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We report on a study of the electrical and optical properties of strained M-plane GaN films grown on LiAlO2 substrates. We have electrically characterized Schottky barrier devices fabricated on strained M-plane GaN films. The current leakage found in these devices is probably related to a surface effect. The influence of the current direction for in-plane transport in M-plane GaN was also studied. Polarized photoluminescence spectroscopy shows the effect of residual biaxial strain on the optical selection rules, allowing to identify intrinsic transitions involving holes from the two uppermost valence bands. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Molecular beam epitaxy of Fe 3 Si films on GaAs (001) is studied in situ by grazing incidence X-ray diffraction. Fe 3 Si grows layer-by-layer. During deposition the growth front roughens as indicated by the damping of the X-ray oscillations and corresponding atomic force micrographs. The X-ray oscillations are modified during growth at substrate temperatures of 180 °C and below.

InxGa1-xAs1-yNy/Al0.33Ga0.67As multiquantum wells grown by plasma-assisted molecular beam epitaxy are studied by resonant inelastic light scattering. Sharp vibration modes have been observed at 323, 402, 454 and 501 cm-1. Their intensities resonate at the barrier bandgap reduced by the presence of N. Their resonance energies reveal the influence of the N concentration on the barrier gap at the multiquantum well interfaces. These peaks are interpreted in terms of local vibrations involving the pairing of N atoms, which seems to occur mostly at the quantum well interfaces due to preferential bonding of N to Al.

A numerical investigation on the transition of surface acoustic waves (SAWs) in AlxGa1−xN/GaN/SiC heterostructures to Lamb-like modes localized in the GaN layers is presented. The significantly large sound velocity in AlN in comparison to that in GaN leads to an expulsion of the SAWs from the surface AlxGa1−xN layer into the buried GaN layer when the SAW wavelength is roughly less than the thicknesses of the AlxGa1−xN and/or GaN layers. The confinement effect is examined as a function of Al composition in the top layer, and is found to be present even when the composition is as low as x ~ 0.3.

Fe 3 Si/GaAs(001) hybrid structures of high crystal and interfacial perfection are fabricated by molecular beam epitaxy at 200 °C. The composition of the films can be tuned over a wide range of Si content. The Fe 3 Si/GaAs(001) films are robust against thermal annealing up to 425 °C. Room-temperature spin injection is demonstrated from the ferromagnetic metal Fe 3 Si into the semiconductor GaAs by analyzing the circular polarization of the electroluminescence intensity emitted by an n-i-p light-emitting diode.

We investigate the effect of hole localization on the magneto-luminescence emission of a two-dimensional electron system formed in a modulation-doped semiconductor heterojunction. The emission of heterojunctions containing a dilute delta-layer of Be acceptors at a large distance from the interface is compared with that of Be-free samples. A narrow Fermi-edge singularity emission line is observed at low temperature in Be-doped samples, involving electrons in the second confined state. The evolution of this line in a perpendicular magnetic field shows common characteristics with previously reported results in high mobility samples not containing Be. A new emission line appears abruptly at filling factor 2 originating by the recombination of electrons in the lowest Landau level with free valence holes, independent of the presence of Be. The abruptness of this transition, which is also observed in some samples at filling factor 1, reveals a simultaneous change in the electron system over a macroscopic sample area. The new optical emission shows marked deviations with respect to the single-particle behavior, which are tentatively interpreted as the formation of a complex state involving a free photocreated hole and the electron system. This complex unbinds when the Fermi level crosses the mobility edge.

We utilize imprint lithography and the epitaxial lift-off technique to fabricate devices in which the photoluminescence (PL) is modulated under irradiation of surface acoustic waves (SAWs). Imprint lithography provides short-wavelength SAW transducers on LiNbO3. A high-quality GaAs–(Al,Ga)As quantum well is bonded in front of the transducers using the epitaxial lift-off technique. The large electromechanical coupling in LiNbO3 enables us to manipulate the carrier dynamics in the quantum well through the SAW-induced piezoelectric fields far more effectively than with transducers prepared directly on GaAs. The influences of the scattering of the SAWs from imperfections in the bonded film are investigated by measuring the quenching of the quantum-well emission by micro-PL.

We restrict the surface acoustic wave (SAW) propagation on a GaAs surface within a waveguide by means of reduction of the SAW velocity realized by metal gratings. The spatial variation of the SAW amplitude is evaluated by mapping the efficiency of the quenching of photoluminescence by the SAWs. Confinement of the SAWs is accomplished at a frequency of 1.4 GHz when the angle of SAW incidence with respect to the waveguide boundary is kept less than ∼10° for SAW velocity reduction of 4%. Mapping also reveals interference stripes associated with the SAWs bounce at the waveguide boundary.

For Si-doped AlxGa1−xAs(311)A with 0⩽x⩽1 grown by molecular-beam epitaxy, a systematic study of electrical properties is performed depending on growth conditions. A phase diagram for the conversion of the conduction type as a function of growth temperature and V/III beam-equivalent pressure ratio is established for AlxGa1−xAs layers. To achieve n-type conduction in AlxGa1−xAs layers, high arsenic flux and growth temperatures not exceeding about 550 °C are required, whereas low arsenic flux and higher growth temperatures lead to p-type conduction. For Si-doped AlxGa1−xAs(311)A layers with x>0.5, p-type conduction is reported with a Si doping efficiency above 90%. Based on these results, all-Si-doped strained-layer In0.2Ga0.8 As/GaAs(311)A quantum-well laser structures with x=0.5 and 0.8 in the cladding layers were grown. Compared with reference laser structures grown on GaAs(100) and doped with Be, a better lasing performance of the all-Si-doped (311)A-oriented single quantum-well laser structures is obtai...

Molecular-beam epitaxial growth and interface formation is investigated and optimized for the material systems Fe-on-GaAs(001) and MnAs-on-GaAs(001), which represent model systems for the integration of magnetic materials with semiconductors and the investigation of spin injection. In view of interface reactions as a key problem for the first system and in order to optimize the surface morphology, the Fe films are grown at reduced temperature. For MnAs-on-GaAs, an abrupt interface with an anisotropic lattice-mismatch accommodation mechanism is observed, which explains the unexpected high quality of the films. It is shown that the phase transition between paramagnetic g MnAs and ferromagnetic f MnAs during cooling after growth plays an important role and leads, at appropriate conditions, to strain-mediated self-organized structures. Electrical injection of spin polarized electrons through the ferromagnet-semiconductor interfaces is observed by analyzing the electroluminescence signal of GaAs/(In, Ga)As lig...

The in-plane polarization properties of strained C- and M-plane GaN films are investigated experimentally by photoreflectance spectroscopy and theoretically using the k . p model. While unstrained and symmetrically strained C-plane films do not show any in-plane polarization anisotropy due to the underlying crystal symmetry, unstrained and strained M-plane GaN films can exhibit a very large in-plane polarization anisotropy. For specific strain values, the transitions between the conduction and the three valence band can be become completely x-, y-, or z-polarized. Consequently, strained M-plane GaN becomes a promising candidate for realizing polarization-sensitive photodetectors.

The dispersion of the Rayleigh wave in a strain-unrelaxed GaAs/AlAs/GaAs heterostructure is evaluated. We determine the wavelengths at which the velocity of surface acoustic waves (SAWs) is maximum or comparable to that in GaAs. The information is utilized to design interdigital transducers to investigate the SAW transmission properties when GaAs micro-beams are realized in the SAW propagation path by selectively etching the buried AlAs layer. A frequency shift as well as an attenuation of the SAW transmission peak is observed for certain SAW wavelengths within the range of the enhanced SAW velocity in the unpatterned heterostructure.

This article describes the fabrication of SiC thin films on top of Si(111) substrates by means of a multiple C-ion implantation and the subsequent growth by plasma-assisted molecular beam epitaxy of GaN layers. The stoichiometry of the top SiC layer is controlled by reactive ion etching. Photoluminescence spectra reveal that all GaN layers are under biaxial tensile strain of thermal origin. The photoluminescence efficiency clearly depends on the stoichiometry of the initial SiC layer and on whether AlN buffer layers are used or not. GaN layers grown directly on bare non-stoichiometric SiC layers exhibit the best photoluminescence efficiency but also a high degree of mosaicity, as measured by X-ray diffraction techniques. The nucleation process involved in the initial stages of the growth leads to the formation of large dislocation-free grains with a high PL efficiency and with a higher tensile strain character. Despite the lack of a perfect monocrystalline SiC substrate lattice, high quality GaN microcrystals are obtained.

We report on the fabrication and characterization of efficient Bragg reflectors, based on 10-periods AlGaN/GaN stacks. Reflectors are designed with a target center wavelength of 510 nm that is applicable to resonant-cavity light emitting diodes for the plastic optical fiber communications. The heterostructures have been grown on GaN/Al 2 O 3 templates by plasma-assisted molecular beam epitaxy using Al concentrations in the range between 15 and 45%. Atomic force microscopy shows smooth surfaces with average roughness of about 19 A, a value well below the starting template one. Crack-free surfaces have been obtained when the Al content is below 30%. X-ray diffraction spectra reveal well-defined multiple satellite peaks which are characteristic for very good interfaces and periodicity. Structural analysis by high-resolution transmission electron microscopy shows a highly periodic structure with abrupt interfaces. In addition, threading dislocations do not affect the superlattice periodicity and quality, even though they run across the whole structure. Reflectivity measurements have also been performed and compared with simulated reflectivity spectra. Measured reflectivity values around 50% are in very close agreement with simulations.

Thick In x Ga 1− x N (0.20 x 2 O 3 templates. The strain and In-content is estimated from high-resolution X-ray diffraction, showing that the bulk samples are not fully relaxed. A bowing parameter of 3.6 eV is obtained from absorption measurements of In x Ga 1− x N layers. Strong In-dependent excitonic localization is observed in these bulk layers, leading to an increase in the absorption band edge with the In content. Regarding the MQWs structures, high-resolution transmission electron microscopy reveals an increase in the interface roughness for high In content. The dominant PL emission of the MQWs shows a red-shift when increasing the well thickness for a given In-content, due to internal piezoelectric field. The excitonic localization is studied and compared between thick layers and MQWs structures.

Static domain formation in doped, weakly coupled semiconductor superlattices results in several current branches separated by discontinuities that exhibit hysteresis. The transition from one branch to its adjacent higher and lower one is studied by time-resolved switching experiments. The distribution function of the relocation time for up and down jumps changes from a simple Gaussian to a first-passage time (FPT) form, when the final voltage on the adjacent branch approaches a discontinuity in the current–voltage characteristic. This observation indicates that for a smaller voltage distance from the discontinuity (FPT distribution) the system reaches the final state via an unstable intermediate state. In contrast, for a larger voltage distance from the discontinuity (Gaussian distribution), the system arrives directly at the final state, because the intermediate state does not exist.

Injection of spin-polarized electrons is observed at a ferromagnet–semiconductor interface by analyzing the electroluminescence signal of a GaAs/(In,Ga)As light emitting diode covered with Fe. The circular polarization degree of the light emission at room temperature reveals a spin injection efficiency of at least 2% from Fe into GaAs. The underlying injection mechanism is explained in terms of a tunneling process.

We investigate the synthesis of (In,Ga)N/GaN multiple quantum wells by plasma-assisted molecular beam epitaxy. For metal-stable growth, reflection high-energy electron diffraction and X-ray diffraction reveal massive In surface segregation which is directly confirmed by In depth profiles recorded by secondary-ion mass-spectrometry. These profiles exhibit a top-hat In distribution and are thus indicative of a zero order segregation mechanism instead of a first order process as observed for other materials systems. The segregation of In during metal-stable growth results in quantum wells with smooth interfaces but larger width than intended, and thus causes blue-shifted transition energies and poor quantum efficiency. This unexpected blue-shift may be the reason for the frequent conclusion that the theoretical polarization fields of Bernardini et al. [Phys. Rev. B 56, R10024 (1997)] are too large for (In,Ga)N. Being in possession of the (at least approximately) correct structural parameters, we find the theoretical fields of Bernardini et al. for (In,Ga)N tube in very satisfactory agreement with the experimental data. Reduction of In segregation by N-stable conditions is possible but inevitably results in rough interfaces.

High-quality ten-period (Al,Ga)N/GaN Bragg reflectors (15%

We have investigated the crystallization of amorphous germanium films on GaAs crystals using nanosecond laser pulses. The structure and composition of the crystallized layers is dominated by nonequilibrium effects induced by the fast cooling process following laser irradiation. Perfect epitaxial films are obtained for fluencies that completely melt the Ge film, but not the substrate. For higher fluencies, partial melting of the substrate leads to the formation of a (GaAs)1−xGe2x epitaxial alloy with a graded composition profile at the interface with the substrate. Since Ge and GaAs are thermodynamically immiscible in the solid phase, the formation of the alloy is attributed to the suppression of phase separation during the fast cooling process. Lower laser fluencies lead to polycrystalline layers with a patterned surface structure. The latter is attributed to the freeze-in of instabilities in the melt during the fast solidification process.

The electrical admittance of rectifying metal–semiconductor contacts on n-type GaAs/Ga(As, N)/GaAs heterostructures depends strongly on frequency and temperature. The distinct dispersion is due to the relatively high diffusion barrier around the Ga(As, N) layer. As long as the admittance is controlled by electrons inside the Ga(As, N) layer, their response to the ac electric field is dependent on frequency and temperature. Under appropriate conditions, capacitance–voltage measurements can be used to examine depth-resolved electrical characteristics of n-type GaAs/Ga(As, N)/GaAs heterostructures. The experimental depth profiles of the carrier concentration are compared with calculations based on self-consistent solutions of the Poisson equation. For 3% GaN mole fraction, the conduction band offset between GaAs and Ga(As, N) is found to be −(0.40±0.01) eV, i.e., about 95% of the total band gap difference. The heterointerfaces are of type I. At the Ga(As, N)-on-GaAs interface, negative charges of about 1.3×1...

An (Al,Ga)As heterostructure consisting of a 10 nm wide GaAs single quantum well and an optimized AlAs/GaAs type-II-superlattice barrier is fused onto a new LiNbO3 substrate by epitaxial lift-off and subsequent wafer bonding. X-electrons formed in the superlattice barrier effectively screen the high mobility electrons in the single quantum well from electronic defects arising at the new hybrid interface. Thus, the electron density as well as its high electron mobility can be preserved in the hybrid system.

Ferromagnetic MnAs layers with abrupt interfaces were grown by MBE on (0 0 1) and (3 1 1)A oriented GaAs substrates. At low growth rates and As-rich conditions, the films are ( 1 1 0 0) oriented on both templates. For MnAs/GaAs(0 0 1) layers it is demonstrated that realizing a well-ordered reconstructed growth surface and the layer-by-layer growth mode on a thin MnAs buffer, films of high structural quality and very promising magnetic properties are obtained. The quality of thick films is limited by processes accompanying the β→αMnAs phase transition during sample cooling. The magnetic domains, their increase in size with increasing external magnetic field and their relation to topographical defects was studied by magnetic force microscopy combined with atomic force microscopy.

Deep-level defects in p-type GaAs/Ga(As,N)/GaAs heterostructures grown by molecular beam epitaxy are investigated by deep-level transient Fourier spectroscopy. Depth-resolved distributions of hole traps are measured in as-grown and annealed heterojunctions in order to identify the defects, which lead to the degradation of the Ga(As,N) properties. Four defects are recognized in the heterostructures studied. Two dominant hole traps are found in Ga(As,N) at energies of about 0.35 and 0.45 eV above the valence band edge. These midgap levels originate from copper- and iron-related defects, the formation of which is connected with operation of the nitrogen plasma cell during Ga(As,N) growth. Both traps, which are removed by annealing, are discussed as the possible nonradiative centers that deteriorate the optical properties. Two other hole traps of intrinsic origin are related to the GaAs growth conditions close to the Ga(As,N)-on-GaAs interface, where the GaAs growth is affected by the nitrogen plasma despite ...

By model quantum calculations we show that the Stern–Gerlach effect can occur for a one dimensional electron gas. We propose the spin filter device based upon this effect and describe fabrication of prototype hybrid ferromagnet–semiconductor microstructures. From the low-temperature transport measurements we derive necessary conditions which must be satisfied in order to detect the spin filtering.

We identify and discuss the essential strategies for the growth of (Al,Ga)N/GaN and GaN/(In,Ga)N heterostructures on SiC(0 0 0 1) by both plasma-assisted and reactive molecular beam epitaxy. Substrate preparation, nucleation, and growth conditions are optimized for simultaneously satisfying the requirement of a high structural, morphological, optical and electrical quality. The results demonstrate that molecular beam epitaxy is a competitive technique for the growth of group III-nitrides.

We describe wave propagation from a partially coherent x-ray source taking into account the free-space wave propagation and the Bragg diffraction on a collimating crystal. We find conditions when the free-space propagation becomes irrelevant and the coherence length is given by the extinction length of the corresponding Bragg reflection. We show that a wide wavelength range can be retained if the collimating crystal is illuminated by a convergent beam. The increase of the coherence length by Bragg diffraction is demonstrated in a laboratory experiment on periodic surface gratings with different periods.

Strained In x Ga 1-x N quantum wells (QWs) on thick GaN base layers were investigated to verify the importance of localized QW excitons in their spontaneous emission mechanisms. A strength of the internal piezoelectric field (F PZ ) across the QW increases with increasing x up to 1.4 MV/cm for x = 0.25, since the in-plain strain increases. For the QWs with the well thickness L greater than 3 nm, F PZ dominates the emission peak energy due to the quantum-confined Stark effect. Absorption spectra of both hexagonal and cubic InGaN QWs exhibited a broad band-tail regardless of the presence of F PZ normal to the QW plane. The luminescence peak energy of the 3 nm thick QWs was higher than the bandgap energy of the unstrained bulk crystal for x < 0.15, showing that doping of Si in barriers or injection of carriers effectively screens the field. The emission lifetime increased with increasing monitoring wavelength, Also, a temperature-induced change in the luminescence peak energy decreased with increasing x. The real-space variation of the luminescence peak energy was confirmed by the spatially resolved monochromatic cathodoluminescence mapping method. The localization depth increases with increasing x, The carrier localization is confirmed to originate from the effective bandgap inhomogeneity due to a fluctuation of the local InN mole fraction, which is enhanced by the large and composition-dependent bowing parameter of InGaN material.

Wurtzite GaN nanocolumns are reproducibly grown by plasma-assisted molecular beam epitaxy on Si(111) and c-sapphire substrates. The nanocolumns density and diameter (600\char21{}1500 \AA{}) are effectively controlled by means of the III/V ratio. The nanocolumns are fully relaxed from lattice and thermal strain, having a very good crystal quality characterized by strong and narrow (2 meV) low-temperature photoluminescence excitonic lines at 3.472\char21{}3.478 eV. In addition, the spectra reveal a doublet at 3.452\char21{}3.458 eV and a broad line centered at 3.41 eV. This broad emission shows a sample-dependent spectral energy dispersion, from 3.40 to 3.42 eV, explained as due to the effect of strain and/or electric fields associated with extended structural defects located at the nanocolumns bottom interface. From cathodoluminescence data, it is concluded that the doublet emission lines originate at the nanocolumns volume, most probably related to ${\mathrm{Ga}}_{\mathrm{I}}$ defects, given the column growth mode (Ga balling).

Using RF plasma-assisted molecular beam epitaxy, we succeeded in the growth of GaN(1 1 0 0) on γ-LiAlO 2 (1 0 0). The crystal orientation and structural properties of 1.5 μm thick GaN films are investigated by means of reflection high-energy electron diffraction, X-ray diffraction, atomic force microscopy and Raman scattering. The layers are shown to be single-phase GaN(1 1 0 0) within the measurements’ sensitivity. The inherent anisotropy of the GaN (1 1 0 0) plane is reflected in the observed anisotropy of in-plane stresses, mosaicities and surface morphology.

We investigate the growth of M-plane GaN(100) on γ-LiAlO2(100) by molecular beam epitaxy. The crystal orientation and structural properties of buffer layers are examined by means of reflection high-energy electron diffraction, high-resolution transmission electron microscopy, X-ray diffraction, Raman scattering, and atomic force microscopy. The layers are shown to be single-phase GaN(100} ) within the measurements' sensitivity. In contrast to the ubiquitous C-plane GaN[0001] orientation, the M-plane of wurtzite nitrides is free of electrical polarization. This is experimentally verified for (Al,Ga)N/GaN heterostructures by continuous-wave and time-resolved luminescence.

In order to distinguish between conflicting interpretations regarding the effect of intralayer insertion at semiconductor junctions, we have carried out synchrotron-radiation photoemission studies of GaAs/AlAs(100) heterojunctions and GaAs/GaAs(110) homojunctions, with and without a Si or a Be intralayer, at room temperature and at low temperature. The synchrotron light induces photovoltage effects at low temperature, which are found to be consistent with the room-temperature band profiles we have previously proposed for these heterojunctions [M. Moreno et al., Phys. Rev. B 58, 13 767 (1998)], assuming a doping role for the intralayer atoms. Band discontinuities play an important role in determining the type of photovoltage effects induced. Our experimental observations can be fully understood in terms of intralayer-induced changes of the bandbending profile, and the occurrence of photovoltage effects at low temperature, calling into question the previous interpretation of room-temperature photoemission results from GaAs/AlAs heterojunctions in terms of intralayer-induced "band-offset" changes.

We show that the phases of surface reflections can be determined by simultaneously exciting a surface and a bulk reflection at grazing incidence. The interference between the surface reflections excited by the incident and the bulk-diffracted waves has been observed for the Ge(113)-(3 × 1) reconstructed surface.

Phase-locked pulse pairs are used for resonant excitation of excitons confined within GaAs/AlGaAs quantum wells. The time-resolved secondary emission displays strong dependence on the relative phases of the two pulses for inter-pulse delays of the order of 1ps or less. Constructive interference is combined with an increase in the rise time of the emission following the second pulse. For longer inter-pulse delays the different phases of inhomogeneous excitons complicate the effect of the second pulse, with the apparent effect changing with time after the second pulse. To model this, the optical Bloch equations for a line of three-level systems are integrated over time and the resulting polarisations summed over a range of angles. The importance of the inhomogeneity in descriptions of coherent control of secondary emission is shown.

The photoluminescence and photoluminescence excitation spectra have been studied in two twin GaAs quantum wells containing a two-dimensional electron gas with high density and high mobility. One of the wells includes a few Be acceptors in a δ -layer at the center of the well, which localize part of the photocreated holes. In the other well the holes are free. Comparison of the optical spectra of both samples shows that the Fermi-edge singularity in emission is only observed for localized holes, while in excitation spectra both Be-bound- and free holes display the same singularity. Thus, localization of holes appears to be relevant mainly by its effect on the final-state density. Magneto-optical measurements confirm the presence of the two kinds of holes and provide estimates of their apparent effective masses.

The effect of ferromagnetic layers on the spin polarization of holes and electrons in ferromagnet–semiconductor superlattices with a fixed Mn δ-layer thickness of 0.11 nm and different GaAs interlayer thicknesses varying in the range from 2.5 to 14.4 nm and a fixed number of periods (40) is studied by means of hot-electron photoluminescence (HPL). Here, our study of the HPL demonstrates that the holes in δ-layers of (Ga,Mn)As DMS occupy predominantly the Mn acceptor impurity band. The width of the impurity band decreases with the increase of the interlayer distance. We also found that an increase in the GaAs interlayer thickness softens the magnetic properties of the ferromagnetic layers as well as reduces the carrier polarization. It is demonstrated that the hole spin polarization in the DMS layers and spin polarization of electrons in nonmagnetic GaAs are proportional to the sample magnetization.

The optical properties of stacked GaAs/(Al,Ga)As quantum wire arrays, with a 0.5 \ensuremath{\mu}m lateral period fabricated by molecular beam epitaxy on patterned $\mathrm{GaAs}(311)A$ substrates, have been investigated. We observe an unexpectedly high quantum wire related contrast in the lateral distribution of the cathodoluminescence (CL) intensity, in particular at 300 K. The temperature dependence of this contrast as well as of the integrated CL intensities in the quantum wires and connecting quantum wells, reveals the loss mechanisms, which cause a reduction of the exciton transfer efficiency from the well into the wire regions. For low and intermediate temperatures, exciton localization and nonradiative recombination within the quantum well regions contribute to the decrease of the transfer efficiency. Near room temperature, the vertical escape of carriers, in particular out of the quantum well regions into the (Al,Ga)As barriers, is the limiting process. Within the framework of a detailed model, we determined the transfer time and the ratio of the radiative recombination times in the well and wire regions by combining the spatially resolved CL measurements with time-resolved photoluminescence spectroscopy.

A new spectroscopic technique is demonstrated on three characteristically different semiconductor quantum-well samples. Coherent excitation spectroscopy reveals coherent coupling and effectively suppresses the signatures of inhomogeneous broadening and incoherent transfer between various excitonic states in quantum wells. Coherently coupled homogeneous subsystems are analyzed in an inhomogeneously broadened excitonic ensemble.

We have studied the temperature dependence of the spin-flip processes of electrons in p-type modulation-doped GaAs quantum wells. We have found that at low temperature (T 30 K) the exchange interaction between electrons and holes is the main mechanism responsible for the spin-flip of the electrons, whereas at higher temperatures the so-called Dyakonov-Perel mechanism dominates. Furthermore, we have found that the recombination time increases considerably with increasing lattice temperature from a value of tr 200 ps at 5 K to tr 400 ps at 60 K. Useful electronic devices rely on the precise control of electronic charge and, in general, the fact that the electrons also have a spin is ignored. However, the scattering processes for electrons depend on their spin state. Spin related effects of carriers are attracting a lot of attention in different fields of condensed matter physics. The interest in the spin mechanisms is due not only to fundamental curiosity but also to possible device applications: semiconductor-based spin-polarized sources have nowadays reached a state where electrons with polarizations of 90% can be obtained. These sources are made out of III‐V compounds grown epitaxially, lifting the degeneracy of the holes at the top of the valence band either by quantum confinement effects and/or by strains

We present the results of in situ reflectance-difference spectroscopy (RDS) investigations of GaAs grown by molecular beam epitaxy at low substrate temperatures (LT-GaAs). The linear electro-optic (LEO) features in the vicinity of the ${E}_{1}$ and ${E}_{1}+{\ensuremath{\Delta}}_{1}$ optical transitions are found to depend on growth temperature ${T}_{g}$ and V:III flux ratio. The observed LEO amplitude was found to decrease linearly with the measured neutral As antisite point defect concentration $[{\mathrm{As}}_{\mathrm{Ga}}^{0}]$ for growth temperatures $200\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}l{T}_{g}l300\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ This behavior relies on the influence of As antisite defects on the optical response of GaAs. The nonstoichiometry of LT-GaAs can be estimated in real time through the LEO amplitude vs $[{\mathrm{As}}_{\mathrm{Ga}}^{0}]$ relation. This makes RDS very attractive not only for in situ growth control but also for real-time tuning of the electronic properties of LT-GaAs.

We study the optical properties of thin (0.4--0.7 \ensuremath{\mu}m ) GaN layers directly grown on $6H\ensuremath{-}\mathrm{SiC}(0001).$ These structures are found to always exhibit a broad luminescence band in the blue spectral range, which is identified as exclusively being due to ${(D}^{0}{,A}^{0})$ transitions in 6H-SiC. It is shown that the SiC substrate is excited indirectly via diffusion of carriers photogenerated in the GaN layer. The drastic differences found between samples of different electron concentration suggest correspondingly large differences in the diffusivity, which cannot be explained simply by different ambipolar carrier mobilities. Instead, different diffusion mechanisms are required, namely, ambipolar and excitonic diffusion at carrier concentrations above and below the Mott density, respectively.

The coherent optical response of bulk GaAs is studied after ultrafast excitation close to the band edge. Pump-probe experiments with 20-fs pulses demonstrate that coherent polarizations between heavy- and light-hole continuum states make a strong contribution to the nonlinear optical response. The beat frequency is determined by the heavy-hole--light-hole energy splitting in the spectral probe window. In addition, excitonic heavy-hole--light-hole quantum beats are observed in the slightly strained GaAs crystal. The different contributions are separated via their polarization selection rules.

GaN epilayers of about 1 μm thick were grown directly on 6H–SiC(0 0 0 1)substrates by direct current plasma assisted molecular beam epitaxy. High-resolution X-ray diffraction and two-dimensional triple axis mapping were used to characterize the GaN epilayers. A FWHM of 11.9 arcmin for an ω scan and 1.2 arcmin for an ω /2 θ scan were observed. A careful study of the rocking curves showed that there were some large mosaics in the GaN epilayers. The two dimensional triple axis mapping showed that the GaN mosaics were disoriented in the (0 0 0 1) plane but rather uniformed in the direction perpendicular to the plane. A mosaic model was deduced to explain the phenomenon, and the dislocation density was estimated to be about 2×10 9 cm −2 according to the model.

The molecular beam epitaxy growth of strained (Ga, In)Sb/GaSb quantum wells is investigated. In a narrow range of growth conditions, (Ga, In)Sb quantum well structures exhibiting excellent structural properties as well as intense and narrow photoluminescence transitions are obtained. Stimulated emission at 1.98 m is observed at room temperature from laser diodes with strained quantum wells as the active zone. The lasers exhibit threshold current densities as low as 280 A and a characteristic temperature of 75 K.

A new detector that integrates energy loss spectroscopy with a standard reflection high-energy electron diffraction (RHEED) detector is used to study MBE growth of GaAs and GaN in real time. The detector is fully compatible with standard MBE machines and does not require any modifications to the system. While maintaining a spatial resolution of better than 200 μm on the phosphorous screen, the energy resolution of the system is typically better than 2 eV. We find that at 20 keV primary beam energy, the energy spectrum is dominated by surface plasmon losses. The shape and phase of RHEED intensity oscillations is not affected by excluding the inelastically scattered electrons. The energy spectrum is sensitive to surface roughness, material composition and layer thickness, providing additional information about the surface during growth.

We review our investigations of the optical properties of β-GaN and β-(In, Ga)N layers grown on GaAs (0 0 1) by plasma-assisted molecular beam epitaxy. For In contents up to 17%, the band gap of β-(In, Ga)N layers, estimated by combining photoluminescence with spectroscopic ellipsometry as well as transmittance spectroscopy, shows good agreement with theoretical predictions from the literature. Using the theoretically predicted band gaps, we calculate 2.75 eV isoenergy contours for both β-(In,Ga)N/GaN and α-(In, Ga)N/GaN quantum wells. Comparing these contours to calculations of the critical thickness, it becomes evident that blue-emitting α-(In, Ga)N/GaN quantum wells are above or at the border to plastic relaxation except for very thin quantum wells of high In content. In contrast, blue-emitting β-(In, Ga)N/GaN quantum wells lie below this border for a wide range of thickness and In content.