G. Bremond

Surface effects on shape, self-organization and photoluminescence of InAs islands grown on InAlAs/InP(001)

InAs nanostructures were grown on In0.52Al0.48As alloy lattice matched on InP(001) substrates by molecular beam epitaxy using specific growth parameters in order to improve island self-organization. We show how the change in InAs surface reconstruction via growth temperature from (2×4) to (2×1) and/or the use of InAlAs initial buffer surface treatments improve the island shape homogeneity (either as quantum wires or as quantum dots). Differences in island shape and in carrier confinement are shown by atomic force microscopy and by photoluminescence measurements, respectively. We point out that such shape amendments induce drastic improvements to island size distribution and discernible changes in photoluminescence properties, in particular concerning polarization.

Optical properties of silicon nanocrystal LEDs

Abstract In this work, we describe how to fabricate good quality 3 nm nc-Si with low size distribution in thermal SiO2 oxides. Photoluminescence, excited photoluminescence, and photocurrent measurements are discussed on the basis of theoretical calculations of the quantified levels in nc-Si. The impact of shape and size in quantum dots on transition energies has been highlighted, thanks to 2D symmetrical self-consistent Poisson–Schrodinger simulations. Both direct and indirect gaps in silicon have been considered in order to carry out a better comparison between simulations and optical measurements. A good agreement is found between simulations and experimental data for the indirect gap of 3 nm dots which show a threshold energy around 2 eV . However, the optical recombinations seems to be related to lower energy states probably due to interfacial radiative defects around 1.58 eV . On the basis of highly luminescent nc-Si, we have fabricated an optimized light emitting device (LED) with a calculated design in order to favour both electron and hole injection. Stable red electroluminescence has been obtained at room temperature and the I–V measurements confirm that the current is related to a pure tunnelling process. A modelling of I–V curves confirms a Hopping mechanism with an average trap distance between 1.4 and 1.9 nm . The Fowler–Nordheim process is not observed during light emission for electric fields below 5 MV / cm . Finally, we have not hot carrier injection and thus it seems possible to develop Si-based LEDs with a good reliability.

From large to low height dispersion for self-organized InAs quantum sticks emitting at 1.55 μm on InP (001)

We show how the height dispersion of self-organized InAs/InP(001) quantum islands emitting at 1.55 μm was reduced by optimizing the epitaxial growth parameters. Low height dispersion was obtained when the InAs deposit thickness was much greater than the critical thickness for two-dimensional/three-dimensional growth mode transition, and when adatom surface diffusion was favored by increasing the growth temperature or reducing the arsenic pressure during the InAs growth. When these growth conditions are not respected, the multicomponent photoluminescence spectrum obtained is explained through the common interpretation of island height varying with monolayer fluctuation. In optimized growth conditions, the multicomponent spectrum obtained is interpreted as emission from fundamental and excited levels of InAs islands with low height dispersion. Transmission electron microscopy (TEM) imaging shows that these InAs islands are stick-like, 50–100 nm in length and 22±1.2 nm in width. Cross-sectional TEM reveals f...

Influence of the Chemical Properties of the Substrate on Silicon Quantum Dot Nucleation

We have studied the influence of SiO 2 surface properties on the nucleation and growth of silicon quantum dots (Si-QDs) deposited by SiH 4 low-pressure chemical vapor deposition (LPCVD). First, the effect of siloxane groups (Si-O-Si) strain at the SiO 2 surface layer, characterized by Fourier transform infrared (FTIR) spectroscopy, is studied. We evidenced an increase of Si-QD nucleation with the strain of siloxane groups in the SiO 2 substrate layer. Second, the Si-QD nucleation strongly depends on the surface silanol group (Si-OH) density. This density, controlled by chemical and thermal treatments, is measured by multiple internal reflexion (MIR) FTIR. Very high Si-QD densities larger than 10 12 /cm 2 are obtained on highly hydroxylated SiO 2 .

Experimental and theoretical investigation of carrier confinement in InAs quantum dashes grown on InP(001)

Carrier confinement in InAs quantum dashes (QDas) grown on InP(001) is investigated both experimentally and theoretically. The aim of these studies is to reconstruct the electronic structure of the QDas. QDas with low size dispersion are achieved by improving growth conditions. Optical transitions between ground and excited states are studied by continuous-wave-photoluminescence and photoluminescence-excitation experiments at low temperature. We also report on infrared spectroscopy of conduction-band intersubband transitions. A simplified theoretical model is developed, yielding results consistent with the experimental data. Combining experimental and theoretical results, we propose an interpretation of the optical transitions occurring in these QDas, and we give a first theoretical absorption spectrum of these structures.

Optical properties of self-assembled InAs quantum islands grown on InP(001) vicinal substrates

We have investigated the effect of misorientated InP(001) substrates on the optical properties of InAs quantum islands (QIs) grown by molecular-beam epitaxy in the Stranski–Krastanow regime. Detailed temperature-dependent photoluminescence (PL), excitation density PL, and polarization of photoluminescence (PPL) are studied. PPL shows a high degree of linear polarization (near 40%) for the nominally oriented substrate n and for the substrate with 2° off miscut angle toward the [110] direction (2° F), while it is near 15% for the substrate with 2° off miscut angle towards [010] direction (2° B), indicating the growth of InAs quantum wires on nominal and 2° F substrates and of InAs quantum dots on 2° B substrate. These island shapes are confirmed by morphological investigations performed by atomic force microscopy. The integrated PL intensity remains very strong at room temperature, as much as 36% of that at 8 K, indicating a strong spatial localization of the carriers in the InAs QIs grown on InP(001).

The acceptor level of vanadium in III-V compounds

The positions of the acceptor level of vanadium in GaAs, GaP, and InP are estimated. This level is found to be at about Ec−0.14 eV in GaAs and about Ec−0.8 eV in GaP. It is above the conduction band edge in InP. These positions in the three hosts are in agreement with the trends proposed by Ledebo and Ridley. The finding of a very high acceptor level of vanadium in GaAs does not allow to explain the semi‐insulating behavior of GaAs:V. Other possible compensating centers are considered.

High quality, relaxed SiGe epitaxial layers for solar cell application

Abstract Epitaxially grown, relaxed Si 1− x Ge x layers with x≤0.1 were grown on a Si (100) substrate by means of reduced pressure chemical vapor deposition at a temperature of 750 or 800°C. The analysis carried out on the grown layers revealed a very high material quality indicated by the low density of dislocations (10 5 cm −2 ) and the high diffusion length which was deduced from the measurements of electron beam induced current (EBIC) performed on the as-grown layers. Transmission electron microscopy (TEM) measurements showed that the threading dislocation segments do not extend inside the layer but are rather confined to the Si/SiGe interface, which results in a low density of dislocations in the material. The processed solar cells made from these SiGe layers showed a higher infrared response than those made of a corresponding Si grown and processed under similar conditions. No degradation of the solar cell performance caused by the dislocations in the SiGe layers has been observed.

Influence of deep levels on CdZnTe nuclear detectors

We report deep level characterization on CdZnTe nuclear detectors grown by the high pressure Bridgman method. Three CdZnTe samples taken from the top, middle and tail of the same ingot were investigated using photo-induced current transient spectroscopy and thermoelectric-effect spectroscopy. Three major traps are detected and their influence on the nuclear detection properties are discussed.

Fe DEEP LEVEL OPTICAL SPECTROSCOPY IN InP

2+ Fe ) deep level have been measured by Deep Level Optical Spectroscopy in n-type Fe doped samples of InP. Optical transitions are interpreted as transitions from the Fe z÷ ground state to the F and L point minima of the conduction band for a°n(hv) and from the valence band to the ground and excited state for FeZ÷ for o°(hv) A theoretical model which accounts for the matn features of the experimental data is proposed.