M. Prost

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.

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...

Analysis of optical gain threshold in n-doped and tensile-strained germanium heterostructure diodes

The optical emission of germanium-based luminescent and/or laser devices can be enhanced by tensile strain and n-type doping. In this work, we study by simulation the interplay between electrical transport and optical gain in highly n-doped and intrinsic germanium p-n heterostructure diodes under tensile strain. The effects of strain and doping on carrier mobilities and energy distribution are taken into account. Whereas the n-doping of Ge enhances the filling of the indirect L and Brillouin zone-center conduction band states, the n-doping also reduces the carrier injection efficiency, which is detrimental for the achievement of optical gain at reduced current densities. For applied biaxial strains larger than 1.25%, i.e., far before reaching the cross-over from indirect to direct band gap regime, undoped germanium exhibits a lower optical gain threshold as compared to doped germanium. We also show that the threshold current needed to reach transparency in germanium heterostructures has been significantly...

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.

Strain engineering in germanium microdisks

The keystone to realize a monolithic integrated source on silicon with germanium is to optimize tensile strain and n-doping. In order to realize an integrated compact source, we demonstrate highly strained n-doped germanium microdisks obtained by two approaches using initially compressed silicon nitride (SiN) deposition. In the first approach, the microdisks are fabricated from relaxed Ge. In a second approach, we use tensile-strained Ge grown on a mismatched buffer layer, thus increasing the global strain in the Ge volume and lowering its gradient. A photoluminescence red-shift up to 450 nm is observed, corresponding to more than 1% biaxial strain.