LÁSER MODIFICATION EFFECTS AND APPLICATIONS

Regrowth Phenomena in Silicon

In this section attention is directed to the structural and compositional property changes produced in silicon surface layers as a result of láser processing. Silicon has been singled out as the vehicle for discussion because of the large volume of study devoted to this important material. Furthermore, many of the phenomena observed in Si can be readily understood in the context of traditional solidification and recrystallization theories that have evolved over the past four decades.

Impurity-Free Si. Láser melting of single-crystal Si wafer sur-faces results in liguid phase epitaxial (LPE) regrowth. However, when ultrashort picosecond pulses are applied, crystalline -* liquid -► amorphous Si transitions can be sequentially induced.

The phenomenon of solid phase epitaxial (SPE) regrowth of amorphous silicon layers upon surface annealing is worth noting. As we shall see later, ion implantation methods can be used to "amorphize," or make amorphous, surface layers of Si.

The latter can be recrystallized by láser annealing, in what amounts to a second surface modification treatment. Better control over SPE can be exercised, however, by means of simple furnace annealing. The result is a well-defíned constant planar regrowth velocity with thermally activated kinetics given by where a is the atomic spacing and v is the lattice frequency. The activation energy (E) for impurity-free SPE is 2.35 eV, a valué that corresponds to cooperative bond breaking and rearrangement at the moving amorphous-crys-talline interface. In its wake, dangling bonds are eliminated and interfacial bond straining and distortion are minimized. In contrast to the several m/sec laser-induced solidification rates, SPE regrowth proceeds at a velocity of only ~ 1 A/sec at 500 °C—a difference spanning some 10 orders of magnitude. Epitaxial regrowth rates of Si-implanted amorphous Si are strongly dependent on substrate orientation with (100) and (111) exhibiting the highest and lowest magnitudes, respectively.

Doped Si

Epitaxial Regrowth. Depending on the type and concentration of impurity atoms, and the thermal processing parameters, a rich assortment of regrowth effects is possible. Consider what happens to an implanted distribution of Bi atoms in Si after irradiation by a 100-nsec ruby láser pulse with an energy density of 2 J/cm2. the distribution, originally centered 1500 A deep, is swept toward the surface during solidification. The reason for this has to do with two facts. The first is that when a liquid and solid are in equilibrium at some temperature, solute generally has greater solubility in the liquid phase; second, solute atoms diffuse rapidly in the liquid phase but are essentially immobile in the solid. When a planar liquid-solid front now passes by the implanted solute, successive partitioning occurs between the two phases in an attempt to maintain a fixed (equilibrium) solute concentration ratio; i.e., k0 = CS/CL, at the interface. Also variably known as the segrega-tion, partition or distribution coefficient, k0 valúes depend on the dopant in question; it can, for example, range from 2.5 X 10~5 for Au to 0.8 for B in Si. A computer simulation of the solidification sequence of events for the case of a solute with k0 = 0.1, where regrowth proceeds at a rate of 2 m/sec. The process resembles zone refining, a technique employed to purify rods of material by directionally sweeping impurities and concentrating them at one end by means of a moving molten zone.

A recent intriguing result is the demonstration of the solid-state analog of zone refining in an amorphous Si layer doped with implanted Au. During epitaxial regrowth of Si, the amorphous Si layer shrinks in extent as the interface moves toward the surface . Simultaneously, Au preferen-tially partitions into the amorphous phase and is concentrated there leaving a virtually Au-free epitaxial Si región behind. Interestingly, Au solubilities in amorphous Si many orders of magnitude greater than in crystalline Si, were measured.