Materials Science Forum Vols. 778-780

Abstract: The carrier velocity is measured as a function of electric field in as-grown and H-intercalaed epitaxial graphene grown on semi-insulating 4H-SiC in order to estimate the low field carrier mobility as a function of temperature. The mobility is also measured on the same samples as a function of temperature in a liquid Helium (He) cooled cryostat. The two temperature dependent measurements are compared in order to deduce the dominant carrier scattering mechanisms in both materials. In as-grown material, acoustic phonon scattering and impurity scattering both contribute, while impurity scattering dominates in H-intercalated material.

Abstract: The energetics for the Si desorption and the C adsorption at a [11-20] step on SiC(0001) surface are studied using the first-principles calculation. It is found that the [11-20] step is stable and nonreactive. The stability of the step is thought to govern the surface morphology during the graphene formation. It is shown that the Si pressure and the temperature are the control parameters for the surface morphology and the graphene quality.

Abstract: Multilayer epitaxial graphene has been grown on the Si-face of 6H-SiC on-axis commercial substrates under high vacuum conditions and at growth temperatures up to 1900 °C, utilizing the standard sublimation growth technique and a modified SiC rapid thermal annealing system which allows for excellent control of heating and cooling ramp rates. The peak growth temperature and total growth time during the graphene growth step, along with the temperature of the initial substrate etch step, were all systematically varied in order to ascertain their effect on the formation of epitaxial graphene films on the SiC surface. Modifying the substrate etch temperature was found to have a significant impact on the morphology of the SiC substrate, with a uniform step structure only developing across the surface within a narrow temperature band. Furthermore, changing the values of the peak temperature or the growth time during the growth step were both shown to have a large effect on the resultant materials properties of the graphene films.

Abstract: SiC crystal is a wide band gap material of high hardness and chemical inertness. Graphene is nowadays a ubiquitous 2D material that would revolutionize many applications. Combining the characteristics of SiC and graphene higher performance and efficiency are expected, e.g. for high frequency electronic devices. The obtaining of graphene directly on SiC substrates by a single step thermal decomposition process is promising, but optimal standardized conditions are not established. We present the use of chemical-mechanical polishing (CMP) as a pre-graphene growth SiC conditioning to enable deep comprehension of the mechanisms of SiC decomposition and control towards selective formation of graphene.

Abstract: This work presents experimental evidence of the formation mechanisms of few-layer graphene (FLG) films on SiC by nickel silicidation. FLG is formed by annealing of a 40 nm thick Ni layer on 6H-SiC at 1035^ºC for 60 s, resulting in a Ni₂Si layer which may be capped by any Ni that did not react during annealing. It has been proposed that FLG forms on top of the Ni during the high temperature stage. In contrast, during cooling, carbon atoms which were released during the silicidation reaction may diffuse back towards the Ni₂Si/SiC interface to form a second FLG film. After annealing, layer-by-layer de-processing was carried out in order to unequivocally identify the FLG at each location using Atomic force microscopy (AFM) and Raman spectroscopy.

Abstract: Raman spectroscopy is commonly applied for studying the properties of epitaxial graphene on silicon carbide (SiC). In principle, the Raman intensity of a single graphene layer is rather low compared to the signal of SiC. In this work we follow an approach to improve the Raman intensity of epitaxial graphene on SiC by recording Raman spectra in a top-down geometry, i.e. a geometry in which the graphene layer is probed with the excitation through the SiC substrate [1]. This technique takes advantage of the fact, that most of the Raman scattered light of the graphene is emitted into the SiC substrate. We analyze in detail the top-down measurement geometry regarding the graphene and SiC Raman intensity, as well as the influence of aberration effects caused by the refraction at the air/SiC interface.

Abstract: We investigated the effect of ion-beam irradiation of the 3C-SiC(111) surface on the growth of graphene by the SiC surface-decomposition method. When a 3C-SiC(111) surface was irradiated by 1 keV Ar⁺ ions at a dose of 4.5 × 10¹⁵ cm² in an ultra-high-vacuum chamber and then annealed at 1200 °C for 1 min, the formation of graphene layers was promoted in comparison with that in the absence of ion-beam irradiation. X-ray photoelectron spectroscopy studies showed that Ar ion bombardment of the 3C-SiC(111) caused breakage of surface bonds and helped Si atoms to desorb from the surface.

Abstract: With power conversion losses endemic in all areas of electricity consumption, broadlycategorized into motion control (accounting for around 50% of total electrical energy use), lighting,air conditioning, and information technology, consumers, governments and utilities are finding waysto achieve higher efficiency. Manufacturers of data servers, telecom systems, solar power invertersand drives for motor control are focused on reducing power conversion losses while simultaneouslyshrinking the size of power systems. Although silicon has historically been the base device materialused by the power conversion industry, it is rapidly reaching its physical performance limits. GaNsemiconductors solutions reduce power conversion loss by over 50% in a significantly smaller formfactor and at a lower cost, when device design, fabrication technology and application design areholistically combined to deliver superior end products.

Abstract: We present temperature dependent magnetoresistance measurements on the 2-dimensional electron gas of epitaxially grown AlGaN/GaN heterojunctions on silicon (Si). We report on the quantum correction to the classical conductance. In particular we found weak localization, electron-electron-interaction, and Shubnikov-de Haas oscillations. The results verify the high material quality of the investigated GaN on silicon.

Abstract: This work reports on the characterization of the decomposition of GaN layers epitaxied on c-plane Sapphire substrate by Differential Scanning Calorimetry. Many configurations have been characterized from two different GaN epilayer providers with a large range of doping concentrations from Non-intentionally Doped layers up to 2x10¹⁹ cm^-3. All intentionally doped layers exhibit an endothermic reaction starting at 200-300 °C while the NiD layer thermogram is the same as the blank experiment. XPS and SEM observations demonstrated that the endothermic reaction is related to the GaN decomposition through Threading Dislocation and nanoPipe.