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Many high-field/attosecond and ultrafast electron diffraction/microscopy experiments on condensed matter require samples in the form of free-standing membranes with nanometer thickness. Here, we report the measurement of the laser-induced damage threshold of 11 different free-standing nanometer-thin membranes of metallic, semiconducting, and insulating materials for 1-ps, 1030-nm laser pulses at 50 kHz repetition rate. We find a laser damage threshold that is very similar to each corresponding bulk material. The measurements also reveal a band gap dependence of the damage threshold as a consequence of different ionization rates. These results establish the suitability of free-standing nanometer membranes for high-field pump-probe experiments. [ABSTRACT FROM AUTHOR]

The structural and optical characteristics of Ge/Si self-assembled quantum dots epitaxially grown on Si(001) were studied. The Ge islands were grown in an industrial 200 mm single-wafer chemical vapor deposition reactor. The islands showed a maximum photoluminescence at 1.55 microns wavelength. The photoluminescence energy was correlated to the three-dimensional quantum confinement energy and to the size and geometry of the clusters.

A study was conducted on GaAs/AlGaAs coupled quantum well mid-infrared intersubband stimulated emission under optical pumping. Two-color picosecond free-electron laser-based time-resolved pump-probe experiments were employed for gain measurements. The stimulated gain exhibited broadening caused by mid-infrared waveguide coupled high intensities. The stimulated gain may be applied in the development of long-wavelength intersubband lasers under optical pumping.

Experimental techniques help investigate the electroluminescence of standard and pseudomorphic (PM) InGaAs high-electron-mobility transistors between 0.7 and 1.6 eV at room and cryogenic temperatures. An interaction between the DX centers and hot holes in the AlGaAs barrier accounts for the broad band at 0.8-0.9 eV for PM transistors. A peak arises due to the GaAs substrate, while the InGaAs layer subbands account for the observed doublet in PM transistors.

The optical emission of germanium-based luminescent and/or laser devices can be enhanced by tensile strain and n-type doping. In this work, we study by simulation the interplay between electrical transport and optical gain in highly n-doped and intrinsic germanium p-n heterostructure diodes under tensile strain. The effects of strain and doping on carrier mobilities and energy distribution are taken into account. Whereas the n-doping of Ge enhances the filling of the indirect L and Brillouin zone-center conduction band states, the n-doping also reduces the carrier injection efficiency, which is detrimental for the achievement of optical gain at reduced current densities. For applied biaxial strains larger than 1.25%, i.e., far before reaching the cross-over from indirect to direct band gap regime, undoped germanium exhibits a lower optical gain threshold as compared to doped germanium. We also show that the threshold current needed to reach transparency in germanium heterostructures has been significantly underestimated in the previous works. [ABSTRACT FROM AUTHOR]

Tensile-strained and n-doped Ge has emerged as a potential candidate for the realization of optoelectronic devices that are compatible with the mainstream silicon technology. Tensile-strained Ge/Si epilayers can be obtained by using the difference of thermal expansion coefficients between Ge and Si. We have combined various surface, structural, and compositional characterizations to investigate the growth mode and the strain state in Ge/Si epilayers grown by molecular-beam epitaxy. The Ge growth was carried out using a two-step approach: a low-temperature growth to produce relaxed and smooth buffer layers, which is followed by a high-temperature growth to get high quality Ge layers. The existence of a substrate temperature window from 260 to 300 °C is evidenced, which allows to completely suppress the Ge/Si Stranski-Krastanov growth. As a consequence of the high temperature growth, a tensile strain lying in the range of 0.22%-0.24% is obtained. Concerning the effect of thermal annealing, it is shown that cyclic annealing may allow increasing the tensile strain up to 0.30%. Finally, we propose an approach to use carbon adsorption to suppress Si/Ge interdiffusion, which represents one of the main obstacles to overcome in order to realize pure Ge-based optoelectronic devices. [ABSTRACT FROM AUTHOR]

We have investigated the optical properties of tensile-strained germanium grown on InGaAs buffer layers as a function of film thickness and buffer layer composition. We study the dependence of the photoluminescence as a function of the strain amplitude and degree of relaxation which are also monitored by X-ray diffraction and Raman spectroscopy. We show that 0.75% biaxially strained germanium can be obtained up to a thickness of 150 nm, a value sufficiently high to allow confinement of the spontaneous emission in a guiding structure. For large thicknesses (>200 nm) and large indium content in the buffer layer, a partial relaxation of the film is observed characterized by a large in-plane anisotropy of the germanium lattice. In this case, a difference of strain magnitude deduced either by microphotoluminescence spectra or by X-ray or Raman measurements is reported. We explain this difference by the sensitivity of microphotoluminescence to the local properties of the material. This study provides guidelines in order to achieve high optical quality and high biaxial tensile strain in Ge films with thicknesses compatible with optical waveguiding. [ABSTRACT FROM AUTHOR]

Germanium is a promising material for electrically pumped light emitters integrated on silicon. In this work, we have investigated the room temperature electroluminescence of pure germanium diodes grown by metal organic chemical vapor deposition. The dependence of the optical response of the p-n diodes is studied as a function of the injected current. Both direct and indirect band gap recombinations are observed at room temperature around 1.6 and 1.8 μm. The amplitude of the direct band gap recombination is equivalent to the one of the indirect band gap. [ABSTRACT FROM AUTHOR]

We have investigated the bound-to-continuum absorption of InGaAs quantum dots as a function of n-doping. We show that the combination of multipass waveguide geometry, large number of quantum dot layers, and spectrally broad absorption leads to significant spectral oscillations on the absorption spectra. The oscillations result from the standing wave pattern caused by the interference of optical beams incident and totally reflected at the sample/air interface. The spectral modulations on the absorption spectra should not be attributed to resonant intersublevel transitions. [ABSTRACT FROM AUTHOR]

We have investigated the band structure of tensile-strained germanium using a 30 band k·p formalism. This multiband formalism allows to simultaneously describe the valence and conduction bands, including the L, Δ, and Γ valleys. We calculate the energy band variation as a function of strain and obtain that the crossover from indirect to direct band gap occurs for a tensile in-plane strain of 1.9%. The effective masses of density of states are deduced from the calculated conduction and valence band density of states. Significant deviations are observed as compared to the effective masses of density of states values of unstrained bulk germanium. We finally calculate the optical gain that can be achieved with tensile-strained bulk germanium. An optical gain larger than 3000 cm-1 is predicted for a carrier density of 1×1018 cm-3 and a 3% in-plane biaxial strain. This optical gain is larger than the one of GaAs calculated with the same formalism and is much larger than the experimental free-carrier absorption losses. This gain should be sufficient to achieve lasing in these structures. [ABSTRACT FROM AUTHOR]

This contribution reports the metal organic vapor phase epitaxy of InAsP/InP(001) quantum dots with a voluntary V-alloying obtained owing to an additional phosphine flux during InAs quantum dot growth. The quantum dots were studied by photoluminescence and transmission electron microscopy. We show that the additional phosphine flux allows to tune quantum dot emission around 1.55 μm while improving their optical properties. The comparison of the optical and structural properties of the InAsP quantum dots allows to deduce their phosphorus composition, ranging from 0% to 30% when the phosphine/arsine flow ratio is varying between 0 and 50. On the basis of the compositions deduced, we discuss on the effects of the phosphine flow and of the alloying on the quantum dot growth, structural, and optical properties. [ABSTRACT FROM AUTHOR]

We report the study of two-dimensional photonic crystals fabricated on silicon-on-insulator substrates. Ge/Si self-assembled islands are embedded as an active internal optical source inside the photonic crystals. We present a detailed analysis of photonic crystal microcavities and waveguides using the room-temperature Ge/Si island photoluminescence. The tunability of the microcavity resonant emission is demonstrated between 1.2 and 1.5 μm. We show that the microcavity photoluminescence is weakly dependent on the temperature. The polarized transmission properties of W1 single-line defect waveguides are investigated using the photoluminescence as an internal source. The transmission spectra are correlated to those given by two-dimensional finite-difference time-domain calculations. [ABSTRACT FROM AUTHOR]

We have performed an electroabsorption spectroscopy of Ge/Si self-assembled islands simultaneously in the near-infrared and in the midinfrared spectral range. The investigated structure consists of self-assembled Ge/Si islands embedded in a p-i-n junction. This active region is inserted into a 3-μm-thick Si0.98Ge0.02 waveguiding layer. Under a positive applied bias, the injected carriers give rise to a current-induced absorption resonant at 185 meV along with an enhanced transmission around 800 meV. The 185-meV resonance is polarized along the growth axis of the islands. The assignment of the optical transitions is made on the basis of a 14-band quantum well k·p calculation. We show that the midinfrared electroabsorption of the islands is associated with a bound-to-continuum transition between the ground states and the wetting layer states. The enhancement of the transmission is correlated to the bleaching of the interband absorption which results from hole injection in the islands. The carrier density and the parameters governing the carrier dynamics in the islands are deduced from the midinfrared modulation amplitude. An Auger recombination coefficient in the islands, C=1.6×10-30 cm6 s-1, is deduced at room temperature. [ABSTRACT FROM AUTHOR]

We report dramatic enhancement of 1.3-1.5 µm room-temperature emission from self-assembled Ge/Si islands in highly confining microcavities on silicon on insulator. The microcavities are fabricated either by creating defects in two-dimensional silicon-based photonic crystals or by etching the silicon layer in order to form isolated micropillars. The optical emission is characterized by nonlinear evolution with pump power, the nonlinearity being more pronounced as the microcavity size is reduced. Both the nonlinearity and luminescence extraction are enhanced in photonic crystals with large air filling factors. The results are interpreted in terms of carrier localization. The luminescence extracted is more than two orders of magnitude higher than that of the unprocessed sample while it is 1% that of a single InGaAs quantum well. This system appears to be a promising alternative for microsources on silicon at telecommunication wavelengths that are fully compatible with silicon-based processing technologies. [ABSTRACT FROM AUTHOR]

We have investigated a silicon-based near-infrared photodetector using a waveguide with strong optical confinement. The high-difference index waveguide is obtained with a silicon-on-insulator substrate. The optically active region consists of self-assembled Ge/Si islands embedded in a p-i-n junction. The Ge/Si islands grown by high-pressure chemical-vapor deposition exhibit a broad photoluminescence and electroluminescence which are resonant around 1.5 µm. The photoluminescence and electroluminescence energies are correlated to the island size and to the island composition using a six-band k-p calculation. The spectral responsivity of the detectors is measured in a front facet coupling geometry with a broadband source and with semiconductor laser diodes. For a 0 V applied bias, responsivities of 25 and 0.25 mA/W are measured at room temperature at 1.3 and 1.55 µm, respectively. [ABSTRACT FROM AUTHOR]

A 30 band k*p formalism which allows to simultaneously describe the valence and conduction bands is utilized to study the band structure of tensile-strained germanium. The calculated optical gain obtained for tensile-strained bulk germanium which is larger than those of GaAs calculated with the same formalism and the experimental free-carrier absorption losses could be sufficient to achieve lasing in the germanium structure.

The metal organic vapor phase epitaxy of InAsP/InP(001) quantum dots with a voluntary V-alloying obtained due to an additional phosphine flux during InAs quantum dot growth is reported. The results from the study offer insight into the effects of the phosphine flow and of the alloying on the quantum dot growth, structural, and optical properties.

A report is presented on the study of two-dimensional photonic crystals fabricated on silicon-on-insulator substrates. A detailed analysis is provided of photonic crystal structures by using the room temperature photoluminescence of Ge/Si self-assembled islands as an internal source.

A combination of Ge/Si self-assembled islands with microcavities on silicon on insulator leads to enhanced emission at room temperature in 1.3-1.55 mu m spectral range. This system appears to be a promising alternative for microsources on silicon telecommunication wavelengths.

Presents a study which investigated the dependence of infrared photoinduced intraband absorption in undoped InAs/GaAs self-assembled quantum dots. Measurement of the intraband absorption by photoinduced spectroscopy; Definition of self-assembled quantum dots; Methodology used to conduct the study; Results of the study.

Investigates the midinfrared absorption between confined levels of undoped indium arsenide/gallium arsenide quantum dots. Analysis of quantum dots as a function of temperature, interband pump photon energy, intensity and infrared polarization; Attribution of infrared resonances to intraband transitions.