Electron mobility in modulation-doped AlGaN-GaN heterostructures

We report on the measurements of the electron mobility in modulation-doped Al0.2Ga0.8N-GaN heterostructures grown on sapphire, conducting 6H-SiC, and insulating 4H-SiC substrates as a function of the sheet electron density, ns, at the heterointerface in a wide temperature range. The mobility increases with an increase in n(s) up to approximately 1 X 10(13) cm(-2) and decreases with a further increase in n(s). This is explained by the electron spillover at high values of n(s) from the two-dimensional states at the AlGaN/GaN heterointerface into the delocalized states in the doped GaN channel. The maximum electron Hall mobility in excess of 2000 cm(2)/V s at room temperature and 11 000 cm(2)/V s at 4.2 K was measured in the heterostructures grown on 6H-SiC at the values of n(s) close to 10(13) cm(-2) and 7 X 10(12) cm(-2), respectively. (C) 1999 American Institute of Physics. [S0003-6951(99)05301-2].