Jan 28, 2010
Green single quantum-dot electroluminescence at 150 K
In recent years, research performed at the Institute of Solid State Physics in Bremen, Germany, has established a new method of InGaN quantum-dot growth. Quantum dots (QDs) have similar optical properties to single atoms, which makes it feasible for them to act as single-photon sources. InGaN compounds are very durable and emit light in the visible spectral range. To achieve single-photon emission in the green spectral range where modern detectors have maximum efficiency, the group has fabricated light-emitting diode structures containing InGaN QDs as active material.
Temperature-dependent measurements performed by the team demonstrate thermal stability of the emission of a single quantum dot up to 150 K. This is an important and promising milestone towards electrically driven single-photon emission at room temperature in durable GaN-based devices, which would allow for applications such as free-space quantum cryptography.
The QDs are grown in the semiconductor epitaxy group by metalorganic vapour phase epitaxy. A two-step growth mode has been developed to create the InGaN QDs inside a GaN matrix. The size of these QDs has to be restrained to a few nanometers in each dimension to provide quantum confinement for the carriers.
Additionally, an advanced sample architecture incorporating an electron-blocking layer has been designed to optimize the stability of the electronic confinement against internal and external influences. During processing of the epitaxial structures to apply electrical contacts, tiny apertures (diameter <1 µm) were prepared to spatially select the emission of a few QDs during operation. However, these remaining QDs differ in their emission energy. Thus, it is possible to investigate the electroluminescence of single QDs in the green spectral region.
One of the most important issues is the temperature stability of QD emission and we have successfully probed such a QD for elevated temperatures up to 150 K. At low temperatures, the QD emission is visible as a sharp line up to 90 K. Beyond this temperature, the QD emission just remains visible up to 150 K as a shoulder on a rising background. Nevertheless, different analysis tools could prove that this shoulder is still related to the single QD investigated.
Full results are available in the journal Nanotechnology.
About the author
Joachim Kalden currently works as a PhD student in the semiconductor optics group of Prof. Dr Jürgen Gutowski at the Institute of Solid State Physics in Bremen, Germany. His research interests include the optical and electro-optical properties of wide-bandgap quantum dots as well as GaN nanorods. The experimental techniques for his investigations are mainly microphotoluminescence and microelectroluminescence in order to separately investigate single nanostructures. Christian Tessarek is a PhD student in the semiconductor epitaxy group of Prof. Dr. Detlef Hommel at the same institute. The aim of his work is the growth of InGaN quantum dots by metalorganic vapour phase epitaxy technique for use in light-emitting structures. Both gratefully acknowledge financial support by the German Research Foundation (DFG) and the Federal Ministry for Education and Research (BMBF).