Advance in modern solid-state physics and information/communication technology have benefited from progress of new material platforms. In particular, diverse confined electronic systems, realized in atomically thin materials, enable to investigate novel solid state phenomena, some of which were already implemented in existing technology: to name a few, two-dimensional electron gases of semiconductor heterostructures for HEMTs and quantum cascade lasers, giant magnetoresistance in metal superlattices for data storage, and Dirac electrons in graphene. Clearly, the precise definition of a new class of heterosturucture materials initiates and advances new physical phenomena and functionalities. The CALDES-Materials Group, led by Associate Director Moon-Ho Jo, focuses on “Heteroepitaxial growth of atomically thin 2D semiconductors in system scales and Photophysics therein”. The Materials Group explores both new materials synthesis and related device physics in a synergic manner. Our Group are particularly interested in (1) heteroepitaxial growth of atomically thin 2D van der Waals (vdW) superstructures, and (2) new types of vdW photophysics and photoresponses therein. Below are the executive summary of such research subjects including some representative works, among others.
We have pioneered the heteroepitaxial integrations of diverse 2D materials by metalorganic chemical vapor depositions (MOCVD). The first example is “polymorphic epitaxy”, where the metallic 1T’ unit cell is seamlessly stitched to the semiconducting 2H one with deterministic crystallographic variants. These coplanar metal-semiconductor contacts are atomically coherent, showing the lowest contact barrier height ever-reported, contributing to the substantial device outperformance, when incorporated into atomically thin field-effect transistors (Nature Nanotechnol., 12, 1064, 2017). We has also explored heteroepitaxial growth of 2D superlattices, composed of more than two kinds of dissimilar transition-metal dichalcogenide monolayers (MLs) with programmable stacking periodicities. Such accurate ML-by-ML stacking was done by precise kinetics-controls in the near-equilibrium limit, creating the new tunable 2D electronic systems.
Another example of superstructures is atomically thin 3D vdW membranes (Science Adv., 5, eaaw3180, 2019), where we have first achieved conformal ML coverage on diverse substrates with nanoscale texturing, and later delaminated from the growth substrates as free-standing membranes. These interesting new meta-material platforms can open up new research avenues for integrated circuitry with extreme scaling, patchable membrane electronics and nonlinear photonics with atomic precisions.
In parallel to new vdW epitaxy of 2D materials, we also explored light-matter interactions in 2D vdW materials. Seamless integration of electronics and photonics on a single chip has been a long-sought goal in information science and technology. Such a hybrid structure may enable ultrafast data operation with minimal power loss, resulting in a paradigm shift for information manipulation and transmission. Despite the intensive efforts toward the goal, realization of miniaturized all optical signal processing units remains elusive, largely due to the weakness of nonlinear optical effects in conventional photonic materials. Distinctive light-matter interactions in atomically thin vdW materials, manifested as layer-number dependent excitations, interlayer coupled excitations, valley-polarization dependent excitations, and etc, may provide promising insights as emerging optical quantum matter. The essence of this research effort is (1) to explore new photoresponses in the 2D vdW materials, and (2) to synthetically integrate them in the circuit levels. One of the earlier example that we reported is interlayer twist-angle dependent light absorption and emission in ML semiconductor stacks (Nature Comm., 6. 7372, 2015). Here, we reported that light absorption and emission in MoS2/WS2 ML stacks can be tunable from indirect- to direct-gap transitions in both spectral and dynamic characteristics, when the constituent ML crystals are coherently stacked without in-plane rotation misfit by epitaxy. Our study suggested that the interlayer rotational attributes determine tunable interlayer excitation as a new set of basis for investigating optical phenomena.
We also discovered an interesting “selective” light-lattice interactions in atomically thin vdW semiconductors, where direct light-lattice interactions can be selective and reversible with the choices of light colors, generating either electron or hole dopants. This new photo-induced doping enables to repeatedly inscribe and erase the carrier types and concentrations of an identical semiconductor channel at room temperature with conventional light sources, thus to achieve monolithic integrated circuitry on atomically thin 2D semiconductors for the first time (Nature Electron., 1, 512 (2018) and Nature Electron. 4, 38 (2021).
The Materials Group also explored more on epitaxy in system scales with atomic precision, and diverse photoexcitation phenomena in 2D heterostructures. Below are the representative publications toward those goals.