Multiskalenmodellierung zum Verständnis der Facettierung und des orientierungsabhängigen Wachstums von Aluminiumnitirid-Volumenkristallen aus der Gasphase
Mechanische Eigenschaften von metallischen Werkstoffen und ihre mikrostrukturellen Ursachen
Zusammenfassung der Projektergebnisse
On the experimental side, the growth mechanisms on the microscopic and macroscopic scale were studied by comparing surface morphology, faceting, and habit of crystals with growth parameters (pressure, orientation, and crucible/set-up materials). The main results are as follows. 1. The growth habit changes from columnar to nearly isometric with increasing growth temperature, while the crystal is completely faceted in the range of temperatures investigated. While the growth directions leading to crystals with stable development, smooth surface, and high structural quality are completely contrasting (only Al-polar growth in W-based set-ups, N-polar growth only possible in TaC-based set-ups), the crystal habit is nearly independent on the set-up material and the corresponding impurity concentrations in the gas phase. 2. The N-polar basal plane is very attractive for growing bulk crystals of low defect density due to the regular step-flow mode on this surface. 3. The decomposition of a flat Al-polar (0001) surface into rhombohedral structure is caused by the high defect density on the surface, which is identified as a network of low angle grain boundaries. 4. No significant differences are found in surfaces of crystals grown at different N2 ambient pressures, which means the supersatureation was nearly the same in all investigated pressures. This result supports the numerical model result that the growth rate is limited by the incorporation of Al from the vapor (see below). 5. For growth in TaC-based set-ups, the growth rate anisotropy and surface morphology do not change significantly with off-oriented seeds. On the numerical modeling side a new “vapor-diffusion” coupled KMC model has been proposed. In the KMC part of the model a most quantitative derivation of the kinetic parameters taking into account all relevant surface constructions was accomplished. On the other hand, via an efficient coupling to a continuum equation for vapor diffusion, long range transport is taken into account in an efficient multi-scale framework, as well. The inner consistency of the model has been established in benchmark simulation as well as in comparison to experiments. Since no such kind of model existed before it may serve as a scratch for a more sophisticated model of the growth of AlN which is indispensible for understanding the orientation-dependent growth and faceting of AlN. The simulation results allow the following conclusions. 1. The rate-limiting step for the PVT growth of AlN is the supply of Al atoms due to the small flow of Al in the vapor phase (Alg) at the steady state under the conditions in this work. 2. The energy barriers for the growth of adsorbed AlN dimers (AlNad) with different neighboring AlN dimers of crystal can influence the growth morphology significantly. 3. For the step-flow growth, the stepbunching caused by the randomness of the terrace widths can be avoided for both homogeneous and linearly inhomogeneous flux of Alg if the rates of the surface events are much faster than the rate of the supply of Al atoms from the vapor. The conclusions 1 and 3 are helpful for improving the growth rate and quality of AlN. Conclusion 2 broadens the conventional idea that the transition from the fractal growth to the compact growth is determined by the ratio of the jumps towards the step and along the step. The cross-linking activities between the experiments and modeling result in the preliminary verifications of the simulation results and the directions of optimizing the growth process, which is beneficial for the both parts.
Projektbezogene Publikationen (Auswahl)
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Faceting in AlN Bulk Crystal Growth and its Impact on Optical Properties of the Crystals, Phys. Status Solidi C 9 (2012) 449-452
M. Bickermann, B. M. Epelbaum, O. Filip, B. Tautz, P. Heimann, A. Winnacker
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Preparation of bulk AlN seeds by spontaneous nucleation of freestanding crystals, Jpn. J. Appl. Phys. Vol 52 [8] (2013) 08JA06
C. Hartmann, J. Wollweber, A. Dittmar, K. Irmscher, A. Kwasniewski, F. Langhans, T. Neugut, M. Bickermann
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A study of the step-flow growth of the PVT-grown AlN crystals by a multi-scale modeling method. CrystEngComm, 2014,16, 6564-6577
W. Guo, J. Kundin, M. Bickermann, H. Emmerich
