Manipulation Of Defects And Band Structure Along With The Optimization Of Electronic Transport Properties Of N-type Bi₂Te₃-based Single-crystal Films

Bi₂Te₃-based compounds are the best and the most widely used commercial thermoelectric（TE）materials around room temperature.Moreover,they are one type of the most important three-dimensional topological insulators.Therefore,the Bi₂Te₃-based compounds,have widespread and promising applications in the following fields,such as TE applications of precise temperature control of laser diodes in 5G/6G communications,the active cooling and precise temperature management of micro-chips and the self-powered wearable electronics using the human body heat,as well as spintronic devices,optoelectronic devices and quantum information storage utilizing their topological properties.Current researches have revealed that,defect engineering is the effective approach for manipulating the electronic band structure of Bi₂Te₃-based compounds,and thus for optimizing their TE performances and for realizing their topological properties.However,to date,the type of defects,their density and distribution are not revealed clearly in Bi₂Te₃,due to the fact that previous discoveries were deduced based on experimental results.Moreover,the effects of defects on electronic band structure and thus on the electronic transport properties are not clear,and lack of supporting evidences from electronic band structure measurements.Herein,we use advanced techniques for the systematic investigations of defects,electronic band structure and electronic transport in Bi₂Te₃.These techniques include the molecular beam epitaxial（MBE）technique for precisely manipulating the defects as well as in-situ scanning tunneling microscopy（STM）that can directly visualize surface defects and angle-resolved photoelectron spectroscopy（ARPES）that is powerful in mapping the electronic band structure.Our research is capable for studying the defects morphologies,their distribution and regulation mechanisms,for clarifying the formation mechanisms and of 0D point defects and 2D planar defects,and for uncovering the tuning effects of electronic band structure via defects engineering.The key research findings of this work are listed as follow:（1）We have grown n-type Bi₂Te₃films on single crystal Al₂O₃substrates with different thickness（d）by MBE at different substrate temperatures(T_sub).Through the combined use of ARPES electronic band structure measurements and electronic transport measurements,this work proposes the new surface upward band bending phenomenon in Bi₂Te₃films,and clarifies two mechanisms regarding to defects formation and transformation.（i）The negatively charged vacancies,V_Te,initially the dominant intrinsic defects,transform gradually during the growth process into positively charged anti-site defects,Bi _Te,driven by thermal annealing from a continuously heated substrate.（ii）From the film’s surface into the inner strata of the film,the density of V_Tedecreases while the density of Bi _Teincreases,leading to a gradient of vacancies and anti-site defects along the film growth direction.As a result,the electron density in Bi₂Te₃films decreases monotonically with increasing the d.Meanwhile,elevating T_subleads to a more significant in-situ annealing effect and an eventual onset of intrinsic excitations that deteriorates electronic transport properties.The thinnest Bi₂Te₃film grown at lower substrate temperature possesses the highest electron concentration of 2.03（?）10²⁰cm^-3,and also the maximum room temperature power factor of 1.6 m Wm^-1K^-2of all grown epitaxial films.（2）This work unambiguously reveals three types of dominant intrinsic point defects in Bi₂Te₃films utilizing the STM technique,including V_Te,Te _Biand Bi _Te.This result is acquired by comprehensive efforts of direct visualization of point defects by STM and the theoretical calculations of STM defects morphologies.Our research confirms the T_suband the Te/Bi flux ratios（R）could effectively regulate the intrinsic point defects,and also uncovers their regulation mechanisms.Lowering the T_suband increasing the R prominently enhance the density of Te _Biand suppress the formation of V_Teand Bi _Te,and vice versa.More importantly,we realize the independent manipulation of Te vacancies V_Teand antisite defects of Te _Biand Bi _Tein MBE grown n-Bi₂Te₃films,which is directly monitored by a scanning tunneling microscope and by virtue of tuning the growth parameters.Based on the above discoveries,we are able to reveal two new important mechanisms regarding how point defects regulating the electronic transport properties,with the aid of ARPES and systematic transport studies.（i）The presence of Bi _Teantisites leads to a reduction of the carrier effective mass in the conduction band,which is not beneficial for optimizing the Seebeck coefficient of n-type Bi₂Te₃.（ii）The intrinsic transformation of V_Teto Bi _Teduring the film growth results in built-in electric field along film thickness direction,and thus is not beneficial for the carrier mobility.Finally,through introducing dominant Te _Biantisites,the n-Bi₂Te₃film can achieve the state-of-the-art thermoelectric power factor of 5.05m Wm^-1K^-2at 240 K,significantly superior to films containing V_Teand Bi _Teas dominant defects.This is one of the best electronic transport properties of n-type Bi₂Te₃thin films so far.（3）In this work,by virtue of advanced techniques of STM,ARPES,STEM and XPS,we are able to identify the structural evolution in the atomic scale for Bi-rich Bi₂Te₃films.The excess of the Bi content will lead to the formation of p-type Bi _Teantisite defects,however,there is a doping limit for the excess of Bi to form Bi _Teantisites.Beyond this limit,the excess of Bi will form the n-type Bi₂planar defects in the van der Waals gap,the excellent electron donors,which can enhance the electron density by one to two orders of magnitude for Bi-rich Bi₂Te₃films.The conversions from n-to p-and to n-type conduction in the Bi₂Te₃films grown at higher substrate temperatures are observed by merely changing the Te/Bi flux ratio.This special phenomenon is resulted from the dominant defects Bi₂,Bi _Teand V_Te,respectively.Benefiting from the remarkable increase in the electron density as well as the suppression of carrier intrinsic excitations,Bi₂Te₃films with Bi₂planar defects possess much improved thermoelectric power factors,with a maximum value of 1.4 m Wm^-1K^-2at 450 K,showing about 130%enhancement than that of the film without Bi₂intercalations.The discovery opens a new avenue to improve the thermoelectric properties of Bi₂Te₃films utilizing the Bi₂planar defects.（4）Through precisely manipulating the growth parameters,this work realizes the fabrication of natural Bi₂-2Bi₂Te₃and Bi₂-Bi₂Te₃superlattice films as well as the complex artificial x Bi₂Te₃-y Bi₂superlattice films.High resolution thin film XRD and STEM measurements verify the formation of Bi₂-2Bi₂Te₃and Bi₂-Bi₂Te₃superlattice structures.ARPES characterizations reveal novel electronic band structure as well as their possible topological features of these two nature superlattices.In the case of the Bi₂layers and Bi₂Te₃layers as the outmost termination layers,the electronic band structure of artificial x Bi₂Te₃-y Bi₂superlattices are apparently different,especially for the dual-cone structure in the band structure when the Bi₂layer are the termination layers.This work finds out that,the strong electron transfer from the Bi₂unit to the Bi₂Te₃unit leads to the Fermi level shifting deep into the conduction band.And,the electron density of Bi₂-Bi₂Te₃based superlattice films is then markedly increased,resulting in high electrical conductivity of up to～49.5（?）10⁴Sm^-1.Benefiting from the suppression of carrier intrinsic excitations and the enhanced Seebeck coefficient,the power factor of the 12Bi₂-6Bi₂Te₃superlattice films is significantly improved,which shows an enhancement of 477%and 243%as compared to that of pure Bi and Bi₂Te₃films.This is an important example for optimizing TE performances by virtue of“dual directions/phases”optimizations.