Department of Materials Science and Engineering (MSE)

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The mission of the Materials Science and Engineering (MSE) Department at Virginia Tech is to lead the multidisciplinary efforts of the College of Engineering, the University, and the Commonwealth in the field of Materials Science and Engineering through our programs of undergraduate and graduate education, research, and continuing education. In service to our many constituencies, we are committed to the excellence of the contributions of faculty members, staff, and students, as judged by the principles and philosophies to which we aspire.

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3D printed graphene-based self-powered strain sensors for smart tires in autonomous vehicles
Maurya, Deepam; Khaleghian, Seyedmeysam; Sriramdas, Rammohan; Kumar, Prashant; Kishore, Ravi Anant; Kang, Min-Gyu; Kumar, Vireshwar; Song, Hyun-Cheol; Lee, Seul-Yi; Yan, Yongke; Park, Jung-Min (Jerry); Taheri, Saied; Priya, Shashank (2020-10-26)
The transition of autonomous vehicles into fleets requires an advanced control system design that relies on continuous feedback from the tires. Smart tires enable continuous monitoring of dynamic parameters by combining strain sensing with traditional tire functions. Here, we provide breakthrough in this direction by demonstrating tire-integrated system that combines direct mask-less 3D printed strain gauges, flexible piezoelectric energy harvester for powering the sensors and secure wireless data transfer electronics, and machine learning for predictive data analysis. Ink of graphene based material was designed to directly print strain sensor for measuring tire-road interactions under varying driving speeds, normal load, and tire pressure. A secure wireless data transfer hardware powered by a piezoelectric patch is implemented to demonstrate self-powered sensing and wireless communication capability. Combined, this study significantly advances the design and fabrication of cost-effective smart tires by demonstrating practical self-powered wireless strain sensing capability. Designing efficient sensors for smart tires for autonomous vehicles remains a challenge. Here, the authors present a tire-integrated system that combines direct mask-less 3D printed strain gauges, flexible piezoelectric energy harvester for powering the sensors and secure wireless data transfer electronics, and machine learning for predictive data analysis.
Application of Sapphire-Fiber-Bragg-Grating-Based Multi-Point Temperature Sensor in Boilers at a Commercial Power Plant
Yang, Shuo; Homa, Daniel S.; Heyl, Hanna; Theis, Logan; Beach, John; Dudding, Billy; Acord, Glen; Taylor, Dwyn; Pickrell, Gary R.; Wang, Anbo (MDPI, 2019-07-21)
Readily available temperature sensing in boilers is necessary to improve efficiencies, minimize downtime, and reduce toxic emissions for a power plant. The current techniques are typically deployed as a single-point measurement and are primarily used for detection and prevention of catastrophic events due to the harsh environment. In this work, a multi-point temperature sensor based on wavelength-multiplexed sapphire fiber Bragg gratings (SFBGs) were fabricated via the point-by-point method with a femtosecond laser. The sensor was packaged and calibrated in the lab, including thermally equilibrating at 1200 °C, followed by a 110-h, 1000 °C stability test. After laboratory testing, the sensor system was deployed in both a commercial coal-fired and a gas-fired boiler for 42 days and 48 days, respectively. The performance of the sensor was consistent during the entire test duration, over the course of which it measured temperatures up to 950 °C (with some excursions over 1000 °C), showing the survivability of the sensor in a field environment. The sensor has a demonstrated measurement range from room temperature to 1200 °C, but the maximum temperature limit is expected to be up to 1900 °C, based on previous work with other sapphire based temperature sensors.
Behavior of Random Hole Optical Fibers under Gamma Ray Irradiation and Its Potential Use in Radiation Sensing Applications
Alfeeli, Bassam; Pickrell, Gary R.; Garland, Marc; Wang, Anbo (MDPI, 2007-05-24)
Effects of radiation on sensing and data transmission components are of greatinterest in many applications including homeland security, nuclear power generation, andmilitary. A new type of microstructured optical fiber (MOF) called the random hole opticalfiber (RHOF) has been recently developed. The RHOFs can be made in many differentforms by varying the core size and the size and extent of porosity in the cladding region.The fibers used in this study possessed an outer diameter of 110 _m and a core ofapproximately 20 _m. The fiber structure contains thousands of air holes surrounding thecore with sizes ranging from less than 100 nm to a few _m. We present the first study ofthe behavior of RHOF under gamma irradiation. We also propose, for the first time to ourknowledge, an ionizing radiation sensor system based on scintillation light from ascintillator phosphor embedded within a holey optical fiber structure. The RHOF radiationresponse was compared to normal single mode and multimode commercial fibers(germanium doped core, pure silica cladding) and to those of radiation resistant fibers (puresilica core with fluorine doped cladding fibers). The comparison was done by measuringradiation-induced absorption (RIA) in all fiber samples at the 1550 nm wavelength window(1545 25 nm). The study was carried out under a high-intensity gamma ray field from a 60Co source (with an exposure rate of 4x104 rad/hr) at an Oak Ridge National Laboratory gamma ray irradiation facility. Linear behavior, at dose values less than 106 rad, was observed in all fiber samples except in the pure silica core fluorine doped cladding fiber which showed RIA saturation at 0.01 dB. RHOF samples demonstrated low RIA (0.02 and 0.005 dB) compared to standard germanium doped core pure silica cladding (SMF and MMF) fibers. Results also showed the possibility of post-fabrication treatment to improve the radiation resistance of the RHOF fibers.
Connection between Carbon Incorporation and Growth Rate for GaN Epitaxial Layers Prepared by OMVPE
Ciarkowski, Timothy; Allen, Noah P.; Carlson, Eric; McCarthy, Robert; Youtsey, Chris; Wang, Jingshan; Fay, Patrick; Xie, Jinqiao; Guido, Louis J. (MDPI, 2019-08-01)
Carbon, a compensator in GaN, is an inherent part of the organometallic vapor phase epitaxy (OMVPE) environment due to the use of organometallic sources. In this study, the impact of growth conditions are explored on the incorporation of carbon in GaN prepared via OMVPE on pseudo-bulk GaN wafers (in several cases, identical growths were performed on GaN-on-Al₂O₃ templates for comparison purposes). Growth conditions with different growth efficiencies but identical ammonia molar flows, when normalized for growth rate, resulted in identical carbon incorporation. It is concluded that only trimethylgallium which contributes to growth of the GaN layer contributes to carbon incorporation. Carbon incorporation was found to decrease proportionally with increasing ammonia molar flow, when normalized for growth rate. Ammonia molar flow divided by growth rate is proposed as a reactor independent predictor of carbon incorporation as opposed to the often-reported input V/III ratio. A low carbon concentration of 7.3 × 10¹⁴ atoms/cm³ (prepared at a growth rate of 0.57 µm/h) was obtained by optimizing growth conditions for GaN grown on pseudo-bulk GaN substrates.
A diffusion-viscous analysis and experimental verification of defect formation in sintered silver bond-line
Xiao, Kewei; Ngo, Khai D. T.; Lu, Guo-Quan (Cambridge University Press, 2014-04-01)
The low-temperature joining technique (LTJT) by silver sintering is being implemented by major manufacturers of power electronic devices and modules for bonding power semiconductor chips. A common die-attach material used with LTJT is a silver paste consisting of silver powder (micrometer- or nanometer-sized particles) mixed in organic solvent and binder formulation. It is believed that the drying of the paste during the bonding process plays a critical role in determining the quality of the sintered bond-line. In this study, a model based on the diffusion of solvent molecules and viscous mechanics of the paste was introduced to determine the stress and strain states of the silver bond-line. A numerical simulation algorithm of the model was developed and coded in the C++ programming language. The numerical simulation allows determination of the time-dependent physical properties of the silver bond-line as the paste is being dried with a heating profile. The properties studied were solvent concentration, weight loss, shrinkage, stress, and strain. The stress is the cause of cracks in the bond-line and bond-line delamination. The simulated results were verified by experiments in which the formation of bond-line cracks and interface delamination was observed during the pressure-free drying of a die-attach nanosilver paste. The simulated results were consistent with our earlier experimental findings that the use of uniaxial pressure of a few mega-Pascals during the drying stage of a nanosilver paste was sufficient to produce high-quality sintered joints. The insight offered by this modeling study can be used to develop new paste formulations that enable pressure-free, low-temperature sintering of the die-attach material to significantly lower the cost of implementing the LTJT in manufacturing.
Dislocation-pipe diffusion in nitride superlattices observed in direct atomic resolution
Garbrecht, Magnus; Saha, Bivas; Schroeder, Jeremy L.; Hultman, Lars; Sands, Timothy D. (Springer Nature, 2017-04-06)
Device failure from diffusion short circuits in microelectronic components occurs via thermally induced migration of atoms along high-diffusivity paths: dislocations, grain boundaries, and free surfaces. Even well-annealed single-grain metallic films contain dislocation densities of about 1014 m-2; hence dislocation-pipe diffusion (DPD) becomes a major contribution at working temperatures. While its theoretical concept was established already in the 1950s and its contribution is commonly measured using indirect tracer, spectroscopy, or electrical methods, no direct observation of DPD at the atomic level has been reported. We present atomically-resolved electron microscopy images of the onset and progression of diffusion along threading dislocations in sequentially annealed nitride metal/semiconductor superlattices, and show that this type of diffusion can be independent of concentration gradients in the system but governed by the reduction of strain fields in the lattice.
Electrical characterization of RuOx/n-GaN Schottky diodes formed by oxidizing ruthenium thin-films in normal laboratory air
Allen, Noah P.; Ciarkowski, Timothy; Carlson, Eric; Chakraborty, Amrita; Guido, Louis J. (2020-01)
Schottky diodes were formed by oxidizing Ru thin films deposited on n-type GaN at 400, 500, and 600 degrees C in normal laboratory air, and their electrical behavior was compared to that of a Ru/n-GaN reference device. The GaN epitaxial layers were grown via metalorganic chemical vapor deposition. The ruthenium films were deposited by electron beam evaporation. The Schottky barriers were characterized via current vs voltage (IV) and deep-level transient spectroscopy (DLTS) measurements between 70 and 400 K. The temperature dependent forward bias IV characteristics were fit, and the extracted temperature dependence of the effective barrier height for each device was shown to be caused by inhomogeneity at the metal/semiconductor interface. It was found that barrier inhomogeneity could be well described by a modified log-normal distribution. In reverse bias, it was shown that the low-energy tail of the barrier distribution is an important factor in determining leakage current. Favorable results occur for diodes oxidized at 400 and 500 degrees C, but raising the oxidation temperature to 600 degrees C results in a drastic increase in leakage current. DLTS measurements reveal one electron trap at E-C - 0.57 eV in each of the samples. It was found that the concentration of this 0.57 eV trap increases substantially at 600 degrees C and that trap-assisted tunneling likely contributes an additional pathway for reverse leakage current. (c) 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
Energy band alignment of atomic layer deposited HfO2 on epitaxial (110)Ge grown by molecular beam epitaxy
Hudait, Mantu K.; Zhu, Y.; Maurya, Deepam; Priya, Shashank (AIP Publishing, 2013-03-01)
The band alignment properties of atomic layer HfO2 film deposited on epitaxial (110)Ge, grown by molecular beam epitaxy, was investigated using x-ray photoelectron spectroscopy. The cross-sectional transmission electron microscopy exhibited a sharp interface between the (110)Ge epilayer and the HfO2 film. The measured valence band offset value of HfO2 relative to (110)Ge was 2.28 +/- 0.05 eV. The extracted conduction band offset value was 2.66 +/- 0.1 eV using the bandgaps of HfO2 of 5.61 eV and Ge bandgap of 0.67 eV. These band offset parameters and the interface chemical properties of HfO2/(110)Ge system are of tremendous importance for the design of future high hole mobility and low-power Ge-based metal-oxide transistor devices. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4794838]
Enhanced Absorption And Electro-Optic Pockels Effect Of Electrostatically Self-Assembled Cdse Quantum Dots
Zhang, F. J.; Zhang, L. M.; Wang, Y. X.; Claus, Richard O. (Optical Society of America, 2005-10-01)
The spectrum and electro-optic properties of CdSe quantum dots are studied. Spectrum wavelength shifts that are due to the quantum size effect and to the electro-optic Stark effect are investigated. It is found that CdSe quantum dot-polymer composites formed by an electrostatic self-assembly (ESA) technique exhibit high internal electric fields. Using the second-order perturbation theory of the ls-ls energy shift (Stark effect), we estimate the internal field of the ESA film to be as high as 2.6 X 10(8) V/m. This value results in a much higher absorption coefficient and electro-optic coefficients for ESA films than for their bulk crystal counterparts or for spin-coated film samples. The relationships among unusual spectra, film structure, and high electro-optic response are analyzed. These results are useful both for understanding the physical mechanisms of semiconductor quantum dots and for developing high-performance photonic devices. (c) 2005 Optical Society of America.
Low temperature processed 0.7SrBi(2)Ta(2)O(9)-0.3Bi(3)TaTiO(9) thin films fabricated on multilayer electrode-barrier structure for high-density ferroelectric memories
Ryu, S. O.; Joshi, Pooran C.; Desu, Seshu B. (AIP Publishing, 1999-10-01)
Thin films of solid-solution material 0.7SrBi(2)Ta(2)O(9)-0.3Bi(3)TaTiO(9) (0.7SBT-0.3BTT) were fabricated on n(+)-polycrystalline (n(+)-poly) Si substrates by a metalorganic solution deposition technique at a low processing temperature of 650 degrees C using a Pt-Rh/Pt-Rh-O-x electrode-barrier structure. The Pt-Rh/Pt-Rh-O-x structure was deposited using an in situ reactive radio frequency sputtering process. The electrodes had a smooth and fine-grained microstructure and were excellent diffusion barriers between the 0.7SBT-0.3BTT thin film and Si substrate. The ferroelectric (0.7SBT-0.3BTT) test capacitors using these electrode-barrier grown directly on Si showed good ferroelectric hysteresis properties, measured through n(+)-poly Si substrate, with 2P(r) and E-c values of 11.5 mu C/cm(2) and 80 kV/cm, respectively, at an applied electric field of 200 kV/cm. The films exhibited good fatigue characteristics (< 10% decay) under bipolar stressing up to 10(11) switching cycles and the leakage current density was lower 10(-7) A/cm(2) at an applied electric field of 200 kV/cm. The good ferroelectric properties of 0.7SBT-0.3BTT solid-solution thin films at a low processing temperature of 650 degrees C and excellent electrode-diffusion barrier properties of a Pt-Rh/Pt-Rh-O-x structure are encouraging for the realization of high-density nonvolatile ferroelectric random access memories on silicon substrates. (C) 1999 American Institute of Physics. [S0003-6951(99)01240-1].
Magnetoelectric quasi-(0-3) nanocomposite heterostructures
Li, Yanxi; Wang, Zhongchang; Yao, Jianjun; Yang, Tiannan; Wang, Zhiguang; Hu, Jia-Mian; Chen, Chunlin; Sun, Rong; Tian, Zhipeng; Li, Jiefang; Chen, Long-Qing; Viehland, Dwight D. (Nature Publishing Group, 2015-12-01)
Magnetoelectric composites of magnetic and ferroelectric components are promising for their use in applications such as information storage. Here, the authors find that magnetic quasiparticles embedded in a ferroelectric film matrix show promising properties compared to the usual thin-film architectures.
Maximum power point tracking for solar panels
(United States Patent and Trademark Office, 2017-06-20)
Approximately one-half of the loss of delivered power from a solar panel having photovoltaic (PV) cells connected in series to form sub-panels due to shading is recovered at low hardware cost by connecting sub-panels in series and providing maximum power point tracking control in common for the series connected sub-panels such that the respective sub-panels produce equal voltages even in the presence of shading of a portion of one or more sub-panels. By doing so, the input voltage of respective power converters which control the voltage at which each sub-panel is operated can be placed close to the maximum power point of each sub-panel regardless of shading and maximum total power harvested even though the respective sub-panels are not operated at optimum voltages.
Method of forming multilayered electrodes for ferroelectric devices consisting of conductive layers and interlayers formed by chemical reaction
(United States Patent and Trademark Office, 1996-02-13)
A ferroelectric device is constructed using a bottom electrode composed of a conducting oxide such as RuO.sub.x, on a substrate such as silicon or silicon dioxide. A ferroelectric material such as lead zirconate titanate (PZT) is deposited on the bottom electrode, and a conducting interlayer is formed at the interface between the ferroelectric and the electrode. This interlayer is created by reaction between the materials of the ferroelectric and electrode, and in this case would be Pb.sub.2 Ru.sub.2 O.sub.7-x. A conductive top layer is deposited over the ferroelectric. This top layer may be a metal, or it may be the same type of materials as the bottom electrode, in which case another interlayer can be formed at the interface. A device constructed in this manner has the property of lower degradation due to fatigue, breakdown, and aging.
A (Permalloy + NiZn Ferrite) Moldable Magnetic Composite for Heterogeneous Integration of Power Electronics
Ding, Chao; Mei, Yunhui; Ngo, Khai D. T.; Lu, Guo-Quan (MDPI, 2019-06-22)
Soft magnetic moldable composites (SM²Cs) would be ideally suited for the integration of magnetic components in power electronic converters because they can be formed into magnetic cores by low-temperature and pressure-less processing. However, most SM²Cs have low relative magnetic permeability, typically less than 30, and high core-loss densities at switching frequencies over 1 MHz. To improve their magnetic properties, we combine powders of Permalloy and a NiZn ferrite with an acrylic polymer to formulate a paste of SM²C. The paste can be molded and then cured below 200 °C without pressure to form cores with a relative permeability over 35 and a core-loss density at 1 MHz, 30% lower than those of commercial cores. The ease of its processing and high-performance properties makes the SM²C a good candidate material for the integration of power magnetics.
Reactive ion etching of lead zirconate titanate and ruthenium oxide thin films
(United States Patent and Trademark Office, 1996-03-05)
A method of reactive ion etching both a lead zirconate titanate ferroelectric dielectric and a RuO.sub.2 electrode, and a semiconductor device produced in accordance with such process. The dielectric and electrode are etched in an etching gas of O.sub.2 mixed with either CClF.sub.2 or CHClFCF.sub.3.
Synthesis kinetics of CdSe quantum dots in trioctylphosphine oxide and in stearic acid
Dickerson, B. D.; Irving, D. M.; Herz, E.; Claus, Richard O.; Spillman, William B. Jr.; Meissner, K. E. (AIP Publishing, 2005-04-01)
A diffusion-barrier model described the early evolution of size-dependent photoluminescence emission from CdSe quantum dots formed by organometallic synthesis. Emission peak widths, emission redshift rates, and nanocrystal growth rates all decreased to a minimum at a reaction completion time. Growth after the completion time by Ostwald ripening was marked by a doubling of the activation energy. The temperature dependence of both reaction completion rates and photoluminescence redshift rates followed Arrhenius behavior governed by activation energies that increased with solvent molecular weight, in this limited case. In stearic acid and in trioctylphosphine oxide, the typical activation energies were 0.6 +/- 0.1 and 0.92 +/- 0.26 eV/molecule, respectively. (c) 2005 American Institute of Physics.
Tailoring a Silver Paste for Additive Manufacturing of Co-Fired Ferrite Magnetic Components
Liu, Lanbing; Ding, Chao; Mei, Yunhui; Lu, Guo-Quan (MDPI, 2019-03-11)
Additive manufacturing (AM), or 3D-printing, has the potential for rapid prototyping of innovative designs of magnetic components used in power electronics converters. In this study, we tailored a silver paste as the metal feedstock of an extrusion 3D printer so that the metal would be compatible with a ferrite paste feedstock for 3D-printing of ferrite magnetic components. We focused on adjusting the metal formulation to match its shrinkage to that of the ferrite and to improve adhesion during the co-sintering process of the printed part. We found that a 5 wt % addition of ferrite powder in the metal paste can achieve matched shrinkage and strong adhesion. Evaluation of the co-sintered magnetic components showed no significant defects, such as cracks, warpage, or delamination, between the metal and ferrite. The shear strength between the two sintered materials was greater than 50 MPa, and the electrical resistivity of the sintered metal winding was less than twice that of the bulk silver, which is lower than those of most 3D-printed winding metals reported in the literature.
Tailoring of surface plasmon resonances in TiN/(Al0.72Sc0.28)N multilayers by dielectric layer thickness variation
Garbrecht, Magnus; Hultman, Lars; Fawey, Mohammed H.; Sands, Timothy D.; Saha, Bivas (Springer, 2017-11-28)
Alternative designs of plasmonic metamaterials for applications in solar energy-harvesting devices are necessary due to pure noble metal-based nanostructures’ incompatibility with CMOS technology, limited thermal and chemical stability, and high losses in the visible spectrum. In the present study, we demonstrate the design of a material based on a multilayer architecture with systematically varying dielectric interlayer thicknesses that result in a continuous shift of surface plasmon energy. Plasmon resonance characteristics of metal/semiconductor TiN/(Al,Sc)N multilayer thin films with constant TiN and increasing (Al,Sc)N interlayer thicknesses were analyzed using aberration-corrected and monochromated scanning transmission electron microscopy-based electron energy loss spectroscopy (EELS). EEL spectrum images and line scans were systematically taken across layer interfaces and compared to spectra from the centers of the respective adjacent TiN layer. While a constant value for the TiN bulk plasmon resonance of about 2.50 eV was found, the surface plasmon resonance energy was detected to continuously decrease with increasing (Al,Sc)N interlayer thickness until 2.16 eV is reached. This effect can be understood to be the result of resonant coupling between the TiN bulk and surface plasmons across the dielectric interlayers at very low (Al,Sc)N thicknesses. That energy interval between bulk and decreasing surface plasmon resonances corresponds to wavelengths in the visible spectrum. This shows the potential of tailoring the material’s plasmonic response by controlling the (Al,Sc)N interlayer thickness, making TiN-based multilayers good prospects for plasmonic metamaterials in energy devices.
Ultra-high frequency photoconductivity decay in GaAs/Ge/GaAs double heterostructure grown by molecular beam epitaxy
Hudait, Mantu K.; Zhu, Y.; Johnston, Steve W.; Maurya, Deepam; Priya, Shashank; Umbel, Rachel (AIP Publishing, 2013-03-01)
GaAs/Ge/GaAs double heterostructures (DHs) were grown in-situ using two separate molecular beam epitaxy chambers. High-resolution x-ray rocking curve demonstrates a high-quality GaAs/Ge/GaAs heterostructure by observing Pendellosung oscillations. The kinetics of the carrier recombination in Ge/GaAs DHs were investigated using photoconductivity decay measurements by the incidence excitation from the front and back side of 15 nm GaAs/100 nm Ge/0.5 mu m GaAs/(100) GaAs substrate structure. High-minority carrier lifetimes of 1.06-1.17 mu s were measured when excited from the front or from the back of the Ge epitaxial layer, suggests equivalent interface quality of GaAs/Ge and Ge/GaAs. Wavelength-dependent minority carrier recombination properties are explained by the wavelength-dependent absorption coefficient of Ge. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4794984]
Void-mediated coherency-strain relaxation and impediment of cubic-to-hexagonal transformation in epitaxial metastable metal/semiconductor TiN/Al0.72Sc0.28N multilayers
Garbrecht, Magnus; Hultman, Lars; Fawey, Mohammed H.; Sands, Timothy D.; Saha, Bivas (2017-08-17)
Bulk metastable phases can be stabilized during thin-film growth by employing substrates with similar crystal structure and lattice parameter, albeit over a thickness range limited by coherency-strain relaxation. Expanding that strategy, growth of superlattices comprising one stable and another metastable compound with similar crystal structure and lattice parameters are known to yield epitaxial stabilization over a few nanometers of thickness. In this work, the high-pressure rocksalt (B1) phase of Al0.72Sc0.28N was stabilized epitaxially in a multilayer with TiN with thicknesses of up to 26 nm. In order to investigate the microstructural changes leading to the phase transformation of the metastable B1 phase to its wurtzite allomorph, we demonstrate a design based on a multilayer architecture with systematically varying thicknesses of the metastable compound within a constant-thickness lattice of stable metallic TiN with the cubic rocksalt structure. The multilayer films show an increasing hardness and elastic modulus for decreasing period thickness, in correspondence with both coherency-strain and Koehler hardening. The phase transition is accompanied by an increase of lattice strain with increasing multilayer periods, and resulting ultimately in coherency-strain relaxation upon phase transformation. Further, we show that the phase transformation is mediated by voids decorating the {130} planes that separate regions of different growth rates and act as additional growth fronts for wurtzite growth during the phase transformation. The TiN/(Al, Sc) N interfaces themselves remain atomically sharp and smooth until the interface structure roughens along with the epitaxial rocksalt to wurtzite transition of (Al, Sc) N. These results show the strong influence of the voids on controlling the target thickness of epitaxially stabilized thin-film growth to the range relevant for applications, such as coatings, plasmonic materials, and electronic device technology, where the mechanical integrity of the material is critical.

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