F. Laruelle

Organized growth of GaAs/AlAs lateral structures on atomic step arrays: what is possible to do?

Abstract Laterally modulated nanostructures, exhibiting large optical and electrical anisotropy, can be grown on vicinal surfaces. However the lateral organization of Ga and Al atoms is far from ideal. We discuss our way of understanding the growth of these structures.

RHEED studies of Ga desorption from GaAs and of As desorption from Si-doped GaAs during growth interruption

Abstract We obtain accurate measurements of Ga desorption rate under conditions of Knudsen evaporation between 600 and 630°C from (001) GaAs or A1As using only RHEED. Within two monolayers from an A1As interface the Ga desorption is reduced by a factor of 2.5−3. In another experiment we observe that a partial surface coverage of a GaAs surface by Si has no effect on Ga desorption but increases strongly the As desorption, leading accordingly to a lowering of the transition temperature to a Ga-rich surface.

Optimization of optical properties of GaAs/GaAlAs quantum wells grown by high temperature migration enhanced epitaxy

Abstract We investigate optical properties of quantum wells grown by migration enhanced epitaxy (MEE) at high temperature (580°C). Our results suggest that Ga droplets that form during the MEE cycle strongly degrade the interface quality. Quantum wells grown by MEE avoiding the formation of Ga droplets exhibit very good optical properties.

The building up of terrace periodicity by MBE growth on (0 0 1) GaAs vicinal surfaces

Abstract The morphology of MBE grown GaAs vicinal surfaces is studied using ex situ AFM. Analysing the statistical distribution of terrace width, we succeed in explaining our measurements either by a thermodynamical equilibrium model taking into account the step by step interaction or by a 2D Monte Carlo simulation of the growth. In the latter case, an anisotropic Schwoebel barrier at the step edge has to be introduced in order to get a good agreement. We argue that growth is the efficient process to get a low disordered step array at the usual time scale and temperature range of MBE. In addition the effect of a GaAs AlAs buffer superlattice, or of pure AlAs layer, or of Si doping in GaAs is also discussed.

Towards the optimization of the growth and properties of modulated electron systems on GaAs vicinal surfaces

The growth of quantum structures with strong in-plane 1D potential modulation (amplitude , the Fermi energy of an electron gas in a selectively doped heterojunction) at short scale (periodicity , its Fermi wavelength) relies on the understanding of both the epitaxy and the electronic properties. Some issues such as the morphology of the step array, the organization of the Ga and Al atoms on the surface, and the effect of the electron gas screening or of the tilt of the lateral modulation on the effective potential, are discussed.

Organized growth of GaAsAlAs lateral superlattices on vicinal surfaces: where are the limits?

Abstract We report on three issues concerning the growth of GaAs AlAs lateral superlattices on vicinal surfaces to obtain quantum wires with negligible inter-wire tunnel coupling. We show that the ledge roughness increases with increasing terrace length and decreases for slower growth rates. The tilt angle of lateral superlattices is not the relevant parameter as long as electronic properties are concerned: one has to consider the product of the coverage error by the lateral superlattice thickness. The Ga flux stability is a critical parameter to be controlled. The use of dual-filament cells improves it by a factor two compared to single-filament ones.

Organised growth on vicinal surfaces:: segregation and disorder effects

The lateral potential of GaAs/AlAs lateral superlattices (LSL) obtained by organised molecular beam epitaxy is modelled by including segregation effects and step array disorder. This model is compared to experimental data of optical spectroscopy and of thermal dependence of the conductivity anisotropy. The LSL periodicity and amplitude are clearly evidenced in optical spectra whereas the lateral interface abruptness shows up in transport experiments.

Zone–edge optical transitions in GaAs/AlAs lateral superlattices grown on vicinal surfaces by molecular beam epitaxy

Abstract We report on new features in the photoluminescence excitation (PLE) spectra and PLE linear polarization spectra of GaAs/AlAs lateral superlattices grown by molecular beam epitaxy (MBE). These lines appear systematically as the tilt angle of the lateral superlattice is varied. They are identified as zone–edge excitonic transitions by comparison between experimental data and detailed numerical calculations of optical transitions including valence-band mixing and tilt effects.

Enhancement of electron conductivity in GaAs/AlAs coupled quantum wires

Abstract The anisotropy of the low-temperature electrical conductivity of laterally modulated electron gases can be analyzed in a semi-classical approximation, taking into account both the anisotropy of the group velocity which was already considered in previous works and that of the collision time which was ignored so far. By doing so, both components of the conductivity tensor changes with the modulation amplitude, in agreement with our measurements. We predict the possibility of observing, in realistic structures, longitudinal conductivities larger than the Drude conductivity of similar but unmodulated electron gases.

Characterisation of buried lateral period superlattices by ballistic electron emission microscopy

Ballistic electron emission microscopy (BEEM), a derivation of scanning tunnelling microscopy (STM), is a recently developed technique that is most often used to measure local potential barrier height variations in metal-semiconductor systems (i.e. Schottky barriers). Here, for the first time, the successful application to the study of buried lateral period superlattices, produced by molecular beam epitaxial growth on a vicinal GaAs(001) substrate, is reported. The structure consists of thin Au and GaAs films on top of the buried AlAs/GaAs superlattice and finally a GaAs substrate. Simultaneously obtained STM topographical images of the Au film and BEEM images of the ballistic electrons reaching the GaAs substrate reveal the expected periodic modulation only in the BEEM image. In addition, spectroscopic examination of the potential barrier at various places over the samples reveals a bimodal distribution consistent with a modulation of Al x Ga 1-x As mole fraction perpendicular to the surface steps. More detailed analysis reveals a compositional modulation larger than previously detected in such samples.