Asim Roy

Incorporation of SnO2 nanoparticles in PMMA for performance enhancement of a transparent organic resistive memory device

We report the electrical bistable characteristics of a hybrid polymer/inorganic nanocomposite device consisting of SnO2 nanoparticles (NPs) embedded in an insulating polymethylmethacrylate (PMMA) layer sandwiched between conductive indium tin oxide (ITO) and aluminium (Al) electrodes. X-ray diffraction measurements were performed for assessment of the crystallographic nature of SnO2 nanoparticles while the microstructural nature of SnO2 nanoparticles embedded in the PMMA matrix was confirmed using transmission electron microscopy. Detailed electrical characterizations suggested an influence of the NP concentration on the switching characteristics of the Al/SnO2-PMMA/ITO memory devices. The highest resistance ratio > 103 (Roff/Ron) was observed in a device with 2 weight% SnO2 NPs. The retention tests on the fabricated device demonstrated the consistency in current of the ON/OFF state even after 104 s. The conduction mechanisms of the fabricated nanocomposite based memory cell were discussed on the basis of experimental data using a charge trapping–detrapping mechanism in the NPs. Our findings offer a feasible and low cost chemical approach to fabricate a transparent and high density RS memory device.

Observation of negative differential resistance and electrical bi-stability in chemically synthesized ZnO nanorods

Zinc oxide nanorods/p-Si heterostructures have been fabricated by depositing the chemically synthesized ZnO nanorods on p-type silicon substrate. Heterostructure shows electrical bi-stability and negative differential resistance (NDR) only at the beginning of the forward bias region, and these phenomena have been explained with the help of energy band diagram. An explanation is proposed for the origin of electrical bi-stability in light of the electric field induced charge transfer across the junction, and the NDR phenomena could be attributed to interfacial traps and defect level that arises due to oxygen and zinc interstitial vacancies. Room temperature photoluminescence measurement of ZnO nanorods exhibits the emission peaks at about 466 nm and 566 nm which are attributed to oxygen vacancies and Zn interstitials. A correlation between NDR and blue emission phenomena in the ZnO nanorods due to defects states has been established.

IMPROVEMENT OF RETENTIVITY IN TiOx/HfOx BILAYER STRUCTURE FOR LOW POWER RESISTIVE SWITCHING MEMORY APPLICATIONS

This paper reports the bipolar resistive switching (BRS) characteristics in Al/Ti/TiOx/HfOx/Pt heterostructure during a DC sweep cycle with current compliance (CC) of 250 μA. The improvement in the switching performance in a CMOS compatible Al/Ti/TiOx/HfOx/Pt memory cell has been observed. The improvement is due to oxygen-rich HfOx layer insertion in simple metal-insulator-metal (MIM) sandwich structure. Analysis of current–voltage (I–V) characteristics revealed the trap-controlled space charge limited current (TC-SCLC) conduction mechanism is the most suitable mechanism signifying the dominant current conduction in all the bias regions and resistance states. Furthermore, this bilayer memory stack exhibits a tight distribution of switching parameters, good switching endurance up to 105 cycles, and good data retention of > 104 s at 85°C.

Enhanced resistive switching phenomena using Al/Ti/TiOx/HfOx/Pt bilayer structure

An improvement in the switching performance is reported in a CMOS compatible Al/Ti/TiOx/HfOx/ Pt memory cell, due to oxygen-rich HfOx layer insertion in simple metal-insulator-metal sandwich structure. TiOx and HfOx embedded in resistive-switching random access memory (RRAM) stack is one of the promising candidates for low power (>2V) bipolar resistive switching memory application. From I-V characteristics, trap-controlled space charge limited current (TC-SCLC) conduction mechanism is found to be the most suitable mechanism signifying the dominant current conduction in all the bias regions and resistance states. This device has shown good uniformity and data retention of >104 s at 85°C with a high resistance ratio of >102.

Optical and electrical properties of undoped and boron doped zinc oxide synthesized by chemical route

We have synthesized and studied the boron doped ZnO nanostructure thin films. The crystallinity of undoped and boron (B) doped ZnO (BZO) has been studied from XRD results. Using the Debye-Scherrer Formula, the grain size has been evaluated, which was found to decrease with increased doping concentration. The optical and electrical properties of (1, 3, 5 wt%) B-doped ZnO (BZO) has been investigated with reference to the undoped counterpart. The UV-VIS spectroscopic analysis revealed that the transmittance for undoped ZnO is maximum and it decreases with doping up to 3% but increases for 5% BZO. The dark as well as photo current–voltage (I–V) characteristics have been investigated in details and the changes occurred in the I-V characteristics with doping concentration as well as under illumination are also quite significant.

Electrical properties of radio-frequency sputtered HfO2 thin films for advanced CMOS technology

The Hafnium oxide (HfO2) high-k thin films have been deposited by radio frequency (rf) sputtering technique on p-type Si (100) substrate. The thickness, composition and phases of films in relation to annealing temperatures have been investigated by using cross sectional FE-SEM (Field Emission Scanning Electron Microscope) and grazing incidence x-ray diffraction (GI-XRD), respectively. GI-XRD analysis revealed that at annealing temperatures of 350°C, films phases change to crystalline from amorphous. The capacitance-voltage (C-V) and current-voltage (I-V) characteristics of the annealed HfO2 film have been studied employing Al/HfO2/p-Si metal–oxide–semiconductor (MOS) structures. The electrical properties such as dielectric constant, interface trap density and leakage current density have been also extracted from C-V and I-V Measurements. The value of dielectric constant, interface trap density and leakage current density of annealed HfO2 film is obtained as 23,7.57×1011eV−1 cm−2 and 2.7×10−5 Acm−2, respec...

Electrical characteristics of ZnO nanorods reinforced polymer nanocomposite thin films

ZnO nanorods have been prepared by simple chemical method, which is used to fabricate organic bistable devices (OBDs). OBDs are fabricated by incorporating different weight percent (wt %) of chemically synthesized Zinc Oxide (ZnO) nanorods into polymethylmethacrylate (PMMA). Current-voltage (I-V) measurements of the spin coated ZnO+PMMA nanocomopsite thin film on indium tin oxide (ITO) coated glass substrate showed current hysteresis behaviour, which is an indication of memory effect. The samples exhibit two distinct resistance states, ON and OFF states, characterised by relatively low and high resistance of the OBDs, respectively. It is also observed that with change in ZnO dopant concentration the value of ON/OFF current changes. Higher ON/OFF current ratio is desired for practical applications. Current conduction mechanism of the devices has been explained invoking various existing models, and it has been found that the trapped-charge-limited conduction mechanism was dominant in our samples.

Thickness and temperature dependent electrical characteristics of SrBi 2 Ta 2 O 2 thin films prepared by pulsed laser ablation

Polycrystalline SrBi2Ta2O9 (SBT) thin films have been prepared by pulsed laser ablation technique on platinum coated silicon substrate. The influence of growth temperature and thickness on microstructure and dielectric properties of films was studied. Crystallite size of SBT films has been found to increase with increasing substrate temperature. With increase of substrate temperature from 500 to 700°C, the dielectric constant gradually increases due to increase in the cystallinity and gain size. Thickness dependent dielectric constant has been studied and discussed in the light of an interfacial dead layer and the finite screening length of the electrode. The C-V measurement of the film deposited at 700°C exhibited butterfly-like curve, which is characteristics of ferroelectric nature of material. The frequency dependent dielectric constant and the dielectric loss of SBT films for various processing temperature was also studied.