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1. Observation of suppressed diffuson and propagon thermal conductivity of hydrogenated amorphous silicon films

   https://doi.org/10.1039/d1na00557j  

   Zhang, Yingying; Eslamisaray, Mohammad Ali; Feng, Tianli; Kortshagen, Uwe; Wang, Xiaojia (December 2021, Nanoscale Advances)

   Hydrogenated amorphous silicon (a-Si:H) has drawn keen interest as a thin-film semiconductor and superb passivation layer in high-efficiency silicon solar cells due to its low cost, low processing temperature, high compatibility with substrates, and scalable manufacturing. Although the impact of hydrogenation on the structural, optical, and electronic properties of a-Si:H has been extensively studied, the underlying physics of its impact on the thermal properties is still unclear. Here, we synthesize a-Si:H films with well-controlled hydrogen concentrations using plasma-enhanced chemical vapor deposition and systematically study the thermal conductivity of these a-Si:H films using time-domain thermoreflectance. We find that the reduction of thermal conductivity of a-Si:H films is attributed to the suppression of diffuson and propagon contributions as the hydrogen concentration increases. At the maximum hydrogen concentration of 25.4 atomic percentage, the contributions from diffusons and propagons to the thermal conductivity are decreased by 40% (from 1.10 to 0.67 W m −1 K −1 ) and 64% (from 0.61 to 0.22 W m −1 K −1 ), respectively. Such a significant reduction in the thermal conductivity of a-Si:H originates from the hydrogen induced material softening, the decrease in density, and phonon-defect scattering. The results of this work provide fundamental insights into the thermal transport properties of a-Si:H thin films, which is beneficial for the design and optimization of amorphous silicon-based technologies including photovoltaics, large-area electronics, and thermoelectric devices. 

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2. Relaxation processes in silicon heterojunction solar cells probed via noise spectroscopy

   https://doi.org/10.1038/s41598-021-92866-w  

   Davenport, Kevin; Trinh, C. T.; Hayward, Mark; Lips, Klaus; Rogachev, Andrey (December 2021, Scientific Reports)

   null (Ed.)

   Abstract We have employed state-of-the-art cross-correlation noise spectroscopy (CCNS) to study carrier dynamics in silicon heterojunction solar cells (SHJ SCs). These cells were composed of a light absorbing n -doped monocrystalline silicon wafer contacted by passivating layers of i - a -Si:H and doped a -Si:H selective contact layers. Using CCNS, we are able to resolve and characterize four separate noise contributions: (1) shot noise with Fano factor close to unity due to holes tunneling through the np-junction, (2) a 1/ f term connected to local potential fluctuations of charges trapped in a-Si:H defects, (3) generation-recombination noise with a time constant between 30 and 50 μs and attributed to recombination of holes at the interface between the ITO and n-a -Si:H window layer, and (4) a low-frequency generation-recombination term observed below 100 K which we assign to thermal emission over the ITO/ ni - a -Si:H interface barrier. These results not only indicate that CCNS is capable of reveling otherwise undetectable relaxation process in SHJ SCs and other multi-layer devices, but also that the technique has a spatial selectivity allowing for the identification of the layer or interface where these processes are taking place. 

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3. Suppressing material loss in the visible and near-infrared range for functional nanophotonics using bandgap engineering

   https://doi.org/10.1038/s41467-020-18793-y  

   Wang, Mingsong; Krasnok, Alex; Lepeshov, Sergey; Hu, Guangwei; Jiang, Taizhi; Fang, Jie; Korgel, Brian A.; Alù, Andrea; Zheng, Yuebing (December 2020, Nature Communications)

   null (Ed.)

   Abstract All-dielectric nanostructures have recently opened exciting opportunities for functional nanophotonics, owing to their strong optical resonances along with low material loss in the near-infrared range. Pushing these concepts to the visible range is hindered by their larger absorption coefficient, thus encouraging the search for alternative dielectrics for nanophotonics. Here, we employ bandgap engineering to synthesize hydrogenated amorphous Si nanoparticles (a-Si:H NPs) offering ideal features for functional nanophotonics. We observe significant material loss suppression in a-Si:H NPs in the visible range caused by hydrogenation-induced bandgap renormalization, producing strong higher-order resonant modes in single NPs with Q factors up to ~100 in the visible and near-IR range. We also realize highly tunable all-dielectric meta-atoms by coupling a-Si:H NPs to photochromic spiropyran molecules. ~70% reversible all-optical tuning of light scattering at the higher-order resonant mode under a low incident light intensity is demonstrated. Our results promote the development of high-efficiency visible nanophotonic devices. 

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4. A multi-layer platform for low-loss nonlinear silicon photonics

   https://doi.org/10.1063/1.5115234  

   MacFarlane, Neil; Kossey, Michael_R; Stroud, Jasper_R; Foster, Mark_A; Foster, Amy_C (November 2019, APL Photonics)

   We demonstrate four-wave mixing (FWM) interactions in a-Si:H waveguides in a multilayer integrated silicon photonic chip. The a-Si:H waveguides are accessed through interlayer couplers from waveguides composed of SiNx. The interlayer couplers achieve a coupling of 0.51 dB loss per transition at the target wavelength of 1550 nm. We observe greater idler power extraction and conversion efficiency from the FWM interaction in the interlayer-coupled multilayer waveguides than in single-material waveguides. 

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5. Hopping charge transport in hydrogenated amorphous silicon–germanium alloy thin films

   https://doi.org/10.1063/5.0077441  

   Stolik, L.; Eslamisaray, M. A.; Nguyen, E.; Kortshagen, U. R.; Kakalios, J. (June 2022, Journal of Applied Physics)

   Measurements of the dark conductivity and thermoelectric power in hydrogenated amorphous silicon–germanium alloys (a-Si 1- x Ge x :H) reveal that charge transport is not well described by an Arrhenius expression. For alloys with concentrations of Ge below 20%, anomalous hopping conductivity is observed with a power-law exponent of 3/4, while the temperature dependence of the conductivity of alloys with higher Ge concentrations is best fit by a combination of anomalous hopping and a power-law temperature dependence. The latter has been attributed to charge transport via multi-phonon hopping. Corresponding measurements of the Seebeck coefficient reveal that the thermopower is n-type for the purely a-Si:H and a-Ge:H samples but that it exhibits a transition from negative to positive values as a function of the Ge content and temperature. These findings are interpreted in terms of conduction via hopping through either exponential band tail states or dangling bond defects, suggesting that the concept of a mobility edge, accepted for over five decades, may not be necessary to account for charge transport in amorphous semiconductors. 

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