Molecular Beam Epitaxy Growth Research Articles

A Hessian Recovery Based Linear Finite Element Method for Molecular Beam Epitaxy Growth Model with Slope Selection

A Hessian Recovery Based Linear Finite Element Method for Molecular Beam Epitaxy Growth Model with Slope Selection

The use of He buffer gas for moderating the plume kinetic energy during Nd:YAG-PLD growth of EuxY2−xO3 phosphor films

We have investigated the He buffer gas process of moderating the kinetic energy of the pulsed laser deposition (PLD) plume during EuxY2−xO3 phosphor film growth. When using a neodymium yttrium aluminum garnet laser for PLD thin film growth, the kinetic energy of the ablation plumes can be high enough to cause the formation of point defects in the film. The buffer gas pressure is an important process parameter in PLD film growth. We find that the presence of the He buffer gas reduces the kinetic energy of the laser deposition plume through many low-angle collisions in the gas phase by a factor of 7 without reducing the deposition rate. This is because He is much lighter than any of the elements in the plume and it does not affect the composition of the oxide films. Consequently, the resputtering of the Y2O3 film surface by the plume was significantly suppressed in the presence of the He gas moderator, leading to a decrease of the defect density in the Y2O3 films. The improvement of the film quality was verified by a systematic analysis of time-resolved photoluminescence (PL) data for EuxY2−xO3 composition–gradient films. The PL lifetime and intensity of Eu0.2Y1.8O3, which shows the highest PL intensity, increased by 13.3% and 36.4%, respectively, when the He gas moderation process was used. The He buffer gas process is applicable to the PLD growth of the other oxide materials as well, where the reduction of the kinetic energy of the plume would bring the PLD process closer to the molecular beam epitaxy growth condition.

Molecular beam epitaxy growth and characterization of GePb alloys

Pb based group-IV alloys such as GePb have been gaining interest as a potential alternative for infrared detectors, quantum materials, and high-speed electronic devices. Challenges remain in their growth due to the extremely low solid solubility of Pb in the Ge–Pb system. This paper reports molecular beam epitaxy growth of GePb alloy thin films on Ge(100) substrates. Effusion cells of Ge and Pb are used to control the flux ratio independently. The optimal substrate temperature is found to be near the thermocouple temperature of 300 °C based on the characterization of the grown films using high-resolution x-ray diffraction. A large change in the Ge:Pb beam equivalent pressure ratio from 10:1 to 1:1 results in only a minimal increase of the Pb composition from 0.74% to 2.84% as estimated from Raman spectroscopy and Rutherford backscattering spectrometry. Scanning electron microscopy images show a large volume of Pb islands on the surface that form into either long trapezoidal rods or uniform droplets, with increasing Pb flux and growth time the density of Pb islands increased.

Low-Temperature Migration-Enhanced Epitaxial Growth of High-Quality (InAs)4(GaAs)3/Be-Doped InAlAs Quantum Wells for THz Applications

This investigation explores the structural and electronic properties of low-temperature-grown (InAs)4(GaAs)3/Be-doped InAlAs and InGaAs/Be-doped InAlAs multiple quantum wells (MQWs), utilizing migration-enhanced epitaxy (MEE) and conventional molecular beam epitaxy (MBE) growth mode. Through comprehensive characterization methods including transmission electron microscopy (TEM), Raman spectroscopy, atomic force microscopy (AFM), pump–probe transient reflectivity, and Hall effect measurements, the study reveals significant distinctions between the two types of MQWs. The (InAs)4(GaAs)3/Be-doped InAlAs MQWs grown via the MEE mode exhibit enhanced periodicity and interface quality over the InGaAs/Be-InAlAs MQWs grown through the conventional molecule beam epitaxy (MBE) mode, as evidenced by TEM. The AFM results indicate lower surface roughness for the (InAs)4(GaAs)3/Be-doped InAlAs MQWs by using the MEE mode. Raman spectroscopy reveals weaker disorder-activated modes in the (InAs)4(GaAs)3/Be-doped InAlAs MQWs by using the MEE mode. This originates from utilizing the (InAs)4(GaAs)3 short period superlattices rather than InGaAs, which suppresses the arbitrary distribution of Ga and In atoms during the InGaAs growth. Furthermore, pump–probe transient reflectivity measurements show shorter carrier lifetimes in the (InAs)4(GaAs)3/Be-doped InAlAs MQWs, attributed to a higher density of antisite defects. It is noteworthy that room temperature Hall measurements imply that the mobility of (InAs)4(GaAs)3/Be-doped InAlAs MQWs grown at a low temperature of 250 °C via the MEE mode is superior to that of InGaAs/Be-doped InAlAs MQWs grown in the conventional MBE growth mode, reaching 2230 cm2/V.s. The reason for the higher mobility of (InAs)4(GaAs)3/Be-doped InAlAs MQWs is that this short-period superlattice structure can effectively suppress alloy scattering caused by the arbitrary distribution of In and Ga atoms during the growth process of the InGaAs ternary alloy. These results exhibit the promise of the MEE growth approach for growing high-performance MQWs for advanced optoelectronic applications, notably for high-speed optoelectronic devices like THz photoconductive antennas.

Interfaces-engineered M-structure for infrared detectors

In this paper, the authors report strain-balanced M-structures InAs/GaSb/AlSb/GaSb superlattice growth on GaSb substrates using two kinds of interfaces (IFs): GaAs-like IFs and InSb-like IFs. The in-plane compressive strain of 60-period and 100-period InAs��/GaSb/AlSb��/GaSb with different InAs (��) and AlSb (��) monolayers are investigated. The M-structures InAs/GaSb/AlSb/GaSb represent type II superlattices (T2SL) and at present are under intensive investigation. Many authors show theoretical and experimental results that such structures can be used as a barrier material for a T2SL InAs/GaSb absorber tuned for long-wave infrared detectors (8 μm–14 μm). Beside that, M-structure can also be used as an active material for short-wave infrared detectors to replace InAs/GaSb which, for this region of infrared, are a big challenge from the point of view of balancing compression stress. The study of InAs/GaSb/AlSb/GaSb superlattice with the minimal strain for GaSb substrate can be obtained by a special procedure of molecular beam epitaxy growth through special shutters sequence to form both IFs. The authors were able to achieve smaller minimal mismatches of the lattice constants compared to literature. The high-resolution X-ray diffraction measurements prove that two types of IFs are proper for balancing the strain in such structures. Additionally, the results of Raman spectroscopy, surface analyses of atomic force microscopy, and differential interference contrast microscopy are also presented. The numerical calculations presented in this paper prove that the presence of IFs significantly changes the energy gap in the case of the investigated M-structures. The theoretical results obtained for one of the investigated structures, for a specially designed structure reveal an extra energy level inside the energy gap. Moreover, photoluminescence results obtained for this structure prove the good quality of the synthesized M-structures, as well as are in a good agreement with theoretical calculations.

Atomic layer molecular beam epitaxy of kagome magnet RMn6Sn6 (R = Er, Tb) thin films

Kagome lattices have garnered substantial interest because their band structure consists of topological flat bands and Dirac cones. The RMn6Sn6 (R = rare earth) compounds are particularly interesting because of the existence of the large intrinsic anomalous Hall effect (AHE), which originates from the gapped Dirac cones near the Fermi level. This makes RMn6Sn6 an outstanding candidate for realizing the high-temperature quantum AHE. The growth of RMn6Sn6 thin films is beneficial for both fundamental research and potential applications. However, most of the studies on RMn6Sn6 have focused on bulk crystals, and the synthesis of RMn6Sn6 thin films has not been reported so far. Here, we report the atomic layer molecular beam epitaxy growth, structural and magnetic characterizations, and transport properties of ErMn6Sn6 and TbMn6Sn6 thin films. It is especially noteworthy that TbMn6Sn6 thin films have out-of-plane magnetic anisotropy, which is important for realizing the quantum AHE. Our work paves the avenue toward the control of the AHE using devices patterned from RMn6Sn6 thin films.

Study on the effects of Si-doping in molecular beam heteroepitaxial β-Ga2O3 films

β-Ga2O3, an emerging wide bandgap semiconductor material, holds significant potential for various applications. However, challenges persist in improving the crystal quality and achieving controllable doping of β-Ga2O3. In particular, the relationship between these factors and the mechanisms behind them are not fully understood. Molecular beam epitaxy (MBE) is viewed as one of the most sophisticated techniques for growing high-quality crystalline films. It also provides a platform for studying the effects of doping and defects in heteroepitaxial β-Ga2O3. In our study, we tackled the issue of Si source passivation during the MBE growth of Si-doped β-Ga2O3. We did this by using an electron beam vaporize module, a departure from the traditional Si effusion cell. Our research extensively explores the correlation between Si doping concentration and film properties. These properties include microstructure, morphology, defects, carrier conductivity, and mobility. The results from these investigations are mutually supportive and indicate that a high density of defects in heteroepitaxial β-Ga2O3 is the primary reason for the challenges in controllable doping and conductivity. These insights are valuable for the ongoing development and enhancement of β-Ga2O3-based device techniques.

Effects of background doping, interdiffusion and layer thickness fluctuation on the transport characteristics of THz quantum cascade lasers

In this work, we investigate the effects of n and p-type background doping, interface composition diffusion (interdiffusion) of the barrier material and layer thickness variation during molecular beam epitaxy (MBE) growth on transport characteristics of terahertz-frequency quantum cascade lasers (THz QCLs). We analysed four exemplary structures: a bound-to-continuum design, hybrid design, LO-phonon design and a two-well high-temperature performance LO-phonon design. The exemplary bound-to-continuum design has shown to be the most sensitive to the background doping as it stops lasing for concentrations around 1.0·1015\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$1.0\\cdot 10^{15}$$\\end{document}–2.0·1015\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$2.0\\cdot 10^{15}$$\\end{document} cm-3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-3}$$\\end{document}. The LO-phonon design is the most sensitive to growth fluctuations during MBE and this is critical for novel LO-phonon structures optimised for high-temperature performance. We predict that interdiffusion mostly affects current density for designs with narrow barrier layers and higher Al\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ extrm{Al}$$\\end{document} composition. We show that layer thickness variation leads to significant changes in material gain and current density, and in some cases to the growth of nonfunctional devices. These effects serve as a beacon of fundamental calibration steps required for successful realisation of THz QCLs.

Investigating the impact of growth temperature on the direct integration of pure phase 2H-MoTe2 with Si(1 1 1) using molecular beam epitaxy

Group-VI transition metal dichalcogenides (TMDCs) have garnered significant attention due to their novel properties in the two-dimensional realm. Among these TMDCs, MoTe2 is a particularly intriguing material with a small difference between Gibbs free energies of its semiconducting and semimetallic phases, rendering it suitable for a wide array of applications. In this study, we report the molecular beam epitaxy (MBE) growth of 2H-MoTe2 directly on a silicon (111) substrate, which holds great significance due to its compatibility with the existing integrated circuit industry. By employing a growth strategy involving a Te-rich silicon surface, we achieved the growth of high-quality, phase-pure 2H-MoTe2 films. The effect of growth temperature on the film quality was investigated, revealing that the growth at 450 °C led to highly crystalline and layered growth. The thickness, roughness and stoichiometry of the grown films were characterized using spectroscopic ellipsometry, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Raman spectroscopy confirmed the presence of the desired 2H-phase for the grown films. The direct integration of MoTe2 with silicon opens up exciting possibilities for its utilization in nanoelectronics and optoelectronics devices.

Molecular beam epitaxy growth of superconducting tantalum germanide

Developing alternative material platforms for use in superconductor–semiconductor hybrid structures is desirable due to limitations caused by intrinsic microwave losses present in commonly used III/V material systems. With the recent reports on tantalum superconducting qubits that show improvements over the Nb and Al counterparts, exploring Ta the superconductor in hybrid material systems is promising. Here, we study the growth of Ta on semiconducting Ge (001) substrates grown via molecular beam epitaxy. We show that at a growth temperature of 400 °C, the Ta diffuses into the Ge matrix in a self-limiting nature resulting in smooth and abrupt surfaces and interfaces with roughness on the order of 3–7 Å as measured by atomic force microscopy and x-ray reflectivity. The films are found to be a mixture of Ta5Ge3 and TaGe2 binary alloys and form a native oxide that seems to form a sharp interface with the underlying film. These films are superconducting with a TC∼1.8−2 K and HC⊥∼1.88 T, HC∥∼5.1 T. These results show this tantalum germanide film to be promising for future superconducting quantum information platforms.

(Ga,Mn)N—Epitaxial growth, structural, and magnetic characterization—Tutorial

The spin control possibility and its application in optoelectronic devices began an intensive research into its utilization, in particular, in the wide-gap semiconductors such as GaN doped with transition metal ions. Due to a strong p–d hybridization in Ga1−xMnxN, the Curie temperature above 300 K was already expected for x = 5%, providing that the free hole concentration necessary for the hole-mediated ferromagnetism exceeds 1020 cm−3. In this context, the development of non-equilibrium techniques enabled the engineering high-quality epitaxial layers of (Ga,Mn)N exhibiting uniform ferromagnetism at low-end cryogenic temperatures. The Tutorial is focused on the molecular beam epitaxy growth method of the Mn-enriched GaN magnetic semiconductors, summarizes the (Ga,Mn)N structural and electronic studies, and explains fundamental ferromagnetic properties, including the determination of the Mn concentration and the Curie temperature based on magnetic measurements. Most studies reveal the homogenous substitution of Mn3+ ions in the GaN matrix. Nevertheless, achieving room-temperature ferromagnetism still remains a challenge. Therefore, in the Tutorial, future research is suggested that can help obtain the homogenous ferromagnetism in (Ga,Mn)N at much elevated temperatures.

Intrinsic insulating transport characteristics in low-carrier density EuCd2As2 films

Searching for an ideal magnetic Weyl semimetal hosting only a single pair of Weyl points has been a focal point for systematic clarification of its unique magnetotransport derived from the interplay between topology and magnetization. Among the candidates, triangular-lattice antiferromagnet EuCd2As2 has been attracting special attention due to the prediction of the ideal Weyl semimetal phase in the ferromagnetic state; however, transport properties of low-carrier density samples have remained elusive. Here, we report molecular beam epitaxy growth of EuCd2As2 films, achieving low-hole density in the range of 1015–1016 cm−3 at low temperature. Transport measurements of such low-carrier density films reveal an insulating behavior with an activation gap of about 200 meV, which persists even in the field-induced ferromagnetic state. Our work provides an important experimental clue that EuCd2As2 is intrinsically insulating, contrary to the previous prediction.

Growth, catalysis, and faceting of α-Ga2O3 and α-(InxGa1−x)2O3 on m-plane α-Al2O3 by molecular beam epitaxy

The growth of α-Ga2O3 and α-(InxGa1−x)2O3 on m-plane α-Al2O3(101̄0) by molecular beam epitaxy (MBE) and metal-oxide-catalyzed epitaxy (MOCATAXY) is investigated. By systematically exploring the parameter space accessed by MBE and MOCATAXY, phase-pure α-Ga2O3(101̄0) and α-(InxGa1−x)2O3(101̄0) thin films are realized. The presence of In on the α-Ga2O3 growth surface remarkably expands its growth window far into the metal-rich flux regime and to higher growth temperatures. With increasing O-to-Ga flux ratio (RO), In incorporates into α-(InxGa1−x)2O3 up to x ≤ 0.08. Upon a critical thickness, β-(InxGa1−x)2O3 nucleates and, subsequently, heteroepitaxially grows on top of α-(InxGa1−x)2O3 facets. Metal-rich MOCATAXY growth conditions, where α-Ga2O3 would not conventionally stabilize, lead to single-crystalline α-Ga2O3 with negligible In incorporation and improved surface morphology. Higher TTC further results in single-crystalline α-Ga2O3 with well-defined terraces and step edges at their surfaces. For RO ≤ 0.53, In acts as a surfactant on the α-Ga2O3 growth surface by favoring step edges, while for RO ≥ 0.8, In incorporates and leads to a-plane α-(InxGa1−x)2O3 faceting and the subsequent (2̄01) β-(InxGa1−x)2O3 growth on top. Thin film analysis by scanning transmission electron microscopy reveals highly crystalline α-Ga2O3 layers and interfaces. We provide a phase diagram to guide the MBE and MOCATAXY growth of single-crystalline α-Ga2O3 on α-Al2O3(101̄0).

In Situ and Real-Time Monitoring of Doping Levels by Reflectance Anisotropy Spectroscopy (RAS) during Molecular Beam Epitaxial (MBE) Growth of III/V Semiconductors

Reflectance anisotropy/difference spectroscopy (RAS/RDS) had been developed for monitoring epitaxial semiconductor growth, especially for the metal-organic chemical vapor deposition (MOCVD) of III/V semiconductors. But RAS is also well suited for the control of III/V growth with molecular beam epitaxy (MBE). Although the work on RAS has already started at least three decades ago, the potential of this in situ and real-time monitoring technique, especially for doping control, is not well known yet. Experimental results are given here on the identification of doping types and concentration during MBE growth, exemplarily for GaAs and AlGaAs. Especially, the dependence of the majority charge carrier concentration (i.e., the doping concentration) on the RAS signal difference between the nondoping and doping cases is addressed here.

Semi-empirical growth model of InSbBi grown by molecular beam epitaxy

The molecular beam epitaxy growth of bump-free indium antimonide-bismide (InSbBi) necessitates precise control of group III and V stoichiometry, as well as the growth temperature and rate. In this research, we propose a semi-empirical growth model approach to enhance our understanding of the involved growth kinetics and predict the bismuth (Bi) content and surface quality of InSbBi films. The model combines the impact of growth rate and the maximum incorporation rate of Bi on the Bi content of InSbBi. Our findings indicate that bump-free InSbBi requires a minimum Bi incorporation rate to Bi adatoms arrival rate (or Bi flux) ratio of 0.96, which closely approaches the ideal value of 1.0. By employing this growth model, we can optimize the growth conditions for achieving high-quality InSbBi films with controlled Bi content.

Atomic construction and spectroscopic characterization of FeSe-derived thin films on SrTiO3 substrates

Controllably fabricating low-dimensional systems and unraveling their exotic states at the atomic scale is a pivotal step for the construction of quantum functional materials with emergent states. Here, by utilizing the elaborated molecular beam epitaxy growth, we obtain various FexSey phases beyond the single-layer FeSe/SrTiO3 films. A synthetic strategy of lowering substrate temperature with superfluous Se annealing is implemented to achieve various stoichiometric FeSe-derived phases, ranging from 1:1 to 5:8. The phase transitions and electronic structure of these FexSey phases are systematically characterized by atomic resolution scanning tunneling microscopy measurements. We observe the long-ranged antiferromagnetic order of the Fe4Se5 phase by spin-polarized signals with striped patterns, which is also verified by their magnetic response of phase shift between adjacent domains. The electronic doping effect in insulating Fe4Se5 and the kagome effect in metallic Fe5Se8 are also discussed, where the kagome lattice is a promising structure to manifest both spin frustration of d electrons in a quantum-spin-liquid phase and correlated topological states with flat-band physics. Our study provides promising opportunities for constructing artificial superstructures with tunable building blocks, which is helpful for understanding the emergent quantum states and their correlation with competing orders in the FeSe-based family.

Improved phosphorus doping in ZnTe by molecular beam epitaxy under alternating source supply

Phosphorus (P) doping in ZnTe grown by molecular beam epitaxy (MBE) under alternating source supply method was investigated to achieve p-type P-doped ZnTe (ZnTe:P) thin films using InP as a P dopant source, and the result was compared with those grown under a simultaneous MBE growth where the source beams were supplied simultaneously. As a result, P concentration in ZnTe thin films was found to increase with increasing the InP flux, and high P concentration up to 6.6 × 1019 cm−3 was confirmed by secondary ion mass spectroscopy (SIMS) analyses. However, In incorporation was also observed in the ZnTe:P thin films, despite that the detected In concentration by SIMS was more than one order of magnitude lower than the P concentration and almost two order of magnitude lower than those grown by the simultaneous MBE. Photoluminescence measurement of ZnTe:P thin film grown under alternating source supply showed a P-related acceptor bound exciton (Ia) peak at 2.37 eV, and the intensity of Ia emission increased after the annealing treatment, indicating the activation of P acceptor. The annealing also decreases the resistivity of the film. The results clearly indicate that the alternating source supply growth is effective to obtain ZnTe:P thin films with better P doping properties.

Molecular Beam Epitaxy Growth of Cadmium Telluride Structures on Hexagonal Boron Nitride.

We investigate the feasibility of the epitaxial growth of a three-dimensional semiconductor on a two-dimensional substrate. In particular, we report for the first time the molecular beam epitaxy growth of cadmium telluride (CdTe) quantum wells on hexagonal boron nitride (hBN). The presence of the quantum wells is confirmed by photoluminescence measurements conducted at helium temperatures. Growth of the quantum wells on two-dimensional, almost perfectly flat hBN appears to be very different from growth on bulk substrates; in particular, it requires 70-100 °C lower temperatures.

Quasi van der Waals Epitaxy of Rhombohedral-Stacked Bilayer WSe2 on GaP(111) Heterostructure.

The growth of bilayers of two-dimensional (2D) materials on conventional 3D semiconductors results in 2D/3D hybrid heterostructures, which can provide additional advantages over more established 3D semiconductors while retaining some specificities of 2D materials. Understanding and exploiting these phenomena hinge on knowing the electronic properties and the hybridization of these structures. Here, we demonstrate that a rhombohedral-stacked bilayer (AB stacking) can be obtained by molecular beam epitaxy growth of tungsten diselenide (WSe2) on a gallium phosphide (GaP) substrate. We confirm the presence of 3R-stacking of the WSe2 bilayer structure using scanning transmission electron microscopy (STEM) and micro-Raman spectroscopy. Also, we report high-resolution angle-resolved photoemission spectroscopy (ARPES) on our rhombohedral-stacked WSe2 bilayer grown on a GaP(111)B substrate. Our ARPES measurements confirm the expected valence band structure of WSe2 with the band maximum located at the Γ point of the Brillouin zone. The epitaxial growth of WSe2/GaP(111)B helps to understand the fundamental properties of these 2D/3D heterostructures, toward their implementation in future devices.

Effect of two-step cubic GaN buffer layer on the radio-frequency plasma-assisted molecular beam epitaxy growth of cubic AlN films grown on MgO (001) substrates

Cubic AlN (c-AlN) films were grown on MgO (001) substrates through the use of a two-step cubic GaN (c-GaN) buffer layer using radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE) technique. The two-step c-GaN buffer layer was found to be crucial for the epitaxial growth of c-AlN, as it showed a specific epitaxial relationship between c-AlN and c-GaN along the MgO [100] zone axis. On the other hand, the c-AlN films grown with the one-step and nonGaN buffer layers showed columnar grain structure. The optical study found that the c-AlN films grown with the two-step c-GaN buffer layer had an indirect bandgap energy of 4.99 ± 0.02 eV, which is consistent with previous experimental data and theoretical calculations. The c-AlN films grown with the one-step and nonGaN buffer layers had indirect bandgap energies of 5.00 ± 0.02 eV and 5.18 ± 0.02 eV, respectively. These results highlight the importance of the two-step c-GaN buffer layer in enhancing the MBE growth of c-AlN on MgO (001) substrate.