Biplob K. Daas

Revealing the electronic band structure of trilayer graphene on SiC: An angle-resolved photoemission study

In recent times, trilayer graphene has attracted wide attention owing to its stacking and electric-field-dependent electronic properties. However, a direct and well-resolved experimental visualization of its band structure has not yet been reported. In this paper, we present angle-resolved photoemission spectroscopy data which show with high resolution the electronic band structure of trilayer graphene obtained on alpha-SiC(0001) and beta-SiC(111) via hydrogen intercalation. Electronic bands obtained from tight-binding calculations are fitted to the experimental data to extract the interatomic hopping parameters for Bernal and rhombohedral stacked trilayers. Low-energy electron microscopy measurements demonstrate that the trilayer domains extend over areas of tens of square micrometers, suggesting the feasibility of exploiting this material in electronic and photonic devices. Furthermore, our results suggest that, on SiC substrates, the occurrence of a rhombohedral stacked trilayer is significantly higher than in natural bulk graphite. (Less)

Polariton enhanced infrared reflection of epitaxial graphene

We show SiC substrate phonon-induced surface plasmon polariton (SPP) formation in epitaxial graphene grown on 4H–SiC, in SiC’s restrahlen band (8–10 μm). By fitting measurement to theory, we extract thickness, momentum scattering time (τ), sheet carrier density (ns), and estimate carrier mobility. By showing that τ∝1ns, we argue that scattering is dominated by short-range interactions at the SiC/graphene interface. SPP formation finds application in nanophotonic devices for optical computing because of graphene’s unique plasmonic properties.

Evidences of electrochemical graphene functionalization and substrate dependence by Raman and scanning tunneling spectroscopies

Electrochemical functionalization and possible hydrogenation of treated epitaxial graphene samples on 6H-SiC are presented. To attract H+ ions to react with the exposed working cathode, a 10% sulfuric acid electrolyte was used with a Pt counter anode. Functionalization was determined using Raman spectroscopy and measured by a marked increase in I(D)/I(G) ratio and introduction of C-H bond peak at ∼2930 cm−1. There was also a marked increase in fluorescence background, which clearly differentiates functionalization from lattice damage in the graphene. Quantifying the fluorescence, we estimate that H-incorporation as high as 50% was achieved based on results on hydrocarbons, although other functional groups cannot be excluded. We further distinguished these functionalization signatures from lattice damage through measurements on nanocrystalline graphene on a and m plane SiC, which displayed very different surface morphologies and no measureable fluorescence. Finally, we show that the extent of functionaliza...

Doping Dependence of Thermal Oxidation on n-Type 4H-SiC

The doping dependence of dry thermal oxidation rates in n-type 4H-SiC was investigated. The oxidation was performed in the temperature range of 1000 °C to 1200 °C for samples with nitrogen doping in the range of 6.5 × 1015 to 9.3 × 1018/cm3, showing a clear doping dependence. Samples with higher doping concentrations displayed higher oxidation rates. The results were interpreted using a modified Deal-Grove model. Linear and parabolic rate constants and activation energies were extracted. Increasing nitrogen led to an increase in the linear-rate-constant preexponential factor from 10-6 to 10-2 m/s and the parabolic-rate-constant preexponential factor from 10-9 to 10-6 m2/s. The increase in the linear rate constant was attributed to defects from doping-induced lattice mismatch, which tend to be more reactive than bulk crystal regions. The increase in the diffusion-limited parabolic rate constant was attributed to the degradation in the oxide quality originating from the doping-induced lattice mismatch. This degradation was confirmed by the observation of a decrease in the optical density of the grown oxide films from 1.4 to 1.24. The linear activation energy varied from 1.6 to 2.8 eV, while the parabolic activation energy varied from 2.7 to 3.3 eV, increasing with doping concentration. These increased activation energies were attributed to the higher nitrogen content, leading to an increase in the effective bond energy stemming from the difference in C-Si (2.82 eV) and Si-N (4.26 eV) binding energies. This paper provides crucial information in the engineering of SiO2 dielectrics for SiC metal-oxide-semiconductor structures, which typically involve regions of very different doping concentrations, and suggests that thermal oxidation at high doping concentrations in SiC may be defect mediated.

Molecular Gas Adsorption Induced Carrier Transport Studies of Epitaxial Graphene Using IR Reflection Spectroscopy

We investigate molecular adsorption doping by electron withdrawing NO2 and electron donating NH3 on epitaxial graphene. Amperometric measurements show conductance changes upon introduction of molecular adsorbents on epitaxial graphene. Conductance changes are a trade-off between carrier concentration and scattering, and manifest at direct current and optical frequencies. We therefore investigate changes in the infrared (IR) reflection spectra to correlate these two frequency domains, as reflectance changes are due to a change of EG surface conductance. We match theory with experimental IR data and extract changes in carrier concentration and scattering due to gas adsorption. Finally, we separate the intraband and interband scattering contributions to the electronic transport under gas adsorption. The results indicate that, under gas adsorption, the influence of interband scattering cannot be neglected, even at DC.

Si-adatom kinetics in defect mediated growth of multilayer epitaxial graphene films on 6H-SiC

We present a quantitative study on the growth of multilayer epitaxial graphene (EG) by solid-state decomposition of SiC on polar (c-plane Si and C-face) and non-polar (a and m planes) 6H-SiC faces, with distinctly different defect profiles. The growth rates are slower than expected from a mechanism that involves Si loss from an open and free surface, and much faster than expected for the nucleation of a defect-free EG layer, implying that defects in the EG play a critical role in determining the growth kinetics. We show that a Deal-Grove growth model, which assumes vertical diffusion of Si through these defects as the limiting factor for EG growth, is unsuitable for describing multilayer growth. Instead, we introduce a lateral “adatom” diffusion mechanism for Si out-diffusion, based on a modified Burton, Cabrera, and Frank model. In this model, defects in epitaxial graphene serve as sinks for Si desorption loss, taking the place of reactive sites, such as step edges for nucleation and growth of crystals p...

Evidence of Electrochemical Graphene Functionalization by Raman Spectroscopy

Electrochemical functionalization of treated epitaxial graphene samples on Si-face 6H-SiC are presented in this work. Three semi-insulating 6H-SiC substrates cut from different boules with varying off cut angle (on axis, 0.5° and 1.0° degrees off axis in the [112‾0] direction) were diced into 10mm x 10mm samples and quality EG grown on top. A home-build electrochemical cell was used with current applied though a 10% H2SO4 solution, with a Pt wire and exposed graphene as the anode and cathode respectively. Functionalization was determined using Raman spectroscopy and measured by an increase in D/G ratio, increase in fluorescence background and introduction of C-H bond peak at ~2930 cm-1. Components of the Raman spectra before and after functionalization of all samples used were analyzed to show a substrate dependent effect on functionalization with values such as D/G ratio and normalized fluorescence slope varying between the substrates.

Study of Epitaxial Graphene on Non-Polar 6H-SiC Faces

We present epitaxial graphene (EG) growth on non-polar a-plane and m-plane 6H-SiC faces where material characterization is compared with that known for EG grown on polar faces. AFM surface morphology exhibits nanocrystalline graphite like features for non-polar faces, while polar silicon face shows step like features. This differing behavior is attributed to the lack of a hexagonal template on the non-polar faces. Non-polar faces also exhibit greater disorder and red shift of all Raman peaks (D, G and 2D) with increasing temperature. This is attributed to decreasing stress with increasing temperature. These variations provide evidence of different EG growth mechanisms on non-polar and polar faces, likely due to differences in surface free energy. We also present differences between a-plane ( ) EG and m-plane ( ) EG in terms of morphology, thickness and Raman characteristics.

Selective multimodal gas sensing in epitaxial graphene by fourier transform infrared spectroscopy

We report the sensing behavior of epitaxial graphene (EG) grown on C-face SiC substrates by infrared reflectance spectroscopy through molecular adsorption of NO2, NH3 and N2 gases. Fourier Transform Infrared Reflection (FTIR) measurements were performed on EG under gas exposure and it clearly exhibits an EG thickness dependence. By comparing the change in Ef under gas adsorption with the adsorbed impurity concentration as a function of EG thickness, the 3 gases were clearly distinguished, enabling a new paradigm for multi-modal gas sensing using optical interrogation of EG surfaces towards EG electronic or optical noses.