Cryogenic Laser Photolithography Finds Quantum Dot Registration

Anne L. Fischer

In quantum dot information processing, it is critical to be able to register the position of a single quantum dot with respect to the antinode of a defect (missing hole) in a photonic crystal to create strong coupling and cavity quantum electrodynamic solid-state devices. Previous cavity quantum electrodynamic devices left the alignment to chance, which was not suitable for production instruments. But researchers at Oxford University and at Hitachi Cambridge Laboratory, both in the UK, recently demonstrated the registration of quantum dots using a cryogenic laser photolithography technique.

The first step was to prepare a quantum dot sample with a tungsten mask made of square apertures. The sample was spin-coated with an SU-8 photoresist. (SU-8 is a commercial negative-tone photoresist from MicroChem Corp. of Newton, Mass., that is used by the microelectromechanical systems community.)

Optical microscope images show the sample before (left) and after (right) reactive ion etching to remove the tungsten.

The researchers used microphotoluminescence spectroscopy to locate a single quantum dot within the aperture. To find the center of the dot, they illuminated the sample with weak pulses from an 80-MHz, 800-nm, mode-locked Ti:sapphire laser from Spectra-Physics of Mountain View, Calif., with an ~100-fs pulse width. They scanned the laser spot over the quantum dot by moving the microscope objective via a piezoelectric stage from Physik Instrumente (PI) GmbH & Co. KG of Karlsruhe, Germany.

Next, they increased the beam power to expose the resist layer so they could define the alignment markers on the surrounding metal. The scanning and writing were controlled by a computer program written in LabView software from National Instruments of Austin, Texas. The challenge was getting the dose just right on the resist, according to Robert A. Taylor, reader in physics at the university. With such a small laser spot, the resist can burn easily, so the researchers had to find a region of intensity such that the resist would be exposed but not damaged.

The sample was mounted in a continuous-flow liquid helium microscope cryostat. Taylor said this was the first time the method had been used at 4 K, which made it possible to find appropriate dots and to register their positions in one operation. This usually is done at room temperature, but then the dots are not visible. Taylor said his group was surprised to discover that it was possible to expose the resist at low temperature.

A benefit of this method is that the researchers can choose quantum dots that have certain wavelengths or lifetimes. They used a Peltier-cooled CCD camera from Andor Technology plc of Belfast, UK, to measure the position and spectral characteristics of the quantum dots. A second simple CCD camera was used to produce images of the sample. After performing eight measurements on a single quantum dot and finding very little drift and uncertainty, the result was a registration accuracy of better than ±50 nm.

Taylor indicated that more work must be done before photonic crystals registered by the markers written using this technique can be etched. He thinks it significant that a commercial microscope can be used to find features that are visible only when samples are cold, and that lithographic patterns can be written around them simply by changing the power of the excitation source.

Applied Physics Letters, May 8, 2006, 193106.