1. Strained MxSi1-x(M=Fe, Mn) layer on virtual substrate: growth, morphology, and magnetic properties
Yuanmin Shao, and Zuimin Jiang
Phys. Dept., Fudan Univ., Shanghai, People’s Republic of China
Motivation
Si-based dilute magnetic semiconductors (DMSs) are regarded as promising function materials for spintronics.
Strain could be used to ajust the distance between impurities and Si atoms in DMSs so as to change the their magnetism. Therefore, it is worth investigating the correlation among the strain, microstructures and physical properties of the films.
I. Fabrication of GexSi1-x virtual substrate
Growth conditions
AFM height image
Raman spectra
excitation wavelength: 325nm
excitation wavelength: 514nm
300nm
Ge 20%
Ge 30%
Ge-Si
Si-Si (substrate)
Ge-Si
Si-Si
Si-Si (GexSi1-x)
250nm LT-Si
Ge-Si
Si (001)
Ge-Si
10 μ m× 10 μm
10 μ m× 10 μm
A low temperature (LT) Si buffer layer was first grown on Si (001) substrates at 450℃ to reduce the threading dislocation density in the SiGe layer. Then a GexSi1-x films was grown on the top of the LT Si buffer at 550℃.
Roughness: 0.67nm
RMS: 0.85nm
Roughness: 1.37nm
RMS: 1.82nm
From the positions of Ge-Si peaks and Si-Si peaks with the 325nm radiation, the GeSi strain relaxation of the Ge0.2Si0.8 sample surface was determined to be 98%, while that of the Ge0.3Si0.7 sample was 89%.
Smooth surfaces with no straight crosshatch lines were fabricated and we are able to keep the surface roughness of Ge0.2Si0.8 layer under 1 nm.
II. Fabrication of strained MxSi1-x(M=Fe, Mn) layer on Ge0.2Si0.8 Virtual Substrate
Raman spectra
Growth conditions
strained
unstrained
Strained MnxSi1-x layer
Unstrained MnxSi1-x layer
FexSi1-x layer
2nm Si
2nm Si
10nm MxSi1-x
10nm MxSi1-x
Si-Si (511.9)
Si-Si (518.5) ×9
excitation wavelength: 325nm ×9
10nm Ge0.2Si0.8
excitation wavelength: 325nm
Si-Si (520.7)
10nm Si
excitation wavelength: 325nm
10nm MxSi1-x
10nm MnxSi1-x
MnSi1.73
300nm
MnSi
MnSi1.73
300nm LT-Si
MnSi
300nm LT-Si
Si substrate
Si-Si (516.3)
amorphous Si
amorphous Si
Si substrate
The MnxSi1-x multilayers were grown at 200 ℃ and then in-situ annealed at 400 ℃. From the Raman spectra of as-grown sample, we find MnSi phase has formed before the crystallization of Si. With the annealing temperature rising to 600 ℃, Si was crystallized and MnSi phase changed into MnSi1.73 phase. Furthermore, even in the unstrained sample, we could still find the Si-Si peak deviating from cm-1, which suggests the Mn-Si alloys introduced tensile strain in Si.
The FexSi1-x multilayers were grown at 400 ℃. However, no Fe-Si phases were found in Fe doped Si multilayers.
On the top of Ge0.2Si0.8 virtual substrate, strained and unstrained M doped Si multilayers were grown.
III. Magnetic properties of strained MxSi1-x(M=Fe, Mn) layer on Ge0.2Si0.8 Virtual Substrate
Unstrained MnxSi1-x layer
Strained MnxSi1-x layer
Unstrained FexSi1-x layer
Strained FexSi1-x layer
After annealing at 700℃, the magnetism of strained sample is stronger than unstrained one. The reason may be the strain in multilayer, after the crystalline of Si, making the magnetism of MnSi1.73phase stronger.
Magnetism of strained multilayer is stronger than unstrained one. The reason of this results is supposed to be that strain will loosen Si crystal structure, which will make more Fe atoms dope into Si lattice. Although this supposition has not been proved, we make sure strain will change the magnetism of DMSs.
IV. Conclusions
Smooth GexSi1-x virtual substrates were frabricated on Si (001) substrate.
The microstructures of strained and unstrained multilayers were determined by Raman spectra. No Fe-Si alloys were found to form at the grown temperature of 400 ℃. However, in MnxSi1-x multilayers, MnSi was found to form before the crystallization of Si, which is harmful to the formation of DMSs. Furthermore, at the annealing temperature of 700℃, the crystallization of MnSi1.73 would introduce extra tensile strain in crystal Si.
Magnetism of strained MnxSi1-x and FexSi1-x multilayer are all stronger than unstrained ones, providing a new way to adjust the magnetism of DMSs by strain.