R. Kucharczyk

A finite superlattice with the embedded quantum well

We present a model of a finite superlattice (SL) with the embedded quantum well (EQW), i.e., with the last well wider or narrower than the rest. The modification of the electronic structure of such a system, resulting from the interaction between the EQW and SL, is studied, and a suggestion is made about experimental investigations of the coupling between the EQW and SL by means of excitonic measurements.

Density-of-states formalism for multi-quantum-barrier structures

Abstract We present the Green function formalism for the density-of-states of the multi-quantum-barrier structure and apply it to a superlattice with the modified terminating layer (the so-called embedded quantum well). The density-of-states distributions calculated for such a system enable to follow the formation of a localized state and its interaction with superlattice extended states.

Internal surface effects in superlattices in contact with substrate

Using the direct matching procedure and the interface response theory, the effects of the internal surface of a superlattice (i.e., the substrate/superlattice interface) are studied as a function of superlattice parameters while the substrate parameters are kept constant. Surface states are found inside mini-gaps and their localization properties are investigated. The crossings of these states with mini-bands are observed and examined by means of the variational density of states.

Density of states of superlattices with multiple layers per period

Abstract Density-of-states (DOS) investigations of polytype superlattices (SLs), whose period consists of more than two different layers, are presented. Local DOS (LDOS) is computed as a function of both the electron energy and the space coordinate, which illustrates — in a direct way — the spatial localization, over the SL period, of states forming the energy minibands. A few examples of AlGaAs-based polytype SLs with a coupled-well, step-well and δ-doped basis are analysed, indicating that various LDOS distributions within SL minibands can be arranged. By changing the basis geometry, the electronic DOS can either be selectively confined to a particular SL layer or extend over the whole SL period, which provides a means of modifying the spatial overlap of states from different SL minibands and thus, tuning the respective interband transitions. In particular, polytype SLs with similar miniband structures may exhibit essentially different LDOS features and, consequently, the different transport and optical characteristics. Therefore, it is emphasized that — for a particular device application of a complex-basis SL — not only the electronic level structure should be appropriately designed, but also the corresponding space-charge distributions should be carefully considered.

Kronig-Penney-type model for semi-infinite superlattices: I. A comment on surface-state energy expression

Abstract Energy spectrum of surface states in a semi-infinite superlattice represented by a Kronig-Penney-type model, calculated via the direct matching procedure and transfer matrix approach, is examined critically. A possibility of appearance of infinite barrier localized states, not corresponding to true surface states, is discussed in detail.

Kronig-Penney-type model for semi-infinite superlattices: II. Density-of-states studies

Abstract A Green function formalism for density-of-states calculations is applied to investigate the electronic structure of a semi-infinite superlattice in contact with a substrate. Density-of-states and space-charge distributions are computed and their modifications due to the varying parameters of the substrate/superlattice interface are studied. The interaction between localized surface states in mini-gaps and extended states in mini-bands is discussed.

Electronic structure of some mesoscopic systems: II. Electronic composites

We study the electronic properties of a mesoscopic system composed of an array of straight, infinite rods made of an isotropic medium and embedded in a regular way in an isotropic background. Such a composite system has two-dimensional periodicity in the plane perpendicular to the rods. Using a Fourier series expansion, the corresponding Schrodinger equation is solved within the effective-mass approximation. The electronic band structure E=E(k) is computed for the wave vector k in the transverse plane, and is illustrated by dispersion curves along the principal directions of the two-dimensional Brillouin zone as well as by the histograms of the density of states. The main result is the appearance of absolute energy gaps in the two-dimensional band structure.

Density of states of a superlattice with a δ defect in the subsurface region

Abstract The effect of a spatially and strongly confined ( δ -like) defect layer on the electronic properties of a semi-infinite superlattice (SL) is investigated, with emphasis placed on local density-of-states (LDOS) modifications at the SL end. The SL energy spectrum and the LDOS distributions, calculated within a modified Kronig–Penney type of approach, are studied for variable δ -defect parameters (i.e. its strength and position with respect to the SL surface) and different SL terminating conditions (depending on the choice of substrate). The possibility of a dramatic redistribution of extended states forming the SL minibands as a result of a single δ -defect sheet insertion in the subsurface SL region is indicated and discussed in detail.

Surface electronic structure of periodically δ-doped superlattice

Abstract Surface electronic structure of a semi-infinite superlattice (SL) intentionally δ-doped in each well- or barrier-layer is studied using a terminated Kronig-Penney-type of model interspersed with a periodic array of δ-function potentials representing the deep-center defect sheets. It is shown that SL minibands can be tailored by varying the properties of inserted δ-defects, e.g., the lowest energetic miniband can be shifted below the bottom of SL quantum wells (as a result, the main SL energy gap becomes smaller than those of the host semiconductors) and miniband alignement can occur (as a result, the bandwith is enhanced by an order of magnitude). Special attention is paid to the effect of δ-doping on the properties of surface states (SSs), i.e., the states appearing within energetic minigaps and localized at the SL/substrate interface. It is found that by an appropriate choice of the δ-defect weight and location within the SL period, SS with a required energy position relative to minigap edges and a desired extension into the SL bulk can be achieved. Moreover, in contrast to standard (i.e., undoped) SLs, a possibility of SS existence for the substrate identical to SL barriers is shown in periodically δ-doped SLs.

Electronic surface states in GaAs/Ga1 − xAlxAs superlattice: effect of surface location

Abstract Two Green function techniques (the direct matching formalism and the interface response theory) are used to investigate the electronic structure of a semi-infinite GaAs/Ga1 − xAlxAs superlattice being in contact with a Ga1 − yAlyAs substrate. The effect of the superlattice termination on the formation of a surface state as well as on the density-of-states distributions is studied in a systematic way by varying the position of the superlattice surface (i.e. substrate/superlattice interface) within a superlattice period. It has been found that the occurrence, the position and the localization properties of surface states are very sensitive to the way the superlattice is terminated—they occur in particular mini-gaps only for some ranges of the outermost layer thickness. When this thickness is close to the value at which surface states merge into the mini-bands, the density-of-states distributions exhibit the most pronounced modifications.