High quality growths of InSb crystals are vital for advancing the fabrication of subwavelength plasmonic nanostructures for Terahertz (THz) applications. InSb is uniquely suited to applications with THz plasmonics because it is the only semiconductor that intrinsically supports the excitation of surface plasmons at THz frequencies.

GaAs (001) is chosen as the substrate because of its low cost and availability. Naturally, the high lattice mismatch between InSb and GaAs of 14.6% leads to high dislocation densities. The effects of substrate misorientation were studied by using substrates with two different offcuts. Sample A was grown with 0° misorientation from the (001) planes, while sample B has a 2° misorientation towards the [010] crystallographic direction.

A synchrotron X-ray beamline, a standard diffractometer, and a scanning electron microscope were used to characterize the two samples of InSb grown by molecular beam epitaxy on GaAs substrates. X-ray diffraction (XRD) and electron channeling contrast imaging (ECCI) were used to, independently, estimate threading dislocation density (TDD) in both samples.

TDD estimates from XRD and ECCI are nearly matched and show that there are significant differences in TDD between the two samples. The TDD of sample B was 63-74% of the TDD in sample A. This reduction in TDD is linked to the substrate misorientation.

ECCI also revealed that a high density of microtwin defects were created during the growth of sample A. From XRD, three-dimensional reciprocal space maps (3D RSMs) were created for both samples. The 3D RSMs for sample A revealed that these microtwins significantly broaden the full width at half maximum (FWHM) of the 004 InSb Bragg peak, but only if the direction of the X-ray beam is parallel to the microtwin boundary lines. Evidence of such microtwin defects was not present in the ECCI or XRD of sample B.

Additionally, a novel method is proposed to visualize the 3D RSMs, allowing for the effects of strain and tilt caused by defects to be independently studied. Measurements from the standard diffractometer revealed that the FWHM of the Bragg peak is anisotropic for both samples. This effect could not be explained by the occurrence of microtwins alone. It is proposed that the anisotropic FWHM is a result of two different types of dislocations, α and β, that form in {111} glide planes. Glide velocities and nucleation energies are not equal in perpendicular directions. This leads to higher densities of β dislocations compared to the density of α dislocations.

Following confirmation of a high carrier concentrated 2D electron gas (2DEG) at the interface of SrNbO₃/BaSnO₃ (SNO/BSO) via interfacial Nb 4d to Sn 5s injection, we investigate the transport properties of this 2DEG. As there can be transport contributions from the BSO 2DEG and the depleted SNO layer, we measure the carrier mobility via both temperature-dependent electronic transport and magnetic THz spectroscopy to decouple the contributions of the conducting pathways. In order to stabilize the material for measurement, we cap the SrNbO₃ film with a layer of SrHfO₃ (SHO), which provides an inert interface in terms of charge transfer. In establishing the transport properties of SNO/BSO, we provide a framework for future SNO interfacial studies.

Composite multiferroics contain both ferromagnetic and ferroelectric layers and are promising candidates for future magnonics applications. These materials have generated much interest recently because they present the opportunity to efficiently control magnon generation and propagation via electrical methods. The ferromagnet Fe_0.75Co_0.25 has many attractive properties, such as a low growth temperature and metallic behavior, making it easier to detect magnetic properties of the material electrically. Fe_0.75Co_0.25 has low ferromagnetic damping, allowing for more straightforward study of magnon propagation, as well as a relatively high magnetoelastic constant.^1–3 This work studies the growth and properties of epitaxial Fe_0.75Co_0.25 on ferroelectric materials by Molecular Beam Epitaxy and using Ferromagnetic Resonance (FMR), Brillouin Light Scattering (BLS), and Magneto-Optical Kerr Effect (MOKE). FMR and MOKE are utilized to determine the magnetic properties including damping parameters and coercivity while BLS illuminates the magnon dynamics and interactions. Leveraging the magnetoelastic nature of Fe_0.75Co_0.25, the multiferroic coupling is investigated by applying a voltage to the ferroelectric substrate, causing a strain unto the magnetic film, and altering the magnetic properties.

References

1. Edwards, E. R. J., Nembach, H. T. & Shaw, J. M. Co₂₅Fe₇₅ Thin Films with Ultralow Total Damping of Ferromagnetic Resonance. Phys. Rev. Appl. 11, 054036 (2019).

2. Lee, A. J. et al. Metallic ferromagnetic films with magnetic damping under 1.4 × 10⁻³. Nat Commun 8, 234 (2017).

3. Schwienbacher, D. et al. Magnetoelasticity of Co₂₅Fe₇₅ thin films. J Appl Phys 126, https://doi.org/10.1063/1.5116314 (2019).

Micro LEDs have emerged as a strong contender for next generation display devices due to their high efficiency, fast response, high brightness, and extended lifetime. For practical applications, it is highly desired that full color LEDs can be monolithically integrated on the same chip, which, however, has remained extremely challenging to achieve via the conventional quantum well based approach. In recent years, N-polar indium gallium nitride (InGaN) based light emitting diodes on the (sub)micron scale, also known as µLEDs, that are synthesized via selective area plasma assisted molecular beam epitaxy, have achieved record levels of efficiency at the (sub)micrometer device scale, with 25% external quantum efficiency (EQE) for green emission and 8% EQE for red. Such advances are enabled by selective area plasma assisted molecular beam epitaxy, in which, unlike thin film epitaxial growths, local kinetics can be controlled by substrate mask patterning. Moreover, the selective area openings on the substrate mask naturally lead to the formation of a photonic crystal. Here, we demonstrate the effect of pattern opening diameters on the InGaN photoluminescence (PL) wavelength. We show that, for a multiple-quantum-disk structure designed for green emission, given a certain photonic crystal lattice constant, the PL peak wavelength can vary over nearly 100 nm as the opening diameter varies over 60 nm, thereby enabling the achievement of multi-color emission for LED structures grown on a single chip in a single epitaxial step. More importantly, we have demonstrated strong coherent emission over a wide wavelength range for such nanowire photonic crystal LED structures. Their emission wavelengths can be precisely controlled and tuned by varying the design and processing parameters. Such nanowire photonic crystal devices not only enable a wide range of wavelength tuning but also lead to high efficiency and highly directional emission which is desired for future near-eye display applications. By further optimizing the design and epitaxial process, the realization of full-color emission for such unique N-polar III-nitride photonic nanostructures can be potentially realized. Work is currently in progress to demonstrate high efficiency micrometer scale green and red LEDs that can exhibit strong coherent emission.

We investigated the reliability of Al_0.34Ga_0.66N/GaN micro-Hall-effect sensors under industry-relevant environmental conditions. The 2DEG formation heterostructure was grown on a GaN/sapphire template by molecular beam epitaxy. The performance and stability of Hall sensor was correlated by monitoring the Hall sensitivity, sheet density of two-dimensional electron gas, and contact resistance while the device was subjected to 200 °C thermal aging for a long-time duration of 2800 hours under atmospheric conditions. The stability and performance of Al_0.34Ga_0.66N/GaN micro-Hall sensors was evaluated by correlating electrical results with the micro-structural evolution of the Al_0.34Ga_0.66N/GaN Hall sensor heterostructure. Overall, we have found that the design Al_0.34Ga_0.66N/GaN Hall-effect sensors structure has yielded a stable response for a prolonged 2800 hours of thermal aging at 200 °C. The output result of Hall device was evaluated in terms of Hall sensitivity and ohmic contacts, data shows very stable performance without any significant degradation. However, at the early stage of thermal aging we notice a small change in performance but after subsequent aging sequence the performance was further stabilized and provided stable output Hall sensitivity for 2800 hours of thermal aging at 200 °C.

Newly theorized hybrid II-IV-N₂/III-N heterostructures, based on current commercialized (In,Ga)N light-emitting diodes (LEDs), are predicted to significantly advance the design space of highly efficient optoelectronics in the visible spectrum, specifically in the green to amber regions where LED efficiencies are orders of magnitude lower than other colors. Yet, there are few epitaxial studies of II-IV-N₂ materials. ZnGeN₂, a ternary analogue of the wide bandgap material GaN, is explored as a potential green-to- amber emitter which can be integrated into existing GaN LED heterostructures due to structural similarity. Cation-ordered ZnGeN₂ has a theoretical band gap of 3.4 eV, which is expected to be reduced with cation disorder. ZnGeN₂ is wurtzite when disordered, and is structurally and electronically similar to GaN, possessing a lattice mismatch of ~0.8%. Past work by this group has demonstrated epitaxial growth of ZnGeN₂ on GaN and AlN via molecular beam epitaxy (MBE) [1,2]. Here we present the first growth of well-defined quantum wells of ZnGeN₂ within GaN by nitrogen plasma-assisted MBE, including successful five-layer multiple quantum well (MQW) structures.

Detailed structural and elemental analysis of the heterostructures was performed, including X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (STEM-EDS), and atom probe tomography (APT). These methods demonstrate high-quality and abrupt interfaces in the heterostructures, even after multiple repeating heterointerfaces. Through changes in growth methodology, we also demonstrate methods to improve unintentional incorporations, including associated improvements in structural quality. We include reports of a full LED stack growth, including n- and p-type GaN for carrier injection, an InGaN/GaN short-period superlattice, the ZnGeN₂/GaN active region, and an AlGaN electron blocking layer. Together, this data demonstrates both the promise of heteroepitaxially integrated hybrid ternary/binary nitride systems along with the challenges associated with growing such systems, including an outlook on methods to improve the materials and devices.

References

[1] M. B. Tellekamp et al. Heteroepitaxial integration of ZnGeN₂ on GaN buffers using molecular beam epitaxy. Crys. Growth Des. 2020; 20, 3, 1868–1875.

[2] M. B. Tellekamp et al. Heteroepitaxial ZnGeN₂ on AlN: Growth, Structure, and Optical Properties. Crys. Growth Des. 2022; 22, 2, 1270–1275.

There is considerable interest in applying machine learning techniques to optimize the synthesis of crystalline materials. Already, Bayesian optimization has been employed to optimize the molecular beam epitaxy (MBE) synthesis of SrRuO₃ and TiN thin films. Also, dimensionality reduction techniques and clustering algorithms have been applied to identify significant features in reflection high–energy electron diffraction (RHEED) patterns for a broad range of material systems, and convolutional neural networks have been shown to be useful in the classification of RHEED spot patterns for arsenide materials. Here, we explore how supervised machine learning techniques can be utilized to understand the relationships between the plasma–assisted molecular beam epitaxy (PAMBE) growth parameter space and the quality of GaN and InN thin film samples.

Data from over 100 PAMBE growth runs of GaN and InN (each) have been organized into material–specific data sets, including substrate temperature, metal source effusion cell temperature, initial N₂ pressure, and RF power. These variables were selected, as they are the direct system parameters a machine learning model would control. Each run took place in a Perkin–Elmer 430 MBE system equipped with an Oxford Applied Research HD–25 RF plasma source. RHEED was used as the primary quality metric, with crystallinity represented for the initial study by a binary numerical value (1 for monocrystalline and 0 for polycrystalline). The values of the growth variables were then mapped to this crystallinity label and other structural properties using supervised learning algorithms to perform both inference and prediction.

P–values corresponding to the growth parameters in each data set were measured with respect to the crystallinity; decision tree algorithms were fit to the same data for additional inference. Results from these two separate analyses were found to agree when deciding the most statistically significant synthesis variables: initial N₂ pressure and substrate temperature for GaN, and indium effusion cell temperature and initial N₂ pressure for InN. Supervised learning algorithms were subsequently trained on the synthesis data and used to predict the probability of growing monocrystalline and other metrics including the Bragg–Williams order parameter across a broad range of synthesis parameter values. The resulting machine learning–predicted growth maps agreed with conventional experimental wisdom while also providing new insight on the processing space for these materials.

This work was supported in part by the National Science Foundation (grant number DMR–2003581).

Current vertical-cavity surface-emitting lasers (VCSELs) on cell phones for facial recognition are based on 940 nm VCSELs consisting of GaAs/AlAs distributed Bragg reflectors (DBRs) grown on GaAs(001) substrates. It is desirable to have longer-wavelength VCSELs, however, because the screen of a smart phone is transparent at longer wavelength (1380 nm) and because of eye safety. The maximum permissible exposure to the retina is higher for wavelength longer than 940 nm.

Long-wavelength lasers beyond 1300 nm is commonly fabricated on InP substrates, but InP-based VCSELs present many practical challenges. Thus, there is a great deal of interest in long-wavelength VCSELs, especially at 1550 nm, on GaAs substrates. Several approaches have been developed, including dilute nitrides, quantum dots, and antimonides. Here we explore strain-compensated GaAsP/InGaAs/GaAsSb W-quantum wells (W-QWs).

In this study, we investigate the effect of the Sb composition in GaAsSb on the photoluminescence (PL) wavelength. The tensile-strained GaAsP layer is inserted to compensate the compressive strain from the InGaAs/GaAsSb/InGaAs W-QWs. The samples are grown on GaAs(001) substrates by solid-source molecular beam epitaxy (MBE) with valved cracker cells for group-V materials.

Because of technical issues, our Sb flux is limited. We, therefore, vary the Sb composition in the range of 6-20% by controlling the growth temperature of GaAsSb, while the other parameters (thickness and composition) are kept constant for the In_0.3Ga_0.7As and GaAs_0.66P_0.34 layers. All samples are grown at 520ºC, except during the growth of GaAsSb. The higher Sb composition is realized at lower growth temperature of GaAsSb.

X-ray rocking curve (XRC) measurements and simulations are performed to investigate the material composition and layer thickness. Low-temperature photoluminescence (PL) spectra are consistent with the XRC results. At the lowest Sb composition of 6%, the PL intensity is the strongest, and room-temperature PL is realized at ~1100 nm. By increasing the Sb composition in the GaAsSb layer, low-temperature (20 K) PL emits at longer wavelength up to ~1400 nm at 20% Sb while the PL intensity is the weakest. The XRC is also degraded.

Two-dimensional 1T-phase NbSe₂, a strongly correlated system, has drawn enormous attention since it was predicted to be a candidate to host quantum spin liquid.^[1] However, the insulating mechanism of 1T-NbSe₂ is still unclear, and there is ongoing debate regarding whether the gap is dominated by Mott physics or charge transfer within each charge density wave (CDW).^[2,3] More experimental studies need to be conducted to determine the potential of 1T-NbSe₂ to support a quantum spin liquid.

In this work, monolayer and bilayer 1T-NbSe₂ were grown with molecular beam epitaxy method and investigated with scanning tunneling microscopy (STM). During the growth, the film was monitored by in situ reflection high-energy electron diffraction, and a quenching treatment was applied to ensure retaining of the 1T-phase. The sample was further transferred to 4K-STM via a vacuum suitcase to avoid contamination. At low temperature, 1T-NbSe₂ experienced a CDW transition and displayed ordered triangular superlattice with start of David motifs, which were clearly shown by our 4K-STM. The density of states of monolayer and bilayer 1T-NbSe₂ was measured with scanning tunneling spectroscopy, and the gap character was discussed. Our measurements reveal that the gap feature is very sensitive to local perturbations, including CDW domains, defects, and interlayer coupling. In summary, we achieved controlled growth of monolayer and bilayer 1T-NbSe₂ and shed light on the delicate modulation of correlation-driven insulating states.

Acknowledgment

The work was supported by the National Science Foundation through grant DMR-200378, with partial support from the Gordon and Betty Moore Foundation through EPiQS grant 9465.

References

[1] G. Misguich, C. Lhuillier, B. Bernu, C. Waldtmann, Phys. Rev. B 1999, 60, 1064.

[2] M. Liu, J. Leveillee, S. Lu, J. Yu, H. Kim, C. Tian, Y. Shi, K. Lai, C. Zhang, F. Giustino, Sci. Adv. 2021, 7, eabi6339.

[3] Y. Nakata, K. Sugawara, R. Shimizu, Y. Okada, P. Han, T. Hitosugi, K. Ueno, T. Sato, T. Takahashi, NPG Asia Mater. 2016, 8, e321.

The resistivity of conventional metal interconnects increases rapidly with decreasing size, which greatly limits the performance of devices and causes high energy consumption. Electron scattering at surfaces and grain boundaries are found to be the main causes for this size effect. To solve this size effect issue, we synthesized two types of cobalt-containing materials, topological semimetal CoSi and delafossite metal PtCoO2, which could be promising candidates to replace currently used conventional metals. CoSi, a topological semimetal with multifold fermions, possesses unique topologically protected surface states that are expected to decrease resistivity at scaled dimensions where surface transport dominates. Here we demonstrate the growth of CoSi thin films and single-crystal CoSi nanowires by MBE. Multiple characterization techniques including RHEED, HRXRD, Raman microscopy, and TEM are utilized for optimizing growth conditions and realization of single-phase CoSi growth. Another candidate PtCoO2, a delafossite metal with an anisotropic 2D fermi surface and layered structure, is expected to have a very low in-plane resistivity even at ultra-downscaled dimensions. We achieved highly conductive PtCoO2 thin films by the technique combining shutter-controlled MBE growth and postgrowth annealing. Apart from the characterization techniques mentioned above, XPS and XRF are also utilized to detail chemical information and optimize the stoichiometry. We will show resistivity vs. dimension data for both CoSi and PtCoO2 and provide an outlook for using these materials as scaled interconnects.

Alloying Bismuth into GaAs has been shown to produce a marked alternation of the valence band structure. The increased spin orbit splitting energy in Ga_1-xAsBi_x has been shown to dramatically reduce the excess noise of GaAs APDs^[1]. Higher performing III-V APDs could yet be achieved by Bi alloying with Al_0.8Ga_0.2As potentially promising a new family of ultra-low-noise, photodetectors. Isolating the effect that Al may have on the incorporation of Bi will be of benefit opening up other potential material systems. This could include telecommunication APDs based on the inclusion of Bi into InAlAs, lattice matched to InP.

This study aims to investigate the synthesis and growth optimisation of Al_xGa_(1-x)As_(1-y)Bi_y with a viewto understand how adding Bi to an Al containing alloy affects its material properties. We present a series of Al_xGa_(1-x)As_(1-y)Bi_y structures, grown in an Omicron MBE STM reactor. Crystallographic and optical material quality was assessed using X-ray diffraction, photoluminescence, Rutherford backscattering and time of flightmeasurements.

Samples of Al_xGa_(1-x)As_(1-y)Bi_{y ­}with between 0-80% Al and up to 6.2% Bi were synthesised successfully. The incorporation efficiency of Bi was unaffected by the group III substitution of Ga for Al. The inclusion of 2.5% of Al into the ternary GaAs_0.975Bi_0.025 showed an acute reduction in the optical quality with thePL intensity reduced by a factor of 36, with further degradation at increased Al concentrations up to 30% with loss of optical activity. Improvements to optical quality and wafer homogeneity were observed with annealing for 30s at temperatures between 400-600°C under N₂. Beyond 600°C optical quality decreased by a factor of 0.5. The bandgap reduction caused by Bi incorporation is strikingly similar to GaAs. There is a strong relation between Bi incorporation and the key growth parameters of temperature and Bi flux that is also akin to those observed in GaAs[2]. Growth temperature variation by 60 °C alone altered Bi content in the between 0.8 – 6.2%.

The study has been successful in the synthesis of Al_xGa_(1-x)As_(1-y)Bi_y. However further work remains in the optimization of the epitaxial growth. Optical quality remains limited by the increase in non-radiative recombination centres with alloying of Al. We attribute this increase in part due to the reduced bond stability between Al and Bi. It is however promising that the incorporation of Bi into the group V lattice site showed no sensitivity to the Al content. This reveals a non-trivial relationship between the Bi incorporation into Al containing III-V alloys.

Noncollinear antiferromagnetic Weyl semimetal Mn₃Sn has become fascinating in the current times because it is one of the rare antiferromagnets that exhibits large anomalous Hall and Nernst effects¹. For future device applications, it is necessary to grow high-quality crystalline films, which has been particularly challenging to achieve. Higo et al. reported a large perpendicular switching in an Mn₃Sn (0110) film grown on a MgO substrate with a W buffer layer by MBE². Gao et al. reported the growth of Mn₃Sn (0001) on Al₂O₃(0001) with a Pt buffer layer, while Mn₃Sn (1120) was grown on R-plane Al₂O₃ and MgO (110) substrates using PLD³. In this work, we grew Mn₃Sn (1120) directly on Al₂O₃ (0001) without a buffer layer in our molecular beam epitaxy chamber. Compared to our previous single-step deposition at high temperature, which resulted in a crystalline but rough and discontiguous film, here the growth was carried with a two-step deposition method at room temperature. This method results in a smooth, epitaxial Mn₃Sn (1120) film having a thickness of ~220 nm. The growth is monitored in-situ using reflection high energy electron diffraction (RHEED) and measured ex-situ using X-ray diffraction, Rutherford backscattering, and cross-sectional STEM. We observe that the RHEED patterns are streaky, and the XRD shows a predominant single crystalline (1120)orientation. Additional results pertaining to the growth and structure, as well as empirical models, will be discussed.

Acknowledgment:

The authors acknowledge support from the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-06ER46317. The authors would like to thank Dr. Eric Stinaff and his students for back-coating the sapphire (0001) substrates.

¹ S. S. Zhang et al., "Many-body resonance in a correlated topological Kagome Antiferromagnet,” Physical Review Letters 125, 046401 (2020).

² T. Higo et al., “Perpendicular full switching of chiral antiferromagnetic
order by current”, Nature607, 474 (2022).

³ D. Gao et al., “Epitaxial growth of high quality Mn₃Sn thin films by pulsed laser deposition”, Applied Physics Letters 121, 242403 (2022).

Diluted magnetic III-N semiconductors (DMSs) have attracted significant attention due to their attractive applications for spintronic devices. The introduction of Mn atoms has been used to induce a ferromagnetic behavior in III-nitride materials [1], such as AlN. The presence of doping atoms on the surface during the MBE growth process can significantly influence the properties of the films [2]. In this study, we investigated the surfactant effects of Mn during the MBE growth of AlN.

The heterostructures were grown on Si (111) substrates employing a 200 nm-thick AlN buffer layer grown at 850 ºC. After the buffer growth, clear streak (1Χ1) reflection high-energy electron diffraction (RHEED) patterns were observed (Fig. 1). Subsequently, three layers of AlN were grown with increasing doping levels of Mn (BEP_Mn=1.9, 3.9 and 5Χ10^-9 Torr, respectively). A set of samples were prepared by varying the growth temperature from 790 to 830 °C.

During the growth of AlN:Mn layers at 790 ºC, the streak (1Χ1) RHEED patterns were conserved, and the RMS surface roughness as evaluated by AFM was in the order of nanometers (Fig. 2). Employing secondary ion mass spectrometry (SIMS), we observed that the Mn concentration (Fig. 3), for the AlN layer grown at BEP_Mn= 5Χ10^-9 Torr was in the order of 1Χ10¹⁹ atoms/cm³. On the other hand, we observed a complete distinct behavior for the growth temperature of 830 °C. No significant Mn incorporation was observed by SIMS in the films, regardless of the Mn flux used. However, for this growth temperature, the appearance of a 3Χ RHEED reconstruction was observed in the AlN:Mn growth (Fig. 1). Furthermore, the surface of the AlN:Mn film showed a very flat morphology with a RMS roughness of 0.3 nm.

The absence of Mn incorporation in AlN layers at 830 ºC, coupled with the observed 3Χ surface reconstruction and a very flat surface morphology, suggest a surfactant behavior of Mn in AlN grown at these conditions. These findings contribute to the fundamental understanding of surfactant effects in III-nitride growth on Si substrates and may have implications for the optimization of AlN-based optoelectronic devices.

[1] R. Frazier et al., “Indication of hysteresis in AlMnN,” Appl. Phys. Lett., vol. 83, no. 9, pp. 1758–1760, 2003, doi: 10.1063/1.1604465.

2] T. F. Kuech, “Surfactants in semiconductor epitaxy,” AIP Conf. Proc., vol. 916, pp. 288–306, 2007, doi: 10.1063/1.2751920.

In_xGa_1-xAs/In_xAl_1-xAs quantum wells with high In content have potential advantages over the GaAs/AlGaAs structures for spintronics and topological quantum computing applications. In comparison to GaAs/AlGaAs quantum wells, In_xGa_1-xAs/In_xAl_1-xAs quantum wells possess a lower electron effective mass, higher g-factor, and higher Rashba spin-orbit coupling. Due to a lack of latticed matched substrates, In_xGa_1-xAs/In_xAl_1-xAs quantum wells are grown on lattice mismatched substrates such as GaAs and InP with a metamorphic buffer layers. However, the growth of high mobility In_xGa_1-xAs/In_xAl_1-xAs quantum wells is hampered by enhanced interface roughness scattering from the metamorphic buffer layers and alloy scattering within the well [1].

In this work, we report the growth of modulation doped In_0.81Ga_0.19As/In_0.81Al_0.19As quantum wells grown on semi-insulating InP (001) substrates. The quantum wells are characterized utilizing low temperature magnetotransport, which is performed using gated Hall bars and the van der Pauw geometry structures. Quantum wells with electron mobilities in excess of 380,000 cm²/Vs have been grown. The electron mobility of the In_0.81Ga_0.19As quantum wells is comparable to that of the current state of the art In_0.75Ga_0.25As quantum wells. The role of growth parameters on electron mobility is discussed. The low temperature electron mobility and carrier density of the quantum wells is modeled to extract the dominant scattering mechanisms that limit the mobility. The influence of an InGaAs digital alloy on the electron mobility and alloy scattering of the quantum well is investigated.

[1] Chen, C. et al. Journal of Crystal Growth 425, 70–75 (2015).

1. F. Giorgetta et al., IEEE Journal of Quantum Electronics 45(8), 1039 (2009)
2. M. Giparakis, et al., Appl. Phys. Lett. 120, 071104 (2022)

In recent years, Mn₃Ga has garnered significant attention due to its exotic physical properties and potential applications in spintronic devices [1,2]. One of the most intriguing, yet less explored, phases is the hexagonal antiferromagnetic phase of Mn₃Ga (D0₁₉), which exhibits anomalous Hall effect and topological Hall effect in distinct temperature ranges [2]. In this presentation, we will delve into the growth and surface studies of a thin film of D0₁₉-Mn₃Ga on a Ga polar- GaN (0001) substrate.

The experiments are carried out in an ultra-high vacuum chamber equipped with a molecular beam epitaxy (MBE) system and a room-temperature scanning tunneling microscope (STM). Initially, the GaN epilayer is deposited on a GaN (0001) substrate at 700 ⁰C under gallium-rich conditions, followed by the growth of D0₁₉-Mn₃Ga at 250 ⁰C under manganese-rich conditions. Reflection high-energy electron diffraction (RHEED) is used during growth to monitor the sample, and the in-plane lattice constant is evaluated. Both RHEED and STM confirm that the grown sample exhibits epitaxial growth. Furthermore, STM measurements show atomic resolution images with multiple flat terraces and steps. The ex-situ­ X-ray diffraction (XRD) clearly shows the Mn₃Ga 0002 peak, and the calculated d-spacing matched well with the step heights measured by STM. These measurements are consistent with the theoretically reported c-value of D0₁₉-Mn₃Ga. The concentration of manganese and gallium in the sample is confirmed to be 3.2:1.0 by Rutherford backscattering (RBS). Various in-situ and ex-situ measurements confirm the D0₁₉-Mn₃Ga growth. Further work is planned to refine the sample stoichiometry and investigate the non-collinear antiferromagnetism.

This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-06ER46317.

References:

[1] L. Song, B. Ding, H. Li, S. Lv, Y. Yao, D. Zhao, and J. He, Appl. Phys. Lett. 119, 152405 (2021).

[2] Z. H. Liu, Y. J. Zhang, G. D. Liu, B. Ding, E. K. Liu, H. Mehdi Jafri, Z. P. Hou, W. H. Wang, X. Q. Ma, and G. H. Wu, Scientific Reports 7, 515 (2017).

Understanding the transport processes of ions under external stimuli is critical as they govern the operation and failure mechanisms within energy-conversion systems and microelectronic devices. The atomically precise fabrication of materials through methods such as molecular beam epitaxy or pulsed laser deposition enables the reliable study of novel functional states, which can be probed to characterize relevant fundamental processes at play. Using in situ transmission electron microscopy, we show that oxygen migration in vacancy ordered, semiconducting SrFeO_2.5 epitaxial thin films can be guided to proceed in two different types of diffusion pathways. Depending on the pathway which the oxygen ions undertake, different polymorphs of SrFeO_2.75 can be achieved, which give rise to different ground electronic properties before reaching a metallic, fully oxidized SrFeO₃ phase. Utilization of oxygen tracer exchange experiments and time-of-flight secondary ion mass spectrometry helps probe the characteristics of oxygen ion transport in the system via determination of the oxygen depth profile. Additionally, ab initio calculations are implemented to reveal the diffusion steps and reaction intermediates. Ultimately, the underlying principles of controlling oxygen diffusion pathways and reaction intermediates which we demonstrate can be beneficial to advancing the design of structurally ordered oxides and novel devices for tailored applications.

High-performance materials that cover the mid-wave (3 to 5 µm) and long-wave (8 to 14 µm) infrared atmospheric transmission windows are essential for detection applications such as thermal sensing, gas detection, and infrared spectroscopy. Strain-balanced type-II InAs/InAsSb superlattices provide a high-quality material system with design flexibility in both the mid-wave and long-wave infrared regions that offer long lifetimes, robust absorption, and the ability to grow thick pseudomorphic layers on commercially available GaSb substrates. Despite many advantages, InAs/InAsSb superlattice performance is hindered by the incorporation of unintentional Sb into the tensile InAs layer.

In this work, the impact of unintentional Sb in the tensile InAs electron well of type-II InAs/InAsSb superlattices is investigated. Several coherently strained mid and long wave superlattices are grown on (100) GaSb substrates by molecular beam epitaxy and examined using X-ray diffraction and temperature-dependent photoluminescence. The zero-order diffraction angle provides average strain and hence the average Sb mole fraction in a superlattice period. Analysis of higher order diffraction angles provides period thickness, which along with the individual layer growth times and the average strain, provides the tensile InAs and compressive InAsSb layer thicknesses. Analysis of the photoluminescence measurements provides the ground-state transition energy of the superlattice, which along with simulations of the ground state energies of the electrons and heavy-holes using a Kronig-Penney model, specify the distribution of Sb among the compressive hole well and the tensile electron well, which contains 1.8% (1.2%) unintentional Sb in the mid (long) wave superlattices.

A model of the Sb mole fraction profile in the compressive and tensile layers is developed and fit to the measured average Sb mole fractions of the compressive and tensile layers. The best-fit parameters provide the saturation and depletion rates of surface Sb and the Sb mole fraction. When the Sb shutter is opened, the compressive Sb mole fraction rapidly saturate at 41% in less than 1 s (1 monolayer); when the Sb shutter is closed, the tensile Sb mole fraction decays to a background of 0.6% in less than 3 s. Dilute amounts of Sb in the tensile electron well reduces the tensile strain, requiring a thicker well to achieve a strain balance. Analysis of the electron and heavy hole wavefunctions show that this increases the electron confinement, reducing the wavefunction overlap, and thus the optical absorption performance of the superlattice.

Semiconductor quantum dots fabricated via filling of local droplet etched nanoholes in Al_xGa_1-xAs with GaAs are excellent candidates for on-demand sources of entangled photon pairs due to their low exciton fine structure splitting. However, photon emission in the GaAs/Al_xGa_1-xAs system is limited to wavelengths below 800 nm and long-haul quantum communication via the global fiber network requires sources emitting photons in the optical C-band, i. e., ca. 1550 nm. One way to tackle this challenge, is to transfer the approach of local droplet etching and re-filling to the In_0.53Ga_0.48As/In_0.52Al_0.48As-system lattice matched to InP. In this study we report on the influence of various growth parameters, as etching temperature, metal species and residual As pressure on the shape, areal density and size of the nanoholes, as these properties play an important role for the later quantum dot’s emission characteristics. We present detailed statistical analysis of the nanohole morphology and densities that were obtained by analyzing measurements performed via atomic force microscopy and scanning electron microscopy. The areal density decreases strongly with increasing etching temperature (see Fig. 1) and the hole depth and diameter increase with increasing etching temperature. With increasing etching temperature, the nanoholes also become more and more elongated along the [011]-direction. Overgrowth of the nanoholes with In_0.52Al_0.48As under As₂-atmosphere preserves the holes (see Fig. 2) and we observed that a moderate overgrowth with 50 nm In_0.52Al_0.48As notably improved the number of symmetric nanoholes for samples etched at 410°C and 435°C. We found that filling the nanoholes with In_0.53Ga_0.48As is possible either under As₂- or under As₄-atmosphere but it works significantly better under As₄-atmosphere. We also observed that the shape of the etched holes strongly depends on the metal species used for etching. Under the same etching conditions, the holes etched with pure Al tend to be significantly more elongated than those etched with In, as can be clearly seen in Fig. 3. Photoluminescence measurements on overgrown filled holes show that the emission wavelength shifts with the filling level of the nanoholes and QD emission in the optical C-band can be achieved when filling holes generated at etching temperatures above 435°C (see Fig. 4).

Group IV alloys of Ge and Sn are extensively studied for various electronic and optoelectronic applications on a Si platform. Ge_1-xSn_x with α-Sn concentrations as low as 6% [1] allows for a transition from an indirect bandgap to a direct optical. Higher Sn content makes possible mid and even long-range infrared optical emission and detection [2]. At the same time, due to the low solid solubility of Sn in Ge (~1%), as well as the large lattice mismatch of α-Sn with Ge (~14%), the realization of high-quality Sn-rich Ge_1-xSn_x structures has proved challenging. In this study, we demonstrate enhanced Sn content using molecular beam epitaxy (MBE) growth of compositionally graded Ge_1-xSn_x on Ge (001). High-quality GeSn alloys with Sn composition reaching 6% at constant temperature. The maximal fraction of Sn was further increased to 9.0% when the growth temperature was continuously lowered while increasing the Sn flux. The analysis of surface droplets and SIMS (secondary ion mass spectrometry) profiles of elemental composition give evidence of Sn rejection during the growth, potentially associated with a critical energy of elastic strain. The intentional reduction of the coherent strain by decreasing the Sn flux near the sample surface has shown to trap a higher fraction of Sn in the Ge_1-xSn_x layer and lower surface segregation. Supporting data (Fig.2) shows an approach for XRD spectra simulation was developed for strain and composition characterization.

[1] S. Wirths, D. Buca, S. Mantl, Prog. Cryst. Growth Charact. Mater. 2016, 62 (1), 1−39.

[2] J. Bass, H. Tran, W. Du, R. Soref, S.-Q. Yu, Opt. Exp. 2021, 29 (19),30844-30856.

Aluminum nitride (AlN) is an ultra-wide direct bandgap semiconductor of interest due to its bandgap of ~6.2 eV, large critical electric field breakdown ( >15 MV/cm), high saturation velocity (~2x10⁷ cm/s) and high thermal conductivity. Compared to GaN, it provides higher Baliga’s figure-of-merit for power devices and higher Johnson’s figure-or-merit for RF devices. Realizing the full potential of this material in electronic device applications requires the ability to tailor the electrical conductivity in active AlN layers through impurity dopings.

Due to AlN’s large bandgap, impurity doping is challenging. To-date there are only a few reports on achieving impurity doping of AlN by molecular beam epitaxy (MBE) [1], ion implantation, [2] and metal organic chemical vapor deposition (MOCVD) [3]. Recently, MOCVD was used to grow metal semiconductor field effect transistor structure with n-type AlN channel [4]. Still, there is limited understanding of how to control and implement repeatable impurity doping in AlN-based devices.

In this paper, we report the plasma-assisted MBE growth of ~500 nm thick Si doped AlN films grown on AlN/sapphire templates using a metal modulated epitaxy (MME) approach Specifically, the parameters of growth temperature (760-1060°C), growth rate (3.7-11.1 nm/min), and Si flux (1E17-5E19 cm^-3) were investigated and correlated with the resulting sheet resistance. All films were nucleated using an optimum in-situ cleaning Al-absorption and desorption technique monitoring the evolution of the growth surface with reflection high-energy electron diffraction. This was followed by a ~20 nm unintentionally doped AlN layer and ~500 nm Si doped AlN layers. Hall measurements show that sheet resistance increases with increasing growth rate, while a minimum resistance is attained at a mid-range thermocouple temperature of 860 °C (~688 °C real temperature). Additional results correlating XRD, AFM, and electrical measurements for the full parameter space will be discussed and related to potential defects limiting the conductivity in these films. Si-doping in AlN/sapphire templates will be compared with that on bulk substrates to determine the impact of threading dislocations on conductivity.

References:

1. H. Ahmad et al., J. Appl. Phys. 131, 175701 (2022) and references therein.
2. P. Bagheri et al., Appl. Phys. Lett. 122, 142108 (2023) and references therein.
3. Y. Taniyasu, Appl. Phys. Lett. 85, 4672 (2004).
4. M. Hiroki et al., IEEE Electron Dev. Lett. 43, 350 (2022).

Group-IV alloys are an emerging material system for potential applications in quantum transport and infrared photodetectors while remaining CMOS compatible. By utilization of strain, magnetic fields, and light illumination, the zero-gap, diamond-cubic phase of Sn, α-Sn, is predicted to be a topological insulator, Dirac semimetal, or Weyl semimetal[1]. Focusing on the unexplored alloys with other Group-IV elements, Ge or Si, offers a novel tool to navigate the exciting boundaries of these topological phases. Additionally, SiGeSn is a model material system to demonstrate the momentum(k)-space charge separation (k-SCS) idea[2].Photodetectors with SiGeSn compositions near the indirect-to-direct bandgap transition have broad wavelength range of 2 to 22 µm covering multiple IR spectrum bands.

Both Sn-rich and Ge-rich SiGeSn samples were grown at Arizona State University by molecular beam epitaxy in a VG-V80 chamber equipped with elemental effusion cells of In, Sb, Cd, Te, Sn and Ge, and a Si sublimation source. Complete sample details investigated using quantum PPMS for quantum and magneto transport measurements and RHEED, XRD, SEM, AFM, XPS, FTIR, and TEM methods for optical and structural characterization to be presented at the conference.

For thin film α-Sn(Ge) samples, InSb substrates were chosen for lattice match conditions. The thermal oxide desorption was done under excess Sb flux at a pyrometer temperature of 480 ˚C after which temperature was lowered to 390 ˚C for Sb-rich InSb buffer growth. To separate from the conducting InSb substrate while maintaining lattice match conditions, a semi-insulating Cd-rich CdTe buffer was grown at 280 ˚C. Samples were cooled overnight via contact with LN₂ shroud and thin films of α-Sn and dilute Ge-containing SnGe alloys were grown. Due to heating by the thermal radiation from the Sn and Ge effusion cells during growth, there is a temperature creep on the sample surface. Therefore, to maintain the substrate at a temperature below the α- to β-Sn phase transition, a short-pulse modulated technique, shutter cycles open for 2 seconds and shut for 10 seconds, was employed to grow the pure α-Sn samples but not for the SnGe films.

Ge-rich SiGeSn alloys with thermalization barrier between 0.4k_BT and 3k_BT were grown on Ge and GeSn virtual substrates. Ge substrate surfaces were cleaned using HF and HCl solutions prior to UHV outgas at 550 ˚C, GeSn virtual substrates used HF and H₂O₂. A Ge buffer was grown at a substrate temperature of 500 ˚C before cooling down to 200 ˚C for SiGeSn growth. The Ge cell was held constant while Sn and Si fluxes were altered to obtain designed composition.

The lack of lattice matched substrates for AlGaN is the primary limitation to achieving high-performance power electronics, high-frequency electronics, and deep UV LEDs. This substrate limitation affects both material quality, through the formation of misfit-induced threading dislocations and strain-induced phase separation, and limitations to device geometry due to resistive or insulating electrical behavior. Dislocations and phase separation prevent AlGaN from reaching its full potential, and in the case of semiconducting substrates the primary loss mechanism in a vertically conductive device is resistive loss in the substrate itself. Thus, AlGaN alloys coulddrive disruptive technology iflong-standing substrate issues can be solved [1]. For Al_xGa_1-xN there are competing effects of increasing alloy scattering, increased bandgap with increasing Al fraction, and decreasing dopant activation such that ideal compositions for power devices fall in the range 0.3 < x < 0.85 [2]. For these compositions pseudomorphic growth on GaN and AlN is very difficult or impossible.

Recently we have reported the design of virtual substrates for Al_xGa_1-xNepitaxy consisting of (111) TaC_x grown on sapphire substrates via RF sputtering [3]. The crystallinity is subsequently improved by face-to-face annealing. These substrates offer several opportunities to improve power electronic devices through lattice and thermal conductivity matching, high electrical conductivity, high stability, and epitaxial liftoff.

In this talk we will discuss the growth of AlGaN on TaC templates by molecular beam epitaxy (MBE). Annealed TaC substrates show streaky-smooth reflection high-energy electron diffraction (RHEED) patterns and 6-fold rotational symmetry. The epilayers consist of Al_xGa_1-xN in the range 0.7 < x < 1. Using RHEED, X-ray diffraction, atomic force microscopy, and scanning transmission electron microscopy (STEM) we investigate the impact of nucleating conditions on the structure of the film and interface. During metal-rich growth we observe incommensurate RHEED features associated with laterally contracted bilayers of metal which are not observed in nitrogen-rich growth. For Al_0.7Ga_0.3N we observe relaxed growth on TaC and strained growth on co-loaded AlN templates, and corresponding to this relaxed growth only the film on TaC exhibits a step-terrace structure in AFM observed as spiral hillocks. The impact of TaC defects on the AlGaN epilayer will be discussed, informed by aberration-corrected STEM.

[1] Kaplar et al., ECS J. Solid State Sci. Technol., 6 (2), p. Q3061, 2016.

[2] Coltrin et al., J. Appl. Phys., 121, p. 055706, 2017.

[3] Roberts et al., arXiv, 2208.11769, 2022.

In recent years, there has been significant progress in the development of germanium-tin (GeSn) lasers, which are promising candidates for applications in on-chip photonics, The recent advances in the growth of GeSn alloys have enabled the realization of high-performance GeSn lasers with improved efficiency, power output, and wavelength tunability. The electrically pumped GeSn laser diode is of much interest, as it presents the capacity of heterogeneous integration with the existing Si CMOS platform. However, the electrically pumped GeSn laser diode stops lasing at 90 K[1], due to increased free carrier absorption loss and competing non-radiative recombination at higher temperature. To get a higher operating temperature, introducing carrier confinement with heterostructures is desired[2]. However, the current epitaxy lattice-matched heterostructures, such as SiGeSn/GeSn and Ge/GeSn, shows insufficient electrical confinement to electrically driven GeSn laser at room temperature due to small band offset.

Here, we introduce a semiconductor grafting technology to form an AlGaAs/GeSn heterostructure to provide a viable approach to creating a larger band offset using the AlGaAs confinement layer[3],regardless of their respective lattice constant. In this grafting strategy, an ultrathin oxide (UO) layer is first deposited on the GeSn substrate, serving as a quantum tunneling layer and a double passivation layer. The formation of heterojunction is followed by transferring a single crystalline AlGaAs layer onto the passivated GeSn and finished by a thermal process to chemically bond them together. The introduction of the UO layer exhibits significantly suppressed interfacial density of states, which rivals the one obtained from epitaxy growth.

The grafted AlGaAs/GeSn heterojunction confines the electrons in the active GeSn layer due to the 0.324eV band offset between AlGaAs and GeSn(Figure 1a). It shows the well-passivated interfaces reflected from the uniform diode ideality factor IF~1.5(Figure 3a) in all of the 341 devices, which are similar to the IF obtained from the MBE growth[4]. And I-V measurement also reveals the benefits from a larger band offset with an On/Off ratio of around 4 orders(Figure 3a). Most of the capacity-voltage sweeping measurements are consistent when the frequency changes from 10kHz to 200kHz(Figure 3b). The formation of the high-quality AlGaAs/GeSn diode indicates the feasibility of semiconductor grafting. The preliminary diode performance has also manifested a great potential for room-temperature electrically pumped GeSn laser by employing AlGaAs/GeSn heterojunction with better electrical confinement.

Iron and tin can be alloyed to form different structures of alternating stackings of Kagome Fe₃Sn and honeycomb Sn₂ (stanene) layers. This alternating sequence results in either Fe₃Sn₂ or FeSn depending on whether there are Fe₃Sn bilayers or Fe₃Sn monolayers separating the stanene layers [1,2]. Fe₃Sn₂ and FeSn provide interesting avenues for spintronics with flat bands arising from geometrical frustration leading to novel topological phases [3]. Fe₃Sn₂ and FeSn have both been grown using molecular beam epitaxy on various substrates. [2, 4, 5] In this study, we grow FeSn by MBE following the method described by Hong et al. [2] Namely, we grew our FeSn on LaAlO₃ substrates at a temperature of 500 °C. The choice of using LaAlO₃ is based on the relatively good lattice match with difference of only 1%. Four samples have been grown with Fe:Sn flux ratios of 0.8:1, 1:1, 1.2:1, and 1.5:1. We compare the results of the 4 samples by means of RHEED, XRD, RBS, and AFM. In all cases, smooth streaky RHEED patterns are observed, and from the streak spacing we calculate the in-plane lattice constants which are then complemented by the lattice constants calculated from the XRD spectra. For the case of the 1:1 flux ratio, using RHEED we find an a = 5.290 Å as compared to the expected a_for FeSn = 5.297 Å [2] and using XRD we find c = 4.56 Å as compared to the expected c for_FeSn = 4.481 Å [2]. In this presentation, we will discuss the lattice parameters as functions of the incident flux ratios as well as the phases and phase purity of the resultant samples. We will also present results for the surface smoothness as a function of flux ratios as measured by the AFM images, and we will also address the resultant film stoichiometry as a function of incident flux ratios.

This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-06ER46317.

References:

[1] L A Fenner et. al., J. Phys. Condens. Matter 21 452202 (2009).

[2] Deshun Hong et. al., AIP Advances 10, 105017 (2020).

[3] Yaofeng Xie et. al. Communications Physics 4:240 (2021).

[4] Shuyu Cheng et. al., APL Mater. 10, 061112 (2022).

[5] Igor Lyalin et. al., Nano Lett. 21, 16, 6975–6982 (2021).

The structural adaptability of transition-metal oxides allows for designing different heterostructures emerging unique physical properties at interfaces¹. High-temperature interface superconductivity takes place at the interface between overdoped (metallic) and undoped (insulating) La₂CuO₄ layers grown by oxide molecular beam epitaxy (MBE)². In addition to homo-epitaxial systems^3,4, multilayers of La₂CuO₄ combined with La_2-xSr_xNiO₄⁵, LaNiO₃^6, and LaSrMnO₃⁷ layers revealed the impact of the interface sharpness on the occurrence of superconducting⁵, thermoelectric⁶, and magnetic⁷ properties, respectively. In this work, we designed new cuprate–manganite interfaces using oxide MBE^7,8 and focused on the interface sharpness and superconducting properties compared to cuprate–cuprate interfaces.

We probed the interfaces using scanning transmission electron microscopy (STEM) techniques, including high-angle annular dark-field (HAADF), annular bright-field (ABF) imaging, and electron energy-loss spectroscopy (EELS).

Our findings demonstrate that hetero-epitaxial contacts with manganite layers can realize sharper Sr-doped La₂CuO₄ interfaces. The dopant distribution in La₂CuO₄ is affected by the elemental intermixing at the first atomic monolayer of the interfacial LaMnO₃ contact, and different superconducting behavior (e.g., interface vs filamentary) can be customized with the interfacial design⁸. With such a design, we create interface superconductivity confined down to one monolayer thickness but with a cost of filamentary behavior due to local intermixing. We also demonstrate that structurally sharp interfaces can be chemically rough, and the chemical intermixing dominates the physical properties.⁸

References:

1. Y. E. Suyolcu et al., J. Supercond. Nov. Magn.33, 107–120 (2020).
2. A. Gozar et al., Nature 455, 782–785 (2008).
3. Y. E. Suyolcu et al., Adv. Mater. Interfaces 4, 1700737 (2017).
4. F. Baiutti et al., Nanoscale 10, 8712–8720 (2018).
5. P. Kaya et al., ACS Appl. Mater. Interfaces 10, 22786–22792 (2018).
6. Y.-M. Wu, Y. E. Suyolcu et al., ACS Nano 15, 16228–16235 (2021).
7. G. Kim et al., Phys. Rev. Mater. 3, 084420 (2019).
8. Y. E. Suyolcu et al., submitted, 2023.

Semiconductor nanowires are the materials with one-dimensional structures and are expected to be applied to next-generation optical and electronic devices. Besides, III-V compound semiconductor GaAs has high electron mobility and photoelectric conversion efficiency, and has been used for lasers, solar cells, and transistors. Monolithic structures of GaAs nanowires grown heteroepitaxially on Si substrates are thus promising for future device applications. Among them, dilute nitride GaNAs or GaInNAs are materials of interest since the introduction of few % of N into host matrix Ga(In)As provides efficient tunability of band gap and lattice constant, working at the near infrared wavelengths of solar spectrum. In this study, we report the molecular beam epitaxial growth and the characteristics of GaAs-related GaNAs and GaInNAs core-multishell nanowires on 2-inch Si(111) substrates.

Hybrid interfaces of topological insulators and s-wave superconductors are great candidates for realizing Majorana bound states which have been projected to have paradigm-changing possibilities in quantum computing. The epitaxial FeTe_1-xSe_x/Bi₂Te₃ platform possess the necessary parameters for topological states, high transition temperatures, and a high level of tunability available through doping and interfacial engineering. Recently, monolayer of superconducting FeTe_1-xSe_x (x=0.25) grown on the Bi₂Te₃ was reported to exhibit emergent topological interfacial Dirac states at the Fermi energy. Pushing to lower Se levels reduces disorder which is critical for interrogating Majorana bound states, yet pure FeTe is not superconducting. Here we systematically interrogate how modifications to the molecular beam epitaxy growth of Bi₂Te₃ and the FeTe_1-xSe_x can enable tailoring both superconductivity and topological properties at low Se doping levels. Low temperature transport measurement, angle resolved photoemission spectroscopy and X-ray diffraction are combined to unravel the roles of band structure, crystallinity, and superconductivity which can be tailored as a function of growth conditions. This study will reveal the complex relation of strain and charge at FeTe_1-xSe_x/Bi₂Te₃ interface which will hopefully create a robust platform for Majorana bounds states and advancing quantum devices.

This material was based on work supported by the U.S. DOE, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division, and U.S. Department of Energy, Office of Science, National Quantum Information Sciences Research Centers, Quantum Science Center.

Van der Waals (vdW) heterostructures combining layered ferromagnets and other two-dimensional (2D) crystals such as graphene and transition metal dichalcogenides are promising building blocks for the realization of ultra-compact devices with integrated magnetic, electronic and optical functionalities. Their implementation in various technologies depends strongly on the development of a bottom-up, scalable synthesis approach allowing to realize highly uniform heterostructures with well-defined interfaces between different 2D layered materials. It also requires that each material component of the heterostructure remains functional, which ideally includes ferromagnetic order above room temperature for 2D ferromagnets. In this contribution, we will present our recent results on van der Waals (vdW) epitaxy of the 2D itinerant ferromagnetic metal Fe_5-xGeTe₂ (FGT, x ~ 0) on single crystalline epitaxial graphene using molecular beam epitaxy. For the growth of FGT films (with thickness ranging from 10 to 15 nm), elemental Fe, Ge, and Te were co-supplied from conventional effusion cells, and a growth temperature of 300 °C was employed. As a substrate, epitaxial graphene on 4H-SiC(0001), synthesized via SiC surface graphitization, was employed. Morphological and structural characterization using methods such as atomic force microscopy, synchrotron-based grazing incidence X-ray diffraction, and scanning transmission electron microscopy (STEM) revealed that epitaxial FGT films exhibiting very good surface morphology, high crystalline quality, and a sharp interface to graphene could be realized. Interestingly, stacking faults related to the presence of single FGT layers with thicknesses exceeding those expected for the Fe₅GeTe₂ phase could be identified by STEM. We expect these to be novel FGT metastable phases with Fe composition higher than 5 and potentially enhanced magnetic properties. Temperature-dependent magneto-transport measurements and superconducting quantum interference device (SQUID) magnetometry were employed to assess the magnetic properties of the samples. Ferromagnetic order with a predominant out-of-plane magnetization was shown to persist above 350 K. Furthermore, magneto-transport also revealed that the epitaxial graphene continues to exhibit a high electronic quality. These results represent an important advance beyond non-scalable flake exfoliation and stacking methods, thus marking a crucial step toward the implementation of ferromagnetic 2D materials in practical applications.

Intrinsic Magnetic Topological Insulator (MTI) has been widely recognized as an excellent platform to study topological surface state critical for understanding exotic quantum phenomena, including the Quantum Anomalous Hall effect and Axion insulator states. Using molecular beam epitaxy (MBE), we gain control of high-quality MnBi2Te4 thin films on Si (111) and epitaxial graphene substrates, and Bi2Te3/ MnBi2Te4 heterostructure. By combining several in-situ characterization techniques, we obtain critical insights toward atomical control of MBE growth of MnBi2Te4 and Bi2Te3/ MnBi2Te4 heterostructures. In specific, we extract the free energy landscape for the epitaxial relationship as a function of the in-plane angular distribution. Furthermore, with the optimized layer-by-layer growth, we map out the chemical potential and Dirac point of the thin film grown. Lastly, we observe Mn out-diffusion behavior across the interface on Bi2Te3/ MnBi2Te4 heterostructure with an abrupt Bi2Te3/ MnBi2Te4 heterostructure with an abrupt interface, we observe Mn out-diffusion behavior across the interface. These scientific insights secure the foundation for understanding growth dynamics and pave the way for the future applications of MBE for magnetic topological insulators and their heterostructure for emerging topological quantum materials.

Effect of current induced spin-orbit field (SOF) on the magnetization reversal have investigated in a crystalline (Ga,Mn)(As,P) ferromagnetic layer with perpendicular anisotropy. To study the dependence of SOFs on current direction, two types of Hall devices along the <110> and the <100> crystallographic directions, in which the Rashba-type and the Dresselhaus-type SOFs are collinear and orthogonal to each other have been fabricated. The current scan experiments clearly show magnetization switching in all devices regardless of current direction, which varies in 4 different crystal directions of the film. However, magnetization switching chirality in current scan hysteresis depends on the crystal direction of current flow. The effect of SOF was further studied external field scan experiments, in which Hall resistance hysteresis shows clear difference between current polarity (i.e., positive and negative currents) with increasing magnitude current. The observed SOT switching chirality in current scan hysteresis and the current polarity dependent shift of in field scan hysteresis are consistently explained with the Rashba-type and the Dresselhaus-type spin-orbit fields induced by tensile strain in the (Ga,Mn)(As,P) film. Furthermore, the differences of magnetization switching field between opposite current polarities show clear dependence on the direction of Hall devices (i.e., <110> and <100>). We have systematically measured crystalline dependences of magnetization switching process by varying magnitude of current and external field strength. From the magnitudes of hysteresis shifts between two opposite current polarities measured for the <110> and <100> Hall devices, we are able to quantify magnitudes of the Rashba-type and the Dresselhaus-type spin-orbit fields.

Efforts to achieve above room temperature ferromagnetism in monolayers of transition metal dichalcogenides (TMDs) through substitutional doping with magnetic impurities are actively being pursued for energy-efficient logic and memory devices. However, the current limitations stem from phase separation and multi-layered growth at heavy doping levels, restricting the doping in monolayers to levels well below the threshold established by density functional theory (DFT) for above room temperature Curie temperature. On the other hand, room temperature magnetism has been frequently observed at significantly lower doping levels (0.1-1%), but this magnetism arises from a combination of substitutional dopants, point defects, contaminants, interstitials, or edge states. As a result, the origin of purely substitutional doping-induced ferromagnetism remains a subject of debate.

Toward this end, this study employs molecular beam epitaxy (MBE) to achieve up to 30% substitutional doping of vanadium (V) and iron (Fe) in a monolayer of tungsten diselenide, surpassing the doping requirements (>15%) indicated by DFT for room temperature ferromagnetism. Magnetometry measurements, however, reveal the absence of ferromagnetism down to a temperature of 4 K in these phase-pure films, with only the phase-separated films exhibiting any room temperature ferromagnetic behavior at Fe doping levels exceeding 30%. Structural characterization utilizing plan-view transmission electron microscopy reveals significant dopant clustering, even at modest doping levels (~5%), which serves as the primary factor responsible for the absence of ferromagnetism in phase-pure films. Remarkably, these observations align with DFT calculations, which predict a low formation energy for dopant clustering, leading to a weakened exchange interaction that subsequently suppresses ferromagnetism. The insights gained from this exploratory study offer a promising pathway to attain high doping densities in monolayer TMDs while emphasizing the influence of dopant clustering on the magnetic properties of the films.

Materials with quasi-2D Kagome layers are an ideal platform for studying physics at the junction of non-trivial band topology and magnetism. In recent years, Kagome-structured ternary compounds RMn₆Sn₆ (R = rare earth) have drawn much attention due to their highly tunable physical properties. With different rare earth elements R, the magnetic anisotropy of RMn₆Sn₆ varies from within the Kagome plane (e.g. Gd) to perpendicular direction (e.g. Tb) [1, 2]. Especially for TbMn₆Sn₆, a large anomalous Hall conductance arises from gapped Dirac cones that are close to the Fermi level [1]. In this work, we synthesized (0001)-oriented thin films of ErMn₆Sn₆ and TbMn₆Sn₆ using atomic layer molecular beam epitaxy (AL-MBE). The structure of the sample was characterized by RHEED, AFM, and XRD. The magnetic properties were measured with SQUID, and the transport properties were measured with PPMS. We show that ErMn₆Sn₆ thin films exhibit easy-plane anisotropy up to room temperature, while TbMn₆Sn₆ exhibits uniaxial anisotropy at low temperatures. In general, the AL-MBE growth recipe can be applied to other materials in the RMn₆Sn₆ family. This work establishes RMn₆Sn₆ thin films as a highly tunable system for fundamental research and potential applications in the future.

References

[1]. Yin, et al. "Quantum-limit Chern topological magnetism in TbMn₆Sn₆." Nature 583.7817 (2020): 533-536.

[2]. Ma et al. "Rare Earth Engineering in RMn₆Sn₆ (R= Gd− Tm, Lu) Topological Kagome Magnets." Physical review letters 126.24 (2021): 246602.

In this research, we investigate the growth and transformation of ultrathin Pt-telluride van der Waals (vdW) compounds by vacuum annealing and Pt-vapor exposure. We find that molecular beam epitaxy readily grown PtTe2 thin films can be converted into Pt3Te4- and furthermore Pt₂Te₂-bilayers through vacuum-induced Te-loss. Using scanning tunneling microscopy, x-ray, and angle resolved photoemission spectroscopy, we find that Pt₃Te₄ remains thermally stable up to 350˚C while achieving Pt₂Te₂ requires a higher annealing temperature of 400˚C. Interestingly, bilayer Pt₂Te₂ can be re-tellurized by exposure to Te-vapor. This causes the topmost Pt₂Te₂ layer to transform into two layers of PtTe₂ and, thus synthesis of Pt₂Te₃.

Additionally, we introduce a novel method to transform monolayer PtTe₂ into Pt₂Te₂, using vapor-deposited Pt atoms. This innovative process allows for well-defined metal-semiconductor junctions by nucleating the Pt₂Te₂ phase within PtTe₂. These compositional phase transformations hold significant potential for efficient in-plane metal contacts, particularly in materials with substantial spin-orbit coupling like PtTe₂.The comprehensive understanding of these processes enables the controlled synthesis of all known Pt-telluride vdW compounds in their ultrathin form by precisely managing Te removal or Pt addition.

Furthermore, we investigate the chemical stability of these materials through exposure to oxygen and air. Remarkably, even after extended air exposure, only the surface Te layer is modified by oxygen chemical bonds, leading to a 3-eV shift to the higher binding energy of the Te-3d core levels. However, these oxygen species can be effectively removed through vacuum annealing at 280 ˚C, restoring the pristine state of Pt-telluride samples. This demonstrates the excellent air stability of these materials.

The growth of van der Waals thin films by MBE has exploded in recent years, including Bi₂Se₃,a popular prototypical 3D topological insulator. Despite the interest in these materials, the growth of high-quality Bi₂Se₃ films by MBE with low carrier density and high mobility remains challenging, in part due to a lack of understanding of the influence of the substrate. In this study, we investigate how the preparation of c-plane sapphire substrates influences film quality. Sapphire was chosen as the substrate of investigation due to its widespread use in van der Waals epitaxy. Although Bi₂Se₃ was used as the material of interest, these results are likely applicable to growth of any van der Waals material on c-plane sapphire.

The Bi₂Se₃ thin films were grown using MBE in a DCA Instruments R450 reactor. Bismuthand selenium were supplied using thermal evaporation from standard Knudsen effusion cells. All films showed streaky reflection high energy electron diffraction patterns and the expected x-ray diffraction patterns, indicative of good film growths. Further characterization was done with atomic force microscopy and room-temperature Hall effect measurements.

We explored three significant substrate preparation methods. The first was an ultra-high vacuum anneal of the substrate at 800°C for 10 minutes. This gave a 9.4% increase in mobility without noticeable change to the surface of the substrate. Second, we found that the previous use of Nano-strip®, a stabilized sulfuric acid and hydrogen peroxide mix, reduced the mobility of the film by 5-12%. It was previously thought that this reagent’s ability to eliminate positive and negative resists, remove organic materials, and create an atomically smooth surface would be beneficial to the growth of thin films. However, the use of Nano-strip® likely resulted in a sulfur-terminated surface, as characterized by XPS. This sulfur-terminated surface was detrimental to good Bi₂Se₃ film growth. Third, we found that annealing sapphire at temperatures which formed a terrace-step morphology had approximately a 40% improvement in mobility of the film. Changes in the anneal temperature showed slight changes in the sapphire step heights following previous literature. Use of UV-light to clean the substrate surface showed mixed results with improvement of mobility and carrier density on less terraced surfaces but worse carrier density and mobility of the highly terraced surfaces. AFM characterization of the films showed no considerable changes in RMS roughness values. Further studies can focus on optimizing these step heights to better match Bi₂Se₃ growth conditions.

The development of short-wave infrared Avalanche Photodiodes (APDs) operating at the 1.55 μm wavelength are critical for advancement of remote sensing and optical communication. APDs achieve internal gain through the impact ionization process, which yields a sensitive, high-speed detector that suppresses the system’s circuit noise. The 1.55 μm wavelength is notable in optical communication for its low loss in optical fiber and can also be used in eye-safe LiDAR systems which, along with high atmospheric transmission and low solar background at this wavelength, enable detection at longer distances than conventional systems [1]. Separate Absorption, Charge, and Multiplication (SACM) APDs are specifically well suited to both applications due to the highly tunable device design. By separating the absorption and multiplication regions of the device, each can be optimized individually, resulting in devices with lower dark currents, lower excess noise factors, and higher gains. The InP substrate is well situated for SACM APD applications at 1.55 μm due to the availability of lattice-matched quaternary multipliers that exhibit extremely low excess noise as well as two lattice-matched bulk absorbers for this wavelength; In_0.47Ga_0.53As and GaAs_0.50Sb_0.50 [2].

In this work, lattice-matched In_0.47Ga_0.53As and GaAs_0.50Sb_0.50 alloys are grown on InP substrates by molecular beam epitaxy to compare their optoelectronic properties as a function of doping and evaluate their performance as the absorber volume in SACM APD applications. The band gap and Urbach energy are measured as a function of temperature using steady-state photoluminescence and evaluated using an Einstein single oscillator model to extract the frozen in disorder, average phonon energy, and electron-phonon coupling parameters. The minority carrier lifetime of each material is extracted from time-resolved photoluminescence to assess how doping modifies the minority carrier lifetime, providing insight into the optimal design of an effective absorber in an SACM APD.

Three-dimensional topological insulators (3d-TIs) are a new class of materials with their non-trivial topology giving rise to exotic metallic surface states protected by time reversal symmetry and an insulating bulk. However, exploiting the surface channels is often hindered by the presence of crystal defects, such as antisites, vacancies and twin domains. In particular, twinning is shown to be highly deleterious for terahertz device applications. Twinning reduces helicity dependent topological photocurrent, thus eliminating twinning can provide a path to chip-scale polarimeters, among other devices. In the past, it has been challenging to fully suppress the twin domains. In our previous study, we have demonstrated that the growth of fully twin-free Bi₂Se₃ and other 3D TIs on smooth non-vicinal InP(111)B substrates is feasible by incorporating a newly developed selenium (Se) passivation technique during the oxide removal process of the substrate¹. This technique allows the formation of several quintuple layers of untwined In₂Se₃ on the InP surface that serve as the platform for the growth of twin-free Bi₂Se₃.

In this study, we investigate the structural details of the In₂Se₃ and Bi₂Se₃ layers formed by this novel technique using high resolution transmission electron microscopy (HR-TEM) and scanning transmission electron microscopy (STEM). The data show that well-ordered In₂Se₃ van der Walls layers form over the InP (111)B surface. The interface between the zinc blende InP lattice and the rhombohedral In₂Se₃ layers is abrupt and flat, and largely free of imperfections and defects. Similarly abrupt interfaces are evident at the Bi₂Se₃/In₂Se₃ interface. Additionally, STEM bright field (BF) and dark field (DF) images show clear evidence of significant Se diffusion into the substrate beyond the In₂Se₃/InP interface. The presence of this excess Se does not alter the crystal structure of the InP, which remains zinc blende. This observation suggests that during the In₂Se₃ formation process, the In atoms remain fixed in their lattice sites while Se diffuses into the substrate. When sufficient Se is present at the appropriate temperature, the lattice transforms into the rhombohedral In₂Se₃ lattice, maintaining its registry with the substrate and precluding the formation of twins. Once this twin-free In₂Se₃ layer is formed, it serves as a perfect template for twin-free Bi₂Se₃ layer or other 3D TI formation. This novel approach for forming a high quality twin free 2-dimensional crystal on a 3-dimensional zinc blende crystal lattice may have more general applications to other technologically important substrates.

1. Wickramasinghe et al. Crystals,13(4),677 (2023)