Influence of the GaN capping thickness on the strain and photoluminescence properties of InN/GaN quantum dots

InN quantum dots (QDs) were grown on 1 μm thick GaN/(0001) sapphire substrates by low pressure metal organic chemical vapor deposition. A single crystalline 10-nm thick GaN capping layer was achieved on the InN QDs by the flow-rate modulation epitaxy method at 650 °C. The (002) ω/2θ scans of the X-ray diffraction measurements show that the reduction of the lattice constant with a capping thickness indicate that the GaN capping process exerts a compressive strain on the InN QDs. The residual strain was reduced from 0.245% to − 0.245% as the GaN cap thickness increases from 0 to 20 nm. In addition, the analysis of the photoluminescence peak energy estimated that the free electron concentration (i.e. density of indium (In) vacancy) decreased from 1.62 × 1018 cm− 3 to 1.24 × 1018 cm− 3. The suggestion here is that the increase of the compressive strain on InN QDs due to the increased GaN capping layer thickness provides the high driving force for the interdiffusion of the In atom and gallium (Ga) atoms between the interface of InN QDs and the GaN capping layer. Thus, we believe that more Ga atoms can diffuse from the GaN capping layer and substitute the high density of In vacancy in the InN QDs, resulting in a decrease of the free electron concentration in the InN QDs with the increase in the GaN capping layer thickness.

► Single crystalline GaN cap layer grown on InN quantum dots.
► GaN cap layer grown by flow-rate modulation epitaxy.
► Surface defect density of InN quantum dots reduced by the GaN cap layer.
► Photoluminescence intensity increases with increasing GaN cap layer thickness.