K. Affolter

Properties of laser‐assisted doping in silicon

Ohmic contacts and p‐n junctions in p‐ and n‐type silicon are generated with the aid of a laser. Doping was achieved by covering the surface of the silicon with a layer of dopant and melting locally with pulses from either a Nd : YAG or a CO2 laser. Typical residual resistances of the Ohmic contacts are of the order of 0.1–1 Ω cm2 and backward/forward resistance ratios of 104 were measured for the diodes. A model which takes account of segregation during the cooling process is discussed and shown to agree with the resulting distribution of dopant. Highly doped material was found in a surface layer of a thickness less than 0.5 μm. This thickness was independent of laser parameters.

Interference effects on the surface of Nd:YAG‐laser‐reacted Pd‐silicide

The generation of silicides by laser irradiation of Pd‐coated Si wafers is often associated with circular ripple patterns formed in the irradiated area. These surface modulations are explained by inhomogeneous reaction of Pd with Si due to interference effects in the incident laser beam.

Laser produced amorphous phases in NbSi and VSi thin films

Abstract The binary systems NbSi and VSi are investigated by laser induced melting and quenching of vapor deposited thin films. Glassy phases of various compositions are produced. The established compositional glass forming ranges support that the equilibrium compounds Nb4Si is stable only at high temperature. Thermal decomposition of the amorphous NbSi films proceeds via several intermediate stages. In particular, the AuCu3-type configuration of Nb3Si was found to be formed. In addition, amorphous NbSi films around 20 at.% si show the formation of an undentified metastable compounds upon post-irradiation annealing. Amorphous VSi films proved to be stable up to at least 500°C. Above this temperature they decompose directly into their respective equilibrium phases.

Laser annealing of silicon on sapphire

Silicon‐implanted silicon‐on‐sapphire wafers have been annealed by 50‐ns pulses from a Q‐switched Nd : YAG laser. The samples have been analyzed by channeling and by ω‐scan x‐ray double diffraction. After irradiation with pulses of a fluence of about 5 J cm−2 the crystalline quality of the silicon layer is found to be better than in the as‐grown state.

Dynamics of CO2 laser heating in the processing of silicon

The optical transmission of commerical Si wafers was measured at the wavelength of 10.6 μm as a function of temperature in the interval from about 78 to 850 K. We show that the absorptivity of Si increases with the sample temperature. This temperature dependence is explained by free‐carrier absorption. A transmission measurement at 10.6 μm wavelength has been used to monitor the temperature of a Si sample as a function of time during cw laser heating.

Electrical properties of cw laser‐annealed ion‐implanted polycrystalline silicon

Resistivity and Hall measurements were performed on O.3‐μm‐thick polycrystalline‐silicon film after implantation with 1012–5×1014 boron or phosphorus ions/cm2 and annealing with a cw Kr‐ion laser (green lines). The laser‐induced increase of grain size drastically alters the resistivity and Hall mobility. The largest effect occurs for average doping levels between 4×1016 and 1018 cm−3, where resistivity decreases by up to four and mobility increases by up to two orders of magnitude, compared with unirradiated parts of the same wafers. Values approaching those of single crystals are obtained. The results are interpreted in terms of a published grain‐boundary trapping model.

Glass-Forming Ability in Laser Quenched Transition-Metal Alloys

The glass-forming ability by laser quenching is investigated for several transition metal systems (Au-Ti, Co-Ti, Cr-Ti, Zr-Ti). Metallic glasses are obtained in the systems Au-Ti, Co-Ti, and Cr-Ti. The results are compared to predictions by several semiempirical criteria of glass-forming ability. Our results indicate that at the cooling rates typical to laser quenching glasses in binary metallic systems form whenever diffusionless crystallization is excluded.

Anisotropic melting and epitaxial regrowth of laser‐irradiated silicon

Using Nd‐YAG laser pulses in the 100‐μs regime, melting and regrowth of monocrystalline silicon is studied. It is shown that surface melting proceeds preferentially along the crystallographic axes. This leads to a characteristic surface pattern if uniform irradiation slightly above melting threshold is applied. Regrowth under these circumstances is epitaxial.

Dynamics of Nd : YAG laser annealing of silicon on sapphire

Epitaxial and polycrystalline silicon layers on sapphire have been annealed with Q‐switch pulses from a Nd : YAG laser irradiated on the Si surface. Time‐resolved optical reflectivity measurements have been performed. The annealing process is shown to be induced by melting and subsequent epitaxial regrowth. The best results were obtained if the whole Si layer was melted, thus allowing the (1102) oriented sapphire substrate to act as a seed for recrystallization. In this case commercially available Si‐on‐sapphire (SOS) wafers with additional Si implantation as well as polycrystalline layers of low‐pressure chemical vapor deposited (LPCVD) Si on sapphire could be epitaxially regrown. The same material deposited on amorphous SiO2 did not show epitaxial regrowth, however an increase in size of randomly orientated grains from 60 nm to 1 μm could be achieved.

cw laser annealing of hydrogenated amorphous silicon obtained by rf sputtering

Hydrogenated amorphous silicon obtained by rf sputtering has been annealed using cw lasers (Ar or Kr ion). The annealed films have been microanalyzed for quantitative profiling of hydrogen and argon. TEM analysis was used for structure determination. The release of hydrogen and argon as well as the grain size of the polycrystalline material obtained are related to the deposited laser energy. Comparison with other production methods of such material from the amorphous state is discussed.