Kibum Kang

High-mobility three-atom-thick semiconducting films with wafer-scale homogeneity

The large-scale growth of semiconducting thin films forms the basis of modern electronics and optoelectronics. A decrease in film thickness to the ultimate limit of the atomic, sub-nanometre length scale, a difficult limit for traditional semiconductors (such as Si and GaAs), would bring wide benefits for applications in ultrathin and flexible electronics, photovoltaics and display technology. For this, transition-metal dichalcogenides (TMDs), which can form stable three-atom-thick monolayers, provide ideal semiconducting materials with high electrical carrier mobility, and their large-scale growth on insulating substrates would enable the batch fabrication of atomically thin high-performance transistors and photodetectors on a technologically relevant scale without film transfer. In addition, their unique electronic band structures provide novel ways of enhancing the functionalities of such devices, including the large excitonic effect, bandgap modulation, indirect-to-direct bandgap transition, piezoelectricity and valleytronics. However, the large-scale growth of monolayer TMD films with spatial homogeneity and high electrical performance remains an unsolved challenge. Here we report the preparation of high-mobility 4-inch wafer-scale films of monolayer molybdenum disulphide (MoS2) and tungsten disulphide, grown directly on insulating SiO2 substrates, with excellent spatial homogeneity over the entire films. They are grown with a newly developed, metal–organic chemical vapour deposition technique, and show high electrical performance, including an electron mobility of 30 cm2 V−1 s−1 at room temperature and 114 cm2 V−1 s−1 at 90 K for MoS2, with little dependence on position or channel length. With the use of these films we successfully demonstrate the wafer-scale batch fabrication of high-performance monolayer MoS2 field-effect transistors with a 99% device yield and the multi-level fabrication of vertically stacked transistor devices for three-dimensional circuitry. Our work is a step towards the realization of atomically thin integrated circuitry.

Near-field electrical detection of optical plasmons and single plasmon sources

We demonstrate an efficient nanoscale electrical detector for propagating surface plasmons, tightly confined to nanoscale silver wires. Our technique is based on the near-field coupling between guided plasmons and a nanowire field-effect transistor. We demonstrate that this near-field circuit can efficiently detect the plasmon emission from a single quantum dot that is directly coupled to the plasmonic waveguide.

Maximum Li storage in Si nanowires for the high capacity three-dimensional Li-ion battery

Nanowires can serve as three-dimensional platforms at the nanometer scale for highly efficient chemical energy storage and conversion vehicles, such as fuel cells and secondary batteries. Here we report a coin-type Si nanowire (NW) half-cell Li-ion battery showing the Li capacity of approximately 4000 mAh/g, which nearly approaches the theoretical limit of 4200 mAh/g, with very high Coulombic efficiency of up to 98%. Concomitantly, we provide direct evidence of reversible phase transitions in the Si NW anodes at the full electrochemical cycles, varying from pure Si to Li22Si5 phase, which has been known empirically inaccessible in the bulk limit.

Diameter-Dependent Internal Gain in Ohmic Ge Nanowire Photodetectors

We report a diameter-dependent photoconduction gain in intrinsic Ge nanowire (NW) photodetectors. By employing a scanning photocurrent imaging technique, we provide evidence that the photocarrier transport is governed by the hole drift along the Ge NWs, ensuing the higher internal gain up to approximately 10(3) from the thin NWs. It is found that the magnitudes of both gain and photoconductivity are inversely proportional to the NW diameter ranging from 50 to 300 nm. We attribute our observations to the variation in the effective hole carrier density upon varying diameters of Ge NWs, as a result of field effects from the diameter-dependent population of the surface-trapped electrons, along with a model calculation. Our observations represent inherent size effects of internal gain in semiconductor NWs, thereby provide a new insight into nano-optoelectronics.

The role of NiOx overlayers on spontaneous growth of NiSix nanowires from Ni seed layers.

We report a controllably reproducible and spontaneous growth of single-crystalline NiSix nanowires using NiOx/Ni seed layers during SiH4 chemical vapor deposition (CVD). We provide evidence that upon the reactions of SiH4 (vapor)-Ni seed layers (solid), the presence of the NiOx overlayer on Ni seed layers plays the key role to promote the spontaneous one-dimensional growth of NiSix single crystals without employing catalytic nanocrystals. Specifically, the spontaneous nanowire formation on the NiOx overlayer is understood within the frame of the SiH4 vapor-phase reaction with out-diffused Ni from the Ni underlayers, where the Ni diffusion is controlled by the NiOx overlayers for the limited nucleation. We show that single-crystalline NiSix nanowires by this self-organized fashion in our synthesis display a narrow diameter distribution, and their average length is set by the thickness of the Ni seed layers. We argue that our simple CVD method employing the bilayers of transition metal and their oxides as the seed layers can provide implication as the general synthetic route for the spontaneous growth of metal-silicide nanowires in large scales.

Atomically Thin Ohmic Edge Contacts Between Two-Dimensional Materials

With the decrease of the dimensions of electronic devices, the role played by electrical contacts is ever increasing, eventually coming to dominate the overall device volume and total resistance. This is especially problematic for monolayers of semiconducting transition-metal dichalcogenides (TMDs), which are promising candidates for atomically thin electronics. Ideal electrical contacts to them would require the use of similarly thin electrode materials while maintaining low contact resistances. Here we report a scalable method to fabricate ohmic graphene edge contacts to two representative monolayer TMDs, MoS2 and WS2. The graphene and TMD layer are laterally connected with wafer-scale homogeneity, no observable overlap or gap, and a low average contact resistance of 30 kΩ·μm. The resulting graphene edge contacts show linear current-voltage (I-V) characteristics at room temperature, with ohmic behavior maintained down to liquid helium temperatures.

Layer-by-layer assembly of two-dimensional materials into wafer-scale heterostructures

High-performance semiconductor films with vertical compositions that are designed to atomic-scale precision provide the foundation for modern integrated circuitry and novel materials discovery. One approach to realizing such films is sequential layer-by-layer assembly, whereby atomically thin two-dimensional building blocks are vertically stacked, and held together by van der Waals interactions. With this approach, graphene and transition-metal dichalcogenides—which represent one- and three-atom-thick two-dimensional building blocks, respectively—have been used to realize previously inaccessible heterostructures with interesting physical properties. However, no large-scale assembly method exists at present that maintains the intrinsic properties of these two-dimensional building blocks while producing pristine interlayer interfaces, thus limiting the layer-by-layer assembly method to small-scale proof-of-concept demonstrations. Here we report the generation of wafer-scale semiconductor films with a very high level of spatial uniformity and pristine interfaces. The vertical composition and properties of these films are designed at the atomic scale using layer-by-layer assembly of two-dimensional building blocks under vacuum. We fabricate several large-scale, high-quality heterostructure films and devices, including superlattice films with vertical compositions designed layer-by-layer, batch-fabricated tunnel device arrays with resistances that can be tuned over four orders of magnitude, band-engineered heterostructure tunnel diodes, and millimetre-scale ultrathin membranes and windows. The stacked films are detachable, suspendable and compatible with water or plastic surfaces, which will enable their integration with advanced optical and mechanical systems.

Atomic Layer-by-Layer Thermoelectric Conversion in Topological Insulator Bismuth/Antimony Tellurides

Material design for direct heat-to-electricity conversion with substantial efficiency essentially requires cooperative control of electrical and thermal transport. Bismuth telluride (Bi2Te3) and antimony telluride (Sb2Te3), displaying the highest thermoelectric power at room temperature, are also known as topological insulators (TIs) whose electronic structures are modified by electronic confinements and strong spin-orbit interaction in a-few-monolayers thickness regime, thus possibly providing another degree of freedom for electron and phonon transport at surfaces. Here, we explore novel thermoelectric conversion in the atomic monolayer steps of a-few-layer topological insulating Bi2Te3 (n-type) and Sb2Te3 (p-type). Specifically, by scanning photoinduced thermoelectric current imaging at the 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. Our discovery suggests that the thermoelectric conversion can be interiorly achieved at the atomic steps of a homogeneous medium by direct exploiting of quantum nature of TIs, thus providing a new design rule for the compact thermoelectric circuitry at the ultimate size limit.

Long-Lived Hole Spin/Valley Polarization Probed by Kerr Rotation in Monolayer WSe2.

Time-resolved Kerr rotation and photoluminescence measurements are performed on MOCVD-grown monolayer tungsten diselenide (WSe2). We observe a surprisingly long-lived Kerr rotation signal (∼80 ns) at 10 K, which is attributed to spin/valley polarization of the resident holes. This polarization is robust to transverse magnetic field (up to 0.3 T). Wavelength-dependent measurements reveal that only excitation near the free exciton energy generates this long-lived spin/valley polarization.

Unconventional roles of metal catalysts in chemical-vapor syntheses of single-crystalline nanowires

In this invited contribution at the 29th International Conference on the Physics of semiconductors (ICPS 2008), we review two examples of solid-catalytic nanowire (NW) growth in parallel comparisons to the NW growth from the eutectic liquid catalyst. First, we demonstrated the Cu-catalyzed Ge NW growth using GeH4 vapor precursor at 200 °C, which is far below the Cu–Ge eutectic temperature of 644 °C, with a relatively uniform diameter distribution directly templated from that of the catalysts. We provide evidence that the formation of solid Cu3Ge catalysts and Ge diffusion across the catalysts are responsible for such low-temperature growth of Ge NWs in a size-deterministic manner. Second, we show the spontaneous silicidation of NiSix NWs on continuous Ni bulks using SiH4 vapor precursor at 400 °C. This growth is particularly marked in that NiSix NWs are formed in a self-organized manner without employing the nanocluster catalysts. We discuss this spontaneous growth of NiSix NWs within the frame of the nuc...