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1. Intlp compounds for Underwater Solar Energy Harvesting

   https://doi.org/10.1557/adv.2018.22  

   Zayan, Ahmed; Vandervelde, Thomas E. (January 2018, MRS Advances)

   ABSTRACT With the rising interest in oceanic monitoring, climate awareness and surveillance, the scientific community need for developing autonomous, self-sustaining Unmanned Underwater Vehicles (UUVs) increased as well. Limitations on the size, maneuverability, power consumption, and available on-site maintenance of these UUVs make a number of proposed technologies to power them harder to implement than others; solar energy harvesting stands as one of the more promising candidates to address the need for a long-term energy supply for UUVs due to its relatively small size and ease of deployment. Studies show research groups focusing on the use of Si cells (amorphous and crystalline), InGaP, and more recently Organic Photovoltaics to convert the attenuated solar spectrum under shallow depths (no deeper than 9.1 m) into electrical energy used or stored by the UUV’s power management system (P. P. Jenkins et al. 2014; Walters et al. 2015). In our study, we consider the ternary compound In 1-x Tl x P that allows for varying the quantum efficiency of the cell, and by extension the overall harvesting efficiency of the system by altering the Tl content (x) in the compound. In 1-x Tl x P on InP is a low strain system since the compound exhibits very little change in its lattice constant with changing Tl content due to the comparable atomic size and forces of In and Tl allowing for relatively easy growth on InP substrates. The study focuses on studying the spectral response and comparing the performance of an optimized single junction In 1-x Tl x P cells to In 1-y Ga y P cells while accounting for the optical losses of the solar irradiance underwater for various depths. 

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2. Epitaxial PbGeSe thin films and their photoluminescence in the mid-wave infrared

   https://doi.org/10.1063/5.0250085  

   Xiao, Kelly; Wong, Bryce; Meyer, Jarod; Nordin, Leland; Mukherjee, Kunal (April 2025, Journal of Applied Physics)

   PbSe is a narrow bandgap IV–VI compound semiconductor with application in mid-wave infrared optoelectronics, thermoelectrics, and quantum devices. Alkaline-earth or rare-earth elements such as Sr and Eu can substitute Pb to widen the bandgap of PbSe in heterostructure devices, but they come with challenges such as deteriorating optical and electronic properties, even in dilute concentrations due to their dissimilar atomic nature. We substitute Pb instead with column IV Ge and assess the potential of rocksalt phase PbGeSe as a wider bandgap semiconductor in thin films grown by molecular beam epitaxy on GaAs substrates. Low sticking of GeSe adatoms requires synthesis temperatures below 260 °C to incorporate Ge, but this yields poor structural and compositional uniformity as determined by x-ray diffraction. Consequently, as-grown films in the range Pb0.94Ge0.06Se–Pb0.83Ge0.17Se (6%–17% Ge) show much less bandgap widening in photoluminescence than prior work on bulk crystals using absorption. We observe that post-growth rapid thermal annealing at temperatures of 375–450 °C improves the crystal quality and recovers bandgap widening. Rapid interdiffusion of Ge during annealing, however, remains a challenge in harnessing such PbGeSe materials for compositionally sharp heterostructures. Annealed 17% Ge films emit light at 3–3.1 μm with a minimal shift in wavelength vs temperature. These samples are wider in bandgap than PbSe films by 55 meV at room temperature, and the widening increases to 160 meV at 80 K, thanks to sharply different dependence of bandgap on temperature in PbSe vs PbGeSe. 

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3. Bandgap and strain engineering in epitaxial rocksalt structure (Ti _0.5 Mg _0.5 ) _1−x Al _x N(001) semiconductors

   https://doi.org/10.1039/d0tc03598j  

   Wang, Baiwei; Zhang, Minghua; Adhikari, Vijaya; Fang, Peijiao; Khare, Sanjay V.; Gall, Daniel (September 2020, Journal of Materials Chemistry C)

   null (Ed.)

   Rocksalt structure nitrides emerge as a promising class of semiconductors for high-temperature thermoelectric and plasmonic applications. Controlling the bandgap and strain is essential for the development of a wide variety of electronic devices. Here we use (Ti 0.5 Mg 0.5 ) 1−x Al x N as a model system to explore and demonstrate the tunability of both the bandgap and the strain state in rocksalt structure nitrides, employing a combined experimental and computational approach. (Ti 0.5 Mg 0.5 ) 1−x Al x N layers with x ≤ 0.44 deposited on MgO(001) substrates by reactive co-sputtering at 700 °C are epitaxial single crystals with a solid-solution B1 rocksalt structure. The lattice mismatch with the substrate decreases with increasing x , leading to a transition in the strain-state from partially relaxed (74% and 38% for x = 0 and 0.09) to fully strained for x ≥ 0.22. First-principles calculations employing 64-atom Special Quasirandom Structures (SQS) indicate that the lattice constant decreases linearly with x according to a = (4.308 − 0.234 x ) Å for 0 ≤ x ≤ 1. In contrast, the measured relaxed lattice parameter a o = (4.269 − 0.131 x ) Å is linear only for x ≤ 0.33, its composition dependence is less pronounced, and x > 0.44 leads to the nucleation of secondary phases. The fundamental (indirect) bandgap predicted using the same SQS supercells and the HSE06 functional increases from 1.0 to 2.6 eV for x = 0–0.75. In contrast, the onset of the measured optical absorption due to interband transitions increases only from 2.3 to 2.6 eV for x = 0–0.44, suggesting that the addition of Al in the solid solution relaxes the electron momentum conservation and causes a shift from direct to indirect gap transitions. The resistivity increases from 9.0 to 708 μΩ m at 77 K and from 6.8 to 89 μΩ m at 295 K with increasing x = 0–0.44, indicating an increasing carrier localization associated with a randomization of cation site occupation and the increasing bandgap which also causes a 33% reduction in the optical carrier concentration. The overall results demonstrate bandgap and strain engineering in rocksalt nitride semiconductors and show that, in contrast to conventional covalent semiconductors, the random cation site occupation strongly affects optical transitions. 

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4. Effects of B and In on the band structure of BGa(In)As alloys

   https://doi.org/10.1063/5.0125109  

   Meng, Qian; El-Jaroudi, Rasha H.; White, R. Corey; Dey, Tuhin; Reza, M. Shamim; Bank, Seth R.; Wistey, Mark A. (November 2022, Journal of Applied Physics)

   Highly mismatched semiconductor alloys (HMAs) offer unusual combinations of bandgap and lattice constant, which are attractive for myriad applications. Dilute borides, such as BGa(In)As, are typically assumed to be HMAs. BGa(In)As can be grown in higher alloy compositions than Ga(In)NAs with comparable bandgaps, potentially enabling routes to lattice-matched telecom lasers on Si or GaAs. However, BGa(In)As remains relatively unexplored, especially with large fractions of indium. Density functional theory with HSE06 hybrid functionals was employed to study BGaInAs with 4%–44% In and 0%–11% B, including atomic rearrangement effects. All compositions showed a direct bandgap, and the character of the lowest conduction band was nearly unperturbed with the addition of B. Surprisingly, although the bandgap remained almost constant and the lattice constant followed Vegard's law with the addition of boron, the electron effective mass increased. The increase in electron effective mass was higher than in conventional alloys, though smaller than those characteristics of HMAs. This illustrates a particularly striking finding, specifically that the compositional space of BGa(In)As appears to span conventional alloy and HMA behavior, so it is not well-described by either limit. For example, adding B to GaAs introduces additional states within the conduction band, but further addition of In removes them, regardless of the atomic arrangement. 

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5. Spectroscopic Ellipsometry Investigation of CuInSe2 as a Narrow Bandgap Component of Thin Film Tandem Solar Cells

   https://doi.org/10.1109/PVSC.2018.8548177  

   Sapkota, Dhurba R.; Koirala, Prakash; Pradhan, Puja; Shrestha, Niraj; Junda, Maxwell M.; Phillips, Adam B.; Ellingson, Randy J.; Heben, Michael J.; Marsillac, Sylvain; Podraza, Nikolas J.; et al (June 2018, 2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC))

   Spectroscopic ellipsometry (SE) was performed on CuIn Se 2 (CIS) thin films and solar cells with a goal toward optimizing this low bandgap absorber for tandem applications. The CIS thin films and the absorbers in devices were deposited by one-stage thermal co-evaporation on silicon and on Mo-coated soda-lime glass substrates in a deposition system that has yielded CuIn 1-x Ga x Se 2 (CIGS) cells with > 17% efficiency using standard thickness (2.0 μm)x = 0.3 absorbers and > 13% using 0.7 μm low-Ga absorbers. In this study, a mapping capability for CIS Cu stoichiometry y = [Cu]/[In] over the film area was established based on a y-dependent parametric dielectric function (ε 1 , ε 2 ) with bandgap critical point E g decreasing linearly from 1.030 eV for y = 0.7 to 1.016 eV for y = 1.1. In addition, a full set of (ε 1 , ε 2 ) spectra measured for the CIS cell components enables analysis of SE data in terms of an accurate structural model for the device. With this model, spectra in the external quantum efficiency can be predicted, and deviations from this prediction can be attributed to incomplete collection of photogenerated electrons and holes as simulated with a carrier collection profile. 

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