Quantum-well superlattices

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 purpose of this work is to demonstrate that the techniques of quantum control, which were developed originally to study atoms and molecules, can be applied to the solid state. Previous work considered a simple example, the asymmetric double quantum well (ADQW). Results for this system showed that both the wave paeket dynamics and the THz emission can be controlled with simple, experimentally feasible laser pulses. This work extends the previous results to superlattices and chirped superlattices. These systems are considerably more complicated, because their dynamic phase space is much larger. They also have potential applications as solid-state devices, such as ultrafast switches or detectors. 

H. Morkoc, F. Hamdani, and A. Salvador, Electronic and Optical Properties of IIT-V Nitride based Quantum Wells and Superlattices K. Doverspike and J. /. Pankove, Doping in the 111-Nitrides... 

Indeed we study the two-dimensional systems in Section 5. In this section we will analyze the structural, electronic and, in particular, the optical properties of Si and Ge based nanofilms (Section 5.1), of Si superlattices and multiple quantum wells where CalQ and SiC>2 are the barrier mediums (Sections 5.2 and 5.3). The quantum confinement effect and the role of symmetry will be considered, changing the slab thickness and orientation, and also the role of interface O vacancies will be discussed. 

Figure 41 Left panel calculated 62 first asymmetric peak (- - -) and its Gaussian fit (—) for the (a) Sip]-SiC>2, (b) Si[2]-SiC>2 and (c) S1O2 superlattices. The letter I indicates the interface Gaussian band while the letter Q indicates the bulk-like Gaussian band. Right panel PL spectra of c-Si/Si02 single quantum wells under 488 nm laser excitation at 2 K (a) 1.7 nm, (b) 1.3 nm and (c) 0.6 nm thickness. The asymmetric PL spectra can be fitted by two Gaussian bands, the weak Q band and the strong I band [51],...

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

Many quite different approaches to alleviating the miserable light emission in bulk Si ( 10 4 quantum efficiency at 300 K) have been proposed and are actively being explored.9"14 Some, such as Si i xGex quantum well or Si/Ge superlattice structures, rely on band structure engineering, while others rely on quantum confinement effects in low dimensional structures, as typified by quantum dots or porous Si (rc-Si15). Still another approach is impurity-mediated luminescence from, for example, isoelectronic substitution or by the addition of rare earth ions. An overview of results obtained with some of these methods is given below. However, in order to understand more fully the reasons why such different approaches are necessary, it is appropriate to review first what creates the optical emission problem in crystalline Si (c-Si). 

I. Harrison. Impurity-induced disordering in III-V multi-quantum wells and superlattices// J.Mater.Sci. Mater.Electron.- 1993.- V.4, No.l - P.1-28. 

M. S. Dresselhaus, Y.-M. Lin, T. Koga, S. B. Cronin, O. Rabin, M. R. Black, and G. Dresselhaus, Quantum Wells and Quantum Wires for Potential Thermoelectric Applications D. A. Broido and T. L. Reinecke, Thermoelectric Transport in Quantum Well and Quantum Wire Superlattices G. D. Mahan, Thermionic Refrigeration... 

Nanocomposites in the form of superlattice structures have been fabricated with metallic, " semiconductor,and ceramic materials " " for semiconductor-based devices. " The material is abruptly modulated with respect to composition and/or structure. Semiconductor superlattice devices are usually multiple quantum structures, in which nanometer-scale layers of a lower band gap material such as GaAs are sandwiched between layers of a larger band gap material such as GaAlAs. " Quantum effects such as enhanced carrier mobility (two-dimensional electron gas) and bound states in the optical absorption spectrum, and nonlinear optical effects, such as intensity-dependent refractive indices, have been observed in nanomodulated semiconductor multiple quantum wells. " Examples of devices based on these structures include fast optical switches, high electron mobility transistors, and quantum well lasers. " Room-temperature electrochemical... 

E24.30 (a) Top-down versus bottom-up. The top-down approach requires one to carve out nanoscale features from a larger object. The bottom-up approach requires one to build up nanoscale features from smaller entities. Lithography is a standard approach to top-down and thin film deposition of quantum wells or superlattices is a standard approach to bottom-up. ... 

Quantum films (multiple quantum wells and superlattices) (1-D quantisation)... 

Figure 3.4 Energy levels of (a) multiple quantum wells and (b) superlattices. When the barriers are thick, the wells are isolated, there is no inter-well electronic coupling, and the quantised states are narrow. When the barriers are thin (<4 nm), inter-well electronic coupling occurs, the quantised states broaden, minibands form and electron delocalisation and transport can occur. Source Nozik and Memming (1996).

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