R. Stępniewski

Graphene Epitaxy by Chemical Vapor Deposition on SiC

We demonstrate the growth of high quality graphene layers by chemical vapor deposition (CVD) on insulating and conductive SiC substrates. This method provides key advantages over the well-developed epitaxial graphene growth by Si sublimation that has been known for decades. (1) CVD growth is much less sensitive to SiC surface defects resulting in high electron mobilities of ∼1800 cm(2)/(V s) and enables the controlled synthesis of a determined number of graphene layers with a defined doping level. The high quality of graphene is evidenced by a unique combination of angle-resolved photoemission spectroscopy, Raman spectroscopy, transport measurements, scanning tunneling microscopy and ellipsometry. Our measurements indicate that CVD grown graphene is under less compressive strain than its epitaxial counterpart and confirms the existence of an electronic energy band gap. These features are essential for future applications of graphene electronics based on wafer scale graphene growth.

Luminescence and reflectivity in the exciton region of homoepitaxial GaN layers grown on GaN substrates

Abstract In this work we report results of photoluminescence (PL) and reflectivity measurements in the exciton region of GaN homoepitaxial layers grown by metalorganic chemical vapour deposition on GaN substrates. At low temperature (4.2K), very narrow (FWHM = 1.0meV) PL lines related to excitons bound to neutral acceptor (3.4666eV) and neutral donor (3.4719eV) were observed. The energies of free excitons from reflectivity and PL measurements were found to be: E A = 3.4780eV, E B = 3.4835eV and E C = 3.502eV.

Transmission electron microscopy and scanning tunneling microscopy investigations of graphene on 4H-SiC(0001)

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 A. The TEM results give evidence that the first carbon layer is separated by 2 A from the Si-terminated SiC surface and that subsequent carbon layers are spaced by 3.3 A. 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.

Quasiclassical cyclotron resonance of Dirac fermions in highly doped graphene

Cyclotron resonance in highly doped graphene has been explored in the infrared magneto-transmission experiment. Contrary to previous works, which only focused on the magneto-optical properties of graphene in its quantum regime, here we study the quasi-classical response of this system. We show that it has a character of classical cyclotron resonance, which is linear in the applied magnetic field, with an effective cyclotron mass defined by the position of the Fermi level m = E_F/v_F^2.

Transmission electron microscopy investigations of epitaxial graphene on C-terminated 4H–SiC

Transmission electron microscopy (TEM) investigations of epitaxial graphene, grown on on-axis and 8° off-axis C-terminated 4H–SiC (0001¯) surfaces are presented. The TEM results provide evidence that the first carbon layer is separated by 3.2 A from the C-terminated SiC surface. It was also found that thick graphene layers grown on on-axis SiC (0001¯) 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 A, which is characteristic for graphite, up to 3.7 A. Graphene grown on 8° off-axis SiC (0001¯) 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 (0001¯) substrate.

Structural and Optical Properties of Homoepitaxial GaN Layers

The review of structural and optical properties of homoepitaxial layers grown by MOVCD on single crystals GaN substrates is presented. The TEM technique is used to characterise the structural properties of epi-layers. It is found that the structural properties of GaN homoepitaxial layers are determined by the polarity of the substrate surface on which the growth takes place. It is shown that threading dislocations are present only in the layers grown on the [0001] “smooth” surface. On the other hand the layers grown on the [0001] “rough” surface are free from vertical defects. The characteristic feature of the growth on the “rough” surface are pinholes. The optical properties of homoepitaxial layers are predominantly determined by the growth polarity as well. It is shown also that the reflectivity measurement is the most precise way to determine the exciton energies and that emissions due to free excitons are strongly affected by polariton effects.

Micro-Raman spectroscopy of graphene grown on stepped 4H-SiC (0001) surface

Graphene grown by chemical vapor deposition on 4H-SiC (0001) was studied using micro-Raman spectroscopy and atomic force microscopy (AFM). AFM revealed that the graphene structure grown on on-axis substrates has a stepped morphology. This is due to step bunching, which results from etching in hydrogen as well as from the process of graphene formation itself. It was shown by micro-Raman spectroscopy that the properties of graphene present on step edges and on terraces are quite different. Graphene on terraces is uniform with a relatively small thickness and strain fluctuations. On the other hand, graphene on step edges has a large thickness and strain variations occur. A careful analysis of micro-Raman spatial maps led us to the conclusion that the carrier concentration on step edge regions is lowered when compared with terrace regions.

Impurity-Related Luminescence of Homoepitaxial GaN Studied with High Magnetic Fields

Recent magnetoluminescence results obtained for homoepitaxial GaN layers are presented. The neutral impurity-bound excitons and donor–acceptor pair emission lines have been studied in magnetic fields up to 27 T. Low-temperature luminescence spectra have been measured with the magnetic field parallel and perpendicular to the hexagonal c-axis of the GaN layers. Experimental results allowed us to evaluate diamagnetic shifts, effective g-factors of electrons and holes involved in neutral donor and neutral acceptor complexes as well as the electron–hole exchange constant for close donor–acceptor pairs. Both the fine structure of the neutral acceptor-bound exciton emission and the specific properties of donor–acceptor pair spectra observed in magnetoluminescence experiments are tentatively attributed to the internal structure of the acceptor state.

Structural investigations of hydrogenated epitaxial graphene grown on 4H-SiC (0001)

Structural investigations of hydrogenated epitaxial graphene grown on SiC(0001) are presented. It is shown that hydrogen plays a dual role. In addition to contributing to the well-known removal of the buffer layer, it goes between the graphene planes, resulting in an increase of the interlayer spacing to 3.6 A–3.8 A. It is explained by the intercalation of molecular hydrogen between carbon planes, which is followed by H2 dissociation, resulting in negatively charged hydrogen atoms trapped between the graphene layers, with some addition of covalent bonding to carbon atoms. Negatively charged hydrogen may be responsible for p-doping observed in hydrogenated multilayer graphene.

Growth of Graphene Layers on Silicon Carbide

The so-called “growth” of graphene was performed using a horizontal chemical vapor deposition (CVD) hot-wall reactor. In-situ etching in the mixture (H2-C3H8) was performed prior to growth at 1600oC temperature under 100 mbar. Systematic studies of the influence of the decomposition temperature and time, substrates roughness, etching of the substrates, heating rate, SiC dezorientation and other process parameters on the graphene thickness and quality have been conducted. Morphology and atomic scale structure of graphene was examined by Scanning Tunnelling Microscopy (STM), Transmission Electron Microscopy (TEM) and Raman scattering methods.