United States

We report our recent results on scalable fabrication of helical slow wave structures (SWSs) applicable in millimeter-through-THz traveling wave tube amplifiers (TWTAs) [1]. The structure is unique in that it has microscale diameter dimensions, a helical geometry, and potentially higher electrical/thermal conductivity than other previously reported self-assembled helices. We fabricate the SWSs by self-assembly of a metal nanomembrane (NM) and electroplating of the helical film, thereby overcoming the inherent trade-off between NM thickness and helix diameter/pitch. Here we focus on our recent efforts on understanding and controlling electrodeposition on freestanding and curved NMs. In particular, we investigate the effect of varying current and temperature on the thickness of the deposited film and its uniformity. In addition, we characterize and justify any changes occurring in the geometry of the helix due to electrodeposition. Fabrication of microscale SWSs started with defining NMs strips by optical lithography, metal evaporation, and lift-off. In our work we used Cr/Au/Cr NMs that have an inherent stress gradient across their thickness. We defined the metal strips onto a bulk Si substrate coated with a sacrificial or release layer such as Ge or GeO, . The released NMs selfassembled into helices with a diameter between 100 and 200 mm upon selective etching of the sacrificial layer by a XeF,, plasma. Helix diameter and pitch are controlled by varying the thickness, the elastic modulus, the residual stress, and the in plane geometry of the NMs. The electroplating procedure employs a two-electrode electrochemical setup with the self-assembled helix and a platinized mesh serving as cathode and anode, respectively. A sulphatebased gold solution is used for the process. We use pulsed current source with variable frequencies to obtain uniform electrodeposition of an Au film with a thickness of 1-2 mm depending on the electrodeposition parameters. In all investigated electroplating conditions, we observed uniformly deposited gold with lower roughness than the skin depth of the metal at THz frequencies, which are crucial attributes to minimize losses and heating in a TWTA based on the helical SWS. Reference [1] Prakash, Divya J., Matthew M. Dwyer, Marcos Martinez Argudo, Mengistie L. Debasu, Hassan Dibaji, Max G. Lagally, Daniel W. van der Weide, and Francesca Cavallo. 2020. “Self-Winding Helices as Slow-Wave Structures for Sub-Millimeter Traveling-Wave Tubes.” ACS Nano, 2021, 15, 1229-1239. doi:10.1021/acsnano.0c08296. Work supported by U.S. AFOSR-Award No. FA9550-19-1-0086

EMC 2022

Electroplated Helical Slow-Wave Structures for Millimeter-Through-THz Vacuum Electronic Devices

vacuum electronic device

helix

electroplating

After two years of virtual conferences, we're excited to host EMC 2022 in person at The Ohio State University in Columbus, Ohio. In addition to cutting-edge technical presentations given in multiple parallel sessions, opportunities to network with colleagues will be present throughout the conference. The size and format of the meeting makes networking easy, whether you have been coming to the conference for many years or are a first-time attendee.

EMC is coordinated with the Device Research Conference (DRC), which will be held the same week, from June 26-29. This recognizes the strong interaction between device and electronic materials research and provides fruitful exchange of information between attendees of both Conferences.

For information on the 80th Device Research Conference, please visit the DRC 2022 website.



The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. This year’s event will feature a plenary session, topical sessions, a poster session and more.

technical paper

B-phase MgGaO ternary alloy thin films of different Mg compositions were grown by using oxygen plasma assisted molecule beam epitaxy (MBE). The atomic ratios of Mg and Ga in sample A, B, and C were recorded as 11.15 at.%/88.85 at.%, 23.29 at.%/76.71 at.%, and 36.35 at.%/63.65 at.%, respectively, which were measured by energy-dispersive X-ray (EDX) spectroscopy. The samples were characterized by a scanning electron microscope (SEM), showing that the surface becomes rougher in samples with higher Mg contents. Three typical monoclinic structure diffraction peaks located at 19.12°, 38.50°, and 59.14°, which correspond to the B phases of (-201),(-402), and (-603) were investigated by X-ray diffraction (XRD), and all these diffraction peaks exhibit an obvious blue shift compared with that of reference B-Ga,0.. Raman spectra were investigated by a confocal Raman microscope equipped with a high-resolution laser centered at 532 nm with a power of 60 mW. Four out of ten A_ and four out of five B_ Raman-active optical phonon modes were observed. The reduced peak intensity of A (2)(9) A (10) B (2)- (3), and B 8 indicates that Mg * atoms have replaced the position of Ga * ions in both octahedral and tetrahedral chains. The ultra-wide bandgap absorption and transmittance spectra were measured using a UV-Vis-NIR spectrophotometer. Ultra-wide bandgaps of 4.98, 5.05, 5.20, and 5.30 eV were measured for pure Ga,0x, sample A, B, and C, respectively with all transmittances of light over 80% in the visible spectrum region. Temperature-dependent photoluminescence (PL) spectra were investigated by a 325-nm laser with temperatures ranging from 13 to 300 K, revealing only intermediate level transitions, which decrease with the increase of temperature. A MSM solar-blind ultraviolet photodetector (PD) was fabricated based on MgGaO films with Pt/Au electrodes. Photoresponse has been studied by typical dark and irradiated wavelength dependent I-V curves.

Characterization of β-Phase MgGaO Thin Films Grown by Molecular Beam Epitaxy

Gallium oxide (Ga2o3.) is an ultra-wide bandgap semiconductor (4.85-5.3 eV) with emerging applications in power electronics because of its high thermal stability, large breakdown voltage and high dielectric constant. Ga, 03 occurs in the a, B, y and k(€) polymorphs having trigonal, monoclinic, cubic normal spinel and orthorhombic structures, respectively. B- “Ga, 03 is thermodynamically stable and produced in melt-grown substrates. Recently, k- -Ga,0. has accrued increased interest because it has demonstrated stability at temperatures 2 700 °c! and its large spontaneous polarization could be exploited in heterostructures” for high electron mobility transistors. In this study, metal organic chemical vapor deposition (MOCVD) was used to grow epitaxial Ga,0. films under different growth conditions including temperature, growth rate, and diluent gas flow rate. The growth experiments were conducted in a vertical, low-pressure, cold-wall MOCVD reactor using triethylgallium (TEGa) and 0, precursors and commercially available 0.15 off-axis (toward m-plane) c-plane sapphire. No was used as both the carrier gas for TEGa and the diluent gas. The chamber pressure was held at 20 Torr. The growth rate was controlled by the TEGa flow rate which ranged from 0.29 sccm to 2.1 sccm. The diluent gas flow rate was varied from 4 slm to 8 sim. The O flow rate was maintained at 500 sccm. The temperature was constant for each growth but varied from 470°C to 570 C. Phase composition and microstructural evolution of the films were investigated using different analytical tools including the X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM). XRD and SEM revealed that the top layer varied in phase composition between ~100% k-Ga,0, and ~100% B-Ga,0.; the surface microstructure ranged from poorly coalesced to completely coalesced grains as a function of growth temperature, growth rate, or diluent gas flow rate. In general, it was found that the k-phase tends to grow at lower temperatures and higher growth rates (higher TEGa flow rates). The growth of nominally single- phase k-Ga 90 within the top layer was observed in a temperature range between 500° C and 530° C. Below 470° C, only amorphous Ga 90,was obtained; above 570° Cc only the B-phase was deposited. HRTEM and STEM of a film grown at 530° C revealed the initial pseudomorphic growth of 3-4 monolayers of a-Ga,0., a 20-60 nm transition layer that contained both B- and y-Ga,0., and a top ~700 nm thick layer of phase-pure k-Ga,0,. H.Y. Playford, A.C. Hannon, E.R. Barney, and R.|. Walton, Chem. Eur. J. 19, 2803 (2013). R. Roy, V.G. Hill, and E.F. Osborn, J. Am. Chem. Soc. 74, 719 (1952). Y. Oshima, E.G. Villora, Y. Matsushita, S. Yamamoto, and K. Shimamura, J. Appl. Phys. 118, 085301 (2015). J. Lee, H. Kim, L. Gautam, K. He, X. Hu, V. Dravid, and M. Razeghi, Photonics 8, 17 (2021). P. Ranga, S.B. Cho, R. Mishra, and S. Krishnamoorthy, Appl. Phys. Express 13, 061009 (2020). On K. Jiang, J. Tang, M. J. Cabral, A. Park, L. Gu, R. F. Davis and L. M. Porter, J. Appl. Phys. (2022)

Phase Composition and Microstructure of κ-Ga2O3 Heteroepitaxial Films Grown by MOCVD

Parametric expressions that describe the complex dielectric function of polycrystalline Mg 2n 1 films as a function of the bandgap E_ have been applied in the metrology of cadmium telluride-based solar cell structures. Valid over the range 3.264 < 3.826 eV, these parametric expressions are applied to facilitate mapping spectroscopic ellipsometry of device structures consisting of Mg Zn, _O deposited on TEC™-15 glass. Mapping spectroscopic ellipsometry is shown to provide maps for the Mg 2n,_,0 effective thickness, or volume per planar substrate area, and Mg 2n,_,0 bandgap within confidence limits of + 1 nm and + 0.003 eV, respectively. As a second application of the parametric expressions, an as-deposited cadmium telluride-based device structure has been analyzed by through-the-glass spectroscopic ellipsometry. Such an analysis is also shown to provide the Mg,2n, 0 effective thickness and bandgap. The outcome of the through-the-glass spectroscopic ellipsometry analysis for this device structure enables simulations of the external quantum efficiency spectra of the resulting solar cell assuming different recombination losses within the individual layers of the structure. A comparison of these simulations with corresponding experimental spectra reveals high current collection from the front of the device with the almost complete collection from the cadmium sulfide components of the Mg,2n _O/cadmium sulfide and cadmium sulfide/cadmium telluride interface layers of the structure.

An Optical Response Analysis of Cadmium Telluride-Based Solar Cell Structures Incorporating High Resistivity Transparent Layers of Magnesium-Zinc Oxide

Transition metal multinary chalcogenides are a unique class of material for photovoltaic (PV) and quantum dot light emitting (QLED) applications. They offer a higher level of flexibility from a materials perspective to achieve tunable band gaps through structural and compositional control. We aim to realize new and more sustainable direct band-gap semiconductor materials from multinary compositions of earth-abundant elements. To this end, detailed ab initio calculations are first performed for the parent binary Cu-chalcogenide composition. A series of new p-doped, n-doped, and degenerate semiconductors based on multinary Cu-chalcogenide nanocrystals are predicted using the theoretical investigations of the band gap and density of states. We experimentally realized the new multinary Cu-chalcogenide compositions via novel solution chemistry approaches. A detailed material characterization based on x-ray diffraction, electron microscopy, and ultraviolet-visible spectroscopy is reported for the new Cuchalcogenide nanocrystals and thin films. These new results will provide key theoretical and experimental insights for achieving more sustainable absorber layer materials for inorganic solar cells.

Designing New Sustainable Semiconductor Materials with Cu-Chalcogenide Nanocrystals

A new class of magnetic metamaterials can be produced by placing magnetic molecules between two ferromagnetic electrodes. Magnetic molecules can be designed to create strong exchange coupling between two ferromagnetic electrodes. Magnetic molecule-induced strong exchange coupling that can be harnessed as the means to initiate an automatic spin filtering process to shuffle the spin density of states in the two ferromagnetic electrodes. Our recent preliminary experimental studies showed that iron and nickel atom-based organometallic molecules (FeNiOMC) produced a strong spin filtering effect between NiFe and Co/NiFe ferromagnetic electrodes (Tyagi, P.; Riso, C., Molecular spintronics devices exhibiting properties of a solar cell. Nanotechnology 2019, 30 (49), 49540). Fe-Ni-OMC dramatically modified the magnetic properties of ferromagnetic electrodes present in the magnetic tunnel junction testbed at room temperature. This paper talks about the resulting Fe-NiOMC and magnetic tunnel junctionbased magnetic metamaterial. This metamaterial showed an intriguing solar cell effect. Further experimental studies are conducted to design several other forms of novel magnetic metamaterials by combining a wide range of Singlemolecule magnetic (SMM) and porphyrin-like magnetic molecules with various permutations of magnetic electrodes. To produce a magnetic tunnel junction testbed that is to be used to test various molecules, we made extensive experimental optimization efforts at each stage of device fabrication to address device fabrication challenges associated with molecule-ferromagnet interactions. In this study, we have systematically optimized several factors using multiple Taguchi Design of Experiment approaches to improve the fabrication of the bottom electrode of the magnetic metamaterial. We systematically varied photolithography parameters and plasma treatment process to yield a bottom ferromagnetic electrode with

Solar Cell Effect with Magnetic Metamaterials Produced by Combining Magnetic Tunnel Junction and Magnetic Molecules

Reliable and controllable growth of two dimensional hexagonal boron nitride (h-BN) is essential for its wide range of applications. Here, we carried out molecular beam epitaxy (MBE) growth of monolayer h-BN on nickel (Ni) substrates by using ammonia (NH) and boron oxide (B04) as the sources. It was found that compared with normal growth of h-BN by introducing the NH, and B,0., source at the same time, the pretreatment with B,0, greatly inhibited the following epitaxial growth of h-BN. With the increase of B,0, pretreatment time and then followed by 15 min h-BN growth, the coverage of h-BN exhibited an exponential decay relationship with the treatment time. The samples were analyzed using depth-profiling X-ray photoelectron spectroscopy (XPS) and reflection high-energy electron diffraction (RHEED). The oxygen/Ni peak intensity ratios derived from the XPS showed that the oxygen concentration increased with the increase of B,0, pretreatment time from the surface while decreased gradually with the increase of XPS sputtering depth. Meanwhile, boron incorporation was negligible regardless of treatment time. The epitaxial relationship was studied by using h-BN coverage-dependent RHEED analysis, obtained on samples including Ni substrate after annealing only, Ni substrate with BAO pretreatment, h-BN samples with the B,03 pretreatment and subsequent 50%, and 100% film coverage. Only two sets of patterns were observed and separated by 30, confirming that there is no in-plane rotation between the h-BN and Ni (111) surface. The oxygen incorporated into the Ni substrate not only suppressed h-BN nucleation by passivating the Ni surface active sites, but also decreased the growth rate of single crystal h-BN domains, shifting from larger sizes to smaller shapes. Detailed structural, optical and electrical characterizations of these monolayer h-BN films will also be presented. Our work reveals the influence of oxygen on the catalytic effect of Ni in the MBE growth of monolayer h-BN and provides an efficient way to control the morphology and quality of the 2D h-BN films.

Molecular Beam Epitaxy Growth of Monolayer Hexagonal Boron Nitride on Nickel Substrates Incorporated with Oxygen Atoms

Exchange bias, a magnetic phenomenon emerging at the interface between a ferromagnet (FM) and an antiferromagnet (AFM), originates from the pinning of spins in the FM layer by the adjacent AFM layer, effectively stabilizing the FM against the environmental stray magnetic fields. This phenomenon has been implemented in a wide range of technological applications including magnetic memories, magnetic read heads, and sensors, and plays the foundational role for advanced spintronics. Here, we report an approach to establishing strong exchange bias in the 2D magnet/oxide heterostructure by a single-step process — atomic layer deposition (ALD). The exchange bias we created is controllable in the range of ~300 to 1500 Oe by replacing different oxides in the heterostructure. Hence, this systematic work elucidates the essential role of ALD-enabled oxidization of layered magnets in the formation of exchange bias and establishes ALD of oxides as a facile, controllable, and generally effective approach to creating giant exchange bias in vaW magnets, representing an integral advance towards practical vdW spintronic devices.

Facile Integration of Giant Exchange Bias in 2D Magnet/Oxide Heterostructures

Layered chalcogenides are promising materials for future (opto-)electronic applications. ' 2 The semiconductor tin selenide (SnSe) is a member of the monochalcogenide (MX) family, which feature functionality including domain-switching behavior, ferroelasticity, thermo-, piezo- and ferroelectricity.” ~ Orthorhombic a-SnSe has strong in-plane anisotropy with implications for thermal, electronic, and optical properties. It has been studied for next-generation microelectronics, photonics and energy-conversion technologies, and has been the focus of a theoretical proposal for ultra-fast, non-thermal optical domain switching. ’~ Control of rotational domains (i.e., twins) during SnSe thin-film synthesis is key to enable high-quality, single-crystalline SnSe for the proposed applications. This work focusses on MBE of SnSe. We demonstrate (1) a fast and easy method to probe rotational domains in SnSe thin films and (2) a solution to overcome the formation of rotational domains by focusing on symmetry and lattice-matched vdW epitaxy of SnSe on a-plane sapphire surfaces. When orthorhombic SnSe (in-plane lattice parameters; a = 4.15A and b = 4.44A) is epitaxially grown on cubic MgO (a = 4.21A), various rotational domains are formed as a result of the various symmetry structure of both materials. ~ The 2-fold rotational symmetry of SnSe combined with the 4-fold symmetry of MgO results in the formation 90° SnSe twins, and a high density of grain boundaries upon closure of the epitaxial film. Characterizing rotational domains in ultra-thin films of 2D materials is generally challenging (in-plane HR-XRD, ‘large-area’ cs-TEM imaging, etc.). We show that, thanks to the optical anisotropy of a-SnSe in the visible (An = 0.88) | 5 supported by an optical model based on the reflectivity of linearly-polarized light, 90° SnSe twins are easily distinguishable using polarized optical microscopy. When studied under cross-polarization at 45° with respect to the SnSe optical axis, slightly increasing or decreasing the angle of the polarizer with respect to the analyzer enables the color splitting of 0°-oriented and 90°- oriented domains into blue and yellow, respectively, when illuminated with white light. We first demonstrate this method on thicker SnSe epitaxial crystallites (~5Onm), using polarized Raman spectroscopy for validation, and we then apply it to thinner (~5nm) epitaxial SnSe films. The distribution of blue and yellow pixels in the optical image enables the quantification of rotational domains and confirms the presence of a high density of 90° twins on MgO. To overcome the formation of 90° twins, we grow epitaxially on a-plane sapphire [a-AI,0..(1 1-20)] substrates. Similarly as a-SnSe, the surface unit of OAI,0.3(1 1-20) can be constructed using dissimilar in-plane lattice parameters; a = 8.24A andb=12.99A. This favors single-oriented domain epitaxy of (2 x 3) units of SnSe on a-Al., O,,(11-20), explained by the symmetry match and small lattice mismatch in both in-plane directions, and confirmed 3 experimentally by our polarized reflectivity method. The developed optical method and epitaxy on a-plane sapphire represent a very useful combination of film metrology and growth methodology to realize high-quality, single-crystalline epitaxial SnSe thin films free of rotational twin domains. We will also highlight our epitaxial growth techniques, especially a comparison between solid-source and plasma-assisted methods. Singh A et al ACS Photonics 2020, 7 (12) Liu Y et al Nat. Rev. Mater. 2016, 1 (9) Zhou J et al Nano Lett. 2018, 18 (12) Gomes C et al J. Appl. Phys. 2020, 128 (12). Chang K et al Nano Lett. 2020, 20 (9) Chen G et al Prog. Mater. Sci. 2018, 97 Zhao D et al Nature 2014, 508 (7496) Jo S et al Adv. Photonics Res. 2021, 2100176. 9) Mortelmans W et al Appl. Mater. Today 2021, 22 10) Nguyen Q et al J. Alloys Compd. 2020, 840

Fast and Easy Optical Imaging and Epitaxial Growth Control of Rotational Domains in SnSe Thin Films

Freestanding graphene nano- and microstructures have been shown to exhibit controllable electron emission when carrying a current and immersed in an external accelerating electric field. Through the mechanism of Phonon-Assisted Electron Emission, these structures demonstrate electron emission at electric field intensities and lattice temperatures below what would be expected for carbon to demonstrate field emission or thermionic emission, respectively. Emission currents have been measured primarily out of the plane of the device surface, allowing some directional control before the accelerating field is even considered. Thus far, such demonstrations have tended to involve structural dimensions on the order of only a handful of um, and have commonly been fabricated from CVD transferred graphene, flakes, or carbon nanotubes in various vertical and horizontal arrangements. Applied fields for such devices typically approach hundreds of kV/cm, and emission currents rarely leave the nA range. Herein are presented graphene structures with dimensions up to cm, in fields not exceeding 1 kV/cm, and producing emission currents of nearly 10 UA. Quasi-freestanding epitaxial graphene, synthesized from a silicon carbide substrate, offers several advantages compared to transferred graphene or unzipped nanotubes, including greater structural integrity, lower defect density, ease of handling, and ready compatibility with conventional simple photolithographic fabrication techniques. The devices presented here were fabricated with a low-cost, wafer scalable, single-photomask process that does not require material transfer to a secondary substrate and is easily tailored to create variations in design parameters, which allowed multiple batches of devices to be rapidly manufactured with precise variations for the purpose of performance testing. The devices exhibit recognizable patterns of emission current performance relative to device and substrate temperature, including an identifiable turn-on point and emission current maintenance. Device behavior relative to substrate heating identifies the electron-phonon interactions that enable function and current emission, both when device temperature is supplied internally and externally. Emission currents are also observed to follow a performance pattern derived from purposeful device morphology, even exhibiting emission currents in accelerating fields as low as 50 V/cm. Graphene electron emission devices present an opportunity for electron emission in a planar 2D heterostructure environment, beyond even the dimensional capabilities of field emission devices, and below the high operating temperatures of thermionic sources. Through refinements in the design and packaging, such devices, which are arbitrarily scalable between the micron and wafer level, could provide the electron sources for novel applications in Xray generation or electron microscopy.

Electron Emission Sources from Quasi-Freestanding Epitaxial Graphene Planar Devices

High-brightness photoemitters can be employed as electron sources in ultrafast electron diffraction and X-ray free-electron lasers, energy recovery linacs, electron microscopes, electron beam lithography, space propulsion and high-power microwave devices. To yield high quantum efficiencies, high brightness beams require rapid response times and low mean transverse energies (MTE). Recently, negative electron affinity (NEA) photocathodes, consisting of a GaAs wafer and a Cs/O surface layer, have been reported to exhibit both reasonably low MTE and good external quantum efficiencies (EQE) of ~10 ~-10 “. However, NEA cathodes are rarely used due to their significantly complex fabrication process which includes high temperature processing steps, entirely in-situ activation as well as ultra-high vacuum storage and operating pressures. Their stability is not also up to the mark. In this investigation, we have fabricated a Hot Electron Laser Assisted Cathode (HELAC) using electronically tunable negative electron affinity silicon surface. Briefly, the device consists of a three-layer stack of (i) a semiconductor, which absorbs incident photons, (ii) a monolayer of graphene, which acts as the gate, and (iii) an insulator, which allows a voltage drop between the semiconductor and graphene. In this device, an electric field is generated by applying a bias voltage between the semiconductor and graphene which mimics the surface dipole field in an NEA photocathode. Electrons injected into the graphene do not require transport to the emitting surface due to the monolayer thickness of the graphene. This device is air stable with a simple ex-situ fabrication flow while preserving the advantages of NEA photocathodes. We have demonstrated an initial proof-of-concept HELAC device with a psilicon substrate, silicon oxide insulator and monolayer graphene stack and evaluated its performance using a 405 nm CW laser at ~40 mW of incident power. Despite having a non-optimized structure, this device has exhibited a current of ~70 yA (~1.1 mA/cm?), resulting in an EQE of ~0.5%. Semiconductor modeling indicated that EQEs of ~10% and response times in the 1-10 ps range are plausible for this device. Additionally, it is expected that low-MTE is achievable by optimizing the quality of the insulator, the barrier height, and the tunneling mechanism. This is analogous to Cs-based NEA devices, where low MTEs are attainable despite a large electric field which is developed due to the surface dipole.

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Electroplated Helical Slow-Wave Structures for Millimeter-Through-THz Vacuum Electronic Devices

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Characterization of β-Phase MgGaO Thin Films Grown by Molecular Beam Epitaxy

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