FABRICATION AND CHARACTERIZATION OF SELECTIVELY GROWN SI1-XGEX SI P+/N HETEROJUNCTIONS USING PULSED-LASER INDUCED EPITAXY AND GAS IMMERSIONLASER DOPING/

A pulsed XeCl excimer laser is used to grow ideally strained heteroepi taxial Si1-xGex/Si layers with Ge fractions up to 21% by intermixing a structure of electron beam evaporated a-Ge on Si(100). The rapid regr owth process induces an interfacial grading of the Ge fraction, which results in unusual stability of the layer strain upon heat treatment, as confirmed by MeV-ion channeling along [100] and [110]. Boron dopant is incorporated during the melt process by using a BF3 gas ambient. H all/van der Pauw and SIMS analysis reveal that the incorporated dopant dose scales with the number of laser pulses. The junction depth is co ntrolled by the incident laser fluence. The melt time is monitored in- situ utilizing the transient reflectance of the sample during the phas e transformations. A patterned reflective aluminum mask is used to obt ain spatially selective melting. In-plane Hall mobilities are found to be lower for the heteroepitaxial junctions than for Si homojunctions. We believe this is due to different transport behavior for holes in t he observed doping regime of 10(18)-10(20) cm-3. Quasiplanar p+/n hete rojunction diodes are fabricated and exhibit near-ideal forward I-V ch aracteristics. Heterojunction diodes exhibit lower turn-on voltages th an equivalent Si homojunction diodes, indicative of a lowered bandgap. The turn-on voltages also depend on the B junction depth with respect to the Si1-xGex/Si interface. Both quantities are controlled independ ently by separating the epitaxy from the doping step.