Effects of strain and confinement on the emission wavelength of InAs quantum dots due to aGaAs1−xNxcapping layer

A GaAsN capping layer grown on InAs quantum dots (QDs) induces a strong redshift of the emission wavelength and extends it beyond $1.3\mu \mathrm{m}$. We investigated this effect systematically by changing the nitrogen content in the GaAsN layer, varying the thickness of this layer, and embedding a GaAs spacer layer between the GaAsN layer and the QDs. The samples were grown on GaAs(001) substrates by plasma-assisted solid-source molecular beam epitaxy (MBE). Additionally, we simulated the band structure and the electron and hole energy levels based on $6\times 6\mathbf{k}\mathbf{\bullet }\mathbf{p}$ calculations, including strain and piezoelectric effects. We found that the wavelength extension is caused by the decrease of the confining energy barrier for the electron wave function in the QDs due to the lower conduction band energy of the GaAsN layer with respect to GaAs. The strain inside the QDs is almost unaffected by the overgrowth with the tensilely strained GaAsN layer. The insertion of a GaAsN layer below the QDs yields only a very small change in wavelength compared to the effect produced by a GaAsN capping layer. This difference is attributed to a reduced QD volume due to the growth on GaAsN that is suggested in cross-sectional scanning tunneling microscopy (XSTM) measurements. The blueshift due to this structural change of the QDs compensates for the redshift that is induced by the decreased confinement.