Semiconductor Quantum Wells

Fig. 12.7 InGaAsP/InP multi quantum well semiconductor structure process (a) Si02 etch mask deposition (b) PMMA spin coating (c) E beam lithography and develop (d) Si02 etch (e) PMMA stripping (f) InGaAsP membrane etch (g) Si02 stripping (h) Chip flipping and bonding to sapphire (i) InP substrate etch (j) Adhesive etch...

Kamalov, V. F. Little, R. Logunov, S. L. El-Sayed, M. A. Picosecond electronic relaxation in CdS/HgS/CdS quantum dot quantum well semiconductor nanoparticles, J. Phys. Chem. 1996, 100, 6381. 

Before discussing the experimental evidence, it is worthwhile to consider lasing-related properties of quantum dots from a fundamental point of view. The theoretical description of the optical gain in bulk and quantum well semiconductors is discussed in Datareview C5.3. 

Dawes and Sceats have applied the model to a study of recombination kinetics in quantum-well semiconductors. ... 

Emission linewidths in doped semiconductor laser materials is determined by the distribution of electrons in the conduction and valence bands of the semiconductor material. This is largely determined by the temperature and is of the order of 10 Hz. An exception to this is in quantum well semiconductor laser material, in which the valence and conduction band energy levels are discrete and therefore 5-10 times narrower than ordinary doped semiconductor materials. 

FIGURE 6 Quantum well semiconductor structure, (a) Geometry of the quantum well, (b) Energy level diagram for the conduction and valence bands, (c) Cross-sectional view of the energy versus wavevector relation for electrons and hole traveling in the plane of the quantum well, (d) Density of states of a quantum well structure as compared to a bulk semiconductor. 

Quantum well semiconductor lasers with both single and multiple active layers have been fabricated. Quantum well lasers with one active are called single-quantum-well (SQW) lasers and lasers with multiple quantum well active regions are called multiquantum-well (MQW) lasers. The layers separating the active layers in a multiquantum well structure are called barrier layers. Typical examples of the energy band diagram of both SQW and MQW are schematically represented in Fig. 18. 

SPECTRAL AND DYNAMIC FEATURES OF QUANTUM WELL SEMICONDUCTOR LASERS... 

Most of the revival in this field in the late eighties is due to two factors the recent advent of powerful and efficient monochromatic pumping sources provided ly quantum-well semiconductor lasers and the confinement obtained ly a R-doped light-guiding active medium. This is the basis for the rapid development of R-doped fibers, new sources allowing, in particular, a 3-level CW laser scheme. 

B. Zhao, A. Yariv Quantum well semiconductor lasers. In Semiconductor Losers I Fundamentals, ed. by E. Kapon, P. L. Kelley (Academic Press, San Diego 1999) Chap.1... 

Nozik and Memming have worked extensively on the electrochemistry and photoelectrochemistry of quantum well semiconductor electrodes (14). As discussed in Section 9.5.1, the quantum wells are produced by either MBE or MOCVD method. Both of these techniques are capable of creating epitaxial layers exhibiting quantum size effects. Their properties can be varied by film thickness, interfacial abruptness, and crystalline perfection. 

Previous SLMs have used photoconductors coupled to liquid crystals [237, 238], multiple-quantum-well semiconductors [239] and silver haUde films [240] to exploit such effects as photorefraction [241], photochromism [242, 243] and saturable absorption, etc. The non-linear optical properties of oUgothiophenes make them promising candidates for active photochromic materials in optically-addressed spatial light modulator devices. In particular, speed and spatial resolution can be improved by two orders of magnitude, compared [244] with devices based on liquid crystals or semiconductor heterostructures. 

A logical consequence of this trend is a quantum w ell laser in which tire active region is reduced furtlier, to less tlian 10 nm. The 2D carrier confinement in tire wells (fonned by tire CB and VB discontinuities) changes many basic semiconductor parameters, in particular tire density of states in tire CB and VB, which is greatly reduced in quantum well lasers. This makes it easier to achieve population inversion and results in a significant reduction in tire tlireshold carrier density. Indeed, quantum well lasers are characterized by tlireshold current densities lower tlian 100 A cm . ... 

Epitaxial crystal growth methods such as molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) have advanced to the point that active regions of essentially arbitrary thicknesses can be prepared (see Thin films, film deposition techniques). Most semiconductors used for lasers are cubic crystals where the lattice constant, the dimension of the cube, is equal to two atomic plane distances. When the thickness of this layer is reduced to dimensions on the order of 0.01 )J.m, between 20 and 30 atomic plane distances, quantum mechanics is needed for an accurate description of the confined carrier energies (11). Such layers are called quantum wells and the lasers containing such layers in their active regions are known as quantum well lasers (12). 

The uncertainty principle, according to which either the position of a confined microscopic particle or its momentum, but not both, can be precisely measured, requires an increase in the carrier energy. In quantum wells having abmpt barriers (square wells) the carrier energy increases in inverse proportion to its effective mass (the mass of a carrier in a semiconductor is not the same as that of the free carrier) and the square of the well width. The confined carriers are allowed only a few discrete energy levels (confined states), each described by a quantum number, as is illustrated in Eigure 5. Stimulated emission is allowed to occur only as transitions between the confined electron and hole states described by the same quantum number. 

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. 

A new chapter in the uses of semiconductors arrived with a theoretical paper by two physicists working at IBM s research laboratory in New York State, L. Esaki (a Japanese immigrant who has since returned to Japan) and R. Tsu (Esaki and Tsu 1970). They predicted that in a fine multilayer structure of two distinct semiconductors (or of a semiconductor and an insulator) tunnelling between quantum wells becomes important and a superlattice with minibands and mini (energy) gaps is formed. Three years later, Esaki and Tsu proved their concept experimentally. Another name used for such a superlattice is confined heterostructure . This concept was to prove so fruitful in the emerging field of optoelectronics (the merging of optics with electronics) that a Nobel Prize followed in due course. The central application of these superlattices eventually turned out to be a tunable laser. 

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. 

Do results obtained from ODMR or OPNMR of semiconductor nanostructures such as quantum wells have relevance to the nanoscopic semiconductors synthesized and studied by chemists ... 

Answer. There has been little effective interplay between experimental results obtained on single nanostructures grown as quantum-wells and studied by optical-pumping methods and those obtained on bulk nanoscale semiconductors by more conventional NMR approaches. However, this situation may change, since the former studies can provide information about the effects of, e.g., charge carriers or strain or compositional interfaces upon NMR parameters such as chemical and Knight shifts and EFGs in reasonably well-defined systems. 

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