Engineering Parallel and Perpendicular Polarized Photoluminescence

Jul 6, 2010 - Department of Electronics and Telecommunications. ‡ Department of Physics. Norwegian University of Science and Technology, NO-7491 ...
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Engineering Parallel and Perpendicular Polarized Photoluminescence from a Single Semiconductor Nanowire by Crystal Phase Control Thang Ba Hoang, †,§ Anthonysamy F. Moses, † Lyubomir Ahtapodov, † Hailong Zhou, † Dasa L. Dheeraj, † Antonius T. J. van Helvoort,‡ Bjørn-Ove Fimland, † and Helge Weman*, † †

Department of Electronics and Telecommunications and ‡ Department of Physics, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway ABSTRACT We report on a crystal phase-dependent photoluminescence (PL) polarization effect in individual wurtzite GaAs nanowires with a zinc blende GaAsSb insert grown by Au-assisted molecular beam epitaxy. The PL emission from the zinc blende GaAsSb insert is strongly polarized along the nanowire axis while the emission from the wurtzite GaAs nanowire is perpendicularly polarized. The results indicate that the crystal phases, through optical selection rules, are playing an important role in the alignment of the PL polarization in nanowires besides the linear polarization induced by the dielectric mismatch. The strong excitation power dependence and long recombination lifetimes (∼4 ns) from the wurtzite GaAs and zinc blende GaAsSb-related PL emission strongly indicate the existence of type II band alignments in the nanowire due to the presence of nanometer thin zinc blende segments and stacking faults in the wurtzite GaAs barrier. KEYWORDS Nanowires, photoluminescence, polarization, selection rules, wurtzite, zinc blende.

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ne-dimensional semiconductor nanowires (NWs) have attracted considerable attention because of their potential applications in electronic and optoelectronic devices as well as for holding interesting physical properties.1-3 The realization of these devices, especially III-V NW-based photonic devices, requires detailed understanding of their optical properties. It is well-known that III-V semiconductor NWs with isotropic interband matrix elements, due to their size and shape (length/diameter ∼100), have a special optical property in which the absorption and emission of light in individual NWs are strongly polarized along the long axis of the NWs.4,5 This is due to the large dielectric mismatch between a NW and its surrounding (e.g., for a GaAs NW in vacuum, the relative dielectric constant, εr, is ∼12 and 1, respectively) and that the diameter of the NW is much smaller than the light wavelengths. It has recently been shown that III-V semiconductor NWs, such as GaAs and InP, may exhibit wurtzite (WZ) or zinc blende (ZB) crystal phase depending on growth conditions.6,7 In bulk and thin films, these semiconductors always have the ZB phase. The new WZ crystal phase of these materials not only raises questions about their fundamental

physical properties such as band gap energies, band offsets, effective masses, phonon energies, etc., but also on their optical properties. Mishra et al.7 observed that for WZ InP NWs, the crystal phase, through its interband dipole optical selection rules, plays an important role in the alignment of the linear polarization of the emitted photons. In fact, they showed that the photoluminescence (PL) emission from individual WZ InP NWs is strongly polarized perpendicular to the crystal c-axis, that is, perpendicular to the long axis of the NWs. In this letter, we demonstrate that the linear polarization of the PL emission can be engineered by controlling the crystal phases along the length in a single NW. The core-shell NWs studied were grown by Au-assisted molecular beam epitaxy (MBE) and has an ∼50 nm long ZB GaAsSb insert (∼20% Sb) in the ∼40 nm diameter WZ GaAs core, as schematically illustrated in Figure 1a. A ∼10 nm thick radial AlGaAs shell (∼30% Al) is grown around the GaAs/GaAsSb/ GaAs core in order to increase the quantum efficiency and to have a possibility to tune the amount of strain on the GaAsSb insert. The detailed growth procedure can be found elsewhere.8,9 The crystal structure of single NWs was studied by transmission electron microscopy (TEM). Figure 1b,c depicts a NW with a 55 nm long GaAsSb/AlGaAs core-shell insert. In this NW, the WZ GaAs/AlGaAs core-shell is seen to have stacking faults below the insert, which is also confirmed by the faint streaks in the selected area electron diffraction (SAED) pattern of this region (Figure 1f). The NW was free

* To whom correspondence should be addressed. E-mail: helge.weman@ iet.ntnu.no. §

On leave from Institute of Materials Science, VAST, Hanoi, Vietnam. E-mail: [email protected]. Received for review: 03/28/2010 Published on Web: 07/06/2010 © 2010 American Chemical Society

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structure in Figure 1 by the TEM techniques used here, neither around the GaAs core barrier nor around the GaAsSb core insert. Micro-PL (µ-PL) measurements on single NWs were carried out using a low vibration Janis ST-500 optical cryostat with built-in attocube piezo-driven sample positioners. Samples were kept at a temperature of around 10 K. For single NW measurements, NWs were removed from their as-grown substrate and dispersed on a Si substrate with an average density of ∼0.1 NW/µm2. Single NWs were excited by either 532 nm (for cw excitation) or 735 nm (for pulsed excitation) laser lines. The laser was defocused (spot size ∼5-7 µm) onto a single NW using a Mitutoyo long working distance 50×, 0.65 numerical aperture microscope objective lens. Unless otherwise noted, all the laser excitation power mentioned in this letter was measured in front of the microscope objective. The µ-PL from single NWs was collected by the same lens and dispersed by a 0.55 m focal length Jobin-Yvon spectrograph and detected by an AndorNewton thermo-electric cooled Si charge-coupled device (CCD) camera. The PL polarization measurements were done using a combination of achromatic wave plates and Glan-Thompson polarizers. These polarizing optics components have been carefully chosen to prevent any wavelength dependent polarization effects in our measurement setup. To get a proper PL image of a single NW, the NW is aligned so that its long axis is oriented along the vertical entrance slit of the spectrograph. With this arrangement, the vertical direction of the CCD image corresponds to the spatial position along the NW while the horizontal direction corresponds to the PL emission energy. The spectral resolution of the system is ∼200 µeV. Time-resolved measurements were conducted with a standard time-correlated single photon counting setup using Ortec analyzers and a PerkinElmer avalanche photodiode. The temporal resolution was ∼600 ps. Figure 2a shows the linear polarization of the PL emission from a GaAs/AlGaAs core-shell NW with a GaAsSb/AlGaAs core-shell insert measured with the laser excitation circularly polarized. The false-color image is constructed by PL spectra taken at different analyzer angles with red color indicating high intensity and blue color indicating low intensity. Zero degree angle is chosen to be along the long axis ([111]B for ZB and [0001] for WZ) of the NW. The PL emissions from the GaAsSb core insert and the GaAs NW core barriers are observed at energies ∼1.28 and 1.49 eV, respectively. We note that the PL emission from the GaAs NW core barriers occurs at an energy below the ZB GaAs free exciton energy (1.515 eV). This is due to the existence of stacking faults in the WZ GaAs NW core barriers which lead to the formation of a type II band alignment as will be explained later in this letter. It can be seen that the PL emission from the ZB GaAsSb core insert is strongly enhanced when it is measured parallel to the long axis of the NW, whereas the emission from the WZ GaAs NW core

FIGURE 1. (a) Schematic showing the targeted core-shell NW. Red and blue arrows mark the lower and upper GaAs/GaAsSb core interfaces, respectively. (b) Low-magnification bright-field TEM image of a NW showing the ZB GaAsSb/AlGaAs core-shell insert in dark contrast and the WZ GaAs/AlGaAs core-shell barrier in gray contrast. (c) Dark-field TEM image of the same core-shell insert at higher magnification. Horizontal stripes below the core insert are due to stacking faults in the GaAs/AlGaAs core-shell WZ phase. (d) HRTEM of the upper GaAsSb/AlGaAs core-shell insert interface with ∼1 nm GaAs ZB phase, marked by white vertical arrows. (e) HRTEM of the lower GaAsSb/AlGaAs core-shell interface. Selected area electron diffraction patterns (f) below the lower GaAsSb/AlGaAs core-shell interface, (g) at the upper GaAsSb/AlGaAs core-shell interface, and (h) above the upper GaAsSb/AlGaAs core-shell interface. (i) Fourier transformation of a HRTEM image of the ZB GaAsSb/AlGaAs core-shell insert area only.

of stacking faults above the insert (see also the SAED pattern in Figure 1h). Other NWs of this batch had a stacking faultfree structure below the insert and stacking faults above it. In an earlier work related to this type of heterostructured NW but without an AlGaAs shell, Dheeraj et al.8 have shown the existence of a ZB GaAs twin segment (a few nanometers thick) formed immediately above the upper GaAsSb/GaAs interface. In the NW shown in Figure 1, the twin segment is also present (∼1 nm) as can be seen in the high-resolution TEM (HRTEM) image in Figure 1d. This ZB GaAs twin segment can also be seen in the SAED pattern (Figure 1g). As will be shown later in this letter, because of the energy band offsets between ZB and WZ GaAs, this ZB GaAs segment has significant effect on the PL emission related to the ZB GaAsSb core insert. At the lower interface, there is a sharp transition from WZ GaAs to ZB GaAsSb (Figure 1e). The ZB GaAsSb core insert itself is free of stacking faults (twins) (see Figure 1c-i). Several other studied NWs of this batch depicted a region with a 4H GaAs polytype directly after the ZB GaAs twin segment, similar to what has been observed previously.8 Since the radial AlGaAs shell copies the crystal phases of the NW core (including stacking faults),9 the AlGaAs shell cannot be distinguished from the core © 2010 American Chemical Society

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FIGURE 3. Linear polarization of the PL emission from a single WZ GaAs NW without a GaAsSb core insert as a function of emission energy and the analyzer angle (0° corresponding to the long axis of the NW [0001]). Here again we note that with low laser excitation power (∼20 µW, circularly polarized), the PL emission energy (∼1.50 eV) is lower than the free exciton energy (1.515 eV) of ZB GaAs.

FIGURE 2. Linear polarization of PL emissions from a WZ GaAs/ AlGaAs core-shell NW with a ZB GaAsSb/AlGaAs core-shell insert measured at 10 K using circularly polarized laser excitation with a power of 10 µW. (a) False-color (red indicates high and blue low intensity) map shows the dependence of the PL intensity as a function of emission energy and the linear analyzer angle. Zero degree corresponds to the long axis of the NW ([111]B for ZB and [0001] for WZ). (b) PL spectra measured parallel (0°) and perpendicular (90°) to the NW.

NW core-shell barrier, the polarization effect due to the dielectric mismatch competes against the polarization effect induced by the optical selection rules in the WZ GaAs crystal phase. Using the observed degree of polarization due to the dielectric mismatch from the ZB GaAsSb PL (+90%) also for the WZ GaAs PL polarization, a linear polarization of ∼ -96% (perpendicular to the NW) is estimated to come from the optical selection rules in the WZ GaAs PL polarization, in close agreement with the expected -100% (assuming negligible wavelength dependence of the dielectric mismatch in the range of 1.25 to 1.50 eV). In addition, we would like to mention that the observed value of the degree of PL polarization from the WZ GaAs NW core barrier (∼ -40%) is comparable to what has been observed in WZ InP NWs (-49%).7 As has been reported earlier, single ZB GaAs NWs of similar dimensions as studied here, give rise to a large degree of polarization (> +80%) of the PL emission along the NW axis.11,12 Thus, to confirm that the PL emission from a single WZ GaAs NW is orthogonally polarized with respect to a ZB GaAs NW (perpendicularly to the NW axis), linear polarization measurements on a single WZ GaAs NW without a GaAsSb core insert were performed. As shown in Figure 3, it is clear that the PL emission from the GaAs NW (here at ∼1.50 eV) is strongly enhanced when measured perpendicularly to the NW axis. For this NW, the degree of linear PL polarization is also ∼ -40% (i.e., perpendicular to the NW axis), proving that optical selection rules related to the crystal phase are dominating over the dielectric mismatch effect. To investigate the PL emissions from the GaAs/AlGaAs core-shell NWs with GaAsSb/AlGaAs core-shell inserts in more detail, we performed two-dimensional (2D) (energy vs spatial position along the NW) µ-PL imaging of such indi-

barriers is strongly enhanced when measured perpendicularly. In Figure 2b, we show the PL spectra taken at 0° (parallel to the long axis of the NW) and 90° polarization (perpendicular to the long axis of the NW). It is clear that the PL emissions from the GaAsSb core insert and GaAs NW core barriers have orthogonal directions of linear polarization. In a WZ NW where the crystal c-axis is oriented along the long axis of the NW ([0001]), optical selection rules require that the emission is only allowed if the electric dipole moment is perpendicular to the crystal c-axis.7,10 For a ZB NW, the optical selection rules play no role in the linear polarization of the emitted photons, and the polarization is thus completely dominated by the effect due to the dielectric mismatch as shown in earlier works.5,7,11 Further, we like to note here that the degree of linear polarization, P ) (I| I⊥)/(I| + I⊥) (where I| and I⊥ are the PL polarization intensities detected parallel and perpendicular to the long axis of the NW, respectively), of the PL emission related to the ZB GaAsSb core insert is ∼ +90% (parallel) whereas the WZ GaAs NW core barrier PL polarization is ∼ -40% (perpendicular). We suggest that this difference in the degrees of polarization from what is expected according to the optical selection rules, is mainly due to the influence of the dielectric mismatch between the NW and its surrounding (vacuum). For the ZB GaAsSb/AlGaAs NW core-shell insert, the dielectric mismatch alone is responsible for the strong polarization of the PL emission along the NW axis as expected from theoretical calculations.4 However, for the WZ GaAs/AlGaAs © 2010 American Chemical Society

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FIGURE 5. Power dependent PL spectra from a WZ GaAs/AlGaAs core-shell NW with a ZB GaAsSb/AlGaAs core-shell insert. The observation of higher energy peaks with blue shifts of the peak energies and broadening of the spectra indicates type II recombinations from both the GaAsSb and GaAs related PL emissions. The dashed line indicates the free exciton (FE) energy of ZB GaAs (1.515 eV).

FIGURE 4. 2D PL images from a single WZ GaAs/AlGaAs core-shell NW with a ZB GaAsSb/AlGaAs core-shell insert recorded with the polarization analyzer set to select PL emission (a) parallel and (b) perpendicular to the NW axis (excitation laser circularly polarized, power ∼50 µW).

Nakayama,14 the conduction and valence bands in WZ GaAs are lifted up by 117 and 84 meV, respectively, with respect to those of ZB GaAs. Because of these band offsets, it is expected that in a WZ GaAs NW with ZB GaAs segments, a type II band alignment will occur.14,16 In Figure 5 we show the PL emission from a WZ GaAs/AlGaAs core-shell NW with a ZB GaAsSb/AlGaAs core-shell insert at different excitation powers. The WZ GaAs/AlGaAs core-shell part of the NW contains many stacking faults as was confirmed by TEM measurements. The large energy blue shift and broadening in the PL emissions from the WZ GaAs core barriers and ZB GaAsSb core insert suggest that the radiative recombination occurs between spatially separated electrons and holes. For the WZ GaAs core barrier related emission, electrons are localized in nanometer thin ZB GaAs segments (one stacking fault is equivalent to one unit cell of ZB) while the holes are localized in the WZ GaAs core (see Figure 7a). This is similar to the observation in WZ InP NWs containing stacking faults which has been reported recently by Pemasiri et al.17 As a result, at low excitation power the PL emission from such a system may occur at energies below the free exciton energy of ZB GaAs (1.515 eV). This is also observed in several PL spectra presented in this letter (Figures 2 and 3). When the excitation power is increased, band bending and state filling (of electrons in the ZB GaAs segments) effects occur, resulting in the observation of higher energy peaks with blue shifts and broadening in the PL emission spectrum. At high excitation power (∼100 µW), an additional peak appears at ∼1.54 eV, which is very close to the WZ GaAs free exciton energy.15,18

vidual NWs. In this imaging method,13 a NW is chosen so that its long axis is aligned vertically along the entrance slit of the spectrograph such that the vertical direction of the Si CCD detector image represents the PL emission along the length of the NW while the horizontal axis of the CCD image represents the PL emission energies. In Figure 4a,b, such 2D images taken with two orthogonal (parallel and perpendicular to the NW axis) polarization angles are shown. These images (false-color scale with red indicating high and blue indicating low intensity) clearly show the PL emission from the GaAsSb core insert at ∼1.29 eV while PL emissions from the GaAs NW core barriers range from 1.50 to 1.53 eV. The 2D images also reveal that the PL emission from the ZB GaAsSb core insert is spatially localized, whereas the emission from the WZ GaAs core barriers is more spread along the entire NW. These images once again confirm that the PL emissions from the WZ GaAs NW core barriers and ZB GaAsSb NW core insert have orthogonal polarization directions; the emission from the ZB GaAsSb core insert is strongly enhanced along the NW while the emission from WZ GaAs NW core barriers is perpendicularly polarized. In addition, we note that the emission from the WZ GaAs NW core barriers is more intense near the ZB GaAsSb core insert. This is possibly due to the higher density of stacking faults that appear in the GaAs NW core barrier near the GaAsSb core insert (which will trap the carriers involved in the GaAs PL emission). As has been predicted by theory and experimentally observed in recent PL experiments, the band gap of WZ GaAs is ∼30 meV higher than that of ZB GaAs.14,15 In addition, according to the calculations by Murayama and © 2010 American Chemical Society

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As has been shown earlier,19 the band alignment in ZB GaAs/GaAsSb quantum wells is rather delicate with the holes strongly confined in the GaAsSb quantum well whereas the electrons are weakly confined in either the ZB GaAs barrier (type II) or in the ZB GaAsSb quantum well (type I) depending on the exact Sb composition and strain distribution. In our present NW with ∼20% Sb composition in the GaAsSb core insert, the band alignments is believed to be type I since the conduction band in the GaAs barriers at both ends of the GaAsSb core insert is lifted with ∼120 meV due to the formation of the WZ GaAs crystal phase.14 The situation however becomes modified in our NWs since there is a ZB GaAs core segment at the upper GaAs/GaAsSb core interface. This can recreate a GaAs/GaAsSb type II band alignment at the upper interface (Figure 7b), depending on the Sb composition and strain distribution around this interface. The strong PL energy blue shift and broadening of the emission peak from a single ZB GaAsSb core insert at increasing excitation power (Figure 5) is an indication of a type II recombination where both band bending and state filling effects are taking place.20 To further confirm the type II band alignment in the WZ GaAs NW core barrier and ZB GaAsSb core insert, we measured the recombination lifetimes from a NW sample similar to the ones presented above. For the WZ GaAs NW core-related emission, we observe a recombination lifetime of ∼3.4 ns at 1.49 eV, as shown in Figure 6a. This observed recombination lifetime is even longer than the longest exciton recombination lifetimes (∼1 ns) observed in very high quality bulk (ZB) GaAs.21 It is well-known that due to the large surface recombination velocity and large exciton diffusion coefficient, the exciton recombination lifetime in ZB GaAs is very short (in the order of a few hundred picoseconds). The longest exciton lifetime reported for stacking fault free ZB GaAs/AlGaAs core-shell NWs was also ∼1 ns.22 Therefore the relatively long recombination lifetime observed in our structure can again be explained by the type II band alignment that confines holes in a dominating WZ GaAs and electrons in nanometer thin ZB GaAs segments created by stacking faults, as described above. The longer recombination lifetimes observed at lower PL emission energies is understood as a result of the state filling effect. In Figure 6b, we show also the recombination lifetimes measured at different detection energies of the GaAsSb core insert related PL emission. The long recombination lifetimes observed here (4.3 ns at 1.30 eV) are also strong indication of a type II band alignment at the upper ZB GaAsSb/ GaAs core interface.20 The energy dependence of the recombination lifetimes also shows an indication of state filling effects. Finally, we discuss the origins of the observed linear polarization anisotropy related to the PL emissions from the WZ GaAs NW barriers and ZB GaAsSb core inserts. For © 2010 American Chemical Society

FIGURE 6. The recombination lifetimes in a single WZ GaAs/AlGaAs core-shell NW with a ZB GaAsSb/AlGaAs core-shell insert. The recombination lifetime of the PL emission from the WZ GaAs NW core barriers (a) and the ZB GaAsSb core insert (b) at two different PL emission energies. Solid and dashed lines are exponential fits to each decay.

the WZ GaAs NW related PL emission (perpendicularly polarized), due to the existence of stacking faults causing nanometer thin ZB segments, the band offset between ZB and WZ GaAs results in a type II band alignment with electrons localized in thin ZB GaAs segments and holes in the dominating WZ GaAs regions of the NW (see Figure 7a). In WZ GaAs the p-like holes split into different valence bands with Γ9 and Γ7 symmetries while in ZB GaAs, the s-like electrons in the conduction band has Γ6 symmetry. We would like to emphasize here that for the given physical system with nanometer thin ZB segments created by stacking faults, the electron wave functions are likely to spread into the nearby WZ GaAs regions of the NW where they will overlap with the wave functions of holes that are Coulomb attracted to the trapped electrons in the ZB segment. This means that the electron wave functions may have a mixture of Γ6 and Γ7 symmetries. In this case, the optical selection rule is applied and radiative recombination between the Γ6 (Γ7) electrons in the conduction band and the Γ9 heavy holes in the valence band is dipole allowed only if the electric field is perpendicular to the WZ crystal c-axis. As a result, one should observe perpendicularly polarized PL emission. For the ZB GaAsSb core insert-related PL emission (parallel polarization), due the existence of the nanometer thin ZB 2931

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rules, has significant effect on the polarization of the PL from NWs besides the dielectric mismatch. Strong excitation power dependent energy blue shifts and long recombination lifetimes (∼4 ns) of both the WZ GaAs and ZB GaAsSb related PL emissions strongly indicate the existence of type II band alignment in the studied NWs. Given the fact that the perpendicularly polarized PL emission observed in this work results from a type II band alignment between ZB and WZ GaAs segments in the case of stacking faults, the same results can be expected from stacking fault free WZ GaAs NWs according to the optical selection rules. The method of analyzing the linear polarization of interband transitions from NWs with mixed crystal phases (either intentionally or caused by stacking faults) can be useful as a general optical characterization method to identify from which crystal phase the optical transitions originate (or rather in which crystal phase the holes are localized). With the rapid progress in controlling mixed ZB and WZ crystal phases in NWs our results also indicate that polarization control can be designed into single NWs, which may become useful for future NW based photonic devices.

FIGURE 7. Schematic diagrams of the band alignment and recombination processes for (a) WZ GaAs NW with a nanometer thin ZB GaAs segment created by stacking faults and (b) WZ GaAs NW with a ZB GaAsSb insert with an adjacent nanometer thin ZB GaAs segment at the right interface. The sketched band diagrams are supported by the structural and optical properties of the studied NWs (assuming the NW growth direction is from left to right in (b)).23

Acknowledgment. We would like to thank Fredrik Karlsson for assistance with the time-resolved measurements. This work was supported by the “NANOMAT” program (Grant No. 182091) of the Research Council of Norway.

GaAs core segment at the upper interface directly after the ZB GaAsSb core insert, the electrons are localized in this ZB GaAs segment while the holes are localized in the ZB GaAsSb insert (Figure 7b). Since electrons are located in ZB GaAs, they therefore have Γ6 symmetry and the holes in ZB GaAsSb have Γ8 symmetry. Here we note that without quantum confinement (due to the large dimensions of the ZB GaAsSb core insert) the heavy and light holes are degenerate. The recombination between the s-like electrons (Γ6 symmetry) in the ZB GaAs core segment and the heavy holes in the ZB GaAsSb core insert (Γ8 symmetry) is thus unpolarized. In this case, the PL polarization will be along the NW axis due to the dielectric mismatch effect between the NW and vacuum and is not affected by the optical selection rules. We want to point out that the observed polarization anisotropy in the studied NW would also be expected if the transitions were type I for both the WZ GaAs and the ZB GaAsSb-related PL emissions. On the other hand, since we have strong indications that they are type II the PL polarization results support the relative WZ/ZB GaAs band alignment (holes localized in WZ GaAs and electrons mainly in ZB GaAs)14 and the relative ZB GaAsSb/GaAs band alignment (holes localized in ZB GaAsSb and electrons in ZB GaAs).19 In conclusion, we have shown that the linear polarization of the PL emission can be engineered by controlling the crystal phase (WZ vs ZB) along the length in a single NW. We have demonstrated that the PL emission from WZ GaAs NWs is polarized perpendicularly while from ZB GaAsSb inserts it is polarized parallel to the NW axis. The result indicates that the crystal phase, through the optical selection © 2010 American Chemical Society

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