As recently remarkable developments for MBE-grown GaN have been reported

a high growth rate of GaN by RF-MBE

and the high electron mobility values could be obtained.The polarity control and the threading dislocation reduction in GaN layers grown on sapphire sub-strates by RF-MBE are discussed in this paper.The polarity of GaN was controlled to N-polarity by growing GaN directly on sufficiently nitrided sapphires

and Ga-polar GaN was realized by uti-lizing the high-temperature-grown A1N nucleation layers.For both N-and Ga-polar GaN

the in-troducing high-temperature-grown AlN multiple intermediate layers is effective to suppress the threading dislocation propagation.The reduction of dislocation brought about the improved room temperature electron mobility

332cm

2

/V·s for Ga-polar GaN and 688cm

2

/V·s for N-polar GaN.