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Electron Spin Structures and Designs of Low Dimensional Semiconductor Materials

Author: ZhaoXuGuang
Tutor: TangZheng
School: East China Normal University
Course: Microelectronics and Solid State Electronics
Keywords: Magnetic semiconductors Nanocluster Magnetic manipulating First-principles theory Vanadium disulfide Metal insulator transition(MIT)
CLC: TN304
Type: PhD thesis
Year: 2013
Downloads: 48
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Abstract

Since theoretical prediction of ferromagnetism with Curie temperature as high as room temperature (RT) in transition metal (TM) element manganese doped semiconductors (such as ZnO, GaAs, and so on) by T. Dietl, magnetic semiconductor materials have been extensively studied both theoretically and experimentally for their potential applications (for example, non-volatile memory, spintronic devices, etc).Although RT ferromagnetism has been observed in some doped oxide and nitride semiconductor materials, the materials are still far away from realistic applications. There are some fundamental issues still remaining open. The first problem is the controversial magnetic origin and nature. Dopants aggregations or defects are regarded as magnetic origins in doped semiconductors. The second one is that it is difficult to fabricate p type oxide and nitride semiconductors because oxide and nitride semiconductors are naturally n type. The third is that ferromagnetism observed in oxides and nitrides without any dopants challenges the theory of magnetic origins in diluted magnetic semiconductors (DMSs), and new chances are provided to design and manipulate exotic materials. In this thesis, some new material and low dimensional structure systems aiming at overcoming these key problems are designed. The detailed researches are as follows.1) p type CuAlO2semiconductor material.We carried out both theoretical and experimental studies about TM elements doped CuAlO2to clarify the magnetic origins of DMSs. Meanwhile, purities and unreacted raw materials were excluded to avoid disturbing magnetic measurements, electronic and spin structures of low dimensional nanosized materials without any dopants were calculated. The reason of choosing and studying CuAlO2is that it is natrual p type semiconductors and both p and n type conductivity can be achieved by appropriate doping. Our theoretical and experimental studies on Co and Ni doped CuAlO2indicated that soluble concentrations of Co or Ni in samples are generally lower than5%. Our magnetic measurements and hybrid density functional calculations demonstrated that all the doped systems are nonferromagnetic, however, ferromagnetism was predicted in TM doped CuA102by the conventional density functional theory of Sato et al. The precedings and our researches demonstrate that first-principles theory calculations may overestimate the ferromagnetic interactions among magnetic dopants in the strongly correlated oxide semiconductors.2) Nanostructures without any dopants.Our calculations about low dimensional semiconductors (ZnO, GaN, CdS and MgO) without any dopants indicate that magnetism of nanoclusters mainly originates from the dangling bond spins of2-coordinate anions (for example, oxygen or nitrogen).3-coordinate anions contribute little to the total magnetic moments, however,4-coordinate anions and all the cations in nanoclusters contribute none. The relation between Heisenberg interexchange interations and the numbers of anions with the least coordinate number is nonlinear. These results can be explained in terms of RKKY interaction. The relation between Curie temperature and the numbers of anions with the least coordinate number is similar to that of magnetism in nanoclusters. Meanwhile, some effective schemes for manipulating magnetism of low dimensional nanosized materials were proposed. These schemes include structure designs, hydrogen passivation and injection of electrons or holes.3) Two-dimensionl semiconductor monolayer and its metal insulator transition (MIT).Successes in fabricating grapheme triode and logic circuits, single-layer MoS2transistors and VS2supercapcitors demonstrate promising perspectives of low dimensional nanosized materials in manufacturing electronic devices, and it stimulates the researchers to study these low dimensional materials. MIT characteristics of these monolayer materials are basic for fabricating electronic devices. Moreover, MIT temperature (308K) is as high as RT for VS2monolayer without additional manipulating. This is one of the main causes to study this kind of material. By virtue of the theory of Peierls phase transition and charge density wave (CDW), systematic calculations about monolayer IT-VS2were carried out based on the first-principles theory. These calculations include electronic structures, density of states, Fermi surfaces, phonon dispersion spectra and theoretical derivations of phase transition. Our calculated results indicate that MIT indeed exists in this kind of material and crystal symmetry, electronic structure, the shape of Fermi surface and electron diffraction patterns agree well with the previous experimental results. This indicates that our methodology and theory are feasible; meanwhile, it paves the way for us to study similar low dimensional phase transition materials.

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