In-plane propagation of millimeter waves in periodic magnetoactive semiconductor structures

Non-transmission bands of electromagnetic waves propagating along the layers in periodic structures are studied in the steady magnetic field perpendicular both to the uniaxis and the direction of propagation. The band control range (36÷75 GHz) inn-InSb/Al₂O₃ structures with the carrier densities 4 10¹³ ≤n ≤ 8 10¹⁴ cm⁻³ in magnetic fieldsB _o ≤ 2 T at temperatures 77 ≤T ≤ 200 K is found to agree with the calculated in the effective medium approximation. Attenuation down to −50 dB within the band is observed. The band lineshape is found to indicate additional effects related to the finite layer thickness and periodicity termination predicted by a more rigorous theory of dispersion.     

Higher multipole surface plasmons,plasmon bands in periodic metallic heterostructures,and plasma modes of semiconductor superlattices

The electronic collective excitations are investigated for a number of different systems whose equilibrium electron density n₀(z) is spatially varying. The relation between higher multipole surface modes, waveguide like plasma modes of a low electron density layer embedded in a high density host, and plasmon bands of periodic metallic heterostructures and semiconductor superlattices is pointed out.     

Plasmon and magneto-plasmon excitations in double heterostructures

The plasmon and magneto-plasmon spectra are analyzed for an electron system which consists of two parallel two-dimensional electron layers which are separated from each other by a certain distance2d. Our approach allows to consider these electron liquids to be on different levelsE ₁ andE ₂ and to regard an arbitrary tunneling probabilityt between them. The Coulomb interaction is treated by means of a random-phase approximation. The resulting modes reflect typical features of the system: there are different plasmon branches connected with either the total or the reduced charge density and a further collective mode consisting of electron transitions from one to another electron layer. The influence of a static external magnetic fieldB applied perpendicularly to the electron layers leads to characteristic combinations of the plasmon modes (forB=0) with the cyclotron frequency _c .     

Plasmons and magnetoplasmons in semiconductor heterostructures

The purpose of this review is to survey the status of the theory and experiment which can contribute to our knowledge of plasmon excitations in synthetic semiconductor heterostructures. With increasing sophistication of experimental probes, plasmons are now often used as a diagnostic tool to characterize the electronic structure of new materials. The surface plasmon becoming known as plasmon–polariton is simply an electromagnetic (EM) wave that propagates along the interface separating the two media. The strength of the fields associated with the wave is localized at and decays exponentially away from the interface. In preparing this review we feel that, beyond the presentation of the results achieved, there is a need to examine carefully the methodologies employed to obtain such results. By methodologies, we do not mean to go into the details of the mathematical machinery (this is carefully avoided!). The time has come, however, to look with a critical eye at the development of the principles, tools, and applications of the model theories applied to diverse geometries of heterostructures. Attention is largely given to the non-radiative modes, scrutinizing the effects of an applied magnetic field, carrier collisions, retardation, and interaction with optical phonons. We have mostly confined our attention to the intrasubband modes, which could be investigated through the use of classical as well as quantal approaches. Abiding by the central theme of the review, numerous theoretical results on plasmons and magnetoplasmons in several geometries of practical interest have been gathered and reviewed. This survey is preceded by the basics of several methodologies, both classical and quantal, applicable to a wide variety of systems of varying interest. The report concludes addressing briefly the anticipated implications of plasmon observation in the respective composite systems under a variety of circumstances. Our satisfaction in writing this review, like any other review which covers a considerably longer period,was in bringing together the results of primary research papers and presenting some unified account of progress in the field. The background provided is believed to make less formidable the task of future writers of reviews in this field and hence enable them to deal more readily with particular aspects of the subjects, or with recent advances in those directions in which notable progress may have been made.     

Collective excitations in semiconductor superlattices

Collective excitations in semiconductor superlattices are studied beyond tbe random-phase approximation (RPA). The Singwi, Tosi, Land and Sjölander (STLS) theory, which accounts for exchange and short-range correlations effects through an effective potential depending on the structurc factor, is generalized to the layered electron system described by the model of Visscher and Falicov. The exact numerical solution of the STLS self-consistent equations provides information about intraplane and interplane correlations. The plasmon dispersion curves are evaluated for some typical values of the coupling constant r_s of the electron system and the distance between the planes for GaAs/AlGaAs semiconductor superlattices. For comparison, the RPA and the Hubbard approximation are also considered.     

Opto-thermionic refrigeration in semiconductor heterostructures.

Combining the ideas of laser cooling and thermionic cooling, we have proposed an opto-thermionic cooling process, and investigated its cooling effect caused by the light emission from a quantum well embedded into a semiconductor pn junction. For a GaAs/AlGaAs opto-thermionic refrigerator in which the Auger recombination is the major nonradiative process, cooling can be achieved in a finite range of bias voltage. Using the measured values of the Auger coefficient, our calculated cooling rate is at least several watts/cm(2).     

Wannier-mott excitons in narrow-gap semiconductor heterostructures

Spectra of excitons in a bulk semiconductor and in a thin semiconducting layer are investigated in the two-band Dirac model. The dependences of the exciton binding energy on the energy gap and, in the two-dimensional case, on the layer width are obtained. The fine structures of exciton spectra are revealed.     

Diluted magnetic semiconductor superlattices and heterostructures

Diluted magnetic semiconductors (DMS) are mixed semiconducting crystals whose lattice is made up in part of substitutional magnetic ions. Cd_1−xMn_xTe and Hg_1−xMn_xTe are examples of such materials. Their structural and band parameters can be “tuned” by composition over a wide range. They can thus be exploited in situations completely similar to those involving Ga_1−xAl_xAs. Using molecular beam epitaxy, we have grown Cd_1−xMn_xTe superlattices with alternating Mn content, having up to 150 layers, with layer thickness ranging from 50 to 100 Å. The superlattice structure is clearly revealed by transmission electron microscopy and by zone-folding of the phonon spectrum observed in Raman scattering. Photoluminescence observed on Cd_1−xMn_xTe superlattices is several orders of magnitude greater than that from a Cd_1−xMn_xTe film with uniform Mn content, or from bulk Cd_1−xMn_xTe specimens. The presence of localized magnetic moments in DMS results in a strong exchange interaction between these moments and band electrons. This in turn leads to gigantic Zeeman splittings of impurity states, exciton levels, Landau levels, and the bands themselves. Zeeman splittings as large as 20 meV (which in non-magnetic semiconductors would require unrealistic megagauss fields) are easily achieved in DMS in fields of several kilogauss. Since the magnitude of this exchange-induced splitting in DMS can be comparable to the binding energies and to the minigaps encountered in multiple quantum wells, DMS superlattices hold promise of a host of novel effects of both fundamental and applied interest.     

Long-wavelength nonpolar phonons in semiconductor heterostructures

Employing a phenomenological long-wavelength approach recently developed, both acoustic and optical phonons in nonpolar heterostructures are studied. Phonon modes in an arbitrary direction can be calculated without additional effort respect to high symmetry directions. A simple analytical expression for the dispersion relation in superlattices guides the physical discussion. We apply this to the calculation of phonon modes in unstrained short period isotopic Ge superlattices, in quantum wells and in strained short period Si/Ge superlattices. We find very good agreement with the results of other, more elaborated and costly calculations.Received: 21 July 2004, Published online: 5 November 2004PACS: 63.20.Dj Phonon states and bands, normal modes, and phonon dispersion - 63.22. + m Phonons or vibrational states in low-dimensional structures and nanoscale materials - 68.65.Cd Superlattices     

Interfacial electronic states in semiconductor heterostructures

It is shown that electronic states of a new type, with energy in the band gap can exist at a heterointerface. The interfacial states may be associated with Tamm surface states in the materials forming the heterointerface, but they can appear even if there are no surface states in the initial materials. In the plane of the heterojunction, the energy spectrum of interfacial states forms a two-dimensional band. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 6, 393–398 (25 March 1998)     

Nonpotential surface waves in magnetoactive plasma structures

Conclusions In this paper we have studied the dispersion characteristics of nonpotential surface waves in two- and three-layered magnetized wave-bearing structures. For the case of a vacuum cavity in an infinite plasma, we have obtained an analytic solution for the SW frequency in a regime which is highly nonpotential. We have shown that when the thickness of the plasma layer in a vacuum-plsma-vacuum waveguide is reduced, the dispersion changes its character.We have investigated numerically how magnetic fields, the radii of the cylinders, and the azimuthal wave number affect the LF and HF branches of the spectrum of surface waves.Khar'kov State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 34, No. 6, pp. 639–645, June, 1991.     

Geometry-induced localization phenomena in semiconductor quantum-dot heterostructures

Conduction-band electrons of semiconductor heterostructures described using the theory obey, for wide-bandgap semiconductors, the one-band effective-mass equation. We present, based on the one-band effective-mass equation, electron-state solutions for a quantum-dot heterostructure composed of two material layers (A and B) and identify localization properties of the groundstate. In particular, we show that the groundstate of two-material layer cylindrical quantum-dot systems can be localized in either material A or B depending on the dimensions of the nanostructure. A structure which is axially stacked (configuration A–B–A) has a certain critical radius below which the electron becomes localized in material A if the total axial length is big enough (A is assumed to be the material with the highest conduction-band edge). Similarly, a structure which is radially stacked (configuration B–A) has a certain critical (axial) length below which the electron becomes localized in the high conduction-band edge material A if the radius is big enough. Although results are presented for cylindrical-shaped heterostructure semiconductors, similar localization inversion of the groundstate may occur in other geometries such as rectangular-shaped quantum-dot heterostructures.