Modulating The Optoelectronic Properties Of Two-dimensional Material Mo Te₂through Phase-transition Engineering

Two-dimensional transition metal choriocarides are a class of materials with special quantum properties,which have attracted a lot of attention and extensive research in recent years.Composed of theVIA and VIB groups,these materials usually have semiconductor and metallic structures that can be stable at room temperature.At the same time,thin layer samples are easily obtained by mechanical stripping due to the interaction of weak van der Waals force between the layers.With the thinning of the layers,the band structure and transport properties change greatly,so it shows a broad application prospect in electronic devices,photoelectric detection and other fields.As a member of two-dimensional transition metal chalcogenides,molybdenum ditelluride（MoTe₂）exhibits unique advantages in near-infrared detection due to its band gap ranging from 0.93 eV to 1.1 eV.At the same time,researches have shown that MoTe₂materials can also stably exist a semi-metallic phase during the transition between semiconductor phase and metal phase which exhibits excellent properties in fields such as superconductivity.Therefore,it has great research value to fabricate MoTe₂photodetectors.On the one hand,due to the reversible phase transformation of MoTe₂materials by soft plasma,the influence on material properties during phase transformation can be effectively reduced.On the other hand,MoTe₂’s multiple phases can be stable at room temperature,providing an ideal platform for the design of diverse device structures.In this paper,firstly,by studying the influence of different types of plasma on the electrical conductivity of MoTe₂,combined with theoretical calculations,it is concluded that the main mechanism of MoTe₂phase transition is the formation of defects.Then combined with photolithography,the photodetector with heterophase homojunction was prepared by changing the phase transition region.By adjusting the phase transition degree,the responsibilities of different wavelengths of laser were improved.Finally,based on the diversity of MoTe₂phases,logic devices with different functions are designed to realize the functions of inversion and selection.The main research contents of the paper are as follows:1.The phase transition mechanism of MoTe₂was studied.Firstly,the phase transition effect of hydrogen and oxygen plasma on MoTe₂was compared,and it was found that the transiton of MoTe₂from hexagonal（2H）phase to twisted triangular（1T′）phase could be achieved by both methods.However,oxygen plasma was more controllable and the resulting sample was more stable in air.At the same time,oxygen plasma is more compatible with other subsequent processes such as exposure and development in photolithography,and more suitable for the study of phase transition mechanism.Secondly,TEM characterization of the samples after oxygen plasma treated showed that more vacancy defects were generated in the MoTe₂lattice after the phase transition,and XPS characterization also showed that the atomic ratio of Moand Te in the treated samples increased from 0.48 to 0.57,which suggested that the plasma-induced MoTe₂phase transition was caused by the introduction of defects.Finally,through first-principles calculation,it is found that when some Te atoms are missing in MoTe₂protocell,the band gap decreases from 1.2 eV to 0.03 eV,that is,the transition from semiconductor to semi-metallic property,which is consistent with the experimental conclusion,and further confirms that the loss of Te atoms is the main cause of MoTe₂phase transition.2.The photodetector based on MoTe₂heterophase homojunction was constructed and its performance is improved.Firstly,photolithography was used to mask different areas of MoTe₂,and 1T′-2H-1T′heterophase homogenous MoTe₂photodetector was obtained by oxygen plasma treatment.Then,the electrical and photoelectric performance of the intrinsic MoTe₂photodetector was compared with that of the heterophase homogenous MoTe₂photodetector.It was found that the carrier mobility increases from～4 cm²/V·s to～20 cm²/V·s,the on/off ratio increases from 14869 to 56952.The responsibility of the device after treated can reach539 mA/W at 447 nm,179 mA/W at 520 nm,589 mA/W at 637 nm,and 563 mA/W at 940nm.All of them were higher than intrinsic MoTe₂at the same wavelength（15.2 mA/W at447 nm,9.6 mA/W at 520 nm,17.7 mA/W at 637 nm and 24.5 mA/W at 940 nm）.Finally,by measurement at different temperature,it is found that the decrease of contact resistance is the main factor to improve the electrical performance.Meanwhile,the depth of plasma phase transition was studied by designing the device structure with edge contact.It was found that compared with the thin layer（below 10 nm）of MoTe₂,the thick layer（above 10 nm）needed more time to realize the phase transition completely,indicating that the plasma-induced phase transition was carried out layer by layer.3.The effect of phase transition on the performance of MoTe₂logic devices was studied.Firstly,MoTe₂-MoS₂logic devices were fabricated by dry transfer technology,in which MoS₂was used as channel material of NMOS and MoTe₂was used as channel material of PMOS.Then,the inverting function of MoTe₂-MoS₂logic device was successfully realized by changing the input voltage,and the multi-bit selection function of MoTe₂-MoS₂logic device was realized by changing theV_DDvalue.Finally,the effect of phase transformation on the performance of MoTe₂-MoS₂logic device was studied by phase transformation in different regions of MoTe₂.It was found that the performance of 2H-1T′-2H MoTe₂logic device was greatly reduced compared with the intrinsic logic device,while the gain of 1T′-2H MoTe₂logic device was increased compared with the intrinsic MoTe₂logic device.