Lung-I Huang

Low Carrier Density Epitaxial Graphene Devices On SiC

The transport characteristics of graphene devices with low n- or p-type carrier density (∼10(10) -10(11) cm(-2) ), fabricated using a new process that results in minimal organic surface residues, are reported. The p-type molecular doping responsible for the low carrier densities is initiated by aqua regia. The resulting devices exhibit highly developed ν = 2 quantized Hall resistance plateaus at magnetic field strengths of less than 4 T.

Insulator-quantum Hall transition in monolayer epitaxial graphene

We report on magneto-transport measurements on low-density, large-area monolayer epitaxial graphene devices grown on SiC. We observe temperature (T)-independent crossing points in the longitudinal resistivity ρxx, which are signatures of the insulator-quantum Hall (I-QH) transition, in all three devices. Upon converting the raw data into longitudinal and Hall conductivities σxx and σxy, in the most disordered device, we observed T-driven flow diagram approximated by the semi-circle law as well as the T-independent point in σxy near e2/h. We discuss our experimental results in the context of the evolution of the zero-energy Landau level at low magnetic fields B. We also compare the observed strongly insulating behaviour with metallic behaviour and the absence of the I-QH transition in graphene on SiO2 prepared by mechanical exfoliation.

Controlling the Fermi Level in a Single-Layer Graphene QHE Device for Resistance Standard

NMIJ/AIST and NIST are collaborating on the development of graphene-based quantized Hall resistance (QHR) devices. We formed graphene films on SiC(0001) substrates and processed the samples into Hall bar devices using the NIST clean room facility. The electronic transport properties have been observed at the NIST and NMIJ/AIST. We used two methods to control the Fermi level in the samples. One is hydrogen intercalation and the other is photochemical gating. Using the former technique, the Fermi level moved across the Dirac point. For the latter technique, it moved closer to the Dirac point.

Controlling Fermi level in single layer graphene QHE device for resistance standard

The National Metrology Institute of Japan/ National Institute of Advanced Industrial Science and Technology (NMIJ/AIST) and the National Institute of Standards and Technology (NIST) are collaborating on the development of graphene-based quantized Hall resistance devices. We formed graphene films on silicon carbide (0001) substrates and processed the samples into Hall bar devices using the NIST clean room facility. The electronic transport properties have been observed at the NIST and NMIJ/AIST. Hydrogen intercalation and photochemical gating were employed to control the Fermi level in the samples. For the first method, the Fermi level was observed to move across the Dirac point. For the latter technique, it moved closer to the Dirac point.

Localization and electron-electron interactions in few-layer epitaxial graphene

This paper presents a study of the quantum corrections caused by electron-electron interactions and localization to the conductivity in few-layer epitaxial graphene, in which the carriers responsible for transport are massive. The results demonstrate that the diffusive model, which can generally provide good insights into the magnetotransport of two-dimensional systems in conventional semiconductor structures, is applicable to few-layer epitaxial graphene when the unique properties of graphene on the substrate, such as intervalley scattering, are taken into account. It is suggested that magnetic-field-dependent electron-electron interactions and Kondo physics are required for obtaining a thorough understanding of magnetotransport in few-layer epitaxial graphene.

Dirac fermion heating, current scaling, and direct insulator-quantum Hall transition in multilayer epitaxial graphene

We have performed magnetotransport measurements on multilayer epitaxial graphene. By increasing the driving current I through our graphene devices while keeping the bath temperature fixed, we are able to study Dirac fermion heating and current scaling in such devices. Using zero-field resistivity as a self thermometer, we are able to determine the effective Dirac fermion temperature (TDF) at various driving currents. At zero field, it is found that TDF ∝ I≈1/2. Such results are consistent with electron heating in conventional two-dimensional systems in the plateau-plateau transition regime. With increasing magnetic field B, we observe an I-independent point in the measured longitudinal resistivity ρxx which is equivalent to the direct insulator-quantum Hall (I-QH) transition characterized by a temperature-independent point in ρxx. Together with recent experimental evidence for direct I-QH transition, our new data suggest that such a transition is a universal effect in graphene, albeit further studies are required to obtain a thorough understanding of such an effect.

Unusual renormalization group (RG) flow and temperature-dependent phase transition in strongly-insulating monolayer epitaxial graphene

By changing the measurement temperature (T), one can vary the effective sample size so as to study the renormalization group (RG) (or T-driven) flow of a semiconductor, a topological insulator, or a graphene device in the complex conductivity plane. Here we report RG flow of large-area, strongly disordered monolayer graphene epitaxially grown on SiC, which becomes insulating as T decreases for zero magnetic field. We observe cusp-like RG flow towards (σxy = e2/h, σxx = e2/h) where σxy and σxx are Hall conductivity and diagonal conductivity respectively. Such features, indicative of a fixed-temperature phase transition, have never been observed before and cannot be explained by existing RG models based on a modular symmetry group. Therefore, our results suggest the need for new theoretical models and experimental study leading to an understanding of strongly disordered two-dimensional materials such as graphene, few-layer black phosphorus, WSe2, and so on.

Development of Low Carrier Density Graphene Devices

Epitaxial graphene on SiC(0001) is used to fabricate Hall bar structures for metrological applications with a fabrication process that has been developed to eliminate organic chemical contamination of the graphene. Before any lithographic patterning a metal protection layer of 15 nm thickness is deposited on as-grown graphene. The protection layer on top of the Hall-bar area is etched by diluted fresh aqua regia at the last step of the fabrication, therefore avoiding any contamination from photoresist or photoresist remover. Results including low carrier density and high mobility are reported, along with the appearance of well-developed quantized Hall resistance plateaus for filling factor V=2 at magnetic fields as low as 2 T.