Electronic conductivity quantum wells

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...

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. 

The electronic states in semiconductor quantum wells are determined by the width of the well, the effective masses of electrons and holes, and the height of the potential barriers in the conduction and valence bands. 

Several strategies based on novel device architectures have been developed in an effort to improve overall thermoelectric efficiency, one of the most promising of which is the use of quantum well superlattices. In certain superlattice systems, the electrical conductivity through the wells is dramatically increased due to an increase in the density of electronic states in the two dimensional system. At the same time, in a layered structure such as a superlattice, thermal conductivity is decreased due to enhanced phonon scattering at interfaces. Hicks, et al. have shown that a significant increase in the figure of merit can be achieved using quantum well superlattices synthesized by molecular beam epitaxy (4). 

In the following, we consider in some detail the transition from discrete to continuum spectra for the case of luminescence from highly excited semiconductor nanostructures. We wiU restrict ourselves to undoped semiconductors so that all carriers in conduction and valence band are optically excited. The luminescence is preceded by a fast carrier relaxation [76], so the recombination takes place when the electron and hole gases are in their respective ground states. In quantum wells, luminescence from high-density optically created electron-hole gases was studied in Refs. [77-79]. In confined structures, such as quantum dots, electrons and holes fill size-quantization energy states up to their respective Fermi... 

A quantum well consists of a thin layer of a semiconductor with a smaller band gap within a semiconductor with a larger band gap. The most studied quantum well stmctures are those formed from a layer of gallium arsenide, GaAs, sandwiched in gallium aluminium arsenide, GaAlAs. The electrons in the thin GaAs layer are effectively trapped in the well formed in the conduction band of the composite material. Similarly, the holes in the thin layer of semiconductor are trapped... 

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