Tensile-Strained Germanium Quantum Dots on (111) Surfaces

Si and Ge are ubiquitous in electronics, but their indirect bandgaps make them unsuitable for optoelectronic devices. Theory shows that placing Ge under tensile strain reduces its semiconductor bandgap by reducing the Γ-valley in Ge’s conduction band faster than the L-valley. Once at ~2% tensile strain, Ge should acquire a direct bandgap. Researchers have therefore tried various ingenious methods to create tensile strain in Ge, but these attempts typically generate strain-induced defects and do not result in viable optoelectronic materials. Our approach to this problem is to synthesize Ge quantum dots (QDs) that self-assemble as a result of biaxial tensile strains on (111) surfaces. We have previously developed a method to grow defect-free GaAs(111) QDs at ~4% tensile strain with molecular beam epitaxy (MBE). Since GaAs and Ge have similar lattice constants, we simply replace GaAs with Ge in these structures. Initial data suggest spontaneous formation of Ge QDs under 3.7% tensile strain, leading to optically active Ge with a reduced bandgap. We will present results demonstrating control of the structural and optoelectronic properties of tensile-strained Ge QDs with MBE parameters.