Substrate Patterning for 2D Material Doping and Photodetector Application

Abstract

Nowadays, various electronic devices using semiconductors such as smart phones, smart TVs, and computers have become indispensable parts in our lives. In particular, demand for semiconductors is rapidly increasing with the advent of the fourth industrial revolution era, which is represented by the internet of things (IoT), artificial intelligence (AI), and autonomous vehicles, etc. This demand includes not only for the existing semiconductors, but also for those with high performance and high integration, namely the development of semiconductors having new functions. One of the most important technologies in semiconductor industry is the patterning technique for fabricating desired device structures. Semiconductor patterning techniques, typified by a photolithography, have been developed since the beginning of the semiconductor industry, in parallel with researches to overcome the patterning limits in order to improve the integration degree of semiconductors. Consequently, understanding the semiconductor manufacturing technology (i.e., various patterning techniques) is very important for researchers and engineers in semiconductor fields. On the other hand, researches on new semiconducting materials are actively being carried out to replace silicon (Si, a typical inorganic semiconductor) along with the paradigm shift of next generation’s electronic devices. The most noteworthy materials are two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), black phosphorous (BP)) and hexagonal boron nitride (h-BN). As 2D materials, they are advantageous to be fabricated in stacked structure and also with flexibilities, which are important characteristics required in the next generation’s electronic devices. Although, there remain many issues to be overcome yet, it is expected gradually to replace inorganic semiconductors in the long term. This thesis begins with a review of various existing semiconductor patterning techniques and 2D materials. Chapter 1 will discuss various existing patterning technologies such as photolithography, electron (e)-beam lithography, laser interference lithography (LIL), and nanoimprint lithography (NIL), and the anodic aluminium oxide (AAO)-based patterning techniques. In addition, various two-dimensional (2D) materials, such as graphene (conductor), TMDs and BP (semiconductors), and h-BN (insulator) will be also reviewed, along with the possible application areas. The research is divided largely into three parts (Chapter 2, 3, and 4). Chapter 2 is a study for substrate patterning in sub-100 nm scale using AAO-based technique. The material AAO, which is widely used in nanopatterning area, will be briefly introduced, and the applied study for substrate patterning will also be introduced. One limitation of the previous technique was the high surface roughness after the dry etching. In order to overcome this issue, the polymer interlayer was introduced between AAO template and substrate. It will be seen that the surface roughness can be greatly reduced by the polymer interlayer. Chapter 3 will summarize the doping effect of graphene on nano-porous (NP) substrates. In this chapter, NP substrate fabricated by a LIL process was used, because of its easiness of large area patterning, and also easy controllability of the pore geometries. It will also be seen that the doping can be controlled by patterned substrates without using special dopants. Finally, in chapter 4, we will show that the photo-response of the MoS2 photodetector can be improved when a SiO2 NL array substrate is used. Here, electron (e)-beam evaporation was utilized to fabricate the SiO2 NL array. The e-beam evaporated SiO2 surface showed different properties from the pristine SiO2 substrate. In the former case, it was reported for the SiO2 to have oxygen-deficient characteristics, which might cause the electron-rich properties, and results in the n-doping enhancement of the transferred MoS2 on it. Thus, when MoS2 is transferred onto a SiO2 NL array substrate (composed of two different surfaces, i.e., bare- and e-beam evaporated SiO2 surfaces), the built-in potentials periodically emerged. When irradiated on it, photo-excitons can be effectively separated into individual carriers (i.e., electron and hole) by the effects, and with the drain bias (VD), the carriers effectively move to each electrode.