Epitaxial growth techniques make it possible to control the thickness of semiconductor layers at the atomic level and create carefully designed quantum-well structures that trap electrons. Now, Fang Qian and colleagues from Harvard University and the Georgia Institute of Technology have combined such semiconductor quantum wells with another type of nanoscale structure that is at the heart of much research at present — the nanowire (Nature Mater. 7, 701–706; 2008).

Credit: © 2008 NPG

Nanowires, as their name suggests, are long structures thin enough that the electrons within are confined to the point where quantum effects take hold. The proposed applications of nanowires are numerous because of their unusual electronic and optical properties. To add device functionality, nanowires are becoming increasingly complex. Surrounding the nanowire in a shell made of a different material, for example, has been found to improve the performance of nanowire-based field-effect transistors. This same principle has now been applied to optical structures, specifically GaN nanowire lasers.

To obtain efficient optical properties, it is important to create a material with a flaw-free crystal structure. Qian et al. used metal–organic chemical vapour deposition to ensure the materials reached the required quality. Nanowires of GaN 100–200 nm across and 20–40 μm long were deposited on sapphire substrates. On top of this they grew alternating layers of GaN and InGaN — a material with a smaller bandgap than GaN that acts as the electron confining layer, or well. By varying the growth time and temperature, the well thickness and the fraction of gallium atoms that are replaced by indium atoms in the InGaN crystal can be controlled. The indium content is important for optical applications as it determines the emission wavelength. This represents a significant advantage of the approach taken by Qian et al. Previous nanowire lasers have been based on binary semiconductors, which offer very little scope for bandgap engineering and therefore tunability.

The laser operation of the structure is quite simple: the quantum wells provide the optical gain medium whereas the nanowire acts as the cavity. One of the designs the researchers investigated consisted of 26 quantum wells, each with a thickness of 1.5 nm separated by 1-nm GaN barriers. Four structures were grown, each with wells with a different indium content. As the amount of indium was increased, the wavelength of the optically pumped laser (operating at room temperature) tuned from 383 nm to 478 nm, thereby covering both UV and visible frequencies. An interesting observation was that the lasers with a high indium content were bent, although this did not prevent them from lasing, testifying to the efficient waveguiding within the nanowire. The reason for the bending is likely to be that the high-indium-content quantum wells were not uniform on both sides of the nanowire, leading to a build up of strain.

The demonstration that such complex heterostructures are possible will hopefully act to stimulate further research into nanowire lasers. The next goal will probably be electrically driven structures, adding further shell layers to act as electrical contacts.