Impact of Grain Sizes on Programmable Memory Characteristics in

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Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 20225−20231

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Impact of Grain Sizes on Programmable Memory Characteristics in Two-Dimensional Organic−Inorganic Hybrid Perovskite Memory Dongwoo Lee, Bohee Hwang, and Jang-Sik Lee* Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH) Pohang 37673, Korea

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S Supporting Information *

ABSTRACT: Recently, organic−inorganic hybrid perovskites (OIHPs) have been used in resistive switching memory applications because of current−voltage hysteresis that originated from ion migration in the perovskite film. As the density of the memory devices continues to increase, the size of the devices approaches that of the individual grains of the polycrystalline films. Thus, the effects of the grain boundary and the grain size will become important to investigate the influence on the switching behaviors. Here, we report the effects of grain sizes on the resistive switching property of (C4H9NH3)2PbBr4 (BA2PbBr4) films. The BA2PbBr4 films were formed by using sequential vapor deposition. First, a lead bromide (PbBr2) film was deposited by thermal evaporation, and then the film was exposed to organic vapor to form BA2PbBr4 films. The grain sizes were controlled by changing the transformation temperatures (TT = 100, 150, and 200 °C). When the TT values were 100, 150, and 200 °C, the grain sizes of BA2PbBr4 were ∼180 nm, ∼1, and ∼30 μm, respectively. In the memory device based on BA2PbBr4, the off current decreased from ∼10−4 to ∼10−8 A as the grain size increased from ∼180 nm to ∼30 μm. This method to synthesize BA2PbBr4 films provides a simple way to control the grain sizes, and understanding of the effects of grain sizes on memory characteristics will provide an insight to improve the reliability of the OIHP-based memory as the electronic devices are scaled down to the sizes of grains. KEYWORDS: 2D layered perovskite, resistive switching memory, vapor deposition, ion migration, grain size control

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applications to resistive switching memories (RSMs).19−28 RSMs have advantages of high density and high scalability,29−32 so these are promising candidates for next-generation memory applications. Most of the materials showed resistive switching (RS) in the polycrystalline structure with grains and GBs. As the size of the devices approached that of the individual grains of the polycrystalline films, the effects of grains and GBs on RSM properties would be important for fabrication of the nanoscale device. In oxide-based memories, annealed WO3 films have low-conductivity at the GB as a result of oxidation of oxygen vacancies.33 In RSM that was based on OIHP, the grain size that ranged from 60 to 600 nm affected the electrical properties,21 but the reason for the effect was not fully determined. In this study, we used sequential vapor deposition (SVD) to control the grain sizes by changing the transformation temperature (TT). Adjustments to TT yielded different grain sizes from the nanoscale to microscale. SVD facilitated the control of the deposition rates of organic and inorganic sources

INTRODUCTION Organic−inorganic hybrid perovskite (OIHP) materials have advantages such as a long electron−hole diffusion length, high energy conversion efficiency, and good light absorption characteristics.1−10 OIHP-based solar cells show photocurrent hysteresis behavior, possibly as a result of charge trapping and detrapping at the interface, ferroelectricity, or ion migration.11−14 The evidence of ion migration in the OIHP devices has been observed from the switchable photovoltaic effect in the perovskite layer with a lateral structure.15 As a reversible p−i−n structure was formed by ion migration in the perovskite layer, the photocurrent could be switched repeatedly with a small electric field. Moreover, films with numerous grain boundaries (GBs) could be easily polarized because of vacancy concentration. In addition, GBs could provide shortcuts for ion migration.14 The negatively charged (halide) vacancies moved faster at the GBs compared to the inside of the grain because of higher ionic diffusivity and higher segregation of ions at the GBs.14 As the grain size of OIHPs decreased, the number of GBs increased, so hysteresis also increased.16−18 These hysteretic phenomena have potential applications in memory. Hysteresis behavior that originated from the ion migration under an electric field could extend the OIHP material © 2019 American Chemical Society

Received: March 21, 2019 Accepted: May 10, 2019 Published: May 22, 2019 20225

DOI: 10.1021/acsami.9b05038 ACS Appl. Mater. Interfaces 2019, 11, 20225−20231

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic illustrations of the procedures to deposit the BA2PbBr4 film by thermal evaporation and vapor-assisted transformation. (b) Schematic illustration of the intercalation process of PbBr2 layer with the BABr vapor. (c) Schematic illustration of the BA2PbBr4 structure.

Figure 2. Time-evolution characterization of the perovskite films by exposing the PbBr2 film to BABr vapor at 150 °C in a N2 atmosphere. (a) XRD patterns of the PbBr2 film transformed for 0, 0.5, and 4 h. Top view SEM images of the (b) PbBr2 film at 0 h (inset: wider view), (c) PbBr2 that had been exposed to BABr vapor for 0.5 h (inset: magnified image), and (d) complete transformation to BA2PbBr4 from PbBr2 after exposure to BABr for 4 h.

of the perovskite films, so the effects of grain sizes on memory characteristics need to be clarified.

individually, so the organic and inorganic source could be deposited one material at a time.34 When the organic source is deposited on a pre-deposited inorganic material; the process is called vapor assisted transformation (VAT). Here, we suggest the use of SVD to synthesize the (C4H9NH3)2PbBr4 (BA2PbBr4) film. We controlled the grain sizes of BA2PbBr4 by controlling TT to 100, 150, and 200 °C. To investigate the effects of grain size on RSM based on BA2PbBr4, we compared the RS properties of the devices with different grain sizes. “Off” current (IOFF) increased as the grain size decreased, as a result of the increase in the number of GBs as the grain size decreased. The grain and GBs at the OIHP film may influence the optoelectronic and electronic properties

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RESULTS AND DISCUSSION The two-dimensional (2D) OIHP film was formed using SVD. Lead bromide (PbBr2) was deposited on an indium tin oxide (ITO)/glass substrate by thermal evaporation, then the predeposited PbBr2 film on the ITO/glass substrate was transferred to a glovebox for n-butylammonium bromide (BABr) deposition (Figure 1a). To sublime BABr, a Petri dish was heated to 100, 150, or 200 °C; the PbBr2 film was placed on the top of the glass Petri dish with the BABr powder spread around it. During exposure to BABr vapor, the PbBr2 film was 20226

DOI: 10.1021/acsami.9b05038 ACS Appl. Mater. Interfaces 2019, 11, 20225−20231

Research Article

ACS Applied Materials & Interfaces

Figure 3. Temperature-evolution characterization of the BA2PbBr4 film that had been transformed at different transformation temperatures. (a) XRD patterns of the BA2PbBr4 film that had been transformed at 100, 150, and 200 °C. (b) Average grain sizes of BA2PbBr4 films as a function of transformation temperatures. Top-view SEM images of the BA2PbBr4 film that had been transformed at (c) 100 °C (inset: magnified image), (d) 150 °C (inset: magnified image) and (e) 200 °C.

°C. The uniform layer of BA2PbBr4 films was demonstrated from cross-sectional SEM measurement (Figure S2d,e).To investigate the relation between TT and grain size, we synthesized BA2PbBr4 films at TT = 100, 150, and 200 °C. When the transformation temperature was 100, 150, and 200 °C, the PbBr2 film was exposed to the BABr vapor for 4.8, 4, and 3.5 h to completely transform to the BA2PbBr4 film. The XRD patterns of the three films showed complete transformation to BA2PbBr4 with the absence of PbBr2 peaks (Figure 3a). The grain size was clearly affected by TT . As the grain size increased, the intensities of XRD peaks increased. Also, in the previous study, a higher reaction temperature led to increased crystallinity.39 Our results show that a higher transformation temperature during the vapor deposition process increases the reaction rate, and this may increase the crystallinity of the BA2PbBr4 film. The average grain size was calculated using the ASTM intercept procedure.40 The details of the calculation procedure are in the Experimental Section. The average grain diameter was 180 ± 16 nm at TT = 100 °C, 1 ± 0.08 μm at TT = 150 °C, and 30 ± 2.5 μm at TT = 200 °C, respectively (Figure 3b). The film that was transformed at TT = 100 °C showed densely packed and pinhole-free surface coverage (Figure 3c). At TT = 150 and 200 °C, the BA2PbBr4 films showed microscale grain sizes (Figure 3d,e). The surface roughness of the BA2PbBr4 films at different transformation temperatures was measured by atomic force microscopy (AFM) (scan size = 1 μm × 1 μm) (Figure S3). The root mean square roughness values of BA2PbBr4 films at 100, 150, and 200 °C were 16.9, 22.6, and 24.2 nm, respectively. Three films showed a smooth and uniform film. As the grain size increased, the surface roughness of the films increased as shown in a previous study.41 The XRD patterns and SEM images of the BA2PbBr4 films that were synthesized at 80 °C were demonstrated to check the characteristic below 100 °C (Figure S4). Even after the PbBr2 film was exposed to BABr vapor at 80 °C for 10 h, the conversion from PbBr2 to BA2PbBr4 did not happen, as indicated by the existence of PbBr2 (Figure S4a). The top view SEM image of the PbBr2 film which was exposed to the BABr vapor at 80 °C for 10 h

transformed to BA2PbBr4 (Figure 1b), which was composed of the inorganic layers of [PbBr6]4‑ octahedral sandwiched between the C4H9NH3+ layers.35 During the transformation process, BABr molecules were intercalated into PbBr2 frames, and this phenomenon triggered spontaneous rearrangement34 that yielded a layered structure of an organic layer sandwiched between inorganic layers (Figure 1c). We checked X-ray diffraction (XRD) patterns and the scanning electron microscopy (SEM) images of BA2PbBr4 films that had been synthesized at τ (VAT time) = 0, 0.5, or 4 h at 150 °C (Figure 2). At τ = 0 h, the XRD pattern showed that the film was PbBr2.36 When the τ increased to 0.5 h, PbBr2 and BA2PbBr4 coexisted in the film by the existence of both PbBr2 (near 30°) and BA2PbBr4 (near 6.4°) (Figure 2a). At τ = 4 h, the PbBr2 film had been completely transformed to BA2PbBr4, as confirmed by the absence of PbBr2 peaks. The XRD patterns of the BA2PbBr4 film exhibited (00l) peaks such as (002), (004), and (006) planes.37 The top view of the BA2PbBr4 film that had been synthesized at 0 h exhibited densely packed polygons with pinhole-free surface coverage (Figure 2b). At τ = 0.5 h, the relatively bright grains were PbBr2 that had not interacted with the BABr vapor (Figure 2c).38 The dark grains were BA2PbBr4 that had been transformed by VAT.34 When τ increased to 4 h, the grain sizes of the BA2PbBr4 film increased to the microscale (∼1 μm) (Figure 2d). The cross-sectional SEM image of the BA2PbBr4 films that had been synthesized at 150 °C for different times was demonstrated (Figure S1). We also checked the XRD patterns and SEM images of BA2PbBr4 films that had been synthesized at τ = 3 and 5 h at 150 °C (Figure S2). When the PbBr2 film was exposed to BABr vapor at 150 °C for 3 h, conversion was not completed. Exposing the PbBr2 film for 5 h transformed it to BA2PbBr4 that was confirmed by the XRD patterns (Figure S2a). The grain size started to increase after 3 h, and the grain size of the BA2PbBr4 film that was exposed to BABr vapor for 5 h slightly increased to ∼1.5 μm (Figure S2b,c). Increasing the time at 150 °C led to slightly increased grain size, but the grain size did not increase dramatically like the film that was synthesized at 200 20227

DOI: 10.1021/acsami.9b05038 ACS Appl. Mater. Interfaces 2019, 11, 20225−20231

Research Article

ACS Applied Materials & Interfaces

Figure 4. Electrical properties of Au/BA2PbBr4/ITO devices that had been transformed at (a) 100 °C (inset: schematic structure of RSM device), (b) 150, and (c) 200 °C. Data retention characteristics of the Au/BA2PbBr4/ITO structure that had been transformed at (d) 100, (e) 150, and (f) 200 °C.

of 1 mA was applied to prevent the breakdown of devices. All devices showed bipolar RS behavior. Near FE = 7.5 × 104 V/ cm, the resistances of the devices were changed from the highresistance state (HRS) to the low-resistance state (LRS); this process was defined as the “set” process. However, as the grain size increased, IOFF decreased (Figure 4a−c). As TT increased from 100 to 200 °C, IOFF decreased from ∼10−4 to ∼10−8 A at 0.2 V. A constant on/off current ratio ION/IOFF was achieved up to 1000 s from the data retention measurement (Figure 4d−f). The cycling endurance of Au/BA2PbBr4/ITO devices that had been transformed at different temperatures (100, 150, and 200 °C) was measured using consecutive ac voltage pulses to confirm the electrical stability under Vset = +4 V and Vreset = −4 V (Figure S6). The width of the voltage pulse was 10 ms and the read voltage was 0.2 V. The endurance properties of the devices varied slightly overtime, but LRS and HRS states remained without degradation. The statistical distribution of HRS and LRS levels of the BA2PbBr4 film that had been transformed at 100, 150, and 200 °C was shown (Figure S7). All devices demonstrated a small range of the resistance level distribution in HRS and LRS states. As the temperature increased from 100 to 200 °C, the HRS state decreased, and this led to a larger on/off ratio. Also, increasing the TT, increased the on/off ratio from 5 to 2400 (Figure S7). OIHPs have intrinsic defects such as vacancies and shallow point defects including interstitials. Halide vacancies are considered to be highly mobile because of low activation energy.48 Therefore, in previous works on OIHP based memories, the RS mechanism was attributed to the formation and rupture of conducting filaments (CFs) as a result of migration of halide vacancies.28 Moreover, the GBs have more open space and lack chemical bonds, so the activation energy of ion migration may be lower around the GB than that of grain interior.49 Because of these reasons, we suggest that halide vacancy assisted CFs would dominantly be formed at the GBs in the BA2PbBr4 film.14 In HRS, Pb2+ and Br− may randomly be distributed in grains and GBs. After the set process was conducted, Pb2+ was randomly distributed in the grains, but Br− was accumulated near the GBs. The CFs that

showed densely-packed polygons (Figure S4b) that were similar to Figure 2b, and a uniform layer was confirmed from the cross-sectional SEM image (Figure S4c). It was reported that perovskite crystallization did not occur below 100 °C.42,43 In the case of previous reports related to vapor deposited 2d layered perovskites, the 2d layered perovskites were formed above 100 °C.44,45 Through the results, it is thought to be difficult to form a BA2PbBr4 film below 100 °C. In previous studies, to increase photovoltaic efficiency, the grain size of OIHP materials was engineered by controlling factors such as CH3NH3I concentration or temperature. In the case of modulating the temperature, the film which was formed at high temperatures showed larger critical free energy than that of the lower temperature.46 This led to larger growth of grains due to a decreased number of nuclei. This effect was also observed in our study; the film which was formed at 200 °C exhibited larger grains than the other films with lower temperatures (100 and 150 °C) because of the decreased number of nuclei that grew into BA2PbBr4. The grain size of the perovskite could be controlled by nucleation and growth rates, and these factors were affected by the temperature.47 At TT = 100 °C, the nucleation rate of the perovskite crystal was faster than the growth rate of the grain, so the grain size was of a nanometer size. As TT increased, the growth rate became faster than the nucleation rate, so the grain size was of the micrometer size. Therefore, the grain size of the BA2PbBr4 film was dependent on TT that was an important parameter to control the grain size. To quantify how the grain size affects the RS properties of the BA2PbBr4 film, we fabricated a device with a Au/ BA2PbBr4/ITO structure that had been deposited at TT = 100, 150, and 200 °C. The three BA2PbBr4 films with different TT had slightly different thicknesses (Figure S5), so in this work, the electric field (FE = V/t) was used instead of an applied bias (V) to compensate for this difference. I−FE curves of the Au/BA2PbBr4/ITO device were measured with bias voltages which were applied to the Au top electrodes in the sequence of 0 V → 3 V → 0 V → −3 V → 0 V, while the bottom electrode (ITO) was grounded. A compliance current 20228

DOI: 10.1021/acsami.9b05038 ACS Appl. Mater. Interfaces 2019, 11, 20225−20231

Research Article

ACS Applied Materials & Interfaces were composed of Br− vacancies might be formed at GBs. The reason for different IOFF values in the device could be explained by the number of CFs or the number of remaining filaments after the rupture occurred. Assuming that the CFs that were assisted by GBs were formed between the top and bottom electrodes, numerous CFs would be formed at low TT = 100 °C, at which the grain size was small. However, the number of CFs would decrease at a high TT = 200 °C, at which the grain size was large because an increase in the grain size resulted in a decreased number of GBs, and this trend might lead to low IOFF. When the reset process happened, the number of remaining filaments between the electrodes would increase as the grain size decreased, so IOFF would increase as the grain size decreased. A decrease in the density of ruptured CFs at GB in a device could result in lower IOFF at a larger grain size.

Calculation of Grain Sizes.40 The average grain size was calculated by the ASTM E112 intercept method. The grain size can be calculated by the following equation G = 6.643856 log10 PL − 3.288 where G is the number of grain size and PL is the number of GB intersections per unit length of the test line. The PL is determined by the following equation PL =

where Pi is the total number of intercepts of all test lines, L is the total length of test lines, and M is the magnification. A precision of better than ±0.25 grain size units is expected.

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ASSOCIATED CONTENT

S Supporting Information *

CONCLUSIONS We reported the control of the grain sizes in BA2PbBr4 films by changing the TT of the VAT. The PbBr2 film, which was deposited by thermal evaporation, was transformed to BA2PbBr4 by exposing it to the organic vapor; this process induced complete conversion to the BA2PbBr4 film. As TT increased, the average grain size increased as ∼180 nm at 100 °C, ∼1 μm at 150 °C, and ∼30 μm at 200 °C. RSM devices based on BA2PbBr4 that were deposited at TT = 100, 150, and 200 °C showed bipolar RS behavior. Also, IOFF decreased as the grain size increased. The proposed method to synthesize the BA2PbBr4 film with different grain sizes could extend the applications of 2D OIHP materials to advanced memory applications.

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Pi L /M

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05038.

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Cross-sectional SEM images of the synthesized films with different transformation temperatures, endurance of the fabricated devices with different transformation temperatures, AFM images of the films synthesized at different transformation temperatures, and the statistical distribution (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jang-Sik Lee: 0000-0002-1096-1783 EXPERIMENTAL SECTION

Author Contributions

D.L. and B.H. contributed equally to this work. J.-S.L. conceived and directed the research. J.-S.L., D.L., and B.H. designed and planned the experiment. D.L. and B.H. performed the experiment and acquired the data. B.H. and J.-S.L. revised the manuscript. D.L., B.H., and J.-S.L. wrote the manuscript.

Materials. BABr and PbBr2 powders were purchased from Greatcell solar and Sigma-Aldrich, respectively. We used the powder without further purification. Deposition of OIHP Films and Device Fabrication. ITOcoated glass was cleaned by sonication in deionized water then acetone, sequentially for 10 min, respectively. Then, the substrates were held at 200 °C in isopropanol for 20 min. The PbBr2 powder was sublimed by thermal evaporator to deposit the PbBr2 film (∼80 nm) on the prepared substrates. The vacuum pressure was ∼6 × 10−6 Torr during thermal evaporation. Then, the PbBr2 film which was deposited on the ITO-coated glass substrate was transferred to the top of a glass Petri dish. The film was treated using the BABr vapor at 150 °C for 4 h in a closed Petri dish with the BABr powder (3.6 mg) that was spread over the Petri dish. After this treatment, the final BA2PbBr4 film was formed. To completely transform into the BA2PbBr4 film, the conversion process was conducted on a hotplate at 100, 150, and 200 °C for 4.8, 4, and 3.5 h, respectively. These processes were conducted under a N2 atmosphere. Gold top electrodes (50 nm) were deposited on the transformed BA2PbBr4 perovskite by thermal evaporation through a shadow mask with a diameter of 100 μm (dot shape). Characterization. Images of the surface and cross section were obtained using a high-resolution field emission scanning electron microscope (JSM-7800F Prime, JEOL) at 5 kV acceleration voltage. The crystal structure was measured using XRD (D/MAX-2500, Rigaku) with Cu Kα radiation. The surface roughness of the films was measured using an AFM (VEECO). The current response of the voltage bias was measured using a semiconductor parameter analyzer (4200-SCS, Keithley). All electrical properties were measured in a probe station under vacuum (∼10−2 Torr). The DC voltage bias was applied to the Au electrode as 0 V → 3 V → 0 V→ −3 V → 0 V; the bottom electrode (ITO) was grounded.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (2016M3D1A1027663, 2018R1D1A1B07043368). In addition, this work was partially supported by the Brain Korea 21 PLUS project (Center for Creative Industrial Materials).

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DOI: 10.1021/acsami.9b05038 ACS Appl. Mater. Interfaces 2019, 11, 20225−20231