AlN-based HEMTs grown on silicon substrate by NH3-MBE

In this paper, AlN-based HEMTs on silicon are demonstrated using NH3-MBE. The spirit is to get the highest 2DEG density theoretically achievable in nitrides while keeping thin barrier thickness, which is mandatory to achieve high frequency performances. To do that, the strategy consists in growing a relaxed AlN buffer on silicon, then a compressively strained thin-GaN channel on top of which an almost strain-free AlN barrier is grown. The concept has been demonstrated by D. Jena et al. on AlN-on-sapphire template, as well as on SiC. However 2DEG mobility values obtained are still quite low mainly due to growth issues. In this work, we take advantage of our strong know-how in growing pure AlN on silicon using MBE. Indeed, since the past 20 years, strong efforts have been successfully conducted, in order to develop a suitable AlN nucleation/buffer layer on Si substrate (with smooth AlN surface, high crystalline quality, controlled and reproducible AlN/Si interface). With a total epilayer thickness < 330nm-thick, a 2DEG density (Ns) as high as 2.7x1013 cm-2 is measured before passivation. Results indicate that Ns increases improving the material quality as well as using SiN surface passivation. An original in-situ SiN surface passivation using NH3-MBE is presented. State-of-the-art AlN-based HEMT mobility values above 600 cm²/Vs are measured and recent improvements are on going to lower the sheet resistance as required for RF applications