High-Performance Flexible Organic Nano-Floating Gate Memory Devices Functionalized with Semiconducting Nanoparticles

High-Performance Flexible Organic Nano-Floating Gate Memory Devices Functionalized with Semiconducting Nanoparticles. Ji Hyung Jung. Department of Energy Engineering Graduate School of UNIST.

High-Performance Flexible Organic Nano-Floating Gate Memory Devices Functionalized with Semiconducting Nanoparticles. A thesis Submitted to Department of Energy Engineering of UNIST in partial fulfillment of the requirements for the degree of Master of Science. Ji Hyung Jung. 12. 19. 2014 Approved by ___________________________ Advisor Byeong-Su Kim.

High-Performance Flexible Organic Nano-Floating Gate Memory Devices Functionalized with Semiconducting Nanoparticles. Ji Hyung Jung. This certifies that the thesis of Ji Hyung Jung is approved.. 12. 19. 2014. Signature. __________________________ Byeong-Su Kim Signature. __________________________ Joon Hak Oh Signature. __________________________ Hyunhyub Ko.

Abstract. High-Performance Flexible Organic Nano-Floating Gate Memory Devices Functionalized with Semiconducting Nanoparticles, 2015, Ji Hyung Jung, Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST). Transistor-type nano-floating gate memory (NFGM) devices have recently attracted great interest because of their unique and compatible characteristics as suitable platforms for integrated circuits. Their excellent memory properties with inexpensive fabrication processes make NFGM devices highly promising for the next-generation data storage devices. Herein, novel nonvolatile NFGM devices utilizing semiconducting cobalt ferrite (CoFe 2O4) nanoparticles (NPs) as the charge trap sites with p-type semiconductor, pentacene as active layer on flexible and transparent polymer substrates as well as on conventional silicon wafers have been prepared. Monodisperse CoFe2O4 NPs, which were synthesized in solution from cheap and nontoxic metal-oleate complex precursor, provide facile and fast deposition on the target substrates via simple spin-casting technique. The newly developed NFGM devices exhibit superb mechanical/electrical stability against pure bending and repeated program/erase (P/E) operations without additional tunneling dielectric layer which enhances data retention capacity and helps the charge carriers to be trapped in the NPs. Furthermore, size effect of CoFe2O4 NPs (5, 8, and 11 nm) on electrical memory performance in NFGM devices was investigated.. Keywords: Nano-floating gate memory, cobalt ferrite, nanoparticle, pentacene, organic memory.

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Contents. I. Introduction ------------------------------------------------------------------------------------------------------ 1. 1.1 Organic nano-floating gate memory (NFGM) devices --------------------------------------------- 1. 1.2 Cobalt ferrite (CoFe2O4) nanoparticles (NPs) -------------------------------------------------------- 4. II. Experiments ----------------------------------------------------------------------------------------------------- 7. 2.1 Synthesis of CoFe2O4 NPs ------------------------------------------------------------------------------ 7 2.2 Characterization of CoFe2O4 NPs ---------------------------------------------------------------------- 8 2.3 Fabrication of CoFe2O4 NFGM devices ------------------------------------------------------------- 15. III. Results & discussion ----------------------------------------------------------------------------------------- 19. 3.1 Analysis of CoFe2O4 NFGM devices ---------------------------------------------------------------- 19 3.2 Electrical memory performance of CoFe2O4 NFGM devices ------------------------------------ 22 3.3 Electrical memory performance and mechanical stability test of flexible CoFe2O4 NFGM devices -------------------------------------------------------------------------------------------------------- 33. IV. Conclusion ---------------------------------------------------------------------------------------------------- 36. V. Reference ------------------------------------------------------------------------------------------------------ 37. Acknowledgement ------------------------------------------------------------------------------------------------ 40.

List of figures Figure 1-1. Schematic illustrations and device operation schemes of transistor and floating gate flash memory devices ---------------------------------------------------------------------------------------------------- 3. Figure 1-2. Various applications of magnetic NPs ----------------------------------------------------------- 5. Figure 1-3. Transmission electron microscopy (TEM) images and size distribution of size- and shapecontrollable CoFe2O4 NPs ---------------------------------------------------------------------------------------- 6. Figure 2-1. TEM images of synthesized CoFe2O4 NPs ------------------------------------------------------ 9 Figure 2-2. Size distribution of CoFe2O4 NPs --------------------------------------------------------------- 10 Figure 2-3. Cyclic voltammograms of CoFe2O4 NPs ------------------------------------------------------- 11. Figure 2-4. Plot of transformed Kubelka-Munk function versus the energy of the light absorption from the UV-vis absorption spectra of CoFe2O4 NPs -------------------------------------------------------- 12. Figure 2-5. Energy band diagram of CoFe2O4 NPs --------------------------------------------------------- 13. Figure 2-6. Contact angle measurement of D. I. water on the Si wafer ---------------------------------- 16. Figure 2-7. Schematic configuration of the NFGM devices based on CoFe2O4 NPs ------------------- 17. Figure 2-8. Schematic illustration and photograph of the flexible CoFe2O4 NFGM devices ---------- 18. Figure 3-1. Atomic force microscopy (AFM) phase images of spin-coated CoFe2O4 NPs on the Si wafer --------------------------------------------------------------------------------------------------------------- 20. Figure 3-2. Cross-sectional scanning transmission electron microscopy (STEM) images of 8 nm CoFe2O4 NP-embedded NFGM devices ---------------------------------------------------------------------- 21. Figure 3-3. Transfer curves of the NFGM devices based on CoFe2O4 NPs ------------------------------ 25.

Figure 3-4. Electrical memory characteristics of NFGM devices based on 8 nm CoFe2O4 NPs ------ 27. Figure 3-5. Memory characteristics of the NFGM devices based on 8 nm CoFe2O4 NPs depending on the program/erase (P/E) operation bias voltage -------------------------------------------------------------- 28. Figure 3-6. Schematic energy band diagrams for the charge trap/release mechanism description --- 30. Figure 3-7. Electrical memory performance of flexible NFGM devices based on 8 nm CoFe2O4 NPs ----------------------------------------------------------------------------------------------------------------------- 34. Figure 3-8. Mechanical stability test of flexible NFGM devices based on 8 nm CoFe2O4 NPs ------- 35.

List of tables Table 2-1. Electrochemical and photochemical data of CoFe2O4 NPs ------------------------------------ 14 Table 3-1. Electrical memory characteristics of the NFGM devices with/without CoFe2O4 NPs ----- 26. Table 3-2. Memory window and read current on/off ratio (read Ion/Ioff) of the NFGM devices based on 8 nm CoFe2O4 NPs depending on the program/erase (P/E) operation bias voltage --------------------- 29 Table 3-3. Comparison of electrical memory performance between with/without additional tunneling dielectric layer in NFGM devices ------------------------------------------------------------------------------ 31. Table 3-4. Comparison of data retention test between with/without additional tunneling dielectric layer in NFGM devices ------------------------------------------------------------------------------------------ 32.

I. Introduction. 1.1 Organic nano-floating gate memory (NFGM) devices. As inorganic semiconductor-based electronic devices are processed on rigid and limited types of substrates via expensive fabrication processes requiring high temperature and vacuum condition, researches for organic semiconductor-based electronic devices have been intensively conducted over the last decades for achieving not only low-cost, facile fabrication processes but also commercialization for flexible and large-area electronics. To truly realize flexible electronics, all the components of electronic devices including transistors,1 batteries,2 and displays3 should be intrinsically flexible. Memory devices are also one of most important elements for flexible electronic devices and have been extensively developed so far. Among various kinds of memory devices, transistor-type flash memory devices have been widely studied because of their suitability as platforms for integrated circuits, and superb electrical performances. They can be fabricated by embedding floating gates and/or tunneling dielectric layer between channel and gate dielectric layer in transistor structure. In flash memory devices, electrically bistable behavior can be observed showing different conductivity at the same gate-source bias voltage (VGS = 0 V) depending on the previous memory operation.4 Charge carriers in active layer can be induced and trapped in floating gates during program operation and released during erase operation resulting in threshold voltage shift, also denoted as memory window or hysteresis, via conductance change of the channel as shown in Figure 1-1.5 Among many kinds of flash memory devices, nanoparticle (NP)-embedded nano-floating gate memory (NFGM) devices have attracted tremendous attention as promising next-generation memory devices because they have the merit of facile control of electrical properties of NPs such as energy level by modulating their sizes and shapes. To date, most of studies on organic NFGM devices have been investigated utilizing metallic NPs. 4, 6 Despite their high cost, Au NPs have been extensively exploited because of their high work function and chemical stability, and thermal evaporation has been widely used for the deposition of the NPs. This method, however, requires high-vacuum condition and not a useful method for controlling the size, shape and the density of NPs because of Ostwald ripening, where small Au NPs tend to dissolve and redeposit onto larger NPs during the evaporation.7 On the other hand, semiconducting or metal oxide NPs have been far less utilized for memory devices despite their lower cost compared with metallic NPs, presumably because of comparatively poor electrical memory performance. 8 In this study, the electrical memory characteristics of novel organic NFGM devices based on 1.

semiconducting CoFe2O4 NPs were investigated showing comparable electrical memory characteristics to the NFGM devices based on metallic NPs. In addition, the effect of size of CoFe2O4 on the NFGM devices has been studied thoroughly.. 2.

(a). (b). Figure 1-1. Schematic illustrations and device operation schemes of a) transistor and b) floating gate flash memory device. (Han, S.-T. et al., Adv. Mater. 2013, 25, 5425-5449)5. 3.

1.2 Cobalt ferrite (CoFe2O4) nanoparticles (NPs) Magnetic NPs have been investigated intensively in recent years because of their electrical, optical, and magnetic property changes depending on their sizes, shapes, agglomerations.9 CoFe2O4 NPs are one of the well-known hard magnetic materials having high coercivity, magnetocrystalline anisotropy, and moderate saturation magnetization as well as outstanding physical/chemical stability.10 Owing to their unique properties of the inverse spinel ferrites of ternary composition, CoFe2O4 NPs have various application fields including sensors, 11 data storage,12 and bio-applications such as drugdelivery systems for biological labeling or magnetic hyperthermia. 13 (Figure 1-2) Accordingly, various size- and shape-controllable synthesis methods of monodisperse NPs have been developed. CoFe2O4 NPs can be prepared via sol-gel,14 Langmuir-Blodgett,15 coprecipitation,16 hydrothermal,17 bacterial18 and micellar synthesis,10 combustion,19 aerosol vapor,20 mechanical grinding,21 or high-temperature decomposition of organic precursors.22 In non-hydrolytic reaction method, size and shape of CoFe2O4 NPs can be controlled by adjusting the ratio between the quantity of nano-crystalline seeds and precursors in the solution and regulating NP growth rate.23 (Figure 1-3) Herein, CoFe2O4 NPs were synthesized via modified thermal decomposition method as previously reported.24 Using this method, the size of CoFe2O4 NPs can be controlled simply by varying Ar bubbling rate in the reacting solution. Furthermore, monodisperse NPs can be synthesized through complete separation of nucleation and growth process using Ar bubbles, which absorb the heat generated from exothermic multiple-bonds formation reactions in the nucleation step.25. 4.

Figure 1-2. Various applications of magnetic nanoparticles. (Jun, Y.-W. et al., Chem. Commun., 2007, 1203-1214)26. 5.

Figure 1-3. Size and shape control of CoFe2O4 NPs. TEM images of spherical CoFe2O4 nanoparticles of (a) 5.2 ± 1.1, (b) 7.9 ± 0.5, and (e) 11.8 ± 1.3 nm and cubic nanoparticles of (c) 9.1 ± 0.5 and (d) 10.9 ± 0.6 nm with scale bar as 50 nm. (f) Size distribution of cubic nanoparticles in figure (d). The inset of figure (f) shows the aspect ratio of cubic nanoparticles in figure (c) and (d). (Song, Q. et al., J. Am. Chem. Soc. 2004, 126, 6164-6168)23. 6.

II. Experiments. 2.1 Synthesis of CoFe2O4 NPs In this study, CoFe2O4 NPs in three different sizes (5, 8, and 11 nm in diameter) have been synthesized from low-cost and non-toxic metal-oleate complex precursor modifying the experimental method as reported in previous study. 24 (Co2+Fe23+)-oleate was used as precursor to synthesize CoFe2O4 NPs. To synthesize precursor, iron(Ⅲ) chloride hexahydrate (FeCl3∙6H2O, 32 mmol), cobalt(Ⅱ) chloride hexahydrate (CoCl2∙6H2O, 16 mmol), and sodium oleate (128 mmol) were dissolved in a mixture of 80 ml of ethanol, 80 ml of D. I. water, and 160 ml of n-hexane. Mixed solution was stirred until all reagents were dissolved thoroughly. The reacted (Co2+Fe23+)-oleate was washed with 120 ml of D. I. water for 3 times and residual solvent was evaporated in the rotary evaporator at 80 ˚C. Then, (Co2+Fe23+)-oleate complex (2.5 g), oleic acid (OLA, 0.25 g), and octadecene (10 ml) were mixed and evacuated at 80 ˚C for 1 h. Then, the mixture was heated to 310 ˚C at a heating rate of 1 ˚C min-1 under Ar bubbling and stirred for 1 h. In this step, Ar bubbling rate into reacting solution was adjusted to control the size of CoFe2O4 NPs because Ar bubbles in solution can absorb the local latent heat generated during nucleation step, leading to continuous primary nucleation process for monodisperse and smaller NPs in low temperature. For the synthesis of 5 nm CoFe2O4 NPs, Ar was bubbled vigorously in the reacting solution. For 11 nm ones, on the other hand, the bubbling was stopped before the reacting solution was heated. After growth of the NPs, the dispersion was cooled down to room temperature and rinsed with acetone/ethanol mixture three times. As-prepared CoFe2O4 NPs were finally dispersed in n-hexane for long-term storage.. 7.

2.2 Characterization of CoFe2O4 NPs Physical and electrical properties of monodisperse CoFe2O4 NPs in three different sizes (5, 8, and 11 nm in diameter) have been chracterized using transmission electron microscopy (TEM), cyclic voltammetry (CV), and UV-vis absorption spectroscopy.. TEM images of monodispersely synthesized CoFe2O4 NPs were shown in Figure 2-1. The size estimation of CoFe2O4 NPs showed the values of 5.59 ± 0.65 (size variation ca.10 %), 8.05 ± 0.57 (size variation ca. 7 %), and 11.30 ± 0.76 nm (size variation ca. 5 %) for the 5, 8, and 11 nm NPs, respectively. (Figure 2-2) The particle size distribution of NPs was estimated from 50 random samples in TEM images. The energy level of valence band of CoFe2O4 NPs can be estimated by cyclic voltammograms as shown in Figure 2-3. The electrolyte was 0.1 M of tetabutylammonium hexafluorophosphate (Bu4NPF6) in anhydrous acetonitrile and cyclic votammograms were measured at a scan rate of 100 mV s-1 at room temperature under N2 gas blowing. Indium tin oxide (ITO) glass deposited by each CoFe2O4 NP dispersion was used as a working electrode. Platinum (Pt) wire and Ag/Ag+ electrode containing 0.01 M of AgNO 3 and 0.1 M of tetrabutyl ammonium perchlorate (TBAP) in acetonitrile were served as a counter and a reference electrode, respectively. The Ag/Ag+ reference electrode was internally calibrated by ferrocene/ferrocenium couple (Fc/Fc+) and the valence band energy level of NPs can be estimated using equation, 𝑜𝑛𝑠𝑒𝑡 𝑜𝑛𝑠𝑒𝑡 𝐸𝑣𝑎𝑙𝑒𝑛𝑐𝑒 𝑏𝑎𝑛𝑑 (𝑒𝑉) = −[𝐸(𝑜𝑥) − 𝐸(𝑓𝑒𝑟𝑟𝑜𝑐𝑒𝑛𝑒) + 4.8]. (1). Energy bandgap, the energy level difference between conduction band and valence band, can be measured using a plot of the modified Kubelka-Munk function versus the energy of exciting light derived from UV-vis spectra.27 (Figure 2-4) The spectra were measured for NP dispersion and spincoated NPs on the quartz plate.. The electromchemical/photochemical data of CoFe2O4 NPs (Figure 2-3, 2-4, and Table 2-1) showed that the energy level of valence band increases gradually from -6.56 to -6.52 eV as the size of NPs increases, and that of conduction band and energy bandgap decrease from -3.80 to -3.94 eV and 2.76 to 2.58 eV, respectively. (Figure 2-5). 8.

(a). (b). (c). (d). (e). (f). Figure 2-1. TEM images of (a, b) 5, (c, d) 8, and (e, f) 11 nm CoFe2O4 NPs. 9.

(b). 35. 30. 30. 25. Percentage (%). Percentage (%). (a). 25 20 15 10 5 0 4.0 4.5 5.0 5.5 6.0 6.5 7.0. 20 15 10 5 0 7.0. Diameter (nm). 7.5. 8.0. 8.5. 9.0. 9.5. Diameter (nm). (c). Percentage (%). 50 40 30 20 10 0. 9. 10. 11. 12. 13. 14. Diameter (nm). Figure 2-2. The size distribution of CoFe2O4 NPs in (a) 5.59 ± 0.65 (size variation ca. 10 %), (b) 8.05 ± 0.57 (size variation ca. 7 %), and (c) 11.30 ± 0.76 nm (size variation ca. 5 %).. 10.

(a). (b) onset. onset. Eox. = 1.82 V. = 1.80 V. Current (a.u.). Current (a.u.). Eox. 0 (c). 1. 2. Potential (V vs Fc/Fc onset. 0. 3. 1. 0. = 1.78 V. 1. 2. Potential (V vs Fc/Fc ) +. 2. Potential (V vs Fc/Fc. ). Current (a.u.). Eox. +. 3. Figure 2-3. Cyclic voltammograms of (a) 5, (b) 8, and (c) 11 nm CoFe2O4 NPs.. 11. +. ). 3.

(a). (d). Eg = 2.76 eV. [F(R)*E]. [F(R)*E]. 1/2. 1/2. Eg = 2.79 eV. 2. 3. 4. 2. 5. 4. 5. Energy / eV. Energy / eV (b). 3. (e). Eg = 2.64 eV. [F(R)*E]. [F(R)*E]. 1/2. 1/2. Eg = 2.77 eV. 2. 3. 4. Energy / eV. 2. 5. (c). 3. 4. 5. 4. 5. Energy / eV. (f). Eg = 2.58 eV. [F(R)*E]. [F(R)*E]. 1/2. 1/2. Eg = 2.76 eV. 2. 3. 4. Energy / eV. 2. 5. 3. Energy / eV. Figure 2-4. Plots of modified Kubelka-Munk function versus the energy of the light absorption in the UV-vis absorption spectra of (a ~ c) NP dispersion and (d ~ f) spin-coated NPs on the quartz plate: (a, d) 5, (b, e) 8, and (c, f) 11 nm NPs.. 12.

Figure 2-5. Energy band diagrams of CoFe2O4 NPs in three different sizes.. 13.

Table 2-1. Electrochemical and photochemical data of CoFe2O4 NPs. Size of NPs. Eoxonset (V)[a]. Valence. Conduction. band. band. (eV)[a]. (eV)[b]. Eg, NPs. Eg, dispersion. (eV)[c]. (eV)[c]. 5 nm. 1.82. -6.56. -3.80. 2.76. 2.79. 8 nm. 1.80. -6.54. -3.90. 2.64. 2.77. 11 nm. 1.78. -6.52. -3.94. 2.58. 2.76. [a]. Deduced from the onset oxidation potentials in the cyclic voltammograms.. [b]. Calculated from Econduction (eV) = Evalence + Eg,NPs. [c]. Extracted from UV-vis spectra using Kubelka-Munk function.. 14.

2.3 Fabrication of NFGM devices. n-octadecyltrimethoxysilane (OTS)-treated Si wafer was prepared for the fabrication of NFGM devices based on CoFe2O4 NPs.28 The solution of 3 mM OTS dissolved in trichloroethylene (TCE) was spin-coated on the highly n-doped (100) Si wafer (< 0.005 Ω cm) with thermally grown SiO2 100 nm (Ci = 32.8 nF cm-2). The Si wafer was pre-cleaned in piranha solution (mixture of H2SO4 : H2O2 = 70 : 30 by volume ratio) before washing with D. I. water and UV/ozone plasma treatment. Spincoated Si wafer was exposed to ammonium hydroxide (NH4OH) vapor overnight in the desiccator for the smooth formation of OTS self-assembled monolayer (SAM) on the surface of Si wafer. Subsequently, the wafer was ultra-sonicated in toluene and rinsed with toluene, acetone, and isopropyl alcohol (IPA) and dried with N2 gas. The contact angle of D. I. water on the surface of Si substrate before and after the SAM treatment was ca. 28 and 107 ˚, respectively. (Figure 2-6) The schematic configuration of the NFGM device based on CoFe2O4 NPs is shown in Figure 2-7. The spin-casting technique was applied for the facile, fast deposition of CoFe 2O4 NPs in the NFGM devices. After spin-casting of 2 mg ml-1 of CoFe2O4 NP dispersion at 1000 rpm, the Si wafer was annealed at 60 ˚C in the vacuum oven for the thorough solvent evaporation. P-type semiconductor, pentacene was thermally deposited on the surface of NPs at 0.1 ~ 0.3 Å s-1 at 60 ˚C (substrate temperature) and 40 nm-thick gold source/drain electrodes were also formed via thermal evaporation using shadow masks with 50 μm of channel length (L) and 1000 μm of channel width (W). To confirm the effect of additional tunneling dielectric layer in NFGM devices, 10 nmthick Al2O3 thin-film was deposited between pentacene and NPs utilizing atomic layer deposition (ALD). For the flexible NFGM devices, transparent and flexible polyethylene terephthalate (PET) film was prepared as a substrate. Cr (5 nm) and Au (100 nm) was thermally deposited on PET film and used as adhesion layer and gate electrode, respectively. 100 nm-thick Al2O3 blocking dielectric layer (Ci = 4.0 nF cm-2) was formed by radio frequency (RF) magnetron sputtering technique. The other processes are the same as those in the fabrication on the Si wafer as described above. The schematic illustration of the structure and photograph of flexible NFGM device based on CoFe2O4 NPs is shown in Figure 2-8.. 15.

(a). (b). Figure 2-6. Contact angle of D. I. water (a) before and (b) after OTS treatment on the Si wafer.. 16.

Figure 2-7. Schematic configuration of the NFGM device based on CoFe2O4 NPs.. 17.

(a). (b). Figure 2-8. (a) Schematic image and (b) photograph of the flexible CoFe2O4 NFGM device.. 18.

III. Results & Discussion. 3.1 Analysis of CoFe2O4 NFGM devices The surface images of CoFe2O4 NPs deposited on Si wafer were obtained from atomic force microscopy (AFM) phase images as shown in Figure 3-1. Uniformly coated NPs were observed in AFM images of 5 and 8 nm CoFe2O4 NPs, while for 11 nm NPs, closely packed aggregated NPs were observed. This agglomeration phenomena of 11 nm CoFe2O4 NPs may be attributed to their narrow particle size distribution (standard deviation ca. 5 %), uniform shape, and van der Waals interaction among NPs.29 Large NPs tend to be clustered together because large NPs usually have the stronger attraction force among NPs than that of small NPs30 and it was experimentally verified that van der Waals force increases as the size of CoFe2O4 NP increases because of their permanent dipole moment in spinel structure.31 The agglomeration of 11 nm CoFe2O4 NPs was also confirmed in their TEM images (Figures 2-1(e) and (f)). Figure 3-2 shows cross-sectional bright-field (BF) and high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images of 8 nm CoFe2O4 NPs-embedded NFGM devices showing that NPs can be deposited uniformly in monolayer on target substrates via facile and simple spin-casting technique. The sample for STEM analysis was prepared using dual-beam focused ion beam (FIB, Helios 450 HP, FEI, USA) on the copper grid and analyzed with high resolution-transmission electron microscope (HR-TEM, Cs-corrected JEM-2100F, JEOL, Japan).. 19.

(a). (b). (c). Figure 3-1. AFM phase images of spin-coated (a) 5, (b) 8, and (c) 11 nm CoFe2O4 NPs on the OTStreated Si wafer.. 20.

(a). (b). (c). (d). Figure 3-2. Cross-sectional (a, b) bright-field (BF) and (c, d) high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images of NFGM devices based on 8 nm CoFe2O4 NPs.. 21.

3.2 Electrical memory performances of the NFGM devices based on CoFe2O4 NPs A Keithley 4200 semiconductor parametric analyzer was used to measure all the electrical performance of NFGM devices based on CoFe2O4 NPs in an N2-filled glove box. Figure 3-3 shows transfer curves of the NFGM devices based on 5, 8, and 11 nm CoFe2O4 NPs during dual gate-source voltage sweep and their electrical memory characteristics were summarized in Table 3-1. All the transfer curves showed typical counterclockwise hysteresis loop of p-type NFGM devices resulting from increased memory window of NFGM devices by charge trapping/releasing of NPs during dual gate-source voltage sweep as described in Chapter 1.1. Memory window of 73.84 ± 6.34 V for NFGM devices based on 8 nm CoFe2O4 NPs was slightly larger than that of 68.27 ± 2.77 V for 5 nm NP-based ones. Meanwhile, NFGM devices based on 11 nm CoFe2O4 NPs showed smaller memory window of 62.51 ± 7.16 V. On the basis of energy level differences among CoFe2O4 NPs in 3 different sizes (Figure 2-5), it is expected that charge trap capacity of NPs would be improved as their sizes increase because of their lower energy level of conduction band. However, this experimental result showed that charge carrier trap capacity of 11 nm CoFe2O4 NPs was decreased even though their energy levels are in equivalent or even in better condition for the charge carrier transport than those of 5 or 8 nm CoFe2O4 NPs. This result could be the consequence of the irregular aggregates of 11 nm CoFe2O4 NPs as previously described in chapter 3.1. The agglomerated NPs may disturb the uniform deposition of pentacene leading to interference in charge transfer between not only source and drain electrode but also pentacene and NPs. This may bring about decreasing memory window as well as charge mobility. Consequently, 8 nm CoFe2O4 NPs showed the best electrical memory performance, and therefore the only 8 nm NPs will be embedded in NFGM devices for the efficient, in-depth study of memory effect of CoFe2O4 NPs. Figure 3-4 shows electrical memory performances of the NFGM devices based on 8 nm CoFe2O4 NPs. Typical output curves of p-type transistors were obtained in Figure 3-4(a). The drain current (ID) response for repeating pulse bias of program/erase (P/E) operations was investigated to measure memory switching speed and electrical stability (Figure 3-4(b)). 60 V, 0 V, -60 V, and 0 V were sequentially and repeatedly applied to gate electrode for program, read on, erase, read off operation, respectively. This switching cycle is usually called as write-read-erase-read (WRER) cycle. As shown in Figure 3-4(b) and (c), electrically fast and stable switching ID response with the values of read on/off current ratio (read Ion/Ioff) above 103 were maintained over 1,000 WRER cycles. Good data storage capacity is one of the important factors in non-volatile memory devices and the capacity was estimated from data retention test. (Figure 3-4(d)) The ID response at both programmed 22.

(high conductance) and erased (low conductance) states was measured at VGS = 0 V after program (VGS = 60 V) and erase (VGS = -60 V) operation, respectively. Both read Ion and Ioff values were decreased slightly during first 100 s, but ca. 3 × 103 of read Ion/Ioff was maintained over 1,000 s. To investigate the memory effect induced by P/E operation voltage level, applied bias was gradually increased from ±10 to ±60 V at an interval of 10 V as shown in Figure 3-5 and the results were summarized in Table 3-2. Both values, memory window and read Ion/Ioff remained almost constant in the range from ±10 V to ±30 V but started to increase above ±30 V and reached ca. 76.79 V and 2.72  103 at ±60 V, respectively. It indicates that CoFe2O4 NPs start to function as charge trap sites from ±30 V. Schematic energy band diagrams of the elements of the NFGM devices were illustrated to propose the possible charge trapping/releasing mechanism of the NFGM devices based on CoFe2O4 NPs. (Figure 3-6) Under high positive gate bias (at VGS = 60 V, program operation), electrons in the lowest unoccupied molecular orbital (LUMO) of pentacene can be attracted and trapped in the conduction band of CoFe2O4 NPs through oleates by strong positive external electric field. As a result, negatively charged NPs induced by significant amount of electrons can cause the negative internal electric field and start to attract the holes in pentacene to the interface between pentacene and oleates to form p-channel resulting in positive threshold voltage shift and maintenance of high conductance state in read on operation (at VGS = 0 V) after program operation. In erase operation (at VGS= -60 V), on the other hand, negative threshold voltage shift can be given via released electrons from NPs or recombination with transferred holes by strong negative external electric field resulting in low conductance state in read off operation (at VGS = 0 V) after erase operation. Tunneling dielectric layer, which has been usually deposited in most NFGM devices for good data storage capacity, was not embedded in this study. It has been reported that SAM alkyl chains surrounding NPs can play a role as alternative tunneling dielectric layer. To verify the effect of additional tunneling dielectric layer, 10 nm-thick Al2O3 thin-film was deposited between CoFe2O4 NPs and pentacene by ALD. Al2O3 has been used widely as an insulator in various electronic devices because of its high electrical breakdown field, high dielectric constant, and large bandgap 32 and ALD has been considered as a useful method for thin-film deposition due to the merits of facile and accurate thickness control, making dense and pinhole-free thin-films with excellent thickness uniformity in large area.33 As summarized in Table 3-3 and 3-4, both memory window and data retention capacity were lowered by embedding Al2O3 thin-film in NFGM devices indicating that additional Al2O3 tunneling dielectric layer rather disturbs charge carriers to transfer and oleates capping CoFe2O4 NPs are thick 23.

enough to be a tunneling layer in NFGM devices.. 24.

(b). 10. -5. 10. -6. 10. -7. 10. -8. 10. -9. 10. -10. 10. -11. Initial Erase Program. -ID (A). -ID (A). (a). -60 -40 -20 0. -ID (A). 10. -5. 10. -6. 10. -7. 10. -8. 10. -9. 10. -10. 10. -11. -5. 10. -6. 10. -7. 10. -8. 10. -9. 10. 20 40 60. VGS (V) (c). 10. Initial Erase Program. -10. -60 -40 -20 0. 20 40 60. VGS (V) Initial Erase Program. -60 -40 -20 0. 20 40 60. VGS (V) Figure 3-3. Transfer curves of the NFGM devices based on (a) 5, (b) 8, and (c) 11 nm CoFe2O4 NPs.. 25.

Table 3-1. Electrical memory characteristics of the NFGM devices based on CoFe2O4 NPs in different sizes and pentacene-based organic thin-film transistors (OTFTs) without NPs. Size of. μavg, initial. μmax, initial. CoFe2O4. (cm2. (cm2. NPs. V-1s-1). V-1s-1). 2.3310-3. 2.6110-3. (±1.95. (±1.92. 10 ). 10 ). 1.9610-3. 2.2410-3. (±6.81. (8.44±. 10-4). 10-4). 1.0410-3. 1.1810-3. (±3.05. (±2.56. 10-4). 10-4). 5.1710-1. 5.2110-1. 5 nm. -3. 8 nm. 11 nm. No NPs. (±1.12. (±1.11. 10 ). 10 ). -1. [a]. -3. -1. Vt, initial. Vt, program. Vt, erase. ΔVt. Ion, read[a]. Ioff, read[a]. Read. (V). (V). (V). (V). (-A). (-A). Ion/Ioff. 4.53. 32.46. -35.82. 68.27. 8.6010-7. 1.6410-10. 5.13103. (±8.13). (±1.78). (±2.09). (±2.77). (±7.52. (±3.06. (±3.86. 10 ). 10 ). 103). 5.9710-7. 2.2810-10. 2.98103. (±4.08. (±1.07. (±5.29. 10-7). 10-10). 102). 4.4910-7. 3.2710-10. 1.35103. (±1.61. (±8.87. (±1.99. 10-7). 10-11). 102). 1.3210-10. 2.2510-10. 6.2110-1. (±1.12. (±7.48. (±1.24. 10 ). 10 ). 10-1). -3.38. 31.63. -42.21. 73.84. (±5.39). (±4.25). (±3.48). (±6.34). -6.18. 30.52. -31.99. 62.51. (±3.65). (±3.16). (±4.05). (±7.16). -20.75. -24.91. -32.51. 7.60. (±4.67). (±2.05). (±2.26). (±1.13). -7. I(on/off, read) was obtained at VGS = 0 V after the program operation (VGS = 60 V), and erase operation (VGS = -60 V). 26. -11. -11. -11.

(a). (b) 0.6. VGS. 0.3 0.2 0.1. 0.0 -60 -50 -40 -30 -20 -10. 10. -3. -5. 10. -7. 10. -9. 10. 0 Read OFF. Erase. -50. ION. -100 -150 -200 IOFF. -11. 100. 150. 0. 200. -250 300. 250. Time (s). VDS (V) (c). 100. Read ON. 50. 10. -ID (A). -ID (A). 0.4. 10. Program. VGS (V). 60 V 50 V 40 V 30 V 20 V 10 V 0V. 0.5. -1. (d) 10. -6. 10. -7. 10. 10. 10. -8. -9. Erased state. -10. 0. 200. 400. 600. 800. -6. 10. -7. 10. -8. 10. -9. -ID (A). -ID (A). Programmed state. 10. 1000. 10. -10. 10. -11. Programmed state. Erased state. 0. 200. 400. 600. 800. 1000. Time (s). P/E cycles. Figure 3-4. Electrical memory characteristics of programmable 8 nm CoFe2O4 NP-based NFGM devices. (a) Output curves, (b) drain current response for P/E cycles, (c) electrical endurance for 1,000 repeating P/E cycles, and (d) data retention time test.. 27.

(b). 10. -6. 10. -7. 10. -8. 10. -9. -ID (A). -5. 10. -10. 10. -11. 10 V 20 V 30 V 40 V 50 V 60 V. -60 -40 -20 0. 20 40 60. 80. Memory window ON/OFF ratio. 60 40 20 0 10. P/E voltage (V). 20. 30. 40. 50. 60. 10. 4. 10. 3. 10. 2. 10. 1. 10. 0. 10. -1. ON/OFF ratio. 10. Memory window (V). (a). P/E voltage (V). Figure 3-5. Memory effect depending on P/E bias voltage for NFGM devices based on 8 nm CoFe2O4 NPs. (a) Transfer curves and (b) summarized values of memory window and read Ion/Ioff.. 28.

Table 3-2. Memory window and read Ion/Ioff depending on the program/erase operation bias voltage for NFGM devices based on 8 nm CoFe2O4 NPs. P/E voltage. Memory window. Read Ion/Ioff. (V). (V). (read at 0 V). ±10. 0.30. 9.59  10-1. ±20. 1.19. 9.71  10-1. ±30. 4.91. 7.74  10-1. ±40. 25.93. 8.15  102. ±50. 52.06. 2.69  103. ±60. 76.79. 2.72  103. 29.

(a). (b). (c). Figure 3-6. (a) Schematic energy band diagrams of program and (b) erase operation for the charge trap/release mechanism description and (c) energy band diagram of pentacene and CoFe2O4 NPs in different sizes.. 30.

Table 3-3. Electrical memory performances of the NFGM devices based on 8 nm CoFe2O4 NPs with/without additional Al2O3 tunneling dielectric layer. Thickness of Al2O3. 0 nm. 10 nm. μavg, initial 2. μmax, initial. (cm. (cm2. V-1s-1). V-1s-1). 1.9610-3. 2.2410-3. (±6.81. (±8.44. 10-4). 10-4). 5.7810-2. 5.8410-2. (±1.30. (±1.25. 10 ). 10 ). -2. -2. Vt, initial. Vt, program. Vt, erase. ΔVt. Ion. Ioff. Read. (V). (V). (V). (V). (-A). (-A). Ion/Ioff. 5.9710-7. 2.2810-10. 2.98103. (±4.08. (±1.07. (±5.29. 10-7). 10-10). 102). 1.7110-5. 2.3410-10. 7.47104. (±4.92. (±7.57. (±1.42. 10 ). 10 ). 104). -3.38. 31.63. -42.21. 73.84. (±5.39). (±4.25). (±3.48). (±6.34). -13.38. 20.98. -38.74. 59.72. (±2.81). (±2.61). (±1.05). (±2.10). 31. -6. -11.

Table 3-4. Data retention time test of the NFGM devices based on 8 nm CoFe2O4 NPs with/without additional Al2O3 tunneling dielectric layer. [a]. Thickness of. Ion. Ion. Ion, retention[a]. Ioff. Ioff. Ioff, retention[a]. Al2O3. (at 0 s, -A). (at 103 s, -A). (%). (at 0 s, -A). (at 103 s, -A). (%). 0 nm. 8.2910-7. 1.04  10-7. 12.65. 3.53  10-10. 6.42  10-11. 18.21. 10 nm. 1.42  10-5. 5.53  10-7. 3.87. 7.17  10-10. 6.49  10-11. 9.06. Ion(off), retention = [Ion(off) (at 103 s)/Ion(off) (at 0 s)]  100 (%). .. 32.

3.3 Electrical memory performance and mechanical test of the flexible NFGM devices based on 8 nm CoFe2O4 NPs 8 nm CoFe2O4 NPs were embedded on the bendable and transparent PET film with 100 nm-thick Al2O3 blocking dielectric layer to investigate their charge trap capacity in the flexible NFGM devices. Electrical memory performance of flexible NFGM devices based on 8 nm CoFe2O4 NPs was shown in Figure 3-7. The memory window was enhanced to ca. 31 V by embedding 8 nm CoFe2O4 NPs in flexible NFGM devices (Figure 3-7(a)), compared with flexible organic thin-film transistor (OTFT) without CoFe2O4 NP showing less than 2 V of memory window (Figure 3-7(b)). The value below 2 V of the memory window of OTFTs indicates that there were few charge trap sites in the active layer, blocking dielectric layer, and interface between them. In Figure 3-7(c) and (d), electrical stability of the memory devices was examined by 700 P/E cycles of dual gate voltage sweep from 20 V to -20 V. The memory window remained stable ca. 30 V without considerable electrical degradation. The mechanical reliability of the NFGM devices based on CoFe2O4 NPs has been also tested by measuring electrical memory performance after repeating pure bending. The tensile strain at the surface (εtop) of the flexible memory devices in pure bending can be estimated from the following equation:. 𝜀𝑡𝑜𝑝 =. (𝐷𝐹 +𝐷𝑆 )(1+2𝜂+𝜒𝜂 2 ) 2𝑅(1+𝜂)(1+𝜒𝜂). 𝐷. ≒ 2𝑅𝑆. (2). where η = DF/DS and χ = YF/YS. R is bending radius, D is the thickness and YF and YS are the Young’s modulus of the thin-film (F) and substrate (S), respectively. εtop can be simply calculated as DS/2R.5 As shown in Figure 3-8, the memory window above 26 V was retained with little electrical degradation during dual gate voltage sweep from 20 V to -20 V against ca. 0.53 % of tensile strain over 500 bending cycles, showing their potential possibilities of application in flexible memory devices.. 33.

(a). (b) -7. 10. Program Erase. 10. -5. 10. -6. Erase Program. -8. -9. 10. 10. 10. -7. 10. -8. 10. -9. -ID (A). -ID (A). 10. -10. -20. -10. 0. 10. 10. 20. -10. -20. -15. -10. -5. 0. 5. VGS (V). VGS (V) (c). (d). 10. -8. 20 Programmed state. 10. Vt. - ID (A). 10. 100 200 300 400 500 600 700. 0. -10. -9. -20. -10. 0. 10. VGS (V). 20. -20. 30. Erased state. 0. 200. 400. 600. P/E Cycles. Figure 3-7. Transfer curves of the 1st dual gate voltage sweep (a) with and (b) without 8 nm CoFe2O4 NPs. (c) Transfer curves for electrical stability test and (d) summarized values of threshold voltage in programmed/erased state for 700 P/E cycles of the flexible NFGM devices based on 8 nm CoFe2O4 NPs.. 34.

(a). (b). 10. Initial After 100 After 200 After 300 After 400 After 500. -8. 20. 10. Programmed state. 10. Vt. - ID (A). 10. -7. 0. -9. Erased state. -10 10. -10. -20. -10. 0. 10. -20. 20. 0. 100 200 300 400 500. Bending Cycles. VGS (V). Figure 3-8. (a) Transfer curves and (b) summarized values of threshold voltage in mechanical stability tests against pure bending.. 35.

IV. Conclusion. This study is to fabricate high-performance flexible NFGM devices based on semiconducting CoFe2O4 NPs. Electrical memory performance depended on the size (5, 8, and 11 nm) of the NPs and the dependence was explained in terms of the energy level difference. Monodisperse NPs could be synthesized by facile thermal decomposition using (Co2+Fe23+)-oleate complex as a precursor and the size of NPs can be controlled by regulating Ar bubbling rate in the reacting solution through complete separation of nucleation and growth process. The NFGM devices based on 8 nm CoFe2O4 NPs showed the best electrical memory performance among 3 different sizes of NPs. The devices showed the excellent memory performance: large memory window of ca. 73.84 V, fast and reversible switching behavior, ca. 3 × 103 of high read Ion/Ioff at VGS = 0 V, and outstanding data retention capability with an aid of hydrocarbon chains capping NPs as alternative tunneling dielectric layer. Furthermore, electrical memory operations of the NFGM devices on the flexible PET substrates have been also investigated and they showed superb, stable electrical characteristics in repeating P/E cycles and mechanical stability against pure bending. These results are expected to open-up wide possibilities for the flexible integrated circuits in data storage technologies.. 36.

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Acknowledgement (감사의 글). 어느덧, 울산에서의 짧고도 긴 2년간의 석사연구를 마무리 하는 시기가 다가왔습니다. 석사 연구를 마무리하면서, 우선 항상 촐랑대며 사고만 치고 다니는 막내 아들을 꿋꿋 이 믿고 응원해 주신 가족들에게 감사의 말을 전하고 싶습니다. 가장으로서 항상 멋진 모습 보여주시는 든든한 아버지, 직장 다니랴 집안일 하랴 몸이 10개라도 모자라지만 힘 든 내색 안하며 항상 웃는 모습 보여주시는 엄마, 그리고 서울에서 열심히 꿋꿋하게 미 래를 향해 달려가고 있는 형 모두 사랑합니다. 2년동안 학문적으로, 그리고 인간적으로 큰 가르침을 주신 오준학 교수님께 깊은 감사 드립니다. 불철주야 연구에 힘쓰시는 교수님의 모습을 본받아 포항공과대학교에서의 박 사과정에서도 유니스트 입학 당시의 열정과 초심을 잃지 않고 항상 열심히 하는, number one이 아닌 only one이 될 수 있도록 노력 하겠습니다. 앞으로도 호된 꾸짖음과 질책, 많 은 지도 편달 부탁 드리겠습니다. 그리고 2년 동안 SNDL 연구실에서 동고동락을 함께하며 많은 도움을 준 호정이, 아름 이, 은광이, 문정이와 사랑하는 동기들, 무열이, 윤호, 자연이, 은엽이, 그리고 무럭무럭 자라고 있는 인호와 철희, 그리고 해랑이 모두 고맙습니다. 2년간의 연구실 생활을 뒤돌 아보며 여러분들의 소중함을 다시 한번 더 느낄 수 있었습니다. 앞으로도 부끄럽지 않은 SNDL 연구원으로서 최선을 다할 것을 약속 드립니다. 바쁘신 와중에 흔쾌히 석사학위 논문심사를 허락해 주시고 아낌없는 조언을 해주신 김 병수 교수님, 고현협 교수님 감사 드립니다. 또한 나노입자 합성과 논문작성에 도움을 주 신 박종남 교수님과 김성환 학생께도 감사의 말씀을 드립니다. 위에서 언급 드린 분들뿐 만 아니라, 제 주변 모든 분들의 응원과 격려, 도움 덕분에 제가 이 자리까지 올 수 있지 않았나 생각해 봅니다. 40.

SNDL 연구실에서 2년 동안 배운 지식, 그리고 행복한 추억 모두 영원히 잊지 않겠습 니다. SNDL 출신 연구원으로서, 어디를 가더라도 부끄럼 없는 사람이 되도록 노력하겠습 니다. 항상 곁에서 따뜻한 격려와 응원을 해주시는 모든 분들의 사랑과 은혜 잊지 않고 살아가겠습니다. 감사합니다.. 2014 년 12 월 정 지 형. 41.