Yanfei Yang

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.

Electrical properties and memory effects of field-effect transistors from networks of single- and double-walled carbon nanotubes

We study field-effect transistors made of single- and double-walled carbon nanotube networks for applications as memory devices. The transfer characteristics of the transistors exhibit a reproducible hysteresis which enables their use as nano-sized memory cells with operations faster than 10 ms, endurance longer than 10(+4) cycles and charge retention of a few hours in air. We propose water enhanced charge trapping at the SiO(2)/air interface close to the nanotubes as the dominant mechanism for charge storage. We show that charge storage can be improved by limiting exposure of the device to air.

Epitaxial graphene homogeneity and quantum Hall effect in millimeter-scale devices

Quantized magnetotransport is observed in 5.6 × 5.6 mm2 epitaxial graphene devices, grown using highly constrained sublimation on the Si-face of SiC(0001) at high temperature (1900 °C). The precise quantized Hall resistance of [Formula: see text] is maintained up to record level of critical current Ixx = 0.72 mA at T = 3.1 K and 9 T in a device where Raman microscopy reveals low and homogeneous strain. Adsorption-induced molecular doping in a second device reduced the carrier concentration close to the Dirac point (n ≈ 1010 cm-2), where mobility of 18760 cm2/V is measured over an area of 10 mm2. Atomic force, confocal optical, and Raman microscopies are used to characterize the large-scale devices, and reveal improved SiC terrace topography and the structure of the graphene layer. Our results show that the structural uniformity of epitaxial graphene produced by face-to-graphite processing contributes to millimeter-scale transport homogeneity, and will prove useful for scientific and commercial applications.

Record Endurance for Single-Walled Carbon Nanotube-Based Memory Cell.

We study memory devices consisting of single-walled carbon nanotube transistors with charge storage at the SiO2/nanotube interface. We show that this type of memory device is robust, withstanding over 105 operating cycles, with a current drive capability up to 10−6 A at 20 mV drain bias, thus competing with state-of-the-art Si-devices. We find that the device performance depends on temperature and pressure, while both endurance and data retention are improved in vacuum.

Electronic transport and possible superconductivity at van Hove singularities in carbon nanotubes

Van Hove singularities (VHSs) are a hallmark of reduced dimensionality, leading to a divergent density of states in one and two dimensions and predictions of new electronic properties when the Fermi energy is close to these divergences. In carbon nanotubes, VHSs mark the onset of new subbands. They are elusive in standard electronic transport characterization measurements because they do not typically appear as notable features and therefore their effect on the nanotube conductance is largely unexplored. Here we report conductance measurements of carbon nanotubes where VHSs are clearly revealed by interference patterns of the electronic wave functions, showing both a sharp increase of quantum capacitance, and a sharp reduction of energy level spacing, consistent with an upsurge of density of states. At VHSs, we also measure an anomalous increase of conductance below a temperature of about 30 K. We argue that this transport feature is consistent with the formation of Cooper pairs in the nanotube.

Electrical Stabilization of Surface Resistivity in Epitaxial Graphene Systems by Amorphous Boron Nitride Encapsulation

Homogeneous monolayer epitaxial graphene (EG) is an ideal candidate for the development of millimeter-sized devices with single-crystal domains. A clean fabrication process was used to produce EG-based devices, with n-type doping level of the order of 1012 cm–2. Generally, electrical properties of EG, such as longitudinal resistivity, remain unstable when devices are exposed to air due to adsorption of molecular dopants, whose presence shifts the carrier density close to the Dirac point (<1010 cm–2) or into the p-type regime. Here, we report experimental results on the use of amorphous boron nitride (a-BN) as an encapsulation layer, whereby EG can maintain its longitudinal resistivity and have its carrier density modulated. Furthermore, we exposed 12 devices to controlled temperatures of up to 85 °C and relative humidity of up to 85% and reported that an approximately 20 nm a-BN encapsulation thickness is sufficient to preserve their longitudinal resistivity to within 10% of the previously measured value. We monitored the electronic properties of our encapsulated and nonencapsulated EG samples by magnetotransport measurements, using a neodymium iron boron magnet. Our results have essential importance in the mass production of millimeter-scale graphene devices, with stable electrical properties.

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.

Probing the dielectric response of the interfacial buffer layer in epitaxial graphene via optical spectroscopy

Monolayer epitaxial graphene (EG) is a suitable candidate for a variety of electronic applications. One advantage of EG growth on the Si face of SiC is that it develops as a single crystal, as does the layer below, referred to as the interfacial buffer layer (IBL), whose properties include an electronic band gap. Though much research has been conducted to learn about the electrical properties of the IBL, not nearly as much work has been reported on the optical properties of the IBL. In this work, we combine measurements from Mueller matrix ellipsometry, differential reflectance contrast, atomic force microscopy, and Raman spectroscopy, as well as calculations from Kramers-Kronig analyses and density functional theory (DFT), to determine the dielectric function of the IBL within the energy range of 1 eV to 8.5 eV.