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AlN epilayers are grown directly on sapphire(0001)substrates each of which has a low temperature AlN nucleation layer.The effects of pretreatments of sapphire substrates,including exposures to NH3/H2and to H2only ambients at different temperatures,before the growth of AlN epilayers is investigated.In-plane misoriented crystals occur in N-polar AlN epilayers each with pretreatment in a H2only ambient,and are characterized by six 60°-apart peaks with splits in each peak in(10ˉ12)phi scan and two sets of hexagonal diffraction patterns taken along the[0001]zone axis in electron diffraction.These misoriented crystals can be eliminated in AlN epilayers by the pretreatment of sapphire substrates in the NH3/H2ambient.AlN epilayers by the pretreatment of sapphire substrates in the NH3/H2ambient are Al-polar.Our results show the pretreatments and the nucleation layers are responsible for the polarities of the AlN epilayers.We ascribe these results to the different strain relaxation mechanisms induced by the lattice mismatch of AlN and sapphire.
AlN epilayers are grown directly on sapphire (0001) substrates each of which has a low temperature AlN nucleation layer. The effects of pretreatments of sapphire substrates, including exposures to NH3 / H2 and to H2only ambients at different temperatures, before the growth of AlN epilayers are investigated.In-plane misoriented crystals occur in N-polar AlN epilayers each with pretreatment in a H2only ambient, and are characterized by sixty ° -apart peaks with splits in each peak in (10ˉ12) phi scan and two sets of hexagonal diffraction patterns taken along the [0001] zone axis in electron diffraction. The misoriented crystals can be eliminated in AlN epilayers by the pretreatment of sapphire substrates in the NH3 / H2ambient. AlN epilayers by the pretreatment of sapphire substrates in the NH3 / H2ambient are Al-polar .Our results show the pretreatments and the nucleation layers are responsible for the polarities of the AlN epilayers. We ascribe these results to the different strain relaxation mechanisms induced by the lattice mismatch of AlN and sapphire.