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Defect Characterization in SiGe/SOI Epitaxial Semiconductors by Positron Annihilation.

Ferragut R, Calloni A, Dupasquier A, Isella G - Nanoscale Res Lett (2010)

Bottom Line: The chemical analysis indicates that the interface does not contain defects, but only strongly localized charged centers.In order to promote the relaxation, the samples have been submitted to a post-growth annealing treatment in vacuum.After this treatment, it was possible to observe the modifications of the defect structure of the relaxed film.

View Article: PubMed Central - HTML - PubMed

Affiliation: L-NESS, Dipartimento di Fisica, Politecnico di Milano, via Anzani 42, 22100 Como, Italy.

ABSTRACT
The potential of positron annihilation spectroscopy (PAS) for defect characterization at the atomic scale in semiconductors has been demonstrated in thin multilayer structures of SiGe (50 nm) grown on UTB (ultra-thin body) SOI (silicon-on-insulator). A slow positron beam was used to probe the defect profile. The SiO(2)/Si interface in the UTB-SOI was well characterized, and a good estimation of its depth has been obtained. The chemical analysis indicates that the interface does not contain defects, but only strongly localized charged centers. In order to promote the relaxation, the samples have been submitted to a post-growth annealing treatment in vacuum. After this treatment, it was possible to observe the modifications of the defect structure of the relaxed film. Chemical analysis of the SiGe layers suggests a prevalent trapping site surrounded by germanium atoms, presumably Si vacancies associated with misfit dislocations and threading dislocations in the SiGe films.

No MeSH data available.


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a S parameter as a function of the positron implantation energy in the SOI sample. Error bar is shown for one point only. bDashed lines represent the fractions of positrons implanted into the oxide (blue) and the silicon substrate (red), calculated according to Ref. [11]. The continue and dash-doted lines represent the fractions of positrons that annihilate after diffusion in the oxide (blue), at the buried interface (green) and into the substrate (red). Surface effects are not visible in the picture (important only at low energies). The upper scale gives the mean positron implantation depth calculated according to Eq. 1
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Figure 1: a S parameter as a function of the positron implantation energy in the SOI sample. Error bar is shown for one point only. bDashed lines represent the fractions of positrons implanted into the oxide (blue) and the silicon substrate (red), calculated according to Ref. [11]. The continue and dash-doted lines represent the fractions of positrons that annihilate after diffusion in the oxide (blue), at the buried interface (green) and into the substrate (red). Surface effects are not visible in the picture (important only at low energies). The upper scale gives the mean positron implantation depth calculated according to Eq. 1

Mentions: Figure 1 shows the results of positron implantation into a bare SOI substrate with an extremely thin (~2 nm) Si layer on top. The S parameter evolution with the implantation energy is shown in panel a, while panel b shows the fraction of positrons annihilated into the oxide, the substrate, and into the buried interface as computed by the application of a positron implantation and diffusion algorithm by means of the VEPFIT program [8]. Although positrons are not directly implanted at interfaces, a substantial fraction of positrons should annihilate into the buried interface at implantation energies higher than 2 keV. The evident dip in the S parameter curve at about 3.5 keV is certainly related to strong positron trapping at interface. This is confirmed by the excellent fit of the experimental data with the VEPFIT model (solid line in Fig. 1a). This line is the outcome of several attempts with different models (sets of VEPFIT input data), which in all cases imply the presence of a positron trapping region corresponding to the nominal position of the Si/SiO2 interface. The VEPFIT curve in Fig. 1a was obtained by assuming an oxide surface and four more layers: Si, SiO2, SiO2/Si interface, and a semi-infinite Si layer. The corresponding best-fit values of positron diffusion lengths L+, thicknesses, and S-parameters of the different layers are reported in Table 2. It was necessary to fix some parameters (labeled F in Table 2). In accordance with Refs. [9] and [10], the interface was modeled as a very thin layer (1 nm) with a short positron diffusion length (L+~1 nm). The best-fit value of the depth of the interface coincides within the experimental error with the nominal value. The introduction of an electric field near the interface region is also possible (~300 V/cm), but in this case, it is necessary to fix the interface position at the nominal value (147 nm). Attempts to introduce another absorbing layer at the first Si/SiO2 interface seem arbitrary.


Defect Characterization in SiGe/SOI Epitaxial Semiconductors by Positron Annihilation.

Ferragut R, Calloni A, Dupasquier A, Isella G - Nanoscale Res Lett (2010)

a S parameter as a function of the positron implantation energy in the SOI sample. Error bar is shown for one point only. bDashed lines represent the fractions of positrons implanted into the oxide (blue) and the silicon substrate (red), calculated according to Ref. [11]. The continue and dash-doted lines represent the fractions of positrons that annihilate after diffusion in the oxide (blue), at the buried interface (green) and into the substrate (red). Surface effects are not visible in the picture (important only at low energies). The upper scale gives the mean positron implantation depth calculated according to Eq. 1
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2991171&req=5

Figure 1: a S parameter as a function of the positron implantation energy in the SOI sample. Error bar is shown for one point only. bDashed lines represent the fractions of positrons implanted into the oxide (blue) and the silicon substrate (red), calculated according to Ref. [11]. The continue and dash-doted lines represent the fractions of positrons that annihilate after diffusion in the oxide (blue), at the buried interface (green) and into the substrate (red). Surface effects are not visible in the picture (important only at low energies). The upper scale gives the mean positron implantation depth calculated according to Eq. 1
Mentions: Figure 1 shows the results of positron implantation into a bare SOI substrate with an extremely thin (~2 nm) Si layer on top. The S parameter evolution with the implantation energy is shown in panel a, while panel b shows the fraction of positrons annihilated into the oxide, the substrate, and into the buried interface as computed by the application of a positron implantation and diffusion algorithm by means of the VEPFIT program [8]. Although positrons are not directly implanted at interfaces, a substantial fraction of positrons should annihilate into the buried interface at implantation energies higher than 2 keV. The evident dip in the S parameter curve at about 3.5 keV is certainly related to strong positron trapping at interface. This is confirmed by the excellent fit of the experimental data with the VEPFIT model (solid line in Fig. 1a). This line is the outcome of several attempts with different models (sets of VEPFIT input data), which in all cases imply the presence of a positron trapping region corresponding to the nominal position of the Si/SiO2 interface. The VEPFIT curve in Fig. 1a was obtained by assuming an oxide surface and four more layers: Si, SiO2, SiO2/Si interface, and a semi-infinite Si layer. The corresponding best-fit values of positron diffusion lengths L+, thicknesses, and S-parameters of the different layers are reported in Table 2. It was necessary to fix some parameters (labeled F in Table 2). In accordance with Refs. [9] and [10], the interface was modeled as a very thin layer (1 nm) with a short positron diffusion length (L+~1 nm). The best-fit value of the depth of the interface coincides within the experimental error with the nominal value. The introduction of an electric field near the interface region is also possible (~300 V/cm), but in this case, it is necessary to fix the interface position at the nominal value (147 nm). Attempts to introduce another absorbing layer at the first Si/SiO2 interface seem arbitrary.

Bottom Line: The chemical analysis indicates that the interface does not contain defects, but only strongly localized charged centers.In order to promote the relaxation, the samples have been submitted to a post-growth annealing treatment in vacuum.After this treatment, it was possible to observe the modifications of the defect structure of the relaxed film.

View Article: PubMed Central - HTML - PubMed

Affiliation: L-NESS, Dipartimento di Fisica, Politecnico di Milano, via Anzani 42, 22100 Como, Italy.

ABSTRACT
The potential of positron annihilation spectroscopy (PAS) for defect characterization at the atomic scale in semiconductors has been demonstrated in thin multilayer structures of SiGe (50 nm) grown on UTB (ultra-thin body) SOI (silicon-on-insulator). A slow positron beam was used to probe the defect profile. The SiO(2)/Si interface in the UTB-SOI was well characterized, and a good estimation of its depth has been obtained. The chemical analysis indicates that the interface does not contain defects, but only strongly localized charged centers. In order to promote the relaxation, the samples have been submitted to a post-growth annealing treatment in vacuum. After this treatment, it was possible to observe the modifications of the defect structure of the relaxed film. Chemical analysis of the SiGe layers suggests a prevalent trapping site surrounded by germanium atoms, presumably Si vacancies associated with misfit dislocations and threading dislocations in the SiGe films.

No MeSH data available.


Related in: MedlinePlus