Defect Characterization in SiGe/SOI Epitaxial Semiconductors by Positron Annihilation
© The Author(s) 2010
Received: 2 July 2010
Accepted: 22 September 2010
Published: 24 October 2010
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 SiO2/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.
KeywordsPositron annihilation spectroscopy Ultra-thin body films SiGe semiconductors Point defects
Silicon–germanium (SiGe) has gained much attention in recent years thanks to its promising electrical and material properties. The complete solubility of the two elements enables band gap engineering, and SiGe is relatively easy to integrate into silicon technology. There are, however, still numerous issues regarding the electrical and material properties of SiGe that have to be clarified, e.g. the formation of electrically active defect complexes such as vacancy-type defects.
The positron annihilation technique is an established method for investigating point defects in materials . When a positron is implanted into condensed matter, it annihilates with an electron and emits two 511 keV γ-rays. The energy spectrum of the annihilation γ-rays is broadened due to the Doppler effect associated with the momentum component of the annihilating electron–positron pair. Positrons tend to be localized in vacancy-type defects because of the Coulomb repulsion from ion cores. Since the momentum distribution of electrons in such defects differs from that in bulk materials, one can detect the defects by measuring the Doppler-broadening spectra of annihilation radiation. A frequently adopted parameter used for characterizing the change in the Doppler-broadening spectra is the so-called S parameter, which mainly reflects changes in the low-momentum region of the electron momentum distribution .
Previous investigations comprise the study of semiconductor layers (usually in the range of hundreds of nanometers) and semiconductor/oxide systems. The present contribution deals with positron implantation into UTB-SOI (ultra-thin body-silicon-on-insulator) and SiGe/SOI multilayer structures (partially presented in Ref. ). The position and chemical environment of the SiO2/Si interface in the UTB-SOI was well characterized. SiGe/SOI have been proposed as an efficient way of producing strain-free substrates by strain equalization between the top crystalline layers or by strain transfer to the buried oxide [3, 4]. Chemical analysis of the annihilation site in the SiGe films suggests a prevalent decoration of the trapping sites (vacancy-like defects) with Ge atoms associated with misfit dislocations and threading dislocations. The capabilities of PAS include the identification and analysis of different type of defects in epitaxial SiGe thin films and the UTB-SOI substrate in a nondestructive manner.
The UTB-SOI wafers, purchased from SOITEC™, were prepared according to the SmartCut™ process, based on implantation and wafer bonding . All the SOI wafers used have a (100) crystal orientation and are slightly p-doped (Boron 0.6−1.6 × 1015 cm−3), as was the Si reference sample.
Characteristics of the samples used in current measurements
2 Si/147 SiO2/Si
50 Si0.64Ge0.36/10 Si/147 SiO2/Si
50 Si0.64Ge0.36/10 Si/147 SiO2/Si
33 min at 750°C
with Z m in nanometers when density ρ and positron implantation energy E are expressed in grams per cubic centimeter and keV, respectively .
The gamma rays produced by positron annihilation were detected by means of a high purity germanium detector with resolution (FWHM) of about 1.32 keV at 511 keV. For each implantation energy, approximately 105 counts were accumulated in the annihilation peak. Measurements were taken in high vacuum conditions, ~10−9 mbar. The annihilation energy spectra have been analyzed both by extracting the full annihilation peak shape and by integrating the annihilation peak in the energy interval |E–511 keV| ≤ 0.85 keV (S parameter). The area under the peak (|E-511 keV| ≤ 4.25 keV) was used for normalization. Since the energy of the annihilation radiation is Doppler shifted in the laboratory frame as a consequence of a finite momentum of the positron–electron annihilating pair along the line that connects the sample to the gamma ray detector, the annihilation peak changes its shape according to the momentum distribution of the electron cloud seen by the positron. An increase of the S parameter thus reflects an increased annihilation rate with free (i.e. valence) electrons, while a broadening of the peak can be linked to an enhanced interaction rate with more bound (i.e. core) electrons.
Results and Discussion
Results obtained in the Si/SiO2/Si SOI heterostructure from the positron lineshape profile (Fig. 1) using VEPFIT
L + (nm)
0.508 ± 0.003
17 ± 4
149 ± 3
0.541 ± 0.002
0.525 ± 0.004
220 ± 20
0.551 ± 0.001
The chemical sensitivity of positrons has been demonstrated in metals and semiconductor systems [16, 17]. Following the procedure outlined in , we have reproduced the measured annihilation peak shapes with a linear combination of Doppler spectra of the semiconductor/oxide interface and the Ge signal of a sample saturated of defects. The obtained fits are shown in Fig. 4. As discussed in Ref. , the results of this latter analysis to the data of Fig. 4 show an increment in the germanium component from (22 ± 10)% to (47 ± 8)% after the annealing. The indetermination of this component is associated with the spread of the experimental data, especially at high momentum. In particular, in order to better reproduce the measured Doppler spectra on the whole momentum scale, we have employed, instead of the annihilation peak shape characteristics of annihilation into bulk germanium, the shape measured from a thick (in the microns range) layer of germanium grown on silicon. The abrupt junction between germanium and silicon promotes the formation of misfit dislocations at the interface and threading dislocation that run across the whole germanium layer with an estimated density of about 109 cm−2. Positrons are trapped at defect sites causing a substantial reduction of the diffusion length and a slight increase of the S parameter. This experimental finding can be explained by postulating positron annihilation at vacancies associated with dislocations, or at negatively charged centers associated with dislocations, given that the deposited Ge layer was free of contaminants or dopant atoms, which are known to produce positron trapping defects . The decomposition of the annihilation spectra into two terms (interface and “germanium defects”) gives an acceptable fit (χ2/[degrees of freedom] values of 1.5 in the as-grown sample and 1.2 in the annealed sample) and conveys the idea of a prevalent decoration of positron trapping centers with germanium atoms (as already pointed out by Rummukainen et al.  in bulk SiGe layers), mainly associated with Si vacancies.
The SiO2/Si interface in the UTB-SOI was identified with accuracy. It was possible to estimate the depth where the interface is located with good precision. The chemical analysis at the surface and the interface shows that positrons do not annihilate into large defects (voids), but rather that they are strongly localized close to the silicon surface. The observed momentum distribution is characteristic of annihilation in a relatively well-ordered oxide structure, typical of low quartz , and the strong localization can be explained with annihilation at negatively charged centers, like silicon dangling bonds [7, 14].
The process of strain relaxation in thin SiGe layers grown on SOI substrates has been analyzed. Relaxation of the strained structure has been found to proceed via the introduction of new defects, presumably Si vacancies, able to trap positrons. Chemical analysis of the annihilation site suggests a prevalent decoration of the trapping sites with germanium atoms. The formation of lattice defects in the form of misfit dislocations between two adjacent layers and threading dislocations in the SiGe substrate are certainly associated with the identified defects.
It is demonstrated that the analysis of positron data coming from extremely thin surface layers is possible thanks to the reduced implantation range of positrons at low implantation energy and to the enhanced contrast due to the prevalent annihilation of not trapped positrons with strongly bound surface/interface oxide electrons.
This work has been partially supported by the CARIPLO project MANDIS.
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