Highly Crystalline CVD-grown Multilayer MoSe2 Thin Film Transistor for Fast Photodetector

Smart interface that can provide communication between human beings and digital devices is of great importance in the present and future electronic applications, and intensive research is being carried out in the field of interactive sensing. The latter involves the use of various types of sensors, such as phototransistors that become activated in the presence of light. For instance, active matrix devices consist of at least a driving transistor and switching transistor to afford selective pixel addressing, and active matrix devices incorporating photosensitive thin-film transistors (TFTs) enable the realization of flat panel displays in which specific pixels are locally activated by incident photons1. In this regards, smart interface calls for novel active-channel materials to realize high-speed driving transistors and highly sensitive photodetectors. Layered semiconductors based on transition metal dichalcogenides (TMDs: MX2 = Mo, W; X = S, Se, Te) exhibit desirable device characteristics, including high mobility (>100 cm2 V−1 s−1) and large photoresponsivity (~500 A/W)2,3,4,5, and mechanical flexibility, which make them attractive for active elements in future interactive electronics. Recently, a significant progress has been made in the synthesis of two-dimensional (2D) semiconductors such as single-layer molybdenum disulfide (MoS2) for electronic and optoelectronic applications, which demonstrated high field-effect mobility (>100 cm2/V · s) and large photoresponsivity (~500 A/W)2,3,4,5, making it attractive for phototransistors. However, the growth of single layers is not favorable for the fabrication of large-area flat panels since it cannot provide sufficient coverage over several square meters under current technologies. In light of this, multilayered structures with high carrier mobility and high photosensitivity are required for the practical large-area sensing applications. However, indirect-bandgap MoS2 multilayers exhibit relatively low photoresponse in TFTs6, unlike direct-bandgap MoS2 monolayers, although they can provide high field-effect mobility (>100 cm2 V−1 s−1) and small subthreshold swing (~70 mV/decade)7,8. An advanced local-gate device structure was introduced by Kwon et al.9 to enhance the photoresponsivity of multilayer MoS2 phototransistors. On the other hand, it is known that MoSe2 can provide higher photoresponsivity compared to MoS2 due to the quantum confinement effect during the bandgap transition10, which implies that using an advanced device structure may not be needed for MoSe2 phototransistors to achieve high sensitivity. In addition, Choi et al.6 reported that TMD multilayers have an advantage over the monolayers that photoresponse is achievable over a broad range of the electromagnetic spectrum from ultraviolet to near infrared. Therefore, multilayer MoSe2 can be a strong contender for an active channel material of future phototransistors. In this study, we present the chemical vapor deposition (CVD) growth of MoSe2 multilayers at a relatively high pressure. The CVD methods reported up to date involve low-pressure deposition with slow nucleation rates11,12,13,14,15, resulting in triangular single layers terminated by either transition metal (e.g., Mo) or chalcogen atom (e.g., Se) for TMDs. In contrast, we demonstrate that indirect-bandgap MoSe2 multilayers can be grown by using a high-pressure CVD method. Based on the two-dimensional nucleation theory, a relatively high pressure at a fixed temperature can induce a large nucleation rate before film growth occurs, and thus, the formation of multilayers is promoted. The microstructure of hexagonal MoSe2 grains is examined using X-ray diffraction (XRD), high-resolution transmission electron microscopy (TEM) and Raman spectroscopy. Our multilayer MoSe2 TFTs exhibit ambipolar behaviors with high photoresponsivity (93.7 A/W) and reasonably large field-effect mobility (~10 cm2/V · s). This highly crystalline and photo-responsive multilayer MoSe2 is anticipated to be used in a myriad of potential applications for interactive electronics.