Substrate engineering for high quality emission of free and localized excitons from atomic monolayers in hybrid architectures

Atomic monolayers represent a novel class of materials to study localized and free excitons in two dimensions and to engineer optoelectronic devices based on their significant optical response. Here, we investigate the role of the substrate on the photoluminescense response of MoSe$_2$ and WSe$_2$ monolayers exfoliated either on SiO$_2$ or epitaxially grown InGaP substrates. In the case of MoSe$_2$, we observe a significant qualitative modification of the emission spectrum, which is widely dominated by the trion resonance on InGaP substrates. However, the effects of inhomogeneous broadening of the emission features are strongly reduced. Even more strikingly, in sheets of WSe$_2$, we could routinely observe emission lines from localized excitons with linewidths down to the resolution limit of 70\,$\mu$ eV. This is in stark contrast to reference samples featuring WSe$_2$ monolayers on SiO$_2$ surfaces, where the emission spectra from localized defects are widely dominated by spectral diffusion and blinking behaviour. Our experiment outlines the enormous potential of III-V-monolayer hybrid architectures to obtain high quality emission signals from atomic monolayers, which are straight forward to integrate into nanophotonic and integrated optoelectronic devices.