The Underemphasized Concept of Crystal Polarity in Conventional Semiconductors and Its Device Application

This review focuses on nitrogen (N)‐polar GaN grown on (0001) sapphire substrates by metalorganic vapor phase epitaxy (MOVPE). Although N‐polar GaN growth is often considered challenging, the mechanisms behind its crystal growth are discussed in detail. For the growth of InN with the highest nitrogen equilibrium vapor pressure among nitride semiconductors, N‐polar growth can be considered to be advantageous because N‐polar growth allows for more efficient nitrogen incorporation from the gas phase compared with Ga‐polar growth. MOVPE with a pressurized reactor is introduced as a method for growing high‐quality indium‐rich InGaN. N‐polar growth with this technique also suggests the potential to narrow the miscibility gap in nitride semiconductors. In nitride semiconductor devices, polarization, which is a unique characteristic of these materials, plays a significant role. The interplay between device performance and polarization is explored, highlighting how careful control of polarization enables the fabrication of advanced devices such as “Inverted HEMTs”. In the fabrication process of nitride semiconductor devices, the Mg‐doped p‐type layer must be grown last due to the risk of Mg contamination in the MOVPE reactor, making band engineering crucial. Using solar cells as an example, the advantages of N‐polar growth in device applications are demonstrated.