Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

The self-catalyzed (or Ga-induced) growth of vertical GaAs nanowires on Si(111) by molecular beam epitaxy has offered the possibility to obtain nanowires with fairly good control of the structural polytypism (especially in favor of the zinc-blende phase), without the risk of contamination by foreign elements like Au. The growth is typically performed close to the congruent temperature of GaAs (580-630 °C) in combination with relatively high V/III ratios. Those conditions ensure the efficient surface diffusion of Ga adatoms from the substrate and the nanowire side-walls to the apex of the nanowires, and suppress the thermal decomposition of the {110} side-walls. Nevertheless, the specific growth conditions impose several limitations: 1) the growth temperature exceeds largely the thermal budget limit of the Si-CMOS technology, rendering the future integration of the two material technologies impossible; 2) the interruption of the axial growth cannot be abrupt because of the long shut-off transient of the As-flux, which is typically accompanied by the formation of a segment that is rich in stacking faults below the Ga droplet; 3) significant inter-diffusion at high temperatures will deteriorate the compositional/doping profile of axial heterostructures; 4) it is difficult to avoid the unintentional radial (shell) growth, which is undesirable in axial heterostructures.
Aiming to surmount the inherent limitations of the conventional self-catalyzed growth mode, we developed a new growth scheme that expanded successfully the growth window to temperatures as low as 450 °C, minimized the radial growth, and allowed for accurate and defect-free interruptions of the axial growth. Our scheme is inspired by the migration-enhanced epitaxy of GaAs thin films, where the Ga and As fluxes are supplied alternately and in doses comparable to the atom sheet-density of the growth interface. In the case of nanowires, though, we adapted the alternate supply of Ga and As to achieve targeted delivery of the Ga atoms to the apex of the nanowires, minimizing in that way the radial growth. This work did not concern the nucleation stage, thus nanowires grown in a conventional way were used as templates for all experiments.
The results of our study include quantitative descriptions of the Ga adatom migration kinetics, the incorporation efficiency of As through the Ga droplet, and the dependence of both on the growth temperature. Having constructed a complete picture of the growth kinetics, we were able to identify the optimal growth conditions and beam-shutter sequence for different growth temperatures, covering the range from 550 to 450 °C. The structural analysis of our nanowires by transmission electron microscopy showed a zinc-blende structure with a small number of twin defects (Fig. 1). However, the spacing between the twins is irregular and does not correlate with the periodic beam-shutter sequence. The success of our growth scheme is interpreted on the basis of the atomic arrangement of the non-reconstructed {110} side-facets.
Ongoing work concerns the nucleation stage at 450 °C, as well as the fine tuning of the growth scheme parameters in order to eliminate the formation of twin defects.