Realistic Growth Conditions Research Articles

Facetting during directional growth of oxides from the melt: coupling between thermal fields, kinetics and melt/crystal interface shapes

Implicit in most large-scale numerical analyses of crystal growth from the melt is the assumption that the melt/crystal interface shape and position are determined by transport phenomena. Although reasonable for many materials under a variety of growth conditions, this assumption is incorrect for a number of practical systems under realistic growth conditions. Specifically, the behavior of systems (e.g. certain oxides) which tend to develop facets along the melt/crystal interface is often affected both by transport phenomena and by interfacial attachment kinetics. We present a new modeling approach which accounts for interfacial kinetic effects during melt growth of large single crystals. The isotherm condition, typically employed at the melt/crystal interface, is replaced by an equation accounting for undercooling due to interface kinetics. A finite-element algorithm, designed to accommodate its numerical mesh to the appearance of facetted interfaces, is applied to this problem. Results are presented for the simulated directional growth of oxide slabs. The interplay between evolving thermal fields and anisotropic interface kinetics is investigated. In particular, the evolution of facets and the dependence of their size on growth conditions is explored. Trends reported here are in qualitative agreement with those appearing in the literature. Discrepancies between quantitative predictions of facet sizes using a theory (see Ann. Rev. Mater. Sci. 3 (1973) 397 and references within) and those calculated in this paper can, in a number of cases, be attributed to the simplifications on which this theory is based.

Application of Diluted Chlorine Dioxide to Radish and Lettuce Nurseries Insignificantly Reduced Plant Development

The possible toxicity of a commercial chlorine dioxide preparation (Halox E-100) was evaluated on radish and lettuce seedlings growing in pots under controlled conditions. A single application of various dilutions to radish seedlings growing in a sterile or nonsterile commercial plant substrate only slightly decreased plant dry weight. At the end of the experiments, the plants appeared unaffected by the treatments. Other common plant parameters (root and stem length, number of true leaves) were unaffected or even enhanced. Halox did not reduce the total level of soil bacteria even after four consecutive applications at any dilution rate. In nonsterile soil, high Halox dilution (1:1000) significantly decreased plant dry weight, and the other concentrations (1:10,000; 1:50,000, and 1:100,000) had no apparent effect on the size of the plants. In sterile soil, high concentrations of Halox (1:1000 and 1:10,000) significantly decreased plant growth, but higher dilutions produced no significant reduction in plant dry weight. For radish plants growing in organic matter-free sand only, dilution of 1:10,000 reduced plant growth. On lettuce plants, dilutions from 1:5000 to 1:25,000 did not reduce plant growth. High levels of Halox (1:1000) were toxic to both radish and lettuce seedlings growing in sand and resulted in chlorosis and significant depression of plant growth. Further dilutions of Halox (equivalent to the level used in water disinfection) significantly decreased toxicity for both plant species. Low concentrations of Halox (>1:50,000) had no apparent effect on the appearance of both plant species. In conclusion, this study suggests that chlorine dioxide-treated drinking water can be considered safe for growing plants; this treatment should be further evaluated using other plant species under more realistic growth conditions.

Growth kinetics, impurity incorporation, defect generation, and interface quality of molecular-beam epitaxy grown AlGaAs/GaAs quantum wells: Role of group III and group V fluxes

A systematic knowledge of the influence of molecular-beam epitaxy growth parameters on the properties of AlGaAs/GaAs quantum wells grown under realistic growth conditions is important in order to obtain optimal performance of modern electronic and optoelectronic devices. Photoluminescence (PL) was used to investigate the influence of the Ga-controlled growth rate in the range below standard growth rates of 1 μm/h down to 0.1 μm/h, and of the As:Ga beam equivalent pressure ratio in the range of 10 to 60, on the growth kinetics, the interface quality, and the impurity incorporation, at a substrate temperature Ts =620 °C. As compared to in situ reflection high-energy electron diffraction (RHEED) measurements, where no sample rotation is possible, PL has the advantage that realistic growth conditions can be used. A careful line shape analysis, together with infrared and time-resolved PL measurements gives information on the interface roughness, the impurity incorporation, and the deep trap concentration. Non-negligible desorption of Ga during growth is found for the range of conditions under study. The desorption is found to increase upon a decrease of As:Ga ratio. The interface roughness as well as the impurity and trap incorporation are found to decrease with decreasing growth rate, an optimum interface quality being obtained below 0.5 μm/h. At this optimal growth rate, increasing the As4:Ga ratio leads to a decrease of shallow impurity concentration and thus to a narrower line width, but to a simultaneous increase of defect generation. Optimal growth conditions are found at a beam equivalent pressure ratio of 15. The observed desorption kinetics and interface properties can be explained in accordance with existing theoretical simulations. Finally, growth interruption was found to lead to optimal formation of flat growth islands when the overall growth rate is lowered.

Impurity redistribution during epitaxial growth

The distribution of impurities incorporated in epitaxial layers during their growth is determined by diffusion due to concentration gradients and by drift in the built-in electric field. This problem has been previously treated in the approximation that the impurities reach their equilibrium distribution during growth. We have extended this calculation to treat impurity drift and diffusion under non-equilibrium conditions, a situation much more characteristic of most realistic growth condi-tions. In the new calculation, the solution of the Shockley-Poisson problem is exact and the boundary con-dition at the growth interface is an approximation based on the electrostatics of surface states. The new calcu-lation also permits consideration of outdiffusion from the substrate, a phenomenon of technological significance. It is also capable of modeling variations of source im-purity concentration and growth rate during epitaxial growth. Calculations modeling n-type GaAs epitaxy of practical interest are presented.