V. Le Thanh

Surface electron-diffraction patterns of β-FeSi2 films epitaxially grown on silicon

Semiconducting β‐FeSi2 is drawing much current research interest because of hoped‐for silicon‐based optoelectronics applications. The study of heteroepitaxial film growth on silicon depends heavily upon several transmission and reflection electron‐diffraction techniques. Because of the complicated crystal structure of this material, the possibility of competing heteroepitaxial relationships, the propensity for formation of epitaxial variants by rotation twinning, and the uncertainty in the crystalline surface nets, the analysis of experimental diffraction patterns is complicated. A theoretical reference for a number of fundamental electron‐diffraction patterns is provided and they are illustrated with a broad range of experimentally obtained patterns from the surfaces of epitaxial films. In situ transmission reflection high‐energy electron diffraction (RHEED) (transmission electron diffraction with conventional RHEED instrumentation), from rough but epitaxial films, is of great utility and quite feasible ...

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 550 °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 density ≈2×109 cm−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 2×10−13 cm2.

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.

Midinfrared photoconductivity of Ge/Si self-assembled quantum dots

We have investigated the midinfrared photoconductivity of Ge/Si self-assembled quantum dots. The self-assembled quantum dots were grown by ultra-high-vacuum chemical vapor deposition on Si(001). The photoresponse of the p-type device exhibits resonances in the midinfrared around 10 μm wavelength. The resonance of the photocurrent shifts to lower energy as the applied bias increases. The photocurrent is weakly dependent on the incoming polarization of the infrared light. The photocurrent is analyzed in terms of bound-to-bound and bound-to-continuum transitions in the valence band. The photocurrent peaks are correlated to the photoluminescence of the device.

Control of tensile strain and interdiffusion in Ge/Si(001) epilayers grown by molecular-beam epitaxy

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.

Strain and composition of capped Ge/Si self-assembled quantum dots grown by chemical vapor deposition

We have investigated the composition and the strain profile of Ge/Si self-assembledquantum dots. The quantum dots, grown by low-or high-pressurechemical vapor deposition, were covered by a silicon cap layer. The composition and the strain were measured by the selected area transmission electron diffraction of a singlequantum dot. The self-assembledquantum dots exhibit a quadratic deformation. No lateral relaxation of the lattice is observed from the main part of the quantum dot. An average composition of Ge around 50% is deduced. The average composition is found dependent on the size of the islands. This composition is correlated to the photoluminescence energy.

Interlevel transitions and two-photon processes in Ge/Si quantum dot photocurrent

Photocurrent spectra due to interlevel transitions of holes in Ge/Si quantum dots show several peaks in the range of 60–300 meV, which superlinearly increase with bias, indicating release of carriers by tunneling. The relative peak intensity drastically changes with applied voltage, its polarity, and the measurement system. Lower energy peaks, at 69 and 86 meV, are observed only with a Fourier transform IR (FTIR) spectrometer. The 69 and 86 meV transitions excite holes into intermediate levels from which they are re-excited to shallow levels in a two-photon process. This is observed with FTIR only where the sample is simultaneously exposed to a wide range of energies. Direct band-to-band excitation at 1.25 eV increases the midinfrared signals by orders of magnitude by pumping the intermediate levels. Placing dopants in the barrier greatly increases photocurrent intensity and reduces noise. One-dimensional and three-dimensional numerical analyses confirm our findings.

Experimental evidence for index modulation by carrier depletionin SiGe∕Si multiple quantum well structures

Experimental results for the refractive index variation obtained by hole depletion in SiGe∕Si multiple quantum wells inserted in a reverse-biased p-i-n junction are reported. The electronic contribution to the index variation is unambiguously separated from the thermal one. Measured refractive index changes around 4.2×10−5V−1 are in quite good agreement with modeling.

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.

Polarized Raman spectroscopy of multilayer Ge/Si(001) quantum dot heterostructures

Polarized Raman spectroscopy in backscattering geometry has been applied here for the investigation of Ge∕Si(001) quantum dot multilayer structures (with the number of layers ranging from 1 to 21) grown by the Stranski-Krastanov technique. The characteristic Raman spectra of the dots have been obtained by taking the difference between the Raman spectra of the dot sample and the reference Si substrate, taken under the same excitation/scattering conditions. We found that the Raman spectra of Ge∕Si dots obtained in such a manner are strongly polarized, in particular, for the Ge-Ge (at ∼295cm−1) and Si-Ge (at ∼413cm−1) vibrational modes. The dependence of peak intensity and peak position of the Ge-Ge and Ge Raman bands versus the number of dot layers has been analyzed. It was found that studied quantum dot (QD) systems possess prominent anisotropic intermixing. This results in the Si content in the dots being high and this increases with the number of QD layers. At the same time, the increase of the number of...