Selective area formation of GaN nanowires on GaN substrates by the use of amorphous Al x O y nucleation layer

Influence of Si Substrate Preparation Procedure on Polarity of Self-Assembled GaN Nanowires on Si(111): Kelvin Probe Force Microscopy Studies

The growth of GaN nanowires having a polar, wurtzite structure on nonpolar Si substrates raises the issue of GaN nanowire polarity. Depending on the growth procedure, coexistence of nanowires with different polarities inside one ensemble has been reported. Since polarity affects the optical and electronic properties of nanowires, reliable methods for its control are needed. In this work, we use Kelvin probe force microscopy to assess the polarity of GaN nanowires grown by plasma-assisted Molecular Beam Epitaxy on Si(111) substrates. We show that uniformity of the polarity of GaN nanowires critically depends on substrate processing prior to the growth. Nearly 18% of nanowires with reversed polarity (i.e., Ga-polar) were found on the HF-etched substrates with hydrogen surface passivation. Alternative Si substrate treatment steps (RCA etching, Ga-triggered deoxidation) were tested. However, the best results, i.e., purely N-polar ensemble of nanowires, were obtained on Si wafers thermally deoxidized in the growth chamber at ~1000 °C. Interestingly, no mixed polarity was found for GaN nanowires grown under similar conditions on Si(111) substrates with a thin AlOy buffer layer. Our results show that reversal of nanowires’ polarity can be prevented by growing them on a chemically uniform substrate surface, in our case on clean, in situ formed SiNx or ex situ deposited AlOy buffers.

Chemical bonding of nitrogen formed by nitridation of crystalline and amorphous aluminum oxide studied by X-ray photoelectron spectroscopy

Numerous efforts have already been made to optimize nitridation of crystalline sapphire (c-Al2O3) substrates whereas very little attention has been paid to nitridation of amorphous aluminum oxide layers (a-AlO x ). An extensive analysis of the reaction of amorphous aluminum oxide films with nitrogen species is thus needed to clarify the mechanisms of nitrogen incorporation into such layers and to control their properties. In this work X-ray photoelectron spectroscopy was used to determine the chemical state of nitrogen formed by nitrogen plasma treatment of c-Al2O3 and 15 nm thick a-AlO x layers grown by atomic layer deposition on Si and sapphire substrates. The results show that the nitridation proceeds significantly different for c-Al2O3 and a-AlO x samples, which we correlate with the initial stoichiometry of both materials. At the surface of sapphire O vacancies were found, which are necessary for the formation of AlN-type bonding via diffusion limited replacement of oxygen by nitrogen. This process was slow and involved formation of oxinitride AlN-O. After 80 min of nitridation only ∼3.4 at% of N was incorporated. In contrast, in a-AlO x layers Al vacancies were present before nitridation. This opened a new, more effective path for nitrogen incorporation via accumulation of N in the cation-deficient lattice and creation of the Al(NO y ) x phase, followed by AlN and AlN-O formation. This scenario predicts more effective nitrogen incorporation into a-AlO x than c-Al2O3, as indeed observed. It also explains our finding that more N was incorporated into a-AlO x on Si than on sapphire due to supply of oxygen from the sapphire substrate.