Conventional Molecular Beam Epitaxy Research Articles

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.

Low-Temperature Growth of InGaAs Quantum Wells Using Migration-Enhanced Epitaxy.

The growth of InGaAs quantum wells (QWs) epitaxially on InP substrates is of great interest due to their wide application in optoelectronic devices. However, conventional molecular beam epitaxy requires substrate temperatures between 400 and 500 °C, which can lead to disorder scattering, dopant diffusion, and interface roughening, adversely affecting device performance. Lower growth temperatures enable the fabrication of high-speed optoelectronic devices by increasing arsenic antisite defects and reducing carrier lifetimes. This work investigates the low-temperature epitaxial growth of InAs/GaAs short-period superlattices as an ordered replacement for InGaAs quantum wells, using migration-enhanced epitaxy (MEE) with low growth temperatures down to 200-250 °C. The InAs/GaAs multi-quantum wells with InAlAs barriers using MEE grown at 230 °C show good single crystals with sharp interfaces, without mismatch dislocations found. The Raman results reveal that the MEE mode enables the growth of (InAs)4(GaAs)3/InAlAs QWs with excellent periodicity, effectively reducing alloy scattering. The room temperature (RT) photoluminescence (PL) measurement shows the strong PL responses with narrow peaks, revealing the good quality of the MEE-grown QWs. The RT electron mobility of the sample grown in low-temperature MEE mode is as high as 2100 cm2/V∗s. In addition, the photoexcited band-edge carrier lifetime was about 3.3 ps at RT. The high-quality superlattices obtained confirm MEE's effectiveness for enabling advanced III-V device structures at reduced temperatures. This promises improved performance for applications in areas such as high-speed transistors, terahertz imaging, and optical communications.

Growth of β-Ga2O3 and ϵ/κ-Ga2O3 on AlN(0001) by molecular-beam epitaxy

The heteroepitaxial growth and phase formation of Ga2O3 on Al-polar AlN(0001) templates by molecular-beam epitaxy (MBE) are studied. Three different MBE approaches are employed: (i) conventional MBE, (ii) suboxide MBE (S-MBE), and (iii) metal-oxide-catalyzed epitaxy (MOCATAXY). We grow phase-pure β-Ga2O3(2̄01) and phase-pure ϵ/κ-Ga2O3(001) with smooth surfaces by S-MBE and MOCATAXY. Thin film analysis shows that the crystallographic and surface features of the β-Ga2O3(2̄01)/AlN(0001) and ϵ/κ-Ga2O3(001)/AlN(0001) epilayers are of high crystalline quality. Growth and phase diagrams are developed to synthesize Ga2O3 on AlN by MBE and MOCATAXY and to provide guidance to grow Ga2O3 on several non-oxide surfaces, e.g., AlN, GaN, and SiC, by MBE, S-MBE, and MOCATAXY.

Growing clean crystals from dirty precursors: Solid-source metal-organic molecular beam epitaxy growth of superconducting Sr2RuO4 films

Ultra-high purity elemental sources have long been considered a prerequisite for obtaining low impurity concentrations in compound semiconductors in the world of molecular beam epitaxy (MBE) since its inception in 1968. However, we demonstrate that a “dirty” solid precursor, ruthenium(III) acetylacetonate [also known as Ru(acac)3], can yield single-phase, epitaxial, and superconducting Sr2RuO4 films with the same ease and control as III–V MBE. A superconducting transition was observed at ∼0.9 K, suggesting a low defect density and a high degree of crystallinity in these films. In contrast to the conventional MBE, which employs the ultra-pure Ru metal evaporated at ∼2000 °C as a Ru source, along with reactive ozone to obtain Ru → Ru4+ oxidation, the use of the Ru(acac)3 precursor significantly simplifies the MBE process by lowering the temperature for Ru sublimation (less than 200 °C) and by eliminating the need for ozone. Combining these results with the recent developments in hybrid MBE, we argue that leveraging the precursor chemistry will be necessary to realize next-generation breakthroughs in the synthesis of atomically precise quantum materials.

Growth of α-Ga2O3 on α-Al2O3 by conventional molecular-beam epitaxy and metal–oxide-catalyzed epitaxy

We report the growth of α-Ga2O3 on m-plane α-Al2O3 by conventional plasma-assisted molecular-beam epitaxy and In-mediated metal–oxide-catalyzed epitaxy (MOCATAXY). We report a growth rate diagram for α-Ga2O3( 101¯0 ), and observe (i) a growth rate increase, (ii) an expanded growth window, and (iii) reduced out-of-plane mosaic spread when MOCATAXY is employed for the growth of α-Ga2O3. Through the use of In-mediated catalysis, growth rates over 0.2 μm h−1 and rocking curves with full width at half maxima of Δω ≈ 0.45° are achieved. Faceting is observed along the α-Ga2O3 film surface and explored through scanning transmission electron microscopy.

Black Phosphorus as Tunable Van der Waals Quantum Wells with High Optical Quality

Van der Waals quantum wells, naturally formed in two-dimensional layered materials with nanoscale thickness, possess many inherent advantages over conventional molecular beam epitaxy grown counterparts, and could bring up intriguing physics and applications. However, optical transitions originated from the series of quantized states in these emerging quantum wells are still elusive. Here, we show that multilayer black phosphorus appears to be an excellent candidate for van der Waals quantum wells with well-defined subbands and high optical quality. Using infrared absorption spectroscopy, we probe subband structures of multilayer black phosphorus with tens of atomic layers, revealing clear signatures for optical transitions with subband index as high as 10, far from what was attainable previously. Surprisingly, in addition to allowed transitions, an unexpected series of "forbidden" transitions is also evidently observed, which enables us to determine energy spacings separately for conduction and valence subbands. Furthermore, the linear tunability of subband spacings by temperature and strain is demonstrated. Our results are expected to facilitate potential applications for infrared optoelectronics based on tunable van der Waals quantum wells.

Atomic layer deposition of diethylzinc/zinc oxide on InAs surface quantum dots: Self-clean-up and passivation processes

Passivation capping of molecular beam epitaxy (MBE)-grown InAs surface quantum dots (SQDs) is achieved by ex situ atomic layer deposition (ALD)-grown ZnO using diethylzinc (DEZ) as the zinc precursor, the main passivation agent, and oxygen plasma. Photoluminescence (PL) intensity is enhanced by 2-fold as the DEZ/ZnO passivation cap thickness reached 30 nm. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) show that contrary to conventional wet chemistry passivation or MBE capping, in which InAs SQDs can be damaged or be shrunken by intermixing of In and Ga, DEZ/ZnO passivation capping almost preserves the shape of the underlying SQDs. The cross-sectional analysis with transmission electron microscopy (TEM) reveals the formation of the ZnO grains on top of the sample accompanied by slight SQD height and width reduction, which are presumably related to the removal of the InAs native oxides. The DEZ “self-clean-up” passivation mechanism, where zinc precursor is responsible for reduction of the surface non-radiative recombination sites, is studied by X-ray photoelectron spectroscopy (XPS). The factors that control the DEZ “self-clean-up” efficiency such as the chain of chemical reactions, steric hindrance, decomposition activation energy, Gibbs reactivity, or Lewis acidity, are evaluated. The results are discussed in comparison with trimethylaluminum (TMA), a precursor used for Al2O3 deposition. We find that the “self-cleaning” by DEZ and TMA occurs through processes of different chemical nature.

Crystal quality improvement of ZnTe (1 1 0) thin film prepared on sapphire by increasing nuclei density on substrate surface

ZnTe thin films were prepared on r, S-plane nanofacet substrates to achieve a high-crystal-quality layer with a (110) orientation. Scanning electron microscopy was used to study the nucleation process. Selective formation of growth nuclei can be achieved by controlling the substrate temperature. The low-temperature grown amorphous layers changed to nuclei by annealing. The surface amorphous layer was completely removed when the annealing temperature was too high. Zn beam irradiation during the annealing removed materials selectively from the r-nanofacet. Migration-enhanced epitaxy (MEE) was introduced to enlarge the nuclei size, and a thick-layer growth by conventional molecular beam epitaxy (MBE) followed. The MEE sequence was increased up to 1200 cycles, and the substrate temperature for MEE and MBE was set to higher temperatures than the conventional MBE substrate temperature (the conventional substrate temperature was around 340 °C). The crystal quality of the thin film was analyzed using the X-ray pole figure, θ–2θ measurements and photoluminescence. (110) oriented layer formation perpendicular to the substrate surface was confirmed after introducing those techniques. The low-temperature photoluminescence measurement indicated the high quality of the thick layer.

Step-flow growth of Al droplet free AlN epilayers grown by plasma assisted molecular beam epitaxy

We have investigated an Al modulation epitaxy (AME) method to obtain step-flow growth of Al droplet free AlN layers by plasma assisted molecular beam epitaxy (MBE). At the usual growth temperature of (Al)GaN/AlN heterostructures, Al atom migration and desorption rate are very low and consequently it is very difficult to avoid the formation of Al droplets on AlN growth front by conventional MBE growth method. By adopting the AME growth method, such a difficulty has been effectively overcome and step flow growth mode of AlN has been clearly observed. By optimizing the AME growth time sequence, namely, AlN growth time and N radical beam treatment time, Al droplet free AlN layers with step flow growth characteristics have been obtained, with atomic flat surfaces and an average atomic step width of ∼118 nm at 970 °C–1000 °C, which is still suitable to grow (Al)GaN/AlN heterostructures by MBE.

Cubic InxGa1−xN/GaN quantum wells grown by Migration Enhanced Epitaxy (MEE) and conventional Molecular Beam Epitaxy (MBE)

We report the growth of InxGa1−xN/GaN quantum wells (QWs) in metastable-cubic phase through two well-controlled methods, Migration Enhanced Epitaxy (MEE) and conventional Molecular Beam Epitaxy (conventional MBE) on GaAs substrate. An increment of In mole fraction in the c-InxGa1−xN QWs was found with the decrease of growth temperature for both methods. The MEE implemented in this work, which consists of alternating the atomic flux periods at temperatures, approximately 100 ∘C lower than in conventional MBE, successfully addresses the cubic InxGa1−xN/GaN QW growth challenge. The QWs grown by this method present lower In segregation than those grown using conventional MBE. Excitonic transitions in the visible spectrum range from violet (414 nm) to green (544 nm) wavelengths were obtained in the QWs, by varying In content for both methods. These excitonic emissions are in good agreement with the theoretical calculations performed. Likewise, we identified the chemical bonds present in each c-InxGa1−xN QW, which corresponded to In–N and Ga–N, and their binding energies through In 3d, Ga 3d, and N 1s core levels by x-ray photoelectron spectroscopy (XPS). Therefore, this work provides an important understanding of In segregation, In incorporation mechanism, and radiative excitonic transitions of cubic QWs, that can be considered in complex heterostructures for novel optoelectronic applications.

Wafer-scale epitaxial modulation of quantum dot density

Precise control of the properties of semiconductor quantum dots (QDs) is vital for creating novel devices for quantum photonics and advanced opto-electronics. Suitable low QD-densities for single QD devices and experiments are challenging to control during epitaxy and are typically found only in limited regions of the wafer. Here, we demonstrate how conventional molecular beam epitaxy (MBE) can be used to modulate the density of optically active QDs in one- and two- dimensional patterns, while still retaining excellent quality. We find that material thickness gradients during layer-by-layer growth result in surface roughness modulations across the whole wafer. Growth on such templates strongly influences the QD nucleation probability. We obtain density modulations between 1 and 10 QDs/µm2 and periods ranging from several millimeters down to at least a few hundred microns. This method is universal and expected to be applicable to a wide variety of different semiconductor material systems. We apply the method to enable growth of ultra-low noise QDs across an entire 3-inch semiconductor wafer.

Light-induced circuit parameter variation in self-assembled quantum-dot photovoltaic cell

We investigate the variations of circuit parameters of photovoltaic (PV) cells containing self-assembled InSb and GaSb quantum dots. The solar cell samples are fabricated by conventional molecular beam epitaxy on (001) GaAs substrate. The current–voltage (I-V) characteristics and the frequency response (10 Hz – 100 kHz) of the fabricated cell is then extracted in dark and illuminated environment. The obtained I-V curves show typical PV properties. The static series and shunt resistances can be then extracted from this measurement result. From the Nyquist plot of the device impedance, both dynamic resistance and capacitance can be further extracted. Variations of these values are quantified. Light-induced lowering of the resistance values are explained by the presence of photogenerated carriers. This work provides a useful information for both the basic understanding of PV devices and the design of electronic circuit for energy harvesting and sensing with PV cells.

Sn doping of (010) β-Ga2O3 films grown by plasma-assisted molecular beam epitaxy

Sn doping of (010) β-Ga2O3 grown by conventional plasma-assisted molecular beam epitaxy (PAMBE) and via metal oxide catalyzed epitaxy (MOCATAXY) using a supplied indium flux during molecular beam epitaxy (MBE) growth was investigated. While high Sn concentrations were achievable over a range of growth conditions in conventional PAMBE, Sn doping less than 1019 cm−3 resulted in non-uniform doping profiles for constant Sn cell temperatures, as well as run-to-run variation in doping. Sn doping in MOCATAXY grown β-Ga2O3 allowed for sharp doping profiles and a wide range of donor concentrations from 3.9 × 1016 cm−3 to 2 × 1019 cm−3 and a maximum room temperature Hall mobility of 136 cm2/V s at 3.9 × 1016 cm−3. From temperature-dependent Hall measurements, Sn was found to have a relatively deep donor state at 77 meV below the conduction band edge. The samples showed low electron mobility at cryogenic temperatures, suggesting the existence of high background impurity levels in the MBE grown films and the need for impurity control in the oxide MBE growth environment.

Realization of Epitaxial Thin Films of the Topological Crystalline Insulator Sr3 SnO.

Topological materials are derived from the interplay between symmetry and topology. Advances in topological band theories have led to the prediction that the antiperovskite oxide Sr3 SnO is a topological crystalline insulator, a new electronic phase of matter where the conductivity in its (001) crystallographic planes is protected by crystallographic point group symmetries. Realization of this material, however, is challenging. Guided by thermodynamic calculations, a deposition approach is designed and implemented to achieve the adsorption-controlled growth of epitaxial Sr3 SnO single-crystal films by molecular-beam epitaxy (MBE). In situ transport and angle-resolved photoemission spectroscopy measurements reveal the metallic and electronic structure of the as-grown samples. Compared with conventional MBE, the used synthesis route results in superior sample quality and is readily adapted to other topological systems with antiperovskite structures. The successful realization of thin films of Sr3 SnO opens opportunities to manipulate topological states by tuning symmetries via strain engineering and heterostructuring.

Thermally annealed wafer-scale h-BN films grown on sapphire substrate by molecular beam epitaxy

The effect of high temperature annealing (HTA) on crystalline quality improvement of h-BN films grown on sapphire substrates has been investigated. It is found that BN grown using conventional molecular beam epitaxy is disordered due to the growth temperature below 1000 °C. By annealing at a temperature of 1700 °C, thermodynamically stable crystalline h-BN could be obtained at wafer scale, where diffusion of atoms in the as-grown BN film is enhanced and the structural defect density decreases. The crystalline h-BN has been confirmed by x-ray diffraction, Raman scattering, and atomic force microscopy measurements. This work demonstrates that HTA is a simple and effective way to achieve wafer-scale crystalline h-BN films, which have numerous potential applications in next-generation two-dimensional devices and flexible III-nitride optoelectronic devices.

Structure and magneto-electric properties of Co-based ferromagnetic films grown on the Pb0.71Sn0.29Te crystalline topological insulator

Co, Ni, Co55Fe45 and Co40Fe40B20 layers were grown on Pb0.71Sn0.29Te (111) crystalline topological insulator films by conventional and laser molecular beam epitaxy (LMBE) methods. It was demonstrated that the Co40Fe40B20 ferromagnetic films were grown epitaxially on the crystalline topological insulator surface, with clear epitaxial relations. Obtained Co40Fe40B20 layers have a bcc crystal structure with a crystalline (111) plane parallel to the (111) plane of Pb0.71Sn0.29Te. The use of reciprocal space three-dimensional mapping of reflection high electron diffraction (RHEED) patterns made it possible to determine the epitaxial relations of the film and substrate. Using laser based angle-resolved photoemission spectroscopy (LARPES) the clean Pb0.71Sn0.29Te surface with Dirac-like dispersion of the topological surface states was demonstrated. The stable surface topological state of Pb0.71Sn0.29Te was observed at least at the monolayer thick Co coverage. It was shown that Co (or Ni) and Co55Fe45 may be used as a pair of contacts (injector and detector) with different coercivities for spin transport measurements in the “metal – topological insulator” systems. The measurements of magnetic and transport properties show distinct hysteresis – type dependence of the magnetoresistance in the range of quadratic I(U) dependence.

Nanostructured MoOx films deposited on c-plane sapphire

Molybdenum oxide films were deposited on α-Al2O3 (0001) at 580 °C using MoO3 from a conventional molecular beam epitaxy Knudsen cell. A relatively smooth film (RMS roughness 1.1 nm) was deposited in 1 min at 580 °C using a Knudsen cell temperature of 620 °C; however, after 15 min deposition under these conditions, isolated islands (30–50 nm wide × 10–20 nm tall) develop that are stable to annealing at 600 °C for 60 min. XPS evidenced that the films are oxygen deficient with an average formula of MoO2.7. The authors infer that this oxygen deficiency is responsible for their thermal stability and may have significant effects on their catalytic and electronic properties. In contrast, stoichiometric MoO3 films deposited at 400 °C sublime completely during annealing at 600 °C.

Enhanced doping efficiency of ultrawide band gap semiconductors by metal-semiconductor junction assisted epitaxy

Ultrawide bandgap semiconductors are important for a broad range of electronic and photonic devices, but their practical application has been limited by poor current conduction. Here, we demonstrate that with controlled tuning of the Fermi level by an in situ metal-semiconductor junction during epitaxy, efficient $p$-type conduction, which was otherwise impossible, can be achieved for large bandgap aluminum gallium nitride (AlGaN). During epitaxy, the Fermi level is pinned away from the valence band edge, which fundamentally reduces the formation energy for substitutional Mg-dopant incorporation while simultaneously increasing the formation energy for compensating defects. We have demonstrated that Mg-doped ${\mathrm{Al}}_{0.9}{\mathrm{Ga}}_{0.1}\mathrm{N}$ can exhibit free hole concentration $\ensuremath{\sim}4.5\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$, with a resistivity $l5\phantom{\rule{0.16em}{0ex}}\mathrm{\ensuremath{\Omega}}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{cm}$, which is nearly three orders of magnitude lower compared to previous reports. Ultraviolet 280-nm light emitting diodes grown using this method exhibited nearly an order of magnitude improvement in external quantum efficiency compared to those grown using conventional molecular beam epitaxy. Such a unique technique can be extended for the epitaxy/synthesis of a broad range of wide bandgap semiconductors to achieve efficient current conduction that was not previously possible.

Wafer-scale all-epitaxial GeSn-on-insulator on Si(1 1 1) by molecular beam epitaxy

In this letter, fabrication of all-epitaxial GeSn-on-insulator (GeSnOI) heterostructures is investigated, wherein both the GeSn epilayer and the Gd2O3 insulator are grown on Si(1 1 1) substrates by conventional molecular beam epitaxy. Analysis of the crystal and surface quality by high-resolution x-ray diffraction, cross-sectional transmission electron microscopy, and atomic force microscopy reveals the formation of a continuous and fully-relaxed single-crystalline GeSn epilayer (with a root-mean-square surface roughness of 3.5 nm), albeit GeSn epitaxy on Gd2O3 initiates in the Volmer–Weber growth mode. The defect structure of the GeSn epilayers is dominated by stacking faults and reflection microtwins, which are formed during the coalescence of the initially-formed islands. The concentration and mobility of holes, introduced by un-intentional p-type doping of the GeSn epilayers, were estimated to cm−3 and cm−2 V−1 s−1, respectively. In metal–semiconductor–metal Schottky diodes, fabricated with these GeSnOI heterostructures, the dark current was observed to be lower by a decade, when compared to similar diodes fabricated with GeSn/Ge/Si(0 0 1) heterostructures. The results presented here are thus promising for the development of these engineered substrates for (opto-)electronic applications.

Effect of BST film thickness on the performance of tunable interdigital capacitors grown by MBE

Voltage-tunable, interdigital capacitors (IDCs) were fabricated on Ba0.29Sr0.71TiO3 grown by hybrid molecular beam epitaxy (MBE). In this growth technique, we utilize the metal-organic precursor titanium tetraisopropoxide rather than solid-source Ti as with conventional MBE. Two samples of varying BaxSr(1−x)TiO3 (BST) thicknesses were fabricated and analyzed. High-quality, epitaxial Pt electrodes were deposited by sputtering from a high-purity Pt target at 825 °C. The Pt electrodes were patterned and etched by argon ion milling, passivated with reactively sputtered SiO2, and then metallized with lift-off Ti/Au. The fabricated devices consisted of two-port IDCs embedded in ground-signal-ground, coplanar waveguide (CPW) transmission lines to enable radio-frequency (RF) probing. The sample included open and thru de-embedding structures to remove pad and CPW parasitic impedances. Two-port RF scattering (S) parameters were measured from 100 MHz to 40 GHz while DC bias was stepped from 0 V to 100 V. The IDCs exhibit a high zero-bias radio-frequency (RF) quality factor (Q) approaching 200 at 1 GHz and better than 2.3:1 capacitance tuning for the 300-nm-thick sample. Differences in the Q(V) and C(V) response with varying thicknesses indicate that unknown higher order material phenomena are contributing to the loss and tuning characteristics of the material.