Laser Light Enables 3-D Doping
KONSTANZ, Germany -- Laser diodes, light-emitting diodes and photovoltaic cells all have one thing in common: They consist of semiconductor chips, which necessarily contain electronic bandgaps.
To engineer an electronic bandgap, thin films of electron donors and acceptors -- usually made of elements from Groups III and V of the Periodic Table -- are layered on top of a substrate. For more precise control of emissivity, these layers are sometimes "doped" with other elements. But doing this in three dimensions has been very difficult -- until now.
Light can be used to dope structures at the nanoscale level. When a multicomponent atomic beam containing atoms of chromium and magnesium fluoride encounters a standing wave of light, an element-specific atom/light interaction occurs. The chromium atoms are deflected and deposited periodically on the substrate's surface, while magnesium fluoride is evenly deposited throughout. Courtesy of Dirk Jürgens, University of Konstanz.
A research team at the University of Konstanz has found a technique for engineering such structures on the nanoscale level. By using a single laser, the scientists can control the material composition of each film as it is laid down on a chip, said Dirk Jürgens, a PhD student working on the project.
The researchers directed a 20-mW Gaussian laser beam with a waist of 100 µm at a mirror; the retroreflection caused a standing wave of light to be formed.
When the researchers projected a molecular beam containing a mixture of chromium and magnesium fluoride perpendicular to the wave, the chromium atoms interacted with it. The local laser electrical field caused the atoms to be deflected toward the nodes of the standing light wave, effectively making the waves into a series of regularly spaced lenses. When they placed a substrate at the focal points of the lenses, deposition of chromium atoms occurred at these points in a regular pattern in line with the periodicity of the standing wave.
Because of the resonant enhancement of the induced dipole force, only chromium atoms were affected in this manner, although some magnesium fluoride was deposited homogeneously.
The group's research appeared in the March 19 issue of Applied Physics Letters. Jürgens said that for now the method is limited to using chromium as a dopant material. Before other atomic species can be used as dopants, other laser systems must be developed that can similarly act as light masks.
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