Engineering van der Waals Heterojunctions for Electronic and Optoelectronic Device Applications
Efficient preparation and characterization of layered materials and their van der Waals heterojunctions lay the foundation for various opportunities in both fundamental studies and device applications. The vast library of 2D materials displays a range of electronic properties, including conductors, semiconductors, insulators, semimetal, and superconductors, and shows strong light-matter interaction. The fact that each layer in the layered material is bonded via van der Waals interaction opens up the possibility of assembling different layers arbitrarily without any consideration over the precision of lattice match- ing. This unique stacking with one-atomic-plane precision can unfold diverse van der Waals heterostructure devices by efficiently engineering its energy band alignment. This paves a path to design novel devices such as solar cells, photodetectors, light-emitting diodes and transistors. In this thesis, our motivation is to explore the electronic and optoelectronic characteristics of 2D materials and their heterojunctions. We focus on designing 2D heterostructures for the multi-functional devices including electronic (diode/transistor) and optoelectronic (highly sensitive photodetection) applications. As the initial step, we realized SnSe2 based photoconductor which shows a very high responsivity of 10^3 A/W at 1 mV voltage bias. We investigated the role of trap states present at the channel- substrate interface on the observed gain mechanism in typical planar 2D photoconductors. Next, in order to improve the speed for a photodetector, we designed a heterostructure composed of ITO/WSe2/SnSe2 vertical heterojunction. This novel design helped us to achieve a large responsivity at near IR region while maintaining high operational speed. We achieved a high responsivity of more than 1100 A/W and fast transient response time in the order of 10 us. Considering the interest of broad band detection, we then fabricated a graphene-absorption-based photodetector where graphene can act as the absorbing medium, utilizing its zero-band gap nature. The absorbed photo-carriers are vertically transported in a fast time scale to a floating MoS2 quantum well, providing photo-gating. This structure exhibited the responsivity of 4.4 * 10^6 A/W at 30 fW incident power which is higher than that of any reported graphene absorption-based photodetectors. As a continuation of the study of heterostructure transport characteristics, we realized a backward diode with WSe2/SnSe2 structure which exhibits an ultra-high reverse recti cation ratio of 2.1 *10^4 with an impressive curvature coefficient of 37 V^(-1). Finally, we proposed a novel methodology for the extraction of Schottky Barrier Height (SBH) using a vertical heterojunction of multilayer transition metal dichalcogenide with asymmetric contacts which allow easy and direct quantitative evaluation of SBH for two contacts simultaneously.