Mahesh Ailavajhala

Total-Ionizing-Dose Effects on the Resistance Switching Characteristics of Chalcogenide Programmable Metallization Cells

Programmable metallization cells (PMCs) are emerging ReRAM devices exhibiting resistance switching due to cation transport in a solid-state electrolyte and redox reactions at the electrodes. Their non-volatility and low power requirements have led to increased interest in their development for non-volatile memory applications. Investigation of the total dose response of PMCs will contribute to our understanding of radiation induced effects in these novel memory devices as well as assess their suitability for use in ionizing radiation environments. This work investigates the impact of total ionizing dose on the switching characteristic of silver doped Ge30Se70 PMC memory devices. The results obtained show that the resistance switching characteristic of these cells which use a solid state electrolyte based on Ge30Se70 is not affected by a total dose exposure of up to 10 Mrad( Ge30Se70).

Effects of Cobalt-60 Gamma-Rays on Ge-Se Chalcogenide Glasses and Ag/Ge-Se Test Structures

Solid state electrolytes fabricated with chalcogenide glass (ChG) are considered viable candidates for the next generation of non-volatile memory technologies. These glasses, which are composed of group IV and/or group V elements with those of group VI chalcogens (S, Se, and Te), are excellent metal ion conductors. Because of this property, the resistance across structures composed of ChG films sandwiched between active metal (e.g., Ag) and inert metal (e.g., Ni) electrodes can be switched upon the application of sufficient bias, thereby enabling memristive action. In this paper, the effects of 60Co gamma-ray irradiations on Ag/Ge30Se70 test structures are investigated. The results show that exposure to high-energy photons can trigger the transport of Ag+ & ions from an active Ag top layer into an underlying Ge30Se70 ChG film. Post-irradiation annealing experiments also indicate that this “photo-doping” process is reversible once the radiation stress is removed. Numerical simulations which model the mechanisms of radiation-induced photo-doping and recovery are shown to agree well with the data. The results and analysis presented in this paper suggest the ChG-based memristors may be more susceptible to transient radiation effects than cumulative radiation damage.

Radiation Hardening by Process of CBRAM Resistance Switching Cells

Non-volatile memory (NVM) technology highly resistant to ionizing dose and radiation effects in general continues to be a challenge for space missions. Novel NVM nano-ionic technologies known as conductive bridging random access memory (CBRAM), a resistive circuit technology, exhibits great promise for both high density memory and high total ionizing dose resilience. In this work, it is discovered that CBRAM can be sensitive to high TID levels. However, this novel technology can be radiation-hardened by process, which is demonstrated in this paper.

Flexible Sensors Based on Radiation-Induced Diffusion of Ag in Chalcogenide Glass

In this paper, previous work on chalcogenide-glass (ChG)-based radiation sensors is extended to include the effects of mechanical strain and temperature stress on sensors formed on a flexible polymer substrate. We demonstrate the feasibility of producing inexpensive flexible radiation sensors, which utilize radiation-induced migration of Ag+ ions in germanium selenide ( Ge20Se80) films to produce a decrease in resistance of several orders of magnitude between surface electrodes. This change in resistance can be related to total ionizing dose to give an instantaneous readout of radiation exposure. The ChG films are inherently flexible and this, along with an extremely simple device fabrication process at or near room temperature, allows inexpensive sensor structures to be fabricated on lightweight pliable polymeric substrates such as polyethylene napthalate (PEN). Test samples were irradiated with ionizing radiation (UV light and 60 Cobalt gamma rays). Irradiated samples were subjected to both tensile and compressive stress, and elevated operating temperatures. Stress and exposure to increased ambient temperature had little effect on device resistance. Analysis of the experimental data is supported by the results of COMSOL simulations that model radiation-induced lateral Ag diffusion in ChG.

Dynamics of silver photo-diffusion into Ge-chalcogenide films: time-resolved neutron reflectometry

Silver diffuses into an amorphous (a-) chalcogenide layer while visible light illuminates Ag/a-chalcogenide films and neutron reflectometry is a suitable technique probing time evolution of the depth profiles without damaging the sample by the probe beam itself. In this paper, we report the results of time-resolved neutron reflectivity measurements of a-Ge40Se60/Ag/ Si films taken while the films are exposed to visible light. From the measurements, we found enormous changes in the neutron reflectivity profile, including a loss of total reflection region, with continuous illumination even after forming one homogeneous layer, which occurred about 50 min after starting illumination. At this stage, a clear off-specular scattering was observed by a linear detector and a surface roughness was observed with naked eyes.

New Functionality of Chalcogenide Glasses for Radiation Sensing of Nuclear Wastes

Data about gamma radiation induced effects in Ge40Se60 chalcogenide thin films and radiation induced silver diffusion within these are presented. Blanket films and devices were created to study the structural changes, diffusion products, and device performance. Raman spectroscopy, X-ray diffraction, current vs. voltage (I-V) and impedance measurements expound the behavior of Ge40Se60 glass and silver diffusion within this glass under radiation. Raman study shows that there is a decrease in the area ratio between edge shared and corner shared structural units revealing structural reorganization occurring in the glasses as a result of gamma radiation. X-ray diffraction studies revealed that with sufficiently radiation dose it is also possible to create Ag2Se in selenium-depleted systems. Oxidation of the Ge enriched chalcogenide backbone is confirmed through the electrical performance of the sensing elements based on these films. Combination of these structural and diffusion products influences the device performance. The I-V behavior is characterized by increase in current and then stabilization as a function of radiation dose. Additionally, device modeling is also presented using Silvaco software and analytical methods to shed light on the device behavior. This type of sensor design and material characterizations facilitate in improving the radiation sensing capabilities of silver containing chalcogenide glass thin films.

Gamma Radiation Induced Effects in Floppy and Rigid Ge-Containing Chalcogenide Thin Films

We explore the radiation induced effects in thin films from the Ge-Se to Ge-Te systems accompanied with silver radiation induced diffusion within these films, emphasizing two distinctive compositional representatives from both systems containing a high concentration of chalcogen or high concentration of Ge. The studies are conducted on blanket chalcogenide films or on device structures containing also a silver source. Data about the electrical conductivity as a function of the radiation dose were collected and discussed based on material characterization analysis. Raman Spectroscopy, X-ray Diffraction Spectroscopy, and Energy Dispersive X-ray Spectroscopy provided us with data about the structure, structural changes occurring as a result of radiation, molecular formations after Ag diffusion into the chalcogenide films, Ag lateral diffusion as a function of radiation and the level of oxidation of the studied films. Analysis of the electrical testing suggests application possibilities of the studied devices for radiation sensing for various conditions.

PMOS device performance improvement using buried contact implants

An ultra-low energy high dose B-based implant was processed after source and drain region formed and before metal sillicide contact formed for PMOS devices. B beam-line (BL) implant and plasma doping (PLAD) using either B2H6 or BF3 gases were utilized for this process. PMOS device performance showed significant improvements, including ~70 percent lower contact resistances, similar threshold and sub-threshold characteristics, and ~15 percent higher drive currents without degrading off current. PLAD is preferred on this application because of its much higher throughput in this process regime.

Silver photo-diffusion and photo-induced macroscopic surface deformation of Ge33S67/Ag/Si substrate

Ge-chalcogenide films show various photo-induced changes, and silver photo-diffusion is one of them which attracts lots of interest. In this paper, we report how silver and Ge-chalcogenide layers in Ge33S67/Ag/Si substrate stacks change under light exposure in the depth by measuring time-resolved neutron reflectivity. It was found from the measurement that Ag ions diffuse all over the matrix Ge33S67 layer once Ag dissolves into the layer. We also found that the surface was macroscopically deformed by the extended light exposure. Its structural origin was investigated by a scanning electron microscopy.

TID Impact on Process Modified CBRAM Cells

In this study it is shown that Conductive Bridging Random Access Memory (CBRAM) might be sensitive to Total Ionizing Dose (TID) depending on the manufacturing process. TID levels at which sensitivity occurs for one of the studied processes are still very high, showing that CBRAM technology is a very interesting solution for future Non Volatile Memory (NVM) technologies to be used in space.