Luminescence properties of GaAs nanowires consisting of wurtzite and zincblende segments

The equilibrium modification of bulk III-As semiconductors is the zincblende (ZB) phase. However, these materials crystallize partially or even predominantly in the wurtzite (WZ) structure when grown in the nanowire form. In particular, GaAs nanowires often exhibit a mixed crystal structure with ZB and WZ segments alternating along the nanowire axis. Since these two phases differ from each other in their energy gap, GaAs nanowires represent quantum structures with complex optical properties, the interpretation of which is currently discussed controversially. In particular, the value of the energy gap of WZ GaAs is under debate.

Figure 1 Cathodoluminescence spectral line scan along the axis of a single GaAs nanowire at a temperature of 6 K. The dashed red line marks the energy gap of ZB GaAs at 6 K. Inset: Scanning electron microscopy image of the corresponding nanowire, where the dashed arrow marks the path of the scan. The lower part of the nanowire clearly shows a luminescence signal above EgZB.

Figure 2 Cathodoluminescence spectra with the lowest and highest peak positions along the axis of another nanowire at room temperature. The dashed red line marks gZB for this temperature. On the high energy side of the spectra, the exponential tail reflects the thermal distribution of carriers at the band edge. The spectral shift of this exponential tail when measured at different positions along the NW axis is assigned to the energy gap difference gWZ-gZB.

We have investigated single GaAs NWs by micro-photoluminescence and spatially resolved cathodoluminescence spectroscopy. The luminescence spectrum of GaAs NWs consisting of WZ and ZB segments is spread over a wide spectral range (cf. Fig. 1), where the optical emission occurs either predominantly above or below the energy gap of ZB GaAs (EgZB) depending on the growth conditions. These results are explained by means of a qualitative model assuming that EgWZ is larger than EgZB and that GaAs NWs with alternating ZB and WZ segments along the wire axis establish a type-II band alignment, where electrons captured within the ZB segments recombine with holes of the neighboring WZ segments. Thus, the corresponding transition energy depends on the degree of confinement of the electrons, i.e., on the thickness of the ZB segments and exceeds EgZB only for very thin ZB insertions. At low temperatures, the incorporation of carbon acceptors plays a major role in determining the spectral profile, as these can effectively bind holes in the ZB segments. Our results clarify why in some previous investigations a luminescence signal was only observed for energies below EgZB, while in other studies a luminescence signal was also observed above the energy gap of ZB GaAs. From the cathodoluminescence measurements of single GaAs NWs performed at 300 K, we deduce a lower bound of 55 meV for the difference between the corresponding energy gaps, i.e.EgWZ-EgZB (cf. Fig. 2).

1 Author U. Jahn , J. Lähnemann , C. Pfüller , O. Brandt , S. Breuer , B. Jenichen , M. Ramsteiner , L. Geelhaar , H. Riechert

Luminescence of GaAs nanowires consisting of wurtzite and zinc-blende segments

Source Phys. Rev. B , 85 , 045323 ( 2012 )
DOI : 10.1103/PhysRevB.85.045323 | Download: PDF | 2275 Cite : Bibtex RIS
U. Jahn, J. Lähnemann, C. Pfüller, O. Brandt, S. Breuer, B. Jenichen, M. Ramsteiner, L. Geelhaar, and H. Riechert