Study On The Electronic And Optoelectronic Properties Of Two-dimensional P-n Homojunctions Tuned By Ferroelectrics

With the rapid development of semiconductor technology,traditional technology and devices are facing more and more challenges.Therefore,the research on new materials and structures has become a research focus now.Recently,due to its special structure and physical properties,two-dimensional(2D)materials provide opportunities for the next-generation nano-electronic and optoelectronic devices.As the most important building elements for modern electronic devices,doped semiconductors play an important role in duplicating silicon-based devices in 2D layered materials.However,the atomically-thin nature of these materials precludes the use of traditional ion implantation techniques for carrier doping and further hinders device development.Although the p-n junction of 2D materials can be achieved by many methods such as electrostatic doping,chemical modification,element doping and so on,the programmable,non-volatile,accurate and reversible carrier doping in 2D materials is still challenging.Here,we demonstrate different homojunctions based on 2D materials doped by ferroelectric domains,especially the electrical and optoelectrical properties of p-n junctions.The results are summarized as follows:1.A scanning probe can be used to control the polarization of ferroelectric polymers deposited on 2D materials in order to define carrier injection and achieve ptype and n-type doping.The approach allows 2D material lateral p-n,n-p,n-n,and p-p homojunctions to be arbitrarily formed and altered.2D materials p-n homojunction devices constructed using the method exhibit high current-rectification ratios and good optoelectronic properties.BJT transistors with high current gain are also constructed.An electrical writing and optical reading memory device without the restrictions of physical source,drain or gate electrodes is fabricated.Quasi-non-volatile memory device with a long refresh time and quick write/erase speed is demonstrated with the help of the probe-polarized ferroelectric domains.2.The series connected 2D material p-n junctions defined by probe-polarized ferroelectric domains are demonstrated.With the help of the AFM probe,periodic ferroelectric domains are formed,thus defining a series connected p-n junction in 2D materials,which can be used as photovoltaic devices.The relationship between the open circuit voltage,short circuit current,output power of the device and the number of p-n junctions in series is explored.The open circuit voltage can even exceed the band gap of the 2D semicondutor.3.2D p-n homojunctios are demostrated with the help of ferroelectric domains polarized by split gates.The ferroelectric field presents strong regulation ability on the channel current.The p-n junction shows high rectification ratio,non-volatile,good retention and endurance properties.It can be used as rectifiers,photodetectors and photovoltaic devices.The influence of the series resistance and parallel resistance on its optoelectronic performance is analyzed.The properties of the p-n junction under the electrostatic field and ferroelectric field are also characterized.This method for fabricating p-n junctions is universal and can be applied to other ambipolar 2D materials.All these three works use ferroelectric field to control the band structure of 2D materials to fabricate non-volatile p-n homojunction.In the first two works,the scanning probe of PFM is used to polarize the ferroelectric films,which are characterized by arbitrary design of device patterns and precise control of carrier type in 2D materials without the restrictions of metal electrodes.Therefore,a variety of lateral homojunctions can be constructed.In the third work,split gates are fabricated to polarize the ferroelectric materials.Therefore,the doping concentration in 2D materials is higher,and the current on-off ratio of the p-n junction is larger.