Resonant tunnelling through two impurities has been observed in a vertical double-barrier GaAs/AlGaAs tunnelling diode. It manifests itself as a peak in the I-V characteristic near the conductance threshold. Analysis of the position of the current peak, its shape and the shift in magnetic field parallel to the current provides information about the characteristics of the two impurities. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Both thermally detected optical absorption and photoluminescence as a function of temperature are used to investigate the electronic states of InGaAsN/GaAs quantum wells grown by molecular beam epitaxy. The band structure of InGaAsN is first described within the two-band model, assuming that nitrogen only affects the conduction band through the interaction between the localised nitrogen level and the host matrix conduction band. Taking advantage of the accurate knowledge of strained InGaAs/GaAs layers, a simple model allows the calculation of the energy levels in InGaAsN/GaAs quantum wells. Furthermore, the labelled ten-band k.p model is used to derive the strained conduction band offset. The calculations of transition energies in the wells compare favourably with experimental results. From photoluminescence experiments at different temperatures, the ionisation of impurities together with the transition between localisation–delocalisation of the carriers in the well are evidenced.

The effects of nitrogen fraction on the temperature dependence of GaNxAs1−x/GaAs (x<2.8%) quantum-well emission was investigated using steady-state photoluminescence between 2 and 300 K. At low temperatures, a characteristic S-shape behavior indicative of carrier localization was observed for each of the samples. This is believed to result from the large miscibility gap induced by the nitrogen, which results in structural/compositional fluctuations in the well. In the high temperature regime (T>150 K) where the emission has a linear dependence, a strong reduction in emission temperature dependence was observed with increasing nitrogen. The temperature dependence was modeled using the band anticrossing approach, with the interaction matrix element parameter CNM (VMN=−CMNx) and the nitrogen level parameter γ (EN=EN0−γx) used as fitting parameters.

We have investigated in-plane photovoltage (IPV) and photoluminescence (PL) in sequentially grown Ga0.8In0.2As/GaAs and Ga0.8In0.2N0.015As0.985/GaAs quantum wells. Temperature, excitation intensity, spectral and time dependent study of the IPV, arising from Fermi level fluctuations along the layers of the double quantum well structure, gives valuable information about the nonradiative centers and hence about the optical quality of the GaInNAs quantum well. It also provides information about the radiative transition energies in all the layers. In order to obtain either the trap activation energies and the detrapping rates of photogenerated carriers in the GaInNAs the IPV results are analyzed in terms of a theoretical model based on random doping fluctuations in nominally undoped multilayer structures. The PL results are analyzed in terms of the band anticrossing model to obtain the electron effective mass from the coupling parameter CNM.

The basic operation of a novel GalnNAs/GaAs based light emitting/absorbing device operating at 1250 nm is described. The device is a bi-directional field effect light emitting and absorbing heterojunction (BiFEEAH), which can simultaneously en-lit and detect light. This feature makes it possible to construct a wavelength converter, where one end of the device absorbs incoming light and the other end emits light at a different wavelength. The current device consists of a simple GaAs p-i-n structure, containing a single 90 Angstrom GalnNAs quantum well in its intrinsic region. This is fabricated into a four contact device with separate n and p conducting channels.

We have developed a model for the rapid calculation of interband transitions in dilute nitride quantum well structures. The model assumes parabolic bands with the conduction band modified using the band anticrossing approach. The model is compared to results from k·p models, which while being more accurate and flexible than our model, require extensive computational time. The model predictions are compared to photoluminescence and photomodulated reflectance measurements of interband transitions in a number of GaNAs quantum well samples.

Molecular beam epitaxy of GaAsN/GaAs and GaInAsN/GaAs structures on {111} oriented substrates has been studied. Ga(In)AsN/GaAs thick layers and quantum wells have been grown on (111)A and (111)B GaAs substrates. Nitrogen incorporation has been found to depend on substrate orientation and growth rate. The most promising orientation appears to be the (111)A orientation for GaAsN quantum wells and emission wavelengths up to 1.5 mum have been obtained. For (111)B, a broad emission is systematically observed indicating the presence of defects originating from N incorporation. For (111)A GaInAsN/GaAs quantum wells, the addition of indium leads to a red shift and to a broadening of the emission. It does not have any beneficial effect on (111)B quantum well optical properties.

We have studied the optical quality of sequentially grown undoped Ga0.8In0.2As and Ga0.8In0.2N0.015As0.985 quantum wells (QWs). Spectral and time-resolved in-plane photovoltage (IPV) and photo-induced transient spectroscopy (PITS) techniques were used in this investigation. Two clear peaks have been observed and analysed in the PITS experiment. Spectral and transient IPV in the same samples has been investigated and a selective light was used as the excitation source to separate the GaInNAs IPV from the other layers. IPV can be explained in terms of random fluctuations of the Fermi level in undoped QWs. Spectral and time-resolved IPV measurements can therefore be used to obtain qualitative and quantitative information about interband transitions and trap activation energies.

The photo-induced transient spectroscopy (PITS) technique was used to investigate the capture and emission dynamics of deep levels in Ga0.8In0.2N0.015As0.985/GaAs and Ga0.8In0.2As/GaAs quantum wells, sequentially grown by molecular beam epitaxy. A broadband white light or a filtered light was used to identify and discriminate the trapping centres present in the sample. Among all the features discovered in this experiment two PITS peaks, showing activation energies of 160 and 330 meV, have been associated with deep levels in GaInNAs.

We have investigated the temperature dependence of the band gap energy in GaInNAs, GaNAs and InGaAs quantum wells. In the structures containing nitrogen the well-known S-shaped characteristic was observed at low temperatures. We explain this anomalous temperature behaviour by strong carrier localization in potential fluctuations at low temperatures. In the nitrogen free samples, there was no S-shaped behaviour and the empirical Varshni dependence was followed.

The precipitation of arsenic in GaAs epitaxially grown at low temperature (LT GaAs) has been studied as a function of the post-growth annealing temperature by three independent methods: transmission electron microscopy (TEM), Raman scattering, and for the first time the anomalous small angle X-ray scattering (ASAXS). All the results agree that the average size increases with increasing annealing temperature but the ASAXS data indicate that the precipitates can be divided into two parts described by separate size distributions. The number density of precipitates around 5 nm size has been estimated to be at least two orders of magnitude higher than that of larger precipitates.

We have investigated the temperature dependence of photoluminescence (PL) emission from sequentially grown Ga0.8In0.2As and Ga0.8In0.2N0.015As0.985 quantum wells between 2 K and room temperature. A significant reduction in the temperature dependence of the GaInNAs bandgap compared to nitrogen-free GaInAs is observed. The results are analysed using the band-anticrossing model, which accurately predicts the temperature dependence of the GaInNAs energy gap from the behaviour of the GaInAs energy gap. We also compare the band-anticrossing interaction parameter CNM used to fit our data with other published values for GaNAs and GaInNAs. The results suggest that CNM may not be independent of indium fraction.

We have found that the local density of state fluctuations (LDOSF) in a disordered metal, detected using an impurity in the barrier as a spectrometer, undergo enhanced (with respect to Shubnikov-de Haas and de Haas-van Alphen effects) oscillations in strong magnetic fields, omega(c)tau>/=1. We attribute this to the dominant role of the states near the bottom of Landau bands which give the major contribution to the LDOSF and are most strongly affected by disorder. We also demonstrate that in intermediate fields the LDOSF increase with field B in accordance with the results obtained in the diffusion approximation.

In this work, we compare the kinetic behavior of As precipitates in Low Temperature Molecular Beam Epitaxy (LT-MBE) grown and As implanted As rich GaAs layers. The evolution of the mean radius, density and volume fraction of As precipitates are measured by Transmission Electron Microscopy as a function of the annealing conditions on both type of layers. While for LT-MBE the As precipitates undergo a conservative Ostwald ripening, for implanted layers this growth is frozen. This behavior is tentatively explained by a vacancy deficit due to the presence of interstitial type extended defects in the implanted layers

We have studied the effects of magnetic field on the fluctuations of the electron density of states in a disordered conductor, a contact in the resonant tunnelling structure GaAlAs/GaAs/GaAlAs. The local density of states (LDOS) in the contact is detected by an impurity which acts as a 'spectrometer'. In the regime of weak magnetic field, omega(c)tau less than or equal to 1, a linear increase of the fluctuation magnitude with B-2 is seen, due to the decrease of the volume where the LDOS is formed and a decrease in the statistical averaging. In the regime omega(c)tau greater than or equal to 1, large oscillations of the fluctuation magnitude have been observed. We suggest an explanation of this new effect, de Haas-van Alphen type oscillations in the fluctuation magnitude. The behaviour of impurity spectrometers in a magnetic field and images given by a complex spectrometer are also discussed.

Single layers of self-assembled InAs/InP quantum dots (QD) are studied by Raman scattering excited in resonance with the confined E-1 transition of InAs. Intense periodic oscillations are observed in the low-frequency Stokes and anti-Stokes spectra of both capped and uncapped QD. By using a controlled chemical etching,we progressively reduced the thickness of the InP cap layer. We found that the oscillations period is determined by the sample surface-go layer separation and by the sound velocity of the longitudinal acoustic phonons. A model based on the interaction between confined electronic stales and standing sound waves due to the sample surface showed a reasonable agreement with the measurements. The dependence of the low-frequency scattering on QD size is discussed.

Using the optical phonons as an internal probe, the change in the dielectric permittivity has been analysed by Raman scattering in GaAs grown by MBE at low temperature (LT-GaAs). The screening effect is discussed in terms of a solid solution whereas the strain one is treated in the perfect adhesion hypothesis. Experimentally, the strain and the screening effects have been discriminated by using various scattering geometries. As the LT-layers with large non-stoichiometry have a poor thermal conductivity, it is shown that local heating considerably affects the experimental data. This can be avoided by surface convective exchange then leading to perfect agreement between the recorded phonon frequency shift and the expected ones. As a consequence, this sustains a new method for a quantitative determination of the As excess in LT-layers.

Raman scattering measurements on low temperature GaAs layers are presented. Phonons in both GaAs and As are studied. The transition from diluted As in the GaAs matrix to the small As clusters formed after annealing is analyzed. This is performed by observing the reduction of the GaAs bound charge, i.e., the longitudinal optical GaAs frequency downshift, to the appearance of the crystalline As vibrational mode. From the phonon shifts of crystalline As, the diameter and the strain are derived, accordingly to transmission electron microscopy measurements.

We present an experimental and theoretical study of electron tunnelling through quantum dots which focusses the attention on the amplitude of the current peaks as a function of magnetic field. We demonstrate that the amplitudes of the current peaks in the tunnelling spectra show a dramatically different behaviour as a function of the magnetic field, depending on the angular momentum of the dot state through which tunnelling occurs. This is seen in the non-monotonic behaviour of the current amplitude in magnetic field. Furthermore, the magnetic field severely hinders tunnelling through states with angular momentum parallel to the field, and in some cases it makes it altogether impossible. This type of investigation allows us to directly probe the details of the confined wave functions of the quantum dot.

The structural quality of the layers grown at low temperature on (111)B GaAs is investigated by Raman spectroscopy and transmission electron microscopy. It is shown that both the amorphous layers grown at 150°C and the defect-rich layers (mostly based on multiple twinning) grown at 250°C can recover perfect crystalline quality upon annealing provided, (i) they are not too thick (<350 nm) and, (ii) the annealing temperature is high enough. Both conditions are necessary to allow the reordering of the layer initiated at the interface to propagate towards the surface without being stopped at dislocation nodes