Temperature-Resistance Behavior of Graphene/Lithium Tantalate Field Effect Transistor
Graphene is a two-dimensional material with a hexagonal honeycomb lattice structure composed of carbon atoms periodical-ly arranged.It has excellent optical,electrical,and mechanical properties.The conduction band and valence band of intrinsic gra-phene intersect in the Brillouin zone,which is a zero-band gap semiconductor material.Graphene has little effect on its resistance due to temperature,so it is difficult to be applied to temperature sensors.However,the atomic-scale thickness of the single to several lay-ers of graphene and its adjustable energy band structure make it highly sensitive to the dielectric environment,which makes the gra-phene/dielectric material system sensitive to a variety of external physical quantities,so the band gap of graphene can be regulated by a variety of methods,among which the electric field control is the most common methods in order to regulate the resistance of gra-phene.Lithium tantalate(LiTaO3)is a material with trigonal distortion perovskite structure.Due to its non-centrosymmetric structure,it has good pyroelectric properties.The carrier concentration of graphene can be changed by the polarization of LiTaO3 when graphene is integrated on its surface,so it results in the resistance changing,thereby converting the thermal signal into an electric signal.For the graphene/pyroelectric material composite structure,the surface of the pyroelectric material is affected by the thermal field to pro-duce a higher polarization electric field,which can effectively affect the carrier transport performance of graphene.Therefore,the gra-phene/LiTaO3 material system can be used as the key material for the thermal detector.The graphene/LiTaO3 material system had been studied in this paper.The influence of temperature on the system was theoretically explained and it also showed that the change of the graphene carrier concentration was proportional to the change of temperature in the system.Based on this property,a new type of the thermal detector was made by integrating graphene on the surface of LiTaO3.Au-graphene-Au structure was prepared on the pyroelec-tric material to analyze the influence of temperature changes on the graphene channel resistance.The pyroelectric substrate was a 40μm thick Z-cut single-sided polished LiTaO3 wafer.Oxygen plasma was used to treat the surface of LiTaO3 substrate in order to improve the hydrophilicity of LiTaO3 substrate and its surface cleanliness,also reduce the appearance of cracks and wrinkles on the graphene surface.Then the graphene was transferred to LiTaO3 substrate by wet transfer,and a 30 μm× 100 μm graphene channel was prepared through photolithography and etching(oxygen plasma,20 W for 20 s)processes.Finally,a lift-off process was used to prepare Ti/Au(5 nm/50 nm)electrodes deposited by using thermal evaporation deposition,thereby preparing a back-gate graphene/LiTaO3 field ef-fect transistor.The number of layers and the defect concentration of graphene were characterized by Raman spectroscopy,and the re-sult showed that its D peak position was at 1346.72 cm-1,the G peak position at 1594.58 cm-1 and 2D peak position at 2684.2 cm-1,which was in line with Raman characteristic peak of graphene.2D/G peak intensity ratios,I2D/IG was 1.8,which showed that the gra-phene was a single layer and D peak intensity ID was small,which showed that there were few defects in the graphene.Then the temper-ature response of the device was texted by the semi-conductor parameter analyzer.By analyzing the Volt-Ampere characteristic curves at different temperatures,it was found that the contact between the graphene and the metal electrode was an Ohmic contact.At room temperature,the resistance of the device was 3800 Ω.As the temperature increased,the resistance of the device increased,which was due to the polarization of LiTaO3.The resistance of the device changed with temperature.This was because the polarization strength of lithium tantalate changed with the change of temperature,which changed the hole carrier concentration of graphene surface,and then led to the resistance change.When the temperature was between 30 and 61.2 ℃,the resistance increased with the increase of tempera-ture,and the resistance temperature coefficient was 3.20％·℃-1.When the temperature was between 61.2 and 66.0 ℃,the resistance decreased with the increase of temperature,and the resistance temperature coefficient was-3.94％·℃-1.The inflection point of the re-sistance at 61.2 ℃ was attributed to the polarized electric field on the surface of lithium tantalate which caused that Fermi level of gra-phene passed through the Dirac point from the valence band to the conduction band,which changed the type of charge carriers in the graphene and made the carriers changed from holes to electrons.This experiment provided experimental data support for the theoretical study of graphene/pyroelectric material system,and provided a new idea for the subsequent application of graphene/pyroelectric materi-al system to thermal detectors:there were abundant types of pyroelectric materials,and flexible thermal detectors could also be pre-pared by using polymer pyroelectric materials as the substrate.It was hoped that the study could provide data and theoretical support for the high-sensitivity thermal detectors.
graphenegraphene field effect transistorlithium tantalatepyroelectricitytemperature coefficient of resistance(TCR)
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