(Invited) A Brief Overview and Outlook on the Progress of III-Nitride Semiconductor Materials

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In the last three decades, III-Nitride (GaN, AlN, InN, and its alloys) semiconductors have revolutionized both the optoelectronic and electronic device industries. Despite a previous lack of native substrates, a number of devices exceeding the performance of those fabricated from well-developed semiconductors were realized. More recently, bulk GaN (ammonothermal) and AlN (physical vapor transport) substrates were developed and commercialized. Both substrates are characterized by low dislocation density, a single crystal nature, and high flatness; but they lack full control of the electrical properties. Films deposited by vapor transport techniques (HVPE and CVD) and molecular beam epitaxy allowed the growth of device layers with high crystalline quality and improved electrical properties. These films have intrinsic free carrier concentrations that are several orders of magnitude lower than that of the substrates. This demonstrates the usefulness of this new type of substrate to fabricate highly efficient optoelectronic and electronic devices. Despite that, low acceptor solubility limit and the high activation of energy of the p-type dopants leads to low free hole concertation at room temperature.Many relevant figures of merit related to electronic device performance scale superlinearly with the band gap; hence, ultra-wide band gap (UWBG) materials (Eg > 3.4 eV) have the potential for far superior performance than conventional WBG materials such as GaN and SiC [1]. UWBG materials will be critical enablers of future electronic components and systems, and a fundamental understanding of their synthesis and properties is critical to exploiting their advantages. In particular, cubic boron nitride, among the semiconductor materials in the UWBG group (c-BN, AlN, Ga2O3, and diamond), with a band gap of ~6 eV and thermal conductivity of 13 W/mK (second only to diamond, 20 W/mK) offers the best combination of breakdown voltage and specific on-resistance [2,3]. Another advantage of c-BN over other UWBG semiconductors is that it can be doped with donors and acceptors, enabling the fabrication of bipolar devices.The development of WBG and UWBG substrates will be reviewed and a brief overview of the intrinsic properties of selected semiconductors will be presented.[1] J.Y. Tsao et al., Adv. Electron. Mater. 4, 1600501 (2018)[2] N. Miyata et al., Phys. Rev. B 40, 12028 (1989)[3] L. Vel et al., Mat. Sci. Engineer. B 10, 149 (1991)This work supported by the Office of Naval Research