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Using the density functional theory, we study the structural and lattice dynamical properties of europium sesquioxide ($Eu_{2}O_{3}$) in the cubic, trigonal, and monoclinic phases. The obtained lattice parameters and energies of the Raman modes show a good agreement with the available experimental data. The Eu-partial phonon density of states calculated for the cubic structure is compared with the nuclear inelastic scattering data obtained from a 20 nm thick $Eu_{2}O_{3}$ film deposited on a YSZ substrate. A small shift of the experimental spectrum to higher energies results from a compressive strain induced by the substrate. On the basis of lattice and phonon properties, we analyze the mechanisms of structural transitions between different phases of $Eu_{2}O_{3}$.

Physical review / B 102(19), 195414 (1-9) (2020). doi:10.1103/PhysRevB.102.195414, Self-organized silicide nanowires are considered as building blocks of future nanoelectronics and have been intensively investigated. In nanostructures, the lattice vibrational waves (phonons) deviate drastically from those in bulk crystals, which gives rise to anomalies in thermodynamic, elastic, electronic, and magnetic properties. Hence a thorough understanding of the physical properties of these materials requires a comprehensive investigation of the lattice dynamics as a function of the nanowire size. We performed a systematic lattice dynamics study of endotaxial FeSi$_2$ nanowires, forming the metastable, surface-stabilized $\alpha$ phase, which are in-plane embedded into the Si(110) surface. The average widths of the nanowires ranged from 24 to 3 nm, and their lengths ranged from several micrometers to about 100 nm. The Fe-partial phonon density of states, obtained by nuclear inelastic scattering, exhibits a broadening of the spectral features with decreasing nanowire width. The experimental data obtained along and across the nanowires unveiled a pronounced vibrational anisotropy that originates from the specific orientation of the tetragonal $\alpha$−FeSi$_2$ unit cell on the Si(110) surface. The results from first-principles calculations are fully consistent with the experimental observations and allow for a comprehensive understanding of the lattice dynamics of endotaxial silicide nanowires., Published by Inst., Woodbury, NY

The structure and dynamical properties of the ${\mathrm{Fe}}_{3}\mathrm{Si}/\mathrm{GaAs}(001)$ interface are investigated by density functional theory and nuclear inelastic scattering measurements. The stability of four different atomic configurations of the ${\mathrm{Fe}}_{3}\mathrm{Si}/\mathrm{GaAs}$ multilayers is analyzed by calculating the formation energies and phonon dispersion curves. The differences in charge density, magnetization, and electronic density of states between the configurations are examined. Our calculations unveil that magnetic moments of the Fe atoms tend to align in a plane parallel to the interface, along the [110] direction of the ${\mathrm{Fe}}_{3}\mathrm{Si}$ crystallographic unit cell. In some configurations, the spin polarization of interface layers is larger than that of bulk ${\mathrm{Fe}}_{3}\mathrm{Si}$. The effect of the interface on element-specific and layer-resolved phonon density of states is discussed. The Fe-partial phonon density of states measured for the ${\mathrm{Fe}}_{3}\mathrm{Si}$ layer thickness of three monolayers is compared with theoretical results obtained for each interface atomic configuration. The best agreement is found for one of the configurations with a mixed Fe-Si interface layer, which reproduces the anomalous enhancement of the phonon density of states below 10 meV.

In this chapter we treat the vibrations of atoms in a crystalline lattice in the way introduced in Frenkel (Phys Rev 37:1276, 1931) [31F1] and adopted in many textbooks (e.g., Ashcroft et al. in Solid State Physics. Cengage Learning Asia, Singapore, 2016 [16A1]). We will start with a homogeneous string, proceed to mono-atomic and diatomic chains and finally arrive at the three-dimensional arrangement found in solids. The resulting quanta of these lattice vibrations, the phonons, will be the focus of Chap. 11.

In this chapter we start to discuss topics that are specific to crystalline solids and apply to the large majority of inorganic semiconductors. In particular we lay out the basis for the following chapters where we consider elementary excitations and so-called quasi-particles in semiconductors. These will be needed to describe and understand the linear optical properties. The properties of the quasi-particles themselves are governed by the underlying periodicity of the crystalline lattice. The latter is in particular reflected by the dispersion of the quasi-particles in reciprocal space.

Confinement of excitons in low-dimensional structures leads to a strong enhancement of excitonic effect. They have impact on optical properties of these structures up to room temperature even for materials with low excitonic binding in the bulk. We will start in this chapter with the properties of excitons in quasi-2D structures (quantum wells). In particular we will illustrate the dependence of this properties on the confining potential, e.g., its width. The influence of disorder due to well-width fluctuations will be addressed as well as polariton propagation in quantum wells. We will further elaborate the consequences of coupling between neighboring wells—up to the limit of superlattices—on excitonic properties. This includes superlattices with type-II band alignment separating electrons and holes, strained-layer superlattices with tilted bandstructures due to piezoelectric effects, and modulation-doped superlattices. Two-dimensional properties of excitons are best realized in monolayer semiconductors. The excitonic states are here strongly influenced by the unusual bandstructure leading to specific valley-dependent properties. We then proceed to excitons in quantum wires and rods and finally come to quantum dots. We discuss the various confinement regimes of excitons in quantum dots and their consequences on optical spectra. We start with quantum-dot ensembles but we will then describe in detail the fine structure of excitonic states in single InGaAs quantum dots. The later is accessible via micro-photoluminescence or near-field spectroscopy. Another experimental method illustrated in this chapter is photoluminescence excitation spectroscopy (PLE).

The linear optical properties of heavily doped semiconductors are strongly influenced by the plasma of free carriers present in this samples. This includes of course the (degenerate) occupation of band states as well as correlation effects like collective excitations. We will address the implications of plasmons and pair excitations in the carrier gas and the description of the related optical properties in the Drude–Lorentz model. We illustrate the optical properties related to surface plasmons and of plasmon-phonon mixed states both, in bulk and low-dimensional semiconductors. We close with a discussion of prominent correlation effects like the Burstein–Moss shift and the Fermi-edge singularity.

The properties of electrons in crystalline solids are strongly determined by the periodic lattice potential. As a preparation for the discussion of the semiconductor bandstructure we derive first some general properties of electrons in crystalline lattices. We start with some necessary approximations by ignoring lattice vibrations (adiabatic approximation) and focus on a single electron in an averaged potential (one-electron approximation). We will account for the translational symmetry of lattices by introducing Bloch waves as solutions of the Schrodinger equation which includes a periodic potential. We illustrate the quasi-particle nature of electrons and holes and elaborate the origin and implications of the effective mass. Finally we describe some consequences of carrier-phonon coupling in (at least partially) ionic crystals.

When light propagates in matter it couples to optical excitations like optical phonons, electronic transitions, excitons etc. which leads to a polarization wave accompanying the electromagnetic wave. Quantization of this mixed propagating wave results in new quasi-particles called polaritons. We review here the general properties of polaritons like their dispersion relation and resulting optical properties in the Lorentz-oscillator model for (crystalline) matter including effects of anisotropy. We explicitly discuss the validity range of the polariton concept for different ratios of the distance of Lorentz oscillators and wavelength. Then we consider the consequences of coupling of the oscillators (i.e., spatial dispersion) for the dispersion, optical functions, and experimental spectra. Finally we introduce surface polaritons and their dispersion relation.

Low-dimensional semiconductor structures are nowadays the centerpiece of many electronic and optoelectronic devices. This is a result of their interesting properties like carrier confinement and localization, tunability of electronic energies by geometric dimension, or a spectrally narrow and high density of states. But low-dimensional structures are also perfect model systems for quantum mechanics and lead to new physics phenomena like the quantum Hall effect. We will discuss here the electronic states and wavefunctions in (square-well) potentials of different dimensionalities. We will address the consequences of finite size and depth of the potentials and how the bandstructure characteristics affect the electronic states. Starting with single quantum wells (QWs) we proceed via coupled QWs to superlattices where miniband formation leads to transport perpendicular to the quantization layers. We focus in particular on a novel class of materials—mono-layer semiconductors—like graphen and transition-metal dichalcogenides. These materials show extraordinary electronic properties due to location of their bandgaps at the corners of a hexagonal Brillouin zone leading to the phenomena of valleytronics. We then describe electronic states in quantum wires, rods and dots and how to realize such systems.

In this chapter we describe macroscopic optical properties resulting from the interaction of light with solid matter. The solid is considered here as a homogeneous medium described by the complex dielectric function \(\varepsilon (\omega )\) or by the complex index of refraction \(\tilde{n}(\omega )\). We concentrate especially on the reflection and transmission of light at the plane interface between two media. As an especially simple case we investigate the boundary of matter and vacuum. We deduce the laws of reflection and refraction from boundary conditions for the electric and magnetic fields. The behavior of wave vectors when crossing an interface follows from momentum conservation. Fresnel’s formulae are discussed to describe amplitudes, phases and intensities of the respective light waves. It follows the description of extinction of light by absorption and scattering. Since semiconductors often occur as plane-parallel slabs we deduce the properties of Fabry–Perot cavities. The chapter concludes with a discussion of optical anisotropy including birefringence, dichroism and optical activity.

In this chapter we illustrate the quasi-particle properties of excitons, which are the bound states of electron–hole pair excitations in semiconductors and insulators. We will develop the concept of Wannier excitons to describe the wavefunction and dispersion relation of these excitations. We also introduce triplet excitons which are the (mostly dark) partners of the singlet ones. Then we discuss modifications of the simple exciton model due the phonon-related polarization of the crystalline lattice and introduce the exciton finestructure due to various exchange interactions. We close with an introduction to excitonic molecules (biexcitons) and charged excitons (trions).

Application of external fields is a well suited strategy to reveal the properties of excitons in semiconductors. It further leads to new phenomena which are interesting for device applications. The external field can be applied temporally constant [79E1, 85H1] or as systematic perturbation in modulation techniques [69C1, 73S1, 04G1]. We will restrict our discussion here to the presentation of general features and of some selected topics and examples. We consider the influence of magnetic, electric and strain fields on the optical properties of excitons including their continuum states and we proceed again from bulk materials to structures of lower quasi-dimensionality. Mostly these fields are applied from outside, but there are also cases of internal fields, e.g., strain fields due to lattice misfit in epitaxy or piezo-electric fields resulting from such strain.

In this chapter we treat the properties of phonons in scenarios where the periodicity of the crystalline lattice is perturbed. This is the case for crystals with a random distribution of atoms on lattice sites (alloys) or for local deviations from periodicity (defects). We also consider the situation of reduced periodicity in layered structures like superlattices and quantum wells. Finally we address confinement of phonons in quantum dots.

Deviations from the perfect crystalline periodicity in the form of defects and disorder play an important role in semiconductor optics. Donors and acceptors not only provide as dopants the free carriers in the bands. They also provide states for radiative recombination with photon energies below the band-gap energy. Such transitions are thus prominent in particular in luminescence spectra. This now allows for optical determination of the defect’s characteristic properties. We will describe the wavefunctions and statistics of carriers bound to shallow and deep defects centers as well as typical recombination mechanisms including the ones involving additional phonons. Then we introduce the method of modulation doping in quantum wells. The second section reviews different types of localization mechanisms for carriers. Such localization is found in disordered potentials, e.g., for high levels of doping, in alloy semiconductors, in quantum wells due to well-width fluctuations or in amorphous semiconductors. We elaborate the modifications of the electronic density of states, e.g., in the form of band tails and show the respective consequences on optical spectra. This also includes the identification of regimes with different extend of the wavefunctions like the Urbach tail and the Tauc regime which are separated by the optical or mobility gap. We will close with the concepts of weak localization and percolation.

In the previous chapter we discussed mainly the optical properties of intrinsic free excitons. Here we consider excitons bound to various types of defects and how they are observed in optical spectra (mainly in luminescence). Excitons can also get trapped in localized states in potential fluctuations of disordered bulk materials. We will discuss how this localization affects the optical spectra and present different approaches for the modeling of localization properties. Many of these aspects are also relevant for the structures of reduced dimensionality presented in the next chapter.

Physical review / B covering condensed matter and materials physics 98(12), 121409 (2018). doi:10.1103/PhysRevB.98.121409, We report a systematic lattice dynamics study of the technologically important Fe$_3$Si/GaAs heterostructure for Fe$_3$Si layer thicknesses of 3, 6, 8, and 36 monolayers. The Fe-partial phonon density of states obtained by nuclear inelastic scattering exhibits up to a twofold enhancement of the low-energy phonon states compared to the bulk material for layer thicknesses of 8 monolayers and below. First-principles calculations explain the observed effect by interface-specific phonon states originating from the significantly reduced atomic force constants and allow for achieving a comprehensive understanding of the lattice dynamics of epitaxial strain-free interfaces., Published by APS, Woodbury, NY

In this article we demonstrate a fully CMOS compatible fabrication process for the realization of microdisk resonators based on silicon oxynitride. The layer fabrication using plasma enhanced chemical vapor deposition is optimized in terms of surface roughness and internal material absorption. Resulting surface roughness due to the etching process is reduced by using optimized etching parameters. Whispering gallery modes of the fabricated microdisk resonators have been investigated by tapered fiber coupling and show quality factors as high as 106.

BiOCl and BiOBr nanodiscs (100–150 nm in diameter, 15–25 nm in thickness) are prepared via water-based nucleation and purified by a phase-transfer reaction, including oleylamine-induced transfer of the as-prepared nanodiscs from the polar water phase to the non-polar toluene phase. The oleylamine-capping is then removed by hydrazine treatment, and the BiOCl/BiOBr nanodiscs are redispersed in 2-propanol. The as-prepared nanodiscs are finally deposited as a porous, p -type semiconductor layer to obtain dye-sensitized solar cells (DSSCs). Herein, coumarin 343 is applied as sensitizer together with BiOCl as p -type semiconductor and a KI–I 2 electrolyte. In addition, eosin Y is applied as sensitizer together with BiOBr as p -type semiconductor and a [C 4 MPyr] 2 [Br 20 ] polybromide electrolyte (C 4 MPyr: N -butyl- N -methylpyrrolidinium). Such polybromide electrolyte is firstly applied in a DSSC and allows for a higher redox potential. Both here established p -DSSCs show the characteristic features and function of a solar cell (BiOCl/coumarin 343/KI–I 2 : V oc = 120 mV, J sc = 57 μA cm −2 , FF = 40.6%, η = 0.003; BiOBr/eosin Y/[C 4 MPyr] 2 [Br 20 ]: V oc = 78 mV, J sc = 3.1 μA cm −2 , FF = 28.6%, η = 0.0005) as a result of this conceptual study.

We demonstrate a small molecule solution processed hole interfacial layer approach to improve the dark current characteristics of polymer photodiodes. The two hole blocking materials under investigation 3-phenyl-4(1′-naphthyl)-5-phenyl-1,2,4-triazole (TAZ) and 2-[3,5-bis(1-phenylbenzimidazol-2-yl)phenyl]-1-phenylbenzimidazole (TPBi) are spincoated from ethanol as an orthogonal solvent on top of a P3HT:PCBM active layer. We reduce the dark current at a bias voltage of −1 V by a factor of 17.2 by introducing a TAZ interfacial layer while keeping the responsivity unchanged.

In this work we present regular and inverted organic tandem solar cells from poly[ N -9′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole): [6,6]-phenyl C 70 -butyric acid methyl ester (PCDTBT:PC 71 BM) with power conversion efficiencies of up to 4.5%. The recombination zone comprises an electron conducting, precursor based zinc oxide buffer layer that was applied from solution under ambient conditions and at moderate processing temperatures. Optimized active layer thicknesses in both subcells were derived from optical Transfer Matrix simulations. The short circuit current density of the tandem cell exceeds half the short circuit current density of the single absorber cells indicating a real gain in quantum yield when utilizing the tandem architecture.

We present fully solution processable hybrid electrodes comprising highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) layers and hydrothermally grown zinc oxide nanorods. These electrodes exhibit conductivities of about 1400 S/cm and a transmittance of more than 80%. By incorporating these electrodes into inverted polymer photovoltaic devices, power conversion efficiencies of 1.6% are achieved. Charge carrier lifetime measurements by means of impedance spectroscopy indicate an improved charge carrier extraction from the device upon the incorporation of the nanorod array.

Optical cavities have been fabricated by overgrowth of truncated GaAs pyramids with a distributed Bragg reflector. The success of this overgrowth depends strongly on the crystallographic orientation of the pyramid facets and shows best results for { 1 1 4 } A facets. In order to fabricate mesas with precisely such facets, a wet-chemical etching process including several selective etching steps has been established. To determine the optical properties of these resonators, InAs quantum dots have been used as an internal broad-band light source. The quality factors for optical modes have been determined to range up to 8000 and show a dependency on cavity width.

In this study, we investigated the effect of in situ annealing on InAs quantum dots site-selectively grown on pre-structured GaAs substrates. A morphological transition is observed with original double dots merging into one single dot during annealing. This is accompanied by a reduction of quantum dots originally nucleating between defined sites. The photoluminescence intensity of annealed site-selective quantum dots is compared to annealed self-assembled dots with linewidths of single dot emission of about 170 and 81 μeV, respectively. UV-ozone cleaning is used to optimize the sample cleaning prior to quantum dot growth.

Multilayer ensembles of alumina capped, widely dispersed silicon nanoparticles ( Si -nps) with mean diameter in the range of about 1–4 nm were grown using pulsed laser deposition. With photo-excitation at ~3.82 eV, photoluminescence (PL) was found to emanate from these Si -nps mainly in the UV spectral region centered at about 3.37 eV due to the Γ25–Γ15 transitions. It was found that while the bandgap measured from photoabsorption spectra showed blueshift with decreasing mean size of the Si -nps, spectral position of the PL peaks remained almost insensitive to variation in the mean size. The PL peak at about 3.37 eV was observed to vanish at temperatures higher than 70 K and another one at about 3.31 eV attributed to the TO phonon replica disappeared above 100 K. In general FWHM of both these PL peaks was found to increase monotonically from about 6 to 19 meV and the peak positions were found to undergo redshift with increase in the sample temperature from 10 to 100 K. These observations could be explained by applying Bose statistics for a 6 meV confined phonon mode broadening of the Si -nps and using the Varshni equation, respectively. Si -nps grown in oxygen ambient at 600°C showed significant enhancement in the PL intensity with increasing pressure. These findings elicit that light emission from Si -nps is either due to the nanoparticles of about 1 nm and smaller size primarily driven by the quantum confinement or due to an interface state pumped by these Si -nps.

We investigate nuclear spin polarization in a single InGaAs quantum dot via electrical injection of spinpolarized electrons in a spin-injection light-emitting diode. The Overhauser shift in the single-dot electroluminescence is compared with results obtained for optical spin injection in the same structure. The results clearly demonstrate the possibility of efficient nuclear spin polarization by purely electrical means which might be useful for future spintronic applications. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The research on ZnO has a long history but experiences an extremely vivid revival during the last 10 years. We critically discuss in this didactical review old and new results concentrating on optical properties but presenting shortly also a few aspects of other fields like transport or magnetic properties. We start generally with the properties of bulk samples, proceed then to epitaxial layers and nanorods, which have in many respects properties identical to bulk samples and end in several cases with data on quantum wells or nano crystallites. Since it is a didactical review, we present explicitly misconceptions found frequently in submitted or published papers, with the aim to help young scientists entering this field to improve the quality of their submitted manuscripts. We finish with an appendix on quasi two- and one-dimensional exciton cavity polaritons.

We investigate the properties of random lasing in nanocrystalline ZnO powders. The lowest threshold for lasing occurs for average particle diameters of about 260 nm. Reproducible lasing features are achieved for reduced ensemble sizes. Spatially resolved luminescence spectroscopy is used to probe directly the degree of localization of random laser mode. We find that strongly confined and extended modes can co-exist in the same spatial area. However, localized modes appear for small optical gain while extended modes are only supported in the presence of large optical gain, as is expected from theory.

Pyramidal micro-cavities represent a novel promising class of semiconductor optical cavities. In contrast to our previous approach based on pyramids sitting on distributed Bragg reflectors, we investigate reversed freestanding GaAs pyramids. The latter are achievable by a wet-chemical etching process where an AlAs sacrificial layer in the epitaxially grown layer structure is used. In freestanding GaAs pyramids, light is simply confined by total internal reflection at the interface of the high refractive index material GaAs to the surrounding. Due to strong optical confinement within the pyramidal shape, small mode volumes are expected. Quality factors up to 3000 were measured in first structures. However, simulations suggest the possibility of much higher values. Therefore, these freestanding pyramids are promising for an optimized ratio between quality factor and mode volume, which is crucial for quantum-optical applications.

Nanocrystalline ZnO powders can act as gain and scattering medium in a random laser where the light emission can be strongly amplified. In this work, we compare the luminescence properties of samples with different particle sizes in the regime of linear and nonlinear optics. In the high-excitation regime random lasing is observed in all samples. Here, the lasing threshold depends strongly on the size distribution in the ensemble. Additional characterization of the samples has been done by determining the absolute quantum efficiency of the radiative processes in the powder. The values are in the 10% range and the near-edge luminescence is strongly influenced by the particle sizes. We show that by annealing the nanocrystals coalesce to larger polycrystalline grains, which results in a new emission band at 3.333 eV due to the grain boundaries. Furthermore, it is found that in the annealed samples the threshold for random lasing could be considerably decreased.

Quasi-two-dimensional arrays of nearly parallel hexagonal ZnO nanorods and a three-dimensional cylindrical microstructure consisting of ZnO nanorods have been grown by low pressure chemical vapor deposition (CVD) and carbothermal evaporation technologies, respectively. The technology ensures high optical quality of the produced nanostructures so as to act as a gain medium for stimulated emission in the ultraviolet spectral region in combination with high quality factor laser resonators. Multiple sharp lasing peaks were realized from the produced structures under nanosecond pulse optical excitation. The lasing peaks display successive onset and saturation with increasing excitation power density in accordance with the lasing behavior of guided modes in ZnO nanorods. The produced structures are expected to find applications in integrated nanoscale optoelectronics, photonics, and sensor technologies.

In this work we examine the lasing properties of single ZnO nanorods under ns- and fs-pulsed excitation. By measuring the photoluminescence of individual nanorods excited by a single ns-pulse we demonstrate that several stable lasing modes are emitted by the nanorod. In this quasi-stationary excitation regime lasing up to room temperature can be achieved. Measurements of the temperature dependent laser threshold and of the spectral lasing region are presented. The excitation with fs-pulsed excitation combined with a time resolved photoluminescence measurement gives furthermore an insight in the lasing dynamics. Scanning electron microscope picture of a sample with an array of ZnO nanorods grown perpendicular aligned on the substrate.

We report on the investigation of CdSe/ZnSe heterostructures by transmission electron microscopy (TEM) and photoluminescence spectroscopy (PL). CdSe layers with nominal thicknesses tCdSe between 0.5 and 8 monolayers (ML) were embedded in a ZnSe matrix and grown on a GaAs(001) substrate by molecular-beam epitaxy at 280 °C. The Cd-distribution was obtained from high-resolution TEM lattice fringe images using composition evaluation by lattice fringe analysis technique. The measured minimum, average and maximum Cd-concentrations and the overall CdSe contents in the layers increase with the nominal CdSe layer thickness and reach a constant value at tCdSe = 4 ML. The measured CdSe content in the regions with the maximal Cd-concentration continues to increase for tCdSe ≥ 5 ML. The increasing of measured Cd-concentrations/CdSe contents is correlated with a red shift of PL spectra. In the CdSe layers with tCdSe ≥ 5 ML, formation of defects is observed. The increasing density of defects and decreasing intensity of PL spectra with tCdSe suggest that the critical thickness for defect formation during CdSe growth on the ZnSe(001) is between 4 ML and 5 ML. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The spin of a single electron confined in a semiconductor quantum dot is a possible candidate for a qubit to realize quantum information processing. Here we discuss the initialization of the electron spins in single quantum dots using a semi-magnetic spin aligner layer. We have succeeded in the concurrent initialization of five quantum dots with near-unity fidelity. The electron spin is read-out by observing the polarization of the emitted photons. These originate from excitonic recombination of the electron with an unpolarized hole in the dot. We find dots emitting highly polarized photons over the whole inhomogeneously broadened quantum-dot ensemble emission spectrum. However, a pronounced dependency between the statistical average of the polarization degree and the quantum-dot ground-state energy is found. The influence of charged-exciton recombination due to donors and defects is discussed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Pyramidal microcavities are a new class of optical resonators with potentially small mode volume and high quality factor. Our GaAs pyramids with embedded InGaAs quantum dots are placed on top of GaAs/AlAs distributed Bragg reflectors to increase the optical confinement at the base of the pyramids. The pyramidal shape is achieved by a wet-chemical etching process using an AlAs sacrificial layer. Temperature-dependent micro-photoluminescence measurements are used to verify optical modes.

Results of experimental studies of photoluminescence (PL) and PL excitation (PLE) spectra of MBE grown single quantum wells (QWs) formed by insertion of few CdSe monolayers in the ZnSe matrix are reviewed. PL spectra of such quantum objects originate from the luminescence of CdSe -rich nanoislands. Two types of island emitting states, namely ground and metastable ones, contribute to the low- and high-energy parts of the PL band, respectively. An interplay between these contributions is responsible for the anomalous temperature dependence of the maximum position of the PL band. The optical orientation and optical alignment experiments at resonant excitations allow to elucidate the nature of the two types of the emitting states. PLE spectra of ground and metastable states have strongly differing characters at excitation below some characteristic energy E ME which is identified as the exciton percolation threshold. A theoretical model of the absorption spectra of emitting island states is presented, and practical applications of the model for the characterization of the island lateral concentration profiles are reported.

Single-crystal phosphorus-doped ZnO nanowires were synthesized by using a single-source precursor-based vapor transport method. The photoluminescence spectra of phosphorus-doped ZnO nanowires and undoped nanowires are compared. While both show several shallow bound exciton complexes, the phosphorus-doped nanowires reveal an additional distinct emission feature at 3.316 eV. Additionally, the time-resolved PL measurements were conducted to characterize the recombination dynamics.

We investigate the dynamics of electrons injected into InAs/GaAs quantum dots by initializing and further observing the spin state of the electrons. For this purpose, we use spin polarized light-emitting diodes where the electron spin is set in a semimagnetic ZnMnSe layer. We find that the degree of optical polarization depends strongly on the ground state energy of the quantum dot. A dependence of polarization on dopant concentration in the spin aligner suggests an influence of residual electrons in the quantum dots.

In this paper we investigate cobalt doped ZnO thin films prepared via different sol-gel methods. XRD shows only the diffraction peaks from wurtzite ZnO without secondary phase, as widely reported in literature, for the Co contents of our samples of 3 to 12%. Raman spectroscopy reveals the existence of anti-ferromagnetic cobalt oxides like CoO and Co 3 O 4 already in intermediately doped samples. Furthermore, the Raman shifts of the doped samples exhibit a shift to higher energy with increasing Co doping content. Unfortunately, Raman spectroscopy is not sensitive to metallic Co and, therefore, cannot be employed to explore the presence of Co clusters. However, we demonstrate that XRD is not a suitable method to rule out the existence of magnetic secondary phases in ZnO doped with transition metals.

Electromodulated reflectance (ER) is a standard characterization method to determine critical points such as the band gap in the band structure of semiconductors. These critical points show up as spectrally narrow features in ER and are typically evaluated using Aspnes's third-derivative functional form. ER spectra of stratified semiconductor systems such as thin-film solar cells, however, are significantly distorted by optical interference due to their layered structure. Furthermore, strong built-in electric fields result in a deviation from the typically assumed low-field conditions. We present here simulations of ER spectra from stratified systems based on transfer matrices using the Franz-Keldysh theory in its general form. For realistic thin-film solar cell conditions, distortions of ER line shapes due to the above-mentioned interferences and strong electric fields appear in the simulations. Furthermore, the results show good agreement with measured ER spectra of a structurally well-characterized $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_{2}$ (CIGS) solar cell. Our analysis points out the restrictions on the determination of energetic position and number of critical points from ER spectra of stratified systems.

The influence of the growth conditions during molecular-beam epitaxy on the properties of InAs/GaAs quantum dot structures were systematically investigated by low temperature photoluminescence spectroscopy and transmission electron microscopy. The circular polarization degree (CPD) of the electroluminescence was compared for two quantum-dot spin-injection light-emitting diodes. The CPD depends on the position of the emission energy in the luminescence band. This correlation is similar for both samples despite the strongly different quantum dot morphologies. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigate electrical spin injection from a semi-magnetic n-type ZnMnSe spin aligner into III–V p–i–n diodes with InGaAs quantum dots (QDs) in the active layer. Quantitative transmission electron microscopy techniques are applied to characterize the different dot types used. Analysis of the circular polarization degree (CPD) of the device emission indicates the spin polarization of the injected electrons. Values of more than 70% are obtained for the wetting layer and high-energy QD states. However, the CPD shows a strong spectral dependence due to spin relaxation at the stage, before the electrons are finally captured in the dots. This is, e.g., evidenced by an initial increase of the polarization degree with rising temperature, attributed to motional narrowing effects. As a prerequisite for more detailed studies, we also demonstrate electrical spin injection into single InGaAs QDs, which should provide the basis for future single spin manipulation experiments. Finally, we suggest GaInNAs as optically active material for the realization of spin-polarized light-emitting diodes with long-wavelength emission. First results indicate CPD values of up to 80% for λ = 1130 nm, suggesting this approach to be very promising. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Time resolved spectroscopy carried out on ZnO nanorods shows luminescence and transient stimulated emission related to free carries at the highest pump fluences and at low temperatures. ZnO nanorods have a much higher index contrast with respect to air than optical fibers therefore providing excellent lateral confinement. Consequently, even in relatively thin nanorods higher order waveguide modes become important. The mode structure of guided modes is analyzed numerically by solving the Helmholtz equation for the case of a hexagonal waveguide. Two aspects of the influence of higher order modes, namely the suppression of p-band emission in thin nanorods and the transition from single mode lasing to multimode lasing in thick rods, are discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In weakly confining quantum structures such as interfacial islands or quantum disks the exciton-spin relaxation is governed by two independent electron and hole spin flip processes between the optically active and dark states. A microscopic theory for these transitions is presented which is based on second order spin-orbit and corrier-phonon interaction processes. We found that the sequential relaxation between bright and -dark states leads to much faster exciton-spin relaxation than for strongly confining ("small") quantum dots where the dominant process stems from electron-hole exchange interaction plus hole deformation potential coupling. In addition, the fast exciton spin relaxation implies that the (exciton-bound) electron spin flip time is also much shorter than for a single electron.

We report on the injection of electron spins into InGaAs quantum dots with an efficiency of up to 60%. This injection is observed in p-i-n light-emitting diode structures using the diluted magnetic semiconductor ZnMnSe as spin aligner (spin-LED). The degree of spin polarization is deduced from the circular polarization degree of the photons emitted when the injected electrons recombine in the quantum dots with unpolarized holes. We observe a strong energy dependence of the polarization degree with a strong increase starting from zero to a high value on the high energy side of the emission spectrum. To study the origin of this dependence, we compare results of two quantum-dot samples with emission peaks at 1.2 eV and 1.33 eV, respectively. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We discuss the time-resolved photoluminescence (PL) spectra of single ZnO nanorods taken at excitation fluences above and below the laser threshold. In the latter case, P-band emission related to polariton-polariton scattering is observed for certain rod geometries while stimulated emission occurs within the electron-hole plasma band. We calculate the intensity distribution of low-order waveguide modes as well as their energy dependence for given nanorod geometries to discuss their relevance with respect to nanorod lasing and polariton propagation. Additional finite-element analysis confirms that a gold layer formed at the nanorod-substrate interface under certain growth conditions leads to an enhancement of confinement within the resonator. (2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on electrical spin injection into InGaAs-based nanostructures through a semi-magnetic ZnMnSe layer in a magnetic field. We show an optical polarization degree of the electro-luminescence of up to 35% in InGaAs quantum wells and up to 14% in InGaAs quantum dots. To be able to attribute the polarization of the emitted photons to the spin alignment in the semi-magnetic layer, we have fabricated reference devices and performed all-optical measurements.

We investigate the lineshape of the zero-phonon luminescence of the A-exciton polariton and of the LO-phonon replicas in ZnO and their dynamics. The lineshape verifies details of the A-exciton polariton model and confirms the inverted band structure (AΓ7, BΓ9, CΓ7). The dynamics give information on relaxation, diffusion and recombination processes. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on an experimental study of the temperature dependent band gap shift and the homogeneous line broadening of the excitonic luminescence in ZnO. This data is derived from photoluminescence (PL) measurements at temperatures from T = 10 - 290 K and fitting the PL line shape with a theoretical model for the exciton line and its LO-phonon replica. Additionally, transmission measurements are utilized to extend the data of the temperature dependent band gap into the temperature range from 290 K up to 800 K. The experimentally determined shift is compared with one that is obtained numerically from the phonon density of states, the pressure dependent band gap, the thermal expansion coefficient and the bulk modulus with only a single fit parameter. The interaction of excitons with phonons leads to a homogeneous broadening of the excitonic emission which reaches values of 40 meV at room temperature. This mechanism results in a substantially increased threshold for stimulated emission from every process involving excitons.

In an introductory chapter we shortly review the history of ZnO research and the motivations for the present renaissance of the worldwide interest in this II–VI compound. Then we concentrate on the following topics: band structure, excitons and polaritons, luminescence dynamics, high excitation effects like biexcitons or the transition to an electron–hole plasma, and finally lasing. We finish with a short conclusion and outlook.