J. Phys. Chem. C 2009, 113, 3855–3861
3855
Programmable Digital Memory Characteristics of Nanoscale Thin Films of a Fully Conjugated Polymer Taek Joon Lee,† Samdae Park,† Suk Gyu Hahm, Dong Min Kim, Kyungtae Kim, Jinchul Kim, Wonsang Kwon, Youngtak Kim, Taihyun Chang, and Moonhor Ree* Department of Chemistry, National Research Laboratory for Polymer Synthesis and Physics, Center for Electro-Photo BehaViors in AdVanced Molecular Systems, National Research Laboratory for Polymer Physical Chemistry, Graduate Institute of AdVanced Materials Science, Polymer Research Institute, and BK School of Molecular Science, Pohang UniVersity of Science & Technology, Pohang 790-784, Republic of Korea ReceiVed: NoVember 9, 2008; ReVised Manuscript ReceiVed: January 24, 2009
This paper reports for the first time the programmable digital memory characteristics of the nanoscale thin films of a fully π-conjugated polymer, poly(diethyl dipropargylmalonate) (pDEDPM) in the absence of doping. This π-conjugated polymer was found to exhibit good solubility in organic solvents and to be easily processed to form nanoscale thin films through the use of conventional solution spin-, roll-, or dip-coating and subsequent drying. Films of the π-conjugated polymer with top and bottom metal electrodes exhibit excellent dynamic random access memory (DRAM) characteristics or write-once-read-many-times (WORM) memory behavior without polarity, depending on the film thickness. All the PI films are initially present in the OFF-state. Films with a thickness of 30 nm were found to exhibit very stable WORM memory characteristics without polarity and an ON/OFF current ratio of 106, whereas films with a thickness of 62-120 nm were found to exhibit excellent DRAM characteristics without polarity and an ON/OFF current ratio as high as 108. These memory characteristics are governed by trap-limited space-charge limited conduction and heterogeneously local filament formation. In these polymer films, both the ester units and the conjugated double bonds of the polymer backbone can act as charge trapping sites. The excellent bistable switching properties and processibility of this π-conjugated polymer mean that it is a promising material for the low-cost mass production of high density and very stable digital nonvolatile WORM memory and volatile DRAM devices. Introduction The world market for mobile devices has grown rapidly over the last 20 years.1 As a result of the rapid growth of this market, nonvolatile memory devices are in high demand.1 To date, all nonvolatile memories have been fabricated with inorganic materials by using complementary metal-oxide semiconductor processes.1 From the materials point of view, inorganic, all single-crystal semiconductor technologies are ultimately limited by the fact that single crystals cannot be grown on amorphous substrates. Thus, the memory density in silicon semiconductor devices can only be improved by reducing the feature size in the two-dimensional plane. Further, such devices have some general shortcomings, such as slow programming, limited endurance, and high voltage programming and erase requirements.1 These devices store data by encoding “0” and “1” as the amount of charge stored in the cells of the device. In contrast to inorganic materials, polymer materials exhibit easy processibility, flexibility, high mechanical strength, and good scalability.2–12 They can also be processed at low cost, and with their use the multistack layer structures required for high density memory devices can easily be fabricated. In general, these polymer memory devices store information on the basis of their high and low conductivity responses to applied voltages, a principle that is entirely different from that of silicon devices. As a result, significant research effort is currently invested in the development of polymer switching materials with properties * To whom correspondence should be addressed. Tel.: +82-54-279-2120. Fax: +82-54-279-3399. E-mail:
[email protected]. † T. J. Lee and S. Park contributed equally to this work.
and processibility that meet the requirements of the production of nonvolatile memory devices. Several such polymer materials have been reported.2–12 In particular, π-conjugated polymers are good candidates for nonvolatile memory devices because of their conductivity or semiconductivity. In fact, since polyacetylene was discovered to exhibit high electrical conductivity in the 1970s,13,14 π-conjugated polymers have become the subject of much research attention because of their microelectronics applications.15–18 The applications of π-conjugated polymers in various fields have been investigated, such as light-emitting devices, photodiodes, solar cells, electromagnetic interference shielding, and ion and biosensors, because of their excellent electrical and mechanical properties as well as their easy processibility.15–20 However, the use of π-conjugated polymers in nonvolatile memory devices has rarely been studied. In recent years, only a few applications of π-conjugated polymers in nonvolatile memory devices have been reported.21–29 Thus, the development of π-conjugated polymers for nonvolatile memory devices is still in the early exploration stage. In this paper, we report for the first time the novel digital memory characteristics of the nanometer-scale thin films of a π-conjugated polymer, poly(diethyl dipropargylmalonate) (pDEDPM). This π-conjugated polymer is easily processed with conventional solution spin-, roll-, or dip-coating and subsequent drying. The films of this π-conjugated polymer with top and bottom metal electrodes exhibit excellent dynamic random access memory (DRAM) characteristics or write-once-read-many-times (WORM) memory behavior without polarity, depending on the film thickness. The DRAM memory and WORM devices are
10.1021/jp809861n CCC: $40.75 2009 American Chemical Society Published on Web 02/11/2009
3856 J. Phys. Chem. C, Vol. 113, No. 9, 2009
Figure 1. (a) Chemical structure of a π-conjugated polymer, pDEDPM, and (b) a schematic diagram of the memory devices fabricated with nanometer-scale thin films of the pDEDPM polymer and gold (Au) top and aluminum (Al) bottom electrodes.
electrically stable for a long time, even in air ambient conditions. The devices’ ON/OFF current ratios are very high. The ON/ OFF switching of these devices was found to be mainly governed by trap-limited space-charge limited conduction and the heterogeneous local filament mechanism. Experimental Section The polymer pDEDPM (Figure 1a) was synthesized by carrying out the polymerization of the diethyl dipropargylmalonate monomer with the aid of MoC15- and WCl6-based catalysts according to a previously reported synthetic method.30 The molecular weight was measured at 40 °C with a size exclusion chromatograph (SEC) (Polymer Laboratory) with two mixed columns (Polymer Laboratory, Mixed C, 300 × 8.0 mm I.D.), which was calibrated with styrene standards (American Polymer Standards Co.). SEC chromatograms were recorded with a refractive index detector (Wyatt, Optilab DSP) by using tetrahydrofuran (THF, Samchun, HPLC grade) as the mobile phase at a flow rate of 0.8 mL/min. Polymer samples for the SEC analysis were dissolved in THF at an appropriate concentration (∼1.0 mg/mL), and the injection volume was 100 µL. The glass transition temperatures (Tg) of the films of the polymer were measured in the range -50 to 290 °C with a differential scanning calorimeter (model DSC 220CU, Seiko, Japan). During the measurements, dry nitrogen gas was used for purging; a flow rate of 80 cc/min and a ramping rate of 10.0 °C/min were employed. In each run, a sample of about 5 mg was used. The value of Tg was measured as the onset temperature of the glass transition in the thermogram. The degradation temperatures (Td) of the polymer films were measured in the range 20-800 °C by using a thermogravimeter (model TGA7, Perkin-Elmer). During these measurements, dry nitrogen gas was used for purging, and a flow rate of 100 cc/ min and a ramping rate of 10.0 °C/min were employed. For the fabrication of the memory devices, homogeneous pDEDPM solutions (0.5, 2, and 4.0 wt %) were prepared in cyclopentanone and then filtered with microfilters with a pore size of 0.45 µm based on polytetrafluoroethylene (PTFE) membranes. Single-active-layer memory devices were then fabricated as follows. Gold (Au) and aluminum (Al) were used as metal electrodes. The bottom metal electrode was deposited with electron-beam sputtering onto glass or a 500 nm thick
Lee et al. thermal oxide deposited silicon wafer by using a shadow mask with a line width of 0.5 mm. The bottom metal electrodes were found to have a thickness of 100 nm. The polymer solutions were spun onto Al or Au line electrode deposited glass substrates or silicon wafers, and a 2000 rpm/40 s spin-coating process was employed. The spun films were dried in vacuum on a hot plate at 60 °C for 8 h. The thicknesses of the resulting films were determined by using a spectroscopic ellipsometer (model VASE, Woollam). Al and Au electrodes were deposited onto the polymer films (coated on the substrates) at a pressure below 10-7 Torr by means of thermal evaporation. The top metal electrodes were determined to have a thickness of 300 nm with line widths of 0.5, 1.0, 1.5, and 2.0 mm. The device structure is an active matrix, which means that the top and bottom metal electrodes are aligned perpendicular to each other. A schematic diagram of a 4 (word line) × 4 (bit line) crossbar memory device is shown in Figure 1b, and the configuration of the top and bottom electrodes can be symmetric or asymmetric. These basic memory cells can be integrated into a cross point memory array. All electrical experiments were conducted in air ambient conditions, without any device encapsulation. The current-voltage (I-V) measurements and electrical-stress tests were carried out with forward and reverse voltage scans over the range -4.0 V (or -8.0 V) to +4.0 V (or +8.0 V) at a scan rate of 500 mV/s by using a Keithley 4200-SCS semiconductor parameter analyzer. In addition, the electrical DC conductivities of the films were measured with a four-point probe or a two-point probe connected to a Keithley semiconductor parameter analyzer. The surface roughness and morphology of each film were determined with an atomic force microscope (Multimode AFM Nanoscope IIIa, Digital Instruments). Optical properties were measured with a Scinco ultraviolet-visible (UV-vis) spectrometer (model S-3100). Cyclic voltammetry (CV) measurements were carried out in an 0.1 M solution of tetrabutylammonium tetrafluoroborate in acetonitrile by using an electrochemical workstation (IM6ex impedance analyzer) with a platinum gauze counter electrode and an Ag/AgCl (3.8 M KCl) reference electrode, and the pDEDPM polymer was coated onto the Au bottom electrode, which was deposited on a silicon wafer. A scan rate of 100 mV/s was used. Results and Discussion The synthesized pDEDPM polymer was found with the SEC analysis to have a weight-average molecular weight of 15100 and a polydispersity index of 4.21. The π-conjugated polymer was found to be thermally stable up to 270 °C in a nitrogen atmosphere (Figure 2a). In the DSC measurements, the polymer was found to exhibit a glass transition at 10 °C (Figure 2b), which is attributed to the rearrangement from exocyclic to endocyclic double bonds in the backbone.30 The pDEDPM polymer films coated onto Au and Al electrodes were examined with atomic force microscopy (AFM) and found to have rootmean-square surface roughnesses of 0.72 and 0.89 nm, respectively, over an area of 1.0 × 1.0 µm2 (data not shown). The AFM analysis confirmed that the pDEDPM polymer forms good quality thin films with smooth surfaces through conventional solution spin-coating and drying processes. In addition, the pDEDPM polymer in thin films was investigated by UV-vis spectroscopy and CV analysis. Figure 3 shows UV-vis spectroscopy and CV data for the pDEDPM films. From the UV-vis spectrum in Figure 3a, the band gap (i.e., the difference between the highest occupied molecular orbital (HOMO) level and the lowest unoccupied molecular orbital (LUMO) level)
Nanoscale Thin Films of a Fully Conjugated Polymer
Figure 2. (a) TGA and (b) DSC thermograms of the pDEDPM polymer, which were measured at a rate of 10.0 °C/min under a nitrogen atmosphere.
Figure 3. (a) UV-vis spectrum of a pDEDPM polymer film coated on a quartz substrate. (b) CV response of a pDEDPM polymer film fabricated with a Au electrode supported by a silicon substrate in acetonitrile containing 0.1 M tetrabutylammonium tetrafluoroborate.
for the pDEDPM polymer is estimated to be 1.84 eV. From the CV data (Figure 3b), the oxidation half-wave potential for the pDEDPM polymer is determined to be 1.13 V vs Ag/AgCl. The external ferrocene/ferrocenium (Fc/Fc+) redox standard E1/2 was measured to be 0.59 V vs Ag/AgCl in acetonitrile. Assuming that the HOMO level for the Fc/Fc+ standard is -4.80 eV with respect to the zero vacuum level, the HOMO level for the pDEDPM polymer is determined to be -5.34 eV. Therefore, the LUMO level of the pDEDPM polymer is estimated to be -3.50 eV. Figure 4a shows typical I-V characteristics of the 62 nm thick pDEDPM films with Al top and Au bottom electrodes, which were measured in dual sweep mode with a positive bias
J. Phys. Chem. C, Vol. 113, No. 9, 2009 3857
Figure 4. Typical I-V curves of the Al/pDEDPM (62 nm thick)/Au devices, which were measured with a compliance current set of 0.01 A in dual sweep mode: (a) the applied voltage was swept from 0 V f +4.0 V f 0 V; (b) the applied voltage was swept from 0 V f -4.0 V f 0 V. The electrode contact area was 0.5 × 0.5 mm2.
(0 V f 4.0 V f 0 V) by using a semiconductor parameter analyzer with a compliance current of 0.01 A. Initially, the device is in the OFF-state, which has a current level that is quite low (of the order of 10-13-10-8 A). In the first dual sweep (see the first sweep in Figure 4a), there is an abrupt increase in the current (from 10-9 to 10-2 A) at 1.85 V (which corresponds to the switching-ON threshold voltage), indicating that the film undergoes an electrical transition from an OFF-state to an ONstate. This electrical transition can serve as the “writing” process in a memory device. When a reverse voltage sweep is applied, the film is reset to the initial low conductivity state (i.e., the OFF-state), which can serve as the “erasing” process in the memory device. The erased state can again be written to a stored state when a voltage greater than the switching-ON threshold voltage is applied (see the second sweep in Figure 4a), indicating that this memory device is rewritable. The third sweep was carried out after turning off the power for 2-5 s. It was found that the ON-state had relaxed to the steady OFF-state. However, the film can be further programmed to the ON-state. The short retention time of the ON-state indicates that the memory device is volatile. The above processes can be repeated many times for every cell (Figure 4a). Overall, the 62 nm thick films exhibit interesting and unique DRAM characteristics during positive voltage sweeps. In the case of negative bias sweeps (0 V f -4.0 V f 0 V), the DRAM memory characteristics were found to be the same as in the case of the positive bias sweeps (Figure 4b). The ON/OFF current ratios were determined from the I-V plots in Figure 4 to be 106-108 over the voltage range 0-4.0 V. Figure 5a shows representative results of the retention tests carried out on the 62 nm thick film devices under a positive bias in ambient conditions. Similar retention test results were obtained under a negative bias (data not shown). Once the device is switched to the ON-state by applying a positive voltage pulse (1.0 V), the ON-state is retained without any degradation for more than 10 h under the applied voltage. The effect of continuous read pulses (with a read voltage of 1.0 V) on the ON- and OFF-states was also investigated (Figure 5b). The inset in Figure 5b shows the pulses used in these measurements. No
3858 J. Phys. Chem. C, Vol. 113, No. 9, 2009
Lee et al.
Figure 5. (a) ON/OFF current ratios for the Al/pDEDPM (62 nm thick)/Au device measured during the voltage sweeps with positive bias as a function of the applied voltage. (b) Effect of read pulses on the ON- and OFF-states of the Al/pDEDPM (62 nm thick)/Au device: the inset shows the pulses used for this measurement.
Figure 6. Experimental and fitted I-V curves for the Al/pDEDPM (62 nm thick)/Au device: (a) OFF-state with a combination of the ohmic model and the space-charge-limited model, and (b) ON-state with the ohmic model.
resistance degradation was observed for the ON- and OFF-states after more than one hundred million (109) read cycles. Neither the voltage stress nor the read pulses result in state transitions since the applied voltage (1 V) is lower than the switching-ON threshold voltage (about 1.85 V) and can maintain the current flow in the ON-state. Thus, both the ON- and OFF-states are stable under voltage stress and are insensitive to read pulses. To investigate the observed memory characteristics, we considered the HOMO and LUMO levels of the pDEDPM and the work functions (Φ) of the Au top and Al bottom electrodes: Φ is -5.20 eV for Au and -4.20 eV for Al. In the Al/pDEDPM/ Au devices, the energy barrier between the work function Φ of the Al bottom electrode and the HOMO level of the active pDEDPM layer is 1.14 eV, which is smaller than that (1.7 eV) between the LUMO level of the active pDEDPM layer and the work function Φ of the Au top electrode. Thus, hole injection from the Al bottom electrode into the HOMO level of the pDEDPM layer is more favorable than electron injection from the Au top electrode into the LUMO level of the pDEDPM layer; that is, hole injection dominates the conduction process in the devices. However, as the sweep voltage increases, double injections (i.e., hole and electron injection) can occur because the energy gap between the work function Φ of the Al bottom electrode and the LUMO level of the active pDEDPM layer is quite low (0.7 eV). The measured I-V characteristics of the devices were further analyzed in detail with various conduction models to investigate their electrical switching characteristics. The trap-limited spacecharge limited conduction (SCLC) model was found to satisfactorily fit the I-V data for the OFF-state. As shown in Figure 6a, the logarithmic plot of the I-V data for the OFF-state contains two linear regions for
