Tensile-Strained Ge Quantum Dots on (111) and (110) Surfaces*

Si and Ge are ubiquitous in electronics, but their indirect bandgaps make them unsuitable for light emitting devices. Theory shows that placing Ge under tensile strain will allow us to engineer its electronic band structure. Tensile strains of 2–4% should transform Ge either into a direct band gap semiconductor or a semimetal, with applications from infrared emitters to low-loss tunnel junctions. Researchers have therefore tried various ingenious methods to create tensile-strained Ge, but these attempts typically generate strain-induced defects and do not result in viable optoelectronic materials. Our approach to this problem is to create Ge quantum dots (QDs) that self-assemble as a result of biaxial tensile strains on low-index surfaces. We have previously demonstrated defect-free, tensile GaAs(111) QDs. Given the similar lattice constants of GaAs and Ge, we follow the same approach to produce self-assembled Ge QDs at 3.7% tensile strain, which we anticipate should lead to optically active Ge with a reduced bandgap. We will discuss our control of Ge QD properties with growth parameters, and discuss possible reasons for some of the structural and optical phenomena we observed.