Research On Graphene Devices For Quantum Hall Resistance Standard

Quantized resistance is an intuitive manifestation of the quantum Hall effect,which is universally observed regardless of the material and geometry of the sample.The resistance quantan depends on nothing but the fundamental physical constants,which renders it ideal for resistance standards and metrology in general.Typically,the observation of the quantum Hall effect requires extremely low temperature and strong magnetic field.However,the unique physical properties of graphene enable the quantum Hall effect to be realized under more relaxed experimental conditions,positioning it as an ideal candidate for the next generation of quantum Hall resistance standard devices.Currently,the graphene used for the preparation of quantum Hall resistance standard devices is mainly synthesized using molecular beam epitaxy on a Si C substrate.This thesis focuses on the preparation of quantum Hall devices using graphene synthesized by chemical vapor deposition,and verifies their feasibility for use as the quantum Hall resistance standard.A systematic study has been carried out to unveil the scattering mechanisms that restrict the device performance and the feasible routes to mitigate the impact of scattering.The thesis also goes beyond the standard graphene quantum Hall device by exploring the properties of graphene/antiferromagnet heterojunction,where quantum anomalous Hall effect may emerge.First,we synthesized large-area,high-quality monolayer graphene films on copper foil using liquid carbon precursor,to be more specific,acetone.The film is then transferred and fabricated into devices with a h BN-based dry transfer technique.The quality of oxidization at the graphene/copper interface is vital to ensure successful transfer of graphene.To optimize the transfer success rate,a large set of experiments have been conducted to compare the oxidation capabilities of different solvents on copper foils.Additionally,we discussed the impact of various parameters,such as the crystal orientation of the copper foil and oxidation temperature,on the resulting oxidation.We conclude that the graphene/copper foil sample oxidized at 60-70°C for 24 hours in saturated water vapor environment yields highest success rate for dry transfer of graphene.The dry transfer technique allowed us to obtain graphene with a size exceeding 35μm（limited by the size of h BN）,which is the largest size reported to date.This research has important implications for guiding the dry transfer of continuously grown graphene films.Next,we performed a systematic study of graphene devices prepared using the dry transfer technique.The dry transfer technique offers several advantages over the traditional wet transfer process,including avoiding organic solvent contamination during graphene transfer and device fabrication,resulting in significantly improved electrical performance.The dry-transferred devices exhibited a reduction in doping level by about one order of magnitude,and an increase in room temperature mobility to 12500 cm²V^-1s^-1,almost 5 times higher than that of devices prepared by the wet transfer process.Moreover,the fact that the robust quantum Hall effect can survive up to room temperature further confirms the high quality of the devices.In several devices,precisely quantized Hall resistance plateaus were observed,with the measured value deviating from theoretical value by less than the measurement noise.However,there is still a certain gap before application of the quantum Hall resistance standard,primarily due to the difficulty in tuning the location of the quantum Hall plateau at=2 to a lower magnetic field.Finally,we extracted the quantum scattering time and the transport scattering time by measuring Shubnikov-de Haas oscillations at low magnetic field,and systematically studied the scattering mechanism in our devices.Our results showed that charged impurities located 1.34 nm from the surface of the graphene were the main limitation to our devices performance,which may originate from the wrinkles on the graphene surface and the bubbles introduced during the transfer process.Additionally,we noted that the dry transfer process introduces stress into the graphene,resulting in non-uniform carrier distribution and doping,which can be mitigated by annealing.This work is of great significance for improving the electrical performance of graphene devices.Finally,we utilized the chemical vapor transport method to grow Mn PSe₃crystal,a two-dimensional antiferromagnetic material.Using an improved dry transfer method,we prepared heterojunctions of Graphene/Mn PSe₃（referred to as Gr/Mn PSe₃）,and performed systematic characterizations.Charge transfer effect was induced at the interface between graphene and Mn PSe₃ due to the difference in work functions,resulting in p-type doping of graphene.In Gr/Mn PSe₃ heterojunctions,the quantum Hall effect and quantum oscillations were observed.However,there is no signature of quantum anomalous Hall states or significant impact of Mn PSe₃ on the band structure of graphene.It is speculated that the disorder in Mn PSe₃ crystal and the relatively large distance between Mn PSe₃ and graphene can suppress the proximity effect and quantum anomalous Hall states.Furthermore,we observed a non-monotonic behavior in the temperature-resistance curve of Gr/Mn PSe₃ heterojunctions,with a minimum occurring in the range of 20-40 K,far below the Néel temperature of Mn PSe₃（70 K）,which represents a significant deviation from previous reports on other graphene/antiferromagnet heterostructures.Our data suggests it might be a direct result of Kondo effect arising from the local magnetic order of magnetic impurities in Mn PSe₃.This study provides valuable insight into the interaction between graphene and antiferromagnetic materials.