Investigations On Graphene-based Passive Devices

Radio frequency(RF)inductor,resonator and other passive devices are the key components of microwave circuits and systems.With the rapid development of microwave circuits and systems towards the direction of miniaturization,low power consumption,and highly integrated and intelligent within the millimeter-wave and terahertz frequency band,passive devices based on traditional electromagnetic materials are facing many challenges,including conductivity decrease and device performance degradation due to the emerging nano-effects and even quantum effects,huge power dissipation caused by the increasingly severe skin effects with frequency,thermal and reliability problems resulted from the increasing current carrying density,and the traditional tuning strategies limited by high-frequency loss,low operating frequency and slow response time.On the other hand,frequency selective surfaces(FSSs)are planar periodic arrays that function as spatial filtering elements for incident waves with differernt incident angles and polarizations.FSSs,usually as passive structures,find widely applications in modulators,abosorbers,polarizers,and radar radomes.However,traditional electromagnetic material based FSSs are often with bulky units,and face the similar challenges to the aforementioned issues of passive devices when applying for millimeter-wave,terahertz and tunable situations.Therefore,it is very important to explore novel electromagnetic materials combined with advanced technology to realize the above-mentioned passive devices,such as inductors,resonators and FSSs,that meet the demands of microwave circuits and systems.Graphene has excellent electromagnetic,thermal and mechanical properties,including high carrier mobility,high electrical conductivity with large tuning range,long mean free path,ultra-low electrical noise,high thermal conductivity,light and excellent flexibility,etc.It is an ideal material to realize RF passives.In this dissertation,the application of graphene in these passive devices is systematically investigated by theoretical and experimental approaches,starting from the material characteristics of graphene and the operation mechanism and design method of passive devices.The main research contents and academic contributions of this dissertation are as follows:(1)Suitable graphene materials are selected with corresponding equivalent models and analysis methods built aiming at different device structures and performance requirements,such as the high frequency impedance extraction method for the analysis of graphene inductors,the conductance model of graphene ribbon considering the selfheating effect and the multiconductor transmission line(MTL)model for the analysis of graphene resonator,the transfer matrix method and the equivalent circuit model of graphene structures for the design of graphene FSS.(2)Graphene on-chip inductors with ultra-high inductance density are fabricated successfully for the first time.The existence of skin effect in graphene is verified by the equivalent circuit simplification and circuit parameter extraction.Meanwhile,the technical strategies for improving the performance of graphene inductor are explored,including optimizing the dielectric isolation layer,improving the contact between graphene and metal electrode,and improving the conductivity and kinetic inductance of graphene by intercalation doping.(3)Bromine intercalation doped graphene is investigated from the perspectives of both theoretical analysis and experimental characterization.The p-type bromine intercalation doped graphene compounds with the conductivity improved by about 3 times and the intercalation stage being 3 to 5 are successfully realized.The effects of doping scheme and ribbon structure on the doping performance are also studied.(4)On-chip inductors based on bromine intercalation doping graphene compounds with greatly improved performance are fabricated.For two-turn inductors,the achieved Q-factor is up to 11.8,with the inductance density increased by 116% compared with traditional metal-based inductors.The inductance density of six-turn inductor is 2.7 times higher than that of silver inductor with identical structure size.(5)A terahertz resonator based on As F5 intercalated graphene is proposed,with the performance superior to traditional metal-based terahertz resonator at the same structural dimensions.Meanwhile,the varying of resonator performance with the structure is studied to provide guidelines for the future design.(6)A terahertz tunable resonator is proposed and designed based on the h BN intercalated multilayer graphene by utilizing its relatively large tuning space.The predicted tuning ratio of resonant frequency is up to 21% with the Q-factor more than 20.(7)By applying the properly designed sandwich structure,a graphene FSS with both reconfigurable polarization selection and tunable frequency selection simultaneously is proposed and designed.The proposed FSS extends the functionality of traditional FSS,and is advantageous to the previously reported graphene structures in the aspects of bandpass frequency response,relatively small magnetic field,and large polarization rotation angle.