Highly tensile-strained Ge quantum dots on GaSb by MBE for light sources on Si

Abstract

It is theoretically predicted that biaxial tensile strain as much as 1.4% can make up the 136 meV gap between the Γ and L valley in Ge [1], thereby converting Ge from an indirect-bandgap semiconductor into a direct-bandgap one that can emit light efficiently covering the telecom band. The mobility of both carriers is dramatically increased simultaneously. Therefore, tensile-strained Ge has drawn large interest in the potential for high speed transistors and light sources for Si photonics. We have proposed and demonstrated that tensile-strained Ge quantum dot (QD) on InP is a better solution for the realization of light sources on Si than thin films since it can hold large strain to convert the bandgap and insensitive to structural defects at the same time [2]. In this work, the molecular beam epitaxy (MBE) of tensile-strained Ge QDs on GaSb(001) with thickness ranging from sub-monolayer (ML) to a few MLs is studied. The formation and evolution of the deposited Ge QDs are investigated by the reflection high-energy electron diffraction (RHEED), and the surface morphology is measured by atomic force microscopy (AFM). In FIG. 1, it is shown that the RHEED pattern changed to a dotty one after 1.7 ML of the Ge deposition indicating a Stranski-Krastanov (SK) growth mode with the existence of a wetting layer. FIG. 2 are AFM images of the samples with different Ge thickness. It can be found that when the thickness is below one ML, the Ge atoms nucleate randomly on the GaSb atomic steps, forming sub-ML islands. The two dimensional growth continues to a full coverage of the GaSb surface and up to 1.7 ML. A few QDs can be found before 1 ML, probably due to surface defects. These sub-ML islands and the one ML thick Ge films are fully strained (7.2% tensile strain). When the thickness is larger than 1.7 ML, clear formation of QDs is observed. The QDs are mostly rectangular shape with the edges along the (110) directions. The evolution observed from RHEED and AFM is consistent. Later, samples of the Ge with different thicknesses capped by GaSb were also grown. Further analysis including optical properties are under implement.