Scientists at the Moscow Institute of Physics and Technology have determined the optical properties of ultra-thin gold films, updating data that have not changed since 1972.
Researchers at the Moscow Institute of Physics and Technology, led by Valentin Volkov, have accurately measured the optical constants of ultra-thin gold films with a thickness of 20 to 200 nanometers (a nanometer is a billionth of a meter). Such films are one of the main elements of micro- and nano-optical and optoelectronic devices. The results of the work will be used to create various sensors and advanced electronic devices. The corresponding article by Russian scientists topped the top downloads on the Optics Express website in just a day.
Today, thin metal films are widely used in compact chemical and biological sensors, light detectors, solar cells, and elements of optical computers. The best material for them and nanostructures based on them is gold - a metal with extremely high electrical conductivity and resistance to oxidation. However, in order to manufacture such nanodevices with specified parameters, it is necessary to measure their optical properties extremely accurately. Typically, researchers use tabular data from works published almost half a century ago.
During this time, the technique has changed markedly - not only measurement, but also the production of films is done differently, which affects both their properties and the accuracy of measurements. Physicists from the Moscow Institute of Physics and Technology selected the optimal initial conditions (deposition rate and substrate temperature) to obtain the best optical properties for the films being measured.
They were grown on a purified silicon substrate placed in a vacuum chamber. A container with a micro-fragment of gold was placed opposite it. A beam of electrons accelerated by a magnetic field was directed at it. With its help, gold was melted and evaporated. The noble metal molecules "bounced" towards the substrate, settled on it and solidified. This method of obtaining ultrapure and ultrathin films is called electron beam evaporation.
Preparation of a sample (a thin film of gold) for measurements on a spectral ellipsometer. PHOTO MIPT
As a result, the authors of the work managed to find out that, up to a thickness of up to 80 nanometers, the optical properties of thin gold films practically do not change with a change in thickness. However, with a thickness of less than 80 nanometers, optical loss as well as DC resistivity for gold increases significantly. This data can be used to design and create even thinner metal films. They will help in the creation of fundamentally new optoelectronic devices that are in demand in various fields. Such 40 nm thick gold films are already used to create highly sensitive biosensors.
IVAN ORTEGA
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