Nonvolatile Memory Device Based on Copper Polyphthalocyanine Thin Films.

In this work, we report the fabrication of nonvolatile memory devices based on chemical vapor deposition-grown copper polyphthalocyanine (CuPPc) thin films. The high polymerization degree and crystallinity of the as-obtained films were confirmed by transmission electron microscopy, X-ray photoelectron spectroscopy, and UV-vis studies. It was found that the device with Au/CuPPc/indium tin oxide sandwich structure exhibits good nonvolatile memory performance with a large ON/OFF current ratio of 103 and long retention time of 1.2 × 103 s.

Introduction

Resistance switching memory devices have gained increan class="Chemical">sing attention as a promising candidate for the next generation of nonvolatile memories owing to their stable performance, high storage density, low power consumption, and good scaling capability.[1−5]

So far, various kinds of materials such as inorganic materials, inorganic/organic hybrid materials, organic small-molecule compounds and polymers have already been demonstrated to exhibit switchable resistance performance.[6−9] Extensive studies on the applications of memory devices based on organic small-molecule and polymeric materials have been carried out because of the advantages such as low cost, three-dimensional stacking capability for high-density data storage, and the possibility of modulating their properties through the molecular design and chemical synthesis.[10−12] However, the relatively low thermostability and chemical resistances of organic materials make them susceptible to damage and difficult to show good data-storage performance in harsh environment such as high temperature and solution processing.[13] In order to improve the device switching performance and enhance the applicability, large numbers of functional materials, including conjugated polymers, vinyl polymers with specific pendant groups, polymer/organic molecule blends, polymer/metal complexes, and so forth have been tested, and they achieved good results.

Metal polyphthalocyanines (PPcs), known for many unique properties such as great environmental stability and good conductivity, have attracted attention of researchers.[14−17] However, the applications of PPcs have been hindered by the poor processability because of the insolubility in any organic solvent, and high melting/boiling point makes them difficult to be fabricated into thin film devices.[18] Most reported synthetic methods for PPcs are based on the solid-phase reaction of metal salts with pyromellitic dianhydride, but the bulk PPcs synthesized generally have low polymerization degree and poor structural uniformity.[19−21] In order to achieve more applications, it is necessary to prepare two-dimensional (2D) PPcs in the form of thin films on arbitrary substrates. In a previous work, the researchers showed that through a surface reaction between 1,2,4,5-tetracyanobenzene (TCNB) molecules and Mn atoms on Ag(111), a 2D phthalocyanine polymer can be obtained.[22] However, the experimental steps were complicated (ultrahigh vacuum conditions and atomically clean surfaces were needed) and only nanoscale domains were obtained, which makes it difficult to meet the requirements for the preparation of electronic devices. At present, chemical vapor deposition (CVD) has emerged as a scalable and controllable way to prepare 2D materials in large area, such as graphene, transition-metal dichalcogenides and hexagonal boron nitride, and so forth.[23−25] In comparison with evaporation and solution processing, the CVD method is beneficial for large-scale fabrication, and the obtained materials are easier to transfer onto the required substrate for device fabrication.

In this communication, we report the CVD growth and characterization of n class="Chemical">copper PPc (CuPPc) thin films and the fabrication and performance of nonvolatile memory devices based on it. The obtained CuPPc films with high polymerization degree and crystallinity were confirmed by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV–vis studies. Also, the devices show obvious nonvolatile rewritable memory performance, with a large ON/OFF current ratio of 103 and a retention time of 1.2 × 103 s.

Results and Discussion

The CuPPc thin film was synthesized by CVD setup, following the protocol as reported in the literature.[26] A schematic illustration of the synthetic process and experimental setup are shown in Figure a,b. The reaction was carried out in a two-zone oven. TCNB was evaporated in the first zone at 110 °C and deposited onto the electrochemically polished Cu foil that was placed in the second zone.

Figure 1

(a) Synthesis of CuPPc from Cu and TCNB. (b) Schematic illustration of the CVD system.

(a) Synthesis of n class="Chemical">CuPPc from Cu and TCNB. (b) Schematic illustration of the CVD system.

The obtained CuPPc thin films were transferred to a 300 nm n class="Chemical">SiO2/Si substrate with a poly(methyl methacrylate) (PMMA)-assisted transfer process.[27] Under the optical microscope, in most areas, the color of the film is uniform, which indicates that the film thickness has high uniformity (Figure a). Also, energy-dispersive X-ray spectroscopy (EDX) further confirms the uniform distribution of elements in the film (Figure S1). The thickness and surface roughness of the CuPPc thin films were characterized by an atomic force microscope (AFM). As shown in Figure b, the thin films have a thickness of 106 ± 6 nm. AFM characterization reveals a total surface roughness of 25.8 ± 0.8 nm. Transmission electron microscopy (TEM) study shows that the CuPPc thin films have a polycrystalline structure with the characteristic crystallite size of at least several tens of nanometers (Figure c and inset). The selected-area electron diffraction (SAED) patterns exhibits a four-fold symmetry which agrees well with that expected from the chemical structure of CuPPc. The lattice spacing is 0.260 ± 0.003 nm. Under a polarized microscope, the color of the CuPPc film changed simultaneously from bright to dark when rotating the substrate, which is caused by the crystallinity of the film (Figure S2). XPS was used to further characterize the chemical composition of the films and confirmed the successful formation of CuPPc. As shown in Figure d, the XPS spectrum of the Cu 2p core level has two obvious peaks at 935.4 and 955.3 eV, respectively, which correspond to the electron state of Cu 2p1/2 and Cu 2p3/2, demonstrating the existence of Cu(II).[28,29] The Cu(II) is located at the ligand environment of the nitrogen-enriched structure of the phthalocyanine ring. The N 1s spectrum of CuPPc can be separated into two components, the lower binding energy component centered at 398.7 eV can be attributed to pyridinic-N and the one at 399.8 eV is from pyrrolic-N, which clearly confirms the presence of Cu–N4 moieties in the CuPPc films (Figure e).[22] The C 1s spectra can be fitted with bands at 284.6, 285.8, and 288.4 eV, which are attributed to aromatic C–C bonds, N–C=N and C=O, respectively (Figure S3a).[30] The UV–vis absorption spectra of CuPPc are shown in Figure f. A strong absorption peak around 200 nm is observed, and there is a certain degree of absorption after the wavelength greater than 800 nm reaches the infrared region. The results show that the absorption of the CuPPc films continues far beyond the monomeric CuPc absorption threshold (700 nm), confirming the extended π-conjugation of CuPPc. Another important observation is the absence of the 740 nm absorption band, which is associated with monomeric CuPc and oligomers with low polymerization degree.[31] The 700 nm band is attributed to the lowest energy electronic transition of the CuPc, while in CuPPc, high conjugation leads to merging of molecular orbitals into valence bands and conduction bands, and the small band gap results in absorption extended to the infrared region. The absence of 740 nm band for the polymer provides extra evidence of a high polymerization degree.

Figure 2

(a) OM image of the CuPPc thin film, the green section is CuPPc, the blue section is the SiO2/Si substrate. (b) AFM image of the CuPPc thin film. (c) TEM image of the CuPPc thin film. (Inset: SAED image of the CuPPc thin film). (d,e) XPS spectra of Cu 2p and N 1s. (f) UV–vis absorption spectra of the CuPPc thin film.

(a) OM image of the CuPPc thin film, the green section is n class="Chemical">CuPPc, the blue section is the SiO2/Si substrate. (b) AFM image of the CuPPc thin film. (c) TEM image of the CuPPc thin film. (Inset: SAED image of the CuPPc thin film). (d,e) XPS spectra of Cu 2p and N 1s. (f) UV–vis absorption spectra of the CuPPc thin film.

The memory device with a sandwich structure of Au/n class="Chemical">CuPPc/indium tin oxide (ITO) was fabricated and the performance was investigated under ambient conditions. The electrode material Au with high work function was used as the top electrode instead of the commonly used Al, which is sensitive to oxidation, contributing to a better long-term stability of the device. As shown in Figure a, the CuPPc thin film-based memory device exhibits a nonvolatile rewritable memory performance, with a turn-on voltage of +1.34 V and an ON/OFF current ratio of about 103. When the voltage swept positively from 0 to +8 V, current through the device rapidly changed from 10–5 to 10–3 A at the turn-on voltage of +1.34 V (sweep 1), implying that the device was switched from the OFF state to the ON state. This switch-on process is also called as the writing process. The ON state could be kept unchanged during the second sweep (sweep 2, from +8 V back to 0 V) and even after cutting off the electricity supply, demonstrating that the device possesses a nonvolatile memory characteristic. When the negative voltage was applied to the device (sweep 3), the device still remained in the ON state until reaching the turn-off voltage of −0.79 V, where the device current decreased abruptly from 10–3 to 10–6 A. This indicates that the device has returned to the OFF state with low conductivity. This process corresponds to the erasing process. A relatively high ON/OFF current ratio of about 103 was obtained. The memory I–V loops are reproducible and of 16 consecutive cycles are shown in Figure S4 without clear degradation of the ON and OFF states, which was also reflected from the changes of the resistance values in the high resistance state (HRS)/low resistance state (LRS). To explore the uniformity of the switching of the as-prepared device, I–V curves of 200 devices were measured. The device-to-device distributions of the HRS resistance and LRS resistance range from 8.2 × 106 to 6.2 × 104 Ω and 1.3 × 103 to 4.7 × 101 Ω (read at −0.1 V), respectively, are shown in Figure b. To evaluate the persistence of the ON/OFF states of the Au/CuPPc/ITO devices, the retention characteristic was measured under −0.1 V read voltage at room temperature, as shown in Figure c. The resistance of both the states showed no obvious change for 1.2 × 103 s, indicating that our devices have a relatively good stability.

Figure 3

(a) Typical current–voltage (I–V) characteristics of an ITO/CuPPc/Au device. A compliance current of 10 mA is used to prevent the breakdown of the device in the set process. (b) Device to device distributions of the resistance, where the results are obtained under −0.1 V read voltage from 200 devices. (c) Retention characteristics of both the resistance states under a continuous −0.1 V readout voltage at room temperature.

Because the n class="Chemical">Au electrode is passive, the electrochemical metallization memory mechanism can be ruled out. In order to explore the conduction mechanism of resistive switching, the typical I–V curve of the devices was plotted using a double-logarithmic scale, as shown in Figure a. The I–V characteristic of the LRS showed a linear plot with a slope of 1, indicating the Ohmic behavior. However, the I–V characteristic of the HRS was more complicated and could be divided into two regions. In the low voltage range from 0 to 0.56 V, the lg I–lg V characteristic showed a linear relation, corresponding to Ohmic behavior, whereas in the high voltage region of the HRS (0.56 V to Vset), the lg I–lg V curve showed a nonlinear relation, suggesting a different conducting mechanism. Further analysis indicated a linear relation between ln(I) and V1/2, as shown in Figure b, suggesting that Schottky emission is the dominant conduction mechanism in the high voltage region of the HRS.[32,33]

Figure 4

(a) Typical I–V curves for resistive switching of the ITO/CuPPc/Au devices plotted on a lg–lg scale. (b) Typical ln(I) vs V1/2 plot for the high voltage region of the HRS.

(a) Typical I–V curves for ren class="Chemical">sistive switching of the ITO/CuPPc/Au devices plotted on a lg–lg scale. (b) Typical ln(I) vs V1/2 plot for the high voltage region of the HRS.

Figure a shows the cross section high-resolution TEM (HRTEM) image of the Au/n class="Chemical">CuPPc/ITO device in the ON state. One conductive filament (CF) (marked with red rectangle) is observed in the HRTEM image, as shown in Figure a,b. The elemental mapping images of In, Cu, and Au are shown in Figure c, indicating that the CF is composed of Cu elements. It can be concluded that the CF is formed at the LRS for the device, which is in agreement with that deduced from the I–V fitting. Therefore, we suppose that in the SET process, the Cu2+ ions located at the phthalocyanine ring were reduced to Cu atoms at the CuPPc/ITO interface. The process would continue until the entire filament is formed. When a negative voltage was applied to the top electrode in the RESET process, the Cu atoms were oxidized into Cu2+ ions, and the filament was ruptured.

Figure 5

HRTEM and EDS results of the Au/CuPPc/ITO memristor in the ON state. (a,b) HRTEM image of the ITO/CuPPc/Au device. (c) Elemental mapping images of In, Cu, and Au.

HRTEM and EDS results of the Au/n class="Chemical">CuPPc/ITO memristor in the ON state. (a,b) HRTEM image of the ITO/CuPPc/Au device. (c) Elemental mapping images of In, Cu, and Au.

Conclusions

In summary, we have synthesized CuPPc thin films through a CVD method and studied their composition and structure via TEM, XPS, and UV–vis, which confirms the successful formation of CuPPc with high polymerization degree and crystallinity. This film was used as the active layer for the memory device with sandwich structure Au/CuPPc/ITO, and the devices showed nonvolatile rewritable memory performance, with a large ON/OFF current ratio of 1 × 103 and a retention time of 1.2 × 103 s. Both the I–V analysis and HRTEM and EDX mapping of the cross section of a device in the ON state suggest the formation of conductive metal filaments.

Experimental Section

CVD Growth of CuPPc Films

CVD growth of CuPPc was carried out in a quartz tube reactor, placed in a two-zone oven. TCNB was grounded and pressed into tablets. Then, it was evaporated in the first zone at 110 °C with a flow of Ar (15 sccm), while a Cu foil was placed in the second one. Reaction temperature was 350 °C and the reaction time was 2.5 h. The temperatures in the two zones were controlled by thermocouples. Before the growth reaction, the Cu foil was annealed at 1050 °C in the atmosphere of Ar (200 sccm)/H2 (50 sccm) to remove organics and oxides on the surface. After reaching the reaction time, the sample was cooled to room temperature and then the Ar flow was shut off.

Memory Device Fabrication and Measurements

The memory device with sandwich structure ITO/n class="Chemical">CuPPc/Au was fabricated. The ITO substrate (10 mm × 10 mm) was precleaned sequentially with acetone, isopropyl alcohol, ethanol, and deionized water in an ultrasonic bath for 12 min. The PMMA/CuPPc films were transferred onto the ITO substrate, followed by the removal of PMMA in acetone overnight. The thickness of the active layer was about 106 nm. Finally, the Au top electrode (50 nm in thickness) was deposited onto the polymer film by E-beam evaporation using a shadow mask under ultrahigh vacuum. The performance of the device was characterized with a Keithley 4200 semiconductor parameter analyzer. All measurements were carried out at room temperature under ambient conditions.