Tin Coatings’ Growth Effects Reveal Path to Precise Control

Nanometer-scale coatings with functional materials are used for numerous applications in photonics and electronics, including in sensors and biomedicine. One-dimensional nanostructures, among these materials, attract substantial attention; they can be used as potential carriers of active nanomaterials at the intersection with other functional materials.

Top-down produced silicon nanowires (SiNWs) are prospective nanostructured surfaces that show highly intense visible (red) light emission at room temperature, as well as different vibrational and electronic properties. SiNW templates could be useful for the development of new semiconductor devices.

To optimize their performance, SiNW arrays can be covered, or coated, with functional oxides such as tin oxide via metal-organic chemical vapor deposition (MOCVD). As a result, the precise control over the morphology of semiconducting oxides at the nanometer scale is of high importance. Given industrial and commercial demands, reproducible, low-cost synthetic methods that can be implemented on an industrial scale are sought for these materials.

An international team of researchers has now observed growth effects of tin coatings on silicon nanometer-structured surfaces, reportedly for the first time. From their observations, the researchers coordinated by the Leibniz Institute of Photonic Technology (Leibniz IPHT) showed how the chemical composition of deposited thin films can be precisely controlled and monitored in the future.

The research opens applications in biophotonics, energy generation, and mobility. Tin-containing layers are in demand for a variety of electronic parts and components in the electrical industry as well as in sensor technology and photovoltaics.

Using ultrathin silicon-based nanowires with a diameter of less than 100 nm, the researchers demonstrated a distribution effect of tin along the nanostructures. Tin-containing layers with different degrees of oxidation were formed along the entire length of the semiconductor nanowires by means of MOCVD at a deposition temperature of 600 °C.

Vladimir Sivakov, head of the Silicon Nanostructures Group at Leibniz IPHT, investigated and discovered the growth mechanisms with his team. Creating the conditions for specifically controlling coating processes, Sivakov said, allowed the researchers to very precisely refine the surfaces, and for the surfaces to be equipped with desired functional properties at previously defined positions.

When the researchers investigated the growth dynamics of the tin-based layers on nanostructured surfaces, the surfaces of the semiconductor nanowires were covered with tin-containing crystals of different sizes and shapes over the entire length. This was in contrast to to planar and unstructured silicon surfaces, on which the deposition took place homogeneously.

Specifically, the results identified the formation of tin dioxide (SnO₂) in the upper part of the nanostructured silicon surfaces, tin monoxide (SnO) in the middle part, and metallic tin (Sn) in the lower part.

The amount and distribution of the formed metallic Sn and its SnO and SnO₂ oxides can be explained and effectively controlled by the length, diameter, porosity, and spacing of the silicon-based semiconductor nanostructures, the researchers said. In addition to these geometrical parameters, the researchers revealed the formation of hydrocarbon-containing byproducts as reducing agents for tin oxide reduction as another factor influencing the distribution of the formed tin layers along the semiconductor nanostructures.

The researchers said that thermal conductivity of the silicon structures — and the temperature distribution along the nanowires during the high-temperature vapor deposition as a result — can also have an influence on the formation of different tin oxide phases.

For surface-enhanced Raman spectroscopy (SERS), tin layers can be used as ultraviolet SERS-active surfaces. This spectroscopy method can be applied to determine the molecular fingerprint of biological samples using SERS-active metal nanostructures.

In addition, there are areas of application in gas sensors in which tin reacts to gases as a highly sensitive layer. Application scenarios in high-performance lithium-ion batteries for electromobility and thermal energy storage are also conceivable, in which tin-coated anodes ensure high electronic conductivity, the researchers said.

The research was funded by the German Research Foundation and was published in Small (www.doi.org/10.1002/smll.202206322), (www.doi.org/10.1002/smll.202206318).