Robert P. Devaty

Field effect in epitaxial graphene on a silicon carbide substrate

The authors report a strong field effect observed at room temperature in epitaxially synthesized, as opposed to exfoliated, graphene. The graphene formed on the silicon face of a 4H silicon carbide substrate was photolithographically patterned into isolated active regions for the semimetal graphene-based transistors. Gold electrodes and a polymer dielectric were used in the top-gate transistors. The demonstration of a field effect mobility of 535cm2∕Vs was attributed to the transistor geometry that maximizes conductance modulation, although the mobility is lower than observed in exfoliated graphene possibly due to grain boundaries caused by the rough morphology of the substrate surface.

Absorption coefficient of 4H silicon carbide from 3900 to 3250 Å

We report the values of the absorption coefficient of 4H SiC at room temperature, in the wavelength range from 3900 to 3350 A and at 3250 A. By using the known shift in the band gap with temperature, we also present an estimate of the absorption coefficient of 4H SiC at 2 K.

Photoluminescence and transport studies of boron in 4H SiC

Two distinct boron-related centers are known in silicon carbide polytypes, one shallow (ionization energy ∼300 meV) and the other deep (∼650 meV). In this work, 4H SiC homoepitaxial films are intentionally doped with the shallow boron center by controlling the silicon to carbon source gas ratio during chemical vapor deposition, based on site competition epitaxy. The dominance of the shallow boron center for samples grown with a low Si/C ratio, favoring the incorporation of boron onto the silicon sublattice, is verified by the temperature dependent Hall effect, admittance spectroscopy and deep level transient spectroscopy. In these samples a peak near 3838 A appears in the low temperature photoluminescence spectrum. Further experiments support the identification of this peak with the recombination of a four particle (bound exciton) complex associated with the neutral shallow boron acceptor as follows: (1) The intensity of the 3838 A peak grows with added boron. (2) Momentum conserving phonon replicas are o...

Penetration depths in the ultraviolet for 4H, 6H and 3C silicon carbide at seven common laser pumping wavelengths

Abstract We report the values of the absorption coefficient of 4H, 6H and 3C SiC at room temperature, in the range 3900–2968 A. By using the known shift in the bandgap with temperature, we also present estimates of the absorption coefficient of 4H, 6H and 3C SiC at 2 K. A table is given for penetration depths at 300 and 2 K for seven common lasers used to pump SiC in this region.

Nitrogen donors and deep levels in high‐quality 4H–SiC epilayers grown by chemical vapor deposition

4H‐SiC epilayers grown by chemical vapor deposition were characterized by Hall effect, admittance spectroscopy, low‐temperature photoluminescence, and deep level transient spectroscopy (DLTS). The nitrogen (N) donor activation energies were estimated as 45–65 meV at hexagonal and 105–125 meV at cubic sites from Hall effect investigations in agreement with the data taken by admittance spectroscopy. In low‐temperature photoluminescence, the N bound exciton peaks were dominant, however, free exciton peaks were also observed. DLTS measurements revealed a low concentration of electron traps (∼1013cm−3) for both samples grown on Si and C faces, indicating high‐quality epilayers independent of the substrate polarity.

Anisotropy of the electron Hall mobility in 4H, 6H, and 15R silicon carbide

Hall effect measurements in a Hall‐bar configuration are performed on nitrogen‐doped n‐type bulk 4H, 6H, and 15R SiC single crystals cut into small parallelepipeds with their longest edges either parallel or perpendicular to the c axis. In the temperature range investigated (40–700 K), an anisotropy of the electron Hall mobility is observed in all three polytypes. While the mobility perpendicular to the c axis—with magnetic field perpendicular or parallel to the c axis—is greater than the mobility parallel to the c axis for 6H and 15R SiC, 4H SiC shows the opposite behavior.

Intense erbium‐1.54‐μm photoluminescence from 2 to 525 K in ion‐implanted 4H, 6H, 15R, and 3C SiC

We have observed intense line spectra in the neighborhood of 1.54 μm from erbium‐implanted samples of 4H, 6H, 15R, and 3C SiC. Samples were implanted to a fluence of about 1013 erbium ions/cm2 using four implant energies. An anneal at 1700 °C in a SiC cavity was used. The temperature dependence of the integrated luminescence intensity from 1.49 to 1.64 μm varies very little from 2 to 400 K. No major differences are found for the spectra of the hexagonal and rhombohedral polytypes but there is a difference for cubic SiC (3C SiC).

Determination of the electric field in 4H/3C/4H-SiC quantum wells due to spontaneous polarization in the 4H SiC matrix

We report a low-temperature photoluminescence study of 4H/3C/4H-SiC single quantum wells. A quantum well consists of thirteen 3C-SiC bilayers as displayed in a high-resolution transmission electron microscope image. The optical emission energy of the quantum well is more than 200 meV below the exciton band gap of bulk 3C-SiC. A strong internal electric field on the order of 1 MV/cm leads to the large redshift of the emission energy due to the quantum-confined Stark effect. The origin of this field is discussed in terms of the spontaneous polarization difference between 3C- and 4H-SiC.

Hydrogen incorporation in boron-doped 6H-SiC CVD epilayers produced using site-competition epitaxy

We report on the initial investigations of using site-competition epitaxy to control boron incorporation in chemical vapor deposition (CVD) 6H-SiC epilayers. Also reported herein is the detection of hydrogen in boron-doped CVD SiC epilayers and hydrogen-passivation of the boron-acceptors. Results from low temperature photoluminescence (LTPL) spectroscopy indicate that the hydrogen content increased as the capacitance-voltage (C-V) measured net hole concentration increased. Secondary ion mass spectrometry (SIMS) analysis revealed that the boron and the hydrogen incorporation both increased as the Si/ C ratio was sequentially decreased within the CVD reactor during epilayer growth. Epilayers that were annealed at 1700°C in argon no longer exhibited hydrogen-related LTPL lines, and subsequent SIMS analysis confirmed the outdiffusion of hydrogen from the boron-doped SiC epilayers. The C-V measured net hole concentration increased more than threefold as a result of thel700°C anneal, which is consistent with hydrogen passivation of the boron-acceptors. However, boron related LTPL lines were not observed before or after the 1700°C anneal.

Aluminum acceptor four particle bound exciton complex in 4H, 6H, and 3C SiC

New lines in the low temperature luminescence spectra of lightly aluminum doped p‐type films of 3C, 6H, and 4H SiC are identified and associated with the recombination of a neutral aluminum acceptor four particle bound exciton complex.