“A German-French research team has identified surprising properties of nano-holes in semiconductor materials produced by a promising technique: In their experiments, hot aluminum droplets have etched remarkable smooth holes in an aluminum gallium arsenide (AlGaAs) substrate. The method is suitable, among other things, for the production of so-called quantum dots, which can be used, for example, for light sources with a very sharply defined color or for storage cells in quantum computers. The team from the Center for Hybrid Nanostructures (CHyN) at the University of Hamburg, the DESY NanoLab and the European Synchrotron Radiation Source (ESRF) in Grenoble, France, present their findings in the journal Physical Review Materials.
There are basically two approaches to the production of nanostructures: In a so-called top-down approach, a crystal surface produced from several layers of different materials can be subsequently structured. However, this method, established in the semiconductor industry, has not yet led to success in the manufacturing of optical quantum dots. In an alternative bottom-up approach, physical mechanisms are exploited that lead to the spontaneous formation of nanostructures during crystal growth. The method developed at CHyN combines both approaches. First, metallic nano droplets are spontaneously formed on the crystal surface, which then etch very smooth nano-holes into the crystal surface. “The etching of nano-holes is an exciting technique for nanostructuring, but it is still not fully understood on the interesting length scales,” explains lead author Vedran Vonk from the DESY Nanolab.
In the work now published, the researchers investigated a typical semiconductor material made of gallium arsenide that was mixed with aluminum. Gallium arsenide is the second most important material in the semiconductor industry after silicon. The optical properties of the material can be continuously adjusted by adding aluminum. “The surface properties of the nano-holes are of central importance when, for example, they are filled with gallium arsenide quantum dots,” explains Christian Heyn of CHyN. “The quantum dots produced in this way are perfectly monocrystalline material inclusions in a semiconductor crystal that can be used, for example, to generate entangled photon pairs.”
For their etching experiments, the researchers coated the semiconductor material with aluminum, which forms droplets on the surface. At temperatures of nearly 700 degrees Celsius, the aluminum droplets etched holes into the semiconductor material. “Surprisingly, at temperatures above 665 degrees Celsius, holes with a smooth and angular shape, like a pointed, angular funnel, were formed,” reports Vonk. “That was unexpected, because when etching, one actually imagines ‘fringed’ structures. But the aluminum obviously etches smooth holes.” The smooth, regular shape of the holes became visible during X-ray examinations at the ESRF. Why the hot aluminum leads to these regular shapes is not yet fully understood and is probably due to the thermodynamic properties of the materials used.
Investigations with an atomic force microscope at CHyN and with a scanning electron microscope at the DESY Nanolab showed that the holes are each about 100 nanometres wide and 60 nanometres deep. One nanometre is one millionth of a millimetre. Other metals can also be used in the process, but the dependence of the shape and facets on the material still needs to be clarified. Such holes could serve as templates for quantum dots with tailor-made properties by being filled with a new material. The researchers now hope to find out how the shape of the nano-holes can be understood and controlled in futures analyses.”