Temperature dependent Al-induced crystallization of amorphous Ge thin films on SiO2 substrates

• Al-induced crystallization (AIC) of amorphous-Ge films on SiO2 substrates.
• The crystal orientations and grain sizes strongly depend on the annealing temperature.
• The low-temperature annealing (325 °C) provides large grains (∼30 μmϕ) with (111) orientations.
• The grown Ge layer works as the epitaxial template for the chemical vapor deposition.

High-quality crystalline Ge thin films on low-cost glass substrates are desired to reduce the fabrication cost of high-efficiency tandem solar cells. We applied an Al-induced crystallization technique to amorphous-Ge films (50-nm thickness) on SiO2 glass substrates. The annealing temperature of the sample strongly influenced the grain size and the crystal orientation in the grown polycrystalline Ge layers: low annealing temperatures resulted in large grains and high (111)-orientation fractions. As a result, annealing at 325 °C provided 98% (111)-oriented grains with average diameters of 30 μm. Moreover, the grown Ge layers could be used as an epitaxial template for chemical vapor deposition. This large-grained Ge film on a SiO2 substrate appears promising for use as a Ge light-absorbing layer, as well as an epitaxial buffer layer for group III–V compound semiconductors.