Showing papers by "Eric Tournié published in 2003"

Nanoindentation of Si, GaP, GaAs and ZnSe single crystals

Abstract: We present a systematic study of the mechanical properties of bulk Si, GaP, GaAs and ZnSe semiconducting materials. Nanoindentation tests have been performed on the (001) surface of single crystals and their interpretation has been supported by AFM imaging of the indented surface. For all four materials, hardness and Young’s modulus appear to be independent of the orientation of the Berkovich nanoindenter tip relative to the [100] direction. In contrast, hardness varies significantly with the applied load for ZnSe, which is the only material exhibiting large creep. All mechanical properties—plasticity, hardness, Young’s modulus—depend on both the interatomic distance and the ionicity of the materials. Si and ZnSe are, respectively, the hardest and the softest materials of the series, the hardness of ZnSe being comparable to that of soft metals.

Annealing effects on the crystal structure of GaInNAs quantum wells with large In and N content grown by molecular beam epitaxy

Abstract: The impact of rapid thermal annealing on the optical emission of GaInNAs/GaAs quantum wells (QWs) grown by molecular beam epitaxy with high In and N content is shown to be highly dependent on the crystal structure of the QWs, as determined by transmission electron microscopy. Due to the presence of higher concentrations of nonradiative recombination centers, the annealing temperature required to obtain maximum photoluminescence emission is higher for the QW with strong structural modulation of the upper interface [at the onset of three-dimensional (3D) growth], intermediate for the two-dimensional (2D) grown QW with compositional fluctuations, and lower for the homogeneous 2D grown QW. Moreover, the transition from homogeneous 2D growth, to 2D growth with compositional fluctuations, and finally 3D growth, leads to progressively deeper carrier localization states below the conduction-band edge. Increasing annealing temperatures gradually shifts the localization states closer to the conduction-band edge, pr...

GaInNAs/GaAs quantum wells grown by molecular-beam epitaxy emitting above 1.5 μm

Abstract: We demonstrate that a careful optimization of the molecular-beam-epitaxy growth conditions allows us to obtain high-quality GaInNAs/GaAs quantum-well (QW) heterostructures exhibiting a perfect two-dimensional microstructure at high In and N contents. Room-temperature emission is achieved up to 1.61 and 1.51 μm for as-grown and annealed samples, respectively. High-resolution x-ray diffraction and transmission electron microscopy reveal that post-growth annealing does not affect the QW composition and width. This confirms that the GaInNAs semiconducting material is well suited for emission in the telecommunication wavelength range near 1.55 μm.

Interplay between the growth temperature, microstructure, and optical properties of GaInNAs quantum wells

Abstract: We investigated the influence of the growth temperature (Tgr) on the microstructure and on the optical properties of GaInNAs quantum wells (QWs). By comparing the structural information (transmission electron microscopy) with the optical properties (photoluminescence spectroscopy), we demonstrate that high photoluminescence efficiency of GaInNAs QWs is achieved only when the two-dimensional growth mode is preserved, which can be obtained at a low Tgr even for high In content. We also show composition modulations in the GaInNAs QWs, which can lead to the interface roughness.

Effect of nitrogen on the band structure and material gain of In/sub y/Ga/sub 1-y/As/sub 1-x/N/sub x/-GaAs quantum wells

Abstract: The conduction subband structure of InGaAsN-GaAs quantum wells (QWs) is calculated using the band anticrossing model, and its influence on the design of long-wavelength InGaAsN-GaAs QW lasers is analyzed. A good agreement with experimental values is found for the QW zone center transition energies. In particular, a different dependence of the effective bandgap with temperature when compared to the equivalent N-free structure is predicted by the model and experimentally observed. A detailed analysis of the conduction subband structure shows that nitrogen strongly decreases the electron energies and increases the effective masses. A very small N incorporation is also found to increase the nonparabolicity, but this effect saturates for higher nitrogen contents. Both the In content and well width decrease the effective masses and nonparabolicity of the conduction subbands. Material gain as a function of the injection level is calculated for InGaAsN-GaAs QWs for moderate carrier densities. The peak gain at a fixed carrier density is found to be reduced, compared to InGaAs, for a small N content, but this reduction tends to saturate when the N content is further increased. For the gain peak energy, a monotonous strong shift to lower energies is obtained for increasing N content, supporting the feasibility of 1.55-/spl mu/m emission from InGaAsN-GaAs QW laser diodes.

Percolation-based vibrational picture to estimate nonrandom N substitution in GaAsN alloys

Abstract: The number of N atoms in N-rich regions mostly due to nonrandom N incorporation in GaAsN (N∼4%), referred to as the Nr rate, is studied using a nonstandard Raman setup that addresses transverse symmetry. The Ga–N optical range shows a two-mode signal which discriminates between the N-poor (Np) and N-rich (Nr) regions. This is discussed via a percolation-based picture for Be-chalcogenide alloys, which exhibit mechanical contrast with regard to the shear modulus. This applies to GaAs–GaN even though the contrast is in the bulk modulus. The balance of Nr/Np strength provides a Nr rate of ∼30%, i.e., much larger than the corresponding Be rate of ∼4% in random Be-based alloys.

Isoelectronic traps in heavily doped GaAs:(In,N)

Abstract: GaAs samples doped with low indium and nitrogen contents were investigated by continuous-wave (CWPL) and time-resolved photoluminescence (TRPL) at low temperature (10 K). The simultaneous incorporation of doping amounts of both indium and nitrogen elements in GaAs creates new isoelectronic traps that are not present in indium-free GaAs:N. These traps are built from nitrogen-related defects of GaAs:N now perturbated by one additional indium atom. They are believed to involve a preferential orientation of the In atom and of one N atom in near-neighbor positions. The experiments are consistent with the assumption that the clusters are perturbed by a single In atom but that a definitive assignment cannot be given without addition theoretical modeling. Optical feature characteristics of the exciton bound to these kinds of isoelectronic traps are reported: (i) the exciton fine structure with dipole-forbidden and dipole-allowed states and (ii) the train of phonon replicas that is typical of isoelectronic traps. The observation of the tine structure of the trapped excitons is made possible because of the thermodynamic equilibrium between the two populations of excitons (dipole-allowed and dipole-forbidden states) in the conditions of our measurement. It gives us a direct measurement of the short-range exchange interaction (∼0.7 meV). The TRPL experiments allow us to characterize the exciton recombination dynamics for the different isoelectronic traps and notably the transfer mechanisms of excitons among these various traps.

Photoluminescence spectroscopy of Ga(In)NAs quantum wells for emission at 1.5 μm

Abstract: We report on the effects of post-growth annealing on the photoluminescence properties of Ga(In,N)As/GaAs(N) quantum wells with N and In concentrations in the range of 2.5% and 30–40%, respectively. The low growth temperature used to prevent the decomposition of the unstable alloy is mainly responsible for the high density of non-radiative defects. The concomitant presence of both In and N induces carrier localization in the quaternary alloy and a strong blueshift after post-growth annealing. Annealing reduces the non-radiative recombination centers and allows an atomic reorganization (responsible for the blueshift). After annealing, the smoothing of the band structure and the improved composition homogeneity induce a decrease of the localization. Finally, we demonstrate emission near 1.5 μm at room temperature.

LO phonon plasmon coupling and mechanical disorder-induced effect in the Raman spectra of GaAsN alloys

Abstract: Mechanical and chemical disorders are investigated in GaAsN alloys with N content close to the solubility limit, ∼2–3 %. One attractive characteristic, yet unexplored, of N-based III–V ternary alloys is that they consist in mixed crystals with a sharp contrast in the bulk modulus of the two constituents. Raman scattering is well-suited for its investigation because it addresses the force constant of the bond, which depends on the mechanical properties of the host matrix. In the Ga–N region, we observe a mode at 425 cm −1 , i.e. below the local mode of N in GaAs at 470 cm −1 . This mode has degenerated LO and TO components and seems to grow at fixed frequency with N content. From a direct comparison with Zn–Be chalcogenides, which exhibit a similar mechanical contrast, it is attributed to GaN vibrations in hard-like N-rich bounded regions, dispersed within the soft-like Ga-rich matrix. In the GaAs region we use Si-doping as a sensitive probe to investigate the structural quality close to the solubility limit. At low N content ( n ⩾10 18 cm −3 ), we observe similar LO phonon–plasmon (LOP) coupling as in GaAs:Si. Unexpectedly LOP coupling is disorder-cancelled at higher N contents while the pure LO and disorder-activated theoretically forbidden TO modes substitute for it. Disorder-induced red-side LO asymmetries are fairly accounted for by the spatial correlation model with Gaussian distribution and the fitted value of correlation length is a quantitative indication for the disorder rate.

Coexistence in photoluminescence of free exciton and bound exciton in low nitrogen content GaInNAs layers

Abstract: We study by time resolved photoluminescence (TRPL) low N and In content GaInNAs (GINA) alloy layers grown by molecular beam epitaxy on GaAs substrate. The TRPL experiments show the coexistence and the carrier exchanges between bound- and free-exciton states, in this kind of alloy. Temperature dependent experiments demonstrate the thermal ionization of bound excitons and the high temperature stability of free excitons. For a temperature higher than 60 K the PL spectra are totally dominated by the free exciton line. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)