Quantum wires and dots are unique condensed matter systems where electron and hole localization can be achieved by lateral confinement down to quasi 0-dimensions. Here we review how these nanostructures are realized and their optical properties, mainly from an experimental point of view. We discuss changes in the energy and momentum mechanisms as the lateral dimensions are reduced, such as the so-called photon bottleneck, using the example of GaAs-GaAlAs quantum dots and wires. Examples of strain-relief by deep etching in both nominally lattice-matched and strained materials are given. The emission of hot luminescence or resonant Raman scattering from dots and wires is shown to be a fingerprint of exciton localization. Manu-body effects are shown to be important in nanostructures such as GaAs- GaAlAs and Si-SiGe dots and wires. The emergence of ultrathin submonolayer quantum wells has provided further insights into the role of exciton localization in nanostructures, and we discuss here the case of submonolayers of in-As embedded in a GaAs matrix. Considerations for devices are discussed along with future trends in the field.

The lattice dynamics of superlattices is now very well understood and most of the theoretical predictions have been quantitatively verified by Raman scattering, at least on GaAS/AlAS structures. This system is particularly interesting for several reasons: - samples are currently available at a very high and controlled quality, phonon dispersion curves in the bulk constituents GaAs and AlAs are known, almost perfect lattice matching is achieved, - GaAlAs alloy lattice dynamics is well described by reasonably simple models. All theses conditions allowed a high degree ofsophistication in the comparison between experiments and theory for GaAS/AlAS superlathces1 . We will review these studies in this communication and emphasize the relation between the GaAs and AlAs optical mode frequency and the interface roughness. Finally we will quote some experimental results on other systems. We will stress that the addition of strain make the quantitative interpretation much more difficult and review some recent progress in this direction.

The joint effects of (partial) ordering and biaxial strain due to lattice mismatch are considers for GaInP₂ epilayers grown by organometallic vapor phase epitaxy. on (001) GaAs substrates. Using polarization dependent electromodulation spectroscopy, lattice-matched and moderately mismatched sample of varying degrees of order are characterized in terms of shifted bandgaps, valence band splittings, and the appearance of new, high-energy features that are associated with zone-folded transitions due to a reduction in symmetry of the crystal. Transition energies and selection rules are identified with theoretical estimates tempered by the joint effects of incomplete or partial ordering and biaxial strain due to lattice mismatch. The effects of ordering are also considered in relation to Raman scattering, where it is shown that polarization dependent lineshape anomalies are associated with zone-folded modes, again due to the new (transformed) crystal symmetry.

We present results of an investigation of the details of the disordering process die to P₄ annealing for Ga_0.52In_0.48P grown in ordered configurations with large and small macro-domain sizes. The effects of P₄ annealing (710 degree(s)C) were monitored as a function of isothermal annealing time by transmission electron microscopy (TEM), photoreflectance (PR), and photoluminescence (PL). During the annealing process the band gap and PL emission energies characteristic of the disordered phase of the sample grow while those of the ordered phase decay, rather than undergoing a continuous energy change. The disordering, indicated by TEM photographs, is found to proceed downward from the free surface of the epilayer and to propagate into the bulk with a well-defined boundary rather than occurring simultaneously throughout the bulk of the layer. This disordering process is consistent with the combined results of PR and PL. The rate at which the disordering occurs is found to be much greater for samples initially having larger domains.

This paper reviews the use of modulation spectroscopy for the characterization of a wide variety of semiconductor device structures. Some systems that will be discussed include pseudomorphic GaAlAs/InGaAs/GaAs modulation-doped quantum-well high-electron-mobility transistors (including the 300K determination of the 2D electron gas density), GaAlAs/GaAs, InP/InGaAs, InGaP/GaAs, InAlAs/InGaAs, and InGaAs/GaAs heterojunction bipolar transistors (including the determination of the built-in fields/doping levels in the emitter and collector regions), GaAs/GaAlAs quantum well IR detectors, quantum well lasers, etc. Particular attention will be paid to nondestructive, contactless techniques such as photoreflectance and contactless electroreflectance that can be performed on entire wafers.

The electromodulation method of contactless electroreflectance has been used to characterize strained-layer, modulation-doped Ga_1-yAl_yAs/In_xAs/GaAs quantum wells that contain a 2DEG. Measurements were made over a wide temperature range. A first- principles many-body lineshape model has been developed. We find that manu-body effects play a role at all temperatures, but they are most pronounced at lower temperatures. The thermal broadening of the lineshape of the peaks corresponding to the interband transitions depends sensitively on the Fermi energy E_F and the temperature, thereby providing us with a method to determine E_F from a detailed lineshape fit. The electron density n_2D is in good agreement with the sheet densities obtained from low-temperature Hall measurements. The variations in the quantum well width, alloy composition, and symmetry have been characterized by fitting the lineshapes with appropriate fitting parameters.

We have studied the influence of silicon doping on the electric-field-induced Stark shifts of the subband states for AlGaAs/GaAs/AlGaAs multiple quantum well structures. The investigations were performed applying electroreflectance (ER), photoreflectance (PR), and for comparison photoluminescence (PL) spectroscopy. The analysis of the PR spectra yields the transition energies at approximately zero electric field, which are in good agreement with PL peaks. The shift of the Er ground state heavy hole transition with respect t the PR data and accompanying self-consistent calculations allow the determination of the field strength F at each gate voltage with high accuracy. On this bases, the other transitions can be identified by their expected shift. It will be shown that all transitions involving the first heavy hole subband show a strong increase of their signals with increasing field strength. The plot of F² versus gate voltage for both samples fits a straight line, slope of which provides the corresponding concentrations of the ionized donors.

We demonstrate the possibility of measuring the depth distribution of damage in GaAs using differential reflectance spectroscopy. Damage was intentionally generated by various ion- assisted processes, such as ion implantation and ion-assisted plasma etching. The high sensitivity of the techniques allowed us to measure damage profiles over a large range of ion energies and ion doses.

Photoreflectance (PR) spectroscopy has been used to study the Fermi-level pinning position of chemically modified (100) GaAs surfaces. It is shown that there are two pinning positions for the unmodified 9100) GaAs surface. For n-GaAs the Fermi level pins near midgap, while for p-GaAs the Fermi level pins near the valence band. We used an Ar/Cl₂ plasma generated by an electro-cyclotron resonance (ECR) source and P₂S₅ chemical passivation to change the stoichiometry of the surface. We show that ECR etching makes the surface oxide As rich and that the Fermi-level position for this circumstance is near midgap. The P₂S₅ passivation produces a thin Ga rich oxide which is observed to in the Fermi-level near the valence band. These results allow us to relate the Fermi-level pinning position to the stoichiometry of the GaAs/oxide interface.

The proposed approach is directed to the use of molecular spectrum data for the study of electronic crystal properties for the formation and characterization of low-dimensional structures.

Semiconductor surfaces develop orientation dependent morphologies during growth that can be used for fabrication of nanostructured materials. We have applied RHEED techniques to study during MBE the nanometer-scale morphologies on non-(001)-oriented GaAs surfaces and their evolution during heterogeneous deposition of AlAs. In particular, the growth behavior of the GaAs(311)Ga surface enables a controlled generation of an ordered surface corrugation. We report on two new methods for a direct synthesis of 1D structures that take advantage of an in- situ lateral patterning of the GaAs substrate: the growth of corrugated GaAs/AlAs heterostructures on the GaAs(311)Ga surface and the synthesis of doping wires by the combination lattice step growth on vicinal GaAs(001) surfaces with planar doping.

Optical spectroscopies of semiconductor crystals can be very useful for the optimization of epitaxial growth. This paper discusses two of the most useful optical measurements, photoluminescence and Raman spectroscopy, and their applications to III-IV epitaxial structures grown by molecular beam epitaxy and organo-metallic vapor phase epitaxy.

The preparation of atomically sharp interfaces for the Si-Ge system is of remarkable interest for the preparation of ultrathin layers and superlattices. We investigated the influence of the molecular beam epitaxy (MBE) growth conditions on the properties of five monolayers of germanium embedded in a (001) silicon matrix for a conventional as well as an antimony- mediated growth in the temperature region from 300 degree(s)C to 450 degree(s)C. The layers were analyzed by electroreflectance (ER), Raman spectroscopy, and transmission electron microscopy (TEM); they show corresponding results for all three methods of investigation.

The discovery of visible luminescence from porous silicon has generated significant interest in view of the fact that bulk silicon does not emit light in the visible part of the spectrum. This material differs from bulk silicon in one important way: it consists of interconnected silicon structures that in some cases are on the order of several nanometers. having a very large surface-to-volume ratio. Thus, in a material such as this, both the effect of particle size and surfaces must be examined in order to determine the optical behavior of this system.

We report the results of an extensive optical characterization of the properties light-emitting porous silicon (LEPSi), using optical techniques such as Raman spectroscopy, FTIR, cw photoluminescence (PL) and time-resolved PL spectroscopy. Additional insight is obtained from several nonoptical techniques, such as optical and electron microscopy, atomic force microscopy, and various surface physics tools. We examine how to control the surface passivation of LEPSi and what the consequence for light emission are. Samples with widely different surface chemistry have been prepared by controlling the electrochemical processes during anodization or by selected post-anodization treatments such as low- and high- temperature oxidation. In particular, we discuss the relationship between the presence of Si-H, Si-O-H, and Si-O bonds, and the relative strengths of the red PL line have a microsecond(s) ec decay time and the blue PL having a Nsec decay time. These results are compared to the predictions of the leading models that have been proposed to explain the efficient room-temperature luminescence of porous silicon.

Within the framework of nonlinear optics we present and discuss data of intracavity four-wave mixing within the active volume of semiconductor lasers and amplifiers and demonstrate both their importance for an understanding of the fundamental nonlinear and ultrafast processes in semiconductor waveguide devices and their potential for photonic applications.

Techniques based on optical, x ray, and electron microscopy measurements are applied to characterized a wide variety of semiconductor multilayer structures. Bragg mirrors serve as valuable test structures for evaluating the epitaxial uniformity of crystal growth systems. Careful characterization of half-wave space single quantum wells provides a method for determining their complex refractive indices using reflectance spectroscopy. Comparison of cross-sectional microphotoluminescence to surface-normal photoluminescence, combined with these characterization techniques, allows studies of spontaneous emission in microcavities and elucidates the difficulties with using surface-normal photoluminescence to determine the alloy composition of the mirror layers. The application of these characterization methods to visible- wavelength AlGaAs mirrors, 485-720 nm, enables the development of these mirrors for uses such as optically tailored substrates and visible surface-emitter or detector arrays.

Electroreflectance (ER) and photocurrent spectra of a strongly and a weakly coupled GaAs- AlAs superlattice are investigated in the Wannier-Stark regime. It is shown that both types of spectra can only be described satisfactorily when, in addition to the excitonic transitions of the first heavy and light hole subband with the first conduction subband, band-to-band transition are taken into account. Therefore, a total of four rather just the two excitonic transitions are necessary to fit the experimental ER spectra. Finally, to describe all features in the ER spectra, interferences within the layered structure have to be included.

Lift-off thin films of GaAs/AlGaAs multiple quantum wells (MQW) have been bonded to different transparent substrates that possess either direction-independent or direction-dependent thermal expansion. Duet to the differential thermal expansion between the thin film and the much thicker substrate, the MQW is under a thermally induced in-plane strain. By proper choice of the substrate crystallographic orientation and bonding temperature various forms of in-plane anisotropic strain have been realized. A detailed study of the anisotropy in the complex refractive index resulting from the in-plane anisotropic strain is presented. The electric field dependence of the anisotropic absorption and birefringence has also been studied.

Undulated bonded silicon-on-insulator structures with oxide/nitride/oxide layers are investigated nondestructively using spectroscopic ellipsometry. Optimum measured information for a wavelength range of 250 to 850 NM has been obtained by using a combination of a lens and nearly 1 mm slit width to minimize the light beam divergence. Using Marquardt regression analysis, slight vertical discrepancy is till observed between measured and calculated data, especially in the wavelength range of 250 to 285 nm due to the still unavoidable existing of beam divergence. This problem affects the accuracy of evaluated layer thicknesses from position to position on the investigated samples, especially the uppermost very thin (1 nm) oxide layer thickness. One interesting result is that because of the observed vertical error in the measured data, we have to avoid in this case making fitting on tan (Psi) and cos (Delta) in order to obtain best calculated data close to the nominal values.

We have developed In_0.53Ga_0.47As/InP quantum wires with lateral widths down to 8 nm by high-resolution electron beam lithography and deep wet chemical etching. The wires were studied by cw- and time-resolved photoluminescence spectroscopy at temperatures of 2 K and 11 K respectively. Even from the narrowest obtained wire structures we observe a clear photoluminescence signal.