Titolo della tesi: Optical and Electrical Characterization of VO2 thin film
During the Semiconductor-to-Metal Transition (SMT), vanadium dioxide (VO2)
reveals a dramatic and reversible change in its electrical and optical characteristics.
The scientific community is highly interested in VO2 because of this phase transition,
which takes place at a temperature that is similar to atmospheric conditions, approx-
imately 68°C. Upon the abrupt transition from an insulating monoclinic (M) phase
to a metallic rutile (R) phase, thermal properties, optical reflectance, and electrical
conductivity undergo significant alterations. The distinct electronic and structural
characteristics of these phase transitions, which are influenced by temperature and
electrical bias, make them extremely interesting. This unique behavior opens the
door to numerous applications, including as smart windows, electronic switches,
memory devices, sensors, and other innovative technologies made possible by this
special behavior. It is essential to understand the electrical and optical behavior of
VO2 during this transition in order to create useful devices based on their special
characteristics.
Vanadium dioxide (VO2) has been researched for decades, but several challenges
still remain to hinder its full potential. The material’s challenging phase behavior,
sensitive nature to the environment, and complicated synthesis requirements lead to
the major difficulties of the material. To ensure the appropriate phase transitions
and high quality performance of VO2, films should be synthesized with extreme
precision, good fabrication methods, thickness, and quality. These difficulties are
restricted not only to the quality and production of thin films but also to the deep
understanding of novel approaches in the field of characterization and theoretical
explanation. More interdisciplinary methods are required to overcome these obstacles
and fully utilize VO2 in practical applications.
In this work, the optical and electrical behavior of a polycrystalline VO2 thin film
(410 nm thick, 2 cm × 2 cm in size) deposited on a sapphire substrate is examined
using optical and electrical characterization methodologies.
We employed two distinct approaches for the optical characterization. First, a
purely optical approach that combines phase transition by Continuous Wave Optical
Excitation (PTCWE) and Polarized Raman Mapping provides a quick and econom-
ical way of obtaining the physical characteristics of the VO2 films. A fascinating
stepped behavior throughout the structural and electronic transition is revealed by
this combined technique, which is explained by the gradual stabilization of rutile
metallic domains within the semiconducting monoclinic matrix. (Chapter 2).
Second, we investigated how the polarization state of the optical beam changes
during the phase transition by employing polarization tomography with Stokes
parameters. This method offers comprehensive information on the optical response
that is dependent on polarization as well as on the impact of the changing metallic
and insulating phases. By this configuration, we examined how the VO2 phase
transition affected the transmitted light’s polarization states. (Chapter 3).
We carry out two sets of experiments for characterizing DC electrical behavior of
the sample VO2. Temperature-dependent measurements under constant voltage and
current-biased measurements at room temperature. The temperature-dependent
method uses a controlled temperature device to heat and cool the sample while
applying set voltages in order to capture the hysteretic characteristic of the phase
transition. In the current-biased observations at room temperature condition, We
are able to determine the minimum current required to move the sample into the
metallic phase at the lowest transition temperature, which is considerably lower than
the typical thermal transition point, and to self-sustain it in the metastable metallic
phase of VO2. Additionally, by examining the electrical parameter characteristics
during the transition, we differentiate between the metastable state and the fully
metallic phase. (Chapter 4).
With all aspects considered, this work offers an excellent understanding of the
electrical and optical responses of VO2 films on a sapphire. The results provide
a novel understanding of the dynamics of phase transitions, including the crucial
role of current-induced transitions, self-sustainability, polarization effects, and the
stabilization of metallic domains. Our results highlight the sample’s ability to self-
sustain the metastable state, which is important for energy-efficient phase-transition-
based devices. Furthermore, VO2 is a promising option for non-volatile memory
systems because of the observed hysteresis behavior throughout the heating and
cooling cycles, which emphasizes the memory effect present in the phase transition.