Two-dimensional materials have been actively explored as candidates to break-through the limitations of conventional silicon-based optoelectronic devices due to their excellent optical properties. In, particular, the transition metal dichalcogenides (TMD), a type of two-dimensional material, have various band gaps library depending on the composition elements and have excellent light absorbance characteristics by their two-dimensional structure. Therefore, researches for TMD-based optoelectronics are being carried out to develop high performance next-generation photodetectors in various detection band ranges. However, the method of synthesizing a flat, thin and uniform two-dimensional materials required for practical optoelectronic device applications is the biggest obstacle for practical optoelectronic device applications. To overcome this, various synthesis technologies have been developed. Metalorganic chemical vapor deposition (MOCVD) is a cutting-edge synthesis technology that can overcome the limitations of the existing solid precursor-based chemical vapor deposition while satisfying all of the above requirements.
In this study, we fabricated a 2D/3D heterojunction photodetector. Among the TMDs, molybdenum disulfide (MoS$_2$) having a band gap of 1.2 to 1.8 eV which is suitable for visible light detection was synthesized by MOCVD to form a heterojunction with silicon, and graphene was used as the top transparent electrode. It has rectifying characteristics as a diode by a heterojunction of MoS$_2$/Silicon, and can be utilized as a photodetector under reverse voltage bias. As a photodetector, it showed photodetection performance for visible light band and achieved high responsivity over 1 A/W, which is the better than conventional silicon-based photodetectors. In addition, it also shows a fast photo-response speed of 35 ns response time. Therefore, the potential of application of the heterojunction photodetector developed in this study, as a next-generation image sensor pixel was confirmed.
이차원 물질은 고유의 우수한 광 특성에 기인하여 기존의 실리콘 기반 광전자소자의 한계를 돌파하기 위한 후보 물질로서 활발히 탐구되어 왔다. 그러나, 평평하고 얇고 균일한 이차원 물질을 합성하는 것은 이차원 물질을 실제로 응용하기 위해서 가장 중요하며, 해결하기 어려운 문제이다. 유기금속화학기상증착법은 위와 같은 요구 조건을 모두 충족시키면서, 기존의 고체 전구체 기반 화학기상증착법의 한계를 뛰어넘는 합성기술이다.
본 연구에서는, 가시광선 검출에 적합한 1.2 ~ 1.8 eV의 밴드갭을 갖는 이차원 물질인 이황화몰리브덴을 유기금속화학기상증착법으로 합성하여 실리콘과 이종 접합을 형성하고, 그래핀을 상부전극으로 이용하는 광소자를 제작하였다. 제작 소자는 이황화몰리브덴과 실리콘의 이종 접합에 의해 정류 특성을 보이는 다이오드로서 동작하며, 역방향 바이어스를 인가하여 광 검출기로 응용할 수 있음을 확인하였다. 광 검출기로서, 가시광선 전 대역에 걸쳐 광 검출 성능을 보였으며, 기존의 실리콘 기반 광 소자보다 우수한 광 반응도를 달성하였다 또한, 35 ns의 빠른 광 반응 속도를 확보하였으므로, 본 연구에서 개발된 이종 접합 광소자의 차세대 이미지센서 픽셀 광소자로서 응용 가능성을 확인하였다.
