PCSI2026 Session PCSI-MoA2: Wide Band Gap Semiconductor & DUV Lithography

A new deposition method developed by Goddard Space Flight Center fluorinates the surfaces of freshly evaporated Al mirrors by exposure to XeF2. This produces an ultrathin film of aluminum fluoride which is thought to conformally coat the aluminum. This is followed by an evaporated LiF coating to create a broadband, relatively stable mirror which is termed Al+XeLiF. This in-situ, room-temperature process produces mirrors with noteworthy environmental stability and high reflectance over a broad spectral range from the FUV to the IR, including the desirable, but hard to achieve, 100 to 115 nm range. NASA may use this approach to prepare mirrors and grating coatings for the future Habitable Worlds Observatory. To accomplish its primary mission of imaging and characterizing potentially habitable exoplanets the HWO requires total extinction of the light from an exoplanet’s star to capture the light of dim exoplanets without contaminating starlight. When used with an internal coronograph, however, surface roughness and polarization effects from the telescope mirrors could result in starlight leaking into the coronograph’s dark pit preventing clear detection and characterization of Earth-size planets in a star’s habitable zone. We studied retardance and diattenuation using variable-angle, spectroscopic ellipsometry (VASE) to better understand these effects on lithium-fluoride-coated aluminum mirrors exposed to different temperature/humidity environments.

The physical evolution of roughness on deposited fluoride surfaces is noteworthy. Fluorides, particularly LiF, are observed to roughen in moist air even at relatively low humidity. Water condenses out of the air in any small defects due to Laplace pressure. Condensed water will attack the film. Then, in an Oswald ripening-type process, larger crystals form at the expense of nanometer size crystals. This creates defects even more favorable for the condensation of moisture and the eventual destruction of the film.

The dielectric response of cubic crystals, like Si, Ge, GaAs, InSb, and CaF₂, is commonly assumed to be isotropic. This assumption stems from the fact that, at optical frequencies, the wavelength of light is much larger than characteristic distances inside the crystal (usually of the order of interatomic distances), a/λ<<1. Within this limit, the macroscopic material response is local. This description breaks down mainly in the spectral region near excitations, where the refractive index increases and the ratio a/λ becomes non-negligible. Keeping non-local terms in the dielectric tensor explains the weak anisotropy observed in cubic crystals [1]. The intrinsic birefringence has caused many problems in the semiconductor industry, where cubic crystals are widely used as substrates for layer growth. Substrate anisotropy can make optical characterization of the layers challenging. Another example is the failure of 157nm lithography, where the crystals used for UV laser optics, CaF₂ and BaF₂, exhibit unacceptably large birefringence [2].

In this work, we utilize high-sensitivity Mueller matrix polarimetry to measure the weak anisotropy of cubic crystals. This approach has recently been shown to work well for reflection measurements on silicon [3]. We also present our transmission experimental results that show intrinsic birefringence of Si and CaF₂. In addition, we demonstrate the directional dependence of the intrinsic birefringence by examining crystals with different surface orientations. Rotational symmetries of the crystal axes are clearly visible (Fig. 1), indicating that the measured birefringence is intrinsic and not induced, e.g. by strain.

Particulate contamination requires dust mitigation techniques to provide low-scatter surfaces on sensitive instrumentation in space. We have previously shown that poly(olefin sulfone)s photodegrade in spacelike conditions: in vacuum and with UV light exposure. We now demonstrate that photodegradable polymers can reduce dust accumulation on optical surfaces for space applications. Our research shows that dissociative degradation of poly(olefin sulfone)s significantly decreased the number of dust particles on a dust-coated surface.

Our findings show a viable way to mitigate the collection of extraterrestrial dust on optical surfaces in space, enabling passive removal of particulate contamination without any direct human intervention.

Nitrides, including those grown by MBE, have exciting optical and electrical properties such as superconductivity¹ and magnetism² that make them important for many technological applications. It is often necessary to transfer nitride films between vacuum chambers for various processing and characterization steps, resulting in contamination that must be removed.³ Additionally, surface and interface studies are difficult due to oxygen, carbon, silicon, etc, contamination during exposure to atmosphere. In this talk we discuss the effectiveness and limitations of InN capping layers which are thermally desorbed in vacuum.

To demonstrate the InN capping and decapping procedure we will discuss synchrotron X-ray characterization of high Al-content AlGaN grown with different strain states, capped, and decapped under vacuum at the Advanced Photon Source (APS). InN decomposes at temperatures above 450°C, while AlGaN does not decompose until much higher temperatures (>800°C).³ We take advantage of this difference in decomposition temperature to utilize InN as a temporary, protective capping layer that can be removed in-situ to examine the properties of the underlying film surface without exposure to atmosphere. We first demonstrate the facile removal of the InN capping layer by examining the RHEED pattern before and after InN deposition and removal. Initial InN films were not smooth and uniform across the surface of the GaN films, as indicated by the spotty RHEED pattern and the visible surface roughness shown in Fig. 1. Growth parameters were tuned systematically to optimize the crystallinity and morphology of the capping layer for maximal coverage of the underlying films. This work serves as a proof of concept for this surface protection technique and can be used on other thin films that are more sensitive to oxidation through atmospheric exposure.

[1] B.J. May, S. Regmi, A.R. Khanolkar, V. Buturlim, Z.E. Cresswell, K.D. Vallejo, K. Gofryk, and D.H. Hurley, AIP Adv. 14, 125327 (2024).

[2] P. Wojciechowski and M. Lewandowski, Crystal Growth & Design, 22, 4618–4639 (2022).

[3] Z. J. Biegler, A.A. Allerman, J. S. Speck, Appl. Phys. Lett. 127, 072101 (2025)

[4] E. Dimakis, E. Iliopoulos, K. Tsagaraki, Th. Kehagias, Ph. Komninou, and A. Georgakilas, J. Appl. Phys. 97, 113520 (2005)

β-Ga₂O₃ has emerged as a leading candidate for next-generation power electronics, radio frequency (RF) switches, and extreme environment electronics due to a wide band gap (4.6 – 4.9 eV), high dopability (~40 meV activation energy for an isolated silicon donor), and melt growth characteristics resulting in commercially available 4-inch substrates and commercial demonstrations of 6-inch substrates by multiple techniques.

The (100) surface of Ga₂O₃ is highly desirable from a device and epitaxy standpoint – bulk growth of (100) material is more scalable than (010), the surface is nearly lattice-matched to p-type partner NiO, and Al₂O₃ incorporates at higher concentrations without phase separation. However, the epitaxial growth rate on (100) surfaces is less than 10% of other faces due to weak bonding and favorable desorption. Recent demonstrations have shown growth rate improvements from 0.4 nm/min to 1.5 nm/min by growing on (100) wafers that are offcut 6° in the -c direction.¹ These films show step-flow growth from (-201) step-edges and high electron mobility. Despite these exciting results, offcuts greater than 6° have not been explored due to the waste associated with grinding and polishing large offcuts.

In this talk we will discuss the molecular beam epitaxy (MBE) growth and properties of β-Ga₂O₃ grown on (100) substrates offcut in the -c direction up to 13.4°. These large offcuts are enabled by edge-fed film-defined growth (EFG) where the offcut is grown into the surface by pulling the crystal through the EFG die with the seed crystal rotated by the desired offcut angle. We will demonstrate that 13.4° offcut substrates still exhibit a terraced (100) surface, and that a >10x increase (4.8 nm/min) in growth rate is achieved. As previously reported on lower offcuts, we observe 100% reversal of substrate twin domains around the (001) direction at the substrate-epilayer interface. We will discuss electrical properties including record-low (by MBE) unintentional doping densities of < 5E15 cm^-3.

This study presents a comprehensive characterization of the dynamic interplay between atomistic interface evolution in amorphous SiO₂ projection optics and device-scale optical performance drift under deep ultraviolet (DUV, 193nm) exposure. By integrating large-scale molecular dynamics (MD) simulations with machine learning force fields and density functional theory (DFT) calculations, we unveil how DUV-induced local compaction and defect evolution at interfacial regions modulate both refractive index (n) and wavefront aberrations in operational lithography systems. Specifically, our results demonstrate a quantitative correlation between MD-tracked Si-Si bond fraction/density metrics and DFT-calculated optical constants, establishing a structure-property relationship essential for device-level transport and optical characterization.

Fleet-scale, in-field monitoring of photolithography scanners confirms that progression of Y-direction Zernike coma (WFE, Coma-Y) is aligned with simulation-predicted compaction-driven refractive index shifts, cementing the impact of interface-level atomic changes on macroscopic optical device performance. Notably, the transition from elastic to plastic compaction regimes leads to nonlinear escalation in WFE drift slopes, providing actionable thresholds for predictive maintenance and device calibration.

This work bridges atomistic simulation and high-volume manufacturing field data to deliver direct, actionable insights for the characterization and management of light transport, wavefront propagation, and functional reliability in advanced optical devices.

Fermi-level pinning is a phenomenon that the Schottky barrier of metal/semiconductor junction exhibits weak dependence on the metal work-function. According to the previous study [1], the metal/graphene/Si junction exhibits strong Fermi-level pinning which is expected on an ideal metal/Si junction. It has been reported that the Fermi-level pinning of metal/SiC junction is relatively weak compared with the metal/Si junction due to the ionicity between atomic elements of crystalline structure [2]. With this background, we investigated the Fermi-level pinning in metal/graphene/4H-SiC junctions. The junction was fabricated by first epitaxially growing graphene on a 4H-SiC substrate with the metal-capping method under UHV environment [3] and then depositing circular metal (Al, Ni, Pt) electrodes onto the grown graphene layer. The Fermi-level pinning factor S was extracted from current-voltage (I-V) and capacitance-voltage (C-V) curves, signifying strong Fermi-level pinning. A theoretical model proposed by Kopylov et al. describing the charge transfer at the graphene/SiC interface provides a plausible explanation for the observed strong Fermi-level pinning [4].

[1]Hoon Hahn Yoon et al., Nano Letters 17(1), 44 (2017)
[2] Stephen Kurtin, T. C. McGill, and C. A. Mead, Physical Review Letters 22, 1433 (1969)
[3] Han Byul Jin et al.,Scientific Reports 5, 9615 (2015)
[4] Sergey Kopylov et al., Applied Physics Letters 97, 112109 (2010)

⁺Author for correspondence: kibogpark@unist.ac.kr
NRF-2023R1A2C1006519, NRF-2022M3K2A1083924

The (ultra)wide bandgap semiconductors gallium nitride (GaN) and aluminum gallium nitride (AlGaN) are the materials of choice for enabling power electronic devices with very high switching frequency and superior energy efficiency. Such devices are based on metal-oxide-semiconductor (MOS) gate stacks, where downscaling and leakage control require gate oxides with high dielectric constant, so-called high-k oxides, such as HfO₂ [1]. However, device performance and especially switching frequencies are often limited by the insufficient quality of the (Al)GaN/high-k interface. Ultrathin, conformal high-k layers can be synthesized using atomic layer deposition (ALD), where the choice of oxide material, pre-ALD cleaning methods, and ALD parameters strongly influence film and interface quality. Many important details about the physics and chemistry of the interface formation still remain unknown. Furthermore, until now all efforts to explore the high-k oxide film formation have been based on ex situ approaches, meaning that film deposition and characterization of the resulting interface occur in separate steps.

Here, we present a first time-resolved investigation of the ALD reactions of HfO₂ on (Al)GaN. For this, we implemented the ALD process in a synchrotron ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) setup [2]. Thus, we mapped surface chemistry and electronic properties in situ during subsequent ALD half-cycles, which consisted of tetrakisdimethylamido-hafnium (TDMA-Hf) and water deposition. We observed a rather inefficient first ALD cycle, compared to previous semicon-ductor ALD studies [2], which improved with increasing aluminum content. Thickness and chemical composition of the resulting Hf-oxide film varied strongly if the order of the precursors was changed. Both observations are against the established ligand-exchange ALD model and highlight the importance of in-depth studies for improving the quality of high-k layers on (Al)GaN.

In addition, we have used XPS to systematically investigate the electronic properties and chemical composition of the interface between different (Al)GaN substrates and HfO₂ or Al₂O₃ high-k oxide films, for different ALD temperatures, where HfO₂ resulted in less interfacial oxide. The choice of pre-ALD cleaning methods (HCl or HF etching) was also found to be of importance, which can enhance ALD efficiency but also result in interface contamination. We will discuss how our structural results can be implemented to improve device performance.

[1] P. Gribisch et al., IEEE Trans. Electron Dev. 70, 4101 (2023).
[2] G. D’Acunto et al., ACS Appl. El. Mat. 2, 3915 (2020).