Growth of p-doped 2D-MoS$_2$ on metal oxides from spatial atomic layer deposition

In this letter we report on the synthesis of monolayers of MoS$_2$ via chemical vapor deposition directly on thin films of Al$_2$O$_3$ grown by spatial atomic layer deposition. The synthesized monolayers are characterized by atomic force microscopy as well as confocal Raman and photoluminescence spectroscopies. Our data reveals that the morphology and properties of the 2D material differ strongly depending on its position on the substrate. Close to the material source, we find individual flakes with an edge length of several hundred microns exhibiting a tensile strain of 0.3%, n-doping on the order of $n_e=0.2 \cdot 10^{13}$ cm$^{-2}$ and a dominant trion contribution to the photoluminescence signal. In contrast to this, we identify a mm-sized region downstream, that is made up from densely packed, small MoS$_2$ crystallites with an edge length of several microns down to the nanometer regime and a coverage of more than 70%. This nano-crystalline layer shows a significantly reduced strain of only <0.02%, photoluminescence emission at an energy of 1.86 eV with a reduced trion contribution, and appears to be p-doped with a carrier density of $n_h=0.1 \cdot 10^{13}$ cm$^{-2}$. The unusual p-type doping achieved here in a standard CVD process without substitutional doping, post-processing, or the use of additional chemicals may prove useful for applications.