Erica DeIonno

A 160-kilobit molecular electronic memory patterned at 10 11 bits per square centimetre

The primary metric for gauging progress in the various semiconductor integrated circuit technologies is the spacing, or pitch, between the most closely spaced wires within a dynamic random access memory (DRAM) circuit. Modern DRAM circuits have 140 nm pitch wires and a memory cell size of 0.0408 μm2. Improving integrated circuit technology will require that these dimensions decrease over time. However, at present a large fraction of the patterning and materials requirements that we expect to need for the construction of new integrated circuit technologies in 2013 have ‘no known solution’. Promising ingredients for advances in integrated circuit technology are nanowires, molecular electronics and defect-tolerant architectures, as demonstrated by reports of single devices and small circuits. Methods of extending these approaches to large-scale, high-density circuitry are largely undeveloped. Here we describe a 160,000-bit molecular electronic memory circuit, fabricated at a density of 1011 bits cm-2 (pitch 33 nm; memory cell size 0.0011 μm2), that is, roughly analogous to the dimensions of a DRAM circuit projected to be available by 2020. A monolayer of bistable, [2]rotaxane molecules served as the data storage elements. Although the circuit has large numbers of defects, those defects could be readily identified through electronic testing and isolated using software coding. The working bits were then configured to form a fully functional random access memory circuit for storing and retrieving information.

Two-Dimensional Molecular Electronics Circuits

Addressing an array of bistable [2]rotaxanes through a two-dimensional crossbar arrangement provides the device element of a current-driven molecular electronic circuit. The development of the [2]rotaxane switches through an iterative, evolutionary process is described. The arrangement reported here allows both memory and logic functions to use the same elements.

Radiation Hardness of

Semiconducting TiO2 displays non-volatile multi-state, hysteretic behavior in its I-V characteristics that can be exploited as a memory material in a memristive device. We exposed memristive TiO2 devices in the on and off resistance states to 45 Mrad(Si) of ~1-MeV gamma radiation and 23 Mrad(Si) of 941-MeV Bi-ions under zero bias conditions and none of the devices were degraded. These results suggest that TiO2 memristive devices are good candidates for radiation hard electronics for aerospace.

{\rm TiO}_{2}

Electrochemically switchable bistable main-chain poly[n]rotaxanes have been synthesised using a threading-followed-by-stoppering approach and were incorporated into solid-state, molecular switch tunnel junction devices. In contrast to single-station poly[n]rotaxanes of similar structure, the bistable polymers do not fold into compact conformations held together by donor–acceptor interactions between alternating stacked π-electron rich and π-electron deficient aromatic systems. Films of the poly[n]rotaxane were incorporated into the devices by spin-coating, and their thickness was easily controlled. The switching functionality was characterised both (1) in solution by cyclic voltammetry and (2) in devices containing either two metal electrodes or one metal and one silicon electrode. Devices with one silicon electrode displayed hysteretic responses with applied voltage, allowing the devices to be switched between two conductance states, whereas devices containing two metal electrodes did not exhibit switching behaviour. The electrochemically switchable bistable poly[n]rotaxanes offer significant advantages in synthetic efficiency and ease of device fabrication as compared to bistable small-molecule [2]rotaxanes.

Memristive Junctions

TiO2 memristor devices may be a promising candidate for radiation hardened next-generation memories. They have been shown to be tolerant to both gamma radiation and alpha particles. In this work, we expand on the radiation studies previously done and measure the response of a large number of TiO2 memristor test devices to both neutrons and protons. We also use simulations to estimate the amount of damage done for each type and level of radiation and correlate the number of displacements to the experimentally measured current-voltage characteristics of the devices. We show that the TiO2 thin films are tolerant to high fluences of both neutrons and protons.

A solid-state switch containing an electrochemically switchable bistable poly[n]rotaxane

The impact of ionizing radiation on the retention and endurance of programmable metallization cells (PMC) ReRAM cells is investigated and presented for the first time, with additional work on resistance switching. This study shows that 60Co gamma-ray exposure has a minimal effect on the retention of PMC devices, up to a total ionizing dose (TID) of 2.8 Mrad (Ge30Se70), the maximum TID level tested. The retention of both high resistance states (HRS) and low resistance states (LRS) during exposure was tested. Endurance appears to be slightly reduced with gamma-ray exposure. The endurance was tested to maximum TID of 4.62 Mrad (Ge30Se70). DC response characterizations were also performed on PMC devices after cumulative dose exposures with 50 MeV protons and 100 keV electrons. The data show that PMCs are most sensitive to proton irradiation incident from the backside of the device. For the electron exposures, it is shown that the LRS is mostly unaffected, but the HRS drifts to lower resistance values with an increase in radiation exposure.

Displacement Damage in TiO

Ion-strike-induced single event transients in a type of nonvolatile resistive memory known as conductive bridge resistive memory (CBRAM) are investigated. Experimental data demonstrating bit upsets in 1T-1R devices under heavy ion strike are presented which show evidence of transitions from not only high to low resistance states but also from low to high resistance states. This is reported for such devices here for the first time. Device and circuit level simulations performed under various bias conditions are used to analyze possible upset modes. A crossbar CBRAM architecture without transistor selectors that offers higher density is also analyzed and shown to be susceptible to multiple bit upsets unlike 1T-1R array. Susceptibility of a 256 ×256 crossbar array to strike induced transients under two different bias schemes is simulated.

_{2}

Memristor devices have been identified as potential replacements for a variety of memory applications and may also be suitable for space applications. In this work, we present a review of radiation testing on TiO2-based memristor devices. The experimental results from three previous studies are reviewed and coupled here with modeling to gain a more complete understanding of the energy deposition and resulting effects on the electrical performance of the device. In addition, we discuss the implications of having a nanometer scaled thin film device and how that affects the energy deposition from the various radiation sources.

Memristor Devices

Over the last several years, various research groups have shown that metal oxide-based memristors or resistive random access memory (RRAM) components are tolerant to high levels of both displacement damage and ionizing dose. However, above certain thresholds of either the deposited displacement damage dose or the deposited ionizing dose, devices do respond to radiation. In this paper, the sensitivity of metal oxide memristor materials to radiation damage from various types of incident energetic ions is assessed.

Ionizing Radiation Effects on Nonvolatile Memory Properties of Programmable Metallization Cells

Resistive random access memories (RRAM) can be made of a variety of materials, including metal oxides and metal-doped chalcogenides, that exhibit memristive behavior. RRAMs have been identified as a leading candidate to replace flash memory and may also be suitable as replacements for other types of memory, including DRAM and SRAM. When sandwiched between two electrodes, specially grown thin films can be switched with applied bias between a high (off ) and a low resistance (on) state. Since 2007, these devices have been considered as candidates for space applications due to their device properties. There is a high density of inherent defects in the as-fabricated devices, which is likely to contribute to their tolerance to displacement damage. In addition, the nanometer length scale of the active device layers allows the devices to dissipate the charge deposited from ionizing dose. However, the testing of devices over several different radiation studies has revealed potential reliability concerns, including device to device variation, device sensitivity to measurements and physical device changes that occur with cycling between states. In this paper, we will discuss these concerns and present initial results of biased radiation testing of TaOx and TiO2 devices with ionizing radiation.