Role of Initial Precursor Chemisorption on Incubation Delay for Molybdenum Oxide Atomic Layer Deposition

In an effort to grow metal oxide films (e.g., MoO₃) at low temperatures, a novel molybdenum precursor, Si­(CH₃)₃CpMo­(CO)₂(η³-2-methylallyl) or MOTSMA, is used with ozone as the coreactant. As is often observed in atomic layer deposition (ALD) processes, the deposition of molybdenum trioxide displays an incubation period (∼15 cycles at 250 °C). In situ FTIR spectroscopy reveals that ligand exchange reactions can be activated at 300 °C, leading to a shorter incubation periods (e.g., ∼ 9 cycles). Specifically, the reaction of MOTSMA with OH-terminated silicon oxide surfaces appears to be the rate limiting step, requiring a higher temperature activation (350 °C) than the subsequent ALD process itself, for which 250 °C is adequate. Therefore, in order to overcome the nucleation delay, the MOTSMA precursor is initially grafted at 350 °C, with spectroscopic evidence of surface reaction, and the substrate temperature then lowered to 250 or 300 °C for the rest of the ALD process. After this initial activation, a standard ligand exchange is observed with formation of surface Si­(CH₃)₃CpMo­(η³-2-methylallyl) after precursor and its removal after ozone exposures, resulting in Mo­(O)₂ formation. Under these conditions, the ALD process proceeds with no nucleation delay at both temperatures. Postdeposition X-ray photoelectron spectroscopy spectra confirm that the film composition is MoO₃. This work highlights the critical role of precursor grafting to the substrate as essential to eliminate the nucleation delay for ultrathin ALD grown film deposition

Reaction Mechanisms of the Atomic Layer Deposition of Tin Oxide Thin Films Using Tributyltin Ethoxide and Ozone

Uniform and conformal deposition of tin oxide thin films is important for several applications in electronics, gas sensing, and transparent conducting electrodes. Thermal atomic layer deposition (ALD) is often best suited for these applications, but its implementation requires a mechanistic understanding of the initial nucleation and subsequent ALD processes. To this end, in situ FTIR and ex situ XPS have been used to explore the ALD of tin oxide films using tributyltin ethoxide and ozone on an OH-terminated, SiO₂-passivated Si(111) substrate. Direct chemisorption of tributyltin ethoxide on surface OH groups and clear evidence that subsequent ligand exchange are obtained, providing mechanistic insight. Upon ozone pulse, the butyl groups react with ozone, forming surface carbonate and formate. The subsequent tributyltin ethoxide pulse removes the carbonate and formate features with the appearance of the bands for CH stretching and bending modes of the precursor butyl ligands. This ligand-exchange behavior is repeated for subsequent cycles, as is characteristic of ALD processes, and is clearly observed for deposition temperatures of 200 and 300 °C. On the basis of the in situ vibrational data, a reaction mechanism for the ALD process of tributyltin ethoxide and ozone is presented, whereby ligands are fully eliminated. Complementary ex situ XPS depth profiles confirm that the bulk of the films is carbon-free, that is, formate and carbonate are not incorporated into the film during the deposition process, and that good-quality SnO_<i>x</i> films are produced. Furthermore, the process was scaled up in a cross-flow reactor at 225 °C, which allowed the determination of the growth rate (0.62 Å/cycle) and confirmed a self-limiting ALD growth at 225 and 268 °C. An analysis of the temperature-dependence data reveals that growth rate increases linearly between 200 and 300 °C

Rapid Wafer-Scale Growth of Polycrystalline 2H-MoS₂ by Pulsed Metal–Organic Chemical Vapor Deposition

High-volume manufacturing of devices based on transition metal dichalcogenide (TMD) ultrathin 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 metal–organic chemical vapor deposition (MOCVD) route for MoS₂ film growth in a research-grade single-wafer reactor. Using bis­(<i>tert</i>-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 metal–organic reagents in the controlled synthesis of TMDs

Selective Atomic Layer Deposition Mechanism for Titanium Dioxide Films with (EtCp)Ti(NMe₂)₃: Ozone versus Water

The need for the conformal deposition of TiO₂ thin films in device fabrication has motivated a search for thermally robust titania precursors with noncorrosive byproducts. Alkylamido-cyclopentadienyl precursors are attractive because they are readily oxidized, yet stable, and afford environmentally mild byproducts. We have explored the deposition of TiO₂ films on OH-terminated SiO₂ surfaces by in situ Fourier transform infrared spectroscopy using a novel titanium precursor [(EtCp)­Ti­(NMe₂)₃ (<b>1</b>), Et = CH₂CH₃] with either ozone or water. This precursor initially reacts with surface hydroxyl groups at ≥150 °C through the loss of its NMe₂ groups. However, once the precursor is chemisorbed, its subsequent reactivities toward ozone and water are very different. There is a clear reaction with ozone, characterized by the formation of monodentate formate and/or chelate bidentate carbonate surface species; in contrast, there is no detectable reaction with water. For the ozone-based ALD process, the surface formate/carbonate species react with the NMe₂ groups during the subsequent pulse of <b>1</b>, forming TiOTi bonds. Ligand exchange is observed within the 250–300 °C ALD window. X-ray photoelectron spectroscopy confirms the deposition of stoichiometric TiO₂ films with no detectable impurities. For the water-based process, ligand exchange is not observed. Once <b>1</b> is adsorbed, there is no spectroscopic evidence for further reaction. However, there is still TiO₂ deposition under typical ALD conditions. Co-adsorption experiments with controlled vapor pressures of water and <b>1</b> indicate that deposition arises solely from <b>1</b>/water <i>gas-phase</i> reactions. This striking lack of reactivity between chemisorbed <b>1</b> and water is attributed to the electronic and steric effects of the EtCp group and facilitates the observation of gas-phase reactions