Rajiv K. Singh

University of Florida

Gainesville, Florida

J. Narayan

North Carolina State University

Raleigh, North Carolina

Pulsed nanosecond excimer lasers can be effectively used for fabrication of silicon based solar cells having controlled junction depths and sheet resistivities, and high minority carrier lifetimes. By using pulsed beams, the ion implanted layers are melted and the underlying ''defect-free crystal'' provides a seed for subsequent crystal growth, resulting in 100% electrical activation of the dopant atoms. The understanding of the laser-solid interactions throws insight into the thermal effects of solids irradiated with pulsed laser beams. For metallization of the p-n junctions, the excimer laser beams can also be used to deposit stoichiometric good quality TiN thin films by evaporation of bulk TiN targets.

Pulsed excimer laser beams (wavelength λ = 0.193–308 µm, pulse duration τ = 15–45 × 10^-9 sec) are ideally suited for fabrication of efficient (shallow junctions) solar cells [1,2]. High quality p-n junctions can be fabricated from polycrystalline silicon by ion implantation followed by laser annealing. By using pulsed nanosecond laser beams, the whole process (melting and resolidification) is completed within 200 × 10^-9 sec [3,4]. Since the solidification velocities are typically 4 to 5 m/sec, and the specimens are subjected to very short times at high temperature, impurity segregation at the grain boundaries is completely eliminated. The depth of p-n junctions can be controlled by varying the laser and ion-implantation parameters. This process leads to formation of "defect-free" p-n junctions with response in the blue region close to that of p-n junctions fabricated from single crystals. In addition, since the substrate temperature remains close to the ambient value during laser annealing, minority carrier lifetime (MCL) of the substrate is not degraded as often happens during conventional furnace annealing. This factor leads to higher quantum efficiency for longer wavelengths in the red region. Thus, the overall efficiencies of solar cells are considerably improved. The next important step in the formation of solar cells, after the formation of p-n junctions, is the metallization process. This step assumes additional importance, particularly for shallow junctions. Recently, we have identified a pulsed laser evaporation technique [5–9] for TiN film fabrication at very low temperatures (25–400ºC). The TiN films are polycrystalline with grain size of ~100 Å, and electrical properties close to that of bulk TiN. This technique can be successfully used for metallization of shallow junctions without adversely affecting the junction characteristics.

The understanding of the laser-solid interactions provides information on the effect of laser parameters including pulse energy density, wavelength, pulse duration on the maximum melt depths, melt lifetimes, surface temperatures, and solidification velocities. The photon energy of a

FRONT MATTER (R1-R8)

1 SESSION 1 - WATER TREATMENT: ADVANCED PHOTOOXIDATION PROCESSES (1-34)

2 SESSION 2 - WASTE TREATMENT (35-58)

3 SESSION 3 - MATERIALS PROCESSING AND SYNTHESIS (59-96)

4 SESSION 4 - SOLAR PUMPING OF LASERS (97-108)

5 SESSION 5 - PHOTOCHEMICAL SYNTHESIS (109-116)

6 SESSION 6 - FUEL PROCESSING ADN THERMOCHEMICAL/PHOTOCHEMICAL CYCLES (117-136)

7 SESSION 7 - ADVANCED RESEARCH (137-140)

8 PLENARY SESSION (141-144)

APPENDIX A: WORKSHOP AGENDA (145-152)

APPENDIX B: LIST OF PARTICIPANTS (153-155)