Research Areas: The research work carried out in the laboratory is mainly related to characterization of the structure and radiation induced effects in chalcogenide glasses doped with Ag, as well as formation of electronic devices based on them. This includes nanaioninc conductive bridge nonvolatile memory devices – CBRAM or redoxy memory and radiation sensors.
Further aspect of the research work developed in the team is related to formation of radiation sensors utilizing the radiation sensitivity of ChG and radiation induced ions diffusion in them. These sensors are two-terminal micro devices with a Ag/Cu source region and a ChG based active region. Exposure to ionizing radiation stimulates RIE in the active region which promotes Ag/Cu diffusion and incorporation in the ChG thereby changing the material’s resistivity by orders of magnitude. The amorphous thin film based microelectronic nanoionic memristive devices are low cost, applicable to non-planar substrates. The γ radiation sensors produce an easily measured change in electrical resistance
Our particular interest is related to formation of printed or thin films nano-ionic nonvolatile memory devices – the so called contact bridge CBRAM non volatile memory devices. They are one of the most promising emerging technology non-volatile memory devices presented in the recent edition of the International Technology Roadmap of Semiconductors. The obvious longevity of this technology is impressive. It already meets memory performance requirements as defined by the International Technology Roadmap of Semiconductors, (ITRS) for the year 2020. The presentation of these devices in the ITRS cites works co-authored by M. Mitkova:
The CBRAM devices are built by an electrochemically inert electrode, solid electrolyte based on chalcogenide glasses doped with Ag, and an Ag electrode. Their performance relies on the formation and dissolution of a metal bridge growing in a solid electrolyte between the two electrodes. This resistive switching of the material is used for binary information storage, and defines the main aspects of this innovative improvement in memory technology.
The CBRAM process is characterized by fast switching, information being stored not as a charge but as nanoscopic amounts of metal in the electrolyte, high scalability and reliability. The storage medium can be placed in the interconnect layers above the silicon circuitry in a back-end-of-line (BEOL) sequence, making the technology compatible with CMOS logic processes. One of the best advantages of these devices is that the threshold voltage for the switching to occur is within 0.2-0.5 volts.
Resistance-voltage and current voltage characteristics of 40 nm PMC device based on Ge-Se glasses
Radiation sensors: Further aspect of the research work developed in the laboratory is related to formation of radiation sensors utilizing the radiation sensitivity of chalcogenide glasses (ChG) and radiation induced ions diffusion in them. These sensors are low cost, high performance microelectronic devices that react to g radiation to produce an easily measured change in electrical resistance. They are two-terminal micro devices with an active region consisting of a chalcogenide glass. Exposure to ionizing radiation stimulates radiation induced effects (RIE) in the active region which promotes silver (Ag) diffusion and incorporation in the ChG thereby reducing the material’s resistivity. Since these devices are based on amorphous films, they can be fabricated on flexible and non-planar substrates which increases their range of application. This approach is characterized by completely new principles of operation that offer low power consumption, compatibility with integrated circuit fabrication, and operational reversibility which allows for calibration and reuse. Since the family of the chalcogenide glasses includes a large number of materials, there are extended possibilities to tailor the sensitivity of the sensor to particular use situations. A schematic presentation of these devices is shown on the figure below.
The nanoionic processes and Ag diffusion occurring at radiation bring to more than 4 orders of magnitude difference in the resistivity (conductivity) of the material before and after radiation with g rays:
Equipment: Part of the experiments related to the research are carried out at the Idaho Microfabrication Laboratory (devices fabrication), Department of Physics – Raman and Optical characterization of the materials and the Boise State Center for Materials Characterization – XRD, TEM and SEM. The main equipment available in the laboratory is related to materials synthesis, wet processes with thin films, thin films fabrication and device testing:
High vacuum evaporation system
Device characterization probe station
Signal analyzer HP 4155 B
High temperature rocking furnace and ductless fume hood
Cryostat for low/high temperature characterization measurements
Research group: The team working on this research is formed by a post doc Ping Chen with PhD in Electrical Engineering with emphasis on chalcogenide glasses; he deals with structural materials characterization; five graduate students; Steve Wald – he works on fabrication (as part of the team) and designs for incorporation of CBRAM device in integrated circuits with advisor Dr. N. Rafla; Mahesh Ailavajhala – he works on radiation sensor research; Shwetha Vure – she works on materials for the two types of devices, Muhammad Risvan Latif – works on CBRAM non volatile memory devices, Bhes Pun and Kishor Kc work on their independent studies related to the two types of devices subject of studies in the laboratory and two undergrad students: Kasandra Wolf who makes atom force microscope studies and Steven Livers who makes glass synthesis.
Photo of the research group
D. Nesheva, M. Ailavajhala, P. Chen, M. Mitkova, et al., “Study of gamma radiation induced effects in Ge-rich chalcogenide thin films,” RAD INt. Conference, Nis Serbia, (2012) 19-22. M. Mitkova, Y. Sakaguchi, et al. – Structural Details of Ge-Rich and Silver-doped Chalcogenide Glasses for Nanoionic Nonvolatile Memory – Phys. Status Solidi (a) 207, (2010) 621
This work has been funded by the United States Department of Energy and the Department of Defense.
For more information regarding this area of research, please visit the website of Maria Mitkova