Hard X-ray Technique Sees Below the Surface

A new technique called HARPES (Hard x-ray Angle-resolved PhotoEmission Spectroscopy) can study electronic structures deep below the material surface, including buried layers and interfaces in nanoscale devices. Developed by Lawrence Berkeley National Laboratory (Berkeley Lab) researchers, the method could pave the way for smaller logic elements in electronics, novel memory architectures in spintronics and more efficient energy conversion in photovoltaic cells.

By collecting and comparing HARPES data at room and cryo temperatures, Berkeley Lab researchers could correct for density of state and x-ray photoelectron diffraction influences in determining electronic structures deep below sample surfaces. (Image: Fadley group)

For the past 30 years, one of the most valuable and widely used techniques for studying electronic structures has been ARPES (Angle-Resolved PhotoEmission Spectroscopy), a technique that primarily looks at surfaces. It uses low energy (soft x-rays) and can probe only to a depth of <10 angstrom, whereas the HARPES technique’s hard x-rays can probe as deep as 60 angstrom.

Hard x-rays, which have sufficiently high photon energies, are key to probing bulk electronic structures because they eject photoelectrons from deep beneath the surface of a solid material. The high-energy photons pass on kinetic energies to the ejected photoelectrons, enabling them to travel a longer distance within the solid. Measuring their kinetic energy and the angles at which they are ejected reveals much about the sample’s electronic structure.

With photoemission spectroscopy techniques such as HARPES, a beam of x-rays flashed on a sample transfers energy from the photons to electrons, causing photoelectrons to be emitted. Measuring the kinetic energy of these photoelectrons and the angles at which they are ejected reveals much about the sample’s electronic structure. (Image: Fadley group)

To demonstrate the capabilities of HARPES, physicist Charles Fadley and research team member Alexander Gray used a high-intensity undulator beamline at the SPring8 synchrotron radiation facility in Hyogo, Japan. The samples they use, tungsten and gallium arsenide, contain relatively heavy elements that have small phonon effects (atomic vibrations). To further reduce these effects, the samples were cryogenically cooled. By combining room temperature and cryo data, the researchers corrected for the influence of indirect transitions and photoelectron diffraction in their results.

"Having sufficient photons from the beamline was critical, as was having a high energy resolution that required an undulator source, a special monochromator and a photoelectron spectrometer with both high throughput for intensity and a lens with angle-resolving capability," Fadley said.

Alexander Gray (left) and Charles Fadley at Beamline 9.3.1 of Berkeley Lab’s Advanced Light Source, where they soon will be able to carry out their HARPES experiments. (Image: Roy Kaltschmidt, Berkeley Lab)

Gray added, "Our HARPES technique not only provided us with information about the energies of the emitted photoelectrons, but also with information about the crystal momentum of electrons within the bulk solid. This extra dimension carries a vast amount of information regarding electronic, magnetic and structural properties of materials, and can be used for in-depth studies of such novel phenomena as high-temperature superconductivity and so-called Mott transitions from insulating to conducting states that might be used for logic switching in the future."

Fadley and Gray soon will be able to carry out HARPES experiments much closer to home. At Berkeley Lab's Advanced Light Source, the first of the world's third-generation synchrotron radiation facilities, an experimental chamber for beamline 9.3.1 is scheduled to open this fall that will provide unique hard x-ray angle-resolved photoemission capabilities.

For more information, visit: www.lbl.gov