High-performance Two-dimensional Black Phosphorus Electronic Devices

Black phosphorus（BP）is a single-element layered semiconductor material stacked together by weak van der Waals force.BP field-effect transistors（FETs）show a p-type dominated ambipolar conduction behavior.The hole mobility of BP FETs can reach 1000cm²/V×s at room temperature,which is larger than that of most of the transition metal dichalcogenides（TMDs）such as molybdenum disulfide.It was found that the bandgap of BP strongly depends on the number of layers changing from⁰.3 eV in the bulk to 1.73eV in the monolayer and remains to be the direct bandgap.This wide span of the bandgap is appealing because it fills the energy spectrum gap between zero-bandgap graphene and relatively large-bandgap TMDs.Besides,black phosphorus shows strong anisotropic characteristics.These excellent properties demonstrate the potential of black phosphorus as a new two-dimensional material for applications in future high performance electronic and optoelectronic devices.This thesis mainly focuses on high-performance two-dimensional black phosphorus electronic devices.We make a detailed exploration on the fundamental material and electric transport properties of black phosphorus,and various methods are adopted to improve the device performances.The main content addresses the following issues:1.Fabrication and interface optimization of black phosphorus back-gate devices:We have fabricated BP FETs using atomic layer deposited high-κ HfO₂ as the back-gate dielectric and compared the performance with the BP FETs based on conventional dielectric SiO₂.The interface characteristics of BP/dielectric and the transport properties under low temperature and high electric field were studied.Key figures-of-merit such as mobility,on-state current,saturation velocity,and interface trap density have been studied systematically for a wide temperature range.Transistors based on HfO₂ exhibit greatly improved device performance in comparison to the BP FETs on conventional SiO₂,showing more than 50%performance improvement in mobility,on-state current,and saturation velocity,and over 8 times reduction in interface trap density.A record high drain current of 906mA/mm at 20 K has been achieved for a 100 nm device on HfO₂,demonstrating excellent current-carrying capability and high field strength of BP as a channel material.2.Fabrication and structure optimization of black phosphorus top-gate devices:The instability of black phosphorus in the ambient environment is of paramount concern for practical applications.Here,we demonstrated improved air-stability by capping the BP flake with an atomic layer deposited（ALD）HfO₂ and an e-beam evaporated（EBE）Al₂O₃,and both of the methods were adopted for the critical processing step of the top gate dielectrics.The device extrinsic f _T and f_max are measured to be 2.2 and 8 GHz for the ALD-HfO₂ top-gate device.However,the Al-gate device shows about 50%improvement in f_T and f_max compared to the ALD-HfO₂ top-gate device.To further improve the radio-frequency performances of BP transistors,a Damascene-like planarization process is presented to create an embedded-gate stack with high-κ dielectrics under a gate-first approach,which minimizes surface impurities and traps at the BP/high-κ interface and preserves the high-quality black phosphorus channel.The radio-frequency performances of this structure improve at least twice compared with conventional top-gate structures.A record high extrinsic f_max of 17 GHz at room temperature for the device with 400 nm gate-length is achieved,which further increases to around 31 GHz at 20 K.Besides,BP transistor-based mixers operating at gigahertz frequency are also demonstrated for the first time.3.Black phosphorus/indium arsenide（BP/InAs）heterojunction tunneling device:Since the band-to-band tunneling probability decreases exponentially with the effective mass（m*）and bandgap（E_g）,it is best to have smaller m* and narrower E_g for higher tunneling probability.In such a case,both InAs and BP are desired channel material candidates for highly efficient tunnel devices.Besides,the narrow bandgaps of BP/InAs can create a tunnel junction tunable by a small supply voltage taking into account of the electron affinity difference.In this thesis,by using InAs as the source and thin BP film as the channel,a vertical asymmetric heterostructure is built with broken-gap band alignment.In comparison to the individual BP FETs,the BP/InAs heterostructure shows a smaller subthreshold swing benefited from the tunnel junction.More importantly,we observe a notable negative differential resistance and negative transconductance systematically tunable by the vertical and transverse electric field.Our results represent advances in the fundamental understanding of heterojunctions and promote future applications in advanced electronics.These results provide insights into important device physics of BP based transistors and show great potential for future high-performance electronic devices.