Synopsis: X rays scratch the surface
X rays scatter relatively strongly in a solid or liquid, which makes them ideal for studying the structure of materials when only a small amount is available. But since x rays are typically a 1 kilo-electron-volt or more in energy, it’s hard to see the tiny energy shift in an x ray when it scatters from a lattice vibration (– milli-electron-volts) in a solid. X-ray techniques tend to therefore be “specialized,” with one method being better suited to, say, figuring out the crystal structure of microscopic samples, while another is chosen for studying excitations.
Now, a multinational team lead by Jorge Serrano at ICREA - Polytechnic University of Catalonia, Spain, was able to measure the spectrum of lattice vibrations in a 6-micron-thick sample of indium nitride. The group has combined two well-established techniques—surface scattering and inelastic scattering—to understand the elastic properties of thin semiconductor films. As it would be in a real device, the indium nitride was grown on a sapphire substrate with a gallium nitride buffer layer in between, but Serrano et al. controlled the x rays in their experiment so they only graze the surface, and not the substrate underneath. The team’s results, which are reported in Physical Review Letters, compare favorably with calculations of the elasticity of indium-nitride, a semiconductor that could prove useful for new types of solar cells. – Jessica Thomas