Buffer leakage is a vexing problem in the development of nitride HEMTs. It degrades the on/off ratio in digital devices, decreases the device speed in high-speed analog applications, and compromises the breakdown property in high-voltage switching applications. This problem exists in HEMTs grown on both semi-insulating (SI) SiC and GaN substrates/templates. On SI GaN templates, high-quality Al(Ga)N/GaN heterojunctions can be achieved using just a few hundred nm GaN buffer layer, which greatly reduces required epitaxial resources and time. Recently, we reported a dopant-free approach by engineering the polarization effect. Insertion of an ultrathin AlN nucleation layer (NL) greatly reduces the buffer leakage, and leads to a much steeper subthreshold slope of HEMT devices and the on/off ratio increased from 10^2 to 10^6. Motivated by this prior result, we have performed a systematic study of the buffer leakage dependence on the AlN nucleation layer thickness, and growth conditions. Two series of AlN/GaN HEMTs were grown by MBE to investigate the buffer leakage as a function of a)the metal fluxes and the b)NL thickness in MBE growth.
In the first series (a), the AlN NL thickness was fixed at 1.5 nm, while the Al flux F(Al) for this NL was varied from ~4.0e-8 Torr to ~1.7e-7 Torr. The NL grown in Al metal-rich regime results in high buffer leakage. The NL grown in the intermediate regime shows good buffer insulation at low bias, but breaks down rapidly at high bias. The N-rich growth condition is found to result in the most insulating buffer, where the leakage current density is less than 10 nA/mm at 10 V DC bias. The nitrogen-rich grown AlN NL prohibits the diffusion of the n-type impurities like silicon and oxygen from the substrate into the GaN buffer. Meanwhile, a natural AlN back barrier is formed, which provides better 2DEG confinement and prevents electrons flowing to the regrowth region under bias.
The second series (b) were grown with NL thicknesses 1.5 nm, 3.0 nm, 4.5 nm and 9.0 nm. The buffer leakage mapping across a 1x1 cm2 sample with a 1.5 nm AlN NL shows leakage less than 5 nA/mm in all dies except the one. In samples with thicker AlN NLs, the leakage current has large spans, sometimes varying from nA/mm to mA/mm. It is clear that thick AlN NLs are not suitable for HEMTs.
We conclude that the nucleation layer needs to be well designed to avoid forming new conducting paths in the buffer layer. A 1.5 nm AlN nucleation layer grown in the N-rich regime with Al flux of ~4.1e-8torr achieves highly insulating buffer with a high degree of uniformity. This result presents an attractive route towards GaN-based digital and high voltage devices in the future.