W. H. Christie

Laser annealing of boron‐implanted silicon

The properties of boron‐implanted silicon annealed by high‐power Q‐switched ruby laser radiation are compared with results obtained by conventional thermal annealing. Laser annealing of the implanted layer results in significantly increased electrical activity, as compared to thermally annealed implanted silicon. This correlates well with transmission electron microscopy and ion‐channeling measurements which show a dramatic removal of displacement damage as a result of laser annealing. A substantial redistribution of the implanted boron concentration profile occurs after laser annealing which cannot be explained by thermal diffusion in the solid.

p‐n junction formation in boron‐deposited silicon by laser‐induced diffusion

A technique for p‐n junction formation in silicon, based on deposition of boron on silicon at room temperature followed by laser irradiation is described. Transmission electron microscopy and electrical measurements indicate that as a result of the laser irradiation the boron is dissolved in the silicon and becomes electrically active. Diode characteristics of p‐n junctions produced by this technique are quite good. The dopant profile distribution has been obtained using secondary ion mass spectrometry and is in qualitative agreement with simplified theoretical calculations.

Redistribution of dopants in ion‐implanted silicon by pulsed‐laser annealing

The redistribution of B, P, As, and Sb implanted into single‐crystal silicon and subsequently laser annealed with a Q‐switched ruby laser has been studied by secondary‐ion mass spectrometry and ion backscattering. Substantial alteration of the as‐implanted profiles occurs, which is pulse‐energy‐density dependent. The altered profiles are in good agreement with theoretical calculations to be presented in a subsequent paper which show that redistribution occurs as a result of diffusion in the molten state.

Rapid thermal and pulsed laser annealing of boron fluoride‐implanted silicon

Characteristics of rapid thermal and pulsed laser annealing have been investigated in boron fluoride‐ (BF+2 and BF+3) ‐implanted silicon using cross‐section and plan‐view electron microscopy. The amorphous layers recrystallize by the solid‐phase‐epitaxial growth process, while the dislocation loops below the amorphous layers coarsen and evolve into a network of dislocations. The dislocations in this band getter fluorine and fluorine bubbles associated with dislocations are frequently observed. The secondary‐ion mass spectrometry techniques were used to study concomitant boron and fluorine redistributions. The as‐implanted Gaussian boron profile broadens as a function of time and temperature of annealing. However, the fluorine concentration peak is observed to be associated with dislocation band, and the peak grows with increasing time and temperature of annealing. The electrical properties were investigated using van der Pauw measurements. The electrical activation of better than 90% and good Hall mobilit...

Laser processing for high-efficiency Si solar cells

High‐efficiency silicon solar cells can be fabricated by ion implantation followed by pulsed laser annealing. The proper choice of implantation parameters (energy and dose), laser energy density, substrate temperature, etc., and the improvement of the minority carrier diffusion length of the starting material are important factors in obtaining high efficiency cells. In this paper, we report on experiments which show that substrate heating during pulsed laser annealing can improve the electrical properties of the emitter regions of solar cells. We have also found that the open circuit voltage and the fill factor of ion‐implanted, laser‐annealed cells can be improved by increasing the emitter dopant concentration, whereas the short circuit current remains fairly constant; these results are in only qualitative agreement with theoretical predictions. By using ion implantation followed by laser annealing to form p‐n junctions, laser damage gettering to enhance the minority carrier diffusion length, and laser‐i...

Melting phenomena and pulsed‐laser annealing in semiconductors

Annealing of displacement damage (amorphous as well as layers containing only dislocation loops), dissolution of boron precipitates, broadening of dopant profiles, and the formation of constitutional supercooling cells have been studied in laser annealed silicon. These samples were irradiated with laser pulses (λ = 0.485 μm, E = 0.7–1.25 J cm−2, τ = 9 ns), the same as those used by Compaan and coworkers for Raman temperature measurements. In contrast to their conclusion, present results can be interpreted only on the basis of a melting model.

Ion beam processing of LiNbO 3

Ion implantation and ion beam mixing have been investigated as alternative techniques to hightemperature diffusion for introducing dopants into LiNbO 3 . Heavy ion bombardment at both 77 and 300 K initiated a near-surface decomposition causing Li to diffuse to the surface where it formed a nonuniform agglomerate. The damage and annealing characteristics of this effect were studied by ion scattering/channeling, secondary ion mass spectrometry, and optical microscopy. The origins of the surface decomposition are discussed along with possible solutions, and selected samples were evaluated for waveguide properties.

Pulsed excimer laser annealing of ion implanted silicon: Characterization and solar cell fabrication

A pulsed ultraviolet excimer laser (XeCl, 308‐nm wavelength, ∼41‐ns pulse duration) has been sucessfully used for laser annealing of both boron‐ and arsenic‐implanted silicon, and for formation of high quality p‐n junctions. Transmission electron microscopy, secondary ion mass spectroscopy, and sheet electrical properties measurements are used to characterize ion implanted and XeCl laser annealed specimens. Predictions of thermal melting model calculations of the annealing process are also compared with results of these measurements. Finally, we demonstrate the first use of high repetition rate, scanned, overlapping excimer laser pulses to fabricate large area photovoltaic solar cells with good performance characteristics.

Substrate heating and emitter dopant effects in laser‐annealed solar cells

This letter provides the first experimental evidence that substrate heating during pulsed‐laser annealing (PLA) of ion‐implanted silicon can significantly improve the electrical properties of the laser recrystalized region due to the reduction of the regrowth velocity. It is also shown that by using the optimum PLA condition, the open‐circuit voltage VOC and the fill factor of ion‐implanted, laser‐annealed solar cells are improved by increasing the emitter dopant concentrations, whereas the short‐circuit current JSC remains fairly constant, results which are in qualitative agreement with theoretical predictions.

New techniques for the study and control of grain boundary effects

Abstract Progress in the development of polycrystalline solar cells is hampered by the present poor state of knowledge about electronic and ionic processes at grain boundaries. A major difficulty in studying these processes is that conventional growth and thermal diffusion techniques used for doping semiconductors cause segregation of the dopant at grain boundaries and are unable to provide control of the fast diffusion which is known to occur along grain boundaries. Neutron transmutation doping is a method for circumventing segregation problems in bulk polycrystalline silicon. With such doping, long-range diffusion does not occur in the material and hence the dopants cannot migrate to grain boundaries. Laser annealing and laser-induced diffusion are two other newly developed techniques; in this paper we show that they can be used to control grain boundary diffusion and segregation. With these techniques the near-surface region of a sample actually melts but stays molten for such a short time (approximately 10 −7 s) that significant dopant migration cannot occur. Furthermore, since the grain boundaries as such do not exist while the material is molten, rapid diffusion to them and along them does not occur. These new doping methods do not necessarily ensure better efficiencies of cells made from polycrystalline materials, but they provide a degree of control of dopants that has not been available before and hence make possible more definitive studies of grain boundary effects. In this paper the methods are discussed and the results of grain boundary studies using a variety of techniques such as secondary ion mass spectrometry, transmission electron microscopy and the measurements of electrical properties are given.