Enhanced Infrared Absorption And Photodetectionresponse Of Graphene

Due to its special position of infrared light in the electromagnetic spectrum,infrared detection has been an important research topic in the field of infrared technologies,which has broad application prospects in night vision imaging,non-destructive testing,disease diagnosis,military surveillance,industrial process monitoring and so on.The development of infrared detection technology depends on the innovativations of basic detection materials.As a new emerging material in recent years,graphene exhibits a number of unique optoelectronic properties such as high carrier mobility,wide spectral response,tunable electrostatic doping as arised from its unique Dirac cone band structure,which opens opportunities for exploring new properties and functionalities of infrared detection devices.To address the problems of weak light absorption and limited photodetection performance of graphene,this dissertation investigates enhanced infrared absorption of graphene in cavities and photoresponse of graphene-Si heterojunction.A theoretical model for simulating infrared properties of graphene was first developed,and then a graphene Salisbury screen and a graphene-silicon heterojunction device were designed and fabricated.An infrared light absorption of 40%was achieved for monolayer graphene.A photoresponse as originated from graphene itself was also measured at 1550 nm wavelength.These research findings are helpful for understanding the fundamental interaction between graphene and light,and are beneficial for the development of graphene-based infrared photodetection devices.The main research results obtained in the dissertation include:1.A transfer matrix model for graphene multilayered structure was established,and a 100%perfect absorption at infrared region was predicted in theory.As limited by its single atom layer thickness,graphene exhibits a week light absorption of 2.3%over a broad spectral band,which limits the efficiency and performance of graphene-based optoelectronic devices.Based on the description of graphene with a two-dimensional conductivity,a transfer matrix model of graphene multilayered structure was developed,and a new approach for enhancing light absorption of graphene in thin-film structures was proposed.Our simulation results showed that in a single-layer graphene Salisbury screen,the absorption of graphene increased from 2.3%to 10%under normal incidence,and a 100%perfect absorption was achievable under TE polarization and near grazing incidence angle.This perfect light absorption occurred when the effective impedance of graphene multilayer structure matches that of free-space,i.e.377Ω.In addition,in multilayer graphene structure,its infrared absorption was proportional to the number of graphene layers,and exhibited near-perfect absorption of 80%-100%within the whole range of incidence angles.This theoretical model and simulation results revealed resonance-enhanced perfect absorption of graphene in the infrared region,which laid a foundation for the experimental verification carried out in this dissertation.2.A graphene Salisbury screen structure was designed and fabricated,which exhibited an enhanced infrared absorption of 40%for monolayer graphene.Based on the transfer matrix model for graphene multilayered structure and using thin film deposition technology,a graphene Salisbury screen sample was designed and fabricated,which consisted of three layers of 300 nm copper,2μm SiO₂ and monolayer graphene.Infrared spectroscopy measurements showed that the single-layer graphene Salisbury screen had an absorption value of 9%at wavelength of 1.8μm under normal incidence and an absorption value of 40%under TE polarization and near-grazing incidence angle.Further material characterizations and theoretical analysis indicated that the CVD-grown graphene fabricated in the sample had a carrier mobility of about 900cm²V^-1s^-1,which is more than one order of magnitude smaller compared with the theoretical ideal value.This limited carrier mobility causes the deviation of measured graphene absorption from the ideal perfect total absorption.Our demonstrated graphene Salisbury screen structure overcomes the limitation of graphene’s weak light absorption by increasing its absorption from intrinsic value 2.3%to 40%.This absorption enhancement suggests a great potential for improving the efficiency of graphene-based optoelectronic devices.3.A graphene-silicon heterojunction was fabricated,which revealed an infrared photoresponse originated from the graphene itself.Graphene has been widely reported to exhibit excellent photodetection properties in both photoconductive and photovoltaic devices.However,their main photoresponse originated from other sensitive materials hybrided with graphene,rather than graphene itself.This dissertation examined infrared photoresponse of graphene itself in a heterojunction structure.A graphene-silicon heterojunction structure was fabricated.Photocurrent spectroscopy measurements showed a photovoltaic response peak at 980 nm from silicon,and a second photoresponse band at 1550 nm form graphene.The detectivity of graphene photoresponse was measured to be 1.3×10⁸ cmHz^1/2W^-1.Further analysis of the detection mechanism based on the band structure indicated that the photoresponse of graphene originated from in-direction electronic transition assisted by phonons or defects instead of the regular interband transition without momentum change.These research findings revealed the mechanism of photoresponse for carriers in graphene as well as the great potential of graphene for infrared detection.