Epitaxy of group III-nitride materials using different nucleation schemes

Sammanfattning: Group III-nitride materials, gallium nitride (GaN), aluminum nitride (AlN) and indium nitride (InN) have direct band gaps with band gap energies ranging from the infrared (InN) to the ultraviolet (GaN) and to the deep ultraviolet (AlN) wave-lengths, covering the entire spectral range from 0.7 eV to 6.2 eV upon alloying. The invention of the GaN-based blue LEDs, for which the Nobel prize in Physics was awarded in 2014, has opened up avenues for exploration of III-Nitride mate-rial and device technologies, and has inspired generations of researchers in the semiconductor field. Group III-nitrides have also been demonstrated to be among the most promising semiconductors for next generation of efficient high-power, high-temperature and high-frequency electronic devices. The need to build a sustainable and efficient energy system motivates the development of vertical GaN transistors and diodes for applications with power ratings of 50-150 kW, e.g., in electric vehicles and industrial inverters. The key is to grow GaN layers with low concentration of defects (impurities and dislocations), which enables an expansion in both voltage and current ratings and reduction of cost. Despite intense investigations and impressive advances in the field, defects are still a major problem which hinders exploiting the full potential of GaN in power electronics. The aim of this thesis is to perform an in-depth investigation of the growth of GaN and AlGaN under several nucleation mechanisms provided by different underlying substrates. In that regard, four different epitaxial approaches based on different nucleation schemes have been studied: (i) growth of planar GaN layers trough NWs reformation. We investigated GaN layers with different thicknesses on reformed GaN NW templates and highlight this approach as an alternative to the expensive HVPE GaN substrates. The sapphire used as a substrate limits to some extent the reduction of threading dislocations, however, the resulting GaN material presents smooth surfaces and thermal conductivity close to the value for bulk GaN. (ii) Homoepitaxial GaN growth. We developed a hot-wall MOCVD epitaxial approach that enables low surface roughness and appropriate impurity levels for advanced vertical power device architectures. A comprehensive picture of GaN homoepitaxy on different GaN surfaces, GaN templates on SiC and HVPE GaN substrates, is established on the basis of experimental results and thermodynamic considerations. (iii) GaN growth on GaN NWs templates by hot-wall MOCVD resulted in an atomically flat smooth surface with reduction of threading dislocations when the optimum annealing conditions have been employed. (iv) Heteroepitaxial growth of low Al composition n-AlxGa1-xN on SiC substrates revealed 700 nm crack-free epi-layers for an Al composition up to 12%. The highest mobility corresponds to an Al content of 6.5% where we also get a reduction in screw and edge dislocations. The results show the potential application of AlxGa1-xN(x= 0 - 0.12) as the active material for drift layers. Some of the epitaxial approaches developed in this thesis have been already implemented in the growth of power devices such as quasi-vertical GaN FinFETs on SiC substrates and fully-vertical GaN FinFETs on HVPE GaN substrates.