Papers by Author: Jun Chen

Abstract: The grain evolution of multicrystalline Si was studied using the ingot grown from microcrystalline template. The grain shape evolution and width increase are not monotonic but may have 3 stages. On the other hand, the grain boundary (GB) analysis suggests that there exit 2 reactions, namely random GB annihilation at the initial stage and Σ3 generation and annihilation at the steady state.

Abstract: We have proposed single seed cast Si growth and developed a furnace for 50 cm square ingots. By optimizing growth parameters, improving gas condition, coating, the quality of mono Si ingot has improved. Namely, dislocation density, the concentrations of substitutional carbon and interstitial oxygen have been significantly reduced. The conversion efficiency of cast Si solar cells has become comparable with those of CZ Si wafers.

Abstract: To get the optimized condition and ideal furnace structure, we have performed seed cast growth of mono-crystalline Si by using unidirectional solidification furnace. More than 20 ingots of 10 cm diameter and 10 cm height were grown under different growth conditions. The quality of ingots was characterized by using Fourier transform infrared spectroscopy (FTIR), infrared microscopy, scanning infrared polariscope (SIRP), X-ray topography, etc. We have realized reduction of carbon, residual strain and extended defects, which may contribute the increase of solar cell efficiency.

Abstract: Grain boundaries and dislocations are major crystallographic defects in multicrystalline Si materials for solar cells. Heavily dislocated grains are detrimental to the photovoltaic performance. This paper attempts to clarify the origin of inhomogeneous defect distribution in multicrystalline Si. The impacts of crystal orientation and grain boundary were investigated. The crystal orientation gives an important geometrical effect in the possibility of initiating slip in a grain when subjected to stress. The presence of grain boundary can also affect dislocation distribution depending on boundary character.

Abstract: A synchrotron white x-ray microbeam diffraction method was employed to investigate lattice distortion in multicrystalline silicon for photovoltaic cells. The measurements were carried out by scanning the sample, and transmission Laue patterns were observed at each position on the sample. Intensity and position maps of the Laue spots showed the distribution of the crystalline quality of the grains and the bending of the lattice planes. Strain and bending distributions were extracted from an analysis of Laue spots at diagonal positions, and these were compared with those obtained by other techniques.

Abstract: Small angle grain boundaries (SA-GBs) are known as the most electrically active defects in multicrystalline silicon. These SA-GBs are classified in “general” and “special” by the normal and strong electrical activity at 300K, respectively. In this study, the origins of these electrical activities of SA-GBs were elucidated by using electron beam induced current (EBIC) and transmission electron microscopy (TEM). It was found that both general and special SA-GBs were composed of edge-type and 60 deg / screw dislocations. The fraction of edge dislocation in special SA-GB was higher than that of general one, which suggests that strong electrical activity is mainly originated in edge dislocations.

Abstract: We have characterized optical property of small-angle (SA) grain-boundaries (GBs) in high-pure multicrystalline Si by using cathodoluminescence (CL). Prior to CL measurement, the electrical activity of GBs were evaluated by using electron-beam-induced current (EBIC). The SA-GBs are categorized into two groups with room temperature (RT-) EBIC contrast. The SA-GBs with misorientation angle about 1º give weak RT-EBIC contrast and yield D3 and D4. The SA-GBs with 2.5º show strong EBIC contrast and yield D1 and D2. These correspondences reflect the dislocation density at the SA-GBs. We also found the curious distribution of D1 emission in some special GBs, which is now difficult to explain. It is noticed that large-angle GBs do not show any D-line emissions at all.

Abstract: We report a dynamic and microscopic investigation of electrical stress induced defects in metal-oxide-semiconductor (MOS) devices with high-k gate dielectric by using electron-beam induced current (EBIC) technique. The correlation between time-dependent dielectric breakdown (TDDB) characteristics and EBIC imaging of breakdown sites are found. A systematic study was performed on pre-existing and electrical stress induced defects. Stress-induced defects are related to the formation of electron trapping defects. The origin of pre-existing defects is also discussed in terms of oxygen vacancy model with comparing different gate electrodes.

Abstract: We report the electrical, structural and mechanical properties of grain boundaries (GBs) in multicrystalline Si (mc-Si) based on electron-beam-induced current (EBIC), transmission electron microscope (TEM), and scanning infrared polariscope (SIRP) characterizations. The recombination activities of GBs are clearly classified with respect to GB character and Fe contamination level. The decoration of Fe impurity at boundary has been approved by annular dark field (ADF) imaging in TEM. Finally, the distribution of residual strain around GBs, and the correlations between strain and electrical properties are discussed.