Quantum-well material

We stress that in the case of an isotropic material, the fields Ei and Et are independent. In these materials an s-polarized cavity photon excites a T-exciton (28) in the quantum well material and interacts with it, while ap-polarized cavity photon excites L- and Z-excitons and interacts with them. In contrast, as seen from eqn (10.7), Ei and Et become coupled when exx eyy. Except for waves propagating along the x and y axes, both Ei and Et exist in the cavity material and interact with excitonic resonance on an equal footing. 

It should be noted that the salient difference between the energy-momentum relation between bulk semiconductor and quantum well material is that the k vector associated with Eq takes on discrete, well-separated values. In the quantum well device, the density of states is obtained fi om the magnitude of the two-dimensional k vector associated with the y-z plane, as compared to the three-dimensional wavevector for the bulk semiconductor. As a result, the final density of states for the quantum well structure is given by... 

FIGURE 19 Plot of the optical gain coefficient in inverted semiconductor media, showing the difference between bulk and quantum well material. 

Fig. 5. Energy levels of electrons and heavy holes confined to a 6-nm wide quantum well, Iuq 53GaQ 4yAs, with InP valence band, AE and conduction band, AE barriers. In this material system approximately 60% of the band gap discontinuity Hes in the valence band. Teasing occurs between the confined...

In photoluminescence one measures physical and chemical properties of materials by using photons to induce excited electronic states in the material system and analyzing the optical emission as these states relax. Typically, light is directed onto the sample for excitation, and the emitted luminescence is collected by a lens and passed through an optical spectrometer onto a photodetector. The spectral distribution and time dependence of the emission are related to electronic transition probabilities within the sample, and can be used to provide qualitative and, sometimes, quantitative information about chemical composition, structure (bonding, disorder, interfaces, quantum wells), impurities, kinetic processes, and energy transfer. 

Another strategy reported by Sales links back to the superlattices discussed in Section 7.2.1.4. It was suggested by Mildred Dresselhaus s group at MIT (Hicks et al. 1993) that semiconductor quantum wells would have enhanced figures of merit compared with the same semiconductor in bulk form. PbTe quantum wells were confined by suitable intervening barrier layers. From the results, ZT values of 2 were estimated from single quantum wells. This piece of research shows the intimate links often found nowadays between apparently quite distinct functional features in materials. 

The creation of nanoscale sandwiches of compound semiconductor heterostructures, with gradients of chemical composition that are precisely sculpted, could produce quantum wells with appropriate properties. One can eventually think of a combined device that incorporates logic, storage, and communication for computing—based on a combination of electronic, spintronic, photonic, and optical technologies. Precise production and integrated use of many different materials will be a hallmark of future advanced device technology. 

Nolte, D. D. Melloch, M. R., Photorefractive quantum wells and thin films, In Photorefractive Effects and Materials Nolte, D. D., Ed. Kluwer Academic Publishers, Dordrecht, 1995... 

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. 

Clearly, to increase the enhancement factor, it is necessary to design and fabricate high-Q, small-V microresonators. However, cavity-enhanced LEDs based on the microresonators with high-Q modes must have equally narrow material spontaneous emission linewidths (Fig. 7a), which are not easily realized in bulk or heterostructure quantum-well microresonators. The recently proposed concept of an active material system, semiconductor quantum dots (QDs) (Arakawa, 2002) combines the narrow linewidth... 

A large fraction of the material science research, and an important chapter of solid state physics are concerned with interfaces between solids, or between a solid and a two dimensional layer. Solid state electronics is based on metal-semiconductor and insulator-semiconductor junctions, but the recent developments bring the interface problem to an even bigger importance since band gap engineering is based on the stacking of quasi two dimensional semiconductor layers (quantum wells, one dimensional channels for charge transport). 

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 electronic and magnetic properties of nanolayers are important in devices formed from electronic materials that are more conventional. We have already discussed quantum well lasers (see Chapter 8) and giant magnetoresistance (GMR) devices used for hard disk read heads (see Chapter 9). Quantum well lasers may be an important component of light-based computers. Other possibilities include magnets with unusual properties (Section 11.2). 

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