High-Temperature Growth of GaN Nanowires by Molecular Beam Epitaxy: Toward the Material Quality of Bulk GaN

Abstract

In molecular beam epitaxy, the spontaneous formation of GaN nanowires on Si(111) substrates at elevated temperatures is limited by the long incubation time that precedes nanowire nucleation. In this work, we present three growth approaches to minimize the incubation time and to thus facilitate significantly higher growth temperatures (up to 875°C). We achieve this advancement by (i) using III/V flux ratios of >1 to compensate for Ga desorption, (ii) introducing a two-step growth procedure, and (iii) using an AlN buffer layer to favor GaN nucleation. The GaN nanowire ensembles grown at so far unexplored substrate temperatures exhibit excitonic transitions with sub-meV linewidths comparable to those of state-of-the-art free-standing GaN layers grown by hydride vapor phase epitaxy.

Nature of excitons bound to inversion domain boundaries: Origin of the 3.45-eV luminescence lines in spontaneously formed GaN nanowires on Si(111)

“…Figure 2 shows the evolution of the low-temperature (10 K) PL spectrum from GaN NW ensembles formed on Si(111) (samples B1-B4) under identical conditions except for the substrate temperature. The linewidth of the (D 0 , X A ) transition decreases with T S increasing from 720 to 820 • C, indicating a progressive reduction of micro-strain induced by NW coalescence [44,48,49]. In parallel, the intensity of the 3.45-eV band with respect to that of the (D 0 , X A ) line also decreases with increasing T S .…”

“…7 reveals that the intensity of the (IDB * , X) band is actually not significantly reduced with increasing T S . The decrease of the relative intensity of the (IDB * , X) band with increasing T S is instead caused by the drastic increase in the (D 0 , X A ) emission intensity, reflecting the reduced concentration of nonradiative point defects at high T S [44,52]. .…”

“…Due to the wide range of conditions, the different NW samples exhibited incubation times between a few minutes and hours as well as different NW densities, average NW diameters, and coalescence degrees. Further details on the growth conditions can be found elsewhere (sample A [37], B1-B4 [43], B5-B7 [44], C [30], and D [45]).…”

III-nitride core–shell nanorod array on quartz substrates

“…The NBE of the GaN NRs peaked at 3.49 eV, slightly higher than the typical peak of neutral donor-bound excitons (D 0 X), which appears at 3.47 eV. This slight increase might arise from compressive strain of the GaN NRs as well as Si impurities dissociated from the mask535455. The compressive strain of GaN NRs was confirmed by Raman spectroscopy (see Supplementary Fig.…”

Crystal-phase quantum dots in GaN quantum wires

“…These findings reveal a zero-dimensional character of the confined exciton state and thus demonstrate that I 1 stacking faults in ultrathin nanowires act as genuine quantum dots.Spontaneously formed GaN nanowires are comparable in structural perfection to state-of-the-art freestanding GaN. 1 The nanowire geometry inhibits the propagation of threading dislocations along the nanowire axis, resulting in dislocation-free crystals regardless of the substrate. 2,3 In contrast to group-III arsenide and phosphide nanowires, which are synthesized by vapor-liquid-solid growth and are prone to a pronounced polytypism, 4,5 spontaneously formed GaN nanowires exclusively crystallize in the wurtzite lattice structure with only occasional I 1 basal plane stacking faults (BSFs).…”