Second-harmonic intensity and phase spectroscopy as a sensitive method top robe the space-charge field in Si(100) covered with canged dielectrics

A space-charge region (SCR) can develop in silicon due to the presence of built-in charges in dielectric thin films that are used in silicon-based device architectures. To study both the strength and polarity of the electric field in such a SCR, the authors performed second-harmonic (SH) generation spectroscopy in the vicinity of the E 1 critical point (2.7–3.5¿eV) of silicon. As multiple contributions add coherently to SH intensity spectra, the electric-field-induced contribution cannot always be distinguished unambiguously from the intensity data in the absence of complementary phase information. Combined SH intensity and phase measurements were therefore performed to resolve this ambiguity. Using a coherent superposition of critical-point-like resonances with excitonic line shapes, the intensity and phase spectra of several SiO2- and Al2O3-based samples were simultaneously modeled. This analysis reveals that not only the polarity of the space-charge field can be determined unambiguously but also that the sensitivity to the electric field strength is significantly enhanced

Plasma-enhanced atomic layer deposition of tungsten oxide thin films using (tBuN)2(Me2N)2W and O2 plasma

The growth of tungsten oxide (WO3) thin films by atomic layer deposition (ALD) offers numerous merits including atomic-scale thickness control at low deposition temperatures. In this work, we have developed and characterized a new plasma-enhanced ALD process for WO3 thin films using the metalorganic precursor (tBuN)2(Me2N)2W and O2 plasma as co-reactant over a wide temperature range of 100 °C-400 °C. The influence of deposition temperature on the growth behaviour and film properties is investigated in detail. The WO3 ALD process developed in this work yields a relatively high growth per cycle (GPC) which varies from ~0.7 Å at 100 °C to ~0.45 Å at 400 °C, as-determined by in-situ spectroscopic ellipsometry (SE). Rutherford backscattering spectrometry (RBS) measurements revealed a mass density of 5.9 g/cm3 and near stoichiometric film composition (O/W = 2.9). Both RBS and X-ray photoelectron spectroscopy (XPS) measurements confirmed no detectable C as well as N impurity incorporation. Grazing incidence X-ray diffraction (GI-XRD) measurements indicated that the films deposited at 400 °C were polycrystalline in nature

Simultaneous scanning tunneling microscopy and synchrotron X-ray measurements in a gas environment

Catalysis and Surface Chemistr

Atomic Layer Deposition of 2D Metal Dichalcogenides for Electronics, Catalysis, Energy Storage, and Beyond

2D transition metal dichalcogenides (TMDCs) are among the most exciting materials of today. Their layered crystal structures result in unique and useful electronic, optical, catalytic, and quantum properties. To realize the technological potential of TMDCs, methods depositing uniform films of controlled thickness at low temperatures in a highly controllable, scalable, and repeatable manner are needed. Atomic layer deposition (ALD) is a chemical gas-phase thin film deposition method capable of meeting these challenges. In this review, the applications evaluated for ALD TMDCs are systematically examined, including electronics and optoelectonics, electrocatalysis and photocatalysis, energy storage, lubrication, plasmonics, solar cells, and photonics. This review focuses on understanding the interplay between ALD precursors and deposition conditions, the resulting film characteristics such as thickness, crystallinity, and morphology, and ultimately device performance. Through rational choice of precursors and conditions, ALD is observed to exhibit potential to meet the varying requirements of widely different applications. Beyond the current state of ALD TMDCs, the future prospects, opportunities, and challenges in different applications are discussed. The authors hope that the review aids in bringing together experts in the fields of ALD, TMDCs, and various applications to eventually realize industrial applications of ALD TMDCs.Peer reviewe

Revisiting the growth mechanism of atomic layer deposition of Al 2O 3: A vibrational sum-frequency generation study

The growth mechanism of the prototypical atomic layer deposition (ALD) process of Al2O3 using Al(CH3)3 (TMA) and H2O has been revisited on the basis of insights obtained with the nonlinear optical analysis technique of broadband sum-frequency generation (BB-SFG). With BB-SFG spectroscopy, both the –CH3 and –OH surface groups ruling the growth of Al2O3 by ALD were detected and could be monitored during the ALD process with submonolayer sensitivity. Several remaining questions pertaining to the growth mechanism of Al2O3 were addressed. The reaction kinetics of the H2O half-cycle were studied for ALD between 100 and 300 ºC, and the reaction cross section σ was determined. The cross section at 300 ºC was fairly large (σ=3x10-19 cm2) and it decreased with decreasing temperature. Below 200 ºC, the cross section also clearly varied with the surface coverage. For example, at 100 ºC, the cross section started at σ=1x10-20 cm2 for a full –CH3coverage and decreased to σ=3x10-21 cm2 for a 60% coverage. This coverage dependence of the reaction kinetics also explains the presence of the persistent –CH3groups at low temperatures which are no longer reactive toward H2O. By a dedicated study using x-ray photo-emission spectroscopy, it was demonstrated that the persistent –CH3groups were not incorporated into the film as a contaminant species. The absolute –CH3 coverage was measured for ALD between 100 and 450 ºC. With this data, steric hindrance was ruled out as the cause of the self-limiting behavior in the TMA half-cycle on basis of the decrease observed in the –CH3 coverage with temperature. The self-limiting behavior was attributed to the depletion of under coordinated O during the TMA half-cycle. Moreover, the chemisorption of TMA on the -OH surface groups during the TMA half-cycle was investigated. On average, 1.5 –CH3 ligands remained on the surface per deposited Al atom after the TMA half-cycle at 300 ºC, and this number decreased to 0.8 at 100 ºC. These insights into the underlying growth mechanism augment the understanding of Al2O3 ALD and reveal several nuances in this well-studied ALD process