Quantum well wires

Electron-hole pairs may bind to each other to form excitons, which can be either free or tied to impurities or defects.1218 The decay of such excitons can lead to light emission that may be tunable by, for example, quantum confinement. Such excitonic emission is thus under active investigation in quantum well, wire, and dot... 

P. Harrison, Quantum wells, wires and dots (Wiley, Chichester, 1999). 

IH Tan, R Mirin, T Yasuda, EL Hu, J Browers, CB Prater, PK Hansma, MY He, AG Evans. Study of partial strain release and surface-states formed on the sidewall of InGaAs quantum-well wires. J Vac Sci Technol B 10 1971-1974, 1992. 

P. Harrison, Quantum Wells, Wires and Dots, WUey, New York (2000)... 

Band gap engineetring confined hetetrostruciutres. When the thickness of a crystalline film is comparable with the de Broglie wavelength, the conduction and valence bands will break into subbands and as the thickness increases, the Fermi energy of the electrons oscillates. This leads to the so-called quantum size effects, which had been precociously predicted in Russia by Lifshitz and Kosevich (1953). A piece of semiconductor which is very small in one, two or three dimensions - a confined structure - is called a quantum well, quantum wire or quantum dot, respectively, and much fundamental physics research has been devoted to these in the last two decades. However, the world of MSE only became involved when several quantum wells were combined into what is now termed a heterostructure. 

Fig. 1 Schematic drawing to show the concept of system dimensionality (a) bulk semiconductors, 3D (b) thin film, layer structure, quantum well, 2D (c) linear chain structure, quantum wire, ID (d) cluster, colloid, nanocrystal, quantum dot, OD. In the bottom, it is shown the corresponding density of states [A( )] versus energy (E) diagram (for ideal cases).

Nano-structures comments on an example of extreme microstructure In a chapter entitled Materials in Extreme States , Cahn (2001) dedicated several comments to the extreme microstructures and summed up principles and technology of nano-structured materials. Historical remarks were cited starting from the early recognition that working at the nano-scale is truly different from traditional material science. The chemical behaviour and electronic structure change when dimensions are comparable to the length scale of electronic wave functions. Quantum effects do become important at this scale, as predicted by Lifshitz and Kosevich (1953). As for their nomenclature, notice that a piece of semiconductor which is very small in one, two- or three-dimensions, that is a confined structure, is called a quantum well, a quantum wire or a quantum dot, respectively. 

The methodology used to answer these questions can be classified as either semi-empirical or based on first principles. The confined structure is assumed to be two-dimensional (2D = quantum well), one-dimensional (lD = quantum wire) or zero-dimensional (0D = quantum dot). 

Figure 17.1. (a) Quantum wells, (b) quantum wires, (c) ordered arrays of quantum boxes, (d) random quantum dots, and (e) an aggregate of nanometer-size grains. 

Quantum dots are nanometre scale in three dimensions, but structures that are only nanometre scale in two dimensions (quantum wires) or one dimension (quantum wells or films) also display interesting properties. The quantised nature of the bands in nanostructures can lae seen in the density of states. Schematic, theoretical density of states diagrams for bulk material, quantum wells, quantum wires, and quantum dots are pictured in Figure 11.3. 

The two parallel ID wires are fabricated by cleaved-edge overgrowth (CEO), see Fig. 1. Initially, a GaAs/AlGaAs heterostructure with two closely situated parallel quantum wells is grown. The upper quantum well is 20 nm wide, the lower one is 30 nm wide and they are separated by a 6 nm AlGaAs barrier... 

Keywords Silicon, germanium, carbon, alloys, nanostructures, optoelectronics, light emission, photoluminescence, electroluminescence, quantum well, quantum wire, quantum dot, superlattices, quantum confinement. 

Theoretical calculations of the electronic structure and optical properties of H-passivated Si quantum wires have been reported by a number of research groups (see, for example, Ref. 116 and references therein). First principles calculations show the same band nesting phenomenon and near-flat dispersion along the T-Z symmetry (wire) direction, as described above for Si quantum wells, and the occurrence of direct gaps.116,117... 

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