FORM 2
THE PATENT ACT 1970
(39 OF 1970)
&
The Patents Rules, 2003
PRO^SIONAL/COMPLETE SPECIFICATION
1. TITLE OF THE INVENTION
DEVELOPMENT OF DYE SENSITIZED MEMRISTOR

Description of Invention
1. Introduction
1.1 Field of Invention:
The present invention generally relates to the semiconductor devices, and more particularly to the photoinduced resistance-switching semiconductor devices. Also, this invention related to the resistance switching and memristor device material composition.
1.2 Background of Invention:
Bipolar resistive switch, also often referred to as a 'memristor', is an electrical device that exhibits a switchable resistance state from low resistance state (LRS) to high resistance state (HRS) and can retain the memory states after removing the external voltage. The programming current compliance and RESET voltage modulation can produce different responses resistances state. In the manufacturing of these devices, a switching layer, typically a transition metal oxide (TMO) is positioned between a top electrode and bottom electrode. Various models have been proposed to the explanation of resistive switching conduction behaviors, including the Ohmic, space charge limited current, Fowler-Nordheim tunneling, Poole-Frenkel emission, Schottky emission etc. Based on these models, filamentary and homogeneous/interfacial mechanisms are considered to be responsible for the resistive switching effect. Most of the times, filamentary and homogeneous/interfacial mechanisms are operating separately. However, co-existence of these mechanisms is also possible. In the typical filamentary resistive switching, the non-conducting bulk switching layer becomes conductive by applying a sufficiently large voltage or a forming voltage

across both top and bottom electrodes. This results in the formation of a conduction path within the bulk switching layer. However, the forming voltage generally depends on the quality of a particular material and also the thickness of the switching layer. Once the conduction path is formed, they may be reset (HRS) or set (LRS) by an appropriately applied voltage (also known as switching voltage). In view of this, the bipolar resistive switching effect may be programmed by passing a current through memristor device. A variety of materials shows the resistance-switching behavior or reversible resistance change in which the resistance of the material is a function of the history of the current through, or voltage across the device. For example, Nickel Oxide (NiO), Niobium Oxide (NbxOx), Titanium Dioxide (TiOx), Hafnium Oxide (HfOx), Zinc Oxide (ZnO), Aluminum Oxide (AlxOx), Magnesium Oxide (MgOx), Chromium Dioxide (CrOx), Vanadium Oxide [VO), Boron Nitride (BN), Aluminum Nitride (AIN), Silicon Oxide (SiOx) and many more shows the resistive switching effect. However, only a few materials exhibit the photoinduced resistive switching effect intrinsically.
The conventional electric field induced memristor devices have lower memory window and non-uniformity in the resistive switching states. Therefore, practical usefulness of the conventional memristor are limited for memory, neuromorphic computing and logic applications. Furthermore, they cannot be used as a memory and optical detector at same time. In addition to this, no single report shows the development of opto-memristor which covers entire ultra-violet (UV) + visible region with high memory window and uniformity in the resistive switching states. Therefore, the present invention would provide practical solution for opto-memristor device, which cover total UV + visible region with higher memory window and uniformity in the resistive switching states. The active layer of the

present invention is Ti02 and natural dye is loaded in the active layer. The Ti02 layer is a UV active whereas, natural dye (Betanin) absorbs the visible region. In view of this, the synergetic integration of TiO2 and dye would cover the total UV + visible region.
1.3 The objective of Invention:
1) To develop a dye-sensitized memristor device
2) To demonstrate light induced resistive switching or memristor switching effect
3) To enhance the light-induced resistive switching or memristor switching effect using dye-sensitized memristor
4) To demonstrate excellent endurance and retention non-volatile memory properties using dye-sensitized memristor
2. Description of Method:
2.1 Synthesis of dye sensitized Tio2 thin film memristor device
The hierarchical titanium dioxide (TiO2) nanostructures were grown by using single step hydrothermal method. For the deposition of Ti02 nanorods on thin film, [1:1] volume of concentrated HCL and double distilled water was taken in a beaker and kept for stirring for 10 minutes; followed by addition of 0.5 µL of titanium (IV) isopropoxide (TTIP) into the above solution and stirred to homogenize the whole solution for 25-30 minutes at room temperature. The obtained clear and transparent solution was then transferred into a Teflon lined stainless steel autoclave (50 ml). The precleaned (ultrasonic treatment by using acetone: ethanol (1:1)) fluorine doped tin oxide (FTO) substrate was immersed vertically in this solution. The autoclave was perfectly sealed and placed in a hot air oven at

160 °C for 3 hours. After cooling the autoclave at room temperature, Ti02 thin film was taken out of it. The obtained thin film was washed for several times with deionized water and dried at room temperature. Further film was annealed for 1 hour at 400 °C in muffle furnace to obtain the desire phase of Tio2
2.2 Extraction of Dye from natural beet source
Betanin is a natural dye which was extracted from Beta vulgaris. For the preparation of dye solution firstly Beets were washed with double distilled water. Further, they were peel off and cut into small pieces. These pieces were put into hot distilled water for approximately 15 minutes. After this process, Beet extract was filtered through clean muslin cloth. The extract was collected in a clean beaker and rapped with aluminum foil; also kept away from sunlight without disturbing.
2.3 Loading of Dye on TiO2 thin film
The equal volume of (1:1) of absolute ethanol and beet extract was taken in a 50 ml beaker. The TiO2 thin film obtained from hydrothermal route was placed in a beaker vertically and kept overnight for 6,12,18 and 24 hours. The beaker was placed into the dark (away from the sunlight), without disturbing overnight. After loading of dye, film was dried at room temperature and used for further memristor device measurements.
2.4 Preparation of top electrode
The top electrode for the present memristor device is aluminum (Al). In the present investigation, the top Al electrode was patterned using vacuum deposition technique. The

thickness of top Al electrode was 500 nm. The detailed description of development of dye sensitized memristor is shown in flowchart 1.
3. Detailed Description of the Invention:
The present invention relates to the improvement in resistive switching devices. A memristor may be switched from an ON to OFF state and vice versa by applying sufficient switching voltage to the device. The memristor or bipolar resistive switch may be switched between a specific pair of resistance state. The incident photon field can be used to affect the resistive switching properties of such devices. The bipolar resistive switch layer is sandwiched or disposed between a first or top electrode and second or bottom electrode. By applying programming signal between first and second electrode it affects the resistance of the active layer which results in the non-volatile memory property. The bipolar resistive switch may comprise any organic or inorganic material such as oxides, polymers, biomaterials, ferrites, nitrides etc. as an active layer that can be formed into a layer between a pair of electrodes. For example, titanium oxide (Tio2) may be used as the oxide layer in a bipolar resistive switch. Here we demonstrate the light-induced resistive switching or memristor switching effect using dye-sensitized Ti02 memristor.
Fig. 1 represents the structure of dye-sensitized AI/TiO2/FTO memristor device. A dye-sensitized memristor comprising of a photo-sensitive active layer 100 which could be sandwiched between two electrodes 110 and 120. The electrodes can be formed from any good electron conductor, and also the electrode material need not be the same for both the upper electrode and the lower electrode. Aluminum (Al) 110 is an example of a material that could be used for the top electrodes and can be patterned using vacuum deposition

techniques. However, other materials can be used, including a large variety of metals as well as electron-conducting nonmetals (Pt, Ag, Au, Co, Hf, Pd, Ir). The photo-responsive active layer 100 can be an insulation layer formed from materials having sufficiently large photo-induced material or photoconductivity transformation properties. The inventors believe that the photo sensitive properties of transition metal oxides can play a critical role in the memristor switching phenomena. An important example of a suitable transition metal oxide for use as the photo-responsive active layer is Ti02 100. The dye molecules 130 are responsible for absorption of visible light 170 whereas Tio2 100 absorbs the UV 110 light. The device can be switched using voltage source 140 and current can be measured using appropriate current measurement unit 150. The photo-induced resistive switching can be achieved by UV 160 and visible source 170.
Fig. 2 reveals photo induced resistive switching characteristics of Al/Ti02/FTO memristor device under different illumination conditions such as Dark 180, UV 190, Visible 200 and UV + Visible 210. The memristor I-V characteristic [photocurrent responses) of the device was measured using a memristor characterization system (ArC ONE) under UV (A = 365 nm, intensity = 0.40 mW/cm2) 160 and visible [intensity = 30 mW/cm2) 170 illumination. The memristor device shows better resistive switching characteristic when UV 160 light is incident on the active layer 100. The TiO2 is an n-type semiconducting material and possess UV 160 active characteristics. The resistive switching behavior of Al/TiO2/FTO memristor device under dark 180 and visible light 200 is irregular and nonlinear. Furthermore, resistive switching in the visible condition is also poor. In the case of UV + visible 210 illumination condition, the resistive switching shows excellent characteristics

as compared to the dark 180, visible 200 and UV 190 illumination condition. In this case, I-V characteristics exhibit good nonlinearity and resistive switching properties. However the memory window and current at resistive switching voltage (±2 V) are very low.
In order to improve the memory window and current at resistive switching voltage (±2 V), we have sensitized the natural dye (Betanin) to TiO2 active layer 100. Fig. 3 represents the pinched hysteresis I-V loops of dye-sensitized TiO2 thin film memristor devices with different dye loadings time variations such as 6 220, 12 230, 18 240 and 24 250 h under UV + Visible light 210 illumination. The photoinduced resistive switching characteristics of different dye loaded Al/Dye/Ti02/FT0 memristor device is clearly observed from the results. As the dye loading time changes, the resistive switching characteristics behavior of Al/Dye/Tio2h/FTO memristor device is also changed. The resistive switching improves from the 6 220 h dye loading time to 12 230 h dye loading time and decline for 18 240 h and 24 250 h dye loading time. The performance comparison of bare TiO2 memristor device 260 and 12 h dye loaded TiO2 memristor device 270 under UV + Visible light 210 illumination is shown in Fig. 4. The improvement in the resistive switching is clearly observed form the experimental results.
Fig. 5. represents the room temperature endurance performance of 12 h dye loaded TiO2 memristor device 270 under UV + Visible light 210 illumination condition. The device successfully switches between two well resolved and distinct resistive switching states, i.e. low resistance state (LRS) 280 and high resistance state (HRS] 290. Furthermore, the reliability of the device is confirmed by continuously switching the device for 10 thousand cycles with less than 1 % coefficient of variation for both states (low and high resistance

state) 280 and 290. The ratio of high resistance to low resistance or memory window is found to be 100 and such kind of memory window is a general requirement for the non-volatile memory application.
Fig. 6. represents the room temperature retention performance of 12 h dye loaded TiO2 memristor device 270 under UV + Visible light 210 illumination condition. The retention non-volatile memory characteristic was measured for 105 seconds. The device shows the highly reliable retention performance without degradation in the low resistance state [LRS] 300 and high resistance state (HRS) 310. The results suggested that the dye-sensitized Ti02 memristor could retain the bi-level data up to 10 years or higher. This kind of practical usability is the need of the hour for non-volatile memory devices.

4. Analysis of Results:
In a dye-sensitized memristor, electrons originate from a dye when it absorbs light. The dye contains a conjugated system (alternating single and double bond) that absorbs light in the visible spectrum. The dye molecule absorbs the incident photons and becomes excited. The excited dye molecules inject electrons into a Ti02 layer which is acts as a semiconductor. The generated electrons need to pass from the two interfaces, Dye/Tio2 interface, and another Tio2/FTO interface. After passing these two interfaces electrons collected at FTO and give rise to the resistive switching effect. The natural dye Betanin is used for the sensitization of Ti02 thin film. Betanin dye possesses carboxylic group (-COOH group). This group acts as an anchoring group for the dye on the Ti02 surface. The charge transfer phenomenon takes place in the Betanin dye which is responsible for electron transfer. In addition to this, Ti02 nanorods .are obtained from this reaction. The ID nanorods have a higher surface area which also enhances the photoinduced resistive switching effect. The TiO2 is active in the ultraviolet region while natural dye Betanin covers the visible region. Also, TiO2 is n-type semiconducting material and having a large band gap (nearly 3.2 eV). In view of this, the complete UV-Visible region is covered by the dye-sensitized Tio2 thin film and excellent resistive switching results are obtained under UV + Visible light illumination condition.

CLAIMS FOR PATENT
We claims
1) Controlling the memristor switching property using light-induced effect.
2) Controlling the memristor switching property using dye sensitization and light-induced effect.
3) Enhancement in the memristor switching effect using light-induced effect.
4) Enhancement in the memristor switching effect using light-induced effect and dye-sensitized technique.
5) Excellent endurance non-volatile memory property using dye-sensitized memristor.
6) Excellent retention non-volatile memory property using dye-sensitized memristor.
7) Highly reliable endurance non-volatile memory property up to 10 thousand cycles.
8) Highly reliable retention non-volatile memory property up to 10 years.