Formation and properties of InGaN QDs: Influence of substrates

We examine the formation and properties of InGaN quantum dots (QDs) on free-standing GaN and GaN/sapphire templates, with and without buried InGaN/GaN QD superlattices (SLs). We use scanning tunneling microscopy (STM) and scanning tunneling spectroscopy to image the QDs and measure their electronic states. As the number of layers preceding the QDs increases (i.e., increasing substrate complexity), the total QD density increases. For free-standing GaN, STM reveals a mono-modal QD-size-distribution, consistent with a limited density of substrate threading dislocations serving as heterogeneous nucleation sites. For GaN/sapphire templates, STM reveals a bimodal QD-size-distribution, presumably due to the nucleation of additional ultra-small InN-rich QDs near threading dislocations. For multi-period QD SLs on GaN/sapphire templates, an ultra-high density of QDs, with a mono-modal size distribution is apparent, suggesting that QD nucleation is enhanced by preferential nucleation at strain energy minima directly...