Investigation On Low Temperature Synthesis Assisted By Plasma And Doping Of Graphene Films

Graphene is a new substance composed of sp2-bonded carbon atoms which arrangedin a regular hexagonal pattern, it is a two-dimensional film with thickness of onlyone-atom. Due to its unique crystal structure, graphene owns extraordinary mechanical,optical, thermal and electrical properties, for example, graphene is the hardestnano-material in the word; its unique optical property produces an unexpectedly hightransparency with absorbs2.3%of white light; the near-room temperature thermalconductivity of graphene was measured to be5300W/m·K; experimental results show thatgraphene has a remarkably high electron mobility at room temperature, with valuesof15000cm~2/V·s, while the resistivity of the graphene is only106Ω·cm, which is thelowest resistivity measured at the room temperature until today. Therefore, grapheneappears to be a very promising material for future applications. However, synthesisgraphene films with large size and high quality is a tough task. And to realize theapplications in a wide range, it is very important to precisely control the properties ofgraphene films. So the synthesis and properties modification of graphene attract more andmore interest.To prepare high quality graphene films, various methods have been developed,including thermal decomposition of silicon carbide (SiC) and chemical vapor deposition(CVD). However, the extremly high temperature becomes technical shortcomings of thosemethods. Therefore, it is necessary to develop methods to produce graphene films at lowtemperature. Low temperature plasma is a very useful technique, which can be used formaterial synthesis and etching, becomes a possible tool for synthesizing graphene films. Asa new plasma source, dual-frequency capacitively coupled plasma (DF-CCP) reactor driven by two generator, with one radio frequency (RF) is chosen to be much higher thanthe other, could achieve an independent control of flux and energy of ions onto thesubstrate. Therefore, DF-CCP could be a potential tool for synthesizing graphene films.Ion beam is a group of ions which produced from a plasma source, move towardsalmost the same direction at approximately the same speed. Recently, ion beam becomes astandard doping method in semiconductor manufacturing. Modify properties of graphenewith ion beam doping is of great research and application values.In this thesis, graphene films were synthesized with dual-frequency capacitivelycoupled plasma at low temperature, and its physical and chemical properties wereinvestigated. Meanwhile, mono-layer graphene films were doped by low energy ion beamirradiation, and its chemical compositions, crystal structure and surface morphology werealso studied.(1) Graphene films were deposited on quartz glass substrates without using anymetallic catalyst at low temperature by dual-frequency capacitively coupled plasma(DF-CCP) enhanced chemical vapor (CVD) technique and low temperature annealing.Effects of HF and LF power, growth time and annealing temperature on layers and crystalqualities of graphene films were studied. The results indicate that graphene films can beobtained only at the HF and LF power at165W and35W, respectively. The short growthtime is very important to obtain few-layer graphene films. The higher annealingtemperature can reduce the film damages and improve the crystal structure of graphenefilms.(2) With the C4F8plasma pre-etching silicon carbide (SiC), a method to growfew-layers graphene films at lower temperature (950C) by thermal decomposition of SiCwas developed. Because the roughness and chemical compostions of surface have greatinfluence on graphene films, th effects of etching characteristics of6H-SiC in C4F8/Arplasma was studied. It is found that at the lower LF power, the surface roughness of etchedSiC increased very little and less CxFyresidues lefted on the substrates. Compared withother plasma dry etching methods, DF-CCP could effectively inhibit CxFyfilms deposition, which is very important for the subsequent graphene films growth. Meanwhile, it alsoprovides a technique which could be used as a good method of etching SiC.According to the results of C4F8plasma pre-etching, proper HF and LF power werechosed for pre-etching the SiC, and then the samples were annealed for graphene growth.The results demonstrate that3-5layers graphene films can form on SiC substrate at lowannealing temperature. Annealed at Ar atmosphere could improve the quality of graphenefilms. Although there are few fluorocarbon films left on the SiC substrates, it has veryweak influence on the graphene films.(3) Effects of low energy ion beam irradiation on mono-layer graphene were studied.Mono-layer graphene samples were irradiated with B, N and F ions at different energy andfluence. X-ray photoelectron spectroscopy indicates that foreign ions can dope intographene and form new chemical bonds with carbon atoms even at very low ion energy.The results of Raman measurement indicate that ion beam irradiation causes very weakdefects and disorder to graphene crystal structure compares with high energy ionirradiation. According to the simulation results, the doping rate increases with the increaseof ion mass but decreases with the increase of ion energy.