H.H. Brongersma

Island growth in the atomic layer deposition of zirconium oxide and aluminum oxide on hydrogen-terminated silicon: Growth mode modeling and transmission electron microscopy

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 oxid...

The limiting factor for oxygen exchange at the surface of fuel cell electrolytes

The outermost surface layer of the yttria stabilised zirconia (YSZ) electrolyte in the solid oxide fuel cell plays an important role in the performance of the cell. By combining 16O/18O exchange experiments with quantitative surface analysis by low energy ion scattering spectroscopy (LEIS), the relation between the composition of the outermost atomic layer and the oxygen kinetics at the surface of the electrolyte can be studied. The results suggest a linear relation between an increasing amount of impurity oxides present at the surface and a decreasing oxygen exchange. Ceramics invariably contain glassy impurities, which segregate to the grain boundaries during sintering. Our results, however, show that the accumulation is restricted to the outermost atomic layer and proceeds until complete coverage is obtained. This observation underlines the importance of the used surface analysis technique. The strong accumulation of the impurities at the surface is observed even in the purest YSZ materials available. A decrease of the total bulk impurity concentration by a factor of 10–100 is necessary to ensure that the YSZ surface cannot be covered completely by impurities. The amount of exchanged oxygen at the outermost surface layer reduces from 50% at a clean yttria stabilised zirconia surface to zero at a surface completely covered by impurity oxides. Although the oxygen exchange experiments pertain to a temperature of 500 °C it is believed that the impurity oxides also have a strong influence at higher temperatures.

On the magnetic properties of ultra-fine zinc ferrites

Zinc ferrite belongs to the class of normal spinels where it is assumed to have a cation distribution of Zn2+(Fe3+)2(O2−)4, and it is purported to be showing zero net magnetisation. However, there have been recent reports suggesting that zinc ferrite exhibits anomaly in its magnetisation. Zinc ferrite samples have been prepared by two different routes and have been analysed using low energy ion scattering, Mossbauer spectroscopy and magnetic measurements. The results indicate that zinc occupies octahedral sites, contrary to the earlier belief that zinc occupies only the tetrahedral sites in a normal spinel. The amount of zinc on the B site increases with decrease in particle size. The LEIS results together with the Mossbauer results and the magnetic measurements lead to the conclusion that zinc occupies the B site and the magnetisation exhibited by ultrafine particles of zinc is due to short range ordering.

Atomic layer deposition of hafnium oxide on germanium substrates

Germanium combined with high-κ dielectrics 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 (ALD) of HfO2 high-κ dielectric layers on HF-cleaned Ge substrates. In this contribution, we describe the HfO2 growth characteristics, HfO2 bulk properties, and Ge interface. Substrate-enhanced HfO2 growth occurs: the growth per cycle is larger in the first reaction cycles than the steady growth per cycle of 0.04nm. The enhanced growth goes together with island growth, indicating that more than a monolayer coverage of HfO2 is required for a closed film. A closed HfO2 layer is achieved after depositing 4–5HfO2 monolayers, corresponding to about 25 ALD reaction cycles. Cross-sectional transmission electron microscopy images show that HfO2 layers thinner than 3nm are amorphous as deposited, while local epita...

Indium diffusion in model polymer light-emitting diodes

The diffusion of indium into poly-(phenylenevinylene) (PPV) in model polymer light-emitting diodes (p-LEDs) was studied with Rutherford backscattering spectrometry (RBS), X-ray photoelectron spectroscopy (XPS), low energy ion scattering spectroscopy (LEIS) and particle induced X-ray emission (PIXE). The model p-LEDs consisted of a glass substrate, an indium–tin-oxide (ITO) electrode, a PPV layer obtained by thermal conversion of sulfonium precursor PPV, and a patterned aluminium electrode. From RBS measurements it was concluded that about 0.01 at.% indium was present in the PPV, homogeneously distributed in depth. Annealing at 230°C for 19 h caused the amount of In in the PPV layer to increase by roughly an order of magnitude. Under the patterned aluminium electrode, the annealing treatment resulted in accumulation of In at the PPV/Al interface, whereas the depth distribution of In remained homogeneous in the uncovered region of the model LEDs. XPS spectra on annealed model LEDs show that In was present in the near surface region of the PPV films, although LEIS analysis showed that In was not situated in the outermost atomic layer. LEIS measurements on as-prepared model LEDs showed that the patterned Al electrode had caused surface contamination of the uncovered PPV film with Al, which can have impact on the diffusion of In to the outermost surface during annealing treatments.

Growth mechanism and continuity of atomic layer deposited TiN films on thermal SiO2

In atomic layer deposition (ALD), film thickness control by counting the number of deposition sequences is poor in the initial, nonlinear growth region. We studied the growth of TiN films formed by sequentially controlled reaction of TiCl4 and NH3 on thermal SiO2 during the transient, nonlinear period. Using low-energy ion scattering and Rutherford backscattering spectroscopy analysis, we have found that a three-dimensional growth of islands characterizes the ALD TiN growth on SiO2. Growth at different temperatures (350 °C and 400 °C) affects the extent of the transient region and the rapid closure of the film. At 400 °C, a reduced growth inhibition and an earlier start of three-dimensional growth of islands results in film closure at about 100 cycles, corresponding to a TiN thickness of 24±3 A. At 350 °C the minimum thickness at which the TiN layer becomes continuous is 34±3 A, deposited with 150 cycles.

Silica poisoning of oxygen membranes

Perovskite oxide membranes (La0.6Sr0.4Co0.2Fe0.8O3) are used for the separation of oxygen from air. In oxygen permeation experiments these membranes showed a peculiar behavior. Besides poor performance, a characteristic coloring also appeared on the surface of the membranes. In order to understand what was happening to the surface of the membranes, they were analysed with Low-Energy Ion Spectroscopy (LEIS) and X-ray Photoelectron Spectroscopy (XPS). The analyses showed that the surface of the LSCF membrane was covered with a SiO2 layer, which obviously reduced the performance and caused the coloring. It was established that the source of the silicon was siloxane containing grease that was used in the manual valves of the setup. In a new improved permeation setup, where grease-free valves were used, the LSCF membranes showed remarkably better performance. The LEIS measurements showed also that the permeation experiment of 300 h did not affect the surface composition of the membranes. The contamination-free LSCF membranes only showed the presence of La, Sr and O in the outermost atomic layer. The observed absence of Co and Fe suggests that further improvement of the membrane performance is possible.

The influence of the preparation method on the surface structure of ZnAl2O4

Abstract The effect of preparation method (sol-gel, wet mixing or coprecipitation) on the surface structure of zinc aluminate is reported. Coprecipitated and sol-gel prepared ZnAl2O4 were found to be present spinel structure. If calcium was added during synthesis, it was found to be deposited on top of the spinel surface while, if tin was added, the surface of ZnAl2O4 was reconstructed. These aluminates were impregnated with platinum and tested in the isobutane dehydrogenation. The characterization techniques used were LEIS and X-ray diffraction.

Oxygen exchange and diffusion in the near surface of pure and modified yttria-stabilised zirconia

By studying the oxygen transport through yttria-stabilised zirconia (YSZ), a strategy could be proposed which should lead to a reduction in the operating temperature of the solid oxide fuel cell (SOFC) to the intermediate temperature range without loss in performance. The combination of isotopic exchange depth profiling with low energy ion scattering (LEIS) and elastic recoil detection analysis (ERDA) has shown a complex structure affecting the surface oxygen exchange reaction and self-diffusion in 10 mol% yttria-doped zirconia. Remarkable is the presence of a thin (about 6 nm) layer at the external surface showing resemblance with the monoclinic phase. The results suggest a significant improvement in the surface oxygen exchange with respect to the values reported in literature when impurity oxides are prevented from segregating to the external surface. A possible operating temperature of around 850 °C seems feasible. Improvements in the surface oxygen exchange by addition of a surface catalyst reported in literature are also attributed to the removal of impurities. Further decrease in operating temperature, down to at least 725 °C, should be possible by removing the impurities in the bulk, which should lead to a considerable increase in the grain boundary diffusion and by reduction of the electrolyte thickness.

Subsurface segregation of yttria in yttria stabilized zirconia

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 2O3 and ZrO2 is determined to be 22163 kJ/mol.