Materials Science Forum Vols. 740-742

Abstract: Growth techniques of high quality AlN and GaN bulk crystals on SiC seeds by sublimation sandwich method are presented. GaN crystals were grown in the temperature range of 1100-1250 °C and with addition of ammonia (NH₃) to prevent GaN decomposition. GaN powder or metallic Ga was used as the source. AlN crystals up to 2 inch diameter have been grown on SiC seeds in the temperature range of 1950 -2050 ⁰С. Kinetic mechanisms and transport features in the sandwich cell are discussed. The achieved high crystal quality has allowed producing semiconductor devices on their basis, in particular, ultraviolet LEDs

Abstract: Recently the wide bandgap semiconductors, silicon carbide (SiC) and aluminum nitride (AlN), have acquired increased importance due to the unique properties that make them applicable to a variety of rapidly-emerging, diverse technologies. In order to meet the challenges posed by these applications the materials need to be manufactured with the highest possible quality, both structural and chemical, at increasingly lower cost. This requirement places rather extreme constraints on the crystal growth as the simultaneous goals of high quality and low cost are generally incompatible. Refractory metal carbide technology, particularly, tantalum carbide (TaC), was originally developed for application in highly corrosive and reactive environments. The SiC group of Prof Yuri A Vodakov (for example, [1]) at Karmon Ltd in St Petersburg, Russia was the first to study and utilize the properties of refractory metal carbides, first for the growth of SiC and later for the growth of AlN. We discuss how the refractory metal carbides can answer many of the problems of growing SiC and AlN in a relatively simple and low cost manner.

Abstract: A systematic study on the density and distribution of extended defects in a typical single crystal AlN boule grown by the physical vapor transport (PVT) method has been carried out in order to gain a detailed understanding of the formation of defects such as dislocations and low angle grain boundaries (LAGBs). Boule surface studies reveal that LAGBs are nucleated during initial stages of growth and propagate to the end of growth. Basal plane dislocations (BPDs) are generated during growth due to thermal gradient stresses. Higher BPD densities are found near the LAGBs at the boule edges due to additional stresses from constrained growth. Threading edge dislocations (TEDs) are typically replicated from the seed, and LAGBs composed of arrays of threading dislocation walls are formed to accommodate the c-axis rotation between different groups of threading screw dislocation (TSD) mediated growth centers.

Abstract: AlN bulk crystals were grown by the sublimation “sandwich method” on the SiC substrates. Two types of containers were used: (i) Ta container with a surface layer of TaC created by the special annealing in contact with carbon, (ii) TaC container created by pressing of TaC powder. Cryptocrystalline AlN wafers grown by oversublimation of the original industrial high purity AlN powder were used as a vapor source. So a considerable decrease of oxygen concentration in the source (10 – 30 times) was achieved. 4H and 6H SiC bulk crystals grown by Nitride Crystals, Ltd., which were used as wafers, were crack-free, micropipe-free and have a low dislocation density (1- 4^.10³cm^-2). The method allowed to grow thick AlN bulk crystals up to 5mm height and up to two inches in diameter with smooth mirror-like surface. X-ray diffractometry and topography of the grown AlN layers show that FWHMs of the rocking curves in ω-scan lie in the range of 60-120 arcsec.

Abstract: Growth of AlN single crystals using carbon-polar surface of SiC substrate by PVT growth method has been attempted. AlN growth on the carbon-face was dominated by spiral growth mode under the applied experimental conditions and further, an abrupt interface was observed between AlN layer and the substrate. Broad XRD rocking curve of the sample, taken from bottom part of the crystal, indicates a high density of misfit dislocations near the interface and further a shift of E₂(high) phonon mode in the Raman measurements shows a significant misfit stress. The XRD-RC FWHM values of symmetric 002 and asymmetric 102 reflections (top part of the crystal) are 380 and 300 arcsec respectively, whereas the Raman E₂(high) peak FWHM value is about 23 cm^-1. Decreasing intensity of silicon and carbon LVM peaks with increasing distance from the interface represents the reduction of their incorporation along the crystal length. EPMA analysis confirms the presence of low silicon concentration of 2 wt% in these crystals grown hetero-epitaxially on SiC.

Abstract: The growth of AlGaN solid solutions on 3C-SiC(111)/Si(111) is demonstrated. The residual stress of the grown layer was investigated by high resolution X-ray diffraction (HRXRD) and infrared ellipsometry. Analysis of the HRXRD data showed that the observed lattice distortion was caused partially by hydrostatic pressure and partially by biaxial tension. The residual stress depends on the layer composition and weakly on the growth temperature.

Abstract: The dependence of the layer growth rate on a gas (argon, nitrogen) pressure inside the reactor has been examined in order to analyze the conditions of growth of AlN thick layers and bulk crystals by the sublimation sandwich-method. It is shown that the layer growth rate steadily increases as the pressure in the reactor decreases in a wide pressure interval 1–0.02 bar. This suggests that a key role in the layer growth kinetics is played by the source-to-substrate transfer of the components (Al, N), rather than the adsorption - desorption processes on the source and substrate surface. In addition the growth rate in argon atmosphere is much higher than in nitrogen one for the high pressures and is practically the same for the lowest (0.05 – 0.02 bar).

Abstract: Two dimensional maps of the electronic conductance in epitaxial graphene (EG) grown on SiC were obtained by conductive atomic force microscopy (CAFM). The correlation between morphological and electrical maps revealed the local conductance degradation in EG over the SiC substrate steps or at the junction between monolayer (1L) and bilayer (2L) graphene regions. The effect of steps strongly depends on the charge transfer phenomena between the step sidewall and graphene, whereas the resistance increase at 1L/2L junction is a purely quantum mechanical effect, due to the weak coupling between 1L and 2L electron wavefunctions.

Abstract: We have grown graphene films on 6H-SiC(0001) using propane CVD and evidenced the strong impact of the hydrogen/argon mixture used as the carrier gas on the graphene/SiC interface and on the orientation of graphene layers. By studying a set of samples grown with different hydrogen/argon mixture using Raman spectroscopy and grazing incidence X-ray diffraction, we evidence the links between graphene/SiC interface and strain in graphene.

Abstract: Few-layers graphene films (FLG) were grown by local solid phase epitaxy on a semi-insulating 6H-SiC substrate by annealing Ni films deposited on the Si and C-terminated faces of the SiC. The impact of the annealing process on the final quality of the FLG films is studied using Raman spectroscopy. X-ray photoelectron spectroscopy was used to verify the presence of graphene on the sample surface. We also demonstrate that further device fabrication steps such as dielectric deposition can be carried out without compromising the FLG films integrity.