We report on a strain-induced phase transformation in Ge nanowires under external shear stresses. The resulted polytype heterostructure may have great potential for photonics and thermoelectric applications. ⟨111⟩-oriented Ge nanowires with standard diamond structure (3C) undergo a phase transformation toward the hexagonal diamond phase referred as the 2H-allotrope. The phase transformation occurs heterogeneously on shear bands along the length of the nanowire. The structure meets the common phenomenological criteria of a martensitic phase transformation. This point is discussed to initiate an on going debate on the transformation mechanisms. The process results in unprecedented quasiperiodic heterostructures 3C/2H along the Ge nanowire. The thermal stability of those 2H domains is also studied under annealing up to 650 °C by in situ TEM.

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Single junction Si solar cells dominate photovoltaics but are close to their efficiency limits. This paper presents ideal limiting efficiencies for tandem and triple junction multijunction solar cells subject only to the constraint of the Si bandgap and therefore recommending optimum cell structures departing from the single junction ideal. The use of III-V materials is considered, using a novel growth method capable of yielding low defect density III-V layers on Si. In order to evaluate the real potential of these proposed multijunction designs, a quantitative model is presented, the strength of which is the joint modelling of external quantum efficiency and current-voltage characteristics using the same parameters. The method yields a single parameter fit in terms of the Shockley-Read-Hall lifetime. This model is validated by fitting experimental data of external quantum efficiency, dark current, and conversion efficiency of world record tandem and triple junction cells under terrestrial solar spectra without concentration. We apply this quantitative model to the design of tandem and triple junction solar cells, yielding cell designs capable of reaching efficiencies without concentration of 32% for the best tandem cell and 36% for the best triple junction cell. This demonstrates that efficiencies within a few percent of world records are realistically achievable without the use of concentrating optics, with growth methods being developed for multijunction cells combining III-V and Si materials.

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The growth of germanium at low temperature by ultrahigh vacuum chemical vapor deposition on Si(001) is investigated in real time by reflection high-energy electron diffraction. These observations are complementarily checked by atomic force microscopy, Rutherford backscattering spectrometry, transmission electron microscopy, and x-ray diffraction experiments. It can be seen that the currently observed Stranski–Krastanov-related two-dimensional (2D) to three-dimensional transition is avoided at 330 °C and that the major part of the relaxation process occurs during the deposition of the first two monolayers. Then, the measured in-plane lattice parameter evolves slowly and approaches that of bulk Ge after deposition of 50 monolayers. The corresponding relaxation equals 83%. The resulting surface is flat, with a rms roughness of 0.55 nm. The relaxation is found to be mainly due to misfits dislocations located at the Ge/Si interface. Regrowth experiments at 600 °C show that the low-temperature films are not stable for thicknesses below 27 nm. In spite of the nearly complete relaxation observed for 7.5 nm, a much higher thickness is needed to enable a continuous 2D growth at 600 °C. Finally, a 500-nm-thick film, obtained with a low-temperature Ge buffer and with a Ge regrowth at high temperature, exhibits a channeling-to-random Rutherford backscattering spectrometry ratio (min) of 4%, which indicates a good crystalline quality. ©2005 American Institute of Physic

We have investigated the photoluminescence of single and multiple layers of Ge/Si self-assembled quantum dots as a function of the excitation power density. We show that the photoluminescence of the quantum dots is strongly dependent on the pump excitation power. The photoluminescence broadens and is blueshifted by as much as 80 meV as the power excitation density increases. Meanwhile, the photoluminescence associated with the two-dimensional wetting layers exhibits only a weak dependence on the pump excitation power. This significant blueshift is interpreted in terms of state filling and recombination from the confined excited hole states in the dots. The photoluminescence data are correlated to the density of states as calculated by solving the three-dimensional Schrödinger equation in these islands with a lateral size of the order of 100 nm.

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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.