R. Jakomin

Optical gain in single tensile-strained germanium photonic wire

We have investigated the optical properties of tensile-strained germanium photonic wires. The photonic wires patterned by electron beam lithography (50 μm long, 1 μm wide and 500 nm thick) are obtained by growing a n-doped germanium film on a GaAs substrate. Tensile strain is transferred in the germanium layer using a Si₃N₄ stressor. Tensile strain around 0.4% achieved by the technique corresponds to an optical recombination of tensile-strained germanium involving light hole band around 1690 nm at room temperature. We show that the waveguided emission associated with a single tensile-strained germanium wire increases superlinearly as a function of the illuminated length. A 20% decrease of the spectral broadening is observed as the pump intensity is increased. All these features are signatures of optical gain. A 80 cm⁻¹ modal optical gain is derived from the variable strip length method. This value is accounted for by the calculated gain material value using a 30 band k · p formalism. These germanium wires represent potential building blocks for integration of nanoscale optical sources on silicon.

Control of tensile strain in germanium waveguides through silicon nitride layers

Germanium ridge waveguides can be tensilely strained using silicon nitride thin films as stressors. We show that the strain transfer in germanium depends on the width of the waveguides. Carrier population in the zone center Γ valley can also be significantly increased when the ridges are oriented along the 〈100〉 direction. We demonstrate an uniaxial strain transfer up to 1% observed on the room temperature direct band gap photoluminescence of germanium. The results are supported by 30 band k·p modeling of the electronic structure and the finite element modeling of the strain field.

High quality tensile-strained n-doped germanium thin films grown on InGaAs buffer layers by metal-organic chemical vapor deposition

We show that high quality tensile-strained n-doped germanium films can be obtained on InGaAs buffer layers using metal-organic chemical vapor deposition with isobutyl germane as germanium precursor. A tensile strain up to 0.5% is achieved, simultaneously measured by x-ray diffraction and Raman spectroscopy. The effect of tensile strain on band gap energy is directly observed by room temperature direct band gap photoluminescence.

Direct and indirect band gap room temperature electroluminescence of Ge diodes

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.

Effect of increasing thickness on tensile-strained germanium grown on InGaAs buffer layers

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...

Stimulated emission in single tensile-strained Ge photonic wire

We have investigated the room temperature optical properties of 50 µm length, 11 µm large and 500 nm thick tensile-strained germanium photonic wires. Tensile strain is transferred in the germanium layer using a Si3N4 film stressor. An optical recombination of tensile-strained germanium involving light hole band is observed around 1690 nm at room temperature corresponding to a strain magnitude around 0.4%. We show that the waveguided emission associated with a single tensile-strained germanium wire increases superlinearly as a function of the illuminated length. A decrease of the spectral broadening is observed as the pump intensity is increased. A 80 cm−1 modal optical gain is derived from the variable strip length method.

First results on the apollon project multi-approach for high efficiency integrated and intelligent concentrating PV modules (systems)

Next generation concentrating photovoltaic technologies could have a large-scale impact on world electricity production once they will become economically attractive and grid parity will be reached. To proceed towards this important goal, a new large integrated project, APOLLON, has started in July 2008, within the frame of the 7th European Framework program, having the main objective of substantial decrease the Concentrating Photovoltaic (CPV) technology cost to a target value of 2 Euro/W. This ambitious objective is targeted to be reached after five years of research and technological activities in which, both ¿point focus¿ and ¿dense array¿ CPV technologies will be implemented by facing all the technology-critical issues related to each component of the CPV systems. With this contribution we report the principal results obtained during the first year of the project regarding Multi-Junction (MJ) solar cells, concentrator optics, assembling, tracking and testing.

Tensile-strain and n-type doping of germanium-on-insulator : Towards a Ge laser

Using a 30 band k.p formalism, we calculate the band structure and the optical gain of tensilely-strained germanium. We show that the room temperature emission of germanium-on-insulator can be significantly enhanced by n-doping of germanium.

Mechanical tensile strain engineering of Ge for gain achievement

We show that the recombination energy of the direct band gap photoluminescence of germanium can be controlled by an external mechanical stress. The deformation is provided by an apparatus used for blister test. This strain leads to a significant change of the room temperature direct band gap recombination of germanium. An energy red-shift up to 60 meV is demonstrated for the room temperature photoluminescence of a thin germanium membrane (125 nm wavelength shift from 1535 to 1660 nm). This photoluminescence shift is correlated to the inplane tensile strain present in the film. A biaxial tensile strain larger than 0.6 % is achieved by this method. The wavelength shift is correlated to the predicted band gap shift as obtained from a 30 band k. p formalism. An excellent agreement is obtained between the experimental band gap shift and the theoretical one. This mechanical deformation allows to approach the direct band gap condition for germanium. We will discuss the possibility to achieve lasing with n-doped layers and the application of an external mechanical stress.