Periodic Reporting for period 1 - SELDOM (SElective Deposition Of 2D Materials)
Reporting period: 2016-04-01 to 2018-03-31
The main objective of the SELDOM project is to gain insight in the surface dependence of MX2 deposition processes, and to apply this understanding to enable their selective deposition. The newly found insight is then applied for the realization of a MX2 hetero-stack. This hetero-stack is a potentially new key component for the development of novel Tunnel Field Effect Transistor (TFET), a low energy, and highly promising candidate to replace the conventional metal-oxide-semiconductor field-effect transistors (MOSFETS).
The surface dependency of WS2 CVD (Chemical Vapor Deposition) with WF6/H2S was investigated on different relevant substrates: Depending on the nature of the substrate, the amount of W and S, the quality and the morphology of the deposited WS2 films varies greatly. In particular, for some specific substrates, etching is also observed during deposition. A model for the reaction mechanism of the growth of WS2 with WF6/H2S on metal oxide surface was elaborated: The WS2 growth is surface-mediated hence the adsorption of WF6 is conditioned by the nature of the surface. The etching of specific surfaces arise from the formation of metal-fluoride bonds on the surface that can lead to the formation of volatile species. For the non-etched surface, the metal-fluoride bonds are stable on these surface (SiO2 and SiNx). These results were presented during the 2017 Euro CVD/Baltic ALD conference.
Alternative deposition processes (Mo(CO)6/H2S and W(CO)6/H2S CVD) have also been explored. The influence of the deposition temperature, surface pre-treatment and precursor partial pressure on the selectivity of WS2 or MoS2 CVD were optimized to decrease the nucleation density of MoS2 or WS2 crystals by several order of magnitude. Combining the selective deposition of WS2 by removal of specific substrates and the suppressed deposition with tuned MoS2 or WS2 CVD parameters on SiO2 enables a pathway for selective deposition of MX2 materials.
Selective deposition of MX2 materials was demonstrated with two approaches: deposition of WS2 by conversion reaction of patterned Si nanodots with WF6 and H2S and sulfidation of pre-patterned Mo oxide nanodots under H2S at high temperature. Both approaches allows the selective deposition of MX2 nanodots only where the pre-patterned structures are present and not on the underlying substrate. The etch/patterning processing before MX2 deposition allow the control of the MX2 structures shape and pitch. The dimension of the MX2 nanodots can be tuned by varying the starting nanodots thickness and diameters. The MX2 materials obtain from these approaches are crystalline and with good stoichiometry but are polycrystalline (with grain size ~20nm). In order to further increase the individual MX2 crystal size we have investigated the addition of an extra selective lateral growth step to the deposition: By taking advantage of the higher reactivity of the 2D material edges as compared to their basal plane, the MX2 material can be grown laterally and the MX2 single crystals grain size (> 100nm) increased while maintaining a MX2 layer thickness of a few monolayers. Such controlled seeding approach was also tested to grow MXA/MXB lateral hetero-stack by growing MXB from the edges of MXA crystals:
The selective growth from MX2 crystal edges was demonstrated by the increase of the WS2 nanodots seeds lateral dimentions with WS2 PEALD while the increase of their height remains negligeable. However, the extra WS2 PEALD induces nucleation of parasitic WS2 crystals on the SiNx and SiO2 substrate thus negating the selectivity obtained with the formation of the MX2 seeds. Alternatively, using WS2 CVD with tuned deposition parameters for the lateral growth from the WS2 nanodot seeds shows good selectivity with limited parasitic nucleation. The uniformity and alignment of the resulting MoS2/WS2 crystals was affected by the patterning/etching processes leading to the MX2 seed formation.
Further attempts to grow MX2 selectively from alternative 2D material (pre-patterned graphene) edges revealed a strong impact of the patterning on the selectivity of the MoS2 or WS2 CVD process: nucleation of parasitic MoS2 or WS2 crystals occurs mostly on defect sites (originating from the transfer and patterning processes) on top of the graphene basal plane. These results highlight the need for alternative patterning methods (transfer, selective etching) for the making of devices with MX2A/MX2B heterostack as conventional patterning/etching steps can affect the reactivity of the MX2 material and hinder the selectivity of deposition processes developed on non-patterned substrates. These results were reported as part of the Beyond CMOS research program to Imec’s industrial partners during the Imec’s biyearly Partner’s Technical Weeks