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Stress and doping uniformity of laser crystallized amorphous silicon in thin film silicon solar cells
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Image of FIG. 1.
FIG. 1.

Schematic of the beveled sample showing the extended film thickness (stretching factor) of the thin crystallized a-Si layer as a result of beveling. Raman measurements taken along the wedge increase the resolution of Raman spectroscopy to the nanometer scale.

Image of FIG. 2.
FIG. 2.

(a) Optical image of the beveled calibration sample [ crystallized a-Si on borosilicate glass (Ref. 25)] after Secco etching showing the homogeneity of the sample (no grain boundaries and other defects) and (b) the Raman peak asymmetry map of the beveled calibration sample, acquired using the 633 nm laser, showing the variation in the free hole concentration within the sample depth. Points marked on the mapping indicate the areas for which the symmetry parameter q was extracted and correlated with the corresponding silicon peak position and free carrier concentration (see Figs. 4 and 5).

Image of FIG. 3.
FIG. 3.

First order Raman Si peaks extracted at different points on the beveled calibration sample captured with the 633 nm laser. As the doping increases, the peak asymmetry increases (Fano resonances) while the peak intensity decreases.

Image of FIG. 4.
FIG. 4.

(a) Experimental and (b) theoretical calibration curves correlating the peak shifts due to boron doping in silicon to the symmetry parameter q representing peak asymmetry. Trends between theory and experiment agree. However, there is a slightly larger change in the peak shift exhibited in the experimental data.

Image of FIG. 5.
FIG. 5.

Free hole concentration correlated with the inverse of the symmetry parameter q extracted at different depths in the calibration sample captured with the (a) 488 nm, (b) 514 nm, and (c) 633 nm lasers. Free hole concentration was determined from. (Refs. 18 and 19)

Image of FIG. 6.
FIG. 6.

Optical images of the bevelled and Secco etched laser crystallized a-Si seed layer showing (a) a large crystallized stripe, (b) microcrystals formed at the edge of the stripes, and (c) the distance between successive well separated laser crystallized stripes showing no overlap between individual laser line scans during crystallization.

Image of FIG. 7.
FIG. 7.

(a) Optical image of the Secco etched laser crystallized a-Si seed layer and (b) the Raman intensity map acquired using the 514 nm laser showing multiple grains with different sizes. Grain boundaries in the maps correlate well with the optical image. Points on the maps were used to compare the stress magnitudes acquired using the different lasers.

Image of FIG. 8.
FIG. 8.

Raman peak asymmetry maps taken with (a) 488 nm, (b) 514 nm, and (c) 633 nm lasers, showing the agreement of the distribution of the free hole concentration in the three maps. There is a slight machine table drift between the three maps depicted by a slight change in the captured patterns.

Image of FIG. 9.
FIG. 9.

Raman spectra captured with the 633 nm laser showing the increase in peak asymmetry with increase in doping between points 9 and 10. Free carrier concentration is at point 9 and at point 10. Points are marked in Fig. 7(b). Peak intensities have been normalized for clarity.

Image of FIG. 10.
FIG. 10.

(a) Optical image of the crystallized seed layer showing four points at the rims and centers of the crystallized laser line at which two-point curves were captured and (b) curves taken at four points showing lower electrical conductivity at points 3 and 4 in comparison with points 1 and 2; this fact can well be attributed to the grain sizes involved and the dopant concentration variation at the different measurement points. measurements in the amorphous green stripe, between the laser crystallized lines, are not plotted as the measured current was too low to be visible on the same axis.

Image of FIG. 11.
FIG. 11.

XTEM images showing the presence of extended lattice defects in the laser crystallized seed layer on glass, which cause local stress fields and local variations in dopant distribution.


Generic image for table
Table I.

Stress values (MPa) extracted from selected points in the mappings acquired using 488, 514, and 633 nm lasers. Stress values taken with the three different excitation wavelengths differ due to the slight stage drift between the three measurements resulting in a neighboring region (i.e., not exact points between the three maps) at which the stresses were extracted.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Stress and doping uniformity of laser crystallized amorphous silicon in thin film silicon solar cells