Atomic layer deposition (ALD) is used in applications where inorganic material layers with uniform thickness down to the nanometer range are required. For such thicknesses, the growth mode, defining how the material is arranged on the surface during the growth, is of critical importance. In this work, the growth mode of the zirconium tetrachloride∕water and the trimethyl aluminum∕water ALD process on hydrogen-terminated silicon was investigated by combining information on the total amount of material deposited with information on the surface fraction of the material. The total amount of material deposited was measured by Rutherford backscattering, x-ray fluorescence, and inductively coupled plasma–optical emission spectroscopy, and the surface fractions by low-energy ion scattering. Growth mode modeling was made assuming two-dimensional growth or random deposition (RD), with a “shower model” of RD recently developed for ALD. Experimental surface fractions of the ALD-grown zirconium oxide and aluminum oxide films were lower than the surface fractions calculated assuming RD, suggesting the occurrence of island growth. Island growth was confirmed with transmission electron microscopy (TEM) measurements, from which the island size and number of islands per unit surface area could also be estimated. The conclusion of island growth for the aluminum oxide deposition on hydrogen-terminated silicon contradicts earlier observations. In this work, physical aluminum oxide islands were observed in TEM after 15 ALD reaction cycles. Earlier, thicker aluminum oxide layers have been analyzed, where islands have not been observed because they have already coalesced to form a continuous film. The unreactivity of hydrogen-terminated silicon surface towards the ALD reactants, except for reactive defect areas, is proposed as the origin of island growth. Consequently, island growth can be regarded as “undesired surface-selective ALD.”

Germanium combined with highdielectrics has recently been put forth by the semiconductor industry as potential replacement for planar silicon transistors, which are unlikely to accommodate the severe scaling requirements for sub-45-nm generations. Therefore, we have studied the atomic layer deposition ͑ALD͒ of HfO 2 high-dielectric layers on HF-cleaned Ge substrates. In this contribution, we describe the HfO 2 growth characteristics, HfO 2 bulk properties, and Ge interface. Substrate-enhanced HfO 2 growth occurs: the growth per cycle is larger in the first reaction cycles than the steady growth per cycle of 0.04 nm. The enhanced growth goes together with island growth, indicating that more than a monolayer coverage of HfO 2 is required for a closed film. A closed HfO 2 layer is achieved after depositing 4-5 HfO 2 monolayers, corresponding to about 25 ALD reaction cycles. Cross-sectional transmission electron microscopy images show that HfO 2 layers thinner than 3 nm are amorphous as deposited, while local epitaxial crystallization has occurred in thicker HfO 2 films. Other HfO 2 bulk properties are similar for Ge and Si substrates. According to this physical characterization study, HfO 2 can be used in Ge-based devices as a gate oxide with physical thickness scaled down to 1.6 nm.

The segregation behavior in 3 and 10 mol % polycrystalline yttria stabilized zirconia ͑YSZ͒, calcined at temperatures ranging from 300 to 1600°C, is characterized using low-energy ion scattering ͑LEIS͒. In order to be able to separate the Y and Zr LEIS signals, YSZ samples have been prepared using isotopically enriched 94 ZrO 2 instead of natural zirconia. The samples are made via a special precipitation method at a low temperature. The segregation to the outermost surface layer is dominated by impurities. The increased impurity levels are restricted to this first layer, which underlines the importance of the use of LEIS for this study. For temperatures of 1000°C and higher, the oxides of the impurities Na, Si, and Ca even cover the surface completely. The performance of a device like the solid oxide fuel cell which has an YSZ electrolyte and a working temperature around 1000°C, will, therefore, be strongly hampered by these impurities. The reduction of impurities, to prevent accumulation at the surface, will only be effective if the total impurity bulk concentration can be reduced below the 10 ppm level. Due to the presence of the impurities, yttria cannot accumulate in the outermost layer. It does so, in contrast to the general belief, in the subsurface layer and to much higher concentrations than the values reported previously. The difference in the interfacial free energies of Y 2 O 3 and ZrO 2 is determined to be Ϫ21Ϯ3 kJ/mol.

Mechanical properties of biological molecular aggregates are essential to their function. A remarkable example are double-stranded DNA viruses such as the φ29 bacteriophage, that not only has to withstand pressures of tens of atmospheres exerted by the confined DNA, but also uses this stored elastic energy during DNA translocation into the host. Here we show that empty prolated φ29 bacteriophage proheads exhibit an intriguing anisotropic stiffness which behaves counterintuitively different from standard continuum elasticity predictions. By using atomic force microscopy, we find that the φ29 shells are approximately two-times stiffer along the short than along the long axis. This result can be attributed to the existence of a residual stress, a hypothesis that we confirm by coarse-grained simulations. This built-in stress of the virus prohead could be a strategy to provide extra mechanical strength to withstand the DNA compaction during and after packing and a variety of extracellular conditions, such as osmotic shocks or dehydration.

The growth and thermal stability of an iron oxide overlayer on yttria-stabilized zirconia (YSZ) have been studied using atomic layer deposition (ALD), mainly in combination with low-energy ion scattering (LEIS). These techniques form a powerful combination, where ALD is designed for controlled (sub)monolayer deposition, while LEIS selectively probes the altered outermost atomic layer. The Fe(acac) 3 precursor reacts already at room temperature with YSZ. The reaction proceeds until saturation, which is characteristic for ALD. After the results of repeated ALD cycles, which consist of Fe(acac) 3 deposition followed by an oxidation treatment, have been studied, a model could be proposed which describes the growth mode of the iron oxide layer on YSZ. Oxidation at temperatures of 800 °C and higher causes a migration of Fe 2 O 3 into the bulk, limiting its usefulness in surface catalytic processes at these temperatures. At 800 °C the diffusion coefficient of Fe in YSZ is determined to be 10 -23 m 2 /s. The reaction mechanism of Fe(acac) 3 with the YSZ surface is studied using infrared diffuse reflectance. The results reveal more than one reaction mechanism, but there seems to be a preference for the reaction via coordinatively unsaturated sites.

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