As we move to the QD system, there is no free motion in any direction, and an atom-like delta function is obtained for the density of states. This atom-like character allows many new aspects of physics to be explored. With regard to semiconductor lasers, 0D systems have a number of predicted advantages. Arguably, a key motivator for the development of technologies to realize QD materials was the prediction of a temperature-insensitive threshold current by Arakawa and Sakaki in 1982 [2]. They explained that this would be achieved if only the ground sub-bands were populated (i.e., the state separation was large compared to kBT, where kB is the Boltzmann coefficient and T is the absolute temperature). This is possible by choosing sufficiently small dimensions for their “3D-quantum well” (prior to the adoption of the term dot for 3D confinement/0D carrier systems). In 1986, Asada et al. [3] modeled the electronic dipole for different carrier dimensionality and showed an increase in material gain for a “quantum box” (again, prior to the adoption of “dot” for 0D systems).
While these two reports are not the only theoretical predictions of the benefits of a QD active, they are certainly very compelling. The temperature sensitivity of a semiconductor laser is a major issue in their deployment in real-world applications. In optical communications, there are typically maximum launch power limits, and minimum receiver powers required to achieve efficient data transmission. As such, large variations of the laser power cannot be tolerated. In fact, the temperature sensitivity of lasers can be so large that they simply cannot be operated at two biases (logic level 0 and 1) over the whole temperature range. This temperature sensitivity results in the need for temperature monitoring and control, resulting in large packaging costs for laser modules as this tends to be rather labor intensive. Secondly, an enhanced gain at low current densities has clear advantages in terms of energy consumption, where very low currents also translate to low self-heating. This high gain for low current characteristic is also of importance in the dynamic performance of the laser, where differential gain plays a key role in direct modulation rates, and the differential gain spectrum dictates the change in lasing wavelength during modulation (chirp) [4].
As a consequence of major advantages such as these, there has been a great deal of work focused on the practical realization of such devices. The challenge of this task is not to be underestimated, as we require many factors to be simultaneously realized in our ideal QD laser active. We require the creation of structures with dimensions off ∼10 nm per side (∼30 atoms per side) in order to have good carrier confinement in all three dimensions. These QDs must be inserted within a semiconductor matrix, minimizing the formation of crystal defects as this will not only act as carrier recombination centers but may also pose problems for device reliability and commercial exploitation of the technology. Another major challenge is that in order to harness all the predicted benefits of QDs in a laser device, all QDs should be essentially identical [5]. The QDs required consist of ∼27 000 atoms embedded in a crystal matrix, with an ensemble of these differing in emission energy by only a few millielectron volts.