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1. Composition Quantification of SiGeSn Alloys through Time-of-Flight Secondary Ion Mass Spectrometry: Calibration Methodologies and Validation with Atom Probe Tomography

   https://doi.org/10.1109/JSTQE.2024.3456439  

   Zhao, Haochen; Liu, Shang; Park, Suho; Feng, Xu; Zeng, Zhaoquan; Kolodzey, James; Yu, Shui-Qing; Liu, Jifeng; Zeng, Yuping (September 2024, IEEE Journal of Selected Topics in Quantum Electronics)

   Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) is a powerful technique for elemental compositional analysis and depth profiling of materials. However, it encounters the problem of matrix effects that hinder its application. In this work, we introduce a pioneering ToF-SIMS calibration method tailored for SixGeySnz ternary alloys. SixGe1-x and Ge1-zSnz binary alloys with known compositions are used as calibration reference samples. Through a systematic SIMS quantification study of SiGe and GeSn binary alloys, we unveil a linear correlation between secondary ion intensity ratio and composition ratio for both SiGe and GeSn binary alloys, effectively mitigating the matrix effects. Extracted relative sensitivity factor (RSF) value from SixGe1-x (0.07<0.83) and Ge1-zSnz (0.066<0.183) binary alloys are subsequently applied to those of SixGeySnz (0.011<0.113, 0.863<0.935 and 0.023<0.103) ternary alloys for elemental compositions quantification. These values are cross-checked by Atom Probe Tomography (APT) analysis, an indication of the great accuracy and reliability of as-developed ToF-SIMS calibration process. The proposed method and its reference sample selection strategy in this work provide a low-cost as well as simple-to-follow calibration route for SiGeSn composition analysis, thus driving the development of next-generation multifunctional SiGeSn-related semiconductor devices. 

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2. Characterization of polymeric surfaces and interfaces using time‐of‐flight secondary ion mass spectrometry

   https://doi.org/10.1002/pol.20210282  

   Mei, Hao; Laws, Travis S.; Terlier, Tanguy; Verduzco, Rafael; Stein, Gila E. (August 2021, Journal of Polymer Science)

   Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used for chemical analysis of surfaces. ToF-SIMS is a powerful tool for polymer science because it detects a broad mass range with good mass resolution, thereby distinguishing between polymers that have similar elemental compositions and/or the same types of functional groups. Chemical labeling techniques that enhance contrast, such as deuterating or staining one constituent, are generally unnecessary. ToF-SIMS can generate both two-dimensional images and three-dimensional depth profiles, where each pixel in an image is associated with a complete mass spectrum. This Review begins by introducing the principles of ToF-SIMS measurements, including instrumentation, modes of operation, strategies for data analysis, and strengths/limitations when characterizing polymer surfaces. The sections that follow describe applications in polymer science that benefit from characterization by ToF-SIMS, including thin films and coatings, polymer blends, composites, and electronic materials. The examples selected for discussion showcase the three standard modes of operation (spectral analysis, imaging, and depth profiling) and highlight practical considerations that relate to experimental design and data processing. We conclude with brief comments about broader opportunities for ToF-SIMS in polymer science. 

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3. Colloidal Synthesis of Homogeneous Ge1-x-ySiySnx Nanoalloys with Composition-Tunable Visible to Near IR Optical Properties

   https://doi.org/10.1021/acs.chemmater.3c01644  

   Spera, Drew; Pate, David; Spence, Griffin C; Villot, Corentin; Onukwughara, Chineme Jeanfrances; White, Daulton; Lao, Ka Un; Özgür, Ümit; Arachchige, Indika U (November 2023, Chemistry of Materials)

   Group IV alloy nanocrystals (NCs) are a class of direct energy gap semiconductors that show high elemental abundance, low to non-toxicity, and composition-tunable absorption and emission properties. These properties have distinguished Ge1-xSnx NCs as an intriguing material for near-infrared (IR) optical studies. Achieving a material with efficient visible emission requires a modified class of Group IV alloys and the computational studies suggest that this can be achieved with Ge1-x-ySiySnx NCs. Herein, we report a colloidal strategy for the synthesis of bulk-like (10.3 ± 2.5 – 25.5 ± 5.3 nm) and quantum-confined (3.2 ± 0.6 – 4.2 ± 1.1 nm) Ge1-x-ySiySnx alloys that show strong size confinement effects and composition-tunable visible to near IR absorption and emission properties. This synthesis produces a homogeneous alloy with diamond cubic Ge structure and tunable Si (0.9 – 16.1%) and Sn (1.8 – 14.9%) compositions, exceeding the equilibrium solubility of Sn (<1%) in crystalline Si and Ge. Raman spectra of Ge1-x-ySiySnx alloys show a prominent redshift of the Ge-Ge peak and the emergence of a Ge-Si peak with increasing Si/Sn, suggesting the growth of homogeneous alloys. The smaller Ge1-x-ySiySnx NCs exhibit absorption onsets from 1.21 to 1.94 eV for x = 1.8 – 6.8% and y = 0.9 – 16.1% compositions, which are blueshifted from those reported for Ge1-x-ySiySnx bulk alloy films and Ge1-xSnx alloy NCs, indicating the influence of Si incorporation and strong size confinement effects. Solid-state photoluminescence (PL) spectra reveal core-related PL maxima from 1.77 – 1.97 eV in agreement with absorption onsets, consistent with the energy gaps calculated for ~3–4 nm alloy NCs. With facile low-temperature solution synthesis and direct control over physical properties, this methodology presents a noteworthy advancement in the synthesis of bulk-like and quantum-confined Ge1-x-ySiySnx alloys as versatile materials for future optical and electronic studies. 

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4. Role of tin clustering in band structure and thermodynamic stability of GeSn by atomistic modeling

   https://doi.org/10.1116/6.0003563  

   Karthikeyan, Sengunthar; Hudait, Mantu K (May 2024, Journal of Vacuum Science & Technology B)

   Synthesis of device-quality GeSn materials with higher Sn compositions is hindered by various factors, such as Sn segregation, clustering, and short-range ordering effects. In the present work, the impact of the clustering of Sn atoms in a GeSn semiconductor alloy was studied by density functional theory using SG15 pseudopotentials in a Synopsys QuantumATK tool, where the thermodynamic stability, effective band structure, indirect and direct bandgaps, and density of states (DOS) were computed to highlight the difference between a cluster-free random GeSn alloy and a GeSn alloy with Sn–Sn clusters. A 54-atom bulk Ge1–xSnx (x = 3.71%–27.77%) supercell was constructed with cluster-free and a first nearest neighbor Sn–Sn clustered GeSn alloy at each composition for this work. Computation using the generalized gradient approximation exchange-correlation functional showed that the thermodynamic stability of GeSn was reduced due to the clustering of Sn, which increased the formation energy of the GeSn alloys by increasing the Hartree potential energy and exchange-correlation energy. Moreover, with the effective band structure of the GeSn material at a Sn composition of ∼22%, both direct (Eg,Γ) and indirect (Eg,L) bandgaps decreased by a large margin of 40.76 and 120.17 meV, respectively, due to Sn–Sn clustering. On the other hand, Eg,Γ and Eg,L decrease is limited to 0.5 and 12.8 meV, respectively, for Sn composition of ∼5.6%. Similar impacts were observed on DOS, in an independent computation without deducing from the electronic band structure, where the width of the forbidden band reduces due to the clustering of Sn atoms in GeSn. Moreover, using the energy bandgaps of GeSn computed with the assumption of it being a random alloy having well-dispersed Sn atoms needs revision by incorporating clustering to align with the experimentally determined bandgap. This necessitates incorporating the effect of Sn atoms clustered together at varying distributions based on experimental characterization techniques such as atom probe tomography or extended x-ray absorption fine structure to substantiate the energy bandgap of the GeSn alloy at a particular composition with precision. Hence, considering the effect of Sn clusters during material characterization, beginning with the accurate energy bandgap characterization of GeSn would help in mitigating the effect of process variations on the performance characteristics of GeSn-based group IV electronic and photonic devices such as varying leakage currents in transistors and photodiodes as well as the deviation from the targeted wavelength of operation in lasers and photodetectors. 

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5. Surface Segregation Across Ternary Alloy Composition Space: CuxAuyPd1−x−y

   https://doi.org/10.1021/acs.jpcc.0c02058  

   Yin, Chunrong; Guo, Zhitao; Gellman, Andrew (April 2020, Journal of physical chemistry)

   Surface segregation is a phenomenon common to all multicomponent materials and one that plays a critical role in determining their surface properties. Comprehensive studies of surface segregation versus bulk composition in ternary alloys have been prohibitive because of the need to study many different compositions. In this work, high-throughput low-energy He+ ionscattering spectra and energy-dispersive X-ray spectra were collected from a CuxAuyPd1−x−y composition spread alloy film under ultrahigh vacuum conditions. These have been used to quantify surface segregation across the entire CuxAuyPd1−x−y composition space (x = 0 → 1 and y = 0 → 1 − x). Surface compositions at 164 different bulk compositions were measured at 500 and 600 K. At both temperatures, Au shows the greatest tendency for segregation to the top-most surface while Pd is always depleted from the surface. Higher temperatures enhance the Au segregation. Segregation at most of the binary alloy bulk compositions matches with observations previously reported in the literature. However, surface compositions in the CuPd B2 composition region reveal segregation profiles that are nonmonotonic in bulk alloy composition. These were not observable in prior studies because of their limited resolution of composition space. An extended Langmuir−MacLean model, which describes ternary alloy segregation, has been used to analyze experimental data from the ternary alloys and to estimate pair-wise segregation free energies and segregation equilibrium constants. The ability to study surface segregation across the ternary alloy composition space with high-throughput methods has been validated, and the impact of bulk alloy phase on surface segregation is demonstrated and discussed. 

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