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This chapter discusses the excitons in organic-based nanostructures. In particular, two-dimensional Frenkel-Wannier-Mott (FWM) excitons in hybrid organic-inorganic nanostructures (quantum wells, quantum wires, quantum dots) are considered. Linear and nonlinear optical response of hybrid FWM excitons is calculated. Based on the fast energy transfer, new concept for light-emitting devices is proposed. Exciton energy transfer from organics to semiconductor nanocrystals and its possible application to carrier multiplication in quantum dots is mentioned. Finally, FWM excitons and polaritons in a hybrid microcavity containing crystalline organic layer and a resonant inorganic QW, are considered. First experiments demonstrating properties of hybrid structures are mentioned.

During the 1970s, III-V compounds and epitaxial crystal growth provided the basis for low dimensional structures (or nanostructures). The best known example is a GaAs quantum well within AlGaAs barriers, electrons, and holes being confined in well defined energy levels that determine the optical properties. Quantum wires and dots are also described. The quantum well laser and the vertical cavity laser (VCSEL) show considerable advantages over their heterostructure predecessor. Another exciting development was that of the two-dimensional electron gas (2DEG) at an interface between semiconductors with different band gaps. By doping only the wide gap material so as to separate the doping atoms from the resulting free electrons, ionised impurity scattering can be minimised and extremely high electron mobilities achieved. Such samples led to the discovery of the fractional quantum Hall effect and to high mobility FETs (HEMTs) for microwave applications. Mesoscopic systems and heterojunction bipolar transistors (HBTs) are also described.

This chapter is devoted to laser structures on GaAs substrates, which are capable of operating near the 1.3-um spectral window. Firstly, motivation for long-wavelength emitters on GaAs is discussed and possible semiconductor materials, suitable for 1.3-um application, are compared. The main part of the chapter is focused on long-wavelength quantum dot lasers. Various approaches for epitaxial deposition of long-wavelength QDs are described. The device characteristics of diode lasers comprising quantum dots formed either with atomic layer epitaxy or dots-in-a-well method are then compared. Efficiency, threshold, and temperature characteristics of long-wavelength QD lasers are also discussed. For the sake of comparison, data on non-QD laser structures are presented. InGaAsN quantum wells and diode lasers based on them are also discussed in detail.

Unlike all of the previous chapters, this one is devoted to low-dimensional semiconductor structures (quantum wells, quantum wires and quantum dots-nanocrystals). Basic classification of these structures is outlined and densities of electronic states are described. A detailed theoretical treatment of electronic states and exciton effects in quantum wells, both with infinite and finite barriers, is presented in effective-mass approximation. Strong and weak quantum confinement limits are discussed. Selection rules for optical transitions in quantum wells are outlined and optical absorption and emission spectra are compared. Specificity of exciton behaviour is stressed. Quantum dots with spherically symmetric potential are described in strong and weak quantum confinement regime. Salient luminescence features of quantum dots are summarized and illustrated via typical examples. Briefly mentioned are other interesting luminescence-related phenomena: phonon bottleneck, excitons indirect in real space, enhancement of nano-luminescence by metal nanoparticles and exciton multiplication.

This chapter starts from the Lorentz oscillator model then discusses the optical transitions in semiconductors. After an introduction of Wannier-Mott and Frenkel excitons, the Hopfield equations are used to describe exciton-polaritons in bulk crystals. The non-local dielectric response theory is used to describe the optical response of semiconductor quantum wells, wires, and dots. Finally, the dispersion of exciton-polaritons in microcavities is derived, and the threshold between weak and strong light-matter coupling regime is discussed.

Stimulated emission and lasing can be achieved easily in a number of semiconductor nanostructures. This chapter gives an overview of a series of physical mechanisms that were found experimentally to give rise to positive optical gain in quantum wells, quantum wires and nanocrystals. In quantum wells, these are radiative recombination of localized excitons, LO-phonon assisted exciton recombination and electron–hole plasma luminescence. In quantum wires the data are rather scarce; localized excitons and electron–hole plasma appears to be involved in lasing. Separately treated are the cases of nanocrystals dispersed randomly in a matrix and that of heterostructures with ordered quantum dots (grown by Stranski–Krastanow method). Exciton and biexciton mechanisms of optical gain in quantum dots are analyzed. The crucial competing role of Auger recombination is expressed via the filling factor. Prospects of random lasing in semiconductor nanostructures are outlined.

This chapter reviews attempts to realize Bose-Einstein condensation of excitons in bulk semiconductors and semiconductor nanostructures, emphasizing on indirect (interwell) excitons in GaAs/AlGaAs coupled double quantum wells. Recent experiments with indirect excitons confined in a circular electric-field-induced in-plane trap are discussed. The observed spatially-ordered patterns in the photoluminescence signal and large coherence length are interpreted in terms of Bose-Einstein condensation of indirect excitons.

This chapter reviews recent achievements in the experimental realization of a cold but still dense gas of long-lived indirect excitons in GaAs-based coupled quantum well structures. The unusual dynamics (photoluminescence jump) and pattern formation (inner and external rings, as well as fragmentation of the external ring in circular-shaped spots), which were detected in time-resolved and spatially-resolved photoluminescence associated with indirect excitons, are described in detail.

Low-dimensional structures consisting of semiconductor heterostructures of thickness down to a very few atomic monolayers were grown ideally by MBE and resulted in a surge of interest in MBE itself. Short period superlattices, doping superlattices, quantum wells, wires and dots and two-dimensional electron gases (2DEGs) all proved of significance from scientific and commercial viewpoints. Superlattices and quantum wells showed negative electrical resistance, doping superlattices provided tuneable energy gaps, and quantum wells, quantum wires and quantum dots were characterised by confined energy states which offered photon energies tuneable by varying their dimensions. They also demonstrated increasingly sharp density of state functions, of interest for semiconductor lasers. 2DEGs showed very high electron mobilities by minimising ionised impurity scattering. The 2D localisation in 2DEGs resulted in discovery of the quantum Hall effect and fractional quantum Hall effect.

This chapter examines the basic excitonic interband transitions in a rectangular quantum well (QW) and similar structures. Topics covered include rectangular type I quantum wells, absorption in rectangular type 2 QWs, parabolic QWs, absorption in an indirect gap QW, intersubband absorption, and non-radiative recombination. Exercises are provided at the end of the chapter.

Optical devices grown by MBE include semiconductor lasers, vertical cavity and quantum cascade lasers, photodetectors and solar cells. The GaAs laser is covered in detail, followed by that of quantum well and quantum dot lasers. Threshold current densities came down from 10⁴ Acm^-2 to 20 Acm^-2 between 1962 and 2006. Roughly half of compact disc lasers were grown by MBE, while the only companies selling quantum dot lasers grow them by MBE. Other MBE lasers are based on GaInNAs(Sb) (for 1.5 µm wavelength), AlGaAsSb/GaSb (2.5 µm), various IV-VI compounds for long wavelengths and InGaN for visible wavelengths. MBE has made a major contribution to the development of vertical cavity lasers (VCSELs and VECSELs). MCT photodetectors for staring-array IR imaging systems are described in detail, MBE being the preferred growth method. A triple junction solar cell grown by MBE currently holds the world record for conversion efficiency.

This chapter discusses heterostructures made of semiconducting materials and their band structures as a realization of band engineering in semiconductors. It introduces the basics of doping and remote doping, and discusses surface states and Fermi-level pinning. Finally, metallic Schottky contacts and ohmic contacts on semiconductor surfaces are introduced. Particular emphasis is put on the understanding of remotely doped heterostructures and quantum wells.

This chapter presents a summary of the experimental results on large r _s 2D electron and hole gases (2DEGs and 2DHGs) in clean semiconductor devices. In particular, it focuses on experiments on the following systems: silicon metal-oxide-semiconductor field-effect transistors (MOSFETs), p-GaAs heterojunctions and quantum wells, n-GaAs heterojunctions, p- and n-SiGe quantum wells, and AlAs quantum wells. The typical values of r _s are ~ 10-40, depending on the device. The high r _s regime is easier to reach in Si MOSFETs than the much cleaner n-GaAs-based devices, due to relatively high effective mass, lower average dielectric constant, and the existence of two degenerate valleys in the spectrum, which further increase r _s by a factor of 2. As a result, to reach the same interaction strengths, electron densities two orders of magnitude lower are needed in n-GaAs heterojunctions compared to those in Si MOSFETs. The great similarities in the data obtained on different types of 2D system imply that the observed behaviours are robust, universal, and largely independent of details, in spite of the fact that in various 2D structures there are significant differences between the electronic spectrum and the nature of the disorder potential.

This chapter presents typical band structures for superlattices and quantum wells computed using the methods described in Chapter 9. It identifies important features of the conduction and valence subbands and minibands, their dispersions, optical matrix elements, and characteristic dependences on the materials, thicknesses, and compositions. The changes that occur when the energy gap becomes very small are also discussed. To complete the picture, it considers how the band structure of wurtzite materials differs from their zinc-blende counterparts, as well as the band structure of quantum wires and dots that feature multidimensional confinement.

This chapter develops a theory of excitonic binding in 2D systems and examines how absorption and recombination processes are changed from those in bulk material. It first presents the theory for excitations in a pure 2D system as developed by Shinada and Sugano, then discusses modifications in real quantum well (QW) structures. Mechanisms that broaden the absorption and emission peaks, the recombination lifetime and absorption spectra in indirect gap materials, and impurities in QWs are discussed. Exercises are provided at the end of the chapter.

This chapter examines the effect of an external field on subband energies, wave function, and absorption coefficients in multiple quantum wells (MQWs). Interband, intersubband, and excitonic transitions in MQWs are studied in detail. In superlattices, the field induces a crossover from anisotropic 3D character to a 2D character. This effect, called Warner–Stark localization, is discussed. Exercises are provided at the end of the chapter.

The properties of the states produced by quantum confinement in double heterostructures are obtained by solving the spatial part of Schrödinger’s equation for a one-dimensional potential well with barriers of finite height. The apparently formidable task of dealing with the very large number of atoms in a solid is greatly simplified by exploiting the periodic nature of crystals through Bloch’s theorem and the envelope functions. Schrödinger’s equation gives, first, the wavefunctions that are used to calculate transition rates via the matrix element and, second, the energies of the confined states and hence the photon energies at which light interactions occur. This is used to develop simple models for the energy states in quantum dots with rectangular and parabolic potential profiles. Quantum wells provide confinement in only one dimension. Freedom in the other two dimensions produces a continuum of states characterised by a density-of-states function.

Chapter 10 reviews both homogeneous and inhomogeneous quantum plasma dielectric response phenomenology starting with the RPA polarizability ring diagram in terms of thermal Green’s functions, also energy eigenfunctions. The homogeneous dynamic, non-local inverse dielectric screening functions (K) are exhibited for 3D, 2D, and 1D, encompassing the non-local plasmon spectra and static shielding (e.g. Friedel oscillations and Debye-Thomas-Fermi shielding). The role of a quantizing magnetic field in K is reviewed. Analytically simpler models are described: the semiclassical and classical limits and the hydrodynamic model, including surface plasmons. Exchange and correlation energies are discussed. The van der Waals interaction of two neutral polarizable systems (e.g. physisorption) is described by their individual two-particle Green’s functions: It devolves upon the role of the dynamic, non-local plasma image potential due to screening. The inverse dielectric screening function K also plays a central role in energy loss spectroscopy. Chapter 10 introduces electromagnetic dyadic Green’s functions and the inverse dielectric tensor; also the RPA dynamic, non-local conductivity tensor with application to a planar quantum well. Kramers–Krönig relations are discussed. Determination of electromagnetic response of a compound nanostructure system having several nanostructured parts is discussed, with applications to a quantum well in bulk plasma and also to a superlattice, resulting in coupled plasmon spectra and polaritons.