Yoshiji Miyamura

Gettering of Cu and Ni Impurities in SIMOX Wafers

The gettering of Cu and Ni impurities in intentionally contaminated SIMOX wafers have been studied by means of cross-sectional transmission electron microscopy, nanoprobe energy dispersive x-ray spectroscopy, secondary ion mass spectrometry, and selective etching. The wafers with Cu or Ni surface concentrations ranged from about 10 12 up to 10 17 atom/cm 2 were annealed at various temperatures followed by slow cooling to room temperature. Single and multistep thermal treatments were applied. It has been found that the buried oxide does not prevent the diffusion of both Cu and Ni contaminants from the top silicon layer into the bulk substrate at the whole investigated temperature range from 600 to 950°C. Moreover the effective gettering of Cu and Ni in the thin silicon substrate layer located just beneath the buried oxide has been observed and explained as being due to the heterogeneous impurity precipitation at stacking fault tetrahedra formed there during the SIMOX manufacturing. The gettering process has remained stable during the thermal simulation of CMOS device process of new generation ICs with 0.25 μm feature size.

Evaluation of defects generation in crystalline silicon ingot grown by cast technique with seed crystal for solar cells

Although crystalline silicon is widely used as substrate material for solar cell, many defects occur during crystal growth. In this study, the generation of crystalline defects in silicon substrates was evaluated. The distributions of small-angle grain boundaries were observed in substrates sliced parallel to the growth direction. Many precipitates consisting of light elemental impurities and small-angle grain boundaries were confirmed to propagate. The precipitates mainly consisted of Si, C, and N atoms. The small-angle grain boundaries were distributed after the precipitation density increased. Then, precipitates appeared at the small-angle grain boundaries. We consider that the origin of the small-angle grain boundaries was lattice mismatch and/or strain caused by the high-density precipitation.

Grain boundary interactions in multicrystalline silicon grown from small randomly oriented seeds

Grain boundary (GB) evolution in multicrystalline silicon grown from small randomly oriented seeds was investigated by statistical analysis of GB interactions (triple junction) with respect to growth height. As grain size increased with growth, the number of GB interactions decreased. The fraction of GB annihilation interactions (which decrease the total number of GBs) is higher throughout growth and increases with growth height in comparison with that of GB generation interactions (which increase the total number of GBs). The dominant GB interaction is that involving Σ3 GBs, especially at the later stage of growth. The impact of GB interactions on grain structure evolution is also discussed.

Advantage in solar cell efficiency of high-quality seed cast mono Si ingot

We have grown 50 cm2 mono Si ingots by the seed cast technique. The carbon and oxygen concentrations of the ingots have been significantly reduced by improving the gas flow condition and coating. The dislocation density was also reduced by eliminating the extra dislocation generation sources. Owing to these developments, the lifetime of wafers has reached 465 µs. Finally, the efficiency of 18.7% has been achieved, which is comparable to 18.9% of the reference Czochralski (CZ) Si wafer.

Impact of Light-Element Impurities on Crystalline Defect Generation in Silicon Wafer

In multi-crystalline silicon grown by unidirectional solidification, there are many origins of crystalline defects. In this study, we investigated the effect of light-element impurities on the generation of crystalline imperfections during crystal growth. In order to control the interfusion of impurities, we regulate the Ar gas flow in the atmosphere on the basis of a computer simulation. The etch pit densities in the sample fabricated without and with Ar gas flow control in the atmosphere were 1.5×105–7.0×107 and 5.0×103–4.0×105 cm-2, respectively. In the sample fabricated without Ar gas flow control, the precipitates consisting of light-elements were observed in the region where the etch pit density markedly increased. In the region with the highest etch pit density, there were small-angle grain boundaries consisting of dislocations. We believed that the precipitates consisting of light-element impurities were the potential origins of small-angle grain boundaries. The light-element impurities should affect the crystalline defect generation induced during crystal growth, and thereby should be controlled.

Effect of Fe Impurities on the Generation of Process-Induced Microdefects in Czochralski Silicon Crystals

The enhancement effect of Fe impurities on the generation of surface and bulk microdefects, such as oxidation-induced stacking faults, oxide precipitates, precipitate-dislocation complexes and bulk stacking faults, has been observed in annealed silicon wafers prepared from Czochralski grown crystals intentionally contaminated with iron. The effect remains significant even for an Fe concentration as low as 1012 atoms/cm3. It has been found that Fe facilitates the nucleation of oxide precipitates in silicon. The mechanism of Fe-assisted nucleation of oxide precipitates is discussed. The effect of Fe on the generation of oxidation-induced stacking faults is explained, assuming that both oxide precipitates and Fe:Si precipitates formed near the wafer surface serve as active nucleation sites of these microdefects.

Effect of Crystallinity on Residual Strain Distribution in Cast-Grown Si

We report the correlation between the crystallinity of ingots grown by the directional solidification technique and the residual strain and dislocation distribution. It was found that mono-like ingots have a 20–25% higher averaged residual strain than multicrystalline Si ingots grown under the same conditions. However the existence of local high-strained areas, which were frequently found in multicrystalline Si ingots, is reduced in mono-like Si ingots. In addition, a reduction in dislocation density was observed. This effect and the decrease in local high strain could be attributed to the decrease in grain boundaries in the mono-like ingots.

Characterization of Residual Strain in Si Ingots Grown by the Seed-Cast Method

The residual strain distribution in cast-grown mono-like Si ingots is analyzed. The effect of the crucible during solidification and the influence of different cooling rates is described. To clarify in which process steps residual strain accumulates, several Si ingots were grown in a laboratory scale furnace (100mm) using different cooling conditions after completion of the solidification. For the cooling, two different cooling rates were distinguished: fast cooling (12deg/min) and slow cooling (5deg/min). It was found that changes in cooling gradients greatly influence the amount of residual strain. The results show that slow cooling in any temperature range leads to strain reduction. The greatest reduction could be found when the temperature gradient was changed to slow cooling in the high temperature region.

Numerical analysis of the dislocation density in multicrystalline silicon for solar cells by the vertical bridgman process

We studied the effects of cooling process on the generation of dislocations in multicrystalline silicon grown by the vertical Bridgman process. From the temperature field obtained by a global model, the stress relaxation and multiplication of dislocations were calculated using the Haasen-Alexander-Sumino model. It was found that the multiplication of dislocations is higher in fast cooling processes. It was confirmed that residual stress is low at high temperatures because the movement of the dislocations relaxes the thermal strain, while the residual stress increases with decreasing temperature, because of reduced motion of dislocations and formation of a strain field at lower temperatures.

Silicon bulk growth for solar cells: Science and technology

The photovoltaic industry is in a phase of rapid expansion, growing by more than 30% per annum over the last few decades. Almost all commercial solar cells presently use single-crystalline or multicrystalline silicon wafers similar to those used in microelectronics; meanwhile, thin-film compounds and alloy solar cells are currently under development. The laboratory performance of these cells, at 26% solar energy conversion efficiency, is now approaching thermodynamic limits, with the challenge being to incorporate these improvements into low-cost commercial products. Improvements in the optical design of cells, particularly in their ability to trap weakly absorbed light, have also led to increasing interest in thin-film cells based on polycrystalline silicon; these cells have advantages over other thin-film photovoltaic candidates. This paper provides an overview of silicon-based solar cell research, especially the development of silicon wafers for solar cells, from the viewpoint of growing both single-crystalline and multicrystalline wafers.