B Lambert

Relationship between self‐organization and size of InAs islands on InP(001) grown by gas‐source molecular beam epitaxy

Using the strained‐induced 2D–3D transition, InAs dots have been grown on InP(001) and examined by transmission electron microscopy. Two different modes of island size and spatial distribution have been identified. For deposit of 1.5 and 1.8 monolayers, the islands are about 7 nm high and randomly distributed. Above 2 monolayers, they are about five times smaller in volume and locally self‐organized, with a typical distance of 40 nm independent of the island density. It is suggested that the strong dependence of the island size on the total amount of deposited InAs is mainly due to long range interactions through the substrate.

Electron and hole transport properties in GaAs-AlGaAs superlattices

Abstract Time-resolved photoluminescence has been used to study the carrier transport in superlattice minibands (vertical transport). Transport of electrons and holes can be studied separately by using appropriate excitation densities and doping levels. Their diffusion coefficients are estimated and studied both as a function of the superlattice period and of the lattice temperature. The different behaviour of electrons and hole mobilities as a function of the SL period is evidenced. The temperature behaviour of the transport properties is similar in a superlattice and in the AlGaAs alloy with an equivalent Al concentration.

Influence of transport properties on the excitation spectra of GaAs/AlxGa1-xAs superlattices and bulk layers

In GaAs/AlxGa1-xAs superlattices, GaAs and AlGaAs epitaxial layers, the results of photoluminescence excitation spectroscopy are shown to depend strongly on carrier mobilities when, at a given distance from the surface, an escape mechanism exists, i.e. a possibility for carriers to escape from the active layer after moving through it. When there is no escape mechanism close enough to the surface, excitation spectra are roughly flat apart from the excitonic resonances. When an escape mechanism is present at a distance of around 1 mu m from the surface, if the mobility is high enough as in GaAs and superlattices with periods smaller than about 70 AA (for x approximately 0.3 and nearly equal well and barrier widths), the excitation spectra exhibit a decreasing high-energy tail; on the contrary in AlGaAs and superlattices with greater periods the excitation spectra have the same shape as in similar samples without an escape mechanism. Superlattices with small periods constitute better structures than equivalent AlGaAs layers for device applications.

Semi-insulating InP co-doped with Ti and Hg

The authors have grown semi-insulating InP ingots co-doped with Ti and Hg. The compensation mechanism is the following: the holes due to the shallow Hg acceptor are compensated by the titanium-related deep donor level situated at about Ec-0.5 eV (T approximately=300 K). Temperature-dependent Hall effect measurements are consistently interpreted using concentration data from SSMS and SIMS measurements. In these samples a Ti-related EPR signal is detected (g=1.94 and Delta BPP=450 G) and attributed to resonance into the 2E ground state of Ti3+.

Optical determination of the AlxGa1-xAs energy gap variation versus the Al concentration in MBE-grown samples

The authors have determined the variation of the AlGaAs energy gap (Eg) versus the aluminium concentration x. The x-values are determined by precise X-ray measurements and the values of Eg are deduced from photoluminescence experiments. They find the law, Eg(x)=Eg(O)+1.34x eV, for x<0.45 at T=2 K.

Excitation mechanisms of rare earth (Yb) luminescence in III-V semiconductors (InP)

Thermal dependent Hall effect and time-resolved band edge photoluminescence on rare earth (Yb) doped indium phosphide are reported. The presence of two different traps related to the rare earth clearly demonstrated A very efficient trapping is observed in time resolved photoluminescence. The variations of the band edge luminescence decay time with temperature and excitation density lead the authors to consider the trapping of both electrons and holes on the rare earth site. They propose a model based on the creation of bound exciton trapped on the rare earth site which allows them to account for both the electrical and optical properties of rare earth in III-V compounds. The reasonable quantitative agreement between this model, time-resolved and cw photoluminescence data is shown and discussed.

Electrical behavior of Yb ion in p‐ and n‐type InP

In this letter are reported admittance spectroscopy experiments on Yb‐doped p‐ and n‐type InP grown by the synthesis method. The purpose is to give a clear understanding of the rare‐earth ion electrical behavior. In p‐type material, the results indicate the presence of two peaks in the conductance spectra at low temperature. The activation energies of these levels have been found to be 42±5 and 50±5 meV above the valence band. We attribute them to Mg and Yb, respectively. In n‐type material, the conductance spectra present a peak at low temperature with an activation energy of 29±3 meV below the conduction band and we attribute it to Yb ion. To explain the origin of these Yb‐related traps, we propose that this ion acts as an isoelectronic trap in InP.

Density-dependent transition from electron to ambipolar vertical transport in short-period GaAs-AlGaAs superlattices

Using time-resolved photoluminescence the authors have studied vertical transport in a series of superlattices with a large included 1 mu m away from the sample surface. With a streak camera, photoluminescence (PL) decays for excitation densities as low as a few 1013 cm-3 photocarriers per pulse can be recorded. At such low densities, the electrons are the minority carriers in residual p-type MBE materials. They can move without being slowed down by the holes. By varying the excitation density one is able to observe the transition from an electron transport regime to an ambipolar one. The use of a one-dimensional diffusion model for the photocreated carriers and the excitons enables one to obtain the diffusion coefficients of electrons, holes and excitons in these structures at different temperatures.

High-mobility vertical transport in graded-gap GaAs/AlGaAs superlattices

In compositionally graded superlattices, grown by molecular beam epitaxy, the carrier mobilities in the direction perpendicular to the layers have been experimentally estimated in an all-optical experiment. Mobilities in low-period graded-gap superlattices are estimated to be greater than those of AlGaAs graded-gap alloys.

Characterisation of semi-insulating InP:Fe

The total iron and the Fe3+ concentration are determined by means of SIMS and EPR measurements. The deduced electron concentration is in good agreement with the results given by Hall effect measurements.