Semiconductor quantum dots charge carriers

Charge carriers in semiconductors can be confined in one spatial dimension (ID), two spatial dimensions (2D), or three spatial dimensions (3D). These regimes are termed quantum films, quantum wires, and quantum dots as illustrated in Fig. 9.1. Quantum films are commonly referred to as single quantum wells, multiple quantum wells or superlattices, depending on the specific number, thickness, and configuration of the thin films. These structures are produced by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) [2j. The three-dimensional quantum dots are usually produced through the synthesis of small colloidal particles. 

Figure 2.8 Free charge carriers in a solid have a parabolic dispersion relation ( (k) oc k ). In a semiconductor, the energy bands for free electrons and holes are separated by an energy gap g. In a bulk semiconductor, the states are quasi-continuous, and each point in the energy bands represents an individual state. In a quantum dot (QD), the charges are confined to a small volume. This situation can be described...

Figure 9.10 Three quantization configurations in semiconductors, (a) Confinement of charge carriers in two dimensions called a quantum well. A narrow band gap ( g(2)) is sandwiched between two wide band gap ( g(l)) semiconductors. The charged carriers are t ped in a two-dimensional potential well, (b) Confinement of charge carriers in one dimension called quantum wires, (c) Confinement of charge carriers in zero dimension called quantum dots, w represents width of confined direction, which is of the order of 1-20 nm. Adapted from reference (14).

Semiconductor nanoparticles that arise due to the confinement of electrons/holes in zero spatial dimension are called quantum dots. Quantum wires and quantum films or wells arise when the charge carriers in the corresponding semiconductor are confined in one or two spatial dimensions. These dimensional confinements and corresponding density of states are illustrated in Figures 9.10 and 9.11, respectively. 

Solar cells based on hot carrier extraction and CM rely on precise control of hot carrier relaxation were expected to be realized in nanostructured semiconductors e.g. QDs) because of enhanced carrier arrier interactions and discretized energy levels. As will be shown below, TRTS is capable of probing charge carrier dynamics at early times after photoexdtation, including intraband relaxation and CM in bulk materials and quantum dots. As such, TRTS represents a powerful technique for evaluating novel semiconductor systems that may be used in the design of more efficient solar cells. 

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