Growth And Structure Characteristic Of Graphene On The Surface Of Seminconductor As Well As Oxide And Study Of Mn-doped SiC Diluted Magnetic Semiconductor

Graphene, comprising of a monolayer of carbon atoms packed into a two-dimensional honeycomb lattice, exhibits a series of peculiar electrical, mechanical and thermodynamic properties and will be widely applied in nano-electrical devices, single molecular devices, photoelectric devices, energy storage etc. At present, the controllable production of large area graphene layer, which could be satisfied with the device technical requirements, is still a bottleneck to hinder the development of graphene. Many production methods of graphene have been developed, such as micromechanical cleavage, chemical stripping, metal substrate epitaxy, etc. Although these methods can make the graphene with good quality, the graphene prepared by these methods need to be transferred to other insulative substrates (SiO₂, sapphire, etc.) due to their electrical property research and device application. As to the graphene prepared by annealing single crystal SiC at high temperature, its quality need to be improved and some basic physics problems need to be resolved. In this thesis, we not only in situ study the growth process of graphene on SiC(0001) by annealing SiC at high temperature in UHV, but also firstly explore to directly grow graphene films on the Si-based substrate and the sapphire substrate via depositing solid-state carbon atoms. Moreover, synchrotron radiation (SR) experimental technology and some normal analysis methods have been employed to investigate the structural characteristics of graphene films.Due to the potential application of spintronics devices, diluted magnetic semiconductor (DMS) has been absorbed much attention recently. SiC-based DMS not only has the properties of spintronics, but also can play the advantage of SiC device, which will greatly improve the limit of the device performation. In this thesis, we firstly attempt to grow Mn-doped SiC DMS thin films on Si substrates by co-deposition method with MBE, and study their structural and magnetic properties. The main results are listed as following:1. The epitaxial growth of graphene on 6H-SiC(0001)1) Synchrotron radiation photoelectron spectroscopy (SRPES) and low-energy electron diffraction (LEED) technology were both used to in-situ study the formation process of graphene on 6H-SiC(0001) by annealing SiC at high temperature in UHV. The results showed that the 6H-SiC(0001) surface appeared a series of surface reconstruction evolution from( 3×3)to ( 3×3)R30, and then to ( 6 3×6 3)R30 reconstruction at last. When the ( 6 3×6 3)R30 reconstruction appeared by annealing at 1150℃, graphene started to form on the surface. As the annealing temperature raised, the signal intensity of graphene continued to enhance, and a prominent Fermi edges appeared, which indicated that when the annealing temperature raised the thickness of graphene increased and the metallic properties of the sample strengthened. Moreover, the interface states of similar C-sp3 hybridization between graphene and SiC substrate still existed throughout the whole annealing process. It may be the reason why the electrical properties of epitaxial graphene on Si-terminated SiC are considerably influenced by the substrate.2) In MBE system, the graphene films were prepared by annealing the substrate of 6H-SiC(0001) at high temperature. RHEED, Raman, NEXAFS and AFM are used to study the effect of different annealing time on the structure and morphology of the grown grapheme films. The results revealed that graphene films could be formed under different annealing time. Due to the difference of thermal expansion coefficients between graphene and SiC substrate, there was compressive stress existing inside the epitaxial graphene. However, with the increase of annealing time, the thickness of graphene layer increased, the impact of SiC substrate on the graphene reduced, the stress inside the graphene layers weaken, the voids on graphene surface decreased and the samples were more smooth. At the same time, with the increase of annealing time, the blue-shift of Raman peaks generated by interaction between graphene layer and substrate decreased, and the intensity of characteristic X-ray absorption from graphene enhanced.2. The growth of graphene films on Si-based substrates1) Graphene films were grown on Si(111) at different substrate temperature (600, 700, 800℃) by directly depositing solid-state carbon atoms. The structural properties are characterized by RHEED, FTIR, Raman and NEXAFS. The results indicated that at low temperature the grown films only contained amorphous carbon. Only the temperature reached 800℃graphene films started to form. Meanwhile, a SiC buffer layer was also found to form at 800℃. Therefore, we thought that the substrate temperature played a key role during the formation process of graphene on Si substrates. Simultaneously, a SiC buffer layer was helpful for the formation of graphene because it could prevent reaction between subsequent deposited carbon atoms and substrate Si atoms.2) In order to study the effects of higher temperature on the graphene grown on Si substrates, a SiC layer with good quality was firstly epitaxially grown on Si(111), then the graphene films were grown on SiC/Si surface at different temperature (800, 900, 1000, 1100℃) by directly depositing carbon atoms. The results showed that the graphene could be formed at all substrate temperatures above, but the best one was at 1000℃. Moreover, we found that the structure of graphene films was not AB stacking Bernal structure, which displayed turbostratic stacking graphene structure like that of epitaxial graphene grown on C-terminated 6H-SiC.3) Graphene films were grown on SiO₂/Si substrates at different substrate temperature (500, 600, 700, 900, 1100, 1200℃) by directly depositing carbon atoms. The results showed that 700℃was the initial temperature for the formation of graphene, and 1100℃was the optimal temperature for the growth of graphene on SiO₂/Si substrate. The structure of graphene films obtained by this method also displayed turbostratic stacking graphene structure, like that of epitaxial graphene grown on C-terminated 6H-SiC. As substrate temperature increased, the quality of graphene films improved. However, if substrate temperature is too high, the partial decomposition of the oxide layer would lead to the poor quality of the grown graphene films.3. The growth of graphene films on sapphire substrate1) In SSMBE system, a SiC layer was at first epitaxially grown on sapphire surface, and then the graphene films were obtained by annealing the sample at high temperature. The structural properties of the sample were characterized by RHEED, X-ray Phi scan, Raman and NEXAFS. The results showed that the sample had a sandwich structure of graphene/SiC/sapphire. Moreover, the graphene films displayed Bernal AB stacking structure, which like that of graphene obtained by annealing Si-terminated 6H-SiC.2) Graphene films were grown on sapphire substrates at different substrate temperature by directly depositing solid-state carbon atoms and the influence of substrate temperature on the structure of grown graphene films was investigated. The results indicated that the substrate temperature played a key role during the formation of graphene grown on sapphir substrates. The graphene started to form at 700℃, and had the best quality at 1300℃. In addition, the graphene films displayed turbostratic stacking structure, like that of graphene obtained by annealing C-terminated 6H-SiC. When the substrate temperature was too high, due to the partial decomposition of Al2O3 on substrate surface, the deposited carbon atoms easily reacted with dissolved oxygen to hinder the formation of graphene.4. The growth, structure and magnetic properties of Mn doped SiC DMS films1) By using MBE co-deposited method, Mn-doped 3C-SiC thin films were grown at different substrate temperature (850, 900, 950℃) on Si(111). The structural properties of the samples are characterized by RHEED, XRD and XANES. The results indicated that as the substrate temperature increased, the quality of SiC thin films improved, and the SiC lattice was not obviously affected by Mn doping. Moreover, for all samples with different substrate temperature, the Mn atoms just existed as the phase of Mn4Si7 and no substitutional or interstitial Mn atoms, or Mn cluster were found in the SiC lattice.2) Mn-doped SiC films with different Mn doping concentration (0.5%, 18%) were grown at 950℃on Si(111) by co-deposition method. The sample with Mn doping concentration of 18% displayed room temperature ferromagnetic property. Its Curie temperature could reach 355 K, and the saturation magnetization of Mn was estimated around 1.93μB/Mn at 5 K. RHEED, XRD, XANES and XPS were used to characterize the structural properties of the samples. The results revealed that for all samples with different Mn doping concentrations, Mn atoms existed as the phase of Mn4Si7 and no substitutional or interstitial Mn atoms were found. We thought that in the sample with the Mn doping concentration of 18%, there was a small amount of carbon atoms incorporating into the Mn4Si7 host, which improved the ferromagnetic coupling of Mn-Mn atoms in Mn4Si7, and induced the room temperature ferromagnetic behavior of the sample.