ge NCs with high-performance resistive switching characteristics by low-temperature growth. Therein, the quality of the interfaces; the crystallinity of Fe₃O₄ and the quality of GeO_x were successfully enhanced by annealing under proper condition: low oxygen pressure (2 × 10⁻⁴ Pa). The high quality of interfaces enables effective application of electric field to Fe₃O₄ and GeO_x. Then, the electric field induces positively charged oxygen vacancy movement, leading to the oxidation degree change of HRL composed of Fe oxide and GeO_x. As a result, the thickness of HRL can be changed by applying the electric field effectively. The NCs annealed at 250°C showed the bipolar resistive switching characteristics with higher P_switch of ~90% and higher Off/On ratio of ~58. The memory effect in NCs annealed at 250°C was also observed by C-AFM method successfully. The experimental confirmation of the memory effect existence even in ultrasmall NCs is significant for realizing non-volatile NC memory leading to neuromorphic device.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplemental material

Supplemental data for this article can be accessed here.

Additional information

Funding

This work was supported by a Grant-in-Aid for Scientific Research A Grant No. [16H02078 and 19H00853] and for Exploratory Research [19K22110]; Japan Society for the Promotion of Science.