Development of in situ characterization techniques in molecular beam epitaxy

Ex situ characterization techniques in molecular beam epitaxy(MBE)have inherent limitations,such as being prone to sample contamination and unstable surfaces during sample transfer from the MBE chamber.In recent years,the need for improved accuracy and reliability in measurement has driven the increasing adoption of in situ characterization techniques.These techniques,such as reflection high-energy electron diffraction,scanning tunneling microscopy,and X-ray photoelectron spectroscopy,allow direct observation of film growth processes in real time without exposing the sample to air,hence offering insights into the growth mechanisms of epitaxial films with controlled properties.By combining multiple in situ characterization techniques with MBE,researchers can better understand film growth processes,realizing novel materials with customized properties and extensive applications.This review aims to overview the benefits and achievements of in situ characterization techniques in MBE and their applications for material science research.In addition,through further analysis of these techniques regarding their challenges and potential solutions,particularly highlighting the assistance of machine learning to correlate in situ characterization with other material information,we hope to provide a guideline for future efforts in the development of novel monitoring and control schemes for MBE growth processes with improved material properties.

Pit density reduction for AlN epilayers grown by molecular beam epitaxy using Al modulation method

We have investigated homoepitaxy of AlN films grown by molecular beam epitaxy on AlN/sapphire templates byadopting both the continuous growth method and the Al modulation epitaxy(AME)growth method.The continuous growthmethod encounters significant challenges in controlling the growth mode.As the precise Al/N=1.0 ratio is difficult toachieve,either the excessive Al-rich or N-rich growth mode occurs.In contrast,by adopting the AME growth method,sucha difficulty has been effectively overcome.By manipulating the supply time of the Al and nitrogen sources,we were able toproduce AlN films with much improved surface morphology.The first step of the AME method,only supplying Al atoms,is important to wet the surface and the Al adatoms can act as a surfactant.Optimization of the initial Al supply time caneffectively reduce the pit density on the grown AlN surface.The pits density dropped from 12 pits/μm^(2)to 1 pit/μm^(2)andthe surface roughness reduced from 0.72 nm to 0.3 nm in a 2×2μm^(2)area for the AME AlN film homoepitaxially grownon an AlN template.

Centimeter-Scale Above-Room-Temperature Ferromagnetic Fe_(3)GaTe_(2)Thin Films by Molecular Beam Epitaxy

Fe_(3)GaTe_(2),as a layered ferromagnetic material,has a Curie temperature(T_(c))higher than room temperature,making it the key material in next-generation spintronic devices.To be used in practical devices,large-sized high-quality Fe_(3)GaTe_(2)thin films need to be prepared.Here,the centimeter-scale thin film samples with high crystal quality and above-room-temperature ferromagnetism with strong perpendicular magnetic anisotropy were prepared by molecular beam epitaxy technology.Furthermore,the Tc of the samples raises as the film thickness increases,and reaches 367K when the film thickness is 60 nm.This study provides material foundations for the new generation of van der Waals spintronic devices and paves the way for the commercial application of Fe_(3)GaTe_(2).

Use of the epitaxial MTBs as a 1D gate(Lg=0.4 nm)for the construction of scaling down two-dimensional field-effect transistors

In recent years,there has been a significant increase in research focused on the growth of large-area single crystals.Rajan et al.[1]recently achieved the growth of large-area monolayers of transition-metal chalcogenides through assisted nucleation.The quality of molecular beam epitaxy(MBE)-grown two-dimensional(2D)materials can be greatly enhanced by using sacrificial species deposited simultaneously from an electron beam evaporator during the growth process.This technique notably boosts the nucleation rate of the target epitaxial layer,resulting in large,homogeneous monolayers with improved quasiparticle lifetimes and fostering the development of epitaxial van der Waals heterostructures.Additionally,micrometer-sized silver films have been formed at the air-water interface by directly depositing electrospray-generated silver ions onto an aqueous dispersion of reduced graphene oxide under ambient conditions[2].

Metal-modulated epitaxy of Mg-doped Al_(0.80)In_(0.20)N-based layer for application as the electron blocking layer in deep ultraviolet light-emitting diodes

This work reports the growth and characterization of p-AlInN layers doped with Mg by plasma-assisted molecular beam epitaxy(PAMBE).AlInN was grown with an Al molar fraction of 0.80 by metal-modulated epitaxy(MME)with a thickness of 180 nm on Si(111)substrates using AlN as buffer layers.Low substrate temperatures were used to enhance the incorporation of indium atoms into the alloy without clustering,as confirmed by X-ray diffraction(XRD).Cathodoluminescence measurements revealed ultraviolet(UV)range emissions.Meanwhile,Hall effect measurements indicated a maximum hole mobility of 146 cm^(2)/(V∙s),corresponding to a free hole concentration of 1.23×10^(19)cm^(−3).The samples were analyzed by X-ray photoelectron spectroscopy(XPS)estimating the alloy composition and extracting the Fermi level by valence band analysis.Mg-doped AlInN layers were studied for use as the electron-blocking layer(EBL)in LED structures.We varied the Al composition in the EBL from 0.84 to 0.96 molar fraction to assess its theoretical effects on electroluminescence,carrier concentration,and electric field,using SILVACO Atlas.The results from this study highlight the importance and capability of producing high-quality Mg-doped p-AlInN layers through PAMBE.Our simulations suggest that an Al content of 0.86 is optimal for achieving desired outcomes in electroluminescence,carrier concentration,and electric field.

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.

Enhanced thermal emission from metal-free,fully epitaxial structures with epsilon-near-zero InAs layers

We introduce a novel method to create mid-infrared(MIR)thermal emitters using fully epitaxial,metal-free structures.Through the strategic use of epsilon-near-zero(ENZ)thin films in InAs layers,we achieve a narrow-band,wide-angle,and p-polarized thermal emission spectra.This approach,employing molecular beam epitaxy,circumvents the complexities associated with current layered structures and yields temperature-resistant emission wavelengths.Our findings contribute a promising route towards simpler,more efficient MIR optoelectronic devices.

Reshaping Li–Mg hybrid batteries:Epitaxial electrodeposition and spatial confinement on MgMOF substrates via the lattice‐matching strategy

The emergence of Li–Mg hybrid batteries has been receiving attention,owing to their enhanced electrochemical kinetics and reduced overpotential.Nevertheless,the persistent challenge of uneven Mg electrodeposition remains a significant impediment to their practical integration.Herein,we developed an ingenious approach that centered around epitaxial electrocrystallization and meticulously controlled growth of magnesium crystals on a specialized MgMOF substrate.The chosen MgMOF substrate demonstrated a robust affinity for magnesium and showed minimal lattice misfit with Mg,establishing the crucial prerequisites for successful heteroepitaxial electrocrystallization.Moreover,the incorporation of periodic electric fields and successive nanochannels within the MgMOF structure created a spatially confined environment that considerably promoted uniform magnesium nucleation at the molecular scale.Taking inspiration from the“blockchain”concept prevalent in the realm of big data,we seamlessly integrated a conductive polypyrrole framework,acting as a connecting“chain,”to interlink the“blocks”comprising the MgMOF cavities.This innovative design significantly amplified charge‐transfer efficiency,thereby increasing overall electrochemical kinetics.The resulting architecture(MgMOF@PPy@CC)served as an exceptional host for heteroepitaxial Mg electrodeposition,showcasing remarkable electrostripping/plating kinetics and excellent cycling performance.Surprisingly,a symmetrical cell incorporating the MgMOF@PPy@CC electrode demonstrated impressive stability even under ultrahigh current density conditions(10mAcm–2),maintaining operation for an extended 1200 h,surpassing previously reported benchmarks.Significantly,on coupling the MgMOF@PPy@CC anode with a Mo_(6)S_(8) cathode,the assembled battery showed an extended lifespan of 10,000 cycles at 70 C,with an outstanding capacity retention of 96.23%.This study provides a fresh perspective on the rational design of epitaxial electrocrystallization driven by metal–organic framework(MOF)substrates,paving the way toward the advancement of cuttingedge batteries.

Embedded high-quality ternary GaAs_(1−x)Sb_(x) quantum dots in GaAs nanowires by molecular-beam epitaxy

Semiconductor quantum dots are promising candidates for preparing high-performance single photon sources.A basic requirement for this application is realizing the controlled growth of high-quality semiconductor quantum dots.Here,we report the growth of embedded GaAs_(1−x)Sb_(x) quantum dots in GaAs nanowires by molecular-beam epitaxy.It is found that the size of the GaAs_(1−x)Sb_(x) quantum dot can be well-defined by the GaAs nanowire.Energy dispersive spectroscopy analyses show that the antimony content x can be up to 0.36 by tuning the growth temperature.All GaAs_(1−x)Sb_(x) quantum dots exhibit a pure zinc-blende phase.In addition,we have developed a new technology to grow GaAs passivation layers on the sidewalls of the GaAs_(1−x)Sb_(x) quantum dots.Different from the traditional growth process of the passivation layer,GaAs passivation layers can be grown simultaneously with the growth of the embedded GaAs_(1−x)Sb_(x) quantum dots.The spontaneous GaAs passivation layer shows a pure zinc-blende phase due to the strict epitaxial relationship between the quantum dot and the passivation layer.The successful fabrication of embedded high-quality GaAs_(1−x)Sb_(x) quantum dots lays the foundation for the realization of GaAs_(1−x)Sb_(x)-based single photon sources.

Epitaxial Properties of Co-Doped ZnO Thin Films Grown by Plasma Assisted Molecular Beam Epitaxy

High quality Co-doped ZnO thin films are grown on single crystalline Al2O3（0001） and ZnO（0001） substrates by oxygen plasma assisted molecular beam epitaxy at a relatively lower substrate temperature of 450℃. The epitaxial conditions are examined with in-situ reflection high energy electron diffraction （RHEED） and ex-situ high resolution x-ray diffraction （HRXRD）. The epitaxial thin films are single crystal at film thickness smaller than 500nm and nominal concentration of Co dopant up to 20%. It is indicated that the Co cation is incorporated into the ZnO matrix as Co^2＋ substituting Zn^2＋ ions. Atomic force microscopy shows smooth surfaces with rms roughness of 1.9 nm. Room-temperature magnetization measurements reveal that the Co-doped ZnO thin films are ferromagnetic with Curie temperatures Tc above room temperature.

CeO_2[100]∥MgO[100]in-plane Epitaxy for Bi epitaxial Grain Boundary Junctions

In order to obtain bi epitaxial 45° grain boundary YBa 2Cu 3O 7 (YBCO) junctions, the in plane epitaxy of CeO 2 films on both MgO films and SrTiO 3 substrates was studied. Using magnetron sputtering technique and decreasing the substrate temperature, 100% CeO 2∥MgO in plane orientation for bi epitaxial grain boundary junction can be obtained.

Epitaxially Grown Ru Clusters-Nickel Nitride Heterostructure Advances Water Electrolysis Kinetics in Alkaline and Seawater Media

The epitaxial heterostructure can be rationally designed based on the in situ growth of two compatible phases with lattice similarity,in which the modulated electronic states and tuned adsorption behaviors are conducive to the enhancement of electrocatalytic activity.Herein,theoretical simulations first disclose the charge transfer trend and reinforced inherent electron conduction around the epitaxial heterointerface between Ru clusters and Ni_(3)N substrate(cRu-Ni_(3)N),thus leading to the optimized adsorption behaviors and reduced activation energy barriers.Subsequently,the defectrich nanosheets with the epitaxially grown cRu-Ni_(3)N heterointerface are successfully constructed.Impressively,by virtue of the superiority of intrinsic activity and reaction kinetics,such unique epitaxial heterostructure exhibits remarkable bifunctional catalytic activity toward electrocatalytic OER(226 mV@20 mA cm^(−2))and HER(32 mV@10 mA cm^(−2))in alkaline media.Furthermore,it also shows great application prospect in alkaline freshwater and seawater splitting,as well as solar-to-hydrogen integrated system.This work could provide beneficial enlightenment for the establishment of advanced electrocatalysts with epitaxial heterointerfaces.

Rapid epitaxy of 2-inch and high-quality α-Ga_(2)O_(3) films by mist-CVD method

High thickness uniformity and large-scale films of α-Ga_(2)O_(3) are crucial factors for the development of power devices.In this work, a high-quality 2-inch α-Ga_(2)O_(3) epitaxial film on c-plane sapphire substrates was prepared by the mist-CVD method.The growth rate and phase control mechanisms were systematically investigated. The growth rate of the α-Ga_(2)O_(3) films was limited by the evaporation of the microdroplets containing gallium acetylacetonate. By adjusting the substrate position(z) from 80 to 50 mm, the growth rate was increased from 307 nm/h to 1.45 μm/h when the growth temperature was fixed at 520 °C.When the growth temperature exceeded 560 °C, ε-Ga_(2)O_(3) was observed to form at the edges of 2-inch sapphire substrate.Phase control was achieved by adjusting the growth temperature. When the growth temperature was 540 °C and the substrate position was 50 mm, the full-width at half maximum(FWHM) of the rocking curves for the(0006) and(10-14) planes were 0.023° and 1.17°. The screw and edge dislocations were 2.3 × 10~6 and 3.9 × 10~(10)cm~(-2), respectively. Furthermore, the bandgaps and optical transmittance of α-Ga_(2)O_(3) films grown under different conditions were characterized utilizing UV-visible and near-IR scanning spectra.

Superconductivity in epitaxially grown LaVO_(3)/KTaO_(3)(111)heterostructures

Complex oxide heterointerfaces can host a rich of emergent phenomena,and epitaxial growth is usually at the heart of forming these interfaces.Recently,a strong crystalline-orientation-dependent two-dimensional superconductivity was discovered at interfaces between KTaO_(3)single-crystal substrates and films of other oxides.Unexpectedly,rare of these oxide films was epitaxially grown.Here,we report the existence of superconductivity in epitaxially grown LaVO_(3)/KTaO_(3)(111)heterostructures,with a superconducting transition temperature of~0.5 K.Meanwhile,no superconductivity was detected in the(001)-and(110)-orientated LaVO_(3)/KTaO_(3)heterostructures down to 50 mK.Moreover,we find that for the LaVO_(3)/KTaO_(3)(111)interfaces to be conducting,an oxygen-deficient growth environment and a minimum LaVO_(3)thickness of~0.8 nm(~2 unit cells)are needed.

Review of a direct epitaxial approach to achieving micro-LEDs

There is a significantly increasing demand of developing augmented reality and virtual reality(AR and VR) devices,where micro-LEDs(μLEDs) with a dimension of ≤ 5 μm are the key elements. Typically, μLEDs are fabricated by dry-etching technologies, unavoidably leading to a severe degradation in optical performance as a result of dry-etching induced damages. This becomes a particularly severe issue when the dimension of LEDs is ≤ 10 μm. In order to address the fundamental challenge, the Sheffield team has proposed and then developed a direct epitaxial approach to achievingμLEDs, where the dry-etching technologies for the formation of μLED mesas are not needed anymore. This paper provides a review on this technology and then demonstrates a number of monolithically integrated devices on a single chip using this technology.

Epitaxial Growth of Si(111)/Er2O3(111)Structure on Si(111)by Molecular Beam Epitaxy

The Si overlayers are grown by molecular beam epitaxy on atomically smllth Er_(2)O_(3)(111)films prepared on Si(111)substrates.Single crystalline Si overlayers are achieved and are evident due to the spot-like reflective high energy electron diffraction(RHEED)patterns and x-ray diffraction patterns.The epitaxial relationship of the Si overlayer along the surface with respect to the orientation of Er_(2)O_(3)and the Si substrate is as follows:overgrown Si(111)//Er_(2)O_(3)(111)//Si(111).The rough surface of Si overlayers,as identified by both RHEED patterns and atomic force microscopy images,indicates a three-dimensional growth mode.The reason for this is based on the interfacial energy argument.Further growth of Er_(2)O_(3)films on this rough Si overlayer leads to the polycrystalline nature of the topmost Er2O3 layer.

A first-principles study on remote van der Waals epitaxy through a graphene monolayer on semiconductor substrates

To investigate the mechanism of remote epitaxy, where the overlayer can follow the same crystalline structure as the underlying semiconductor substrate through a thin two-dimensional interlayer, we systematically study the potential fluctuations of graphene covered Si, Ga As, and Ga N substrates from first-principles. We find that the uneven semiconductor surface, the distorted graphene, and the non-uniform interface charge transfer make significant contributions to the potential fluctuation. The semiconductor substrate with different surface reconstructions and orientations will generate different potential fluctuations through the graphene interlayer. We also calculate and compare the adsorption of adatoms on graphene covered substrates. The adsorption energies of adatoms not only depend on their distances to the underlying semiconductor surface, but are also sensitive to the direction of the charge transfer at the graphene/substrate interface. Changing the semiconductor reconstruction or orientation could even reverse the order of the adsorption energies of cation and anion adatoms by reversing the interface charge transfer direction, leading to a change in the growth orientation of the overlayer.Our study improves the understanding of the mechanism of remote epitaxy, and reveals that it is possible to control the initial nucleation and orientation of overlayers by changing the semiconductor reconstructions and/or orientations in remote epitaxy.

Intercalation of hafnium oxide between epitaxially-grown monolayer graphene and Ir(111)substrate

Intercalation of insulating materials between epitaxial graphene and the metal substrates is highly demanded to restore the intrinsic properties of graphene,and thus essential for the graphene-based devices.Here we demonstrate a successful solution for the intercalation of hafnium oxide into the interface between full-layer graphene and Ir(111)substrate.We first intercalate hafnium atoms beneath the epitaxial graphene.The intercalation of the hafnium atoms leads to the variation of the graphene moire superstructure periodicity,which is characterized by low-energy electron diffraction(LEED)and lowtemperature scanning tunneling microscopy(LT-STM).Subsequently,we introduce oxygen into the interface,resulting in oxidization of the intercalated hafnium.STM and Raman's characterizations reveal that the intercalated hafnium oxide layer could effectively decouple the graphene from the metallic substrate,while the graphene maintains its high quality.Our work suggests a high-k dielectric layer has been successfully intercalated between high-quality epitaxial graphene and metal substrate,providing a platform for applications of large-scale,high-quality graphene for electronic devices.

Epitaxial growth triggered core-shell Pd@RuP nanorods for high-efficiency electrocatalytic hydrogen evolution

Ru with Pt-like hydrogen bond strength,knockdown cost(~1/3 of Pt),and eximious stability is a competitive replacement for Pt-based catalysts towards the hydrogen evolution reaction(HER)in water splitting.The design of Ru-based catalysts via interface construction,crystal phase control,and specific light element doping to realize the impressive promotion of limited activity and stability remains challenging.Herein,we report the fabrication of Pd@RuP core-shell nanorods(NRs)via an epitaxial growth method,where ultrathin RuP shells extend the face-centered cubic(fcc)crystal structure and(111)plane of the Pd NRs core.Density functio nal theory results confirm that the core-s hell interface engineering and P doping synergistically accelerate electron transfer and moderate the d-band center to generate a suitable affinity for H*,thus optimizing HER kinetics.Compared with Pd@Ru NRs and Pt/C,the Pd@RuP NRs exhibit preferable electrocatalytic stability and superior activity with a low overpotential of 18 mV at 10 mA cm-2in the alkaline HER process.Furthermore,the integrated Pd@RuP//RuO2-based electrolyzer also displays a low operation potential of 1.42 V to acquire 10 mA cm-2,demonstrating great potential for practical water electrolysis.Our work presents an efficient avenue to design Ru-based electrocatalysts via epitaxial growth for extraordinary HER performance.

Multiple SiGe/Si layers epitaxy and SiGe selective etching for vertically stacked DRAM

Fifteen periods of Si/Si_(0.7)Ge_(0.3)multilayers(MLs)with various Si Ge thicknesses are grown on a 200 mm Si substrate using reduced pressure chemical vapor deposition(RPCVD).Several methods were utilized to characterize and analyze the ML structures.The high resolution transmission electron microscopy(HRTEM)results show that the ML structure with 20 nm Si_(0.7)Ge_(0.3)features the best crystal quality and no defects are observed.Stacked Si_(0.7)Ge_(0.3)ML structures etched by three different methods were carried out and compared,and the results show that they have different selectivities and morphologies.In this work,the fabrication process influences on Si/Si Ge MLs are studied and there are no significant effects on the Si layers,which are the channels in lateral gate all around field effect transistor(L-GAAFET)devices.For vertically-stacked dynamic random access memory(VS-DRAM),it is necessary to consider the dislocation caused by strain accumulation and stress release after the number of stacked layers exceeds the critical thickness.These results pave the way for the manufacture of high-performance multivertical-stacked Si nanowires,nanosheet L-GAAFETs,and DRAM devices.