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A metastable photoinduced electron paramagnetic resonance (EPR) signal at low temperatures is reported for GaN alloyed with boron (i.e., BxGa1−xN) epitaxial layers grown at temperatures ranging from 840 to 1090° C. An isotropic EPR line with g = 2.004 is observed with an intensity depending on the growth temperature for all the samples with boron content between 0.73% and 2.51%. The temperature dependence of EPR intensities is compared with the results of high-resolution photoinduced transient spectroscopy. This enables a link between particular traps and the EPR signal. The activation energies of these traps are consistent with the theoretical position of the VN–VGa complex. Thermal annihilation of the EPR signal with 30 meV activation energy corresponds to shallow donor ionization. A model explaining the light-induced EPR signal that involves redistribution of electrons between deep and shallow donors, mediated by photoionization to the conduction band, is proposed. [ABSTRACT FROM AUTHOR]

Efficient control of intercalation of epitaxial graphene by specific elements is a way to change properties of the graphene. Results of several experimental techniques, such as X-ray photoelectron spectroscopy, micro-Raman mapping, reflectivity, attenuated total reflection, X-ray diffraction, and X-ray reflectometry, gave a new insight into the intercalation of oxygen in the epitaxial graphene grown on 4H-SiC(0001). These results confirmed that oxygen intercalation decouples the graphene buffer layer from the 4H-SiC surface and converts it into the graphene layer. However, in contrast to the hydrogen intercalation, oxygen does not intercalate between carbon planes (in the case of few layer graphene) and the interlayer spacing stays constant at the level of 3.35-3.32 Å . Moreover, X-ray reflectometry showed the presence of an oxide layer having the thickness of about 0.8Å underneath the graphene layers. Apart from the formation of the nonuniform thin oxide layer, generation of defects in graphene caused by oxygen was also evidenced. Last but not least, water islands underneath defected graphene regions in both intercalated and non-intercalated samples were most probably revealed. These water islands are formed in the case of all the samples stored under ambient laboratory conditions. Water islands can be removed from underneath the few layer graphene stacks by relevant thermal treatment or by UV illumination. [ABSTRACT FROM AUTHOR]

We present optical, electrical, and structural properties of nitrogen-doped graphene grown on the Si face of 4H-SiC (0001) by chemical vapor deposition method using propane as the carbon precursor and N2 as the nitrogen source. The incorporation of nitrogen in the carbon lattice was confirmed by X-ray photoelectron spectroscopy. Angle-resolved photoemission spectroscopy shows carrier behavior characteristic for massless Dirac fermions and confirms the presence of a graphene monolayer in the investigated nitrogen-doped samples. The structural and electronic properties of the material were investigated by Raman spectroscopy. A systematical analysis of the graphene Raman spectra, including D, G, and 2D bands, was performed. In the case of nitrogen-doped samples, an electron concentration on the order of 5-10 × 1012 cm–2 was estimated based upon Raman and Hall effect measurements and no clear dependence of the carrier concentration on nitrogen concentration used during growth was observed. This high electron concentration can be interpreted as both due to the presence of nitrogen in graphitic-like positions of the graphene lattice as well as to the interaction with the substrate. A greater intensity of the Raman D band and increased inhomogeneity, as well as decreased electron mobility, observed for nitrogen-doped samples, indicate the formation of defects and a modification of the growth process induced by nitrogen doping. [ABSTRACT FROM AUTHOR]

The principal structural defects in graphene multilayers synthesized on the carbon-terminated face of a 4H-SiC (0001) substrate were investigated using the high-resolution transmission electron microscopy. The analyzed systems include a wide variety of defected structures such as edge dislocations, rotational multilayers, and grain boundaries. It was shown that graphene layers are composed of grains of the size of several nanometres or larger; they differ in a relative rotation by large angles, close to 30°. The structure of graphene multilayers results from the synthesis on a SiC (0001) surface, which proceeds via intensive nucleation of new graphene layers that coalesce under various angles creating an immense orientational disorder. Structural defects are associated with a built-in strain resulting from a lattice mismatch between the SiC substrate and the graphene layers. The density functional theory data show that the high-angular disorder of AB stacked bilayers is not restoring the hexagonal symmetry of the lattice. [ABSTRACT FROM AUTHOR]

Transmission electron microscopy (TEM) investigations of epitaxial graphene, grown on on-axis and 8° off-axis C-terminated 4H–SiC [formula] surfaces are presented. The TEM results provide evidence that the first carbon layer is separated by 3.2 Å from the C-terminated SiC surface. It was also found that thick graphene layers grown on on-axis SiC [formula] are loosely bound to the SiC substrate. Moreover, the structural observations reveal a certain degree of disorder between the graphene planes, which manifests itself in a rotation of the layers and in an increase in the interplanar spacing between certain carbon layers from 3.35 Å, which is characteristic for graphite, up to 3.7 Å. Graphene grown on 8° off-axis SiC [formula] substrates covers the steps of SiC and as a result disorder seems to be not as pronounced as it is on the on-axis SiC [formula] substrate. [ABSTRACT FROM AUTHOR]

Transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and micro-Raman investigations of epitaxial graphene on 4H-SiC on-axis and 4° off-axis are presented. The STM images show that there is superimposed on 1×1 graphene pattern the carbon nanomesh of honeycomb 6×6 structure with the lattice vector of 17.5 Å. The TEM results give evidence that the first carbon layer is separated by 2 Å from the Si-terminated SiC surface and that subsequent carbon layers are spaced by 3.3 Å. It is also found in TEM that the graphene layers cover atomic steps, present on 4° off-axis SiC(0001) surface, indicating a carpetlike growth mode. However, a bending of graphene planes on atomic steps of SiC apparently leads to generation of stress which leads to creation of edge dislocations in the graphene layers. [ABSTRACT FROM AUTHOR]

Room temperature electroreflectance measurements on δ-doped low-dimensional structures are presented. Previously proposed electroreflectance bias-wavelength mapping is used for characterization of (1) a modulation Si δ-doped pseudomorphic InGaAs/GaAs quantum well and (2) a Sn δ-doped GaAs layer. An electric field above and below the δ-doping plane found from the Fourier transform applied to Franz-Keldysh oscillations was used to find the δ-dopant concentration in investigated structures. The position of the δ-doping plane and a Schottky barrier height are also determined. [ABSTRACT FROM AUTHOR]

Discusses a study which examined the application of tight-binding total energy calculations of the two possible states of the neutral oxygen-vacancy center in silicon oxide within the bond orbital approximation. Role played by oxygen vacancy; Composition of a neutral oxygen vacancy; Cause of the spin-paired electron configuration; Possibility in obtaining a spin-paired electron configuration.