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

Abstract

Examples are presented that application of amorphous Al x O y nucleation layer is an efficient way of controlling spatial distribution of GaN nanowires grown by plasma-assisted molecular beam epitaxy. On GaN/sapphire substrates Al x O y stripes induce formation of GaN nanowires while a compact GaN layer is formed outside the stripes. We show that the ratio of nanowire length h to the thickness of the compact layer d can be tailored by adjusting impinging gallium and nitrogen fluxes. Calculations of the h/d aspect ratio were performed taking into account dependence of nanowire incubation time on the growth parameters. In agreement with calculations we found that the value of h/d ratio can be increased by increasing the N/Ga flux ratio in the way that the N-limited growth regime determines nanowire axial growth rate while growth of compact layer remains Ga-limited. This ensures the highest value of the h/d aspect ratio. Local modification of GaN growth kinetics caused by surface diffusion of Ga adatoms through the boundary separating the Al x O y stripe and the GaN/sapphire substrate is discussed. We show that during the nanowire incubation period gallium is transported out of the Al x O y stripe, which delays nanowire nucleation onset and leads to reduced length of GaN nanowires in the vicinity of the stripe edge. Simultaneously the growth on the GaN/sapphire substrate is locally enhanced, so the planar GaN layers adopts a typical edge shape of mesa structures grown by selective area growth. Ga diffusion length on a-Al x O y surface of ∼500 nm is inferred from our results.

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

“…Recently, there is an increasing interest in the application of amorphous AlO y films deposited by ALD as nucleation layers for the PAMBE growth of GaN nanostructures. Such buffers effectively induce selective area formation of GaN NWs on sapphire [33] and GaN [69,70]. As shown in previous studies [34,41,71,72], AlO y buffer layers significantly enhance the nucleation rate of GaN with respect to nitridated Si without a loss of structural and optical quality [34].…”

Surface Diffusion of Gallium as the Origin of Inhomogeneity in Selective Area Growth of GaN Nanowires on AlxOy Nucleation Stripes

“…As discussed above, this nonuniformity initially originates from the diffusion barrier which prevents Ga diffusion onto the mask surface and is further enhanced by nucleation and growth of GaN islands and NWs within the stripe. According to ref , MBE growth of GaN NWs on Al x O y layers on Si(111) involves very long nucleation times which are required to nucleate the first GaN islands from which the NW growth subsequently starts. ,,,, Nucleation of GaN islands is limited by the kinetics of Ga adatoms. ,,,, Difficult nucleation is observed when DσN 0 t des ≪ 1, where D is the diffusion coefficient of Ga adatoms on a-Al x O y , σ as the capture coefficient, N 0 is the surface concentration of nucleation centers, and t des is the mean stay time of Ga adatom on the surface before desorption. Under such conditions, formation of GaN islands occurs in the incomplete condensation regime, and most probably by heterogeneous nucleation .…”

“…It is remarkable that the whole set of data can be reasonably fitted with the same parameters, including the Ga diffusion length λ on a-Al x O y of 600 nm. This value nicely correlates with the one of ref , where it was roughly estimated at 500 nm. As for the Ga diffusion length on SiN x (λ 0 ), it equals to approximately 1000–1100 nm from λ 0 /λ = 1.7–1.9.…”

“…Amorphous Al x O y (a-Al x O y ) buffer layers effectively induce catalyst-free nucleation of GaN NWs on sapphire 22 and GaN 57 substrates. As shown in previous studies, 24,26,27 a-Al x O y buffer layers significantly enhance the nucleation rate of GaN NWs with respect to nitridated Si without loss of the structural and optical properties.…”

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

“…4,6,7 Recently there is an increasing interest in application of amorphous AlO x lms grown by ALD as nucleation layers for plasma-assisted molecular beam epitaxy (PAMBE) of GaN nanostructures. Such buffers effectively induce catalyst-free nucleation of GaN nanowires on sapphire 8 and GaN 9 substrates. As shown in previous studies, [10][11][12][13] AlO x buffer layers signicantly enhance the nucleation rate of GaN with respect to nitridated Si (the most common substrate for growing GaN nanowires), without loss of structural and optical properties.…”