M. V. S. Chandrashekhar

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...

High Purity Semi-Insulating 4H-SiC Epitaxial Layers by Defect-Competition Epitaxy: Controlling Si Vacancies

Thick, high-purity semi-insulating (SI) homoepitaxial layers on 4H-SiC were demonstrated using a novel compensation scheme controlled by defect-competition epitaxy at C/Si ratios of 1.3–1.5. These showed resistivity of ~109 Ω cm. Comparison of secondary ion mass spectra between low-doped epilayers grown at C/Si ratio 1.3 showed little difference in residual impurity concentrations. A reconciliation of impurity concentration with measured resistivity indicated a compensating trap concentration of ~1015 cm-3 present only in SI epilayers. High-resolution photoinduced transient spectroscopy (HRPITS) identified them as Si vacancy related centers, with no detectable EH6/7 and Z1/2 levels. Recombination lifetimes of ~5 ns suggest application in fast-switching power devices.

Evidence of minority carrier injection efficiency >90% in an epitaxial graphene/SiC Schottky emitter bipolar junction phototransistor for ultraviolet detection

In this letter, we report the UV detection characteristics of an epitaxial graphene (EG)/SiC based Schottky emitter bipolar phototransistor (SEPT) with EG on top as the transparent Schottky emitter layer. Under 0.43 μW UV illumination, the device showed a maximum common emitter current gain of 113, when operated in the Schottky emitter mode. We argue that avalanche gain and photoconductive gain can be excluded, indicating minority carrier injection efficiency, γ, as high as 99% at the EG/p-SiC Schottky junction. This high γ is attributed to the large, highly asymmetric barrier, which EG forms with the p-SiC. The maximum responsivity of the UV phototransistor is estimated to be 7.1 A/W. The observed decrease in gain with increase in UV power is attributed to recombination in the base region, which reduces the minority carrier lifetime.

Comparison of 4H Silicon Carbide Epitaxial Growths at Various Growth Pressures Using Dicholorosilane and Silane Gases

SiC epitaxial films grown in an inverted chimney CVD reactor using silane-propane-hydrogen and dichlorosilane (DCS)-propane-hydrogen systems are compared for growth rates and doping concentrations at various growth pressures. Parasitic depositions in the gas injector tube using these precursor gases are also compared for precursor depletion. Virtual Reactor, a commercial software, is employed to predict growth rates and compare them to experimental results for the same growth conditions using DCS and silane gases.

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.

Epitaxial growth of graphene on SiC by Si selective etching using SiF4 in an inert ambient

Epitaxial graphene growth on SiC by Si selective etching using tetrafluorosilane (SiF4) is introduced, where SiF4 in Ar ambient selectively etches Si from the SiC surface at temperatures 1400 °C or above, leaving the C as graphene. Raman spectra of SiC treated in Ar for 60 min at 300 Torr did not show a graphene G-peak. However, with the addition of SiF4, a clear G-peak was observed for the surface treated for only 1 min, demonstrating faster Si removal using SiF4. Si selective etching of SiC is explained by the Gibbs free energy, where Si removal is more favorable compared to C removal by SiF4.

Behavior of Particles in the Growth Reactor and their Effect on Silicon Carbide Epitaxial Growth

Formation of particles and their effect on SiC epitaxial growth in the CVD reactor is investigated. Particle induced defects in the epilayer at different gas decomposition conditions are discussed. A higher number of pits with larger diameters are observed in the epilayer for conditions where gases decompose later in the gas injector tube (i.e. nearer to the substrate). On the other hand, the number and size of these pits reduce for the condition where gas decomposes earlier in the tube. To investigate the effect of particles during the growth, various particles with different size, shape and compositions are intentionally placed on the substrate surface before epitaxial films are grown. Samples are mapped and compared at similar locations in the pre-growth, post growth and post-etch (by molten KOH) conditions. It is found that the nature of particle induced defects depends primarily on size and shape of particles.

High-speed solar-blind UV photodetectors using high-Al content Al0.64Ga0.36N/Al0.34Ga0.66N multiple quantum wells

We demonstrate high-external quantum efficiency (~50%) solar-blind AlGaN p–n junction photodetectors with high-Al content multiple quantum wells (MQWs). A peak responsivity of 0.1 A/W at 250 nm, which falls >103 by 280 nm, indicates that the optical absorption is dominated by the MQW structures. At a reverse bias of 0.5 V, the dark current is <0.1 pA. The readout RC-limited time response is measured as 0.4 µs, and an achievable detector RC-limited time response of 2 ns is estimated. The devices do not show internal gain, which accounts for their high speed.