Fabrication and characterization of selectively grown Si_1-xGe_x/Si p⁺/n heterojunctions using pulsed laser induced epitaxy and gas immersion laser doping

Abstract

A pulsed XeCl excimer laser is used to grow ideally strained heteroepitaxial Si_1-xGe_x/Si layers with Ge fractions up to 21% by intermixing a structure of electron beam evaporated a-Ge on Si(100). The rapid regrowth 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 meltprocess by using a BF₃ gas ambient. Hall/van der Pauw and SIMS analysis reveal that the incorporated dopant dose scales with the number of laser pulses. The junction depth is controlled by the incident laser fluence. The melt time is monitored in-situ utilizing the transient reflectance of the sample during the phase transformations. A patterned reflective aluminum mask is used to obtain 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 the observed doping regime of 10¹⁸–10²⁰ cm^-3. Quasiplanar p⁺/n heterojunction diodes are fabricated and exhibit near-ideal forward I-V characteristics. Heterojunction diodes exhibit lower turn-on voltages than equivalent Si homojunction diodes, indicative of a lowered bandgap. The turn-on voltages also depend on the B junction depth with respect to the Si_1-xGe_x/Si interface. Both quantities are controlled independently by separating the epitaxy from the doping step.