Atmospheric-pressure atomic layer deposition: recent applications and new emerging applications in high-porosity/3D materials.

Atomic layer deposition (ALD) is a widely recognized technique for depositing ultrathin conformal films with excellent thickness control at Ångström or (sub)monolayer level. Atmospheric-pressure ALD is an upcoming ALD process with a potentially lower ownership cost of the reactor. In this review, we provide a comprehensive overview of the recent applications and development of ALD approaches emphasizing those based on operation at atmospheric pressure. Each application determines its own specific reactor design. Spatial ALD (s-ALD) has been recently introduced for the commercial production of large-area 2D displays, the surface passivation and encapsulation of solar cells and organic light-emitting diode (OLED) displays. Atmospheric temporal ALD (t-ALD) has opened up new emerging applications such as high-porosity particle coatings, functionalization of capillary columns for gas chromatography, and membrane modification in water treatment and gas purification. The challenges and opportunities for highly conformal coating on porous substrates by atmospheric ALD have been identified. We discuss in particular the pros and cons of both s-ALD and t-ALD in combination with their reactor designs in relation to the coating of 3D and high-porosity materials.

Encapsulation of epitaxial silicene on ZrB2 with NaCl.

Silicene and other two-dimensional materials, such as germanene and stanene, have chemically reactive surfaces and are prone to oxidation in air, and thus require an encapsulation layer for ex situ studies or integration in an electronic device. In this work, we investigated NaCl as an encapsulation material for silicene. NaCl was deposited on the surface of epitaxial silicene on ZrB2(0001) thin films near room temperature and studied using synchrotron-based high-resolution photoelectron spectroscopy. The deposition of NaCl resulted in dissociative chemisorption, where the majority of epitaxial silicene reacted to form Si-Clx species.

A nitride-based epitaxial surface layer formed by ammonia treatment of silicene-terminated ZrB2.

We present a method for the formation of an epitaxial  surface layer involving B, N, and Si atoms on a ZrB2(0001) thin film on Si(111). It has the potential to be an insulating growth template for 2D semiconductors. The chemical reaction of NH3 molecules with the silicene-terminated ZrB2  surface was characterized by synchrotron-based, high-resolution core-level photoelectron spectroscopy and low-energy electron diffraction. In particular, the dissociative chemisorption of NH3 at 400 °C leads to surface  nitridation, and subsequent annealing up to 830 °C results in a solid phase reaction with the ZrB2 subsurface layers. In this way, a new nitride-based epitaxial  surface layer is formed with hexagonal symmetry and a single in-plane crystal orientation.

On the feasibility of silicene encapsulation by AlN deposited using an atomic layer deposition process.

Since epitaxial silicene is not chemically inert under ambient conditions, its application in devices and the ex-situ characterization outside of ultrahigh vacuum environments require the use of an insulating capping layer. Here, we report on a study of the feasibility of encapsulating epitaxial silicene on ZrB2(0001) thin films grown on Si(111) substrates by aluminum nitride (AlN) deposited using trimethylaluminum (TMA) and ammonia (NH3) precursors. By in-situ high-resolution core-level photoelectron spectroscopy, the chemical modifications of the surface due to subsequent exposure to TMA and NH3 molecules, at temperatures of 300 °C and 400 °C, respectively, have been investigated. While an AlN-related layer can indeed be grown, silicene reacts strongly with both precursor molecules resulting in the formation of Si-C and Si-N bonds such that the use of these precursors does not allow for the protective AlN encapsulation that leaves the electronic properties of silicene intact.

Interaction of epitaxial silicene with overlayers formed by exposure to Al atoms and O2 molecules.

As silicene is not chemically inert, the study and exploitation of its electronic properties outside of ultrahigh vacuum environments require the use of insulating capping layers. In order to understand if aluminum oxide might be a suitable encapsulation material, we used high-resolution synchrotron photoelectron spectroscopy to study the interactions of Al atoms and O2 molecules, as well as the combination of both, with epitaxial silicene on thin ZrB2(0001) films grown on Si(111). The deposition of Al atoms onto silicene, up to the coverage of about 0.4 Al per Si atoms, has little effect on the chemical state of the Si atoms. The silicene-terminated surface is also hardly affected by exposure to O2 gas, up to a dose of 4500 L. In contrast, when Al-covered silicene is exposed to the same dose, a large fraction of the Si atoms becomes oxidized. This is attributed to dissociative chemisorption of O2 molecules by Al atoms at the surface, producing reactive atomic oxygen species that cause the oxidation. It is concluded that aluminum oxide overlayers prepared in this fashion are not suitable for encapsulation since they do not prevent but actually enhance the degradation of silicene.

Low-temperature deposition of high-quality silicon oxynitride films for CMOS-integrated optics.

The growth of silicon oxynitride thin films applying remote inductively coupled, plasma-enhanced chemical vapor deposition is optimized toward high optical quality at a deposition temperature as low as 150°C. Propagation losses of 0.5±0.05 dB/cm, 1.6±0.2 dB/cm, and 0.6±0.06 dB/cm are measured on as-deposited waveguides for wavelengths of 1300, 1550, and 1600 nm, respectively. Films were deposited onto a 0.25 µm technology mixed-signal CMOS chip to show the application perspective for three-dimensional integrated optoelectronic chips.

Ultra-thin atomic layer deposited TiN films: non-linear I-V behaviour and the importance of surface passivation.

We report the electrical resistivity of atomic layer deposited TiN thin films in the thickness range 2.5-20 nm. The measurements were carried out using the circular transfer length method structures. For the films with thickness in the range of 10-20 nm, the measurements exhibited linear current-voltage (I-V) curves. The sheet resistance R(sh) was determined, and the resistivity was calculated. A value of 120 microohms-cm was obtained for a 20 nm TiN layer. With decreasing film thickness, the resistivity slightly increased and reached 135 microohms-cm for a 10 nm film. However, the measurements on 2.5-5.0 nm thick films revealed non-linear I-V characteristics, implying the dependence of the measured resistance, and therefore the resistivity, of the layers on applied voltage. The influence of the native oxidation due to the exposure of the films to air was taken into account. To fully eliminate this oxidation, a highly-resistive amorphous silicon layer was deposited directly after the ALD of TiN. The electrical measurements on the passivated 2.5- and 3.5 nm TiN layers then exhibited linear I-V characteristics. A resistivity of 400 and 310 microohms-cm was obtained for a 2.5- and 3.5 nm TiN film, respectively.

A difference in using atomic layer deposition or physical vapour deposition TiN as electrode material in metal-insulator-metal and metal-insulator-silicon capacitors.

In this work, metal-insulator-metal (MIM) and metal-insulator-silicon (MIS) capacitors are studied using titanium nitride (TiN) as the electrode material. The effect of structural defects on the electrical properties on MIS and MIM capacitors is studied for various electrode configurations. In the MIM capacitors the bottom electrode is a patterned 100 nm TiN layer (called BE type 1), deposited via sputtering, while MIS capacitors have a flat bottom electrode (called BE type 2-silicon substrate). A high quality 50-100 nm thick SiO2 layer, made by inductively-coupled plasma CVD at 150 degrees C, is deposited as a dielectric on top of both types of bottom electrodes. BE type 1 (MIM) capacitors have a varying from low to high concentration of structural defects in the SiO2 layer. BE type 2 (MIS) capacitors have a low concentration of structural defects and are used as a reference. Two sets of each capacitor design are fabricated with the TiN top electrode deposited either via physical vapour deposition (PVD, i.e., sputtering) or atomic layer deposition (ALD). The MIM and MIS capacitors are electrically characterized in terms of the leakage current at an electric field of 0.1 MV/cm (I leak) and for different structural defect concentrations. It is shown that the structural defects only show up in the electrical characteristics of BE type 1 capacitors with an ALD TiN-based top electrode. This is due to the excellent step coverage of the ALD process. This work clearly demonstrates the sensitivity to process-induced structural defects, when ALD is used as a step in process integration of conductors on insulation materials.

Replication molds having nanometer-scale shape control fabricated by means of oxidation and etching.

A means of accurate control of the curvature radius of molds that are used in nanostructure replication techniques is presented. The local non-uniform growth of SiO2 at regions with high curvature is used to fabricate molds with a curvature radius ranging anywhere between 10 and 250 nm. The mold radius is predicted by numerical simulation as a function of oxidation temperature and time and confirmed by a series of oxidation and etching experiments. The silicon, silicon dioxide, and polymer nanostructures are analyzed by scanning electron microscopy and compared with the theory. Replication into photo-plastic polymer from various sharp and round molds is performed, and their properties are discussed. Our results are useful for designing nanostructures in the area of soft lithography and nanoprobe engineering.