저차원 칼코겐 화합물의 기상 이종 접합 에피탁시 성장

Abstract

The field of low-dimensional materials has experienced tremendous progress in the past decade as researchers around the world work to increase our understanding of the growth, fabrication, and fundamental properties of individual crystals. While each material offers new and exciting applications, the combination of several materials into heterostructure have address uniquely tailored properties in recent years as which demonstrated on monocrystalline flakes, thereby it has become the focus of related research community. Nevertheless, before heterostructures reach maturity as viable candidates for technology applications, there are a number of challenges that must be addressed. In the thesis, we address these challenges for two kinds of nanomaterial systems, semiconductor 1-D nanowire and 2-D layered chalcogenides. They currently present various interesting phenomena themselves, and also serves as a model system for the realization of more complicated heterostructures. We suggest our original methods for controllable synthesis of an individual component and with careful focus on their heteroepitaxial integration into complex architecture. In the chapter 2, I suggest a simple synthetic route for the planar-defect-free II–VI NWs via tunable alloying, i.e., single ZB phase Cd1–xZnxTe NWs (0 ≤ x ≤ 1). Metal-catalytic growth of compound semiconductor nanowire typically has shown large density of stacking faults and twin defect in zinc blende phase or mixture of zincblende (ZB) and wurtzite (WZ) phase. We found that ZnTe and CdTe nanowires synthesized by Au catalyst possess large density stacking faults and twin defect in zinc blende phase. It was found that the eutectic alloying of Cd and Zn in Au catalysts immediately alleviates the interfacial instability during the catalytic growth and forms homogeneous ZB crystals as opposed to unwanted ZB/WZ mixtures. Thereby we have achieved the growth of ZB single-crystalline phase Cd1–xZnxTe NWs, whose relative composition, and thus the energy band gap, is controllably tuned over the entire alloying range. In the Chapter 3, I report a simple synthetic route for straight 1D heterointerface formation during catalytic NW growth by controlling the residue within catalysts in gas-phases. Specifically, we have heteroepitaxially grown Ge NWs on the top of ZnSe NWs with atomically sharp interfaces by eliminating Zn residues within Au catalysts with Se vapour treatments prior to the Ge NW growth. We argue that Ge nucleation can be facilitated in the absence of residual atomic species, and this promotes straight 1D heteroepitaxy over the low energy barrier with atomically sharp interfaces. Based on such straight 1D Ge/ZnSe NWs, we also demonstrate that the local 1D potential can be encoded within an individual NW to form 1D heterojunction photodetectors in an individually addressed manner. In the Chapter 4, I proposed 2D vertical stacking and lateral stitching growth of monolayer (ML) hexagonal transition-metal dichalcogenides (TMDCs) i.e. MoS2 and WS2. The 2D heteroepitaxial manipulation of MoS2 and WS2 MLs either vertical stacking or lateral stitching configuration is deterministically achieved by the control of the 2D nucleation kinetics during the sequential vapor-phase growth. We show WS2MoS2 growth sequence yields lateral stitching growth, when growth sequence is reversed into MoS2WS2, second WS2 growth prefer to proceed onto pre-existing MoS2 ML. This different growth behavior can be explained in terms of different nucleation energy barriers which can be overcome by supersaturation degrees that is determined by growth temperature. Microscopic structural analysis confirms the creation of hexagon-on(by)-hexagon unit-cell stacking (stitching) without interlayer rotation misfits. Furthermore, photoluminescence measurement indicates that light emission at 1.5eV indicates photon generation via electron-hole recombination across the vertical stacks, suggesting strong interlayer coupling in our hetero-stacks. In the chapter 5, we report a polymorphic phase-deterministic and phase-integrated growth of distinct polymorphic 2H (semiconducting) and 1T’ (metallic) MoTe2 and WTe2 few-layer crystals. Polymorphism in atomically thin TMDC crystals offers opportunities to exploit the corresponding electronic phases as circuit components in the ultra-thin two-dimensional (2D) electronics and optics. The specific phase in a certain class of 2D TMDCs is determined by the atomic coordination types of the TM ions with chalcogen atoms.. We found that two polymorphic phases can be deterministically synthesized at different growth temperature. While metastable 1T’ phase is formed at higher growth temperature with stabilization with subsequent quenching process, stable 2H phase grows at lower temperature with characteristic rectangular and hexagonal facet respectively. Further we suggest that sequential growth leads to the heteroepitaxial nucleation of second phase from the first phase edge, thereby it successfully forms lateral polymorphic heterojunction between 1T’ and 2H phases. Lastly, we show structurally coherent and electrically near-transparent edge-contact properties at such polymorphic semiconductor-metal interfaces. Our demonstration of synthetic integration of atomically thin metals and semiconductors by polymorphic heteroepitaxy establishes a new way of 2D semiconductor integrated circuitry. In the chapter 6, I report the two different growth mechanisms of vdw heteroepitaxial Bi2Te3/Sb2Te3 few-layers stacking by choosing different substrates. During the sequential gas-phase growth of Bi2Te3-Sb2Te3 vertical stacking heterostructure, first we observed different edge shape of few quintuple layers (QLs) Bi2Te3 crystal depending on growth substrates i.e. SiO2/Si and h-BN. Further, in the stacking growth of Sb2Te3 on Bi2Te3 crystal as a second step, we show that the growth mode becomes the layer-by-layer growth (Volmer-Weber growth) on h-BN substrates, and the 3D island growth (Frank-van der Merwe Growth) on SiO2/Si. We find that the compressive strain in the substrates imposed by the lattice mismatch plays a crucial role to determine different growth modes in these 2D nucleation kinetics model. This misfit strain controls surface diffusion kinetics of Sb2Te3 precursor onto Bi2Te3 surface which consequently determine different growth modes in these 2D nucleation kinetics. Our result, conformal layer-by-layer growth in vdW heterostructures, suggests general implication with the key role of 2D substrate for large-area 2D stacking growth of various layered materials. Throughout the Appendix, I introduced collaboration research related on my thesis especially on two dimensional metal chalcogenides of chapter 4 and 6. In the Appendix A, we have investigated light absorption and emission at the heterointerfaces of ML stacks composed of MoS2 and WS2 MLs with and without inter-ML rotational misfit in both spectroscopic and time-resolved manners. In the Appendix B, we explore novel thermoelectric conversion in the atomic monolayer steps of a-few-layer topological insulating Bi2Te3 (n-type) and Sb2Te3 (p-type). We found few unit layer step (1-3QLs) boundaries within an individual Bi(Sb)2Te3 grown by gas-phase growth. With these monolayer step in single 2D crystals, specifically by scanning photoinduced thermoelectric current imaging at these monolayer steps, we show that efficient thermoelectric conversion is accomplished by optothermal motion of hot electrons (Bi2Te3) and holes (Sb2Te3) through 2D subbands and topologically protected surface states in a geometrically deterministic manner.