Yan Wen-Sheng

Growth of Few-Layer Graphene on Sapphire Substrates by Directly Depositing Carbon Atoms

Few-layer graphene (FLG) is successfully grown on sapphire substrates by directly depositing carbon atoms at the substrate temperature of 1300°C in a molecular beam epitaxy chamber. The reflection high energy diffraction, Raman spectroscopy and near-edge x-ray absorption fine structure are used to characterize the sample, which confirm the formation of graphene layers. The mean domain size of FLG is around 29.2 nm and the layer number is about 2–3. The results demonstrate that the grown FLG displays a turbostratic stacking structure similar to that of the FLG produced by annealing C-terminated α-SiC surface.

Graphene films grown on sapphire substrates via solid source molecular beam epitaxy

A method for growing graphene on a sapphire substrate by depositing an SiC buffer layer and then annealing at high temperature in solid source molecular beam epitaxy (SSMBE) equipment was presented. The structural and electronic properties of the samples were characterized by reflection high energy diffraction (RHEED), X-ray diffraction Φ scans, Raman spectroscopy, and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The results of the RHEED and Φ scan, as well as the Raman spectra, showed that an epitaxial hexagonal α-SiC layer was grown on the sapphire substrate. The results of the Raman and NEXAFS spectra revealed that the graphene films with the AB Bernal stacking structure were formed on the sapphire substrate after annealing. The layer number of the graphene was between four and five, and the thickness of the unreacted SiC layer was about 1–1.5 nm.

Defects mediated ferromagnetism in a V-doped 6H?SiC single crystal

Undoped and V-doped 6H—SiC single crystals have been grown by the physical vapor transport method. The V concentration is determined to be 3.76 × 1017 at/cm3 and 6.14 × 1017 at/cm3 by secondary ion mass spectrometry for low V-doped and high V-doped SiC samples, respectively. The undoped 6H—SiC shows diamagnetism, while the V-doped 6H—SiC exhibits weak ferromagnetism. The lower V-doped sample shows stronger ferromagnetism compared to that of the higher V-doped sample. However, the structural characterization indicates that the lower V-doped SiC has a relative poor crystalline quality. It is found that both V dopants and defects are essential for introducing ferromagnetic exchange in V-doped SiC single crystals.

Formation mechanism of Ge nanocrystals embedded in SiO2 studied by fluorescence x-ray absorption fine structure

This paper reports that the Ge nanocrystals embedded in SiO2 matrix are grown on Si(100) and quartz–glass substrates, and the formation mechanism is systematically studied by using fluorescence x-ray absorption fine structure (XAFS). It is found that the formation of Ge nanocrystals strongly depends on the properties of substrate materials. In the as-prepared samples with Ge molar content of 60%, Ge atoms exist in amorphous Ge (about 36%) and GeO2 (about 24%) phases. At the annealing temperature of 1073 K, on the quartz–glass substrate Ge nanocrystals are generated from crystallization of amorphous Ge, rather than from the direct decomposition of GeO2 in the as-deposited sample. However, on the Si(100) substrate, the Ge nanocrystals are generated partly from crystallization of amorphous Ge, and partly from GeO2 phases through the permutation reaction with Si substrate. Quantitative analysis reveals that about 10% of GeO2 in the as-prepared sample are permuted with Si wafer to form Ge nanocrystals.