R.G. Lee

Compression of a crystalline ZnO nanotube: An experimental exploration of the B4 to B1 transition mechanism

The high-pressure behavior of zinc oxide (ZnO) nanotube has been investigated to 38.7 GPa by in situ synchrotron x-ray diffraction in a diamond anvil cell at room temperature. The transformation from the hexagonal wurtzite (B4) to the cubic rocksalt (B1) phase started at 10.5 GPa and completed at 18.4 GPa. The initial transition pressure of nanotube is found identical to that in bulk crystal but lower than in nanodots, while the completion pressure of nanotube is identical to that in nanodots but higher than in bulk crystal. This indicates that the c-direction of hexagonal ZnO crystal plays a more important role in the initiation of the phase transition, and the a-direction controls its completion. These prove that the B4-B1 transition undergoes a hexagonal path. It is also found that the c/a ratio of the B4 phase decreases slightly before the phase transition and tends to increase during the phase transition, which is also consistent with the theoretical hexagonal-path model. The bulk moduli of B4 and B1...

Modeling of residual thermal stresses for aluminum nitride crystal growth by sublimation

Residual thermal stress distribution in AlN single crystal, grown on tungsten as a crucible material, was investigated using a numerical study. It has been demonstrated that a three-dimensional, instead of a two-dimensional, formulation predicts significantly greater values of stress. Dimensionless coordinates were used to essentially simplify the stress analysis and reduce the number of calculations. In addition, thermoelasticity approach simplifies the study of stresses for a nonstationary temperature field. The stress in the AlN film along the thickness or [0001] growth direction is essentially zero but the in-plane stress is large. The stress at the corner of the film is much higher due to stress concentration and could cause formation of microcracks. The stress in the film is tensile while that in the substrate is compressive, which causes a reversal of the stress across the interface. Separation or delamination of the film from the substrate could occur due to this reversal of the stress at the inte...

Modeling of the effects of different substrate materials on the residual thermal stresses in the aluminum nitride crystal grown by sublimation

A three-dimensional numerical finite element modeling method is applied to compare interfacial residual thermal stress distribution in AlN single crystals grown by using different substrates such as silicon carbide, boron nitride, tungsten, tantalum carbide, and niobium carbide. A dimensionless coordinate system is used which reduces the numbers of computations and hence simplifies the stress analysis. All components of the stress distribution, both in the film and in the substrate, including the normal stress along the growth direction as well as in-plane normal stresses and shear stresses are fully investigated. This information about the stress distribution provides insight into understanding and controlling the AlN single crystal growth by the sublimation technique. The normal stress in the film at the interface along the growth direction and the shear stresses are zero except at the edges, whereas in-plane stresses are nonzero. The in-plane stresses are compressive when TaC and NbC substrates are use...

Oxidation of Aluminum Nitride for Defect Characterization

The thermal oxidation of aluminum nitride was developed as a means to study defects in bulk aluminum nitride crystals. The oxidation kinetics was established for the dry oxidation of highly textured AlN polycrystals produced by sublimation-recombination crystal growth in a tungsten furnace. Despite seeding on polycrystalline tungsten, the grains were predominantly [0001] oriented as verified by electron backscattering diffraction (EBSD). The oxidation rate is dependent on the crystal’s orientation, polarity, stress, and surface condition, thus oxidation decorates grain boundaries, polishing scratches, and inversion domains by producing oxide layers of different thicknesses. Low temperature (800 °C) dry oxidation produced an amorphous oxide layer and generated a high density of defects (vacancies, stacking faults, and dislocations) in the nitride near the oxide/nitride interface, as observed by cross-sectional transmission electron microscopy. In contrast, high temperature oxidation (1000 °C) produced a crystalline oxide layer, and left the nitride free of observable defects.