A. Ghrib

Tensile Ge microstructures for lasing fabricated by means of a silicon complementary metal-oxide-semiconductor process

In this work we study, using experiments and theoretical modeling, the mechanical and optical properties of tensile strained Ge microstructures directly fabricated in a state-of-the art complementary metal-oxide-semiconductor fabrication line, using fully qualified materials and methods. We show that these microstructures can be used as active lasing materials in mm-long Fabry-Perot cavities, taking advantage of strain-enhanced direct band gap recombination. The results of our study can be realistically applied to the fabrication of a prototype platform for monolithic integration of near infrared laser sources for silicon photonics.

Strain analysis in SiN/Ge microstructures obtained via Si-complementary metal oxide semiconductor compatible approach

We have analyzed the strain distribution and the photoluminescence in Ge microstructures fabricated by means of a Si-CMOS compatible method. The tensile strain in the Ge microstructures is obtained by using a SiN stressor layer. Different shapes of microstructure, allowing the Ge layers to freely expand into one, two, or three dimensions, resulted in different strain distribution profiles. Maximal equivalent biaxial tensile strain values up to ∼0.8% have been measured. Room temperature photoluminescence emission has been observed and attributed to direct-band gap recombination spectrally shifted by tensile strain.

Tensile-strained germanium microdisks

We show that a strong tensile strain can be applied to germanium microdisks using silicon nitride stressors. The transferred strain allows one to control the direct band gap emission that is shifted from 1550 nm up to 2000 nm, corresponding to a biaxial tensile strain around 1%. Both Fabry-Perot and whispering gallery modes are evidenced by room temperature photoluminescence measurements. Quality factors up to 1350 and limited by free carrier absorption of the doped layer are observed for the whispering gallery modes. We discuss the strain profile in the microdisks as a function of the disk geometry. These tensile-strained microdisks are promising candidates to achieve Ge laser emission in compact microresonators.

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.

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.

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...

Schottky electroluminescent diodes with n-doped germanium

n-doped germanium can be used as an active material for the realization of an optical source under electrical pumping. We propose to use Schottky contacts for germanium electroluminescent devices, and we show that carrier injection and electroluminescence in these Schottky devices can be optimized by depositing a thin Al2O3 interfacial layer on top of n-doped germanium. In the latter case, hole injection is optimized due to the drastic decrease of interface trap densities and room-temperature electroluminescence can be observed at small current injection with a higher differential efficiency as compared to the standard Schottky sample.

Making germanium, an indirect band gap semiconductor, suitable for light-emitting devices*

Germanium (Ge) is a group-IV indirect band gap semiconductor but the difference between its direct and indirect band gap is only 140 meV. It has been shown that when Ge is subjected to a tensile strain and a heavy n-doping level, room-temperature photoluminescence (PL) can be greatly enhanced. Among these two factors, achieving a heavy n-doping level in Ge (i.e., electron concentrations higher than 1 × 1019 cm−3) is a challenge since the solubility of most group-V elements (P, As, Sb) in Ge is very low. We report here Ge growth on silicon substrates using molecular beam epitaxial (MBE) technique. To enhance the n-doping level in Ge, a specific n-doping process based on the decomposition of the GaP compound has been implemented. The GaP decomposition allows producing P2 molecules, which have a higher sticking coefficient than that of P4 molecules. We show that phosphorus doping at low substrate temperatures followed by flash thermal annealing are essential to get a high doping level. We have obtained an activate phosphorus concentration up to 2 × 1019 cm−3 and room-temperature PL measurements reveal an intensity enhancement up to 50 times. This result opens a new route for the realization of group-IV semiconductor optoelectronic devices.

Tensile Strained Ge Layers Obtained via a Si-CMOS Compatible Approach

Although rapid advances in Si photonics over the last decade has enabled mass production of higher functionality and lower cost photonic components (such as waveguides, couplers, modulators, photodetectors, etc..) integrated with both digital and analog circuitry in silicon complementary metal oxide semiconductor technology (Si-CMOS), an efficient electrically-pumped light emitter integrated in the Si-CMOS has so far been considered the Holy Grail of the monolithic electronics-photonics integration.