R. S. Ajimsha

Maxwell–Wagner relaxation dominated Al2O3/TiO2 nanolaminates (ATA NLs) have recently demonstrated their potential for high-density energy storage applications. In this report, we have unraveled the defect-mediated transport mechanisms prevailing in Al2O3/TiO2 sub-nanometric laminates. Temperature-dependent ac conductivity measurements revealed the signature of small polaron hopping in TiO2 active layers and trap-assisted tunneling transport through Al2O3 barrier layers, which was corroborated by resonant photoelectron spectroscopy and temperature-dependent current–voltage measurement. The polaronic defect states, found ∼1 eV below the Fermi level, served as the hopping centers and leakage paths for current. The signature of quantum tunneling transport and the negative differential conductance observed toward higher electric field was attributed to the splitting of delocalized minibands. These transport properties of Al2O3/TiO2 nanolaminates will help in tailoring these materials for next-generation storage capacitors.

The tunability and controllability of conductance quantization mediated multilevel resistive switching (RS) memory devices, fabricated in crossbar geometry can be a promising alternative for boosting storage density. Here, we report fabrication of Cu/TiO₂/Pt based RS devices in 8 × 8 crossbar geometry, which showed reliable bipolar RS operations. The crossbar devices showed excellent spatial and temporal variability, time retention and low switching voltage (μA). Furthermore, during the reset switching, highly repeatable and reliable integral and half-integral quantized conductance (QC) was observed. The observed QC phenomenon was attributed to the two dimensional confinement of electrons as lateral width of the conducting filament (CF) matches the fermi wavelength. The magnitude and number of the QC steps were found to increase from ∼2.5 to 12.5 and from 5 to 18, respectively by increasing the compliance current (I _C) from 50 to 800 μA which also increased the diameter of the CF from ∼1.2 to 3.3 nm. The enhancement in both number and magnitude of QC states was explained using electrochemical dissolution mechanism of CF of varying diameter. A thicker CF, formed at higher I _C, undergoes a gradual rupture during reset process yielding a greater number of QC steps compared to a thinner CF. The realisation of QC states in the crossbar Cu/TiO₂/Pt device as well as I _C mediated tunability of their magnitude and number may find applications in high-density resistive memory storage devices and neuromorphic computing.

A self-powered UV sensor with high responsivity was developed by coupling ZnO based piezoelectric nanogenerator and photodetector for uninterrupted real-time monitoring of UV radiation. Unlike conventional UV photodetectors, our approach couples a ZnO based metal-semiconductor-metal photodetector as a variable load resistor with a flexible ZnO:PVDF piezoelectric nanogenerator through impedance matching to realize self-powering of photodetector. The as fabricated UV photodetector exhibits a dark resistance of approximately 55 MΩ and change of resistance from 12 to 2 MΩ in response to UV illumination ranging from 1.5 to 14.5 mW/cm2. Under UV exposure of 1.5 mW/cm2, the output voltage of self-powered UV sensor drops from ∼14.6 V (dark) to ∼3.88 V. With increasing UV intensity from 1.5 to 14.5 mW/cm2, the output voltage gradually decreases from ∼3.88 to ∼1.04 V due to reduced ZnO photodetector resistance. This self-powered UV sensor exhibits excellent performance with a sensitivity of ∼93%, a voltage responsivity of 7.14 V (mW−1 cm−2), a response time of 0.67 s, and a recovery time of 4 s, which are better than the previous reports. The environmentally stable and biocompatible nature of ZnO based system makes it a promising candidate for self-powered real time ultraviolet remote sensing applications.

In the past decade resistance-based memory devices, or the resistive random access memory Devices (RRAM) have emerged as a potential candidate for multi-state memory storage and non-conventional computing applications. Reports on conduction quantization (QC) have added an interesting layer to the utility of these RRAM devices for ultra-dense memory and neuromorphic computing applications due to the occurrence of integral and half-integral conduction (resistive) states. Since the first reports of QC phenomena in RRAM devices, there have been detailed studies on the nature of the conducting filaments, switching mechanisms, and tunability of the QC states, but there exists a scarcity of studies exploring controllability of QC phenomena in scalable device geometries. In this work, we report compliance current controlled tunable QC phenomena in crossbar RRAM cells based on electrochemical metallization switching mechanism. The devices exhibited robust bipolar resistive switching, with well separated high and low resistance states.  The magnitude and number of the QC states were found to increase from ~2.5 to 3.5 and from 4 to 6, respectively as the IC increased from 50 to 200μA. The Cu/Ta2O5/Pt device structure was chosen to strategically govern the metallic nature of the conduction filament (CF) formation, which helped postulating factors contributing to the tunability of the states via compliance current. We report the lateral dimension variability as the main factor governing the magnitude and number of quantized steps observed in RRAM devices, where we also discuss a numerical method to approximate the diameter of the CFs. The increase in number and magnitude of QC steps with IC was explained considering the fact that thicker CF obtained at higher ICC , when undergoes a gradual rupture during reset process, results in larger number of QC steps compared to a thinner CF.

In this work we investigate the conductance quantization (QC or CQ) phenomenon in the Cu/Ta2O5/Pt resistive random access memory (RRAM) devices. The Ta2O5 film was deposited on Pt-Si substrate using RF magnetron sputtering, followed by patterning of Cu top electrodes. The devices demonstrate robust bipolar resistive switching behavior, with low set and reset voltages (< 1V), and high resistance contrast (~103 ), owing to the electrochemical metallization mechanism. Quantized conduction was observed during the reset operation with well-defined conduction steps. Conductance (G) histograms revealed Gaussian distribution of G/G0 around the integral and half-integral values, where G0 is the fundamental unit of conductance (77.6 μS). The maximum conductance state observed was 310G0. Moreover, the currentvoltage curves of QC reset cycles clearly revealed that multiple conduction states appear at the same current value which opens potential avenues for developing multi-state memory devices based on QC states. The findings in this work contribute to the understanding of quantized conduction in RRAM devices for non-conventional computing applications such as in-memory and neuromorphic computation.

Considering the potential applications of Al2O3/TiO2 nanolaminates (ATA NLs) in storage capacitors, device-grade ATA NLs are fabricated using an ALD system, wherein the effect of precursor purging time on interfacial, and dielectric properties is thoroughly investigated. With an increase in half-cycle purging time from 2 to 4 s, the observed improvement in interface quality and sublayer density of these NLs is ascribed to the efficient removal of reaction by-products and impurities. Moreover, with an increase in purge time from 2 to 4 s, the increase in dielectric constant and concurrent decrease in dielectric loss from ∼132 to 154 and from ∼0.29 to 0.2, respectively, are primarily assigned to the improvement in sublayer conductivity contrast assisted Maxwell–Wagner interfacial polarization across Al2O3/TiO2 interfaces. The NL based devices fabricated at 4 s purging time, exhibited a capacitance density of ∼18.94 fF/μm2, low equivalent oxide thickness of ∼1.82 nm, and reduced leakage current density of ∼3.04 × 10−5 A/cm2 at 2 V applied bias, which demonstrates its suitability as high-k materials for energy storage applications. Furthermore, this study not only gives an insight of the purging time induced growth chemistry of ATA NLs but also explores the possibility of improving its dielectric performance essential for multifaceted applications.

Multilevel cell (MLC) capability of a memory device is an attractive alternative to realise high integration density of memory arrays. In addition, power efficient memory cell along with MLC capability has huge potential in neuromorphic computing and data storage applications. In this study low power, high speed and 3-bit multilevel bipolar resistance switching operation is reported in TiO ₂ thin films where copper is used as counter electrode. On account of using an oxidisable electrode, low programming voltages (∼+0.2 V, −0.1 V) along with low reset current (∼10 µ A) are observed, thereby ensuing low power (∼ µ W) memory operation. Additionally, observation of fast switching (∼250 ns) translated to ultra-low energy (∼pJ) consumption. By varying compliance current ( I _C ) from 25 µ A to 1.6 mA during the set process, eight distinct non-volatile resistance states are reliably obtained, which showed statistically significant and non-overlapping resistance distribution. The multilevel resistive switching in Cu/TiO ₂ /Pt device is explained by considering filamentary the widening phenomenon in the electrochemical metallization memory cell. Numerical calculations revealed that increase in the diameter of the metallic conducting filament from ∼3 nm to 70 nm with increasing I _C is accountable for multiple low resistance states. The results of this study are promising towards realisation of emerging memory devices with low operating power, fast switching operation and high spatial density.

We have observed disorder driven weak localization with phase coherent electron transport in Ga doped (Zn:V)O thin films grown by pulsed laser deposition. All the Ga doped (Zn:V)O thin films with fixed 1 at% V and varying (0–5 at%) Ga concentration exhibit metal like behavior by Mott's definition while temperature coefficient of resistivity (TCR) systematically changes its sign with variation of temperature. This anomaly has been understood by considering quantum corrections to classical Boltz mann conductivity where significant contribution of weak localization has been confirmed from the observation of negative magneto resistance (MR) in temperature dependent magneto-transport measurements at varying magnetic fields. Phase coherent length of Ga doped (Zn:V)O thin films varies with both Ga concentration and measurement temperature and attains a maximum value ∼320 nm at 5K for 2 at% of Ga concentration. The nature of temperature dependence on phase coherent length demonstrates electron-electron scattering as the dominant phase breaking mechanism in the Ga doped (Zn:V)O thin films. Phase coherent electron transport and its tunability with Ga concentration in (Zn:V)O thin films is expected to have potential in futuristic phase coherent electron transport devices.

The transient characteristics of resistive switching processes have been investigated in TiO ₂ thin films grown by atomic layer deposition (ALD) to study the temporal evolution of the switching processes and measure the switching times. The reset and set switching times of unipolar Au/TiO ₂ /Pt devices were found to be ~250 µ s and 180 ns, respectively in the voltage windows of 0.5–0.9 V for reset and 1.9–4.8 V for set switching processes, obtained from quasi-static measurements. The reset switching time decreased exponentially with increasing amplitude of applied reset voltage pulse, while the set switching time remained insensitive to the amplitude of the set voltage pulse. A fast reset process with a switching time of ~400 ns was achieved by applying a reset voltage of ~1.8 V, higher than that of the quasi-static reset voltage window but below the set voltage window. The sluggish reset process in TiO ₂ thin film and the dependence of the reset switching time on the amplitude of the applied voltage pulse was understood on the basis of a self-accelerated thermal dissolution model of conducting filaments (CFs), where a higher temperature of the CFs owing to enhanced Joule heating at a higher applied voltage imposes faster diffusion of oxygen vacancies, resulting in a shorter reset switching time. Our results clearly indicate that fast resistive switching with switching times in hundreds of nanoseconds can be achieved in ALD-grown TiO ₂ thin films. This may find applications in fast non-volatile unipolar resistive switching memories.

The resistive switching dynamics of NiO thin films in Au/NiO/Pt device configuration have been investigated to measure the switching times of set and reset events and their dependence on compliance current and switching voltages. The set switching time was found to be ~10 ns at the set voltage of ~1.8 V, while reset switching time was much longer ~150 µ s at reset voltage of 0.8 V. With increasing compliance current from 5 to 75 mA during set process, although the resistance contrast of two states improved due to the decrease in the resistance of the low resistance state, the reset switching time increased substantially up to ~3 ms while set time remained nearly unchanged. The fast reset switching time of ~27 ns, comparable to that of set switching time, was achieved by applying a higher reset voltage of ~1.2 V. The observed dependence of reset time on compliance current and reset voltage in NiO thin films was explained in light of the conducting filamentary model in which reset process is of thermal nature and involves dissolution of conducting filaments as a consequence of Joule heating generated by the reset current.