Fabrication Of New Graphene Field Effect Transistors With Naturally Oxidized Aluminum Dielectric And Research On Turning-off-able Characteristics

Due to remarkable properties in electrics,mechanics,thermology and optics,graphene has attracted considerable attention and becomes a research hotspot since its discovery in 2004.One of the most promising applications of graphene in electronics is to be used as a channel material for field effect transistors.While,the device geometry and dielectric will affect or even change the performance of graphene field effect transistor?GFET?.In this thesis,based on naturally oxidized Aluminum?Al?dielectric,simple preparation processes are proposed,and GFETs with new structures are fabricated.The influences resulting from the structure and dielectric on the properties of GFET are studied.The main work and achievements are summarized as follows:1.It is necessary to deposit dielectric in the fabrication of GFET.Al can be naturally oxidized in air,which can be used as dielectric for GFET.Hence,the fabrication process of the Al embedded-gate?EG?GFET is simplified due to the elimination of depositing dielectric.Due to the thin dielectric layer resulting from naturally oxidized Al,an Al-EG-GFET with a high gate capacitance?875nF/cm²?is fabricated.And based on a single Al-EG-GFET,a frequency doubler with a high conversion gain is realized.2.In conventional GFETs,there are two large un-gated channel regions?L_A?between the gate and source/drain electrodes.The access resistance?R_A?resulting from the L_A is one of key factors that limit the performance of GFET.It will reduce the drain current and trans-conductance,decrease the frequency performance.The L_A is mainly caused by the alignment precision and operating skill.Moreover,in conventional GFETs,usually,the length of the L_A between the gate and source is not as same as that between the gate and drain.Namely,it has an asymmetry in structure.When the source and drain electrodes exchanged,the characteristics of the GFET is also asymmetric.The asymmetric characteristics is analyzed by using the common source circuit model with source negative feedback resistor and the total resistance formula of GFET.3.By improving the source-drain current model and using the simulation,the influence of the access resistance resulting from the connection region on the performance of the GFET is studied.In order to reduce the L_A or R_A as much as possible,it is more feasible to devise self-aligned?SA?structures and fabricate self-aligned GFETs.A new idea for preparing SA-GFETs is proposed,that is,a source-drain electrode is formed first,and then a self-aligned gate electrode is formed.The principle of fabricating self-aligned GFETs is that the thin-layer metal deposited by electron beam evaporation has a weak step coverage.Hence,during the fabrication process,the source and drain electrodes are aligned to the gate edge.Consequently,the R_A is greatly reduced by minimizing the L_A.4.Previous SA structures and fabrication approaches of SA-GFETs are investigated.Then,two new SA-GFETs are proposed and fabricated,namely,the metal-stacked gate electrode SA-GFET and the metal-stacked source/drain electrodes SA-GFET.Compared with previous SA-GFET fabrication approaches,the fabrication process of the two SA-GFETs is simple,and damages to graphene channel resulting from the plasma are avoided.The key fabrication process of the metal-stacked gate electrode SA-GFET is as follows:firstly,multi-layer metal stacked gate electrode is formed.Then,the top layer metal acts as an etch stop layer,while the bottom layer metal is slightly etched by a selective etchant.Therefore,the T-shaped gate electrode with lateral undercut is formed.While,the key fabrication process of the metal-stacked source/drain electrodes SA-GFET is as follows:firstly,source/drain electrodes made of three layers of metal are formed.Then,the second layer of metal is slightly etched by a selective etchant,while the first layer and the third layer of metal are not etched.Therefore,source/drain electrodes with lateral recessing are formed.Due to the ingenious structure,for the metal-stacked source/drain electrodes SA-GFET,the level of the lateral recessing of source/drain electrodes does not change the value of the access resistance.5.Due to zero bandgap,GFET has a very low On/Off ratio and can not be turned off.Researchers have been struggling to find ways for achieving a high On/Off ratio.To obtain a high On/Off ratio,a new approach that is different from that of the existing GFETs is explored.That is,based on the dielectric material,we try to find a new idea to achieve a high On/Off ratio of GFET.It has been found that the GFET with naturally oxidized Al dielectric has an On/Off ratio of 5×10⁷ and it can be turned off and on.Firstly,the mechanism of turning-off and turning-on of the GFET is qualitatively interpreted.Due to the limitation on the number of carriers in per unit area(10¹¹¹0¹²/cm²)in graphene channel and the slight conductivity of the dielectric,when a positive voltage is applied at the gate,and the ability of extracting electrons by the gate via the dielectricis stronger than that of inducing electrons,the number of carriers in the channel becomes very small or almost zero.Hence,the GFET is turned off.It is inferred that the bandgap of graphene is opened at this time.Conversely,when a negative voltage is applied at the gate and carriers are re-injected,the turned-off GFET is turned back on.Secondly,based on the carrier re-injection principle,it is possible that the number of carriers in the channel can be controlled by the negative gate voltage.Then,I_dss can be controlled.As a result,according to P=IV,the power consumption of the GFET will be reduced by orders of magnitude compared to that of conventional GFETs.Thirdly,the storage of information can be achieved between the turning-off and turning-on of the GFET,which can be used to implement a non-volatile memory.In addition,the gate current assistance gain is found,which is different from the working principle of conventional FET devices.Finally,the GFET with HfO₂ dielectric is prepared to verify the interpretation of turning-off and turning-on.