Growth and performance of n++ GaN cap layer for HEMTs applications

• Published in: Materials science in semiconductor processing Vol. 185; p. 108959
• Main Authors: Kuzmík, J., Blaho, M., Gregušová, D., Eliáš, P., Pohorelec, O., Hasenöhrl, S., Haščík, Š., Gucmann, F., Zápražný, Z., Dobročka, E., Kyambaki, M., Konstantinidis, G.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-01-2025
• Subjects: Current collapse; InAlN/GaN HEMT; Nitride materials; Normally-off; InAlN/GaN HEMT; Current collapse; Normally-off; Nitride materials
• ISSN: 1369-8001
• DOI: 10.1016/j.mssp.2024.108959

Apart from providing high-quality ohmic contacts to III-N devices, n++ GaN cap layers can eliminate surface-related current collapse effects in high-electron-mobility transistors (HEMTs). Various metal-organic chemical vapor deposition conditions and n-type doping are tested in GaN growth on sapphire by using triethylgallium as a Ga precursor, replacing more conventional trimethylgallium. Consequently, despite relatively low temperature of the growth at 800 °C to facilitate InAlN barrier capping, optimized growth conditions and SiH4 flow provided free electron concentration of 7.2 × 1019 cm−3 and mobility of 103 cm2/Vs. Moreover, electron concentration in the range of 1020 cm−3 is demonstrated by using flow modulation epitaxy. On the other hand, as calibration growths indicated, excessive SiH4 flow alone can lead to high density of edge dislocations and surface undulations without enhancing free electron concentration. 6 nm (Si: 4.2 × 1019 cm−3) and 8 nm thick (Si: 7.2 × 1019 cm−3) n++ GaN cap is implemented in normally-off n++ GaN/InAlN/AlN/GaN HEMTs grown on Si, with a collapse-free performance for the latter case. By calculating energy band and free carrier concentration profiles of the heterostructures it is shown, that free electrons of a sufficiently thick and doped n++ GaN cap can shield the channel from the unstable surface potential.