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- Creators: Brahms, Johannes, 1833-1897
Description
Advances in software and applications continue to demand advances in memory. The ideal memory would be non-volatile and have maximal capacity, speed, retention time, endurance, and radiation hardness while also having minimal physical size, energy usage, and cost. The programmable metallization cell (PMC) is an emerging memory technology that is likely to surpass flash memory in all the listed ideal memory characteristics. A comprehensive physics-based model is needed to fully understand PMC operation and aid in design optimization. With the intent of advancing the PMC modeling effort, this thesis presents two simulation models for the PMC. The first model is a finite element model based on Silvaco Atlas finite element analysis software. Limitations of the software are identified that make this model inconsistent with the operating mechanism of the PMC. The second model is a physics-based numerical model developed for the PMC. This model is successful in matching data measured from a chalcogenide glass PMC designed and manufactured at ASU. Matched operating characteristics observable in the current and resistance vs. voltage data include the OFF/ON resistances and write/erase and electrodeposition voltage thresholds. Multilevel programming is also explained and demonstrated with the numerical model. The numerical model has already proven useful by revealing some information presented about the operation and characteristics of the PMC.
ContributorsOleksy, David Ryan (Author) / Barnaby, Hugh J (Thesis advisor) / Kozicki, Michael N (Committee member) / Edwards, Arthur H (Committee member) / Arizona State University (Publisher)
Created2013
Description
The dissolution of metal layers such as silver into chalcogenide glass layers such as germanium selenide changes the resistivity of the metal and chalcogenide films by a great extent. It is known that the incorporation of the metal can be achieved by ultra violet light exposure or thermal processes. In this work, the use of metal dissolution by exposure to gamma radiation has been explored for radiation sensor applications. Test structures were designed and a process flow was developed for prototype sensor fabrication. The test structures were designed such that sensitivity to radiation could be studied. The focus is on the effect of gamma rays as well as ultra violet light on silver dissolution in germanium selenide (Ge30Se70) chalcogenide glass. Ultra violet radiation testing was used prior to gamma exposure to assess the basic mechanism. The test structures were electrically characterized prior to and post irradiation to assess resistance change due to metal dissolution. A change in resistance was observed post irradiation and was found to be dependent on the radiation dose. The structures were also characterized using atomic force microscopy and roughness measurements were made prior to and post irradiation. A change in roughness of the silver films on Ge30Se70 was observed following exposure. This indicated the loss of continuity of the film which causes the increase in silver film resistance following irradiation. Recovery of initial resistance in the structures was also observed after the radiation stress was removed. This recovery was explained with photo-stimulated deposition of silver from the chalcogenide at room temperature confirmed with the re-appearance of silver dendrites on the chalcogenide surface. The results demonstrate that it is possible to use the metal dissolution effect in radiation sensing applications.
ContributorsChandran, Ankitha (Author) / Kozicki, Michael N (Thesis advisor) / Holbert, Keith E. (Committee member) / Barnaby, Hugh (Committee member) / Arizona State University (Publisher)
Created2012
Description
Chalcogenide glass (ChG) materials have gained wide attention because of their applications in conductive bridge random access memory (CBRAM), phase change memories (PC-RAM), optical rewritable disks (CD-RW and DVD-RW), microelectromechanical systems (MEMS), microfluidics, and optical communications. One of the significant properties of ChG materials is the change in the resistivity of the material when a metal such as Ag or Cu is added to it by diffusion. This study demonstrates the potential radiation-sensing capabilities of two metal/chalcogenide glass device configurations. Lateral and vertical device configurations sense the radiation-induced migration of Ag+ ions in germanium selenide glasses via changes in electrical resistance between electrodes on the ChG. Before irradiation, these devices exhibit a high-resistance `OFF-state' (in the order of 10E12) but following irradiation, with either 60-Co gamma-rays or UV light, their resistance drops to a low-resistance `ON-state' (around 10E3). Lateral devices have exhibited cyclical recovery with room temperature annealing of the Ag doped ChG, which suggests potential uses in reusable radiation sensor applications. The feasibility of producing inexpensive flexible radiation sensors has been demonstrated by studying the effects of mechanical strain and temperature stress on sensors formed on flexible polymer substrate. The mechanisms of radiation-induced Ag/Ag+ transport and reactions in ChG have been modeled using a finite element device simulator, ATLAS. The essential reactions captured by the simulator are radiation-induced carrier generation, combined with reduction/oxidation for Ag species in the chalcogenide film. Metal-doped ChGs are solid electrolytes that have both ionic and electronic conductivity. The ChG based Programmable Metallization Cell (PMC) is a technology platform that offers electric field dependent resistance switching mechanisms by formation and dissolution of nano sized conductive filaments in a ChG solid electrolyte between oxidizable and inert electrodes. This study identifies silver anode agglomeration in PMC devices following large radiation dose exposure and considers device failure mechanisms via electrical and material characterization. The results demonstrate that by changing device structural parameters, silver agglomeration in PMC devices can be suppressed and reliable resistance switching may be maintained for extremely high doses ranging from 4 Mrad(GeSe) to more than 10 Mrad (ChG).
ContributorsDandamudi, Pradeep (Author) / Kozicki, Michael N (Thesis advisor) / Barnaby, Hugh J (Committee member) / Holbert, Keith E. (Committee member) / Goryll, Michael (Committee member) / Arizona State University (Publisher)
Created2013
Description
Programmable metallization cell (PMC) technology is based on an electrochemical phenomenon in which a metallic electrodeposit can be grown or dissolved between two electrodes depending on the voltage applied between them. Devices based on this phenomenon exhibit a unique, self-healing property, as a broken metallic structure can be healed by applying an appropriate voltage between the two broken ends. This work explores methods of fabricating interconnects and switches based on PMC technology on flexible substrates. The objective was the evaluation of the feasibility of using this technology in flexible electronics applications in which reliability is a primary concern. The re-healable property of the interconnect is characterized for the silver doped germanium selenide (Ag-Ge-Se) solid electrolyte system. This property was evaluated by measuring the resistances of the healed interconnect structures and comparing these to the resistances of the unbroken structures. The reliability of the interconnects in both unbroken and healed states is studied by investigating the resistances of the structures to DC voltages, AC voltages and different temperatures as a function of time. This work also explores replacing silver with copper for these interconnects to enhance their reliability. A model for PMC-based switches on flexible substrates is proposed and compared to the observed device behavior with the objective of developing a formal design methodology for these devices. The switches were subjected to voltage sweeps and their resistance was investigated as a function of sweep voltage. The resistance of the switches as a function of voltage pulse magnitude when placed in series with a resistance was also investigated. A model was then developed to explain the behavior of these devices. All observations were based on statistical measurements to account for random errors. The results of this work demonstrate that solid electrolyte based interconnects display self-healing capability, which depends on the applied healing voltage and the current limit. However, they fail at lower current densities than metal interconnects due to an ion-drift induced failure mechanism. The results on the PMC based switches demonstrate that a model comprising a Schottky diode in parallel with a variable resistor predicts the behavior of the device.
ContributorsBaliga, Sunil Ravindranath (Author) / Kozicki, Michael N (Thesis advisor) / Schroder, Dieter K. (Committee member) / Chae, Junseok (Committee member) / Alford, Terry L. (Committee member) / Arizona State University (Publisher)
Created2011
Description
The field of flexible displays and electronics gained a big momentum within the recent years due to their ruggedness, thinness, and flexibility as well as low cost large area manufacturability. Amorphous silicon has been the dominant material used in the thin film transistor industry which could only utilize it as N type thin film transistors (TFT). Amorphous silicon is an unstable material for low temperature manufacturing process and having only one kind of transistor means high power consumption for circuit operations. This thesis covers the three major researches done on flexible TFTs and flexible electronic circuits. First the characterization of both amorphous silicon TFTs and newly emerging mixed oxide TFTs were performed and the stability of these two materials is compared. During the research, both TFTs were stress tested under various biasing conditions and the threshold voltage was extracted to observe the shift in the threshold which shows the degradation of the material. Secondly, the design of the first flexible CMOS TFTs and CMOS gates were covered. The circuits were built using both inorganic and organic components (for nMOS and pMOS transistors respectively) and functionality tests were performed on basic gates like inverter, NAND and NOR gates and the working results are documented. Thirdly, a novel large area sensor structure is demonstrated under the Electronic Textile project section. This project is based on the concept that all the flexible electronics are flexible in only one direction and can not be used for conforming irregular shaped objects or create an electronic cloth for various applications like display or sensing. A laser detector sensor array is designed for proof of concept and is laid in strips that can be cut after manufacturing and weaved to each other to create a real flexible electronic textile. The circuit designed uses a unique architecture that pushes the data in a single line and reads the data from the same line and compares the signal to the original state to determine a sensor excitation. This architecture enables 2 dimensional addressing through an external controller while eliminating the need for 2 dimensional active matrix style electrical connections between the fibers.
ContributorsKaftanoglu, Korhan (Author) / Allee, David R. (Thesis advisor) / Kozicki, Michael N (Committee member) / Holbert, Keith E. (Committee member) / Kaminski, Jann P (Committee member) / Arizona State University (Publisher)
Created2012
ContributorsBrahms, Johannes, 1833-1897 (Composer)
ContributorsBrahms, Johannes, 1833-1897 (Composer)
ContributorsBrahms, Johannes, 1833-1897 (Composer)
ContributorsBrahms, Johannes, 1833-1897 (Composer)
Description
Programmable metallization cell (PMC) technology employs the mechanisms of metal ion transport in solid electrolytes (SE) and electrochemical redox reactions in order to form metallic electrodeposits. When a positive bias is applied to an anode opposite to a cathode, atoms at the anode are oxidized to ions and dissolve into the SE. Under the influence of the electric field, the ions move to the cathode and become reduced to form the electrodeposits. These electrodeposits are filamentary in nature and persistent, and since they are metallic can alter the physical characteristics of the material on which they are formed. PMCs can be used as next generation memories, radio frequency (RF) switches and physical unclonable functions (PUFs).
The morphology of the filaments is impacted by the biasing conditions. Under a relatively high applied electric field, they form as dendritic elements with a low fractal dimension (FD), whereas a low electric field leads to high FD features. Ion depletion effects in the SE due to low ion diffusivity/mobility also influences the morphology by limiting the ion supply into the growing electrodeposit.
Ion transport in SE is due to hopping transitions driven by drift and diffusion force. A physical model of ion hopping with Brownian motion has been proposed, in which the ion transitions are random when time window is larger than characteristic time. The random growth process of filaments in PMC adds entropy to the electrodeposition, which leads to random features in the dendritic patterns. Such patterns has extremely high information capacity due to the fractal nature of the electrodeposits.
In this project, lateral-growth PMCs were fabricated, whose LRS resistance is less than 10Ω, which can be used as RF switches. Also, an array of radial-growth PMCs was fabricated, on which multiple dendrites, all with different shapes, could be grown simultaneously. Those patterns can be used as secure keys in PUFs and authentication can be performed by optical scanning.
A kinetic Monte Carlo (KMC) model is developed to simulate the ion transportation in SE under electric field. The simulation results matched experimental data well that validated the ion hopping model.
The morphology of the filaments is impacted by the biasing conditions. Under a relatively high applied electric field, they form as dendritic elements with a low fractal dimension (FD), whereas a low electric field leads to high FD features. Ion depletion effects in the SE due to low ion diffusivity/mobility also influences the morphology by limiting the ion supply into the growing electrodeposit.
Ion transport in SE is due to hopping transitions driven by drift and diffusion force. A physical model of ion hopping with Brownian motion has been proposed, in which the ion transitions are random when time window is larger than characteristic time. The random growth process of filaments in PMC adds entropy to the electrodeposition, which leads to random features in the dendritic patterns. Such patterns has extremely high information capacity due to the fractal nature of the electrodeposits.
In this project, lateral-growth PMCs were fabricated, whose LRS resistance is less than 10Ω, which can be used as RF switches. Also, an array of radial-growth PMCs was fabricated, on which multiple dendrites, all with different shapes, could be grown simultaneously. Those patterns can be used as secure keys in PUFs and authentication can be performed by optical scanning.
A kinetic Monte Carlo (KMC) model is developed to simulate the ion transportation in SE under electric field. The simulation results matched experimental data well that validated the ion hopping model.
ContributorsYu, Weijie (Author) / Kozicki, Michael N (Thesis advisor) / Barnaby, Hugh (Thesis advisor) / Diaz, Rodolfo (Committee member) / Goryll, Michael (Committee member) / Arizona State University (Publisher)
Created2015