Magneto-Optical Properties of Wurtzite-Phase InP Nanowires - Nano

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Magneto-Optical Properties of Wurtzite-Phase InP Nanowires M. De Luca,† A. Polimeni,*,† H. A. Fonseka,‡ A. J. Meaney,§ P. C. M. Christianen,§ J. C. Maan,§ S. Paiman,‡,⊥ H. H. Tan,‡ F. Mura,∥ C. Jagadish,‡ and M. Capizzi† †

Dipartimento di Fisica and CNISM, Sapienza Università di Roma, Piazzale A. Moro 2, 00185 Roma, Italy Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 0200, Australia § High Field Magnet Laboratory, Institute of Molecules and Materials, Radboud University Nijmegen, Toernooiveld 7, NL-6525 ED Nijmegen, The Netherlands ∥ Dipartimento di Scienze di Base e Applicate per l’Ingegneria, Sapienza Università di Roma, Via Scarpa 16, 00185 Roma, Italy ‡

ABSTRACT: The possibility to grow in zincblende (ZB) and/or wurtzite (WZ) crystal phase widens the potential applications of semiconductor nanowires (NWs). This is particularly true in technologically relevant III−V compounds, such as GaAs, InAs, and InP, for which WZ is not available in bulk form. The WZ band structure of many III−V NWs has been widely studied. Yet, transport (that is, carrier effective mass) and spin (that is, carrier g-factor) properties are almost experimentally unknown. We address these issues in a wellcharacterized material: WZ indium phosphide. The value and anisotropy of the reduced mass (μexc) and g-factor (gexc) of the band gap exciton are determined by photoluminescence measurements under intense magnetic fields (B, up to 28 T) applied along different crystallographic directions. μexc is 14% greater in WZ NWs than in a ZB bulk reference and it is 6% greater in a plane containing the WZ ĉ axis than in a plane orthogonal to ĉ. The Zeeman splitting is markedly anisotropic with gexc = |ge| = 1.4 for B⊥ĉ (where ge is the electron g-factor) and gexc = |ge − gh,//| = 3.5 for B//ĉ (where gh,// is the hole g-factor). A noticeable Binduced circular dichroism of the emitted photons is found only for B//ĉ, as expected in WZ-phase materials. KEYWORDS: Semiconductor nanowires, wurtzite-phase InP, magneto-photoluminescence, effective mass, gyromagnetic factor

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electronic devices exploiting spin-polarized transport in NWs rely on the Zeeman effect, whose strength is governed by the carrier g-factor.19 Recently, hybrid superconductor-semiconducting nanowire devices using III−V materials have been employed in the field of the quest for Majorana Fermions.20,21 In particular, the condition for the observation of these elusive particles is determined, among other factors, by the Zeeman gap EZ = gμBB/2 (μB is the Bohr magneton and B is the magnetic field intensity). Optical spectroscopy under magnetic field (B) is a powerful tool to monitor and tune the orbital and spin degrees of freedom of carriers in semiconductors and allows determination of their carrier effective mass and g-factor. For instance, recent magneto-photoluminescence experiments on WZ In0.1Ga0.9As NWs have shown an increase in the exciton reduced mass μexc with respect to that expected for ZB.22 Sizable differences were observed in the values of μexc and exciton Zeeman splitting depending on the orientation of the magnetic field relative to the NW ĉ axis,22 however, such differences have not been reported in mixed-phase quantum disks formed in GaAs NWs.23 Magneto-PL measurements have been also recently

ndium phosphide nanowires (NWs) represent a model system in which the interplay between structural and electronic properties has been widely investigated.1−5 Onedimensional growth of InP NWs occurs preferentially in the hexagonal wurtzite (WZ) phase, contrary to the bulk case where the cubic zincblende (ZB) phase is ubiquitous. Furthermore, a switch from WZ to ZB and vice versa, or even wires where both phases coexist, can be obtained under suitable vapor−liquid−solid growth conditions.6,7 Several theoretical studies addressed the fundamental electronic properties of WZ crystals,8,9 more recently with emphasis on NWs.10−13 In contrast to WZ GaAs nanowires for which debate is still ongoing,14 the electronic properties of WZ InP nanowires are well assessed. An energy difference of +70 meV between the WZ and ZB band gap has been determined1−5,15 and higherenergy critical points have been established by optical techniques.3,16,17 However, no experimental estimate of the carrier effective mass and g-factor of WZ InP is available even though these key parameters are very important for implementing novel devices based on WZ NWs and on onedimensional crystal-phase homostructures.7 As an example, the value of the carrier effective mass is essential for the design of NW-based electronic and thermoelectric18 applications that are strongly influenced by carrier mobility. Also, magneto© XXXX American Chemical Society

Received: March 7, 2014 Revised: June 10, 2014

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dx.doi.org/10.1021/nl500870e | Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. (a,b) Top and 45° tilted scanning-electron-microscopy images, respectively, of the sample. (c) High-resolution transmission electron microscopy image taken along the [112̅0] zone axis of the nanowire shown in the top-left inset (close to the wire basis, as indicated by the red square). This image and the diffraction pattern of the selected area (bottom-right inset) confirm WZ crystal structure.

Figure 2. (a) Temperature-dependence of macro-PL spectra recorded on an ensemble of 2.5 × 105 NWs in backscattering configuration at laser power density P = 6.4 W/cm2. The WZ band gap free exciton is indicated as A. The lower-energy bands are likely due to impurities and/or type-II transitions. The A exciton exhibits a regular Varshni-like energy shift from T = 10 to 310 K; see inset. (b) T = 80 K micro-PL spectra (lines and symbols) recorded at P = 3.2 kW/cm2 on a single NW transferred onto a Si substrate by mechanical friction. The experimental configuration is sketched in the inset; laser excitation and PL collection are perpendicular to the NW symmetry axis (ĉ), and PL emission is filtered by a linear polarizer. A linear polarization degree of −0.65, which corresponds to light polarization perpendicular to ĉ is found; see text. The thick gray line refers to the T = 90 K PL spectrum shown in panel a and here normalized to the ε⊥ spectrum recorded on a single NW for comparison purpose.

used to characterize the optical properties of ZB GaAs/AlAs core/shell24 and WZ GaN25 nanowires. In this work, we study Au-catalyzed InP NWs with highpurity WZ phase. Figure 1a,b shows top and 45° tilted views of an ensemble of NWs, respectively. The wires are all parallel to each other with an average density of 8 NWs/μm2. The NWs have a hexagonal cross-section arising from growth on a (111)B-oriented InP substrate. Each NW exhibits a tapered shape with approximate tip and base diameters of 30 and 200 nm, respectively. The straight nontapered WZ nanowires obtained under suitable conditions in our previous works tend to show poor crystal and optical quality with high density of defects and broad emission spectra related to impurities and other crystal imperfections.26,27 The tapered InP NWs used in the present study have almost pure WZ phase, as demonstrated by the transmission electron microscope image shown in Figure 1c. The NWs do not contain any ZB insertion and very few stacking faults are seen along the NWs, except at the NW tips, which are formed during the cooling down after growth. NW tips do not contribute sizably to the PL emission because of the

probable presence of nonradiative recombination centers and because of a negligible contribution to the volume of the NWs investigated here. The diffraction pattern in the bottom-right inset of Figure 1c shows the typical hexagonal geometry of the NW lattice. We point out that the most important characteristics for the current study are the crystal and optical qualities of the nanowires and that the tapered morphology does not alter the outcome, as the dimensions of these nanowires are large enough to let us disregard quantum confinement effects.28 The WZ phase is confirmed by the optical properties of the wires. Figure 2a shows photoluminescence (PL) spectra taken at different temperatures (from 10 to 90 K) on an ensemble of NWs (macro-PL). As previously reported by other authors,1,2,4,7,29 the T = 10 K spectrum exhibits different contributions with low-energy bands likely due to acceptorrelated states (or type-II WZ-ZB recombinations). The highest energy (1.493 eV) transition is ascribed to the band gap free exciton of WZ, usually referred to as A exciton, which involves the recombination of an electron and a hole at the ΓC7 and ΓV9 points of the Brillouin zone, respectively.30 The high optical quality of the samples allowed us to collect PL emission up to B


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Figure 3. (a) Peak-normalized PL spectra, taken at T = 77 K, of an ensemble of about 600 InP nanowires for different values of the magnetic field in the Voigt geometry (2 T steps, laser power density P = 130 W/cm2). “A” indicates the band gap exciton and “I” refers to the peak ascribable to an impurity or a type II transition. The arrows indicate the two split components of the A exciton. The experimental configuration is sketched in the inset. The field is perpendicular to the emitted light wave-vector, k, which is parallel to the NW symmetry axis, ĉ. (b) The same as (a) in the Faraday geometry (2 T steps, P = 360 W/cm2). Red and black lines refer to σ− and σ+ circular polarization, respectively. The arrows indicate the two split components of the A exciton. The experimental configuration is depicted in the inset: Magnetic field B, light wave vector k and NW ĉ axis are parallel to each other and perpendicular to the sample substrate. A variable retarder (followed by a linear polarizer, not shown in the figure) allows collecting light with opposite circular polarizations. In both panels, laser direction is antiparallel to ĉ and k.

easier and have a statistical meaning hardly achieved by studies on a single nanostructure. We now illustrate the effects of a magnetic field on the NW electronic properties. As a preliminary remark, we again stress that the pronounced tapering of our NWs does not influence the transport and spin properties in our samples, whose large wire diameter excludes sizable quantum confinement effects.28 Magneto-PL measurements were performed by probing an ensemble of about 600 NWs at T = 77 K using a long focal length objective (laser spot diameter ∼10 μm). The relatively high temperature employed permitted us to reduce the contributions from lower-energy bands that largely dominate the spectra at low T (see Figure 2a), while maintaining a narrow line width and high intensity for the A exciton emission. Figure 3 shows the peak-normalized PL spectra for selected values of the magnetic field. Parts (a) and (b) refer to the two configurations where B is applied perpendicular and parallel to the NW ĉ axis, respectively (see sketches in the insets). The luminescence wave vector k is always parallel to ĉ and antiparallel to laser excitation (backscattering configuration). We refer to the B⊥k configuration as the Voigt configuration and to the B//k configuration as the Faraday configuration. In both cases, with increasing field the contribution from the lowenergy bands increases with respect to the A exciton contribution. This supports the attribution of those bands to loosely bound carriers, whose recombination rate is more effectively enhanced by B.34 In the Voigt configuration, the PL spectrum blueshifts as the field increases (Figure 3a). The shoulder appearing on the highenergy side of the A exciton peak for B ≥ 21 T indicates a line splitting increasing with B. The PL band (i.e., its intensity and line shape) is insensitive to circular polarization filtering (not shown here). In the Faraday configuration, the PL line shape changes with increasing B to a much larger extent, as displayed in Figure 3b. The PL spectrum blueshifts and shows a marked

room temperature and thus to obtain the energy shift of the A transition from 10 to 310 K, as shown in the inset of Figure 2a. At low temperature (20 nm,28 two main mechanisms contribute to the PL polarization:32,33 (i) confinement of the electromagnetic field that is due to the dielectric contrast between the NW and the surrounding air; (ii) optical selection rules, which are related to the symmetry of the electronic levels of the recombining carriers. According to the first mechanism solely,32,33 ρ ≥ 0 values should be observed in NWs with dielectric constant εr = 9 (close to that of InP) having diameter smaller than 260 nm. However, in wurtzite radiative recombination of the exciton ground state (band A) is permitted only for photons having polarization vector orthogonal to ĉ (namely, the second mechanism mentioned above). This corresponds to a highly anisotropic internal emission within the NW that leads finally to the negative, large value of ρ here reported.32,33 It is worth noting that the PL fullwidth at half-maximum (fwhm = 9 meV) found on the single NW is very close to the fwhm determined by macro-PL (thick solid gray line) on the ensemble of 2.5 × 105 NWs. Therefore, measurements on an ensemble of NWs well characterize the NW optical properties. Moreover, these measurements are C


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line splitting, as highlighted in the spectra taken under σ+ and σ− circular polarization. In order to analyze these results quantitatively, we first assume that the A exciton properties are those pertaining to bulk WZ crystals. Indeed, the NW typical dimensions (>30 nm) permit to disregard theories developed for WZ nanocrystals. At B = 0 T, the electron−hole exchange interaction splits A excitons into dipole-forbidden (dark) excitons with Γ6 symmetry and energy E6, and dipole-allowed (bright) excitons with Γ5 symmetry and energy E5 = E6 + Δ56.22,35,36 The analytic part of the exchange interaction, Δ56, is usually very small for bulk WZ crystals (0.1 meV in highly polar GaN37) and will be set to zero in the quantitative analysis of the data. In the Voigt configuration (B⊥k//ĉ), the magnetic field mixes bright and dark excitons resulting in two states Γ±5/6 with energies22,35 ± E5/6 (B) = E6 +

1 1 2 Δ56 + ΔEd, ⊥(B) ± Δ56 + ge2μB2 B2 2 2 (1)

where ge is the electron g-factor, μB is the Bohr magneton, and ΔEd,⊥(B) is the exciton diamagnetic shift for B⊥k. As a result of the dark-bright state mixing, Γ±5/6 states are insensitive to circular polarization. Notice that the electron contribution is isotropic, while the hole contribution is anisotropic, as gh = gh,//cos ϑ, where ϑ is the angle between the magnetic field and the ĉ axis.37,38 This angle is 90° in the Voigt configuration, therefore the hole contribution does not appear in the last term of eq 1. In the Faraday configuratin (B//k//ĉ), no mixing of states is expected and the energy of the bright states is given by22,35 E5±(B) = E6 + Δ56 + ΔEd,//(B) ±

1 |g − gh,//|μB B 2 e

Figure 4. (a) T = 77 K PL spectra recorded in the Voigt geometry (laser power density P = 130 W/cm2). Dashed (solid) line refers to the 0 T (28 T) spectrum. The up-pointing arrows in the 28 T spectrum indicate the two states Γ±5/6 of the A exciton. In both spectra, label “I” refers to the peak ascribable to an impurity or a type II transition. (b) PL spectra taken at T = 77 K and recorded in the Faraday geometry (P = 360 W/cm2). The dashed line shows the spectrum at 0 T. The impurity (or type II transition) is labeled “I”. Black and red solid lines refer to spectra recorded for σ+ and σ− circular polarization at 28 T, respectively. Label “I−” (“I+”) refers to the low (high)-energy state of the impurity (or type II transition). In both spectra, the highest energy peaks indicated by arrows identify the A exciton emissions (Γ±5 ). Different intensities can be observed: Stronger (weaker) intensity corresponds to emission from lower (higher)-energy exciton states. This results in a degree of circular dichroism as high as 20% for the A exciton at 28 T.

(2)

E±5

where gh,// is the hole g-factor and states have opposite circular polarizations. ΔEd,// (B) is the diamagnetic shift of the exciton energy for B //k. Typically, ΔEd(B) depends on the exciton reduced mass and the relative dielectric constant of the material εr. By using a variational method one finds39,40 ⎛ μexc ⎜ m0 ΔEd(B ; μexc , εr) = 13.6 × 10 ⎜ 2 ⎜ εr ⎝ 3

symmetry in this configuration.35 As a matter of fact, the splitting of the A exciton into Γ+5/6 and Γ‑5/6 states can be observed for B ≥ 21 T only, as also indicated by the arrows in Figure 3a. The transition indicated as “I” is due to an impurityrelated (or to a type-II) recombination, as discussed in Figure 2a. Similar to the A exciton, its intensity does not depend on the versus of circular polarization. In Figure 4b (Faraday configuration), the 28 T spectrum exhibits major differences with respect to the previous configuration. The A exciton band is made of two contributions, whose relative spectral weight strongly depends on the versus of the circular polarization detected. Following eq 2, we ascribe the more intense peak in the σ+ (σ−) spectrum to the Γ+5 (Γ−5 ) state. Furthermore, the intensity of the A exciton band depends on circular polarization, thus resulting in a 20% degree of circular dichroism at 28 T, another evidence of the WZ phase of the NWs. We also note that transition “I” has a behavior very similar to that of transition A upon circular polarization filtering: It splits into “I+” and “I−”, as indicated in Figure 4b. Although the “I+” energy nearly coincides with that of Γ−5 , this does not impede a line shape analysis, because the intensity of the two states depends on σ± in an opposite way. The energy shift with magnetic field of the various PL components in the Voigt and Faraday configurations are shown in Figure 5a,b, respectively. For ease of comparison, the energy shift is displayed relative to B = 0 T.

( ) ⎞⎟⎟ ⎟ ⎠

⎡ ⎛ ⎢ ⎜ ε 6 − ∑ A p⎢4.26 × 10 ⎜ μ r ⎢ ⎜ exc p=1 ⎢⎣ ⎝ m0 9

( )

⎞2 ⎤ ⎟ ⎥ ⎟ B⎥ meV ⎟ ⎥ ⎠ ⎥⎦ p

(3)

where the coefficients Ap are determined by solving numerically the problem and m0 is the electron mass in vacuum. We now assign the various spectral components of the PL bands on the ground of the equations just discussed. In Figure 4, the PL spectra in the energy range of the A exciton are shown for different field configurations at B = 0 and 28 T. In panel a (Voigt configuration), a shoulder is clearly seen on the high-energy side of the 28 T spectrum. According to eq 1, we ascribe the two higher energy components of the spectrum (uppointing arrows) to Γ+5/6 and Γ‑5/6 states. Note that the data analysis is challenging due to the absence of any dependence of PL on the versus of circular polarization, as expected for WZ D


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Figure 5. (a) Magnetic-field dependence of the A exciton energy in the Voigt configuration with respect to zero field (the experimental geometry is sketched in Figure 3a). Full black and red triangles represent the energy shift of the two components Γ+5/6 and Γ−5/6 of the Γ5/6 exciton. Open circles show the average shift of the two components. Open squares represent the diamagnetic shift of the free exciton measured in a bulk ZB InP (100) substrate. Solid lines are fits of eq 3 to the data that allow extraction of the reduced masses of the free excitons involved. Notice that the bulk ZB InP data were collected with B//k. (b) Magnetic-field dependence of the A exciton energy in the Faraday configuration with respect to zero field (the experimental geometry is sketched in Figure 3b). Full black and red triangles correspond to the two components Γ+5 , and Γ−5 of the Γ5 exciton, as determined by selecting luminescence of circular polarization σ+ and σ−, respectively. Open circles show the average shift of the two opposite polarized components. The solid line is a fit of eq 3 to this shift.

Voigt Configuration (Figure 5a). For B ≥ 21 T, open circles represent the relative energy shift of the A exciton as averaged between the two Γ+5/6 and Γ−5/6 components (black and red full triangles, respectively) in which the A exciton splits at these high fields; see Figures 3a and 4a. For B < 21 T, these two components are not resolved and, therefore, the shift of the single A peak observed is also represented by open circles. The diamagnetic shift ΔEd,⊥ (B; μexc, εr) given by eq 3 (solid blue line) reproduces very well the experimental relative shift of [E+5/6(B) + E−5/6(B)]/2 (open circles) with μ//,⊥ exc = (0.072 ± 0.001)m0 and εr = 11.93.41 μ//,⊥ exc refers to the exciton motion in a plane containing the ĉ axis with electron and hole masses * × m⊥*)1/2, where m// * and m⊥* are the given by m//,⊥ = (m// carrier effective masses parallel and orthogonal to ĉ, respectively.42 Finally, the thin green line is the fit to the diamagnetic shift measured on a ZB(100) InP substrate (open small squares) by using eq 3. In this case μexc = (0.063 ± 0.001) m0, which is 14% less than the value derived for WZ InP NWs. Faraday Configuration (Figure 5b). Black and red full triangles give the relative energy shift of the Γ5+ and Γ5− components in which the A exciton splits; see Figures 3b and 4b. The exciton diamagnetic shift in the Faraday configuration, ΔEd,// (B; μexc, εr), can thus be obtained by [E+5 (B) + E−5 (B)]/2, see eq 2, namely, by the average between these two Γ5± components (open circles). A very good fit (solid line) of ΔEd,// (B; μexc, εr) is obtained via eq 3 with μ⊥exc = (0.068 ± 0.001)m0, which is the reduced mass of excitons moving in a plane orthogonal to ĉ. In the Faraday configuration, therefore, the exciton effective mass is 6% smaller than that determined in the Voigt configuration (μ//,⊥ exc ). This points to a small but measurable mass anisotropy with respect to the NW axis, as also directly indicated in Figure 5 by the smaller diamagnetic shift for B⊥k//ĉ with respect to B//k//ĉ. For both configurations, we have repeated the measurements on different nanowire samples from the same growth and with laser power densities differing by about 1 order of magnitude. The results are very similar to those shown in Figure 5. Furthermore, μexc values estimated from measurements carried

out at T = 4.2 K confirm those estimated at T = 77 K. It should be noted, however, that the analysis of the 4.2 K data involves an element of uncertainty due to the weighty presence of the low-energy recombination bands seen in Figure 2a. The present findings agree with the common prediction that the carrier effective mass in the WZ crystal phase is higher than that in the ZB phase.11−13,43 In particular, the values we find confirm that the conduction band minimum has ΓC7 character rather than ΓC8 (an issue still debated in WZ GaAs9,11,12), as the predicted effective masses for excitons involving ΓC8 states are heavier than those determined here.11−13 Moreover, our experimental results qualitatively agree with the mass anisotropy predicted in WZ between carrier motion parallel (heavier mass) and perpendicular (lighter mass) to the ĉ axis.11,12,43 An opposite mass anisotropy is calculated, however, in ref 13. In Figure 6, the splitting of the PL spectra for different orientations of B relative to ĉ is now considered. The field dependence of the Zeeman splitting ZS = E+5/6 − E−5/6 in the Voigt configuration is shown for two different data sets (blue squares and red diamonds). The solid line is a fit with |ge| = 1.4 ± 0.2 of the relation ZS = |ge|μB B, derived from eq 1 with Δ56 = 0, to the average (gray circles) of the two sets of experimental values. The obtained value of the gyromagnetic factor is slightly larger than that measured in bulk ZB InP (|gbulk e | = 1.21 at T = 80 K).44 The field dependence of the Zeeman splitting between the Γ±5 states of the A exciton in the Faraday configuration ZS = E+5 − E−5 is shown in Figure 6 by full red circles. The relation ZS = |ge − gh,//|μBB obtained by eq 2 fits well the data up to 17 T with |ge − gh,//| = 3.5 ± 0.1. At higher fields, a marked slowdown is observed. We then infer that the g-factor of holes is not constant with magnetic field, as found also in other semiconductor systems,45,46 provided that the electron g-factor is independent of B, as indicated by gray circles. In conclusion, we characterized the transport and spin properties of ensembles of high-purity wurtzite InP nanowires by photoluminescence under high magnetic fields. We found a E


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K using a liquid N-bath cryostat. PL was excited by a frequencydoubled Nd:YVO4 laser focused using a long focal length objective (spot diameter ∼10 μm), collected by the same objective, dispersed by a 0.30 m monochromator and detected by a liquid N-cooled Si CCD. Polarization of the emitted light was analyzed using a combination of a linear polarizer and a liquid crystal variable retarder in order to make all measurements insensitive to the polarization response of the optical setup. All PL spectra were normalized by the setup responses.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Figure 6. Magnetic field dependence of the Zeeman splitting, ZS, measured in the B⊥k//ĉ (gray circles) and B//k//ĉ (red circles) configurations at T = 77 K. In B⊥k//ĉ (or Voigt) geometry, the ZS has been determined by the average between the splittings extracted from the spectra shown in Figure 3b (red diamonds) and by another data set recorded under the same geometry and P = 1030 W/cm2 (blue squares). The error bars take into account the uncertainty in the determination of ZS values in each data set. The ZS is linearly dependent on the magnetic field and the solid line is a fit of eq 1 to the data. The electron g-factor is |ge| = 1.4 ± 0.2. In the B//k//ĉ (or Faraday) geometry, the ZS has been determined by the PL spectra shown in Figure 3b. It is not linear with the magnetic field for B ≥ 18 T. The solid line is a fit of eq 2 to the linear region of the data. The exciton g-factor is |ge − gh,//| = 3.5 ± 0.1.

Present Address ⊥

(S.P.) Department of Physics Faculty of Science Universiti Putra Malaysia 43400 Serdang, Selangor, Malaysia.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support of HFML-RU/FOM, member of the European Magnetic Field Laboratory (EMFL). Part of this work has been supported by EuroMagNET II under the EU contract number 228043. M.D.L. acknowledges funding by Sapienza Università di Roma under the “Avvio alla Ricerca” grant. A.P. acknowledges financial support from “Ateneo 2013” funding. The Australian authors acknowledge the Australian Research Council for the financial support and Australian National Fabrication facility and Australian Microscopy and Microanalysis Research Facility for providing access to some of the equipment used in this work.

sizable increase (14%) in the exciton reduced mass with respect to that measured in zincblende bulk and confirmed the ΓC7 symmetry of the bottom of conduction band in WZ InP. A small degree (6%) of anisotropy of the exciton reduced mass with respect to ĉ axis is derived, which is in contrast to previous results in WZ In0.1Ga0.9As NWs (higher anisotropy22) and mixed-phase GaAs quantum disks (null anisotropy23). A stronger anisotropy has been found in the Zeeman splitting, which for B⊥ĉ is nearly half of that found for B//ĉ. The electron g-factor (|1.4|) is slightly greater than its counterpart in ZB and the g-factor for holes appears to depend on magnetic field for B > 17 T. Finally, under the effect of magnetic field a 20% circular polarization dichroism of the emitted photons is observed for the Faraday configuration only, in agreement with the WZ phase of NWs. These results enrich the understanding of the electronic properties of nanowires and prompt further investigations of this material system that has great potential in many nanoelectronic applications. Experimental methods. The samples were grown by metal organic chemical vapor deposition on semi-insulating InP (111)B substrates. Au nanoparticles of 30 nm diameter were used to catalyze the vapor−liquid−solid growth of NWs. The precursors were trimethylindium (TMIn) and phosphine (PH3). NW growth was carried out for 20 min at 480 °C with a V/III ratio of 350. The crystal structure of the NWs was investigated using a Jeol 2100F transmission electron microscopy operated at 200 kV. Photoluminescence (PL) measurements on NW ensembles were performed at different temperatures using a He closed-cycle cryostat. PL was excited by a frequency-doubled Nd:YVO4 laser (λ = 532 nm) with a spot diameter of ∼200 μm, dispersed by a 0.75 m monochromator, and detected by a liquid N-cooled Si CCD. PL measurements on single nanowires lying on a Si substrate were performed in a liquid N-flow cryostat using a 50× microscope objective with a 0.55 numerical aperture (spot diameter ∼1 μm). For PL measurements under magnetic field, samples were placed in a water-cooled Bitter magnet at T = 77

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