The research work carried out in the laboratory is mainly related to synthesis and characterization of the composition, structure and physicochemical properties of pure chalcogenide glasses and chalcogenide glasses doped with electronegative metals, including radiation induced effects, as well as formation of electronic devices based on them. This includes nanaioninc conductive bridge nonvolatile memory devices – memristors and radiation and gas sensors as well as their inclusion in integrated circuits.
Chalcogenide glasses synthesis
Chalcogenide glasses are synthesized in the laboratory by alloying at high temperature the necessary quantities of the elements building the glasses in evacuated quartz ampoules in a rocking furnace.
Materials’ composition characterization
Using Energy Dispersive Spectroscopy (EDS) we encounter the composition of the chalcogenide glasses for a precise idea about the composition of the thin films deposited from chalcogenide glasses synthesized in the laboratory.
Raman spectroscopy is the most useful method for studying the structure of chalcogenide glasses. It allows defining the structural units building the glasses and was the tool with which we have established the changes occurring in the chalcogenide matrix after introduction of Ag in it. This method was the key and helped us to be the first to understand the structure of Ge very rich glasses. The importance of this study is supported by the fact that the sputtered chalcogenide films and particularly Ge containing films used in the industry for formation of non volatile memory devices are usually Ge rich and the chalcogenide matrix forming after Ag diffusion is also very Ge rich.
X-ray diffraction (XRD)
We apply this method for understanding the molecular structure of the chalcogenide glasses. Our greatest success in application of the XRD studies was in detailed information that we got as how does Ag reside in the chalcogenide matrix after it diffuses in it. Using the Debay-Scherrer equation we also calculated the size of the microcrystals that form after Ag diffusion in the chalcogenide glasses.
X-ray photoelectron spectroscopy (XPS)
This is the best method to understand the molecular structure of the surface and also applying Auger spectroscopy in depth of the films. We realized using this method that even the X-ray radiation during the measurement made with the XPS instrument contributes towards structural changes in the glass films and Ag diffusion in the chalcogenide matrix and that prolonged illumination of the Ge rich chalcogenide films with visible light contributes to formation of an oxide film on their surface.
Atom Force Microscopy (AFM)
We use the AFM method to examine the surface of the films that we are depositing in terms of their roughness. This type of characterization combined with other data helps understand better the processes and effects going on with the films. For example, we found out through the study of the surface roughness that the chalcogenide glass films rich in Ge oxidize easily during the light illumination. This changes dramatically the structure and composition of the chalcogenide network. All this affects the films surface which becomes smoother.
Scanning electron microscopy (SEM)
We use the scanning electron microscopy for surface and patterning characterization.
Transmission electron microscopy
We are using this technique in order to produce cross sectional images of our devices and check the quality of the processing. For example using TEM, we check the quality of our lift off process at device formation, the via filling and the quality of the side walls of the devices.
Our particular interest is related to formation of printed or thin films nano-ionic nonvolatile memristive devices – the so called conductive bridge nanoionic non volatile memory devices. They are one of the most promising emerging technologies for non-volatile memory devices presented in the recent edition of the International Technology Roadmap of Semiconductors. These devices can be built by an electrochemically inert electrode, solid electrolyte based on ChG 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 (1 and 0), and defines the main aspects of this innovative improvement in memory technology. The conductive bridge nanoionic non volatile memory device 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 sides of the conductive bridge nanoionic non volatile memory memristive devices is that the threshold voltage for the switching to occur is within 0.2-0.5 volts. The presentation of these devices in the ITRS cites works co-authored by M. Mitkova:
Application of the nanoionic memory devices in integrated circuits:
Simulations based on the electronic characteristics of the nanoionic devices produced in the laboratory for formation of hybrid CMOS/Memristive circuits, neuromorphic architectures and big arrays.
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:
Part of the experiments related to the research are carried out at the Idaho Microfabrication Laboratory (this is the clean room at Boise State) – mainly devices fabrication; Department of Physics – Raman, XPS and Optical characterization of the materials and the Center for Materials Characterization – TEM, AFM and SEM. The equipment in the Nanoionic Materials and Devices Laboratory is organized in such manner that one can start with measurements of the materials for synthesis of bulk glasses, these glasses can be synthesized in the laboratory in the rocking furnace available, further, there is a vacuum evaporation system for formation of thin films from the synthesized glasses, there is a wet bench for chemical processes, optical station for realization of photoinduced processes, vacuum oven for keeping the materials and devices insulated from the air, probe station for devices characterization with signal analyzer and capability for work in the temperature region -70C – 140C.
Some examples for the equipment in the laboratory are presented below:
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 designs for incorporation of the nanoionic memristors in integrated circuits; Mahesh Ailavajhala – he works on radiation sensor research; Shwetha Vure – she works on materials for the two types of devices, Muhammad Rizvan Latif – works on improved materials solutions for non volatile memory devices and devices characterization, Bhes Pun works on non-standard Si based technologies, Kishor Kc works on gas sensors based on chalcogenide glasses; and two undergrad students: Kasandra Wolf who makes atom force microscope studies and Bryan Wright who makes glass synthesis and literature search.