C.R. Eddy

Hall effect mobility of epitaxial graphene grown on silicon carbide

Epitaxial graphene (EG) films were grown in vacuo by silicon sublimation from the (0001) and (0001¯) faces of 4H-SiC and 6H-SiC. Hall effect mobilities and sheet carrier densities of the films were measured at 300 and 77 K and the data depended on the growth face. About 40% of the samples exhibited holes as the dominant carrier, independent of face. Generally, mobilities increased with decreasing carrier density, independent of carrier type and substrate polytype. The contributions of scattering mechanisms to the conductivities of the films are discussed. The results suggest that for near-intrinsic carrier densities at 300 K epitaxial graphene mobilities will be ∼150 000 cm2 V−1 s−1 on the (0001¯) face and ∼5800 cm2 V−1 s−1 on the (0001) face.

Bilayer graphene grown on 4H-SiC (0001) step-free mesas.

We demonstrate the first successful growth of large-area (200 × 200 μm(2)) bilayer, Bernal stacked, epitaxial graphene (EG) on atomically flat, 4H-SiC (0001) step-free mesas (SFMs) . The use of SFMs for the growth of graphene resulted in the complete elimination of surface step-bunching typically found after EG growth on conventional nominally on-axis SiC (0001) substrates. As a result heights of EG surface features are reduced by at least a factor of 50 from the heights found on conventional substrates. Evaluation of the EG across the SFM using the Raman 2D mode indicates Bernal stacking with low and uniform compressive lattice strain of only 0.05%. The uniformity of this strain is significantly improved, which is about 13-fold decrease of strain found for EG grown on conventional nominally on-axis substrates. The magnitude of the strain approaches values for stress-free exfoliated graphene flakes. Hall transport measurements on large area bilayer samples taken as a function of temperature from 4.3 to 300 K revealed an n-type carrier mobility that increased from 1170 to 1730 cm(2) V(-1) s(-1), and a corresponding sheet carrier density that decreased from 5.0 × 10(12) cm(-2) to 3.26 × 10(12) cm(-2). The transport is believed to occur predominantly through the top EG layer with the bottom layer screening the top layer from the substrate. These results demonstrate that EG synthesized on large area, perfectly flat on-axis mesa surfaces can be used to produce Bernal-stacked bilayer EG having excellent uniformity and reduced strain and provides the perfect opportunity for significant advancement of epitaxial graphene electronics technology.

Epitaxial growth of AlN films via plasma-assisted atomic layer epitaxy

Thin AlN layers were grown at 200–650 °C by plasma assisted atomic layer epitaxy (PA-ALE) simultaneously on Si(111), sapphire (112¯0), and GaN/sapphire substrates. The AlN growth on Si(111) is self-limited for trimethyaluminum (TMA) pulse of length > 0.04 s, using a 10 s purge. However, the AlN nucleation on GaN/sapphire is non-uniform and has a bimodal island size distribution for TMA pulse of ≤0.03 s. The growth rate (GR) remains almost constant for Tg between 300 and 400 °C indicating ALE mode at those temperatures. The GR is increased by 20% at Tg = 500 °C. Spectroscopic ellipsometry (SE) measurement shows that the ALE AlN layers grown at Tg ≤ 400 °C have no clear band edge related features, however, the theoretically estimated band gap of 6.2 eV was measured for AlN grown at Tg ≥ 500 °C. X-ray diffraction measurements on 37 nm thick AlN films grown at optimized growth conditions (Tg = 500 °C, 10 s purge, 0.06 s TMA pulse) reveal that the ALE AlN on GaN/sapphire is (0002) oriented with rocking curve f...

Fabrication of top-gated epitaxial graphene nanoribbon FETs using hydrogen-silsesquioxane

Top-gated epitaxial graphene nanoribbon (EGNR) field effect transistors (FETs) were fabricated on epitaxial graphene substrates which demonstrated the opening of a substantial bandgap. Hydrogen silsesquioxane (HSQ) was used for the patterning of 10 nm size linewidth as well as a seed layer for atomic layer deposition (ALD) of a high-k dielectric aluminum oxide (Al2O3). It is found that the resolution of the patterning is affected by the development temperature, electron beam dose, and substrate materials. The chosen gate stack of HSQ followed by Al2O3 ALD permits stable device performance and enables the demonstration of the EGNR-FET.

Vertical conduction mechanism of the epitaxial graphene/n-type 4H-SiC heterojunction at cryogenic temperatures

Vertical diodes of epitaxial graphene on n− 4H-SiC were investigated. The graphene Raman spectra exhibited a higher intensity in the G-line than the 2D-line, indicative of a few-layer graphene film. Rectifying properties improved at low temperatures as the reverse leakage decreased over six orders of magnitude without freeze-out in either material. Carrier concentration of ∼1016 cm−3 in the SiC remained stable down to 15 K, while accumulation charge decreased and depletion width increased in forward bias. The low barrier height of 0.08 eV and absence of recombination-induced emission indicated majority carrier field emission as the dominant conduction mechanism.

Nucleation of in-grown stacking faults and dislocation half-loops in 4H-SiC epitaxy

Ultraviolet photoluminescence, transmission electron microscopy and KOH etching were used to characterize extended defects in 4H-SiC epilayers grown at high growth rates (18 μm/h). Layers exhibited high densities of in-grown stacking faults and dislocation half-loops. The stacking faults were 8H Shockley-type faults. The Burgers vector of the dislocation half-loops was in the (0001) basal plane. Both defects nucleate within the epilayer at early stages of growth. Defect nucleation is directly correlated with high initial growth rate and is not related to any defects/heterogeneities in the substrate or epilayer. Epilayer growth by nucleation of two-dimensional islands is proposed as a possible mechanism for the formation of both defects, through nucleation of faulted Si-C bilayers.

Step edge influence on barrier height and contact area in vertical heterojunctions between epitaxial graphene and n-type 4H-SiC

Vertical rectifying contacts of epitaxial graphene grown by Si sublimation on the Si-face of 4H-SiC epilayers were investigated. Forward bias preferential conduction through the step edges was correlated by linear current density normalization. This phenomenon was observed on samples with 2.7–5.8 monolayers of epitaxial graphene as determined by X-ray photoelectron spectroscopy. A modified Richardson plot was implemented to extract the barrier height (0.81 eV at 290 K, 0.99 eV at 30 K) and the electrically dominant SiC step length of a Ti/Al contact overlapping a known region of approximately 0.52 μm wide SiC terraces.

Membrane amplitude and triaxial stress in twisted bilayer graphene deciphered using first-principles directed elasticity theory and scanning tunneling microscopy

Departments of Marine Engineering, Material Science and Engineering,Texas A&M University, College Station TX, 77843 USA(Dated: July 9, 2014)Twisted graphene layers produce a moir´e pattern (MP) structure with a predetermined wavelengthfor given twist angle. However, predicting the membrane corrugation amplitude for any angle otherthan pure AB-stacked or AA-stacked graphene is impossible using first-principles density functionaltheory (DFT) due to the large supercell. Here, within elasticity theory we define the MP structureas the minimum energy configuration, thereby leaving the height amplitude as the only unknownparameter. The latter is determined from DFT calculations for AB and AA stacked bilayer graphenein order to eliminate all fitting parameters. Excellent agreement with scanning tunneling microscopy(STM) results across multiple substrates is reported as function of twist angle.I. INTRODUCTION

Site Specific TEM Specimen Preparation for Characterization of Extended Defects in 4H-SiC Epilayers

Silicon carbide (SiC) is an important semiconductor for high temperature, high voltage and high power electronic applications. Unfortunately, SiC still has not reached its potential in these applications due to the presence of extended defects in epilayers, which degrade the performance of SiC-based devices. In this work, focused ion beam (FIB) is employed to prepare site-specific cross-sectional transmission electron microscopy (TEM) specimens allowing direct characterization of unwanted in-grown stacking faults (IGSFs) and basal plane dislocations (BPDs). IGSFs are known to decrease breakdown voltages and increase leakage currents in SiC diodes [1]; however, their nucleation mechanism is currently not established. BPDs can form recombination-induced stacking faults, which lead to device performance degradation over time [2]. Recently, KOH etching of the substrate was reported to cause conversion of BPDs to electrically benign threading edge dislocations (TEDs) in the epilayer. However, for some BPDs this conversion was also associated with a shift in the locations of etch pits [3]. In this study, FIB and TEM are used to explain the nucleation mechanism of IGSFs and the previously observed shift in the locations of BPDs.

Burgers Vector Determination of Threading Screw Dislocations in 4H-SiC via Forescattered Electron Channeling Contrast Imaging

Forescattered electron channeling contrast imaging (ECCI) offers the potential of imaging and analyzing extended defects in a scanning electron microscope (SEM) equipped with a commercial electron backscatter diffraction (EBSD) system. The ability of ECCI to quickly image threading dislocations over large areas of thin film samples, without the difficult sample preparation needed for transmission electron microscopy (TEM), has already been demonstrated [1]. The somewhat more elusive goal of using ECCI to determine the Burgers vector of threading dislocations, however, requires advances in both the determination of experimental parameters and accompanying image simulations. In order to reach this goal we have recorded and simulated ECCI images of an sample with features that are relatively easily studied and modeled: those of specially engineered 4H-SiC mesas (Fig.1) [2]. Imaging of threading screw dislocations (TSDs) penetrating the (0001) surface revealed dark-to-light contrast, the direction of which depends on the acting Bragg reflection, the deviation from the Bragg condition sg, and the dislocation Burgers vector (Fig.2). Burgers vector identification was confirmed through observations of the rotational direction of atomic step spirals associated with various screw dislocations [3].