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Abstract
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.
Highlights
Optical gain has been recently observed by pump-probe measurement in n-doped tensilestrained germanium at room temperature [1]
The germanium layers were grown on GaAs substrate by metal-organic chemical vapor deposition (MOCVD) [11, 12]
We have introduced a new method to impose a tensile stress on germanium and succesfully fabricated tensile-strained germanium photonic wires using Si3N4 straining layers
Summary
Optical gain has been recently observed by pump-probe measurement in n-doped tensilestrained germanium at room temperature [1] This observation was shortly followed by the demonstration of a germanium laser under pulsed optical pumping, establishing a new paradigm for the integration of an efficient optical source on a silicon platform [2,3]. The tensile strain lifts the degeneracy of the valence band, pushing the light hole band at higher energy Another possibility to impose a tensile stress is to use silicon nitride layers as external stressors. This local strain engineering method was pioneered by the microelectronics industry to enhance the mobility properties of transistors. The demonstration of optical gain with germanium photonic wires opens new perspectives for future integration of efficient nanoscale optical sources on silicon
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