R. F. Wood

Theoretical analysis of thermal and mass transport in ion‐implanted laser‐annealed silicon

Experimentally observed laser‐induced redistributions of ion‐implanted dopants in silicon are explained theoretically in terms of diffusion in the molten state. Calculations of thermal and mass diffusion in silicon show that the redistribution of a dopant file after pulsed‐laser annealing is dependent on the time the dopant region remains molten and on the value of the mass‐diffusion coefficient for the particular dopant.

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.

Model for nonequilibrium segregation during pulsed laser annealing

Highly nonequilibrium thermodynamic processes occur during the ultrarapid recrystallization characteristic of pulsed laser annealing. Values of interface segregation coefficients are observed to differ from equilibrium values by as much as three orders of magnitude and equilibrium solubility limits may be exceeded by similar magnitudes. In this letter, a model is developed which accounts quantitatively for these effects.

Bulk nucleation and amorphous phase formation in highly undercooled molten silicon

Solidification of undercooled liquid (l) Si formed by pulsed laser melting of amorphous (a) layers has been studied experimentally and theoretically. Bulk nucleation apparently occurs at a temperature higher than that of the l→a phase transition. Release of latent heat on nucleation is crucial in determining the depth of melting. It is emphasized that bulk nucleation implies that the l→a transition cannot be explained by purely thermodynamic considerations.

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...

Electrical and structural characteristics of laser‐induced epitaxial layers in silicon

We have used pulsed‐laser radiation to grow homoepitaxial p‐n junctions in silicon. Doped amorphous silicon was deposited on (100) and (111) silicon substrates and annealed with a Q‐switched ruby laser. By this technique, perfect epitaxial layers with good electrical characteristics and controlled dopant profiles can be achieved. The technique can potentially be competitive with or replace ion implantation for many semiconductor‐device applications.

High‐efficiency Si solar cells by beam processing

Utilizing two recently developed beam processing techniques, i.e., gas discharge implantation and XeCl excimer laser annealing, p‐n junction silicon solar cells with total area (∼2 cm2) AM1 efficiencies as high as 16.5% have been made. These cells are of a particularly simple structure, fabricated without any sophisticated processing steps, and subjected to no high‐temperature treatment.

Pulsed neodymium: yttrium aluminum garnet laser (532 nm) melting of crystalline silicon: Experiment and theory

Time‐resolved reflectivity measurements have been used to determine both the time of onset‐of‐melting and the duration of melting resulting from frequency‐doubled neodymium: yttrium aluminum garnet (532 nm) pulsed‐laser irradiation of crystalline silicon. Substantially shorter surface melt durations were obtained with increasing energy density El than were reported earlier by others. Thermal melting model calculations, which take into account the temperature‐dependent optical and thermal properties of silicon, are in substantial agreement with the observed El dependence of the onset of melting and surface melt duration. Inclusion of intensity‐dependent absorption in the modeling further improves this agreement.

Radiation damage in neutron transmutation doped silicon: Electrical property studies

Radiation damage in neutron‐transmutation‐doped (NTD) silicon, irradiated to introduce 5×1013 to 6×1016 phosphorus cm−3, has been studied by electrical property measurements. The experimental results indicate that thermal‐neutron‐induced (n,γ) recoil‐type damage can be annealed at 400 °C. The nature of any remaining lattice defects and their annealing behavior above 400 °C is a function of the fast‐neutron fluence. Small defect clusters are present in Si irradiated with a light‐to‐moderate fast‐neutron fluence (?5×1018 n cm−2), and temperature‐dependent Hall coefficient measurements indicate that at least two deep acceptor levels and one deep donor level are formed during annealing. One of these acceptor levels anneals at ∼450 °C, and the other two levels anneal at ∼550 °C. A shallow acceptor level near the valence band that anneals at 750 °C is also observed. Larger defect clusters which reduce the electron mobility tremendously and distort the band structure are formed in heavily irradiated Si (5×1018 t...

Computational modeling of physical processes during laser ablation

Abstract A combined theoretical and experimental effort to model various physical processes during laser ablation of solids using a variety of computational techniques is described. Currently the focus of the modeling is on the following areas: (a) rapid transformations through the liquid and vapor phases under possibly nonequilibrium thermodynamic conditions induced by laser-solid interactions; (b) breakdown of the vapor into a plasma in the early stages of ablation through both electronic and photoionization processes; (c) hydrodynamic behavior of the vapor/plasma during and after ablation; and (d) the effects of initial conditions in the vapor, in particular, the nature of the initial velocity distribution, on the characteristics of subsequent vapor expansion. The results from the modeling will be compared with experimental observations where possible.