Mapping the Ge/InAl(Ga)As interfacial electronic structure and strain relief mechanism in germanium quantum dots

Abstract

Tensile-strained germanium (ε-Ge) has attracted significant interest due to its unique properties in emerging optoelectronic devices. High tensile-strained Ge materials with superior quality are still being investigated due to the intrinsic instability of ε-Ge against the formation of stacking faults (SFs). This work seeks to improve the understanding of these limits by closely examining, experimentally, the mechanisms by which tensile strain is relaxed in Ge. Here, ε-Ge layers were grown on highly mismatched In_0.53Ga_0.47As and In_0.51Al_0.49As virtual substrates (f = 3.4%), formed as quantum dots (QDs) by molecular beam epitaxy, and their strain relaxation mechanism was analyzed. Both In_0.51Al_0.49As and In_0.53Ga_0.47As growth templates were created using an Al_0.49In_0.51x(Ga_0.51)_1−xAs linearly graded metamorphic buffer on GaAs(001)/2° and InP(001)/0.5° substrates, respectively. Fully 3D growth (Volmer–Weber growth mode) due to high tensile strain resulted in Ge QDs with an average diameter and height of ∼50 nm and ∼20 nm, respectively, and a uniform density of ∼320 μm⁻². Analysis of the interfacial electronic structure using high-resolution transmission electron microscopy collected from the Ge QDs indicated that minimal tensile strain was retained in Ge due to SF formation, corroborated via the Raman results. All Ge QDs contain multiple SFs of the close-packed {111} planes nucleated by Shockley partial dislocations with Burger vectors b = ⅙〈112〉. The presence of additional misfit dislocations at the Ge/In_0.51Al_0.49As or Ge/In_0.53Ga_0.47As heterointerface, not associated with SFs, indicates further relaxation by perfect dislocations with Burger vectors b = ½〈110〉. The tensile misfit of 3.4% in Ge revealed instability against SF formation, and the availability of a defect type must have the effect of lowering the critical layer thickness for ε-Ge layers. Thus, the above results suggest that a maximum tensile strain amount >3.4% is not achievable in Ge without the formation of Shockley partial dislocations.