13th International Conference on Defects--Recognition, Imaging and Physics in Semiconductors: Defects in Silicon
Program Organizers: Marek Skowronski, Carnegie Mellon University; Robert Stahlbush, Naval Research Laboratory; Michael Dudley, State University of New York at Stony Brook
Tuesday AM
September 15, 2009
Room: Glessner Auditorium
Location: Oglebay Resort and Conference Center
Session Chair: Piotr Edelman, SemilabSDI
10:30 AM Invited
Imaging of Metal Precipitates in Silicon by Luminescence Spectroscopy and Synchrotron Techniques: Martin Schubert1; 1Fraunhofer Institute for Solar Energy Systems ISE
Metallic impurities in silicon, precipitated or dissolved, frequently limit the conversion efficiency of solar cells. Impurities, commonly in interaction with crystallographic defects reduce the recombination lifetime of free carriers. Furthermore, shunts as well as reverse breakdown sites may be induced. Detection techniques for dissolved and precipitated transition metals in silicon by fast luminescence imaging and by luminescence spectroscopy with highest resolution are discussed. Band to band recombination as well as defect luminescence is considered. Results on the precipitate distribution in both, industrial multicrystalline silicon, and model systems of intentionally contaminated monocrystalline silicon with defined dislocation networks are presented. X-ray fluorescence spectroscopy measurements at the ESRF synchrotron complement the detection of precipitates.
11:00 AM
Photoluminescence Analysis of Iron Contamination Effect in Multicrystalline Silicon Wafers for Solar Cells: Michio Tajima1; Masatoshi Ikebe1; Yoshio Ohshita2; Atsushi Ogura3; 1Institute of Space and Astronautical Science / JAXA; 2Toyota Technological Institute; 3Meiji University
We investigated the effect of iron contamination on the electronic properties of dislocation clusters and oxygen precipitates in multicrystalline silicon (mc-Si). Photoluminescence (PL) spectroscopy at 300K and 4.2K and monochromatic PL intensity mapping at 300K were performed on mc-Si wafers before and after iron contamination. PL spectra consisted of the band-edge emission, the 0.78 eV emission associated with oxygen precipitates, and the dislocation-related D-lines. Intensity mapping of the band-edge emission revealed the dislocation clusters as dark lines, and the majority part appeared as bright lines in mapping of the 0.78 eV emission, suggesting that the majority part of the dislocation clusters act as preferential oxygen precipitation sites. The iron contamination increased the electrically active dislocation clusters. The electronic property of the dislocation clusters was changed as indicated by the spectral change of D-lines. In contrast, the oxygen precipitates along the dislocation clusters were not influenced by the iron contamination.
11:15 AM
Quantitative Photoelastic Characterization of Residual Strains in Grains of Multi-Crystalline Silicon: Masayuki Fukuzawa1; Masayoshi Yamada1; Rafiqul Islam2; Jun Chen3; Takashi Sekiguchi3; 1Kyoto Institute of Technology; 2Khulna University of Enginerring and Technology; 3National Institute of Material Science
The residual strain and its variation in the multicrystalline Si wafers (mc-Si) for solar cell were quantitatively characterized by scanning infrared polariscope (SIRP). The phase retardation δ and the principal axes Ψ of strain-induced birefringence were measured at each point in grains. The crystallographic orientations of the grains were also characterized by electron-back-scatter-diffraction (EBSD) technique. With these data, the strain was calculated by considering the anisotropy of photoelastic coefficients. It was clarified that the residual strain was large at the grains with multi-twin boundaries and the vicinity of small-angle grain boundaries, which reached to the order of 10-4, corresponding to 10 MPa in stress.
11:30 AM
Observation of Two-Dimensional Distribution of Lattice Inclination and Strain in Strained Si Wafers by Synchrotron X-Ray Topography: Takayoshi Shimura1; Tomoyuki Inoue1; Daisuke Shimokawa1; Takuji Hosoi1; Heiji Watanabe1; Atsushi Ogura2; Masataka Umeno3; 1Osaka University; 2Meiji University; 3Fukui University of Technology
Strained Si technology has attracted substantial attention as a means of enhancing carrier mobility in MOSFETs and thereby extends the limits of device miniaturization and performance. One of the methods available for the fabrication of strained Si devices involves the use of strained Si wafers. However, the crystalline quality of the strained Si wafers fabricated to date remains poor compared to conventional SOI wafers. It is therefore important to evaluate the crystalline quality of the strained Si wafers in order to continue improving this technology. In this paper we show two-dimensional distributions of lattice inclination and strain in strained Si wafers obtained by synchrotron x-ray topography. We observed a series of x-ray topographs obtained by changing the incident angle and derived rocking curves at each pixel of CCD detector. Lattice inclination and strain distributions were estimated by comparing the rocking curves measured at different azimuth angles.
11:45 AM
Correlation between Oxygen Precipitation and Generation of Extended Defects in Czochralski Silicon: Investigation by Means of Scanning Infrared Microscopy: Yuheng Zeng1; Deren Yang1; Xiangyang Ma1; Jiahe Chen1; Duanlin Que1; 1Zhejiang University
Oxygen precipitates and secondary defects in Czochralski silicon (CZ-Si) have received extensive and intensive investigations in the past decades. However, the critical size of oxygen precipitates for inducing secondary defects remains somewhat unclear. In this paper, size effect of oxygen precipitates on secondary defects in the CZ-Si specimens subjected to different isothermal annealing was investigated by scanning infrared microscopy (SIRM). It was found that secondary defects generated in the case that oxygen precipitates grew to a certain size, as it was believed that interstitial Si (I) atoms preferred to aggregate to oxygen precipitates larger than a certain size, which we denoted as the critical size to induce secondary defects. Upon generation of secondary defects, the critical size remains nearly unchanged in the extending annealing. However, the critical size would reduce with annealing at higher temperatures or with higher I-atom concentration. Finally, the reason for the above results was discussed.
12:00 PM
Characteristic Aspects of Low-Temperature Elastic Softening Due to Vacancies in Boron-Doped FZ Silicon Crystals: Hiroshi Yamada-Kaneta1; Hajime Watanabe1; Yuta Nagai1; Shotaro Baba1; Mitsuhiro Akatsu1; Yuichi Nemoto1; Terutaka Goto1; 1Niigata University
We recently found that the vacancy causes the elastic softening at low temperatures whose magnitude gives the vacancy concentration. The softening of B-doped silicon was characterized by the following behaviors: (1) The softening suddenly starts at around 5 K in cooling the sample, and (2) the softening is weakened by the applied magnetic field to vanish at nearly 3 T, in contrast to the softening of the non-doped silicon. Here, we confirm that these characteristics in the softening is general for (and inherent to) the B-doped silicon crystals containing the positively charged vacancies. From various positions in an ingot of B-doped FZ silicon crystal grown by SUMCO TECHXIV CORPORATION, we cut out many samples for the ultrasonic measurements for the softening. For all the samples, the observed softenings exhibited the above-mentioned characteristics, although their magnitudes varied on the ingot position. The origin of this characteristic behavior will also be addressed.
12:15 PM
Optimization of Silicon Ingot Quality by the Numerical Prediction of Bulk Crystal Defects: Fabrice Loix1; François Dupret2; Arnaud de Potter1; Wu Liang1; Roman Rolinsky1; Nathalie Van den Bogaert1; 1FEMAGSoft S.A.; 2UCL
We will here focus on the prediction of Si ingot quality grown by Czochralski process and its optimization by means of numerical simulation. We present a fully time-dependent model devoted to predict the global heat transfer in a furnace, the solid-liquid interface shape, and the resulting distributions of point- and micro-defects as calculated from the Sinno-Dornberger (S-D) model together with an extension of the lumped model of Voronkov and Kulkarni. In addition to the classical point-defect evolution mechanisms, a new lumped model is developed to calculate the formation and growth of micro-defects in order to predict their densities and size distributions anywhere in the crystal. Another key issue in Czochraslki Si growth is to control the density of oxygen and any other species (including dopants and impurities) inside the crystal. Modeling issues will be here again detailed.