Molecular Insight into Real-Time Reaction Kinetics of Free Radical Polymerization from the Vapor Phase by In-Situ Mass Spectrometry.

The combination of organic chemistry and chemical vapor deposition enables a unique way to deposit conformal, high quality polymer thin films from the vapor phase. Particularly initiated chemical vapor deposition (iCVD) has recently shown its great potential in many different application fields. With the ever-increasing demands on the process, the need for additional process refinement is also growing. In this study the enhancement of the iCVD process by in-situ mass spectrometry is presented. The approach enables insight into real-time reaction kinetics during the deposition process as well as identification of reaction pathways. Furthermore, the composition of the gas phase can be precisely controlled and spontaneously adjusted if necessary. Particularly the deposition of thin films with thicknesses in the low nanometer range and the deposition of copolymers can benefit from this approach. The presented approach enables enhanced process control as well as the ability to perform extensive kinetic studies.

III-Nitride Magnetron Sputter Epitaxy on Si: Controlling Morphology, Crystal Quality, and Polarity Using Al Seed Layers.

Group III-nitride semiconductors have been subject of intensive research, resulting in the maturing of the material system and adoption of III-nitrides in modern optoelectronics and power electronic devices. Defined film polarity is an important aspect of III-nitride epitaxy as the polarity affects the design of electronic devices. Magnetron sputtering is a novel approach for cost-effective epitaxy of III-nitrides nearing the technological maturity needed for device production; therefore, control of film polarity is an important technological milestone. In this study, we show the impact of Al seeding on the AlN/Si interface and resulting changes in crystal quality, film morphology, and polarity of GaN/AlN stacks grown by magnetron sputter epitaxy. X-ray diffraction measurements demonstrate the improvement of the crystal quality of the AlN and subsequently the GaN film by the Al seeding. Nanoscale structural and chemical investigations using scanning transmission electron microscopy reveal the inversion of the AlN film polarity. It is proposed that N-polar growth induced by Al seeding is related to the formation of a polycrystalline oxygen-rich AlN interlayer partially capped by an atomically thin Si-rich layer at the AlN/Si interface. Complementary aqueous KOH etch studies of GaN/AlN stacks demonstrate that purely metal-polar and N-polar layers can be grown on a macroscopic scale by controlling the amount of Al seeding.

Monitoring of the Initial Stages of Diamond Growth on Aluminum Nitride Using In Situ Spectroscopic Ellipsometry.

The high thermal conductivity of polycrystalline diamond makes it ideally suited for thermal management solutions for gallium nitride (GaN) devices, with a diamond layer grown on an aluminum nitride (AlN) interlayer atop the GaN stack. However, this application is limited by the thermal barrier at the interface between diamond and substrate, which has been associated with the transition region formed in the initial phases of growth. In this work, in situ spectroscopic ellipsometry (SE) is employed to monitor early-stage microwave plasma-enhanced chemical vapor deposition diamond growth on AlN. An optical model was developed from ex situ spectra and applied to spectra taken in situ during growth. Coalescence of separate islands into a single film was marked by a reduction in bulk void fraction prior to a spike in sp2 fraction due to grain boundary formation. Parameters determined by the SE model were corroborated using Raman spectroscopy and atomic force microscopy.

Real-time insight into nanostructure evolution during the rapid formation of ultra-thin gold layers on polymers.

Ultra-thin metal layers on polymer thin films attract tremendous research interest for advanced flexible optoelectronic applications, including organic photovoltaics, light emitting diodes and sensors. To realize the large-scale production of such metal-polymer hybrid materials, high rate sputter deposition is of particular interest. Here, we witness the birth of a metal-polymer hybrid material by quantifying in situ with unprecedented time-resolution of 0.5 ms the temporal evolution of interfacial morphology during the rapid formation of ultra-thin gold layers on thin polystyrene films. We monitor average non-equilibrium cluster geometries, transient interface morphologies and the effective near-surface gold diffusion. At 1 s sputter deposition, the polymer matrix has already been enriched with 1% gold and an intermixing layer has formed with a depth of over 3.5 nm. Furthermore, we experimentally observe unexpected changes in aspect ratios of ultra-small gold clusters growing in the vicinity of polymer chains. For the first time, this approach enables four-dimensional insights at atomic scales during the gold growth under non-equilibrium conditions.

Revealing the growth of copper on polystyrene-block-poly(ethylene oxide) diblock copolymer thin films with in situ GISAXS.

Copper (Cu) as an excellent electrical conductor and the amphiphilic diblock copolymer polystyrene-block-poly(ethylene oxide) (PS-b-PEO) as a polymer electrolyte and ionic conductor can be combined with an active material in composite electrodes for polymer lithium-ion batteries (LIBs). As interfaces are a key issue in LIBs, sputter deposition of Cu contacts on PS-b-PEO thin films with high PEO fraction is investigated with in situ grazing-incidence small-angle X-ray scattering (GISAXS) to follow the formation of the Cu layer in real-time. We observe a hierarchical morphology of Cu clusters building larger Cu agglomerates. Two characteristic distances corresponding to the PS-b-PEO microphase separation and the Cu clusters are determined. A selective agglomeration of Cu clusters on the PS domains explains the origin of the persisting hierarchical morphology of the Cu layer even after a complete surface coverage is reached. The spheroidal shape of the Cu clusters growing within the first few nanometers of sputter deposition causes a highly porous Cu-polymer interface. Four growth stages are distinguished corresponding to different kinetics of the cluster growth of Cu on PS-b-PEO thin films: (I) nucleation, (II) diffusion-driven growth, (III) adsorption-driven growth, and (IV) grain growth of Cu clusters. Percolation is reached at an effective Cu layer thickness of 5.75 nm.