| AVS 65th International Symposium & Exhibition | |
| Thin Films Division | Tuesday Sessions |
| Session TF+AM+EM+PS-TuM |
| Session: | Atomic Layer Processing: Area Selective Deposition |
| Presenter: | Timothy N. Walter, The Pennsylvania State Univeristy |
| Authors: | T.N. Walter, The Pennsylvania State Univeristy S. Lee, The Pennsylvania State Univeristy M. Chubarov, The Pennsylvania State University X. Zhang, The Pennsylvania State Univeristy T.H. Choudhury, The Pennsylvania State University J.M. Redwing, The Pennsylvania State University T.N. Jackson, The Pennsylvania State Univeristy S.E. Mohney, The Pennsylvania State Univeristy |
| Correspondent: | Click to Email |
Inducing nucleation on the inherently passivated surfaces of 2D materials such as transition metal dichalcogenides (TMDs) can be challenging for atomic layer deposition (ALD); however, this situation also presents an opportunity for selective growth by ALD. Additionally, ZnO/TMD heterojunctions have favorable band alignments for catalysis and offer possibilities for nanoscale electronic devices including transparent electronics, photodiodes, and piezo-phototronics. This work explores the growth of ZnO on the TMDs MoS2 and WSe2 using thermal ALD, thermal ALD with UV-O3 surface pre-treatment, and plasma enhanced ALD (PEALD). Depositions were performed on both few-layer exfoliated flakes and coalesced single-layer films (with scattered 2- or 3-layer islands) that were already grown by gas source chemical vapor deposition (CVD). Samples were characterized by atomic force microscopy (AFM), Raman spectroscopy, photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS) before and after deposition of ZnO. For both MoS2 and WSe2, thermal ALD of ZnO using diethyl zinc (DEZ) and water at 125 °C resulted in a long nucleation delay on the TMD surfaces, showing selectivity against ZnO growth on TMDs compared to the surrounding SiO2/Si substrate. Even after hundreds of cycles, very little change was detected by XPS, Raman spectroscopy, or AFM; however, nucleation did occur at defects and caused surface roughness to increase. UV-O3 pre-treatment before thermal ALD yielded different results on MoS2 compared to WSe2. UV-O3 functionalizes MoS2 for nucleation and subsequent growth of ZnO without destroying the underlying MoS2; however, UV-O3 fully oxidized regions of the WSe2 surface and promoted nucleation. PEALD using DEZ and N2O on both TMDs resulted in a conformal and smooth film, but it oxidized the top layer of the TMDs according to XPS. In conclusion, UV-O3 pre-treatment and plasma-enhanced deposition allow for nucleation and growth of ZnO on TMD substrates, sometimes to the detriment of the top layer of material; however, the planar surfaces of 2D materials resist nucleation for hundreds of cycles of thermal ALD.