P. Boucaud

Band‐edge and deep level photoluminescence of pseudomorphic Si1−x−yGexCy alloys

Photoluminescence of strained Si1−x−yGexCy alloys grown by rapid thermal chemical vapor deposition on Si(100) is investigated. Two dominant features are reported: At low pump intensities, the photoluminescence is dominated by a deep level broad luminescence peak around 800 meV whereas at high pump intensities, a well‐resolved band‐edge luminescence (no phonon and transverse optic replica) is observed. At 77 K, we attribute this band‐edge feature to an electron‐hole plasma luminescence of the ternary alloy. The dependences of the deep level and band‐edge peaks versus the excitation power density are, respectively square‐root‐like or superlinear. A blue shift of the energy gap of Si1−x−yGexCy alloys with respect to Si1−xGex alloy is observed. The blue shift increase with carbon content corresponds to what is expected for the bulk alloy. An eventual influence of the strain relaxation cannot be excluded.

Intraband absorption in Ge/Si self-assembled quantum dots

We have observed intraband absorption in Ge/Si self-assembled quantum dots. The self-assembled quantum dots are grown at 550u200a°C by chemical vapor deposition. Atomic force microscopy shows that the quantum dots have a square-based pyramidal shape (≈100 nm base length) and a densityu2002≈2×109u200acm−2. Intraband absorption in the valence band is observed around 300 meV (4.2 μm wavelength) using a photoinduced spectroscopy technique. The intraband absorption is in-plane polarized. It is attributed to bound-to-continuum transitions since the intraband energy corresponds to the energy difference between the Si band gap and the photoluminescence energy of the quantum dots. The magnitude of the intraband absorption saturates when the ground level of the quantum dots is filled. This feature allows the measurement of the in-plane absorption cross section of the intraband transition which is found as large as 2×10−13u200acm2.

Two-dimensional photonic crystals with Ge/Si self-assembled islands

Two-dimensional photonic crystals were fabricated on silicon-on-insulator waveguides with self-assembled Ge/Si islands deposited on top of the upper silicon layer. The photonic crystals consist of triangular lattices of air holes designed to exhibit a forbidden band around 1.5 μm. Different hexagonal photonic crystals microcavities were processed whose optical properties are probed at room temperature with the Ge/Si island photoluminescence. Quality factors larger than 200 are measured for hexagonal H3 cavities. A significant enhancement of the Ge/Si island photoluminescence is achieved in the 1.3–1.55 μm spectral region using the photonic crystal microcavities. We show that the energy resonance of the defect modes can be tuned with the filling factor of the photonic crystal.

Intersubband stimulated emission in GaAs/AlGaAs quantum wells: Pump-probe experiments using a two-color free-electron laser

Intersubband stimulated emission under optical pumping has been observed in the conduction band of GaAs–AlGaAs quantum wells. The asymmetric coupled quantum wells which exhibit three conduction bound levels are designed to exhibit population inversion under optical pumping. An optical excitation at λ=9.2u2009μm is used to bleach the absorption between the ground and second excited subband. The population inversion between excited subbands is pumped and probed on a picosecond time scale by a tunable two-color free-electron laser. The stimulated amplification is studied at low temperature in infrared waveguides as a function of the waveguide length and of the probe wavelength. A stimulated gain ≈80u2009cm−1 is measured at 12.5 μm in agreement with calculations.

Photoluminescence of strained Si1−yCy alloys grown at low temperature

The photoluminescence of strained Si1−xCx alloys grown at low temperature by rapid thermal chemical vapor deposition is investigated. The photoluminescence spectra are mainly characterized by a deep level broadband at low energy which subsists up to room temperature. This low energy emission is associated with the low‐temperature growth process required for the incorporation of carbon into substitutional sites. The stability of the layers after thermal annealing is monitored using this low energy radiative recombination. A blue shift of the photoluminescence energy peak is observed and the peak intensity presents a maximum versus annealing time. The blue shift and the associated increased linewidth is explained in terms of the local strain induced band‐gap fluctuations. Infrared transmission spectra of the annealed samples suggest that silicon carbide precipitates appear during the anneal and that oxygen and carbon complexes contribute to the formation of the deep level band.

Photoluminescence and intersubband absorption spectroscopy of interdiffused Si/SiGe quantum wells

The interdiffusion and thermal stability of narrow Si/SiGe multi‐quantum wells is investigated by photoluminescence and intersubband spectroscopy. The photoluminescence exhibits a blueshift as a function of the temperature of annealing. The activation energy of the intermixing process and the interdiffusion coefficient are deduced from the photoluminescence shift versus temperature of the anneal. The intersubband absorption is measured by photoinduced infrared spectroscopy on the interdiffused samples for light polarized perpendicular (z polarization) or parallel (x polarization) to the layer plane. In z polarization, the absorption of annealed samples exhibits a redshift respective to the as‐grown sample which is enhanced as more levels are confined in the well. The magnitude of this shift is in good agreement with simulations based on the data obtained by photoluminescence experiments. The redshift of the intersubband absorption in x polarization is lower than in z polarization due to the lower dependen...

SECOND-HARMONIC GENERATION IN ASYMMETRIC SI/SIGE QUANTUM WELLS

The observation of infrared second‐harmonic generation in asymmetric Si/SiGe p‐doped quantum wells is reported. The generated signal stems entirely from valence intersubband transitions, since bulk Si, with an inversion symmetric crystal structure, has a zero second‐order susceptibility. The experiments were performed using a Q‐switched CO2 laser operating at 10.56 μm and give a nonlinear susceptibility of 5×10−8 m/V.

Intersubband absorption of GaAs/AlGaAs quantum wells in MBE grown mid-infrared slab waveguides

A structure to enhance the absorbance due to intersubband transitions in GaAs/AlGaAs quantum wells is discussed. Mid-infrared slab waveguides including 30 quantum wells were grown using molecular-beam epitaxy (MBE). Photoluminescence experiments revealed an excellent uniformity of the samples. Absorption measurements over the whole 9-13.4- mu m spectral range were performed for the first time using the combination of CO/sub 2/ and NH/sub 3/ lasers. Effective absorbance due to intersubband transitions as high as 14 dB were measured for 3-mm-long waveguides. The waveguide structure is expected to be a good candidate for optoelectronic devices in the 10- mu m region.<<ETX>>

Strong 1.3–1.5 μm luminescence from Ge/Si self-assembled islands in highly confining microcavities on silicon on insulator

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.

Low temperature electroluminescence spectroscopy of high electron mobility transistors on InP

Electroluminescence spectroscopy of short gate high‐electron‐mobility transistors (HEMTs) on InP substrates is performed at cryogenic temperatures. Electroluminescence is a reliable tool to investigate impact ionization as compared to studies based on gate current which depend on the weakness of the intrinsic gate current intensity. In on‐state biased devices, a low energy (0.7–0.9 eV) recombination band is observed which is related to radiative recombination of carriers created by impact ionization in the low band gap InGaAs channel. The evolution of the luminescence intensity versus bias applied to the device shows that the electroluminescence intensity and impact ionization depend on two competing parameters: the electric field in the gate–drain access area and the drain current intensity. We show that the so‐called ‘‘kink’’ effect, which is a noticeable increase of the output conductance and which is observed at relatively moderate drain bias (600–750 mV) in our devices, is not correlated with impact ...