G. Kowalski

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.

A study of defects generated in Czochralski-grown Si during two-step annealing

A variety of oxygen-related micro-defects (spherical and octahedral precipitates, their agglomerations, dislocations and dislocation loops) with dimensions in the range 0.1-100 mu m were revealed using X-ray topography and transmission electron microscopy after two-step annealing of Czochralski-grown silicon. The defects are distributed in a non-uniform way across the samples with the gradient of their density normal to the crystal surface.

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.

Lattice relaxation and metastability of the EL2 defect in semi-insulating GaAs and low temperature GaAs

Using the two-dimensional reciprocal space mapping, we have clearly established the relative positions of the arsenic atom (EL2 defect) corresponding to its stable and metastable configurations. It is shown that lowering only the temperature of the sample, before transition, induces transformation of the lattice. Characteristic temperature (120–140 K) below which such transformation takes place is the same as for thermal recovery of the defect. It is suggested that this temperature related lattice relaxation might promote a further transition to the metastable state. Full reciprocal space maps of the x-ray diffracted intensity are shown for the whole substrate-layer system.

On the role of nitrogen in stiffening the diamond structure

Three large (4–7 mm) natural diamonds, each half brown and half white, from the Argyle Diamond Mine in Western Australia, have been studied for growth history and crystal perfection. Scanning electron microscopy and X-ray single-crystal topography showed the diamonds to have good octahedral morphologies but poor internal perfection. X-ray double-crystal topography quantified this lack of perfection, rocking-curve widths taking unusually large values of 300 and 75 s of arc for the brown and white regions, respectively, in a diffraction geometry for which 3 s of arc would be expected for a perfect diamond. Fourier transform infrared spectroscopy revealed significant differences in nitrogen concentration between the brown and white regions of the diamonds. The white regions, with 400 to 600 p.p.m. nitrogen, contained over 1.6 times more nitrogen than the brown regions. It is concluded that the extra nitrogen (in A and B forms) in the white regions stiffens the lattice against distortion by natural plastic deformation.

Reciprocal-space mapping of synthetic and natural diamond

High-resolution reciprocal-space maps of synthetic and natural diamonds have been recorded for the first time on the new six-circle diffractometer at the Synchrotron Radiation Source of the Daresbury Laboratory, UK. Variations in lattice spacing (typically 10 ppm) have been measured within specific growth sectors of synthetic diamonds as well as lattice tilts (typically 1 arcmin). Similar measurements have been made on diamonds containing strain-producing metallic inclusions. Platelet radii in a natural diamond have also been estimated, giving results (such as 140 A) agreeing with measurements from other techniques.

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.

Mn impurity lattice location in the ferromagnetic zincblende gallium manganese arsenide layer structure

Very weak Bragg reflections have already been used successfully to investigate dopant positions in the structure of the host lattice. The intensity of the x-ray diffracted beam is a function of the structure factor, F, which in turn is modified by the presence of the additional dopant atoms in the lattice. That change can be measured by carefully analysing the integrated intensity of the relevant weak reflections. New materials like low temperature GaMnAs layers are the subject of recent vigorous investigations into their various properties. A theoretical model based on detailed calculations of the structure factor is a way of answering the basic question about the lattice arrangements of the Mn atoms in the host lattice of the GaAs layer. Understanding the structural properties of these materials is essential, and investigations into the local arrangements around the dopant atoms are vital. One can do this through measurements of the quasi-forbidden (very weak) x-ray reflections. Our evaluation clearly indicates the need for the highest possible temperature control and stability of the growth process of such materials.