Synthesis And Optimization Of The Silicon-Based Graphene Optoelectronic Devices

Semiconductor industry prospers as the economy boosts in these years,putting new demand on optoelectronic properties of the semiconductor,especially of silicon.However,silicon has indirect bandgap,which limits its performance in the photon-electron convertion.As a typical two-dimensional material,graphene has exotic electrical and optical properties.Besides,graphene-silicon optoelectric devices can be easily fabricated by merely transferring.Therefore,fabrication and optimization of the graphene-silicon optoelectric device have become a heated topic for study.This thesis focuses on the synthesis of silicon-based graphene,the structure and performance of graphene-silicon optoelectronic devices.In this thesis,in situ growth of silicon-based graphene has been achieved,and novel device structure has been designed.Follow-ing results will be discussed in detail in this thesis:(1)Growth of graphene quantum dots(GQDs)directly on monocrystalline silicon by chemical vapor deposition method has been achieved.Due to the catalysis of copper,methane can be decomposed by copper vapor under high temperature(about 800 degree)and deposites on silicon surface.Raman and X-ray photo-electron spectroscopy(XPS)confirm that the deposited material is graphene quantum dots without organic groups,which has luminescence feature.In the experiments,the emission peaks of graphene quantum dots are independent of the size of GQDs or the testing temperature.In the experiments,it is also found that the growth of graphene quantum dots on silicon are self-limited with this method,which may be attributed to the large contact angle of carbon tissue on silicon surface.(2)Catalyst-free growth of graphene directly on silicon with plasma-enhenced chemical vapor deposition and atomic layer deposition have been achieved.To solve to problem of lacking catalysis ability,plasma is induced to the deposition process instead of copper catalyst.On the other hand,hydroquinone treatment modifies the silicon surface,increasing the nucleation density of graphene.With modest growth time,large area graphene is eventually grown on silicon without metal catalyst.The same growing process can be realized with atomic layer deposition as well.With fine control of the growing conditions,growth process of graphene is detailed observation and analysed.J-Vcurves suggest that graphene grown on silicon forms high quality Schottky junction.(3)A novel approach for optimizing graphene-silicon photodetector with two-dimensional Pt fractal nanoparticles.High diameter-thickness ratio fractal nanoparticles are synthesized with oleic-water method.Due to the large ratio aspect,aggregation of nanoparticle is overcomed.Uniformly coverage of Pt nanoparticles can be obtatined by spincoating.With?5 nm branch width,plasmon oscillates strongly in the nanoparticles,which improves the external quantum efficiency(EQE)of graphene-silicon photodetectors to over 80%.Therefore,responsivity of the detector is greatly enhanced to 26 A W-'.Same as in the graphene-silicon solar cell,the high work function of Pt induces p-type doping to the graphene and strengthens the Schottky junction,which increases the switching speed and suppresses the noise of the detector.As a result,detectivity of graphene-silicon photodetector rises significantly to 7.5 × 1010 Jones.(4)A novel approach for optimizing graphene-silicon solar cells with Pt cubic nanoparticles.By covering Pt cubic nanoparticles of?7 nm,the photon absorption and the carrier separation of graphene-silicon solar cell can be improved simultaneously.Plasmon resonance in the metal nanoparticles significantly reduces the reflection of graphene-silicon solar cells.High work function of Pt offers stable p-type doping in the graphene and strengthens the built-in electric field of graphene-silicon junction.With these effects,open-circuit voltage(Voc),shorct-circuit current density(Jsc)and fill factor(FF)are comprehensively enhanced.In addition to these effects,continuous photon-assisted optimization of Pt cubic nanoparticles is observed.A new optimization mechanism related to electron affinity has been proposed based on this phenomenon.This novel optimization technique shows excellent stability in the experiment.