Methods are discussed for producing single-crystal shapes on amorphous materials. A first method deposits a layer of Germanium-Tin (GeSn) alloy comprising between three and sixteen atomic-percent tin on material incapable of seeding crystal formation, the layer is photolithographically defined into a shape having a point having radius less than 100 nanometers; and the shape is annealed by heating to a temperature below 450 degrees Celsius. A second method also photolithographically defines a shape on a layer of GeSn, then uses a laser to heat and crystalize seed spot on the shape; and anneals the shape by heating and thereby crystalizing additional GeSn alloy of the shape. In embodiments, the crystalized GeSn serves to seed InGaP and/or InGaAs layers that may serve together with the GeSn as layers of a tandem photovoltaic cell.