Polymethyl methacrylate (PMMA) is the most commonly used electron-beam resist. While it is cost effective and easily available and offers the highest resolution among any polymeric resist, PMMA suffers from poor dry-etch resistance. For that reason, other resists like hydrogen silsesquioxane and ZEP520A are typically preferred. In this work, we present the fabrication of uniform silicon nanopillar arrays using PMMA as a direct etch mask, emphasizing the process optimizations, particularly in reactive-ion etching (RIE) chemistry, required to overcome key roadblocks. Electron-beam lithography was used to write patterns in PMMA positive-tone resist. To transfer the patterns to the substrate and achieve nanopillars of sidewalls with smooth sides, RIE was employed with SF6/O-2 chemistry. The fabricated structures were characterized by atomic force microscopy and scanning electron microscopy.

We report surface smoothing mediated by the bismuth (Bi) surfactant in homoepitaxial InSb(100) thin films grown by molecular beam epitaxy. Across a temperature range of 280-385 degrees C, Bi-assisted growth yields significantly reduced surface roughness compared to control samples, as confirmed by atomic force microscopy and power spectral density analysis. X-ray photoelectron spectroscopy shows no detectable Bi above the instrument threshold, consistent with the established Bi-exclusion regime in InSbBi alloys, where incorporation is suppressed at substrate temperatures above similar to 240 degrees C and under Sb-rich conditions. Within this exclusion window, Bi remains confined to the growth front, where it modifies surface kinetics and suppresses the onset of three-dimensional roughening, thereby expanding the usable growth window for InSb. This surfactant-mediated smoothing is particularly valuable for heterostructures that impose low-temperature growth constraints, enabling InSb layers to be incorporated at reduced temperatures while maintaining high interface quality.

Optical metasurfaces are gaining attention as a new technology because they can perform the tasks of several traditional optical elements simultaneously, while at a fraction of the size and weight. Their performance is dictated by the subwavelength structured surface, which also makes them extremely sensitive to morphology changes after fabrication. Oxidation and plastic deformation (edge rounding) are two dominant types of surface changes that occur in high-temperature environments, which limit optical metasurfaces from being a useful technology for those applications. Iridium is a refractory metal that is stable in these types of environments. The lack of reactivity, extremely high melting temperature, and high hardness are what makes iridium an ideal choice for the application. However, these features also present major challenges for lithographic pattern transfer at the nanoscale. We present several techniques for etching nanoscale patterns into iridium films, demonstrating the potential for use of optical metasurfaces in high-temperature environments.

Three Ge(1-x)Snx films were measured by spectroscopic ellipsometry to extract their optical properties. The Sn contents of the films were 3.6%, 6.5%, and 8.4%, and all were fully strained to a Ge (001) substrate. Optical constants were collected from 0.39-4.116 eV, at temperatures between 78 K and 475 K. Critical point energies in the band structure were red-shifted with increasing Sn content and increasing temperatures. An extra critical point appears between E-0+Delta and E-1 transitions in GeSn samples that does not appear in Ge.

This study reports the discovery of reentrant step-flow growth behavior in homoepitaxial GaSb(100) films grown via molecular beam epitaxy under bismuth surfactancy. Using ex situ atomic force microscopy, we observe a superposition of short-scale mound and large-scale step-flow morphologies at temperatures significantly below the conventional threshold for step-flow growth. The findings suggest that bismuth surfactant atoms facilitate long-range adatom kinetics, enabling step-flowlike features despite reduced thermal energy. This behavior diverges from traditional epitaxial growth models and aligns with previously reported reentrant phenomena in other material systems. The implications for surface morphology control and device fabrication are discussed, highlighting the role of surfactants in enabling nonequilibrium growth pathways.

We investigate the surface morphologies of two series of homoepitaxial GaSb(100) thin films grown on GaSb(100) substrates by molecular beam epitaxy in a Veeco GENxplor system. The first series was grown at temperatures ranging from 290 to 490°C and serves as a control. The second series was grown using the same growth parameters with bismuth used as a surfactant during growth. We compared the two series to examine the impacts of bismuth over the range of growth temperatures on the GaSb surface morphologies using atomic force microscopy and the film properties using Raman spectroscopy and scanning electron microscopy. High-resolution x-ray diffraction was performed to confirm that bismuth was not incorporated into the films. We found that the morphological evolution of the GaSb series grown without bismuth is consistent with the standard surface nucleation theory and identified the 2D-3D transition temperature as close to 290° C. In contrast, the presence of a Bi surfactant during growth was found to significantly alter the surface morphology and prevent undesired 3D islands at low temperatures. We also observed a preference for hillocks over step morphology at high growth temperatures, antistep bunching effects at intermediate temperatures, and the evolution from step-meandering to mound morphologies at low temperatures. This morphological divergence from the first series indicates that bismuth significantly increases in the 2D Erlich–Schwöebel potential barrier of the atomic terraces, inducing an uphill adatom flux that can smoothen the surface. Our findings demonstrate that bismuth surfactant can improve the surface morphology and film structure of low-temperature grown GaSb. Bismuth surfactant may also improve other homoepitaxial III-V systems grown in nonideal conditions.

Microlens arrays (MLA) are a critical component of avalanche photodiode (APD) technology, but their performance is rarely characterized or investigated independent of the photodetector. To understand how best to improve coupling, the MLA must be characterized and compared to other technologies on a fair and consistent basis. In order to investigate new designs and compare different types of microlens technologies, a custom microscope was built. This microscope was designed to image the MLA surface as well as its focused beam which required solutions to the following challenges: 6-axis manipulation of the sample, sub-micron positional resolution, perpendicular travel to the optical axis, brightfield imaging, multispectral imaging, image processing, uniform irradiance over the sample area, and calibrated absolute optical power measurements. This microscope design enables the focal length range to be mapped with sub-micron accuracy along the paraxial ray. The imaged beam spot is quantitatively analyzed in MATLAB for 1/e2 diameter and encircled energy, as well as qualitatively with complementary data sets. It was shown that Fresnel lens and plano-convex refractive MLA designs from various material groups could be compared equally in a side-by-side comparison.

Iridium (Ir) is a refractory metal commonly seen in industrial applications, but has great potential for optical applications including metasurfaces. Metasurfaces are used to control the optical properties of an interface via sub-wavelength surface structures. These patterns require sharply defined features to create precise optical phase interactions. For high-temperature environments, most materials are insufficient candidates for metasurfaces because the sharpness of the surface structures are lost due to edge-rounding or oxidation. Ir is better suited for metasurface applications in high-temperature environments but the patterning of Ir using nanofabrication techniques has not been thoroughly investigated. In this work, Ir metasurfaces were fabricated and characterized for optical applications in the infrared.

Variable angle spectroscopic ellipsometry (VASE) was used to determine the thicknesses of polymethyl methacrylate (PMMA) on Si before and after etching with two different etchants (CF4 + O-2 and Argon). Once a complete optical model for a base PMMA on Si sample was created, it was applied to all etched samples to determine thicknesses. Despite some minor changes to the optical behavior of PMMA caused by the Ar etching, our ability to fit to observed interference peaks remained unaffected. This technique allows for nanometer accurate thickness measurements, which is an improvement from current thickness measurement methods such as stylus profilometry.

Variable‐angle spectroscopic ellipsometry is used to determine the room temperature complex refractive index of molecular beam epitaxy grown GaSb 1− x Bi x films with x ≤ 4.25% over a spectral range of 0.47–6.2 eV. By correlating to critical points in the extinction coefficient k , the energies of several interband transitions are extracted as functions of Bi content. The observed change in the fundamental bandgap energy ( E 0 , −36.5 meV per %Bi) agrees well with previously published values; however, the samples examined here show a much more rapid increase in the spin‐orbit splitting energy (Δ 0 , +30.1 meV per Bi) than previous calculations have predicted. As in the related GaAsBi, the energy of transitions involving the top of the valence band are observed to have a much stronger dependence on Bi content than those that do not, suggesting the valence band maximum is most sensitive to Bi alloying. Finally, the effects of surface droplets on both the complex refractive index and the critical point energies are examined.

We demonstrate that when highly lattice-mismatched GaSb layers are grown on GaAs(111)A substrates, the strain can be relieved by a self-assembled array of interfacial misfit (IMF) dislocations. This 2D array consists of periodically spaced, pure 60 degrees dislocations that lie in the plane of the GaSb/GaAs(111)A interface. The efficient strain relief provided by the IMF means that the GaSb exhibits good material quality, with threading dislocation densities in the 2-3 x 10(8) cm(-2) range. Other through-film defects associated with twinned GaSb(111) regions have densities between 0.2 and 2 x 10(8) cm(-2). The ability to grow GaSb on GaAs substrates with a (111) orientation creates research opportunities for the integration of dissimilar materials.

Two- or three-dimensionally patterned subwavelength structures, also known as metamaterials, have the advantage of arbitrarily engineerable optical properties. In thermophotovoltaic (TPV) applications, metamaterials are commonly used to optimize the emitter’s radiation spectrum for various source temperatures. The output power of a TPV device is proportional to the photon flux, which is proportional to the emitter size. However, using 2D or 3D metamaterials imposes challenges to realizing large emitters since fabricating their subwavelength features typically involves complicated fabrication processes and is highly time-consuming. In this work, we demonstrate a large-area (78 cm2) thermal emitter. This emitter is simply fabricated with one-dimensional layers of silicon (Si) and chromium (Cr), and therefore, it can be easily scaled up to even larger sizes. The emissivity spectrum of the emitter is measured at 802 K, targeting an emission peak in the mid-infrared. The emissivity peak is ∼0.84 at the wavelength of 3.75 μm with a 1.2 μm bandwidth. Moreover, the emission spectrum of our emitter can be tailored for various source temperatures by changing the Si thickness. Therefore, the results of this work can lead to enabling TPV applications with higher output power and lower fabrication cost.

We report on changes in Ge1−xSnx films (0.065 ≤ x ≤ 0.144) after short high-temperature anneals. Films were grown by molecular beam epitaxy on (001) Ge wafers, rapidly annealed, and characterized by x-ray diffraction, Raman spectroscopy, and optical microscopy. Sn segregated to the surface after a maximum temperature is inversely related to the Sn content. Lower content films showed little to no improvement in crystal quality below segregation temperatures, while higher content and partially relaxed films demonstrated improved uniformity for moderate annealing.

The bandgap of germaniumtin alloys extends from 0.8 e V into the metallic as the Sn composition increases, which overlaps the strongly absorbing range of the dominant 111- V systems used for thermophotovoltaic (TPV) power collection and for narrow bandgap diodes in multijunction PV cells. Si and Ge wafers are more physically resilient than III - V wafers and material abundance is higher than indium-containing cells. Si wafers are also available in larger formats, with a broader array of processing technologies. In fully relaxed GeSn films with greater than 6% Sn, the bandgap transitions to direct; in compressively strained films, the transition is higher. For high-stress and cost-limited applications, Group-IV devices have the potential to be a useful substitute for existing options. We designed a Gel-xSnx device and simulated its performance in Silvaco Atlas using material properties obtained from preliminary growths. We deposited the Gel-xSnx TPV device by molecular beam epitaxy, targeting 3 % Sn to achieve a similar bandgap to GaSb. Boron and antimony were used as p and n dopants. Material quality was characterized by XRD, spectroscopic ellipsometry, photoluminescence, and microscopy techniques. Component material electrical characterization was performed via Hall effect.

Due to its refractory properties and higher oxidation resistance, iridium (Ir) exhibits great potential for applications such as thermophotovoltaic emitters or contamination sensing. However, the lack of its temperature-dependent optical data prevents accurate modeling of Ir-based optical devices operating at higher temperatures. In this work, refractive indices of as-deposited and annealed Ir films, sputter-deposited, are characterized at between room temperature and 550°C over 300 nm to 15 µm of wavelength. The extinction coefficients of both as-deposited and annealed Ir films tend to decrease as temperature increases, with the exception of as-deposited Ir at 550°C due to significant grain growth. Under 530°C, optical constants of as-deposited Ir are less sensitive to temperature than those of annealed Ir. These characteristics of Ir films are correlated with their microstructural changes.

Thermophotovoltaic (TPV) devices enable energy harvesting from waste heat. The oxidation resistance of TPV emitters is one of the most important criteria for stable and sustainable applications of TPV devices. However, most TPV emitters in the literature are tested in nonoxidizing atmosphere. This indicates that those emitters may not be directly deployable without further testing in air. This work performs radiation and thermal stability tests of a reported TPV emitter in air.

As the majority of the input energy in power generation or energy consumption processes goes to waste as heat, thermophotovoltaic (TPV) devices enable energy recovery from (waste) heat. In TPV devices, the power output and conversion efficiency are impacted by thermal emitters. Since TPV devices operate at higher temperatures, emitters that can withstand hot environments without significant degradation of their emission performance are required. Refractory metals are commonly used as the emitter material due to their higher melting point and optical properties. This paper reviews physical and chemical properties of 15 refractory metals that may affect the emitter's performance at high temperatures: melting point, crystal structure, lattice constant, standard reduction potential, diffusion coefficient, Young's modulus, thermal expansion coefficient, and refractive index. Biological hazards and prices of the metals are also explored. Then, selective TPV emitters fabricated with the refractory metals are compared regarding their thermal stability. Finally, material properties are discussed toward achieving thermally robust TPV emitters.

Owing to their tunable electromagnetic properties, subwavelength structures such as metamaterials have enabled novel applications across fields of engineering. In particular, metal-insulator-metal (MIM) plasmonic metamaterials have demonstrated efficient light energy absorption based on localized surface plasmon resonances. Due to these properties, MIM plasmonic resonance structures present its potential applications for photon absorption or emission at elevated temperatures, such as thermophotovoltaics. However, majority of reported MIM plasmonic structures are built with materials with a lower melting point such as gold or silver. Therefore, there are needs to explore how refractory materials affect the resonance properties of MIM plasmonic structures for high-temperature applications. In this work, we numerically report MIM plasmonic metamaterials built with highly refractory materials. Based on finite-difference timedomain (FDTD) simulation results, light absorption of these metamaterials peaks as high as 99.9% at the wavelength of 8.3 μm. This strong, selective absorption is attributed to the localized surface plasmon resonance. The results of this study suggest that the applications of MIM plasmonic devices may be extended for higher-temperature environments.

Plasmonic metamaterials (MMs) have enabled their novel applications across numerous engineering fields ranging from photodetecting to bio-imaging, cancer diagnosis, and photovoltaics. However, as a vast majority of reported plasmonic MMs are based on materials with a relatively low melting point such as Au or Ag, their utilities are limited for lower-temperature applications. Thus, realizing plasmonic resonance based on refractory materials will further extend their applications to higher-temperature environments. In this work, we numerically report plasmonic MMs built with Ir and HfO2, which are well-known highly refractory materials. These plasmonic MMs resonate selectively based on the excitation of localized surface plasmon polaritons and magnetic resonances. The wavelength where resonance occurs can be modulated by changing the width of the top metal of the structure. Moreover, we find that the resonance wavelength and the top metal’s width are in a polynomial relationship over the wavelength range studied. Therefore, with the tunable properties, the refractory plasmonic MMs in this work present a potential to extend the utilities of plasmonic devices for higher-temperature applications.

Metamaterials (MMs) have created selective emitters for numerous wavelengths, owing to their engineerable electromagnetic properties. However, practical deployment of large-area MM emitters may be challenging due to their fabrication difficulty. In this work, we demonstrate wafer-sized selective emitters with refractory metal Ir based on optical interference effects. The simple structure of our emitters provides ease for large-area fabrication. Thus, the results of this study present a great potential to advance applications where large-area emitters are desired, such as thermophotovoltaic energy harvesting.