The scanning tunneling microscopy and spectroscopy of GaSb_(1-x)Bi_(x) films of a few-nanometer thickness grown by molecular beam epitaxy

The ultrahigh vacuum scanning tunneling microscope(STM)was used to characterize the GaSb_(1-x)Bi_(x) films of a few nanometers thickness grown by the molecular beam epitaxy(MBE)on the GaSb buffer layer of 100 nm with the GaSb(100)substrates.The thickness of the GaSb_(1-x)Bi_(x) layers of the samples are 5 and 10 nm,respectively.For comparison,the GaSb buffer was also characterized and its STM image displays terraces whose surfaces are basically atomically flat and their roughness is generally less than 1 monolayer(ML).The surface of 5 nm GaSb_(1-x)Bi_(x) film reserves the same terraced morphology as the buffer layer.In contrast,the morphology of the 10 nm GaSb_(1-x)Bi_(x) film changes to the mound-like island structures with a height of a few MLs.The result implies the growth mode transition from the two-dimensional mode as displayed by the 5 nm film to the Stranski-Krastinov mode as displayed by the 10 nm film.The statistical analysis with the scanning tunneling spectroscopy(STS)measurements indicates that both the incorporation and the inhomogeneity of Bi atoms increase with the thickness of the GaSb_(1-x)Bi_(x) layer.

Molecular Beam Epitaxy Growth and Scanning Tunneling Microscopy Study of Pyrite CuSe2 Films on SrTiO3

We perform molecular beam epitaxy growth and scanning tunneling microscopy study of copper diselenide （CuSe2 ） films on SrTiO3 （001）. Using a Se-rich condition, the single-phase pyrite CuSe2 grows in the Stranski-Krastanov （layer-plus-island） mode with a preferential orientation of （111）. Our careful inspection of both the as-grown and post-annealed CuSe2 films at various temperatures invariably shows a Cu-terminated surface, which, depending on the annealing temperature, reconstructs into two distinct structures 2 ×√3 and √x ×√3-R30°. The Cu termi- nation is supported by the depressed density of states near the Fermi level, measured by in-situ low temperature scanning tunneling spectroscopy. Our study helps understand the preparation and surface chemistry of transition metal pyrite dichalcogenides thin films.

Electrical properties and structural optimization of GaN/InGaN/GaN tunnel junctions grown by molecular beam epitaxy

The InGaN films and GaN/InGaN/GaN tunnel junctions(TJs)were grown on GaN templates with plasma-assisted molecular beam epitaxy.As the In content increases,the quality of InGaN films grown on GaN templates decreases and the surface roughness of the samples increases.V-pits and trench defects were not found in the AFM images.p++-GaN/InGaN/n++-GaN TJs were investigated for various In content,InGaN thicknesses and doping concentration in the InGaN insert layer.The InGaN insert layer can promote good interband tunneling in GaN/InGaN/GaN TJ and significantly reduce operating voltage when doping is sufficiently high.The current density increases with increasing In content for the 3 nm InGaN insert layer,which is achieved by reducing the depletion zone width and the height of the potential barrier.At a forward current density of 500 A/cm^(2),the measured voltage was 4.31 V and the differential resistance was measured to be 3.75×10^(−3)Ω·cm^(2)for the device with a 3 nm p++-In_(0.35)Ga_(0.65)N insert layer.When the thickness of the In_(0.35)Ga_(0.65)N layer is closer to the“balanced”thickness,the TJ current density is higher.If the thickness is too high or too low,the width of the depletion zone will increase and the current density will decrease.The undoped InGaN layer has a better performance than n-type doping in the TJ.Polarization-engineered tunnel junctions can enhance the functionality and performance of electronic and optoelectronic devices.

Comparison of resonant tunneling diodes grown on freestanding GaN substrates and sapphire substrates by plasma-assisted molecular-beam epitaxy

AlN/GaN resonant tunneling diodes(RTDs)were grown separately on freestanding Ga N(FS-GaN)substrates and sapphire substrates by plasma-assisted molecular-beam epitaxy(PA-MBE).Room temperature negative differential resistance(NDR)was obtained under forward bias for the RTDs grown on FS-GaN substrates,with the peak current densities(Jp)of 175-700 kA/cm^(2)and peak-to-valley current ratios(PVCRs)of 1.01-1.21.Two resonant peaks were also observed for some RTDs at room temperature.The effects of two types of substrates on epitaxy quality and device performance of GaN-based RTDs were firstly investigated systematically,showing that lower dislocation densities,flatter surface morphology,and steeper heterogeneous interfaces were the key factors to achieving NDR for RTDs.

Nanoelectronic devices resonant tunnelling diodes grown on InP substrates by molecular beam epitaxy with peak to valley current ratio of 17 at room temperature

This paper reports that InAs/In0.53Ga0.47As/AlAs resonant tunnelling diodes have been grown on InP substrates by molecular beam epitaxy. Peak to valley current ratio of these devices is 17 at 300K. A peak current density of 3kA/cm^2 has been obtained for diodes with AlAs barriers of ten monolayers, and an Ino.53Ga0.47As well of eight monolayers with four monolayers of InAs insert layer. The effects of growth interruption for smoothing potential barrier interfaces have been investigated by high resolution transmission electron microscope.

Molecular beam epitaxy and superconductivity of stoichiometric FeSe and K_x Fe_(2-y)Se_2 crystalline films

Our recent progress in the fabrication of FeSe and KxFe2_ySe2 ultra thin films and the understanding of their superconductivity properties is reviewed. The growth of high-quality FeSe and KxFe2_ySe2 films is achieved in a well controlled manner by molecular beam epitaxy. The high-quality stoichiometric and superconducting crystalline thin films allow us to investigate the intrinsic superconductivity properties and the interplay between the superconductivity and the film thickness, the local structure, the substrate, and magnetism. In situ low-temperature scanning tunneling spectra reveal the nodes and the twofold symmetry in FeSe, high-temperature superconductivity at the FeSe/SrTiO3 interface, phase separation and magnetic order in KxFe2_ySe2, and the suppression of superconductivity by twin boundaries and Fe vacancies. Our findings not only provide fundamental information for understanding the mechanism of unconventional superconductivity, but also demonstrate a powerful way of engineering superconductors and raising the transition temperature.

Molecular beam epitaxy growth of iodide thin films

Study of two-dimensional(2D)magnetic materials is important for both fundamental research and application.Here we report molecular beam epitaxy growth of iodides,candidates for exhibiting 2D magnetism.Decomposition of CrI_(3)is utilized to produce stable gaseous I_(2)flux.Growth of MnI_(2),GdI_(3),and CrI_(2)down to monolayer is successful achieved by co-depositing I2 and corresponding metal atoms.The thin films of the three materials are characterized by scanning tunneling microscope and found to be insulators with bandgaps of 4.4 e V,0.6 e V,and 3.0 e V,respectively.The film growth paves the way for further study of magnetic properties at the 2 D limit.

Direct Imaging of Molecular Orbitals of Metal Phthalocyanines on Metal Surfaces with an O2-Functionalized Tip of a Scanning Tunneling Microscope

High-resolution scanning tunneling microscope images of iron phthalocyanine and zinc phthalocyanine molecules on Au（111） have been obtained using a functionalized tip of a scanning tunneling microscope （STM）, and show rich intramolecular features that are not observed using clean tips. Ab initio density functional theory calculations and extended Huckel theory calculations revealed that the imaging of detailed electronic states is due specifically to the decoration of the STM tip with O2. The detailed structures are differentiated only when interacting with the highly directional orbitals of the oxygen molecules adsorbed on a truncated, [111]-oriented tungsten tip. Our results indicate a method for increasing the resolution in generic scans and thus, have potential applications in fundamental research based on high-resolution electronic states of molecules on metals, concerning, for example, chemical reactions, and catalysis mechanisms.

Measuring fine molecular structures with luminescence signal from an alternating current scanning tunneling microscope

In scanning tunneling microscopy-induced luminescence(STML),the photon count is measured to reflect single-molecule properties,e.g.,the first molecular excited state.The energy of the first excited state is typically shown by a rise of the photon count as a function of the bias voltage between the tip and the substrate.It remains a challenge to determine the precise rise position of the current due to possible experimental noise.In this work,we propose an alternating current version of STML to resolve the fine structures in the photon count measurement.The measured photon count and the current at the long-time limit show a sinusoidal oscillation.The zero-frequency component of the current shows knee points at the precise voltage as the fraction of the detuning between the molecular gap and the DC component of the bias voltage.We propose to measure the energy level with discontinuity of the first derivative of such a zero-frequency component.The current method will extend the application of STML in terms of measuring molecular properties.

Scanning tunneling microscopic observation of enhanced superconductivity in epitaxial Sn islands grown on SrTiO3 substrate

Recent experimental and theoretical studies of single-layer FeSe film grown on SrTiO_3 have revealed interface enhanced superconductivity, which opens up a pathway to promote the superconducting transition temperature. Here, to investigate the role of SrTiO_3 substrate in epitaxial superconducting film, we grew a conventional superconductor b-Sn(bulk T_c～ 3.72 K) onto SrTiO_3 substrate by molecular beam epitaxy. By employing scanning tunneling microscope and spectroscopic measurements, an enhanced Tcof 8.2 K is found for epitaxial b-Sn islands, deduced by fitting the temperature dependence of the gap values using the BCS formula. The observed interfacial charge injection and enhanced electron–phonon coupling are responsible for this Tcenhancement. Moreover, the critical field of 8.3 T exhibits a tremendous increase due to the suppression of the vortex formation. Therefore, the coexistence of enhanced superconductivity and high critical field of Sn islands demonstrates a feasible and effective route to improve the superconductivity by growing the islands of conventional superconductors on perovskite-type titanium oxide substrates.

Influence of spacer layer thickness on the current-voltage characteristics of pseudomorphic AlAs/In_(0.53)Ga_(0.47)As/InAs resonant tunnelling diodes

This paper investigates the dependence of current voltage characteristics of AlAs/In0.53Ga0.47As/InAs resonant tunnelling diodes （RTDs） on spacer layer thickness. It finds that the peak and the valley current density J in the negative differential resistance （NDR） region depends strongly on the thickness of the spacer layer. The measured peak to valley current ratio of RTDs studied here is shown to improve while the current density through RTDs decreases with increasing spacer layer thickness below a critical value.

基于Surfscan的椭圆缺陷测量

为满足快速准确测量分子束外延(MBE)生长的GaAs表面椭圆缺陷,提出了基于表面颗粒度扫描仪(Surfscan)测量椭圆缺陷的方法。根据理论计算,将Surfscan的测试模型由球形优化为更适用于椭圆缺陷的椭球形测试模型。由于椭圆缺陷的长轴基本都沿[1■0]方向,通过旋转晶片,分别沿着外延片的[1■0]和[110]晶向上进行激光扫描,测量外延片表面缺陷的尺寸、位置和数量,并通过Surfscan的数据分析系统读取选取椭圆缺陷的长轴和短轴尺寸,然后利用椭球形测试模型计算出所选取椭圆缺陷的长轴和短轴尺寸,与光学显微镜测试结果比较发现,Surfscan测试的原始数据与光学显微镜测试结果差别较大,而经椭球测试模型优化后的结果与光学显微镜测试结果一致;利用两次扫描的缺陷尺寸和数量的变化以及椭圆缺陷的长短轴比,能够计算出椭圆缺陷的数量和占总缺陷的比例,与光学显微镜测试结果较为一致。相比于光学显微镜测试外延片缺陷用时几十分钟,Surfscan测试只需要10 min左右就可完成,缩短了测试时间,并且可以扫描样品的整个表面,减少了人为因素的影响,重复性更好,满足生产需要。

Manipulation and control of a single molecular rotor on Au(111) surface

Three different methods are used to manipulate and control phthalocyanine based single molecular rotors on Au （111） surface： （1） changing the molecular structure to alter the rotation potential; （2） using the tunnelling current of the scanning tunnelling microscope （STM） to change the thermal equilibrium of the molecular rotor; （3） artificial manipulation of the molecular rotor to switch the rotation on or off by an STM tip. Furthermore, a molecular ＇gear wheel＇ is successfully achieved with two neighbouring molecules.

A bistable, self-latching inverter by the monolithic integration of resonant tunnelling diode and high electron mobility transistor

This paper reports that the structures of AlGaAs/InGaAs high electron mobility transistor （HEMT） and AlAs/GaAs resonant tunnelling diode （RTD） are epitaxially grown by molecular beam epitaxy （MBE） in turn on a GaAs substrate. An Alo.24Gao.76As chair barrier layer, which is grown adjacent to the top AlAs barrier, helps to reduce the valley current of RTD. The peak-to-valley current ratio of fabricated RTD is 4.8 and the transconductance for the 1-μm gate HEMT is 125mS/mm. A static inverter which consists of two RTDs and a HEMT is designed and fabricated. Unlike a conventional CMOS inverter, the novel inverter exhibits self-latching property.

Epitaxial growth of antimony nanofilms on HOPG and thermal desorption to control the film thickness

Group-V elemental nanofilms were predicted to exhibit interesting physical properties such as nontrivial topological properties due to their strong spin-orbit coupling,the quantum confinement,and surface effect.It was reported that the ultrathin Sb nanofilms can undergo a series of topological transitions as a function of the film thickness h:from a topological semimetal(h>7.8 nm)to a topological insulator(7.8 nm>h>2.7 nm),then a quantum spin Hall(QSH)phase(2.7 nm>h>1.0 nm)and a topological trivial semiconductor(h<1.0 nm).Here,we report a comprehensive investigation on the epitaxial growth of Sb nanofilms on highly oriented pyrolytic graphite(HOPG)substrate and the controllable thermal desorption to achieve their specific thickness.The morphology,thickness,atomic structure,and thermal-strain effect of the Sb nanofilms were characterized by a combination study of scanning electron microscopy(SEM),atomic force microscopy(AFM),and scanning tunneling microscopy(STM).The realization of Sb nanofilms with specific thickness paves the way for the further exploring their thickness-dependent topological phase transitions and exotic physical properties.

Dependence of current-voltage characteristics of pseudomorphic AlAs/In_(0.53)Ga_(0.47)As/InAs resonant tunnelling diodes on quantum well widths

This paper studies the dependence of I - V characteristics on quantum well widths in AIAs/In0.53Ga0.47As and AIAs/In0.53Ga0.47As/InAs resonant tunnelling structures grown on InP substrates. It shows that the peak and the valley current density in the negative differential resistance region are closely related with quantum well width. The measured peak current density, valley current densities and peak-to-valley current ratio of resonant tunnelling diodes are continually decreasing with increasing well width.

利用扫描隧道显微术对SrTiO_(3)(001)上生长CoSe/FeSe异质界面结构和谱学研究

硒化铁/钛酸锶(001)(FeSe/SrTiO_(3)(001),FeSe/STO)中界面增强的超导电性一直是近些年凝聚态物理领域的热点问题之一,基于FeSe薄膜构筑异质界面是调控其超导电性和构筑新奇量子物态的重要手段。本文报道了利用分子束外延在SrTiO_(3)(001)衬底上制备了高质量硒化钴-硒化铁(CoSe/FeSe)纵向异质结的方法,利用扫描隧道显微镜和扫描隧道谱详细研究了该异质界面的电子性质和电子态密度的实空间分布。由于CoSe/FeSe界面存在强烈的电荷转移,FeSe处于过掺杂状态,导致FeSe/STO界面相互作用已不满足非绝热近似,以及FeSe中超导电性的消失。空间依赖的扫描隧道谱显示,CoSe/FeSe的异质界面边缘处出现了晶向依赖的边缘电子态和边角电子态,这些边界上的电子态受到CoSe/FeSe界面电荷转移效应的调制。该研究为基于单层FeSe/STO界面超导电性的调控及量子结构的构筑提供了借鉴。

Bimodal growth of Fe islands on graphene

Magnetic metals deposited on graphene hold the key to applications in spintronics. Here, we present the results of Fe islands grown on graphene/Si C(0001) by molecular beam epitaxy, which are investigated by scanning tunneling microscopy. The two types of islands distinguished by flat or round tops are revealed, indicating bimodal growth of Fe. The atomic structures on the top surfaces of flat islands are also clearly resolved. Our results may improve the understanding of the mechanisms of metals deposited on graphene and pave the way for future spintronic applications of Fe/graphene systems.

Single-layer CrI3 grown by molecular beam epitaxy

Single-and few-layer chromium triiodide(CrI3),which has been intensively investigated as a promising platform for two-dimensional magnetism,is usually prepared by the mechanical exfoliation.Here,we report direct growth of single-layer CrI3 using molecular beam epitaxy in ultrahigh vacuum.Scanning tunneling microscopy(STM),together with density functional theory(DFT)calculation,revealed that the iodine trimers,each of which consists of three I atoms surrounding a three-fold Cr honeycomb center,are the basic units of the topmost I layer.Different superstructures of single-layer CrI3 with periodicity around 2–4 nm were obtained on Au(111),while only the 1×1 structure was observed on the graphite substrate.At an elevated temperature of 423 K,single-layer CrI3 began to decompose and transformed into single-layer chromium diiodide.Our bias-dependent STM images suggest that the unoccupied and occupied states are spatial-separately distributed,consistent with the results of our DFT calculation.We also discussed the role of charge distribution in the super-exchange interactions among Cr atoms in single-layer CrI3.

Atomically constructing a van der Waals heterostructure of CrTe_(2)/Bi_(2)Te_(3) by molecular beam epitaxy

A 2D heterostructure with proximity coupling of magnetism and topology can provide enthralling prospects for hosting new quantum states and exotic properties that are relevant to next-generation spintronic devices.Here,we synthesize a delicate van der Waals(vdW)heterostructure of CrTe_(2)/Bi_(2)Te_(3) at the atomic scale via molecular beam epitaxy.Low-temperature scanning tunneling microscopy/spectroscopy measurements are utilized to characterize the geometric and electronic properties of the CrTe_(2)/Bi_(2)Te_(3) heterostructure with a compressed vdW gap.Detailed structural analysis reveals complex interfacial structures with diversiform step heights and intriguing moirépatterns.The formation of the interface is ascribed to the embedded characteristics of CrTe_(2) and Bi_(2)Te_(3) by sharing Te atomic layer upon interfacing,showing intercoupled features of electronic structure for CrTe_(2) and Bi_(2)Te_(3).Our study demonstrates a possible approach to construct artificial heterostructures with different types of ordered states,which may be of use for achieving tunable interfacial Dzyaloshinsky–Moriya interactions and tailoring the functional building blocks in low dimensions.