Effect of AlN buffer layer properties on the morphology and polarity of GaN nanowires grown by molecular beam epitaxy

Abstract

Low-temperature AlN buffer layers grown via plasma-assisted molecular beam epitaxy on Si (111) were found to significantly affect the subsequent growth morphology of GaN nanowires. The AlN buffer layers exhibited nanowire-like columnar protrusions, with their size, shape, and tilt determined by the AlN V/III flux ratio. GaN nanowires were frequently observed to adopt the structural characteristics of the underlying AlN columns, including the size and the degree of tilt. Piezoresponse force microscopy and polarity-sensitive etching indicate that the AlN films and the protruding columns have a mixed crystallographic polarity. Convergent beam electron diffraction indicates that GaN nanowires are Ga-polar, suggesting that Al-polar columns are nanowire nucleation sites for Ga-polar nanowires. GaN nanowires of low density could be grown on AlN buffers that were predominantly N-polar with isolated Al-polar columns, indicating a high growth rate for Ga-polar nanowires and suppressed growth of N-polar nanowires under typical growth conditions. AlN buffer layers grown under slightly N-rich conditions (V/III flux ratio ¼ 1.0 to 1.3) were found to provide a favorable growth surface for low-density, coalescence-free nanowires.

GaN based nanorods for solid state lighting

“…Brubaker et al 43 showed that the AlN buffer layer growth has significant impact on the morphology and polarity of GaN nanorods, which the GaN nanorods were frequently observed to adopt the structural characteristics of the underlying AlN columns, including the size and the degree of tilt. The GaN nanorods are found to be with Ga-polarity and the GaN matrix has the same polarity of the underlying low temperature AlN buffer layer, depending on the growth condition.…”

“…39 A similar growth model of nanorod morphology depending on the MBE system geometry was also proposed by Galopin et al 40 In the last 2 years, the polarity of GaN nanorods became an attractive topic. [41][42][43][44][45] To determine the polarity of the GaN nanorods, many methods were employed, for example, convergent beam electron diffraction (CBED), 46,47 electron energy loss spectra (EELS), 48 probe Cs corrected transmission electron microscopy measurement, 49 Kelvin probe force microscopy (KPFM), 50 piezoresponse force microscopy (PFM), 51 coaxial impact-collision ion scattering spectroscopy (CAICISS) analysis, 52 circular photogalvanic effect (CPGE), 53 and wet chemical etching. 54 Most of them are also suitable for polarity measurement of the nanorod structure.…”

“…Slightly N-rich conditions during AlN growth provide a favorable growth surface for straight, low-density, and coalescence-free nanorods. 43 From a theoretical point of view, Lymperakis et al performed a calculation based on density-functional theory to study the diffusion barriers of Ga and N adatoms on the nonpolar a-and m-plane surfaces. 62 They found that there is a strong in-plane anisotropy of the diffusion barriers for both surfaces, as shown in Table I: larger diffusion lengths parallel to the c axis are found for the a-plane surface.…”

Nucleation of single GaN nanorods with diameters smaller than 35 nm by molecular beam epitaxy

“…Some reports show that dislocations and plastic relaxation were necessary [15][16][17][18] for the nucleation. However, reports based on the observations of the polarity of nanorods suggested that nanorods were formed as a continuation of underlying pedestals, 8,12,19 as evidenced from the statistics of the rod-diameters in comparison with the underlying island-diameters.…”

“…Ga-polar, 20 N-polar, 12,21,22 or the coexistence of both polarities 19,23 of nanorods have all been reported. Direct evidence other than the rod-polarity is therefore needed to clarify the origin of the nucleation mechanism.…”