A New Phase of Silicon and a Study of Its Magnetic and Optoelectronic Properties

The pursuit of ferromagnetism in materials outside of transition metals and rare earths has excited scientists worldwide for a long time. This is because spin-polarized electrons can be used to process and store information with atomic resolution. However, materials with an even number of electrons such as carbon and silicon without unpaired spins were not considered seriously in terms of bulk ferromagnetism. The dangling bonds in bulk carbon and silicon materials usually reconstruct and eliminate sources of unpaired electrons. However, at the free surfaces of covalently bonded materials, steps and kinks can provide sources of dangling bonds and unpaired spins, which can lead to paramagnetism and ferromagnetism, provided these spins can achieve long-range ordering. Our recent discovery showed the occurrence of robust ferromagnetism in Q-carbon, which consisted of randomly packed diamond tetrahedra. The bonding within the tetrahedra in Q-carbon was determined to be sp³ with no dangling bonds. However, the bonding between the tetrahedra was a mixture of sp³ and sp² with an overall fraction of about 85% sp³ and 15% sp². Thus, dangling bonds and unpaired spins between the tetrahedra played a critical role in producing robust ferromagnetism in Q-carbon. It should be mentioned that defect-induced intrinsic ferromagnetism has been observed in oxides and nitrides and other related materials with laser annealing, vacuum thermal annealing, and ion implantation. In oxides, such as ZnO, ferromagnetism is related to paramagnetic Zn vacancies, which are coupled through oxygen vacancies for room-temperature ferromagnetism.<br/>Herein, we report the outstanding magnetic, and optoelectronic properties in a new phase of silicon (Q-silicon) formed by nanosecond pulsed laser melting and quenching of amorphous silicon. Through detailed structure-property correlations in Q-silicon films, we show that the bonding characteristics in Q-silicon are the same as crystalline silicon, but with a 60% higher atomic density. Consequently, Q-silicon is shown to have a narrow bandgap of 0.6 eV (laser-annealed at 0.3 Jcm^-2) with the enhanced density of states near the Fermi level. The isothermal field-dependent magnetization plots confirm room-temperature ferromagnetism in Q-silicon with a finite coercivity of ~ 100 Oe at 300 K, which is characteristic of long-range ferromagnetic ordering. Furthermore, the blocking temperature estimated from the temperature-dependent magnetization plots is greater than 400 K, confirming the robust ferromagnetic interactions in Q-silicon. It should be noted that amorphous silicon before laser annealing shows a diamagnetic behavior, where silicon dangling bonds between tetrahedra are saturated. The tetrahedra in amorphous silicon are not as closely packed as in Q-silicon, resulting in the reconstruction of sp³ dangling bonds. The Curie temperature of Q-silicon is estimated to be over 500 K, obtained by the extrapolation of fits to experimental data using modified Bloch’s law. Thus, the discovery of Q-silicon with exceptional functionalities opens a new frontier for spin-based computing and atomic-level storage.