Phononic engineered materials and devices

Disorder in hydrogenated amorphous silicon leads to short electronic diffusion lengths and poor electronic lifetimes relative to crystalline silicon. Interestingly this same disorder can lead to longer phonon diffusion lengths and lifetimes. These properties suggest disorder-induced truncation of the Brillouin zone results in fewer phonon decay possibilities thereby reducing Umklapp scattering. Phonon propagation speed is a function of the mechanical properties of the matrix and therefore a function of hydrogen content. Therefore, it is possible to engineer structures to guide phonons using methods previously developed to pattern the refractive index. Taken together these properties offer a means of explaining observed phononic behavior of amorphous silicon and offer a platform for engineering devices.