Importance of point defect reactions for the atomic-scale roughness of III–V nanowire sidewalls

Abstract

The surface morphology of III–V semiconductor nanowires (NWs) protected by an arsenic cap and subsequently evaporated in ultrahigh vacuum is investigated with scanning tunneling microscopy and scanning transmission electron microscopy. We show that the changes of the surface morphology as a function of the NW composition and the nature of the seed particles are intimately related to the formation and reaction of surface point defects. Langmuir evaporation close to the congruent evaporation temperature causes the formation of vacancies which nucleate and form vacancy islands on {110} sidewalls of self-catalyzed InAs NWs. However, for annealing temperatures much smaller than the congruent temperature, a new phenomenon occurs: group III vacancies form and are filled by excess As atoms, leading to surface AsGa antisites. The resulting Ga adatoms nucleate with excess As atoms at the NW edges, producing monoatomic-step islands on the {110} sidewalls of GaAs NWs. Finally, when gold atoms diffuse from the seed particle onto the {110} sidewalls during evaporation of the protective As cap, Langmuir evaporation does not take place, leaving the sidewalls of InAsSb NWs atomically flat.

Insights into the Arsenic Shell Decapping Mechanisms in As/GaAs Nanowires by X-ray and Electron Microscopy

“…This is in good agreement with the As 3d core-level XPS spectra (Figure c), which indicate a GaAs surface without traces of oxidation. However, the As 3d peaks are broad, which could be explained by the presence of surface defects such as As Ga antisite defects as observed by STM . Moreover, the high-resolution TEM image in Figure e shows that the NW surface is crystalline and strongly facetted, comparable to as-grown NWs, opening the way for further epitaxial growth.…”

“…However, the As 3d peaks are broad, which could be explained by the presence of surface defects such as As Ga antisite defects as observed by STM. 30 Moreover, the high-resolution TEM image in Figure 6e shows that the NW surface is crystalline and strongly facetted, comparable to as-grown NWs, opening the way for further epitaxial growth. These results can be compared with NWs that have been exposed to air, and which exhibit a thin amorphous oxide shell.…”

“…27−31 Despite its practical importance, the physical mechanism behind the NW decapping has not been much studied, except by reflective high-energy electron diffraction (RHEED), X-ray photoelectron spectroscopy (XPS), 26 and STM. 30 It has been shown that even if the capping protects the NW facets well enough for subsequent epitaxial shell growth, it may induce surface defects that require nanometer resolution analysis on a single NW, such as transmission electron microscopy (TEM), compared to characterization techniques operated at larger scales such as RHEED and XPS, which intrinsically probe a NW assembly. Therefore, the objective of this work is to combine thermal annealing using in situ TEM and scanning photoelectron microscopy (SPEM) to determine how the morphology, structure, and surface chemistry evolve during the decapping of an As shell wrapping a single GaAs NW.…”

“…, in GaAs or InAs, and consequently, this approach does not require the introduction of any additional element, which could modify the nature of the surfaces. Second, the As decapping is obtained through heating at rather moderate temperature (less than 400 °C), and can be performed in standard growth reactors or in situ in scanning tunneling microscopes (STM). − Despite its practical importance, the physical mechanism behind the NW decapping has not been much studied, except by reflective high-energy electron diffraction (RHEED), X-ray photoelectron spectroscopy (XPS), and STM . It has been shown that even if the capping protects the NW facets well enough for subsequent epitaxial shell growth, it may induce surface defects that require nanometer resolution analysis on a single NW, such as transmission electron microscopy (TEM), compared to characterization techniques operated at larger scales such as RHEED and XPS, which intrinsically probe a NW assembly.…”

Sb-saturated high-temperature growth of extended, self-catalyzed GaAsSb nanowires on silicon with high quality

“…For Ga adatoms on such an inversely tapered sidewall the conditions are similar to those during growth on vicinal surfaces, where the substrate is slightly misoriented (few degrees) from a lowindex plane creating steps with single or few-layer atomic terraces. Hence, a tapered NW can be well approximated by an arrangement of steps on the { ̅ 11 0} sidewall, as found in recent scanning tunneling microscopy (STM) [36]. Generally, the Ga diffusion length during VLS growth is large, especially in the early growth stages it can be assumed that it is longer than the NW length.…”