4.1.1 What are they?
The research of microelectronic materials is driven by the need to tailor electronic and optical properties for specific component applications. Progress in epitaxial growth and advances in patterning and other processing techniques have made it possible to fabricate âartificialâ dedicated materials for microelectronics. In these materials, the electronic structure is tailored by changing the local material composition and by confining the electrons in nanometer-size foils or grains. Due to quantization of electron energies, these systems are often called quantum structures. If the electrons are confined by a potential barrier in all three directions, the nanocrystals are called quantum dots (QDs). This review of quantum dots begins with discussion of the physical principles and first experiments and concludes with the first expected commercial applications: single-electron pumps, biomolecule markers, and QD lasers.
In nanocrystals, the crystal size dependency of the energy and the spacing of discrete electron levels are so large that they can be observed experimentally and utilized in technological applications. QDs are often also called mesoscopic atoms or artificial atoms to indicate that the scale of electron states in QDs is larger than the lattice constant of a crystal. However, there is no rigorous lower limit to the size of a QD, and therefore even macromolecules and single impurity atoms in a crystal can be called QDs.
The quantization of electron energies in nanometer-size crystals leads to dramatic changes in transport and optical properties. As an example, Fig. 4.1 shows the dependence of the fluorescence wavelength on the dimensions and material composition of the nanocrystals. The large wavelength differences between the blue, green, and red emissions result here from using materials having different band gaps: CdSe (blue), InP (green), and InAs (red). The fine-tuning of the fluorescence emission within each color is controlled by the size of the QDs.
The color change of the fluorescence is governed by the âelectron in a potential boxâ effect familiar from elementary text books of modern physics. A simple potential box model explaining the shift of the luminescence wavelength is shown in the inset of Fig. 4.1. The quantization of electron states exists also in larger crystals, where it gives rise to the valence and conduction bands separated by the band gap. In bulk crystals, each electron band consists of a continuum of electron states. However, the energy spacing of electron states increases with decreasing QD size, and therefore the energy spectrum of an electron band approaches a set of discrete lines in nanocrystals.
As shown in Sec. 4.4, another critical parameter is the thermal activation energy characterized by kBT. For the quantum effects to work properly in the actual devices, the spacing of energy levels must be large in comparison to kBT, where kB is Boltzmann's constant, and T the absolute temperature. For room-temperature operation, this implies that the diameter of the potential box must be at most a few nanometers.