Molecular Beam Epitaxy Growth Conditions Research Articles

Epitaxial (AlxGa1−x−yIny)2O3 alloys lattice matched to monoclinic Ga2O3 substrates

We have epitaxially stabilized a series of monoclinic (AlxGa1−x−yIny)2O3 alloys by careful choice of molecular beam epitaxy growth conditions, which balance alloy growth with suboxide desorption. The films are pseudomorphic to (010) β-Ga2O3 substrates at thicknesses up to 150 nm with compositions ranging from (Al0.01Ga0.83In0.16)2O3 to (Al0.24Ga0.75In0.03)2O3. The absorption edge shifts from approximately 4.62–5.14 eV with coincidently increasing Al and decreasing In mole fractions. J–V measurements reveal an increase in resistivity over four orders of magnitude with a maximum value of 4.2 × 105 Ω-cm for (Al0.17Ga0.76In0.07)2O3, which has nearly identical lattice parameters (both in-plane and out-of-plane) to the underlying β-Ga2O3. Scanning transmission electron microscopy of this sample reveals a mostly uniform and single crystalline film, though we identify areas of non-uniform In incorporation and some γ-phase inclusions. This work demonstrates the feasibility of thick layers lattice-matched to β-Ga2O3 with increased bandgap compared to phase-separation limited (Al,Ga)2O3. These alloys can enable higher bandgap epitaxial dielectrics and high sheet charge density transistors by increasing the conduction band offset with respect to β-Ga2O3.

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.

Growth of InGaAs/InAlAs superlattices for strain balanced quantum cascade lasers by molecular beam epitaxy

This study investigated the molecular beam epitaxy (MBE) growth conditions of strain-balanced (SB) In0.669GaAs/In0.362AlAs superlattices (SLs) for SB quantum cascade lasers (QCLs). The growth modes and properties of SB SLs are strongly affected by the growth conditions. The properties of the SB SLs were evaluated using atomic force microscopy (AFM) and high-resolution X-ray diffraction (HRXRD) analysis. Following the establishment of optimized conditions for SB SLs growth, SB QCL were grown. The HRXRD analysis and transmission electron microscopy (TEM) measurements showed that the grown samples exhibited abrupt interfaces and good structural quality. An epitaxial wafer was processed into a Fabry-Perot cavity with a ridge structure. The device operated in pulsed mode emitting ∼4.7 µm at room temperature with a peak power of 650 mW and a slope efficiency of 870 mW/A.

Material Defects and Dark Currents in InGaAs/InP Avalanche Photodiode Devices

An In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</sub> As/indium phosphide (InP) avalanche photodiode (APD) with a separate absorption, grading, charge, and multiplication (SAGCM) structure is epitaxially grown by molecular beam epitaxy (MBE). The resulting material is studied using X-ray diffraction (XRD), photoluminescence (PL), and scanning transmission electron microscopy. The activation energy extracted from the dark current of the APD indicates that it is dominated by the generation–recombination (G–R) process. Deep low-temperature PL peaks reveal the existence of an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{v} + 0.42$ </tex-math></inline-formula> -eV deep energy level defect in the indium–gallium–arsenide (InGaAs) absorber layer, which is considered to be a result of point defects caused by Ga-poor or In-poor MBE growth conditions. The effect of the defect on the dark current is confirmed using numerical calculation methods.

Bulk-like magnetic properties in MBE-grown unstrained, antiferromagnetic CuMnSb

A detailed study of the influence of molecular beam epitaxial growth conditions on the structural and magnetic characteristics of CuMnSb films on lattice matched GaSb is presented. For a set of nine 40 nm thick layers, the Mn and Sb fluxes are varied to produce material with different elemental compositions. It is found that the layers grown under a relative Mn to Sb flux ratio of ΦMn/ΦSb=1.24±0.02 are closest to the stoichiometric composition for which the Néel temperature (TN) attains its maximum values. Mn-related structural defects are believed to be the driving contribution to changes in the vertical lattice parameter. Having established the optimum growth conditions, a second set of samples with CuMnSb layer thickness varied from 5 to 510 nm is fabricated. We show that for sufficiently large thicknesses, the magnetic characteristics (TN≃62K, Curie–Weiss temperature ΘCW=−100 K) of the stoichiometric layers do correspond to the parameters reported for bulk samples. On the other hand, we observe a reduction of TN as a function of the CuMnSb thickness for our thinnest layers. All findings reported here are of particular relevance for studies aiming at the demonstration of Néel vector switching and detection in this noncentrosymmetric antiferromagnet, which have been recently proposed.

Broadband Quantum Dot Superluminescent Diode with Simultaneous Three-State Emission.

Semiconductor superluminescent light-emitting diodes (SLEDs) have emerged as ideal and vital broadband light sources with extensive applications, such as optical fiber-based sensors, biomedical sensing/imaging, wavelength-division multiplexing system testing and optoelectronic systems, etc. Self-assembled quantum dots (SAQDs) are very promising candidates for the realization of broadband SLED due to their intrinsic large inhomogeneous spectral broadening. Introducing excited states (ESs) emission could further increase the spectral bandwidth. However, almost all QD-based SLEDs are limited to the ground state (GS) or GS and first excited state (ES1) emission. In this work, multiple five-QD-layer structures with large dot size inhomogeneous distribution were grown by optimizing the molecular beam epitaxy (MBE) growth conditions. Based on that, with the assistance of a carefully designed mirror-coating process to accurately control the cavity mirror loss of GS and ESs, respectively, a broadband QD-SLED with three simultaneous states of GS, ES1 and second excited-state (ES2) emission has been realized, exhibiting a large spectral width of 91 nm with a small spectral dip of 1.3 dB and a high continuous wave (CW) output power of 40 mW. These results pave the way for a new fabrication technique for high-performance QD-based low-coherent light sources.

WSe2 growth on hafnium zirconium oxide by molecular beam epitaxy: the effect of the WSe2 growth conditions on the ferroelectric properties of HZO

Direct integration of transition metal dichalcogenides on a ferroelectric such as hafnium zirconium oxide (HZO) using an industrially scalable technique is important for realizing various ferroelectric-based device architectures. The interface formed due to the processing conditions during direct deposition is the focus of the current study. In this work, molecular beam epitaxy (MBE) is used to directly deposit WSe2 on HZO substrates, and the effects of the MBE growth conditions, specifically high temperature and a high Se flux, are examined. Anneals of HZO under a Se flux, which serve to replicate the conditions during actual WSe2 deposition, result in the crystallization of amorphous as-deposited HZO substrates and incorporation of Se into the HZO. The crystallinity and composition of the HZO substrates affect the degree of Se incorporation. Some of the Se found in the HZO is an adsorbed layer that can be thermally desorbed, but it also has a chemisorbed component fully incorporated within the HZO lattice. Measurement of the electrical properties of the HZO films did not provide evidence that the incorporated Se was detrimental to the functionality of the HZO as a ferroelectric layer.

Even-Odd Layer-Dependent Anomalous Hall Effect in Topological Magnet MnBi2Te4 Thin Films.

Recently, MnBi2Te4 has been demonstrated to be an intrinsic magnetic topological insulator and the quantum anomalous Hall (QAH) effect was observed in exfoliated MnBi2Te4 flakes. Here, we used molecular beam epitaxy (MBE) to grow MnBi2Te4 films with thickness down to 1 septuple layer (SL) and performed thickness-dependent transport measurements. We observed a nonsquare hysteresis loop in the antiferromagnetic state for films with thickness greater than 2 SL. The hysteresis loop can be separated into two AH components. We demonstrated that one AH component with the larger coercive field is from the dominant MnBi2Te4 phase, whereas the other AH component with the smaller coercive field is from the minor Mn-doped Bi2Te3 phase. The extracted AH component of the MnBi2Te4 phase shows a clear even-odd layer-dependent behavior. Our studies reveal insights on how to optimize the MBE growth conditions to improve the quality of MnBi2Te4 films.

Anisotropic Piezoelectric Response from InGaN Nanowires with Spatially Modulated Composition and Topography over a Textured Si(100) Substrate.

An anisotropic piezoelectric response is demonstrated from InGaN nanowires (NWs) over a pyramid-textured Si(100) substrate with an interfacet composition and topography modulation induced by stationary molecular beam epitaxy growth conditions, taking advantage of the unidirectional source beam flux. The variations of InGaN NWs between the pyramid facets are verified in terms of morphology, element distribution, and crystalline properties. The piezoelectric response is investigated by electrical atomic force microscopy (AFM) with a statistic analyzing method. Representative pyramids from the ensemble, on top of which InGaN NWs grown with a substrate held at an oblique angle, were characterized for understanding and confirming the degree of anisotropy. The positive deviated oscillation of the peak force error is identified as a measure of the effective AFM tip/NW interaction with respect to the electrical contact and mechanical deformation. The Schottky contact between the metal-coated AFM tip and the NWs on the different facets reveals distinctions consistent with the interfacet composition variation. The interfacet variation of the piezoelectric response of the InGaN NWs is first evaluated by electrical AFM under zero bias. The average current monotonically depends on the scan frequency, which determines the average peak force error, that is, mechanical deformation, with a facet characteristic slope. A piezoelectric nanogenerator device is fabricated out of a sample with an ensemble of pyramids, which exhibits anisotropic output under periodic directional pressing. This work provides a universal strategy for the synthesis of composite semiconductor materials with an anisotropic piezoelectric response.

Heteroepitaxial growth of β-Ga2O3 films on SiC via molecular beam epitaxy

β-Ga2O3 is a promising ultrawide bandgap semiconductor for next generation radio frequency electronics. However, its low thermal conductivity and inherent thermal resistance provide additional challenges in managing the thermal response of β-Ga2O3 electronics, limiting its power performance. In this paper, we report the heteroepitaxial growth of β-Ga2O3 films on high thermal conductivity 4H-SiC substrates by molecular beam epitaxy (MBE) at 650 °C. Optimized MBE growth conditions were first determined on sapphire substrates and then used to grow β-Ga2O3 on 4H-SiC. X-ray diffraction measurements showed single phase (2¯01) β-Ga2O3 on (0001) SiC substrates, which was also confirmed by TEM measurements. These thin films are electrically insulating with a (4¯02) peak rocking curve full-width-at-half-maximum of 694 arc sec and root mean square surface roughness of ∼2.5 nm. Broad emission bands observed in the luminescence spectra, acquired in the spectral region between near infrared and deep ultraviolet, have been attributed to donor-acceptor pair transitions possibly related to Ga vacancies and its complex with O vacancies. The thermal conductivity of an 81 nm thick Ga2O3 layer on 4H-SiC was determined to be 3.1 ± 0.5 W/m K, while the measured thermal boundary conductance (TBC) of the Ga2O3/SiC interface is 140 ± 60 MW/m2 K. This high TBC value enables the integration of thin β-Ga2O3 layers with high thermal conductivity substrates to meliorate thermal dissipation and improve device thermal management.

Effect of molecular beam epitaxy growth conditions on phase separation in wide-bandgap InAlAsSb lattice-matched to InP

InxAl1-xAs1-ySby is the only III-V material lattice-matched to InP with a direct bandgap energy range as large as ~1.45–1.80 eV, making it interesting for multiple optoelectronic applications. However, inherent material challenges in this immature quaternary alloy have thus far precluded the growth of InxAl1-xAs1-ySby with device-quality properties at the InP lattice constant. We have investigated how molecular beam epitaxy growth conditions affect the photoluminescence intensity, spectrum, temperature- and power-dependence, as well as the surface morphology of In0.23Al0.77As0.75Sb0.25, which is lattice-matched to InP with bandgap energy of ~1.68 eV as measured by ellipsometry. We find that reducing the adatom mobility, via a lower substrate temperature and higher group-V overpressure, diminishes the tendency for phase separation, but also quenches the photoluminescence intensity. These findings provide a step toward identifying an optimal growth window for realizing high-quality, wide-bandgap InxAl1-xAs1-ySby lattice-matched to InP.

Influence of strain on the indium incorporation in (0001) GaN

The incorporation of indium in GaN (0001) surfaces in dependence of strain is investigated by combining molecular-beam epitaxy (MBE) growth, quantitative transmission electron microscopy, and density-functional theory (DFT) calculations. Growth experiments were conducted on GaN, as well as on $30%\ifmmode\pm\else\textpm\fi{}2%$ partially relaxed $\mathrm{I}{\mathrm{n}}_{0.19}\mathrm{G}{\mathrm{a}}_{0.81}\mathrm{N}$ buffer layers, serving as substrates. Despite the only $0.6%$ larger in-plane lattice constant of GaN provided by the buffer layer, our experiments reveal that the In incorporation increases by more than a factor of two for growth on the $\mathrm{I}{\mathrm{n}}_{0.19}\mathrm{G}{\mathrm{a}}_{0.81}\mathrm{N}$ buffer, as compared to growth on GaN. DFT calculations reveal that the decreasing chemical potential due to the reduced lattice mismatch stabilizes the In--N bond at the surface. Depending on the growth conditions (metal rich or N rich), this promotes the incorporation of higher In contents into a coherently strained layer. Nevertheless, the effect of strain is highly nonlinear. As a consequence of the different surface reconstructions, growth on relaxed $\mathrm{I}{\mathrm{n}}_{x}\mathrm{G}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{N}$ buffers appears more suitable for metal-rich MBE growth conditions with regard to achieving higher In compositions.

Strain-driven quantum dot self-assembly by molecular beam epitaxy

Research into self-assembled semiconductor quantum dots (QDs) has helped advance numerous optoelectronic applications, ranging from solid-state lighting to photodetectors. By carefully controlling molecular beam epitaxy (MBE) growth parameters, we can readily tune QD light absorption and emission properties to access a broad portion of the electromagnetic spectrum. Although this field is now sufficiently mature that QDs are found in consumer electronics, research efforts continue to expand into new areas. By manipulating MBE growth conditions and exploring new combinations of materials, substrate orientations, and the sign of strain, a wealth of opportunities exist for synthesizing novel QD nanostructures with hitherto unavailable properties. As such, QDs are uniquely well positioned to make critical contributions to the development of future quantum technologies. In this tutorial, we summarize the history of self-assembled QDs, outline some examples of quantum optics applications based on QDs, discuss the science that explains the spontaneous formation of QDs, and provide recipes for successful QD growth by MBE for some of the most commonly used semiconductor materials systems. We hope that compiling this information in one place will be useful both for those new to QD self-assembly and for experienced researchers, ideally supporting the community’s efforts to continue pushing the boundaries of knowledge in this important field.

Aluminum metallization of III–V semiconductors for the study of proximity superconductivity

Managing the interaction of materials with insertion layers and nonconventional molecular beam epitaxy growth conditions allows for interfaces that are more precise but requires judicious examination of the multiple possible design variables. Here, we show a comparison between As- and Sb-containing insertion layers between Al and two binaries with different group V elements and demonstrate that antimonide layers greatly improve the interface. In addition to depositing Al at extremely slow growth rates onto cold (below 0 °C) substrates, the reactivity is particularly minimized with AlSb insertion layers, which improves interface abruptness, preserves the underlying semiconductor layer’s crystalline properties, and produces flatter superconductor surfaces.

Influence of strain on the InAs1 – xSbx composition

The composition of III–V semiconductor alloys with multiple group V elements results from a complex interaction of each group V species with each other and with group IIIs. Molecular beam epitaxy growth conditions, such as the group V absolute fluxes and flux ratios, substrate temperature, group III growth rate, the presence of surfactants, and the alloy's strain state, all affect the composition of InAs1 – xSbx. These factors are codependent in a manner that is far from completely established. In this work, the authors examine how the sign and degree of strain affects the film’s composition. In this study, the authors show that InAs1 – xSbx alloys have some ability to resist the creation of strain by self-adjusting the incorporation of the group V elements that would otherwise be changed in an unfavorable way (by, for example, a substrate temperature change). This self-latching to the substrate lattice constant is beneficial, since it means that the fluxes may not have to be adjusted as precisely as otherwise needed.

Effects of molecular beam epitaxy growth conditions on grain size and lattice strain in a-axis-oriented BaSi2 films

We grow 0.5 μm thick a-axis oriented BaSi2 films on Si(111) substrates by a conventional two-step growth method and compare their properties with those grown by a three-step growth method. Both grow methods consist of the initial growth of a BaSi2 template layer and the following molecular beam epitaxy (MBE) to form BaSi2 films. In the two-step growth method, Ba-to-Si deposition rate ratios (RBa/RSi) were varied in the range 0.4–4.7 during MBE. On the other hand, in the three-step growth method, MBE-grown BaSi2 layers are composed of BaSi2 films grown first under Ba-rich conditions (RBa/RSi = 4.0), followed by those grown under Si-rich conditions (RBa/RSi = 1.2). The grain size of BaSi2 films by the three-step growth method was much smaller than those grown by the two-step growth method. To our surprise, however, much higher photoresponsivity was obtained for BaSi2 films grown by the three-step growth method.

Low-density patterned InAs quantum dot arrays

The authors present work on patterned InAs quantum dot (QD) arrays using periodicities between 0.25 and 10 μm, with tailored molecular beam epitaxy growth conditions to promote high QD occupancy in the 10 μm array for further device implementation. The gallium-deoxidation method was used to maintain the pattern integrity. At high growth temperatures and reduced arsenic overpressure conditions, the authors have shown increasing QD occupancy as the patterned spacing increases. The 10 μm array was found to have 89% single QD occupancy. A low density of QDs in known locations enables the QDs to be patterned singly into quantum device applications.

AMMONIA MOLECULAR BEAM EPITAXY OF AlGaN HETEROSTRUCTURES ON SAPPHIRE SUBSTRATES

In order to develop a technology for the growth of Al(Ga)N-based heterostructures, the effect of different molecular beam epitaxy growth conditions on the properties of AlN and AlGaN layers was studied. The optimal conditions for the growth of AlN buffer layers were established, which made it possible to achieve a root mean square roughness as small as 0.7 nm. It was shown that an increase of AlN layer thickness leads to a decrease of density of edge dislocations, while no explicit dependence of the screw dislocation density on the layer thickness was observed. The minimal obtained dislocations density values for 1.25μm-thick AlN layer were n edges = 5.9×10 9 cm -2 and n screw = 2.2×10 7 cm -2 for edge and screw dislocations respectively. As a result of optimization of the AlGaN growth temperature, a series of 0.15μm-thick layers was grown, which showed stimulated emission at wavelengths λ = 330 nm, 323 nm, 303 nm, and 297 nm with threshold power densities of 0.7 MW/cm 2 , 1.1 MW/cm 2 , 1.4 MW/cm 2 and 1.4 MW/cm 2 , respectively. The determined optimal epitaxy conditions for AlN and AlGaN layers were used to grow the AlGaN/GaN high electron mobility transistor structure on a sapphire substrate with two-dimensional electron gas, which had a mobility of 1950 cm 2 /(Vs) at a concentration of 1.15×10 13 cm -2 . The obtained results are important for creating of nitride-based UV-emitting optoelectronic semiconductor devices, as well as high-power and high-frequency electronic devices.

Optimization of MBE Growth Conditions of In0.52Al0.48As Waveguide Layers for InGaAs/InAlAs/InP Quantum Cascade Lasers.

We investigate molecular beam epitaxy (MBE) growth conditions of micrometers-thick In0.52Al0.48As designed for waveguide of InGaAs/InAlAs/InP quantum cascade lasers. The effects of growth temperature and V/III ratio on the surface morphology and defect structure were studied. The growth conditions which were developed for the growth of cascaded In0.53Ga0.47As/In0.52Al0.48As active region, e.g., growth temperature of Tg = 520 °C and V/III ratio of 12, turned out to be not optimum for the growth of thick In0.52Al0.48As waveguide layers. It has been observed that, after exceeding ~1 µm thickness, the quality of In0.52Al0.48As layers deteriorates. The in-situ optical reflectometry showed increasing surface roughness caused by defect forming, which was further confirmed by high resolution X-ray reciprocal space mapping, optical microscopy and atomic force microscopy. The presented optimization of growth conditions of In0.52Al0.48As waveguide layer led to the growth of defect free material, with good optical quality. This has been achieved by decreasing the growth temperature to Tg = 480 °C with appropriate increasing V/III ratio. At the same time, the growth conditions of the cascade active region of the laser were left unchanged. The lasers grown using new recipes have shown lower threshold currents and improved slope efficiency. We relate this performance improvement to reduction of the electron scattering on the interface roughness and decreased waveguide absorption losses.

MBE AlGaN/GaN HEMT Heterostructures with Optimized AlN Buffer on Al2O3

The effect of molecular-beam epitaxy (MBE) growth conditions on properties of AlN epitaxial layers was investigated resulting in determination of optimal substrate temperature and ammonia flow. Optimal substrate temperature for growth of GaN and AlGaN layers was determined analyzing thermal decomposition rate of GaN. Based on the information, high electron mobility transistor heterostructures were grown on sapphire substrates using both ammonia and combined plasma-assisted/ammonia MBE modes. The highest achieved 2DEG mobility was 1992 cm2/(V s) (at 2DEG density of 1.17 × 1013 cm–2) which is the current state-of-the-art level.