Molecular Beam Epitaxy Method Research Articles

Wafer‐Scale Growth of Vertical‐Structured SnSe2 Nanosheets for Highly Sensitive, Fast‐Response UV–Vis–NIR Broadband Photodetectors

AbstractOwing to the strong light–matter interactions, 2D semiconducting thin films have shown their great potential in the development of high‐performance ultraviolet–visible–near infrared (UV–Vis–NIR) broadband photodetectors (PDs). However, the planar 2D semiconducting thin films via conventional growth method are often in the form of isolated flakes and/or suffering from low absorption efficiency of light, which hinders the UV–Vis–NIR optoelectronics from widespread applications. Herein are reported high‐performance nonplanar UV–Vis–NIR broadband PD arrays, based on wafer‐scale, vertical‐structured SnSe2 nanosheet arrays (NSAs) via low‐temperature molecular beam epitaxy method. The vertical‐structured SnSe2 NSAs possess light absorption efficiency of >90% covering the wave range from 340 to 650 nm. Benefiting from the excellent light trapping ability and uniformity of waferscaled SnSe2 NSAs as well as relatively short channel, the broadband PD arrays exhibit superior comprehensive performance. They achieve a high responsivity of 31.94 A W‐1, fast speed (≈63 μs), and high uniformity. In addition, the responsivity of the broadband PD arrays is further improved via Zn‐doping technique.

Resonant x-ray scattering method for measuring cation stoichiometry in BaSnO3 thin films

We develop a resonant scattering technique to measure cation stoichiometry of lanthanum-doped BaSnO3 (BSO) thin films on a DyScO3 substrate. Samples are grown by a hybrid molecular beam epitaxy method and display high room-temperature carrier mobilities. The measured thin films are grown with widely differing cation arrival rates, with Ba being evaporated from an elemental source and Sn from a SnO2 source. Differences in mobilities in these films may arise from differences in Ba/Sn cation stoichiometry. Owing to the similar scattering strength of the Ba and Sn cations, odd-order Bragg peaks of BSO are particularly sensitive to the material's cation stoichiometry, i.e., the Ba/Sn ratio. Sensitivity to cation stoichiometry is further enhanced using the technique of resonant x-ray scattering, which changes the scattering strength of a single element across the Ba L absorption edges. We determine that the Ba/Sn cation stoichiometry varies from unity by less than 1% for films of mobility ranging from 84.8 to 144 cm2/(V s) and conclude that the mobility of these films scales with film thickness and growth rate rather than stoichiometry.

Effects of alkali-metal incorporation into epitaxial Cu(In,Ga)Se2 solar cells prepared by molecular beam epitaxy

Cu(In,Ga)Se2 (CIGS) solar cells with energy conversion efficiencies of more than 20% have been reported by several groups; most studies that reported such high efficiencies employed alkali-metal fluoride postdeposition treatments (PDTs) and Na doping. In this study, we investigated the effects of KF-PDT and NaF doping on epitaxial CIGS solar cells grown on GaAs substrates. We fabricated epitaxial CIGS layers without grain boundaries by molecular beam epitaxy methods at a substrate temperature of 520 °C. NaF doping and KF-PDT had positive effects on conversion efficiencies, even when using Cu stoichiometric CIGS layers. A conversion efficiency of more than 20% was achieved using Cu poor CIGS with NaF doping and KF-PDT after heat light soaking. Time-resolved photoluminescence was used to compare the minority carrier lifetimes of CdS-capped epitaxial CIGS layers with and without alkali-metal doping. NaF doping and KF-PDT increased minority carrier lifetimes similarly as in polycrystalline CIGS solar cells.

Modelling of epitaxial growth of two-dimensional film

The study is devoted to the theory of epitaxial growth of two-dimensional materials (in particular, the group IV elements) and its applications in creating a virtual program for modeling this growth by the molecular beam epitaxy (MBE) method.

Separation of AlN layers from silicon substrates by KOH etching

In this work, the AlN/Si(111) epitaxial structures grown consistently by plasma assisted molecular beam epitaxy (PA MBE) and hydride vapour phase epitaxy (HVPE) methods were studied. The PA MBE AlN buffer layers were synthesized via coalescence overgrowth of self-catalyzed AlN nanocolumns on Si(111) substrates and were used as templates for further HVPE growth of thick AlN layer. It was shown that described approaches can be used to obtain AlN layers with sufficiently smooth morphology. It was found that HVPE AlN inherited crystallographic polarity of the AlN layer grown by PA MBE. It was demonstrated that the etching of such AlN/Si(111) epitaxial structures results in partial separation of the AlN epilayers from the Si(111) substrate and allows to form suspended structures. Moreover, the avoidance of surface damage and backside overetching was achieved by use thin Cr film as surface protective coating and by increasing the layer thickness accordingly.

Observation of symmetry-protected Dirac states in nonsymmorphic α -antimonene

Two-dimensional (2D) Dirac states with linear band dispersion have attracted enormous interest since the discovery of graphene. However, to date, 2D Dirac semimetals are still very rare due to the fact that 2D Dirac states are generally fragile against perturbations such as spin-orbit couplings. Nonsymmorphic crystal symmetries can enforce the formation of Dirac nodes, providing a new route to establishing symmetry-protected Dirac states in 2D materials. Here we report the symmetry-protected Dirac states in nonsymmorphic alpha-antimonene. The antimonene was synthesized by the method of molecular beam epitaxy. Two Dirac cones with large anisotropy were observed by angle-resolved photoemission spectroscopy. The Dirac state in alpha-antimonene is of spin-orbit type in contrast to the spinless Dirac states in graphene. The result extends the 'graphene' physics into a new family of 2D materials where spin-orbit coupling is present.

Efficient and Anisotropic Second Harmonic Generation in Few‐Layer SnS Film

AbstractGroup‐IV monochalcogenide ferroelectric materials possess fascinating nonlinear optical properties and electro‐optical effects which play a vital role in 2D ferroelectrics, multiferroics and nonlinear optoelectronics. However, the studies of nonlinear optical properties of group‐IV monochalcogenides are still in their infancy stage. Herein, the preparation of few‐layer SnS by the molecular beam epitaxy (MBE) method is reported and the efficient second harmonic generation (SHG) of SnS monochalcogenide is systematically scrutinized by using ultrafast nonlinear optical spectroscopy. The obtained thickness‐dependent SHG is highly related with the coherence length. Second‐order nonlinear susceptibility of few‐layer SnS is also obtained. Polarization‐dependent SHG studies reveal its intrinsic anisotropy pattern and can be utilized to identify the crystalline orientation of few‐layer SnS film.

Large‐Area Tellurium/Germanium Heterojunction Grown by Molecular Beam Epitaxy for High‐Performance Self‐Powered Photodetector

AbstractAs an attractive elemental semiconductor material, p‐type tellurium (Te) with a narrow bandgap provides high carrier mobility, strong light–matter interactions in a wide spectral range, and good chemical stability, which enlightens the potential in optoelectronic devices. However, the applications are impeded by weak carrier separation and vague potential in scaling‐up. In this work, the integration of Te and conventional semiconductor germanium (Ge) is designed. Through molecular beam epitaxy (MBE) method, large‐area and uniform Te films with high crystallinity are directly deposited on the Ge substrates. The difference in work function between Te and Ge layer leads to a built‐in electric field, which can effectively enhance the carrier separation. As a result, a self‐powered splendid photovoltaic performance is observed in the MBE grown Te/Ge vertical heterojunction with current on/off ratio over 103, responsivity (R) 523 mA W−1, and specific detectivity (D*) 9.50 × 1010 cm Hz1/2 W−1 when illuminated by near‐infrared light (980 nm, 2.15 µW cm−2). Furthermore, excellent stability and high response speed of the ultrathin heterostructure offer a significant application value for multipurpose photoelectric devices.

Resolving the intrinsic bandgap and edge effect of BiI3 film epitaxially grown on graphene

Two-dimensional materials with layered structures, appropriate bandgap, and high carrier mobilities have gathered tremendous interests due to their potential applications in optoelectronic and photovoltaic devices. Here, we report the growth of BiI3 thin film with controllable atomic thickness on graphene-terminated 6H–SiC(0001) substrate by molecular beam epitaxy (MBE) method. The growth kinetic processes and crystalline properties of the BiI3 film are studied by scanning tunneling microscopy (STM). The scanning tunneling spectroscopy (STS) reveals a bandgap of 2.8 eV for monolayer BiI3 with a weak dependence on film thickness for few-layer BiI3, which greatly exceeds the previous reported values identified by macroscopic optical measurements. This discrepancy originates from the edge effect of BiI3 that renders the bandgap downshift to 1.5–1.6 eV, as identified by the STS curves and the further confirmed by density functional theory (DFT) calculations. Our work provides a method to fabricate high-quality monolayer BiI3 film and resolves its intrinsic bandgap as well as the edge effect on reduction of bandgap, benefitting not only to fundamental researches but also to nanoelectronic and optoelectronic applications.

Structural and optoelectronic properties of combining Nb-doped SrTiO3/ITO films on (0 0 1)-YSZ substrate

Functional oxides combined with conventional conductive films are competitive alternative candidates as electrode and electron transport layer. This work explored the film and interface structures and the optoelectronic properties of the combined films of Nb-doped SrTiO3 (Nb-STO) as a classical functional oxide and ITO. The combined films were deposited on a (001)-YSZ substrate via laser molecular beam epitaxy method. In-situ high-energy electron diffraction, X-ray diffraction, and transmission electron microscopy shew [011]Nb-STO azimuth orientation and a two-fold symmetry rotation of the Nb-STO layer. Interestingly, the Nb-STO film growth at 650 °C or higher was able to induce cracks in ITO film. Besides, the growth temperature of Nb-STO from 400 °C to 700 °C changed its structures from amorphousness to polycrystal, the Nb-STO/ITO interface relationships, sheet resistance (from 25 to 83 Ω/sq) and electron mobility (40 to 2 cm·V−1·s−1); however, this had little influence on the transmittance. This work provided an insight into the combination of functional oxides and ITO for the fabrication of the electron transport layer.

Two‐dimensional magnetic transition metal chalcogenides

AbstractThe field of two‐dimensional (2D) magnets has expanded rapidly during the past several years since the first demonstration of intrinsic 2D magnetism in atomically thin CrI3 and Cr2Ge2Te6 in 2017. 2D transition metal chalcogenides (TMCs), a class of strongly correlated materials, have exhibited a wide variety of novel electronic and optical properties, and more recently magnetism. Here, we review recent experimental progress achieved in the growth of 2D magnetic TMC materials using chemical vapor deposition and molecular beam epitaxy methods. Outstanding examples include the demonstration of room temperature intrinsic and extrinsic ferromagnetism in monolayer VSe2, MnSe2, Cr3Te4, V‐doped WSe2, and so on. A brief discussion on the origin of the exotic magnetic properties and emergent phenomena is also presented. Finally, we summarize the remaining challenges and future perspective on the development of 2D magnetic materials for next‐generation spintronic devices.

Structural characterizations and electronic properties of CuSe monolayer endowed with triangular nanopores

CuSe monolayer possesses intrinsically patterned triangular nanopores with uniform size and can serve as a template for selective adsorptions for molecules and nanoclusters. Here, we prepare the CuSe monolayer on Cu(111) substrate by molecular beam epitaxy method and characterize CuSe monolayer in detail by bond-resolved scanning tunneling microscopy and non-contact atomic force microscopy. The results further confirm the honeycomb feature and triangular nanopores existence of CuSe monolayer. In addition, scanning tunneling spectroscopy measurements reveal the semiconducting features of CuSe monolayer with a band gap of 2.40 eV. This work helps to understand the structure and electronic properties of those intrinsically patterned two-dimensional materials.

The Study of Dark Currents in HgCdTe Heterostructure Photodiodes

Dark current is the main factor that influences photodiode performance. It should be minimal to reduce noise and ensure a high level of photoelectric parameters. In order to identify the predominant causes of generation–recombination in photodiodes based on a mercury–cadmium–tellurium (MCT) ternary compound in the given voltage range, a calculation model is proposed for dark currents caused by fundamental and other current mechanisms. The components of dark current in photodiodes based on MCT heterostructures grown by molecular beam epitaxy (MBE) and liquid-phase epitaxy (LPE) methods are determined in the reverse bias voltages range from 0 to 40 mV. In the range from 0 to 20 mV, these characteristics are reduced to the diffusion component. When the reverse bias voltage exceeds 30 mV, an increase in the Shockley–Read–Hall (SRH) generation–recombination current and tunneling current through trap levels in the band gap is observed.

Single-photon avalanche diode detectors based on group IV materials

Today there are several types of photodetectors that can cope with the task of detecting a single photon, however, avalanche photodiodes are most widely used for applications in fiber-optic communication systems and quantum cryptography. In the past 5 years experimental fabrication of single-photon avalanche detectors within conventional complementary metal–oxide–semiconductor technology and by the method of molecular beam epitaxy have made possible many demonstrations of avalanche photodetectors with exceptional performance characteristics and opened wide perspectives for development of devices of new generation. In this work, we review state-of-the-art designs of single-photon avalanche photodiodes based on group IV materials, their operating characteristics, and achieved values of basic parameters, as well as the methods of their synthesis, and outline the perspectives of using such structures in emerging device applications.

Ultra-low Young’s modulus and high super-exchange interactions in monolayer CrN: A promising candidate for flexible spintronic applications* *Project supported by the National Natural Science Foundation of China (Grant No. 61888102), the National Key Research and Development Program of China (Grant No. 2016YFA0202300), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB30000000).

Monolayer CrN has been predicted to be half-metallic ferromagnet with high Curie temperature. Due to bulk CrN’s biocompatibility, the monolayer is a promising candidate for bio-related devices. Here, using first-principles calculations based on density functional theory, we find that the formation energy of the bulk CrN stacking from layers with square lattice is only 68 meV/atom above the convex hull, suggesting a great potential to fabricate the monolayer CrN in a square lattice by using molecular beam epitaxy method. The monolayer CrN is then proved to be a soft material with an ultra-low Young’s modulus and can sustain very large strains. Moreover, the analysis of the projected density of states demonstrates that the ferromagnetic half-metallicity originates from the splitting of Cr-d orbitals in the CrN square crystal field, the bonding interaction between Cr–N, and that between Cr–Cr atoms. It is worth noting that the super-exchange interaction is much larger than the direct-exchange interaction and contributes to the ultra-high Curie temperature, which is obtained from Monte Carlo simulations based on Heisenberg model. Our findings suggest that the monolayer CrN can be an indispensable candidate for nanoscale flexible spintronic applications with good biocompatibility and is considerable appealing to be realized in experiment.

Структурные свойства тонких пленок кристаллического топологического изолятора Pb-=SUB=-0.7-=/SUB=-Sn-=SUB=-0.3-=/SUB=-Te на Si(111)

In this work, the selection and optimization of technological growth parameters of thin Pb0.7Sn0.3Te layers up to 300 nm thick, grown on the Si(111) surface at temperatures of 230−400◦C by the method of molecular beam epitaxy was carried out. The surface morphology of the films was studied, and the epitaxial relations were determined. It was shown that, depending on the growth temperature, the surface morphology ranges from relatively narrow terraces to smooth micrometer-sized islands with monoatomic steps on their surface.

Epitaxial Growth of Single-Phase 1T'-WSe2 Monolayer with Assistance of Enhanced Interface Interaction.

The WSe2 monolayer in 1T' phase is reported to be a large-gap quantum spin Hall insulator, but is thermodynamically metastable and so far the fabricated samples have always been in the mixed phase of 1T' and 2H, which has become a bottleneck for further exploration and potential applications of the nontrivial topological properties. Based on first-principle calculations in this work, it is found that the 1T' phase could be more stable than 2H phase with enhanced interface interactions. Inspired by this discovery, SrTiO3 (100) is chosen as substrate and WSe2 monolayer is successfully grown in a 100% single 1T' phase using the molecular beam epitaxial method. Combining in situ scanning tunneling microscopy and angle-resolved photoemission spectroscopy measurements, it is found that the in-plane compressive strain in the interface drives the 1T'-WSe2 into a semimetallic phase. Besides providing a new material platform for topological states, the results show that the interface interaction is a new approach to control both the structure phase stability and the topological band structures of transition metal dichalcogenides.

Structural Quality and Magnetotransport Properties of Epitaxial Layers of the (Ga,Mn)(Bi,As) Dilute Magnetic Semiconductor.

Structural analysis of epitaxial layers of the (Ga,Mn)(Bi,As) quaternary dilute magnetic semiconductor (DMS), together with investigations of their magnetotransport properties, has been thoroughly performed. The obtained results are compared with those for the reference (Ga,Mn)As layers, grown under similar conditions, with the aim to reveal an impact of Bi incorporation on the properties of this DMS material. Incorporation of Bi into GaAs strongly enhances the spin-orbit coupling strength in this semiconductor, and the same has been expected for the (Ga,Mn)(Bi,As) alloy. In turn, importantly for specific spintronic applications, strong spin-orbit coupling in ferromagnetic systems opens a possibility of directly controlling the direction of magnetization by the electric current. Our investigations, performed with high-resolution X-ray diffractometry and transmission electron microscopy, demonstrate that the (Ga,Mn)(Bi,As) layers of high structural quality and smooth interfaces can be grown by means of the low-temperature molecular-beam epitaxy method, despite a large difference between the sizes of Bi and As atoms. Depending on the applied buffer layer, the DMS layers can be grown under either compressive or tensile misfit strain, which influences their magnetic properties. It is shown that even small 1% Bi content in the layers strongly affects their magnetoelectric properties, such as the coercive field and anisotropic magnetoresistance.

Influence of AlN buffer layer on molecular beam epitaxy growth of wurtzite Al1−xScxN thin films

Wurtzite–Al1−xScxN thin films deposited by solid-state alloying of AlN with ScN exhibit high piezoelectric coefficient and large band gap that makes it a promising material for a variety of applications in piezo-electronics, electronic, acoustoelectric devices, etc. Research on epitaxial Al1−xScxN growth in wurtzite crystal structure is still at an early stage and achieving high scandium (Sc) concentrations in epitaxial films without any phase separation or secondary phase formation is still a critical challenge. Moreover, as most of the reports of wurtzite–Al1−xScxN growth thus far relies on low-vacuum growth techniques, such as magnetron sputtering that are prone to large impurities and contaminants detrimental for device applications, high-vacuum deposition techniques, such as molecular beam epitaxy method needs to be developed. In this paper, we report the epitaxial growth of wurtzite–Al1−xScxN on sapphire (Al2O3) substrates under different Sc fluxes using ultra-high vacuum plasma-assisted molecular beam epitaxy. To prevent ScN phase separation, a 30 nm AlN buffer layer is deposited in situ on GaN epilayers as well as Al2O3 substrates that result in phase-pure wurtzite–Al1−xScxN thin films without any phase separation or secondary phase formation. The structural and compositional analyses performed with high-resolution X-ray diffraction (HRXRD) and secondary ion mass spectroscopy (SIMS), reveal epitaxial wurtzite–Al1−xScxN growth with 0001 orientations on (0001) Al2O3 substrates and the presence of cubic ScN. Demonstration of phase-pure Al1−xScxN on AlN buffer layers will enable the development of devices with improved efficiencies.

Structural and dynamic properties of short-period GaN/AlN superlattices grown by submonolayer digital epitaxy

Structural and dynamic properties of short-period GaN/AlN superlattices with the thicknesses of the constituent layers varying from two to several monolayers, grown using the method of submonolayer digital molecular beam epitaxy, are experimentally and theoretically studied. It is established that in the grown samples there are two types of periodicity. One type is formed by periodic sequence of GaN and AlN layers in the superlattice, and the second one is related with periodic interruptions in the growth of superlattice for the evaporation of excess Ga metal. The dependences of the positions and intensities of the lines in the Raman spectra on the period of the superlattice are determined, and microscopic nature of optical phonon modes is established. The doublet structure of the E(TO) lines localized in the GaN and AlN layers of superlattice, genetically related to the E 2(high) and E1 phonon branches of the bulk crystal, is first discovered and explained. A strong dependence of the polar modes localized in the AlN layer on the thickness of layer forming the superlattice is revealed. The results of complex studies will improve the accuracy of quantitative estimation of important parameters of superlattice structures and can be used to optimize growth parameters for the fabrication of structurally perfect short-period GaN/AlN superlattices.