An arrangement of self-assembled GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy on a Si(111) substrate is studied as a function of the temperature at which the substrate is nitridized before GaN growth. We show that the NWs grow with the c-axis perpendicular to the substrate surface independently of nitridation temperature with only a slight improvement in tilt coherency for high nitridation temperatures. A much larger influence of the substrate nitridation process on the in-plane arrangement of NWs is found. For high (850 °C) and medium (450 °C) nitridation temperatures angular twist distributions are relatively narrow and NWs are epitaxially aligned to the substrate in the same way as commonly observed in GaN on Si(111) planar layers with an AlN buffer. However, if the substrate is nitridized at low temperature (~150 °C) the epitaxial relationship with the substrate is lost and an almost random in-plane orientation of GaN NWs is observed. These results are correlated with a microstructure of silicon nitride film created on the substrate as the result of the nitridation procedure.

Nucleation kinetics of GaN nanowires (NWs) by molecular beam epitaxy on amorphous AlxOy buffers deposited at low temperature by atomic layer deposition is analyzed. We found that the growth processes on a-AlxOy are very similar to those observed on standard Si(111) substrates, although the presence of the buffer significantly enhances nucleation rate of GaN NWs, which we attribute to a microstructure of the buffer. The nucleation rate was studied vs. the growth temperature in the range of 720–790 °C, which allowed determination of nucleation energy of the NWs on a-AlxOy equal to 6 eV. This value is smaller than 10.2 eV we found under the same conditions on nitridized Si(111) substrates. Optical properties of GaN NWs on a-AlxOy are analyzed as a function of the growth temperature and compared with those on Si(111) substrates. A significant increase of photoluminescence intensity and much longer PL decay times, close to those on silicon substrates, are found for NWs grown at the highest temperature proving their high quality. The samples grown at high temperature have very narrow PL lines. This allowed observation that positions of donor-bound exciton PL line in the NWs grown on a-AlxOy are regularly lower than in samples grown directly on silicon suggesting that oxygen, instead of silicon, is the dominant donor. Moreover, PL spectra suggest that total concentration of donors in GaN NWs grown on a-AlxOy is lower than in those grown under similar conditions on bare Si. This shows that the a-AlxOy buffer efficiently acts as a barrier preventing uptake of silicon from the substrate to GaN.

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.

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We report on plasma-assisted molecular beam epitaxial growth of GaN nanowires (NWs) on Si(111) substrates with a thin amorphous Al2O3 buffer layer deposited by atomic layer deposition. Comparison of nucleation kinetics shows that presence of amorphous Al2O3 buffer significantly enhances spontaneous nucleation of GaN NWs. Slower nucleation was observed on partially amorphous silicon nitride films. No growth of NWs was found on sapphire substrate under the same growth conditions which we explain by a low density of defects on monocrystalline substrate surface where NWs may nucleate. Our finding shows that tuning of substrate microstructure is an efficient tool to control rate of self-induced nucleation of GaN NWs.

We present a comprehensive description of the self-assembled nucleation and growth of GaN nanowires (NWs) by plasma-assisted molecular beam epitaxy on amorphous Al x O y buffers (a-Al x O y ) prepared by atomic layer deposition. The results are compared with those obtained on nitridated Si(111). Using line-of-sight quadrupole mass spectrometry, we analyze in situ the incorporation of Ga starting from the incubation and nucleation stages till the formation of the final nanowire ensemble and observe qualitatively the same time dependence for the two types of substrates. However, on a-Al x O y the incubation time is shorter and the nucleation faster than on nitridated Si. Moreover, on a-Al x O y we observe a novel effect of decrease in incorporated Ga flux for long growth durations which we explain by coalescence of NWs leading to reduction of the GaN surface area where Ga may reside. Dedicated samples are used to analyze the evolution of surface morphology. In particular, no GaN nuclei are detected when growth is interrupted during the incubation stage. Moreover, for a-Al x O y , the same shape transition from spherical cap-shaped GaN crystallites to the NW-like geometry is found as it is known for nitridated Si. However, while the critical radius for this transition is only slightly larger for a-Al x O y than for nitridated Si, the critical height is more than six times larger for a-Al x O y . Finally, we observe that in fully developed NW ensembles, the substrate no longer influences growth kinetics and the same N-limited axial growth rate is measured on both substrates. We conclude that the same nucleation and growth processes take place on a-Al x O y as on nitridated Si and that these processes are of a general nature. Quantitatively, nucleation proceeds somewhat differently, which indicates the influence of the substrate, but once shadowing limits growth processes to the upper part of the NW ensemble, they are not affected anymore by the type of substrate.

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The growth mode and structural and optical properties of novel type of inclined GaN nanowires (NWs) grown by plasma-assisted MBE on Si(001) substrate were investigated. We show that due to a specific nucleation mechanism the NWs grow epitaxially on the Si substrate without any Si(x)N(y) interlayer, first in the form of zinc-blende islands and then as double wurtzite GaN nanorods with Ga-polarity. X-ray measurements show that orientation of these nanowires is epitaxially linked to the symmetry of the substrate so that [0001] axis of w-GaN nanowire is directed along the [111]Si axis. This is different from commonly observed behavior of self-induced GaN NWs that are N-polar and grow perpendicularly to the surface of nitridized silicon substrate independently on its orientation. The inclined NWs exhibit bright luminescence of bulk donor-bound excitons (D(0)X) at 3.472 eV and exciton-related peak at 3.46 eV having a long lifetime (0.7 ns at 4 K) and observable up to 50 K.