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Introduction 1,2

GaN-based light-emitting diodes (LEDs) and AlGaN/GaN heterostructures3–5 grown on silicon substrates appeared in the late 1990s and early 2000s. In the early days, two main challenges for growing GaN on Si were identified: (1) nucleation of the AlN buffer layer, grown to prevent Ga-Si alloy formation, and quality/reproducibility of the resulting AlN/Si interface, and (2) epilayer cracking because of a larger thermal expansion coefficient for nitrides compared to silicon.6–8 Because of this critical thickness for the onset of cracking (∼1 μm), it appeared challenging to reduce the threading dislocation density (TDD), which is the main defect in GaN layers and decreases when the layer thickness increases. Now, GaN-on-Si technology has experienced significant improvements in terms of layer growth quality.9 However, mass adoption of GaN-on-Si for lighting is still controversial, mainly because epilayers on Si are more complex (and more expensive) to grow compared to, for example, GaN on Al2O3 (sapphire).10 Concerning electronic applications, the context is more favorable for GaN on Si because of the poor thermal conductivity of Al2O3 (sapphire) and the high cost of SiC, both of which are commonly used substrates for GaN.10 GaN on Si thus seems to be a promising contender for the fabrication of low cost and highly efficient power devices.11 However, despite promising performance achieved using AlGaN/GaN heterostructures on Si,12–15 this technology is

facing reproducibility and reliability issues.16 The purpose of this article, with the previous background, is to present some of the challenges of epitaxial growth of GaN on Si, with an emphasis on basic aspects that are important to consider for power electronics. Power electronics is the application of solid-state electronics for the control and conversion of electric power.

Understanding the AlN nucleation process Despite large lattice mismatch (+19%), AlN buffer layers having smooth surface morphologies (typically with rms

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