Mateusz Tokarczyk

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.

Epitaxial Growth on 4H-SiC on-Axis, 0.5°, 1.25°, 2°, 4°, 8° Off-Axis Substrates – Defects Analysis and Reduction

A good selection of growth parameters (in-situ etching, C/Si ratio, growth rate) enables obtaining of ~1nm high steps of epitaxial layers, which are comparable to the size of an elementary cell (8°off-axis) and achieve the density of BPD=8•103/cm2. Due to crystallization on substrates with low misorientation (<2°off-axis) it is possible to obtain epitaxial layers substantially lacking in BPD dislocations. However, a slightly more developed surface with Ra=1-2.5nm (1.25°, 2°off-axis) characterizes these layers. By lowering the C/Si ratio, morphology of layers crystallized on substrates with low misorientation was improved. Extending growth rate improved both the crystallographic quality of the grown layers and their polytype stability. Nevertheless, growth without BPDs, also referred to as the homogeneous (4H) polytypic growth on 4H-SiC on-axis substrates, is the most efficient way of defect elimination.

New X-ray insight into oxygen intercalation in epitaxial graphene grown on 4H-SiC(0001)

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 A. Moreover, X-ray reflectometry showed the presence of an oxide layer having the thickness of about 0.8 A underneath the graphene layers. Apart from the formation of the nonuniform thin oxide layer, generation of defects in graphene caused by...

Multilayer graphene stacks grown by different methods-thickness measurements by X-ray diffraction, Raman spectroscopy and optical transmission

X-ray diffraction, Raman spectroscopy and Optical absorption estimates of the thickness of graphene multi layer stacks (number of graphene layers) are presented for three different growth techniques. The objective of this work was focused on comparison and reconciliation of the two already widely used methods for thickness estimates (Raman and Absorption) with the calibration of the X-ray method as far as Scherer constant K is concerned and X-ray based Wagner-Aqua extrapolation method.

Hydrostatic-pressure-induced changes of magnetic anisotropy in (Ga, Mn)As thin films

The impact of hydrostatic pressure on magnetic anisotropy energies in (Ga, Mn)As thin films with in-plane and out-of-plane magnetic easy axes predefined by epitaxial strain was investigated. In both types of sample we observed a clear increase in both in-plane and out-of-plane anisotropy parameters with pressure. The out-of-plane anisotropy constant is well reproduced by the mean-field p-d Zener model; however, the changes in uniaxial anisotropy are much larger than expected in the Mn-Mn dimer scenario.

Epitaxial graphene perfection vs. SiC substrate quality

Polytype instability of SiC epitaxial films was the main focus of attention in the experiment performed since this factor has a decisive influence on graphene growth, which was the second stage of the experiment. Layers deposited in various initial C/Si ratios were analyzed.Our observations indicate that the initial C/Si ratio in epitaxial growth is a crucial parameter determining which polytype will be grown, in particular for cubic (3C) or hexagonal (4H) polytypes. If the initial C/Si ratio was close to its final value, the dominant polytype was 4H. On the other hand, when the initial C/Si ratio was close to zero, 3C became the major polytype in spite of a non favourable growth temperature.The results for graphene growth on an epi-SiC layer and a bulk substrate, in which case the dominant polytype was 4H, are also presented. These results indicate that layers on epitaxial 4H-SiC are thicker, more relaxed and have better quality in comparison with samples on 4H-SiC substrates.Morphology and defects in SiC epilayers were analyzed using Nomarsky optical microscopy, scanning electron microscopy (SEM) and high resolution X-ray diffraction (XRD). Graphene quality was characterized by Raman spectroscopy.