Low-Temperature Silicon Liquid Phase Epitaxy

LPE has been widely used at high-temperature and has proved its capacity to grow high-quality epitaxial layers. But for low-temperature, it appears to be more difficult. However, it presents strong interest for low-cost applications (especially for solar cells) as it broadens the choice of the substrate. Actually, it will be possible to use glass substrate, if a suitable seed layer is previously deposited on the surface. But for such process, new solvents have to be defined. Main difficulties, compared to conventional LPE at 900-1,000°C, are the low solubility of silicon in usual solvent and the presence of native silicon oxide, which cannot be removed under H2 flow. 

A variety of solvents have been proposed like Au, Al, Ga [29,30], Pb [31], Sn [32] and others alloys as Au-Pb, Au-Zn, Al-Sn, Al-Zn [33], Au-Bi [32], Al-Ga [34], and Sn-Pb [35]. Resulting layers presented high doping level up to 1018 at. cm-3 (when using doping element like Al, Ga), uncompleted coverage of the surface (non-miscibility of Si in some alloys at low-temperature) or resulted in layers with electrical defects (Au). 

At 800° C, Si solubility increases up to 2Wt% with this alloy (with pure Sn melt), Si solubility is only 0.16Wt% at the same temperature. On a Si substrate, they were able to obtain flat and homogeneous 30 /im thick 

---