Study Of Mn-doped Ⅳ Group Diluted Magnetic Semiconductors By X-ray Absorption Fine Structure

This thesis presents a comprehensive investigation on the Mn-doped IV group (Ge and Si) diluted magnetic semiconductors (DMSs) prepared by the magnetron co-sputtering method, using the fluorescence X-ray absorption fine structure (XAFS), X-ray diffraction (XRD), atomic force microscope (AFM), and Raman spectrum methods. The local structure parameters including valence, position, distribution, and coordination environment about Mn atoms in the Mn-doped IV group DMS thin films are obtained. We have attempted to model the representative Mn positions in the Mn-doped IV group DMSs, using the FEFF8 code based on an effective one-electron, relativistic, self-consistent, real-space Green's-function formula with the final-state potentials including a screen core hole. Combined with the experimental XAFS results, the existence formation and occupation of the Mn atoms in the matrix have been discussed in detail. Results show that most of Mn atoms occupy the substitutional Si sites in the Mn-doped Si DMS thin films, while the formation of Mn atoms in the Mn-doped Ge DMS thin films depend on the Mn doping concentration. Furthermore, the measurements of magnetic property are also carried out. Careful research indicates that the magnetic property of Si1-xMnx with Mn atoms substituting for Si atoms is in a slightly paramagnetic nature at 50K. The relationship between the structures and magnetism of Mn-doped Si is discussed. This work provides theoretical and experimental support to find IV group diluted magnetic semiconductors with more valuable applications.The ultra high vacuum magnetron co-sputtering system used to grow diluted magnetic semiconductor thin films has been installed in this work. This system consists of a growth chamber (base pressure 6.6×10-6 Pa) with four permanent magnetic targets and a sample preparation chamber (base pressure 6.6×10-5 Pa) with a magnetic sample transfer facility. The growth rate can be controlled within 0.3~3.0 nm/min. It can realize co-sputtering of various materials, such as magnetic metals like Fe, Ni, Co; metals like Cu, Mn, Cr; and semiconductors like Ge, Si, InSb, GaSb etc. Highly qualified Mn-doped IV group thin films have been deposited by varying the growth and postgrowth conditions, such as substrate temperature, sputtering pressure, rotating speed and annealing temperatures.The main content in this thesis is as follows:1. Structures and magnetism of Mn-doped Si DMSsThe structures and magnetism of Mn-doped Si DMSs (Si1-xMnx x=0.03, 0.05, 0.08) were investigated by XRD, XAFS, and SQUID. No diffraction peaks of Si-Mn compounds or Mn metal can be detected by XRD for the Si1-xMnx thin films. And the XAFS results of Mn K-edge of Si1-xMnx thin films with all Mn doping concentration show that only one strong peak of the first Mn-Si shell at about 1.90 ? can be found in the radial structure function (RSF) spectra, which indicate that the Mn atoms in the Si1-xMnx thin films may be incorporated into the Si thin film, and predominantly surrounded by Si atoms. EXAFS fittings indicate that the coordination number NMn-Si≈4.0 and bond length RMn-Si≈2.35 ? of the first shell Mn-Si for the Si1-xMnx thin films are correspondingly close to that NMn-Si (4.0) and RMn-Si (2.35 ?) for the substitutional structure MnSi. To determine the real location of Mn atoms in the Si1-xMnx thin films, we have attempted to calculate the EXAFS spectra for three representative models: the substitutional site (MnSi), the tetrahedral interstitial site (MnIt), and the hexagonal interstitial site (MnIh). The calculations were performed with the FEFF 8 code at the Mn K-edge. Compared the radial structural functions from theoretical calculations and the experimental radial structural functions of the Si1-xMnx thin films, we can deduce that most of Mn atoms occupy the substitutional positions of the Si sites in Si1-xMnx DMS thin films. From XRD and EXAFS results we notice that Si1-xMnx DMS thin films are lack of long-range structural order around Mn atoms, the EXAFS fitting results show that the Debye-Waller factorσ2Mn-Si (0.008~0.010?2) of the first nearest neighbor in the Si1-xMnx thin films are larger than that of the crystal Si, these suggest that the local structure of our co-sputtering grown Si1-xMnx thin films are distorted. The magnetization measurements show that the magnetic property of Si1-xMnx with Mn atoms substituting for Si atoms is in a slightly paramagnetic nature at 50K. The electronic structure calculations show that the 3d states of Mn create deep levels within the band gap of the host, and thus could not hybridize with the 3sp orbitals of Si. The absence of the p-d hybridization makes the ferromagnetic interactions, such as RKKY-like, double exchange, and superexchange interactions, hardly function, leading to the observation of a paramagnetic behavior in the Si1-xMnx DMS samples.2. Structure investigation of Mn-doped Ge DMSsFluorescence XAFS, XRD, AFM and Raman spectrum were used to study the structures and morphology of Ge1-xMnx (x=0.07, 0.25, 0.36) DMS thin films. The roughness of as-grown films rises with the content of doped Mn, as revealed by the AFM measurements. The XRD results show that in the Ge1-xMnx thin film with low Mn doping concentration (x=0.07), only diffraction peaks attributed to crystalline Ge can be observed. In samples with high Mn doping concentration (x=0.25, 0.36), the secondary phase Ge3Mn5 appears, and its content enhances with Mn doping concentration. The Raman spectrum and XAFS spectroscopy indicates that the ordering degree of the as-grown films is gradually deteriorated with increasing Mn content, and that the Ge lattice is slightly expanded for samples with high Mn content. Furthermore, the quantitative analysis of XAFS indicates that in Ge1-xMnx thin film with low Mn doping concentration (x=0.07), Mn atoms are mainly incorporated into the Ge lattice and located at the substitutional sites of Ge atoms with the ratio of 75%, while for the high Mn concentration (x=0.25, 0.36), most of Mn atoms aggregate to form Ge3Mn5.