New Physics and Device Applications of Quantum Dots
Associate Professor Ewa M. Goldys
An increasing need for sources and detectors for mid and far infrared applications such as infrared spectroscopy for chemical analysis, remote sensing and atmospheric communications provides the driving force to develop improved infrared light detectors. At present, commercial infrared light detectors are principally based on HgCdTe, and while their performance parameters such as detectivity and responsivity remain excellent, their deficiencies such as nonuniformity of HgCdTe wafers, important for imaging, as well as difficult manufacturing technology remain well known. Therefore the motivation arose to seek alternatives, preferably based on GaAs-type materials where advanced growth technology such as the molecular beam epitaxy (MBE) is widely available. Since over ten years the quantum well intersubband detectors (QWIPS) based on GaAs-type materials are being developed, and while this work still continues, much of the underlying science has been well established.
Recently, new fundamental optical properties of nanostructures have been discovered. These include significant changes in the energy level assignment and in the selection rules for optical absorption. The relaxed selection rules, and particularly absorption at normal incidence (forbidden in most commonly used n-type GaAs/AlGaAs quantum wells[1]. Interestingly, the modified optical properties also can arise in larger nanostructures (that is not quantised in the growth plane) due to stress gradients in the quantum dots[2], these lead to normal incidence operation of quantum dot light detectors.
In the recent two years the quantum dot infrared detectors emerged at the forefront of light detector research, In comparison with QWIPS, the quantum dot detectors offer important >advantages in regard to the performance parameters such as responsivity, detectivity and normal incidence operation. Standard quantum dot detectors, similarly to QWIPs respond to a single radiation wavelength or to a narrow spectral band.
The modified properties of quantum dots significantly influence the key light detector parameters, such as detectivity and responsivity. Compared to quantum wells used in QWIPs, quantum dots are characterised by slowing of the intersubband relaxation time due to a reduced electron-phonon interaction. The reduced phonon scattering due to a discrete density of states in a quantum dot leads to long lifetime and long dephasing time and therefore to an increased radiative efficiency. Quantum dot detectors are also expected to exhibit lower dark current and noise than a quantum well detector.
It has therefore been anticipated that the success achieved in using quantum well structures in novel optoelectronic and electronic devices may be extended by using quantum dots instead of quantum wells due to significant improvements in the infrared detector performance.
The significance of quantum dot light detectors lies in the fact that they are an emerging class of infrared detectors that will complement the HgCdTe detectors and QWIPs with commensurate or higher detectivity and fast response time. HgCdTe are traditionally the only high detectivity far infrared detector on the market today. Investigations of quantum dot light detectors have just started to appear in the recent literature. These devices offer scope for improved performance compared to quantum well light detector devices (QWIPs), and hence they are significant, while relatively unexplored.