Study On Optoelectronic Devices Based On Graphene-silicon Heterojunction

With the rapid development of information technology,people’s demand for information transmission bandwidth,processing speed,and data capacity is increasing.In order to meet the needs of the development of information technology,people have been committed to promoting the miniaturization of chips and reducing the size of transistors in the microelectronic industry,to achieve higher integration in recent years.According to Moore’s law,the integration of microelectronic technology can be doubling every two years,which is close to the physical limit of integrated circuits.In order to break through the size limitation of silicon integrated circuits,silicon-based optoelectronic technology emerges.Then,when people were focusing on the research of silicon-based photonic devices,graphene had set off a new research upsurge.The excellent optical,electronic,and thermal properties of graphene are expected to inject new vitality and power into many research fields and bring a new dawn for the development of silicon photonics.Taking advantage of graphene’s single-atomic-layer structure,graphene can be combined with various silicon photonic structures through van der Waals force,and forming graphene-silicon heterojunction structure.And,it has various excellent properties such as controllable energy band structure,high nonlinear coefficient,and ultra-fast carrier mobility,which bring more possibilities for the development of integrated optoelectronics.In this paper,we will focus on the theoretical and experimental study of optoelectronic structures based on graphenesilicon heterojunction.The main research contents are summarized as follows:(1)The characteristics and advantages of graphene-silicon heterostructure were analyzed,and its application background and significance in the field of integrated optoelectronic devices were described.Starting from the discovery and properties of graphene,the wide application of graphene-silicon hybrid structure in optoelectronic devices and the importance of multi-functional integration of optoelectronic devices were discussed.The development status of graphene-silicon heterojunction photodetectors and modulators was emphatically analyzed.In order to improve the light detection response and light modulation efficiency,the characteristics and advantages of various graphene-silicon hybrid structures were reviewed.At the same time,we analyzed the advantages and potential of photonic crystal waveguides and microcavities used in multifunctional optoelectronic devices and summarized their applications.(2)Based on the basic theoretical analysis model of graphene,combined with Poisson equation and free carrier drift-diffusion equation,current equation and heat conduction equation,the optical-thermal-electrical characteristics of graphene-silicon heterostructure were numerically calculated and analyzed,and the photoelectric detection and optical modulation functions were simulated.Then the fabrication process of graphene-silicon heterojunction optoelectronic devices was introduced,including the preparation of graphene,the fabrication of silicon-based optoelectronic structures,and the wet transfer of graphene.(3)A carbon quantum dots coupled graphene-silicon heterojunction photodetector was proposed.Carbon quantum dots were prepared on the surface of graphene-silicon heterojunction by carbon evaporation.The concentration and distribution of carbon quantum dots was confirmed by the test of scanning electron microscopy and energy dispersive spectrometer.Then,the photoelectric characteristics of the device were tested,and the energy band structure of the device was analyzed.It was found that when the carbon quantum dots were coupled to the graphene-silicon heterojunction,the electrons transferred from graphene to carbon quantum dots,resulting in the decrease of graphene’ Fermi level and the increase of the Schottky barrier.Thus,the built-in electric field became stronger,which was conducive to the separation of electron-hole pairs.At the same time,the carbon quantum dots on the surface of the sample enhanced the scattering of the incident light and enhanced the light absorption of the device.Therefore,the light responsivity of the detector was improved.(4)The graphene-silicon photonic crystal waveguide thermo-optic switch based on bandedge tuning was studied experimentally and numerically.With the heating power increasing,the bandedge of photonic crystal waveguides can redshift continuously.Theoretically,the optical bandwidth is not limited.In the experiment,～20 nm tuning bandwidth with only 28 K temperature increase was realized.By combining the slow light effect of photonic crystal waveguide with graphene nanosheets,the extinction ratio and modulation depth were significantly improved,and the maximum extinction ratio could reach 22 d B.Graphene nanosheets are used as thermo-electrodes to generate and transmit heat power,which can reduce heat loss and improve thermal tuning efficiency.At the same time,we also carried out temperature test and thermal and optical simulation of the device,analyzed the influence of thermal power on the refractive index and deformation of the structure in detail,and the result proved that both the thermo-optic effect of the material and the overall thermal deformation of the structure lead to the redshift of the transmission spectrum,so the device can have better thermal tuning efficiency.(5)A multifunctional device based on graphene/p-i-n silicon photonic crystal microcavity hybrid structure was proposed,which was featuring triple light detection,modulation,and bistable switching.Through depositing single-layer graphene onto the Ph C cavity,the interaction between light and material can be enhanced greatly,and significant light detection and light modulation at the resonant wavelength can be realized.Based on this structure,we achieve a photocurrent responsivity of ～ 14 m A/W at 1530.8 nm(cavity resonant wavelength),which is an order of magnitude higher than that at the off-resonant wavelength.At the same time,based on the thermo-optic effect of silicon and graphene,the electro-optic and all-optical modulation functions of the device are effectively realized.In addition,the photonic crystal side-coupled cavity structure has a high quality-factor,thus,the device can achieve low threshold optical bistability and fast response speed.The rise time and decay time of the all-optical bistable switch are about 0.49 μs and 0.56 μs.The rise time and decay time of electrooptic hybrid bistable switch are about 0.52 μs and 0.50 μs.The multi-functionality of a single device can greatly reduce the photonic overhead and provide potential applications for future integrated optoelectronics.