The transformation of an oxide substrate by its reaction with a chemical precursor during atomic layer deposition (ALD) has not attracted much attention, as films are typically deposited on top of the oxide substrate. However, any modification to the substrate surface can impact the electrical and optical properties of the device. We demonstrate herein the ability of a precursor to react deep within an oxide substrate to form an interfacial layer that is distinct from both the substrate and deposited film. This phenomenon is studied using a scandium precursor, Sc(MeCp)2(Me2pz) (1, MeCp = methylcyclopentadienyl, Me2pz = 3,5-dimethylpyrazolate), and five oxide substrates (SiO2, ZnO, Al2O3, TiO2, and HfO2). In situ Fourier transform infrared (FTIR) spectroscopy shows that at moderate temperatures (∼150 °C) the pyrazolate group of 1 reacts with the surface hydroxyl groups of OH-terminated SiO2 substrates. However, at slightly higher temperatures (≥225 °C) typically used for the ALD of Sc2O3, there is a direct reaction between 1 and the SiO2 layer, in addition to chemisorption at the surface hydroxyl groups. This reaction is sustained by sequential exposures of 1 until an ∼2 nm thick passivating interface layer is formed, indicating that 1 reacts with oxygen derived from SiO2. A shift of the Si 2p core level position, measured by ex situ X-ray photoelectron spectroscopy, is consistent with the formation of a ScSixOy layer. Similar observations are made following the exposure of a ZnO substrate to 1 at 275 °C. In contrast, Al2O3, TiO2, and HfO2 substrates remain resistant to reaction with 1 under similar conditions, except for a surface reaction occurring in the case of TiO2. These striking observations are attributed to the differences in the electrochemical potentials of the elements comprising the oxide substrates to that of scandium. Precursor 1 can react with SiO2 or ZnO substrates, since the constituent elements of these oxides have less-negative electrochemical potentials than do aluminum, titanium, and hafnium. Additionally, Sc2O3 and surface carbonates are deposited on all substrates by gas-phase reactions between 1 and residual water vapor in the reactor. The extent of gas-phase reactions contributing to film growth is governed by the relative pressure of water vapor in the presence of 1. These results suggest caution when using very reactive, oxophilic precursors such as 1 to avoid misinterpreting unconventional film deposition as that resulting from a standard ALD process.
Abstract This work reports on high current density 1.2 kV class HfO 2 -gated vertical GaN trench metal-oxide-semiconductor field-effect transistors (MOSFETs). An output current density of 330 mA mm −1 is reported at a drain bias of five volts, which, to our knowledge, is over ten-times the highest reported values for 1.2 kV class GaN or SiC MOSFETs. This work also showcases a significant achievement in demonstrating substantially thick (100 nm) HfO 2 on GaN with simultaneous low leakage current (0.5 nA at 2 MV cm −1 ), a high breakdown strength (5.2 MV cm −1 ), and a high recorded dielectric constant (22.0).
Electric fields in a surface acoustic wave in a piezoelectric substrate can pattern charge in an adjacent graphene film via the acousto-electric effect and thus reconfigure the optical transmission in an unpatterned graphene metasurface
Abstract This review describes metal‐organic precursors for the growth of metal‐containing thin films by chemical vapor deposition (CVD)‐based methods. The major emphasis is on precursors that have been reported since 2004, which corresponds to a time of major growth in this field. Progress in the development of metal‐organic precursors is documented for the main group, lanthanide, and group 4– 11 elements. In the main group elements, there has been considerable research activity directed toward the identification of strontium and barium precursors, due both to the technological importance of mixed oxide phases and the inherent difficulties in obtaining volatile, stable thermally complexes of these large metal ions. Aluminum, gallium, and indium have also been the subject of intense investigation because of the importance of many phases containing these elements. The group 4 and 5 elements titanium, zirconium, hafnium, niobium, and tantalum have been the subject of considerable precursor development activity because of the importance of several mixed oxide phases and the applications of zirconium oxide and hafnium oxide as high‐permittivity gate materials in microelectronic devices. Growth of metal nitride films of these elements has also been an active area of research for use as barrier materials in microelectronic devices. The deposition of copper and other first‐row transition‐metal films from metal‐organic precursors is driven by the urgent need for copper metalization procedures in microelectronics device manufacturing. The atomic layer deposition (ALD) growth of the noble metals ruthenium, rhodium, iridium, palladium, and platinum has been a very active research area. The current state of metal‐organic precursor development is presented for each of these metallic elements.
The atomic layer deposition (ALD) of cobalt metal films is described using the precursor bis(1,4-di-tert-butyl-1,3-diazadienyl)cobalt and tert-butylamine or diethylamine. Platinum, copper, ruthenium, Si(100) with native oxide, thermal SiO2, hydrogen-terminated silicon, and carbon-doped oxide substrates were used with growth temperatures between 160 and 220 °C. Plots of growth rate versus pulse lengths showed saturative, self-limited behavior at ≥3.0 s for bis(1,4-di-tert-butyl-1,3-diazadienyl)cobalt and ≥0.1 s for tert-butylamine. An ALD window was observed between 170 and 200 °C, with a growth rate of 0.98 Å/cycle on platinum substrates. A plot of thickness versus the number of cycles at 200 °C on platinum substrates was linear between 25 and 1000 cycles, with a growth rate of 0.98 Å/cycle. A 98 nm thick film grown at 200 °C showed crystalline cobalt metal by X-ray diffraction. Atomic force microscopy of 10 and 98 nm thick cobalt metal films grown on platinum substrates at 200 °C showed rms roughness values that were ≤3.1% of the film thicknesses. X-ray photoelectron spectroscopy analyses were performed on 49 and 98 nm thick films grown on platinum substrates at 170 and 200 °C, respectively. Both samples showed oxidized cobalt on the film surface but revealed cobalt metal upon argon ion sputtering. The films showed >98% pure cobalt, with ≤0.9% each of oxygen, carbon, and nitrogen. On copper substrates, a plot of thickness versus the number of cycles was linear between 25 and 500 cycles, with a growth rate of 0.98 Å/cycle. In contrast, analogous growth studies on ruthenium substrates showed no films after 25 and 50 cycles, a small amount of growth at 100 cycles, and a growth rate of 0.98 Å/cycle at 200 and 500 cycles. No film growth was observed at 200 °C on Si(100) with native oxide, 100 nm thermal SiO2, hydrogen-terminated silicon, and carbon-doped oxide substrates after 500 cycles. Similarly, no growth was observed on these insulating substrates after 200 cycles at temperatures between 160 and 220 °C. Accordingly, this process affords inherently selective cobalt metal growth on metal substrates between 160 and 220 °C. Lower purity nitrogen carrier and purge gas (<99.9%) afforded a much lower growth rate, likely because of the formation of surface cobalt oxides and attendant reduced cobalt metal nucleation. A mechanism for cobalt metal growth is proposed in which 1 and tert-butylamine form an adduct, which then decomposes thermally to cobalt metal, 1,4-di-tert-butyl-1,3-diazadiene, and tert-butylamine.
