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Preparation, Structure and Magnetic Properties of SiC-based Diluted Magnetic Semiconductors

Author: LvZhiCong
Tutor: ZhengHaiWu
School: Henan University
Course: Condensed Matter Physics
Keywords: SiC Diluted magnetic semiconductors Structure Magnetic properties
CLC: O472.5
Type: Master's thesis
Year: 2013
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Abstract

Diluted magnetic semiconductors can be formed by doping transition metal or rare-erarh ions intoconventional semiconductors. In recent years, DMSs have attracted considerable attention due to theirpossibility of manipulating charge and spin degrees of freedom in a single material. DMS are one of themost promising materials for potential application in spintronic device due to their favorable magnetic,magneto optical, and magneto electrical properties. As one of the most important wide-gap semiconductors,SiC was considered as a promising DMSs matrix material due to its outstanding intrinsic characters such ashigh thermal conductivity, high breakdown field, and high saturation velocity. Ferromagnetic ordering （FM）of SiC based DMSs prepared by various methods has been established at a wide temperature range, and theorigin of the FM is still unclear. Therefore, for the practical application of DMSs, a crucial prerequisite isthat the ferromagnetism should be retained at room temperature.Recently, miniaturization of electronic devices in industrial applications and scientific research makes1D semiconductor materials more promising than films. In view of this, the nanowires are currently beingexplored as possible building blocks for electronic and opto-electronic devices. DMS nanowires would beimportant for application in nanoscale spintronic devices using electronic spin as an additional degree offreedom. In this work, we have synthesized Fe-doped3C-SiC nanowires by gas-solid growth method. Themicrostructure and magnetic properties of Fe-doped3C-SiC nanowires are detailed analysised.Ion implantation is an efficient way of introducing magnetic ions into host semiconductors, and theimplant process is also attractive for its ability to create selective magnetic regions. In this work, we haveimplemented N-ion implantation and Cu-ion implantation in6H-SiC single crystal, respectively.The main content of this thesis is as follows:（1） Undoped and Fe doped SiC nanowires were prepared by gas-solid growth method underappropriate pressure, gas flowing and temperature. High purity SiO, high purity graphite powders, highpurity carbon nanotube and high purity Fe powders were used as starting materials. The phase and valencestate of the nanowires were examined by powder X-ray diffraction and X-ray photoelectron spectroscopy.Morphology and microstructure were observed by field emission scanning electron microscopy and transmission electron microscopy. The magnetic properties were measured with a commercial PhysicalProperty Measurement System. The results demonstrated that the nanowires had single-crystallineFe-doped cubic silicon carbide （3C-SiC） structure. The diameter of the nanowires ranged from50to200nm, with a length up to tens of micrometers. The high resolution transmission electron microscopy andselected area electron diffraction indicated that the nanowire grew along the [111] direction with the latticespacing of0.248nm. The magnetic properties measurement showed that the nanowires exhibited roomtemperature ferromagnetism behavior. We proposed that Fe doping along with the defects have greatinfluence on local moment formation and collective magnetization. In addition, the FM may also originatefrom the existence of uncompensated spins and surface anisotropy since the nanowires have a very highsurface-to-volume ratio.（2） n-type6H-SiC （0001） single crystal implanted with N+ions with an energy of160keV and withtwo different doses:2×10¹⁶cm^-2and1×10¹⁷cm^-2at room temperature. During the implantation, the waferwas tilted7ofrom the normal to minimize the channeling defect. The wafer was subsequently rapid thermalannealed at850oC for10min under the protection of flowing N₂. The phase, the morphology, evolution ofthe defects and magnetic characteristics of6H-SiC single crystal samples were studied by means of X-raydiffraction, atomic force microscopy, positron annihilation spectroscopy and superconducting quantuminterference device magnetometer, respectively. The absence of magnetic clusters or secondary phases wasconfirmed by X-ray diffraction. The morphological characteristics of implanted sample deteriorated asshown by atomic force microscopy analysis. Positron annihilation spectroscopy analysis indicated the maindefect type was silicon vacancy (V_Si). It is speculated that the vacancy defects together with somesubstituted N atoms should jointly be responsible for the room temperature ferromagnetism.（3） n-type6H-SiC （0001） single crystal implanted with Cu+ions with an energy of200keV and a doseof8×10¹⁵cm^-2at room temperature. During the implantation, the wafer was tilted7ofrom the normal tominimize the channeling defect. The wafer was subsequently rapid thermal annealed at850oC for15minunder the protection of flowing N₂. No ferromagnetism （FM）-related secondary phases was confirmed byX-ray diffraction and X-ray photoelectron spectroscopy. Positron annihilation lifetime spectroscopyanalysis indicated the main defect type was silicon vacancy (V_Si) and the concentration of it increased afterCu implantation. The substituted Cu along with the irradiation induced vacancies type defects play an important role in room temperature ferromagnetism.

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