PhD defence of Maya Wehbe

Modeling and characterization of nano-compliance effects for localized epitaxial growth of GaN on Si substrates

Maya Wehbe conducted her PhD research at CEMEF in the framework of the ANR project PEGADIS under the supervision of Patrice Gergaud and Matthew Charles (CEA Leti) as well as Daniel Pino Munoz, MSR team.

Maya Wehbe defends her PhD in Physics of condensed matter and radiation on November 29th, 2024 in front of the following jury:

– Stéphanie ESCOUBAS, Aix-Marseille Université

– Nicolas GRANDJEAN, École Polytechnique Fédérale de Lausanne (EPFL)

– Vincent CONSONNI, Grenoble INP

– Stéphane MOREAU, CEA Grenoble

– Maud NEMOZ, CRHEA – CNRS

– Eirini SARIGIANNIDOU, Grenoble INP – UGA

Abstract:

Gallium nitride (GaN) is a promising semiconductor for µLEDs, but heteroepitaxial growth of GaN generates dislocations that reduce their emission efficiency. To improve the quality of the GaN, we propose novel approach based on growing GaN pyramids on top of GaN/AlN/Si(111)/SiO₂/Si(001) etched nano-pillars. The approach relies on the excess surface energy at each pyramid’s interface and the viscoelastic properties of SiO₂ to allow the pillars to tilt/twist, coalescing and aligning the GaN on top. The main objective of this work is to gain a physical understanding of the processes operating during the GaN coalescence for a better control, determine the GaN quality, investigate the tilt/twist of the pillars and propose an optimal pillars pattern. Therefore, different samples were studied by electron backscatter diffraction, cathodoluminescence and three advanced X-ray diffraction techniques at the European Synchrotron Radiation Facility. The results demonstrated the rotation of pillars by 0.1° using SXDM measurements on sets of three pillars. Additionally, we showed that with our growth method we were able to obtain homogenous GaN layers in lines of pillars with very low dislocation density (1.2 x 10⁷ cm^-2). We were able to follow the behavior of the GaN at the early stage of coalescence, in fact, the initially misoriented GaN pillars, were found to coalesce into larger well-defined GaN domains with a unique orientation distribution within each domain and a tilt limit of 0.1° between neighboring pillars was found. Geometrically necessary dislocations were found at the grain boundaries of the GaN domains and estimated to be around ~10¹¹ cm^-2. To complete the work, finite element simulations using abaqus software are performed to identify the optimal parameters that make the pillars rotation energetically feasible; the radius (r) of the pillar was identified as the parameter with the greatest impact on the energy required to rotate the pillars as the latter is proportional to r⁴. These results allowed the realization of new optimized pillars pattern that showed promising results and will allow the fabrication of high quality GaN islands suitable for µLEDs.

Keywords: Gallium nitride, X-ray diffraction, synchrotron, crystallography, microLEDs, mechanical modeling