Polarized recombination of acoustically transported carriers in GaAs nanowires
© Möller et al.; licensee Springer. 2012
Received: 10 January 2012
Accepted: 17 April 2012
Published: 14 May 2012
The oscillating piezoelectric field of a surface acoustic wave (SAW) is employed to transport photoexcited electrons and holes in GaAs nanowires deposited on a SAW delay line on a LiNbO3 crystal. The carriers generated in the nanowire by a focused light spot are acoustically transferred to a second location where they recombine. We show that the recombination of the transported carriers occurs in a zinc blende section on top of the predominant wurtzite nanowire. This allows contactless control of the linear polarized emission by SAWs which is governed by the crystal structure. Additional polarization-resolved photoluminescence measurements were performed to investigate spin conservation during transport.
Semiconductor nanowires offer new perspectives for low-dimensional semiconductor devices since the small radius favors mesoscopic size effects and lifts the epitaxial constraints associated with the growth of dissimilar materials. In addition, the geometry of nanowires enables them to function as active device elements and interconnects, which can lead to highly integrated optoelectronic device structures. In this respect, the crystal structure of the nanowires is of importance since it controls the optoelectronic properties. Exploitation of these properties normally requires doping and contacting of non-planar, nanometer-sized structures for the application of electric control fields. Here, we applied surface acoustic waves (SAW) to remotely control the carrier recombination in GaAs/AlGaAs core-shell nanowires.
We have recently demonstrated that the oscillating piezoelectric field of a SAW can transport photoexcited charge carriers in GaAs nanowires as well as control the spatial location of exciton recombination along the nanowire axis . In this contribution, we use polarized photoluminescence (PL) spectroscopy with spatial resolution to address the linear polarized emission characteristics of the photoexcited carriers along the nanowire. In addition, we face the question whether optically generated electron spins can be maintained during acoustic transport in nanowires.
The results shown here were obtained at low temperatures (20 K) by mounting the sample in a microscope cryostat for spatially resolved photoluminescence measurements (see Figure 1a). A 757-nm diode laser yielding 150 ps wide pulses was used as excitation source, the repetition rate of the laser being synchronized with the SAW frequency. The linearly polarized laser beam was focused onto the nanowire by using a 50× objective (spot diameter of approximately 1.5 μ m on the sample). The PL emission of the nanowire, collected by the same objective, was split by a birefringence prism into two orthogonally polarized rays and imaged on the entrance slit of a spectrometer. The rays with polarization parallel and perpendicular to the nanowire axis are then detected with spatial resolution (about 1 μ m) on the upper and lower regions of a cooled charge-coupled device camera (CCD), respectively. For the spin transport measurements, a λ/4 plate was put into the beam path before the objective in order to convert the linear polarized light into a left-handed circularly polarized one and vice versa (see Figure 1b).
Results and discussion
Without acoustic power, the broad emission band at G is highly polarized perpendicular to the nanowire axis (see Figure 2a,c). When a SAW is applied, the emission at G maintains its polarization behavior while the recombination at R is polarized parallel to the nanowire axis (see Figure 2b,d). The emission at G is consistent with the optical selection rules expected for the wurtzite GaAs nanowires  where the band edge emission is dipole allowed only if the electric dipole moment is perpendicular to the wurtzite c-axis . The emission at R, however, does not coincide with these selection rules. Since the recombination of the transported charge carriers take place at a remote position the crystal structure of that region may be different. Indeed, the emission energy of approximately 818 nm - which is exactly the band gap energy of zinc blende bulk GaAs  - suggests that recombination takes place in a zinc blende region at the top of the nanowire, opposite to the acoustic transducer. This is supported by transmission electron microscopy measurements, which reveal a zinc blende section in the top region of the nanowires probably created during the cooling down after growth, as previously observed in CBE- and MBE-grown GaAs nanowires [6, 7]. The emission polarized parallel to the nanowire axis at R is consistent with the emission characteristics observed in zinc blende GaAs nanowires [8, 9]. Here, the emission is preferentially polarized along the nanowire axis due to the dielectric mismatch between the nanowire and its surroundings. The degree of linear polarization, P=(I⊥−I∥)/(I⊥ + I∥) - where I⊥ and I∥ are the PL polarization intensities detected perpendicular and parallel to the nanowire axis, respectively - at the excitation spot G, is approximately + 88%(perpendicular) in the absence of a SAW, whereas at the remote position R the polarization becomes equal to approximately −70%(parallel) with acoustic power.
We have investigated the acoustic transport of charge along GaAs nanowires. We have shown that the transported electrons and holes recombine in zinc blende sections within the predominant wurtzite nanowire. In addition, spin polarization-resolved photoluminescence measurements have revealed that the spin polarization is lost during transport.
MM is PhD student, MMLJ is senior researcher and AC is the director of Materials Science Institute of the University of Valencia. AHM, SB and CP are post-doc researchers. LG, OB and PVS are senior researchers and HR is the director of the Paul-Drude-Institut für Festkörperelektronik, Berlin.
We acknowledge the expertise of W Seidel and S Krauß in the preparation of the SAW samples. MM, MMLJ and AC thank the Spanish Ministry of Science and Innovation (grant Nos. TEC2009-12075 and MAT2009-10350) for financial support. AHM thanks the support of the ‘Comissionat per a Universitats i Recerca del Departament d’Innovació, Universitats i Empresa de la Generalitat de Catalunya.’
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