Kinetic study of H-terminated silicon nanowires oxidation in very first stages

Oxidation of silicon nanowires (Si NWs) is an undesirable phenomenon that has a detrimental effect on their electronic properties. To prevent oxidation of Si NWs, a deeper understanding of the oxidation reaction kinetics is necessary. In the current work, we study the oxidation kinetics of hydrogen- terminated Si NWs (H-Si NWs) as the starting surfaces for molecular functionalization of Si surfaces. H-Si NWs of 85-nm average diameter were annealed at various temperatures from 50°C to 400°C, in short-time spans ranging from 5 to 60 min. At high temperatures (T ≥ 200°C), oxidation was found to be dominated by the oxide growth site formation (made up of silicon suboxides) and subsequent silicon oxide self-limitation. Si-Si backbond oxidation and Si-H surface bond propagation dominated the process at lower temperatures (T < 200°C)

Rapid Cryogenic Electrical Characterization of Materials and Devices Using Gifford-McMahon Cryocoolers

Thin-film heterostructures are necessary building blocks for superconducting and phononic quantum computing devices. Many new generations of quantum hardware demand extensive materials research to optimize performances at cryogenic temperatures (below 10 K). Here, we demonstrate compact cryogenic measurement systems capable of reaching sub-10K temperatures in less than three hours with the ability to measure AC/DC resistance and dielectric properties of thin-film materials. Our platform utilizes Gifford-McMahon (GM) cryocoolers as effective tools for providing high throughput cooling-warming cycles. We successfully used the GM-based measurement systems to measure 1) the superconducting transition temperature for Nb thin films (Tc ~7.8 K), and 2) the temperature dependence of the dielectric constant in SiO2 thin films down to 10 K. The fast electrical characterization feedback will be critical in developing robust materials and components for cryogenic computing devices

Nanoscale Surface and Interface Characterization of Earth-Abundant Thin-Film Solar Cells

Thin-film kesterites have been explored as promising absorbers in future photovoltaic devices due to their earth-abundant and non-toxic constituents, which do not impose any future production limitations. However, the current record conversion efficiency of polycrystalline kesterite devices is 12.6%—i.e., at least 2.4% short of the efficiency threshold needed to make this material competitive with chalcogenide-based thin film technologies. This shortage in conversion efficiency has been in part ascribed to the large extent of carrier recombination by defects at the grain boundaries and contact/absorber interfaces. In this work, methods nanoscale compositional and electrical characterization of grain boundaries and contact/absorber interfaces in kesterite solar cells have been developed, using a unique combination of advanced nano-characterization tools including Auger Nanoprobe Microscopy (NanoAuger), Kelvin Probe Force Microscopy (KPFM) and Cryogenic Focused Ion Beam (Cryo-FIB). NanoAuger and KPFM measurements on high-performance CZTSSe thin film PV devices revealed that the presence of SnOx at the grain boundaries is essential to the high VOC. This passivation layer needs to be formed by an air anneal process performed after the film deposition. In contrast to the oxide at the grain boundary, oxide layer on the top surfaces of the grains has been found to be (Sn,Zn),O. A new cross-sectioning method via grazing angle of incidence Cryo-FIB milling, has been developed where smooth cross-sections with at least 10x scale expansion have been prepared. These surfaces were characterized for CIGSe monitor films confirming the presence of MoSe2 interlayer acting as a proper hole contact on the back surface

Grazing Incidence Cross-Sectioning of Thin-Film Solar Cells via Cryogenic Focused Ion Beam: A Case Study on CIGSe

Cryogenic focused ion beam (Cryo-FIB) milling at near-grazing angles is employed to fabricate cross-sections on thin Cu­(In,Ga)­Se₂ with >8x expansion in thickness. Kelvin probe force microscopy (KPFM) on sloped cross sections showed reduction in grain boundaries potential deeper into the film. Cryo Fib-KPFM enabled the first determination of the electronic structure of the Mo/CIGSe back contact, where a sub 100 nm thick MoSe_<i>y</i> assists hole extraction due to 45 meV higher work function. This demonstrates that CryoFIB-KPFM combination can reveal new targets of opportunity for improvement in thin-films photovoltaics such as high-work-function contacts to facilitate hole extraction through the back interface of CIGS