NAMBE 2025 Session NAMBE-MoP: NAMBE Poster Session

The growth of functional oxides on semiconductors holds significant potential for various electronic and optoelectronic devices. Gallium arsenide is one of the toughest platforms for the integration of functional oxides, due to interface stability and lattice mismatch. Current strategies require multi-step growth schemes which take time and are hard to automate.

In this work we present a simple new method for the functional oxide epitaxy on GaAs. We demonstrate it with the growth of SrTiO₃ on GaAs, and compare the structural characteristics of the resulting films to films grown using the conventional growth scheme.

X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis of SrTiO₃ films grown by conventional method and new one are comparable. However, the new method is simpler, faster and easier to implement. We further demonstrate doping of the oxide films using the new method.

The work therefore offers comparable results using a highly-simplified molecular beam epitaxy (MBE) process, opening routes for automation and scalable production of functional oxides/GaAs heterostructures.

This work was supported by the Pazy Foundation and the PMRI – Peter Munk Research Institute – Technion.

The hybrid molecular beam epitaxy (HMBE) technique for oxide growth has opened access to a self-regulated growth regime for complex oxide thin films with a wide range of chemistries, including titanates, vanadates, stannates, and ruthenates. While the favorable growth kinetics is often linked to the volatility of metalorganic (MO) precursors, their thermal decomposition and surface reaction mechanisms remain intricate and not yet fully elucidated. For instance, the widely used metalorganic precursor titanium(IV)-isopropoxide (TTIP), essential for titanate growth via HMBE, is generally believed to decompose thermally through C–O bond dissociation via the β-hydride elimination mechanism. However, alternative reaction pathways may also play a significant role. We present a hybrid computational framework of quantum mechanics (QM) calculations, ReaxFF reactive force field molecular dynamics (ReaxFF-MD) and metadynamics simulations that challenge this conventionally assumed scenario for thermal pyrolysis of TTIP. Utilizing the newly developed QM-informed ReaxFF force field, this study introduces a complete reaction scheme for TTIP decomposition, along with the statistical analysis and the reaction barriers for the various ligand liberation steps obtained from ReaxFF-MD and metadynamics simulations, respectively. Our combined approach show that the initial organic ligand separation step is spontaneous and occurs pre-dominantly via C-O bond dissociation, albeit not always via β-hydride elimination. Additionally, there is non-negligible contribution from the pathway of Ti-O bond breaking. During the thermal decomposition, the oxidation state of Ti plays a crucial role in directing the reaction pathways, along with other contributing factors; reactants with Ti in its equilibrium oxidation state are prone to undergo β-hydride elimination via H-transfer reactions to stabilize Ti’s oxidation state in a degraded molecule, which is also confirmed by the lower activation barriers extracted from Metadynamics. Thenew MOdesign strategy presented here constitutes a predictive and cost-effective framework for molecular design of MO precursors with engineered decomposition and tailored reaction pathways, thus affording rapid and cost-effective advancements in the existing and future applications for chemical vapor deposition-based thin film growth and coating processes.

Mid-wavelength infrared (MWIR) materials, such as GaInAsSbBi alloys, are gaining attention for next-generation infrared applications due to their tunable band gaps that can, in principle, reach very long wavelengths (E_G < 0.125 eV) while lattice matching to GaSb substrates. In this study, we investigated the optical properties of a GaInAsSbBi quinary grown by molecular beam epitaxy (MBE). The sample consists of a 3 µm thick Ga_0.141​In_0.859As_0.774​Sb_0.223​Bi_0.003 epilayer grown on a 20 nm InAs_0.91​Sb_0.09​ buffer layer on an n-GaSb substrate. The optical characterization was performed using two J.A. Woollam variable angle spectroscopic ellipsometers (IR-VASE and UV-VASE) covering the spectral range from 0.03 eV to 6.5 eV.

The IR-VASE data indicate strong absorption above ∼0.25 eV, while below this energy, the epilayer remains transparent, exhibiting prominent interference oscillations in the 0.1-0.25 eV range. Initial fitting using an exciton absorption model did not yield an accurate match with the experimental data. We therefore used a B-spline fit of the pseudo-dielectric function for improved modeling. Additionally, depolarization effects peaked at approximately 6% in the 0.2–0.25 eV range, with notable effects across 0.1–0.28 eV.

To eliminate backside reflections, backside roughening was performed via sandblasting, followed by remeasurement of the quinary sample. The data showed an improvement where depolarization was reduced to 4% in the 0.2–0.25 eV range, and approached zero above this energy, confirming the effectiveness of the roughening process.

For UV-VASE analysis, a second-derivative fit of the pseudo-dielectric function was performed using MATLAB, identifying critical point transitions, (E₁, E₁+∆_1, E₀^`, and E₂​) characteristic of typical zincblende materials. However, achieving an optimal fit in the UV range remained challenging due to surface-related effects. To address this, AFM measurements were conducted to quantify surface roughness (~1nm), followed by sample cleaning to reduce the oxide layer, and then UV-VASE measurements were repeated to improve the agreement of the model with data.

These findings provide valuable understanding of the optical properties of MBE-grown GaInAsSbBi alloys and highlight their potential for MWIR detector applications and modeling for quantum efficiency assessments.

Currently, HgCdTe focal plane arrays provide exceptional performance at the intersection of low dark current, radiation tolerance, and wavelength cutoff tunability. However, HgCdTe suffers in manufacturability; specifically, large format HgCdTe arrays struggle with total thickness variation leading to poor yield following hybridization to the readout integrated circuit, a problem that is not prevalent in III-V focal plane array technologies. Metamorphic InAsSb and type-II superlattices are attractive alternatives for large format focal plane arrays but ultimately do not deliver the same level of performance (in terms of dark current, radiation tolerance, and cutoff tunability) as state-of-the-art HgCdTe, due in large part to the fact that the HgCdTe solution is a random alloy with a lattice-matched (albeit bespoke) substrate. Incorporation of Bi (notably the group-V analog to Hg) in random alloys of InAsSbBi offers a means of tuning this random alloy’s cutoff across the mid and long-wave infrared with a lattice-matched GaSb substrate, however, III-V-Bi alloys have historically been plagued by degraded optoelectronic quality due to the difficulty in incorporating Bi into the alloy.

In recent work demonstrating the synthesis of mid-wave infrared GaInAsSbBi alloys with smooth surface morphologies and long minority carrier lifetimes, Bi’s tendency to incorporate in clusters was found to be suppressed with the inclusion of Ga in the quinary GaInAsSbBi. With this new-found link offering a means to suppress Bi clustering and a large set of bright photoluminescence test structures exhibiting long minority carrier lifetimes, there is now great potential to gain insight into the nature of Bi incorporation and clustering, and its effect on minority carrier lifetime. This work focuses on examination of the low temperature photoluminescence, where localization effects are dominant, to characterize Bi clustering through its impact on the density of tail states below the bandgap and minority carrier lifetime. The density of tail states are characterized by the Urbach energy slope of the low energy side of the photoluminescence, the temperature-dependence of which yields an evaluation of the frozen-in lattice disorder. The lifetime is evaluated with time-resolved photoluminescence down to 4 K where localization acts to increase the lifetime, and its effect is characterized with a 3-state recombination model. Materials with varying Ga content and with heavy, moderate, and sparse Bi droplet coverage are compared to gain insight into what growth conditions are most conducive to producing the highest optoelectronic quality GaInAsSbBi.

[1] J. Appl. Phys. 137, 065702 (2025).

The delafossite PdCoO2 consists of alternating layers of highly conductive palladium and Mott insulating octahedral cobalt oxides. While PdCoO2 is diamagnetic in bulk crystals, both Co and Pd layers exhibit magnetic metastability. We have recently demonstrated [1] that small perturbations, such as strain, can induce itinerant magnetism in helium-implanted thin films. This positions PdCoO2 as a promising material for future spintronics, serving as a platform for itinerant and emergent magnetism through helium microscope patterning [2] of the surface. A key question remains regarding the locality of the emergent magnetization relative to the helium implantation sites, in thin films of finite depth: after helium is implanted, are there other extrinsic effects that may affect the magnetism, such as vacancies or micro-strain distortions [3]? In this talk, we examine data from muon spectroscopy measurements alongside simulated depth profiles from TRIM [4] calculations, correlating the helium implantation depth profile with the muon magnetic volume fraction. Our analysis reveals extraneous magnetic sites located 10 to 30 nm from the surface that cannot be accounted for by helium implantation alone. This suggests that helium implantation does not fully explain the induced magnetization in PdCoO2, necessitating additional surface-sensitive measurements to characterize the remaining magnetization.

[1] Brahlek, M. et al. Emergent Magnetism with Continuous Control in the Ultrahigh-Conductivity Layered Oxide PdCoO2. Nano Letters 23, 7279–7287 (2023).

[2] Toulouse, C. et al. Patterning enhanced tetragonality in BiFeO3 thin films with effective negative pressure by helium implantation. Physical Review Materials 5, 024404 (2021).

[3] Das, S., Liu, W., Xu, R. & Hofmann, F. Helium-implantation-induced lattice strains and defects in tungsten probed by X-ray micro-diffraction. Materials and Design 160, 1226–1237 (2018).

[4] Ziegler, J. F., Ziegler, M. D., & Biersack, J. P. Srim – the stopping and range of ions in matter. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 268 (11–12), 1818–1823 (2010). https://doi.org/10.1016/j.nimb.2010.02.091

In this work, we investigate interface electric field of GaAs/AlGaAs quantum dot (QD). The annealing temperature effects on interface electric for low-temperature growth (LTG) GaAs/AlGaAs QD. The LTG GaAs QDs were annealed at temperature of 650, 700 and 750 ℃. From the photoluminescence (PL) results, we found that the PL intensity were enhanced as increasing the annealing temperature and the emission wavelength became blue shift with increasing the annealing temperature. We confirm that the crystal quality of LTG QD could be improved due to the thermal quarrying effect and the GaAs QD size could be smaller due to the Ga out-diffusion and Al inter-diffusions during the thermal annealing process. In photoreflectance (PR) spectra, the Franz-Keldysh oscillations (FKO) above GaAs band gap are getting stronger with increasing annealing temperature. The interface electric field strength also increases due to the decreasing of the defect density. As the results, we found that the defect density could be decreased as increasing the annealing temperature increased.

Oxide-wide bandgap semiconductors are suitable hosts for rare-earth (RE) ions owing to their availability at low cost, ease of synthesis, chemical stability, and, above all, the tunability of their optical and electrical properties. Wide bandgap semiconductors doped with RE ions combine the luminescence efficiency of RE ions with the electrical properties of the semiconductor to serve as a great inorganic luminescent material in the optoelectronic industry for light and display applications. ZnO is a wide bandgap II-VI semiconductor with an exciton binding energy of 60 meV. It is established that in Mg alloyed ZnO, the bandgap of resultant Zn_1-xMg_xO semiconductor ternary alloy increases with the Mg content, x. The alloyed concentration of Mg should be chosen accordingly to retain the ZnO's wurtzite crystal structure and to gain the desired optical and electrical properties. Doping Europium (Eu) ions can potentially generate red luminescence with high colour purity through the intra 4f ⁵D₀ → ⁷F_{2, 3 or 4} transitions. In this work, oxygen plasma-assisted molecular beam epitaxy was utilised to grow Zn_1-xMg_xO epilayers doped with Eu at the level ~ 0.6 ± to 0.2%. The as-grown materials were investigated using X-ray Diffraction analysis, Scanning Electron Microscopy, Photoluminescence (PL), PL lifetime and PL excitation analyses. The photo-excitation emissions of Eu³⁺ ions are found to be dependent on the Mg content and bandgap energy of Zn_1-xMg_xO:Eu epilayers. Eu³⁺ ions are found to occupy multiple optical sites in the ZnMgO host. Highly efficient host-to-dopant energy transfer from the ZnMgO to the Eu ions enhances the red luminescence from Eu³⁺ ions. In the Zn_1-xMg_xO host, Eu³⁺ ions exhibit a long luminescence lifetime ranging up to tens of milliseconds and a stable, endurable luminescence, favouring light-emitting device applications.

Acknowledgement: This work was supported in part by the Polish National Science Centre, Grant No. 2019/35/B/ST8/01937.

Co-introducing Mg in rare earth (RE)-doped host matrices is known to enhance the RE luminescence activity. Recently, we reported that Mg enhances Eu³⁺ red emission and activates higher energy radiative transitions of Eu³⁺ in Eu-doped ZnMgO matrices^1-3. In this work, Oxygen plasma-assisted molecular beam epitaxy was utilised to grow30 x (ZnMgO/ ZnO:Eu)/ZnMgO and also 30 × (Zn_1-xMg_xO/Zn_1-yMg_yO:Eu)/Zn_1-xMg_xO (x > y) multi-quantum-well structures (MQWs) on a-Al₂O₃ and c-ZnO substrates. Two Mg effusion cells were used to grow ZnMgO barriers with a high Mg concentration. The barrier and well thicknesses were varied in the range of 9 to 1 nm. The structural quality of crystals was investigated by high-resolution X-ray diffraction (HR-XRD) and transmission electron microscopy (TEM) techniques. Luminescence characteristics were analyzed via photoluminescence (PL), low-temperature cathodoluminescence (CL) and PL excitation (PLE) measurements. Intra 4f transitions from Eu³⁺ ions were readily visible in {Zn_1-xMg_xO/Zn_1-yMg_yO:Eu} MQWs. However, Eu³⁺ luminescence in {ZnMgO/ZnO:Eu} MQWs was weak and recovered using post-growth rapid thermal annealing at 800°C in oxygen ambient. Nevertheless, PL and PLE analysis show the excitation of Eu³⁺ centers via exciton trapping in the quantum wells and subsequent energy transfer to Eu³⁺ ions. The optical coupling of Eu³⁺ ions with both quantum barrier excitons and quantum well excitons leads to multiple energy transfers, amplifying the dopant luminescence.

References:

1. Mathew, J. A., et al. Journal of Luminescence 251 (2022): 119167.

2. Mathew, J. A., et al. ACS Applied Optical Materials 1.9 (2023): 1575-1585.

3. Mathew, J. A., et al. Materials Research Bulletin (2025): 113403.

Acknowledgement: This work was supported in part by the Polish National Science Centre, Grant No. 2019/35/B/ST8/01937.

Event-based sensors have sparked a revolution in space-surveillance, offering an innovative solution to the constraints of traditional frame-based systems where power consumption is an ever-worsening constraint as arrays get larger and temporal resolution demands increase. In addition to reduced power consumption, low latency, and wide dynamic range, the event-based sensor fundamentally only produces data when there is a change in illumination from which events are generated; no data is produced if the scene remains static. With their event-based datastream being inherently focused on the dynamic information of the scene, they are particularly well-suited to machine vision and autonomous sensing applications. These sensors have many compelling advantages, however, there are presently no commercial event-based sensors made to cover the mid-wave infrared spectral range. Since many space-based sensing applications are primarily concerned with this waveband to see through the 3 - 5 µm atmospheric transmission window, there is great motivation to investigate how mid-wave infrared III-V detectors will function with the event-based sensor unit cell to assess their performance and potential utility for space-based sensing missions.

Non-ambient stages for measuring epitaxial films with x-ray diffraction enable temperature-dependent characterization of epitaxial films not otherwise available in bulk form or as powders. To demonstrate the strain evolution and thermal properties of epitaxial films at different temperatures, we use an Anton-Paar DCS 350 stage on a PAN’alytical Empyrean diffractometer to measure antimonide-based strained-layer superlattices (SLS) and epitaxially stabilized α-Sn_1-xGe_x films.

For SLSs we discuss methods for modeling strain in multilayer films under temperature changes whose composite diffraction patterns qualitatively behave differently than those for single-layer films. The modeled diffraction pattern evolves in a predictable fashion as the substrate and layers thermally expand or contract. This modeling includes the estimation of the lattice parameters, coefficients of thermal expansion, and elastic properties of ternary compounds that may not be experimentally known and remains a limitation in creating accurate structural models.

We also investigate films that are difficult or impossible to produce in bulk or powder form. As an example, we measure some thermal properties of single-crystal, α-Sn_1-xGe_x films. In bulk form, Sn and Ge are practically insoluble (<1 at%), and Sn undergoes the α →β phase transition at 13°C. Despite these apparent limitations on film stability, we produce films with up to 6% Ge compositions and phase stability over 100°C by pseudomorphically growing these alloys on nearly lattice-matched substrates of CdTe and InSb. We exploit our ability to grow these films on two different substrates with sufficiently different coefficients of thermal expansion to determine the relaxed lattice constants, thermal expansion coefficients, and biaxial relaxation constants.

We address basic challenges in calibrating a non-ambient stage without established standards and make recommendations for standards based on our methodology.

Actinide-based compounds exhibit unique physics due to the presence of 5f electrons and serve as important technological materials. The thermophysical, magnetic, and topological properties of actinide systems have been studied in a range of chemistries, albeit far fewer than most classes of materials due to limited source material availability and associated safety constraints. Thin film synthesis of actinides has been performed by various techniques and enabled the study of the unique electron configuration, strong mass renormalization, and nuclear decay in actinide metals and compounds. However, the synthesis of monocrystalline actinide compounds with high purity, low defect and controllable dopant densities, high surface smoothness, and potential epitaxial integration with other materials could enable deeper understanding and exploitation of complex physics and strongly spin-orbit coupled (SOC) systems for novel technologies. This work presents the molecular beam epitaxy of single crystal, epitaxial uranium and uranium nitride thin films. The actinide films are not stable in ambient environments, necessitating the employment of an in-situ capping layer to protect against oxidation. Structural properties of the as-grown films are investigated using reflection high energy electron diffraction and X-ray diffraction. This work provides a platform for detailed investigation into actinide behavior and epitaxial integration of high SOC materials with other systems.

Gallium selenide (GaSe) is a layered semiconductor with potential applications for single photon emission and ultrathin field effect transistors. A fundamental challenge for layered materials is to control stacking disorder, which can limit carrier mobility and optical properties.Here we map the adsorption-controlled growth window for GaSe films grown by molecular beam epitaxy on GaAs (111)B substrates, as a function of Se/Ga flux ratio and sample temperature. We observe broad windows of adsorption-controlled growth for GaSe, and compare the regions of stability to the bulk Ellingham diagrams. Using a combination of magnetotransport, photoluminescence, Raman spectroscopy, and transmission electron microscopy, we correlate changes in the carrier mobility and optical properties with changes in layer stacking disorderOur work is an essential step towards controlling phase purity for GaSe, and for future devices based on these materials.

Josh Eickhoff^1,2, Wendy Sarney², Ibrahim Boulares², Sina Najmaei², Daniel Rhodes¹, Jason Kawasaki¹

¹University of Wisconsin Madison-Materials Science and Engineering

²DEVCOM Army Research Laboratory

This work was supported by NSF QLCI HQAN and ARL

2D semiconducting monolayer transition metal dichalcogenides (TMDs) have multiple applications in the growing field of quantum information science. In particular, monolayer TMDs are a promising platform for single-photon emitters due to their low dimensionality and high controllability. In this work, we utilize molecular beam epitaxy (MBE) to grow monolayer MoSe₂ with the goal of creating and controlling defects for single-photon applications. We aim to systematically investigate defect creation and passivation in these materials, comparing the defect properties and emission characteristics with those of exfoliated and CVD-grown monolayers. Additionally, we will grow MoSe₂ using MBE in bulk and few-layer forms and investigate the role of defects in emission properties after passivation in these materials. We will be looking at the interplay between the inherent defects in the grown monolayer and external processes.

The submonolayer technique is an alternative method for growing quantum dots (QDs) in a flexible manner, achieving high surface density without the presence of a wetting layer, which is typical of conventional quantum dots obtained in the Stranski-Krastanov (SK) growth mode. InAs/GaAs submonolayer quantum dots (SMLQDs) are formed through the cyclic deposition of a fractional monolayer of InAs (typically 30–70%), followed by the growth of a few monolayers (MLs) of GaAs. The small two-dimensional (2D) InAs islands that nucleate in successive InAs submonolayers tend to vertically stack due to the local strain field caused by lattice mismatch, eventually forming individual QDs, the height of which depends on the number of cycle repetitions. Our recent studies [1, 2] indicate that growing InAs/GaAs SMLQDs in the presence of a (2x4) surface reconstruction at a higher temperature than usually (525 °C instead of 480-510 °C) enhances the performance of infrared photodetectors, when compared to the common c(4x4) reconstruction obtained at lower temperature.

In the present work, we used these new conditions to check their influence on the performance of solar cells. Five p-i-n junctions were grown by molecular beam epitaxy (MBE), processed, and tested. Sample A was a reference device consisting of GaAs only, without any quantum dots. Samples B, C, and D contained ten layers of InAs/GaAs QDs embedded in the intrinsic layer of sample A, and differing solely in their growth conditions. Sample B had conventional InAs SKQDs obtained by depositing 2.0 MLs of InAs at 515 °C. Sample C had SMLQDs formed in the presence of a c(4x4) reconstruction of the GaAs(001) surface by repeating ten times a basic cycle consisting of 0.5 ML of InAs followed by 2.5 MLs of GaAs at 490 °C. Sample D was identical but was grown at higher temperature (525 °C) in order to keep a (2x4) reconstruction that is supposed to provide better 2D InAs islands than the c(4x4) reconstruction [3,4]. Finally, Sample E contained ten InGaAs quantum wells having the same nominal width, average In composition, and silicon doping as the SMLQDs. The experimental results of all devices will be presented and interpreted on the basis of the internal strain, amount of InAs material inside the QDs, and segregation of In atoms in the InAs/GaAs system.

[1] R. S. R, Gajjela et al., Physical Review Materials 4, 114601, (2020).

[2] A. Alzeidan et al., Sensors and Actuators: A. Physical 374, 115464, (2024).

[3] G. R. Bell et al., Physical Review B 61, R10551, (2000).

[4] J. G. Belk et al., Surface Science 387, 213, (1997).

The 3D topological insulator Bi₂Se₃ has attracted considerable interest due to the potential to exploit its topological surface states in various devices, including polarization sensitive photodetectors. However, implementation by using MBE-grown films is limited by the presence of crystalline defects. In particular, axially rotated twin domains may generate photocurrents that effectively cancel one another. The quality of epitaxial growth is often facilitated by a two-step process involving deposition at higher temperatures on a nucleation layer grown at relatively low temperature. The suppression of twinning in films grown via a two-step method should be aided by the reduction of twins in the nucleation layer.

We investigated the influence of relative deposition rate on twinning in Bi₂Se₃ films grown at a low temperature, as for a nucleation layer. The films were co-deposited from a conventional thermal effusion source for Bi and a valved cracking source for Se, on pre-annealed Al₂O₃ (0001) substrates. The fluxes for both materials were measured prior to deposition using a beam flux monitor and a series of films were grown by varying the Bi beam flux while maintaining a constant flux of Se and adjusting the deposition time accordingly to maintain consistent thickness. Films were annealed in Se flux at 270 °C to obtain a film consistent with preparation for a primary layer growth as in a two-step process. The twinning ratio of the low temperature films was characterized by x-ray diffraction phi-scans and atomic force microscopy. The results indicate that the absolute rate of Bi₂Se₃ growth plays a significant role in twin formation.

Ever since the first demonstration of ferroelectric switching in ScAlN,^[1] the III-nitride research community has devoted tremendous efforts into the development of ScAlN/GaN ferroelectric high mobility electron transistors (HEMTs). Two generally pursued research targets are: 1) reduction of subthreshold sway (SS) in transfer characteristics by taking advantage of the negative capacitance effect during ferroelectric flipping, and 2) reconfigurable Enhance-/Depletion-mode HEMTs enabled by the large residual polarization charge density. J. Casamento et al. reported SS=28.1mV/dec for I_DS changing from 0.1 to 0.001 mA/mm during backward scan of V_GS, while a very high SS was observed during forward scan.^[2] Similar phenomenon was demonstrated by P. Wang et al., with a SS<50 mV/dec for I_DS changing from 0.004 to 1×10^-5mA/mm during backward scan and a SS>100 mV during forward scan.^[3] To date, ferroelectric reduction of SS was widely observed in “OFF” state, i.e., for I_DS in the μA/mm range. Herein, we demonstrate a ScAlN/GaN ferroelectric transistor with a SS<30 mV/dec for backward scan and a SS<50 mV/dec for forward scan. The ON/OFF state I_DS is 38/0.0017 mA/mm, respectively, and the ON/OFF transition occurs within 0.1V, promising drastic reduction of power consumption during HEMT ON/OFF switching.

In Fig. 2(a), unambiguous ferroelectric switching current peaks were observed at applied voltage of ±9.7V, corresponding to a coercive field of 0.97 MV/cm. P-E curves in Figs. 2(b) and (c) demonstrate a clear wake-up process in the first 20 scans, and afterwards, the residual polarization (P_r) stabilized at 54μC/cm². Several groups have reported different values of P_r,^[1-5] which could be related to ScAlN deposition method, defects and even interaction between ferroelectric flipping with the underlying 2DEG as unintentionally doped GaN buffer was grown in this case. Further investigation on the discrepancy in P_r is ongoing. The transfer characteristics of HEMT in Fig. 2(d) demonstrates hysteresis curves, with sharp turn off gate voltage (V_GS) at -11.6 V and sharp turn on V_GS at -5.2V while V_DS is 10V. We found that the threshold voltage turning range shrinks as V_DS decreases. At V_DS=0.5V, almost no hysteresis was observed. This work paves path towards ‘digital’ GaN HEMTs without a subthreshold zone.

GaN promises significant application in high power RF devices. It is critical to reduce dislocation density in GaN epilayers for reliable HEMT device operation. This study investigates the epitaxial growth of GaN on AlN buffer layers by MBE. The growth started with a 100-nm thick AlN buffer layer on SiC substrate. For sample 240812B(S1), only an AlN buffer layer was grown. Two methods were utilized for the subsequent growth of GaN at 780℃. For Sample 240815A (S2), 640nm GaN was grown under a Ga/N flux ratio of 1.8, while for Sample 240822A(S3), 70nm GaN buffer layer was first grown at Ga/N flux ratio of 0.9 prior to the growth of 540nm GaN at Ga/N flux ratio of 1.8. For both samples, an AlGaN barrier layer and a GaN cap layer were grown on top.

XRD study reveals that FWHMs of AlN(002) in S1 and S3 are comparable, much lower than that of Sample S2 (130 vs. 530 arcsec), indicating severe nucleation of screw-type or mixed-type dislocations in AlN buffer layer upon Ga-rich growth of GaN. The FWHMs of GaN (002)/(102) in Sample S3 (228/1109 arcsec) are significantly improved vs. those of S2(860/1360 arcsec). Pits were observed in AFM of S2, potentially due to higher density of c-component dislocations. For heteroepitaxial growth of GaN on AlN, it has been widely studied that 3D nucleation of GaN together with subsequent transition to 2D growth of GaN is an effective approach for strain relaxation and dislocation annihilation in GaN.However, we identified that, for the first time, Ga-rich growth of GaN on AlN has a detrimental effect on the underlying AlN buffer layer.

TEM study reveals that excessive screw-type and mixed-type dislocations nucleate at the GaN/AlN interface, extending across the interface downwards into AlN and upwards into GaN. We postulate that the liquid metallic Ga bi-layer on AlN at the start of GaN growth could facilitate misfit dislocation nucleation and inject point defects into the underlying AlN buffer layer, leading to threading dislocation propagation both upwards and downwards. T-dependent Hall measurement reveals that the carrier mobilities of S3 at RT and T<100K are 1050 cm²/V·s and 2420 cm²/V·s, much higher than those of S2(782 and 1470 cm²/V·s). Improvement in carrier mobility is related to the reduction of dislocation density and surface pits. This work provides crucial insights into interfacial engineering for GaN RF device applications.

Years of the MBE community working to improve the optoelectronic quality of mid-wave infrared strained layer superlattice material has recently paid off with its transition into the F-35’s EODAS system [1]. However, while superlattice material is now routinely produced with relatively long minority carrier lifetimes and higher absorption, the gains in these parameters have diminished over time and the performance of mid-wave infrared nBn detectors is now largely limited by the properties of the barrier layer. Specifically, while typical nBn detectors are diffusion-limited at higher temperatures, their performance below ~130 K is often limited by depletion and tunneling dark currents. Elimination of these current mechanisms would reduce noise and improve sensitivity under low temperature operation, as is typical of space-born sensors. This motivates a more comprehensive study of the barrier properties.

In this work, 5.5 µm cutoff InGaAs/InAsSb superlattice nBn photodetectors with different barrier-absorber configurations are grown by molecular beam epitaxy, and the presence of a charged defect layer near the “barrier on active region” interface of nBn photodetectors is evidenced by electrical and optical characterizations. Three nBn structure configurations are evaluated: a conventionalnBn (barrier on active region), an invertednBn (active region on barrier), and a symmetricnBn where an active region is grown on both sides of the barrier (active region on barrier on active region). For the symmetric structure, a positive bias depletes the top layer while the negative bias depletes the bottom layer. In each structure, a sizeable depletion region is exhibited near the “active region on barrier” junctions (invertednBn and top layer of the symmetricnBn) but not for the conventional “barrier on active region” junctions (conventional nBn and bottom layer of the symmetric nBn). As a result, the inverted and symmetric nBn (under positive bias) exhibit significantly higher dark currents that are dominated by the depletion current mechanism compared to the conventional andsymmetric nBn (under negative bias), which is dominated by diffusion current. Silvaco TCAD modeling is used to demonstrate that these observed effects are consistent with the presence of a positive sheet charge density near the “barrier on active region” interface.

The origin of this charged interface is investigated via transmission electron microscopy imaging, which shows that that the “barrier on active region” interface giving rise to this effect exhibits an abrupt region of AlAs-like material, which may source this behavior.

[1] https://www.rtx.com/raytheon/what-we-do/air/eodas

We present an MBE-grown GaAs/AlAs multilayer structure optimized for optical edge detection at multiple wavelengths. This 20-layer aperiodic stack is designed to selectively manipulate spatial frequency components of incident light, enhancing high-frequency features while suppressing low-frequency intensity variations. The fabrication via molecular beam epitaxy ensures precise layer thickness, high-quality interfaces, and minimal deviations from the design thicknesses. Simulated and experimental angle-resolved reflectance measurements confirm a strong numerical aperture (NA)-dependent reflectance transition, demonstrating the feasibility of high-contrast edge enhancement in imaging applications. The multilayer interference structure provides an energy-efficient, real-time optical processing solution with multi-wavelength operation, scalable for integration into advanced imaging and computational optics. Our findings underscore the potential of MBE-grown multilayer stacks as compact, hardware-based alternatives to conventional digital and metasurface-based edge detection techniques.

Various characterizations were performed to analyze the samples, including Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), Secondary Ion Mass Spectrometry (SIMS), Reflection High-Energy Electron Diffraction (RHEED), Raman Spectroscopy, Vibrating Sample Magnetometry (VSM), and X-ray Diffraction (XRD).

Our results show that ferromagnetism in the samples is enhanced when the samples exhibit nanostructured features, with a more pronounced effect at smaller particle sizes, consistent with theoretical calculations

In conclusion, this study provides a detailed understanding of the magnetic behavior of Mn-doped AlN. Experimental characterizations and theoretical calculations showed that magnetism is more pronounced in smaller particle sizes. The analysis of the Mn adsorption process on AlN (0002) helped identify the conditions under which magnetism increases, which has implications for the design of magnetic materials at the nanoscale, especially in spintronic applications.

Acknowledgment: This work was partially supported by CONAHCYT under project CF-2023-G-426.

Topological states are predicted to exist in Ge-Sn digital alloys composed of atomically-controlled, monolayer-digitized structures of α-Sn and Ge grown on Ge (111). However, realizing these structures is nontrivial. The Ge-Sn system is metastable at Sn compositions above 1.1 % due to the difference in the two constituent elements’ lattice constants of 14.7 %. Additionally, α-Sn, the diamond crystal lattice phase of Sn, is unstable at temperatures above 13.2 °C. Here, we demonstrate the monolayer-by-monolayer growth of Ge-Sn digital alloys using low-temperature molecular beam epitaxy, which enables growth of the metastable α-Sn phase near the critical temperature for the crystal-amorphous transition and allows atomically precise control of layering within the structure. We investigate the progression of the surface morphology of these structures via scanning tunneling microscopy, from which we find that Sn forms quasi-2D island structures at thicknesses beyond a wetting layer. We quantify the extent of interdiffusion of the atomic layers through cross-sectional transmission electron microscopy and atom probe tomography.As the structure grows, we provide insight into the breakdown of epitaxial growth, driven by both the metastability of the Ge-Sn system and slow growth kinetics at the low growth temperatures. Finally, we measure the band structure of select digital alloy materials through angle-resolved photoelectron emission spectroscopy. This work exhibits the feasibility of digital alloy growth in the Ge-Sn system, emphasizing the growth kinetics and morphology of metastable Ge-Sn digital alloys.

Step meandering and step bunching are two common morphological phenomena observed in epitaxial growth of semiconductor materials. Computational studies suggest that the Ehrlich-Schwoebel (ES) potential is responsible for these formations—an energetic barrier limiting adatom diffusion between adjacent layers of the epitaxial growth surface. Depending on the location of the barrier relative to the terrace, the ES potential can limit diffusion on to or off of layers, resulting in different morphologies. Studies exploring this effect primarily utilize kinetic Monte Carlo (kMC) simulations [1, 2, 3], although variants such as a hybrid kMC and continuum model [4] and a cellular automata-based model [5] have also been proposed. However, kinetic Monte Carlo-based methods have limitations, including the difficulty of implementing the kinetics as well as computational demand that scales with adatom density.

We propose a purely continuum model based on extending the Cahn-Hilliard equations, which have been successfully applied in modeling a variety of phase separation problems, for multilayer epitaxial growth. Here, the presence and absence of adatoms are considered the two phases, replicating the effects of interatomic interactions. By introducing an interlayer stepping term into the simulation model, we are able to replicate the effects of an ES potential and produce step meandering and step bunching structures. Finally, we show that our simulation matches empirical growths where the ES potential is present, such as Bismuth surfactancy studies on GaSb grown via MBE [6].

[1] F. F. Leal, S. C. Ferreira, S. O. Ferreira, J. Phys.: Condens. Matter. 23, 292201 (2011).

[2] J. G. Amar, F. Family, MRS Online Proceedings Library. 440, 229–240 (1996).

[3] M. Rost, P. Šmilauer, J. Krug, Surface Science. 369, 393–402 (1996).

[4] J. P. DeVita, L. M. Sander, P. Smereka, in Multiscale Modeling in Epitaxial Growth, A. Voigt, Ed. (Birkhäuser, Basel, 2005), pp. 57–66.

[5] M. Załuska-Kotur, H. Popova, V. Tonchev, Crystals. 11, 1135 (2021).

[6] T. P. Menasuta, K. A. Grossklaus, J. H. McElearney, T. E. Vandervelde, Journal of Vacuum Science & Technology A. 42, 032703 (2024).

Bottom-up synthesized carbon nanostructures, particularly graphene nanoribbons (GNRs), possess atomically precise architectures and tunable electronic properties, making them promising candidates for next-generation electronic devices and advanced technologies. On-surface synthesis of atomically precise GNRs has been demonstrated using molecular beam epitaxy (MBE), which involves the deposition of monomer precursors and subsequent polymerization on the surface. However, their synthesis typically relies on metal substrates (e.g., Au, Cu, and Ag), which must be transferred to insulating substrates to enable device integration. Conventional transfer methods, such as chemical etching and hydrogen bubble delamination, face significant challenges in maintaining the structural and functional integrity of the materials. Chemical etching often introduces irreversible chemical damage, degrading material quality, while hydrogen bubble delamination is problematic for metal thin films due to weak adhesion and mechanical fragility. These limitations restrict the scalability and reliability of current transfer techniques. Here, we present a wafer-scale, etch-free transfer method using low-melting-point metals (LMPMs), Field’s metal (51% In, 32.5% Bi, 16.5% Sn), to address these challenges. This method provides robust mechanical support for metal thin films during hydrogen bubble delamination, enabling the transfer of carbon nanostructures without the use of harsh etchants while preserving their structural integrity. As a demonstration, 7-armchair graphene nanoribbons (7-AGNRs) were synthesized on Au(111)/mica substrates and successfully transferred onto SiO₂/Si substrates using this method. Raman spectroscopy confirmed the preservation of structural uniformity and minimal defect density after transfer. The scalability of this method was further validated through wafer-scale transfer of Au thin films from 100-mm sapphire substrates. Additionally, field-effect transistors (FETs) fabricated using transferred GNRs exhibited stable electronic performance. This work establishes LMPM-assisted transfer as a versatile and scalable platform for integrating carbon nanostructures into various technological applications.

Heterostructures of a 2-dimensional electron gas (2DEG) semiconductor and a superconductor are prime candidates for various applications including quantum computing and topological superconducting circuits [1,2]. It is required that the 2DEG layer will be in close proximity to the superconductor and the layers will have an Ohmic contact. A high 2DEG mobility as well as high quality interface are often needed for device applications. The system of near-surface InAs (InGaAs/InAs/InGaAs) quantum well (QW) is a prime candidate. It has a narrow bandgap with the Fermi level close to the conduction band. Typically, a thin epitaxial Al layer is grown above the QW. However, Al superconduct at 1.2K, and in order to use it in a qubit, it is necessary to cool the structure to 30mK to limit unwanted thermal population of states. Another material to take into consideration is Nb that superconducts at 9K and can possibly used in a structure at a higher temperature of 1K.

In this work, we aim to use Nb as the superconductor in the structure. The common way to grow today is by ex-situ sputtering Nb on InAs/Al structure. MBE growth of Nb is not as well studied as Al, and we will show our approach to use Nb in growth and fabrication. We will review the differences between the methods and review the effect of As capping on the InAs layer prior to Nb sputtering. The growth process and characterization of the samples will be presented.

[1] H. Kroemer, Physica E 20, 196 (2004)

[2] J. A. del Alamo, Nature 479, 317 (2011)

Experimentally, GeSn has been reported to possess a direct band gap when the Sn content exceeds 8% [1,2], creating exciting possibilities for optics on silicon platform. More recently, growth of GeSn on Ge substrates has attracted added interest due to the potential impact of short-range ordering between Ge and Sn on the GeSn band structure [3]. Scanning Tunneling Microscopy and Spectroscopy (STM/S) and MBE serves as an ideal tools for observing the quality of the Ge buffers at the atom scale and potentially for directly observing short-range ordering. In this presentation, we will discuss optimizing the growth conditions for a Ge buffer using MBE and STMS, monitored in real-time via in situ RHEED. STM/S analysis confirms the atomic scale quality of the Ge buffer, which serves as a template for the subsequent growth of high-quality GeSn layers. Observed atomic-resolution STM/S reveals well-defined surface reconstructions in both the Ge buffer and the overlying GeSn layer, confirming their quality. Additionally, XRD-RSM analysis verifies the high structural quality of the bulk material, while ARPES studies reveal sharp and well-defined electronic bands, further corroborating the excellent crystalline quality of the material. We will also address strategies to mitigate surface oxidation, a critical challenge even under UHV conditions due to residual oxygen. Understanding and controlling surface structure is discussed as a key feature for the study of ordering using STM/S.

References:

[1] S. A. Ghetmiri et al., Direct-Bandgap GeSn Grown on Silicon with 2230 nm Photoluminescence, Appl. Phys. Lett. 105, 151109 (2014).

[2] N. von den Driesch et al., Direct Bandgap Group IV Epitaxy on Si for Laser Applications, Chem. Mater. 27, 4693 (2015).

[3] B. Cao, S. Chen, X. Jin, J. Liu, and T. Li, Short-Range Order in GeSn Alloy, ACS Appl. Mater. Interfaces 12, 57245 (2020).

SiSn and SiGeSn alloys have emerged as promising candidates for next-generation electronic and optoelectronic devices due to their tunable band structures, enhanced carrier mobility, and potential compatibility with Si-based platform. In fact, SiGeSn presents the special opportunity for synthesis of strain-free SiGeSn on a Ge substrate or Ge/Si substrate creating the opportunity for strain-free Ge/SiGeSn and GeSn/SiGeSn structures. This advancement holds great potential for the development of devices such as Si-based light sources and quantum cascade lasers. While the growth of SiGeSn is still challenging due to issues such as phase separation, strain relaxation, defect formation.

In this study, we report the successful growth of SiSn with Sn content up to 5.5% on Si substrates using molecular beam epitaxy (MBE). X-ray diffraction (XRD) 004 rocking curves revealed the crystalline structure with a well-defined peak corresponding to the SiSn alloy. Atomic force microscopy (AFM) measurements indicated an atomically smooth surface with minimal roughness, further validating the high-quality epitaxial growth. We pursued the growth of SiSn as a first step followed by the growth of lattice-matched Si_0.42GeSn_0.10 bulk materials and Si_0.25 GeSn_0.09/Ge superlattice structures by MBE. XRD supports lattice-matched strain-free growth for both SiGeSn and Ge/SiGeSn on Ge. Meanwhile, SIMS demonstrates high Si and Sn composition up to 42%Si and 10%Sn and 25%Si and 10%Sn respectively. Additionally, a strong PL peak around 1850 nm is observed in the Ge/SiGeSn SLs structure, which demonstrates its high quality.

This study establishes a robust framework for high-quality SiSn and SiGeSn epitaxy, addressing critical challenges in material growth by MBE. The demonstrated material quality and optoelectronic performance provide a foundation for advancing Si-integrated photonic devices, such as lasers, detectors, and quantum systems.

Recent theoretical studies suggest that screw dislocations (SDs) could be repurposed as one-dimensional topological matter [1,2], with promising applications in quantum information science. Inspired by these prospects, our work focuses on scalable synthesis of SDs in single-crystalline semiconductors.

Here we present our recent results on the controlled synthesis of screw dislocations (SDs) in epitaxially grown (001) GaAs nanomembranes (NMs) or ultra-thin sheets where the lateral size-to-thickness ratio exceeds 10². Our approach to SDs’ synthesis involves stacking GaAs NMs at a non-zero twist angle to form twisted bicrystals (TBiCs). This configuration generates an array of twist boundaries at the interface, which act as seeds for the growth of SDs along the thickness direction in the bonded crystals. The propagation of SDs from the interfacial twist boundary is driven by thermal energy during annealing.

We used 200 nm GaAs NMs epitaxially grown onto Al_0.8Ga_0.2As-coated (001) bulk GaAs substrates. The 500 nm Al_0.8Ga_0.2As layer served as a sacrificial layer during the release process of the NMs. GaAs NMs were patterned into 2D arrays of 250 × 250 μm² square pixels and released in place by selective etching of the Al_0.8Ga_0.2As layer in diluted HCl. Pixelated NMs were then dr-transferred at a non-zero twist angle onto unpatterned NMs. The resulting TBiCs were annealed at 500°C for 40 hours in Ar atmosphere. We counteracted degassing during annealing by capping the TBiCs with bulk GaAs substrates.

High-resolution cross-sectional transmission microscopy (TEM) showed gap- and oxide-free interfaces within the annealed TBiCs, which are vital to the templated growth of SDs. Plan-view TEM detected arrays of SDs in weak-beam conditions. The measured spacing between the line defects was 13.23 nm, corresponding to a twist angle of 1.6° between the two bonded GaAs crystals, as measured by selected area electron diffraction (SAED).

[1] L. Hu, H. Huang, Z. Wang, W. Jiang, X. Ni, Y. Zhou, V. Zielasek, M. G. Lagally, B. Huang and F. Liu, “Ubiquitous Spin-Orbit Coupling in a Screw Dislocation with High Spin Coherency”, Phys. Rev. Lett, 121, 066401 (2018).

[2] Y. Ran, Y. Zhang, and A. Vishwanath, “One-Dimensional Topologically Protected Modes in Topological Insulators with Lattice Dislocations”, Nat. Phys. 5, 298 (2009). + Author for correspondence: ruhin@unm.edu

ACKNOWLEDGEMENT. Work at the University of New Mexico was supported by NSF CAREER award No. 2144944. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government.

MnSb exhibits ferromagnetism with a high Curie temperature (Tc = 587 K) and a notably large magneto-optical Kerr rotation, making it a promising candidate for spintronic applications. Here, we demonstrate the synthesis of MnSb on the GaAs (111) surface using molecular beam epitaxy (MBE). We then characterize the nanoscale magnetic ordering and spin fluctuations in the films using a scanning nitrogen-vacancy magnetometer. Additionally, detailed ex-situ crystallographic investigations, including transmission electron microscopy (TEM) and X-ray diffraction (XRD), are performed to assess the structural integrity of the grown crystals. The results of detailed experimental characterizations will be presented, along with analysis of the new observations within advanced theory and modeling. Overall, this study highlights the potential of MnSb for future quantum technology applications. Authors acknowledge support from the Massachusetts Technology Collaborative (award number 22032), and the National Science Foundation (award number OSI 2329067).

Keywords: Ferromagnetic MnSb, Molecular Beam Epitaxy, Momentum Microscopy, NV Magnetometry, Quantum Materials.

The development of quantum computing stillrelies on the development of advanced material platforms. Among the most promising candidates are semiconductor-superconductor hybrid systems, such as Andreev quantum bits and Kitaev transmons. These systems are based on high-quality superconducting thin films with transparent interfaces to low-dimensional semiconductors, offering the potential for extended coherence times and robust qubit-qubit coupling [1].

To this end, a metamorphic growth protocol has been employed,and low-temperature electron mobilities up to 8×10⁵cm²/Vs have been achieved in undoped deep InAs/In_0.81Ga_0.19Astwo-dimensional electron gases (2DEGs) grown on GaAs (001) [2]. Additionally, superconducting proximity effects have been observed in Josephson junctions between shallow InAs 2DEGs and epitaxial Al layers [3]. Optimal mobilities were achieved by tuning the thickness t of a strain-relieving In_0.84Al_0.16As layer beneath the quantum well (QW) region [2].

Here, discuss the strain relaxation dynamics for varying t and their impact on electron scattering mechanisms in the InAs 2DEGs. Two-dimensional XRD reciprocal space maps of the (004) and (224) reflections (Fig. 1) reveal how increasing t from 50 nm to 300 nm leads to near-complete strain relaxation and a reduction in mosaicity in both the InAs QW and the surrounding barriers. Mobility measurements from gated Hall bars (Fig. 2) reveal striking gains in electron mobility and a reduced anisotropy between the [110] and[-110] orientations as t grows. This improvement stems from diminished anisotropic scattering mechanisms, linked to the cross-hatch roughness pattern—a memory of the buried dislocation network in the buffer layer (see insets of Fig. 2). These features, shaped by strain and composition fluctuations, highlight the interplay between structural engineering and electronic properties.

[1] J. S. Lee et al. Nano Lett. 19, 3083 (2019)

[2] A. Benali et al., J. Cryst. Growth 593, 1267681 (2022)