아르곤 플라즈마 처리를 통한 2차원 이황화몰리브덴(MoS2) 박막의 구조상전이 거동에 관한 연구

국문 초록 (Abstract)

2차원(Two Dimensional) 물질은 그래핀의 발견 이후 차세대 반도체 소자로 널리 연구되어왔다. 특히 이황화몰리브덴 (MoS2)은 우수한 모빌리티 특성과 층 수에 따라 다른 밴드갭 특성을 가지는 차세대 반도체 소자 후보 중 하나이다. 하지만 이황화몰리브덴(MoS2)은 metal contact과 높은 접촉 저항을 가지며 그에 따른 소자 성능 저하의 문제점을 가지고 있다. 이 문제를 해결하기 위해 최근 연구되고 있는 분야 중 하나로 밴드갭 엔지니어링이 있으며, 이는 이황화몰리브덴(MoS2)의 구조상에 따른 밴드갭 차이를 이용하는 방법이다. 이황화몰리브덴(MoS2)은 동질이상이라는 복수의 구조상을 가지며 각 구조상에 따라 도체, 반도체 등 서로 다른 전기적 특성을 가진다. 이를 통해 반도체 성질을 가지는 육방형(Hexagonal, 2H) 구조상 대신 도체 성질을 가지는 팔면체(Octahedral, 1T) 구조를 이용하면 접촉 저항을 줄일 수 있다. 이를 구현하기 위한 연구도 꾸준히 이루어 지고 있으며 아르곤 플라즈마 처리도 이황화몰리브덴(MoS2)의 구조상전이 방법 중 하나이다. 아르곤 플라즈마 처리는 아르곤 이온의 물리적 충격을 이용한 운동 에너지 전달을 통해 이황화몰리브덴 (MoS2)의 구조상전이를 일으킨다. 본 연구에서는 물리적으로 박리된 단층 혹은 2층의 이황화몰리브덴(MoS2)에 축전 결합 플라즈마(Capacitively Coupled Plasma, CCP) 방식의 아르곤 플라즈마 처리를 통해 플라즈마의 RF전력과 처리 시간에 따른 이화몰리브덴(MoS2)의 구조상전이 현상 및 거동에 대해 알아보고자 하였다. 이황화몰리브덴(MoS2)의 구조상 및 층 수는 라만분광법 및 PL로 판별하였다. 본 연구를 통해 이황화몰리브덴(MoS2)을 disordered 및 식각없이 육방형 (2H) 상에서 팔면체(1T)상으로 구조상전이 시키기 위해서는 특정 조건의 아르곤 플라즈마 처리가 필요함을 알 수 있었다.

다국어 초록 (Multilingual Abstract)

Two-dimensional (2D) materials have been widely researched as next generation
semiconductor devices after graphene was discovered. Especially, MoS2 is one of the
potential candidates for 2D channel material due to its high mobility and unique band gap
property depending on the number of layers. Mono layered MoS2 has direct band gap of
1.8 eV which can decrease energy loss induced by phonon emission while bulk MoS2 and
Si has indirect band gap of 1.2 eV and 1.14 eV, respectively. Despite of the forementioned
advantages, high contact resistance between MoS2 and metal contact caused
by van der Waals gap is one of the critical problems in terms of transistor performance.
Phase transition of MoS2 can be a solution for decreasing contact resistance since MoS2
has several crystal phases which have different electrical properties to each other. For
example, 2H phase of MoS2 is semiconducting, whereas its 1T phase is metallic.
Various researches already introduced how to transform the phase of MoS2 and
experimentally proved that 1T phase can effectively reduce the contact resistance. Argon
plasma-treatment is one method to change the phase of MoS2 by kinetic energy transfer
through ion bombardment. However, the effect of the plasma conditions such as RF
(Radio Frequency) power on phase transition still lacks systematic investigation. Also,
process condition of argon plasma-treatment should be set carefully because plasmatreatment
process may create disordered phase or etch MoS2 layer.
In this study, we demonstrated phase transition of MoS2 in various plasma conditions.
Plasma RF power and time were controlled as variables and phase transition or etching
were distinguished by Raman spectroscopy and PL. Mechanically exfoliated mono- or bilayered
2H-MoS2 from bulk MoS2 was used. Argon plasma-treatment was processed in
CCP (Capacitively Coupled Plasma) system with 27.12 MHz RF generator. During argon
plasma-treatment, temperature, pressure and argon gas flux were fixed to 30 ℃, 2 torr
and 900 sccm, respectively. RF power was performed from 20W to 100W and time were
controlled from 1s to 500s. Samples were measured before and after argon plasmatreatment
by Raman spectroscopy and PL. 532nm ND:Yag laser was used both in Raman
and PL.
Raman and PL spectra of MoS2 has changed from 2H-phase to 1T-phase according to
Ar plasma treatment condition. First, the phase transition of MoS2 from 2H to 1T is
depending on RF power under the same plasma treatment time. As RF power is higher,
phase transition occurred more quickly. As well as etching of MoS2 occurred at high RF
power. Second, the phase transition is also plasma treatment time dependent. Under 10s
of plasma treatment at various RF power, the phase has not changed. However, the phase
of MoS2 starts to change after 20s. Until 50s of plasma-treatment, Raman peak of 2H is
almost disappears and 1T peak is only detected. After 100s, crystallinity is broken and
peak shows disordered phase. After 500s, monolayer MoS2 is etched.