Functional surface finishing: targeted control of ultra-fine tin coatings

A team of researchers has succeeded in observing novel growth effects of tin layers on silicon nanometer-structured surfaces. This allows the chemical composition of deposited thin films to be precisely controlled and monitored.

Nanometer-scale coatings with functional materials play an important role in many sensory, electronic and photonic applications. An international team of researchers - coordinated by the Leibniz-IPHT in Jena - has succeeded for the first time in observing novel growth effects of tin coatings on silicon nanometer-structured surfaces. With the knowledge gained, the chemical composition of deposited thin films can be precisely controlled and monitored in the future, opening up new applications in the fields of biophotonics, energy generation or mobility.

Tin-containing layers are in demand for a wide variety of electronic parts and components in the electrical industry as well as in sensor technology or photovoltaics. Researchers from the Leibniz Institute for Photonic Technologies (Leibniz-IPHT), together with scientists from Germany, Russia and Great Britain, investigated the formation process of nanoscale tin layers, the results of which they summarize in the renowned journal Small.

The starting material for the observed growth processes of tin-containing thin films is formed by ultrathin silicon-based structures in the form of nanowires with a diameter of less than 100 nanometers. In experimental studies, the researchers were able to demonstrate for the first time a specific distribution effect of tin along these silicon nanostructures: Tin-containing layers with different degrees of oxidation formed along the entire length of the semiconductor nanowires using metal-organic chemical vapor deposition at a deposition temperature of 600 degrees Celsius. "By understanding how tin coatings grow and which factors influence this growth process, we create the conditions for controlling coating processes in a targeted manner. This allows surfaces to be refined very precisely and endowed with desired functional properties at previously well-defined positions," explains Dr. Vladimir Sivakov, head of the Silicon Nanostructures group at Leibniz-IPHT, who researched and uncovered the growth mechanisms together with his team.

Nanometer-thin coatings with tin enable specific optical and electrical properties and allow, among other things, to further improve the research and development of optical and biophotonic methods. In surface-enhanced Raman spectroscopy (SERS), with which the molecular fingerprint of biological samples can be determined using SERS-active metal nanostructures, tin coatings can be used as UV-SERS-active surfaces. 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 investigated the growth dynamics of the observed tin-based layers on nanostructured surfaces using microscopic and spectroscopic methods. They found that the surfaces of the semiconductor nanowires - in contrast to planar and unstructured silicon surfaces, on which the deposition was homogeneous - were covered with tin-containing crystals of different size and shape over the entire length. The results presented in the journal Small show the formation of different tin oxide phases along the nanostructured silicon surfaces, which could be identified with tin dioxide (SnO2) in the upper part, tin monoxide (SnO) in the middle part and with metallic tin (Sn) in the lower part.

The amount and distribution of the resulting 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. In addition to these geometrical parameters, the researchers were able to reveal the formation of hydrocarbonaceous byproducts as reducing agents for tin oxide reduction as another factor influencing the distribution of the formed tin layers along the semiconductor nanostructures. The thermal conductivity of the silicon structures and thus the temperature distribution along the nanowires during the high-temperature vapor deposition can also influence the formation of different tin oxide phases.

The project "Development and atomic and electronic structure characterization of functional Sn/SnOx surfaces for SERS-based analysis of misfolded proteins" 448666227 (SI1893/27-1) was funded by the German Research Foundation (DFG). (OM-10/23)