Defect And Magnetism Properties In Ion-implanted Semiconductors Studied By Positron Annihilation Technique

Dilute magnetic semiconductors(DMSs),which utilize both the charge and the spin freedom of electrons are attracting great interest in the field of materiel science and information technology.Due to the promising technological applications in spintronics,DMSs can change information transmission and processing as well as storage mode and size.They are usually achieved in the semiconductors doped by magnetic ions which are partially filled with d or f electrons.However,unexpected high temperature ferromagnetism(FM)recently was observed in some oxide semiconductors without any magnetic ions.The so called "d0 ferromagnetism"provides an opportunity to search for new high temperature DMSs,while it also confuses people about magnetism origin and theoretical modes.In this thesis,we use ion implantation method to introduce d0 ferromagnetism in the third-generation semiconductors such as GaN and SiC,and thoroughly investigate the relationship between the defects and magnetic properties by using positron annihilation technology,which was proved to be a sensitive way for study atomic-scale defects in materials.Moreover,the first-principle calculation is also developed to study the electron spin polarization states of crystals with different ions doping or defects containing.We try to give original insight in d0 ferromagnetism origin and coupling mechanism of non-magnetic ion implanted semiconductors.The major research content and achievements of this dissertation are as follows:1)At first we discuss the relationship between ferromagnetic property and defects in GaN material,and then analyze immanent difference of FM origin between light-elements p-type C atom and n-type O atom doped GaN.We use N-ion implantation method to introduce crystal defect in GaN single crystal films experimentally.Raman spectrum and positron annihilation spectrum show that the room temperature FM origin from Ga-vacancies which produced by ion-implantation.This conclusion was confirmed by first-principle calculation.For C and O ions implantation,the FM origin of implanted GaN is very different.The observed room temperature FM in experimental O-implanted sample arises from ferromagnetic coupling of Ga vacancies instead of O atoms.However,the C adjacent with VGa in GaN material mainly stabilizes the ferromagnetic behavior at room temperature,instead of increasing the magnetic moments.2)Defect-induced ferromagnetic materials with only s or p electrons have extensive application and broad prospects because of its bio-compatibility.But reach room temperature FM and how to understand this kind of defect-induced FM based on the experiment still with challenge.The various experiment phenomenon also results researchers' continues debate.SiC material is a good defect-induced FM carrier.In this thesis,we use carbon ion implanted into 6H-SiC single-crystal to obtain room temperature FM,and change the defect structure of sample by the annealing.The effect defect on FM property is analyzed by positron annihilation spectrum combine with Raman spectrum,and fair explanation of ferromagnetism and paramagnetism origin in the sample is also given.The results confirm that Si-related vacancies are responsible for the ferromagnetic single obtained in C-implanted sample.The theoretic calculations indicate that the room temperature FM properly origin from the coupling effect of Si-vacancy like VSi-VSi and VSi-VC,The magnetic moment was caused by C2p dangling bond spin polarization and coupling.Paramagnetism in samples may related to spare isolated C dangling bond that defects at low concentration and C atoms can form spin-polarized local moment,which unable to achieve long-range magnetic coupling interactions.3)How to increase ferromagnetic moment in the DMSs become a key problem to be solved as the coupling of the defect-induced FM usually very week.The way of doping with non-magnetic ions to control charge or spin state of the defect may lead to a large ferromagnetic moment in the defect.Through the contrastive experiments that B/N ions implanted into SiC single-crystal,we discuss the possibility of improve FM performance by control the spin order in the way of doping.The result indicates that the doped B atom has no effect on FM,while the negative charge ground introduced by N atom will improve the total magnetism of 6H-SiC under the almost the same defect situation.This result is confirmed by the experiment that O ion implantation can lead to stronger room temperature FM.The simulation indicates that OC-VSi complex-defect can provide focalized moment and donor O can control the spin polarization and magnetic coupling of the defect.We also obtain strong ferromagnetic property in the Cu-implanted SiC semiconductors,which ferromagnetic signal can't be totally eliminated by annealing process.The simulation show that the implanted Cu ion can contribute localized ferromagnetic moment and form long range ferromagnetic coupling,except for the effect of all the complex-defect induced by the implantation.