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As archaeologists, we seek to understand variation and change in past human societies. This goal necessitates a comparative approach, and comparisons justify the broad cross-cultural and diachronic scope of our work. Without comparisons we sink into the culture-bound theorizing against which anthropology and archaeology have long sought to broaden social science research. By undertaking comparisons that incorporate long-term social variability, archaeologists not only improve our understanding of the past, but also open the door to meaningful transdisciplinary research. Archaeologists have unique and comprehensive data sets whose analysis can contribute to dialogues surrounding contemporary issues and the myriad challenges of our era.
In the past two decades, the pendulum seems to have swung away from comparative research in archaeology. Many archaeologists focus on detailed contextual descriptions of individual cases, and only a few have dedicated themselves to explicit comparative work. Yet in that same time span, fieldwork has expanded tremendously throughout the world, leading to an explosion of well-documented diachronic data on sites and regions. We now have substantial detail on the variation inherent in phenomena such as cultural assemblages, settlement patterns, and economic activity. New methods, from dating techniques to digital data processing, promote comparative analysis and greatly advance our understanding of human societies and change. The time is ripe for a renewed commitment to comparative research in archaeology.
Systematic experiments have been carried out to characterize the yttria containing zirconia thin films on sapphire substrates by 248nm KrF excimer laser ablation. The deposition rate as a function of laser fluence and O2 pressure at room temperature was measured with a quartz crystal microbalance. The results show different threshold fluences for deposition in vacuum vs. oxygen. While the deposition rate increases with increasing fluence at a given oxygen pressure, the rate eventually saturates at a higher laser fluence. At a given fluence, the oxygen pressure dependence of the deposition rate shows a radical reduction when the O2 pressure increases from 10 mTorr to 1 Torr. Rutherford backscattering spectrometry (RBS) and x-ray photoelectron spectroscopy were used to obtain stoichiometric information. A very strong pressure dependence of the O/Zr ratio was observed. While the trend of increasing O/Zr and Zr/Y ratio with increasing O2 pressure is apparent, the correlations between O/Zr as well as Zr/Y ratio and other processing conditions are less obvious. RBS results indicate an increasing roughness at the interface between the ZrO2 film and the sapphire substrate as the oxygen pressure exceeds 50 mTorr. The structural information obtained from x-ray diffraction patterns indicates broadening of peak width with increasing laser fluence as well as decreasing substrate temperature. For the film deposited at a lower substrate temperature, a strong (002) texture was observed.
Measurements are presented which show the effect of proton irradiation on the irreversibility line and critical current in Tl2 CaBa2Cu2O8 thin films. These data show that the irreversibility line is dependent on the defect structure and that the pinning energy is increased by proton irradiation. This leads to an increase in the critical current density at 60 K for the lowest radiation dose. Further irradiation reduces the critical current, even while the irreversibility line is enhanced.
Bombardment damage produced by Si+ ions in AlxGa1−xAs/GaAs layer structures has been studied using transmission electron microscopy and ion channeling and backscattering spectrometry. The damage resistance of A1xGa1−xAs alloy layers increases with Al concentration. In particular, by comparison of complementary Si+ ion doses yielding similar nuclear displacement densities at 150keV and 2MeV bombardment energies, it is demonstrated for the first time that the local concentration of implanted Si impurity is likely to be a significant factor in controlling lattice damage build-up, especially for the highest Si+ ion implantation doses. It is also shown that, in a manner analogous to A1As, the alloy layers can confer a significant protection from ion damage upon adjacent, epitaxially-bonded narrow zones of crystalline GaAs.
Chemical processing routes to advanced ceramic materials are gaining importance as a convenient approach to control the stoichiometry, purity, microstructure and final form of the ceramic products . The pyrolytic conversion of organometallic molecules and polymers is one such chemical processing route that has been widely applied in ceramic fiber technology [1,2], in coating processes [1,2], and in the sintering of bulk ceramic objects . Despite these advances in practical applications, there is a continuing need in this area for a better fundamental understanding of the chemistry involved during the precursor-to-ceramic conversion process and for the development of new precursors which yield the desired ceramic(s) in high yield and purity.
The organometallic chemical vapor deposition (OMCVD) of transition metal carbides (M = Ti, Zr, Hf, and Cr) from tetraneopentyl-metal precursors has been carried out. Metal carbides can be deposited on Si, A120 3, and stainless steel substrates from M[CH 2C(CH3)3]4 at temperatures in the range of 300 to 750 "C and pressures from 10-2 to 10-4 Torr. Thin films have also been grown using a carrier gas (Ar, H2). The effects of variation of the metal center, deposition conditions, and reactor design on the resulting material have been examined by SEM, XPS, XRD, ERD and AES. Hydrocarbon fragments generated in the deposition chamber have been studied by in-situ mass spectrometry. Complimentary studies examining the UHV surface decomposition of Zr[CH2C(CH3)3]4 have allowed for a better understanding of the mechanism leading to film growth.
Silicon nucleation on silicon dioxide and selective silicon epitaxial growth (SEG) were studied in an ultra high vacuum rapid thermal chemical vapor deposition (UHV-RTCVD) reactor. Experiments were performed using 10% Si2H6 in H2 in a pressure range of 10 - 100 mTorr at 760°C. Under these conditions, the growth rate ranged from 75 to 330 nm/minute. Loss of selectivity via Si island formation on SiO2 was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM) revealing a strong dependence on deposition pressure. Cross sectional transmission electron microscopy (XTEM) was employed to study the vertical oxide/epitaxy interface where faceting can occur. The incubation time for nucleation was found to increase from 10s to 70s as pressure is reduced from 100 mTorr to 10 mTorr, allowing thicker selective epitaxial film growth in spite of the reduced growth rates. This was attributed to the reduction in gas phase supersaturation of the Si containing species resulting in a lower density of adsorbed atoms on the SiO2 surface. This process shows a potential for chlorine free selective epitaxial growth and provides insight to the surface morphology of polycrystalline films deposited at low pressures.
In this paper, we report our results on surface preparation for the growth of epitaxial Si films. Hydrogen passivated surfaces are currently being investigated for application in Si epitaxy to eliminate the high temperature in-situ bake necessary to remove the native oxide. Hydrogen passivation is obtained by a dilute HF dip before the substrate is loaded in the process chamber. However the passivation is partially lost when the HF dip is followed by a water rinse which results in oxygen absorption on the substrate. It was found that the peak oxygen concentration at the epitaxy substrate interface increase by an order of magnitude due to a five minute water rinse. We report here that oxygen and carbon at the epitaxy substrate interface can be desorbed during initial stage of epitaxial growth by reducing epitaxial growth rate. In this work, epitaxial Si films were deposited over a wide range of growth rates obtained by varying Si2H6 flow rates. The peak oxygen concentration decreases by an order of magnitude by changing the growth rate from 3000 to 700Å/kminute for a deposition temperature of 800°C. We believe that at higher growth rates Si overgrows on absorbed oxygen maintaining epitaxial alignment reflected in the good electrical quality of the epitaxial films. However, at low growth rates oxygen has sufficient time to desorb before overgrowth can take place, improving the epitaxy substrate interface quality.
Temporally and spatially resolved photoluminescence has been used to study patterned structures prepared by focused ion beam (FIB) implantation of multiple quantum wells (MQWs) followed by rapid thermal annealing (RTA). Exciton lifetimes at different positions across the interface between implanted and unimplanted regions suggest that the interface between these two regions is of good quality. Anisotropic exciton diffusion is also observed, suggesting that these structures provide lateral confinement for excitons.