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Abstract:

The III-Nitrides of AlN, GaN and InN, their solid solutions, and ZnO form in the wurtzite crystal structure.  Deviation from the ideal c/a ratio for a hexagonal crystal of 1.633 and the absence of a center of symmetry dictate that dipole orientation and therefore spontaneous polarization will occur and induce electrostatic fields along the <0001> polar axes that spatially separate electrons and holes within quantum well structures.  This separation causes a reduction in oscillator strength and a shift in the optical transitions due to reduced recombination efficiency in optoelectronic devices grown along the polar c-axis.  Piezeoelectric polarization is also present in multilayer heterostructures.  The use of non-polar GaN-based layers circumvents these effects.
Boules and large wafers of the III-Nitrides having a low density of dislocations are not available.  As such, essentially all nitride films and device structures are grown on either sapphire or silicon carbide (SiC) substrates containing a buffer layer of GaN, AlN or AlGaN.  These films grow via complex thermodynamically- and kinetically-controlled mechanisms and contain significant residual stresses and densities of defects that affect the properties of all optoelectronic and microelectronic devices produced in this materials system.
The results of fundamental studies concerned with the mechanisms of the initial and the subsequent stages growth of GaN on the polar (0001) and the non-polar (1120) planes of an AlN/SiC substrate and of ZnO on the analogous planes of GaN and ZnO substrates will be detailed in the seminar.  Briefly, films of GaN grow on AlN/SiC(0001) substrates via the Stranski-Krastanov mode, i.e., via the formation of a 1-1.5 nm thick wetting layer and the subsequent growth and coalescence of islands.  Copious threading dislocations are generated to relieve the biaxial stress.  Gallium nitride films deposited on AlN/4H-SiC(1120) grow via the Volmer-Weber mode with rapid growth of islands along (1100) to near surface coverage at a thickness of 2 nm.  Continued deposition results in both faster vertical growth along [1120] relative to the lateral growth along [0001] and an [1100]-oriented microstructure containing rows of GaN. Fully dense GaN films develop between 100 and 250 nm of growth, and the preferred in-plane orientation changes to [0001].  Reduction in both the residual stresses and the dislocation density has been achieved in these films via lateral overgrowth techniques.
Fully dense GaN films develop between 100 and 250 nm of growth, and the preferred in-plane orientation changes to [0001].  Reduction in both the residual stresses and the dislocation density has been achieved in these films via lateral overgrowth techniques.
Dense ZnO thin films have been achieved on GaN(0001) epilayers and ZnO substrates via the repetition of an iterative sequence involving the growth at 480°C of needles having a decreasing diameter as a function of height followed by the lateral growth at 800°C from the sidewalls of these needles and coalescence of the growth fronts. The relationships among the growth chemistry and process parameters and the resulting microstructure of and impurity chemistry within the various ZnO films will be presented in concert with results from microscopy and spectrometry studies.

Bio: Robert Davis received his Ph.D. from the University of California at Berkeley.  He is a member of the National Academy of Engineering.  He was formerly on the faculty of North Carolina State University as the Kobe Steel, Ltd. Distinguished University Professor of Materials Science and Engineering.  Professor Davis' research interests include the growth and characterization of semiconductor thin films, nanolithography and the fabrication and characterization of optoelectronic devices and their associated components.