Density control of GaN quantum dots on AlN single crystal

Abstract

Full control over the density and emission properties of GaN quantum dots (QDs) should be feasible, provided that the growth proceeds in the Stranski-Krastanov (SK) growth mode. In this work, we derive the phase diagram for GaN QD formation on AlN by NH3-molecular beam epitaxy and analyze the corresponding optical signature by micro-photoluminescence (μ-PL). Interestingly, the growth window for SK-GaN QDs is very narrow due to the relatively small lattice mismatch of the GaN/AlN system (2.5%), constituting a fundamental challenge for QD growth control. By relying on bulk AlN single crystal substrates, we demonstrate QD density control over three orders of magnitude, from 108 to 1011 cm−2 by changing the growth rate. In contrast, the QD density is pinned to 2 × 1010 cm−2 when growing on AlN/sapphire templates, which exhibit dislocation densities on the order of 1010 cm−2. Thanks to QD densities as low as 108 cm−2 on bulk AlN, we can probe the emission of spatially isolated single GaN QDs by μ-PL on unprocessed samples.