Florence Nelson

Optical properties of large-area polycrystalline chemical vapor deposited graphene by spectroscopic ellipsometry

Spectroscopic ellipsometry was used to characterize the complex refractive index of chemical vapor deposition (CVD) graphene grown on copper foils and transferred to glass substrates. Two ellipsometers, with respective wavelength ranges extending into the ultraviolet and infrared (IR), have been used to characterize the CVD graphene optical functions. The optical absorption follows the same relation to the fine structure constant previously observed in the IR region, and displays the exciton-dominated absorption peak at ∼4.5 eV. The optical functions of CVD graphene show some differences when compared to published values for exfoliated graphene.

Enhanced Optical Second-Harmonic Generation from the Current-Biased Graphene/SiO2/Si(001) Structure

We find that optical second-harmonic generation (SHG) in reflection from a chemical-vapor-deposition graphene monolayer transferred onto a SiO2/Si(001) substrate is enhanced about 3 times by the flow of direct current electric current in graphene. Measurements of rotational-anisotropy SHG revealed that the current-induced SHG from the current-biased graphene/SiO2/Si(001) structure undergoes a phase inversion as the measurement location on graphene is shifted laterally along the current flow direction. The enhancement is due to current-associated charge trapping at the graphene/SiO2 interface, which introduces a vertical electric field across the SiO2/Si interface that produces electric field-induced SHG. The phase inversion is due to the positive-to-negative polarity switch in the current direction of the trapped charges at the current-biased graphene/SiO2 interface.

Electronic Excitations in Graphene in the 1–50 eV Range: The π and π + σ Peaks Are Not Plasmons

The field of plasmonics relies on light coupling strongly to plasmons as collective excitations. The energy loss function of graphene is dominated by two peaks at ∼5 and ∼15 eV, known as π and π + σ plasmons, respectively. We use electron energy-loss spectroscopy in an aberration-corrected scanning transmission electron microscope and density functional theory to show that between 1 to 50 eV, these prominent π and π + σ peaks are not plasmons, but single-particle interband excitations.

Optical and structural characterization of epitaxial graphene on vicinal 6H-SiC(0001)–Si by spectroscopic ellipsometry, Auger spectroscopy, and STM

The authors report results of spectroscopic ellipsometry (SE) measurements in the near-IR, visible, and near-UV spectral ranges using a Woollam dual rotating-compensator ellipsometer, analyzing data in terms of both epitaxial graphene and interface contributions. The SiC samples were cleaned by standard methods of CMP and HF etching prior to mounting in UHV and growing epitaxial graphene by thermal annealing at ∼1400 °C. Most samples were vicinally cut 3.5° off (0001) toward [11−20]. STM measurements show that the initial regular step edges were replaced by somewhat irregular edges after graphene growth. From growth-temperature and Auger data the authors estimate that the graphene is ∼3–4 ML thick. The authors find significant differences among the spectral features of the interface “buffer” layer and those of graphene. Specifically, the hyperbolic-exciton peak reported previously at ∼4.5 eV in graphene shifts to a similarly shaped peak at ∼4 eV in the interface buffer layer. The authors attribute this sh...

Spectroscopic Ellipsometry of CVD Graphene

Hydrocarbon-based CVD on metallic substrates provides a means of scalable fabrication of graphene thin films. CVD graphene is known to have grain boundaries which impact carrier mobility and optical response. Here, we use spectroscopic Ellipsometry (SE) to characterize the complex refractive index of Chemical Vapor Deposition (CVD) graphene grown on copper (Cu) foils and transferred to glass substrates. Two ellipsometers, with respective wavelength ranges extending into the ultra-violet (UV) and infrared (IR), have been used to characterize the CVD graphene optical constants. Our data follows the same relation to the fine structure constant observed in IR region by Heinz, et. al. We also observe the exciton dominated absorption peak at ~4.5 eV previously reported for exfoliated graphene by . The optical constants of CVD graphene show some differences to published values for exfoliated graphene. A Monte Carlo fit analysis is subsequently used for uncertainty determination. Wafer maps of 300 mm silicon wafers show the capability of ellipsometry to provide process control during manufacturing.

Spectroscopic Ellipsometry of Nanoscale Materials for Semiconductor Device Applications

Several years ago, the semiconductor industry began to refer to integrated circuits as nanoelectronic devices [1]. Now, most realize that nanoelectronics is the most prevalent nanotechnology. The continued decrease in device feature size has challenged spectroscopic ellipsometry (SE) with nano-films, nanowires, and nano-dots. There are many examples of the measurement of thin dielectric films [2, 3], and now there are examples of crystalline semiconductor nanowires in the form of the Fin in the transistor known as a Fin-FET [4]. The semiconductor industry is also working on materials for “beyond CMOS” devices.

Aberration Corrected Microscopy of CVD Graphene and Spectroscopic Ellipsometry of Epitaxial Graphene and CVD Graphene for Comparison of the Dielectric Function

Graphenes importance in post-CMOS device research drives the need for a variety of metrology methods for film characterization. Chemical Vapor Deposition (CVD) on metallic foils and the thermal decomposition of SiC have become two of the dominant fabrication methods of large area graphene due to their industrial scalability. The former method has required transfer of the graphene film to secondary substrates (i.e. SiO2/Si) for electrical and optical characterization, but is conducive to TEM imaging due to the fact that the film can be etched from the growth foil and directly transferred to support grids. The latter method does not require film transfer for optical or electrical characterization and can control layer number by selection of process parameters as well as Si-face vs. Cface growth. The work will present STEM imaging of CVD graphene performed at 60 kV with atomic resolution achieved through aberration correction. Single-crystal areas are identified as well as defect structures.

Simulation Study Of Transmission Electron Microscopy Imaging Of Graphene Stacking

Graphene is the subject of intense study due to its high mobility and mechanical integrity. These properties make it an attractive material for the “beyond CMOS” technology that will replace today’s transistor. Acceleration of process and device technology requires considerable advances in the imaging and characterization of graphene. The physical dimensions of available single and multi‐layer samples are not large enough for many metrology methods. For example, the spot size of ellipsometry is typically larger than available samples. Electron microscopy of graphene is also challenging. Carbon is a difficult element to image with electron microscopy because of its low atomic number. The high mobility of single layer and misoriented two and three layer graphene make it attractive for nanoelectronics. The current investigation explores HRTEM simulations of graphene stacking configurations AAA/ABA/ABC as well as bilayers with misorientations between the individual layers. HAADF (High Angle Annular Dark Field...

Graphene metrology and devices

Abstract The unusual electronic properties of graphene make it a prime candidate material for extending nanoelectronics and designing new types of switches. Graphenes unusual properties are a result of the unusual band structure associated with the hexagonal bonding pattern and the electron/hole transport through the pi orbitals. Graphene samples are frequently more than one layer, or few-layer graphene, and the change in electronic properties with each layer depends on the stacking configuration and the rotational misorientation between the layers. Transport measurements of single layer graphene (SLG) show that graphene exhibits the quantum Hall effect. In addition, Berry Phase corrections to carrier transport measurements are now widely recognized. Because graphene is a single layer of carbon atoms, it is difficult to find, manipulate, and measure. We review the status of physical and electrical characterization of graphene and discuss the remaining challenges. We discuss results from optical microscopy, transmission electron microscopy, low energy electron microscopy, nano-Raman, and several scanned probe methods. Issues such as determination of the number of layers of graphene and rotational stacking misorientation are emphasized.