Al:ZnO Transparent Resistive Switching Devices Grown by

Article pubs.acs.org/Langmuir

ZnO/Al:ZnO Transparent Resistive Switching Devices Grown by Atomic Layer Deposition for Memristor Applications Rajeh Mundle, Christian Carvajal, and Aswini K. Pradhan* Center for Materials Research, Norfolk State University, 700 Park Avenue, Norfolk, Virginia 23504, United States ABSTRACT: ZnO has intrinsic semiconductor conductivity because of an unintentional doping mechanism resulting from the growth process that is mainly attributable to oxygen vacancies (VO) positioned in the bandgap. ZnO has multiple electronic states that depend on the number of vacancies and the charge state of each vacancy. In addition to the individual electron states, the vacancies have different vibrational states. We developed a high-temperature precursor vapor mask technique using Al2O3 to pattern the atomic layer deposition of ZnO and Al:ZnO layers on ZnO-based substrates. This technique was used to create a memristor device based on Al:ZnO thin films having metallic and semiconducting and insulating transport properties ZnO. We demonstrated that adding combination of Al2O3 and TiO2 barrier layers improved the resistive switching behavior. The change in the resistance between the high- and low-resistivity states of the memristor with a combination of Al2O3 and TiO2 was approximately 157%. The devices were exposed to laser light from three different laser diodes. The 450 nm laser diode noticeably affected the combined Al2O3 and TiO2 barrier, creating a highresistivity state with a 2.9% shift under illumination. The high-resistivity state shift under laser illumination indicates defect shifts and the thermodynamic transition of ZnO defects.

1. INTRODUCTION Modern displays utilize transparent conducting oxides (TCOs) in thin-film transistors to modulate pixels while allowing back light through.1 Transparent electronics can be combined with liquid crystal display (LCD) technology to fabricate transparent devices. To create such devices, transparent ZnO, Al2O3, and Al:ZnO have been grown by atomic layer deposition and radiofrequency (RF) sputtering to create transparent thin-film transistor (TTFT) device arrays.2,3 One research group used Ga-doped ZnO as the transparent top and bottom contacts in a ZnO-based memristor as a substitute for more expensive indium tin oxide (ITO) TCOs.4 The emergence of headmounted displays and plans for their expansion to larger markets require low-cost, high-quality TCOs. Head-mounted displays placed near the eye5,6 and equipped with augmented reality algorithms will help revolutionize medical surgery7 and mechanical repair8 by overlaying information that is relevant to the task at hand over the wearer’s line of sight. Display technology for near-eye display will use nanogratings to increase the light originating from LCD microdisplays that propagates through the waveguides.9,10 These new display devices and near-eye display applications could benefit from transparent memory, transparent switches, and transparent optical sensors. Cross-point memristive switching devices can act as synaptic elements to create neural computing machines mimicking the neural networks created by neurons in a biological brain.11 ZnO can be doped with elements such as Al or Ga to increase its conductivity, making its electron-transport behavior relatively metal-like.12,13 Undoped ZnO has an intrinsic conductivity similar to that of a semiconductor © XXXX American Chemical Society

because of an unintentional doping mechanism resulting from the growth process14 that is mainly attributable to oxygen vacancies. ZnO grown via atomic layer deposition (ALD) has reported resistivity values ranging from insulating to on the order of 10−3 Ω·cm.15,16 The conductivity of a semiconductor is related to the intrinsic carrier concentration, ni. Undoped silicon can maintain an intrinsic carrier concentration on the order of 1 × 1010 cm−3 at 300 K because of its low bandgap energy (1.1 eV). Undoped Ge and GaAs have bandgap energies of 0.7 and 1.4 eV, respectively, corresponding to intrinsic carrier concentrations of 2.3 × 1013 and 2.1 × 106 cm−3 at 300 K.17 Equation 1 is the mass action law and is used to calculate ni, where NC and NV are the densities of states of the conduction band and valence band, respectively, which are defined in eqs 2 and 3. ni 2 = NCNV e−(Eg / kT )

(1)

⎛ 2πm*kT ⎞3/2 e ⎟ NC = 2⎜ 2 ⎠ ⎝ h

(2)

⎛ 2πm *kT ⎞3/2 h NV = 2⎜ ⎟ ⎝ h2 ⎠

(3)

Received: March 15, 2016 Revised: April 19, 2016

A

DOI: 10.1021/acs.langmuir.6b01014 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 1. Configuration of ALD grown thin films for each memristor. (a) Base structure with a bottom electrode made of Al:ZnO with 30:1 (Zn:Al) cycle ratio. The second layer is Al:ZnO with (Zn:Al) 20:1 cycle ratio. The third layer is insulating ZnO grown with O3 and diethylzinc. The final layer is Al:ZnO with (30:1) cycle ratio. Insulating barrier layers are grown between the Al:ZnO (20:1) and ZnO layers. (b) Al2O3, (c) Al2O3 + TiO2, and (d) TiO2.

At T = 300 K, based on the reported effective masses of electrons and holes for ZnO (0.28m 0* and 0.59m 0* , respectively)18 and its bandgap (3.37 eV), ZnO oxide should effectively have ni = 0. The thermal energy at room temperature is not sufficient to promote electron migration into ZnO’s conduction band. The origin of the generic n-type behavior is attributed to ZnO defects, with the most studied defects believed to be responsible for the n-type behavior being oxygen vacancies, VO, and hydrogen ions, H+.19,14 In ZnO, the primary point defects are Zn vacancies (VZn), VO, zinc interstitials (Zni), and oxygen interstitials (Oi).20−23 The corresponding energy levels are located in the ZnO bandgap. VZn and VO defects each −2 2+ have three charge states, which are V0Zn and V−1 Zn ; VZn ; and VO , + 0 VO, and VO, respectively. The order of the charges relates to the relaxation of the surrounding atoms, which shift from a relaxed state to a less relaxed state because of the added charges.24 Point defects in metal oxides are currently being used to explain another interesting property called resistive switching.25 These devices are called memristors and consist of a metal oxide between two metal electrodes, which is typically known as an MIM structure. This structure was first devised in 1971 by Leon Chua, who performed a symmetry analysis of the fundamental electronic components of electronic circuits by theorizing a connection between the magnetic flux, φ, and the charge, q.26 The current understanding is that a memristor remembers its resistance from the last voltage applied and has two distinct states: a low-resistive state (LRS) and a highresistive state (HRS).26 The application of a switching voltage

to the MIM structure can cause switching between the two resistive states of the metal oxide through oxygen vacancy migration and the creation of oxygen vacancy filament-like conduction paths between the metal electrodes.27−30 The current vs voltage curve of the memristor takes on the appearance of a loop with the LRS and HRS curves crossing at the origin. The LRS-to-HRS transition at a negative voltage is called the switching voltage, and the opposite transition is called the positive switching voltage. ZnO is capable of multiple electronic states that depend on the number of vacancies and the charge state of each vacancy. According to density functional studies, the first charge state is located near the valence band (VB) at approximately 0.6−0.9 eV and corresponds to two electrons. The next charge state is located 2.6 eV above the VB and corresponds to a single added electron. The third charge state is located 2.9 eV above the VB and is closer to the conduction band (CB).21−24 In the case of ZnO, which has a bandgap of 3.38 eV, the transition energy of electrons moving from V0O to the nearby V+O would be approximately 2 eV. The transition energy of an electron moving to V2+ O would be 2.3 eV, whereas 2.78 eV is required to reach the CB. An electron transitioning from V+O to V2+ O would require 0.3 eV, and one transitioning to the CB would require 0.78 eV. The thermal barrier for electrons escaping from V+O is 0.30 eV. In addition to the individual electron states, each vacancy has different vibrational states because of the atoms surrounding it. The dangling Zn−O bonds around the vacancy are able to relax to different configurations depending on its B


Article

Langmuir

Figure 2. Process flow for creating resistive switching MOS. Steps a−f are for creating the Al2O3 vapor mask. Steps g−m are for depositing and pattering the Al:ZnO (Zn:Al) 20:1. Steps n−p show how to prevent high temperature Al2O3 from forming on the Al2O3 mask. In step o TiO2 is deposited in place of Al2O3 or on top of Al2O3 as needed. Steps q−v show how to deposit the ZnO and top Al:ZnO (Zn:Al) 30:1 electrode. Steps w−z show how to remove the Al2O3 mask to allow contact with bottom Al:ZnO (Zn:Al) 30:1. (aa) Final device.

charge state.24 In the V0O charge state, the surrounding Zn−O bond angles around the vacancy are not relaxed, and the configuration is similar to the structure in the presence of oxygen. The fully relaxed state is V2+ O and is near the CB 0 minimum. A vacancy can transition from V2+ O to VO through thermodynamic transitions and is limited by only the thermodynamic transition energy, ε(q/q′), where q′ = q + 1e− and represents the energy required for an individual defect in charge state q to transition to the next charge state q′.20,21,31 VZn has luminescence peaks of 2.5 eV, which are associated with the energy level position in the bandgap.20,23 The thermodynamic transition has an energy ε(0/−1) = 0.18 eV above the VB, and the ε(−1/−2) transition is located at 0.9 eV above the VB. The thermodynamic transitions for VO are in the range of 2.69−2.9 eV for ε(2+/+), 1.81−1.94 eV for ε(+/0), and 2.25− 2.42 eV for ε(2+/0). If interactions between defects in a variety of states, the CB, and the VB are allowed, ZnO’s electro-optical properties can become complex. ZnO in an MIM structure including oxygen vacancies can behave similarly to Al2O3, which exhibits Fowler−Nordheim electron tunneling from electrodes into the Al2O3 defect states.32 Interactions between the vacancies create an intermediate band.33,34 If VO could form an intermediate band (V+O) in ZnO, electrons could be transported across ZnO in a continuous band or through hopping from vacancy to vacancy. Because VO is the main defect responsible for memristors’ switching mechanisms, manipulating the electronic properties of the VO and the applied electric field redistribution of vacancies in ZnO should modulate the memristor’s current vs voltage curve. These modulations would arise from the

manipulation of the vacancy charge states through optical or thermodynamic interactions. An electric field applied to the MIM structure would redistribute the oxygen vacancies, resulting in variation in the conductivity of the metal oxide.30,35 Interactions between vacancies are believed to generate conduction paths. The removal of vacancies from one of the metal−metal oxide interfaces in the MIM structure creates an insulating barrier, resulting in electron tunneling across that barrier.36 Existing studies focus on the creation of defect states and the migration of defect states under the influence of electric field. In this study we will explore the memristor behavior when the defect charge states are manipulated. Engineering a memristor with transparent electrodes and active layer allows for the manipulation of the vacancy charge state using an external optical source.

2. EXPERIMENTAL SECTION To explore the effect of light on the electron-transport mechanisms of the resistive switching process, we developed a transparent resistive switching device consisting of an Al:ZnO TCO. The Al:ZnO growth procedures and electrical properties were established in previous studies.37,38 Transparent resistive switching devices, including fully ZnO-based ones, have been constructed previous using various techniques, such as sputtering, sol−gel, metal−organic chemical vapor-phase deposition (MOCVD), and pulsed laser deposition.39−41 These techniques require premade material sources with set empirical ratios. All the layers in the resistive switching device studied here were grown using the ALD system, and they are transparent.37,38 The Al:ZnO was grown using the Cambridge NanoTech Savannah 100 Atomic Layer Deposition System, utilizing the cycle ratio doping technique to control the Al concentration. The metal−organic precursors used were diethylzinc (DEZ) as a Zn source, trimethylC


Article

Langmuir

Figure 3. (a) SEM image of Al2O3 vapor mask (light shade) with 400 μm diameter holes deposited on the Al:ZnO (Zn:Al) 30:1 bottom electrode (dark shade). (b) Fully formed device with Al2O3 and TiO2 barrier layers. (c) Cross section of 1 cm × 1 cm device with Al2O3 barrier layer. (d) Transmission spectroscopy of 1 cm × 1 cm device with Al2O3 barrier layer grown on sapphire. aluminum (TMA) as an Al source, and H2O as an oxidizer. The cycle ratio relates the number of ALD cycles with DEZ pulses to that with TMA and is represented as “(Zn:Al)”. A single ALD cycle is defined as when the metal−organic precursor pulse is introduced to the growth chamber by N2 as a carrier gas, followed by a timed N2 purge. Next, an oxidizer is pulsed into the chamber. The Al:ZnO (AZO) TCO with a 30:1 cycle ratio was used for the top and bottom electrodes (Figure 1) because of its metallic electron-transport properties. All active layers were grown at a substrate temperature (Ts) of 250 °C, except for the ZnO layer. The AZO(30:1) layer was grown on a glass substrate, followed by the deposition of AZO (20:1), which has a semiconducting transport property.37 In Figure 1a−d, the ZnO layer is grown using DEZ and O3 as an oxidizer at Ts = 220 °C. We established that O3-grown ZnO is insulating.42 A layer of ALD grown with either Al2O3 or TiO2 or both was added between the AZO (20:1) and ZnO. The insulating Al2O3 was grown using TMA and H2O at Ts = 250 °C. The other insulating oxide was TiO2, which was grown using tetrakis(dimethylamido)titanium(IV) (TDMATi) and H2O as an oxidizer at Ts = 250 °C. In this study, the TiO2 + Al2O3 MOS, as shown in Figure 1c, consisted of TiO2 grown using an ALD exposure mode, with the TDMA Ti precursor vapor pulse extended for an additional 8 s in the growth chamber, thereby increasing the coverage and growth rate. The TiO2 was grown via a total of 20 ALD cycles. The growth rate of TiO2 in the Savannah 100 ALD system has been documented to be approximately 0.40 Å/cycle in the without-exposure mode. The AZO(30:1) layer with metallic electron-transport property, insulating ZnO, and semiconducting AZO(20:1) (MOS) structure was made as a 1 cm × 1 cm multilayer film for the absorption and transmission optical studies. To investigate the electrical resistive switching and optical properties simultaneously, the metal-oxide semiconductor (MOS) structure was created in a circular shape with a diameter of 400 μm. To create a patterned ALD, ZnO-doped and undoped films were grown on top of another AZO or ZnO film at Ts = 250 °C, which required a new high-temperature masking technique for ALD. In ALD the precursor molecules can go under solid masks changing the shape of the intended pattern. If Ts is kept at 150 °C or below

polymer resist like PMMA, poly(vinylpyrrolidone) (PVP), and poly(dimethylsiloxane) (PDMS) can be used to pattern ALD films.43−46 At temperatures above 150 °C the use of these polymers is not feasible due to decomposition of polymer or cross-linking of polymer chains complicating or preventing etching of polymer. To solve this problem, we used the process shown in Figure 2 to pattern ZnO-based layers on ZnO-based substrates, while also creating proper electrical isolation of multiple MOS structures. The main aspect of the technique is the patterning of poly(methyl methacrylate) (PMMA) spin-coated film using Cr shadow mask pattern, deposited on quartz, to create a positive pattern. The Cr shadow mask shields PMMA from a 185 and 254 nm UV light combination generated by a Novascan PSD UV cleaner. Next, Al2O3 grown at 150 °C with 150−200 ALD cycles is deposited on the patterned PMMA. After Al2O3 growth UV light is illuminated through the transparent Al2O3 to develop the PMMA pattern below. The Al2O3 on top of the developed PMMA is disrupted forming fractures, which allows the Al2O3/PMMA layer to be etched away by sonication of sample in acetone. The Al2O3 layer is kept thin to keep Al2O3 conformed to PMMA surface,47 keeping stress high. After the sonication process a negative pattern Al2O3 vapor mask remains, and it protects the ZnO-based substrate from ALD precursor reactions while also acting as an etching stop in a future step. On top of the patterned Al2O3 structure ALD ZnO-based film is deposited; then a PMMA film is spin coated on top followed by UV patterning with Cr mask. The pattern is same as the Al2O3 vapor mask pattern and was aligned to pattern using in lab technique consisting of magnetically locking Cr shadow mask to patterned substrate. The multilayer structure is then placed in diluted HCl. The PMMA pattern protects the second ZnO based layer underneath from the etching process. The final structure is a patterned ZnO based film grown on a ZnO-based film. For large AZO (30:1) bottom contact area polyimide tape was used to protect AZO(30:1) from high temperature Al2O3 growth as shown in Figure 2 (n and o). It is noted that PMMA could not be used for patterning due its temperature limit. Patterned holes were left in the low temperature Al2O3 layer for a later step where 0.4 M NaOH is used48 for the low temperature Al2O3 lift off in steps y and D


Article

Langmuir z of Figure 2. Multiple structures were created on AZO(30:1)-coated glass substrate. The MOS devices were tested using a probe station and a semiconductor characterization system. The array of MOS devices grown on a glass substrate, as described in Figure 2, was placed on a Signatone 1160 series probe station. Two Signatone model S-725-PLV micropositioners loaded with tungsten probes were used for the measurement. One probe was placed on the bottom AZO(30:1) contact, and the other was placed on the 400 μm diameter AZO (30:1) top contact. The probes were connected to the source monitor units of a Keithley 4200-SCS semiconductor characterization system for current vs voltage measurements, with the voltage polarized relative to the top AZO(30:1) contact. The 4200-SCS was also equipped for capacitance vs bias voltage measurements via the 4200-CVU unit.

silicone adhesive was used to prevent the growth of hightemperature Al2O3 on the 200-cycle, low-temperature Al2O3, as shown in Figure 2n,o. This polyimide tape is, however, prone to degassing and can leave behind silicone residue. The lowtemperature Al2O3 was removed from the AZO (30:1) as shown in Figure 2z using a NaOH solution to cleave the bonds between the two layers.48 The electrical properties of the MOS devices were measured by connecting it to the top Al:ZnO electrode and the exposed bottom Al:ZnO blanketing electrode. Following fabrication, the ZnO resistive switching MOS (rMOS) was subjected to a forming process to give it the ability to switch between resistive switching states. The initial state of the rMOS with an Al2O3 + TiO2 insulator barrier is shown in Figure 4, which shows that the device behaved as a capacitor

3. RESULTS AND DISCUSSION Each step of the ALD-based fabrication process of the MOS structure was studied to confirm the deposition of the desired film and determine the versatility of the Al2O3 mask layer. The presence of the Al2O3 high-temperature mask on the AZO (30:1), as shown in Figure 2a−f, can be seen in the scanning electron microscope (SEM) images of Figure 3a. The dark-gray circles are holes in the Al2O3 that reveal the AZO (30:1) substrate. Next, following the process flow in Figure 2, we obtained the final MOS structure, as seen in the field emission SEM (FESEM) images in Figure 3b, which show the MOS with an Al2O3 + TiO2 barrier layer from Figure 1c. Figure 3c presents a 1 cm2 MOS with an Al2O3 barrier layer cross section, as depicted in Figure 1b, and the other layers, which are marked by a white arrow going from the bottom to the top AZO (30:1) TCO electrodes. The measured thickness values are consistent with what was expected based on the ALD cycles and growth rates. The AZO (30:1) bottom electrode resulting from 810 ALD cycles was approximately 65 nm thick. The next layer is the AZO (20:1), which was grown with 760 ALD cycles and had a thickness of approximately 63 nm and a clear boundary with the AZO (30:1) layer. The third layer is the Al2O3 layer, represented by a dark band, which was grown via 300 ALD cycles and had a thickness on the order of 25 nm. On top of the Al2O3 is a layer of ZnO grown with 1400 ALD cycles of DEZ and O3 at Ts = 220 °C, as established in previous works.42 The thickness ranged from 46 to 50 nm, and a boundary between the 63 and 68 nm thick fifth AZO (30:1) layer is present, but it was not as defined as that between the AZO (20:1) and AZO (30:1) layers, so we have marked the inteface location with a white line. In Figure 3d, we show the transmission spectroscopy of the MOS structure grown on a sapphire substrate with an Al2O3 barrier. The technique shown in Figure 2 was timeconsuming, requiring 60 min to develop poly(methyl methacrylate), PMMA, and more experiments are needed to determine its nanoscale feasibility. This technique allowed us to pattern ZnO and Al:ZnO films grown at 250 °C, a temperature that would damage most polymers used in masking. However, it requires additional steps to protect the AZO(30:1) bottom electrode during the deposition of additional layers and then to expose the electrode for measurements after fabrication. The PMMA technique demonstrated in Figure 2a−f cannot be used for protection because of the 250 °C temperature, and thus, the PMMA cannot be removed by etching. Additionally, the steps in Figure 2h−m cannot be used because Al2O3 is resistant to etching. Hydrofluoric acid could be used to etch Al2O3 but will result in damage to and the removal of all oxide layers if the etching rate is not controlled. To protect the AZO (30:1) electrode, a high-temperature-resistant polyimide tape with

Figure 4. Capacitance vs bias voltage of memristor with Al2O3 and TiO2 barrier showing curves measured consecutively starting with 10 V and then 11, 12, and 13 V. Top inset: current vs bias voltage for same device cycled from low voltage to 8 V, then back to low voltage, and is repeated for 9, 10, and 11 V. Bottom inset: shown is the resistive switching behavior of the device activated following the application of voltage near the 13 V forming voltage. The resistive switching device current vs voltage loop is measured at voltages below the forming voltage. The smaller inset shows how the LRS and HRS cross at the origin.

with capacitance in the 10−8−10−9 F range. In Figure 4, the voltage applied to the rMOS was cycled from −5 to 10 V and then from −5 to 11 V. This process was repeated for 12 and 13 V. After each bias voltage cycle, the capacitance curve shifted to the right, indicating a redistribution of charges. The current vs voltage curve in Figure 4 (top inset) shows the evolution of the current loop for different voltage curves. The loop had a flat current profile below 5 V, and subsequently, the current rose and then dipped. The current continued to increase exponentially with intermediate dips, and then, upon reaching Vmax, the bias voltage decreased. The current exhibited E


Article

Langmuir

Figure 5. Resistive switching current vs voltage curves of devices, activated further with voltages above the rMOS configuration forming voltage. (a) The barrier layer in the devices measured is Al2O3. A rMOS with 700 ALD cycles and one with 1400 ALD cycles of O3 grown ZnO are studied. (b) A rMOS with Al:ZnO (Zn:Al) 30:1 electrode grown with 1500 ALD cycles and one with 810 cycles. Above a current threshold rMOS undergoes an irreversible resistive switching event with voltages of magnitude above 9 V. (c) Resistive switching MOS current vs voltage curves of devices with and without Al2O3 barrier. (d) Compare device with Al2O3 barrier to one with Al2O3 + TiO2 barrier; (e) compare device with Al2O3 barrier to one with TiO2 barrier.

the oxygen deficient regions connected with shared metal electrode. Formation of Schottky barrier at metal and oxygen rich oxide interface suppresses current in a voltage range related to barrier heights. Our device in the second state then has more than one ion conducting region and Schottky barriers for electrons to overcome. Further increasing the voltage above 13 V and switching the polarity resulted in another shift in the resistive curve to a higher current state in the mA range. The threshold we used for the resistive switching device studied here was that the resistive switching loops must be repeatable for at least five cycles. The third resistive switching state was used here. During the first state the device behaves like a MOS capacitor; however, at higher voltages it switches to the second state with an anomalous behavior, which occurs when two memristors are put together in series. In the third state the

exponential decay in a separate curve shifted to the right of the rising voltage curve. An increase in Vmax thus resulted in only a small shift in the rising high-current curve, with Vmax = 11 V showing the largest shift; however, a gradual increase in the positive shift of the return curve occurred. The forming process was triggered by voltages above 13 V, at which the I−V curve developed a memristor loop with HRS and LRS curves with currents on the order of 10−5 A. The red and blue curves in the bottom inset of Figure 4 represent half of the resistive switching loop. The HRS state (red) above 0 V became the LRS state below 0 V, and the opposite occurred for the LRS state curve (blue). This deviation from the model can be resolved by noting the HRS and LRS states in the bottom inset of Figure 4, which cross before and after the origin. The behavior in the bottom inset is seen in devices with two bipolar memristor combined in series.49−51 The memristors are combined with F


Article

Langmuir

Figure 6. Ridged electronic band structure of rMOS with Al2O3 + TiO2 barrier. Oxygen vacancy positions with corresponding charge states are shown for each metal oxide.

2 orders of magnitude change in current after switching; however, the device was not reversible due to the high current destroying the area around the contact of the measuring probes. Thinner Al:ZnO electrodes are used to avoid this problem. More than five measurements were performed for individual device and multiple devices were tested on a single substrate. We next explored the effect of the metal oxide barrier layers on the resistive switching behavior. In Figure 5c, rMOS devices with and without Al2O3 are compared. The rMOS demonstrates resistive switching without Al2O3, but the ΔR is greater when Al2O3 is deposited over 300 ALD cycles. The Al2O3based MOS devices were more stable after multiple cycles than those without the Al2O3 layer. In Figure 5d, MOS devices with Al2O3 are compared to those with Al2O3 + TiO2. Devices with the Al2O3 barrier layer have larger ΔR values than those with TiO2 added. MOS devices with Al2O3 and 20 ALD cycles of TiO2 performed better than those with only a TiO2 barrier layer because the barrier height of TiO2 with ZnO is lower than that with Al2O3. The bandgap of Al2O3 has been established to be 8.8 eV, and the TiO2 bandgap is 3.1 eV.52 The ZnO conduction band edge is 0.35 eV below the TiO2 CB edge and 3.35 eV below the Al2O3 CB edge.52−56 The TiO2 layer has a dielectric constant of 80, and Al2O3 has a dielectric constant of 9; thus, TiO2 can hold a larger electric field.52 Thus, TiO2 could effectively move charge toward or away from the tunnel barrier at a faster rate, but the low barrier height leads to electrons transitioning over the TiO2 barrier. The addition of the Al2O3 layer adds a barrier that electrons cannot transition over. Figure 5e shows the improved resistive switching behavior compared

device runs at higher current and is the most stable state in regards to higher handling increasing current. Varying the parameters of the rMOS layers, including the thicknesses of the ZnO and Al:ZnO (30:1) layers, can modify the resulting resistive switching behavior. The rMOS contains ZnO layers of two different thicknesses and an Al2O3 barrier layer. The rMOS grown via 1400 cycles of O3-grown ZnO in Figure 5a exhibits a change in resistance, ΔR, between the HRS and LRS states that is greater than that of the 700-cycle ZnO sample. The curves in Figure 5b establish a limit for the Al:ZnO contact resistivity. We demonstrated that the 1500-cycle AZO (30:1) has more metal-like electronic properties and lower resistivity than sub-1000 cycle films, such as the 810-cycle one.38 The 1500-cycle AZO (30:1) had lower resistivity but was prone to breakdown at the probe when high voltages were applied, triggering an irreversible resistive switching event, as seen in Figure 5b. The rMOS device ZnO layer was grown over 1400 ALD cycles to improve its resistive switching, and the Al:ZnO (30:1) contacts were grown via 810 ALD cycles to prevent contact breakdown above 9 V bias voltage. The difference between the 1500 cycle Al:ZnO and the 810 cycle Al:ZnO sample is the magnitude in electrical conductivity. The 1500 cycle Al:ZnO has the higher conductivity,37,38 so it can pass more current at low voltages, but there is a current limit around 70 mA where Al:ZnO breaks down. The current device uses Al:ZnO transparent conductor as electrodes. The probes used for current vs voltage (I−V) measurement make contact with top and bottom electrodes. In Figure 5b, the devices made with Al:ZnO consisting of 1500 ALD cycles initially show about G


Article

Langmuir

increase in current is evident. The TiO2 grown with 100 ALD cycles had a greater ΔR and a more-defined resistive switching curve. In Figure 7b, the MOS devices with insulator barriers that demonstrate the best resistive switching performance are compiled. Figure 6 shows that Al2O3 has oxygen vacancies with electronic states 2.57 eV above its VB maximum, which is positioned at −9.8 eV; these correspond to locations that are 0.48 eV above the ZnO VB maximum at −7.71 eV. In addition to acting as a barrier layer, Al2O3 may also contribute to the resistive switching by blocking electrons from moving between the conduction bands of the ZnO and Al:ZnO (20:1) layers, directing them to tunnel into the Al2O3 defect levels. TiO2 has a V0O that is 0.9 eV below its CB minimum (−4 eV relative to the vacuum level).57 The TiO2 VO is in a relaxed state and is occupied by two electrons. The other vacancy charge states are located in the CB of TiO2. Because the filled singular defect state in the TiO2 bandgap cannot accept electrons, the TiO2 does not contribute to the resistive switching behavior. TiO2 increases the current of the rMOS by donating electrons from the VO to the CB.26 The increase in the CB carrier concentration would allow electron transport to bypass vacancies. The MOS devices in Figure 7b were used for optoelectronic studies. To determine the effect of laser diode light on the rMOS structure’s resistive switching properties, the I−V measurements were repeated six times. The laser illumination sequence started with three measurements to establish the baseline memristor behavior without an optical stimulus. The fourth and fifth measurements were taken during illumination by a singlewavelength laser diode. Following the fifth measurement, the laser diode stimulus was removed, and then, the sixth measurement was performed to determine whether the device would reset back to the baseline loop current. In Figure 8a, the curves for the rMOS with an Al2O3 + TiO2 barrier is shown. In the positive bias voltage range, the LRS is approximately 1.57 times higher than the HRS when the bias voltage is below 0.5 V and 1.26 times higher near 6 V. The rMOS was next illuminated with 405 nm laser light. Then, the LRS was only 1.43 times higher than the HRS at 0.05 V and 0.88 times higher at 6 V. The HRS curve shifts to 22−40% higher currents from 0.05 to 4.75 V but then decreases to 11% at higher voltages, as seen in the inset of Figure 8a. The decrease in the difference between the LRS and HRS during the laser illumination could indicate that the electron density in the CB increased; however, the difference between the response of the HRS and that of the LRS could indicate the modification of another conduction mechanism, such the defects’ charge states. The inset in Figure 8a shows that the memristor postillumination curve, shown in green, nearly returns to its initial I−V behavior, with a slight shift to higher current indicating the restoration of the conduction mechanism with residual effects. When the barrier was changed to Al2O3, the 405 nm laser light also altered the resistive switching curve, as shown in Figure 8b. The HRS in the Al2O3 barrier layer rMOS showed a clear current change under 405 nm laser light illumination that follows the illumination pattern. The shift was 13% at a bias voltage of 0.01 V, with a maximum of 24% at 4.07 V and subsequent decrease to values below 1%. The postillumination measurement revealed that the HRS curve returned to its initial state. The LRS curves shifted before illumination, with the current decreasing after each initial measurement. The laser shifts the LRS curve to higher currents. Measurements following the removal of the laser illumination returned to the initial state

to that of the rMOS devices with TiO2 grown via only 20 ALD cycles, which behaves more ohmically at higher voltages. The insertion of TiO2 does not enable low voltage operation it instead limits us to low voltage operation due to random ruptures in device layer. The ridged electronic band structure of the MOS device with Al2O3 + TiO2 barrier layers is presented in Figure 6, including the positions of the oxygen vacancies for all metal oxides.34,57 The transform voltage is related to the electroforming process at the metal semiconductor interface,36 and these (Vset and Vreset) are relatively large. At the forming voltage the oxygen is pulled toward the interface. The interface can heat up due to the electroforming process, Joule heating at the Ohmic junctions (metal/n-type semiconductor interface).58 The heat generated from this can lower boundaries for creating vacancies. The high transform voltages could be due to barrier heights or the conductivity of the Al:ZnO electrode layers. To further explore the influence of the TiO2 layer, we created devices with different thicknesses grown over 20, 100, and 200 ALD cycles. These devices were then compared, as shown in Figure 7a, to an MOS with no TiO2, constructed as indicated in Figure 1a. Comparing these MOS devices to one with no TiO2 reveals that their ΔR values do not differ by much, but an

Figure 7. (a) Resistive switching behavior of rMOS withTiO2 barrier layers grown with different number of ALD cycles. (b) rMOS configurations with best performance. H


Article

Langmuir

Figure 8. Current vs voltage curve of rMOS resistive switching device with barrier layers (a) Al2O3 + TiO2 barrier, (b) Al2O3 barrier, and (c) TiO2 barrier before, during, and then after illumination with 405 nm (3.06 eV) wavelength laser diode. The insets are a close-up of the LRS and HRS response to the laser diode.

electrons from V0O to another oxygen vacancy in the +1 or +2 charge state. If the Franz−Keldysh effect59 is considered, the band bending could allow for transitions from V0O to the CB at higher bias voltages. The change in the HRS caused by the 450 nm laser light was not as large as the response produced by the 405 nm laser light. It is noted that for all measurements with laser light we measure the I−V characteristics 3 times without laser stimulation. Next we do two measurements with laser stimulations. Between the measurements with laser illumination the laser is turned off. The final measurement is without any laser stimulation and is expected to have levels the same as before measurements with laser illumination; that is established in the first three measurements. We also performed the I−V curves for the first three measurements with no laser stimulation. Under 520 nm wavelength laser illumination, as shown in Figure 10a, the rMOS with an Al2O3 + TiO2 barrier did not follow the laser illumination sequence pattern. Instead, a shift to lower current occurred in the postillumination HRS curve, but such shifts often follow individual measurements under no illumination. A 520 nm laser has a photon energy of 2.38 eV,

with slight deviations. Figure 8c, which presents the results obtained for the rMOS with a TiO2 barrier, indicates that the MOS responds to the 405 nm light, but frequent fluctuations and shifts in the curves and a large asymmetry between the polarizations of the measurements occurred. Subsequently, we applied a 450 nm laser with a photon energy of 2.76 eV. The resistive switching curves in Figure 9 illustrated the interactions between the rMOS devices and the 450 nm laser. In Figure 9a,b, we see shifts in the resistive switching HRS curve, but they are random and do not fit the illumination sequence pattern. In Figure 9c, the shifts fit the illumination sequence pattern, with changes of 2.0% at 0.05 V increasing to 4.5% at 6.8 V and then decreasing to 2.9% as the bias voltage increased to 10 V. In the inset in Figure 9c, the postillumination curve shifts back toward the initial state, with a slight shift below the initial conditions. The photon energy of 2.76 eV is sufficient to cause the individual oxygen vacancy defects in ZnO to undergo thermodynamic transitions. Thus, a 0 vacancy with a V+O or a V2+ O charge state could achieve the VO charge state, thereby increasing the MOS device’s resistance. This photon energy would also allow for the promotion of I


Article

Langmuir

Figure 9. Current vs voltage curve of rMOS resistive switching device before, during and then after illumination with 450 nm (2.76 eV) wavelength laser diode: (a) Al2O3 barrier layer; (b) TiO2 barrier layer; (c) Al2O3 + TiO2 barrier layer. The insets are close-ups of the LRS and HRS response to the laser diode.

which is sufficient to produce ε(+/0) thermodynamic transitions, which could explain the shifts in the HRS toward lower currents. Some transitions between defects in different states were possible. First, the electron transition was from V0O to V+O, but this transition results in the formation of the starting defect states. The transition of an electron from V0O to V2+ O would result in the creation of 2V+O after relaxation, which would increase the conduction of injected electrons. Electron transitions from V+O to V2+ O result in the formation of the starting defect states. A transition of an electron from V+O to the CB results in the creation of a V2+ O , thereby reducing the injected electron propagation between defects but increasing the transport through the CB. The 520 nm laser light did not completely shift all charge states to the V0O charge state through thermodynamic transition because the resistive switching loop did not collapse. Neither the 520 nm nor the 450 nm laser light shifted the HRS curve as substantially as the 405 nm laser light, which resulted in a shift of 24%. The number of photons impinging on the rMOS per second was approximate 9.20 × 1015, with lower-energy laser photons increasing exhibiting values 11% and 28% higher,

which should have a stronger effect on the defect states. A possible explanation for the shifts in the LRS and HRS curves is that VZn acts as a buffer, maintaining all defects in the V+O charge state without a need for the absorption of laser photons. The VZn is an acceptor capable of creating holes in the VB of the 23 ZnO as it shifts to the V−2 Zn charge state. These holes may act as recombination sites for single electrons from the V0O charge state, thereby creating a V+O charge state. V2+ O can transition to VO+ through a ε(2+/+) thermodynamic transition or by −1 accepting electrons from V−2 Zn . The resulting VZn can be reversed by the promotion of an electron from the VB, which would also replenish the holes. An effective way of creating V+O is by sending a trapped electron from V0O to the CB. Any thermodynamic transition of V+O after excitation by laser light would be reversed by direct or indirect interaction with VZn. Upon illuminating the rMOS structure with laser light, the HRS curves shifted, and we demonstrated that the difference between the HRS and photoresponse HRS has a maximum. This maximum in the Gaussian curve is attributable to the changed dependence of the rMOS on the bias voltage after laser illumination. The percent changes in the current of the J


Article

Langmuir

relationship at voltages above 4.05 V, but a decrease in slope occurred between 5.26 and 6.5 V. The I−V curve above 6.5 V and below 5.26 V exhibited a quadratic relationship. The TiO2 barrier samples maintained a quadratic relationship after laser illumination. A cubic relationship is often associated with electrons being injected into oxides with filled traps or trapfilled-limited (TFL) conduction, whereas the quadratic relationship is associated with space-charge-limited conduction (SCLC), in which electrons are injected into oxides with defect traps.27,39,60 In the ZnO active layer, the traps would be VO, which would undergo a change in their charge state when filled. Additionally, the energy level position in the ZnO bandgap transitions to approximately 0.6 eV above the VB maximum when it is filled, increasing the energy required for electrons to be ejected to electrodes and, coupled with the nonuniform distribution of VO in ZnO, making it more insulating. Nonuniform distribution of vacancies has an impact due to the presence of barrier between oxygen vacancy high and low concentration regions.61 During laser light illumination, V0O may change to V+O, creating a path for injected electrons to traverse, which would indicate that the shift of HRS to higher currents is caused by changes in the VO charge state.

4. CONCLUSIONS In conclusion, we developed a high-temperature precursor vapor mask using Al2O3 to pattern the ALD of ZnO and Al:ZnO layers grown on ZnO-based substrates. This technique was used to create memristor devices based on Al:ZnO thin films with metallic electron-transport properties, Al:ZnO with semiconducting electron-transport properties, and insulating ZnO grown with ozone. We demonstrated that adding Al2O3 and TiO2 barrier layers improved the resistive switching behavior. The percent change in the resistance between the HRS and LRS of the memristor with Al2O3 + TiO2 was approximately 157%. The devices were exposed to laser light from three different laser diodes. The 405 nm laser diode induced the largest shift in the HRS current of 40% for the memristor with the Al2O3 + TiO2 barrier, 24% for the Al2O3 barrier, and 23% for the TiO2 barrier. The 450 nm laser diode only noticeably affected the Al2O3 + TiO2 barrier HRS, resulting in a shift of 2.9% under illumination. The HRS shift of the memristor under 520 nm laser illumination did not fit the 3 × off, 2 × on, and then off laser illumination pattern. Considering all possible between-defect shifts and the thermodynamic transition of ZnO defects, the resistive switching loop should have shown significant shifts in the LRS and HRS states for all three wavelengths. This discrepancy could be attributable to null electron transitions between defects, simultaneous electron transport not involving oxygen vacancies, or a mechanism that buffers the V+O charge state concentration. Understanding the transitions between defects in the ZnO and their interactions with thermodynamic transitions could facilitate understanding n-type conduction in ZnO and lead to the development of new devices based on defect transitions. More studies are needed to determine the exact positions of the Zn vacancy single electron states; although the neutral and −1 charge state positions in the bandgap could be derived from photoluminescence and density functional theory studies of thermodynamic transitions, the −2 charge state has not been well documented. The ability to use a single deposition system and the same base precursor chemicals to create multilayer resistive switching devices consisting of

Figure 10. (a) Current vs voltage curve of rMOS resistive switching device with Al2O3 + TiO2 barrier layers before, during, and then after illumination with 520 nm (2.38 eV) wavelength laser diode. The inset is a close-up of the HRS response to the laser diode. (b) Percentage change in current vs voltage curve of the high resistive state (HRS) due to laser diode illumination for different combinations of barrier layer oxides and laser diode wavelengths.

HRS curve after laser illumination are shown in Figure 10b for different barrier layer configurations and laser wavelengths. When exposed to 405 nm laser light, the Al2O3 + TiO2 barrier MOS exhibited a maximum shift in HRS at 4.75 V and moves to 6.8 V in response to 450 nm laser light. The Al2O3 barrier MOS had a maximum at approximately 4.07 V. The percent change of the TiO2 barrier MOS was relatively constant down to 2.65 V and then increased to 23% at its maximum. To better understand the HRS I−V curves before and after illumination, they were fitted with polynomial functions to determine the extent of the linear relationship between the voltage and the current, I ∝ VP, where P is 1, 2, 3, ..., n, n + 1. The HRS I−V curves in the initial states had a cubic relationship, except for the TiO2 barrier samples, which exhibited quadratic relationships. The Al2O3 + TiO2 barrier rMOS curve develops a quadratic relationship under 405 nm laser light illumination did not change under 450 nm laser illumination. The HRS curve of the Al2O3 barrier rMOS showed no change in its voltage K


Article

Langmuir

Prepared By Atomic Layer Deposition. J. Appl. Phys. 2013, 114, 024308. (16) Saha, D.; Das, A. K.; Ajimsha, R. S.; Misra, P.; Kukreja, L. M. Effect of Disorder on Carrier Transport in ZnO Thin Films Grown by Atomic Layer Deposition at Different Temperatures. J. Appl. Phys. 2013, 114, 043703. (17) Kasap, S. O. Principles of Electronic Materials and Devices, 3rd ed.; McGraw-Hill: Boston, 2006. (18) Meyer, B. K.; Alves, H.; Hofmann, D. M.; Kriegseis, W.; Forster, D.; Bertram, F.; Christen, J.; Hoffmann, A.; Straßburg, M.; Dworzak, M.; Haboeck, U.; Rodina, A. V. Bound Exciton and Donor−Acceptor Pair Recombinations in ZnO. Phys. Status Solidi B 2004, 241, 231− 260. (19) Lee, Y. E. Microstructural Evolution and Preferred Orientation Change of Radio-Frequency-Magnetron Sputtered ZnO Thin Films. J. Vac. Sci. Technol., A 1996, 14, 1943. (20) McCluskey, M. D.; Jokela, S. J. Defects in ZnO. J. Appl. Phys. 2009, 106, 071101. (21) Gallino, F.; Pacchioni, G.; Di Valentin, C. Transition Levels of Defect Centers in ZnO by Hybrid Functionals and Localized Basis Set Approach. J. Chem. Phys. 2010, 133, 144512. (22) Ahn, C. H.; Kim, Y. Y.; Kim, D. C.; Mohanta, S. K.; Cho, H. K. A Comparative Analysis of Deep Level Emission in ZnO Layers Deposited by Various Methods. J. Appl. Phys. 2009, 105, 013502. (23) Janotti, A.; Van de Walle, C. G. Fundamentals of Zinc Oxide as a Semiconductor. Rep. Prog. Phys. 2009, 72, 126501. (24) Janotti, A.; Van de Walle, C. G. Oxygen Vacancies in ZnO. Appl. Phys. Lett. 2005, 87, 122102. (25) Yang, J. J.; Pickett, M. D.; Li, X.; Ohlberg, D. A. A.; Stewart, D. R.; Williams, R. S. Memristive Switching Mechanism for Metal/Oxide/ Metal Nanodevices. Nat. Nanotechnol. 2008, 3, 429−433. (26) Strukov, D. B.; Snider, G. S.; Stewart, D. R.; Williams, R. S. The Missing Memristor Found. Nature 2008, 453, 80−83. (27) Park, S.-J.; Lee, J.-P.; Jang, J. S.; Rhu, H.; Yu, H.; You, B. Y.; Kim, C. S.; Kim, K. J.; Cho, Y. J.; Baik, S.; Lee, W. In Situ Control of Oxygen Vacancies in TiO2 by Atomic Layer Deposition for Resistive Switching Devices. Nanotechnology 2013, 24, 295202. (28) Tedesco, J. L.; Gergel-Hackett, N.; Stephey, L.; Herzing, A.; Hernández-Mora, M.; Kopanski, J.; Hacker, C.; Richter, C. Flexible Memristors Fabricated Through Sol-Gel Hydrolysis. ECS Trans. 2011, 35, 111−120. (29) Williams, R. How We Found The Missing Memristor. IEEE Spectrum 2008, 45, 28−35. (30) Joglekar, Y. N.; Wolf, S. J. The Elusive Memristor: Properties of Basic Electrical Circuits. Eur. J. Phys. 2009, 30, 661−675. (31) Van de Walle, C. G. First-Principles Calculations for Defects and Impurities: Applications to III-Nitrides. J. Appl. Phys. 2004, 95, 3851. (32) Hickmott, T. W. Temperature-Dependent Fowler−Nordheim Tunneling and a Compensation Effect in Anodized Al-Al2O3‑Au Diodes. J. Appl. Phys. 2005, 97, 104505. (33) Nigo, S.; Kubota, M.; Harada, Y.; Hirayama, T.; Kato, S.; Kitazawa, H.; Kido, G. Conduction Band Caused by Oxygen Vacancies in Aluminum Oxide for Resistance Random Access Memory. J. Appl. Phys. 2012, 112, 033711. (34) Momida, H.; Nigo, S.; Kido, G.; Ohno, T. Effect of VacancyType Oxygen Deficiency On Electronic Structure in Amorphous Alumina. Appl. Phys. Lett. 2011, 98, 042102. (35) Savel’ev, S. E.; Alexandrov, A. S.; Bratkovsky, A. M.; Stanley Williams, R. Molecular Dynamics Simulations of Oxide Memory Resistors (Memristors). Nanotechnology 2011, 22, 254011. (36) Pickett, M. D.; Strukov, D. B.; Borghetti, J. L.; Yang, J. J.; Snider, G. S.; Stewart, D. R.; Williams, R. S. Switching Dynamics in Titanium Dioxide Memristive Devices. J. Appl. Phys. 2009, 106, 074508. (37) Mundle, R. M.; Terry, H. S.; Santiago, K.; Shaw, D.; Bahoura, M.; Pradhan, A. K.; Dasari, K.; Palai, R. Electrical Conductivity And Photoresistance of Atomic Layer Deposited Al-Doped ZnO Films. J. Vac. Sci. Technol., A 2013, 31, 01A146.

layers with varying electrical properties would lower the cost of memristor-based computing technologies.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (A.K.P.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We are thankful to M. Bahoura and Bo Xiao for useful discussions. This work is supported by the NSF-CREST (CNBMD) Grant HRD 1036494 and partially by DoD (CEAND) Grant W911NF-11-1-0209 (US Army Research Office).

■

REFERENCES

(1) Street, R. A. Thin-Film Transistors. Adv. Mater. 2009, 21, 2007− 2022. (2) Carcia, P. F.; McLean, R. S.; Reilly, M. H.; Nunes, G. Transparent ZnO Thin-Film Transistor Fabricated by Rf Magnetron Sputtering. Appl. Phys. Lett. 2003, 82, 1117. (3) Mourey, D. A.; Zhao, D. A.; Sun, J.; Jackson, T. N. Fast PEALD ZnO Thin-Film Transistor Circuits. IEEE Trans. Electron Devices 2010, 57, 530−534. (4) Zheng, K.; Sun, X. W.; Zhao, J. L.; Wang, Y.; Yu, H. Y.; Demir, H. V.; Teo, K. L. An Indium-Free Transparent Resistive Switching Random Access Memory. IEEE Electron Device Lett. 2011, 32, 797− 799. (5) Ho, C. P.; Yam, Y. Computational Design and Prototype Development of Optical Prism for Augmented Reality Projection. Proceedings of ISOT’12, Int. Symposium on Optomechatronic Technologies, 2012; pp 1−4. (6) Cheng, D.; Wang, Q.; Wang, Y.; Jin, G. Lightweight SpatialMultiplexed Dual Focal-Plane Head-Mounted Display Using Two Freeform Prisms. Chin. Opt. Lett. 2013, 11, 031201−31204. (7) Fuchs, H.; Livingston, M. A.; Raskar, R.; Colucci, D.; Keller, K.; State, A.; Crawford, J. R.; Rademacher, P.; Drake, S. H.; Meyer, A. A. Augmented Reality Visualization for Laparoscopic Surgery; Wells, W. M., Colchester, A., Delp, S., Eds.; Medical Image Computing and Computer-Assisted Interventation MICCAI’98; Springer-Verlag: Berlin, 1998; Vol. 1496, pp 934−943. (8) Henderson, S.; Feiner, S. Exploring the Benefits of Augmented Reality Documentation for Maintenance and Repair. IEEE Transactions on Visualization and Computer Graphics 2011, 17, 1355−1368. (9) Shechter, R.; Amitai, Y.; Friesem, A. A. Compact Beam Expander with Linear Gratings. Appl. Opt. 2002, 41, 1236−1240. (10) Saarikko, P. Diffractive Exit-Pupil Expander for Spherical Light Guide Virtual Displays Designed for Near-Distance Viewing. J. Opt. A: Pure Appl. Opt. 2009, 11, 065504. (11) Choi, H.; Jung, H.; Lee, J.; Yoon, J.; Park, J.; Seong, D.-J.; Lee, W.; Hasan, M.; Jung, G.-Y.; Hwang, H. An Electrically Modifiable Synapse Array of Resistive Switching Memory. Nanotechnology 2009, 20, 345201. (12) Bamiduro, O.; Mustafa, H.; Mundle, R.; Konda, R. B.; Pradhan, A. K. Metal-Like Conductivity in Transparent Al:ZnO Films. Appl. Phys. Lett. 2007, 90, 252108. (13) Banerjee, P.; Lee, W.-J.; Bae, K.-R.; Lee, S. B.; Rubloff, G. W. Structural, Electrical, and Optical Properties of Atomic Layer Deposition Al-doped ZnO films. J. Appl. Phys. 2010, 108, 043504. (14) Chen, L.-Y.; Chen, W.-H.; Wang, J.-J.; Hong, F. C.-N.; Su, Y.-K. Hydrogen-Doped High Conductivity ZnO Films Deposited by RadioFrequency Magnetron Sputtering. Appl. Phys. Lett. 2004, 85, 5628. (15) Wu, Y.; Hermkens, P. M.; van de Loo, B. W. H.; Knoops, H. C. M.; Potts, S. E.; Verheijen, M. A.; Roozeboom, F.; Kessels, W. M. M. Electrical Transport and Al Doping Efficiency in Nanoscale ZnO Films L


Article

Langmuir (38) Mundle, R.; Pradhan, A. K. Electrical Response in Atomic Layer Deposited Al:ZnO with Varying Stack Thickness. J. Appl. Phys. 2014, 115, 183503. (39) Kim, A.; Song, K.; Kim, Y.; Moon, J. All Solution-Processed, Fully Transparent Resistive Memory Devices. ACS Appl. Mater. Interfaces 2011, 3, 4525−4530. (40) Cao, X.; Li, X.; Gao, X.; Liu, X.; Yang, C.; Yang, R.; Jin, P. AllZnO-Based Transparent Resistance Random Access Memory Device Fully Fabricated at Room Temperature. J. Phys. D: Appl. Phys. 2011, 44, 255104. (41) Seo, J. W.; Park, J.-W.; Lim, K. S.; Yang, J.-H.; Kang, S. J. Transparent Resistive Random Access Memory and Its Characteristics for Nonvolatile Resistive Switching. Appl. Phys. Lett. 2008, 93, 223505. (42) Mundle, R.; Terry, H.; Bahoura, M.; Pradhan, A. K. OzoneAssisted Atomic Layer Deposited ZnO Thin Films for Multifunctional Device Applications. J. Phys. D: Appl. Phys. 2013, 46, 475101. (43) Färm, E.; Kemell, M.; Ritala, M.; Leskelä, M. Selective-Area Atomic Layer Deposition Using Poly(methyl methacrylate) Films as Mask Layers. J. Phys. Chem. C 2008, 112, 15791−15795. (44) Ellinger, C. R.; Nelson, S. F. Selective Area Spatial Atomic Layer Deposition of ZnO, Al2O3, and Aluminum-Doped ZnO Using Poly(vinyl pyrrolidone). Chem. Mater. 2014, 26, 1514−1522. (45) Park, K. S.; Seo, E. K.; Do, Y. R.; Kim, K.; Sung, M. M. Light Stamping Lithography: Microcontact Printing without Inks. J. Am. Chem. Soc. 2006, 128, 858−865. (46) Biercuk, M. J.; Monsma, D. J.; Marcus, C. M.; Becker, J. S.; Gordon, R. G. Low-Temperature Atomic-Layer-Deposition Lift-Off Method for Microelectronic and Nanoelectronic Applications. Appl. Phys. Lett. 2003, 83, 2405. (47) Wilson, C. A.; Grubbs, R. K.; George, S. M. Nucleation and Growth during Al 2 O 3 Atomic Layer Deposition on Polymers. Chem. Mater. 2005, 17, 5625−5634. (48) Gu, D.; Baumgart, H.; Abdel-Fattah, T. M.; Namkoong, G. Synthesis of Nested Coaxial Multiple-Walled Nanotubes by Atomic Layer Deposition. ACS Nano 2010, 4, 753−758. (49) Strukov, D. B.; Kohlstedt, H. Resistive Switching Phenomena in Thin Films: Materials, Devices, and Applications. MRS Bull. 2012, 37, 108−114. (50) Lee, M.-J.; Lee, C. B.; Lee, D.; Lee, S. R.; Chang, M.; Hur, J. H.; Kim, Y.-B.; Kim, C.-J.; Seo, D. H.; Seo, S.; et al. A Fast, HighEndurance and Scalable Non-Volatile Memory Device Made from Asymmetric Ta2O5−x/TaO2−x Bilayer Structures. Nat. Mater. 2011, 10, 625−630. (51) Linn, E.; Rosezin, R.; Kügeler, C.; Waser, R. Complementary Resistive Switches for Passive Nanocrossbar Memories. Nat. Mater. 2010, 9, 403−406. (52) Robertson, J. High Dielectric Constant Gate Oxides for Metal Oxide Si Transistors. Rep. Prog. Phys. 2006, 69, 327−396. (53) Liu, Y. L.; Liu, Y. C.; Yang, H.; Wang, W. B.; Ma, J. G.; Zhang, J. Y.; Lu, Y. M.; Shen, D. Z.; Fan, X. W. The Optical Properties of ZnO Films Grown on Porous Si Templates. J. Phys. D: Appl. Phys. 2003, 36, 2705−2708. (54) Cagin, E.; Chen, D. Y.; Siddiqui, J. J.; Phillips, J. D. Hysteretic Metal−Ferroelectric− Semiconductor Capacitors Based on PZT/ZnO Heterostructures. J. Phys. D: Appl. Phys. 2007, 40, 2430−2434. (55) Van de Walle, C. G.; Neugebauer, J. Universal Alignment of Hydrogen Levels in Semiconductors, Insulators and Solutions. Nature 2003, 423, 626−628. (56) Robertson, J. Band Offsets of Wide-Band-Gap Oxides and Implications for Future Electronic Devices. J. Vac. Sci. Technol., B: Microelectron. Process. Phenom. 2000, 18, 1785. (57) Janotti, A.; Varley, J. B.; Rinke, P.; Umezawa, N.; Kresse, G.; Van de Walle, C. G. Hybrid Functional Studies of the Oxygen Vacancy in TiO2. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 085212. (58) Mehonic, A.; Cueff, S.; Wojdak, M.; Hudziak, S.; Jambois, O.; Labbé, C.; Garrido, B.; Rizk, R.; Kenyon, A. J. Resistive Switching in Silicon Suboxide Films. J. Appl. Phys. 2012, 111, 074507.

(59) Lach, E.; Schuh, K.; Schmidt, M. Application of Electroabsorption Modulators for High-Speed Transmission Systems. Journal of Optical and Fiber Communications Reports 2005, 2, 140−170. (60) Morrison, F. D.; Zubko, P.; Jung, D. J.; Scott, J. F.; Baxter, P.; Saad, M. M.; Bowman, R. M.; Gregg, J. M. High-Field Conduction in Barium Titanate. Appl. Phys. Lett. 2005, 86, 152903. (61) Sun, Y.; Yan, X.; Zheng, X.; Liu, Y.; Zhao, Y.; Shen, Y.; Liao, Q.; Zhang, Y. High On−Off Ratio Improvement of ZnO-Based FormingFree Memristor by Surface Hydrogen Annealing. ACS Appl. Mater. Interfaces 2015, 7, 7382−7388.

M


Al:ZnO Transparent Resistive Switching Devices Grown by

Recommend Documents