Quantum heterostructures

The time has come to make a quantum leap, both figuratively and literally. Short-term solutions may exploit organic semiconductors [31-33] and semiconductor quantum heterostructures [34, 35]. A long-term solution will be provided by molecular electronics [36-40] in which single molecules are employed as integrated circuits [41, 42]. 

Inhomogeneous media with micro- and nanosized structural features are known to strongly alter the character of various physical processes compared to common homogeneous materials. For example, photonic band gap (PBG) structures and metamaterials can be used for subwavelength light control [1]. Similarly, quantum heterostructures such as quantum dots and quantum wells have shown great promise in nano- and optoelectronics, as well as in quantum computing. 

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. 

The ability to create and observe coherent dynamics in heterostructures offers the intriguing possibility to control the dynamics of the charge carriers. Recent experiments have shown that control in such systems is indeed possible. For example, phase-locked laser pulses can be used to coherently amplify or suppress THz radiation in a coupled quantum well [5]. The direction of a photocurrent can be controlled by exciting a structure with a laser field and its second harmonic, and then varying the phase difference between the two fields [8,9]. Phase-locked pulses tuned to excitonic resonances allow population control and coherent destruction of heavy hole wave packets [10]. Complex filters can be designed to enhance specific characteristics of the THz emission [11,12]. These experiments are impressive demonstrations of the ability to control the microscopic and macroscopic dynamics of solid-state systems. 

Calculating the exact response of a semiconductor heterostructure to an ultrafast laser pulse poses a daunting challenge. Fortunately, several approximate methods have been developed that encompass most of the dominant physical effects. In this work a model Hamiltonian approach is adopted to make contact with previous advances in quantum control theory. This method can be systematically improved to obtain agreement with existing experimental results. One of the main goals of this research is to evaluate the validity of the model, and to discover the conditions under which it can be reliably applied. 

Kim S, Fisher B, Eisler HJ, Bawendi M (2003) Type-11 Quantum Dots CdTe/CdSe (core/sheU) and CdSe/ZnTe(core/shell) heterostructures. J Am Chem Soc 125 11466-11467 Aharoni A, Mokaii T, Popov 1, Banin U (2006) Synthesis of InAs/CdSe/ZnSe core/ shelll/shell2 structures with bright and stable near-infrared fluorescence. J Am Chem Soc 128 257-264... 

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. 

Stripe—geometry gain—guided AlGaAs-GaAs quantum well heterostructure lasers have been fabricated from masked hydrogenation to produce the resistive regions necessary to current confinement. 

F. Capasso, Graded-Gap and Superlattice Devices by Band-gap Engineering W. T. Tsang, Quantum Confinement Heterostructure Semiconductor Lasers... 

R.J. Matyi, in Heterostructures and Quantum Devices (VLSI Electronics Microstructure Science), W.R. Frensley, N.G. Einspruch (eds.), Academic Press, San Diego, 1994, Ch. 2. 

The external quantum efficiency of the EL from PS-based devices has been increased from low initial values of 0.001% [Ko9] to values close to 1% [Ni4, La6, Co5]. This, however, is still about one order of magnitude smaller than the maximum quantum efficiency of state-of-the-art LEDs based on III-V semiconductor heterostructures. 

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... 

E, Kapon, Lateral Patterning of Quantum Well Heterostructures by Growth of Nonplanar... 

R. Cingolani, Optical Properties of Excitons in ZnSe-Based Quantum Well Heterostructures A. Ishihashi and A. V. Nurmikko, II-VI Diode Lasers A Current View of Device Performance... 

Interfaces are of critical importance in determining the electronic and optical properties of quantum well heterostructures. It is necessary to... 

New physics such as the fractional quantum Hall effect has emerged from non-magnetic semiconductor heterostructures. These systems have also been a test bench for a number of new device concepts, among which are quantum well lasers and high electron mobility transistors. Ferromagnetic 111-Vs can add a new dimension to the III-V heterostructure systems because they can introduce magnetic cooperative phenomena that were not present in the conventional III-V materials. 

Fig. 3. The lattice-matched double heterostructure, where the waves shown in the conduction band and the valence band are wave functions, T(x), representing probability density distributions of carriers confined by the barriers. The chemical bonds, shown as short horizontal stripes at the AlAs—GaAs interfaces, match up almost perfecdy. The wave functions, sandwiched in by the 2.2 eV potential barrier of AlAs, never see the defective bonds of an external surface. When the GaAs layer is made so narrow that a single wave barely fits into the allotted space, the potential well is called a quantum well. Because of the match in the atomic spacings between GaAs and AlAs, 99.999% of the interfacial chemical bonds are saturated.

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