Molecular Beam Epitaxy Method Research Articles

A broad-band investigation of ferromagnetic resonance in exchange coupled permalloy/phthalocyanine bilayer heterostructures at room temperature

The phenomenon of ferromagnet/molecule interfacial electronic hybridization enables the possibility of modifying magnetic characteristics of downscaled systems for spintronic applications. Ferromagnetic resonance (FMR) at the nanoscale level through a coplanar waveguide (CPW) over a wide frequency range is a remarkably reliable technique for investigation of the magnetic heterostructure’s interfacial properties and their feasibility in applications as spintronic devices. Here, magnetron sputtering and organic molecular beam epitaxy methods are used to fabricate the following nanoscale heterostructures: permalloy (Py)/Fe phthalocyanine (Pc), Py/ZnPc, and Py/Cr. Magnetization dynamics of the bilayer heterostructures is investigated in a CPW by studying frequency dependence (5–50 GHz) of the resonance field and linewidth with parallel and perpendicular geometries of the static applied magnetic field, thereby providing insights into possible intrinsic and extrinsic sources of FMR damping. Planar Pc molecules at the Py layer interface are found to have the potential to contribute to magnetic stabilization based on calculated magnetic parameters. This is evidenced by the larger results of surface anisotropy of the Py/MPc bilayer heterostructures induced by exchange coupling interaction at room temperature as well as the lower values of damping constant.

Strong Covalent Coupling in Vertically Layered SnSe2/PTAA Heterojunctions Enabled High Performance Inorganic-Organic Hybrid Photodetectors.

Controllable large-scale integration of two-dimensional (2D) materials with organic semiconductors and the realization of strong coupling between them still remain challenging. Herein, we demonstrate a wafer-scale, vertically layered SnSe2/PTAA heterojunction array with high light-trapping ability via a low-temperature molecular beam epitaxy method and a facile spin-coating process. Conductive probe atomic force microscopy (CP-AFM) measurements reveal strong rectification and photoresponse behavior in the individual SnSe2 nanosheet/PTAA heterojunction. Theoretical analysis demonstrates that vertically layered SnSe2/PTAA heterojunctions exhibit stronger C-Se covalent coupling than that of the conventional tiled type, which could facilitate more efficient charge transfer. Benefiting from these advantages, the SnSe2/PTAA heterojunction photodetectors with an optimized PTAA concentration show high performance, including a responsivity of 41.02 A/W, an external quantum efficiency of 1.31 × 104%, and high uniformity. The proposed approach for constructing large-scale 2D inorganic-organic heterostructures represents an effective route to fabricate high-performance broadband photodetectors for integrated optoelectronic systems.

Carrier mobility dependence of indium antimonide-bismide on carrier concentration, temperature, and bismuth composition grown on semi-insulating gallium arsenide substrate

We explore the impact of carrier concentration, temperature, and bismuth (Bi) composition on the carrier mobility of indium antimonide-bismide (InSb1−x Bi x ) material. Utilizing the molecular beam epitaxy method, we achieved high Bi composition uniformity. This method also enables the InSb1−x Bi x to be grown on semi-insulating GaAs substrate, effectively preventing parallel electrical conduction during Hall effect measurement. Our findings reveal that InSb1−x Bi x doped with silicon (Si) and tellurium (Te) consistently exhibit n-type conductivity. In contrast, InSb1−x Bi x doped with beryllium (Be) exhibit a transition from n to p type conductivity, subjected to the Be doping level and the measurement temperature. Based on these observations, we proposed an empirical model describing the dependence of InSb1−x Bi x electron mobility on carrier concentration, temperature, and Bi composition, specifically for Si and Te-doped InSb1−x Bi x samples. These insights gained from this study hold potential application in photodetector device simulations.

Hybrid Molecular Beam Epitaxy for Single-Crystalline Oxide Membranes with Binary Oxide Sacrificial Layers.

The advancement in thin-film exfoliation for synthesizing oxide membranes has led to possibilities for creating artificially assembled heterostructures with structurally and chemically incompatible materials. The sacrificial layer method is a promising approach to exfoliate as-grown films from a compatible material system, allowing for their integration with dissimilar materials. Nonetheless, the conventional sacrificial layers often possess an intricate stoichiometry, thereby constraining their practicality and adaptability, particularly when considering techniques such as molecular beam epitaxy (MBE). This is where easy-to-grow binary alkaline-earth-metal oxides with a rock salt crystal structure are useful. These oxides, which include (Mg, Ca, Sr, Ba)O, can be used as a sacrificial layer covering a much broader range of lattice parameters compared to conventional sacrificial layers and are easily dissolvable in deionized water. In this study, we show the epitaxial growth of the single-crystalline perovskite SrTiO3 (STO) on sacrificial layers consisting of crystalline SrO, BaO, and Ba1-xCaxO films, employing a hybrid MBE method. Our results highlight the rapid (≤5 min) dissolution of the sacrificial layer when immersed in deionized water, facilitating the fabrication of millimeter-sized STO membranes. Using high-resolution X-ray diffraction, atomic-force microscopy, scanning transmission electron microscopy, impedance spectroscopy, and scattering-type near-field optical microscopy (SNOM), we demonstrate single-crystalline STO membranes with bulk-like intrinsic dielectric properties. The employment of alkaline earth metal oxides as sacrificial layers is likely to simplify membrane synthesis, particularly with MBE, thus expanding the research and application possibilities.

Atomic and electronic structure of monolayer ferroelectric GeS on Cu(111)

Two-dimensional (2D) ferroelectric materials are important materials for both fundamental properties and potential applications. Especially, group Ⅳ monochalcogenide possesses highest thermoelectric performance and intrinsic ferroelectric polarization properties and can sever as a model to explore ferroelectric polarization properties. However, due to the relatively large exfoliation energy, the creation of high-quality and large-size monolayer group Ⅳ monochalcogenide is not so easy, which seriously hinders the integration of these materials into the fast-developing field of 2D materials and their heterostructures. Herein, monolayer GeS is successfully fabricated on Cu(111) substrate by molecular beam epitaxy method, and the lattice structure and the electronic band structure of monolayer GeS are systematically characterized by high-resolution scanning tunneling microscopy, low-energy electron diffraction, <i>in-situ</i> X-ray photoelectron spectroscopy, Raman spectra, and angle-resolved photoelectron spectroscopy, and density functional theory calculations. All atomically resolved STM images reveal that the obtained monolayer GeS has an orthogonal lattice structure, which consists with theoretical prediction. Meanwhile, the distinct moiré pattern formed between monolayer GeS and Cu(111) substrate also confirms the orthogonal lattice structure. In order to examine the chemical composition and valence state of as-prepared monolayer GeS, <i>in-situ</i> XPS is utilized without being exposed to air. The measured spectra of XPS core levels suggest that the valence states of Ge and S elements are identified to be +2 and –2, respectively and the atomic ratio of Ge/S is 1∶1.5, which is extremely close to the stoichiometric ratio of 1∶1 for GeS. To further corroborate the quality and lattice structure of the monolayer GeS film, <i>ex-situ</i> Raman measurements are also performed for monolayer GeS on highly oriented pyrolytic graphene (HOPG) and multilayer graphene substrate. Three well-defined typical characteristic Raman peaks of GeS are observed. Finally, <i>in-situ</i> ARPES measurement are conducted to determine the electronic band structure of monolayer GeS on Cu(111). The results demonstrate that the monolayer GeS has a nearly flat band electronic band structure, consistent with our density functional theory calculation. The realization and investigation of the monolayer GeS extend the scope of 2D ferroelectric materials and make it possible to prepare high quality and large size monolayer group Ⅳ monochalcogenides, which is beneficial to the application of this main group material to the rapidly developing 2D ferroelectric materials and heterojunction research.

On-surface synthesis of two types of cyano-substituted polyfluorene derivatives via Ullmann coupling on Au(111).

Using 4-(3,6-dibromo-9H-carbazol-9-yl)benzonitrile (DBCB) precursors, we successfully constructed two types of cyano-substituted polymers on Au(111) by the molecular beam epitaxy method. According to the geometry, the two polymers are referred to as w-type polymers composed of cis-dimers and z-type polymers composed of trans-dimers. The intermediate dimers and final polymers were well characterized by high-resolution scanning tunneling microscopy (HR-STM). Moreover, the productivities of these two polymers can be controlled by adjusting the heating rate and different treatment methods. High heating rates and hot deposition can provide more ample space and time for molecular diffusion, which is conducive to the formation of w-type polymers with relatively low density. In addition, by combining scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations, we have shown that the addition of CN groups reduces the band gap of the two polymers. Our investigation thus shows the controllable construction of nanostructures through efficient surface synthesis parameters and reveals the potential of using functional groups as tools to modify the electronic properties of polymers.

Overview of the process technology for the preparation of ultrahigh purity indium required for the fabrication of indium phosphide related epitaxial structures based devices needed for advanced electronic applications

Indium, a rare metal, is created incidentally during the zinc refining process. When a zinc metal is produced, valuable elements, such as indium, are recovered and reused. The sulfide minerals, sphalerite, galena, and chalcopyrite, are all common hosts for indium metals. Indium metals of varying purities (from 99% to 99.9%) are used in many different commercial, other exclusive, specialty, dentistry, and research and development settings. In the production of indium phosphide and related select bulk single crystals, such as InP, InAs, InSb, etc., and select multilayered epitaxial material-systems based device structures, such as InGaAs/InP, InGaAsP/InP, etc., an ultrahigh purity (99.99999%) indium metal is used as one of the initial and primary input materials. light-emitting diodes, infrared detectors, lasers, and other components cannot be made without these device topologies. Triple junction solar cells made of GaInP, GaAs, and Ge with 40% conversion efficiency are being developed for use in space. Metal-organic and molecular beam epitaxial methods utilize trimethyl/triethyl-indium-epi-precursors, the high purity indium derivatives, as starting materials to develop and manufacture multilayered structures of InGaAs/InP, InGaAsP/InP, InGaN/InP AlInN, etc. The purpose of this review is to quickly touch on indium mineral sources, important uses for different indium metal grades, and the processes needed to refine, purify, and ultrahigh purify indium to higher purity levels using a zone refining–melting–leveling process, as well as impurity segregation considerations. The use of vacuum, inert gas environments, and an external electromagnetic field to efficiently segregate, levitate, stir, homogenize, and mix the molten zone/melt interface area (region) as well as purity analyses at ppb levels, class clean room, and packaging concepts were also discussed. This review also touched briefly on the use of ultrahigh purity indium in the preparation of TMIn, TEIn, and InCl precursors necessary for the growth of device structures by molecular beam, metal-organic vapor phase, atomic layer epitaxial, and chemical vapor deposition processes. Purifying and preparing polycrystalline indium to a type 7 N purity level as well as standardization and criticality testing for fine-tuning system parameters are essential parts of developing the purification process technology. It also highlights various compound semiconductors and epitaxial systems, such as high purity indium compounds, such as indium phosphide, for cutting-edge electronic applications. Material yield enhancement, impurity management (including C, O, N, and others), consistent results, impurity reduction (down to the ppb level), and class clean packaging are all active topics of research and development. There has been a rise in demand for ultrapure metals (7–10 N) with stringent purity criteria in the aerospace and defense sectors, where they are used in cutting-edge nanoelectronic applications. This literature review delves into these and related topics regarding the production of ultrahigh purity indium. The major objective of this review is to provide a concise summary of the research and development progress made toward the ultrahigh purity (7N-99.99999%) indium preparation and its epitaxial electronics application considerations as of the time of this writing.

Interface-enhanced superconductivity in monolayer 1T′-MoTe2 on SrTiO3(001)

Introducing superconductivity into two-dimensional (2D) films with nontrivial topology has been intensively pursued as one of the feasible scenarios to realize 1D topological superconductor. Prevailing endeavors mostly exploit the external gating or proximity effect of a traditional superconductor, by which the critical temperatures (T_{mathrm{c}}) are limited to several Kelvin range. Here, we report on the discovery of interface-enhanced superconductivity in monolayer 1T′-MoTe2 film. A thermally driven phase transition from Mo6Te6 nanowires to 1T′-MoTe2 films, grown on SrTiO3(001) surface by the molecular beam epitaxial methods, is demonstrated. A combined study of scanning tunneling microscopy/spectroscopy, electrical transport and magnetization measurements indicates the T_{mathrm{c}} of MoTe2 film is around 30 K, two orders of magnitude larger than its 3D counterpart crystal. This study shows that interfacial engineering is an efficient way to tune monolayer 1T′-MoTe2 film into superconducting states, and thus may pave the way toward higher-T_{mathrm{c}} 1D intrinsic topological superconductivity.

Structural and optical properties of InP1-xSbx/n-InAs epilayers grown by gas source molecular beam epitaxy

High-resolution x-ray diffraction (HR-XRD), photoluminescence (PL), synchrotron radiation extended x-ray absorption fine-structure (SR-EXAFS) measurements are methodically analyzed for assessing the optical and structural properties of InP1−xSbx/n-InAs epifilms grown by gas-source molecular-beam epitaxy method. For InP0.63Sb0.37/n-InAs sample, the PL study has revealed two A1, A2 energy bands. The A1 band prevalent at low temperature is attributed to the recombination of carriers trapped in the tail states. The A2 band with nearly constant transition energy ∼ 0.46 eV is virtually temperature independent. In InP1−xSbx alloys, the energy peak of A1 band redshifted with temperature and showed strong compositional disorder. The A2 band, dominant at higher temperature with Gaussian-like line shape is ascribed to the deep-level transition as its behavior coincided with the signature of a configuration coordinate model. The deep level responsible for A2 band is possibly linked to the “vacancy-impurity” like complexes having ground and excited states in the energy band gap. The composition dependent analysis of SR-EXAFS data for InP1−xSbx/InAs samples has confirmed the maintenance of nearest neighbor In-P, In-Sb shell distances within the range of bulk binary materials’ bond lengths. We feel that our results of HR-XRD, PL and SR-EXAFS techniques have provided valuable information on the structural and optical characteristics of the InP1−xSbx/n-InAs (001) epilayers and can be extended to many other technologically important materials.

Wavelength-switchable ultraviolet light-emitting diodes.

Dual-wavelength ultraviolet light-emitting diodes (UV-LEDs) exhibiting two discrete emission peaks of comparable intensities are reported in this work. Furthermore, this is the first report where complete switching between these two peaks was achieved by simply changing the duty cycle of the pulsed-mode excitation. While earlier reports on dual-wavelength emission were based on complex multi-stage devices, our device layer-structure was nominally similar to single-wavelength LEDs, and the special properties were realized solely through the use of specifically designed AlGaN alloys. The molecular beam epitaxy (MBE) method was chosen for this work, which can operate at significantly wider growth-parameter ranges than other more commonly used techniques. The dual-wavelength nature of our LEDs was brought about by the deliberate incorporation of nanometer-scale alloy fluctuations in the quantum well based active regions by modulating the relative surface diffusion rates of Ga/Al adatoms. The wavelength selectivity was linked to the variation in thermally assisted carrier delocalization, transport, and subsequent recombination processes in regions of different compositional inhomogeneities.

Epitaxial Electrodeposition of Ordered Inorganic Materials.

ConspectusThe quality of technological materials generally improves as the crystallographic order is increased. This is particularly true in semiconductor materials, as evidenced by the huge impact that bulk single crystals of silicon have had on electronics. Another approach to producing highly ordered materials is the epitaxial growth of crystals on a single-crystal surface that determines their orientation. Epitaxy can be used to produce films and nanostructures of materials with a level of perfection that approaches that of single crystals. It may be used to produce materials that cannot be grown as large single crystals due to either economic or technical constraints. Epitaxial growth is typically limited to ultrahigh vacuum (UHV) techniques such as molecular beam epitaxy and other vapor deposition methods. In this Account, we will discuss the use of electrodeposition to produce epitaxial films of inorganic materials in aqueous solution under ambient conditions. In addition to lower capital costs than UHV deposition, electrodeposition offers additional levels of control due to solution additives that may adsorb on the surface, solution pH, and, especially, the applied overpotential. We show, for instance, that chiral morphologies of the achiral materials CuO and calcite can be produced by electrodepositing the materials in the presence of chiral agents such as tartaric acid.Inorganic compound materials are electrodeposited by an electrochemical-chemical mechanism in which solution precursors are electrochemically oxidized or reduced in the presence of molecules or ions that react with the redox product to form an insoluble species that deposits on the electrode surface. We present examples of reaction schemes for the electrodeposition of transparent hole conductors such as CuI and CuSCN, the magnetic material Fe3O4, oxygen evolution catalysts such as Co(OH)2, CoOOH, and Co3O4, and the n-type semiconducting oxide ZnO. These materials can all be electrodeposited as epitaxial films or nanostructures onto single-crystal surfaces. Examples of epitaxial growth are given for the growth of films of CuI(111) on Si(111) and nanowires of CuSCN(001) on Au(111). Both are large mismatch systems, and the epitaxy is explained by invoking coincidence site lattices in which x unit meshes of the film overlap with y unit meshes of the substrate.We also discuss the epitaxial lift-off of single-crystal-like foils of metals such as Au(111) and Cu(100) that can be used as flexible substrates for the epitaxial growth of semiconductors. The metals are grown on a Si wafer with a sacrificial SiOx interlayer that can be removed by chemical etching. The goal is to move beyond the planar structure of conventional Si-based chips to produce flexible electronic devices such as wearable solar cells, sensors, and flexible displays. A scheme is shown for the epitaxial lift-off of wafer-scale foils of the transparent hole conductor CuSCN.Finally, we offer some perspectives on possible future work in this area. One question we have not answered in our previous work is whether these epitaxial films and nanostructures can be grown with the level of perfection that is achieved in UHV. Another area that is ripe for exploration is the epitaxial electrodeposition of metal-organic framework materials from solution precursors.

Optimization of large magnetoresistance of polycrystalline Bi film

The oriented growth and morphology of polycrystalline bismuth (Bi) films deposited on insulating substrates is investigated based on the molecular beam epitaxy method. The film thickness effect on the resistance-temperature characteristics and magnetoresistance properties is analyzed. The Bi film resistivity with thickness range [25, 65] nm is investigated using the four-probe method at 2–300 K. The magnetic field contributes to opening the energy gap of Bi thin films and a more obvious opening of the energy gap is observed in the thinner films. The Bi film has the weak antilocalization (WAL) effect and the thickness increase affects the WAL effect. The large magnetic field is beneficial to the observation of the WAL effect. The temperature-dependent resistivity of Bi thin films is described based on the Ziman-Faber theory. The competition between the metal surface state and the bulk state explains the semimetal-semiconductor transition and the magnetoresistance effect in the thickness range [25, 65] nm. In addition, the thickness effect on the magnetoresistance property is researched, which provides a new approach to specify the Bi thin film magnetoresistance.

Magnetotransport Study of Dirac Metal FeSn Thin Films Grown on Silicon Substrates

Thin films of iron–tin alloy FeSn are grown on silicon substrates and their structural and transport properties are investigated for the first time. Herein, the molecular beam epitaxy method is used to grow 50 and 30 nm thick FeSn films on silicon substrates containing 10 nm of MgO as a buffer layer. The films are characterized structurally using an X‐ray diffractometer, showing a hexagonal crystal structure with the space group P6/mmm (191). The results from electrical and magnetotransport measurements show these films exhibit characteristics close to metals. Herein, the magnetotransport properties of the thin films which show positive magnetoresistance and sample‐dependent Hall effect with possible multiband transport are further measured.

Superconducting phase diagram of lanthanum films on the substrate of Si(100)

Lanthanum films were grown on the substrate of Si(100) (La/Si(100)) by the molecular beam epitaxy (MBE) method and their superconducting properties were studied via the magnetic and electrical transport measurements. The superconducting transition temperature (Tc) is around 5.3 K. Besides, the second magnetization peak (SMP) occurs well below the upper critical field Hc2(T) due to the thermomagnetic flux-jump instabilities. In addition, we also calculated the critical current density (Jc) and studied the vortex-pinning behavior. The Jc is higher than 10 MA/cm2 at certain temperatures. The pinning analysis suggests the dominant role is the δl pinning. Moreover, the comprehensive superconducting phase diagram is offered. Our study completes the superconducting properties of lanthanum, which would facilitate the understanding of superconducting mechanism in the type Ⅱ superconductors.

Новые центры рекомбинации в слоях КРТ МЛЭ на подложках (013) GaAs

A large inhomogeneity of the minority lifetime from 1 to 10 µs at 77 K over the area is observed in some experiments when high-quality HgCdTe layers of the electronic type of conductivity are grown on GaAs substrates with a diameter of 76.2 mm with the (013) orientation by the method of molecular beam epitaxy. As a rule, the such lifetimes are determined by carrier recombination at Shockley-Hall-Read (SHR) centers. Modern studies and ideas about the nature of the SHR centers do not allow us to explain the observed results. The measurements of HgCdTe layers by the second harmonic generation showed the existence of a quasi-periodic change in the signal at the minima of the azimuthal dependence, which is associated with the appearance of misoriented microregions of the crystal structure. The amplitude of the quasi-periodic change in the signal decreases with increasing lifetime and completely disappears for regions with higher lifetime values. Similar dependences are observed during etching of HgCdTe layers, which indicates the existence of misoriented microregions in the bulk. Thus, misoriented microregions of the crystal structure have a significant effect on the lifetime and are new centers of Shockley-Hall-Read recombination.

New recombination centers in MBE MCT layers on (013) GaAs substrates

A large inhomogeneity of the minority lifetime from 1 to 10 μs at 77 K over the area is observed in some experiments when high-quality HgCdTe layers of the electronic type of conductivity are grown on GaAs substrates with a diameter of 76.2 mm with the (013) orientation by the method of molecular beam epitaxy. As a rule, the such lifetimes are determined by carrier recombination at Shockley-Hall-Read (SHR) centers. Modern studies and ideas about the nature of the SHR centers do not allow us to explain the observed results. The measurements of HgCdTe layers by the second harmonic generation showed the existence of a quasi-periodic change in the signal at the minima of the azimuthal dependence, which is associated with the appearance of misoriented microregions of the crystal structure. The amplitude of the quasi-periodic change in the signal decreases with increasing lifetime and completely disappears for regions with higher lifetime values. Similar dependences are observed during etching of HgCdTe layers, which indicates the existence of misoriented microregions in the bulk. Thus, misoriented microregions of the crystal structure have a significant effect on the lifetime and are new centers of Shockley-Hall-Read recombination. Keywords: HgCdTe layers, lifetime, second harmonic, azimuthal angular dependences, recombination centers, misoriented microregions.

Single crystal growth of topological semimetals and magnetic topological materials

Topological materials have attracted much attention due to their novel physical properties. These materials can not only serve as a platform for studying the fundamental physics, but also demonstrate a significant potential application in electronics, and they are studied usually in two ways. One is to constantly explore new experimental phenomena and physical problems in existing topological materials, and the other is to predict and discover new topological material systems and carry out synthesis for further studies. In a word, high-quality crystals are very important for studying quantum oscillations, angle resolved photoemission spectra or scanning tunneling microscopy. In this work, the classifications and developments of topological materials, including topological insulators, topological semimetals, and magnetic topological materials, are introduced. As usually employed growth methods in growing topological materials, flux and vapour transport methods are introduced in detail. Other growth methods, such as Bridgman, float-zone, vapour deposition and molecular beam epitaxy methods, are also briefly mentioned. Then the details about the crystal growth of some typical topological materials, including topological insulators/semimetals, high Chern number chiral topological semimetals and magnetic topological materials, are elaborated. Meanwhile, the identification of crystal quality is also briefly introduced, including the analysis of crystal composition and structure, which are greatly important.

Single crystal growth of topological semimetals and magnetic topological materials

Topological materials have attracted much attention due to their novel physical properties. These materials can not only serve as a platform for studying the fundamental physics, but also demonstrate a significant potential application in electronics, and they are studied usually in two ways. One is to constantly explore new experimental phenomena and physical problems in existing topological materials, and the other is to predict and discover new topological material systems and carry out synthesis. In a word, high-quality crystals are very important for studying quantum oscillations, angle resolved photoemission spectra or scanning tunneling microscopy. In this work, the classifications and developments of topological materials, including topological insulators, topological semimetals, and magnetic topological materials, are introduced. As usually employed growth methods in growing topological materials, flux and vapour transport methods are introduced in detail. Other growth methods, such as Bridgman, float-zone, vapour deposition and molecular beam epitaxy methods, are also briefly mentioned. Then the details about the crystal growth of some typical topological materials, including topological insulators/semimetals, high Chern number chiral topological semimetals and magnetic topological materials, are elaborated. Meanwhile, the identification of crystal quality is also briefly introduced, including the analysis of crystal composition and structure, which are greatly important.

Crystalline and optoelectronic properties of Ge1−x Sn x /high-Si-content-Si y Ge1−x−y Sn x double-quantum wells grown with low-temperature molecular beam epitaxy

In this study, we investigated the impact of the growth temperatures of molecular beam epitaxy method for the Si y Ge1−x−y Sn x barrier with a Si content over 20% of Ge1−x Sn x /Si y Ge1−x−y Sn x single-quantum well (QW) on their crystalline and photoluminescence (PL) properties. As a result, we found that lowering T SiGeSn down to 100 °C achieves the superior crystallinity and the higher PL efficiency at room temperature. It was owing to the suppression of the Sn segregation according to the surface morphology observation. Based on this finding, we realized the epitaxial growth of Ge1−x Sn x /Si y Ge1−x−y Sn x double-QWs at 100 °C. We verified the superior crystallinity with the abrupt interface by x-ray diffraction and scanning transmission electron microscopy. In this study, we discussed the optical transition mechanism of the single- and double-QWs based on the band alignment simulation. Finally, we found that the double-QW grown at 100 °C can sustain its crystalline structure against annealing at the N2 atmosphere up to 350 °C, and the PL performance can be also improved by the thermal treatment at around 350 °C.

First-principle study on the effect of S/Se/Te doping and VZn-Hi coexistence on ZnO electrical conductivity

Abstract In a vacuum environment, when ZnO is prepared using the chemical vapor deposition method and the molecular beam epitaxial growth method, H-gap impurities inevitably remain in the ZnO system, which is often ignored. The study of Zn vacancies under experimental conditions poses a challenge. Second, as an n-type semiconductor, ZnO is characterized by a self-compensation of natural donor defects and poor stability, which severely limit the acquisition of p-type ZnO. Based on the above problems, the conductive properties of S/Se/Te doped and VZn-Hi coexisting ZnO were investigated by first principle to acquire high-stability and high-quality p-ZnO. The study found that Zn35SO35, Zn35SeO35, and Zn35SHiO35 all have good p-type conductivity, which can effectively improve hole mobility and electrical conductivity. Among them, Zn35SO35 has the largest hole concentration at 2.80×1021 cm−3, as well as the best conductivity. The choice of Zn35SO35 provides a reference for obtaining new high-quality p-type ZnO semiconductors.