Transparent Memory Devices Research Articles

Structural, Optical, and Electrical Properties of Sputtered ZnO Thin Films and Their Application in Transparent Memory Devices

Structural, Optical, and Electrical Properties of Sputtered ZnO Thin Films and Their Application in Transparent Memory Devices

Band tailoring by annealing and current conduction of Co-doped ZnO transparent resistive switching memory

The switching characteristics of ITO/Zn1-xCoxO/ITO transparent resistive random access memories were studied. 5 mol% cobalt doped ZnO resistive layer improves bipolar switching properties. In addition, the redshift in band energy caused by doping of cobalt (Co) was studied. The doped memory device also showed a change in band energy by 0.1 eV when subjected to annealing of 400 °C. Annealing below 400 °C temperature did not show any characteristic changes. The film morphology analysis suggested the increase in roughness with annealing temperature, which can be seen from FESEM and AFM images. In this study annealing and Co doping effect on ZnO based non-volatile memory device is presented. Moreover, transparent memory devices with 90% transmittance at 550 nm wavelength have been reported. At low field and high field region Schottky emission and ionic conduction are dominated respectively.

A Non-Volatile Memory Based on NbOx/NbSe2 Van der Waals Heterostructures

Two-dimensional (2D) van der Waals (vdW) layered transition metal dichalcogenides (TMDs) materials have been receiving a huge interest due to atomically thin thickness, excellent optoelectronic properties, and free dangling bonds. Especially the metallic TMDs, such as MoTe2 (1T’ phase), NbS2, or NbSe2, have shown fascinating physical properties through various applications, such as superconductor and charge density wave. However, carrier transport of metallic TMDs would be degraded due to the poor stability in ambient conditions. To date, achieving both high device performance and long-term stability is still a huge challenge. Thus, an alternative way to develop both unavoidable native oxide and metallic TMDs is under consideration for new era research. In this respect, 2D metallic TMD materials have attracted high attention due to their great potential in neuromorphic-based devices with metal-insulator-metal structures, making it possible to produce scalable, flexible, and transparent memory devices. Herein, we experimentally demonstrated a synthesized metallic NbSe2 by a chemical vapor deposition method with a highly uniform, good shape distribution and layer controller ranging from 2–10 layers. Together, for the first time, we proposed the NbOx/NbSe2 heterostructure memristor device based on the native NbOx oxide on the interface of multi-layer NbSe2 flakes. The ultra-thin native NbOx oxide of 3 nm was formed after a period of oxidation time under air condition, which acts as a memristive surface in the Au-NbOx-Au lateral memristor device, in which oxygen vacancies form a conductive filament. Our NbOx/NbSe2 hetero-tructured memristor exhibits a stable memory window, a low-resistance-state/high-resistance-state ratio of 20, and stable endurance properties over 20 cycles at a low working voltage of 1 V. Furthermore, by the retention property test, non-volatile characteristics were confirmed after over 3000 s in our best data. Through a systematic study of the NbOx/NbSe2 heterostructured memristor device, this report will open new opportunities for next-generation memory devices application.

Flexible and fully transparent WORM memory devices based on Ag nanoparticles blended with poly(ethylene-co-vinyl acetate)

Silver nanoparticles (Ag NPs) blended with poly(ethylene-co-vinyl acetate) (EVA) were used as an active layer to fabricate flexible and fully transparent memory devices by using the spin-coating method followed by the thermal roll lamination technique with the structure of ITO/EVA:Ag NPs/ITO. The devices exhibited a non-volatile write-once-read-many-times (WORM) memory type and possessed current bi-stability with ON/OFF current ratio within the range of 104-105 at a reading voltage of +1 V. The data continuous read operations reached more than 10 h, which revealed the stability of the memory devices over a long period of time. The conduction mechanisms of the memory device could be explained by theoretical models and proposed by electron trapping at the trapping center of Ag NPs inside the EVA matrix. In addition, the memory device showed a fast response of 231 ns and was fully transparent with maximum transmittance of the device at 83.2%. Further, the devices could be operated under the bending condition of more than 0.83% strain.

Characteristics of flexible and transparent Eu2O3 resistive switching memory at high bending condition

The characteristics of ITO/Eu2O3/ITO/PET transparent and flexible resistive switching memory are studied. The device exhibits superior characteristics such as device area-independent and forming-free resistive switching behavior with a resistance on/off ratio of 104, good retention of >104 s and high AC endurance of >107 cycles. The conduction mechanism of the high-resistance state is the Poole–Frenkel mechanism, while that of the low-resistance state is ohmic conduction. The electrical characteristics of the flexible device have shown excellent results up to 5 mm bending radius, at which a degradation in the on/off ratio of the memory window is observed, due to the change in the dielectric layer resistance. The resistive switching characteristics can be improved during bending up to the radius of 2 mm by the incorporation of an aluminum-doped zinc oxide layer in the device as the bottom electrode, proving its application in future flexible and transparent memory devices.

Transparent write-once-read-many-times memory devices based on an ITO/EVA:rGO/ITO structure

Fully transparent electronic devices play an important role in electronic applications. Well-known examples are the wearable devices and transparent displays; their applications are transparent memory devices of interest. The transparent write-once-read-many-times (WORM) memory devices based on synthesized graphene oxide (GO) blended in poly (ethylene-co-vinyl acetate) (EVA) had been fabricated by a thermal roll lamination technique in which the synthesized GO inside the EVA matrix was reduced into reduced graphene oxide (rGO) during the lamination process. The memory devices exhibited an optical transmittance of more than 60% in the visible light region. The conduction mechanisms of the memory devices were identified using theoretical models based on the current–voltage (I–V) curves. The fitted I–V results were described by the conductive filaments in which electrons were transported through the rGO direct contacts. In addition, the maximum ON/OFF current ratio of the transparent WORM memory device was approximately 5 × 105, and the retention time tests showed two current states with good data retention properties under different temperatures.

Memristive properties of transparent oxide semiconducting (Ti,Cu)Ox-gradient thin film

The paper presents the results of the analysis of memristive properties observed in (Ti,Cu)-oxide thin film with gradient distribution of elements, prepared using the multi-source reactive magnetron co-sputtering process. The performed electrical measurements showed the presence of pinched hysteresis loops in the voltage–current plane for direct and alternating current bipolar periodic signal stimulation. Investigations performed using a transmission electron microscope equipped with an energy dispersive spectrometer showed that the elemental composition at the cross section of the thin film was very well correlated with the gradient V-shaped profile of the powering of the magnetron source equipped with a Cu target. The prepared samples were transparent in the visible part of optical radiation. The obtained results showed that the prepared gradient (Ti,Cu)Ox thin film could be an interesting alternative to the conventional multilayer stack construction of memristive devices, which makes them a promising material for manufacturing transparent memory devices for transparent electronics.

Fully transparent, non-volatile bipolar resistive memory based on flexible copolyimide films

Partially aliphatic homopolyimides and copolyimides were prepared from rel-(1′R,3S,5′S)-spiro[furan-3(2H),6′-[3]oxabicyclo[3.2.1]octane]-2,2′,4′,5(4H)-tetrone (DAn), 2,6-diaminoanthracene (AnDA), and 4,4′-oxydianiline (ODA) by varying the molar ratio of AnDA and ODA. We utilized these polyimide films as the resistive switching layer in transparent memory devices. While WORM memory behavior was obtained with the PI-A100-O0-based device (molar feed ratio of DAn : AnDA : ODA = 1 : 1 : 0), the PI-A70-O30-based device (molar feed ratio of DAn : AnDA : ODA = 1 : 0.7 : 0.3) exhibited bipolar resistive switching behavior with stable retention for 104 s. This result implies that the memory properties can be controlled by changing the polyimide composition. The two devices prepared from PI-A100-O0 and PI-A70-O30 showed over 90% transmittance in the visible wavelength range from 400 to 800 nm. The behavior of the memory devices is considered to be governed by trap-controlled, space-charge limited conduction (SCLC) and local filament formation.

Uniform resistive switching properties of fully transparent TiO2-based memory devices

Uniform resistive switching properties were observed in fully transparent indium-tin-oxide (ITO)/TiO2 film/F-doped SnO2 (FTO) devices where the TiO2 films were fabricated by chemical solution deposition method. In addition to high transmittance of above 70% for visible light, good resistive switching parameters, such as centralized reset and set voltages, stable resistance values at high and low resistance states (read at 0.2 V), and good retention data (up to 10000 s) also presented in the transparent memory devices. The dominant conducting mechanisms were Ohmic behavior and Schottky emission at low resistance state and high resistance state. The resistive switching phenomenon was explained by formation and rupture of the filaments, in which oxygen ions migration and current-induced thermal effect play important roles. Our results show the potential application of the ITO/TiO2/FTO cell in transparent electronics devices.

Charge Trap Memory Based on Few-Layer Black Phosphorus and High-K Materials

Due to the unique properties of the two dimensional material, it has great potential for future information technology devices. Based on field-effect transistors with large hysteresis, consisting of a few-layer black phosphorus channel and a three dimensional (3D) Al2O3/HfO2/Al2O3, we successfully made the nonvolatile charge-trap memory devices. Because of the extraordinary trapping ability of the high-k HfO2, an unprecedented memory window exceeding 12 V is obtained. The device shows a high endurance of over 120 cycles and a stable retention of ∼30% charge loss after 10 years, even lower than the reported MoS2 flash memory. The high program/erase current ratio, large memory window, stable retention and high on/off current ratio, provide a promising route towards flexible and transparent memory devices utilizing atomically thin two-dimensional materials. The combination of 2D materials with traditional high-k charge-trap gate stacks opens up an exciting field of nonvolatile memory devices[1]. References (1) Qi Feng, Faguang Yan, Wengang Luo and Kaiyou Wang, Nanoscale, 8(2016)2686.

Improvement of reliability of polymer nanocomposite based transparent memory device by oxygen vacancy rich ZnO nanorods

Different weight percent (wt%) of Zinc Oxide (ZnO) nanorods, prepared by chemical route, are used into polymethylmethacrylate (PMMA) to fabricate organic bistable devices (OBDs). The devices utilize spin coated polymer nanocomopsite thin film, which exhibit electrical bistability and WORM characteristics in switching between high conductive (ON) and low conductive (OFF) states and it has been observed that along with ZnO nanorod concentration, oxygen vacancy content is also an important factor for the achievement of ON-OFF conductive states. The maximum ON/OFF current ratio is of the order of 5×103 for Al/ZnO nanorods:PMMA/ITO heterostructures with 0.5wt% of ZnO nanorods. Statistical results indicate that the operating stability of devices with high retention time of 103s. We demonstrate that the enhancement of memory parameters are controlled by the oxygen vacancies existed in the engineered ZnO nanorods. These native defects are credited as basis of resistive switching behaviour of the devices. By comparing the C–V curves to the current–voltage (I–V) curves, dielectric constant, electron mobility and oxide trap densities have been estimated.

Highly transparent and flexible bio-based polyimide/TiO2 and ZrO2 hybrid films with tunable refractive index, Abbe number, and memory properties.

The novel bio-based polyimide (4ATA-PI) and the corresponding PI hybrids of TiO2 or ZrO2 with excellent optical properties and thermal stability have been prepared successfully. The highly transparent 4ATA-PI containing carboxylic acid groups in the backbone could provide reaction sites for organic-inorganic bonding to obtain homogeneous hybrid films. These PI hybrid films showed a tunable refractive index (1.60-1.81 for 4ATA-PI/TiO2 and 1.60-1.80 for 4ATA-PI/ZrO2), and the 4ATA-PI/ZrO2 hybrid films revealed a higher optical transparency and Abbe's number than those of the 4ATA-PI/TiO2 system due to a larger band gap of ZrO2. By introducing TiO2 and ZrO2 as the electron acceptor into the 4ATA-PI system, the hybrid materials have a lower LUMO energy level which could facilitate and stabilize the charge transfer complex. Therefore, memory devices derived from these PI hybrid films exhibited tunable memory properties from DRAM, SRAM, to WORM with a different TiO2 or ZrO2 content from 0 wt% to 50 wt% with a high ON/OFF ratio (10(8)). In addition, the different energy levels of TiO2 and ZrO2 revealed specifically unique memory characteristics, implying the potential application of the prepared 4ATA-PI/TiO2 and 4ATA-PI/ZrO2 hybrid films in highly transparent memory devices.

Charge trap memory based on few-layer black phosphorus.

Atomically thin layered two-dimensional materials, including transition-metal dichalcogenide (TMDC) and black phosphorus (BP), have been receiving much attention, because of their promising physical properties and potential applications in flexible and transparent electronic devices. Here, for the first time we show nonvolatile charge-trap memory devices, based on field-effect transistors with large hysteresis, consisting of a few-layer black phosphorus channel and a three dimensional (3D) Al2O3/HfO2/Al2O3 charge-trap gate stack. An unprecedented memory window exceeding 12 V is observed, due to the extraordinary trapping ability of the high-k HfO2. The device shows a high endurance of over 120 cycles and a stable retention of ∼30% charge loss after 10 years, even lower than the reported MoS2 flash memory. The high program/erase current ratio, large memory window, stable retention and high on/off current ratio, provide a promising route towards flexible and transparent memory devices utilising atomically thin two-dimensional materials. The combination of 2D materials with traditional high-k charge-trap gate stacks opens up an exciting field of nonvolatile memory devices.

Fully transparent nonvolatile resistive polymer memory

ABSTRACTWe present a fully transparent nonvolatile resistive polymer memory device based on an anthracene‐containing partially aliphatic polyimide along with indium tin oxide (ITO) top and bottom electrodes. High transmittance of over 90% in the wavelength range of 400 to 800 nm is accomplished with an ITO/polyimide/ITO/glass device. The device shows unipolar write‐once‐read‐many times (WORM) memory behavior with an ON/OFF current ratio of ∼2 × 103, and the ratio remained without any significant degradation for over 104 s. The memory behavior of the device is considered to be governed by trap‐controlled space‐charge limited conduction (SCLC) and local filament formation. Based on molecular simulation of the polyimide, the location of energy states is different from that in the conventional charge transfer (CT) mechanism. Despite the relatively low ON/OFF current ratio, our results can give insight into the development of fully transparent memory device. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 918–925

Controlled charge trapping by molybdenum disulphide and graphene in ultrathin heterostructured memory devices

Atomically thin two-dimensional materials have emerged as promising candidates for flexible and transparent electronic applications. Here we show non-volatile memory devices, based on field-effect transistors with large hysteresis, consisting entirely of stacked two-dimensional materials. Graphene and molybdenum disulphide were employed as both channel and charge-trapping layers, whereas hexagonal boron nitride was used as a tunnel barrier. In these ultrathin heterostructured memory devices, the atomically thin molybdenum disulphide or graphene-trapping layer stores charge tunnelled through hexagonal boron nitride, serving as a floating gate to control the charge transport in the graphene or molybdenum disulphide channel. By varying the thicknesses of two-dimensional materials and modifying the stacking order, the hysteresis and conductance polarity of the field-effect transistor can be controlled. These devices show high mobility, high on/off current ratio, large memory window and stable retention, providing a promising route towards flexible and transparent memory devices utilizing atomically thin two-dimensional materials.

Nonvolatile multibit Schottky memory based on single n-type Ga doped CdSe nanowires

Nonvolatile resistive switching has been observed for the first time in CdSe nanowire (NW)/Au Schottky barrier diodes, where a Schottky contact electrode and an Ohmic contact electrode were formed at the Au/CdSe NW and CdSe NW/In interfaces, respectively. The CdSe NWs Schottky devices were found to possess multibit storage ability in an individual nanowire, and exhibited excellent memory characteristics, with a resistance on/off ratio exceeding four orders of magnitude, a long retention time of over 104 s and a lower operating voltage of 2 V. By replacing the SiO2/Si substrate with a poly ethylene terephthalate substrate, flexible and transparent memory devices with superior stability under strain were realized. The resistive switching of CdSe NW/Au Schottky devices is understood by electron trapping and detrapping in the interfacial oxide layer. Our findings provide a viable way to create new functional high-density nonvolatile multibit memory devices compatible with simple processing techniques for normal one-dimensional nanomaterials.

Endurance Characteristics of Amorphous-InGaZnO Transparent Flash Memory With Gold Nanocrystal Storage Layer

The amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT)-based nonvolatile transparent Flash memory devices were fabricated with gold (Au) nanocrystal storage layer. The performance and the reliability of transparent memory devices have been characterized by experiment and technology computer-aided design simulation. This memory device shows a large-enough memory window ΔV = 4.7 V at the program/erase (P/E) voltage VPGM/VERS = 35/-35 V for the P/E time TPGM/ TERS = 3/25 s. The memory window was kept almost the same after 1050 P/E cycles; however, the center voltage of the memory window (VC) was shifted in a negative direction. The cycling effect was explained by the change in the density of states (DOS) and the acceptor-like interface-trap density DitA(E) in the a-IGZO channel layer with increasing P/E cycles. The main mechanism for the change in VC was found to be the accelerated injection of holes into the gate insulator due to the energy band bending during the erase operation.

Transparent organic thin-film transistors and nonvolatile memory devices fabricated on flexible plastic substrates

Transparent and flexible organic thin-film transistors (OTFTs) and nonvolatile memory devices were developed. The memory device is based on OTFTs by incorporating self-assembled gold nanoparticles in the organic dielectric layers as the charge storage elements. The organic memory devices exhibited good programmable memory properties with respect to the program/erase operations, resulting in controllable and reliable threshold voltage shifts by hole trapping/detrapping in the gold nanoparticles. The organic memory devices could be electrically programmed and erased, leading to a large memory window of around 15 V. Additionally, the long term data retention (>105 s) and the stable cyclic bending tests (>2000 cycles) confirmed that the flexible memory devices exhibited good electrical reliability as well as mechanical stability. The organic memory devices were composed of a solution-processed organic dielectric layer/metallic nanoparticles and a low-temperature processed organic semiconductor layer. Additionally, transparent indium-tin oxide (ITO) deposited at room temperature was used for the gate and source–drain electrodes to fabricate transparent memory devices. Therefore, this approach could potentially be applied to advanced transparent and flexible electronic devices as well as integrated organic device circuits.

Transparent indium gallium zinc oxide transistor based floating gate memory with platinum nanoparticles in the gate dielectric

A transparent memory device has been developed based on an indium gallium zinc oxide thin film transistor by incorporating platinum nanoparticles in the gate dielectric stack as the charge storage medium. The transfer characteristics of the device show a large clockwise hysteresis due to electron trapping and are attributed to the platinum nanoparticles. Effect of the gate bias stress (program voltage) magnitude, duration, and polarity on the memory window characteristics has been studied. Charge retention measurements were carried out and a loss of less than 25% of the trapped elec-trons was observed over 104 s indicating promising application as nonvolatile memory.