Theoretical Design And Properties Of Doped Graphene Nanoribbon Heterojunction Devices

For half a century,the continuous upgrading of computer and the continuous improvement of their performance have benefited from the continuous development of miniaturization of electronic devices.In order to implement the concept of sustainable development,researchers are devoted to building higher performance,lower energy,lower cost,and small-sized electronic devices on the nanoscale.Graphene is a kind of two-dimensional single-atom-layer material but with zero bandgap,which is disadvantageous for using graphene designing functional devices.To make graphene better used in electronic devices,its zero bandgap can be opened by doping other atoms in graphene or trimming it into graphene nanoribbon(GNR).Since there are infinite varieties of doping atoms and sites,GNR can produce a variety of electronic and spintronic characteristics,which has broad research prospects.Based on the previous studies of GNRs,different kinds of electronic and spintronic devices can be designed based on GNR.In order to further inverstigate and understand the performances of doping graphene-based spintronic devices,the band structure of doped zigzag GNR(z GNR)and the electron-transport properties of heterojunction devices formed of doped z GNR are studies by using the non-equilibrium Green's function method and density functional theory.The main research contents and conclusions are as follows:(1)In z GNR heterojunction device,the band structures of the electrodes play a crucial role in electronic and spintronic transport properties.Therefore,we design several electrode structures by doping boron and nitrogen atoms in z GNRs.In the study,the influence of doping type,doping site,doping concentration and whether the edge doped atoms are hydrogenated is analyzed in detail.The numerical results show that the effect of doping sites on the energy band structuues cannot be underestimated.When the dopant atoms are at the edge of z GNR,the band is spin-separated near the Fermi level.When the dopant atoms are in the middle of z GNR,the band is spin-degenerate.Under the same size supercell structure,the higher the doping atoms concentrate,the higher the spin degenerate.The largest band gap can reach 1e V in the studied structures,and the Fermi level almost locates at the middle position between the bottom of the conduction bands and the top of the valence bands.In addition,when the edge doped with boron atoms is not hydrogenated,a localized spin-up subband which is nearly flat appears near the Fermi level.According to the existence of the localized sub-bands,the conduction and valence bands are asymmetrical relative to the Fermi level,which makes it have the potential to construct heterojunction devices.In the calculated electrode structure,the single edge doped z GNR still maintains the metallic property,while the double edge doped z GNR shows semi-conductivity,and the edge doped z GNR is close to semi-metallic system when the dopent atom is not hydrogenated.(2)Basied on the studies of the boron and nitrogen doped z GNR electrode,two kinds of heterojunction devices were designed with different doping concentrations of electrodes.Numerical analysis shows that both heterojunction exhibit spin rectification behavior.Under positive bias,due to the mismatch of the energy bands on the left and right sides of the bias window,there is no transmission peak,which further causes the curren disappear.On the contrary,under negative bias,the energy bands on both sides overlap each other in the bias window,which result in large transmission peaks and further the obvious current through the heterojunction.However,comparing the two heterojunctions we found that the electrode with lower doping concentration shows larger spin rectification ratio,for which the highest spin-down rectification ratio reaches 10~6.Judging from the relationship between current and bias voltage,the two heterojunction devices both show negative differential conductance in the calculated bias regime,in which the current decreases as the bias voltage increases.In addition,both heterojunctions exhibit spin filtering effect.The heterojunction with low electrode doping concentration exhibits spin filtering efficiency of more than 90%under low bias voltage.while the highest spin filtering efficiency for the heterojunction device with high electrode doping concentration is less than 80%.(3)A local subband appears near the Fermi level when the non-hydrogenated boron atoms are doped at one edge of z GNR.Use it as one electrode of the heterojunction,the system exhibits large spin filtering performance.Therefore,the z GNR with the nitrogen atoms doped on the upper or lower edges is selected as the other electrode to design two kinds of heterojunction devices to investigate the influence of doping sites on the spin transport properties of the heterojunction.The results show that both heterojunctions exhibit dual-spin filtering effects,accompanied by spin rectification behavior.When nitrogen atoms are doped on the lower edge of z GNR,negative differential conductance also appears under negative bias.Therefore,the difference for the nitrogen atom doping on the upper edge from lower edge of z GNR has little effect on the electronic transport of the device.The thesis is divided into six chapters.The first chapter introduces the research background of spintronics and some spintronic devices.The second chapter focuses on theoretical method,which introduces the method of calculating the transport properties of the heterojunction systems;The third chapter discusses the energy band properties z GNR with boron and nitrogen dopant atoms.In chapter four,the influence of electrode doping concentration on the spin rectification of heterojunction devices is investigated.Chapter five study the influence of different dopant positions on the dual-spin filtering behavior of the heterojunctions.The sixth chapter summarizes the previous work and looks into the future.