Paper PS+TF-ThA6
Optimizing MoO₃ Plasma-enhanced ALD Thin Films for use in Controllable 2D Material Synthesis

Thursday, November 2, 2017, 4:00 pm, Room 23

Monolayer MoS₂ is a direct bandgap semiconductor with promising properties for novel devices. It has been shown that valley polarization can be achieved in MoS₂ monolayers with circularly polarized light, which would allow the realization of novel information processing architectures through manipulation of the valley pseudo-spin. However, current production methods of MoS₂ monolayers are either low yielding, or of relatively poor quality for valleytronic applications. To control the layer number, defectivity, and crystallinity of MoS₂, a novel method for limiting growth through the sulfurization of wafer-scale MoO₃ thin films has been developed.

Thorough characterization of the MoO₃ plasma-enhanced ALD process was performed to understand the effect of MoO₃ process parameters on the resultant MoS₂. MoO₃ films of 20-35 nm were deposited utilizing (NtBu)₂(NMe₂)₂Mo as the organometallic precursor and O₂ plasma for the oxygen source. Variations in Mo precursor dose time and O₂ plasma exposure time show the expected trends, but substrate temperature effects are more significant. Growth per cycle increases with substrate temperature, from 0.88 Å/cycle at 100 ^oC, to 1.32 Å/cycle at 350 ^oC, with the highest GPC of 1.4 Å/cycle at 300 ^oC. Raman spectroscopy shows that films grown at low temperature are amorphous, while polycrystalline film growth occurs above 250 ^oC. These results are consistent with previously demonstrated MoO₃ growth utilizing the same process and precursor¹. Surface roughness, as measured by AFM, also increases with temperature, which is consistent with a transition to polycrystalline film growth. Through XPS analysis, the deposited films were determined to be sub-stoichiometric in all deposits, averaging an O/Mo ratio of 2.6, regardless of substrate temperature.

MoS₂ films were grown on bare sapphire wafers by placing MoO₃ source wafers face-to-face with growth wafers. The wafers were oriented horizontally and enclosed in a graphite susceptor to enable inductive heating. Spacing between the MoO₃ source wafer and sapphire growth wafer was varied from 0 to 1 mm, and wafers were reacted at 700 ^oC in H₂S gas. Initial reactions have shown increasing film deposition with decreased spacing between growth and source wafers. These results suggest that MoS₂ film growth by this method is vapor-phase transport limited. XPS data confirms the formation of MoS₂ on the growth wafer, however several layers of growth are observed. The effect of MoO₃ source wafer crystallinity on MoS₂ film quality is under investigation and will be presented.

[1] Vos, M., Macco, B., Thissen, N., Bol, A., Kessels, W. JVST A, 2015, 34(1), 01A103-1-7.