UNN scientists achieve a hundredfold improvement in luminescent properties of silicon
Lobachevsky University researchers have managed to enhance significantly the light-emitting properties of silicon (Si) by optimising the synthesis of the 9R-Si hexagonal phase. The study was published in an article in the highly ranked journal Applied Physics Letters.
Nizhny Novgorod scientists proposed a unique method for producing hexagonal silicon nanoinclusions. It is based on ion implantation, a traditional technology in microelectronics,which is widely used in industry to deposit impurity atoms into semiconductors in the process of fabrication of diodes and transistors.
The progress of microelectronics, the mainstay of modern information technology, has been based on the production of silicon integrated circuits. Today, when technologies are changing from electronic to photonic circuits, an essential disadvantage of silicon - its low light-emitting properties - becomes more evident. However, abandoning silicon as the basic material of microelectronics will only slow down the development of technology. Therefore, one of the most important tasks is to develop the required light-emitting properties of silicon. Solving this task would allow a revolutionary leap forward in the processing and transmission of enormous amounts of information.
One of the ways to preserve silicon as an electronics material of the future is nanostructuring, which consists in forming Si nanocrystals in wide-gap matrices (oxides). In their study, Lobachevsky University scientists have revealed optimal modes for ion-beam synthesis of optically active 9R-Si phase nanocrystals in silicon-silicon dioxide (SiO2/Si) structures. The formation of such inclusions under ion irradiation of SiO2/Si systems was first discovered at Lobachevsky University several years ago. Actually, silicon in its hexagonal phase is not a single material. It is some "family" of crystals with similar structure, which differs from the traditional cubic silicon phase by its properties along one of the atomic axes, which changes both the electrical and optical characteristics of the material.
Lobachevsky University researchers have managed to develop a method for synthesizing silicon with the 9R structure, when silicon atoms are arranged in nine layers (with a period of 9 atomic layers) along the selected direction. Scientists have now proved that these inclusions have better emission properties than ordinary (cubic) silicon. A relationship between the conditions of synthesis and luminescence of the nanostructures was discovered and a mechanism was proposed to form this silicon phase. The mechanism involves the mechanical stress arising in the SiO2 film during irradiation, as well as the stress associated with the penetration of ions and recoil atoms from the film into the substrate.
Silicon with 9R phase inclusions emits at longer wavelengths if compared with cubic silicon (cubic silicon emits at 1130 nm in the infrared band while the one developed by the UNN scientists - at 1240 nm). The intensity of the emission increases considerably - a hundredfold, according to the estimates of the authors of the study. The emisssion is also retained at elevated temperatures. The structural transformation of silicon to a hexagonal crystal occurs through the impact of noble gas ions. Moreover, the impact occurs not even on the silicon itself but on a silicon oxide layer about a hundred nanometers thick on top of the silicon. It turns out that in this case, the emission intensity of the formed inclusions of hexagonal silicon is the greatest.
The scientists attribute this effect to two factors: the size of the formed inclusions and the amount of radiation defects in them. After all the procedures, the oxide layer can be removed by etching. The resulting silicon with hexagonal inclusions near the surface can be used to create data transmission circuits using light. The ion implantation method is one of the basic technological methods in microelectronics. It is easily scalable in industrial applications. Lobachevsky University researchers plan to implement the proposed approach in silicon photonics technology. Their short-term goal is to produce homogeneous layers and control their thickness.
The project was carried out by a research group of the Laboratory of Physics and Technology of Thin Films at the UNN Physics and Technology Research Institute. The team of authors represents the school of ion implantation, which was founded at Lobachevsky University over 60 years ago by one of the pioneers of this method in our country, Professor David Tetelbaum, who continues to lead this school widely recognized in the world.
Reference: Photoluminescence of ion-synthesized 9R-Si inclusions in SiO2/Si structure: Effect of irradiation dose and oxide film thickness / A. Nikolskaya, A. Belov, A. Mikhaylov, A. Konakov, D. Tetelbaum, D. Korolev // Appl. Phys. Lett. – 2021. – Vol. 118, № 21. – P. 212101. – DOI: 10.1063/5.0052243 / IF = 3.597 Q1 (JCR 2019)