Heteroepitaxial Growth of GaN on Unconventional Templates and LayerTransfer Techniques for LargeArea, Flexible/Stretchable LightEmitting Diodes

temperature. This wide spectral range enables potential applications from the infrared to the deep ultraviolet regions of the electromagnetic spectrum. This makes GaN-based nitrides one of the most important materials for light sources such as light-emitting diodes (LEDs). [ 1–5 ] Sapphire has long been used as a substrate for GaN-based LEDs, and silicon (Si) is becoming its strong competitor due to the advantages of its scalability and lower production costs. [ 6–10 ] Lattice mismatch values (defi ned here as (a GaN –a sub )/a sub ) of GaN on sapphire and Si are as high as 16.1 and –16.9%, respectively. As a consequence, the resulting GaN layers are highly defective, with a typical threading dislocation density (TDD) in the range of about 1×10 8 –2×10 10 cm −2 , [ 11–13 ] which is fi ve orders of magnitude larger than that for conventional AlGaAs or AlInGaP LED materials. Nevertheless, GaN-based materials, including In x Ga 1− x N, exhibit highly effi cient photoluminescence (PL) (more than 50% effi ciency), which poses interesting questions as to the role of the defects. [ 14–18 ] Even polycrystalline In x Ga 1–x N materials have intriguing PL characteristics. [ 19,20 ] Such PL characteristics, which are insensitive to lattice mismatch or the TDD, motivate further studies into the growth of GaN on unconventional substrates such as glass, which are scalable to much larger areas with lower costs than Si. [ 21,22 ] However, there remain two major challenges for exploiting glass as a growth template for GaNbased optoelectronic devices: its amorphous nature and its low melting point. The former impedes epitaxial growth, and the latter results in poor crystal quality, yielding randomly oriented, polycrystalline GaN. Here, we review recent progress in heteroepitaxial growth of GaN, both on amorphous and on large-scale single-crystalline substrates through the use of appropriate interlayers (ILs), as well as progress in low-temperature GaN growth which has enabled successful demonstrations of GaN-based red–green– blue (RGB) LEDs on glass substrates. [ 23–28 ] We also review layer-transfer techniques for GaN thin fi lms grown on ILs onto foreign substrates for high-performance fl exible/stretchable lighting applications. Large-scale, fl exible inorganic LEDs could be applied for fl exible LED TV displays, large-scale Heteroepitaxial Growth of GaN on Unconventional Templates and Layer-Transfer Techniques for Large-Area, Flexible/Stretchable Light-Emitting Diodes