Due to its large piezoelectric and spontaneous polarization coefficients combined with the possibility of being grown lattice-matched with GaN, wide bandgap ScAlN is becoming a promising material in III-nitride semiconductor technology. In this work, and for the first time, ScAlN growth has been performed by molecular beam epitaxy with ammonia source as nitrogen precursor. High electron mobility transistor heterostructures with a 26 nm thick Sc0.15Al0.85N barrier have been grown on GaN-on-sapphire substrates. The effect of growth temperature, ranging between 620 and 800 °C, was carefully investigated. A smooth surface morphology with a mean roughness below 0.5 nm is obtained whatever the temperature while for 670 °C the (0002) and (101̄3) x-ray diffraction rocking curves show minimum full-width at half-maximum of 620 and 720 arc sec, respectively. Furthermore, two-dimensional electron gases with a high density of 3-3.5 × 1013/cm2 were evidenced in the heterostructures grown below 720 °C.