Effect of an electric field during the deposition of silicon dioxide thin films by plasma enhanced atomic layer deposition: an experimental and computational study.

The growth, chemical, structural, mechanical, and optical properties of oxide thin films deposited by plasma enhanced atomic layer deposition (PEALD) are strongly influenced by the average-bias voltage applied during the reaction step of surface functional groups with oxygen plasma species. Here, this effect is investigated thoroughly for SiO2 deposited in two different PEALD tools at average-bias voltages up to -300 V. Already at a very low average-bias voltage (< -10 V), the SiO2 films have significantly lower water content than films grown without biasing together with the formation of denser films having a higher refractive index and nearly stoichiometric composition. Substrate biasing during PEALD also enables control of mechanical stress. The experimental findings are supported by density functional theory and atomistic simulations. They demonstrate that the application of an electric field during the plasma step results in an increased energy transfer between energetic ions and the surface, directly influencing relevant surface reactions. Applying an electric field during the PEALD process leads to SiO2 thin films with significantly improved properties comparable to films grown by ion beam sputtering.

Initial Growth Study of Atomic-Layer Deposition of Al2O3 by Vibrational Sum-Frequency Generation.

The initial growth during the atomic-layer deposition (ALD) of Al2O3 using trimethylaluminum (TMA) and water was studied on two starting surfaces: SiO2 and -H-terminated Si(111) [H/Si(111)]. In situ spectroscopy ellipsometry (SE) showed virtually immediate growth of Al2O3 on both surfaces, although for H/Si(111) a reduced growth-per-cycle was observed in the initial 20 cycles. The underlying surface chemistry during the initial cycles of ALD was monitored with in situ broadband sum-frequency generation (BB-SFG) spectroscopy. For the SiO2 surface, the -CH3 surface groups were followed revealing that only the first TMA half-cycle deviates from the steady-growth regime. The reaction cross section of the initial TMA half-cycle (σTMA = 2.0 ± 0.2 × 10-18 cm2) was a factor of 3 lower than the cross section of the TMA half-cycle during the steady-growth regime of ALD (σTMA = 6.5 ± 0.6 × 10-18 cm2). All H2O half-cycles, including the first, showed steady-growth behavior with a corresponding reaction cross section (σH2O = 4.0 ± 0.4 × 10-20 cm2). Therefore, only the first ALD cycle was affected by initial growth effects on the SiO2 starting surface, in line with the SE data. For the H/Si(111) surface, the Si-H groups were monitored with BB-SFG spectroscopy, revealing a reaction cross section of σTMA = 3.1 ± 0.3 × 10-18 cm2 for the first TMA half-cycle on H/Si(111); a factor two lower than that during the steady regime of Al2O3. These results demonstrate that the chemistry during the initial growth regime of Al2O3 ALD on SiO2 and H/Si(111) shows subtle but measurable differences compared to the steady-growth regime.

Atomic Layer Deposition of Cobalt Using H2-, N2-, and NH3-Based Plasmas: On the Role of the Co-reactant.

This work investigates the role of the co-reactant for the atomic layer deposition of cobalt (Co) films using cobaltocene (CoCp2) as the precursor. Three different processes were compared: an AB process using NH3 plasma, an AB process using H2/N2 plasma, and an ABC process using subsequent N2 and H2 plasmas. A connection was made between the plasma composition and film properties, thereby gaining an understanding of the role of the various plasma species. For NH3 plasma, H2 and N2 were identified as the main species apart from the expected NH3, whereas for the H2/N2 plasma, NH3 was detected. Moreover, HCp was observed as a reaction product in the precursor and co-reactant subcycles. Both AB processes showed self-limiting half-reactions and yielded similar material properties, that is, high purity and low resistivity. For the AB process with H2/N2, the resistivity and impurity content depended on the H2/N2 mixing ratio, which was linked to the production of NH3 molecules and related radicals. The ABC process resulted in high-resistivity and low-purity films, attributed to the lack of NH x,x≤3 species during the co-reactant exposures. The obtained insights are summarized in a reaction scheme where CoCp2 chemisorbs in the precursor subcycle and NH x species eliminate the remaining Cp in the consecutive subcycle.

Characterization of nano-porosity in molecular layer deposited films.

Molecular layer deposition (MLD) delivers (ultra-) thin organic and hybrid materials, with atomic-level thickness control. However, such layers are often reported to be unstable under ambient conditions, due to the interaction of water and oxygen with the hybrid structure, consequently limiting their applications. In this contribution, we investigate the impact of porosity in MLD layers on their degradation. Alucone layers were deposited by means of trimethylaluminium and ethylene glycol, adopting both temporal and spatial MLD and characterized by means of FT-IR spectroscopy, spectroscopic ellipsometry, and ellipsometric porosimetry. The highest growth per cycle (GPC) achieved by spatial MLD resulted in alucone layers with very low stability in ambient air, leading to their conversion to AlOx. Alucones deposited by means of temporal MLD, instead, showed a lower GPC and a higher ambient stability. Ellipsometric porosimetry showed the presence of open nano-porosity in pristine alucone layers. Pores with a diameter in the range of 0.42-2 nm were probed, with a relative content between 1.5% and 5%, respectively, which are attributed to the temporal and spatial MLD layers. We concluded that a correlation exists between the process GPC, the open-porosity relative content, and the degradation of alucone layers.

Low-temperature plasma-enhanced atomic layer deposition of 2-D MoS2: large area, thickness control and tuneable morphology.

Low-temperature controllable synthesis of monolayer-to-multilayer thick MoS2 with tuneable morphology is demonstrated by using plasma enhanced atomic layer deposition (PEALD). The characteristic self-limiting ALD growth with a growth-per-cycle of 0.1 nm per cycle and digital thickness control down to a monolayer are observed with excellent wafer scale uniformity. The as-deposited films are found to be polycrystalline in nature showing the signature Raman and photoluminescence signals for the mono-to-few layered regime. Furthermore, a transformation in film morphology from in-plane to out-of-plane orientation of the 2-dimensional layers as a function of growth temperature is observed. An extensive study based on high-resolution transmission electron microscopy is presented to unravel the nucleation mechanism of MoS2 on SiO2/Si substrates at 450 °C. In addition, a model elucidating the film morphology transformation (at 450 °C) is hypothesized. Finally, the out-of-plane oriented films are demonstrated to outperform the in-plane oriented films in the hydrogen evolution reaction for water splitting applications.