Electronic Structure And Magnetism Of Ge-based And HfO₂-based Ferromagnetic Semiconductors

The electron has two fundamental degrees of freedom,its charge and its spin.As one of the greatest achievements of humankind in the 20^th century,microelectronics based on electronic charges and transportation has greatly promoted the advancement of society and brought people into information era.However,in conventional microelectronics,we only take advantage of the charge property of the electron,while the spin degree of freedom of the electron is neglected.Until 1980s the discovery of Giant magnetoresistance had revolutionized applications in magnetic recording and memory.Thus,spin-dependent electrical transport was inspired because of its huge potential application.And this launched the new field of spin electronics-'spintronics', which is centered on the spin of electrons including their generation,transport and detection.Ferromagnetic semiconductor is one of the key materials of spintronics.It is usually synthesized by transition metal atoms doping semiconductors,and the ferromagnetism is induced by the exchange interaction between transition metal atoms.In the early research,the Curie temperature of ferromagnetic semiconductors is mostly lower than 10 K,which limits its practical applications.Much effort has been devoted to search for high Curie temperature ferromagnetic semiconductors.In 1990s, the Curie temperature of Ga_1-xMn_xAs materials reaches up to 170 K.Subsequently,a lot of results onâ…¡-â…¥compounds(concentrated on oxide) and groupâ…£magnetic semiconductors develop rapidly.Ge is group-â…£semiconductor.It has similar electronic structure with silicon, and it is compatible with the current mainstream semiconductor processing technology.Many different explanations about the origin of ferromagnetism in Mn_xGe_1-x have been reported,and the Curie temperature differs greatly.There are a lot of ferromagnetic alloy phases composed by Mn and Ge,which need to be excluded from the intrinsic ferromagnetism.Besides the single crystal Mn_xGe_1-x magnetic semiconductor,researches on other phases,such as polycrystal,amorphous,and nanocrystal,also have been reported.First-principles calculation results show that Mn atoms tend to cluster in Ge and couple antiferromagnetically,which is a great challenge to achieve high Curie temperature ferromagnetism.Accompanied by the rapid development of semiconductor technology,the conventional gate electrode SiO₂ is about to reach its limit scale in quantum mechanics.The insulated oxides HfO₂ and ZrO₂ have much larger dielectric constant than SiO₂,which are potential substitution to SiO₂ materials.In spintronies,d⁰ ferromagnetism was first reported in undoped HfO₂,and different lattice defects are considered as the source of magnetism.However,some reports show that the ferromagnetism should be attributed to the unintentional dopants.In transition metal doped HfO₂,some experimental results show extrinsic ferromagnetism,and few theoretical investigations have been reported to verify the intrinsic ferromagnetism.To theoretically study the magnetic semiconductors,two approaches are usually employed:(a) model Hamiltonian or(b) first-principles calculation based on density functional theory.The latter one has been used in this dissertation.We calculated the electronic structures of Ge-based and HfO₂-based magnetic semiconductors using first-principles calculation software.According to the band structures,we discuss magnetic exchange interaction between transition metal atoms and the origin of ferromagnetism.Profiting from the development of density functional theory and computer technology,many first-principle calculation software packages,such as Vasp,Castep,Siesta,PWscf(Quantum-Espresso),emerged in the last few decades. Our calculations are carried out using Espresso package.It is a set of programs for electronic structure calculations with density functional theory,using a plane-wave basis set and pseudopotentials.In the Mn doped Ge magnetic semiconductor,Mn dopants tend to cluster in the Ge host and the AFM ordering is much more stable than the FM ordering between the nearest-neighbor substitutional Mn atoms,which results in a low Curie temperature. In the spin distribution pictures,it is found that the spin polarization always undergoes a sign reversal along the Mn-Mn and Ge-Mn bonds for both AFM and FM coupling configurations.H interstitials in Mn_xGe_1-x tend to approach the Mn atoms,and the H:ls state in Mn-H-Mn complexes can strongly hybridize with the valence states of Mn and change the spin polarization of Mn atoms.Although the doped Mn atoms tend to form the nearest neighboring Mn atomic pairs with antiferromagnetic coupling in Mn-doped Ge without H,the unsymmetrical configurations of Mn-H-Mn complexes show ferromagnetic ground state in Mn-doped Ge with H interstitials.Therefore, Mn-doped Ge with H interstitials is predicted to be ferromagnetic semiconductor with higher Curie temperature and larger magnetization than Mn-doped Ge without H.First-principles calculations of undoped HfO₂ and cobalt doped HfO₂ have been carried out to study the magnetic properties of the dielectric material.In contrast to previous reports,it was found that the native defects in HfO₂ couldn't induce strong ferromagnetism.However,the cobalt substituting hafnium is the most stable defect under oxidation condition,and the ferromagnetic coupling between the cobalt substitutions is favorable in various configurations.We found that the ferromagnetic coupling is mediated by the threefold-coordinated oxygen atoms in monoclinic HfO₂ and could be further enhanced in electron-rich condition.Because the local density approximation often suffers the underestimation of the band gap and the overestimation of 3d states energy,the LDA+U method is implemented to amend the calculated results.The Hubbard U can be calculated based on the constrained density functional method and the linear response approach.In this calculation,the electronic structure is slightly influenced by LDA+U.