The optical and structural properties of InAs/GaAs quantum dots (QD) are strongly modified through the use of a thin (~
5 nm) GaAsSb(N) capping layer. In the case of GaAsSb-capped QDs, cross-sectional scanning tunnelling microscopy
measurements show that the QD height can be controllably tuned through the Sb content up to ~ 14 % Sb. The increased
QD height (together with the reduced strain) gives rise to a strong red shift and a large enhancement of the
photoluminescence (PL) characteristics. This is due to improved carrier confinement and reduced sensitivity of the
excitonic bandgap to QD size fluctuations within the ensemble. Moreover, the PL degradation with temperature is
strongly reduced in the presence of Sb. Despite this, emission in the 1.5 μm region with these structures is only achieved
for high Sb contents and a type-II band alignment that degrades the PL. Adding small amounts of N to the GaAsSb
capping layer allows to progressively reduce the QD-barrier conduction band offset. This different strategy to red shift
the PL allows reaching 1.5 μm with moderate Sb contents, keeping therefore a type-I alignment. Nevertheless, the PL
emission is progressively degraded when the N content in the capping layer is increased.
InAs nanostructures formed on InP substrates allow the realization of devices working in telecommunication wavelength range between 1.4 and 1.65 μm. However due to the low lattice mismatch existing between InAs and InP, the self assembling process in InP is more complex than on GaAs substrates. First high density quantum wires obtained on InP(001) have been integrated in laser. Lasers emitting at room
temperature have been achieved. For an infinite length cavity, a threshold current density per QD plane as low as 45 A/cm2 is deduced. This result compares favourably with those obtained on quantum wells lasers. However, the stability of the threshold current with temperature, predicted for quantum dots laser is not
observed. Thus, growth on non standard substrates such as miscut substrates or high index substrates have been investigated in order to achieve QDs on InP. On (113) B substrates, quantum dots in high density and with size comparable with those achieved on GaAs(001) have been obtained. Lasers with record threshold current have been obtained. However the modulation properties of the laser are not as good as predicted for ideal quantum dots lasers. Finally we present the attempts to extend the QD emission wavelength in the 2-3
Theoretical analysis of the electron energy spectrum and the magnetization in a strained InxGa1-xAs/GaAs selfassembled
quantum ring (SAQR) is performed using realistic parameters, determined from the cross-sectional
scanning-tunneling microscopy characterization. The Aharonov-Bohm oscillations in the persistent current have
been observed in low temperature magnetization measurements on these SAQRs. The effect of the Coulomb
interaction on the energy spectra of SAQRs is studied for rings with two electrons and with an exciton. Our
analysis of the photoluminescence spectrum in magnetic fields up to 30 T shows that the excitonic properties
strongly depend on the anisotropic shape, size, composition and strain of the SAQRs and is in a good agreement
with the experimental data.
Shallowly formed InAs quantum dots (QDs) embedded in GaAs are investigated by Optically Detected Microwave
Resonance (ODMR) technique. The low temperature (1.6 K) photoluminescence (PL) spectrum reveals a two-peak
structure which is attributed to two different classes of QDs: smaller and larger in size. V-band (60 GHz) ODMR is
selectively detected in each of the peaks and depending on the PL detection energy, a different ODMR spectrum is
obtained. Detection in the high-energy band reveals a low-field negative signal which is ascribed to cyclotron resonance
of the electron in the two-dimensional wetting layer, corresponding to an effective mass of 0.067 m0. The microwave-induced
signal at higher fields (~1.1 T) is tentatively attributed to magnetic resonance transitions between spin states of
the holes confined in the smaller QDs. When monitoring the emission of the larger QDs, the obtained microwave-induced
signal is negative while the resonance line at low field, associated with the cyclotron resonance, is no longer
present. The V-band ODMR spectra are compared with W-band (94 GHz) measurements obtained for the same QD structure.
ihe (1 10) face in Ill-V binaiy compounds is a non-polar surface and allows the determination of valence and conduction hand edges as well as energy gap at the clean ultra-high vacuum cleaved surface. In epitaxial 111-V semiconductor multilayers this (1 10) face forms a cross section to the preferential 001 growth direction. With scanning tunneljng microscopy we have been able to observe the atomic arrangement in MBE-grown GaAs and A1GaAs layers as well in their interfaces. The binary and ternary cornpoupds can be clearly distinguished and we also find indications for composition fluctuations in the ternary. The atomic resolution images show that the material transition occurs over 2 unit cells. From the current-voltage characteristics across the GaAs-AlGaAs interface the valence band edge is determined and compared with theoretical calculations. The electronic valence band transition occurs over a length scale of less than 4 urn.