Growth of aluminum nitride bulk crystals by sublimation

The research work of this thesis is driven by the fact that the lack of nitride bulk crystals has hindered the full-realization of III-nitride devices. AIN bulk crystals were grown on a resistively heated furnace by sublimation and characterized by optical microscopy, XRD, AFM, SEM, and Raman spectroscopy.; First the properties, synthesis, advantages and disadvantages of the seven most commonly/potentially employed substrates (sapphire, 6H-SiC, Si, GaAs, LiGaO₂, Al, and GaN) for epitaxy are presented and consequences are discussed, including the crystallographic orientation and polarity, surface morphology, stress, and defects in the GaN films.; Subsequently, the transport effect and surface kinetics for the sublimation growth were investigated. Theoretical predictions from a detailed two-dimensional model accounting transport only agree well with the experimental data at pressure above 100 Torr and seed temperature ranging from 1700∼1900°C while the activation energy of the growth rate was estimated as 681KJ/mol. Consequently, a global model accounting for both the surface kinetics and transport in the vapor phase is described to explain the mismatches between transport-only model predictions and experiments below 100 Torr. The model parameters for the sticking coefficient of N₂ were identified from the experimental data. The refined model more accurately predicts the growth rate over a wider pressure range.; Then, the effects of substrate misorientation and buffer layers (both SiC and AlN) on the morphology and growth mode of AlN deposited on 6H-SiC were explored. The AlN sublimated on the on-axis and off-axis 6H-SiC substrate without any treatment proceeded by island growth, producing a high density of screw dislocations. This produced individual AlN grains and rough surface morphologies. These problems were largely eliminated by first depositing an AlN layer on the 6H-SiC by MOCVD before starting sublimation growth. 2-dimensional growth was achieved on the 6H-SiC with AlN-buffered layers and no screw dislocations were observed. At last, growth on off-axis SiC with 6H-SiC epilayers had the step flow growth mode, but with the increasing of step width, hexagonal nucleation hillocks deposited on terraces.; Finally, AlN crystals (thickness >200 μm) grown on the 6H-SiC substrates by sublimation were characterized by Raman spectroscopy. Good agreement was found between the theoretical frequency shifts on E₁(TO) mode calculated from model predicted stress and experimental ones taken along the edges of crystal samples. Stress has linear distribution along the depth of samples, and changes from compressive at growing surface to tensile at interface (up to 1 GPa) between AlN and SiC.