Modulation Of The Electrical Properties Of Bismuth-based Semiconductor Thin Films/PMN-PT Ferroelectric Single Crystal Heterostructures

Topological insulators have a unique energy band structure,since 3D topological insulators Sb₂Te₃,Bi₂Se₃ and Bi₂Te₃ were predicted theoretically,the doping and modulation based on these materials have emerged in an endless series.In 2013,professor Xue Qikun's research group of Tsinghua university discovered the famous quantum anomalous hall effect in the band modulated and magnetic-doped Cr_0.15(Bi_0.1Sb_0.9)_1.85Te₃,making the research on topological insulators become one of the main directions of condensed matter physics in recent years.In the performance research and device design of 3D topological insulators,it is one of the most important experimental methods to construct a field effect transistor with gate material and apply an electric field to modulate the carrier concentration,magnetoresistance and other physical properties of topological insulators.The commonly used gate materials include Si O₂,Al₂O₃,Hf O₂,h-BN,Si Nx,Sr Ti O₃,etc.However,the modulation range of these gate materials on the surface carrier concentration of the film less than the magnitude of 10¹³/cm²,and the modulation is volatile.By contrast,using ferroelectric as the gate material,the ferroelectric material could be polarized and the polarization direction could be reversed by the applied voltage,then the ferroelectric polarization could remain after removing the voltage.Therefore,it is possible to control the carrier concentration and the electrical properties of the films adjacent to the ferroelectric material in a nonvolatile manner.?1-x?Pb(Mg_1/3Nb_2/3)O₃-x Pb TiO₃?PMN-x PT?single crystal with the composition near the morphotropic phase boundary?x=0.29-0.33?is an excellent ferroelectric and piezoelectric material.The Curie temperature of PMN-x PT is about 140 ^oC,if PMN-x PT is polarized under the applied voltage at room temperature,there will still be a large residual polarization?30-40mC/cm²?even if the voltage is removed,this residual polarization could produce the surface carrier concentration modulation with the magnitude of approximately 2?10¹⁴/cm²,which is 1-2 orders of magnitude higher than that of the commonly used gate materials mentioned above.Therefore,combining the topological films with the PMN-x PT single crystal,constructing“topological insulator film/ferroelectric single crystal”heterostructure.The lattice strain and the polarization charges could be induced in PMN-x PT by the applied electric field.It is expected to modulate the physical properties of the topological insulator films in a nonvolatile manner using the lattice strain or the polarization charges,such as the energy band structure,carrier concentration and type,magnetoresistance and so on.In this way,we can deeply understand the nonvolatile modulation mechanism of the quantum state and quantum transport of topological insulator,and enrich the means and research connotation of the physical property modulation of the topological insulator materials.Based on the above analysis,in this paper,we deposited the 3D topological insulators Bi₂Se₃,Bi₂Se₂Te,Bi₂Te₃ and the Bi_1-xSb_x?x=0,0.07,0.17?alloy thin film on the PMN-0.29PT ferroelectric single crystal,constructing the bismuth-based semiconductor/ferroelectric single crystal heterostructure.We investigated the modulation of the electrical transport properties of the bismuth-based semiconductor film by the external electric field,the following is the main research contents:?1?Bi₂Se₃ films with different thickness?t=12,18,24 nm?were deposited on PMN-0.29PT?111?single crystal substrate by pulsed laser deposition.By studying the changes of the resistance and carrier concentration of the films induced by electric field,it is proved that the interface coupling mechanism of“topological insulator film/ferroelectric single crystal”heterojunction is the interfacial charge effect?i.e.ferroelectric field effect?,rather than the lattice strain effect.The in-situ and nonvolatile modulation of physical properties such as the resistance and magnetoresistance were realized using the interfacial charge effect,implying that the interfacial charge effect could effectively modulate the Fermi energy level and carrier concentration and their related electrical transport properties.The interfacial charge effect weakened with the increase of the film thickness.At low temperature,there was an upturn in the film resistance with the decrease of temperature,which could be well explained by the 2D Dirac model.After further considering the contribution of the body state to the conductance,the conductance behavior of the film in a wider temperature range?2.5-50 K?could be understood well.HLN Equation could fit the magnetoconductance data under the low magnetic field?B?0.3 T?well,the results show that there is 2D anti-weak localization effect in the film.The magnetoconductance data under the high magnetic field could be explained well by the modified 2D Dirac model after considering the contribution from the body state.The similar phenomenon was observed in the Bi₂Se₂Te/PMN-0.29PT?111?heterostructure.?2?Bi₂Te₃ film with the thickness of 6 nm and the c-axis orientation were grown on PMN-0.29PT?111?single substrate using pulsed laser deposition.The influence of the polarization charges induced by the polarization inversion and the influence of ultraviolet light on the electrical properties of the films were studied.The results show that the polarization charge could modulate the physical properties of the film in a nonvolatile manner,such as the resistance,magnetoresistance and the temperature dependence conductive behavior,etc.Hall measurements show that this modulation originates from the polarization charges of the PMN-0.29PT,which modulate the Fermi level and the carrier concentration of the film.Equation HLN could fit the magneoconductance data under low magnetic field well for the positively poled and negatively pole PMN-0.29PT,respectively,within the temperature range of 2-50 K.When the PMN-0.29PT is in the negatively poled state,the fitting parameter ?_??^-p/2?=1?,demonstrating that the surface state dominates the conductance of the Bi₂Te₃ film for negatively poled state.When the PMN-0.29PT is in the positively poled state,the fitting parameter ?_??^-p/2?=1.5?,demonstrating that the body state dominates the conductance of the Bi₂Te₃ film for positively poled state.Moreover,the resistance of the film decreases under ultraviolet light ??=405 nm?,this is because ultraviolet light could excite the valence band electrons to the conduction band and increase the carrier concentration of the film.?3?Bi_1-xSb_x?x=0,0.07,0.17?films with different thickness?85,170,255 nm?were grown on PMN-0.29PT?111?single crystal substrate by pulsed laser deposition,the influence of the lattice strain of the PMN-0.29PT on the electrical transport properties of the film and the influence of ultraviolet irradiation on the resistance of Bi_0.93Sb_0.07?85nm?film were studied.The results show that the lattice strain induced by electric field plays a major role in modulating the electrical properties of the film,while the interfacial charge effect could be neglected.By modulating the strength of the applied electric field accurately,nonvolatile strain states with different amplitude could be obtained in the substrate,these strain states are reversible,the induced strain transfers to the film through the interface and induced the resistance change of the film.Multiple nonvolatile resistive states corresponding to the different strain states could be obtained in the film.Whether the substrate was in the positively poled state or the negatively poled state,the resistance of Bi_0.93Sb_0.07 film was reduced by ultraviolet irradiation,this phenomenon is attributed to the fact that ultraviolet light excites free electrons,increasing the carrier concentration of the film.In addition,Bi and Bi_0.93Sb_0.07 films both exhibit the semiconductor conductive behavior,however,the conductive behavior of Bi_0.83Sb_0.17 films has a semimetal-semiconductor transition.Both the high magnetic field?9 T?and the in-plane tensile strain could increase the temperature of the semimetal-semiconductor transition.