Papers by Author: Wlodek Strupiński

Abstract: 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.

Abstract: Transmission Electron Microscopy (TEM) investigations of graphene layers synthesized on Si and C-terminated on-axis oriented 4H-SiC are presented. The high-resolution TEM (HRTEM) revealed distinctive distance differences between the first carbon graphene layer and SiC surface for both polarities. The prolonged annealing of SiC with carbon face shows, that in addition to the increase of number of graphene layers, there is also observed splitting between stack of graphene layers and the surface of SiC substrate. In addition, the density of so called “puckers” increases.

Abstract: 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.

Abstract: Optical transmission and transmission electron microscopy studies of epitaxial graphene structures grown on the carbon terminated face of 4H-SiC(000-1) on-axis substrates are presented. Several samples obtained using different growth conditions were studied. Optical microscope showed regions of micrometer size with different layer number. The exact number of layers was obtained from transmission electron microscope studies. Optical transmission spectra showed no wavelength dependence and allowed us to obtain the average number of graphene layers.

Abstract: The electron backscatter diffraction (EBSD) detector placed inside a commercial scanning electron microscope (SEM) has been used to study of different SiC polytypes. Different growth conditions in chemical vapor deposition (CVD) method were applied to obtain the 3C- and 4H-SiC polytypes epitaxial layers. Growth processes were conducted on the Si-face on-axis 4H-SiC substrates. The growth temperature was in the range of 1300-1620°C and the reactor pressure was 75mbar. The initial C/Si ratio was varied from 0.075 reaching final value of 1.8. It was observed that intentional ramping of the C/Si ratio at the first stage of the growth clearly influences the 4H/3C factor. The growth temperature and ramping of the C/Si ratio were the main parameters to achieve a homogeneous 3C and 4H-SiC epitaxial layers.

Abstract: The influence of surface preparation of 4H-SiC substrates on structural properties of GaN grown by low pressure metalorganic vapour phase epitaxy was studied. Substrate etching has an impact on the crystallographic structure of epilayers and improves its crystal quality. The GaN layers were characterized by RBS/channelling and HRXRD measurements. It was observed that on-axis 4H-SiC is most suitable for GaN epitaxy and that substrate etching improves the crystal quality of epilayer.

Abstract: Crystallographic quality of the epitaxial layers depends on the process temperature, partial pressures of active components and the surface polarity and also on the crystallographic quality of the subsurface layer resulting from the preparation of the substrate. The polishing etching in hydrogen-propane atmosphere of 4H-SiC substrate of different orientations and polarity was studied. The optimization of the polishing etching has been achieved with respect to the flow of C3H8, the duration and the temperature of the process. The investigation of the surface of SiC substrate before and after in situ polishing-etching in H2+C3H8 atmosphere was carried out by Nomarski interference contrast microscopy (DIC) and atomic force microscope (AFM).

Abstract: Transmission Electron Microscopy (TEM) investigations of graphene layers on Si terminated 4H-SiC(0001) are presented. The graphene layers have been grown in a standard method using decomposition of silicon carbide. Two kind of graphene layers have been investigated: 1) grown on substrates with on-axis orientation, 2) grown on substrates with 4° and 8° off-axis orientation in respect of c-axis of SiC. In the case of 0° orientation the high resolution TEM micrographs revealed that a thin layer graphene is present: 1-3 monolayers were obtained. It was found that the first carbon layer was about 2Å from the SiC surface. This result indicates that a strong covalent bonds between carbon layer and silicon atoms on the SiC surface exist. The subsequent graphene layers have been found spaced by 3.4 Å - similar as in the graphite. That indicates a weak van der Waals bonding between subsequent carbon layers. In the case of 4° and 8° off-axis orientation a thicker layer of about 5-6 monolayers of graphene were obtained. Relative spacings of graphene layers were the same as in the case of on-axis orientation.

Abstract: 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.

Abstract: The influence of in situ etching of Si-face n-4H-SiC wafers in H2 and propane on the surface morphology of the grown epi-layers were examined using differential interference contrast (DIC) optical microscopy and atomic force microscope (AFM). Two defect-selective etching techniques were applied in order to reveal the type and spatial distribution of defects in the substrates and epi-layers. It was found that for the flow applied in this experiment propane plays a significant role for the etching process. Depending on temperature and etching time we obtained completely different picture of substrate surface morphology. The propane etching was verified as a tool for substrate surface improvement.