InxGal-xAs x025-0.35 grown at low temperature on GaAs by molecular beam epitaxy is characterized by Hall effect, transmission electron microscopy, and ultrafastoptical testing. As with low temperature LT GaAs, the resistivity is generally higher after a brief anneal at 600 deg C. High-resolution transmission electron micrography shows all the as-grown epilayers grown directly on GaAs to be heavily dislocated due to the large lattice mismatch 2- 3. Annealed layers also show precipitate formation, in addition to the dislocations. Like LT GaAs, InxGa1-xAs lifetimes shorten as growth temperatures are reduced and LT InxGal-xAs lifetimes are generally shorter in as-grown samples than in annealed samples. The metal-semiconductor-metal photodetectors we fabricated on the material exhibit response times of 1-3 picoseconds, comparable to results reported on GaAs grown at low temperature, and the fastest ever reported in the wavelength range of 1.0-1.3 microns. To improve the crystalline quality and to distinguish detector speed and responsivity limitations due to dislocations versus defects induced by LT growth, we have grown 3microns-thick graded layers of InxAll-xAs between the GaAs substrates and In0.35Ga0.65As films. The InxAll-xAs layers are heavily dislocated, with the dislocation density increasing with distance from the GaAs substrate, and abruptly terminating at or below the In0.35Ga0.65As layer. Epitaxy, AlGaAs-InGaAs-GaAs, Pseudomorphic heterostructures, Strained layer supperlattices, Dislocations, Photoluminescence, Hall effect, Electron diffraction, Photoreflectance.