Abstract: Any mechanical strain that alters the length and angle of atomic bonds in a crystalline semiconductor will change how it interacts with electrons, photons, and phonons. Ge is unique among all semiconductors in that large biaxial tensile strains of 2-4% are predicted to change it from an indirect-gap to a direct-gap semiconductor. However, no conclusive demonstrations of direct-gap Ge have been made, and indirect- to direct-gap conversion remains an outstanding fundamental challenge. In this talk, I will describe a new method to grow nanocomposite layers consisting of a dense array of tensile-strained Ge nanowires (NWs) embedded in an In0.52Al0.48As matrix. The nanocomposites are formed by surface-mediated phase separation that occurs during molecular beam epitaxy (MBE) growth. While the low mutual solubility of Ge and III-V compounds provides the driving force for phase separation, we show that the degree of phase separation is strongly controlled by growth kinetics. The structure and composition of the nanocomposites can also be strongly tuned using growth conditions, allowing the controllable formation of Ge quantum dots, nanowires, and nanobelts.