Coulomb-blockade peak spacing statistics of graphene quantum dots on SiO2

Extrinsic disorder strongly affects the performance of graphene-based quantum dots. The standard SiO2 substrate is generally considered to be one major factor besides edge-induced disorder. In this report we present the fabrication of lithographically defined quantum dots on SiO2 with short and narrow constrictions and different central island sizes. Low temperature transport measurements display distinct Coulomb-blockade peaks with amplitudes exceeding what is commonly observed experimentally. The analysis of the normalized Coulomb-blockade peak spacing shows a size dependence, which has not previously been observed for devices on SiO2. Furthermore, a quantitative comparison of the peak spacing distribution to the literature shows that one of the two devices compares favorably to a similar sized dot placed on hexagonal boron nitride, which is known to reduce the substrate disorder. Our findings suggest that the other sources of extrinsic disorder, such as lithography residues, may play an important role ...