Nanopillars shine better without gold

Aging method with gold catalyst leads to performance-reducing impurities

Gallium arsenide nanopillars grow on a silicon substrate. © PDI
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Gallium arsenide nanopillars are considered to be a promising material for efficient semiconductors, such as solar cells. Now researchers have demonstrated for the first time that the common breeding method of this nanostructure with the aid of a gold catalyst sensitively disturbs crucial properties. As they report in the journal "Nano Letters", the gold-free nanoparticles they produced a hundred times more light than those produced by standard methods and even proved to be superior to the classical layer semiconductors.

Everyone wants it - just not the semiconductor physicists. For them, gold is not the beloved precious metal, but a troublemaker who worsens the electronic properties of their materials. Nevertheless, they often use gold as a catalyst for the growth of nanopillars - because it's easy. The gold is deposited on a surface and heated. As a result, it flows into small droplets that serve as seeds for column growth. The pillars push the gold up during growth like a small cap. Probably the gold also migrates into the column material and contaminates it, but experimentally proving that is difficult.

"For a direct detection of gold in the semiconductor one would need a method with very high sensitivity and spatial resolution, " explains Lutz Geelhaar from the Paul Drude Institute (PDI) in Berlin. After all, only a few nanometers away from the measuring point is the golden hood, which can falsify the measurements.

Columns in photoluminescence comparison

However, for the first time, PDI researchers have found indirect evidence for the incorporation of gold in gallium arsenide (GaAs) columns. Gallium arsenide is used, among other things, as a material for light emitting diodes and laser diodes as well as for solar cells of high efficiency, as used in satellites. In order for semiconductors such as gallium arsenide to grow on silicon, researchers are increasingly working to breed them not as a layer but as a pillar. Here, tensions due to different crystal lattice do not affect so much.

The researchers compared the properties of gallium arsenide pillars grown with gold and those using pure gallium as the germ. Using photoluminescence measurements, they determined how much light the columns emitted after excitation by a laser beam. The measuring method is based on semiconductors after energetic excitation electrons and holes display

form, which then reunite and emit light.

Hundredfold more light without a gold catalyst

It turned out that the gold-free pillars emitted more than a hundred times more light than gold-grown pillars. The lifespan of the charge carriers in the gold-free semiconductor was also much longer. The columns were irradiated for three nanoseconds. By contrast, gold was all over after only eight picoseconds, and thus after a roughly four hundred times shorter period of time. The intensity of the light and the decay time are a measure of the optoelectronic properties of the material, the so-called internal quantum efficiency.

"We would not have expected these big differences, " says Geelhaar. He emphasizes that the method is not direct evidence for gold atoms in the semiconductor. But everything speaks in favor of the drastic changes in properties attributable to the gold. The researchers also compared the lifetime of the charge carriers with values ​​from the literature f Similar layers: "Our gold-free pillars even exceed the layers on silicon, " says the physicist. There is therefore much to be said for the fact that columns can replace the layers once. On one condition: you release the gold. (Nano Letters 2011, 11, 1276 1279)

(Forschungsverbund Berlin, 28.06.2011 - NPO)