Scholarly Works, Geosciences

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On the role of inherited rock fabric in critical zone porosity development: Insights from seismic anisotropy measurements using surface waves
Eppinger, Benjamin J.; Holbrook, W. Steven; Flinchum, Brady A.; Grana, Dario; Richter, Daniel de B.; Hayes, Jorden L.; Riebe, Clifford S.; Harman, Ciaran J.; Carr, Bradley J. (Wiley, 2025-07)
Within Earth's critical zone, weathering processes influence landscape evolution and hillslope hydrology by creating porosity in bedrock, transforming it into saprolite and eventually soil. In situ weathering processes drive much of this transformation while preserving the rock fabric of the parent material. Inherited rock fabric in regolith makes the critical zone anisotropic, affecting its mechanical and hydrological properties. Therefore, quantifying and studying anisotropy is an important part of characterising the critical zone, yet doing so remains challenging. Seismic methods can be used to detect rock fabric and infer mechanical and hydrologic conductivity anisotropy across landscapes. We present a novel way of measuring seismic anisotropy in the critical zone using Rayleigh and Love surface waves. This method leverages multi-component surface seismic data to create a high-resolution model of seismic anisotropy, which we compare with a nuclear magnetic resonance log measured in a nearby borehole. The two geophysical data sets show that seismic anisotropy and porosity develop at similar depths in weathered bedrock and both reach their maximum values in saprolite, implying that in situ weathering enhances anisotropy while concurrently generating porosity in the critical zone. We bolster our findings with in situ measurements of seismic and hydrologic conductivity anisotropy made in a 3 m deep soil excavation. Our study offers a fresh perspective on the importance of rock fabric in the development and function of the critical zone and sheds new insights into how weathering processes operate.
Seismicity zoning at Coso geothermal field and stress changes from fluid production and migration
Tung, Sui; Kaven, Ole; Shirzaei, Manoochehr; Masterlark, Timothy; Wang, Herbert F.; Huang, Wei-Chuang; Feigl, Kurt L. (Elsevier, 2024-11-15)
The Coso geothermal field is a major geothermal power production site in the western United States. It has been observed that low-magnitude seismic events (M < 3.71) are unevenly distributed in three distinct zones, namely, nearfield (<3 km), midfield (3-6 km), and farfield (> 6 km) from the Coso geothermal plant. These zones exhibit distinct changes in earthquake location before and during geothermal production episodes that began in 1986. After 1986, the midfield region of the main flank experiences a significantly lower seismicity rate than the surrounding areas before production episodes. During 2014-2019, the farfield earthquakes cluster in the eastern and western parts of the greater Coso area, which is discernably different from how those pre-production earthquake events were distributed along the conjugate NW-SE and SW-NW trending structures across the main flank. Here, we analyze the stage of stress with finite-element-based poroelastic simulations to illustrate how the spatiotemporal evolution of the seismicity is associated with the pattern of stress perturbations caused by fluid migration amid the operations of geothermal power plants. Generally, similar to 70% of co-production seismicity is found in zones of increased Coulomb stress between 2014 and 2019 at >99% confidence. Meanwhile, the midfield zone of seismic paucity overlaps with the zone of decreasing pore-fluid pressure. Overall, the results provide a physical explanation of how decadal geothermal operations at Coso have perturbed stress-field changes and contributed to the evolving characteristic seismic pattern, shedding insights into assessing the seismic hazard in other geothermal settings.
A Cnidarian affinity for Salterella and Volborthella: implications for the evolution of shells
Vayda, Prescott J.; Xiao, Shuhai; Keller, Noah D.; Hagen, Amy P. I.; Strauss, Justin V.; Hagadorn, James W.; Lonsdale, Mary C.; Selly, Tara; Schiffbauer, James D. (Cambridge University Press, 2025-10-13)
The Cambrian Explosion saw the widespread development of mineralized skeletons. At this time, nearly every major animal phylum independently evolved strategies to build skeletons through either agglutination or biomineralization. Although most organisms settled on a single strategy, Salterella Billings, 1865 employed both strategies by secreting a biocalcitic exterior shell that is lined with layers of agglutinated sediments surrounding a central hollow tube. The slightly older fossil, Volborthella Schmidt, 1888, shares a similar construction with agglutinated grains encompassing a central tube but lacks a biomineralized exterior shell. Together these fossils have been grouped in the phylum Agmata Yochelson, 1977, although no phylogenetic relationship has been suggested to link them with the broader metazoan tree, which limits their contribution to our understanding of the evolution of shells in early animals. To understand their ecology and place them in a phylogenetic context, we investigated Salterella and Volborthella fossils from the Wood Canyon and Harkless formations of Nevada, USA, the Illtyd Formation of Yukon, Canada, and the Shady Formation of Virginia, USA.Thin-section petrography, acid maceration, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, and X-ray tomographic microscopy were used to provide new insights into these enigmatic faunas. First, morphological similarities in the aperture divergence angle and ratio of central tube diameter to agglutinated layer thickness suggest Salterella and Volborthella are related. Second, both fossils exhibit agglutinated grain compositions that are distinctive from their surrounding environments and demonstrate selectivity on the part of their producers. Finally, the calcitic shell composition and simple layers of blocky prismatic shell microstructure in Salterella suggest a possible cnidarian affinity. Together these data point to these organisms being sessile, semi-infaunal filter or deposit feeders and an early experimentation in cnidarian biomineralization chronicling a hypothesized transition from an organic sheath in Volborthella to a biomineralized shell in Salterella.
Tapping into the success of Science on Tap New River Valley: Lessons learned from eight years of a community-centered science outreach program
Hagen, Amy P. I.; DeNunzio, Maria; Kroehler, Carolyn J.; Chen, S.; Goh, Ying-Xian; Burke, K. L.; Oyedele, E.; Hockman, C.; Vayda, P.; Bracci, Nicole R.; Pagani, M.; Good, Deborah J.; Pfeiffer, Douglas G.; Allen, E.; Raun, Patricia (2025-10)
Communicating scientific research to the public through outreach programs is beneficial but faces many challenges. These include impediments to designing and implementing sustainable outreach programming as well as communication challenges between scientists and audience. Here we present Science on Tap New River Valley (NRV), a community-centered monthly science outreach program designed to reduce barriers to effective science outreach and share research with residents of the New River Valley in Virginia. The program places special emphasis on interactivity with community members and provides support for speakers to ensure that this goal is met. With a relatively small budget and a primarily graduate student-led volunteer organizing team, Science on Tap NRV serves as a model of a sustainable outreach program that provides opportunities for community members to engage with scientists, researchers to build and practice their science communication skills, and graduate students to learn to coordinate and implement effective outreach events. We share the details of the factors that have contributed to the success of Science on Tap NRV since 2017.
Precambrian Earth: Co-evolution of life and geodynamics
Westall, Frances; Xiao, Shuhai (Elsevier, 2024-11-01)
The Precambrian covers 80% of the history the Earth. In this timespan, the Earth developed from an anaerobic planet to the oxygenic planet dominated by Wilson-style plate tectonics that we know today. Concomitant with geological evolution, life emerged and evolved, gradually colonising all known aqueous habitats. Until the Palaeoarchaean, life was largely dominated by its geological environment. However, as of the Mesoarchaean, when there were major changes in geodynamics leading to continental erosion and runoff of essential nutrients, the effects of life started to impinge on the geological environment. The interaction of life and Earth was and is reciprocal, hence the term biogeodynamics. In this review, we trace the evolution of geology and life in parallel, thus highlighting the gradual buildup of the importance of life on terrestrial processes, and the importance of changes in the geological environment on the evolution of life. We do not attempt to make an exhaustive review of all the occurrences of life in the Precambrian but use selected examples to illustrate key events and changes. We conclude by addressing certain aspects of the evolution of life that require more in-depth study and show how the finding of extra-terrestrial life would advance our understanding of life on Earth.
Osteohistological signal from the smallest known phytosaur femur reveals slow growth and new insights into the evolution of growth in Archosauria
Goldsmith, Erika R.; Barta, Daniel E.; Kligman, Ben T.; Nesbitt, Sterling J.; Marsh, Adam D.; Parker, William G.; Stocker, Michelle R. (Wiley, 2024-12-03)
Fossils of embryonic and hatchling individuals can provide invaluable insight into the evolution of prenatal morphologies, heterochronies, and allometric trajectories within Archosauria but are exceptionally rare in the Triassic fossil record, obscuring a critical aspect of archosaurian biology during their evolutionary origins. Microvertebrate sampling at a single bonebed in the Upper Triassic Chinle Formation within Petrified Forest National Park has yielded diminutive archosauriform femora (PEFO 45274, PEFO 45199) with estimated and measured femoral lengths of similar to 31 mm and similar to 37 mm, respectively. These new specimens provide the unique opportunity to assess the preservation, body size, and growth dynamics of skeletally immature archosauriforms in North America and compare the growth dynamics of archosauromorphs within an evolutionary and ontogenetic context. We assign PEFO 45199 and PEFO 45274 to Phytosauria (Archosauriformes) based on their strongly sigmoidal shape in lateral view, the presence of proximal anterolateral and posteromedial tubera, the absence of an anteromedial tuber of the proximal end, a teardrop-shaped proximal outline, and a fourth trochanter that is not confluent with the proximal head. Osteohistological analyses of PEFO 45274 reveal a cortex comprising low vascularity, parallel-fibered bone composed of primary osteons that lacks a hatching line and any lines of arrested growth. We interpret PEFO 45274 as a slow-growing, post-hatching individual of less than 1 year of age. Surprisingly, osteohistology of some larger phytosaur femora implies faster growth rates in comparison to PEFO 45274 based on the occasional presence of woven bone and overall higher degrees of vascular density, suggesting the ontogenetic shift from rapid-to-slow growth rates might not occur simply or uniformly as expected in Phytosauria and that non-archosaurian archosauriforms may exhibit size-dependent histological characteristics. This study highlights the importance of including osteohistology from multiple body sizes to investigate non-archosaurian archosauriform ancestral growth rates given the phylogenetic position of phytosaurs near the divergence of Archosauria.
Thermodynamics of calcium binding to heparin: Implications of solvation and water structuring for polysaccharide biofunctions
Knight, Brenna M.; Gallagher, Connor M. B.; Schulz, Michael D.; Edgar, Kevin J.; McNaul, Caylyn D.; McCutchin, Christina A.; Dove, Patricia M. (National Academy of Sciences, 2025-08-26)
Heparin sulfates are found in all animal tissues and have essential roles in living systems. This family of biomacromolecules modulates binding to calcium ions (Ca²⁺) in low free energy reactions that influence biochemical processes from cell signaling and anticoagulant efficacy to biomineralization. Despite their ubiquity, the thermodynamic basis for how heparans and similarly functionalized biomolecules regulate Ca²⁺ interactions is not yet established. Using heparosan (Control) and heparins with different positions of sulfate groups, we quantify how SO₃⁻ and COO⁻ content and SO₃⁻ position modulate Ca²⁺ binding by isothermal titration calorimetry. The free energy of all heparin-Ca²⁺ interactions (ΔGrxn) is dominated by entropic contributions due to favorable water release from polar, hydrophilic groups. Heparin with both sulfate esters (O-SO₃⁻) and sulfamides (N-SO₃⁻) has the strongest binding to Ca²⁺ compared to heparosan and to heparin with only O-SO₃⁻ groups (~3X). By linking Ca²⁺ binding thermodynamics to measurements of the interfacial energy for calcite (CaCO₃) crystallization onto polysaccharides, we show molecule-specific differences in nucleation rate can be explained by differences in water structuring during Ca²⁺ interactions. A large entropic term (-TΔSrxn) upon Ca²⁺–polysaccharide binding correlates with high interfacial energy to CaCO₃ nucleation. Combining our measurements with literature values indicates many Ca²⁺–polysaccharide interactions have a shared thermodynamic signature. The resulting enthalpy–entropy compensation relationship suggests these interactions are generally dominated by water restructuring involving few conformational changes, distinct from Ca²⁺–protein binding. Our findings quantify the thermodynamic origins of heparin-specific interactions with Ca²⁺ and demonstrate the contributions of solvation and functional group position during biomacromolecule-mediated ion regulation.
Sinking Airports: A Glance at the State of US Transport Infrastructure
Dasho, Oluwaseyi; Shirzaei, Manoochehr (American Geophysical Union, 2025-07-28)
Land subsidence poses a growing challenge to the operational safety and structural integrity of global air transport infrastructure. This study assesses the impact of differential land subsidence on airport runways using cutting‐edge Interferometric Synthetic Aperture Radar (InSAR) data across 15 major U.S. airports, providing an estimate of potential foundational damage caused by settlement due to natural and anthropogenic factors. Our findings show San Francisco International Airport experiences the fastest subsidence rate of 9.2 ± 0.2 mm/year, while Los Angeles International Airport has the slowest subsidence rate of 2.0 ± 0.2 mm/year. While 96.1% of runway areas fall under low damage risk, 3.9% are at medium to veryhigh (VH) risk, with 3.5 million m2 exposed to subsidence rates exceeding 5 mm/year and 13,950 m2 classified as being at high to VH damage risk. Although no accidents have been directly linked to subsidence, increasing maintenance costs underscore the need for proactive monitoring. InSAR provides a near real‐time, costeffective solution for detecting infrastructure vulnerabilities, offering a non‐intrusive approach to enhancing airport resilience and operational safety.
Insights into the in-situ degradation and fragmentation of macroplastics in a low-order riverine system
Gray, Austin D.; Gore, Beija; Gaesser, Megan; Sequeira, Luisana Rodriguez; Thibodeau, Tessa; Montgomery, Allison; Purvis, Sam; Ouimet, Kathryn; Dura, Tina; Mayer, Kathleen (Oxford University Press, 2025-05)
Inland riverine systems are major conduits of microplastics to coastal environments. Plastic materials that pass through riverine systems are subjected to various degradation processes that facilitate their fragmentation into microplastics (MPs). Low-order streams, a critical yet understudied part of river networks, significantly influence the fate and transport of MPs. Here, we investigate the in situ degradation of common macroplastic polymers (e.g., low-density polyethylene, polyethylene terephthalate, and polystyrene) and their fragmentation into MPs in urban and forested streams. We deployed macroplastic items and a natural biodegradable polymer (cellulose) into a stream habitat for 52 weeks. We found that regardless of stream type (forested or urban), macroplastic polymers produced MPs in two weeks, with polystyrene having the highest fragmentation rate (8 particles/week). We explored several degradation indices (carboxyl index, hydroxyl index, and vinyl index), which revealed that photooxidation played a role in macroplastic degradation over time. Another driver of degradation was biofilm formation observed on the surface of all items, mainly composed of diatoms. Lastly, we found that field-aged macroplastics can leach plastic-derived dissolved organic. Our study narrows the knowledge gap regarding MP degradation and fragmentation in freshwater by providing real-time in situ data on the rate of polymer fragmentation in a low-order riverine system.
Land subsidence risk to infrastructure in US metropolises
Ohenhen, Leonard O.; Zhai, Guang; Lucy, Jonathan; Werth, Susanna; Carlson, Grace; Khorrami, Mohammad; Onyike, Florence; Sadhasivam, Nitheshnirmal; Tiwari, Ashutosh; Ghobadi-Far, Khosro; Sherpa, Sonam F.; Lee, Jui-Chi; Zehsaz, Sonia; Shirzaei, Manoochehr (Springer Nature, 2025-05-08)
Land subsidence is a slow-moving hazard with adverse environmental and socioeconomic consequences worldwide. While often considered solely a coastal hazard due to relative sea-level rise, subsidence also threatens inland urban areas, causing increased flood risks, structural damage and transportation disruptions. However, spatially dense subsidence rates that capture granular variations at high spatial density are often lacking, hindering assessment of associated infrastructure risks. Here we use space geodetic measurements from 2015 to 2021 to create high-resolution maps of subsidence rates for the 28 most populous US cities. We estimate that at least 20% of the urban area is sinking in all cities, mainly due to groundwater extraction, affecting ~34 million people. Additionally, more than 29,000 buildings are located in high and very high damage risk areas, indicating a greater likelihood of infrastructure damage. These datasets and information are crucial for developing ad hoc policies to adapt urban centers to these complex environmental challenges.
Increased flood exposure in the Pacific Northwest following earthquake-driven subsidence and sea-level rise
Dura, Tina; Chilton, William; Small, David; Garner, Andra J.; Hawkes, Andrea; Melgar, Diego; Engelhart, Simon E.; Staisch, Lydia M.; Witter, Robert C.; Nelson, Alan R.; Kelsey, Harvey M.; Allan, Jonathan C.; Bruce, David; DePaolis, Jessica; Priddy, Michael; Briggs, Richard W.; Weiss, Robert; La Selle, SeanPaul; Willis, Michael; Horton, Benjamin P. (2025-05)
Climate-driven sea-level rise is increasing the frequency of coastal flooding worldwide, exacerbated locally by factors like land subsidence from groundwater and resource extraction. However, a process rarely considered in future sea-level rise scenarios is sudden (over minutes) land subsidence associated with great (>M8) earthquakes, which can exceed 1 m. Along the Washington, Oregon, and northern California coasts, the next great Cascadia subduction zone earthquake could cause up to 2 m of sudden coastal subsidence, dramatically raising sea level, expanding floodplains, and increasing the flood risk to local communities. Here, we quantify the potential expansion of the 1% floodplain (i.e., the area with an annual flood risk of 1%) under low (~0.5 m), medium (~1 m), and high (~2 m) earthquake-driven subsidence scenarios at 24 Cascadia estuaries. If a great earthquake occurred today, floodplains could expand by 90 km² (low), 160 km² (medium), or 300 km² (high subsidence), more than doubling the flooding exposure of residents, structures, and roads under the high subsidence scenario. By 2100, when climate-driven sea-level rise will compound the hazard, a great earthquake could expand floodplains by 170 km² (low), 240 km² (medium), or 370 km² (high subsidence), more than tripling the flooding exposure of residents, structures, and roads under the high subsidence scenario compared to the 2023 floodplain. Our findings can support decision-makers and coastal communities along the Cascadia subduction zone as they prepare for compound hazards from the earthquake cycle and climate-driven sea-level rise and provide critical insights for tectonically active coastlines globally.
Estimating Wind Direction and Wind Speed Over Lakes With Surface Water Ocean Topography and Sentinel‐1 Satellite Observations
McQuillan, Katie A.; Allen, George H.; Fayne, Jessica; Gao, Huilin; Wang, Jida (American Geophysical Union, 2025-03-25)
Wind at the water‐air interface is an important driver of hydrologic and biogeochemical processes in lakes. Satellite synthetic aperture radar (SAR) is commonly used over the ocean to retrieve wind fields using backscatter coefficients which are sensitive to wind‐driven surface water roughness; however, its application to lakes has been largely unexplored. Here we assess the utility of SAR to retrieve wind fields specifically for lakes. We estimated wind direction from SAR backscatter using the Modified Local Gradient method for Surface Water Ocean Topography (SWOT) and Sentinel‐1 data. The estimated wind direction was then used as an input into a C‐band geophysical modeling function (GMF) to invert wind speed from Sentinel‐1 data. Comparisons between SWOT backscatter and in situ wind speeds were used to provide a foundation for understanding how SWOT could be used to study wind speeds. Using buoy data for validation, we found wind direction (1 km) mean absolute error (MAE) ranged from 31° to 40° for Sentinel‐1 and 28° to 38° for SWOT. Sentinel‐1 wind speed (100 m) MAE ranged from 1.05 to 2.09 m/s. These retrievals were more accurate and at higher resolution compared to global reanalysis dataset ERA5 (0.25°), with wind direction MAE from 23° to 50° and wind speed MAE from 1.49 to 2.35 m/s. SWOT backscatter sensitivity to wind speed depended on incidence angle, and demonstrated utility for developing a GMF for lakes. These methods could be used to better understand wind dynamics globally, especially over small lakes and in data poor regions.
Elucidating the role of InGaAs and InAlAs buffers on carrier dynamics of tensile strained Ge double heterostructures
Bhattacharya, Shuvodip; Johnston, Steven W.; Bodnar, Robert J.; Hudait, Mantu K. (ACS, 2024-06-06)
Extensive research efforts of strained germanium (Ge) are currently underway due to its unique properties, namely, (i) possibility of band gap and strain engineering to achieve a direct band gap, thus exhibiting superior radiative properties, and (ii) higher electron and hole mobilities than Si for upcoming technology nodes. Realizing lasing structures is vital to leveraging the benefits of tensile-strained Ge (ε-Ge). Here, we use a combination of different analytical tools to elucidate the effect of the underlying InGaAs/InAlAs and InGaAs overlaying heterostructures on the material quality and strain state of ε-Ge grown by molecular beam epitaxy. Using X-ray analysis, we show the constancy of tensile strain in sub-50 nm ε-Ge in a quantum-well (QW) heterostructure. Further, effective carrier lifetime using photoconductive decay as a function of buffer type exhibited a high (low) defect-limited carrier lifetime of ∼68 ns (∼13 ns) in 0.61% (0.66%) ε-Ge grown on an InGaAs (InAlAs) buffer. These results correspond well with the measured surface roughness of 1.289 nm (6.303 nm), consistent with the surface effect of the ε-Ge/III–V heterointerface. Furthermore, a reasonably high effective lifetime of ∼78 ns is demonstrated in a QW of ∼30 nm 1.6% ε-Ge, a moderate reduction from ∼99 ns in uncapped ε-Ge, alluding to the surface effect of the overlying heterointerface. Thus, the above results highlight the prime quality of ε-Ge that can be achieved via III–V heteroepitaxy and paves a path for integrated Ge photonics.
Carrier Recombination Dynamics of Surface Passivated Epitaxial (100)Ge, (110)Ge, and (111)Ge Layers by Atomic Layer Deposited Al₂O₃
Hudait, Mantu K.; Johnston, Steven W.; Das, Manash R.; Karthikeyan, Sengunthar; Sahu, Partha P.; Das, Jagat; Zhao, Jing; Bodnar, Robert J.; Joshi, Rutwik (ACS, 2023-05-24)
Germanium (Ge) and its heterostructures with compound semiconductors offer a unique optoelectronic functionality due to its pseudo-bandgap nature, that can be transformed to a direct bandgap material by providing strain and/or mixing with tin. Moreover, two crystal surfaces, (100)Ge and (110)Ge, that are technologically important for ultralow power fin or nanosheet transistors, could offer unprecedented properties with reduced surface defects after passivating these surfaces by atomic layer deposited (ALD) dielectrics. In this work, the crystallographically oriented epitaxial Ge/AlAs heterostructures were grown and passivated with ALD Al2O3 dielectrics, and the microwave photoconductive decay (μ-PCD) technique was employed to evaluate carrier lifetimes at room temperature. The X-ray photoelectron spectroscopy analysis reveals no role of orientation effect in the quality of the ALD Al2O3 dielectric on oriented Ge layers. The carrier lifetimes measured using the μ-PCD technique were benchmarked against unpassivated Ge/AlAs heterostructures. Excitation wavelengths of 1500 and 1800 nm with an estimated injection level of ∼1013 cm-3 were selected to measure the orientation-specific carrier lifetimes. The carrier lifetime was increased from 390 ns to 565 ns for (100)Ge and from 260 ns to 440 ns for (110)Ge orientations with passivation, whereas the carrier lifetime is almost unchanged for (111)Ge after passivation. This behavior indicates a strong dependence of the measured lifetime on surface orientation and surface passivation. The observed increase (>1.5×) in lifetime with Al2O3-passivated (100)Ge and (110)Ge surfaces is due to the lower surface recombination velocity compared to unpassivated Ge/AlAs heterostructures. The enhancement of carrier lifetime from passivated Ge/AlAs heterostructures with (100)Ge and (110)Ge surface orientations offers a path for the development of nanoscale transistors due to the reduced interface state density.
Lattice matched GeSn/InAlAs heterostructure: Role of Sn in energy band alignment, atomic layer diffusion and photoluminescence
Karthikeyan, Sengunthar; Joshi, Rutwik; Zhao, Jing; Bodnar, Robert J.; Magill, Brenden A.; Pleimling, Yannick; Khodaparast, Giti A.; Hudait, Mantu K. (Royal Society Chemistry, 2023-07-20)
Germanium alloyed with α-tin (GeSn) transitions to a direct bandgap semiconductor of significance for optoelectronics. It is essential to localize the carriers within the active region for improving the quantum efficiency in a GeSn based laser. In this work, epitaxial GeSn heterostructure material systems were analyzed to determine the band offsets for carrier confinement: (i) a 0.53% compressively strained Ge0.97Sn0.03/AlAs; (ii) a 0.81% compressively strained Ge0.94Sn0.06/Ge; and (iii) a lattice matched Ge0.94Sn0.06/In0.12Al0.88As. The phonon modes in GeSn alloys were studied using Raman spectroscopy as a function of Sn composition, that showed Sn induced red shifts in wavenumbers of the Ge-Ge longitudinal optical phonon mode peaks. The material parameter b representing strain contribution to Raman shifts of a Ge0.94Sn0.06 alloy was determined as b = 314.81 ± 14 cm−1. Low temperature photoluminescence measurements were performed at 79 K to determine direct and indirect energy bandgaps of Eg,Γ = 0.72 eV and Eg,L = 0.66 eV for 0.81% compressively strained Ge0.94Sn0.06, and Eg,Γ = 0.73 eV and Eg,L = 0.68 eV for lattice matched Ge0.94Sn0.06 epilayers. Chemical effects of Sn atomic species were analyzed using X-ray photoelectron spectroscopy (XPS), revealing a shift in Ge 3d core level (CL) spectra towards the lower binding energy affecting the bonding environment. Large valence band offset of ΔEV = 0.91 ± 0.1 eV and conduction band offset of ΔEC,Γ-X = 0.64 ± 0.1 eV were determined from the Ge0.94Sn0.06/In0.12Al0.88As heterostructure using CL spectra by XPS measurements. The evaluated band offset was found to be of type-I configuration, needed for carrier confinement in a laser. In addition, these band offset values were compared with the first-principles-based calculated Ge/InAlAs band alignment, and it was found to have arsenic up-diffusion limited to 1 monolayer of epitaxial GeSn overlayer, ruling out the possibility of defects induced modification of band alignment. Furthermore, this lattice matched GeSn/InAlAs heterostructure band offset values were significantly higher than GeSn grown on group IV buffer/substrates. Therefore, a lattice matched GeSn/InAlAs material system has large band offsets offering superior carrier confinement to realize a highly efficient GeSn based photonic device.
High-κ Gate Dielectric on Tunable Tensile Strained Germanium Heterogeneously Integrated on Silicon: Role of Strain, Process, and Interface States
Hudait, Mantu K.; Clavel, Michael B.; Karthikeyan, Sengunthar; Bodnar, Robert J. (ACS, 2023-08-17)
Tensile strained germanium (ϵ-Ge) layers heterogeneously integrated on Si substrates are of technological importance for nanoscale transistors and photonics. In this work, the tunable tensile strained (0% to 1.2%) ϵ-Ge layers were grown by solid source molecular beam epitaxy using GaAs and linearly graded InxGa1-xAs as intermediate buffers, and their structural and metal-oxide semiconductor capacitor (MOS-Cs) properties were analyzed as a function of strain and process conditions. X-ray topography measurements displayed no visible thermal crack and a low thermal stress of ∼50 MPa. Temperature dependent strain relaxation properties, studied by X-ray and Raman analyses, confirmed that the tensile strain amount of 1.2% was well preserved within the ϵ-Ge layer when annealed up to 550 °C. Further, transmission electron microscopic study revealed a good quality 1.2% strained ϵ-Ge/In0.17Ga0.83As heterointerface. In addition, unstrained Ge (0% ϵ-Ge) MOS-Cs with atomic layer deposited Al2O3 and thermally grown GeO2 composite gate dielectrics of varying oxidation times (0.25-7.5 min) at 550 °C exhibited a low interface state density (Dit) of ∼2.5 × 1011 eV-1 cm-2 at 5 min oxidation duration. The minimum oxidation time needed for good capacitance-voltage (C-V) characteristics on 0.2% ϵ-Ge is inadequate to accomplish similar C-V characteristics on 1.2% ϵ-Ge MOS-C, due to the higher strain field impeding the formation of the GeO2 interface passivation layer at lower oxidation duration. In addition, with the trade-off between the minimum Dit and minimum equivalent oxide thickness values, ∼1.5 min is found to be an optimum oxidation time for good quality 1.2% ϵ-Ge MOS-C. The minimum Dit values of 1.36 × 1011 and 2.06 × 1011 eV-1 cm-2 for 0.2% and 1.2% ϵ-Ge, respectively, were determined for 4 nm Al2O3 with 5 min thermal oxidation at 550 °C. Therefore, the successful monolithic integration of tunable tensile strain Ge on Si with structural defects and MOS-Cs analyses offer a path for the development of tensile strained Ge-based nanoscale transistors.
GeSn-on-GaAs with photoconductive carrier lifetime >400 ns: role of substrate orientation and atomistic simulation
Karthikeyan, Sengunthar; Johnston, Steven W.; Gayakwad, Dhammapriy; Mahapatra, Suddhasatta; Bodnar, Robert J.; Zhao, Jing; Joshi, Rutwik; Hudait, Mantu K. (Royal Society Chemistry, 2024-04-04)
Group IV GeSn quantum material finds application in electronics and silicon-compatible photonics. Synthesizing these materials with low defect density and high carrier lifetime is a potential challenge due to lattice mismatch induced defects and tin segregation at higher growth temperature. Recent advancements in the growth of these GeSn materials on Si, Ge, GaAs, and with substrate orientations, demonstrated different properties using epitaxial and chemical deposition methods. This article addresses the effect of GaAs substrate orientation and misorientation on the materials’ properties and carrier lifetimes in epitaxial Ge0.94Sn0.06 layers. With starting GaAs substrates of (100)/2°, (100)/6°, (110) and (111)A orientations, Ge0.94Sn0.06 epitaxial layers were grown with an intermediate Ge buffer layer by molecular beam epitaxy and analyzed by several analytical tools. X-ray analysis displayed good crystalline quality, and Raman spectroscopy measurements showed blue shifts in phonon wavenumber due to biaxial compressive strain in Ge0.94Sn0.06 epilayers. Cross-sectional transmission electron microscopy analysis confirmed the defect-free heterointerface of Ge0.94Sn0.06/Ge/GaAs heterostructure. Minority carrier lifetimes of the unintentionally doped n-type Ge0.94Sn0.06 epilayers displayed photoconductive carrier lifetimes of >400 ns on (100)/6°, 319 ns on (100)/2°, and 434 ns on (110) GaAs substrate at 1500 nm excitation wavelength. On the other hand, Ge0.94Sn0.06 layer showed poor carrier lifetime on (111)A GaAs substrate. The observed differences in carrier lifetimes were correlated with the formation energy of the Ge on (100)/6° and (100)/2° GaAs heterointerface using Stillinger-Weber interatomic potential model-based atomistic simulation with different heterointerfacial bonding by Synopsys QuantumATK tool. Total energy computation of 6280-atom Ge/GaAs supercell on (100)/6° leads to lower formation energy than (100)/2°, making it more thermodynamically stable. Hence, the growth of the GeSn/III-V material system using misoriented (100) substrates that are more thermodynamically stable will enhance the performances of optoelectronic devices.
Elucidating the role of InGaAs and InAlAs buffers on carrier dynamics of tensile strained Ge double heterostructures
Bhattacharya, Shuvodip; Johnston, Steven W.; Bodnar, Robert J.; Hudait, Mantu K. (American Chemical Society, 2024-06-06)
Extensive research efforts of strained germanium (Ge) are currently underway due to its unique properties, namely, (i) possibility of band gap and strain engineering to achieve a direct band gap, thus exhibiting superior radiative properties, and (ii) higher electron and hole mobilities than Si for upcoming technology nodes. Realizing lasing structures is vital to leveraging the benefits of tensile-strained Ge (ϵ-Ge). Here, we use a combination of different analytical tools to elucidate the effect of the underlying InGaAs/InAlAs and InGaAs overlaying heterostructures on the material quality and strain state of ϵ-Ge grown by molecular beam epitaxy. Using X-ray analysis, we show the constancy of tensile strain in sub-50 nm ϵ-Ge in a quantum-well (QW) heterostructure. Further, effective carrier lifetime using photoconductive decay as a function of buffer type exhibited a high (low) defect-limited carrier lifetime of ∼68 ns (∼13 ns) in 0.61% (0.66%) ϵ-Ge grown on an InGaAs (InAlAs) buffer. These results correspond well with the measured surface roughness of 1.289 nm (6.303 nm), consistent with the surface effect of the ϵ-Ge/III-V heterointerface. Furthermore, a reasonably high effective lifetime of ∼78 ns is demonstrated in a QW of ∼30 nm 1.6% ϵ-Ge, a moderate reduction from ∼99 ns in uncapped ϵ-Ge, alluding to the surface effect of the overlying heterointerface. Thus, the above results highlight the prime quality of ϵ-Ge that can be achieved via III-V heteroepitaxy and paves a path for integrated Ge photonics.
Mapping the Ge/InAl(Ga)As interfacial electronic structure and strain relief mechanism in germanium quantum dots
Hudait, Mantu K.; Bhattacharya, S.; Karthikeyan, S.; Zhao, J.; Bodnar, Robert J.; Magill, Brenden A.; Khodaparast, Giti A. (Royal Society Chemistry, 2024-09-12)
Tensile-strained germanium (ϵ-Ge) has attracted significant interest due to its unique properties in emerging optoelectronic devices. High tensile-strained Ge materials with superior quality are still being investigated due to the intrinsic instability of ϵ-Ge against the formation of stacking faults (SFs). This work seeks to improve the understanding of these limits by closely examining, experimentally, the mechanisms by which tensile strain is relaxed in Ge. Here, ϵ-Ge layers were grown on highly mismatched In0.53Ga0.47As and In0.51Al0.49As virtual substrates (f = 3.4%), formed as quantum dots (QDs) by molecular beam epitaxy, and their strain relaxation mechanism was analyzed. Both In0.51Al0.49As and In0.53Ga0.47As growth templates were created using an Al0.49In0.51x(Ga0.51)1−xAs linearly graded metamorphic buffer on GaAs(001)/2° and InP(001)/0.5° substrates, respectively. Fully 3D growth (Volmer-Weber growth mode) due to high tensile strain resulted in Ge QDs with an average diameter and height of ∼50 nm and ∼20 nm, respectively, and a uniform density of ∼320 μm−2. Analysis of the interfacial electronic structure using high-resolution transmission electron microscopy collected from the Ge QDs indicated that minimal tensile strain was retained in Ge due to SF formation, corroborated via the Raman results. All Ge QDs contain multiple SFs of the close-packed {111} planes nucleated by Shockley partial dislocations with Burger vectors b = ⅙〈112〉. The presence of additional misfit dislocations at the Ge/In0.51Al0.49As or Ge/In0.53Ga0.47As heterointerface, not associated with SFs, indicates further relaxation by perfect dislocations with Burger vectors b = ½〈110〉. The tensile misfit of 3.4% in Ge revealed instability against SF formation, and the availability of a defect type must have the effect of lowering the critical layer thickness for ϵ-Ge layers. Thus, the above results suggest that a maximum tensile strain amount >3.4% is not achievable in Ge without the formation of Shockley partial dislocations.
A framework for developing a real-time lake phytoplankton forecasting system to support water quality management in the face of global change
Carey, Cayelan C.; Calder, Ryan S. D.; Figueiredo, Renato J.; Gramacy, Robert B.; Lofton, Mary E.; Schreiber, Madeline E.; Thomas, R. Quinn (Springer, 2024-09-20)
Phytoplankton blooms create harmful toxins, scums, and taste and odor compounds and thus pose a major risk to drinking water safety. Climate and land use change are increasing the frequency and severity of blooms, motivating the development of new approaches for preemptive, rather than reactive, water management. While several real-time phytoplankton forecasts have been developed to date, none are both automated and quantify uncertainty in their predictions, which is critical for manager use. In response to this need, we outline a framework for developing the first automated, real-time lake phytoplankton forecasting system that quantifies uncertainty, thereby enabling managers to adapt operations and mitigate blooms. Implementation of this system calls for new, integrated ecosystem and statistical models; automated cyberinfrastructure; effective decision support tools; and training for forecasters and decision makers. We provide a research agenda for the creation of this system, as well as recommendations for developing real-time phytoplankton forecasts to support management.

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