Density-functional study of Mn monosilicide on the Si(111) surface: Film formation versus island nucleation

Abstract

The stability of thin films and of small crystallites of Mn monosilicide (MnSi) on the Si(111) surface is investigated by density-functional theory calculations. Extending previous studies of MnSi/Si(001), our calculations indicate that MnSi films on Si(111) have similar electronic and magnetic properties, i.e., large magnetic moments at the Mn atoms near the surfaces and interfaces and a high degree of spin polarization at the Fermi level. Hence, such MnSi films could be interesting as a spintronics material compatible with silicon. Moreover, from our calculated total energies we conclude that the Si(111) substrate should be more suitable to grow MnSi layers than the Si(001) substrate. This result is obtained by analyzing the conditions for the formation of three-dimensional (3D) MnSi islands, either in the B20 crystal structure or as pseudomorphic islands in the B2 structure: On Si(001), 3D islands, even if they are just a few lattice constants wide, are found to be already more stable than a homogeneous MnSi film. A bipyramidal “iceberg” island consisting of MnSi in the B20 structure on the Si(001) substrate is found to be most stable among the structures investigated. For MnSi on Si(111), however, our calculations show that the nucleus for forming a 3D island is larger. Therefore, Mn deposition initially leads to the formation of flat 2D islands. On Si(111), the lowest-energy structure for such islands is found to be similar to the B20 structure of bulk MnSi, whereas on Si(001) this structure is incompatible with the substrate lattice. Our results are in agreement with the experimental observations, formation of an almost closed film with (√3x√3) structure on Si(111), and 3D island formation on Si(001)

Si-Based Magnetic Semiconductors

“…Magnano et al have also explored the co-deposition of Mn and Si to create both ultrathin magnetic films (as thin as two monolayers), and surprisingly these films displayed a hint of magnetism appearing at temperatures exceeding room [148,149]. This additional magnetic signal is thought to be due to Mn atoms with reduced coordination at surfaces or near the film-substrate interface as predicted in [145]. Other investigations of RDE films include [150][151][152] where thicker films were formed.…”

“…Using density functional calculations, Hortamani et al predicted that ultrathin films of MnSi on Si(111) would have large spin polarizations (50 %) at the Fermi level and may be an efficient source of spinpolarized carriers for injection into silicon [144,145]. Motivated by this work, Higashi et al explored the growth of few monolayer MnSi films grown on Si(111) surfaces produced by the deposition of three monolayers of Mn at room temperature followed by a 250 C anneal for more than 5 min [146,147].…”

Growth process and surface structure of MnSi on Si(111)

“…For example, Fe and Co films deposited on a Si substrate react with Si and form nonmagnetic metal silicides. In contrast, in a study based on density functional theory, it was found that a MnSi film has a large magnetic moment owing to Mn atoms residing near the surface and interface [3], which makes MnSi a promising candidate for realizing magnetic material/Si systems. So far, its morphology and electronic structure have been investigated [4][5][6].…”

Local structure and magnetic properties of B2- and B20-like ultrathin Mn films grown on Si(001)

“…36 The drop seen in the 2-and 3-ML films is then attributed in part to the formation of bulk moments, and due to the presence of defects, which possibly explain the decrease in the moment of MnSi / Si(111) films. 22 A second possibility is the formation of an antiferromagnetic state.…”