Carrier transport mechanism in aluminum nanoparticle embedded AlQ3 structures for organic bistable memory devices

Organic Electronics 10 (2009) 138–144

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Carrier transport mechanism in aluminum nanoparticle embedded AlQ3 structures for organic bistable memory devices V.S. Reddy, S. Karak, S.K. Ray, A. Dhar * Department of Physics and Meteorology, IIT Kharagpur, Kharagpur 721 302, India

a r t i c l e

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Article history: Received 4 July 2008 Received in revised form 2 October 2008 Accepted 16 October 2008 Available online 31 October 2008

PACS: 72.80.Le 73.61.Ph 85.30.De 85.65.+h Keywords: Organic semiconductor Memory device Conduction mechanism Nanoparticle

a b s t r a c t Electrical bistability is demonstrated in organic memory devices based on tris-(8-hydroxyquinoline)aluminum (AlQ3) and aluminum nanoparticles. The role of the thickness of middle aluminum layer and the size of the nanoparticles in device performance is investigated. Above a threshold voltage, the device suddenly switches from a low conductivity OFF state to a high conductivity ON state with a conductivity difference of several orders of magnitude. The OFF state of the device could be recovered by applying a relatively high voltage pulse. The electronic transition is attributed to an electric field induced transfer of charge between aluminum nanoparticles and AlQ3. The type of charge carriers responsible for conductance switching is investigated. The charge carrier conduction mechanism through the device in ON and OFF states is studied by temperature dependent current–voltage characteristics and analyzed in the framework of existing theoretical models. The conduction mechanism in the OFF state is dominated by field-enhanced thermal excitation of charge carriers from localized centers, whereas it changes to Fowler–Nordheim tunneling of charge carriers in the ON state. The device exhibited excellent stability in either conductivity states. The results indicate the strong potential of the device towards its application as a nonvolatile electronic memory. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction During the past fifteen years, tremendous work has been done in the field of organic semiconductor based electronic devices. Devices such as light emitting diodes [1–3], field effect transistors [4–6] and solar cells [7–9] have shown great potential towards future technologies. Organic semiconductor based devices exhibit unique advantages such as low fabrication cost, high mechanical flexibility and versatility of the material chemical structure compared with the inorganic semiconductor devices. However, to widen the application of organic semiconductors to varieties of electronics systems, organic based memory devices are essential [10–16]. The basic feature of a memory device is to exhibit bistable behavior having two different resistance states at the same applied voltage. Ma et al. have * Corresponding author. Tel.: +91 3222 283830; fax: +91 3222 255303. E-mail address: [email protected] (A. Dhar). 1566-1199/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2008.10.014

demonstrated memory effects in a three layer organic/metal nanoparticle/organic structure embedded between two electrodes [17]. Bozano and coworkers have studied the important role of different metal nanoparticles in a three layer memory structure [18]. During the device operation, when applied voltage exceeds a certain value, the device suddenly switches from a low conductivity ‘OFF’ state to a high conductivity ‘ON’ state, with a conductivity difference of several orders of magnitude. This high conductivity state will be memorized until an ‘OFF’ voltage is applied to erase it. Despite the superior performance of organic memory devices, the charge carrier transport mechanism in the ‘ON’ and ‘OFF’ states and the type of carriers responsible for bistability are not understood clearly. In this paper, we describe the fabrication and operation of the tri-layer organic/metal/organic and the single layer memory structures sandwiched between two electrodes. The effect of the thickness of intermediate metal layer and the size of the nanoparticles in device performance is

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investigated. The charge carrier conduction mechanism through the device in ‘ON’ and ‘OFF’ states is studied by temperature dependent current–voltage characteristics and analyzed in terms of existing theoretical models. The type of carriers responsible for conductance switching is also investigated. 2. Experimental In present investigation, three different types of device structures were studied. The first type (type-I) is the trilayer AlQ3/Al nanoislands/AlQ3 structure sandwiched between two electrodes (hereafter called tri-layer device), the second one (type-II) is a single AlQ3 layer embedded between two electrodes (hereafter called single layer device), and the third type (type-III) is the ITO/Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)/ AlQ3/Al nanoislands/AlQ3/Al device. For all types of devices, patterned indium tin oxide (ITO) (SPI Inc.) with sheet resistance 10 X/h was used as the bottom electrode. Prior to AlQ3 deposition, ITO substrates were cleaned thoroughly in acetone, isopropyl alcohol and de-ionized water in sequence by an ultrasonic cleaner followed by drying with nitrogen gas. For the fabrication of tri-layer structure, both the AlQ3 and Al middle layers were deposited by thermal evaporation at a base pressure of 5  106 mbar. The thickness of the each AlQ3 layer was about 50 nm and that of the middle Al layer was varied between 5 and 20 nm. The deposition rate of the middle Al layer was controlled by a quartz crystal thickness monitor. In order to obtain the nanoscale metal islands, a low evaporation rate of less than 0.1 nm/s was used. For the single layer device structure, AlQ3 (100 nm) was thermally evaporated on ITO electrodes. For the fabrication of ITO/PEDOT:PSS/AlQ3/Al nanoislands/AlQ3/Al device, a 40 nm thick PEDOT:PSS layer was spin coated on ITO substrates from a 2.8 wt.% water solution (Aldrich). Post baking was done for 1 h at around 110 °C to remove the solvent completely. Deposition of all other layers of this device was done in the same manner as the tri-layer device. Finally, aluminum top electrode strips were deposited by thermal evaporation through a shadow mask. The thickness of different layers was monitored by quartz crystal monitor and was verified by stylus profilometer (Veeco Dektak3). Cross sectional scanning electron

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microscopy (SEM) images of the device were taken using a ZEISS SUPRA 40 field emission (FE) microscope. Atomic force microscopy (AFM) (Veeco Nanoscope-IV) in tapping mode was used to evaluate the surface morphology of the middle Al layer. Fourier-transform infrared (FTIR) spectra of the middle Al layer in the wave number range 400–4000 cm1 were obtained using a Nexus 870 Thermo Nicolet spectrometer. The optical transmission spectra were measured in the wavelength range 300–1100 nm using a UV–vis–NIR spectrophotometer (Perkin–Elmer Lambda 45). The dc current–voltage characteristics of the devices were obtained using a Keithley 485 Pico ammeter and an Advantest R6144 programmable dc voltage generator. 3. Results and discussion Fig. 1 shows the cross sectional view of a typical trilayer memory structure (type-I) sandwiched between ITO and Al electrodes. The FE–SEM micrograph clearly indicates ITO, two AlQ3 layers and Al layer with sharp interfaces. The thickness of bottom AlQ3, top AlQ3, ITO and Al layers are estimated to be 70 nm, 50 nm, 400 nm and 200 nm, respectively. The performance of organic bistable device is sensitive to metal nanoparticles and the thickness of the middle metal layer. The ON/OFF current ratio, for example, depends strongly on the size and density of the nanoparticles. Morphology and the thickness of the middle Al layer depends on the evaporation rate and is controlled by quartz crystal monitor. Films deposited with a higher deposition rate led to continuous films and, hence, non-switching devices. A deposition rate less than 0.1 nm/s resulted in island like growth and bistable devices. Detailed information about the nanostructure of the middle Al layer has been obtained by AFM. Fig. 2a illustrates the surface morphology of the middle Al layer of thickness 5 nm. It clearly shows disconnected metal islands of various sizes. The size distribution of nanoislands is estimated from this AFM image and is shown in Fig. 2b. The island size varies from 5 nm to about 25 nm with peak of the distribution at 15 nm. When the thickness of the film is increased to 10 nm, the size of nanoislands increases slightly as shown in Fig. 2c. For this film the island size distribution ranges from 8 nm to about 35 nm and peaks at 20 nm, as shown in Fig. 2d. The gaps

Al Top AlQ 3 Bottom AlQ 3

ITO

Glass substrate Fig. 1. Cross sectional SEM image of the tri-layer memory device, ITO/AlQ3/Al/AlQ3/Al (type-I).

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Fig. 2. Surface morphology and island size distribution of the middle aluminum layer of thickness (a, b) 5 nm; (c, d) 10 nm and (e, f) 15 nm.

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between the islands are of the order of 3–7 nm. Coalescence of nanoislands is observed with further increase in film thickness as shown in Fig. 2e. Here the thickness of the middle Al layer is about 15 nm. A large variation in the size of the nanoislands (from 15 to 60 nm) is observed in Fig. 2f, with the peak of the distribution at 30 nm. Though the deposition of Al nanoparticles is performed at 5  106 mbar, there is a time delay between the deposition of nanoparticles and top AlQ3 layer. During this time a thin oxide shell is formed on the nanoparticles. Since the deposition is done under high vacuum, a large fraction of the aluminum remains as a metallic core surrounded by a thin oxide shell [18]. Formation of oxide shell was also mentioned by other authors. Ma et al. have performed in situ X-ray photoemission spectroscopy measurements and confirmed the formation of 16 Å oxide shell [11]. Fig. 3a shows the FTIR spectra of the middle Al layer of thickness 10 nm in the range 400–2000 cm1. The strong band observed at 538 cm1 (labeled A) is assigned to octahedral Al–O stretching vibrations, whereas the broad band centered at 812 cm1 (labeled B) is assigned to tetrahedral Al–O stretching vibrations [19–21]. These results clearly indicate the presence of oxide shell on the metal islands. In addition to AFM and FTIR, UV–vis transmission spectra have been used to study the middle Al layer deposited on AlQ3. Fig. 3b shows the transmission spectra of the middle Al layer of different thickness deposited on 50 nm thick AlQ3 film. Transmission spectrum of pure AlQ3 film is also shown as a reference. For the Al film that appears in normal metallic color, the transmission is relatively low in the UV–vis region. It indicates that most of the radiation from the light source is reflected away by the continuous metal film. The film containing Al nanoislands appears in deep blue color and shows relatively high transparency in the UV–vis region, as shown in Fig. 3b. The devices fabricated using this deep blue color Al films have shown bistable behavior. Fig. 4 shows typical current–voltage (I–V) characteristics of the tri-layer nanoparticle memory device (type-I) on a semi logarithmic scale obtained by sweeping the volt-

a

age from –5 to +5 V and then back to –5 V. Here the thickness of the middle Al layer is about 15 nm. Almost symmetric I–V characteristics are observed for both the polarities of the voltage. This figure can be used to define the key parameters of the device. Initially, the device remains in the high resistance ‘OFF’ state at a smaller bias. When the applied voltage reaches a threshold value (Vth), resistance of the device decreases abruptly and the device current increases by several orders of magnitude. The value of Vth is in the range 1.7–2.3 V. As shown in Fig. 4, the device current switches between the states of high and low resistance at the threshold, reaches the maximum (VON), and then goes through a negative differential resistance (NDR) region to a minimum (till VOFF), after which it increases again almost exponentially. When the voltage is decreased towards zero, the current follows the upper curve and the device is set in its ‘ON’ state until the voltage near VOFF is applied. An ‘‘N-shaped” I–V curve is obtained in the ‘ON’ state of the device. The low voltage region below Vth is the region of bistability, in which the state of the device can be read (Vread). The ‘ON’ state of the device is obtained by applying a voltage near VON and the ‘OFF’ state is recovered by applying a voltage beyond VOFF. Fig. 5 shows the current–voltage characteristics of the tri-layer devices ITO/AlQ3/Al/AlQ3/Al (type-I) with different thickness of middle Al layer. The characteristics of the device without middle Al layer, is also shown for comparison. Both the devices show bistable properties. The bistability is attributed to the presence of metal nanoparticles in the AlQ3 matrix. For tri-layer devices, the middle thin Al layer contains the nanoparticles. In case of single layer devices, metal nanoparticles are incorporated in AlQ3 layer by diffusion during the evaporation of top electrode [22]. The threshold voltage is in the range 1.7–2.3 V for the devices with different thickness of middle Al layer. The ON/OFF current ratio is very small (10) for single layer device and increases with increasing thickness of the middle Al layer till 10 nm. The devices with 10 nm thick middle Al layer have shown better switching properties with large ON/OFF current ratio (>105) at a read volt-

b 100 2.0

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Fig. 3. (a) FTIR spectra of the middle Al layer of thickness 10 nm, and (b) transmission spectra of Al films of different thickness deposited on 50 nm thick AlQ3 film.

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applications. Stress tests have been performed by subjecting the device to a low bias of 1 V over prolonged periods of time in both ON and OFF states and by recording the currents at different times. When subjected to stress test, the device in the OFF state retains its conductivity and do not undergo any transition to the ON state, as shown in Fig. 6. Similar tests have been performed on the device in the ON state. The current in both ON and OFF states remains constant even after a prolonged period of continuous stress test. It shows that there is no significant degradation of the device in both ON and OFF states, indicating the stability of both material and material/electrode interfaces. To understand the conduction mechanism through the device in both ON and OFF states, the current–voltage characteristics at different temperatures have been measured and analyzed in the framework of existing theoretical models. Fig. 7 shows the current–voltage characteristics of the device, ITO/AlQ3/Al(10 nm)/AlQ3/Al (type-I), at different temperatures. The threshold voltage of the device decreases with increasing temperature. The device current in the OFF state has shown strong temperature dependence. This thermally activated charge transport may be either due to Poole–Frenkel (PF) emission of charge carriers or to the Schottky contact at the Al/AlQ3 interface. The PF emission is due to the field-enhanced thermal excitation of charge carriers from localized centers or potential wells within the bulk of the semiconductor, whereas the Schottky effect results from the injection of charge carriers from the electrode into the semiconductor. In order to differentiate between these two conduction mechanisms, current–voltage characteristics are measured at both the polarities of voltage. In present study, an asymmetric electrode structure (ITO and Al electrodes) is used for the fabrication of all the devices. The difference between the work functions of ITO and Al is 0.6 eV (the work functions of ITO and Al are 4.9 and 4.3 eV, respectively). For the Schottky controlled conduction in an asymmetric electrode structure, the current–voltage characteristics should also be asymmetrical when the voltage is reversed. However, all

Fig. 5. Current–voltage characteristics of the single layer device and trilayer device, ITO/AlQ3/Al/AlQ3/Al, with different thickness of middle Al layer (type-I and type-II).

ON state -5

10

Current (A)

age of 1 V. If the thickness of the middle Al layer is increased beyond 10 nm, the ON/OFF ratio decreases sharply and the switching properties almost vanish at a thickness of 20 nm as shown in Fig. 5. At higher thicknesses of middle Al layer, for example at 15 nm, Al nanoparticles grow in size and join with the neighboring particles as shown in AFM image (Fig. 2f). Then the number of isolated particles that can store charge becomes less and hence the ON/OFF ratio goes down. Single layer devices exhibit lower current because of the mismatch of electrode work functions to the HOMO and LUMO levels of AlQ3. The current in both ON and OFF states increases with increasing thickness of the middle Al layer. These results suggest that the nanoparticles not only induce bistability, but also contribute to the conduction process. The retention of the ON and OFF states and device performance under electrical stress is important for practical

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Time (sec) Fig. 6. Voltage stress test of the device in the ON and OFF states at a read voltage of 1 V.

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served in the OFF state of the device as shown in Fig. 8a. The experimental activation energy obtained from the temperature dependant current characteristics is 1.6 eV, which is in close agreement with the theoretical ionization energy of PF emission. These results confirm the predominance of PF emission in the OFF state of the device. The current–voltage and current–temperature relations have changed after electrical transition to the ON state. A linear relation is observed between log(I/V2) and 1/V as shown in Fig. 8b. In addition, the current in the ON state is insensitive to temperature. These results suggest that the current conduction in the ON state is probably due to Fowler–Nordheim tunneling of charge carriers through a triangular barrier. The current in case of Fowler–Nordheim tunneling is given by [24]

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4dðqUÞ3=2 ð2m Þ1=2 I ¼ C 1 V exp  3qhV

Voltage (V) Fig. 7. Current–voltage characteristics of ITO/AlQ3/Al(10 nm)/AlQ3/Al device at different temperatures.

q I ¼ CV exp  kT



qV U pel

1=2 !# ð1Þ

where q is the electronic charge, U is the ionization energy of the traps, e is the dynamic permittivity of AlQ3, k is the Boltzmann constant, T is the temperature, and C is a constant. A linear relation between log(I/V) and V1/2 is ob-

Current/Voltage (A/V)

a

ð2Þ

where U is the energy barrier height, d is the tunneling distance, m* is the reduced mass of the charge carrier, and C1 is a constant. Thus, the current conduction mechanism changed from the Pool–Frenkel emission in the OFF state to Fowler–Nordheim tunneling in the ON state. In order to investigate the type of carriers (either electrons or holes) responsible for conductance switching, a hole injecting material PEDOT:PSS is deposited between ITO electrode and bottom AlQ3 layer. The current–voltage characteristics of ITO/PEDOT:PSS/AlQ3/Al(10 nm)/AlQ3/Al device exhibit slight hysteresis and no obvious switching behavior as shown in Fig. 9. Moreover, emission of light from the device is observed after the incorporation of PEDOT:PSS layer. In case of ITO/AlQ3/Al(10 nm)/AlQ3/Al device, electrons injected from the Al electrode are trapped inside Al nanoislands and produces bistability. Probability of electron-hole recombination is very small because of the less number of available holes. PEDOT:PSS enhances the injection of holes into the AlQ3 layer and hence increases the probability of electron-hole recombination [25]. The process of recombination results in smearing

the devices have shown symmetric current–voltage characteristics in the OFF state as shown in Fig. 4. Therefore, the current conduction is dominated by PF emission in the OFF state of the device. This PF mechanism is further confirmed by analyzing the current–voltage characteristics. The reported values of HOMO and LUMO levels of AlQ3 are around 5.8 and 3.0 eV, respectively [18,23]. In case of PF emission, the theoretical energy barrier for the emission of electrons is 1.3 eV. The current through the device in case of PF emission is given by [24]

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Fig. 8. (a) Variation of log(I/V) with V1/2 in the OFF state of the device, and (b) variation of log(I/V2) with 1/V in the ON state of the device. Symbols are the experimental data and the solid lines are the fitting data using theoretical models.

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the framework of existing theoretical models. The charge conduction for the device in the OFF state is mainly due to the field-enhanced thermal excitation of charge carriers from localized centers, whereas it changes to Fowler– Nordheim tunneling of charge carriers in the ON state. Present study reveals that the electrons are majority carriers in these organic bistable devices. The devices exhibit a repeatable bistable behavior and stability in either state. These characteristics indicate that the tri-layer device has a strong potential towards its application as nonvolatile electronic memory.

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Acknowledgements

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Voltage (V) Fig. 9. Current–voltage characteristics of ITO/AlQ3/Al(10 nm)/AlQ3/Al device with and without PEDOT:PSS layer between ITO and bottom AlQ3.

out of bistability and emission of light. Thus the electrons are majority carriers and holes are minority carriers in case of our organic bistable devices. In the light of above experimental results, we propose that the electronic transition is due to an electric field induced transfer of charge between aluminum nanoparticles and AlQ3. When the applied electric field is high enough, electrons in the AlQ3 may gain enough energy and tunnel through aluminum oxide shell into the core of aluminum nanoparticles. The charge on the aluminum nanoparticles may be stable due to the presence of insulating aluminum oxide shell, which prevents the recombination of charge after the removal of external electric field. The OFF state of the device can be recovered by applying a voltage near VOFF, which is due to the removal of charge from the core of the aluminum nanoparticles. 4. Conclusions In conclusion, electrically bistable devices using AlQ3 and aluminum nanoparticles have been demonstrated. The effect of the thickness of middle aluminum layer and the size of the nanoparticles on device performance is investigated. It is observed that the bistability is very sensitive to nanostructure of the middle aluminum layer. For obtaining the devices with well controlled bistability, the middle aluminum layer should have island like structure, instead of continuous metal layer. These nanoislands act as the charge storage elements, which enable the nonvolatile electrical bistability when biased to a sufficiently high voltage. The charge carrier conduction mechanism through the device in ON and OFF states is studied by temperature dependent current–voltage characteristics and analyzed in

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