Anas Mazady

Memristor PUF—A Security Primitive: Theory and Experiment

Physical unclonable functions (PUFs) have emerged as a promising hardware security primitive, but existing CMOS PUFs suffer from issues related to reliability and susceptibility to modeling attacks. Memristors, leveraging nanotechnology fabrication process and highly nonlinear behavior, pose as an interesting alternative. In this paper, we report the first demonstration of memristor based PUFs. The six memristors we fabricated show a 50% uncertainty of PUF response and high reliability upon repeated interrogation. A physics based circuit model of memristors was also implemented to accurately determine the simulation time required for randomly selected polyominoes from a 3-D array of memristors. The proposed model provides higher degree of complexity and results in seven orders of increase in simulation time for an attacker than that reported by Rajendran , 2012.

Memristor: Part I—The Underlying Physics and Conduction Mechanism

Memristor switching and observed I-V characteristics are explained using the underlying physics of the device in terms of the formation and rupture of filaments. Three different conduction mechanisms, namely-filament-assisted tunneling current, bulk tunneling current, and currents flowing through low and high conductivity filaments give rise to the total current in memristive systems. Heating of filaments during current conduction may reduce the ROFF/RON ratio of the device by increasing its ON resistance. In case of an organic layer, this issue can be circumvented by suitably increasing the organic layer thickness. Effects of different parameters on lifetime of memristors as resistance-switching random access memory have also been investigated.

Memristor: Part II–DC, Transient, and RF Analysis

The dc and RF circuit performance of memristor circuits, including transient behavior, is developed by considering current contributions arising from different conduction mechanisms, namely, filament-assisted and bulk tunneling currents and currents flowing through low and high conductivity filaments. The dc circuit model explains the observed I-V hysteresis and most importantly allows scaling and optimization. A transient circuit model of memristive system is developed, based upon the dynamics, incorporating underlying electrochemistry that suggests SET and RESET transitions for a 50-nm TiO2-based memristor take approximately 120 ps to stabilize. A faster READ/WRITE operation would require appropriate conditioning circuitry. RF analysis suggests for a maximum allowable frequency of 7.5 GHz beyond which memristors can no longer be used as RRAM. These values can be pushed to higher limits by increasing the device cross-sectional area or choosing lower permittivity materials. The developed model allows its incorporation in commercial circuit simulators. The derived model is validated by incorporating it in Chuas chaotic circuit.

Transient Circuit Model of Memristors

We report a transient circuit model of Pt/TiO2(40nm)/Pt memristor that accurately represents the underlying switching mechanism and can be extended to any other material platforms. The analysis is based on the analytic formulation of dynamical characteristics. The transient circuit model is simulated in Cadence using HSPICE. Rise time and fall times of the 40 nm TiO2 memristor were calculated to be 11.4 ps and 11.6 ps, respectively. The settling time during ON and OFF switching was determined to be 1.04 ns and 0.78 ns, respectively.

ZnMgO/ZnO Core-Shell Structures for Gas Sensing

Co-axial Zn1−xMgxO core, ZnO shell structures were grown using metal organic chemical vapor deposition (MOCVD), with Mg mole fractions of 2, 5 and 10%. The co-axial core shell structure, with the respective Mg concentration is verified using scanning electron microscope (SEM), transmission electron microscope (TEM) and energy dispersive spectroscopy (EDS). The response times (ṟise time and fall time) of the devices, after being exposed to methanol, varied with Mg mole fraction at the core, r-0.17s and, f-0.37s & f-0.48s for 2% Mg, r-0.81s and, f-5.98s & f-0.89s for 5% Mg and r-0.33s and f-0.13s for 10% Mg. The sensitivity of the devices at room temperature increased with the increment of Mg mole fraction at the core, 25%, 48% and 50% with Mg concentration of 0.02, 0.05 and 0.1, respectively, under high concentration of methanol. The estimated activation energy, corresponds to doubly charged oxygen vacancy (Vo2+).

ZnMgO solar blind detectors: from material to systems

Zinc oxide (ZnO) is a unique wide bandgap biocompatible material system exhibiting both semiconducting and piezoelectric properties that has a diverse group of growth morphologies. Bulk ZnO has a bandgap of 3.37 eV that corresponds to emissions in the ultraviolet (UV) spectral band. Highly ordered vertical arrays of ZnO nanowires (NWs) have been grown on substrates including silicon, SiO2, GaN, and sapphire using a metal organic chemical vapor deposition (MOCVD) growth process. The structural and optical properties of the grown vertically aligned ZnO NW arrays were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL) measurements. Compared to conventional UV sensors, detectors based on ZnO NWs offer high UV sensitivity and low visible sensitivity, and are expected to exhibit low noise, high quantum efficiency, extended lifetimes, and have low power requirements. The photoresponse switching properties of NW array based sensing devices have been measured with intermittent exposure to UV radiation, where the devices were found to switch between low and high conductivity states at time intervals on the order of a few seconds. Envisioned applications for such sensors/FPAs potentially include threat detection and threat warning.

DC circuit model of a memristor

We present a dc circuit model of a memristor based on the reported I-V data [1]. The method described here can easily be implemented to realize memristor based circuitry that serve different application platforms fabricated using any material combination.

Growth dependent optical properties of ZnMgO at THz frequencies

A relatively high Mg mole fraction of 7% is achieved using the cavitation effect under sonication to overcome the low solubility of ZnO-MgO at low temperature. The Mg mole fraction is confirmed by shift in the near band emission of free exciton under photoluminescence spectroscopy at room temperature. The x-ray diffraction pattern has a large peak associated to ZnO (002) from which the c-lattice constant is calculated to be 5.1967Ǻ. The nanorods (NRs) grown via sonochemical are compared to nanowires (NWs) grown using metal organic chemical vapor deposition (MOCVD) and hydrothermal synthesis. Also, the effect of the ZnO film used as seed layer is described and compare to a simple spin coated layer. Terahertz (THz) index of refraction and dielectric constant of wurtzite Zn1-xMgxO NWs with Mg mole fraction of 7% via sonochemical are determined using THz time domain spectroscopy (THz-TDS). The results are compared with ZnO and ZnMgO NWs with 10% Mg mole fraction grown using MOCVD. The successful growth of Zn1-xMgxO with wurtzite structure at low temperature permits realization of the growth of heterostructures, quantum well, nanowires and nanorods on flexible substrates providing lower cost, optical and carrier confinement necessary in advanced light emitting diodes (LEDs), laser diodes (LDs) and high efficiency solar cells.

Optimized Growth of ZnO Nanowires and Nanorods Using MOCVD

ZnO NWs were grown on Si (111) and Al2O3 substrates using MOCVD. ZnO (002) NWs on Si were randomly orientated while the NWs grown on sapphire were mostly vertically aligned as evident from scanning electron microscope images. The c-axis lattice constant corresponding to ZnO (002)/Si(111) demonstrated a dominant peak at 34.47°(2θ) attributed to ZnO along (002) with a FHWM of 0.0948°(θ) with corresponding c and a-lattice axes constants of 5.1996A and 3.2456A, respectively. The c-axis lattice constant for ZnO/sapphire was estimated to be 5.205A resulting in an out of plane strain of only 0.03%. Photoluminescence of ZnO nanowires grown on sapphire shows absorption peaks associated to exciton-exciton recombination and native defect such as zinc interstitial. In the case of NWs grown on Si, only exciton to exciton recombination was observed.

Energy Harvesting Leveraging Piezoelectric Property of ZnO Nanorods

Vertical ZnO nanorods (NRs) were grown on flexible plastic substrates at low temperature using hydrothermal synthesis method. An energy scavenging piezoelectric device was constructed using two flexible substrates with the NR sides facing each other providing a maximum open-circuit voltage of 1.4 V (peak). Two sets of three piezoelectric devices connected in series in a half-diode-bridge circuit configuration was demonstrated to turn on a commercial red LED.