講演情報
[15a-K401-8]N-polar AlN-based GaN channel on sapphire for high electron mobility transistors
〇Markus Pristovsek1, Ituski Furuhashi1, Xu Yang1, Chengzhi Zhang2, Matthew D. Smith2, Martin Kuball2 (1.Nagoya Univ., 2.Univ. of Bristol)
キーワード：
HEMT、N-polar AlN
We have therefore investigated N-polar AlN-based HEMTs with GaN channels on sapphire. The N-polar AlN buffer is relaxed and on top up to about 9 nm strained GaN channels can be grown. The 2DEG is at the bottom GaN/AlN interface and thus away from the surface and highly confined in the channel. The challenging growth of smooth N-polar AlN in metal-organic vapour phase epitaxy (MOVPE) was only achieved recently using very low V/III ratios. The resulting 2DEGs show low mobilities compared to GaN-based HEMTs. We have compared several samples and growth strategies with different interface/surface roughness and relaxation. For typical mobilities around 100 cm²/Vs, the roughness has no impact (as long as the rms roughness is smaller than 1 nm). However, as soon as the GaN channel relaxes, the mobilities are strongly reduced. Using tri-ethyl gallium as Ga precursor and very high V/III ratios, we could reduce both oxygen and carbon in the GaN channel to values below 1017 cm-3 even for the low growth temperatures ~750°C needed to avoid step bunching. Thus, point defects in GaN cannot explain the low mobilities. However, the oxygen level in N-polar AlN is near 1019 cm-3 and could limit mobilities via charged point defects close to the channel. An intermediate AlN layer using higher V/III ratios and grading the temperature towards GaN channel had improved the mobilities, further supporting this assumption. Thus, dislocation from the AlN buffer continuing in the GaN channel and oxygen point defects in AlN are the main suspects for limiting the 2DEG mobilities and are the current focus of investigation.
Using these samples, first HEMT devices have been processed successfully at the University of Bristol.
Using these samples, first HEMT devices have been processed successfully at the University of Bristol.