| Since graphene was first discovered in 2004,various two-dimensional materials have sprung up in large numbers.With their unique properties and excellent performance,two-dimensional materials show extraordinary application potential in the field of electronic and photoelectric functional devices.As a zero-band-gap semiconductor,graphene has good conductivity and high mobility,but the on/off ratio of traditional graphene field effect devices is very low.The band gaps of two-dimensional transition metal chalcogenides（TMDCs）are generally 1 e V to 2 e V.Their field effect devices generally have high on/off ratio.However,the Schottky barrier between semiconductor and electrode determines that the TMDCs based field effect devices have high power consumption.And the pinning effect of Fermi level will also limit the on/off ratio,which is an important factor limiting the performance of traditional TMDCs field effect devices.Therefore,according to the characteristics of two-dimensional materials such as graphene and TMDCs,in order to further improve the performance of TMDCs based electronic devices,this dissertation focuses on the electrical and photoelectric characteristics of graphene-TMDCs vertical tunneling heterojunction devices and metal-TMDCs contacting field effect devices from the two aspects of constructing graphene-TMDCs vertical van der Waals tunneling heterojunction and optimizing metal TMDCs contact.Whether it is tunneling or metal contact,the core is the regulation of Fermi level,so as to realize the regulation of barrier height.The main research contents of this dissertation are as follows:（1）The properties of two-dimensional materials such as graphene and TMDCs are summarized,and the advantages and limitations of two-dimensional materials in electronic devices are introduced in detail.In order to solve the problems of traditional electronic devices and improve the performance of devices,we propose two research ideas:one is to construct graphene-TMDCs-graphene vertical tunneling heterojunction field effect devices and study their tunneling electrical characteristics.The second is to study the contact characteristics between different metals and semiconductor Mo S2 in order to reduce the Schottky barrier and improve the performance of the Mo S2 device.Then,the basic theories of two-dimensional materials based field effect transistors and two-dimensional material photodetectors are summarized.At the same time,the research background and application of two-dimensional material vertical tunneling heterojunction and the related background of metal-TMDCs contact are introduced.In chapter 2,the methods and processes of preparation,characterization and testing of heterojunction devices based on two-dimensional materials in our experiments are introduced.（2）Graphene-WS2-graphene vertical tunneling heterojunction field effect devices were prepared and their tunneling characteristics were studied.Tungsten disulfide（WS2）is a semiconductor with relatively wide band gap in TMDCs family.Its single layer has a direct band gap of 2.1 e V and multi-layer has an indirect band gap of 1.3 e V,which is suitable for tunneling layer.We fabricated graphene-WS2-graphene（GWG）vertical tunneling heterojunction field effect devices with different thickness of WS2 and tested their electrical properties.For the GWG device（8 nm WS2）,the on/off ratio is about 2×107 and 6×103 at 5 K and 280 K.Moreover,the device shows n-type under small bias voltage（0.15 V）and bipolar under large bias voltage（2 V）.The carrier polarity can be adjusted only by adjusting the bias voltage,which is very unusual in semiconductor devices.Furthermore,we establish a complete carrier tunneling model in WS2.At 5 K,with the voltage Vb increasing from 0,the carrier tunneling transport model changes from direct tunneling（DT）transport under small bias to Fowler Nordheim tunneling（FNT）transport under large bias.The transition bias voltage VD-FN of DT to FNT is extracted,and the FN tunneling barrierφB can be calculated.As the temperature rises above 200 K,we find that the tunneling current under small bias gradually shows an obvious temperature dependence.We deduce that this is related to the thermal transport of carriers,where the transport modes of carriers include thermal transport and direct tunneling transport.The transition temperature Tc can be clearly extracted from the Arrhenius curve of conductivity near zero bias.When the temperature is higher than Tc,the carrier transport is mainly thermal transport;When the temperature is lower than Tc,the carrier transport is mainly direct tunneling transport.In order to further improve the device performance,we prepared graphene WS2 graphene vertical tunneling heterostructure field effect devices with h-BN as back gate,and achieved 1.5×106 on/off ratio at 300 K and 5×108 at 5 K.And the subthreshold swing at 300 K is as low as 0.45 V/dec.（3）A high performance tunneling Mo Te2 photodetector was fabricated.Firstly,the relationship between photoconductive gain（G）and various parameters is summarized,and a method to shorten the channel distance（L）to improve the performance of photodetector is proposed.Using vertical tunneling structure to prepare photodetector can effectively shorten the channel width.We fabricated a planar Mo Te2 photodetector with a low responsivity of 10 m A/W（532 nm,0.7μW）and 0.21 m A/W（1064 nm,1.65μW）by applying-40 V gate and 1 V bias.In addition,the photosensitive area is narrow and uneven,and the performance is very unsatisfactory.In order to improve the performance of Mo Te2 based photodetector,we prepared graphene-Mo Te2-graphene vertical tunneling photodetector with a high responsivity of 350 m A/W（532 nm,0.7μW）and 150 m A/W（1064 nm,1.65μW）by applying-40 V gate and 1 V bias.The photosensitive area is large and the light response is uniform.The responsivity increased by 35 times（532 nm）and700 times（1064 nm）respectively and the responsivity is greatly improved.（4）High performance Mo S2 field effect devices are fabricated,and three important conditions for metal evaporation in the process of fabricating high performance Mo S2field effect devices are summarized.The electrical characteristics of Mo S2 field effect devices in contact with six kinds of metal:Yb（3.1 e V）,Sc（3.5 e V）,Mg（3.6 e V）,Bi（4.1e V）,Ti（4.3 e V）and Pd（5.1 e V）with different work functions are studied.We found that Mo S2 field effect devices using Bi as contact metal can achieve Ohmic contact by applying>20 V gate.And the I-V curve shows good linearity,and the devices show high mobility(230 cm2v-1s-1)and large open state current（35μA/μm）and higher on/off ratio（107）.We have prepared Sc-Mo S2 field effect devices,and observed a phenomenon similar to Ohmic contact.The comprehensive performance of the device is similar to that of Bi contact Mo S2.For comparison,we have prepared Mo S2 field effect devices with Yb,Mg,Ti and Pd as contant layers.The Ohmic contact characteristics that do not appear under the back gate control are Schottky contacts,and the open state current,on/off ratio and other performance parameters are not good.By summarizing the electrical characteristics of six different metal contact Mo S2 field effect devices,we summarized three requirements that the evaporated metal should meet in the preparation of high-performance Mo S2 field effect devices:stable chemical properties,strong oxidation and corrosion resistance.The relation between work function of metal and electron affinity of semiconductor is Wm<χ.Low melting point and boiling point.Realizing the Ohmic contact of TMDCs semiconductor field effect devices and reducing the contact resistance are of great significance in the field of semiconductor applications.It can effectively enhance the electrical performance of semiconductor devices.Moreover,the contact layer metal evaporation process has a good fit with high integrated circuits and can be put into application very quickly. |
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