Research On Charge Transport In Graphene/Silicon Schottky Diode

Extensive experimental studies have discovered a wide range of two-dimensional(2D)materials successfully integrated into existing semiconductor technologies to develop devices that outclass conventional electronics.Graphene is an ideal representative twodimensional material with a unique band structure and excellent optical and electrical properties,which expands its way to uncover charge transport at higher switching frequencies.The research in the thesis shows that the combination of two-dimensional materials and silicon-based devices makes full use of mutual advantages to push the performance limitations of conventional devices.The thesis mainly studies graphene-based Schottky devices.Conventionally,the primary purpose of the Graphene/Si(Gr/Si)Schottky diode has been employed as a photodiode and other sensing applications.We explored the underlying charge transport mechanism for applications beyond the traditional photodetection domain.In this thesis,we demonstrated an electromechanical method to enhance photovoltaic and power conversion efficiency(PCE).The influence of silicon surface states on the electrical properties of the device is studied,as these states are closely related to the device stability.The systematic in-depth study of the Gr/Si junction was lacking for fast dynamic switching and electrostatic discharge(ESD)conditions to explore the junction robustness and reliability for the on-chip protection and high operating voltage application.There was a need for an on-chip protection device that bridges 2D-based devices and CMOS-integrated circuits.Thus,Gr/Si Schottky diode was explored and analyzed for such dynamic conditions.The main contents are as follows:(1)Graphene-Silicon Schottky junction(GSJ)has the potential for large-scale manufacturing and integration,which introduces new opportunities to Schottky solar cells for photovoltaic(PV)power conversion.However,these devices faced low power conversion efficiency(PCE)due to the small open-circuit voltage(Voc).In this study,we demonstrated an electromechanical method based on the flexoelectric effect to enhance the PV efficiency in GSJ.By atomic force microscope(AFM)tip-based indentation and insitu current measurement,the current-voltage(I-V)response under flexoelectric strain gradient was obtained.By experimental validation,we found that the open-circuit voltage(Voc)can be enhanced from 0.38 to 0.46 V by the applied forces,and the Schottky barrier height can be increased from 0.65 to 0.7eV.Consequently,the power conversion efficiency(PCE)was enhanced up to 20%with the assumption that the short circuit current(Isc)is identical and has a similar fill factor(FF).(2)Broadening the scope of the Gr/Si Schottky diode,the excellent charge absorption capability of the device finds its innovative application for on-chip protection of surgeinitiated failures of sensitive 2D materials-based devices.In this study,dynamic characteristics for the failure of the Gr/Si Schottky diode are systematically investigated for their potential for electrical stress applications.Dynamic biasing leads to Joule heatingwhich is one of the major causes of electrical breakdown.Such events generate a high surge current and significantly affect the electrical performance of the device,resulting in permanent damage in severe cases.Our findings revealed that Gr/Si diode could absorb high surge current,has a fast reverse recovery time(～10ns),and a highly robust junction(VR-BD～80V)at higher dynamic switching.Benefitting from these advantage,we demonstrated a strategy to protect 2D material-based devices from electrical stress.It has been observed that electrical input spikes are the primary reasons for electrical failure in 2D heterostructures.The ultra-fast response of the Gr/Si Schottky diode can effectively bypass electrical spikes beyond thresholds and increase the lifetime of the 2D-PN diodes(i.e.,WSe2/MoS2).This study provides an overall picture of the failure mechanisms of Grbased Schottky diode under dynamic biasing conditions,leading to protection circuits for 2D heterostructures and extending the device lifetime.(3)To extend our scientific study,we employed the electrostatic discharge approach to explore the charge transport at 2D-3D contacts using Gr/Si Schottky diode as a representative structure.In this study,we investigated the dominating and limiting factors of Gr/Si interfaces designed for high light absorption,paying particular attention to the nature of the contact failure under high electrostatic discharge(ESD)conditions.Our results indicated that current crowing(CC)is the dominant source of device failure in graphene-based devices.Current crowding caused by highly electrostatic fluctuations at the 2D/3D interfaces significantly limits the device lifetime and performance.Reducing the current crowding effects at the contact edges of graphene by engineering device structure can substantially enhance the current density(up to～5×103 A/cm2)and breakdown voltages(forward/reverse～100/181V)of Gr/Si Schottky junctions.Benefitting from broadband photodetection and Gr/Si junction robustness,it may find excellent potential for applications where high current carrying capability and avalanche photodetection at significant high voltage biases are desired.The research in this thesis advocates the integration of 2D and 3D materials for enhanced photovoltaic effect and junction robustness for the Gr/Si Schottky diode for futuristic on-chip protection at high operating voltages.