In recent years, various energy-harvesting devices or systems have attracted much attention due to the explosive growth of portable electronic devices and the gradual depletion of fossil-fuel energy. Among more advanced energy harvesting devices or systems, thermoelectric-based devices have been in the spotlight, particularly because they can be used for many practical applications and adapted easily to be used for small-scale implementations. Over the past years, there have been many thermoelectric-based energy-harvesting researches, most of which have been focused largely on various bulk types of thermoelectric semiconductor materials. Unfortunately, those bulk types of thermoelectric devices or systems cannot be used for the applications of the integrated circuit (IC) chips and human body. Moreover, the bulk types of thermoelectric semiconductor materials with high efficiency mostly contain toxic components such as bismuth (Bi) and lead (Pb).
More recently, it was known that nanostructured thermoelectric materials could have even higher efficiency compared to those bulk types of thermoelectric semiconductor materials. Reportedly, there have been other intensive researches on the nano-wires (NWs) and quantum dots (QDs), which are not easily adaptable to industry or have some limitations for commercial or practical applications.
From this point of view, this thesis presents a study on the thermoelectric properties of Si/$Mg_2Si$ hetero-structure thin films deposited by an RF magnetron sputtering method. Especially in this study, the Si/$Mg_2Si$ hetero-structure thin film layers were fabricated that are environmentally friendly and non-toxic Si/Silicide ($Mg_2Si$) thermoelectric materials. Also, the formation of the hetero-structure silicide/Si thin film layers was designed based on the so-called “bandgap engineering” concept, which was adopted in an effort to filter out the low-energy electrons by using potential barriers and thus decrease the thermal conductivity. For better performance, an optimization of the barrier length and barrier height of the silicide/Si thin film layers was made by using Landauer-Buttiker formalism in the ballistic regime. The figure-of-merit (ZT) was improved as the silicide thin film layers with a barrier height of 0.1 - 0.6eV were inserted individually layer-by-layer between the Si thin film layers, while the barrier length was fixed at 10nm. Among the various candidates of silicide layers, the $Mg_2Si$ silicide material was selected due to its electron affinity ($e\chi$) of 3.4 - 3.7 eV which leads to the work function difference ($e\phi_{Mg2Si-Si}$) of 0.3 - 0.6 eV between the $Mg_2Si$ silicide and the heavily doped Si film layers.
First, both the magnesium (Mg) and silicon (Si) thin film layers, stacked alternatively on top of each other, were sputter-deposited with various process conditions on the $SiO_2$/Si substrates. Then, the thermal annealing for the $Mg_2Si$ formation was made all at an optimal condition.
Finally, those Si/$Mg_2Si$ thin film layers showed more improvements in thermoelectric and electrical properties, compared to the amorphous silicon (a-Si) thin film layer. They showed an improved power factor which is 2 times higher than that of the a-Si thin film layer. Their thermal conductivity was also reduced to two thirds of that of the a-Si layer.
에너지 하베스팅 기술의 전폭적 관심으로 인하여 에너지 하베스팅 기술중의 하나인 열전(Thermoelectric) 기술 또한 최근 많은 연구와 개발이 진행되고 있다. 그러나 많은 연구가 Bismuth Telluride 기반의 독성 물질을 사용하고 있다. 또한 Bismuth Telluride 나 Skutterudite 물질기반의 열전 소재는 나노 사이즈 (nano-size) 기반의 박막 증착에 한계점이 있으며, 물질의 희소성과 높은 수입의존도로 인하여 다른 대안이 필요한 상태이다.
본 논문에서는 인체에 무해한 실리콘 (Si) 기반의 물질을 이용한 친환경적인 열전기술에 대한 연구를 소개하고자 한다. 본 연구에서 Si 자체는 제백 계수가 낮고 열전도도가 크기 때문에 이를 제어하기 위하여 Bandgap engineering 을 적용하였다. Bandgap engineering 을 통하여 저에너지 전자의 이동을 제어하고 phonon scattering 을 억제하여 제백 계수를 높이고 열전도성을 낮추었다. 이를 위하여 CMOS 공법에 많이 사용되는 Silicide 물질을 선택하였으며 RF magnetron sputtering 방법을 사용하여 증착 하였다.
