Photonics Theory Group

Theory of III-Nitride Quantum Well and Quantum Dot structures.

Nitride-based semiconductor materials InN, GaN, AlN and their alloys attract great attention due to their promising applications in optoelectronic devices such as light emitting devices (LEDs) and laser diodes (LDs)[6]. Depending on the alloy composition, these systems could in principle cover a wide wavelength range from red through yellow and green to blue[7].

InGaN systems are especially promising candidates for energy-efficient solid state lighting (which combines output from red, green and blue LEDs) since the inclusion of phosphide binaries are not required to generate a white light source. Additionally, owing to quantum confinement effects, fabrication and studies of nitride-based quantum dots (QDs) have recently attracted a great deal of interest for novel applications due to their unique physical properties [8]. For example, GaN/AlN QDs have shown great promise for single-photon sources operating at much higher temperature compared to QD systems based on conventional III-V materials [8]. Despite their potential for optoelectronic applications, the electronic and optical properties of group-III-nitride QDs are still far less known than those of the more traditional III-V-semiconductor QD systems. Here at Tyndall we both model the electronic, optical and electromagnetic properties in the Photonics theory group, but also we have a growing experimental capacity both in growth and characterisation.

Electrostatic built-in fields in nitride based QDs

QDs based on III-nitride semiconductors have been grown mainly in the hexagonal phase. In comparison with conventional III-V materials, the wurtzite group III-nitrides exhibit very strong electrostatic built-in fields [9]. As a consequence the optical properties of nitride-based nanostructures are significantly modified by these contributions. For instance the built-in field gives rise to a strong separation of electron and hole wave functions [1]. Consequently, the optical recombination rate in these structures is drastically reduced.

To circumvent these problems, there has been a rapid increase in studies of non-polar growth of III-nitride structures, where the [0001] direction lies in the growth plane[10,11] Growth of quantum wells (QWs) along a non-polar direction can eliminate polarization-induced fields, and hence give improved radiative recombination rates[8]. However, and in contrast to the case in non-polar QWs, QDs are three dimensional objects, and therefore still retain facets along the [0001] direction, even when grown on a non-polar substrate. Therefore, it is not directly obvious how the polarization potential should behave in such a system.