Analysis of Incubation Times for the Self-Induced Formation of GaN Nanowires: Influence of the Substrate on the Nucleation Mechanism

Abstract

Surprisingly long incubation times for the self-induced formation of GaN nanowires on different substrates can reach hundreds of minutes and remain a mystery in GaN crystal growth. Herein, we examine the incubation times of GaN islands that subsequently give rise to nanowires on amorphous Al x O y /Si and SiN x /Si substrates versus the temperature and gallium flux. Experimental data are obtained by in situ monitoring of the surface morphology by reflection high energy electron diffraction during plasma-assisted molecular beam epitaxy. We develop a model that confirms an inverse power-law dependence of the incubation time on the gallium flux and the Arrhenius-type temperature dependence. The power exponent p and the activation energy E inc are related to the nucleation mechanism and the island growth regime. We find the values p ≅ 1, E inc = 6.0 eV for Al x O y , and p ≅ 2, E inc = 10.2 eV for SiN x buffer. The dominant nucleation mechanism on amorphous Al x O y should be heterogeneous. Homogeneous nucleation dominates on SiN x , while the diffusion growth regime of GaN islands occurs in both cases. Overall, the long incubation times are attributed to extremely low effective diffusion lengths of gallium adatoms such that the squared diffusion length times the gallium bonding rate ranges from 10–4 to 1 nm2 in typical cases.

Analysis of incubation time preceding the Ga-assisted nucleation and growth of GaAs nanowires on Si(111)

“…The value of 699 °C for the maximum temperature at which nucleation is possible is consistent with the discussion of Fig. 1(b [22]. It is seen that the incubation time for Ga droplets gives the main contribution into the total incubation time.…”

“…According to nucleation theory, any material k ( = k Ga and GaAs, where nucleation of GaAs NWs from Ga droplets is limited by the As flux) can start growing only if its concentration k c is larger than the equilibrium, [44]. When nucleation is difficult, corresponding to the interesting situations with long incubation times, the concentration of the metastable mother phase quickly reaches its maximum due to a balance of the incoming flux and desorption, and then slowly decreases due to nucleation (the so-called regime of incomplete condensation [22,44]). In this case, the concentration of material k at the beginning of nucleation is given by (…”

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

“…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. 10 Additionally, AlO x buffers prevent diffusion of silicon from the substrate 10 facilitating growth of GaN nanostructures at high temperatures without incorporating any impurities, 14 thus potentially leading to exceptional optical properties.…”

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

“…This allows us to conclude that no mixed polarity with a certainty above 99.8% (more than 400 NWs analyzed) was observed for GaN NWs grown on Si(111) substrates covered by a thin amorphous AlO y buffer layer. previous studies [34,41,71,72], AlOy buffer layers significantly enhance the nucleation rate of GaN with respect to nitridated Si without a loss of structural and optical quality [34]. Additionally, AlOy buffers prevent diffusion of silicon from the substrate [34], facilitating the growth of GaN nanostructures at high temperatures without incorporating any impurities [73], thus potentially leading to exceptional optical properties.…”