Development of oxide electronics based on two-dimensional materials derived from liquid metals
Creators: name: Aukarasereenont, Patjaree
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401605
subject: Functional materials
classificationCode: 510499
subject: Condensed matter physics not elsewhere classified

description: Transparent and flexible electronics have been integrated in our daily life in many forms of applications. The contributions of such technologies range from displays, wearables, automotive circuits, solar cells, and the list goes on. To date, several materials have been investigated to be used in a thin-film transistor (TFT), a key component enabling these applications. Among them, two-dimensional (2D) oxides possess outstanding chemical and physical properties as well as offer a wide range of bandgaps i.e., they can be studied as conductors, semiconductors, and insulators.

Semiconducting oxide materials can also be subdivided into n-type and p-type semiconductors, in which the former has electrons as a majority charge carrier while the latter relies on holes. A great body of research has been carried out to explore and study n-type oxides while very few p-type oxides have been reported. Despite challenges in the search and synthesis of high mobility p-type oxides, both n-type and p-type semiconductors are required to realise the full spectrum of functional oxide electronics. This thesis focuses on studying a novel high mobility p-type oxide, namely tellurium oxide (β-TeO2).
Aside from the general need for the development of high-quality p-type oxide materials, in recent years, 2D semiconductors have also become a key research focus. 2D materials offer various notable and outstanding characteristics including remarkable carrier mobility, optical transparency, mechanical flexibility, lateral strength, etc. Moreover, their properties can be further modified through varying the thickness and surface functionalisation. Layering different 2D materials on top of one another also leads to a heterostructure which results in hybrid properties that are not observed in each individual material. 

Several approaches have been established to isolate 2D oxide materials, yet some still possess limitations in cost, time, as well as high fabrication temperature. Nonetheless, liquid metals and their alloys have now emerged as another route to produce 2D materials at relatively low temperature with simplicity. The rapid self-limiting oxidation process of liquid metals in response to exposure to an oxygen-environment, following the Cabrera-Mott model, results in the formation of an ultrathin oxide layer on the surface of molten metal. This oxide skin can be exfoliated from the parent liquid metal with ease and transferred onto a desired substrate. To date, a variety of liquid metal-based transfer techniques based on this mechanism have been developed, which led to the development of a number of 2D oxides. 

This thesis employed liquid metal-based synthesis principles to further expand the 2D oxides library. The material of interest in this thesis is β-TeO2, which has been predicted based on DFT calculations to be a high mobility wide bandgap p-type semiconductor. The first technical chapter of this work reports the synthesis of 2D β-TeO2 by isolating it from a molten selenium-tellurium alloy through a specifically developed synthesis technique. The material exhibited indeed a high hole mobility which was assessed through both back-gated field-effect transistor (FET) and Hall effect measurements. 

The crystal structure of ultrathin β-TeO2 was also found to be non-centrosymmetric, rendering it a potential piezoelectric material. As such, piezoresponse force microscopy (PFM) analysis was subsequently performed to broaden the versatility of β-TeO2, and the results are in a good agreement with the hypothesis. Nanosheets with three different thicknesses were analysed and exceptionally high vertical piezoresponse coefficients (d33) were observed, with the highest value being obtained from the thinnest β-TeO2 sheet. 

Another interesting 2D material that has been successfully isolated using liquid metal-based techniques and which is now widely studied is 2D Ga2O3. Recently, 2D Ga2O3 derived from liquid gallium has been reported to be a promising dielectric candidate as well as being capable of protecting materials underneath the oxide layer from further fabrication steps. Therefore, the material was integrated in H-diamond MOSFETs where two layers of the material were required to fully encapsulate an active region. The results have shown that double-printed dielectric Ga2O3 worked effectively as a gate dielectric material and exhibited excellent stability. The final stage of this work investigated the fabrication of a top-gated FET utilising β-TeO2 as a semiconducting channel and Ga2O3 nanosheets as a dielectric material. Despite considerable effort being dedicated towards realising this device architecture, a functioning device has not been acquired yet. The challenges that were encountered are discussed in detail and further suggestions that may allow realising this device in future work are provided.

Overall, this thesis introduces 2D β-TeO2 as one of the first high mobility semiconducting p-type oxides and high d33 piezoelectric material. These results also emphasise the potential of liquid metal-based synthesis routes in providing opportunities to study stratified and non-stratified 2D materials. The findings of this thesis will open up new avenues for exploring β-TeO2 and other liquid metal derived 2D materials as well as their potential application in oxide electronics technologies.