First-principles Investigation On Two-Dimensional AlN Doped With Nonmagnetic Elements

Spintronics is an interdiscipline of microelectronics and magnetic physics,aim-ing to add the spin degree of freedom to convention al charge-based electronic devices.These spintronic devices in which both the charge and spin degrees of the carriers can be exploited simultaneously,have the potential advantages of increased data processing speed,increased integration densities,decreased electric power consum p-tion,and nonvolatility.Meanwhile,the discovery of graphene with its excellent properties not only opened a new field in theoretical physics but also triggered the developments of many other two dimensional materials.The ultrathin structures of the two dimensional materials with the atoms in a plane are quite suitable for the mai n-stream planar technology of the microelectronics.And the novel properties induced by quantum confinement effect makes the spintronic devices multifunctional.So we be-lieve that the spintronic devices based on two dimensional materials will lead the wave of the next generation nanodevices.Theoretical predictions show high curie temperature ferromagnetism can be obtained in wide band gap III-V and II-VI binary semiconductors by doping.As a typical direct bandgap semiconductor,we choose the AlN monolayer to study the magnetism.Bulk AlN,with a direct band gap of 6.2 e V,was widely used in the field of blue and ultraviolet optical devices.Because of its ex-traordinary physical properties including small thermal expansion coefficient,high thermal conductivity and a reasonable thermal match to Si and Ga As,bulk AlN was also used in various electronic devices.High-quality ultrathin AlN nanosheets was successfully fabricated recently.Therefore our studies of the magnetism of the AlN monolayer are worthwhile and significant.Based on spin-polarized density functional theory calculations,we studied the nonmagnetic elements doped AlN and Mo atom doped AlN,Ga N and In N.We studied the magnetism of the doping systems by analyzing the geometric structures and elec-tronic structures.The whole paper is organized as follows:In chapter 1,the foundational conceptions of diluted magnetic semiconductors,spintronics and the magnetic exchange interactions are simply introduced.And the development of the two dimensional materials,the current state of the AlN nanosheets research are systematically introduced.In chapter 2,the first-principles calculation method was introduced step by step from the quantum mechanical calculations based on wave function to density func-tional theory calculations.At the end of this chapter,the computing software VASP and the commonly used exchange-correlation potentials are simply introduced.In chapter 3,the geometric structures,electronic structures and the magnetic properties of the AlN monolayers doped with the typical alkali metal?Li,Na and K?atoms.The substitutional atoms located above the pristine AlN monolayers with C _3vv symmetric local structures.Acceptor levels,which formed mainly by N-2p electronic states,are induced in the band gap of the pristine AlN monolayers by these three atoms doping.All the three alkali metal atoms induced total magnetic moments of2?_B in the doped systems independently.A common feature of the doped systems is that the strong localization of the spin density around the dopant atom versus the percolation of the spin density to the whole supercell.The strong localization impurity states lead to a spin-polarized ground state and the formation of local moments,and the extension of the impurity states mediate the long-ranged magnetic interactions between the local magnetic moments.The magnetic coupling calculations show that the magnetic interactions in Li and Na doped systems are short-ranged,while that of the K doped system is long-ranged which to the benefit of the room temperature fer-romagnetism with low impurity concentration.The estimated curie temperature of the K doped AlN monolayer is higher than 1000K by Heisenberg mean-field model.In chapter 4,the structural,electronic and magnetic properties in AlN monola y-ers doped with nonmagnetic atoms?Mg,Ca,Zn and Sr?are theoretically studied.Mg and Zn atoms locate in the AlN monolayer planes with a D _3h local symmetry while Ca and Sr atoms locate above the AlN monolayer planes with a C _3v local symmetry.Shallow acceptor levels are induced in the pristine AlN monolayer band gap by all four atoms doping,while the spin-splitting of the states around the Fermi level is re-markable.The calculations based on GGA-PBE show all the doped systems are half-metallic with the same net magnetic moments of 1?_B,and the calculations within HSE06 scheme show that Mg,Zn and Sr doped AlN monolayers remain half-metallic while Ca doped case is magnetic semiconductor.The total magnetic moments mainly stemmed from the spin-polarized holes resided on the three nearest neighboring N atoms,while other N atoms in the whole supercell contribute little spin moments.Compared to the alkali metal doped AlN monolayers,Mg,Ca,Zn and Sr doped sy s-tems seem to more easiler to form long-range ferromagnetism.All the four doped systems posses ferromagnetic ground states,with curie temperatures at least 600K,pronouncing the potential applications for the spintronic devices in the future.In chapter 5,we studied the AlN,Ga N and In N monolayer doped with Mo atom,systematically and comparatively analyzed the various magnetic behaviors in different doped systems.The Mo atom locates above the AlN monoayer and lift up one of the three nearest neighboring N atoms.This Jahn-Teller distortion could be benefical to reduce the total energy of the system.The local structures around the doping atom keep D_3h symmetric in Mo doped Ga N and In N monolayers.After the Mo doping,the doped AlN monolayer changes to magnetic semiconductor,while the doped Ga N and In N monolayers change to nonmagnetic metal and magnetic meta l respectively.The net magnetic moments of the Mo doped AlN and In N monolayers are 1.0?_B and 0.54?_B,respectively.The spin electronic states are very localized,no magnetism percol a-tion is observed.Therefore,almost no magnetic exchange interaction arise in the Mo atoms doped AlN monolayer even two Mo atoms locate at adjacent lattice sit es in the supercell.In the doped In N monolayer,magnetic interaction is only observed when two Mo atoms locate at the adjacent lattice sites,regretfully,this doping configuration is not the ground state of the doped system.Broadly speaking,Mo atoms doped AlN and In N nanosheets are not suitable for ferromagnetic applications.Surprisingly,two Mo atoms doped Ga N monolayer shows relatively long-range ferromagnetism even if the Ga N monolayer with one Mo atom is nonmagnetic.If two Mo atoms was im-planted into Ga N monolayer with a separation less than 8?,the ferromagnetism could be stabilized under room temperature.Even the distance between two doped atoms up to 10?,the estimated curie temperature is close to room temperature.The research of Mo atom doped AlN,Ga N and In N monolayers demonstrate that an appropriate mag-nitude of localization?or delocalization?is what the key point to produce room tem-perature ferromagnetism.