Date of Final Oral Examination (Defense)
Type of Culminating Activity
Doctor of Philosophy in Electrical and Computer Engineering
Electrical and Computer Engineering
Maria I. Mitkova, Ph.D.
For decades, various radiation-detecting materials have been extensively researched, to find a better material or mechanism. Recently, there has been a growing need for smaller, and more effective materials or devices that are Integrated Circuits (IC) compatible, and can perform similar functions as bulkier Geiger counters, and other measurement options, which fail the requirement for easy, cheap, and accurate radiation dose measurements. Here arises the use of thin films of chalcogenide glasses, which have unique properties of high thermal stability along with high sensitivity towards short wavelength radiation.
In this work, the effect of γ-rays, generated from a 60Co source, on the properties of thin films chalcogenide glasses was studied. Various film compositions from different germanium containing chalcogenide glass systems, i.e., Ge-S, Ge-Se, and Ge-Te, were investigated. These materials are the most thermally stable among the chalcogenide glasses, therefore they were studied to get a broad perspective of the development of structures, and the effect of chemical bonding under different radiation doses.
Study of the bare films provided an insight into the structural changes, and allowed the creation of different device designs, which take advantage of these changes. The bare film investigations were performed using Raman spectroscopy, and Energy Dispersive X-ray Spectroscopy (EDS). The result of these studies revealed that the destruction, and reorganization of the structure that occurred depends on the original structure of the host material. Gamma radiation-induced new structural formation were discovered, and related to the film structural organization, and the chemical bonding within the specific films. Additionally, X-ray Photoelectron spectroscopy (XPS), and Atomic Force Microscopy (AFM) provided insight into the topological transformation associated with the underlying structural changes. Along with the bare films, radiation-induced silver diffusion was studied to understand the role, and effect of silver during a radiation event. The introduction of silver creates different silver containing products that aid or hinder the increase in the film conductivity. These silver containing films were investigated using X-ray diffraction, and elemental mapping to determine the silver containing products, crystal sizes, rate of silver diffusion, and the oxidation rate due to radiation dose. These results were discussed based on the particular structures of the glasses, and the existing models. This information was also used as inputs in order to model, and simulate the real time diffusion of silver using COMSOL multiphysics software. Combined, these results provided a partial view of the mechanisms contributing to the device performance.
After careful considerations of the various effects on the conductivity of the films, several device designs were fabricated, and their electrical performances are presented as a function of radiation dose. Three distinct generations of devices were created, each of which has offered a different methodology for amplifying the effects determined in the film analysis. Two generations of devices (Gen. 1 and Gen. 2) were fabricated using a laterally diffusing silver source while Gen. 3 devices were created with a specific structure where the vertical diffusion of silver contributed to changes in conductivity. The structure of the Gen. 2 devices was derived through electric field simulations, and then was fabricated using conventional photolithography processes. The conductivity of the three types of devices was measured by performing current vs. voltage measurements after discrete doses, after all the dynamic effects had ceased. Some devices show greater than four orders of magnitude change in current from pre radiation to post irradiation. This is a substantial change, which can be detected using significantly lower voltages when compared to the current dosimeters, allowing these sensors to be used in low power or energy saving applications. Additionally, a special circuit has been designed, which allows the capability to detect these changes in current.
Ailavajhala, Mahesh Satyanarayana, "Nano-Ionic Radiation Sensor: Materials Engineering, Device Design, and Testing" (2014). Boise State University Theses and Dissertations. 814.