High volume manufacturing of devices based on transition metal dichalcogenide (TMD) ultra-thin films will require deposition techniques that are capable of reproducible wafer-scale growth with monolayer control. To date, TMD growth efforts have largely relied upon sublimation and transport of solid precursors with minimal control over vapor phase flux and gas-phase chemistry, which are critical for scaling up laboratory processes to manufacturing settings. To address these issues, we report a new pulsed metalorganic chemical vapor deposition (MOCVD) route for MoS2 film growth in a research-grade single-wafer reactor. Using bis(tert-butylimido)-bis(dimethylamido)molybdenum and diethyl disulfide we deposit MoS2 films from ≈ 1 nm to ≈ 25 nm in thickness on SiO2/Si substrates. We show that layered 2H-MoS2 can be produced at comparatively low reaction temperatures of 591 °C at short deposition times, approximately 90 s for few-layer films. In addition to the growth studies performed on SiO2/Si, films with wafer-level uniformity are demonstrated on 50 mm quartz wafers. Process chemistry and impurity incorporation from precursors are also discussed. This low-temperature and fast process highlights the opportunities presented by metalorganic reagents in the controlled synthesis of TMDs.

Tertbutylallylcobalttricarbonyl (tBu-AllylCo­(CO)3) is shown to have strong substrate selectivity during atomic layer deposition of metallic cobalt. The interaction of tBu-AllylCo­(CO)3 with SiO2 surfaces, where hydroxyl groups would normally provide more active reaction sites for nucleation during typical ALD processes, is thermodynamically disfavored, resulting in no chemical reaction on the surface at a deposition temperature of 140 °C. On the other hand, the precursor reacts strongly with H-terminated Si surfaces (H/Si), depositing ∼1 ML of cobalt after the first pulse by forming Si–Co metallic bonds. This remarkable substrate selectivity of tBu-AllylCo­(CO)3 is due to an ALD nucleation reaction process paralleling a catalytic hydrogenation, which requires a coreactant that acts as a hydrogen donor rather than a source of bare protons. The chemical specificity demonstrated in this work suggests a new paradigm for developing selective ALD precursors. Namely, selectivity can be achieved by tailoring the ligands in the coordination sphere to obtain structural analogues to reaction intermediates for catalytic transformations that exhibit the desired chemical discrimination.

We investigated the atomic layer deposition (ALD) of indium oxide (In2O3) thin films using alternating exposures of trimethylindium (TMIn) and a variety of oxygen sources: ozone (O3), O2, deionized H2O, and hydrogen peroxide (H2O2). We used in situ quartz crystal microbalance measurements to evaluate the effectiveness of the different oxygen sources and found that only O3 yielded viable and sustained In2O3 growth with TMIn. These measurements also provided details about the In2O3 growth mechanism and enabled us to verify that both the TMIn and O3 surface reactions were self-limiting. In2O3 thin films were prepared and characterized using X-ray diffraction, ultraviolet–visible spectrophotometry, spectroscopic ellipsometry, X-ray photoelectron spectroscopy, and scanning electron microscopy. The electrical transport properties of these layers were studied by Hall probe measurements. We found that, at deposition temperatures within the range of 100–200 °C, the In2O3 growth per cycle was nearly constant at 0.46 Å/cycle and the films were dense and pure. The film thickness was highly uniform (<0.3% variation) along the 45 cm length of our tubular ALD reactor. At higher growth temperatures the In2O3 growth per cycle increased due to thermal decomposition of the TMIn. The ALD In2O3 films showed resistivities as low as 3.2 × 10–3 Ω cm, and carrier concentrations as large as 7.0 × 1019 cm–3. This TMIn/O3 process for In2O3 ALD should be suitable for eventual scale-up in photovoltaics.

The initial stages of cobalt metal growth by atomic layer deposition are described using the precursors bis(1,4-di-tert-butyl-1,3-diazadienyl)cobalt and formic acid. Ruthenium, platinum, copper, Si(100), Si-H, SiO, and carbon-doped oxide substrates were used with a growth temperature of 180 °C. On platinum and copper, plots of thickness versus number of growth cycles were linear between 25 and 250 cycles, with growth rates of 0.98 Å/cycle. By contrast, growth on ruthenium showed a delay of up to 250 cycles before a normal growth rate was obtained. No films were observed after 25 and 50 cycles. Between 100 and 150 cycles, a rapid growth rate of ∼1.6 Å/cycle was observed, which suggests that a chemical vapor deposition-like growth occurs until the ruthenium surface is covered with ∼10 nm of cobalt metal. Atomic force microscopy showed smooth, continuous cobalt metal films on platinum after 150 cycles, with an rms surface roughness of 0.6 nm. Films grown on copper gave rms surface roughnesses of 1.1-2.4 nm after 150 cycles. Films grown on ruthenium, platinum, and copper showed resistivities of <20 μΩ cm after 250 cycles and had values close to those of the uncoated substrates at ≤150 cycles. X-ray photoelectron spectroscopy of films grown with 150 cycles on a platinum substrate showed surface oxidation of the cobalt, with cobalt metal underneath. Analogous analysis of a film grown with 150 cycles on a copper substrate showed cobalt oxide throughout the film. No film growth was observed after 1000 cycles on Si(100), Si-H, and carbon-doped oxide substrates. Growth on thermal SiO substrates gave ∼35 nm thick layers of cobalt(ii) formate after ≥500 cycles. Inherently selective deposition of cobalt on metallic substrates over Si(100), Si-H, and carbon-doped oxide was observed from 160 °C to 200 °C. Particle deposition occurred on carbon-doped oxide substrates at 220 °C.

Ruthenium (Ru) films are deposited using atomic layer deposition (ALD), promoted by a self-catalytic reaction mechanism. Using zero-valent, η 4 -2,3-dimethylbutadiene Ruthenium tricarbonyl (Ru(DMBD)(CO) 3 ) and H 2 O, Ru films are deposited at a rate of 0.1 nm/cycle. The temperature for steady deposition lies between 160 and 210 °C. Film structure and composition are confirmed via X-ray diffraction, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. The room-temperature electrical resistivity of 10 nm Ru films is found to be 39 μΩ• cm. In situ quadrupole mass spectrometry and density functional theory are used to understand ALD surface reactions. The ligand, dimethylbutadiene dissociatively desorbs on the surface. On the other hand, the carbonyl ligand is catalyzed by the Ru center. This leads to the water gas shift reaction, forming CO 2 and H 2 . Modulating deposition temperature affects these two ligand dissociation reactions. This in turn affects nucleation, growth, and hence, Ru film properties. Self-catalyzed reactions provide a pathway for low-temperature ALD with milder co-reactants.