Important Structural Parameters

Quantum dots have been grown in a wide range of material systems, including elemental [Ge(Si) on Si] [24, 25], III-V (such as InGaAs on GaAs [12], InP on GaAs [26], InAs on InP [27], and nitride [28-31]), and II-VI [9, 32, 33] semiconductors. They can be produced in many ways, including by high-temperature precipitation in molten silicate glass matrices [34-37], by molecular precursor pyrolysis reactions for III-V semiconductor QDs [38, 39], by colloidal chemistry techniques [9, 32, 33, 40-42], from size fluctuations in conventional quantum wells [43-45], by mechanochem-ical synthesis [46, 47], and by lithographic patterning and etching [48-52]. However, the most important technique for QD growth is coherent island formation through heteroepi-taxial growth of lattice mismatched semiconductor materials [24, 25], because this approach combines QD growth with well-established semiconductor integration techniques.

Just as the dimensions of a quantum well and the geometry of the well influence the energy levels of the well, the size and composition profile of a QD and the material of the QD and the matrix can influence the energy levels

Figure 1. The density of the electronic states D(E) in (a) bulk materials, (b) quantum wells, (c) quantum wires, and (d) QDs. E represents the energy levels.

of the QD. Consequently, the opto-electronic properties of a QD are sensitive not only to all these parameters, but also to the shape of the QD [53-64]. The calculated values for QD ground-state and excited-state emission, and the intersublevel energies, are dependant upon the shape and aspect ratio (i.e., height to base diameter) [65] assumed in the calculation (e.g., conical shaped [66], pyramid-shaped [53, 67-69], and lens-shaped [70] QDs). As a consequence, knowledge of the geometry and chemical composition of QDs is a prerequisite if models are to be used to calculate their electronic and optical properties. It follows from this that understanding the growth mechanisms that control these parameters is crucial for producing device-applicable QDs.

In this review article, the terms "QD" and "island" are used interchangeably, as they often are in the literature, although generally those islands that are of sufficiently small size are properly named QDs.

0 0

Post a comment