Semiconductor Germanium and Germanium-Tin Thin Films by Simultaneous Laser Sintering and Crystallization

The crystallization process of germanium (Ge) and germanium-tin (GeSn) thin films by a continuous wave (CW) and a pulsed laser is very effective for producing smooth, homogeneous, and crack-free polycrystalline films to use in transistors, photodetectors, and photovoltaic applications. However, little progress has been made to directly crystallize Ge and GeSn films based on micro/nanoparticles (NPs) using the laser sintering (LS) process. In this thesis work, a simultaneous LS and crystallization process of Ge and GeSn micro/nanoparticles thin films on silicon substrates are demonstrated. Silicon substrates with a silicon dioxide (SiO₂) insulating layer on top were considered for compatibility with complementary metal-oxide-semiconductor (CMOS) technology. The LS process was applied to solution deposited film of micro/nanoparticles, of 4-5 µm in thickness of Ge and GeSn using both CW infrared (IR) laser of wavelength 1070 nm and pulsed ultraviolet (UV) laser of 355 nm wavelength. After the LS process, around 2-2.5 µm thin films of polycrystalline Ge and GeSn were achieved with optical and electrical properties comparable to traditionally deposited films. The crystallinity of the polycrystalline Ge and GeSn films was evaluated by Raman spectroscopy and X-Ray diffraction (XRD). The laser-sintered Ge films exhibited a Raman peak at 300 cm⁻¹ and XRD 2Θ peak at 27.35, which indicated the poly-crystalline structure. The fabricated Ge film showed high hole mobility of 203 cm²/V.s, without any doping and film electrical resistivity value of 6.24x10⁵ Ω-cm. The mobility increased after incorporating 12% Sn in the film. The laser-sintered GeSn films showed hole mobility of 240 cm²/V.s. The developed LS process allows the quick deposition of thick polycrystalline films, removing surface porosity and voids, increasing films adhesion with the substrate, and faster thermal annealing.