Research On The Theory And Experiment Of Gallium Nitride Based Dilute Magnetic Semiconductors

As good alternative materials for spintronics, Dilute Magnetic Semiconductors (DMSs) has become a hot point for spintronic material research. DMSs have a band structure of semiconductors and also a similar lattice constant to the host material. In addition they can be well compatible with existing semiconductor manufacture technologies. DMSs also have the characteristics of magnetic materials.The major obstacles for DMSs research are to obtain the low Curie temperature and the limitation of solubility of magnetic elements. In 2000, T. Dietl and his colleague predicted the Curie temperatures (TC) for Mn doped GaN can be above the room temperature using the Zener's model. These predictions for GaN have led to the great experimental efforts. GaN-based DMSs can be synthesized by the diffusion method, molecular beam epitaxy(MBE), metal organic chemical vapor deposition (MOCVD) and ion implantation. The diffusion of magnetic elements into GaN is hampered by their solubility and also requires high temperature and long processing time, making it impractical. For the MBE and MOCVD, doping solubility of magnetic elements is always limited in GaN. It is well known that ion implantation has many advantages including independent control of the doping level, selective area doping, and the ability to fabricate planar devices and self aligned structures without any limitation of dopant solubility. Ion implantation is a very effective method for the formation of DMSs. At present, although some experimental studies on the magnetic characteristics of GaN-based DMSs prepared by ion implantation have been reported, little is known about unintentionally doped GaN epilayers implanted with Mn ions.The magnetic ion related properties and the effect of microstructure change on both magnetic and optical properties of Mn-implanted GaN are still not clear and required to be investigated further.Under this background, the electronic structure and optical properties of Mn-doped GaN have been calculated and analyzed using the first-principles plane-wave pseudo-potential approach based on density functional theory. The microstructure, optical, magnetic and electrical properties of GaN based DMSs prepared by Mn ion implantation have been studied systematically. The main results and contributions of this dissertation are as follows:1) The band structure, the density of states, and the optical properties of Mn-doped GaN were calculated and analyzed using the first-principles plane-wave pseudo-potential approach based on density functional theory. It has shown that Mn-doping changes the properties of the films. The calculated results also reveal a spin polarized impurity band in the band structure of GaMnN due to hybridization of Mn 3d and N 2p orbitals. This band renders the material half metallic. Carriers injected from the GaMnN layer will have high spin-polarization if charge carries within it are sufficiently mobile. The dopants can provide large numbers of carriers near the Fermi energy and change the properties of the interband transition of electrons. This unique feature, together with the previously suggested high Curie temperature and inherent compatibility with GaN technology, makes GaMnN a potentially ideal material for spin injection applications.2) Based on the analysis of ion-implantation, the ion implantation range, location of peak concentration and longitudinal straggling of Mn are calculated with the Monte Carlo simulator TRIM. The process to form GaN based DMSs with Mn ion implantation has been proposed, including the implantation energy and dose, annealing conditions.3) The structural and optical properties of Mn-Ion implanted GaN have been investigated. First, the properties of Mn-Ion implanted unintentionally doped GaN have been studied. From XRD measurements, the samples display the characters of Mn-Ion implanted GaN with a contribution of the Mn occupying the Ga sites or the Ga-Mn phases and the Mn-N compounds. Micro-Raman spectra have demonstrated that in addition to GaN like phonon modes, the most significant features of Raman scattering from the present Mn doped GaN layers are new phonon modes and the appearance of left and right shoulders (SL, SR) around E 2high. The new phonon modes have been analyzed firstly by applying the Lorentzian formula to be useful for multi-peaks cases, and it can be seen that there are at least two peaks constituting each shoulder SL and SR respectively. Using the reduced mass model, the phonon modes constituting each shoulder are attributed to the LVM of different MnxNy localized structures and the LVM of Mn atoms in the (Ga,Mn)N. The results of photoluminescence(PL) measurement showed that optical transitions related to ion-implantation appear at 2.53eV and 2.92eV. It can be obtained from the analysis that the peak at 2.92eV is ascribed to a conduction band to deep acceptor transition or a shallow donor to deep acceptor transition, where the deep acceptor level is located about at Ev+0.4eV. It is also believed that the peak at 2.53eV is a transition between a shallow dopant level and a deep acceptor level.The properties of Mn-Ion implanted Mg doped GaN have also been investigated. Micro-Raman spectra showed the similar results as Mn-Ion implanted unintentionally doped GaN. The results of PL measurement showed that a new emission peaking at 1.69eV besides the transitions at 2.54eV and 2.9eV. Considering the characteristics of Mg-doped GaN, the peak at 2.9eV is ascribed to a MgGa-VN complex related deep donor(Dd) to Mg related shallow acceptor transition. It is believed that the peak at 1.69 eV is a transition between the a MgGa-VN complex related deep donor(Dd) level and a VGa related deep acceptor (Ad)level.4) The surface damage induced by ion implantation and damage repaired by annealing were studied. The results showed that the thermal decomposition of GaN surface at high temperatures limits the application of a higher annealing temperature for repairing the damage of the ion-implanted samples. The study also showed that Hot Target in ion implantation process is an effective manner to reduce ion implantation damage.Using different annealing temperatures we have observed the gradual recovery of the crystalline features based on the Raman spectra of Mn-implanted GaN samples. The evolution of the Raman spectra with annealing temperature(TA) suggests the existence of three stages in the lattice recovery process. Firstly, for TA below 800°C, the implanted sample starts its recrystallization and restores the crystalline quality. Then, for TA from 800°C to 900°C, there is better recovery of the crystalline quality and the decrease of some lattice imperfections in Mn-implanted samples after annealing. Finally, for TA above 900°C, the surface of GaN epilayers begins to decompose.Our results suggest that the optimal lattices recovery and ferromagnetic properties are achieved by RTA at 800°C~900°C.5) The magnetic and electrical properties of unintentionally doped GaN have been studied. The room-temperature FM in unintentionally doped GaN epilayers implanted with Mn ions was achieved after being annealed at 700°C, 800°C and 900°C. The highest magnetization was obtained in the samples annealed at 800°C. We believe that the (Ga,Mn)N can be mainly responsible for the observed FM behavior of Mn-implanted unintentionally doped GaN epilayers. The GaxMny phases produced after annealing Mn-implanted unintentionally doped GaN epilayers at 700°C and 800°C provide holes and also enhance the FM. It is further confirmed by the temperature dependency of the Magnetization that Curie temperature of the material is higher than the room temperature. The tests showed that the trends of temperature dependency of the Magnetization has been divided into two parts, which also approved the deduction of the contribution of (Ga,Mn)N and GaxMny phases for the observed FM. The results of C-V and Hall tests showed that the defects introduced by ion implantation have an impact on the concentration of carrier on one hand, and also reduce the carrier's mobility on the other hand. All of these have a significant impact on the properties of DMSs prepared by ion implantation, so should be considered in order to obtain good properties of the materials.6) The magnetic and electrical properties of Mn-Ion implanted Mg doped GaN have also been studied. The characteristics of the magnetic test showed similar results to Mn-Ion implanted unintentionally doped GaN. The room-temperature FM in the sample was achieved after annealing. The highest magnetization was also obtained after annealing at 800°C. The results show that the magnetizations are significantly higher than Mn ion implantation unintentionally doped GaN. Although the trend of M-T curves of the samples is divided into two parts, the change is much smaller. It is believed that the difference between the Mn-ion implanted Mg-doped GaN and the Mn-ion implanted undoped GaN was mainly due to the high concentration of holes in Mn-implanted Mg-doped GaN epilayers, which ensures that the (Ga,Mn)N can be mostly responsible for the observed FM behavior. A weak activation of Mg-doped GaN epitaxial films was carried out at 700℃. When the samples were annealed after ion implantation, there will be a second activation of the Mg ions. So after being annealed at 800℃and 900℃, the concentration of carriers has increased to some extent. As the annealing temperature above 900°C, the surface of GaN begins to decompose, leading to the formation of N vacancies. Thus, some parts of the holes compensate with electrons generated from N vacancies, resulting in the reduction of the hole concentration. This characteristic is also confirmed by the electrical test.