Design, Theoretical, and Experimental Investigation of Tensile-Strained Germanium Quantum-Well Laser Structure

Strain and band gap engineered epitaxial germanium (epsilon-Ge) quantum-well (QW) laser structures were investigated on GaAs substrates theoretically and experimentally for the first time. In this design, we exploit the ability of an InGaAs layer to simultaneously provide tensile strain in Ge (0.7-1.96%) and sufficient optical and carrier confinement. The direct band-to-band gain, threshold current density (Jth), and loss mechanisms that dominate in the epsilon-Ge QW laser structure were calculated using firstprinciples-based 30-band k.p electronic structure theory, at injected carrier concentrations from 3 x 10(18) to 9 x 10(19) cm(-3). The higher strain in the epsilon-Ge QW increases the gain at higher wavelengths; however, a decreasing thickness is required by higher strain due to critical layer thickness for avoiding strain relaxation. In addition, we predict that a J(th) of 300 A/cm(2) can be reduced to <10 A/cm(2) by increasing strain from 0.2% to 1.96% in epsilon-Ge lasing media. The measured room-temperature photoluminescence spectroscopy demonstrated direct band gap optical emission, from the conduction band at the G-valley to heavy-hole (0.6609 eV) from 1.6% tensile-strained Ge/In0.24Ga0.76As heterostructure grown by molecular beam epitaxy, is in agreement with the value calculated using 30-band k.p theory. The detailed plan-view transmission electron microscopic (TEM) analysis of 0.7% and 1.2% tensile-strained epsilon-Ge/InGaAs structures exhibited well-controlled dislocations within each epsilon-Ge layer. The measured dislocation density is below 4 x 10(6) cm(-2) for the 1.2% epsilon-Ge layer, which is an upper bound, suggesting the superior epsilon-Ge material quality. Structural analysis of the experimentally realistic 1.95% biaxially strained In0.28Ga0.72As/13 nm epsilon-Ge/In0.28Ga0.72As QW structure demonstrated a strained Ge/ In0.28Ga0.72As heterointerface with minimal relaxation using X-ray and cross-sectional TEM analysis. Therefore, our monolithic integration of a strained Ge QW laser structure on GaAs and ultimately the transfer of the process to the Si substrate via an InGa(Al)As/III-V buffer architecture would provide a significant step toward photonic technology based on strained Ge on a Si platform.