UC Santa Barbara

Recently reported N-polar gallium nitride (GaN) high electron mobility transistors (HEMTs) have shown exceptional W-band (75-110 GHz) performance, surpassing current Ga-polar device technology in terms of output power performance. This is in part due to the incredible progress made in the growth and optimization of N-polar GaN in the last decade, despite the many problems associated with the unstable N-polar surface. Top performing GaN devices, reported today, are all grown by MOCVD on foreign substrates such as SiC. Growing on foreign substrates where lattice mismatch is present leads to a high density of threading dislocations (TDs) which generate at the substrate/epi interface and propagate in the growth direction, terminating on the surface. These TDs can act as coulombic scattering centers, and, in the case of MBE, lead to vertical leakage and roughen the GaN surface. Although growing thick MOCVD buffer layers has shown to decrease the density of these TDs by almost two orders of magnitude, the resulting TD-density is still typically two orders of magnitude higher than what is found in high-quality bulk-GaN substrates.

Amplifiers biased in class-A mode operate with at a high source-to-drain quiescent current. High channel current in a material with a high density of scattering centers may lead to power dissipation in the form of heat which can impede device efficiency – something which could be improved by growing devices on bulk GaN substrates with an order of magnitude or lower TDD. In addition to dislocation scattering, alloy scattering has shown to be a significant inhibitor of mobility in GaN HEMTs. Using MBE, which typically utilized growth temperatures much lower than MOCVD, thicker coherently strained AlN interlayers or back-barriers can be realized to mitigate the effects of alloy scattering.

Although it has been shown in the past that GaN grown on bulk GaN by MBE resulted in a surface with a high density of pits and depressions, the work presented here demonstrates how smooth high-quality GaN can be grown on non-miscut bulk-GaN substrates. Although the pits seen on the surface of GaN grown on bulk GaN may come from a variety of factors, evidence presented in this work reveals that surface impurities play a significant role in the generation of these pit. By employing an ex-sity UV-ozone surface clean and initiating growth with a thin layer of AlN grown under Ga-rich conditions, high-quality, pit-free, N-polar GaN surfaces were obtained by MBE.

Several device structures using pure AlN back-barriers with thicknesses ranging from 2-6 nm were simulated and it was found that using a 30 nm graded AlGaN layer behind a 2 nm or 6 nm AlN back-barrier resulted in a 2DEG sheet charge density around 4x1013 cm-2, approximately 3-4x larger than what has been reported in current N-polar HEMT structures. Whether or not these devices can be realized will depend on how much strain the AlN backbarrier and graded AlGaN region can sustain without relaxation – something with is subject to further study. From this work, however, we now have the tools to grow more complex N-polar HEMT structures on bulk GaN by MBE which may one day surpass current N-polar HEMT technology.