Karl F. Ludwig

Tailoring nanoporous materials by atomic layer deposition

Atomic layer deposition (ALD) is a cyclic process which relies on sequential self-terminating reactions between gas phase precursor molecules and a solid surface. The self-limiting nature of the chemical reactions ensures precise film thickness control and excellent step coverage, even on 3D structures with large aspect ratios. At present, ALD is mainly used in the microelectronics industry, e.g. for growing gate oxides. The excellent conformality that can be achieved with ALD also renders it a promising candidate for coating porous structures, e.g. for functionalization of large surface area substrates for catalysis, fuel cells, batteries, supercapacitors, filtration devices, sensors, membranes etc. This tutorial review focuses on the application of ALD for catalyst design. Examples are discussed where ALD of TiO(2) is used for tailoring the interior surface of nanoporous films with pore sizes of 4-6 nm, resulting in photocatalytic activity. In still narrower pores, the ability to deposit chemical elements can be exploited to generate catalytic sites. In zeolites, ALD of aluminium species enables the generation of acid catalytic activity.

Heteroepitaxy, polymorphism, and faulting in GaN thin films on silicon and sapphire substrates

The structure of GaN films grown by electron‐cyclotron‐resonance‐assisted molecular beam epitaxy on Si(111), Si(001), basal‐plane sapphire, a‐plane sapphire, and r‐plane sapphire substrates was studied with four‐circle x‐ray diffractometry. Phase content, domain size, inhomogeneous strain, and in‐plane and out‐of‐plane domain misorientations were measured and compared for films grown on each type of substrate. Wurtzite and zinc blende polymorphs were found to coexist in films grown on Si(111). The two structures grow in the (0002) and (111) orientations, respectively, so that they may transform into each other via stacking faults on close‐packed planes. Smaller amounts of zinc blende material were also found in predominately (0002) wurtzitic films on a‐plane sapphire and (1120) wurtzitic films on r‐plane sapphire.

PHASE SEPARATION AND ORDERING IN INGAN ALLOYS GROWN BY MOLECULAR BEAM EPITAXY

In this study, we investigated phase separation and long-range atomic ordering phenomena in InGaN alloys produced by molecular beam epitaxy. Films grown at substrate temperatures of 700–750 °C with indium concentration higher than 35% showed phase separation, in good agreement with thermodynamic predictions for spinodal decomposition. Films grown at lower substrate temperatures (650–675 °C) revealed compositional inhomogeneity when the indium content was larger than 25%. These films, upon annealing to 725 °C, underwent phase separation, similar to those grown at the same temperature. The InGaN films also exhibited long-range atomic ordering. The ordering parameter was found to increase with the growth rate of the films, consistent with the notion that ordering is induced at the growth surface. The ordered phase was found to be stable up to annealing temperatures of 725 °C. A competition between ordering and phase separation has been observed, suggesting that the driving force for both phenomena is lattice...

LONG RANGE ORDER IN ALXGA1-XN FILMS GROWN BY MOLECULAR BEAM EPITAXY

The first observation of atomic long range ordering in AlxGa1−xN thin films grown by electron cyclotron resonance assisted molecular beam epitaxy on sapphire and 6H-SiC substrates is reported. The phenomenon was investigated by studying the superlattice peaks (0001), (0003), and (0005) using x-ray diffraction. The relative intensity of these peaks was found to be largest for Al content in the 30%–50% range in qualitative agreement with expectations for an ordered structure of ideal Al0.5Ga0.5N stoichiometry. The average size of the ordered domains in the films was found to be within a factor of 4 of the films’ thicknesses. The degree of ordering depends on the III/V flux ratio and exhibits a weaker dependence on Si doping.

Real-time x-ray studies of Mo-seeded Si nanodot formation during ion bombardment

The formation of self-organized Si nanostructures induced by Mo seeding during normal incidence Ar+ ion bombardment at room temperature is reported. Silicon surfaces without Mo seeding develop only power-law roughness during 1000eV ion bombardment at normal incidence, in agreement with scaling theory expectations of surface roughening. However, supplying Mo atoms to the surface during ion bombardment seeds the development of highly correlated, nanoscale structures (“dots”) that are typically 3nm high with a spatial wavelength of approximately 30nm. With time, these saturate and further surface roughening is dominated by the growth of long-wavelength corrugations.

In situ x‐ray diffraction analysis of the C49–C54 titanium silicide phase transformation in narrow lines

The transformation of titanium silicide from the C49 to the C54 structure was studied using x‐ray diffraction of samples containing arrays of narrow lines of preformed C49 TiSi2. Using a synchrotron x‐ray source, diffraction patterns were collected at 1.5–2 °C intervals during sample heating at rates of 3 or 20 °C/s to temperatures of 1000–1100 °C. The results show a monotonic increase in the C54 transition temperature by as much as 180 °C with a decreasing linewidth from 1.0 to 0.1 μm. Also observed is a monotonic increase in (040) preferred orientation of the C54 phase with decreasing linewidth. The results demonstrate the power of in situ x‐ray diffraction of narrow line arrays as a tool to study finite size effects in thin‐film reactions.

Chemical ordering in AlGaN alloys grown by molecular beam epitaxy

Aluminum gallium nitride alloys were grown by molecular beam epitaxy and their film composition, structure, and microstructure were investigated by Rutherford backscattering spectroscopy, atomic force microscopy, x-ray diffraction, and transmission electron microscopy. It was found that the ratio of group-III to group-V fluxes influences the relative incorporation of gallium and aluminum in the films. The transmission electron microscopy and x-ray diffraction studies revealed the existence of three types of spontaneously formed superlattice structures with periodicities of 2, 7, and 12 ML. While the 2 ML ordering is preferred under group-V rich conditions of growth, the 7 and 12 ML orderings were observed under group-III rich conditions of growth.

High contrast x-ray speckle from atomic-scale order in liquids and glasses

The availability of ultrafast pulses of coherent hard x rays from the Linac Coherent Light Source opens new opportunities for studies of atomic-scale dynamics in amorphous materials. Here, we show that single ultrafast coherent x-ray pulses can be used to observe the speckle contrast in the high-angle diffraction from liquid Ga and glassy Ni(2)Pd(2)P and B(2)O(3). We determine the thresholds above which the x-ray pulses disturb the atomic arrangements. Furthermore, high contrast speckle is observed in scattering patterns from the glasses integrated over many pulses, demonstrating that the source and optics are sufficiently stable for x-ray photon correlation spectroscopy studies of dynamics over a wide range of time scales.

Texture of TiSi2 thin films on Si (001)

The texture of blanket C49 and C54 phase TiSi2 films on Si (001) substrates was examined with x-ray pole figure analysis. Textures were studied both in films with initial Ti deposited by sputtering and by low-temperature, plasma assisted chemical vapor deposition. Detailed differences were observed between silicide films grown by subsequently annealing films deposited with the two different methods. However, in both cases, the dominant C49 phase orientational relationships develop with respect to the substrate (001) planes. In contrast, we find that the dominant crystallographic relationships of the C54 phases in both cases are not with the surface (001) plane of the substrate, but instead with the Si (111) planes. Texturing in the case of the sputtered films is particularly complex with one prominent orientation having C54 (11¯0)‖(1¯11) Si with C54 (331)‖(011) Si and a second having C54 (1¯03¯)‖(111) Si with C54 (040)‖(11¯0) Si. These orientations, which suggest local epitaxy of the disilicide with...

In situ x-ray studies of native and Mo-seeded surface nanostructuring during ion bombardment of Si(100)

Native and Mo-seeded nanostructuring of the Si(100) surface during Ar+ ion bombardment is investigated by means of real-time grazing-incidence small-angle x-ray scattering and atomic force microscopy. During off-axis bombardment at room temperature, the native early-stage growth kinetics of nanoripples on the surface is found to be in reasonable overall agreement with theoretical predictions, particularly when an ion impact induced lateral mass redistribution term is included. For normal-incidence bombardment at room temperature, a native short wavelength smoothing of the amorphized Si surface is observed, suggesting that ion impact induced lateral mass redistribution dominates the Bradley–Harper instability. During 5% Mo-seeded normal-incidence bombardment at temperatures up to 450 °C, nanodots form with heights decreasing as the substrate temperature increases. This trend is counter to that typically observed for the growth of large cone structures on metals and suggests that the primary effect of thermal energy here is in promoting surface smoothing, rather than increasing diffusion of seed atoms to form protective clusters. During seeded bombardment at 650 °C the surface remains crystalline and surface corrugations exhibit dynamic scaling characteristic of surface diffusion-driven instabilities. This is the same behavior as is found in the absence of seeding and its presence suggests that at this concentration seeding does not play a large role during normal-incidence bombardment of the Si surface at high temperatures.