Impact of surfaces on the optical properties of GaAs nanowires

Abstract

The effect of surfaces on the optical properties of GaAs nanowires is evidenced by comparing nanowires with or without an AlGaAs capping shell as a function of the diameter. We find that the optical properties of unpassivated nanowires are governed by Fermi-level pinning, whereas, the optical properties of passivated nanowires are mainly governed by surface recombinations. Finally, we measure a surface recombination velocity of 3 ϫ 10 3 cm s −1 one order of magnitude lower than values previously reported for ͕110͖ GaAs surfaces. These results will serve as guidance for the application of nanowires in solar cell and light emitting devices. © 2010 American Institute of Physics. ͓doi:10.1063/1.3519980͔Semiconducting nanowires ͑NWs͒ are a topic of intense research as they offer the opportunity to explore properties of one-dimensional electronic systems. The understanding and the mastering of the electronic properties of such onedimensional systems are essential to achieve high efficiency devices. In particular, with the increase of the surface/volume ratio, the electronic properties of NW based devices become strongly dependent on the surface electronic states that can alter their electronic properties. Two main surface effects are reported in literature: surface states can act as recombination centers for free carriers 1,2 or as surface charged traps. 3,4 The first effect has recently been quantified by an original optical method for silicon NWs. 5,6 The surface charge traps induce a pinning of the Fermi-level at the surface. A depletion shell appears and the electronic channel available for carriers is narrowed. Experimental proof of this effect in semiconducting NWs was observed by photoconductivity or capacitancevoltage measurements. [7][8][9][10] In this letter, we present our results on the influence of surfaces on GaAs NWs measured by low temperature microphotoluminescence ͑-PL͒ spectroscopy. A systematic comparison is done between unpassivated NWs and those that were capped with a shell of Al 0.4 Ga 0.6 As. Moreover, we take advantage of the tapered shape of the NWs to understand the role of diameter and surface/volume ratios in the luminescence efficiency. We demonstrate that capping directly modifies the recombination velocity and that passivated NW electronic properties are governed by surface recombinations, whereas, unpassivated NWs are strongly depleted by the Fermi-level pinning at the surface induced by charged surface trap states. This work will serve as a guidance for the passivation of the NW surfaces, which is of crucial importance for applications in laser and solar cell technology. 11GaAs NWs were synthesized by molecular beam epitaxy. The growth conditions are described elsewhere. 12-14The NWs have a prismatic shape and exhibit a pure zincblende GaAs crystal structure oriented along the ͑111͒ direction. Surfaces are ͕110͖ oriented. 15,16 The NWs are extremely long ͑16 m͒ and present a tapered shape ͑diameters go from 140 nm at the base to 70 nm at the tip͒. We investigated two samples which di...

Directional and Dynamic Modulation of the Optical Emission of an Individual GaAs Nanowire Using Surface Acoustic Waves

“…Remarkably, η is almost identical for both frequencies which can be readily explained by the fast PL decay for uncapped, zinc blende phase GaAs NWs of τ PL < 30 ps ∼ 0.02 · f −1 3 governed by surface recombination 16 which is almost two orders of magnitude faster than the period of the SAW. Due to this fast decay the SAW represents a quasi-static perturbation and the observed modulation reveals the dependency of the SAW-driven exciton dissociation as a function of the local phase of the SAW.…”

“…by using surface-passivated core-shell NW structures. 16 Moreover, a wide range of sophisticated architectures such as SAW-tunable crystal phase superlattices, 12 interdot couplings 18 and advanced memory devices 19 or acoustically triggered generation of single photons 20,21 and polarization entangled photon pairs 22 can be realized.…”

Lasing from individual GaAs-AlGaAs core-shell nanowires up to room temperature

“…To determine the vital role of the lattice-matched AlGaAs shell for the NW lasers, we have performed detailed characterizations exploring the influence of the passivation shell on the optical yield from the NW core. Essentially, by employing thin AlGaAs shells carrier diffusion to the NW surface is suppressed 47,48 . Our studies indicate that already o10-nm-thick Al 0.3 Ga 0.7 As shell is entirely sufficient to prevent carrier diffusion to the surface and thereby enhance the internal quantum efficiency of the NW core.…”

Residual strain and piezoelectric effects in passivated GaAs/AlGaAs core-shell nanowires

“…In-situ epitaxial AlGaAs layers produced prior to GaAs exposure to air provide an efficient passivation scheme. Therefore, GaAs/AlGaAs combinations have long been used in two-dimensional layers, and were lately applied to nanowires [10][11][12][13][14]. In this work, we report on the optical properties of ensembles of GaAs/Al 0.35 Ga 0.65 As core shell nanowires, and we show by a combination of optical spectroscopy and theoretical calculations that the AlGaAs shell not only provides excellent passivation of the GaAs core, but also changes its band structure as a result of the small lattice mismatch between AlGaAs and GaAs.…”