Abstract
Tensile-strained Ge is a possible laser material for Si integrated circuits, but reports of lasers using tensile Ge show high threshold current densities and short lifetimes. To study the origins of these shortcomings, Ge ridge waveguides with tensile strain in three dimensions were fabricated using compressive silicon nitride (SiNx) films with up to 2 GPa stress as stress liners. A Raman peak shift of up to 11 cm−1 was observed, corresponding to 3.6% hydrostatic tensile strain for waveguides with a triangular cross-section. Real time degradation in tensile-strained Ge was observed and studied under transmission electron microscopy (TEM). A network of defects, resembling dark line defects, was observed to form and propagate with a speed and density strongly correlated with the local strain extracted from both modeled and measured strain profiles. This degradation suggests highly tensile-strained Ge lasers are likely to have significantly shorter lifetime than similar GaAs or InGaAs quantum well lasers.
Highlights
Tensile-strained germanium has been studied as a possible laser material due to its nearly-direct bandgap [1,2] and its compatibility with conventional silicon integrated circuit fabrication [3,4,5,6].Theoretically, a tensile strain of 0.7% or 1.4% could produce a direct bandgap inGe [7,8,9,10]
Cross-section high-resolution transmission electron electron microscopy microscopy (TEM) (HRTEM) images were taken in a FEI Titan 80–300
The use of SiNx stress liners induced tensile strains in the Ge of up to 3.0–3.6%, as measured by Raman and HRTEM strain analysis, and PL peak emission shifted to lower energies, PL emission was very weak
Summary
Tensile-strained germanium has been studied as a possible laser material due to its nearly-direct bandgap [1,2] and its compatibility with conventional silicon integrated circuit fabrication [3,4,5,6].Theoretically, a tensile strain of 0.7% (hydrostatic) or 1.4% (biaxial) could produce a direct bandgap inGe [7,8,9,10]. Tensile-strained germanium has been studied as a possible laser material due to its nearly-direct bandgap [1,2] and its compatibility with conventional silicon integrated circuit fabrication [3,4,5,6]. A tensile strain of 0.7% (hydrostatic) or 1.4% (biaxial) could produce a direct bandgap in. The hydrostatic component of strain is responsible for the shift from an indirect to a direct bandgap. Biaxial strain has a weaker hydrostatic component due to Poisson contraction along the out-of-plane direction. The first optically pumped Ge laser used 0.15% biaxial strain plus degenerate n-type doping to partly populate the Γ valley with electrons despite the indirect bandgap [2]
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