R. Bożek

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.

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.

Structure and magnetism of MnAs nanocrystals embedded in GaAs as a function of post-growth annealing temperature

Self-organized Ga(Mn)As nanoclusters, embedded in GaAs, were formed during post-growth thermal annealing of Ga1−xMnxAs layers. Structural and magnetic properties of such composites were systematically studied as a function of the annealing temperature. Small (∼3 nm) Mn-rich zinc-blende Mn(Ga)As clusters, coherent with the GaAs matrix, were formed at the annealing temperature of 500 °C. An increase of the annealing temperature of up to 600 °C led to the creation of 10–20 nm large NiAs-type hexagonal MnAs nanocrystals. Magnetization measurements showed that the MnAs nanoprecipitates were superparamagnetic, with a distribution of blocking temperatures that depended on the MnAs cluster size. Some intermediate paramagnetic clusters (structurally disordered clusters) were also observed.

Different paths to tunability in III–V quantum dots

Tunability in the concentration and average dimensions of self-forming semiconductor quantum dots (QDs) has been attained. Three of the approaches examined here are: variations with temperature, group V partial pressure and with substrate miscut angle. Thermally activated group III adatom mobilities result in larger diameters and lower concentrations with increasing deposition temperatures. These variations are presented for InGaAs/GaAs and AlInAs/AlGaAs, where striking differences were seen. Tunability in the InGaAs/GaAs QD concentration was also obtained in metalorganic chemical vapor deposition by varying the arsine flow. The latter gave widely varying concentrations and similar sizes. Substrate orientation was found to also be a key factor in island nucleation: Changes in vicinal orientation near (100) can be used to exploit the preferential step edge nucleation at mono and multi-atomic steps, so varying miscut angle (θm) can be used to change island densities and sizes. Anisotropies in island nucleat...

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.

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.

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.

Growth Rate and Thickness Uniformity of Epitaxial Graphene

The paper provides a deeper understanding of key-parameters of epitaxial graphene growth techniques on SiC. At 16000C, the graphene layer is continuous and covers a large area of the substrate. Significant differences in the growth rate could be observed for different reactor pressures and the polarity of SiC substrates as well as for the substrate miscut and surface quality. In addition, graphene thickness uniformity and mechanism of ridges creation was examined.

Light-induced narrowing of excitonic absorption lines in GaN

The fundamental absorption edge of GaN has been investigated in GaN/AlxGa1−xN heterostructures. A broad excitonic line is observed after the sample has been cooled in darkness. A metastable narrowing occurs and three excitonic absorption lines are observed after the sample has been illuminated. Measurements of this effect have been made as a function of temperature and photon energy. It seems that the broadening is induced by an electric field present around dislocations in GaN. This field is reduced by free carriers created during illumination, which results in narrowing of the excitonic lines.

Optical determination of the dopant concentration in the δ-doping layer

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.