Growth of III-nitrides by molecular beam epitaxy: Unconventional substrates for conventional semiconductors

III-nitrides are intriguing materials with the potential to be used in various interdisciplinary applications, including high-power optoelectronics, energy conversion, green technologies, single-photon emitters, quantum computing, optical communications, biosensing, solar water-splitting systems, and sensing in challenging high-temperature conditions. Commercial planar III-nitrides are hetero epitaxially grown on Si, silicon carbide (SiC), sapphire, etc. (conventional substrates); however, due to several reasons, including lattice and thermal mismatch with the substrate, the quality of the materials, and the performance of the resultant devices are constrained. The issue can be addressed using unusual or unconventional substrates such as flexible metal foil, oxides, diamonds, and 2D materials. The mismatches (lattice and thermal expansion coefficient mismatch) between unconventional substrates and nitrides are significantly lesser than conventional substrates. Furthermore, these substrates exhibit potential advantages over their planar equivalents, such as wavelength tunability to the near-infrared and lower efficiency droop. Nitride epitaxy on such unusual substrates is also a good candidate for upcoming applications like the Internet of Things and artificial intelligence because of its low profile and low power consumption. This perspective critically examines and discusses the most recent advances in III-nitride epitaxy on unconventional substrates. We assess and explore the molecular beam epitaxy growth conditions and processes that proceeded to its synthesis. The relationships between optical, electrical, and thermal properties, growth conditions, and relevant applications are described on numerous unconventional substrates. Therefore, the challenges and constraints of effectively utilizing the promise of III-nitride epitaxy for practical implementations are emphasized.